An activatable lithium/bromine trifluoride electrochemical cell comprising an anode of lithium or lithium alloy, a positive current collector based on carbon black, and an electrolyte based on bromine trifluoride, wherein the quantity of bromine trifluoride is not less than 200 ml per dm.sup.2 of anode area, and that means are provided for causing the electrolyte to circulate through said cell at a linear speed of not less than 0.5 cm/second. Discharge yield is doubled [curve B] compared with a prior art cell [curve A].

The present invention relates to an activatable lithium/bromine trifluoride 
electrochemical cell. 
BACKGROUND OF THE INVENTION 
In a very wide variety of applications there is a permanent demand for 
energy sources of ever increasing performance, having higher energy then 
primary cells having a liquid cathode of the thionyl chloride type. 
Potentially the most advantageous electrochemical couple is the couple 
lithium/fluorine giving a theoretical potential of about 5.9 volts and a 
theoretical energy density of more than 5000 Wh/kg. However it is 
difficult to envisage using fluorine directly because of the major 
handling problems encountered. Fluorine is gaseous at ambient temperature 
and under atmospheric pressure and is extremely reactive. 
In contrast, it is possible to envisage using fluorine in association with 
some other element giving compounds that are more easily handled at a 
temperature close to ambient temperature. This applies when fluorine is 
associated with a halogen, such as bromine, to obtain a compound that is 
liquid, such as bromine trifluoride, and which can be used as a catholyte 
in liquid cathode lithium primary cells. 
The problem posed by such cells is that anode discharge is limited by a 
passivation phenomenon. 
An object of the present invention is to mitigate this drawback. 
SUMMARY OF THE INVENTION 
The present invention provides an activatable lithium/bromine trifluoride 
electrochemical cell comprising an anode of lithium or lithium alloy, a 
positive current collector based on carbon black, and an electrolyte based 
on bromine trifluoride, wherein the quantity of bromine trifluoride is not 
less than 200 ml per dm.sup.2 of anode area, and that means are provided 
for causing the electrolyte to circulate through said cell at a linear 
speed of not less than 0.5 cm/second, thereby preventing anode reaction 
products such as LiF and/or LiBrF.sub.4 precipitating. 
The bromine trifluoride may contain at least one soluble compound such as 
lithium trifluoromethane sulfonate at a concentration lying in the range 
10.sup.-2 moles per liter to 1 mole per liter. 
By way of example, said means may comprise a PTFE pump external to the 
cell. 
The dispositions of the present invention make it possible to avoid lithium 
discharge being limited, and consequently nearly all of the lithium (80% 
to 90%) can be used. This doubles the discharge yield of the cell.

DETAILED DESCRIPTION 
Two cells A and B were prepared comprising: 
a lithium anode; 
an electrolyte based on bromine trifluoride, and including a compound such 
as lithium trifluoromethane sulfonate at a concentration of 0.1 moles per 
liter, there being 200 ml of electrolyte per dm.sup.2 of anode area; and 
a cathode current collector made of porous carbon. 
Those cells did not contain conventional separators such as organic or 
inorganic felts. The interelectrode distance which was about 1 mm was 
maintained by Teflon studs, having a diameter of 1 mm and a thickness of 1 
mm. 
After activation, the cell A was discharged at 10 mA/cm.sup.2. Curve A of 
FIG. 1 was obtained. 
Cell B was, FIG. 3 associated with a pump 2 within loop 4, external of the 
cell B and being made of PTFE to cause the electrolyte E to circulate 
inside the cell at a linear speed of 0.5 cm per second controlled by a 
valve 3 within the loop 4. It was discharged under the same conditions as 
cell A, thereby obtaining curve B. The discharge yield was thus doubled, 
and the faraday efficiency of the lithium anode reached 80%. The discharge 
potential was 4.8 volts during 3.5 hours. 
Thereafter, an Li/SOCl.sub.2 type prior art cell C was discharged at 25 
mA/cm.sup.2, as was a cell D of the invention and analogous to the cell B. 
The corresponding discharge curves are given in FIG. 2. 
It can be seen that the increase in discharge potential was of the order of 
50% and that the increase in discharge time under high current density was 
of the order of 30%. 
These results give rise to twice the energy density per unit volume and to 
a 60% increase in power per unit volume for the cell of the invention 
compared with the Li/SOCl.sub.2 cell. 
Naturally, the invention is not limited to the embodiment described above. 
Thus, bromine trifluoride can be used on its own, or together with 
additives other than that specified above.