Solid electrolyte for a lithium-ion electrochemical cell

Abstract

The invention relates to a compound of the formula Li.sub.7-xPS.sub.6-xX.sub.x-z(BH.sub.4).sub.z, in which x is selected from the group comprising Cl, Br, I, F and CN, 0≤x≤2, 0≤z≤0.50. This compound can be used as a solid electrolyte of a lithium-ion electrochemical element.

Claims

1. A crystalline compound of formula Li.sub.7-xPS.sub.6-xX.sub.x-z(BH.sub.4).sub.z wherein: X is selected from the group consisting of Cl, Br, I, F and CN 0<x≤2 0<z≤0.50.

2. The crystalline compound as claimed in claim 1, wherein x=1.

3. The crystalline compound as claimed in claim 1, wherein X is I or Cl.

4. The crystalline compound as claimed in claim 1, where 0.1≤z≤0.35.

5. The crystalline compound as claimed in claim 1, where 0.1≤z≤0.20.

6. The crystalline compound as claimed in claim 1, where 0.15≤z≤0.20.

7. A process for preparing a crystalline compound as claimed in claim 1, comprising the steps of: a) providing a mixture comprising Li.sub.2S, P.sub.2S.sub.5, LiBH.sub.4 and LiX wherein X is selected from the group consisting of Cl, Br, I, F and CN; b) grinding the mixture for a period of at least 10 hours to allow the incorporation of LiBH.sub.4 into the compound Li.sub.7-xPS.sub.6-xX.sub.x-z(BH.sub.4).sub.z.

8. The preparation process as claimed in claim 7, wherein the grinding step b) is carried out for a period of at least 15 hours.

9. The preparation process as claimed in claim 8, wherein the grinding step b) is carried out for a period of at least 20 hours.

10. An electrochemical cell comprising a solid electrolyte comprising the crystalline compound as claimed in claim 1.

11. The electrochemical cell as claimed in claim 10, wherein the solid electrolyte does not contain LiBH.sub.4.

12. The electrochemical cell as claimed in claim 10, further comprising: at least one negative electrode comprising an active material selected from the group consisting of carbon, tin, silicon, lithium and indium; at least one positive electrode comprising an active material selected from the group consisting of lithiated transition metal oxides and sulfur compounds.

13. The electrochemical cell as claimed in claim 12, wherein: the active material of the negative electrode is selected from the group consisting of lithium and indium; the active material of the positive electrode is selected from the group consisting of S, TiS.sub.2, TiS.sub.3, TiS.sub.4, NiS, NiS.sub.2, CuS, FeS.sub.2, Li.sub.2S, MoS.sub.3, polyacrylonitriles-sulfur, dithiooxamide and disulfur compounds.

14. A process for manufacturing a solid-electrolyte electrochemical cell, said process comprising the steps of: a) preparing a mixture containing a positive electrochemically active material; b) depositing on the mixture obtained in step a) a layer of the crystalline compound as claimed in claim 1 to form a solid electrolyte; c) application of at least one layer of a mixture containing a negative electrochemically active material on a free side of the compound layer forming the solid electrolyte.

15. A method comprising the step of substituting a boron-containing anion for a halide ion in a crystalline compound of formula Li.sub.7-xPS.sub.6-xX.sub.x to obtain a crystalline compound of formula Li.sub.7-xPS.sub.6-xX.sub.x-z(BH4).sub.z, where 0<z≤0.50, where X is selected from the group consisting of Cl, Br, I, F and CN and 0<x≤2 in order to increase the ionic conductivity of this compound.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 schematically represents the structure of a lithium-ion electrochemical cell as manufactured in the examples.

(2) “Li” and “In” refer to the lithium layer and the indium layer, respectively.

(3) “SE” refers to the solid electrolyte layer.

(4) “Positive” means the layer containing the positive active material.

(5) FIG. 2 shows the ionic conductivity of compounds of formula Li.sub.7-xPS.sub.6-xI.sub.x-z(BH.sub.4), for different values of the degree of substitution of the halide ion I.sup.− by the borohydride ion: 0%, 10%, 17%, 33% and 50%.

(6) FIG. 3 shows two X-ray diffraction spectra. The top spectrum is obtained with the compound in Example 2. The bottom spectrum is obtained with the compound from Reference Example 1.

(7) FIG. 4 shows the X-ray diffraction spectra of samples A, B and C described in the experimental section. The lower spectrum is obtained from sample A. The spectrum of the medium is obtained from sample B. The top spectrum is obtained from sample C.

(8) FIG. 5 shows the discharge curve at room temperature at regime C/20 of a lithium-ion electrochemical cell comprising a solid electrolyte of formula Li.sub.6PS.sub.5Cl.sub.0.83(BH.sub.4).sub.0.17.

DISCLOSURE OF EMBODIMENTS

(9) The compound according to the invention has the formula Li.sub.7-xPS.sub.6-xX.sub.x-z(BH.sub.4).sub.z wherein:

(10) X is selected from the group consisting of Cl, Br, I, F and CN

(11) 0<x≤2

(12) 0<z≤0.50.

(13) Preferably, element X is I or Cl.

(14) This compound is characterized by a substitution of part of the halide ion X.sup.− by the borohydride ion BH.sub.4.sup.−. This substitution has the effect of increasing the ionic conductivity compared with that of the unsubstituted Li.sub.7-xPS.sub.6-xX.sub.x compound.

(15) In an embodiment, x is greater than or equal to 0.1.

(16) In an embodiment, z is greater than or equal to 0.05.

(17) In an embodiment, z is less than or equal to 0.35.

(18) The Applicant surprisingly observed that the increase in ionic conductivity was greatest when the substitution rate was in the range of 10 to 20% (0.1≤z≤0.20), preferably in the range of 15 to 20% (0.15≤z≤0.20). The ionic conductivity can be multiplied by seven thanks to this substitution.

(19) It was also observed that the increase in ionic conductivity was more pronounced when the compound was in an amorphous state. The advantages of an amorphous structure are isotropic conductivity, ease of fabrication in dense thin films. The compound can be subjected to a grinding step to increase its amorphous character.

(20) It is preferable not to subject the compound to a heat treatment, such as annealing, as this promotes the appearance of a crystalline structure. The examples in the experimental section illustrate the effect of the degree of crystallinity of the compound on its ionic conductivity.

(21) The compound according to the invention is the result of a chemical reaction between LiBH.sub.4 and Li.sub.2S, P.sub.2S.sub.5 and LiX. The process for preparing the compound according to the invention comprises the steps of:

(22) a) providing a mixture comprising Li.sub.2S, P.sub.2S.sub.5, LiBH.sub.4 and LiX wherein X is selected from the group consisting of Cl, Br, I, F and CN;

(23) b) grinding the mixture for a period of time sufficient to allow the incorporation of LiBH.sub.4 into the compound Li.sub.7-xPS.sub.6-xX.sub.x-z(BH.sub.4).sub.z.

(24) It should be noted that according to the invention, the borohydride ions BH.sub.4.sup.− are integrated into the Li.sub.7-xPS.sub.6-xX.sub.x structure during grinding. The grinding step is therefore carried out as long as lithium borohydride LiBH.sub.4 remains in the mixture, i.e. not yet incorporated into Li.sub.7-xPS.sub.6-xX.sub.x The grinding time depends on the conditions under which the grinding is carried out (number of balls, internal volume of the jar, speed of the mill, quantity of starting mixture, etc.). However, it is easy for the skilled person to determine by routine testing whether lithium borohydride remains in the mixture. The X-ray diffraction technique can be used for this purpose to detect the presence of residual lithium borohydride.

(25) Preferably, the grinding is carried out over a period of at least 10 hours, preferably at least 15 hours, and more preferably at least 20 hours.

(26) The grinding step is usually carried out under inert atmosphere, for example under argon, and under dry atmosphere.

(27) Preferably, the grinding step is conducted at room temperature.

(28) According to the invention, the grinding is carried out in a single operation on a mixture containing all the reagents Li.sub.2S, P.sub.2S.sub.5, LiBH.sub.4 and LiX, unlike the process for manufacturing the solid electrolyte of document JP 2016-134316 wherein the compound Li.sub.6PS.sub.5X is manufactured first, then the solid solution of LiX-LiBH.sub.4 is manufactured second, and finally the mixture of Li.sub.6PS.sub.5X with LiX-LiBH.sub.4 is manufactured.

(29) The compound according to the invention can be used as solid electrolyte. The thickness of the solid electrolyte layer can vary between 10 μm and 1 mm.

(30) The compound according to the invention can also be used in mixture with a negative active material of the electrochemical cell and/or in mixture with a positive active material of the electrochemical cell. Preferably, the compound according to the invention used in mixture with the negative active material or with the positive active material is identical to the compound used as solid electrolyte.

(31) The positive active material may be selected from the group consisting of: a sulfur-containing compound i) selected for example from S, TiS.sub.2, TiS.sub.3, TiS.sub.4, MoS.sub.2, MoS.sub.3, FeS, FeS.sub.2, CuS, NiS, NiS.sub.2, Ni.sub.3S.sub.2, Li.sub.2S; a compound ii) of formula Li.sub.xMn.sub.1-y-zM′.sub.yM″.sub.zPO.sub.4 (LMP), wherein M′ and M″ are different from each other and are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Mn, Zn, Y, Zr, Nb and Mo, with 0.8≤x≤1.2; 0≤y≤0.6; 0≤z≤0.2; compound iii) of formula Li.sub.xM.sub.2-x-y-z-wM′.sub.yM″.sub.zM′″.sub.wO.sub.2(LMO2), wherein M, M′, M″ and M′″ are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo, provided that M or M′ or M″ or M″′ is selected from Mn, Co, Ni, or Fe;

(32) M, M′, M″ and M″′ being different from each other; with 0.8≤x≤1.4; 0≤y≤0.5; 0≤z≤0.5; 0≤w≤0.2 and x+y+z+w≤2; compound iv) of formula Li.sub.xMn.sub.2-y-zM′.sub.yM″.sub.zO.sub.4 (LMO), wherein M′ and M″ are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo;

(33) M′ and M″ being different from each other, and 1≤x≤1.4; 0≤y≤0.6; 0≤z≤0.2; compound v) of formula Li.sub.xFe.sub.1-yM.sub.yPO.sub.4 wherein M is selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; and 0.8≤x≤1.2; 0≤y≤0.6; compound vi) of formula xLi.sub.2MnO.sub.3; (1-x)LiMO.sub.2 where M is selected from Ni, Co and Mn and x≤1;

(34) and a mixture of these compounds.

(35) The negative active material may be selected from the group consisting of:

(36) i) a carbon-based compound, such as graphite;

(37) ii) a lithium oxide of titanium, such as Li.sub.4Ti.sub.5O.sub.12;

(38) iii) a metal selected from lithium, indium, aluminum, silicon, tin and alloys containing

(39) these metals, preferably an alloy of lithium and indium.

(40) One or more binders may be added to the mixture containing the positive active material and the compound according to the invention. This binder may be selected from the group consisting of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene rubber (EPDM), styrene-butadiene rubber (SBR), polyvinyl alcohol, carboxymethylcellulose (CMC). Likewise, one or more binders may be added to the mixture containing the negative active material and the compound according to the invention. These binders may be the same as those chosen for the positive active material.

(41) A good electronically conductive compound, such as carbon, may also be added to the mixture containing the positive active material and the compound according to the invention or be added to the mixture containing the negative active material and the compound according to the invention.

(42) The mixture containing the positive active substance and optionally one or more binders as well as the electronically conductive compound can be deposited on a current collector to form a positive electrode. Likewise, the mixture containing the negative active material and optionally one or more binders as well as the electronically conductive compound can be deposited on a current collector to form a negative electrode.

(43) An “all-solid” electrochemical cell is obtained by superimposing at least one positive electrode, the solid electrolyte comprising the compound according to the invention and at least one negative electrode. The assembly can be obtained by compression.

EXAMPLES

(44) Different argyrodite type compounds were synthesized. Their composition is shown in Table 1 below.

(45) TABLE-US-00001 TABLE 1 Compositions tested Example Composition x z X Reference 1 Li.sub.6PS.sub.5I 1.00 0.00 I Ex. 1 Li.sub.6PS.sub.5I.sub.0.90(BH.sub.4).sub.0.10 1.00 0.10 I Ex. 2 Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17 1.00 0.17 I Ex. 3 Li.sub.6PS.sub.5I.sub.0.67(BH.sub.4).sub.0.33 1.00 0.33 I Ex. 4 Li.sub.6PS.sub.5I.sub.0.50(BH.sub.4).sub.0.50 1.00 0.50 I Reference 2 Li.sub.6PS.sub.5Cl 1.00 0.00 Cl Ex. 5 Li.sub.6PS.sub.5Cl.sub.0.83(BH.sub.4).sub.0.17 1.00 0.20 Cl

(46) By way of counter-example, a mixture comprising 83 mol % Li.sub.6PS.sub.5I and 17 mol % LiBH.sub.4 was prepared.

(47) For the examples, the compounds are prepared by mechanosynthesis, i.e. high-energy mechanochemical grinding. The powders of the initial reagents Li.sub.2S (Sigma Aldrich, 99.98%), P.sub.2S.sub.5 (Sigma Aldrich, 98%), LiBH.sub.4 (Rockwood Lithium, 97.8%), LiCl and LiI (Sigma Aldrich 99.99%) are mixed in stoichiometric quantities. For each synthesis, 1 g of mixture is placed in a 45 cm.sup.3 stainless steel jar. 25 balls of 7 mm diameter are also placed in the jar. The latter is tightly closed under argon in glove box. The equipment used for grinding is a Fritsch™ Pulverisette 7 planetary mill. The grinding time of the compounds according to the invention is 20 hours at the rotational speed of 600 rpm. These grinding conditions allow the chemical reaction between the different constituents to take place.

(48) For the counter-example, the compound Li.sub.6PS.sub.5I is prepared as described above and then mixed in stoichiometric proportions with LiBH.sub.4 for 10 min at a rate of 300 rpm. These grinding conditions do not allow the substitution of part of I.sup.− by BH.sub.4.sup.−.

(49) When the samples undergo heat treatment, this consists of heating them to 550° C. for 5 hours in a sealed autoclave. This heat treatment causes recrystallization of the compound.

(50) X-ray diffraction analyses are performed on a Bruker™ D8 Advanced diffractometer using the Kα line of copper or molybdenum. A waterproof protection allows the analysis to be carried out under argon atmosphere.

(51) Ion conductivity measurements are performed on pellets made from solid electrolyte powder. The preparation of pellets consists of pressing solid electrolyte powder into a pellet mold under a pressure of 2 tons. The diameter of the pellet is 7 mm. The prepared electrolyte pellet is then inserted between two lithium metal discs and placed in a Swagelok™ type electrochemical cell. Conductivity measurements are carried out using an Autolab™ PGSTAT30 type potentiostat using a sinusoidal voltage of variable frequency between 1 Hz and 1 MHz and an amplitude of 10 mV.

(52) Assembly of the Electrochemical Cells:

(53) The “all-solid” electrochemical cell is obtained by pressing three layers: the first consists of a mixture containing the positive active material and the solid electrolyte, the second consists of solid electrolyte only (this layer acts as a separator), and the third consists of a negative electrode based on lithium and indium.
Preparation of the Mixture Containing the Positive Active Material:

(54) The positive active material used is titanium sulfide, TiS.sub.2. Its theoretical capacity is 239 mAh/g. This is mixed manually with solid electrolyte powder in an agate mortar in a glove box. As the TiS.sub.2 compound is electronically conductive, the addition of conductive carbon is not necessary. The percentage of solid electrolyte in the mixture is 60%.

(55) Preparation of the Li—In Negative Electrode:

(56) This consists of a 200 μm layer of lithium metal on which a 100 μm layer of indium is deposited.

(57) Assembly Step:

(58) A thin layer of mixture containing the positive active material is placed in a 9 mm diameter mold. A layer of solid electrolyte is then deposited. A pressure of 2 tons is exerted by means of a press. A pellet is thus obtained. On the electrolyte layer, the indium foil is then deposited followed by the lithium foil. The assembly is tested in a Swagelok™ type electrochemical cell. The structure of the resulting electrochemical cell is shown schematically in FIG. 1.

(59) Results:

(60) Table 2 below shows the results of the ion conductivity measurements. The results obtained for iodide-containing compounds (Reference Examples 1 and Examples 1 to 4) are shown graphically in FIG. 2. These results show that for a substitution rate of 10, 17, 33 and 50%, the ionic conductivity of the compound is increased.

(61) TABLE-US-00002 TABLE 2 Result of ion conductivity measurements (S/cm) % increase in Ion conductivity conductivity compared at room with the un- temperature substituted Example Compound (S/cm) compound Reference 1 Li.sub.6PS.sub.5I 1.0 × 10.sup.−4 0 Ex. 1 Li.sub.6PS.sub.5I.sub.0.90(BH.sub.4).sub.0.10 2.1 × 10.sup.−4 110 Ex. 2 Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17 7.5 × 10.sup.−4 650 Ex. 3 Li.sub.6PS.sub.5I.sub.0.67(BH.sub.4).sub.0.33 1.3 × 10.sup.−4 30 Ex. 4 Li.sub.6PS.sub.5I.sub.0.50(BH.sub.4).sub.0.50 1.1 × 10.sup.−4 10 Reference 2 Li.sub.6PS.sub.5Cl 1.5 × 10.sup.−5 0 Ex. 5 Li.sub.6PS.sub.5Cl.sub.0.83(BH.sub.4).sub.0.17 6.5 × 10.sup.−5 333 Counter- 83% Li.sub.6PS.sub.5I + .sup. 8 × 10.sup.−5 −20 example 17% LiBH.sub.4

(62) For the Li.sub.6PS.sub.5I.sub.1-z(BH.sub.4).sub.z family of compounds, it can be seen that the ion conductivity shows a maximum as a function of the rate of substitution of the I.sup.− ion by the BH.sub.4.sup.− ion. The optimal value of the substitution rate is between 10% and 33%, and close to 17%.

(63) The increase in ionic conductivity is also observed when element X is chlorine. The compound in Example 5 has an ionic conductivity of 6.5×10.sup.−5 S/cm while the compound in Reference Example 2 has an ionic conductivity of only 1.5×10.sup.−5 S/cm. The substitution of 17% of the Cl.sup.− ions by BH.sub.4.sup.− ions tripled the ion conductivity.

(64) These results were compared with the results obtained for the counter-example which was prepared by simply mixing the two compounds Li.sub.6PS.sub.5I and LiBH.sub.4. In this case, the conductivity of the mixture is lower than that of the Li.sub.6PS.sub.5I compound alone.

(65) In order to demonstrate that the BH.sub.4.sup.− ion is incorporated into the structure of the compound Li.sub.6PS.sub.5I, an X-ray diffraction pattern was performed on the compound in Example 2 and on the compound in Reference Example 1. Both compounds were subjected to a heat treatment in order to increase their crystallinity. FIG. 3 compares the spectrum of the compound in Example 2 wherein 17% iodide was replaced by the BH.sub.4.sup.− ion with that of the compound in Reference Example 1.

(66) The spectrum of the compound in Reference Example 1 (bottom spectrum) shows peaks attributable to the presence of the argyrodite phase of cubic structure.

(67) The spectrum of the compound in Example 2 (upper spectrum) differs from that of Reference Example 1 primarily in that it shows low intensity peaks due to a small amount of unreacted Li.sub.2S used as a reactant in the starting mixture. Peaks attributable to Li.sub.2S are marked with asterisks (*). This spectrum also shows the absence of the LiBH.sub.4 phase, which proves that the borohydride ion has integrated into the crystallographic structure of the compound Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17 during mechanosynthesis.

(68) Study of the Influence of the Degree of Crystallinity of the Compound on the Ionic Conductivity of the Compound Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17 (Example 2):

(69) A sample A was prepared. It is produced by grinding the mixture of reagents Li.sub.2S, P.sub.2S.sub.5, LiI and LiBH.sub.4 for 20 hours at a rotational speed of 600 rpm in the Fritsch Pulverisette 7 planetary mill under the conditions described above. Grinding led to the formation of the compound in Example 2. An X-ray diffraction spectrum was performed on this sample A. This spectrum is shown in FIG. 4 (bottom spectrum).

(70) Sample A was then heat treated at 550° C. for 5 hours in a sealed autoclave to induce crystallization. A sample B is thus obtained. An X-ray diffraction spectrum was performed on this sample B. This spectrum is shown in FIG. 4 (middle spectrum).

(71) Sample B was then subjected to grinding to reduce its crystallinity. A sample C was thus obtained. An X-ray diffraction spectrum was performed on this sample C. This spectrum is shown in FIG. 4 (top spectrum).

(72) The spectrum of sample A shows only low intensity peaks corresponding to the presence of the Li.sub.2S phase.

(73) The spectrum of sample B shows the low intensity peaks attributable to the presence of the Li.sub.2S phase as well as well-defined high intensity peaks attributable to crystalline Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17.

(74) The spectrum of sample C shows that the Li.sub.2S phase has almost disappeared. The peaks attributable to Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17 decreased markedly in intensity, indicating that the grinding step has rendered amorphous a significant amount of Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17.

(75) The ion conductivity of samples A, B and C was measured. The ion conductivity values are shown in Table 4:

(76) TABLE-US-00003 TABLE 4 Effect of crystallinity on the conductivity of the compound Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17 Width at half-height of the peak Conductivity located at at room Synthesis Crystalline an angle of temperature Sample conditions state 20°(*) (S/cm) A ground amorphous — 7.5 × 10.sup.−4  B ground and cubic 0.2° 8 × 10.sup.−6 then heat- structure treated thin lines C ground then cubic 0.8° 9 × 10.sup.−5 heat treated structure then ground broad lines (*)the angle is measured using the wavelength of molybdenum

(77) Measurements show that the highest ionic conductivity is obtained for the amorphous sample A. Conversely, the lowest ionic conductivity is obtained for sample B wherein Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17 is well crystallized. An intermediate conductivity value is observed for the sample C which has an intermediate degree of crystallinity between the amorphous and crystalline state.

(78) The use of the compound according to the invention as solid electrolyte makes it possible to reduce the voltage drop induced by the resistance of the separator. The following calculation demonstrates this advantage. In a lithium-ion electrochemical cell with an electrode surface capacity of 4 mAh/cm.sup.2 and a 25 μm thick separator layer consisting of the compound of Reference Example 1 (Li.sub.6PS.sub.5I), the voltage drop induced by the separator during a discharge at regime 10C is about 1 V according to the equations R=1/σ. e/S with

(79) R: separator resistance (Ohm),

(80) σ: electrolyte conductivity (S/m),

(81) e: separator thickness (m),

(82) S: separator surface area (m.sup.2) and the voltage drop across the separator is equal to ΔU=R×I, where I is the current flowing through the separator. This voltage drop is very significant because it represents 27% of the open circuit voltage of a lithium-ion electrochemical cell comprising a positive electrode whose active material would consist of a lithium oxide of nickel, cobalt and aluminum (NCA) and comprising a negative electrode whose active material would consist of graphite. Indeed, the open circuit voltage of such a cell is of the order of 3.6 V. This voltage drop decreases to 0.13 V when the separator consists of the compound from Example 2: Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17 This value of 0.13 V is quite acceptable as it represents only 3.6% of the open circuit voltage. Reducing this voltage drop allows the lithium-ion electrochemical cell to deliver a higher voltage for a given discharge regime.

(83) TABLE-US-00004 TABLE 3 Comparison between the voltage drop induced by a separator comprising the Reference Compound 1 and the voltage drop induced by a separator comprising the compound Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17 of Example 2. voltage separator drop due to conductivity resistance the separator (S/cm) (ohm .Math. cm.sup.2) (V) Reference 1.0 × 10.sup.−4 25 1 Example 1 Li.sub.6PS.sub.5I Example 2 7.5 × 10.sup.−4 3.33 0.13 Li.sub.6PS.sub.5I.sub.0.83(BH.sub.4).sub.0.17

(84) For information purposes, FIG. 5 shows the discharge curve at regime C/20 at room temperature of an electrochemical cell comprising: a TiS.sub.2-based positive active material; a solid electrolyte consisting of the compound Li.sub.6PS.sub.5Cl.sub.0.83(BH.sub.4).sub.0.17; a negative active material based on indium and lithium.

(85) The mass capacity value measured in discharge at C/20 is 238 mAh/g. It is almost equal to the theoretical capacity of TiS.sub.2 (239 mAh/g), which shows that the electrolyte works very well at room temperature.