WO2014064392A1 - Iron and lithium hydroxysulfate - Google Patents

Abstract

The invention relates to a material with monoclinic structure, having formula Li_xFeSO₄OH, in which x is less than or equal to 1, the production method thereof, an electrode comprising such a material as an active electrode material, and an electrochemical cell comprising such an electrode.

Description

 Lithium and iron hydroxysulfate

 The present invention relates to a hydroxysulfate of lithium and iron useful as electrode active material, and a process for its preparation.

 Lithium batteries have become indispensable components in most electronic devices and are widely studied for use in electric vehicles as well as in the field of energy storage.

 Lithium batteries have a cathode in which the active ingredient is a compound capable of reversibly inserting lithium ions, an electrolyte comprising an easily dissociable lithium salt, and an anode whose active ingredient is lithium Li-foil. or a lithium alloy, or a compound capable of reversibly inserting lithium ions at a potential lower than that of the active material of the cathode.

 The various components of a lithium battery are chosen so as to produce at the lowest possible cost batteries that have a high energy density and operate safely.

In the most commonly used batteries, the active material of the cathode is LiCoI / 3Nii _{/ 3} Mn _1/3 O2, LiMn ₂ O ₄ or LiFePO ₄ , the electrolyte is a solution of lithium salt (for example LiPF ₆ in a polar aprotic organic solvent), and the active material of the anode is a lithium insertion material (for example graphite). In this type of battery, called "lithium ion battery", the energy density and the cost are essentially related to the choice of the active ingredient of the cathode.

 Many studies concern cathode materials consisting of lamellar oxides whose capacity reaches 250 niAh / g for a redox potential of the order of 3.8 V. However, the cost of these materials remains high. they contain Co and / or Ni.

The iron-based compounds, in particular LiFePO ₄ , Li ₂ FeSiO ₄ , and LiFeBO ₃ , are economically interesting especially because of the abundance of Fe sources. However, these materials generate a lower energy density than those of lamellar oxides.

It has been proposed to remedy this drawback by using fluorinated compounds such as, for example, LiCoPO ₄ F which has a redox potential of the order of 5.2 V. However, the electrolytes generally used are not stable at such high potential. Other fluorinated compounds such as fluoro sulfate of formula A _y s MSO ₄ F wherein A is Li, Na or K, M is a 3d metal and y is between 0 and 1 were studied. These compounds have a high redox potential with respect to Li ° / Li ⁺ . For example; LiFeSO ₄ F has a redox potential of 3.6 V in the tavorite form and a redox potential of 3.9 V in the triplite form. The triplite form is the most interesting, and it can be obtained by solid route. However, this compound has the disadvantage of containing fluorine.

A non-fluorinated compound was obtained by reacting Li ₂ SO ₄ and FeSO ₄ at 300 ° C under an inert atmosphere, preferably under argon (M. eynaud, M. Ati, BC Melot, MT Sougrati, G. Rousse, J. Chotard, J.-M. Tarascon, Electrochem Comm 2012, 21, 77-80). This compound corresponds to the formula Li ₂ Fe (SO ₄ ) ₂ and has a redox potential of 3.8 V vs. Li. However, it has a low specific capacity because two polyanion groups (SO ₄ ) are associated with a single redox pair Fe ³⁺ / Fe ²⁺ ).

The use of a LiFeSO ₄ OH compound has also been considered. However, the preparation of this compound proved impossible under normal conditions of temperature and pressure using the usual synthetic routes by ceramic, solvothermally, or ionothermally.

 The object of the present invention is to provide a material usable as a cathode active material in a lithium battery and producing a high energy density in good safety conditions, said material being inexpensive and fluorine-free.

The material of the present invention has the formula Li _x FeSO ₄ OH in which x is less than or equal to 1, which has a monoclinic structure with the following conditions:

when x = l, the mesh parameters are a = 9.516 (2) Å, b = 5.4951 (9) λ, c = 7.377 (l) λ, and β = 109.209 (5) ° (giving a volume V = abc * sin (P) = 364,27A ³ ), and

 when x decreases, the mesh parameters decrease.

According to the invention, the expression "when x decreases, the mesh parameters decrease" means that when x decreases, the parameters a, b and c decrease. Indeed, the material of the present invention is lamellar, that is to say that it has a crystallographic structure that can be schematized by a stack of 2D sheets along the z axis, the z axis being characterized by the parameter c. It is therefore understood that when x decreases, that is to say when lithium ions are removed in said material, the mesh parameters a, b and c decrease.

 Figure 1 shows an X-ray diffraction pattern for the compound in which x = 0.01 (upper curve) and the compound in which x = 1 (lower curve). It shows that the peaks move to higher angle values as the Li content decreases, indicating that the mesh parameters and the volume decrease, while the symmetry remains.

The material of the invention has an average redox potential of 3.6 V vs. Li. This redox potential, combined with its ability to reversibly deink 0.9 Li ⁺ , gives it an energy density equivalent to that of the LiFeSO ₄ F material and greater than that of the Li ₂ Fe (SO ₄ ) _{2 material} due to of the molar mass of the latter although its redox potential is higher.

 The material of the invention is useful as the active material of an electrode for forming the cathode of a lithium battery.

The process for preparing the Li _x FeSO ₄ OH material of the invention in which x = 1 consists in preparing a mixture of FeSO ₄ powder and LiOH powder, and in subjecting said mixture to grinding, and is characterized in that that: the molar ratio LiOH / FeSO ₄ in the powder mixture is greater than 0.5;

 the grinding is carried out in a ball mill without adding heat and keeping the powder mixture at a temperature below 300 ° C.

When the FeSO ₄ and LiOH powders are subjected to grinding, a Li ₂ SO ₄ + Fe ₃ (SO ₄ ) ₂ .2OH + LiOH (amorphous) mixture is first formed, followed by the LiFeSO ₄ monoclinic structure compound. OH of the invention according to the following reaction:

Fe ₃ (SO ₄ ) ₂ (OH) ₂ + Li ₂ SO _{4 +} LiOH → 3 LiFeSO ₄ OH

Traces of amorphous FeSO ₄ may be present in the material thus obtained.

The intermediate material Fe (SO ₄ ) ₂ (OH) ₂ can be isolated as a single phase by oxidizing the mixture Li ₂ SO ₄ + Fe (SO ₄ ) ₂ .2OH + LiOH (amorphous) with NO ₂ BF ₄ which has the advantage of eliminating Li ₂ SO ₄ and H ₂ O. Fe (SO ₄ ) ₂ (OH) ₂ is a new material, which is isostructural with Mg ₃ (SO ₄ ) ₂ (OH) ₂ and which possesses the the following mesh parameters: a = 8.074Â, b = 6.524Â, c = 6.3027Â, β = 1 1 1.533Â °, V = 308.884 (0.134) to ³ .

Maintaining the powders at a temperature below 300 ° C during grinding is intended to avoid various parasitic reactions, including an internal oxidation-reduction reaction between the two pairs S ⁶⁺ / S ⁴⁺ and Fe ³⁺ / Fe ²⁺ , especially when the grinding jar contains traces of water.

The molar ratio LiOH / FeSO ₄ in the powder mixture subjected to grinding is preferably from 1 to 1.5.

In a preferred embodiment, a ball mill containing a mass of balls M _b such that the ratio <M _b / M _p <60 is used, M _p being the mass of the powder to be ground.

 Preferably, the volume occupied by the mixture of powders to be ground is less than 1/3 of the volume of the grinding jar which contains it.

 The method of the invention can be implemented in a ball mill operating by a vibratory movement of the balls, or in a ball mill operating by a centrifugal movement of the balls.

As an example of a ball mill operating by vibratory movement, mention may be made of the Spex P8000 mill in which a metal jar having a volume of 25 cm ³ contains the powders to be ground and the grinding balls, the jar being subjected to a vibratory movement. following the three directions of space with a vibration frequency of up to 14 Hz.

As an example of a mill operating by a centrifugal movement of the balls, one can mention the Retsch PM 100 mill. This mill operates with a gear ratio of 1 / (-1) and a speed of up to 650 rpm. Under the effect of the centrifugal forces generated by the rotation, the balls move and crush the powders to grind against the inner wall of the jar (which has a volume of 50 cm ³ ). The grinding is then essentially carried out by pressure and friction. The combination of shock forces and friction forces thus created ensures a high and highly efficient grinding degree of planetary ball mills.

The duration of grinding depends on the energy developed by the mill and the amount of powder to grind. In order to avoid an increase of the temperature above 300 ° C, it may be necessary to carry out the grinding in several sequences, separated by pause times to cool the grinding jar and the powders it contains. . For example, in a Spex P8000 grinder or grinder etsch PM 100, a series of "30 minutes grinding" / 15 minute break sequences can be implemented.) In said grinders, the actual grinding time (excluding break time) to obtain the phase (Fe ₃ (SO ₄ ) 2 (OH) ₂ ) can vary between 30 minutes and 3 hours, preferably between 1 and 3 hours, and even more preferably between 30 minutes and 1 hour, and then the actual grinding time to obtain the phase LiFeSO ₄ OH may be at least 2 hours, and preferably at least 90 minutes, using a bead mass / powder mass ratio of> 30.

A compound according to the invention is useful as a cathode active material in a battery operated by lithium ion circulation between an anode and a cathode. Another object of the invention is a cathode containing hydroxysulfate LiFeSO ₄ OH as active material, as well as a battery containing it.

 A cathode according to the invention comprises a current collector carrying a layer of cathode material which contains the hydroxysulfate of the invention. The cathode material may further contain a binder polymer and / or an electronic conduction agent. Preferably, the cathode material contains at least 80% by weight of hydroxy sulfate.

 When charging the battery, the active material of the cathode releases lithium ions which are transferred to the anode through the electrolyte. During the discharge of the battery, lithium ions are inserted in the active material of the cathode. Insertion / de-insertion reactions occur at the redox potential of the cathode active material.

A Li _x FeSO ₄ OH material in which x <1 is obtained when the LiFeSO ₄ OH material is used as the active material of an electrode used as a positive electrode in an electrochemical cell operating as a lithium battery. The lithium content decreases during the charging phase of said electrochemical cell.

In a so-called "lithium" battery, the anode consists of a lithium Li Li sheet, a lithium alloy or an intermetallic lithium compound. In a so-called "lithium ion" battery, the anode comprises an anode material on a current collector, said anode material containing a compound in which the lithium ions can be reversibly inserted at a lower potential. the redox potential of the active material of the cathode, and optionally a polymeric binder. The electrolyte of a battery according to the invention comprises a lithium salt dissolved in a solvent. The solvent may be a polar aprotic liquid solvent, a solvating polymer optionally plasticized with a liquid solvent or an ionic liquid, or a gel consisting of a gelled liquid solvent by addition of a solvating or non-solvating polymer.

The lithium salt may be chosen from those conventionally used in lithium or lithium-ion batteries, in particular salts having a ClO ₄ ^" , BF ₄ ^" , PF ₆ ^" anion, and salts having a perfluoroalkanesulfonate anion, bis (perfluoroalkylsulfonyl) imide, bis (perfluoroalkylsulfonyl) methane or tris (perfluoroalkylsulfonyl) methane.

 The present invention is illustrated by the following examples, to which it is however not limited.

Example 1

Preparation of FeS0 ₄

10 g of FeSO ₄ .7H ₂ O and 50 g of ascorbic acid were dissolved in 50 ml of distilled water under an argon atmosphere at room temperature. Then, an equivalent volume of anhydrous ethanol was added, which caused the formation of a precipitate which was isolated by centrifugation. The recovered compound was subjected to the same treatment three times. Then, the precipitate was separated by centrifugation and dried under vacuum at 70 ° C. The green powder thus obtained was heated under argon flow at a rate of 1 ° C per minute to 290 ° C, and held at 290 ° C for 5 hours. Then, it was allowed to cool to room temperature.

X-ray diffraction analysis (XRD) shows that it is a-FeSO ₄ stable at room temperature.

The α-FeSO ₄ powder was stored under an argon atmosphere until it was used.

Example 2

 Preparation of LiFeSC ^ OH in an SPEX mill

An SPEX-8000® mill was used comprising a 25 cm ³ stainless steel reactor containing 6 stainless steel balls each having a weight of 7 g and a diameter of 12 mm.

By operating under an argon atmosphere, 1.2988 g of FeSO ₄ obtained according to Example 1 and 0.221 g of LiOH (corresponding to one LiOH / FeSO ₄ molar ratio of 1.08), and was milled for different times.

 Characterization by X-ray diffraction (XRD)

 Characterization by XRD (Ka cobalt line) was carried out on the product obtained after 5, 30, and 90 minutes of grinding, respectively.

 The diagrams are shown in FIG. 2. The curves 5, 30 and 90 correspond to the diagram of the product obtained respectively after 5 minutes, 30 minutes and 90 minutes of grinding. The intensity I (in arbitrary units) is indicated on the ordinate, as a function of wavelength 2Θ (in degrees) on the abscissa.

 After 5 minutes, the material essentially contains the pristine phase

After 30 minutes, we note:

peak growth at 29 = 26 ° corresponding to Li ₂ SO ₄ ;

a peak at 2Θ = 30 ° corresponding to the Fe ₃ (SO ₄ ) ₂ .2OH, isostructural phase of the Mg ₃ (SO ₄ ) _2. 2OH phase described in particular in: "Crystal structure of the new natural magnesium sulfate Mg ₃ ( SO ₄ ) ₂ (OH) ₂ "Yamnova, NA; Pushcharovskii, D.Yu .; Apollonov, VN, Moskovskogo Universiteta Vestnik, Geologiya (1989) 44, p73-p75.

 After 90 minutes, the DRX curve is totally different from the previous ones. Indexation of the peaks reveals a monoclinic structure with the mesh parameters a = 9.516 (2) Å, b = 5.4951 (9) Å, c = 7.377 (1) Å = 109.209 (5) ° which has never has been listed in the prior art.

 Characterization by High Resolution Transmission Electron Microscopy (METHR)

The product obtained after 90 minutes of grinding was also characterized by METHR. The electron diffraction patterns made it possible to deduce the Plilc space group and, together with the X-ray diffraction measurements, to compare the simulated diagram with the measured diagram (FIG. 3). The good agreement indicates the robustness of the glare and consequently the validity of the structure. Example 3

Preparation of LiFeSO ₄ OH in an SPEX mill

The procedure of Example 2 was repeated, but using different LiOH / FeSO ₄ molar ratios of 0.5, 1.08 and 1.2. The products obtained after 90 minutes of grinding were characterized by XRD. The corresponding curves are shown in FIG. 4. They show that the material of the invention LiFeSO ₄ OH can only be obtained for a molar ratio LiOH / FeSO ₄ greater than 0.5.

Example 4

Preparation of LiFeSO ₄ OH in a RETSCH mill

A RETSCH mill was used comprising a 50 cm ³ stainless steel reactor containing 8 balls of 7 g each made of stainless steel.

Working under an argon atmosphere, 1.2988 g of FeSO ₄ obtained according to Example 1 and 0.221 1 g of LiOH (corresponding to a molar ratio of LiOH / FeSO ₄ of 1.08) were introduced into the reactor, and it was subjected to grinding for different durations, with a rotational speed of 600 rpm.

 Characterization by XRD

 Characterization by XRD was performed on the product obtained after different grinding times.

The diagrams are shown in FIG. 5. The intensity I (in arbitrary units) is indicated on the ordinate, as a function of wavelength 2Θ (in degrees) on the abscissa. FIG. 5 shows that, at the beginning of grinding, the Is ₃ (SO ₄ ) ₂ (OH) ₂ isostructural to Mg ₃ (SO ₄ ) ₂ (OH) ₂ phase which is transformed into the LiFeSO ₄ OH phase 2 hours. The process is therefore slower when a RETSCH mill is used instead of a Spex 8000 mill.

Example 5

 Electrochemical characterization

An electrochemical cell, under an argon atmosphere, was assembled from a lithium metal disc forming the anode, a borosilicate glass fiber film (Whatman GF / D) impregnated with a 1 M solution of LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate (DMC) (mass ratio 1/1) forming the electrolyte, and a cathode comprising the material of Example 1. The cathode was developed from 8 g of a mixture containing 80% by weight of LiFeSO ₄ OH powder obtained according to Example 2 and 20% carbon powder, the mixture being stirred for 15 minutes in a Spex-8000 mill under an argon atmosphere.

The electrochemical cell thus obtained was cycled in a static galvano mode at 20 ° C. using a VMP system (Biology SA, Claix, France). The cycling was carried out between 4.2 and 2.5 V vs. Li ⁺ / Li °, with exchange of 1 Li ⁺ ion per period of 10 hours. The cycling curves are shown in FIG. 6. The potential U (in Volts) is given as a function of the x-rate of lithium ions in the cathode material. Figure 6 shows that during the ^era charging up to 4.2 V vs. Li + / Li °, the structure of the LiFeSO ₄ OH compound lost 0.75 Li ⁺ ion. The discharge causes the re-insertion of about 0.6 Li ⁺ ion into the structure of the LiFeSO ₄ OH compound, so that the reversible capacity is HO mAh / g, representing 75% of the theoretical capacity which is 152.4 mAh /boy Wut.

Example 6

 Characterization of Lio ^ FeSC ^ OH

The electrochemical cell as obtained in Example 5 was subjected to galvanostatic cycling at 20 ° C. using a potentiostat sold under the trade name VMP by Bio-Logic SA (Claix, France). Cycling was performed at 4.5 V vs. Li ⁺ / Li °, with a current regime of C / 30. The resulting solution was washed with DMC and dried to afford Li _0; iFeSO ₄ OH. The electron diffraction patterns of the compound obtained Li _0; iFeSO ₄ OH allowed to deduce the space group Plilc and the XRD curve revealed a monoclinic structure having the following parameters of mesh: a = 9.481 (3) À, b = 5.296 (2) À, c = 7.207 (2 ), Β = 1 10.55 (3) ° and V = 338.9 (2) to ³ .

Claims

1. Material having the formula Li _x FeSO ₄ OH wherein x is less than or equal to 1, which has a monoclinic structure with the following conditions:  when x = 1, the mesh parameters are a = 9.516 (2) Å, b = 5.4951 (9) λ, c = 7.377 (1) λ, and β = 109.209 (5) °, and  when x decreases, the mesh parameters decrease. 2. Process for preparing the Li _x FeSO ₄ OH material according to claim 1 wherein x = 1, consisting of preparing a mixture of FeSO ₄ powder and LiOH powder, and subjecting said mixture to grinding, characterized in that than : the molar ratio LiOH / FeSO ₄ in the powder mixture is greater than 0.5;  the grinding is carried out in a ball mill without adding heat and keeping the powder mixture at a temperature below 300 ° C. 3. Method according to claim 2, characterized in that the molar ratio LiOH / FeSO ₄ is from 1 to 1.5. 4. The method of claim 2, characterized in that a ball mill containing a mass of balls M _b such that the ratio 30 <M _b / M _w <60, M _w being the mass of the powder to be ground .  5. Method according to claim 2, characterized in that the volume occupied by the mixture of powders to be ground is less than 1/3 of the volume of the grinding jar which contains it.  6. Method according to claim 2, characterized in that it is implemented in a ball mill operating by a vibratory motion of the balls, or in a ball mill operating by a centrifugal movement of the balls. An electrode comprising a current collector carrying a layer of electrode material, characterized in that the electrode material contains the Li _x FeSO ₄ OH material of claim 1. 8. Electrode according to claim 7, characterized in that the content of Li _x FeSO ₄ OH in the electrode material is greater than or equal to 80% by weight.  An electrode according to claim 7, characterized in that the electrode material further contains a binder polymer and / or an electronic conduction agent.  An electrochemical cell comprising a cathode and anode separated by an electrolyte and functioning as a lithium battery, characterized in that the cathode is an electrode according to claim 7.  1. An electrochemical cell according to claim 10, characterized in that the anode is constituted by:  a lithium Li Li sheet, a lithium alloy or an intermetallic lithium compound, or by  an anode material on a current collector, said anode material containing a compound in which the lithium ions can reversibly insert at a potential lower than the redox potential of the active material of the cathode, and optionally a polymer binder.  12. Electrochemical cell according to claim 10, characterized in that the electrolyte comprises a lithium salt dissolved in a solvent, said solvent being chosen from:  polar aprotic liquid solvents,  the solvating polymers optionally plasticized with a liquid solvent or an ionic liquid,  gels constituted by a gelled liquid solvent by adding a solvating or non-solvating polymer. 13. Electrochemical cell according to claim 10, characterized in that the lithium salt is chosen from salts having an anion ClO ₄ ^" , BF ₄ ^~ , PF ₆ ^~ , and salts having an anion perfluoroalkanesulfonate, bis (perfluoroalkylsulfonyl) imide bis (perfluoroalkylsulfonyl) methane or tris (perfluoroalkylsulfonyl) methane. 14. Process for the preparation of a Li _x FeSO ₄ OH material in which x <1, characterized in that it consists in subjecting a charge cell to an electrochemical cell according to claim 10.