USE OF A TRANSITION METAL SULPHIDE COMPOUND IN A POSITIVE ELECTRODE FOR SOLID STATE BATTERIES

Abstract

The present invention generally relates to the use of a transition metal sulphide compound in a positive electrode for solid state batteries, to a transition metal sulphide compound, to a device or a material incorporating said compound, such as a composite material, an electrode, an electrochemical energy storage cell or a device such as an all-solid-state battery. It further relates to a method to manufacture and/or to use such a compound, material or device and to a process to manufacture said compound, material and/or device.

Claims

1. A method of manufacturing a positive electrode of a solid state battery, comprising providing a transition metal sulphide compound in association or combination with an oxide compound, as an active material for said positive electrode of a solid state battery.

2. The method according to claim 1, wherein said transition metal of said transition metal sulphide is chosen in the group consisting of Titanium, Iron, Manganese, Nickel, Vanadium and a combination thereof.

3. The method according to claim 1, wherein said oxide compound is of the empiric formula A.sub.xC.sub.cO.sub.z wherein A is an alkali, C comprises at least one element from the Group 2 to Group 13, O is oxygen, x is a number such as 0<x<3, c is a number ranging from 1 to 3 and z is equal to x+c.

4. A composite material comprising a transition metal sulphide compound and oxide compound, eventually mixed with an electrolyte material, and preferably in a powder, or compacted powder, form.

5. An electrochemical cell comprising at least a positive and a negative electrode in contact with an electrolyte material, wherein said positive electrode comprises, consists or essentially consists of the composite material according to claim 4.

6. A solid state battery which comprises at least one electrochemical cell as claimed in claim 5 and external connections; preferably said battery is a solid state lithium battery.

7. The solid state battery according to claim 6, wherein said battery is cyclable in an allowed cut-off voltage range of 1.5-4.5 V vs. Li/Li.sup.+, preferably at a range of 1.9-4.3 V vs. Li/Li.sup.+, more preferably at a range of 2-4 V vs. Li/Li.sup.+.

8. A method of manufacturing a positive electrode of a solid state battery, comprising providing a ternary transition metal sulphide compound comprising an alkali metal as an active material for said positive electrode of a solid state battery

9. The method of claim 8, wherein said transition metal of said ternary transition metal sulphide is chosen in the group consisting of Titanium, Iron, Manganese, Nickel, Vanadium and a combination thereof.

10. The method according to claim 8, wherein the method includes mixing said ternary transition metal sulphide compound in a powder form with a solid electrolyte material which is also in a powder form.

11. A positive electrode for a solid state battery, said electrode comprising a ternary transition metal sulphide and, eventually, a proportion of a solid electrolyte material.

12. An electrochemical cell which comprises at least one positive electrode and one negative electrode in contact with an electrolyte material, wherein said positive electrode is as defined in claim 11.

13. A method of manufacturing a positive electrode of an electrochemical cell, comprising providing a ternary transition metal sulphide compound of formula III: Li.sub.1.33−2y/3M.sub.0.67-y/3N.sub.yS.sub.2 (III), wherein MN is a combination of two or more transition metals, Li, M and N have each an atomic content dependent upon y, as shown in formula III, y being a number strictly superior to 0 and not superior to 0.5, such as 0.3, preferably N is Ti and N is Fe, as an active material of said positive electrode.

14. An electrochemical cell comprising at least a positive and a negative electrode in contact with an electrolyte, wherein said positive electrode comprises, consists or essentially consists of a ternary transition metal sulphide compound of formula III: Li.sub.1.33−2y/3M.sub.0.67-y/3N.sub.yS.sub.2 (III), wherein MN is a combination of two or more transition metals, Li, M and N have each an atomic content dependent upon y, as shown in formula III, y being a number strictly superior to 0 and not superior to 0.5, such as 0.3, preferably N is Ti and N is Fe.

15. A solid state battery which comprises at least one electrochemical cell as claimed in claim 14 and external connections, preferably said battery comprises more than one of said cells.

16. The composite material according to claim 4, wherein said transition metal of said transition metal sulphide is chosen in the group consisting of Titanium, Iron, Manganese, Nickel, Vanadium and a combination thereof.

17. The composite material according to claim 4, wherein said oxide compound is of the empiric formula A.sub.xC.sub.cO.sub.z wherein A is an alkali, C comprises at least one element from the Group 2 to Group 13, O is oxygen, x is a number such as 0<x<3, c is a number ranging from 1 to 3 and z is equal to x+c.

18. The positive electrode of claim 11, wherein said transition metal of said ternary transition metal sulphide is chosen in the group consisting of Titanium, Iron, Manganese, Nickel, Vanadium and a combination thereof.

19. The positive electrode of claim 11, wherein said use includes to mix said ternary transition metal sulphide compound in a powder form with a solid electrolyte material which is also in a powder form.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] A particular embodiment of the present invention will now be described with reference to the following figures in which:

[0059] FIG. 1a shows an exploded perspective view of a device used to carry electrochemical measurements from an electrochemical cell, such as chronoamperometry, galvanostatic cycling and EIS measurements; FIG. 1b is a cross sectional view of the device taken along the line AA.

[0060] FIG. 2 shows the powder X-ray diffraction (XRD) patterns of an NMC 622/LTFS powdered mix, the same composite after a heat treatment, and LTFS and NMC 622 powders.

[0061] FIG. 3 shows the current against time plot of a cell of an NMC 622/LTFS mix pellet (1:1 weight ratio) before (black line) and after (red line) a heating treatment.

[0062] FIG. 4 shows the plot of stationary current determined from FIG. 3, against the applied potential, for the NMC 622/LTFS mix pellet.

[0063] FIG. 5 shows a galvanostatic cycling curve at C/50 rate of a cell made from a mixture of LTFS and Li.sub.3PS.sub.4 in70:30 weight ratios, cycled at different temperature regimes (room temperature, 45, 75, 105° C.).

[0064] FIG. 6 shows a galvanostatic cycling curve (Potential against specific capacity) at C/50 rate of the test battery A.

[0065] FIG. 7 shows a galvanostatic cycling curve (Potential against specific capacity) at C/50 rate of the test battery B.

[0066] FIG. 8 shows a specific capacity (mAh.Math.g.sup.−1) evolution over cycles for test battery B obtained in the same condition as FIG. 7.

[0067] FIG. 9 shows a galvanostatic cycling curve (Potential against specific capacity) at C/50 rate of the proof batteries A and B.

[0068] FIG. 10 shows electrochemical impedance spectroscopy (EIS)spectra of Proof battery A and of Proof battery B obtained after the first charge (at a delithiated state).

EXAMPLES

[0069] Examples 1 to 5 relate to the manufacture of various compounds, materials and electrochemical cells and a description of the device used to make and test them.

[0070] Examples 6 to 10 demonstrate, inter alia, that the Li-active sulphide material is electrochemically active and remains chemically stable and electrochemically active despite a temperature change, is electrochemically stable over the operating voltage range of a positive electrode (e.g. 4.0 V vs. Li/Li+) and that oxide/SE interface was improved when the oxide is mixed with a LTFS of the invention.

Example 1: Method to Synthesize a Lithium Sulphide (LTFS) of the Invention

[0071] The compound Li.sub.1.13Ti.sub.0.3Fe.sub.0.57S.sub.2, hereafter called LTFS, was prepared by solid state reaction (Liu et al). For this purpose, 259.6 mg of Li.sub.2S (Alfa Aesar, 99 w/w %), 728.4 mg of TiS.sub.2 (Sigma Aldrich, 99.9 w/w %) and 263.7 mg of FeS (Alfa Aesar, 99 w/w %) were weighed and homogeneously mixed and hand-grinded for 30 minutes. Afterwards, these precursor powders were filled in quartz tubes in an Ar-filled glovebox (O.sub.2 and H.sub.2O level<1 ppm) followed by sealing the tube under vacuum (˜10.sup.−5 mbar). The sealed tubes were subsequently annealed at 750° C. for 36 h followed by quenching in water. The resulting compound was collected inside a glovebox and hand-grinded for 10 minutes prior to storage and to further use. When the tube was exposed to ambient atmosphere, the air contact was avoided by sealing. Otherwise, the whole process was done inside the Ar-filled glovebox.

Example 2: Preparation of the Solid Electrolyteβ-Li.SUB.3.PS.SUB.4

[0072] The β-Li.sub.3PS.sub.4 solid electrolyte was obtained from Li.sub.2S and P.sub.2S.sub.5 via solution synthesis in tetrahydrofuran (THF):328.07 mg of Li.sub.2S (Alfa Aesar, purity 99 w/w %) and 529 mg of P.sub.2S.sub.5 (Acros Organics, purity >98 w/w %) were weighed and homogeneously mixed by hand grinding for several minutes in an argon-filled glove box (O.sub.2 and H.sub.2O level<1 ppm). The mixture was collected and suspended in 25 mL of THF (Sigma Aldrich, purity 99.9%) at room temperature.

[0073] The mixture was left for 3 days under continuous stirring. The solid was then recovered by centrifugation at 600 rpm for 3 minutes, and rinsed with pure THF. After rinsing, the wet powder was transferred to a glass tube, which was further introduced in a sand bath placed on top of a hot plate. The glass tube was then connected to Schlenk line (vacuum gas manifold) inside the glove box, and evacuated to a final pressure of around 0.1 mbar, in order to dry the sample.

[0074] The tube was then left at room temperature for 2 hours, and then heated at 100° C. for 24 hours to remove remaining THF. Finally, it was heated at 140° C. for 18 hours to obtain the final product of β-Li.sub.3PS.sub.4.

Example 3: Preparation of Positive Electrode Composite Materials According to the Invention

[0075] The layered oxide used in the examples to make a positive electrode composite material is LiNi.sub.0.6Mn.sub.0.2Co.sub.0.2O.sub.2 (called hereafter NMC 622) having an average particle size of 10 μm with 99.9 w/w % of purity. It was provided by the company Umicore™′ (Bruxelles, Belgium).

[0076] Within an argon-filled glove box, the LTFS obtained according to Example 1 and NMC 622 were mixed together (1:1 weight ratio) with hand grinding for 30 min, and further ball milling at 140 rpm for 30 min (powder to ball weight ratio 1:36, 1 g of power against 36 g of zirconia made balls, i.e. 12 balls of 3 g) in a planetary ball mill. The ball milling was performed outside the glovebox with the composite contained in two air tightened zirconia ball mill vessels.

[0077] Then, the Li-active sulphideβ-Li.sub.3PS.sub.4 of example 2 was weighed to be added to the NMC622/LTFS mixture in the desired weight ratio (70% of NMC 622/LTFS and 30% of (β-Li.sub.3PS.sub.4). These components were then mixed by hand grinding with a mortar and pestle for 1 hour. This composite material was used to assemble the test battery B.

[0078] Another composite material according to the invention was obtained following the same steps used for the test battery B material, but without the use of a layered oxide material. Hence, the LTFS was added to the Li-active sulphideβ-Li.sub.3PS.sub.4 of example 2 according to a weight ratio (70:30). This composite material was used to assemble the test battery A.

[0079] Another composite material according to the invention was obtained following the same steps as for test battery B, but with a different ratio of NMC 622/LTFS. In this material the weight ratio is not 1:1 as for test battery B, but 9:1. The mixture NMC 622/LTFS was added to the Li-active sulphideβ-Li.sub.3PS.sub.4 of example 2 according to weight ratio 70:30 (70% of NMC 622/LTFS and 30% of (β-Li.sub.3PS.sub.4). This composite material was used to assemble the proof battery B.

[0080] Another composite material was obtained following the same steps, but without the use of LTFS. Hence, the NMC 622 was added to the Li-active sulphideβ-Li.sub.3PS.sub.4 of example 2 according to a 70:30 weight ratio (70% of NMC 622/LTFS and 30% of (β-Li.sub.3PS.sub.4). This composite material was used to make the proof battery A.

Example 4: Pellet Assembling for Testing (See Example 6)

[0081] 60 mg of each of the positive electrode composite materials of Example 3 were placed into a dye set and compressed applying 4 tons.Math.cm.sup.−2 with a hydraulic press during 5 minutes to form a pellet of 8 mm diameter and 0.3 mm thick. The pellets were then removed from the dye set.

Example 5: Cell Assembly

[0082] A home-made cell is used for battery assembling and testing (cycling and EIS) and for the heating and characterization (chronoamperommetry) of pelletized LTFS/NMC mixtures. The battery casing enables pressure monitoring and to operate at a temperature ranging from ambient temperature (about 20° C.)up to 170° C. (the maximum temperature inside the sleeve). The casing was used to provide the aforementioned heating treatment in situ to the sample instead of another ex situ heating treatment implying the direct manipulation of the sample, which could alter the results.

[0083] The Casing Structure

[0084] The battery casing 1 can be described as the assembly of three cylinder-shaped separate elements better shown in FIG. 1a. These are: [0085] an upper 3 and lower 5 pistons; and [0086] a central body part 7.

[0087] The upper piston 3 comprises a top plunger 9 and the lower piston 5 comprises a lower plunger 11, both having a shape of a cylindrical rod, which extend along the longitudinal and central axis A. At the top of the upper piston 13 and at his centre, a nut 13 is located.

[0088] At the centre of the central bodypart 7, a hole 15 can accommodate the plungers 9, 11 from each piston 3 and 5. The three elements 3, 5 and 7 are assembled by introducing the plungers 9,11 into a hole 15 (provided within the central body part and having a diameter of about 8 mm)from both sides of the central body part 7, along the longitudinal axis A. The casing 1 can be closed or opened by simply inserting (or removing) the at least one of the plungers 9, 11 into (or from) the hole 15.

[0089] As shown in FIG. 1b, the upper piston 3 comprises, in a stacked relationship the nut β, a force sensor 17, the upperplunger 9. The upper plunger 9 comprises a top electrical contact 19.

[0090] The lower piston 5 comprises, in a stacked relationship the bottom to the top, a base 21, a body and the lower plunger 11. The lower plunger 11 further comprises a bottom electrical connector 23.

[0091] The central bodypart 7 comprises, around the hole 15, an internal insulation sleeve 25 in a polymer material. The sleeve internal void is about of 8 mm diameter. When the plungers 9, 11 of each piston 3, 5 enter from the both sides of the body 7 into the hole 15, a cylindrical space 27 is created between the ends of each plungers 9, 11. The positive electrode material, a solid-electrolyte (SE) and a negative electrode forming an electrochemical cell can be placed within this space 27. A heating sleeve 29 is provided around internal sleeve 25 and is adapted to apply heat to space 27 and the electrochemical cell when located within the space 27.

[0092] Assembly Steps of the Electrochemical Cells

[0093] All the cells assemblies and tests were carried out in argon-filled glove box (O.sub.2 and H.sub.2O level<1 ppm).

[0094] The cell was assembled by sequential loading and pressing of each of its element at room temperature, without heating. The electrochemical cell comprises: [0095] a solid-electrolyte (SE); [0096] a positive electrode composite, comprising the materials according to the invention and the commercial oxide; and [0097] a negative electrode made of indium-copper disk.

[0098] The indium-copper disk was made by spreading 10-15 mg of indium foil (Sigma Aldrich, purity 99%, thickness=0.127 mm) on one side of copper disk cut into 8 mm of diameter from a cupper foil (Goodfellow, purity 99.99%, thickness=0.02 mm). The negative electrode is originally made of indium, and is free of lithium at a pristine state. However, after the first charge, due to the delithiation of the positive electrode, lithium ions from the lithiated positive electrode form an InLi inter-metallic phase with indium, according to following reaction: In +Li.sup.++e.sup.−=InLi and provide lithium at the negative electrode.

[0099] This way, when cell measurements are made, the negative electrode acts at the same time as counter and reference electrode, and the positive electrode is considered as the working electrode.

[0100] At the beginning of the assembly, the upper piston 3 of the casing 1 was removed. First, 30 to 35 mg of solid electrolyteβ-Li.sub.3PS.sub.4, prepared from Example 2, were loaded inside the sleeve 25 and were homogeneously distributed onto the lower plunger 11 and within the space 27. The variation of the mass of the solid electrolyte in the above-mentioned range had no appreciable impact in the final result. Then, the casing 1 was closed by inserting the upper plunger 9 inside the sleeve 25. The casing 1 was vertically positioned in an hydraulic press (not represented) and a pressure of 1 ton.Math.cm.sup.−2 was applied for one minute (and monitored by a pressure sensor 17), and applied for 1 minute. A pellet of the solid-electrolyte was thus obtained.

[0101] Next, the upper piston 3 of thecasing 1 was removed, and 5-10 mg of one of the positive electrode composite materials were loaded and homogeneously spread onto the surface of the pre-compressed solid-electrolyte pellet. This amount was enough to cover the entire surface of the pellet. The casing 1 was closed and the plunger 9 was slowly pressed against the positive electrode composite materials with the hydraulic press (pressing rate: 0.4 tons.Math.cm.sup.−2 every 10 min) until reaching a value of 4 ton.Math.cm.sup.−2. This final pressure was maintained for 2 hours.

[0102] The casing 1 was taken out from the press, and the lower piston 5 was removed from the casing 1. The casing was turned over to itself and an indium-copper disk was placed within the space 27 so that the indium layer of the disk contacted the solid-electrolyte layer. The lower piston 5 was repositioned and the pressure needed to compact the cell applied: 0.5 ton/cm.sup.2.

[0103] Once the battery is assembled within the sleeve, the cell is closed and the casing 1 is placed in a stainless steel frame (not represented). A bolt (not shown) was provided within the nut 13 and tightened with a screwdriver. This way, the casing was fixed to the frame and the pressure needed for cycling was provided.

[0104] Testing of the Electrochemical Cells

[0105] To carry out the cycling, once the desired pressure was set, the casing was connected to the Biologic (Seyssinet-Pariset, France) potentiostat/galvanostatmodelVMP-3 via the cables linking the inside and the outside of the glove box.

[0106] The conversion between V vs.(In—InLi)/Li.sup.+ and V vs. Li/Li+ scale was done by assuming that In—Li electrode potential remains stable once lithiated, and remains at 0.6 V vs. Li/Li.sup.+.

[0107] The Following Cells were Assembled and Tested. Namely:

Test Battery a

[0108] Positive electrode composite: LTFS/β-Li.sub.3PS.sub.4 composite (70:30 weight percent ratio) [0109] Solid electrolyte: β-Li.sub.3PS.sub.4, and [0110] Negative electrode: Li—In alloy (Indium onto a copper disk).

Test Battery B

[0111] Positive electrode composite: NMC 622—LTFS (1:1 weight ratio) mixed by hand grinding with β-Li.sub.3PS.sub.4 in a 70:30 weight ratio (70% of NMC 622/LTFS mixture, 30% of (β-Li.sub.3PS.sub.4) [0112] Solid electrolyte: β-Li.sub.3PS.sub.4, and [0113] Negative electrode: Li—In alloy (Indium onto a copper disk).

Proof Battery a

[0114] Positive electrode composite: NMC 622 mixed by hand grinding with β-Li.sub.3PS.sub.4 in a 70:30 weight ratio (70% of NMC 622, 30% of β-Li.sub.3PS.sub.4) [0115] Solid electrolyte: β-Li.sub.3PS.sub.4, and [0116] Negative electrode: Li—In alloy (Indium onto a copper disk)

Proof Battery B

[0117] Positive electrode composite: NMC 622—LTFS (9:1 weight ratio) mixed by hand grinding with β-Li.sub.3PS.sub.4 in a 70:30 weight ratio (70% of NMC 622/LTFS mixture, 30% of (β-Li.sub.3PS.sub.4) [0118] Solid electrolyte: β-Li.sub.3PS.sub.4, and [0119] Negative electrode: Li—In alloy (Indium onto a copper disk).

Example 6: Preliminary Testing 1—Chemical Stability of the Interface Between Li-Active Oxide and Li-Active Sulphide

[0120] The suitability and the chemical stability of the material was tested by measurements of X-ray diffraction analysis and chronoamperometry on a pellet made from the mixture NMC 622 and LTFS in a weight ratio of 1:1 (see Example 5), without β-Li.sub.3PS.sub.4,before and after being subjected to a heating treatment of 100° C. for 5 days under inert atmosphere (Argon gas). This temperature was chosen to accelerate the interface decomposition reactions.

[0121] Once the heat treatment was completed, the cell was left for cooling down to room temperature, and then chronoamperometry measurements were carried out on the heated pellet. Powders of pristine LTFS and NMC were used as XRD reference.

[0122] Heated and non-heated pellets of NMC 622 and LTFS were grinded and X-ray diffraction patterns were measured on the resulting powders.

[0123] Thus, the criterion used to define a stable interface is based on the comparison of X-ray diffraction patterns and electrical resistance of the pellet prior and after the heating treatment.

Structural Characterization of the Synthesized Powdered Compounds by X-Ray Diffraction (XRD) Analysis

[0124] X-ray diffraction (XRD) measurements provide information on the crystal structure of characterized materials. The XRD patterns were collected using Bruker d8 advanced diffractometer. The following parameters were set to collect X-ray pattern data: [0125] Detector slit=9.5 mm; [0126] Beam slit=0.6 mm; [0127] Range: 2θ=10°-50°; [0128] X-ray wavelength=1.5406 Å(Angstroms) (CuKα); [0129] Speed: 211 s/step; [0130] Increment: 0.015°.

[0131] As shown in FIG. 2, no significant changes in the structure of the materials (“Fresh NMC 622— LTFS 1:1” and “Heated NMC 622— LTFS 1:1”) were evidenced, before and after heating, in terms of number of peaks, position, width and intensity.

[0132] Therefore, this result shows that the crystal structure of the characterized material remains stable after the heat treatment.

Chronoamperometry (CA)Measurements

[0133] In chronoamperometry, the current is measured as a function of time after application of a potential step. A pellet made from the mixture NMC 622 and LTFS in a weight ratio of 1:1 was submitted to potential which was increased every 10 seconds by jumps of 0.01 V, this for 10 times. At each potential jump, a stationary current was determined.

[0134] The results are shown in FIG. 3. A small shift in current from the third potential step may be due to a change in the temperature inside the glovebox (as each curve was measured on different day (5 days separating the two measurements). However, the variation is small enough to be ascribed to this temperature change. Therefore, the resistance is not substantially impacted.

[0135] FIG. 4 shows of the plotted graph of stationary current against the applied potential. The electrical resistance was determined as the inverse of the slope of the graph of FIG. 4. No significant changes in the electrical resistance of the pellets were observed before (2.48Ω) and after heating (2.57Ω) at 100° C. The specific conductivity a was determined using the following relationship σ=L/(A*R), where L and A are respectively the thickness and area of the pellet, R is the electrical resistance of the pellet determined from the slope. The dimensions of the pellets remain the same (diameter=8 mm, thickness=0.3 mm) during the measurements. The specific conductivity σthus remains stable at 0.02 S.cm.sup.−1 as it only varies of 0.001 S.cm.sup.−1 before and after the heat treatment. An unaltered electrical conductivity would mean no formation of resistive interlayer of decomposition products and therefore, no interface reaction. The stability of electric conductivity despite the heat treatment is therefore indicative of the stable interface between the oxide NMC 622 and the NTFS.

[0136] In view of the results of XRD and CA measurements, it can be considered that the interface between the oxide NMC 622 and LTFS remains stable despite the heating.

Example 7: Preliminary Testing of a Material According to the Invention—Galvanostatic Cycling to Test Electrochemical Activity of the Li-Active Sulphide

[0137] In order to test the electrochemical activity of LTFS, the positive electrode composite used in Test Battery A was assembled according to example 5, using the positive electrode comprising the mixture of LTFS and of β-Li.sub.3PS.sub.4 in a 70:30 weight ratio,β-Li.sub.3PS.sub.4 as solid electrolyte and Li—In alloy as the negative electrode.

[0138] After a resting period of 5 hours at open circuit, the cell was cycled at a rate of C/50 between 1.9V and 3.0V vs. Li/Li+, 4 cycles each at following temperatures: room temperature, 45° C., 75° C., and 105° C.

[0139] As shown in FIG. 5, an irreversibility, (which can be possibly ascribed to the formation of side products at the solid electrolyte-electrode interface) was observed only during the first cycle and this phenomenon was observed in all the first cycles at all of the temperatures tested. The overvoltage was measured at a plateau observed at the potential around 2.4 V. This value was at: [0140] Room Tp: 196 mV [0141] 45° C.: 135 mV [0142] 75° C.: 100 mV [0143] 105° C.: 61 mV.

[0144] The overvoltage decreases as the temperature increases. It does not exceed 200 mV regardless of the above-mentioned temperature regimes. Furthermore, the capacity retention observed remains above 90% over 16 cycles. Hence reversible cycling was demonstrated and shown to be favoured by the increase of temperature as this is evidenced by smaller polarization (i.e. overpotential).

Example 8: Electrochemical Characterization 1—Galvanostatic Cycling to Test Electrochemical Stability of the Li-Active Sulphide LTFS Over the Operating Voltage Range of a Commercialized Li-Active Oxide

[0145] In this example, the galvanostatic cycling was carried out over the operating voltage range in two steps, in order to test: [0146] the electrochemical stability of solely the LTFS combined with the solid-electrolyte β-Li.sub.3PS.sub.4 (test battery A); and [0147] the electrochemical stability of the LTFS mixed with solid-electrolyte β-Li.sub.3PS.sub.4 and with a commercialized oxide (test battery B).

[0148] For this purpose, two solid state batteries were assembled following the same procedure for battery assembling which is described in Example 5.

[0149] The electrochemical analyses were carried out in an argon-filled glovebox (O.sub.2 and H.sub.2O level <1 ppm) with Biologic (Seyssinet-Pariset, France) potentiostat/galvanostatmodelVMP-3.

Test Battery a

[0150] This battery was tested to show the electrochemical stability and suitability of the selected sulphide material per se at the operational voltage of the oxide material. The details of each component of such a cell, already described in example 5, are as follows. [0151] Positive electrode composite: LTFS-β-Li.sub.3PS.sub.4 composite (70:30 weight percent ratio) [0152] Solid electrolyte: β-Li.sub.3PS.sub.4 [0153] Negative electrode: Li—In alloy (Indium onto a copper disk)

[0154] After a resting period of 5 hours at open circuit, this battery was cycled at room temperature, at a cycling rate of C/50 within the voltage window of 1.9V-4.3 V vs. Li/Li+.

[0155] As shown in FIG. 6, except for the above-mentioned irreversibility from the first cycle, no significant variation in terms of the specific capacity (mAh.Math.g.sup.−1) is observed and the shape of the curve remains stable. In particular the capacity (˜200 mAh.Math.g.sup.−1) at the upper limit potential (near 4.3 V vs. Li/Li+) remains stable over 3 cycles.

[0156] This result illustrates that LTFS can withstand the voltage window of the commercialized oxide NMC 622.

Test Battery B

[0157] This battery was tested to show the electrochemical stability of the selected Li-active sulphide LTFS in presence of a Li-active layered oxide at the operational voltage of said oxide. The selected Li-active oxide was LiNi.sub.0.6Mn.sub.0.2Co.sub.0.2O.sub.2 (NMC 622).

[0158] The details of each component of such a cell, already described in example 5, are as follows: [0159] Positive electrode composite: NMC 622— LTFS (1:1 weight ratio) mixed by hand grinding with β-Li.sub.3PS.sub.4 in a 70:30 weight ratio (70% of NMC 622/LTFS mixture, 30% of (β-Li.sub.3PS.sub.4); [0160] Solid electrolyte: β-Li.sub.3PS.sub.4; [0161] Negative electrode: Li—In alloy (Indium onto a copper disk).

[0162] After a resting period of 5 hours at open circuit, this battery was cycled at room temperature, at a cycling rate of C/50 and in a voltage window of 1.9V-4.3 V vs. Li/Li+.

[0163] In reference to FIG. 7, the potential against capacity plot exhibits two zones. Between 1.9-2.5 V vs. Li/Li.sup.+, LTFS is electrochemically active, while NMC 622 is active in the range 3-4.3V vs. Li/Li+. In both well differentiated potential ranges, a good reversibility of the materials (LTFS and NMC) that are part of the electrode of the test battery B is observed.

[0164] FIG. 8 shows the evolution of the specific capacity evolution of the test battery B at delithiated state over 10 cycles. But for the irreversibility between the first and second cycles, the capacity remains stable at 125 mAh.Math.g.sup.−1. the specific capacity evolution of the test battery B, at delithiated state over 10 cycles, is represented.

[0165] These results demonstrate that Li-active sulphide LTFS is electrochemically stable, when mixed with the commercialized Li-active oxide, over the operating voltage range of the oxide.

Example 9: Electrochemical Characterization 2—Superiority of the Commercialised Li-Active Oxide Mixed with the Li-Active Sulphide LTFS Over the Oxide without the LTFS

[0166] Two solid state batteries were built, one of them not containing LTFS in the positive electrode material (Proof battery A) and the other one having a mixture of both Li-active oxide and Li-active sulphide(LTFS) materials as a positive electrode (Proof battery 8). In both cases, the electrochemical testing was done in the operational voltage of the oxide NMC 622, i.e. 2.6-4.3 V vs. Li/Li+.

[0167] The details of each component of such cells, already described in example 5, are as follows:

Proof Battery a without LTFS [0168] Positive electrode composite: NMC 622 mixed by hand grinding with β-Li.sub.3PS.sub.4 in a 70:30 weight ratio (70% of NMC 622, 30% of (β-Li.sub.3PS.sub.4) [0169] Solid electrolyte: β-Li.sub.3PS.sub.4 [0170] Negative electrode: Li—In alloy (Indium onto a copper disk)

Proof Battery B

[0171] Positive electrode composite: NMC 622—LTFS (9:1 weight ratio) mixed by hand grinding with β-Li.sub.3PS.sub.4 in a 70:30 weight ratio (70% of NMC 622/LTFS mixture, 30% of (β-Li.sub.3PS.sub.4) [0172] Solid electrolyte:β-Li.sub.3PS.sub.4 [0173] Negative electrode: Li—In alloy (Indium onto a copper disk)

[0174] After a resting period of 5 hours at open circuit, both batteries Proof A and Proof B were tested at room temperature, for 5 cycles each, at a cycling rate of C/50 and within a voltage window of 2.1-4.3 V vs. Li/Li+. FIG. 9 shows that the NMC 622/LTFS 9:1 mixture (proof battery B) exhibited an improved cycling performance in terms of reversible capacity (111 mAh.Math.g.sup.−1 for Proof Battery B against 71 mAh.Math.g.sup.−1 for Proof Battery A) and overvoltage (polarization at 130 mAh/g: 180 mV for Proof Battery B and 300 mV for Proof Battery A). The beneficial effect of LTFS is therefore demonstrated.

Example 10: Electrochemical Characterization 3—Electrochemical Impedance Spectroscopy (EIS)

[0175] Proof Battery A and Proof Battery B were used to carry out electrochemical impedance spectroscopy (EIS). The EIS measurements consist of exciting an electrochemical system with a sinusoidal electric signal around at a fixed value, i.e. a fixed potential or current. By detecting the response of such a sinusoidal signal of the system, it is possible to observe the impedance evolution (Z=V/I, in complex unit). The higher the impedance, the more resistive the interface is. Another advantage of this technique lies in possibility to distinguish phenomena occurring within the electrochemical cell by frequency ranges. Furthermore, the EIS can be considered as non-destructive for a low amplitude signal.

[0176] In this example, the amplitude of excitation signal was chosen to be 10 mV, potential mode. In order to avoid the evolution of the electrochemical system during the measurements, the EIS spectra were obtained in a frequency range of 100 KHz-100 MHz to probe the cathode-solid-electrolyte interfaces at the end of first charge (at delithiated state) at C/50 rate, 16 points per decade. The measurement was done after 15 minutes of relaxation and at an OCV of around 3.9 V for proof battery A (NMC) and around 4.1V for proof battery B (NMC/LTFS). FIG. 10 shows the evolution of impedance Z in Nyquist plan (-Im(Z) vs. Re(Z)). A semi-circle represents a contribution from different region of the electrochemical cell according to the frequency range.

[0177] For example, [0178] At a middle frequency range (frequency ranging from 100 mHz to 100 kHz): phenomena occurring at solid-electrolyte and positive electrode interfaces [0179] At a lower frequency range (lower than 100 mHz): phenomena occurring at solid-electrolyte and negative electrode interface (not represented) [0180] At a higher frequency range higher than 100 kHz): phenomena occurring near the electrolyte (not represented) (cf. Zhang et al.).

[0181] The curves shown in FIG. 10 are the ones corresponding to the middle frequency range and thus to the interfaces between the solid-electrolyte and the positive electrode-composite. The result shows clearly that the radius and the shape of the curves is smaller for proof battery B compared to that of proof battery A, indicating a less resistive cathode/SE interface for proof battery B. These results are in line with the galvanostatic measurements results carried out in example 9.

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