WO2025010842A1 - Cathode electrode sheet and battery having same - Google Patents

Abstract

A cathode electrode sheet, comprising a current collector and a composite active coating arranged on the surface of the current collector. In the thickness direction of the composite active coating, the composite active coating comprises, successively combined, a layered oxide active coating, a protective layer and a phosphate active coating; the phosphate active coating comprises a phosphate material; the protective layer comprises a lithium-rich material; the layered oxide active coating comprises layered oxide; the ratio of the thickness of the protective layer to the thickness of the phosphate active coating is 6-60:40-150; the surface density of the protective layer is 10-90 g/m², and the surface density of the phosphate active coating is 90-280 g/m².

Description

A cathode electrode and a battery thereof

This application claims priority to the Chinese patent application filed with the China Patent Office on July 13, 2023, with application number 202310862745.8. The entire contents of the above application are incorporated by reference into this application.

Technical Field

The present invention belongs to the technical field of batteries, and in particular, relates to a cathode electrode sheet and a battery thereof.

Background Art

Phosphate materials have good cycle performance and safety performance, and layered oxides have high capacity and compaction density; using phosphate materials and layered oxides simultaneously in the cathode electrode can achieve a cathode electrode with better comprehensive performance in cycle performance, safety performance, capacity, and compaction density. At present, there are co-doped electrodes on the market that directly mix and coat the above two materials. This design is easy to manufacture, but the particle size and density of the above two materials are very different, and it is difficult to achieve uniform mixing, so that the phosphate material and the layered oxide cannot fully exert their synergistic advantages; there are also patents that point out that phosphate materials and layered oxides can be layered and coated to obtain composite electrodes, but no matter whether the phosphate material is located in the upper or lower layer, the composite electrode produced cannot match the application of high voltage, which easily leads to excessive delithiation of the electrode. This is mainly because the working voltage of the phosphate material is low, while the working voltage of the layered oxide is high, and the use of separate coating cannot achieve stable dynamic performance under high voltage.

Technical issues

How to give full play to the synergistic advantages of phosphate materials and layered oxides in the same cathode electrode and ensure the structural stability and cycle stability of the cathode electrode under high voltage is a technical problem that technical personnel in this field urgently need to solve in their research.

Technical Solution

The object of the present invention is to provide a cathode electrode sheet in which the synergistic advantages of phosphate materials and layered oxides can be fully utilized, and the cathode electrode sheet as a whole can be ensured to have excellent structural stability and cycle stability under high voltage.

According to one aspect of the present invention, a cathode electrode sheet is provided, comprising a current collector and a composite active coating disposed on the surface of the current collector; in the thickness direction of the composite active coating, the composite active coating comprises a layered oxide active coating, a protective layer and a phosphate active coating which are composited in sequence, the phosphate active coating comprises a phosphate material, the protective layer comprises a lithium-rich material, and the layered oxide active coating comprises a layered oxide; the thickness ratio of the protective layer to the thickness of the phosphate active coating is 6-60:40-150; the surface density of the protective layer is 10-90 g/m ² , and the surface density of the phosphate active coating is 90-280 g/m ² . For example, the ratio of the thickness of the protective layer to the thickness of the phosphate active coating is 6:40, 6:60, 6:100, 6:150, 30:40, 30:60, 30:100, 30:150, 60:40, 60:60, 60:100 or 60:150, etc.; for example, the surface density of the protective layer is 10g/ ^m2 , 30g/ ^m2 , 50g/ ^m2 , 70g/ ^m2 or 90g/ ^m2, etc.; for example, the surface density of the phosphate active coating is 90g/ ^m2 , 150g/ ^m2 , 200g/ ^m2 , 250g/ ^m2 or 280g/ ^m2 , etc.

According to another aspect of the present invention, a battery is provided, the battery comprising the above-mentioned cathode electrode plate.

Beneficial Effects

Since the working voltage of phosphate materials is relatively low, they are easily overcharged under high voltage, causing unstable electrode structure. In addition, the working voltage of phosphate materials and layered oxides do not match. The cathode electrode integrating the above two materials is easily unstable and has performance degradation due to the loss of active lithium under high voltage. The cathode electrode provided by the present invention separates the layered oxide and phosphate materials with different working voltages by setting a protective layer between the phosphate active coating and the layered oxide active coating. Moreover, on the one hand, by adopting a high-voltage-resistant lithium-rich material to form the protective layer, the protective layer can preferentially delithiate under high voltage, thereby compensating for the active lithium loss of the phosphate material under high voltage, avoiding excessive delithiation of the phosphate material, and playing a protective role of the lithium-rich material on the phosphate material. On the other hand, by reasonably setting the thickness ratio and surface density of the protective layer and the phosphate material active coating, the content of the lithium-rich material in the protective layer and the phosphate material in the phosphate active coating is controlled within a suitable range, and on the basis of ensuring that the protective layer effectively plays its preferential delithiation role under high voltage, the active lithium loss is effectively compensated, and at the same time, it helps to stabilize the composite effect of the phosphate active coating and the protective layer, thereby preventing the poor structural stability caused by the different working voltages of the protective layer and the phosphate active coating, and avoiding the problems of increased surface resistance, cycle deterioration, and cost increase caused by excessive surface density of the protective layer. Based on this, the cathode electrode provided by the present invention is conducive to preventing the problem of excessive lithium desorption caused by the working voltage drop between the layered oxide and the phosphate material, thereby realizing stable lithium desorption of the cathode electrode; thereby fully utilizing the high cycle performance advantages of the phosphate material and the high capacity advantages of the layered oxide in the same cathode electrode, and ensuring that the cathode electrode as a whole has excellent structural stability and cycle stability under high voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the structure of the cathode electrode sheet obtained in Examples 1 to 10;

FIG2 is a schematic diagram of the structure of the cathode electrode piece obtained in Examples 11-12;

FIG3 is a schematic diagram of the structure of the cathode electrode sheet obtained in Comparative Examples 1 to 4;

FIG4 is a schematic diagram of the structure of the cathode electrode piece prepared in Comparative Examples 5-6;

Reference numerals: 1 layered oxide active coating, 2 protective layer, 3 phosphate active coating, 4 current collector.

Embodiments of the present invention

In one embodiment, the chemical formula of the phosphate material is LiMn _x Fe _y A _{( 1-xy )} PO ₄ , wherein 0≤x≤1, 0≤y≤1, and the element A refers to one or two of Mg, Al, Ni, Co, V, Nb, Ca, Ti, Zn, Cu, Zr, Sr, Sn, Y, W, B, Si, Na, and K;

In one embodiment, the chemical formula of the layered oxide is Li _a Ni _b Co _c M _{( 1-bc )} O ₂ , wherein 0.8≤a≤1.4, 0.2≤b≤1, 0≤c≤0.3, and the M element refers to one or two of Mn, Mg, Al, Zr, Ti, V, W, B, Nb, Ca, Zn, Cu, Sr, Sn, Y, Si, Na, and K.

Preferably, on the same side of the current collector of the cathode electrode sheet, the layered oxide active coating, the protective layer, and the phosphate active coating are sequentially arranged in a direction away from the surface of the current collector.

In one embodiment, the thickness of the protective layer is 6 to 60 μm; the thickness of the phosphate active coating is 40 to 150 μm. For example, the thickness of the protective layer is 6 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm or 60 μm; the thickness of the phosphate active coating is 40 μm, 60 μm, 90 μm, 120 μm or 150 μm, etc. On the one hand, if the thickness of the protective layer is too small, the content of the lithium-rich material in the protective layer is small, and it is difficult to achieve a good lithium removal and lithium replenishment effect; if the thickness of the protective layer is too large, the content of the lithium-rich material in the protective layer is large, so that the protective layer has a lower conductivity and a higher impedance, which reduces the cycle performance of the electrode. Therefore, the cathode electrode provided by the present invention can effectively compensate for the loss of active lithium of the phosphate material under high voltage while ensuring that the impedance of the battery is at a low level by reasonably setting the thickness of the protective layer, and realize the protective effect of the lithium-rich material on the phosphate material, thereby improving the electrochemical performance of the cathode electrode. On the other hand, if the phosphate active coating is too thin, the content of phosphate material is low, which reduces the cycle performance of the cathode electrode and weakens the safety performance; if the phosphate layer is too thick, the electrode becomes thicker as a whole, the lithium ion transmission path becomes longer, and the interface impedance of the electrode is increased, which reduces the cycle performance of the electrode. Therefore, the present invention further improves the energy density and cycle performance of the cathode electrode by reasonably setting the thickness of the phosphate active coating.

In one embodiment, the thickness of the layered oxide active coating is 5-200 μm, and the surface density is 30-200 g/m ² . For example, the thickness of the layered oxide active coating is 5 μm, 20 μm, 60 μm, 100 μm, 150 μm or 200 μm, etc., and the surface density is 30 g/m ² , 60 g/m ² , 90 g/m ² , 120 g/m ² , 150 g/m ² or 200 g/m ² , etc. If the layered oxide active coating is too thin, the content of the layered oxide in the layered oxide active coating is small, so that the capacity and energy density of the pole piece are reduced; if the layered oxide active coating is thick, the transmission path of lithium ions is long, so that the impedance of the pole piece is increased and the cycle performance is reduced.

In one embodiment, the thickness of the layered oxide active coating is 60-150 μm.

In one embodiment, D represents the sum of the thicknesses of the layered oxide active coating, the protective layer, and the phosphate active coating that are compounded in sequence, and D satisfies: 50μm≤D≤300μm. If the thickness of the cathode electrode is too thin, the content of the three cathode active materials contained is too small, the electrode is difficult to process, and the capacity and energy density are low; if the thickness of the electrode is too thick, the electrolyte is difficult to infiltrate, resulting in an increase in the impedance of the electrode and a decrease in the cycle performance. In the present invention, by controlling the thickness of the cathode electrode within the above range, the cycle performance and energy density of the electrode can be further improved while ensuring the processing performance of the electrode.

In one embodiment, the lithium-rich material includes at least one of lithium-rich manganese-based materials, lithium ferrite, and lithium nickelate. The chemical formula of lithium ferrite is Li ₅ FeO ₄ , and the chemical formula of lithium nickelate is Li ₂ NiO ₂ .

When the lithium-rich material is charged to 4.6V for the first time, the charging capacity contributed by the unit Li content is recorded as C, where the charging capacity contributed by the unit Li content in the lithium-rich manganese-based material is recorded as C1, where 3000mAh/g≤C1≤3600mAh/g; the charging capacity contributed by the unit Li content in the lithium ferrite material is recorded as C2, where 2100mAh/g≤C2≤3200mAh/g; the charging capacity contributed by the unit Li content in the lithium nickel oxide material is recorded as C3, where 2400mAh/g≤C3≤3100mAh/g. If the charging capacity contributed by the unit Li content is too low, the lithium replenishment effect of the lithium-rich material is poor, and it is difficult to achieve stable lithium deintercalation of the cathode electrode; if the charging capacity contributed by the unit Li content is too high, the oxidation of the lithium-rich material increases, catalyzing the electrolyte reaction, which will deteriorate the cycle performance.

In one embodiment, the general chemical formula of the lithium-rich manganese-based material is sLi ₂ MnO ₃ •(1-s)LiNi _p Mn _q B _(1-pq) O2, wherein 0.1≤s≤0.4, 0.1≤p≤0.5, 0.1≤q≤0.8, and the B element refers to one or two of Co, Mg, Al, Zr, Ti, V, W, B, Nb, Ca, Zn, Cu, Sr, Sn, Y, Si, Na, and K.

In one embodiment, the lithium-rich material is a lithium-rich manganese-based material, the thickness of the protective layer is 6-60 μm, and the surface density is 10-90 g/m ^2. The lithium-rich manganese-based material has excellent lithium supplementation effect and cycle performance. The lithium-rich manganese-based material is used to form the protective layer, and the thickness and surface density of the protective layer are controlled, so that the protective layer can have a good composite effect with the phosphate active coating and the layered oxide active coating. At the same time, the protective layer can be preferentially de-lithiumed at high voltage, so that the cathode electrode can achieve stable lithium deintercalation, thereby improving the energy density and cycle performance of the electrode.

In one embodiment, the lithium content of the lithium-rich manganese-based material is 7-11wt% calculated as a percentage by mass. If the lithium content of the lithium-rich manganese-based material is too low, the lithium replenishing effect of the lithium-rich manganese-based material is poor, and it is difficult to achieve stable lithium deintercalation of the cathode electrode; if the lithium content of the lithium-rich manganese-based material is too high, the resistance of the lithium-rich manganese-based material increases, the oxidizability of the material increases, and the electrolyte reaction is catalyzed, which will deteriorate the cycle performance. Therefore, the cathode electrode provided by the present invention improves the cycle performance of the cathode electrode by reasonably setting the lithium content of the lithium-rich manganese-based material in the protective layer while ensuring that the lithium replenishing effect of the lithium-rich manganese-based material can enable the electrode to achieve stable lithium deintercalation.

In one embodiment, the particle size of the lithium-rich manganese-based material is 2-13 μm, and the specific surface area is 0.5-5 g/m ² . The present invention controls the particle size and specific surface area of the lithium-rich manganese-based material within the above range, which can improve the compaction density and structural stability of the cathode electrode sheet while ensuring the processing performance of the cathode electrode sheet, thereby improving the electrochemical performance and cycle performance of the electrode sheet.

In one embodiment, the lithium-rich material is lithium ferrite or lithium nickelate, and the thickness of the protective layer is 10-50 μm, and the surface density is 1-70 g/m ² . Lithium ferrite (Li ₅ FeO ₄ ) and lithium nickelate (Li ₂ NiO ₂ ) have excellent lithium supplementation effects. The protective layer is formed by Li ₅ FeO ₄ and Li ₂ NiO ₂ , and the thickness and surface density of the protective layer are controlled, so that the composite effect of the protective layer and the phosphate active coating and the layered oxide active coating can be good. At the same time, the protective layer can be preferentially de-lithiumed at high voltage, so that the cathode plate can achieve stable lithium de-insertion, thereby improving the energy density and cycle performance of the cathode plate.

Example 1

This embodiment provides a battery, and the preparation method thereof comprises the following steps:

1. Preparation of cathode electrode

The phosphate material LiFePO ₄ , the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 94:3:3, and then the solvent NMP was added to prepare the first cathode slurry; the lithium-rich material 0.34Li ₂ MnO ₃ •0.67LiMn _0.5 Ni _0.5 O ₂ (the particle size of the lithium-rich material is 10μm, the specific surface area is 4.5g/m ² , and the lithium content of the lithium-rich material is 9.6wt%), the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 94:3:3, and then the solvent NMP was added to prepare the second cathode slurry; the layered oxide LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 94:3:3, and then a solvent NMP is added to prepare a third cathode slurry; the third cathode slurry is coated on the surface of the current collector 4, dried to form a layered oxide active coating 1 with a thickness of 40 μm and a surface density of 130 g/m ² , then the second cathode slurry is coated on the surface of the layered oxide active coating 1, dried to form a protective layer 2 with a thickness of 10 μm and a surface density of 20 g/m ² , then the first cathode slurry is coated on the surface of the protective layer 2, dried to form a phosphate active coating 3 with a thickness of 60 μm and a surface density of 120 g/m ^2. Then, the cathode pole piece is obtained by rolling and slicing. In the cathode pole piece, D represents the sum of the thicknesses of the layered oxide active coating 1, the protective layer 2, and the phosphate active coating 3, which are compounded in sequence, and D=110 μm; the structural schematic diagram of the cathode pole piece prepared above is shown in FIG1.

2. Anode plate preparation

The anode active material graphite, the conductive agent acetylene black, the binder carboxymethyl cellulose (CMC), and SBR (styrene butadiene rubber) are mixed in a mass ratio of 94:1:2:3, and then deionized water is added as a solvent and stirred evenly to obtain an anode slurry; the anode slurry is evenly coated on the surface of the copper foil, and then dried, rolled, and sliced to obtain an anode sheet.

3. Preparation of electrolyte

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then the fully dried lithium salt LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol/L.

4. Preparation of diaphragm

The diaphragm is made of polyethylene diaphragm;

5. Battery cell assembly

The cathode electrode sheet, the isolation film, and the anode electrode sheet are stacked in order, the isolation film is placed between the cathode electrode sheet and the anode electrode sheet to play an isolation role, and then they are wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, and after drying, the electrolyte is injected, and after vacuum packaging, standing, forming, shaping and other processes, the battery of this embodiment is obtained.

Example 2

This embodiment prepares a battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the phosphate material is LiMn _0.6 Fe _0.4 PO _4. In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

Example 3

This embodiment prepares a battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the layered oxide is Li _1.10 Ni _0.5 Mn _0.5 O ₂ . Except for the above difference, the materials and process operation used in this embodiment are strictly consistent with those in the embodiment 1.

Example 4

This embodiment prepares a battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the lithium-rich material is Li ₅ FeO ₄ . In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

Example 5

This embodiment prepares a battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the lithium-rich material is Li ₂ NiO ₂ . In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

Example 6

This embodiment prepares a battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the thickness of the protective layer is 30 μm and the surface density is 50 g/m ² . In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

Example 7

This embodiment prepares a battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the thickness of the protective layer is 60 μm and the surface density is 80 g/m ² . In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

Example 8

This embodiment prepares a battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the thickness of the protective layer is 6 μm and the surface density is 8 g/m ² . In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

Example 9

This embodiment prepares a battery with reference to the embodiment 4. The difference between this embodiment and the embodiment 4 is that the thickness of the protective layer is 90 μm and the surface density is 90 g/m ² . In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

Example 10

This embodiment prepares a battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the thickness of the protective layer is 2.4 μm and the surface density is 10 g/m ² . In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

Embodiment 11

This embodiment provides a battery, and the preparation method thereof comprises the following steps:

1. Preparation of cathode electrode

The phosphate material LiFePO ₄ , the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 94:3:3, and then the solvent NMP was added to prepare the first cathode slurry; the lithium-rich material 0.34Li ₂ MnO ₃ •0.67LiMn _0.5 Ni _0.5 O ₂ (the particle size of the lithium-rich material is 10μm, the specific surface area is 4.5g/m ² , and the lithium content of the lithium-rich material is 9.6wt%), the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 94:3:3, and then the solvent NMP was added to prepare the second cathode slurry; the layered oxide LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , acetylene black as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder are mixed in a mass ratio of 94:3:3, and then a solvent NMP is added to prepare a third cathode slurry; the first cathode slurry is coated on the surface of the current collector 4, dried to form a phosphate active coating 3 with a thickness of 60 μm and a surface density of 120 g/m ² , and then the second cathode slurry is coated on the surface of the phosphate active coating 3, dried to form a protective layer 2 with a thickness of 10 μm and a surface density of 20 g/m ² , and then the third cathode slurry is coated on the surface of the protective layer 2, dried to form a layered oxide active coating 1 with a thickness of 40 μm and a surface density of 130 g/m ^2. Then, the cathode pole piece is obtained by rolling and slicing, in which D represents the sum of the thicknesses of the phosphate active coating 3, the protective layer 2 and the layered oxide active coating 1 which are compounded in sequence, and D=110 μm; the structural schematic diagram of the cathode pole piece prepared above is shown in FIG2.

2. Anode plate preparation

The anode active material graphite, the conductive agent acetylene black, the binder carboxymethyl cellulose (CMC), and SBR (styrene butadiene rubber) are mixed in a mass ratio of 94:1:2:3, and then deionized water is added as a solvent and stirred evenly to obtain an anode slurry; the anode slurry is evenly coated on the surface of the copper foil, and then dried, rolled, and sliced to obtain an anode sheet.

3. Preparation of electrolyte

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then the fully dried lithium salt LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol/L.

4. Preparation of diaphragm

The diaphragm is made of polyethylene diaphragm;

5. Battery cell assembly

The cathode electrode sheet, the isolation film, and the anode electrode sheet are stacked in order, the isolation film is placed between the cathode electrode sheet and the anode electrode sheet to play an isolation role, and then they are wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, and after drying, the electrolyte is injected, and after vacuum packaging, standing, forming, shaping and other processes, the battery of this embodiment is obtained.

Example 12

This embodiment provides a battery, and the preparation method thereof comprises the following steps:

1. Preparation of cathode electrode

The phosphate material LiMn _0.6 Fe _0.4 PO ₄ , the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 94:3:3, and then the solvent NMP was added to prepare the first cathode slurry; the lithium-rich material 0.34Li ₂ MnO ₃ •0.67LiMn _0.5 Ni _0.5 O ₂ (the particle size of the lithium-rich material is 10μm, the specific surface area is 4.5g/m ² , and the lithium content of the lithium-rich material is 9.6wt%), the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 94:3:3, and then the solvent NMP was added to prepare the second cathode slurry; the layered oxide LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , acetylene black as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder are mixed in a mass ratio of 94:3:3, and then a solvent NMP is added to prepare a third cathode slurry; the first cathode slurry is coated on the surface of the current collector 4, dried to form a phosphate active coating 3 with a thickness of 60 μm and a surface density of 120 g/m ² , and then the second cathode slurry is coated on the surface of the phosphate active coating 3, dried to form a protective layer 2 with a thickness of 10 μm and a surface density of 20 g/m ² , and then the third cathode slurry is coated on the surface of the protective layer 2, dried to form a layered oxide active coating 1 with a thickness of 40 μm and a surface density of 130 g/m ^2. Then, the cathode pole piece is obtained by rolling and slicing, in which D represents the sum of the thicknesses of the phosphate active coating 3, the protective layer 2 and the layered oxide active coating 1 which are compounded in sequence, and D=110 μm; the structural schematic diagram of the cathode pole piece prepared above is shown in FIG2.

2. Anode plate preparation

The anode active material graphite, the conductive agent acetylene black, the binder carboxymethyl cellulose (CMC), and SBR (styrene butadiene rubber) are mixed in a mass ratio of 94:1:2:3, and then deionized water is added as a solvent and stirred evenly to obtain an anode slurry; the anode slurry is evenly coated on the surface of the copper foil, and then dried, rolled, and sliced to obtain an anode sheet.

3. Preparation of electrolyte

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then the fully dried lithium salt LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol/L.

4. Preparation of diaphragm

The diaphragm is made of polyethylene diaphragm;

5. Battery cell assembly

The cathode electrode sheet, the isolation film, and the anode electrode sheet are stacked in order, the isolation film is placed between the cathode electrode sheet and the anode electrode sheet to play an isolation role, and then they are wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, and after drying, the electrolyte is injected, and after vacuum packaging, standing, forming, shaping and other processes, the battery of this embodiment is obtained.

Comparative Example 1

This comparative example provides a battery, and its preparation method comprises the following steps:

1. Preparation of cathode electrode

The phosphate material LiFePO ₄ , the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 94:3:3, and then the solvent NMP is added to prepare the first cathode slurry; the layered oxide LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 94:3:3, and then the solvent NMP is added to prepare the third cathode slurry; the third cathode slurry is coated on the surface of the current collector 4, and dried to form a layered oxide active coating 1 with a thickness of 40 μm and an area density of 130 g/m ² , and then the first cathode slurry is coated on the surface of the layered oxide active coating 1, and dried to form a phosphate active coating 3 with a thickness of 60 μm and an area density of 120 g/m ^2. Then the cathode pole piece is obtained by rolling and slicing; the structural schematic diagram of the cathode pole piece obtained above is shown in FIG3 .

2. Anode plate preparation

The anode active material graphite, the conductive agent acetylene black, the binder carboxymethyl cellulose (CMC), and SBR (styrene butadiene rubber) are mixed in a mass ratio of 94:1:2:3, and then deionized water is added as a solvent and stirred evenly to obtain an anode slurry; the anode slurry is evenly coated on the surface of the copper foil, and then dried, rolled, and sliced to obtain an anode sheet.

3. Preparation of electrolyte

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then the fully dried lithium salt LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol/L.

4. Preparation of diaphragm

The diaphragm is made of polyethylene diaphragm;

5. Battery cell assembly

The cathode electrode sheet, the isolation film, and the anode electrode sheet are stacked in order, the isolation film is placed between the cathode electrode sheet and the anode electrode sheet to play an isolation role, and then they are wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, and after drying, the electrolyte is injected, and the battery of this comparative example is obtained through vacuum packaging, standing, forming, shaping and other processes.

Comparative Example 2

This comparative example prepares a battery with reference to comparative example 1, and the difference between this comparative example and comparative example 1 is that the phosphate material is LiMn _0.6 Fe _0.4 PO ₄ . In addition to the above differences, the materials and process operations used in this comparative example are strictly consistent with those in comparative example 1.

Comparative Example 3

This comparative example prepares a battery with reference to Example 1. The difference between this comparative example and Example 1 is that the thickness of the protective layer is 1.5 μm and the surface density is 5 g/m ² . In addition to the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

Comparative Example 4

This comparative example prepares a battery with reference to Example 1. The difference between this comparative example and Example 1 is that the thickness of the protective layer is 100 μm and the surface density is 100 g/m ² . In addition to the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

Comparative Example 5

This comparative example provides a battery, and its preparation method comprises the following steps:

1. Preparation of cathode electrode

The phosphate material LiFePO ₄ , the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 94:3:3, and then the solvent NMP is added to prepare the first cathode slurry; the layered oxide LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 94:3:3, and then the solvent NMP is added to prepare the third cathode slurry; the first cathode slurry is coated on the surface of the current collector 4, and dried to form a phosphate active coating 3 with a thickness of 60 μm and an area density of 120 g/m ² , and then the third cathode slurry is coated on the surface of the phosphate active coating 3, and dried to form a layered oxide active coating 1 with a thickness of 40 μm and an area density of 130 g/m ^2. Then, the cathode pole piece is obtained by rolling and slicing; the structural schematic diagram of the cathode pole piece obtained above is shown in FIG4 .

2. Anode plate preparation

The anode active material graphite, the conductive agent acetylene black, the binder carboxymethyl cellulose (CMC), and SBR (styrene butadiene rubber) are mixed in a mass ratio of 94:1:2:3, and then deionized water is added as a solvent and stirred evenly to obtain an anode slurry; the anode slurry is evenly coated on the surface of the copper foil, and then dried, rolled, and sliced to obtain an anode sheet.

3. Preparation of electrolyte

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then the fully dried lithium salt LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol/L.

4. Preparation of diaphragm

The diaphragm is made of polyethylene diaphragm;

5. Battery cell assembly

The cathode electrode sheet, the isolation film, and the anode electrode sheet are stacked in order, the isolation film is placed between the cathode electrode sheet and the anode electrode sheet to play an isolation role, and then they are wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, and after drying, the electrolyte is injected, and the battery of this comparative example is obtained through vacuum packaging, standing, forming, shaping and other processes.

Comparative Example 6

The battery of this comparative example is prepared with reference to comparative example 5. The difference between this comparative example and comparative example 5 is that the phosphate material is LiMn _0.6 Fe _0.4 PO ₄ . In addition to the above difference, the materials and process operations used in this comparative example are strictly consistent with those in comparative example 5.

Test Case

1. Participants

In this test example, the cathode plates and batteries prepared in Examples 1 to 12 and Comparative Examples 1 to 6 were used as test objects to conduct relevant performance tests.

2. Test content

(1) Surface resistance

The resistance of the cathode electrode was tested and recorded using the diaphragm resistance test equipment TT-ACCF-G2 at a pressure of 0.4 T. The surface resistance of the cathode electrode was calculated according to the following formula: surface resistance = resistance/area of the electrode.

(2) Cycle performance

The test battery was placed in a 45℃ constant temperature box, charged at 1C constant current and constant voltage, cut off at 0.02C, and then discharged at 1C, cycled to 80% SOH, and the number of cycles was recorded.

(3) Energy density

The test battery was charged to 4.25V at 0.33C constant current and constant voltage, cut off at 0.02C, and then discharged to 2.8V at 0.33C. The capacity, average voltage and cell mass were recorded. The energy density of the battery was calculated according to the following formula: Energy density = capacity * average voltage / battery mass.

3. Test results

Table 1. Performance test results of the batteries of Examples 1 to 12 and Comparative Examples 1 to 6

Group Sheet resistance (mΩ/mm2) Number of cycles (cycles) Energy density (Wh/kg) Example 1 0.05 3000 240 Example 2 0.07 2400 245 Example 3 0.06 3000 220 Example 4 0.05 3000 235 Example 5 0.05 3000 230 Example 6 0.05 3400 250 Example 7 0.06 3200 255 Example 8 0.07 2400 240 Example 9 0.07 2000 225 Example 10 0.05 2000 225 Embodiment 11 0.05 2900 240 Example 12 0.07 2300 245 Comparative Example 1 0.04 1800 220 Comparative Example 2 0.04 1500 230 Comparative Example 3 0.05 1500 210 Comparative Example 4 0.1 1600 220 Comparative Example 5 0.04 1600 220 Comparative Example 6 0.04 1300 230

The relevant performance test results of the batteries prepared in Examples 1 to 12 and Comparative Examples 1 to 6 are shown in Table 1.

The performance test results corresponding to Examples 1 and 11 are compared with those of Comparative Examples 1 and 5. As shown in Table 1, under the same conditions of preparing the phosphate material and other materials for the battery, compared with Examples 1 and 11, the cathode electrode pieces prepared in Comparative Examples 1 and 5 are only provided with a phosphate active coating and a layered oxide active coating, but no protective layer. Since the working voltage of the phosphate material is relatively low, it does not match the working voltage of the layered oxide. The cathode electrode piece integrating the above two materials is prone to lose active lithium at high voltage, resulting in unstable electrode structure and deterioration of performance. The cycle performance of the battery obtained is significantly lower than that of the battery prepared in Examples 1 and 11, and the energy density is lower than that of the battery prepared in Examples 1 and 11. The performance test results corresponding to Examples 2 and 12 are compared with those of Comparative Examples 2 and 6. It can be seen from Table 1 that under the same conditions of preparing the phosphate material and other materials for the battery, compared with Examples 2 and 12, the cathode electrode pieces prepared in Comparative Examples 2 and 6 are only provided with a phosphate active coating and a layered oxide active coating, but no protective layer is provided. Since the working voltage of the phosphate material is relatively low, it does not match the working voltage of the layered oxide. The cathode electrode piece integrating the above two materials is prone to unstable electrode structure and deterioration of performance due to the loss of active lithium under high voltage. The cycle performance of the battery obtained thereby is significantly lower than that of the battery prepared in Examples 2 and 12, and the energy density is lower than that of the battery prepared in Examples 2 and 12. This shows that, relative to comparative examples 1-2 and 5-6, the cathode pole pieces provided in embodiments 1-2 and 11-12 introduce a protective layer, and by setting a protective layer between the phosphate active coating and the layered oxide active coating, the layered oxide and phosphate materials with different working voltages are separated. The protective layer can preferentially delithium under high voltage, thereby compensating for the loss of active lithium of the phosphate material under high voltage, avoiding excessive delithiation of the phosphate material, playing a protective role of the lithium-rich material on the phosphate material, and realizing stable lithium deintercalation between the layered oxide and the phosphate material; thereby, the high cycle performance advantages of the phosphate material and the high capacity advantages of the layered oxide are fully utilized in the same cathode pole piece, and it is ensured that the cathode pole piece as a whole has excellent structural stability and cycle stability under high voltage.

The performance test results corresponding to Examples 1, 6-10 and Comparative Examples 3-4 are compared. It can be seen from Table 1 that, under the same conditions of other materials and operations for preparing the battery, the thickness ratio of the protective layer to the phosphate active coating in the cathode electrode sheet prepared in Comparative Example 3 does not meet 6-60:40-150, the surface density of the protective layer is <10g/m ² , and the cycle performance of the battery obtained thereby is significantly lower than that of the battery prepared in Examples 1, 6-10, and the energy density is lower than that of the battery prepared in Examples 1, 6-10; the thickness ratio of the protective layer to the phosphate active coating in the cathode electrode sheet prepared in Comparative Example 4 does not meet 6-60:60-150, the surface density of the protective layer is >90g/m ² , and the cycle performance of the battery obtained thereby is significantly lower than that of the battery prepared in Examples 1, 6-10, and the energy density is lower than that of the battery prepared in Examples 1, 6-10. This shows that, relative to comparative examples 3 to 4, the cathode plates provided in Examples 1 and 6 to 10 reasonably set the thickness ratio and the surface density of the protective layer to the phosphate active coating, so that the contents of the lithium-rich material in the protective layer and the phosphate material in the phosphate active coating are controlled within a suitable range. On the basis of ensuring that the protective layer effectively exerts its preferential lithium removal effect under high voltage, the composite effect of the phosphate active coating and the protective layer is improved, and the problems such as poor structural stability caused by the different working voltages of the protective layer and the phosphate active coating are prevented, thereby improving the electrochemical performance and cycle performance of the prepared cathode plate under high voltage.

The performance test results of Example 1 are compared with those of Examples 6 to 10. As shown in Table 1, under the same conditions of other materials and operations for preparing the battery, the thickness of the protective layer set in Example 9 is greater than 60 μm, and the content of the lithium-rich material in the protective layer is high, so that the protective layer has a lower conductivity and a higher impedance, which reduces the cycle performance of the electrode piece, and the energy density and cycle performance of the battery obtained are lower than those of the battery prepared in Example 1; the thickness of the protective layer set in Example 10 is less than 6 μm, and the content of the lithium-rich material in the protective layer is small, which is difficult to achieve a good lithium removal and lithium replenishment effect, and the energy density and cycle performance of the battery obtained are lower than those of the battery prepared in Example 1. This shows that, compared with Examples 9 and 10, the cathode electrode pieces provided in Examples 1, 6 to 8 can effectively play the protective role of the lithium-rich material on the phosphate material by reasonably setting the thickness of the protective layer while ensuring that the impedance of the battery is at a low level, avoiding excessive lithium removal of the phosphate material, so that the prepared battery has excellent electrochemical performance and cycle performance.

Claims (11)

A cathode electrode sheet, comprising a current collector and a composite active coating disposed on the surface of the current collector; in the thickness direction of the composite active coating, the composite active coating comprises a layered oxide active coating, a protective layer and a phosphate active coating which are composited in sequence, the phosphate active coating comprises a phosphate material, the protective layer comprises a lithium-rich material, and the layered oxide active coating comprises a layered oxide; The thickness ratio of the protective layer to the phosphate active coating is 6-60:40-150; The surface density of the protective layer is 10-90 g/m ² , and the surface density of the phosphate active coating is 90-280 g/m ² . The cathode electrode according to claim 1, wherein the thickness of the protective layer is 6-60 μm; and the thickness of the phosphate active coating is 40-150 μm. The cathode electrode according to claim 2, wherein the thickness of the layered oxide active coating is 5-200 μm and the surface density is 30-200 g/m ² . The cathode electrode according to any one of claims 1 to 3, wherein D represents the sum of the thicknesses of the layered oxide active coating, the protective layer and the phosphate active coating that are composited in sequence, and D satisfies: 50 μm ≤ D ≤ 300 μm. The cathode electrode as claimed in claim 1, wherein the lithium-rich material includes at least one of lithium-rich manganese-based materials, lithium ferrite, and lithium nickel oxide. The cathode electrode according to claim 5, wherein the lithium-rich material is a lithium-rich manganese-based material, the thickness of the protective layer is 6-60 μm, and the surface density is 10-90 g/m ² . The cathode electrode as claimed in claim 6, wherein the lithium content of the lithium-rich manganese-based material is 7-11wt% calculated as a percentage by mass. The cathode electrode according to claim 7, wherein the particle size of the lithium-rich manganese-based material is 2-13 μm, and the specific surface area is 0.5-5 g/m ² . The cathode electrode according to claim 5, wherein the lithium-rich material is lithium ferrite or lithium nickelate, the thickness of the protective layer is 10-50 μm, and the surface density is 1-70 g/m ² . In the cathode electrode as claimed in claim 5, when the lithium-rich material is charged to 4.6V for the first time, the charging capacity contributed by the unit Li content is recorded as C, wherein: If the lithium-rich material is a lithium-rich manganese-based material, the charging capacity contributed by the unit Li content in the lithium-rich manganese-based material is recorded as C1, where 3000mAh/g≤C1≤3600mAh/g; If the lithium-rich material is a lithium ferrite material, the charging capacity contributed by the unit Li content in the lithium ferrite material is recorded as C2, where 2100mAh/g≤C2≤3200mAh/g; If the lithium-rich material is a lithium nickel oxide material, the charging capacity contributed by the unit Li content in the lithium nickel oxide material is recorded as C3, wherein 2400mAh/g≤C3≤3100mAh/g. A battery comprising the cathode electrode as claimed in any one of claims 1 to 10.