Anodes including methylated amorphous silicon for lithium ion batteries

Abstract

The use of a methylated amorphous silicon alloy as the active material in an anode of Li-ion battery is described. Lithium storage batteries and anodes manufactured using the material, as well as a method for manufacturing the electrodes by low-power PECVD are also described.

Claims

1. An electrode comprising: at least one substrate comprising at least one layer based on a metal selected from the group consisting of copper, nickel, iron, stainless steel, molybdenum, tungsten and an alloy thereof, on which is deposited a coating including at least one material based on methylated amorphous silicon, wherein the methylated amorphous silicon is doped by inclusion of atoms chosen among atoms of the group consisting of phosphorus, boron, arsenic, antimony, aluminum, gallium, indium and bismuth.

2. The electrode according to claim 1, wherein the material based on methylated amorphous silicon comprises 50 to 100% of methylated amorphous silicon in mass fraction.

3. The electrode according to claim 1, wherein the coating based on methylated amorphous silicon is in the form of a continuous layer having a thickness of between 30 and 70 nm.

4. The electrode according to claim 1, wherein the material based on methylated amorphous silicon is deposited directly on the layer based on a metal or an alloy of metals.

5. The electrode according to claim 1, wherein the substrate has a rough surface.

6. The electrode according to claim 1, wherein the methylated amorphous silicon has an infrared spectrum wherein: i) a peak associated with a symmetrical deformation vibration of a methyl group, whose maximum is located between 1230 and 1260 cm.sup.−1, at a height higher than three quarters of an absorption band associated with elongation vibrations of the C—H links between 2800 and 3100 cm.sup.−1; ii) no absorption is observed, associated with elongation vibrations of C—H links between 3000 and 3100 cm.sup.−1, whose amplitude exceeds a quarter of a height of an absorption band associated with elongation vibrations of the C—H links between 2800 and 3000 cm.sup.−1.

7. The electrode according to claim 1, wherein, if α refers to a quotient of a concentration of carbon atoms [C], related to a sum of atom concentrations [C] and [Si] in the at least one material based on methylated amorphous silicon: $\alpha =\frac{\left[C\right]}{\left[C\right]+\left[\mathrm{Si}\right]}$ $5\%\le \alpha \le 40\%.$

8. The electrode according to claim 1, wherein the material based on methylated amorphous silicon is deposited directly on the layer based on a metal which is a metallic film.

9. The electrode according to claim 1, wherein the methylated amorphous silicon has the chemical formula (II):
a-Si.sub.1-x(CH.sub.3).sub.x-y(CH.sub.2).sub.y:H with 0<x≦0.4 and y≦x  (II).

10. The electrode according to claim 9, wherein the methylated amorphous silicon has the chemical formula (I):
a-Si.sub.1-x(CH.sub.3).sub.x:H with 0<x≦0.4  (I).

11. The electrode according to claim 1, wherein the coating based on methylated amorphous silicon is in the form of a continuous methylated amorphous silicon layer, or of a superimposition of a plurality of continuous methylated amorphous silicon layers, said continuous methylated amorphous layer or the superimposition of the plurality of continuous methylated amorphous silicon layers having a thickness higher than or equal to 30 nm.

12. The electrode according to claim 11, wherein the thickness of the continuous methylated amorphous silicon layer, or the superimposition of the plurality of continuous methylated amorphous silicon layers, goes up to 10 μm.

13. The electrode according to claim 11, wherein the thickness of the continuous methylated amorphous silicon layer, or of the superimposition of the plurality of continuous methylated silicon layers, is greater than or equal to 100 nm.

14. The electrode according to claim 11, wherein the thickness of the continuous methylated amorphous silicon layer, or of the superimposition of the plurality of continuous methylated amorphous silicon layers, is greater than or equal to 500 nm.

15. A lithium storage battery comprising: as an anode an electrode which comprises: at least one substrate, the at least one substrate comprising at least one layer based on a metal selected from the group consisting of copper, nickel, iron, stainless steel, molybdenum, tungsten and an alloy thereof; and a coating including at least one material based on methylated amorphous silicon is deposited on the at least one substrate.

16. The lithium storage battery according to claim 15, wherein the methylated amorphous silicon has the chemical formula (I):
a-Si.sub.1-x(CH.sub.3).sub.x:H with 0<x≦0.4  (I).

17. The lithium storage battery according to claim 15, wherein the material based on methylated amorphous silicon comprises 50 to 100% of methylated amorphous silicon in mass fraction.

18. The lithium storage battery according to claim 15, wherein the coating based on methylated amorphous silicon is in the form of a continuous layer having a thickness comprised between 30 and 70 nm.

19. The lithium storage battery according to claim 15, wherein the material based on methylated amorphous silicon is deposited directly on the layer based on a metal or an alloy of metals.

20. The lithium storage battery according to claim 15, wherein the substrate has a rough surface.

21. The lithium storage battery according to claim 15, wherein the methylated amorphous silicon is doped by inclusion of atoms chosen among atoms of phosphorus, boron, arsenic, antimony, aluminum, gallium, indium or bismuth.

22. The lithium storage battery according to claim 15, wherein the methylated amorphous silicon is has an infrared spectrum wherein: i) a peak associated with the symmetrical deformation vibration of a methyl group, whose maximum is located between 1230 and 1260 cm.sup.−1, at a height higher than three quarters of a height of an absorption band associated with elongation vibrations of C—H links between 2800 and 3100 cm.sup.−1; and ii) no absorption is observed, associated with elongation vibrations of C—H links between 3000 and 3100 cm.sup.−1, whose amplitude exceeds a quarter of a height of an absorption band associated with elongation vibrations of C—H links between 2800 and 3000 cm.sup.−1.

23. The lithium storage battery according to claim 15, wherein the methylated amorphous silicon has the chemical formula (II):
a-Si.sub.1-x(CH.sub.3).sub.x-y(CH.sub.2).sub.y:H with 0<x≦0.4 and y≦x  (I).

24. The battery according to claim 23, wherein, if a refers to a quotient of a concentration of carbon atoms [C], related to a sum of the atom concentrations [C] and [Si] in the at least one material: $\alpha =\frac{\left[C\right]}{\left[C\right]+\left[\mathrm{Si}\right]}$ $5\%\le \alpha \le 40\%.$

25. A method for manufacturing the electrode according to claim 1, comprising at least the steps of: (i) providing the at least one substrate; and (ii) depositing the coating on the substrate by PECVD using a power lower than 0.3 W/cm.sup.2.

26. The method according to claim 25, wherein the coating based on methylated amorphous silicon is in the form of a continuous methylated amorphous silicon layer, and the continuous methylated amorphous silicon layer, or a superimposition of a plurality of continuous methylated amorphous silicon layers, has a thickness higher than or equal to 30 nm.

27. The method according to claim 25, wherein the coating based on methylated amorphous silicon is in the form of a continuous methylated amorphous silicon layer, and the continuous methylated amorphous silicon layer, or a superimposition of a plurality of continuous methylated amorphous silicon layers, has a thickness higher than or equal to 10 μm.

28. The method according to claim 25, wherein the temperature of the substrate is comprised between 150 and 320° C.

29. The method according to claim 25, wherein the temperature of the substrate is comprised between 180 and 280° C.

30. The method according to claim 25, wherein step (ii) is carried out by PECVD using the power lower than 0.25 W/cm.sup.2.

31. The method of claim 25, wherein step (ii) is carried out by PECVD using the power comprised between 0.03 and 0.15 W/cm.sup.2, the gaseous flow of methane is comprised between 1 and 200 cm.sup.3/min.

32. The method according to claim 25, wherein: a temperature of the substrate is between 100° C. and 350° C., the power is lower than 0.3 W/cm.sup.2, a gaseous flow rate of silane is between 0.02 and 100 cm.sup.3/mn under standard conditions, and a gaseous flow rate of methane is between 1 and 200 cm.sup.3/min under standard conditions.

33. The method according to claim 32, wherein step (ii) is carried out using the gaseous flow rate of silane is between 0.1 and 50 cm.sup.3/min under standard conditions, and the gaseous flow rate of methane is between 2 and 100 cm.sup.3/min under standard conditions.

34. The method according to claim 32, wherein the gaseous flow rate of silane is between 0.5 and 30 cm.sup.3/min under standard conditions, and the gaseous flow of methane rate is between 4 and 50 cm.sup.3/min under standard conditions.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) FIG. 1: Typical infrared spectra of various layers of a-Si.sub.1-xC.sub.x:H obtained by PECVD deposition in low-power regime

(2) FIG. 2: Schematic representation of the cell constitution

(3) FIG. 3: Sectional representation of the sealed casing

(4) FIG. 4: Graphic representation of the variation of the potential as a function of the charge during the two first cycles of two cells

(5) FIG. 5: Graphic representation of the variation of the mass capacity of a cell as a function of the number of cycles

(6) FIG. 6: Graphic representation of the variation of the initial reversible mass capacity of a measured cell as a function of the carbon content

(7) FIG. 7: Graphic representation of the life span of the cells as a function of the carbon content

(8) FIG. 8: Graphic representation of the variation of the capacities of two cells with and without carbon

(9) FIG. 9: Graphic representation of the life span of the cells as a function of the thickness for carbon contents of 0% and 20%.

DETAILED DESCRIPTION OF THE INVENTION

(10) 1. Method of Depositing the Material on the Substrate:

(11) The layers of methylated amorphous silicon with which the presented results have been obtained have been made by plasma-enhanced chemical vapor deposition (PECVD) on a metallic substrate made of stainless steel. During the deposition, the substrate is heated to a temperature of 250° C. The electromagnetic excitation of the plasma at 13.56 MHz is capacitively coupled to the deposition chamber. Just before the deposition, the substrate is subjected to a plasma of hydrogen (H.sub.2) (pressure 200 mTorr, power density 100 mW/cm.sup.2) during 5 minutes. During the deposition, the pressure of the gaseous mixture in the chamber is comprised between 35 and 45 mTorr, and the flow rate of the gaseous mixture is fixed at 20 cubic centimeters per minute in the standard conditions (20 sccm). The power density provided to the plasma is comprised between 60 and 100 mW/cm.sup.2. These conditions allow being in the above-described “low-power” regime for which the speed of deposition not depends much of the carbon concentration of the methylated amorphous silicon obtained. In our condition, this speed of deposition is of about 20 nm per minute. The carbon content x of the material of composition a-Si.sub.(1-x)(CH.sub.3).sub.x:H depends on the composition of the silane/methane mixture. To reach a given value x, the proportion of methane g in the gaseous mixture (i.e. the ratio of the partial pressure of methane to the pressure of the gas) is given by the following formula (I. Solomon et al., Physical Review B 38 (1988) 9895-9901): g=11.4×/(1+9x).

(12) 2. Study of the Material by Infrared Spectroscopy:

(13) A deposition of various layers of a-Si.sub.(1-x)(CH.sub.3).sub.x:H by PECVD deposition in low-power regime as described hereinabove at §1 has been performed on a single-crystal silicon substrate so as to measure simply the transmission infrared absorption. The composition of the gaseous mixture calculated from the formula given at the preceding paragraph allowed to make layers of material with a carbon content of 10%, 20%, 30%, 37%. A high-power deposition (500 mW/cm.sup.2) has been made by way of comparison with a gaseous mixture having the same composition as that used to deposit a layer with a content of 20% in low-power regime.

(14) The corresponding infrared spectra are presented in FIG. 1: the carbon content x is indicated on each curve. The curve at the bottom of the figure represents the spectrum of the layer deposited in high-power regime.

(15) In this later case, the highest proportion of CH.sub.2-type and unsaturated carbon (═CH.sub.z, indicated sp.sup.2) is observed, in comparison with the corresponding curve deposited in low-power regime. The spectra have been recorded under non-polarized light and with an angle of incidence of 45°, and the absorbance has been reduced to a layer thickness of 1 micron.

(16) 3. Constitution of the Cells

(17) The test experiences are executed in silicon-lithium cells, prepared in a glove box, and placed in sealed casings.

(18) The active part of the cell is a stacked structure shown in FIG. 2 and consisted of: a layer (2.1) of the material to be studied (methylated amorphous silicon) deposited on a stainless steel substrate (2.2) of 1 mm thick and of 2 cm.sup.2. a separator (2.3) composed of a sheet of glass fiber of about 0.5 mm thick and of substantially the same size as the stainless steel substrate, impregnated with an electrolyte solution, for example a solution concentrated at 1 mol/L of LiClO.sub.4 in polypropylene carbonate (PC). a sheet of lithium (2.4) of 300 μm thick and of the same size as the separator. a copper collector (2.5) of 100 μm thick.

(19) The sealed casing (3) is consisted of three main parts shown in FIG. 3: a stainless steel lower element (3.1), comprising a circular receiver (3.2) able to contain the active part of the cell (3.3). This lower element is in direct contact with the stainless steel substrate (3.4) of the cell, allowing a good electric contact with this electrode. a stainless steel lid (3.5), in which is fixed an element (3.6) comprising four retractable brass pins (3.7) ensuring the electric contact with the copper collector (3.8) of the cell. an intermediate washer (3.9) made of polytrifluorochloroethylene (Kel′F), ensuring the electric insulation between the two stainless steel parts of the casing.

(20) The stainless steel lower element (3.1) and the stainless steel lid (3.5) are fixed by four screws (3.10) to the Kel′F washer (3.9), with the screws not passing through the washer and hence not making contact with the opposite stainless steel element.

(21) Two gaskets (3.11) made of a fluoroelastomer (Viton) ensure the seal of the casing (3), respectively between the Kel′F washer (3.9) and the stainless steel lower element (3.1) of the casing, and between the washer (3.9) and the lid (3.5).

(22) 4. Preparation of the Cells The methylated amorphous silicon, deposited as described above at §1, is used without particular treatment after deposition. The cells are prepared in a glove box, whose water content is about 100 ppm. The various elements of the cell are dried under vacuum, at ordinary temperature, in the lock of the glove box, during about twenty hours.

(23) 5. Characteristic Parameters of the Layers of Materials: Thickness, Carbon Content, Charge-Discharge Current

(24) The active materials of the cells are tested for various thicknesses and various carbon contents.

(25) Tested Thicknesses:

(26) Thin (30-70 nm) or thick (290 nm) layers of amorphous silicon (0% of carbon) or of methylated amorphous silicon, deposited on stainless steel.

(27) Tested Carbon Contents:

(28) 0, 10, 15, 20, 25, 33% of carbon.

(29) Various charge/discharge currents have been used to cycle the cells. In accordance with the common practice, these charge/discharge currents are indicated in a unit normalized based on the initial reversible capacity of the cell: a current of C/2 corresponds to a charge/discharge of the cell within two hours, a current of C/10 corresponds to a charge/discharge of the cell within ten hours, etc. The currents used are presented in Table 1.

(30) TABLE-US-00001 TABLE 1 Charge/discharge currents Thickness % carbon 30-70 nm 290 nm 0 C/10-C/1.5 10 C/10-C/1.5 15 C/10-C/1.5 20 C/1.5-C/0.5 C/10 25 C/10-C/1.5 33 C/10-C/1.5

(31) The cells have all been cycled at ordinary temperature (20-25° C.).

(32) 6. Cycling Results

(33) a) Cycling Behavior of the Cells

(34) FIG. 4 shows the evolution of the potential during the two first cycles of two cells of 65 nm and 50 nm thick, for methylated amorphous silicon containing 10% of carbon, and 20% of carbon, respectively, cycled at C/10, as a function of the charge applied to the electrode. The counter-electrode is a metallic lithium electrode, such that the potential is always positive, and the charge state corresponds to a potential close to zero.

(35) This figure highlights irreversible capacities of 25% of the capacity of first charge for the methylated amorphous silicon containing 10% of carbon, and of 34% for the methylated amorphous silicon containing 20% of carbon. The arrows show the direction of variation of the potential.

(36) FIG. 5 shows the evolution of the mass capacity of a cell with a methylated amorphous silicon electrode containing 15% of carbon, for a thickness of 60 nm, cycled at C/1.5.

(37) b) Effect of the Carbon Content

(38) FIG. 6 shows the variation of the initial reversible mass capacity measured as a function of the carbon content.

(39) FIG. 7 shows the number of cycles after which the capacity is reduced to 80%, 60% and 40% of the initial capacity, as a function of the carbon content, for layers of 30-70 nm thick. These numbers of cycles highly increase as a function of the carbon content.

(40) c) Effect of the Thickness

(41) FIG. 8 compares the variations of reversible capacity of two cells: one with an electrode of 70 nm thick made of pure amorphous silicon (without carbon), the other with an electrode of 290 nm thick made of methylated amorphous silicon containing 20% of carbon: despite a lower mass capacity, the thick layer with 20% of carbon shows a better total capacity (and surface capacity) than the thin layer without carbon. Its evolution with the number of cycles also shows a slower degradation of the capacity. The reversible capacity of the thick cell reaches a maximum after a few cycles. This “forming” period is not shown here.

(42) FIG. 9 illustrates the durations after which the capacities of the cells are reduced to 80%, 60%, and 40% for cells made of pure amorphous silicon (without carbon), of 30 and 70 nm thick, and with 20% of carbon, of 50 and 290 nm thick.

(43) The capacity of the cells with a methylated amorphous silicon electrode is less degraded as a function of the thickness than that of the cells without carbon. The cells of 290 nm thick with a methylated amorphous silicon electrode containing 20% of carbon have a greater surface capacity and a better cyclability than the cells without carbon of 70 nm thick.

(44) d) High Charge/Discharge Speeds

(45) The material supports correctly high charge/discharge speeds. A cell of 50 nm thick with a methylated amorphous silicon layer of 20% of carbon content has been cycled at C/0.5. The number of cycles after which its residual capacity has been reduced to 80, 60 et 40% was respectively:

(46) 80% initial C: 60 cycles

(47) 60% initial C: 480 cycles

(48) 40% initial C: 1200 cycles.

(49) The capacity decreases more rapidly to 80% of the initial capacity than for a slower charge-discharge speed. Beyond this initial decrease, the values are of the same order, or even slightly better than those obtained for less rapid cycling processes.

(50) Finally, the coulombian efficiency is particularly high for this high charge-discharge speed: higher than 90% after the 3.sup.rd cycle, 95% after the 4.sup.th cycle and tending to 99-99.8% after the 60.sup.th cycle.