SULFIDE-TYPE CERAMIC ELECTROLYTES

Abstract

SULFIDE CERAMIC ELECTROLYTES The present invention relates to solid sulfide electrolytes having improved conductivity, their preparation process and the electrochemical cells and batteries containing them. Figure for abstract: None

Description

Title of the invention: SULFIDE-TYPE CERAMIC ELECTROLYTES

[0001] The present invention relates to the field of batteries, and in particular solid electrolyte batteries, of the sulfide type.

[0002] Solid sulfide electrolytes are reaching sufficient maturity to consider their industrial use. Their high ionic conductivity values ​​combined with their ductility and limited density make them serious candidates for the first generations of all-solid-state batteries capable of competing with the energy densities of current Li-ion accumulators with liquid electrolytes.

[0003] Recent progress in the field of sulfide electrolytes concerns in particular the discovery of new chemical compositions and new crystallographic structures making it possible to improve performance, in particular in terms of chemical and electrochemical stability, ductility, conductivity, etc.

[0004] US 2016 / 0149258 relates to solid sulfide electrolytes and describes in particular electrolytes of formula Li3x+2y+3P|XS4 and Li5x+3Pi_xS4, i.e. depleted in phosphorus.

[0005] Indeed, increasing the Li / P ratio is generally sought to improve conductivity. However, these lithium-rich materials are in fact more expensive.

[0006] New compounds based on sulfur, lithium and phosphorus supplemented with phosphorus have now been discovered. These compounds correspond to phases that have not yet been described.

[0007] Thus, according to a first object, the present invention relates to a compound of formula (I):

[0008] (Li3P1+xS4)i_y(LiX)y

[0009] (I)

[0010] in which

[0011] 0 < x < 0.2 ;

[0012] 0 < y < 0.3 ;

[0013] Each X, identical or different for each LiX group, represents a halogen atom chosen from Cl, I, Br, F.

[0014] In particular, the conductivity measurements have shown that this composition range makes it possible to improve the conductivity, compared to that of the compound Li3PS4, in particular for compounds of formula (I) and Li3PS4 synthesized under the same conditions.

[0015] The following embodiments may be cited, each of the embodiments being able to be considered in isolation or according to each of their possible combinations:

[0016] According to one embodiment, these phases notably imply the existence of a singularity within the composition domain Li2S - P(2+X)S5.

[0017] More particularly, the DRX structure analysis carried out on this composition range is marked by the appearance of diffraction peaks not corresponding to the precursors P2S5 and Li2S, nor to the known reference compounds Li3PS4 or Li4P2S6.

[0018] According to one embodiment, in formula (I) y=0.

[0019] According to one embodiment, x is between 0.04 and 0.14.

[0020] According to one embodiment, the compounds of formula (I) are chosen from Li3Pij04 S4, Li3Pi o9S4, and mixtures thereof, more particularly the compound LI5P109S4

[0021] According to one embodiment, the compounds of formula (I) are in crystalline or partially crystalline form.

[0022] In particular, the compounds of formula (I) exhibit in particular an X-ray diffraction (XRD) peak at 20 = 19.1° + / - 0.25 obtained with the K(alpha) line of copper.

[0023] Furthermore, according to one embodiment, the ratio between the maximum intensity of the diffraction spectrum in the interval Imax [17°; 18.5°] relative to the maximum intensity of the spectrum in the interval Imax [18.5°; 19.5°] is greater than 0.1, preferably between 0.1 and 1.00.

[0024] Furthermore, according to one embodiment, the intensity ratio of the signal at 1=34.00° relative to the maximum intensity of the diffraction spectrum in the interval Imax [29.5°; 31°] is greater than 0.04, preferably between 0.04 and 1.00.

[0025] The intensity values ​​correspond to the difference between the value of the spectrum signal and the value of the signal corresponding to the background noise of the spectrum.

[0026] According to another object, the present invention also relates to the process for preparing a compound as defined above, said process comprising:

[0027] the step of mixing powders of the precursors P2S5 and Li2S,

[0028] the addition of phosphorus of zero oxidation state, then

[0029] the treatment of the mixture thus obtained.

[0030] P2S5 and Li2S precursors are commercially available, for example, these materials are available from Aldrich or Alfa Aesar.

[0031] Typically, the precursors are in crystalline form. According to one embodiment, the treatment can be carried out by mechanical grinding or by heating in particular.

[0032] According to one embodiment, the heating is carried out at a temperature below 300°C, typically at a temperature between 175 and 225°C.

[0033] According to one embodiment, the co-grinding can be carried out by mixing said precursors in the desired proportions, typically, according to the proportions respecting the molar ratios required by formula (I).

[0034] According to one embodiment, the co-grinding can be carried out at room temperature.

[0035] According to one embodiment, the co-grinding can be carried out by means of a ball mill. Typically, the co-grinding can be carried out by a mill marketed by Fritsch (Fritsch 7), with balls of diameter between 0.1 and 15 mm, in bowls of 10 to 50 ml, during cycles lasting between 1 min and 2 hours for a total duration of between 5 and 100 h, at a rotation speed of between 100 and 1000 rpm. Typically, the particle size of the mixture after co-grinding is less than 20 μm, in particular less than 5 μm.

[0036] According to one embodiment, the precursors P2S5 and Li2S are mixed in contents such that the respective molar ratio n(P2S5) / n(Li2S) is between 2.5 and 2.98%.

[0037] Typically, the phosphorus is added in an amount such that the molar ratio n(phosphorus at oxidation state 0) / (nP2S5 + nLi2S) is between 0.01 and 0.10.

[0038] According to another object, the present invention relates to a solid sulfide electrolyte for a battery comprising a compound of formula (I) according to the invention.

[0039] More particularly, said solid sulfide electrolyte has a lithium ion conductivity value at room temperature greater than that of Li3PS4, in particular when said compound of formula (I) and Li3PS4 are synthesized under the same conditions.

[0040] According to one embodiment, said electrolyte is suitable for “all solid” type batteries.

[0041] According to another object, the present invention relates to an electrochemical element comprising an electrolyte according to the invention. More particularly, said electrochemical element is an all-solid element comprising a cathode layer, an anodic layer and an electrolyte layer between the anodic and cathodic layers, such that said electrolyte layer contains the solid sulfide electrolyte according to the invention.

[0042] The electrochemical element according to the invention is particularly suitable for lithium accumulators, such as Li-ion, primary Li (non-rechargeable) and Li-S accumulators as well as their equivalents with other alkaline elements (Na-ion, K-ion, etc.) for the corresponding formulations.

[0043] The invention also relates to a module comprising the stack of at least two electrochemical elements according to the invention, each element being electrically connected with one or more other element(s).

[0044] The term “module” therefore designates here the assembly of several electrochemical elements.

[0045] According to another object, the present invention also relates to a battery comprising one or more modules according to the invention.

[0046] The term "battery" or "accumulator" therefore means here the assembly of several modules, said assemblies can be in series and / or parallel. The invention preferably relates to accumulators whose capacity is greater than 100 mAh, typically 1 to 100 Ah. Brief description of the drawings

[0047] [Fig-1] [Fig.l] represents the ternary diagram of sulfur, lithium and phosphorus (A) and the X-ray diffraction spectrum of compositions according to the binary domain Li2S - P(2+X)S5(B).

[0048] [Fig.2] [Fig.2] represents the conductivity and activation energy measurements according to the binary domain Li2S - P(2+X)S5.

[0049] [Fig.3] [Fig.3] illustrates the stability in a symmetrical cell for electrolytes according to the invention (x=0.04 / 0.09 / 0.14) compared to Li3PS4.

[0050] [Fig.4] [Fig.4] represents cycling curves in all-solid-state battery, in NCA cell (LixNio.8Coo.i5Alo.o502) / graphite for electrolytes according to the invention (x=0.04 (A), x=0.09 (B); x=0.14 (C) compared to Li3PS4; and (D) the polarization for electrolytes according to the invention (x=0.04 / 0.09 / 0.14) compared to Li3PS4 Examples

[0051] The following examples illustrate in a representative and non-limiting manner an embodiment according to the invention.

[0052] Firstly, the precursors are weighed, mixed and ground in a mortar (2.5g in total) according to the following proportions: Li2S p2s5 P° Counter example Li3PS4 0.95691 g 1.54308 g 0g Example 1 Li3Pi.o4S4 0.95010 g 1.53210 g 0.01778 g Example 2 Li3Pi.o9S4 0.94280 g 1.52034 g 0.03684 g Example 3 LI3Pi.i4S4 0.93498 g 1.50772 g 0.05729 g Example 4 Li3P11gS4 0.92655 g 1.49413 g 0.07930 g

[0053] The mixture of precursors is placed in a 20 mL zirconia bowl containing 4 balls of 10 mm diameter and then these bowls are placed in a planetary mill (Fritsch Pulverisette 7). The grinding conditions are as follows: 500 rpm, 30 min grinding, 5 min break, 30 cycles, i.e. 15 hours of effective grinding. At the end of the first grinding, the powder that tends to stick to the walls must be removed with a spatula in a glove box. This operation is repeated 3 times (i.e. 45 hours of effective grinding) to obtain a homogeneous and amorphous compound.

[0054] The powder is heat treated in a sealed tube with a carbon coating which is carried out as follows: in a quartz tube, 2 ml of acetone are introduced then the tube is heated. The decomposition of the acetone will generate carbon which is deposited on the walls of the tube. In a glove box, 1 g of the amorphous compound is pressed to 160 MPa then placed in a carbon crucible. The whole is placed in the tube which is then put under vacuum before being sealed. The heat treatment of the sample is carried out in a furnace with a heating rate of 100°C / h up to 300°C, then maintained for 4 hours at this temperature and cooled to room temperature with a ramp of 100°C / h. After cooling, the tube is opened in a glove box under argon.

[0055] Figure 1A represents the composition of the examples in the lithium-sulfur-phosphorus ternary diagram. The examples of the invention are located on a straight line segment which passes between the composition Li3PS4 and pure phosphorus.

[0056] The crystallographic structures of the examples were analyzed by X-ray diffraction on sample powder using the K-alpha line of copper. The analyses were carried out in an air-free environment to avoid any parasitic reaction. (See Figure 1B). The values ​​of the spectrum intensities for the calculation of the ratios I[34°] / Imax [29.5°;31°] and Imax [17°;18.5°] / Imax [18.5°;19.5°] correspond to the difference between the intensity of the overall signal of the spectrum and the signal corresponding to the background noise of the spectrum.

[0057] The intensities were calculated relative to a baseline taking into account the slope of each diffraction spectrum considered.

[0058] The ratios I[34°] / Imax [29.5°;31°] and Imax [17°;18.5°] / Imax [18.5°;19.5°] are gathered in the table below: I[34°] / Imax [29.5° ;31°] Imax [17° ;18.5°] / Imax [18.5° ;19.5°] Counter example <0.02 0.052631579 Example 1 0.9 0.6 Example 2 0.32 0.214285714 Example 3 0.6 0.526315789

[0059] Table 2: ratio I[34°] / Imax [29.5°;31°] and Imax [17°;18.5°] / Imax [18.5°;19.5°]

[0060] The conductivity measurements were carried out by impedance spectroscopy in imposing an alternating current I between the 2 faces of an electrolyte pellet of diameter 7mm and thickness e placed between 2 stainless steel electrodes. The densification of the electrolyte pellet is prepared either by uniaxial compression or by isostatic compression. The value of the ionic conductivity ni is estimated from the relationship:

[0061] oionic = e / (R*S)

[0062] where R is the resistance measured on the Nyquist diagram and its value corresponds to the intersection of the signal relating to the blocking electrodes with the real axis.

[0063] The conductivity measurements are carried out at 25°C, 45 and 60°C, thus making it possible to estimate the activation energy.

[0064] Ea= - 1 / R * In [ o(Ti) / (o(T2) ] / ( 1 / Ti - 1 / T2 ), with R =8.314 and T is the measurement temperature in Kelvin.

[0065] Production of electrochemical cells:

[0066] The electrolyte layer acting as a separator is prepared by compressing powder in a die under a pressure of 300 MPa. Then a positive electrode mixture consisting of electrolyte powders and the cathode material LiNi0.s0Co0.15Al0.15 02 is placed on the solid electrolyte layer and then compressed under a pressure of 300 MPa. The negative electrode mixture consisting of electrolyte powder and graphite is placed on the other side of the solid electrolyte layer. The entire accumulator is then compressed to 400 MPa. The sealed cell containing the accumulator makes it possible to maintain the mechanical pressure under 100 MPa.

[0067] For symmetrical cells, the 2 positive and negative electrodes are replaced by lithium films which are compressed onto the electrolyte layer under a pressure of 100 MPa.

[0068] XRD analyses show the structural changes caused by the addition of phosphorus. These are characterized by changes in peak intensity compared to the Li3PS4 compound as shown in [Fig.2] and in Table 2.

[0069] The conductivity measurements of Examples 1 to 3 and the counterexample are gathered in [Fig. 2]. These show that when the phosphorus content increases in the Li3Pi+xS4 compounds, the conductivity of the electrolyte is improved and the activation energy is reduced.

[0070] The symmetrical lithium-based electrochemical cells were cycled at different current densities. [Fig. 3] shows the evolution of the polarization of the symmetrical cells. For a current density of 0.05 mA / cm2, the examples of the invention have polarizations much lower than those of the material of the counter-example Li3PS4 (2 to 3 times lower).

[0071] Electrochemical cells assembled with graphite electrodes and cathode material LiNio.8oCo0.i5Alo.i502 were cycled at a C / 40 regime.

[0072] The charge and discharge curves (Figures 4A, B and C) show a voltage unstable when charging the compound of the counter-example, Li3PS4. This instability is characteristic of the formation of micro short circuits. Unlike the Li3PS4 compound, the materials of the invention have a very regular charging curve. On the other hand, it can be noted that the irreversible capacity (difference between the charged and discharged capacity) is lower for the materials of the invention.

[0073] Similarly, the polarization during charging and discharging, characterized for example by the voltage difference between charging and discharging for the composition Lio.6oNio.8oCoo.i5 AI0.15O2 (see Figure 4D), is significantly lower for the materials of the invention

[0074] Therefore, in summary, the materials of the invention exhibit higher conductivity, lower cycling polarizations, lower irreversible capacities and more regular charge curves than the Li3PS4 material.

Claims

Claims

1. Compound of formula (I): (Li3P1+xS4ky(LiX)y (I) in which 0 < x < 0.2; 0 < y < 0.3; Each X, identical or different for each LiX group, represents a halogen atom chosen from Cl, I, Br, F.

2.

3. A compound of formula (I) according to claim 1 such that y=0. A compound of formula (I) according to claim 1 or 2 such that x is between 0.04 and 0.

14.

4. A compound of formula (I) according to any one of the preceding claims, selected from Li3Pi o4S4, Li3Pi>09S4, and mixtures thereof.

5. A compound of formula (I) according to any one of the preceding claims as it comprises the compound Li3Pi o9S4.

6. A compound of formula (I) according to any one of the preceding claims, in crystalline or partially crystalline form.

7. Compound of formula (I) according to any one of the preceding claims having an X-ray diffraction (XRD) peak at 20 = 19.10° + / - 0.25° obtained with the k(alpha) line of copper.

8. Compound of formula (I) according to any one of the preceding claims such that the ratio between the maximum intensity of the diffraction spectrum in the interval Imax [17°; 18.5°] relative to the maximum intensity of the spectrum in the interval [18.5°; 19.5°] is greater than 0.1, preferably between 0.1 and 1.00

9. Compound of formula (I) according to any one of the preceding claims such that the intensity ratio of the X-ray diffraction (XRD) spectrum at 1=34.00° relative to the maximum intensity signal in the interval Imax [29.5°; 31°] is greater than 0.04, preferably between 0.04 and 1.

00.

10. A method of preparing a compound as defined in any one of the preceding claims, comprising: the step of mixing powders of the precursors P2S5 and Li2S, adding phosphorus of zero oxidation state, then mechanically grinding or heating the mixture thus obtained, such that the molar ratio n(phosphorus of zero oxidation state) / (nP2S5 + nLi2S ) is between 0.01 and 0.

10.

11. A method according to claim 10, such that the heating step is carried out at a temperature below 300°C.

12. A solid sulfide electrolyte comprising a compound of formula (I) according to any one of claims 1 to 9.

13. A solid sulfide electrolyte according to claim 12, has a lithium ion conductivity value at room temperature higher than that of Li3PS4.

14. An all-solid electrochemical element comprising a cathode layer, an anodic layer and an electrolyte layer between the anodic and cathodic layers, such that said electrolyte layer contains the solid sulfide electrolyte according to claim 12 or 13.

15. Module comprising the stack of at least two electrochemical elements according to claim 14.

16. Battery comprising one or more modules as defined in claim 15.

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