SULFIDE CERAMIC ELECTROLYTES

Abstract

The present invention relates to sulfide solid electrolytes having improved conductivity, a process for the preparation thereof, and electrochemical elements and batteries containing same.

Claims

1. Compound of formula (I): $\begin{array}{cc}{\left({\mathrm{Li}}_3{P}_{1+x}{S}_4\right)}_{1-y}{\left(\mathrm{Li}X\right)}_y& \left(I\right)\end{array}$ where: 0<x<0.2; 0y0.3; Each X, the same or different for each group LiX, represents a halogen atom chosen from among Cl, I, Br, F.

2. The compound of formula (I) according to claim 1, such that y=0.

3. The compound of formula (I) according to claim 1, such that x lies between 0.04 and 0.14.

4. The compound of formula (I) according to claim 1, chosen from among Li.sub.3P.sub.1.04S.sub.4, Li.sub.3P.sub.1.09S.sub.4, and mixtures thereof.

5. The compound of formula (I) according to m such that it comprises the compound Li.sub.3P.sub.1.09S.sub.4.

6. The compound of formula (I) according to claim 1, in crystalline or partially crystalline form.

7. The compound of formula (I) according to claim 1, having an X-ray (XRD) diffraction peak at 2=19.100+/0.25 obtained with the copper K(alpha) line.

8. The compound of formula (I) according to claim 1, such that the ratio between the maximum intensity of the diffraction pattern in the range I.sub.max [17;18.5] and the maximum intensity of the pattern in the range [18.5;19.5] is higher than 0.1 preferably between 0.1 and 1.00.

9. The compound of formula (I) according to claim 1, such that the ratio of the intensity of the X-ray (XRD) diffraction pattern at I=34.00 relative to the signal of maximum intensity in the range I.sub.max [29.5;31] is higher than 0.04, preferably between 0.04 and 1.00.

10. A process for preparing a compound such as defined in claim 1 comprising: the step of mixing the precursor powders P.sub.2S.sub.5 and Li.sub.2S, the addition of phosphorus at oxidation state zero, then mechanical grinding or heating of the mixture obtained.

11. The process according to claim 10, such that the heating step is conducted at a temperature lower than 300 C.

12. A sulfide solid electrolyte comprising a compound of formula (I) according to claim 1.

13. The sulfide solid electrolyte according to claim 12 has a lithium ion conductivity value at ambient temperature higher than that of Li.sub.3PS.sub.4.

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

15. A module comprising a stack of at least two electrochemical elements according to claim 14.

16. A battery comprising one or more modules such as defined in claim 15.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG. 1 gives the ternary diagram of sulfur, lithium and phosphorus (A), and the X-ray diffraction pattern of compositions in the binary domain Li.sub.2SP.sub.(2+x)S.sub.5 (B).

[0047] FIG. 2 gives conductivity and activation energy measurements in the binary domain Li.sub.2SP.sub.(2+x)S.sub.5.

[0048] FIG. 3 illustrates stability in a symmetric cell for electrolytes of the invention (x=0.04/0.09/0.14) in comparison with Li.sub.3PS.sub.4.

[0049] FIG. 4 gives the cycling curves in an all-solid-state battery, in NCA cell (Li.sub.xNi.sub.0.8Co.sub.0.15Al.sub.0.05O.sub.2)/graphite for electrolytes of the invention (x=0.04 (A), x=0.09 (B), x=0.14 (C) in comparison with Li.sub.3PS.sub.4; and (D) the polarization for electrolytes of the invention (x=0.04/0.09/0.14) in comparison with Li.sub.3PS.sub.4

DETAILED DESCRIPTION

[0050] The following examples are representative, nonlimiting examples of one embodiment of the invention.

[0051] At a first stage, the precursors are weighed, mixed and ground in a mortar (total of 2.5 g) in the following proportions:

TABLE-US-00001 Li.sub.2S P.sub.2S.sub.5 P Counter- Li.sub.3PS.sub.4 0.95691 g 1.54308 g 0 g example Example 1 Li.sub.3P.sub.1.04S.sub.4 0.95010 g 1.53210 g 0.01778 g Example 2 Li.sub.3P.sub.1.09S.sub.4 0.94280 g 1.52034 g 0.03684 g Example 3 LI.sub.3P.sub.1.14S.sub.4 0.93498 g 1.50772 g 0.05729 g Example 4 Li.sub.3P.sub.1.19S.sub.4 0.92655 g 1.49413 g 0.07930 g

[0052] The mixture of precursors is placed in a 20 mL zirconium grinding bowl containing 4 balls of diameter 10 mm, and these bowls are placed in a planetary ball mill (Fritsch Pulverisette 7). The grinding conditions are as follows: 500 rpm, 30 min grinding, 5 min pause, 30 cycles i.e. 15 h of effective grinding. After the first grinding, the powder tending to adhere to the walls must be detached with a spatula in a glovebox. This operation is repeated 3 times (i.e. 45 h of effective grinding) to obtain a homogeneous, amorphous compound.

[0053] The powder is heat treated in a sealed tube with carbon lining carried out as follows: 2 mL of acetone are charged in a quartz tube, and the tube is heated. Decomposition of the acetone will generate carbon which is deposited on the walls of the tube. In a glovebox, 1 g of the amorphous compound is pressed at 160 MPa, then placed in a carbon crucible. The whole is disposed in the tube which is placed under a vacuum before being sealed. Heat treatment of the sample is performed in an oven at a heating rate of 100 C./h up to 300 C., held for 4 h at this temperature, and then cooled to ambient temperature at a ramp of 100 C./h. After cooling, the tube is opened in a glovebox under argon.

[0054] FIG. 1A illustrates the compositions of the examples in the ternary lithium-sulfur-phosphorus diagram. The examples of the invention lie in a right-hand segment which passes between the composition Li.sub.3PS.sub.4 and pure phosphorus.

[0055] The crystallographic structures of the examples were analyzed by X-ray diffraction on the powder of the samples using the copper K-alpha line. The analyses were performed under protection against air to prevent any parasitic reaction. (See FIG. 1B). The pattern intensity values to calculate the ratios I[34]/I.sub.max [29.5;31] and I.sub.max [17;18.5]/I.sub.max [18.5;19.5] correspond to the difference between the intensity of the overall signal of the pattern and the signal corresponding to the background noise of the pattern.

[0056] The intensities were calculated in relation to a baseline taking into account the slope of each diffraction pattern under consideration.

[0057] The ratios I[34]/I.sub.max [29.5;31] and I.sub.max [17;18.5]/I.sub.max [18.5;19.5] are grouped together in the Table below:

TABLE-US-00002 TABLE 2 ratio I[34]/I.sub.max [29.5; 31] and I.sub.max [17; 18.5]/I.sub.max [18.5; 19.5] Imax [17; 18.5]/Imax I[34]/Imax [29.5; 31] [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

[0058] Conductivity measurements were performed by impedance spectroscopy by imposing an alternating current I between the 2 sides of an electrolyte pellet having a diameter of 7 mm and thickness e placed between 2 electrodes in stainless steel. Densification of the electrolyte was prepared either by uniaxial compression or by isostatic compression. The value of ionic conductivity i is estimated from the relationship:

.sub.ionic=e/(R*S)

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

[0059] Conductivity measurements were performed at 25 C., 45 and 60 C. thereby allowing estimation of activation energy.

E.sub.a=1/R*ln[(T.sub.1)/((T.sub.2)]/(1/T.sub.11/T.sub.2),

with R=8.314 and T is the measuring temperature in Kelvin.

Preparation of Electrochemical Cells:

[0060] The electrolyte layer acting as separator was prepared by compressing the powder in a die under a pressure of 300 MPa. A mixture of positive electrode, composed of powders of electrolyte and of cathode material LiNi.sub.0.80Co.sub.0.15Al.sub.0.15O.sub.2, was deposited on the layer of solid electrolyte and compressed under a pressure of 300 MPa. The mixture of negative electrode, composed of electrolyte powder and graphite, was placed on the other side of the solid electrolyte layer. The entire secondary battery was then compressed at 400 MPa. The sealed cell containing this battery allowed the maintaining of mechanical pressure at 100 MPa.

[0061] For symmetric cells, the two positive and negative electrodes were replaced by lithium films which were compressed onto the electrolyte layer under a pressure of 100 MPa.

[0062] XRD analyses show the structural changes caused by the addition of phosphorus. These are characterized by changes in peak intensities compared with the Li.sub.3PS.sub.4 compound as shown in FIG. 2 and in Table 2.

[0063] The conductivity measurements in Examples 1 to 3 and in the counter-example are grouped together in FIG. 2. These show that when the phosphorus content is increased in the Li.sub.3P.sub.1+xS.sub.4 compounds, the conductivity of the electrolyte is improved and activation energy is reduced.

[0064] The lithium-based symmetric electrochemical cells were cycled at different current densities. FIG. 3 shows the change in polarization of the symmetric cells. At a current density of 0.05 mA/cm.sup.2, the examples of the invention exhibit much lower polarizations than those of the material in the counter-example Li.sub.3PS.sub.4 (2 to 3 times lower).

[0065] The electrochemical cells assembled with graphite electrodes and LiNi.sub.0.80Co.sub.0.15Al.sub.0.15O.sub.2 cathode material were cycled at a rate of C/40.

[0066] The charge and discharge curves (FIGS. 4A, B and C) show unstable voltage when charging the compound of the counter-example Li.sub.3PS.sub.4. This instability is characteristic of the formation of micro-short circuits. Contrary to compound Li.sub.3PS.sub.4, the materials of the invention exhibit a very regular charge curve. Additionally, it can be noted that the irreversible capacity (difference between charge and discharge capacity) is lower for the materials of the invention.

[0067] Similarly, polarization on charge and discharge, characterized for example by the voltage difference between charge and discharge for composition Li.sub.0.60Ni.sub.0.80Co.sub.0.15Al.sub.0.15O.sub.2 (see FIG. 4D), is significantly lower for the materials of the invention.

[0068] Consequently, to summarize, the materials of the invention exhibit higher conductivity, lower cycling polarizations, lower irreversible capacities, and more regular charge curves than the Li.sub.3PS.sub.4 material.