Conventional battery systems are typically based on lead-acid. nickel-cadmium and mercury and are associated with toxic heavy metal waste in both manufacture and subsequent disposal. In addition. lead acid secondary batteries are bulky and heavy, whilst nickel cadmium secondary batteries are of relatively low specific energy, comparatively high self-discharge characteristics and suffer undesirable"memory'effects that reduce recharging efficiency.

More recently developed primary batteries use lithium and sodium electrodes.

These tend to have relatively good performances but then-are not re-chargeable.

Lithium and sodium also suffer from being expensive and difficult to work with. This makes manufacturing time consuming and costly. In addition. lithium and sodium are highly reactive metals, which limits their potential use in transport applications. where safely in the event of an accident is important.

Furthermore. these primary batteries have a fairly short shelf life as a result of their self-discharge characteristics.

As an alternative to lithium and sodium. it is know n thai aluminium can be used as an anode in an electrolytic cell. Aluminium has a volumetric energy density of 8.0-iO Ah/dm3, which is about 4 times as large as that of lithium, and a gravimetric charge density of 2980 Ah'kg. which is second only to lithium (3860 Ah/kg). It is lightweight, inexpensive, non-toxic, easy to fabricate and

readily available. In addition, it is anticipated that aluminium will be capable of recycling from battery components at the end of their useful life.

Whilst conventional batteries use acid as the electrolyte. some batteries use molten salts. Of particular interest in the field of electrochemical cells are fused salt compositions that are molten at low temperature. Such molten salt compounds are ionic and liquid at temperatures below the individual melting points of the component compounds. These melts can form liquids simultaneously upon mixing the components together.

One known group of low temperature molten salts comprises an admixture of a metal halide (a compound consisting of a metal and a halogen. generally covalently bonded together) and an organic salt. Examples of such molten salts are mixtures of aluminium trichloride and organic onium compounds such as N- (n-buta, l) pyridinium chloride (BPC) and l-meths-eths limidazolium chloride (MEIC). These salts are, however, toxic, expensive and sensitive to the presence of oxygen and moisture, with which they react and decompose readily.

This is disadvantageous.

An object of the invention is to provide a batten-that is less harmful to the environment than those available at present. In particular. it is an object of the present invention to provide a secondary. re-charaeable battery that avoids the heavy metal pollution that is associated with conventional primary batteries.

According to one aspect of the invention. there is provided a battery having an anode. a cathode and a molten electrolyte that comprises a mixture of a metal halide and a tetraalkyl/tetraphenyl ammonium salt that has the form R4N. X-.

wherein R4N includes four identical alkyls of from I to 12 carbon atoms or four identical phenyl groups, and X-is a halide or complex halogen-containing ion.

A suitable mole ratio of tetraalkyStetraphenyl ammonium salt to metal halide may range from about 2: 1 to about 1: 2. The composition may have a mole ratio of tetraalkyl/tetraphenyl ammonium salt to metal halide of about 1: 1 to about 1: 2.

According to another aspect of the invention, there is provided a battery having an anode, a cathode and a molten electrolyte that comprises a mixture of a metal halide and an alkylan lammonium salt that has the form R1R2R3R4N+ .X-, wherein Ri, R) and R3 are alkyl groups of from 1 to 12 carbon atoms, at least one of Rl. R) and R, having from 5 to 12 carbon atoms. R4 is a benzyl or phenyl group and X-is a halide or complex halogen-containing ion.

A suitable mole ratio of alkylarylammonium salt to metal halide can range from about 2 : 1 to about 1 : 2. The composition may have a mole ratio of about 1: 1 to about 1 : 2.

According to another aspect of the invention. there is provided a battery having an anode. a cathode and a molten electrolyte that comprises a mixture of a metal halide and an alkyltriphenylphosphonium salt that has the form Rl R, R « R, Pt . X-. wherein Ri is an alkyl group of from 1 to 12 carbon atoms. and R,. R3 and R4 are phenyl groups, and X-is a halide or complex halogen-containing ion.

A suitable mole ratio of alkyltriphenylphosphonium salt to metal halide can be in the range from about 2: 1 to about 1: 2. The composition may have a mole ratio of alkyltriphenylphosphonium salts to metal halide of about 1: 1 to about 1 : 2.

According to another aspect of the invention, there is provided a battery having an anode. a cathode and a molten electrolyte that comprises a mixture of a metal halide and a alkyltriphenylphosphonium salt that has the form XRIR, R3 P+ . X-. wherein XR ; is a haloalkyl group of from 1 to 12 carbon atoms, and R2, R3 and R4 are phenyl groups, and X-is a halide or complex halogen-containing ion.

A suitable mole ratio of haloalkyltriphenylphosphonium salt to metal halide can range from about 2: 1 to about 1: 2. The composition may have a mole ratio of haloalkyltriphenylphosphonium salts to metal halide of about 1 : 1 to about 1: 2.

According to yet another aspect of the invention, there is provided a battery having an anode, a cathode and a molten electrolyte that comprises a mixture of a metal halide and an alkylarylsulfonium salt that has the form Rl R. R-S-. X-. wherein R, is an alky ! group having from 1 to 12 carbon atoms, and R, and Ra are alkyl or phenyl groups and X-is a halide or complex halogen-containing ion.

A suitable mole ratio of alkylarylsulfonium salt to metal halide can range from about 2: 1 to about 1: 2. Preferably, the composition comprises a mole ratio of alkylarylsulfonium salt to metal halide of about 1 : 1 to about 1: 2.

The halide may be an aluminium halide.

The complex halogen-containing ion may be one of the BF4-or PF6- methylsulfonate group.

The anode may be aluminium. The cathode may be glassy carbon.

The molten electrolyte composition may have three or more components.

According to a vet further aspect of the present invention, there is provided an electrochemical cell comprising an anode, a cathode and a molten electrolyte composition comprising a mixture of a first metal halide, a second metal halide and an organic onium salt.

Preferably. the molten electrolyte composition is a low temperature molten electrolyte composition. By"low temperature"what is meant is that the molten compositions are in liquid form below about l00 ZC. includino ambient temperature of between 20 to 35 °C at standard pressure.

The melt may have four or more components.

Preferably, the anode is aluminium. An advantage of this is that a high energy. lightweight, economic and environmentally friendly secondary battery can be implemented. The cathode may be glassy carbon.

The first metal halide may be an aluminium halide, for example aluminium chloride or aluminium bromide or a gallium halide.

The second metal halide is preferably a halide of a metal other than aluminium such as iron, nickel or copper halides, for example iron. nickel or copper chloride.

The second metal halide or halogen containing ion may be selected from one of the following: Inorganic aluminium salts, such as aluminium phosphate, aluminium tungstate.

Alkali metal halides, such as, LiCI, KCl, NaCl, RuCl, CsCl.

# Heavy metal halides or transit metal halides. such as. CuCl, CuCl2, FeCl2, FeCl3, SnCl2. zu Alkali metal complex halogen-containino salts. such as. KPF6, KBF4.

The organic onium salt may be an organic ammonium. phosphonium or sulphonium salt. such as trimethylphenylammonium chloride (TMPAC), trimethylsulponium chloride (TMSC) and benzyltrimethylammonium (BTMAX), with or without an inorganic salt.

The organic onium salt mas be selected from the following : R4N. X. where R4N includes four identical alkyls of I to 12 carbons or four identical pheny ! groups and X-is a halide or a complex halogen containing ion.

R1R2R3R4N+.X-, where Rl, R, and R3 are alkyl groups of 1 to 12 carbons, R is a benzyl or phenyl group and X-is a halide or a complex halogen containing ion.

# R1R2R3R4P+. X-, where R, is an alkyl group of 1 to 12 carbons, and . R3 and R4 are phenyl groups and X-is a halide or a complex halogen containing ion.

XR, R2R3R4P+. X-, where XR, is a haloalkyl group of 1 to 12 carbons, and R2, R3 and R4 are phenyl groups and X-is a halide or a complex halogen containing ion.

RIR2R3S+-X-, where R, is an alkyl group of 1 to 12 carbons, and R, and R3 are alkyl or phenyl groups and X-is a halide or a complex halogen containing ion.

The electrochemical cells described above may be primary or secondary cells.

Various electrochemical cells in which the invention is embodied will be described by way of example only.

The electrochemical cells in which the invention is embodied have electrolytes that include at least two separate components that are mixed together to form a melt. Examples of low temperature molten etectroiytes that comprise two components. i. e. binary melts. are mixtures of aluminium halide or gallium halide and an organic onium salt. These mixtures have one of the following formulas: (1) tetraalkyl/tetraphenvl ammonium salts : R4N-.X-, where R4 are four identical alkyls that have from 1 to 12 carbon atoms or four identical phenyl groups. and X-is a halide or a complex halogen-containing ion. such as BF- and PF6-

A suitable mole ratio of tetraalkyl/tetraphenyl ammonium salts to metal halide can range from about 2 : 1 to about 1: 2. The composition may have a mole ratio of tetraalkyl/tetraphenyl ammonium salts to metal halide of about 1: 1 to about 1: 2.

As used herein."alkyl"means saturated hydrocarbyl groups.

(2) alkylarylammonium salts: RlR2R3R4N+. X-, where R1, R2 and Rs are alkyl groups of from 1 to 12 carbon atoms, R4 is a benzyl or phenyl group, and X-is independently a halide or a complex halogen-containing ion. such as BF4-and PF6-methylsulfonate group.

A suitable mole ratio of alkylarylammonium salts to metal halide can range from about 2: 1 to about 1: 2. The composition may have a mole ratio of alkylarylammonium salts to metal halide of about 1 : 1 to about 1: 2.

(3) alkyltriphenylphosphonium salts: RlRrR3R4P. X-. where R, is alkyl group of from 1 to 12 carbon atoms, and R2, R3 and R4 are phenyl groups. and X-is a halide or a complex halogen-containing ion such as BF4-and PF6-.

A suitable mole ratio of alkyltriphenylphosphonium salts to metal halide can range from about 2 : 1 to about 1: 2. The composition may have a mole ratio of alkyltriphenylphosphonium salt to metal halide of about 1 : 1 to about 1: 2.

(4) haloalkyltriphenylphosphonium salts: XR1R2R3R4P+ .X-, where XR1 is haloalkyl group of from 1 to 12 carbon atoms, R,. Ro and R4 are phenyl groups, and X-is a halide or a complex halogen-containing ion such as BF4- and PF6-.

A suitable mole ratio of haloalkyltriphenylphosphonium salts to metal halide can range from about 2: 1 to about 1: 2. Preferably, the composition has a mole ratio of haloalkyltriphenylphosphonium salt to metal halide of about 1: 1 to about 1: 2.

(5) alkylarylsulfonium salts: R, RrR3S+. X-, where R, is an independent alkyl group of 1 to 12 carbons, and R2 and R3 are alkyl or phenyl groups, and X-is an independent halide or a complex halogen-containing ion. such as BF4-and PF6-.

A suitable mole ratio of alkylarylsulfonium salt to metal halide can range from about 2: 1 to about 1: 2. The composition has a mole ratio of alkylarylsulfonium salt to metal halide of about 1: 1 to about 1: 2.

The melts described above can be synthesized by a simple process in a glove box filled with nitrogen. For example, when the metal halide is aluminium trichloride. an appropriate onium salt is weighed and the correct stoichiometric amount of aluminium trichloride is slowly added to the onium salt to form a 2 : 1 molar ratio fused salt composition that is fluid at ambient temperature. This causes a spontaneous exothermic reaction, resulting in a clear viscous colored solution. If a melt does not form spontaneous the solids can be heated until a substantially homogeneous liquid forms. The reaction n picallv takes place in a closed vessel and the evolved heat can be controlled by periodically cooling the reaction vessel. After complete addition of AICI, the room temperature melt is heated to up to 70 °C to expedite dissolution of any remaining solid. After cooling down. the melt remains a liquid at temperatures below 70 °C. Most of

the AlCl3 and onium salt melts, in fact, remain liquid at 50 °C and some of them below about 20 °C.

As a specific example. a 2: 1 molar ratio of AICI ; : diphenvlethylsulfonium tetrafluoroborate (DPES+ BF4-) melt was prepared as follows. Ten grams of aluminium trichloride was slowly added to 11. 32 g of DPES+ BF4-in a 100 ml flask. This caused a spontaneous exothermic reaction to occurr, resulting in a clear brown solution having a 2: 1 molar ratio fused salt composition that was fluid at 50 °C. After twenty four hours of heating at 75 °C. all solids were dissolved and the melt was cooled down to ambient temperature.

This method can also be used in the preparation of 2 : 1 molar ratio of GaCl3:trimethylphenylammonium methylsulfonate (TMPA+ CH3SO3-) and 2: 1 molar ratio of AlCl ; : chloromethyl triphenylphosphonium chloride (CMTPP+ Cl).

In addition, to using binary melts as an electrolyte for a battery, ternary or quarternarx melts could be used. The composition of ternary tow temperature molten electrolytes comprises a mixture of an onium salt and a metal halide. such as. although not exclusively. used for the binary melts described above. plus one of the following inorganic salts: 1. Inorganic aluminium salts, such as aluminium phosphate. aluminium tungstate.

2. Alkali metal halides, such as, LiCI, KCI. NaCI. RuCI. CsCI.

3. Heavy metal halides or transit metal halides. such as. CuCI. CuCI.

FeCI"FeCI3. SnCI.

4. Alkali metal complex halogen-containing salts, such as. KPF6, KBF.

Other examples of onium salts that can be used in ternary melts include trimethylphens lammonium chloride (TMPAC), trimethylsulponium chloride (TMSC) and benzyltrimethylammonium (BTMAX).

Generally, the salts listed in (1) above have the same cations Alj+ as the binary melts described previously. The salts listed in (2) have the same halide anions X-as the previously described. When not involved in battery reactions, these salts play a role to buffer the Lewis acidity of the binary melts. and to lower the freezing points. The salts listed in (3) are cathodic activating materials. As supporting electrolytes. salts (4) are useful for increasing the conductivities of the melts.

Each of the melts described above can be used in a battery. For example. an AlCl3 : trimethylphenylammonium chloride melt was used by the inventors to form an Al/Fe secondary battery.

The Al/Fe battery consists of a 2mm diameter aluminium rod anode and a 4mm diameter giassy carbon rod cathode, introduced through the side ports of a 20 ml H-shaped cell comprising of two tubes that are separated by a glass frit. One tube contains approximately 8 ml of the molar ratio of' : 1 AlCl3 : trimethylphenylammonium chloride and contacts the Al anode. The other tube contains approximately 8 ml of the molar ratio of 2 : 1 : 0. 2 AlCl3 : trimethylphenstammonium chloride : FeCI,. This is in contact with the glassy carbon cathode. Both electrodes extended approximately 5 cm into the electrolyte.

The H cell described above is typically constructed in an araon-filled Schlenk line and all experiments are carried out with the Schlenk line under argon atmosphere at or near 20 °C. This cell initially shows an open-circuit voltage of 1.7 V. The cell is charged at room temperature by increasing the applied voltage in 0.05 V steps from about 1.0 V to 2.5 V. The open-circuit voltage after charging is, typically, 2.05 V. The cell is then discharged in 0.1 V steps down from about 2.0 V to 0.8 V. A gradually sloped cell voltage is observed on discharge, having an average voltage of 1.87 V. Following the eight cycles described above. the cell can be repeatedly charged for I hour at 2.5 V and discharged for 1 hour at 0.5 V. The cell repeatedly displays a charging voltage of 2.1 V and gave an average discharge voltage of 1. 8 V.

In another example, the cathode used is a 2mm diameter tunasten rod and the anode is again Al. The Al and tungsten rods are introduced through the side ports of a 15ml tube cell. The cell contains approximatelv,10 ml of electrolte. which is AICI- : trimethvlphenviammonium chloride : Fez) havina a molar ratio of 2 : 1: 0.2. Both the Al anode and tungsten cathode extended approximated 5 cm into the electrolyte. The cell is constructed in an argon tilled Schlenk line and all experiments are carried out with the Schlenk line under argon atmosphere at or near 20 °C.

The tube cell described above initially shows an open-circuit voltage of 1. 4 V.

The cell is charged at room temperature by increasing the applied voltage in 0.05 V steps from about 1. 0 V to 2.5 V. The cell is then discharged in 0.1 V steps down from about 2.0 V to 0.8 V. The open-circuit voltage after charging was 1. 65 V. A gradually sloped cell voltage is observed on discharge. having

an average voltage of 1. 55V. Following ten of the cycles described above. the cell was repeatedly charged for 1 hour at 2.0 V and discharged for I hour at 0.5 V. After this, the cell displays a charging voltage of 1.55 V and gives an average discharge voltage of 1.46 V.

In another example. a battery consists of a 5 x 10cm aluminium foil anode and a 20mm diameter graphite rod. The AI foil and graphite rod are introduced into a tube containing approximately 8 ml of 2: 1: 0.1 (mole ! mole/mole) AIC13 : TMPAC : SnCI) and each extend approximately 5 cm into the electrolyte. The cell is constructed under argon atmosphere and all experiments are carried out at or near 20 °C.

The open-circuit voltage of this cell before charging is 1.2 V. The cell is charged at room temperature by increasing the applied voltage in 0.05 V steps from about 1.0 V to 2.5 V. The cell is then discharged in 0. 1 V steps down from about 2. 0 V to 0.8 V. The open-circuit voltage after charging is 1.65 V.

% ith loxv temperature especially ambient temperature molten electrolytes, a secondary battery using an aluminium or an aluminium alloy anode in combination with an appropriate cathode material and operating at low temperature, particularly at room temperature. is provided.