GB2197529A - Electrode modification - Google Patents

Abstract

A solid state electrochemical cell may comprise a Li or Li-based anode, a lithium ion conducting polymeric electrolyte, and a composite cathode comprising an insertion electrode material such as V6O13 and polymer electrolyte material. To improve contact between the electrode material and the electrolyte material in the composite cathode, the electrode material is functionalised by providing a plurality of silylated hydroxyl groups, the silyl groups having one or more reactive groups capable of bonding to the polymer electrolyte material. For example, unmodified electrode material is reacted with a silylating reagent of the formula R3Si(CH2)nX where R is alkoxy; n is 0, 1 or 2; and X is alkyl, aryl, heterocyclic, amino, or halogeno. Use of such reagents may also reduce the extent of decomposition of the polymer electrolyte.

Description

SPECIFICATION Electrode modification This invention relates to a modified electrode material and its use in solid state electrochemical cells, and to the manufacture of such material.

A solid state electrochemical cell for use in a secondary battery and comprising a Li or Libased anode, a lithium ion conducting polymeric electrolyte, and a cathode based on an insertion electrode material such as V60,3, V205 or TiS2 is known for example from European Patent Application Publication No 0 013 199 (corresponding to US Patent No 4 303 748), an insertion electrode material being a material that is capable of acting as an electrode in a solid state electrochemical cell by virtue of its ability to allow certain ions to be physically inserted into its structure during charging of the cell and to be subsequently removed therefrom during discharging thereof.In order to achieve high active cathode utilizations at realistic current densities, the cathode may be constructed as a composite structure comprising the insertion electrode material, polymer electrolyte material and, if required, an electronically conducting medium such as graphite.

In operation of the above cell, lithium ions are physically inserted into the structure of the electrode material during charging and are subsequently removed therefrom during discharging.

The overall cell reaction when the electrode material is V6013 is as follows:x Li +xe V6o13=LixV6o13 for x in the range 0 to 8.

The electrode material expands and contracts as lithium ions are inserted and removed causing contact with the polymer electrolyte in the composite cathode to weaken; the observed cell capacity therefore falls after a number of charge/discharge cycles.

The invention is concerned with modifying the insertion electrode material to ameliorate the above problem and, in one aspect, provides a modified insertion electrode material suitable for use in a solid state electrochemical cell having a plurality of silylated hydroxyl groups, the silyl groups having one or more reactive groups capable of bonding to a polymer electrolyte (as defined herein). Preferably, the reactive group is amino, which may be unsubstituted, substituted or quaternary, since there is evidence that its presence may catalyse silylation of the hydroxyl groups. It is also preferred that each silyl group is organic and has one or more methylene groups between said reactive group(s) and the silicon atom.

Tests to be described hereinafter on solid state cells having a composite cathode including the material of the invention show that the long term capacity of the cells does not fall. This may be due, at least in part, to improved contact between the modified electrode and the polymer electrolyte in the composite cathode possibly due to the presence of strong, flexible bonds between the electrode material and the polymer involving the silyl groups. Also, where the insertion electrode material is V60l3, it is possible that silylation reduces the oxidation state of the vanadium at the surface to V(IV) which, unlike higher oxidation states of vanadium, does not generate free radicals which, if present, would initiate decomposition of the polymer in the composite cathode.In other words, silylation according to this invention may have a stabilising effect on the cathode material-polymer interface and hence on the long-term capacity of the cell.

An insertion electrode material for modification according to this invention must have a plurality of -OH or =O-H+ groups, for example on its surface or within its body or pores.

Examples of such materials are vanadium oxides such as the aforementioned V6013 and derivatives thereof such as LiV2O5. The modified material may, for example, be represented as

shown for functionalisation of two hydroxyl groups only, where the vertical line represents the surface of the electrode material; R is lower alkoxy (such as methoxy or ethoxy), aroxy, 0 heterocylic, lower alkyl aryl or heterocylic; n is 1 or 2 or a higher integer; and X is lower alkyl, aryl, heterocyclic, amino, halogeno, or hydroxy and is capable of bonding to a polymer electrolyte (as defined herein), "lower" meaning having from 1 to 4 carbon atoms. Preferably, R is ethoxy and (CH2)nX is (CH2)3NH2 or (CH2)3NH(CH2)3NH2.

The modified electrode material of the invention may be made of silylating unmodified electrode material by methods known in the art, for example by reacting unmodified electrode material having acidic OH groups with a silylating reagent of the formula R3Si(CH2)nX where n and X are defined as above and R is an alkoxy group having from 1 to 4 carbon atoms.

The silylation reaction replaces surface OH groups with covalently bound organic groups. The OH groups of V6013 are strongly acidic because of the high oxidation states of the vanadium, i.e.

V(IV) and V(V), and therefore react readily with silylating reagents. However the silyl groups are probably retained by the vanadium oxide when the vanadium is in a low oxidation state. It is not a requirement for the silylation that the vanadium is in a low oxidation state.

It has been found that the degree of silylation is important; experiments may have to be done to determine the optimum degree of silylation to obtain good performance when thematerial of the invention has been incorporated into an electrochemical cell. For example, material subjected to a high degree of silylation may give rise to a cell thatis difficult to charge, suggesting that the suface layers resulting from silylation are too thick. Conversely, material subjected to a low degree of silylation may give rise to cells with irreproducible properties.

For use in a solid state electrochemical cell, the modified electrode material may be incorporated into a composite cathode in association with a polymer electrolyte material in the form of an alkali metal ion conducting polymeric material, which composite cathode may be fabricated into a cell by methods known in the art. The cell may have an alkali metal or alkali metal-based anode and a polymer electrolyte in the form of an alkali metal ion conducting polymeric material.

Preferably, the alkali metal is lithium. Such polymer electrolyte materials are ionically conducting complexes of plastics polymeric materials with alkali metal salts wherein the polymeric material forms a donor:acceptor type bond with the alkali metal ion and wherein the polymeric material is capable of existing in an ionically conducting amorphous form. Examples of such polymeric materials are poly(ethene oxide) and poly(propene oxide), commonly known as poly(ethylene oxide) and poly(propylene oxide) respectively, and examples of such alkali metal salts are lithium salts, the anion of which may, for example be I-, Br-, C104 -, SCN - or CF3SO3-. The composite cathode, the cell and their methods of manufacture are further aspects of the invention.

The invention will now be particularly described, by way of example only, as follows. Reference will be made to the accompanying drawings wherein Figure 1 is a graph of the relationship between theoretical cell capacity and number of charge/discharge cycles for a cell having a composite cathode including a material of the invention; and Figure 2 is a similar graph but wherein the material of the invention is functionalised to a lesser degree.

EXAMPLE 1 Preparation of Modified Electrode Material V60,3(20g) was wetted with toluene (15 ml) that had been previously dried by calcium chloride and distillation in a Dean-Stark apparatus, and ball-milled for two hours. The toluene was pumped off under vacuum and the fraction of V6013 below 250 #m retained for further use after drying at 1200C for 12 hours.

The milled and dried V6013 (15 g) was placed in a conical flask (250 ml) together with hydrochloric acid (0.25 molar, 100 ml). The flask was shaken for three hours at ambient temperature. The activated oxide was filtered and washed thoroughly with distilled water until the washings were at pH 6. The solid was dried at 120"C for twelve hours. The sample had coagulated a little so it was remitted for two hours and the portion below 250 ism was retained for further use.

The V6013 (25 g) was placed in a two-necked flask (250 ml) together with dry toluene (120 ml). A silylating reagent, 3-aminopropylethoxysilane (20 g), in dry toluene (50 ml) was added dropwise to the refluxing mixture. The reaction time was six hours. The oxide was filtered off, washed with dry toluene, placed in a Soxhlet apparatus and the free silylating reagent removed with toluene by continuous extraction for 8 hours. The sample was vacuum dried at room temperature for 12 hours.

Preparation and Testing of Cell The electrode material prepared as above was fabricated into a composite cathode in association with a polymer electrolyte (poly(ethene oxide))xLiF3CSO3 and carbon which in turn was fabricated into an electrochemical cell in association with a lithium anode and a (poly(ethene oxide))xLiF3CSO3 electrolyte, where x is in the range 9 to 20. The above fabrications were carried out by the methods described in UK Patent Specification No 2 139 410A (corresponding to US Patent No 4 547 440).

The discharge capacity of the cell was measured as a function of number of charge/discharge cycles under the following conditions: Discharge Current 0.25 mA Charge Current 0.13 mA Temperature 1 200C The results, expressed as a percentage of theoretical cell capacity (1.49 mAL), are shown in Fig. 1 from which it will be seen that, after an initial sharp fall, the cell capacity becomes relatively steady.

EXAMPLE 2 Preparation of Modified Electrode Material Milled and dried V60,3, prepared as for use in Example 1, (lOg) was activated by refluxing with hydrochloric acid (100 ml) for 4 hours. The solid was filtered and washed with distilled water until free from acid. It was noted that some blue vanadium compound was leached. The solid V60,3 (15 g) was placed in a two necked flask (200 ml) and dry toluene (100 ml). The reagent, 3-aminopropylethoxysilane, (10 g) in dry toluene (50 ml) was added dropwise to the refluxing mixture. The# total reaction time was six hours. The product was filtered, washed with dry toluene and then treated as described in Example 1.

Preparation and Testing of Cell The electrode material prepared as above was fabricated into an electrochemical cell and the cell tested as described in Example 1.

The results are again expressed as a percentage of theoretical cell capacity (2.01 mAL) and are shown in Fig. 2. It will be seen that the cell capacity becomes very steady after the initial drop.

EXAMPLE 3 Preparation of Modified Electrode Material Milled and dried V6013 (5 g), as prepared for use in Example 1, was placed in a two necked flask (100 ml) together with dried toluene. The mixture and apparatus was flushed with nitrogen.

Then a silylating reagent, 3-aminopropylethoxysilane (4 g), in dry toluene (25 ml) was added dropwise at room temperature. The mixture was stirred for three hours after which time the V60,3 was filtered and dried under vacuum at room temperature. The solid was placed in a Soxhlet apparatus and the free silylating reagent was removed with toluene by continuous extraction for eight hours. The sample was redried under vacuum.

It should also be noted that further experiments have been carried out in which solid state electrochemical cells utilising the present invention have been found to have higher % theoretical capacities than those shown in Figs. 1 and 2, namely about 60%

Claims (19)

1. A modified insertion electrode material suitable for use in a solid state electrochemical cell having a plurality of silylated hydroxyl groups, the silyl groups having one or more reactive groups capable of bonding to a polymer electrolyte (as defined herein).

2. A material according to claim 1 comprising a silylated oxide of vanadium.

3. A material according to claim 2 wherein the oxide of vanadium is V6013.

4. A material according to any of the preceding claims wherein the reactive group is an unsubstituted, substituted or quaternary amino group.

5. A material according to any of the preceding claims wherein each silyl group has one or more methylene groups between said reactive group(s) and the silicon atom.

6. A material according to any of the preceding claims wherein the silylated hydroxyl groups have the general formula

wherein R is a lower alkoxy, aroxy, 0 heterocyclic, lower alkyl, aryl or heterocyclic group; n is an integer; and X is a lower alkyl, aryl, heterocyclic, amino, halogeno or hydroxy group capable of bonding to a polymer electrolyte (as defined herein), "lower" meaning having from 1 to 4 carbon atoms.

7. A material according to claim 6 wherein R is a methoxy or ethoxy group; n is 1, 2 or 3; and X is an amino group

8. A modified electrode material comprising V6013 having a plurality of silylated hydroxyl groups of the formula

9. A modified electrode material substantially as described herein with reference to any of the examples.

10. A method of making a modified insertion electrode material according to any of the preceding claims comprising silylating an unmodified electrode material having acidic OH groups with a silylating reagent of the formula R3Si(CH2)nX where n and X are defined as in claim 6 and R is an alkoxy group having from 1 to 4 carbon atoms.

11. A method of making a modified insertion electrode material substantially as described herein with reference to any of the examples.

12. A composite cathode material comprising a modified insertion electrode material according to any of claims 1 to 10 in association with a solid, alkali metal ion conducting electrolyte material comprising a complex of a solid, plastics macromolecular material and a salt of an alkali metal, said macromolecular material and the cation of the salt being capable of forming a donor acceptor type bond to produce the complex.

13. A composite cathode material according to claim 12 wherein the macromolecular material is a lithium ion conducting polymeric material.

14. A composite cathode material according to claim 13 wherein the polymeric material is poly(ethene oxide) or poly(propene oxide).

15. A composite cathode according to any of claims 12 to 14 wherein the salt is a lithium salt, the anion of which is I , Br , ClO4 , SCN or F3CSO3

16. A composite cathode substantially as described herein with reference to any of the examples.

17. A solid state electrochemical cell comprising an alkali metal or alkali-metal based anode; an alkali metal ion conducting electrolyte comprising a complex of a solid, plastics macromolecular material and a salt of an alkali metal, said macromolecular material and the cation of the salt being capable of forming a donor:acceptor type bond to produce the complex; and a composite cathode according to any of claims 12 to 16.

18. A solid state electrochemical cell according to claim 17 wherein the alkali metal is lithium.

19. A solid state electrochemical cell substantially as described herein with reference to any of the examples.