GB2134697A - Iron/carbon current collector for lithium/thionyl chloride cell - Google Patents

Abstract

A primary electrochemical reserve cell of the type comprising an anode of an electropositive metal (particularly lithium) and an electrolyte comprising a non-metal oxy-halide (particularly thionyl chloride) is provided with a cathodic current collector composed of an intimate physical mixture of carbon and metallic iron.

Description

SPECIFICATION Liquid Cathode primary cell The present invention relates to primary electrochemical power cells comprising an anode of an electro-positive metal, an electrolyte comprising a non-metal oxy-halide present either as a liquid or in solution, and a cathodic current collector in contact with the non-metal oxy-halide.

Such cells will subsequently be referred to as of the type specified. The non-metal oxy-halide acts as a liquid cathode and in use is reduced to halide ions and other products. Phosphoryl chloride, sulphuryl chloride and thionyl chloride (the last of these being the preferred material) are commonly used liquid cathodes. Lithium is the preferred metal for the anode, at least when thionyl chloride constitutes the liquid cathode, although calcium anodes have also been used. Lithium/thionyl chloride cells are capable of achieving high power:weight and energy:weight ratios and have many potential uses for powering portable electrical equipment.

Hitherto the cathodic current collector in cells of the type specified has usually been made of inert conducting material such as finely divided carbon (which tends to become clogged in use by inert reaction products), although it has been proposed to make a lithium/thionyl chloride cell incorporating a porous iron cathode. Such a cathode rapidly corrodes in use to form soluble reaction products which do not hinder the cell discharge. However it appears that such a pure iron cathode must be formed with a high specific surface area in order to be effective.

We have unexpectedly found that a cell of the type specified provided with a cathodic current collector composed of an intimate mixture of finely divided metallic iron and finely divided carbon has a greater current capacity and a higher mean voltage during discharge than similar cells provided with either a pure iron or a pure carbon cathode and discharged under similar conditions.

In accordance with the present invention, a cell of the type specified is provided with a cathodic current collector composed of an intimate physical mixture of carbon and metallic iron.

Preferably the cathodic current collector incorporates between 5% and 50% of iron by mass (relative to the total mass of metallic iron and carbon present). Preferably the iron and carbon are both in particulate form.

Further objects and advantages of the invention will become clearer by way of example on consideration of Flgures 1 to 7 of the accompanying drawings, of which: Figure 1 is a diagrammatic representation of a reserve battery in accordance with the invention; Figure 2 is a voltage:time plot for a lithium/thionyl chloride cell provided with a pure iron cathode; Figure 3 is a set of plots of voltage:capacity (in ampere-hours) per gram of cathode material at a discharge rate of 50mA/cm2 for various cathodes under various conditions in known cells and in cells in accordance with the invention; Figure 4 is a set of plots of voltage::capacity (in ampere-hours) per gram of cathode material for lithium/thionyl chloride cells having a Lewis acidic electrolyte and a pure carbon cathode discharge at various rates; Figure 5 is a similar set of plots for a cell in accordance with the invention having 15% iron/85% carbon electrode; Figure 6 is a set of plots of voltage:capacity per gram of cathode material at various discharge rates for cells similar to those of Figure 3 except that a neutral electrolyte was used, and Figure 7 is a set of plots of voltage:capacity per gram of active cathode material at various discharge rates for cells in accorance with the invention provided with a 15% iron/85% carbon cathode and a neutral electrolyte.

Figure 1 is a diagrammatic sectional elevation of a bipolar reserve battery in accordance with the invention. The battery comprises an evacuated container 1 which in use communicates with a reservoir 2 filled with an electrolyte solution of lithium aluminium chloride (1 M) and aluminium chloride (2M) dissolved in thionyl chloride.

Reservoir 2 (which is generally in the form of a spiral tube) is provided with a chemical gas generator 3 which can be activated electrically, causing the pressurised electrolyte solution to rupture a membrane 4 and fill the container 1. The positive terminal 5 of the battery is hermetically sealed into the container and is incorporated into a nickel plate on which a layer 6 of cathode material is deposited. Layer 6 consists of a mixture of 15% by weight finely divided iron relative to 85% by weight Shawinigan 50% compressed acetylene black, together with P.T.F.E. binder. The negative terminal 8 of the battery is similarly hermetically sealed into the container and is incorporated into a similar nickel plate which supports a lithium foil 9.

Bipolar electrodes are located intermediate the positive and negative plates by glass fibre separators 10, and comprise pairs of nickel plates 7 pierced by central registering holes 1 1 and held in contact and supporting layers of cathode material 6 and lithium foils 9 on their outwardly-facing surfaces. In use the gas generator 3 is activated electrically (by means not shown) and the electrolyte solution bursts the membrane 4 and rushes into the spaces between the bipolar electrodes, causing an e.m.f. to be generated across terminals 5 and 8.

Each cell corresponding to the plots of Figures .2 to 6 contained approximately 1 smug of active cathodic material, the cathode being formed by making an intimate mixture of Shawinigan 50% compressed acetylene black, P.T.F.E. powder and (if appropriate) iron powder in known proportions by agitating these materials in an inert liquid, applying the resulting paste to a weighed strip of nickel foil (which serves as an inert support) and allowing the liquid to evaporate. The dried cathode was then weighed in order to determine the mass of active cathodic material (carbon + iron) by subtraction.

The plot shown in Figure 2 is for a cell comprising a lithium anode, and electrolyte of 0.5M lithium aluminium chloride and 4M aluminium chloride (which acts as a Lewis acid) in thionyl chloride, and a cathodic current collector of pure iron sheet. The cell was discharged at a rate of 5mA/cm2 of true cathode surface area. It can be seen that the cell voltage tails off rapidly after about 130 seconds and we have found that this 1 30 second period is not significantly affected by the mass of the cathode. Thus iron acts essentially as a "two dimensional" cathode in this cell.

Clearly it is disadvantageous in practical electrochemical power cells to employ a cathode whose current capacity is limited by its surface area.

Figure 3 shows four plots of cell voltage: capacity/gram of cathode material for cells comprising a lithium anode, and electrolyte of lithium aluminium chloride dissolved (in the concentrations indicated in the key) in thionyl chloride, and a cathodic current collector composed of iron and/or carbon in proportions by mass as indicated in the key. In two cases the electrolyte solution additionally contained 2.OM aluminium chloride, which functioned as a Lewis acid.

The mean particle size of the iron powder was approximately 7 ym. A number of cells of the type indicated in the key were constructed and discharged at a rate of 50mA/cm2 of geometric cathode area to a voltage of 2.OV. It can be seen that a Lewis acid tends to sustain the e.m.f. of the lithium/thionyl chloride/carbon cell but increases the rate of decline of voltage of the lithium/thionyl chloride/carbon-iron cell. This strongly suggests that the addition of iron to a carbon cathode in such a cell changes the cell mechanism. It can be seen that the voltage of the neutral carbon-iron cathode cell is virtually constant for over 90% of the discharge, and this is a very useful characteristic.

Figures 4 and 5 show the effect of increased current density on the discharge curves of lithium/thionyl chloride/carbon cells and lithium/thionyl chloride iron-carbon cells respectively, a Lewis-acidic electrolyte/molar in lithium aluminium chloride and 2 molar in aluminium chloride being employed in each case.

Although the carbon cathode cell has a greater capacity at discharge rates below about 50mA/cm2, the cell in accordance with the invention has a significantly increased capacity at a discharge rate of 200mA/cm2.

Figures 6 and 7 show that an even greater improvement in cell performance results at high discharge rates when an iron/carbon cathode is used in a neutral electrolyte (1 .8M LiAICI4). It will be appreciated that since iron/carbon cathodes are less bulky than pure carbon cathodes of the same mass, the present invention provides an even greater improvement in cell power when compared with prior art carbon cathode cells on a volume basis.

Although the reactions occurring in lithium/thionyl chloride cells are poorly understood it is believed that in cells in accordance with the invention the iron reacts to form iron (III) chloride, which is a Lewis acid. Since Lewis acids in thionyl chloride attack lithium and similar metals, cells in accordance with the present invention, particularly if they initially contain Lewis acids, in the eletrolyte solution, are best suited for reserve applications (in which the electrolyte is added immediately prior to use). Reserve cells in accordance with the invention provided with a neutral electrolyte have the advantage that the less heat is evolved when the electrolyte is added to the electrodes than if a Lewis acid were present.

Claims (8)

1. A cell of the type specified provided with a cathodic current collector composed of an intimate physical mixture of carbon and metallic iron.

2. A cell according to Claim 1, wherein the cathodic current collector incorporates between 5% and 50% iron by mass, relative to the total mass of iron and carbon present.

3. A cell according to Claim 1 or Claim 2, wherein the iron and carbon are both in particulate form.

4. A reserve cell according to any preceding Claim.

5. A cell according to any preceding Claim, wherein the electrolyte is at least initially substantially free of Lewis acid.

6. A cell according to any preceding Claim, wherein said non-metal oxy-halide is thionyl chloride, said electro-positive metal is lithium and said electrolyte further comprises dissolved lithium aluminium chloride.

7. A cell according to Claim 6, wherein the cathodic current collector incorporates 10% to 20% iron and 80% to 90% carbon by mass, relative to the total mass of carbon and iron present.

8. A cell as claimed in Claim 1. constructed substantially as described hereinabove with reference to Figure 1 of the accompanying drawings.