NANOPOROUS ELECTRODE

Abstract

The present application relates to an electrode comprising pillars of conductors covered with at least two layers for improving the deposition of lithium, and the electrochemical cells and batteries comprising same.

Claims

1. A nanoporous negative electrode comprising conducting pillars disposed on the current collector, such that the porosity of said negative electrode is such that:
E×ε>4.85×C where C is the surface capacity of said positive electrode (in mAh/cm.sup.2); E is the thickness of said negative electrode in the discharged state, expressed in μm; and ε is the porosity of said negative electrode in the discharged state (the porosity being defined as the ratio of the difference between the total volume of the electrode (excluding the current collector) and the volume of said material divided by the volume of the electrode), said electrode being characterized in that the surface of said pillars is at least partially covered with a layer of a material consisting of at least one element forming alloys with lithium.

2. The negative electrode according to claim 1 comprising a second nanometric conducting layer for the lithium deposited on at least a portion of the surface of said layer.

3. The negative electrode according to claim 1 wherein the conducting pillars are selected from copper pillars, carbon nanotubes or microporous carbons.

4. The negative electrode according to claim 3 wherein the carbon nanotubes are vertically aligned carbon nanotubes (VACNT).

5. The negative electrode according to claim 2 wherein the second layer comprises a polymer, a ceramic or a gel.

6. The negative electrode according to claim 2, wherein the thickness of the second layer has a thickness comprised between 0 and 100 nm.

7. The negative electrode according to claim 1, wherein the lithiophilic element is selected from silver, zinc and magnesium.

8. The negative electrode according to claim 2 as same further comprises a third layer comprising an electrolyte.

9. The electrode according to claim 1 having a thickness in the charged state (Ec) and a thickness in the discharged state (Ed), such that:
Ec−Ed<4.85×C×h, where C is the surface capacity of the positive electrode (in mAh/cm.sup.2); h is a dimensionless number comprised between 0 and 0.3.

10. A method for preparing an electrode according to claim 1 comprising the step of successively depositing a first layer and, where appropriate, a second layer, each of the depositing steps being carried out by a physical or chemical process in the vapor phase (PVD or CVD, respectively), or by a wet process.

11. An electrochemical cell comprising a negative electrode according to claim 1, characterized in that same relates to an all-solid-state or hybrid battery.

12. The electrochemical cell according to claim 11, such that same relates to an Li free battery, in that same does not contain metallic lithium during assembly.

13. An electrochemical module comprising the stack of at least two elements according to claim 11, each element being electrically connected to one or a plurality of other elements.

14. A battery comprising one or more modules according to claim 13.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] FIG. 1 is a schematic drawing of the electrode structure according to the invention.

[0055] FIG. 2 is a schematic drawing of the structure of an electrode according to the aforementioned FIG. 1, in the charged state, where Li (5) is present around the pillars.

DETAILED DESCRIPTION

[0056] The current collector (1), such as a metal strip, has a flat surface, on which pillars of conducting material (2) rise, such as copper pillars or carbon nanotubes.

[0057] The pillars (2) are covered with at least one layer: [0058] first, in direct contact with the pillar, a first layer (3) made of a material apt to form alloys with lithium; and [0059] on this first layer and in contact with the electrolyte: if appropriate, a second layer (4) conducting Li.sup.+ ions.

[0060] In operation, during charging, Li.sup.+ ions arriving from the solid electrolyte layer separating the 2 positive and negative electrodes, react at the ends of the pillar. When the pillar is made of carbon, the latter forms a lithiated compound of variable formula (e.g., in the case of graphite, the composition thereof is CLI.sub.0.17), when the potential of the negative electrode reaches potentials less than 0V, a deposition of lithium should form; nevertheless, the formation of metallic lithium requires a nucleation energy which may be relatively high on the carbon and corresponds to an overvoltage. Supersaturation of the carbon with lithium can thus take place and lithium will thus diffuse into the pillar; adding a material forming alloys with lithium to the surface of the CNT will reduce the metallic lithium formation overvoltage. Lithium in the structure of the supersaturated lithiated carbon will thus be able to be converted into metallic lithium on the layer deposited on the surface of the CNT. It should be incidentally noted that the alloy-forming material will already have been lithiated before the precipitation of metallic lithium because the formation potential thereof is greater than that of metallic lithium. In parallel with this process, the lithium ions can also flow through said possible second layer so as to be deposited under this layer, in the form of metallic Li.

[0061] The following examples are given as a non-limiting example of an embodiment of the present invention.

EXAMPLES

[0062] Producing Electrochemical Cells:

[0063] Relating to the preparation of the negative electrode, the deposition of VACNT can be performed as described in the article by Arcila-Velez et al., Nano Energy, Volume 8, 2014, 9-16: VACNT tubes are made of a quartz tube with a diameter of 5 cm; the 2 ends of the tube are partially closed with stainless steel. The tube is placed in an oven with 2 hot zones, the first zone serving for preheating, while the reaction takes place in the second zone. A pump is used to inject the precursor (solution of ferrocene in xylene, containing e.g. 0.5 at % of iron) in the center of the preheating zone. Copper sheets cleaned with acetone, a few cm wide and long, are placed in the oven, in the center of the reaction zone. The system is brought to 600° C. under a stream of argon and hydrogen (17% by volume of H.sub.2). The precursor is injected at 600° C. with a low flow rate, e.g. from 0.1 to 1.5 ml/h, into a stream of C.sub.2H.sub.2 with a flow rate of 30 cm.sup.3/min. The treatment time varies from 5 to 50 min depending on the desired length of the CNT.

[0064] The depositions of the first and second layers can be made according to the following methods:

TABLE-US-00001 TABLE 1 Methods First layer Second layer 1 PECVD in 2 steps: ALD ALD deposition for depositing a layer of AgO or ZnO; and a reduction heat treatment under hydrogenated argon (between e.g. 300 and 500° C.) 2 PECVD ALD 3 PECVD in 2 steps: MLD ALD deposition for depositing a layer of AgO or ZnO; and followed by a reduction heat treatment under hydrogenated argon (between e.g. 300 and 500° C.) 4 PECVD ALD 5 ALD ALD 6 ALD immersion in a solution containing the gel constituents dissolved in a solvent or a solvent mixture containing, e.g., acetonitrile and/or acetone.

[0065] The preparation of the electrolyte layer and the positive electrode is carried out under an argon atmosphere (<1 ppm H.sub.2O).

[0066] The electrolytic membrane is obtained in a plurality of steps. A first step of mixing a sulfide electrolyte such as argyrodite Li.sub.6PS.sub.5Cl with 2% by mass of a copolymer binder containing polyvinylidene fluoride is carried out in a planetary mill. The mixing is carried out at a rate of 1000 rpm for 10 min with a plurality of solvents: xylene and isobutyl isobutyrate which are priorly dried using molecular sieves (pore size of 3 Å). In a second step, the ink thus obtained is coated on a PET film allowing the membrane to be detached after drying. The thickness of the membrane is 50 μm.

[0067] Similarly, the positive electrode is made from a material such as NMC with the composition LiNi0.60Mn0.20Co0.20O.sub.2 covered with a 10 nm layer of LiNbO.sub.3, mixed with solid electrolyte Li.sub.6PS.sub.5Cl, carbon fibers (VGCF) and a copolymer binder containing polyvinylidene fluoride, in the NMC mass proportions:Li.sub.6PS.sub.5Cl:VGCF:binder 70:30:3:3. These materials are dispersed in a mixture of xylene and isobutyl isobutyrate solvents. A homogeneous ink is obtained after passing through the planetary mixer. The ink is then coated on an aluminum current collector covered beforehand with a thin layer of carbon. The grammage of the electrode is varied between 15 and 95 mg/cm.sup.2.

[0068] After drying, a 12 mm diameter disk is cut from the electrolyte membrane along with a 10 mm diameter positive electrode disk. The two discs are compressed against each other in a mold under a pressure of 5.6 t.

[0069] For a given example, a 10 mm diameter disk is cut out from the negative electrode corresponding to the example and placed on the other side of the electrolytic membrane. This stack is then compressed at a pressure of 1 t/cm.sup.2 and can undergo a heat treatment between 80 and 130° C. for 12 h.

[0070] The stack is then placed in a Swagelok cell compressed at a pressure comprised between 1 and 5 MPa. For electrical tests, charging and discharging are performed at a speed of C/20.

Counter Examples

[0071] For the comparative example No. 1, the CNT powder with a diameter of 40 nm and a length comprised between 20 and 50 μm is dispersed in an organic solvent (e.g. NMP) in the presence of 2% PVDF. The mixture is deposited on a copper collector, then dried at 120° C. and compressed; the thickness of the layer is 25 μm. A deposition of silver is then carried by PECVD on the layer thus produced, followed by an LiPON deposition by ALD. The negative electrode is then produced by cutting a 10 mm diameter coated collector disc.

[0072] For the comparative examples 2 to 5, the method of preparation of the negative electrodes is similar to that used for examples 1 to 6.

[0073] The electrochemical cells are then prepared in an identical manner to examples 1 to 6.

[0074] The examples described in Tables 2 and 3 show that the thickness variations are significantly less than in the comparative examples 1 and 4 described in Tables 4 and 5; indeed, the negative electrodes of the comparative examples 4 and 5 correspond to an increase in thickness corresponding to more than 60% of the initial thickness.

[0075] The comparative examples 2, 3 and 5 have too large inter-CNT distances which give rise to problems of mechanical strength in the CNTs under pressure, associated with a heterogeneous lithium deposition resulting in a shorter service life.

TABLE-US-00002 TABLE 2 Average CNT CNT distance Thickness capacity Diameter Length between Composition 1.sup.st layer Composition Thickness Example (mAh/cm.sup.2) (nm) (μm) CNT (nm) 1.sup.st layer (nm) 2.sup.nd layer 2.sup.nd layer 1 4 40 25 236 Ag 10 LiPON 10 2 4 10 30 82 Si 2 Li.sub.2ZrO.sub.3 2 3 6 20 50 30 Zn 5 PEO/LiTFSI 20 4 6 50 40 30 Mg 5 LLZO 5 5 6 10 60 4 Al2O3 2 6 10 100 60 188 SiO2 4 PC-PVDF 10 LiTFSI gel

[0076] The following particularities are observed for the embodiments of Table 2 above: [0077] Example 1: Inter-CNT distance on the order of 300 nm; [0078] Example 2: small thickness of the surface layers; [0079] Example 3: large surface capacity; [0080] Example 4: Ec−Ed>0 but <0.2×4.85° C.; [0081] Example 5: small pore size=small inter-CNT distance; [0082] Example 6: very large surface capacity.

TABLE-US-00003 TABLE 3 Thickness Thickness CNT in discharged in charged density porosity 4.85*C Ec − Ed Example state (*) state (*) per cm.sup.2 % Ed(μm)*poro (mAh/cm.sup.2) (μm) 4.85*0.2*C 1 25 25.0 1.00E+09 95 23.74 19.40 0.00 3.88 2 30 30.0 1.00E+10 97 29.24 19.40 0.00 3.88 3 50 50.0 1.00E+10 62 30.76 29.10 0.00 5.82 4 40 44.5 1.00E+10 62 24.61 29.10 4:49 AM 5.82 5 60 60.0 2.00E+11 49 29.46 29.10 0.00 5.82 6 60 60.0 1.00E+09 87 52.28 48.50 0.00 9.70 (*) negative electrode thickness without collector thickness for one electrode side

Comparative Examples

[0083] The following examples were obtained from cells comprising negative electrodes, the characteristics of which are given in Tables 4 and 5.

TABLE-US-00004 TABLE 4 Average CNT CNT distance Thickness comparative capacity Diameter Length between Composition 1.sup.st layer Composition Thickness example (mAh/cm.sup.2) (nm) (μm) CNT (nm) 1.sup.st layer (nm) 2.sup.nd layer 2.sup.nd layer 1 4 40 25 236 AG 10 LIPON 10 2 4 40 25 276 — 0 — 0 3 4 40 25 960 AG 10 LIPON 10 4 6 40 30 5 AG 5 LIPON 5 5 6 2000 50 1162 AG 5 LIPON 5

[0084] The following particularities are observed for the embodiments of Table 4 above: [0085] Example 1: CNT powder deposited on a Cu collector=>all Li is deposited between CNT and copper; [0086] Example 2: Poor penetration of lithium into the porosity=>poor service life and swelling of the electrode; [0087] Example 3: Inter-CNT distance>300 nm=>low mechanical strength; [0088] Example 4: E*poro>4.85.C and Ec−Ed>0.2×4.85×C: strong electrode swelling corresponding to more than 60% of the electrode thickness=>poor service life; [0089] Example 5: Poor penetration of lithium into the porosity and small developed surface of carbon leading to dendrite formation.

TABLE-US-00005 TABLE 5 Thickness Thickness CNT comparative in discharged in charged density porosity 4.85*C Ec − Ed example state (*) state (*) per cm.sup.2 % Ed(μm)*poro (mAh/cm.sup.2) (μm) 4.85*0.2*C 1 25 45 95 23.74 19.40 20 3.88 2 25 40.0 1.00E+09 99 24.69 19.40 0.00 3.88 3 25 25.0 1.00E+08 100 24.97 19.40 0.00 3.88 4 30 47.9 5.00E+10 37 11:15 AM 29.10 17.95 5.82 5 50 50.0 1.00E+07 69 34.29 29.10 0.00 5.82

[0090] It thus appears in particular that when E*poro>4.85.C (example 4), a strong swelling of the electrode is observed.