WO2025106666A1

WO2025106666A1 - Li 2zrcl 6-li 6ps 5cl solid electrolyte pairing for dual solid electrolyte solid-state batteries

Info

Publication number: WO2025106666A1
Application number: PCT/US2024/055916
Authority: WO
Inventors: Elias SEBTI; Tyler PENNEBAKER; Raphaele CLEMENT
Original assignee: University of California Berkeley; University of California San Diego UCSD
Current assignee: University of California Berkeley; University of California San Diego UCSD
Priority date: 2023-11-14
Filing date: 2024-11-14
Publication date: 2025-05-22
Anticipated expiration: 2026-05-14

Abstract

Pairing of Li₂ZrCl₆ (LZC) and Li₆PS₅Cl (LPSC) solid electrolytes, which increases the effective electrochemical and chemical stability window of the electrolyte of lithium-based solid-state batteries, and enables stable cycling with a high voltage cathode and a metallic anode, with the potential to greatly enhance solid-state battery energy density.

Description

Li2ZrCl6-Li6PS5Cl Solid Electrolyte Pairing for Dual Solid Electrolyte Solid-State Batteries CROSS REFERENCE TO RELATED APPLICATIONS This application claims the benefit under 35 U.S.C. Section 119(e) of the following co-pending and commonly-assigned application: U.S. Provisional Application Serial No.63/598,827 filed on November 14, 2023, by Elias Sebti, Tyler Pennebaker and Raphaële Clément, entitled “Li2ZrCl6- Li₆PS₅Cl Solid Electrolyte Pairing for Dual Solid Electrolyte Solid-State Batteries,” attorneys’ docket number G&C 30794.0851USP1 (UC-2024-868-1); which application is incorporated by reference herein. BACKGROUND OF THE INVENTION 1. Field of the Invention. This invention relates to a Li₂ZrCl₆-Li₆PS₅Cl solid electrolyte pairing for dual solid electrolyte solid-state batteries. 2. Description of the Related Art. (Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.) When put together, the limited electrochemical stability window of solid electrolytes (SEs) and the implementation of high voltage cathodes or high energy density metallic anodes seem mutually exclusive, casting doubt on the possibility of energy density improvements in solid-state batteries (SSBs). No known SE composition is stable against both metallic anodes and high voltage oxide cathodes. Work to date has mainly focused on pairing electrolytes within the same chemical family (i.e. pairing of two sulfide-based SEs) and with comparable electrochemical stability windows, limiting the potential for use in high voltage SSBs as degradation will occur at one or both of the electrodes. Over the past couple of years, however, several groups have reported on the combined use of halide and sulfide electrolytes, with mixed results. Wu et al. found success with a Na-based SSB with a NaCrO₂+Na_2.25Y_0.25Zr_0.75Cl₆ cathode composite and a Na₃PS₄ SE layer, displaying record-breaking 89.3% capacity retention over 1000 cycles, as the chloride is stable at high potentials while the sulfide passivated against a Na-Sn anode. [21] Other groups have used a thin layer of sulfide SE to prevent contact between a chloride SE and the metallic anode, resulting in stable cell cycling due to the passivating behavior of the LPSC/anode interface. [2, 14, 16, 12, 13, 15, 26, 27] Those studies, however, did not assess the compatibility of the sulfide and chloride solid electrolytes, which can also affect device-level performance. In fact, chloride- sulfide interfaces are not always chemically compatible, as highlighted by Samanta et al. [28] for various chloride-argyrodite Li6PS5Cl (LPSC) combinations. Further, Rosenbach et al. employed a bilayer approach with Li₃InCl₆ (LIC) and LPSC in LiNi0.8Mn0.1Co0.1O2:LIC|LIC|LPSC|Li-In cells. During cycling, they observed significant impedance growth at the LIC|LPSC interface that was attributed to reactivity between LIC and LPSC, evidenced by the observation of InS- moieties with time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). [4] What is needed, then, are chemically (and electrochemically) stable solid electrolyte pairings that can be implemented with high voltage cathodes and high energy density anodes. The present invention satisfies this need. SUMMARY OF THE INVENTION A solid electrolyte (SE) pairing of Li₂ZrCl₆-Li₆PS₅Cl (LZC-LPSC) exhibits stable behavior under standard cycling operations in single layer and bilayer solid- state batteries (SSBs). LZC is used as the catholyte material, withstanding the highly oxidative potentials in the cathode composite due to its stability up to >4V vs. Li/Li⁺ [1,2]. An LPSC SE layer is placed in contact with the metallic anode, preventing it from contacting the easily reduced LZC, and forms a passivated interface against the anode through decomposition into ionically conductive and electronically insulating products (LiCl, Li3P, and Li2S) [3]. As such, interfaces at both electrodes are stabilized, promoting stable cycling of a high-energy density SSB. While maintaining stable interfaces with limited resistance growth is paramount to capacity retention in SSBs, the use of two SEs entrains the possibility of resistance growth at the SE/SE interface since halide and sulfide SEs can react, forming insulating interphases that limit Li conduction across the interface. [4] The LZC-LPSC pairing, however, does not decompose until 260°C, and is shown to behave stably without the large interfacial impedance growth even after a 24-hour heat treatment at 110 °C. This lack of decomposition at the interface constitutes an important demonstration of a kinetically stable interface with facile Li transport across the phase boundary. The LZC-LPSC combination also demonstrates stable cycling in single layer and bilayer SSBs, which is highly relevant to real world applications. Overall, the LZC-LPSC system is a conductive, scalable, and kinetically- stabilized system with high promise for implementation in energy-dense dual SE SSBs with steady long-term cycling. BRIEF DESCRIPTION OF THE DRAWINGS The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. Figure 1: Schematic of a solid electrolyte in a single layer configuration and bilayer configuration. Figure 2: Scanning electron microscopy (SEM) images obtained at an accelerating voltage of 2 kV on the unmixed solid electrolytes after a 12 h heat treatment at 300°C: (a) LZC HT-300, (b) LPSC HT-300, and (c) LPS-HT300. The results indicate a significant spread in particle size and shape in all samples. Figure 3: (a) XRD patterns, (b) ⁶Li Nuclear Magnetic Resonance (NMR) spectra, and (c) electrochemical impedance spectroscopy (EIS) spectra collected on as milled and heat treated LZC. Figure 4: Analysis of oxygen (O) content in heat treated LZC (LZC HT-300) (a) SEM micrograph of LZC particles coated with platinum (Pt). The probed region is delineated in white. (b) Windowless EDS map showing distribution of O in the same probed region likely encompassing multiple, agglomerated LZC particles. Regions comprising more or less O than average are delineated in white. An acceleration voltage of 3 kV was used. (c) Weight fraction of various elements obtained from the spectrum collected on the entire region shown in (b), indicating negligible O contamination of the sample (average O content of 2.9 wt.%). Lithium (Li) is not observed due to the extremely low efficiency of Li X-ray production in bonded states and high absorption. Carbon (C) contamination in the SEM chamber is evident, while silicon (Si) arise from the substrate, and gallium (Ga) is implanted during the preparation of the cross-section. (d) Variation in average O content across different regions of the sample. The average O contents in O-rich and O-poor regions, delineated in Figure (b), do not differ by more than 1.2 wt.%, within error of the measurement. Figure 5: (a) XRD patterns, (b) ⁶Li NMR spectra, and (c) EIS spectra collected on as received and heat treated LPSC. Figure 6: Analysis of O content in heat treated LPSC (LPSC HT-300). (a) SEM micrograph of LPSC particle coated with Pt. The particle is delineated in white. (b) Windowless EDS map showing distribution of O in the same particle of LPSC. Regions comprising more or less O than average are delineated in white. An acceleration voltage of 3 kV was used. (c) Weight fraction of various elements obtained from the spectrum collected on the entire region shown in (b), indicating negligible O contamination of the sample (average O content of 1.0 wt.%). The rather small amount of Cl in this sample cannot be probed at a low accelerating voltage of 3 kV, while Li is not observed due to the extremely low efficiency of Li X-ray production in bonded states and high absorption. C contamination in the SEM chamber is evident, while Si arises from the substrate, and Ga is implanted during the preparation of the cross-section. (d) Variation in average O content across different regions of the sample. The average O contents in O-rich and O-poor regions, delineated in Figure (b), do not differ by more than 2.2 wt.%, within error of the measurement. Figure 7: (a) XRD patterns, (b) ³¹P NMR spectra, and (c) EIS spectra collected on as received and heat treated LPS. Figure 8: Analysis of O content in heat treated LPS (LPS HT-300). (a) SEM micrograph of LPS particle coated with Pt. The particle is delineated in white. (b) Windowless EDS map showing distribution of O in the same particle of LPS. Regions comprising more or less O than average are delineated in white. An acceleration voltage of 3 kV was used. (c) Weight fraction of various elements obtained from the spectrum collected on the entire region shown in (b), indicating negligible O contamination of the sample (average O content of 3.5 wt.%). Li is not observed due to the extremely low efficiency of Li X-ray production in bonded states and high absorption. C contamination in the SEM chamber is evident, while Si arises from the substrate, and Ga is implanted during the preparation of the cross-section. (d) Variation in average O content across different regions of the sample. The average O contents in O-rich and O-poor regions, delineated in Figure (b), differ by no more than 5 wt.%. Figure 9: Results from synchrotron XRD with in-situ heating experiments. Diffraction patterns obtained for a) LZC+LPS and b) LZC+LPSC mixed samples as a function of temperature. Patterns were acquired during a temperature ramp from room temperature up to 300 °C and during a subsequent 300 °C temperature hold. c) Rwp values resulting from sequential refinement of the LZC+LPSC diffraction patterns during the in-situ experiment using a two-phase (LZC and LPSC) model. The Rwp value increases starting at approximately 260 °C, indicating sample degradation. The LZC+LPS patterns could not be refined due to the sample’s low crystallinity. Figure 10: Analysis of annealed LZC and LPS samples, and mixed pellets heat treated for 24 hours at various temperatures (e.g., HT-110 stands for a treatment at 110 °C). (a) XRD patterns obtained on all samples. Degradation phases observed in the HT-300 sample are marked with * for LiCl and ° for LixZryP2S6 phases. (b) ⁶Li and (c) ³¹P ss-NMR spectra obtained on the same samples. The unmixed spectrum in panel (a) is the sum of two spectra, where each spectrum was obtained on one of the individual phases without mixing. Low intensity, unidentified signals in the ³¹P spectrum are indicated with a circle (°). (d) Zr 3d XPS spectra obtained on pristine Li₂ZrCl₆, and on the HT-110, HT-150, and HT-300 mixed pellets. (e) Raman spectra collected at various spots on the LZC-LPS HT-300 sample as well as the annealed unmixed LPS. Spectra for ZrS₂ and ZrS₃ are digitized from reports: ZrS₂ data from Mañas-Valero et al. [29] , ZrS3 data from Jin et al. [30] Figure 11: Analysis of annealed LZC and LPSC samples, and mixed pellets heat treated for 24 hours at various temperatures (e.g., HT-110 stands for a treatment at 110 °C). (a) XRD patterns obtained on all samples. Degradation phases observed in the HT-300 sample are marked with * for LiCl and ° for LixZryP2S6 phases. (b) ⁶Li and (c) ³¹P ss-NMR spectra obtained on the same samples. The unmixed spectrum on panel (b) is the sum of two spectra, where each spectrum was obtained on one of the individual phases without mixing. (d) Zr 3d XPS spectra obtained on pristine Li₂ZrCl₆, and on the HT-110, HT-150, and HT-300 mixed pellets. (e) Raman spectra collected at various spots on the LZC-LPSC HT-300 sample as well as the annealed unmixed LPSC. Spectra for ZrS₂ and ZrS₃ are digitized from reports: ZrS₂ data from Mañas-Valero et al.[29] , ZrS3 data from Jin et al. [30] Figure 12: Analysis of annealed LZC and small-particle LPSC samples, and mixed pellets heat treated for 24 hours at various temperatures (e.g., HT-110 stands for a treatment at 110 °C). (a) XRD patterns obtained on all samples. Degradation phases observed in the HT-300 are marked with * for LiCl and ° for LixZryP2S6 phases. (b) ⁶Li and (c) ³¹P ssNMR spectra obtained on the same samples. The "unmixed" spectrum on panel (b) is the sum of two spectra collected on pure LZC and pure LPSC (after annealing), scaled to match the 1:1 molar ratio. Figure 13: EIS Nyquist plots for samples made with LPSC and LZC, before and after a heat treatment at 110 °C for 24 hours under 70 MPa of applied pressure (HT-110). All spectra were obtained at 70 MPa of applied pressure with an excitation voltage of 30 mV. These spectra were collected on (a) a bilayer pellet made from pristine LPSC and pristine LZC, (b) a bilayer pellet made from LPSC and LZC separately annealed at 300 °C for 12 hours (labeled “HT-300”), and (c) a pristine LZC sample (no annealing). Figure 14: ⁶Li VT-NMR on the LZC-LPSC HT-25 sample. All spectra were obtained at 18.8 T and 30 kHz of spinning speed. In these spectra, the LPSC resonance was fixed to 1.3 ppm to avoid ppm drift from shim coil heating effects. Figure 15: Galvanostatic cycling curves for (a) single and (b) bilayer solid- state batteries using LZC and LPSC. Cells were cycled at C/3 between 4-2.5 V vs. Li/Li⁺ after a first cycle at C/10. (c) Capacity is plotted as a function of cycle number to track capacity retention. The cathode was made of 40% wt. LFP, 57% wt. LZC, 3% wt. VGCF. Fluctuations in the cell capacity are due to laboratory (diurnal) temperature variations. For electrochemical performance evaluation, cells were cycled under 50 MPa at room temperature with Li0.5In acting as the counter electrode. Capacity utilization and cell cycling was evaluated using Neware Instrument cyclers (https://newarebattery.com incorporated by reference herein) starting with 0.1C by long cycling at 0.3C. All cells were cycled at room

evidenced by the capacity fluctuations due to the temperature fluctuations correlated to time of day." Figure 16A-16B. Flowcharts illustrating methods of making a solid electrode, electrode, or battery. Figure 17. Schematic of an example cold pressing apparatus that can be used to manufacture pelletized layers described herein. Figure 18. Schematic of a vice that can be used during an example hot pressing process according to one or more embodiments. Obtained from Figure 1 of [25]. Figure 19. Anode-less battery according to one or more embodiments. Figure 20. Example battery heating and operating circuit. DETAILED DESCRIPTION OF THE INVENTION In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. Technical Description Example Solid Electrolyte Configurations Figure 1 illustrates example configurations of a solid electrolyte in a lithium solid state battery. The battery comprises a composite cathode layer comprising Li2ZrCl6 (LZC), wherein the LZC is a catholyte; and a solid electrolyte layer comprising Li6PS5Cl (LPSC). The composite cathode layer and the solid electrolyte layer are in direct contact in the case of the single layer system. The composite cathode layer and the solid electrolyte layer are coupled via the catholyte layer in the bilayer system. Figure 1A illustrates the single layer configuration wherein the solid electrolyte layer is between the composite cathode layer and the anode of the battery. Figure 1B illustrates a bilayer configuration comprising the solid electrolyte layer in contact with a separate catholyte layer comprising LZC, wherein the catholyte layer is between the solid electrolyte layer and the composite cathode layer. As illustrated in Figures 1A and 1B, the catholyte layer and the composite cathode layer each comprise a first plurality of particles comprising the LZC, and the solid electrolyte layer comprises a second plurality of particles comprising the LPSC. An average particle size and size distribution of the particles are such that physical contact between the first plurality of particles and the second plurality of particles promotes lithium transport between the LPSC and LZC while suppressing detrimental reactivity between the LZC and the LPSC during normal battery operation. In one or more embodiments, the catholyte layer and/or the composite cathode layer each comprise a first plurality of particles comprising the LZC and a first diameter D1 in a range 10 nm ≤ D1 ≤ 2 microns. The solid electrolyte layer comprises a second plurality of particles comprising the LPSC and a second diameter D2 in a range of 10 nm ≤ D2 ≤ 10 microns. In one or more embodiments, the catholyte layer and/or the composite cathode layer each comprise ball milled particles comprising the LZC. In yet further embodiments, the catholyte layer and/or the composite cathode layer each comprise particles comprising the LZC having a size distribution, disorder, crystallinity, and conductivity characteristic of, or equivalent to, that formed by ball milling a powder comprising the LZC. In some embodiments, the LZC is crystalline with defects and/or disorder and comprises a conductivity greater than 0.1 mS per cm. Figure 1 further illustrates the battery comprises a current collector (e.g., metal) on top of the composite cathode layer and underneath the anode layer. In the illustrated embodiment, the composite cathode layer further comprises the LZC mixed with a cathode active material and a carbon conductive additive. In one or more examples, the cathode active material comprises a lithium transition metal compound and the anode layer comprises a lithium alloy or lithium metal. Example lithium transition metal compounds include, but are not limited to, a lithium transition metal oxide, a lithium transition metal oxyfluoride, or a lithium transition metal polyanionic compound. First Example: Evaluation of LZC-LPSC stability a. LZC preparation LZC was synthesized by mechanochemical synthesis (3 hours at 550 rpm) from a 2:1 molar ratio of LiCl and ZrCl₄ with the inclusion of 10% weight excess of ZrCl4 to compensate for its preferential adhesion to the milling media. An X-ray diffraction (XRD) pattern (Figure 3(a)) and ⁶Li NMR spectrum (Figure 3(b)) obtained on the as-milled material confirm its identity as the P3^തm1 polymorph with a small LiCl impurity. [5] A Li-ion conductivity of 0.167 mS cm^-1 and 0.431േ0.008 eV activation energy is measured with EIS (Figure 3(c)). Samples intended for chemical reactivity testing were annealed at 300 °C for 12 hours in a sealed ampule under vacuum. The heat treatment preserves the LZC crystal structure and causes LiCl to be uptaken by the LZC as evidenced by its disappearance from the XRD pattern and ⁶Li NMR spectrum (Figures 3(a) and 3(b)). Following the 300°C anneal, the particles appear to have a wide range of sizes, from approximately 10 nm to 2 microns, as observed with scanning electron microscopy (SEM) (Figure _{2(a)). The ionic conductivity of the annealed LZC decreases to 5.26ൈ 10ିସ mS cm-1} and the activation energy increases to 0.495േ0.009 eV (Figure 2(c)). Given that the chemical stability of chloride and sulfide solid electrolytes has been found to depend sensitively on the impurity O content, the O content of the LZC sample annealed to 300°C was probed using windowless EDS. This technique allows for high resolution analysis of light elements. Here, an air-free workflow was adopted to minimize sample contamination, leveraging a SemiLab shuttle for sample transfers. The solid electrolyte powder was first deposited onto a Pt/Pd coated Si wafer inside an Ar-filled glovebox. A protective Pt layer was then deposited on top of the particles. Cross-section preparation was conducted using a focused ion beam (FIB). A trench was milled to clear out the cross-sectional face, and the cross-section was further polished with a lower current beam. To increase the EDS mapping resolution, the sample volume from which X-rays are generated is minimized, and as much as possible kept smaller or on the order of the particle size. For this, a low accelerating voltage of 3 kV was used, limiting the probing depth to less than ~1 µm, which likely encompasses multiple, agglomerated particles of LZC. The low accelerating voltage used for O analysis means that most of the other elements’ X-ray K-lines are only weakly excited or not excited at all, and the resulting EDS spectra cannot be used for traditional EDS quantification. The results from this analysis are shown in Figure 4, and indicate minimal O contamination of the sample (average O content of 2.9 wt.%), and minimal variation in O content throughout the sample (difference in O content between O-rich and O-poor regions is no more than 1.2 wt.%, which is within error of the measurement). It is therefore safe to assume that O plays a minimal role in the stability of the LZC electrolyte. b. LPSC preparation LPSC was purchased from a commercial vendor. Its XRD pattern and ³¹P _{NMR spectra (Figures 5(a) and 5(b)) confirm its F4} ^ത _{3m crystal structure and agree} well with previous reports. [6] Its ionic conductivity and activation energy are 2.19 mS cm^-1 and 0.362േ0.007 eV, respectively (Figure 5(c)). Samples intended for chemical reactivity testing were annealed at 300°C for 12 hours in a sealed ampule under vacuum. The heat treatment preserves the LPSC crystal structure as evidenced by the retention of all reflections in the XRD pattern and identical ³¹P NMR spectrum (Figures 5(a) and 5(b)). Following heat treatment, the particles appear to have a wide range of sizes, from approximately 10 nm to 10 microns, as observed with SEM (Figure 2(b)). The ionic conductivity and activation energy of the annealed LPSC increase to 2.60 mS cm^-1 and 0.395േ0.006 eV, respectively (Figure 4(c)). The O content of the LPSC sample annealed to 300°C was probed using windowless EDS (similar sample preparation as for LZC) and a low accelerating voltage of 3 kV was used, limiting the probing depth to less than ~1 µm. The probed volume is approximately within the size of a single LPSC particle, as shown in Figure 6(a). The windowless EDS results, shown in Figure 6 (b)-(d) indicate minimal O contamination of the sample (average O content of 1.0 wt.%), and minimal variation in O content throughout the sample (difference in O content between O-rich and O-poor regions is no more than 2.2 wt.%, which is within error of the measurement). It is therefore safe to assume that O plays a minimal role in the stability of the LPSC electrolyte. c. Li₃PS₄ (LPS) preparation Li3PS4 was purchased from a commercial vendor. Its XRD pattern and ³¹P NMR spectra (Figures 7(a) and 7(b)) confirm its Pnma crystal structure and agree well with previous reports. [7,8] Its ionic conductivity and activation energy are _{7.26ൈ 10ିଶ mS cm-1 and 0.392േ0.005 eV, respectively (Figure 7(c)).} Samples intended for chemical reactivity testing were annealed at 300 °C for 12 hours in a sealed ampule under vacuum. The heat treatment leads to a slight decrease in crystallinity (Figure 7(a)) and increase in the fraction of isolated ^^-type PS4 tetrahedra in the ³¹P NMR spectrum (peak at 86.5 ppm in Figure 7(b)). [9] Following heat treatment, the particles appear to have a wide range of sizes, from approximately 10 nm to 2 microns, as observed with SEM (Figure 2(c)). After _{annealing, the ionic conductivity decreases to 5.47ൈ 10ିଶ mS cm-1 and the activation} energy increases to 0.421േ0.012 eV (Figure 7(c)). The O content of the LPS sample annealed to 300°C was probed using windowless EDS (similar sample preparation as for LZC) and a low accelerating voltage of 3 kV was used, limiting the probing depth to less than ~1 µm. The probed volume is approximately within the size of a single LPS particle, as shown in Figure 8(a). The windowless EDS results, shown in Figure 8 (b)-(d) indicate minimal O contamination of the sample (average O content of 3.5 wt.%), and minimal variation in O content throughout the sample (difference in O content between O-rich and O-poor regions is no more than 5.0 wt.%). It is therefore safe to assume that O plays a minimal role in the stability of the LPS electrolyte. d. Heat treatment procedure As described above, LZC, LPSC, LPS were individually annealed at 300 °C for 12 hours under vacuum ahead of the mixed heat treatment testing in order to avoid evolution of the individual phases during reactivity tests. In order to test the reactivity between the SEs, powders were ground together in a mortar and pestle for 20 min to create a homogeneous mixture and subsequently pelletized to generate the most contact surface area between the phases. The contact area between LZC and LPS(C) in these pellets is more significant than that would be present in a SSB as contact area would be limited to a 2D plane within the cell (either at cathode composite-SE layer interface in a single layer cell or at the interface between SEs in a bilayer configuration), accelerating the formation of decomposition products. While the focus of this exploration is the LZC-LPSC pairing, the LZC-LPS pairing serves as a foil here, highlighting the impressive stability of the LZC-LPSC system. As an initial test of chemical compatibility, 1:1 molar ratios of as-milled LZC and as-received LPS(C) were mixed and examined using synchrotron X-ray diffraction (XRD) during in-situ heating. The experiment involved a temperature ramp from room temperature up to 300 °C, followed by a 300 °C temperature hold. The results, presented in Figure 9, indicate that while the LZC-LPS mixture starts to degrade at approximately 90 °C, the LZC-LPSC mixture is stable until approximately 260 °C. To further understand the degradation mechanisms of these mixtures, 1:1 molar ratios of LZC and LPS(C), both previously annealed at 300 °C for 12 hours, were mixed and exposed to a 24-hour heat treatment at 25, 70, 110, 150, and 300 °C. Pellets were sealed in dried quartz ampules under vacuum before heat treatment. Mixed-phase heat treated samples (HT-) are labeled according to the temperature they were exposed (i.e. HT-110 experienced 24 hours at 110 °C). For the LZC-LPS system, the onset reaction temperature of 90 °C observed by in-situ heating synchrotron XRD is corroborated by the analysis of heat-treated mixed pellets. XRD analysis of the mixed pellets (Figure 10(a)) reveals the appearance of LiCl in samples treated at 110°C and above, indicating degradation of the original LZC and LPS phases, with (an) additional Li_xZr_yP₂S₆ phase(s) related to Li₂MP₂S₆ (M = V, Mn, Fe, Co, Ni, and Zn) compounds reported by Sundaramoorthy et al.[31] present in the HT-300 sample. The ⁶Li and ³¹P ss-NMR spectra obtained on the LZC- LPS pristine and heat-treated mixed pellets are shown in Figures 10(b) and 10(c). The ⁶Li LPS resonance begins to broaden for the HT-110 sample while another small broad resonance, assigned to ^^-LPS, is observed at 1.37 ppm with a corresponding growth of the ³¹P resonance at 89.1 ppm in Figure 10(c), indicating a degradation of the LPS phase. After 24 hr at 300 °C, some LZC remains while LPS has completely decomposed, generating LiCl and LixZryP2S6, in good agreement with the XRD results. While the LZC ⁶Li resonance does not obviously evolve up to 150 °C, the Zr 3d XPS spectra presented in Figure 10(d) point to the formation of ZrS₂ and ZrS₃ species, indicating LZC is reacting with the LPS. Raman spectra shown in Figure 10(e) confirm the presence of S₈, P₂S₆ ^4-, and ZrS_x species in the sample after degrading during the 300 °C heat treatment. The in-situ heating synchrotron XRD results presented earlier indicated that the LZC-LPSC combination is remarkably stable up to approximately 260 °C, findings that are once again corroborated by the analysis of heat-treated mixed pellets. Figure 11(a) shows the XRD results obtained on pristine LZC and LPSC, and on mixed pellets. Only after a 300°C heat treatment does the pellet indicate any sign of degradation, with the formation of both LiCl and LixZryP2S6 phases similarly to the LZC-LPS system. In Figures 11(b) and 11(c), the ⁶Li and ³¹P ss-NMR spectra obtained on LZC-LPSC samples do not show any signs of evolution for all samples heat treated at or below 150 °C. Heat treatment at 300 °C decomposes the LPSC completely, leaving behind ^^-Li3PS4, LixZryP2S6, and LiCl. Stability of the LZC- LPSC combination up to 150 °C is further confirmed by Zr 3d XPS in Figure 11(d) as the pristine LZC signal is retained for the HT-150 sample. LZC, ZrS2, ZrS3, PS4^3-, and P₂S₆ ^4- moieties are all detected by Raman spectroscopy (see Figure 11(e)) in the HT-300 sample, confirming that a reaction between LZC and LPSC occurred during the heat treatment. Neither of the two SE pairings tested here is thermodynamically stable, but the LPSC-LZC pairing does not react up to approximately 260 °C when the powders are mixed, pelletized, and heated together, while the LPS-LZC pairing reacts at around 90 °C. The higher chemical compatibility of the LPSC-LZC pairing is expected to translate to a higher (electro)chemical stability when the dual electrolyte is cycled in an SSB. Although the dual-electrolyte chemistry plays a key role in reactivity (thermodynamic factors), the limited kinetics of the decomposition reactions are important as well in stabilizing the LZC-LPSC pairing. Although the SEs of interest have rather different particle sizes (see Figures 2(a), 2(b), 2(c)), the inventors note that particle size does not seem to play a major role in the stability of the LZC-LPSC system. Indeed, further tests were conducted using ~1 μm-sized LPSC particles (as specified by vendor), and yielded similar results to those obtained with larger LPSC particles (> 1 μm), as shown in Figure 12. No reactivity is observed in the HT-150 sample, and degradation is only observed after a heat treatment at 300 °C. Second Example: Evaluation of LZC-LPSC conductive properties Bilayer pellet analysis Bilayer pellets were pressed by sequential layer densification of LPSC (pressed first at 350 MPa), then LZC (pressed at 175 MPa), in a custom Ti/PEEK/Ti plunger cell. Two dense bilayer pellets were formed from the as-milled or as-received phases and the annealed phases (denoted as HT-300) with relative densities of 94% and 96%, respectively. In a first instance, we compare the room temperature total conductivity and activation energy of the LZC/LPSC and LZC HT-300/LPSC HT-300 pellets to determine the impact of a 300°C anneal. We then investigate the impact of a 24-hour heat treatment at 110 °C under 70 MPa of applied pressure (mimicking industrially relevant hot pressing) on the room temperature total conductivity and activation energy of the LZC HT-300 / LPSC HT-300 bilayer pellet. The EIS data obtained on the LPSC/LZC pellets in Figure 13 is fit with a parallel constant phase element (CPE) and resistor (mimicking conduction through the bulk of the material) in series with a CPE (blocking electrode behavior), indicating that an interfacial component cannot be separated from the bulk even at low temperatures (EIS system limited to -30 °C). We observe a significant decrease in total conductivity from 0.304 mS cm^-1 for the LZC/LPSC bilayer pellet to 1.30 ^S cm^-1 for the LZC HT-300/LPSC HT-300 bilayer pellet (Figure 13(a)). This decrease is attributed to a large decrease in the conductivity of the LZC component upon crystallization, as the conductivity of this component alone decreases from 0.167 mS cm^-1 to 0.526 ^S cm^-1 after a 24-hour 300°C anneal, as shown in Figure 13(b)). The activation energy increases from 0.417 േ 0.008 eV for the LZC/LPSC bilayer to 0.561േ0.015 eV for the LZC HT-300 /LPSC HT-300 bilayer. For the LZC HT-300 / LPSC HT-300 pellet, this activation energy is larger than either of the activation energies for the individual HT-300 phases (LZC HT-300: 0.495േ0.009 eV; LPSC HT-300: 0.395േ0.006 eV), indicating that the activation energy barrier associated with interfacial transport is significant. After a 110 °C heat treatment at 70 MPa for 24 hours, the conductivity of the LPSC HT-300/LZC HT-300 bilayer increases from 1.30 ^S cm^-1 to 1.52 ^S cm^-1 (Figure 13(c)), consistent with the relative density increase (96 to 100%). The activation energy decreases from 0.561േ0.015 eV to 0.493േ0.008 eV, which is very close to the value obtained for pure LZC HT-300 (0.495േ0.009 eV). This suggests that the interfacial activation energy is small, and significantly reduced compared to prior to the 110°C heat treatment. These results lead the inventors to believe that hot calendering of thin SE membranes could be a viable and scalable way to form conductive bilayer SE components from LZC and LPSC for incorporation into an SSB. The pressure, speed, and temperature of the calenders should be selected to limit potential reactivity or conductivity decreases of individual phases. Third Example: NMR characterization Variable temperature (VT) ⁶Li NMR on the HT-25 sample (both phases previously annealed separately at 300°C, mixed, and heat treated at 25 °C for 24 hours), shown in Figure 14, displays signs of Li chemical exchange between the two phases. In NMR, as the sample temperature increases and the rate of chemical exchange between distinct local environments increases, the resonances corresponding to the sites in exchange grow slightly closer together, broaden, and eventually coalesce at their weighted average position when the crossover point is reached. [10,11] As such, ⁶Li VT-NMR on the mixed LZC-LPSC system can provide insight into the conductive properties of the interface between the two phases through observation of Li chemical exchange. For the HT-25 sample, at low temperatures of 308 K, asymmetric tails on the LZC (-1.1 ppm) and LPSC (1.3 ppm) signals point towards one another, indicating a small degree of exchange. As the sample temperature is further increased up to 341 K, each signal progressively broadens and shifts towards the other, confirming that the interface between the phases allows for inter-phase Li hops. Despite the large particle size of each individual phase, which had each been previously heat treated, and the low densification pressures (measurements conducted on a powder that was ground in a mortar and pestle from the densified HT-25 pellet and densified by hand when filling the rotor), the detection of Li hopping across the phase boundary demonstrates that the interface between the phases is highly conductive. Fourth Example: Evaluation of cycling behavior Dual-SE SSBs were assembled in single layer and bilayer configurations with a Li-In anode and a cathode composite comprised of the LZC catholyte, LiFePO4 (LFP) cathode active material, and vapor-grown carbon fibers (VGCF) conductive additives. In the single layer cells, the LPSC SE layer was placed in direct contact with the composite cathode. In bilayer cells, a thin buffer layer of LZC was placed between the LPSC SE layer and the cathode composite, preventing contact between the oxidation prone sulfide SE and the LFP. Diagrams for both cell architectures are provided in Figure 15(c). Both cells display similar first cycle capacities of approximately 100 mAh g^-1 at a C/10 current rate, with little overpotential (see Figures 15(a) and 15(b)). Upon increasing the rate to C/3 in the second cycle, cells overpotentials increase and discharge capacities decrease to approximately 75 mAh g^-1. After 140 cycles, the discharge capacities for the single layer and bilayer cells are 56.8 and 69.6 mAh g^-1, respectively. The faster capacity fade observed for the single layer configuration is due to sulfide SE oxidation from contact between the LPSC layer and cathode composite, highlighting the long-term cycling benefits of preventing contact between the CAM and SE layer with the LZC buffer layer. The stable cycling performance of the LZC/LPSC bilayer cell indicates that the SE pairing could be a viable option for further bilayer SSB development. Nevertheless, the cell capacity is low relative to the theoretical capacity of LFP (170 mAh g^-1) even during the initial C/10 cycle, indicating that further optimization of the cell construction is required to achieve greater cathode utilization. In summary, the experimental results presented herein demonstrate: 1. LZC-LPSC pairing is a highly conductive, stable solid electrolyte pairing with high potential for implementation in energy-dense solid-state batteries. While LZC-LPSC is not thermodynamically stable against decomposition, its large particle sizes enable a kinetic stabilization that preserves the bulk structure and conductive properties of both constituents, even under exposure to high temperature conditions. 2. LZC-LPSC forms near 100% dense bilayer pellets from cold pressing, highlighting the promising mechanical properties of the pairing. 3. The LZC-LPSC phase boundary is highly conductive to Li, even under cold pressing or low-pressure conditions. 4. Cells assembled with the LZC-LPSC solid electrolyte pairing display stable cycling behavior over 150 cycles at room temperature. Fifth Example: Particle Size analysis A rough estimate of the particle size distribution of the LZC, LPSC and LPS solid electrolytes after annealing to 300°C was obtained using a GeminiSEM560 scanning electron microscope (SEM), an accelerating voltage of 2 kV, an electron current of 50-100 pA and an SE2 Everhart-Thornley detector. The Zeiss SmartSEM software was used for SEM image collection and for driving the microscope. Example Process Steps for manufacturing a solid electrolyte useful in a lithium ion battery. Figure 16A illustrates a method of making a solid electrolyte combined with a catholyte, comprising the following steps. Block 1600 represents obtaining a first powder comprising LZC and a second powder comprising LPSC. The first powder can be manufactured by ball milling or an equivalent method. Block 1602 represents densifying the first powder to form a first pelletized layer or densifying the second powder to form the second pelletized layer – i.e., densifying the first (second) powder to form a first (second) pelletized layer. Block 1604 represents depositing the second (first) powder on the first (second) pelletized layer and densifying the second (first) powder on the first (second) pelletized layer to form the second (first) pelletized layer. In one embodiment, the densifying comprises cold pressing. Figure 17 illustrates a cold pressing process wherein pressure is applied with a hydraulic press at room temperature. In one embodiment, the cold pressing at room temperature comprises applying a pressure of at least 350 MPa for at least 1 minute to the second powder to form the second pelletized layer; and applying a pressure of a least 175 MPa for at least 1 minute to the first powder comprising the LZC when the first powder is on top of the second pelletized layer. In one or more embodiments, the densifying comprises or further comprises hot pressing or hot rolling (calendering) to form a more intimate interfacial contact between the pelletized layers. Figure 18 illustrates a hot-pressing process wherein the sample is previously cold pressed in a cell, then the sample cell is clamped into a vice (e.g., at 70 MPa), and then the sample cell is placed in environmental chamber for 24 hours at 110°C. Refs [21-23] describe example processes for cold and hot pressing processes in solid electrolytes that can be used for the cold and hot pressing described herein. Ref. [23] describes an example hot calendaring process for solid-state batteries that can be used for the densifying. Block 1606 represents the solid electrolyte and catholyte formed by the process, wherein the first pelletized layer and the second pelletized layer form a bilayer, or wherein the first pelletized layer is a solid electrolyte that is part of a densified composite cathode in a single layer configuration. Block 1608 represents optionally combining the electrolyte with the electrodes of a battery. In one embodiment, the combining comprises pressing the first pelletized layer to a cathode and pressing the second pelletized layer to an anode. Figure 16B illustrates a method of making solid electrolyte combined with a catholyte according to another embodiment. Block 1610 represents combining an LZC powder comprising the LZC compound with a first binder in a LZC solution; Block 1612 represents combining an LPSC powder comprising the LPSC compound with a second binder in an LPSC solution; Block 1614 represents drying the solutions to form an LZC layer and an LPSC layer; and Block 1616 represents contacting the LZC layer with the LPSC layer. Illustrative embodiments of the present invention include, but are not limited to, the following (referring also to Figs.1-20).. 1. A device structure 100, 200 useful in a lithium ion solid state battery, comprising: a composite cathode layer 102 comprising an LZC compound (LZC in Fig.1A and 1B) comprising at least lithium, zirconium, and chlorine, optionally fluorine and/or oxygen, wherein the LZC compound is a catholyte; and a solid electrolyte layer 104, 106 comprising an LPSC compound (LPSC in Fig.1A and 1B) comprising at least lithium, phosphorus, sulfur, and chlorine (optionally also oxygen and/or fluorine) arranged in an argyrodite structure, and wherein the composite cathode layer and the solid electrolyte layer are in direct contact or coupled via a catholyte layer 108. 2. The device structure of claim 1, wherein the LZC compound has a trigonal structure (P-3m1) or a monoclinic structure (C2/m). 3. The device structure of claim 1 or 2, wherein the LZC compound comprises Li2ZrCl6 and the LPSC compound comprises Li6PS5Cl. 4. The device structure of any of the claims 1-3, wherein: the LZC compound comprises Li2-xZrCl6-x (0 ≤ x ≤ 2) or Li2Zr1-xCl6-4x (0 ≤ x ≤ 1) or a variant thereof swapping out at least one of Li, Zr or Cl by another element; and the LPSC compound comprises Li_6-xPS_5-xCl_1+x (0 ≤ x ≤ 1)_. 5. The device structure of any of the clauses 1-4, wherein the composite cathode layer comprises the LZC compound mixed with a cathode active material 116 and a carbon conductive additive 118. 6. The device structure of any of the clauses 1-5, wherein the LZC compound and the LPSC compound consist of a chemical composition resulting in a capacity retention of at least 87% over 100 cycles, with each cycle comprising a charge and discharge of the cell 110 under C/3 conditions (charging over 3 hours and discharging over 3 hours) or at slower rates, when the electrochemical cell comprises the solid electrolyte layer in contact with the anode, and between the anode and the composite cathode layer (see e.g. Fig.15c). 7. Fig.1B illustrates an example of the device structure of any of the clauses 1-5, further comprising: a bilayer comprising: a catholyte layer 108 comprising an LZC compound, and the solid electrolyte layer 106 comprising an LPSC compound, wherein the catholyte layer 108 is between the solid electrolyte layer 106 and the composite cathode layer 102. 8. The device structure of clause 7, wherein the LZC compound and the LPSC compound consist of a chemical composition resulting in a capacity retention of at least 97% over 125 cycles, with each cycle comprising a charge and discharge of the cell 202 under C/3 conditions (charging over 3 hours and discharging over 3 hours) or at slower rates, when the electrochemical cell comprises the bilayer between and in contact with the anode and the composite cathode layer. 9. The device structure of any of the clauses 1-9, wherein a pelletized mixture of the LZC compound and LPSC compound at a temperature of at least 200 degrees Celsius is characterized by a measurement as being electrochemically stable without decomposition, degradation or reaction products associated with a reaction of the LZC compound with the LPSC compound, wherein the measurement comprises at least one of an X-ray Diffraction measurement, X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance (NMR) spectroscopy, or Raman spectroscopy. 10. The device structure of any of the clauses 1-9, wherein a pelletized mixture of the LZC compound and LPSC compound at a temperature of at least 200 degrees Celsius does not contain LixZryP2S6, ^^-Li3PS4, ZrS2, LiCl, ZrS3, PS4^3-, or P₂S₆ ^4- moieties. 11. The device structure of any of the clauses 1-10, wherein: the catholyte layer 108 and the composite cathode layer 102 each comprise a first plurality of particles 112 comprising the LZC compound, the solid electrolyte layer comprises a second plurality of particles 114 comprising the LPSC compound, and the average particle size (e.g., diameter, or largest diameter) and size distribution of the particles are such that physical contact between the first plurality of particles and the second plurality of particles promotes lithium transport between the LPSC and LZC compounds while suppressing detrimental reactivity between the LZC and the LPSC compounds during normal battery operation. 12. The device structure of any of the clauses 1-11, wherein: the catholyte layer and/or the composite cathode layer each comprise a first plurality of particles comprising the LZC compound and a first diameter D1 in a range 10 nm ≤ D1 ≤ 2 microns, and the solid electrolyte layer comprises a second plurality of particles comprising the LPSC compound and a second diameter D2 in a range of 10 nm ≤ D2 ≤ 10 microns. 13. The device structure of any of the clauses 1-12 wherein the catholyte layer and/or the composite cathode layer comprise ball-milled particles 112 comprising the LZC compound. 14. The device structure of any of the clauses 1-13 wherein the catholyte layer and/or the composite cathode layer comprise particles 112 comprising the LZC compound having a size distribution, disorder, crystallinity, and conductivity characteristic of, or equivalent to, that formed by ball milling a powder comprising the LZC compound. 15. The device structure of any of the clauses 1-14, wherein the LZC compound is crystalline with defects and/or disorder and comprises a conductivity greater than 0.1 mS per cm. 16. A lithium ion solid state battery 110 comprising the device structure of any of the clauses 1-6 and 8-15, wherein: the battery comprises the composite cathode layer and an anode 116; the solid electrolyte layer is a single electrolyte layer 104 between the composite cathode layer and the anode 116. 17. A lithium ion solid state battery 110, 202 comprising the device structure of any of the clauses 1-16 wherein: the battery comprises the composite cathode layer and an anode 116; the composite cathode layer comprises particles 112 of the LZC compound mixed with particles of the cathode active material 116 and particles of the carbon conductive additive 118; the catholyte layer 108 contacts the composite cathode layer and the solid electrolyte layer 106; and the solid electrolyte layer 106 is between the anode 116 and the catholyte layer 102. 18. A lithium ion solid state battery system 2000 comprising the device structure of any of the clauses 1-17: an electrochemical cell 110, 202 comprising the solid electrolyte layer 106, 104 between an anode 116 and the composite cathode layer 102; a heating element 2004 coupled to the cell for heating the cell to a desired temperature; and a circuit 2006 configured to charge the electrochemical cell at the desired temperature T of up to at least 70 degrees Celsius (250 degrees Celsius upper bound, e.g., room temperature ≤ T ≤ 250 degrees Celsius). 19. A lithium ion solid state battery system comprising the device structure of any of the clauses 1-18, comprising: an electrochemical cell 110.202 comprising the solid electrolyte layer 104, 106 between an anode 116 and the composite cathode layer 102; and a circuit 1500, 2004 configured to or operable to charge and/or operate the electrochemical cell at an increased voltage (e.g., above 3.4 V) applied across the anode and the composite cathode layer that is larger than that applied in an electrochemical cell comprising the solid electrolyte layer comprising the LPSC compound without the LZC compound or the LZC compound without the LPSC compound. 20. The battery of any of the clauses 15-19, wherein the cathode active material 102 comprises a lithium transition metal compound or sulfur and/or the anode 116 comprises a lithium alloy or lithium metal. 21. The battery of clause 20 wherein the lithium transition metal compound comprises a lithium transition metal oxide, a lithium transition metal oxyfluoride, a lithium transition metal polyanionic compound, a lithium transition metal fluoride, or a lithium transition metal sulfide. 22. The device structure of any of the clauses 1-22, wherein the solid electrolyte layer 106, 104, the composite cathode layer 102, and the catholyte layer 108 each comprise pelletized or densified layers or layers cast from a solution. 23. An anode-less battery 1900 comprising the device structure of any of the clauses 1-14, comprising the solid electrolyte layer between the composite cathode layer and a current collector such that lithium extracted from the composite cathode layer upon initial charging of the battery is plated directly onto the current collector 1902 to form an anode. 24. A method of making a solid electrolyte in contact with a catholyte, comprising: contacting a first layer comprising an LZC compound comprising at least lithium, zirconium, and chlorine, with a second layer comprising an LPSC compound comprising at least lithium, phosphorus, sulfur, and chlorine arranged in an argyrodite structure. 25. The method of clause 24, further comprising combining an LZC powder comprising the LZC compound with a first binder in a LZC solution; combining an LPSC powder comprising the LPSC compound with a second binder in an LPSC solution; drying the solutions to form an LZC layer and an LPSC layer; and contacting the LZC layer with the LPSC layer. 26. The method of clause 24, wherein the method comprises: obtaining an LZC pelletized layer comprising the LZC compound; depositing an LPSC powder comprising the LPSC compound on the LZC pelletized layer; and densifying the LPSC powder on the LZC pelletized layer, or depositing an LZC powder comprising the LZC compound on an LPSC pelletized layer and densifying the LZC powder on the LPSC pelletized layer. 27. The method of clause 26 wherein the densifying comprises cold pressing. 28. The method of clause 27, wherein the cold pressing at room temperature comprises: applying a pressure (e.g., but not limited to, of at least 350 MPa for at least 1 minute) to the LPSC powder to form the LPSC pelletized layer; and applying a pressure (e.g., but not limited to, of a least 175 MPa for at least 1 minute) to the LZC powder when the LZC powder is on top of the LPSC pelletized layer. 29. The method of any of the clauses 24-28, wherein the densifying or casting further comprises hot pressing or hot rolling (calendering) to form a more intimate interfacial contact between the pelletized layers. 30. The method of any of the clauses 24-29 further comprising: pressing the first layer comprising a first pelletized layer to, or casting the first layer comprising a cast layer on a cathode; and pressing the second layer comprising a second pelletized layer to, or the second layer comprising a cast layer on, an anode. 31. The method of any of the clauses 24-30, wherein the first layer and the second layer form a bilayer, or wherein the first layer comprises the LZC compound, cathode active material, and carbon conductive additive in a single layer configuration. 32. The method of any of the clauses 24-31, wherein the first layer is formed from an LZC powder comprising the LZC compound, the LZC powder comprising particles formed by ball milling or having a size distribution, disorder, crystallinity, and conductivity characteristic of, or equivalent to, that formed by ball milling a powder comprising the LZC compound. 33. The method of any of the clauses 24-32, wherein: the first layer is formed from an LZC powder and the second layer is formed from an LPSC powder, and the LZC powder is deposited on a cathode when forming the first layer prior to the second layer; or the LPSC powder is deposited on an anode when the forming the second layer prior to the first layer. 34. The method of any of the clauses 24-33 performed in an oxygen-free and moisture-free chamber or environment. 35. The method of clause 34, wherein the chamber or environment comprises an inert (e.g. Ar) gas. 36. The device structure or battery of any of the clauses 1-23 manufactured using the method of any of the clauses 24-34. 37. The battery of any of the clauses 15-20, wherein the solid electrolyte layer forms a passivating interface against the anode through decomposition into ionically conductive and electronically insulating products (LiCl, Li3P, and Li2S). 38. The device, method, system, or battery of any of the clauses 1-37 wherein selection of the chemical composition of the LZC compound and LPSC compound means both elemental selection (i.e. lithium, zirconium, etc.) and stoichiometry / relative amounts of those elements in the compound. 39. The device, method, system or battery of clause 38 wherein the stability of the catholyte in contact with the high voltage cathode depends on the anion. In one embodiment, chlorine (Cl) is selected because it is fairly electronegative and unlikely to give up an electron (be oxidized) upon contact with the cathode. Substitution of chlorine (Cl) by fluorine (F) or oxygen (O) may further increase the high voltage stability of this catholyte. 40. The device, method, system, or battery of clause 38 wherein the solid electrolyte is selected to be Li6PS5Cl because it is stable against a low voltage anode. It does not contain any element that can easily be reduced (take up an electron on contact with the anode). 41. The device, method, system, or battery of any of the clauses 1-40 wherein the LZC compound and/or the LPSC compound further include at least one of O, F, Si, Ge, Sn, Se, or Y. 42. The device structure of any of the clauses, wherein the LZC compound and the LPSC compound each have an oxygen content of less than 5 wt.% as measured in an LZC sample and an LPSC sample using a process comprising: preparing the LZC (LPSC) sample by: depositing particles of the LZC compound (LPSC) onto a Pt/Pd coated Si wafer inside an Ar-filled glovebox and depositing a protective Pt layer on top of the particles; using a focused ion beam (FIB) to mill out a trench in the Pt layer exposing a cross-sectional face of the particles; and polishing the cross-sectional face with a e.g., lower current beam; performing Energy-dispersive X-ray spectroscopy (EDS) comprising: accelerating an electron beam with an accelerating voltage below 3 kV onto the cross-sectional face, limiting a probing depth to less than 1 µm below the cross- sectional face; and determining the oxygen content from an analysis of X-rays emitted from the LZC (LPSC) sample in response to the electron beam. 43. The device structure of any of the clauses 1-42, wherein the LZC compound and the LPSC compound each have an oxygen content below a threshold value (e.g., less than 10 wt.% oxygen). 44. The device structure of any of the clauses 1-43, manufactured and handled in an inert and moisture free atmosphere (containing less than 10 ppm of water and less than 10 ppm of oxygen). 45. The device structure of any of the clauses 1-44 wherein the combination of the LZC compound and the LPSC compound is chemically and electrochemically stable for operating conditions of an electrochemical cell comprising the device structure. 46. The device structure of any of the clauses wherein the LZC compound and LPSC compound comprise a composition different from Li2ZrCl6-Li6PS5Cl Advantages and Improvements Implementation of the Li₂ZrCl₆-Li₆PS₅Cl (LZC-LPSC) SE pairing in a single SSB presents an opportunity to achieve high energy densities surpassing those of today’s conventional lithium ion batteries, and significant safety benefits due to the replacement of flammable organic electrolytes. Energy density and safety are the two most important metrics for the development of batteries for consumer products and with sufficiently small form factors to power mobile devices such as smartphones and electric vehicles. Hence, any battery technology that can enhance either of those two metrics carries enormous commercial advantages. When put together, the limited electrochemical stability window of SEs and the implementation of high voltage cathodes or high energy density metallic anodes seem mutually exclusive, casting doubt on the possibility of energy density improvements in SSBs. To date, no known SE composition is stable against both metallic anodes and high voltage oxide cathodes. However, the present disclosure has demonstrated that appropriate combination of different SEs in a single SSB can withstand the electrochemical environments on both sides of the battery. By implementing an oxidation-resistant catholyte and reduction- resistant SE layer, a dual SE approach leverages the differing electrochemical stability windows of the two electrolytes to maximize the stability of SE interfaces at both the cathode and anode, enabling stable high voltage cycling over a wide potential range. Alternatively, a bilayer architecture can also be implemented, where a thin layer of the catholyte is introduced between the SE layer and the cathode composite, preventing any possible contact between the cathode active material and oxidation- prone SE layer. The observed compatibility of the LZC-LPSC system signifies that SSBs incorporating >4V oxide cathodes and metallic anodes can be cycled stably. [2, 13, 15, 5, 17]. This disclosure further presents data contrasting the stability of two solid electrolyte combinations (LZC and LPSC, as well as LZC and LPS) at different temperatures. The (electro)chemical stability of the SE pairings of interest under normal SSB operating conditions was first tested using a simple proxy, that is, chemical stability under various heat treatment conditions from 25°C to 300°C. The underlying assumption here is that a more stable SE pairing under normal SSB operating conditions should also be stable up to higher heat treatment temperatures. The inventors’ investigations demonstrate that the phase boundary between the SE materials is highly conductive to Li-ions, with the potential for further improvements when the materials are densified together, under either cold or hot- pressing conditions. While the data presented herein shows that the LZC-LPSC SE pairing exhibits stable behavior under standard cycling operations in single layer and bilayer SSBs assembled with LiFePO₄ as the cathode active material and Li-In as the anode, these qualities are expected to transfer to other cathode active materials and anodes. Investigations of additional cathode and anode materials with this pairing are expected to demonstrate potential to stabilize interfaces with high voltage cathodes and high-capacity anode materials generally. Overall, the LZC-LPSC system is a conductive, scalable, and kinetically stabilized system with high promise for implementation in energy-dense dual SE SSBs with steady long-term cycling. In addition, both LZC and LPSC are deformable, which greatly facilitates their processing and enables full cell fabrication with cold pressing, lowering manufacturing costs relative to SSBs incorporating hard oxide SE materials. While cold pressing is the cheapest manufacturing option, we also demonstrate that their chemical stability up to 260 °C allows them to be hot pressed together to form near 100% dense layers, facilitating Li conduction across the interface. Finally, LZC and LPSC are both composed of earth-abundant elements with robust supply networks meaning that sourcing of their synthetic precursor materials is less subject to market volatility. Both classes of materials (halides and sulfides) have also been manufactured with scalable wet synthesis procedures, facilitating their respective production. [18, 14, 19] References The following publications are incorporated by reference herein: [1] Wang, S.; Bai, Q.; Nolan, A. M.; Liu, Y.; Gong, S.; Sun, Q.; Mo, Y. Lithium Chlorides and Bromides as Promising Solid-State Chemistries for Fast Ion Conductors with Good Electrochemical Stability. Angew. Chem. Int. Ed.2019, 58 (24), 8039–8043. https://doi.org/10.1002/anie.201901938. [2] Cronk, A.; Chen, Y.-T.; Deysher, G.; Ham, S.-Y.; Yang, H.; Ridley, P.; Sayahpour, B.; Nguyen, L. H. B.; Oh, J. A. S.; Jang, J.; Tan, D. H. S.; Meng, Y. S. Overcoming the Interfacial Challenges of LiFePO4 in Inorganic All-Solid-State Batteries. ACS Energy Lett.2023, 8 (1), 827–835. https://doi.org/10.1021/acsenergylett.2c02138.

C.; Zeier, W. G.; Janek, J. Interfacial Reactivity and Interphase Growth of Argyrodite Solid Electrolytes at Lithium Metal Electrodes. Solid State Ion.2018, 318 (July 2017), 102–112. https://doi.org/10.1016/j.ssi.2017.07.005. [4] Rosenbach, C.; Walther, F.; Ruhl, J.; Hartmann, M.; Hendriks, T. A.; Ohno, S.; Janek, J.; Zeier, W. Visualizing the Chemical Incompatibility of Halide and Sulfide‐Based Electrolytes in Solid‐State Batteries. Adv. Energy Mater.2022, 2203673. https://doi.org/10.1002/aenm.202203673. [5] Wang, K.; Ren, Q.; Gu, Z.; Duan, C.; Wang, J.; Zhu, F.; Fu, Y.; Hao, J.; Zhu, J.; He, L.; Wang, C.-W.; Lu, Y.; Ma, J.; Ma, C. A Cost-Effective and Humidity-Tolerant Chloride Solid Electrolyte for Lithium Batteries. Nat. Commun. 2021, 12 (1), 4410. https://doi.org/10.1038/s41467-021-24697-2. [6] Hanghofer, I.; Brinek, M.; Eisbacher, S. L.; Bitschnau, B.; Volck, M.; Hennige, V.; Hanzu, I.; Rettenwander, D.; Wilkening, H. M. R. Substitutional Disorder: Structure and Ion Dynamics of the Argyrodites Li 6 PS 5 Cl, Li 6 PS 5 Br and Li 6 PS 5 I. Phys. Chem. Chem. Phys.2019, 21 (16), 8489–8507. https://doi.org/10.1039/C9CP00664H. M.; Kobayashi, T.; Nagao, M.; Hirayama, M.;

Transitions of the Lithium Ionic Conductor Li3PS4. Solid State Ion.2011, 182 (1), 53–58. https://doi.org/10.1016/j.ssi.2010.10.001. Eck, E. R. H. van; Cui, G.; Kentgens, A. P.

M. Aluminium Ion Doping Mechanism of Lithium Thiophosphate Based Solid Electrolytes Revealed with Solid-State NMR. Phys. Chem. Chem. Phys.2023, 25 (6), 4997–5006. https://doi.org/10.1039/D2CP04670A. [9] S.; Porcheron, B.; Salager, E.;

T. L.; Fleutot, B.; Braida, M.-D.; Masquelier, C. Structural Details in Li3PS4: Variety in Thiophosphate Building Blocks and Correlation to Ion Transport. Energy Storage Mater.2022, 44, 168–179. https://doi.org/10.1016/j.ensm.2021.10.021. S.; Control of Structure and Conduction Properties in Na–Y–Zr–Cl Solid Electrolytes. J. Mater. Chem. A 2022, 10 (40), 21565–21578. https://doi.org/10.1039/D2TA05823E. [12] Kwak, H.; Kim, J.-S.; Han, D.; Kim, J. S.; Park, J.; Kwon, G.; Bak, S.- M.; Heo, U.; Park, C.; Lee, H.-W.; Nam, K.-W.; Seo, D.-H.; Jung, Y. S. Boosting the Interfacial Superionic Conduction of Halide Solid Electrolytes for All-Solid-State Batteries. Nat Commun 2023, 14 (1), 2459. https://doi.org/10.1038/s41467-023- 38037-z. [13] Luo, X.; Zhong, Y.; Wang, X.; Xia, X.; Gu, C.; Tu, J. Ionic Enhancement of Li2ZrCl6 Halide Electrolytes via Mechanochemical Synthesis for All-Solid-State Lithium–Metal Batteries. ACS Appl. Mater. Interfaces 2022, 14 (44), 49839–49846. https://doi.org/10.1021/acsami.2c14903. [14] Wang, C.; Liang, J.; Luo, J.; Liu, J.; Li, X.; Zhao, F.; Li, R.; Huang, H.; Zhao, S.; Zhang, L.; Wang, J.; Sun, X. A Universal Wet-Chemistry Synthesis of Solid-State Halide Electrolytes for All-Solid-State Lithium-Metal Batteries. Science Advances 2021, 7 (37), eabh1896. https://doi.org/10.1126/sciadv.abh1896. [15] Chen, S.; Yu, C.; Chen, S.; Peng, L.; Liao, C.; Wei, C.; Wu, Z.; Cheng, S.; Xie, J. Enabling Ultrafast Lithium-Ion Conductivity of Li2ZrCl6 by Indium Doping. Chinese Chemical Letters 2022, 33 (10), 4635–4639. https://doi.org/10.1016/j.cclet.2021.12.048.

Sann, J.; Zeier, W. G.; Janek, J. Lithium‐ Metal Anode Instability of the Superionic Halide Solid Electrolytes and the Implications for Solid‐State Batteries. Angew. Chem. Int. Ed.2021, 60 (12), 6718– 6723. https://doi.org/10.1002/anie.202015238.

H.; Bao, W.; Sreenarayanan, B.; Doux, J.-M.; Li, W.; Lu, B.; Ham, S.-Y.; Sayahpour, B.; Scharf, J.; Wu, E. A.; Deysher, G.; Han, H. E.; Hah, H. J.; Jeong, H.; Lee, J. B.; Chen, Z.; Meng, Y. S. Carbon-Free High-Loading Silicon Anodes Enabled by Sulfide Solid Electrolytes. Science 2021, 373 (6562), 1494–1499. https://doi.org/10.1126/science.abg7217.

[18] Rajagopal, R.; Subramanian, Y.; Jung, Y. J.; Kang, S.; Ryu, K.-S. Rapid Synthesis of Highly Conductive Li6PS5Cl Argyrodite-Type Solid Electrolytes Using Pyridine Solvent. ACS Appl. Energy Mater.2022, 5 (8), 9266–9272. https://doi.org/10.1021/acsaem.2c01157. [19] Li, X.; Liang, J.; Chen, N.; Luo, J.; Adair, K. R.; Wang, C.; Banis, M.

S.; Huang, H.; Li, R.; Sun, X. Water‐Mediated Synthesis of a Superionic Halide Solid Electrolyte. Angew. Chem.2019, ange.201909805. https://doi.org/10.1002/ange.201909805. [20] Wu, E.A., Banerjee, S., Tang, H. et al. A stable cathode-solid electrolyte composite for high-voltage, long-cycle-life solid-state sodium-ion batteries. Nat Commun 12, 1256 (2021). https://doi.org/10.1038/s41467-021-21488-7. [21] Wang, Y.; Hoang, B.; Hoerauf, J.; Lee, C.; Lin, C.-F.; Rubloff, G. W.; Lee, S. B.; Kozen, A. C. Hot and Cold Pressed LGPS Solid Electrolytes. J. Electrochem. Soc.2021, 168 (1), 010533. https://doi.org/10.1149/1945-7111/abdb44. [22] Kotobuki, M.; Lei, H.; Chen, Y.; Song, S.; Xu, C.; Hu, N.; Molenda, J.; Lu, L. Preparation of Thin Solid Electrolyte by Hot-Pressing and Diamond Wire Slicing. RSC Adv.2019, 9 (21), 11670–11675. https://doi.org/10.1039/C9RA00711C. [23] Huang, B.; Xu, B.; Zhang, J.; Li, Z.; Huang, Z.; Li, Y.; Wang, C.-A. Li- Ion Conductivity and Stability of Hot-Pressed LiTa2PO8 Solid Electrolyte for All- Solid-State Batteries. J Mater Sci 2021, 56 (3), 2425–2434. https://doi.org/10.1007/s10853-020-05324-9.

Throughput Manufacturing Method for Composite Solid-State Electrolytes. iScience 2021, 24 (2), 102055. https://doi.org/10.1016/j.isci.2021.102055.

[26] https://doi.org/10.1002/anie.201909805 [27]https://doi.org/10.1039/d0ee01017

9, 3683−3693, doi:10.1021/acsenergylett.4c01084 [29] https://doi.org/10.3390/app6090264 [30]

Conclusion This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto

Claims

WHAT IS CLAIMED IS: 1. A device structure useful in a lithium ion solid state battery, comprising: a composite cathode layer comprising an LZC compound comprising at least lithium, zirconium, and chlorine, wherein the LZC compound is a catholyte; and a solid electrolyte layer comprising an LPSC compound comprising at least lithium, phosphorus, sulfur, and chlorine arranged in an argyrodite structure, and wherein : the composite cathode layer and the solid electrolyte layer are in direct contact or coupled via a catholyte layer; and the combination of the LZC compound and the LPSC compound is chemically and electrochemically stable for operating conditions of an electrochemical cell comprising the device structure.

2. The device structure of claim 1, wherein the LZC compound has a trigonal structure (P-3m1) or a monoclinic structure (C2/m).

3. The device structure of claim 1, wherein the LZC compound comprises Li₂ZrCl₆ and the LPSC compound comprises Li₆PS₅Cl.

4. The device structure of claim 1, wherein: the LZC compound comprises Li2-xZrCl6-x (0 ≤ x ≤ 2) or Li2Zr1-xCl6-4x (0 ≤ x ≤ 1) or a variant thereof swapping out at least one of Li, Zr or Cl by another element; and the LPSC compound comprises Li6-xPS5-xCl1+x (0 ≤ x ≤ 1).

5. The device structure of claim 1, wherein the composite cathode layer comprises the LZC compound mixed with a cathode active material and a carbon conductive additive.

6. The device structure of claim 5, wherein the LZC compound and the LPSC compound consist of a chemical composition resulting in a capacity retention of at least 87% over 100 cycles, with each cycle comprising a charge and discharge of the cell under C/3 conditions (charging over 3 hours and discharging over 3 hours) or at slower rates, when the electrochemical cell comprises the solid electrolyte layer in contact with the anode, and between the anode and the composite cathode layer.

7. The device structure of claim 1, further comprising: a bilayer comprising: a catholyte layer comprising an LZC compound, and the solid electrolyte layer comprising an LPSC compound, wherein the catholyte layer is between the solid electrolyte layer and the composite cathode layer.

8. The device structure of claim 7, wherein the LZC compound and the LPSC compound consist of a chemical composition resulting in a capacity retention of at least 97% over 125 cycles, with each cycle comprising a charge and discharge of the cell under C/3 conditions (charging over 3 hours and discharging over 3 hours) or at slower rates, when the electrochemical cell comprises the bilayer between and in contact with the anode and the composite cathode layer.

9. The device structure of claim 1, wherein a pelletized mixture of the LZC compound and LPSC compound at a temperature of at least 200 degrees Celsius is characterized by a measurement as being electrochemically stable without decomposition, degradation or reaction products associated with a reaction of the LZC compound with the LPSC compound, wherein the measurement comprises at least one of an X-ray Diffraction measurement, X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance (NMR) spectroscopy, or Raman spectroscopy.

10. The device structure of claim 1, wherein a pelletized mixture of the LZC compound and LPSC compound at a temperature of at least 200 degrees Celsius does not contain Li_xZr_yP₂S₆, ^^-Li₃PS₄, ZrS₂, LiCl, ZrS₃, PS₄ ^3-, or P₂S₆ ^4- moieties.

11. The device structure of claim 1, wherein: the catholyte layer and the composite cathode layer each comprise a first plurality of particles comprising the LZC compound, the solid electrolyte layer comprises a second plurality of particles comprising the LPSC compound, and the average particle size and size distribution of the particles are such that physical contact between the first plurality of particles and the second plurality of particles promotes lithium transport between the LPSC and LZC compounds while suppressing detrimental reactivity between the LZC and the LPSC compounds during normal battery operation.

12. The device structure of claim 1, wherein: the catholyte layer and/or the composite cathode layer each comprise a first plurality of particles comprising the LZC compound and a first diameter D1 in a range 10 nm ≤ D1 ≤ 2 microns, and the solid electrolyte layer comprises a second plurality of particles comprising the LPSC compound and a second diameter D2 in a range of 10 nm ≤ D2 ≤ 10 microns.

13. The device structure of claim 1 wherein the catholyte layer and/or the composite cathode layer comprise ball-milled particles comprising the LZC compound.

14. The device structure of claim 1 wherein the catholyte layer and/or the composite cathode layer comprise particles comprising the LZC compound having a size distribution, disorder, crystallinity, and conductivity characteristic of, or equivalent to, that formed by ball milling a powder comprising the LZC compound.

15. The device structure of claim 1, wherein the LZC compound is crystalline with defects and/or disorder and comprises a conductivity greater than 0.1 mS per cm.

16. A lithium ion solid state battery comprising the device structure of claim 5, wherein: the battery comprises the composite cathode layer and an anode; the solid electrolyte layer is a single electrolyte layer between the composite cathode layer and the anode.

17. A lithium ion solid state battery comprising the device structure of claim 1 wherein: the battery comprises the composite cathode layer and an anode; the composite cathode layer comprises particles of the LZC compound mixed with particles of the cathode active material and particles of the carbon conductive additive; the catholyte layer contacts the composite cathode layer and the solid electrolyte layer; and the solid electrolyte layer is between the anode and the catholyte layer.

18. A lithium ion solid state battery system comprising the device structure of claim 1: an electrochemical cell comprising the solid electrolyte layer between an anode and the composite cathode layer; a heating element coupled to the cell for heating the cell to a desired temperature; and a circuit configured to charge the electrochemical cell at a temperature of up to at least 70 degrees Celsius.

19. A lithium ion solid state battery system comprising the device structure of claim 1, comprising: an electrochemical cell comprising the solid electrolyte layer between an anode and the composite cathode layer; and a circuit configured to charge and/or operate the electrochemical cell at an increased voltage applied across the anode and the composite cathode layer that is larger than that applied in an electrochemical cell comprising the solid electrolyte layer, the catholyte, and the composite cathode layer comprising the LPSC compound without the LZC compound or the LZC compound without the LPSC compound.

20. The battery of claim 19, wherein the cathode active material comprises a lithium transition metal compound or sulfur and/or the anode comprises a lithium alloy or lithium metal.

21. The battery of claim 20 wherein the lithium transition metal compound comprises a lithium transition metal oxide, a lithium transition metal oxyfluoride, a lithium transition metal polyanionic compound, a lithium transition metal fluoride, or a lithium transition metal sulfide.

22. The device structure of claim 1, wherein the solid electrolyte layer, the composite cathode layer, and the catholyte layer each comprise pelletized or densified layers or layers cast from a solution.

23. An anode-less battery comprising the device structure of claim 1, comprising the solid electrolyte layer between the composite cathode layer and a current collector such that lithium extracted from the composite cathode layer upon initial charging of the battery is plated directly onto the current collector to form an anode.

24. A method of making a solid electrolyte in contact with a catholyte, comprising: contacting a first layer comprising an LZC compound comprising at least lithium, zirconium, and chlorine, with a second layer comprising an LPSC compound comprising at least lithium, phosphorus, sulfur, and chlorine arranged in an argyrodite structure.

25. The method of claim 24, further comprising combining an LZC powder comprising the LZC compound with a first binder in a LZC solution; combining an LPSC powder comprising the LPSC compound with a second binder in an LPSC solution; drying the solutions to form an LZC layer and an LPSC layer; and contacting the LZC layer with the LPSC layer.

26. The method of claim 24, wherein the method comprises: obtaining an LZC pelletized layer comprising the LZC compound; depositing an LPSC powder comprising the LPSC compound on the LZC pelletized layer; and densifying the LPSC powder on the LZC pelletized layer, or depositing an LZC powder comprising the LZC compound on an LPSC pelletized layer and densifying the LZC powder on the LPSC pelletized layer.

27. The method of claim 26 wherein the densifying comprises cold pressing.

28. The method of claim 27, wherein the cold pressing at room temperature comprises: applying a pressure to the LPSC powder to form the LPSC pelletized layer; and applying a pressure to the LZC powder when the LZC powder is on top of the LPSC pelletized layer.

29. The method of claim 24, wherein the contacting further comprises hot pressing or hot rolling (calendering) to form a more intimate interfacial contact between the pelletized layers.

30. The method of claim 24 wherein the contacting further comprises: pressing the first layer comprising a first pelletized layer to, or casting the first layer comprising a cast layer on a cathode; and pressing the second layer comprising a second pelletized layer to, or the second layer comprising a cast layer on, an anode.

31. The method of claim 24, wherein the first layer and the second layer form a bilayer, or wherein the first layer comprises the LZC compound, cathode active material, and carbon conductive additive in a single layer configuration.

32. The method of claim 24, wherein the first layer is formed from an LZC powder comprising the LZC compound, the LZC powder comprising particles formed by ball milling or having a size distribution, disorder, crystallinity, and conductivity characteristic of, or equivalent to, that formed by ball milling a powder comprising the LZC compound.

33. The method of claim 24, wherein: the first layer is formed from an LZC powder and the second layer is formed from an LPSC powder, and the LZC powder is deposited on a cathode when forming the first layer prior to the second layer; or the LPSC powder is deposited on an anode when the forming the second layer prior to the first layer.

34. The method of claim 24 performed in an oxygen-free and moisture- free chamber or environment.

35. The method of claim 33, wherein the chamber or environment comprises an inert (e.g. Ar) gas.

36. The device structure or battery of any of the claim 1 manufactured using the method of clam 24.

37. A battery comprising the electrochemical cell of claim 1, comprising the solid electrolyte layer between the composite cathode layer and an anode, wherein the solid electrolyte layer forms a passivating interface against the anode through decomposition into ionically conductive and electronically insulating products (comprising at least one of LiCl, Li3P, or Li2S).