Nano-scale/nanostructured Si coating on valve metal substrate for lib anodes

Abstract

An improved structure of nano-scaled and nanostructured Si particles is provided for use as anode material for lithium ion batteries. The Si particles are prepared as a composite coated with MgO and metallurgically bonded over a conductive refractory valve metal support structure.

Claims

1. An electrically active electrode material for use with a lithium ion cell, the electrochemically active electrode material comprising a substrate material consisting of individual filaments of a valve metal selected from the group consisting of tantalum, niobium, an alloy of tantalum, an alloy of niobium.[.,.]. .Iadd.and an alloy of .Iaddend.hafnium, .[.titanium and aluminum,.]. not larger than about 10 microns across, which filaments are adhered together to form a mat or porous sheet and wherein the individual filaments of the mat or porous sheet are coated with a coating of crystalline silicon .[.inside.]. .Iadd.in .Iaddend.a stabilizing magnesium oxide .[.coating.]. .Iadd.matrix.Iaddend., wherein the silicon is metallurgically bonded to the valve metal filaments.

2. The electrically active electrode material of claim 1, wherein the valve metal filaments have a thickness of less than 10 microns.

3. The electrically active electrode material of claim 1, wherein the valve metal filaments have a thickness below about 1 micron.

4. The electrically active electrode material of claim 1, formed into an anode.

5. A method of forming an electrode substrate useful for forming a lithium ion battery comprising the steps of: (a) providing valve metal substrate material formed of individual filaments of a valve metal selected from the group consisting of tantalum, niobium, an alloy of tantalum, an alloy of niobium.[.,.]. .Iadd.and an alloy of .Iaddend.hafnium, .[.titanium and aluminum,.]. not larger than about 10 microns across; (b) forming the individual filaments of step (a) into a mat or porous sheet; and (c) subjecting the mat or porous sheet of step (b) and silica to a simultaneous magnesiothermic co-reaction with magnesium to produce a coating of crystalline silicon .[.inside.]. .Iadd.in .Iaddend.a stabilizing magnesium oxide .[.coating.]. .Iadd.matrix.Iaddend., wherein the silicon coating is metallurgically bonded to the individual valve metal filaments.

6. The method of claim 5, wherein the magnesiothermic co-reaction is conducted under vacuum or in an inert gas at elevated temperature of 800-1200° C.

7. The method of claim 6, wherein the elevated temperature is 900-1100° C.

8. The method of claim 6, wherein the magnesiothermic co-reaction is conducted for 2-10 hours.

9. The method of claim 5, wherein the filaments have at thickness of less than 10 microns.

10. The method of claim 5, wherein the filaments have a thickness below about 1 micron.

11. A lithium ion battery comprising a case containing an anode and a cathode separated from one another, and an electrolyte, wherein the anode is formed of electrically active electrode material as claimed in claim 1.

12. The method of claim 6, wherein the elevated temperature is 950-1050° C.

13. The method of claim 6, wherein the magnesiothermic co-reaction is conducted for 4-8 hours.

14. The method of claim 6, wherein the magnesiotheimic co-reaction is conducted for 5-6 hours.

15. The electrically active electrode material of claim 1, wherein the valve metal filaments have a thickness of less than 5 microns.

16. The method of claim 5, wherein the filaments have a thickness of less than 5 microns.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the present invention will be seen from the following detailed description, taken in conjunction with the accompanying drawings, wherein:

(2) FIG. 1 is a schematic block diagram of a process for providing anode material in accordance with the present invention;

(3) FIGS. 2 and 3 are SEM photographs at two different magnifications showing nanoscaled nanostructure of Si particles metallurgically bonded to Ta support particles in accordance with the present invention;

(4) FIG. 4 plots capacity versus time of anode material made in accordance with the present invention;

(5) FIG. 5 plots coulomb efficiency versus time of anode material made in accordance with the present invention;

(6) FIG. 6 plots differential capacity versus cell voltage for a lithium ion battery anode made in accordance with the present invention;

(7) FIG. 7 is a cross-sectional view of a rechargeable battery in accordance with the present invention; and

(8) FIG. 8 is a perspective view of a battery made in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(9) In one embodiment of the invention, the refractory metal is formed of micron size (e.g. not larger than about 10 microns in across) tantalum filaments formed as described for example, in my earlier U.S. Pat. Nos. 9,155,605, 5,869,196, 7,146,709, and PCT WO2016/187143 A1, the contents of which are incorporated herein by reference.

(10) Referring to FIG. 1, the production process starts with the fabrication of valve metal filaments, preferably tantalum, by combining filaments or wires of tantalum with a ductile material, such as copper to form a billet at step 10. The billet is then sealed in an extrusion can in step 12, and extruded and drawn in step 14 following the teachings of my '196 U.S. patent. The extruded and drawn filaments are then cut or chopped into short segments, typically 1/16th-¼th inch long at a chopping station 16. Preferably the cut filaments all have approximately the same length. Actually, the more uniform the filament, the better. The chopped filaments are then passed to an etching station 18 where the ductile metal is leached away using a suitable acid. For example, where copper is the ductile metal, the etchant may comprise nitric acid.

(11) Etching in acid removes the copper from between the tantalum filaments.

(12) After etching, one is left with a plurality of short filaments of tantalum. The tantalum filaments are then washed in water in a washing station 20, and the wash water is partially decanted to leave a slurry of tantalum filaments in water. The slurry of tantalum particles in water is then mixed with fine, e.g. 4 to 200 micron size silica particles in water, in a coating station 22, forming a spongy mass. The coated spongy mass is then dried and subjected to magnesiothermic reaction by treating under vacuum or in an inert gas at 800 to 1200° C., preferably 900 to 1100° C., more preferably 950 to 1050° C., for 2 to 10 hours, preferably 4 to 8 hours, more preferably 5 to 6 hours at a reaction station 24. The magnesium reduces the silica and the oxide impurities within the tantalum fibers simultaneously permitting silicon to metallurgically bond to the tantalum fibers. Any magnesium oxide which results may remain, but preferably is removed for example by acid etching. On the other hand, it is not necessary to completely remove any copper which may be left over from the extrusion and drawings steps, since the copper also would metallurgically bond to the silicon. The resulting structure is a spongy, high surface area, conductive electrochemically stable tantalum metal substrate mass coated with a composite of sub-micron Si particles coated with a MgO matrix. The resulting spongy mass may then be mixed with water, and cast as a mat at a rolling station 26. The resulting mat is then further compressed and dried at a drying station 28.

(13) As an alternative to coating and rolling a thin sheet may be formed by spray casting the slurry onto to a substrate, excess water removed and the resulting mat pressed and dried as before.

(14) There results a highly porous thin sheet of Si/MgO composite or Si coated tantalum filaments substantially uniform in thickness.

(15) As reported in my aforesaid PCT application, an aqueous slurry of chopped filaments will adhere together sufficiently so that the fibers may be cast as a sheet which can be pressed and dried into a stable mat. This is surprising in that the metal filaments themselves do not absorb water. Notwithstanding, as long as the filaments are not substantially thicker than about 10 microns, they will adhere together. On the other hand, if the filaments are much larger than about 10 microns, they will not form a stable mat or sheet. Thus, it is preferred that the filaments have a thickness of less than about 10 microns, and preferably below 1 micron thick. To ensure an even distribution of the filaments, and thus ensure production of a uniform mat. the slurry preferably is subjected to vigorous mixing by mechanical stirring or vibration.

(16) The density or porosity of the resulting tantalum mat may be varied simply by changing the final thickness of the mat.

(17) Also, if desired, multiple layers may be stacked to form thicker mats that may be desired, for example, for high density applications.

(18) The resulting tantalum mat comprises a porous mat of sub-micron size Si or Si/MgO composite coated tantalum filaments in contact with one another, forming a conductive mat.

(19) Alternatively, in a preferred embodiment of the invention, the raw tantalum filaments may be formed as mats of electrode material by casting and rolling above described are then coated with silicon nanoparticles by magnesiothermic reduction as above described, e.g., by dipping the tantalum mat into an aqueous based solution containing fine silica in water, and then heating under vacuum or inert gas as above described.

(20) The Si/Ta structure as shown in FIGS. 2 and 3 is that of valve metal structure that is coated with a layer of nanoscaled nanostructure Si particles. The MgO can act as a stabilizing buffer against the degradation of the Si during cycling as the LIB anode. Although it is preferred that the MgO matrix is removed, using mineral acids, to reveal a nanoscaled nanostructure of the Si particles which are metallurgically bonded to the Ta support particles.

(21) The resulting materials are tested for capacity over time, coulomb efficiency over time and differential capacity over cell voltage, and the results shown in FIGS. 4-6.

(22) The resulting Si coated refractory material can be formed into useful LIB anodes via any standard manufacturing method, including, but not limited to: thin wet-lay methods deposited on a current collector, with or without conductive carbon additive; calendared fabrics; coins; etc. For example, referring to FIGS. 7 and 8, the coated mats are then assembled in a stack between separator sheets 36 to form positive (anode) and negative (cathode) electrodes 38, 40. The electrodes 38, 40 and separator sheets 36 are wound together in a jelly roll and inserted in the case 42 with a positive tab 44 and a negative tab 46 extending from the jelly roll in an assembly station 48. The tabs can then be welded to exposed portions of the electrode substrates, and the case filled with electrolyte and the case sealed. The result is a high capacity rechargeable battery in which the electrode material comprises extremely ductile fine metal composite filaments capable of repeated charging and draining without adverse effects. Other methods are also contemplated.

(23) Various changes may be made in the above invention without departing from the spirit and scope thereof. For example, the invention has been described particularly in connection with silicon, other materials such as germanium advantageously may be employed. Still other changes may be made without departing from the spirit and scope of the invention.