GB2266179A

GB2266179A - Electrochemical cells with complex of sulphur trioxide and tertiary amine polymer as cathode

Info

Publication number: GB2266179A
Application number: GB9207397A
Authority: GB
Inventors: Alexander Gilmour
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-04-03
Filing date: 1992-04-03
Publication date: 1993-10-20
Anticipated expiration: 2012-04-03
Also published as: GB9207397D0; GB2266179B

Abstract

Electrochemical cells of the non-aqueous type have a cathode composition consisting of a solid complex of sulphur trioxide and a polymeric tertiary amine eg. polyvinyl pyridine or polymethyl pyrrole. High energy density is achieved by virtue of the high EMF generated in a cell which has an alkali or an alkaline earth metal as the anode. The compositions are especially suitable for secondary cells since the micro-structure of the polymeric complex is essentially maintained after the incorporation of metal ions. The utilisation of sulphur trioxide as a solid complex obviates the presence of the highly corrosive liquid material in contact with the alkali or alkaline earth metal anode and the cell container and results in a battery system having long storage and service life.

Description

ELECTROCHEMICAL CELLS WITH A NOVEL CATHODE COMPOSITION This invention is concerned with a novel positive electrode composition for electrochemical cells of the non-aqueous type.

A high energy and power density together with superior charge retention over prior art systems are salient features of the new electrode composite as is the relatively low cost and nontoxicity of the starting materials.

An outstanding need exists for more efficient storage of electrical energy by redox chemical conversion. At present, most applications for a rechargeable or secondary battery system are met by either the lead acid or the nickel cadmium types, these being capable of storing a maximum of around 35 Watt.hours per Kg. of battery weight. For electric vehicles to be viable, the energy density available from existing systems is totally inadequate and this restricts the development of vehicles to compete with those powered by internal combustion engines. In the field of portable communications equipment, the need for greater amounts of electrical energy to be stored in handsets is a top priority.

A target energy density of 100 Watt.hours per Kg. is generally regarded as being an acceptable and realistic value for the new generation of rechargeable batteries. In order to achieve this threefold increase over existing electro- chemical systems, it is necessary to resort to non-aqueous media. Primary or non-rechargeable battery systems based on lithium metal as anode are capable of delivering over 1,000 Watt.hours per Kg. are now well established. The lithium thionyl chloride system is a good example of a high energy primary couple but it is not rechargeable. Sodium/sulphur and sodium/metal chloride systems under development are being reported to have good reversibility and to give energy densities in excess of 100 Watt.hours per Kg.These systems have an operating temperature around 3500C,, which limits their viability and casts doubts on safety in use It is an important object of the present invention that the chosen electrochemical system is operational at ambient temperatures, obviating the need for an external heating source. This is achieved by the use of electrolytes based on non-aqueous solvents which are liquid over a wide temperature range and have good ionic conductivity. Suitable electrolytes consist of organic or inorganic solvents or mixtures thereof, with salts of the alkali or alkaline earth metal which forms the negative electrode dissolved therein.

A non-rechargeable lithium battery system, well known as a power source for heart pacemakers, comprises a positive electrode which is a complex compound of iodine and poly2-vinyl pyridine (PVP.I). In this system, the PVP polymer material forms a solid solution with iodine and regulates the latter's reaction with lithium metal since there is no discrete separator present. This system is not reversible and has very low rate capability. By contrast, a very high rate primary lithium system uses liquid sulphur dioxide (SO2) ås active cathode material, the SO, also serving as electrolyte solvent or co-solvent. This system can be discharged at high rates down to -40 C. but extensive development work aimed at making it rechargeable, has met with only limited success.It is a major object of the present invention to provide a polymeric material having functional groups which will form an insoluble complex with a highly active Lewis acid such as SO, or S03 and confer much higher rate capability than that achieved in the PVP.Iodine complex but be electrochemically reversible.

It has been known for some considerable time that a solution of sulphur trioxide containing an ionically conducting lithium salt generates an EMF of around 4.5 Volts Attempts to utilise such an electrochemical cell have been unsuccessful mainly because of the high reactivity of SO, to other cell constituents, including the separator and the container as well as the lithium anode itself.Solid complexes are formed by combination of SO3 with monomeric tertiary amines, including heterocyclic compounds such as pyridine, these being stable in the absence of moisture to around 2000C. These complexes are unsuitable as cathode materials because of their ready solubility in excess SO SO3 and other highly polar solvents, liquid amine being generated on reaction of lithium with SO3.

The present invention provides an electrochemical cell in which sulphur trioxide is immobilised on a polymeric network having a matrix of tertiary nitrogen sites available for complex formation. The requisite source materials are derived from the polymerisation of tertiary amines having an unsaturated side chain or the oxidative polymerisation of conjugated heterocyclic compounds containing tertiary nitrogen. A solid insoluble composite is formed by direct combination of SO3 with such polymers.

Although many polymeric structures fall within the scope of this ihvention, the examples given below are selected mainly because they are available, performance being non- optimised: poly-(4-vinylpyridine) poly-(2-vinylpyridine) poly-(N-vinylpyrrole) and the oxidised form poly-(N-methylpyrrole) The composite cathode active material is formed by the combination of SO3 with the polymer at the tertiary amine site in the monomer units in a 1:1 ratio. For the intended application, it is essential that the polymer and the SO3 used in preparation of the composite are completely anhydrous.

In accordance with the invention, there is provided an electrochemical cell, the positive electrode of which consists of a polymer/SO3 complex as described above in contact with a suitable current collector. Certain polymers of the "conjugated" type, an example of which is poly-(N-methyl pyrrole) , are electronically conductive to a sufficient degree to carry current from the reactive sites in the complex to the positive terminal. Non conductive polymers require the addition of carbon black or similar finely divided inert conductive material to be in intimate contact with the polymer complex. The anode (negative electrode) of the cell may be a light metal or light metal alloy. Lithium is the preferred anode material, giving the highest energy density and voltage.

Alloys of lithium with carbon or aluminium may be used to improve the electrochemical stability to the electrolyte. For high rate applications, anhydrous liquid electrolytes are required by virtue of their good ionic conductivity. Solid polymer electrolytes may also be used for low rate applications.

Lithium ion conductivity is achieved in both instances by the incorporation of suitable salts such as LiCF3 SO3. Liquid electrolytes may consist of mixtures of aprotic organic solvents and/or liquid sulphur dioxide. For use with liquid electrolytes, a chemically resistant microporous separator is essential. Preferred materials are fluoropolymers such as Tefzel (Dupont) or polyvinylidene fluoride which resist attack by the strongly oxidising SO3/polymer cathode complex.

EXAMPLE:- Poly-(4-vinylpyridine), crosslinked with 2% divinylbenzene was added to an equal weight of freshly distilled sulphur trioxide in a glass stoppered conical flask under dry nitrogen. The polymer complex was filtered in a dry nitrogen atmosphere and dried on a glass frit filter funnel. A slurry consisting of 92% P#VP.SO3 complex, 6% Vulcan XC72 carbon black and 2% polyvinylidene fluoride in cyclohexane was prepared and coated onto 18 micron aluminium foil. Solvent was removed by gently heating under vacuum. The coated foil was placed between rollers in order to consolidate the cathode layer and improve its adhesion to the aluminium foil. Discs of 20mm diameter were prepared for making up into test cells. The anode in the test cells was of pure lithium metal and the separator consisted of a microporous Tefzel (EFTE polymer) material. A 1 Molar solution of lithium trifluoromethane sulphonate (LiCF3SO3) in equal parts by volume of propylene carbonate and ethylene carbonate was used for the electrolyte. The open circuit voltage of the cells was 1.6 Volts. The test cells were 2 cycled at a current density of 1 ma./cm down to 3.0 volts ma./cm2 and recharged at 0.5 ma./cm up to 1.7 volts cutoff. The average running voltage under these conditions was 3.8 volts.

Under these conditions the non-optimised test cells gave over 30 charge/discharge cycles.

Claims

1. An electrochemical cell characterised in that the positive electrode consists of a macromolecular complex of sulphur trioxide and a polymeric substance containing tertiary linked nitrogen as the recurring structural unit represented by the formula:

where R is an alkylene group and R1 and R2 are alkyl groups or R, R1 and R2 may constitute a heterocyclic 5 or 6 membered ring structure having at least one tertiary bound nitrogen atom, the said positive electrode being used in conjunction with a non-aqueous electrolyte and a negative electrode consisting of a light metal or light metal alloy.

2. An electrochemical cell according to claim 1, in which the polymeric substance is poly-(4-vinylpyridine).

3. An electrochemical cell according to claim 1, in which the polymeric substance is poly-(N-methylpyrrole).

An An electrochemical cell according to claims 1,2 and 3, in which the preferred ratio of sulphur to nitrogen in the S03/polymer complex is 1:1.

5. An electrochemical cell according to c ] aim 1, in which the polymer has a conjugated structure which confers inherent electronic conduction and obviates the need to include carbon or other conductive materials in the positive electrode.

6. An electrochemical cell according to claims 1 to 5, in which the negative electrode is lithium metal and the electrolyte is a solution of a lithium salt in aprotic solvents.

7. An electrochemical cell according to claims 1 to 5, in which the negative electrode is magnesium metal and the electrolyte is a solution of a magnesium salt in aprotic solvents.

8. An electrochemical cell according to claims 1 to 6, in which the electrolyte consists essentially of a solution of lithium salts in liquid sulphur dioxide.

9. An electrochemical c e 1 1 according to claims 1 to 5, in which the electrolyte consists of a solid polymer composition made i oni call y conductive by the incorporation of appropriate lithium salts.

10. An electrochemical cell according to claims 1 to 9, which forms the unit for a rechargeable battery.

11. An electrochemical cell according to claims 1 to 9, which forms the unit for a super or ultra-capacitor.