High specific energy electrochemical cell with low initial impedance

A high specific energy electrochemical cell comprises: a negative active material (4) based on an alkali metal or on an alkaline-earth metal; a separator (8); a non-aqueous liquid electrolyte; and a positive electrode comprising a solid active mass (5). The positive mass is provided, on its electrolyte side, with a porous and electron conductive metal covering (12) which does not participate in the electrochemical reaction, whose thickness is of the order of a few microns, and which is in contact with the metal portions serving as a positive current collector. The covering greatly reduces the initial impedance of the cell.

The present invention relates to a high specific energy electrochemical 
cell with low initial impedance. 
It relates to a cell whose negative active material is based on an alkali 
metal or on an alkaline-earth metal, the electrolyte is a non-aqueous 
liquid which may organic or inorganic, and its positive electrode includes 
a solid active mass. This mass may be an inert solid material (a 
cathecter) and a solid active material based on oxides (of lead, copper, 
manganese, etc . . . ), on sulfides (of iron, etc . . . ) or on oxidized 
salts (of chromium, molybdenum, etc . . . ). 
BACKGROUND OF THE INVENTION 
Positive active materials do not, in general, conduct electrons and it is 
necessary to add an electron conductor to them to provide the positive 
electrode with sufficient electron conductivity to convey the required 
currents, while avoiding presenting an initial voltage which is so high as 
to be incompatible with the utilization. 
French published patent specification No. 2 316 756 describes various 
compositions of positive active masses intended to solve this problem. In 
particular, reducing agents which participate in the electrochemical 
reaction are added to the positive active masses, in proportions in the 
range of 10% to 20% by weight. However, in some special applications it is 
necessary to immediately obtain as low an impedance level as possible, or 
an impedance level which is close to the average discharge level, and the 
previously described means as inadequate for such applications since the 
initial impedance of the cells made in that way is high. 
The present invention proposes to remedy this drawback and to supply an 
electrochemical cell having low initial impedance and whose initial 
voltage is practically the same as its operating voltage. 
SUMMARY OF THE INVENTION 
The present invention provides a high specific energy electrochemical cell 
comprising a negative active material based on an alkali metal or on an 
alkaline-earth metal, a separator, a non-aqueous liquid electrolyte, and a 
positive electrode comprising a solid active mass, wherein the said 
positive mass is provided on its electrolyte side with a porous and 
electron conductive metal covering which does not participate in the 
electrochemical reaction, whose thickness is of the order of a few 
microns, and which is in contact with the metal portions serving as a 
positive current collector. 
The metal covering covers the solid mass of the electrode regardless of the 
surface appearance thereof; it thus extends over "bumps" and "dents". 
Such a covering which provides uninterrupted electron conduction while 
retaining a degree of porosity may advantageously be made by vacuum 
metallization or by any other method known to the person skilled in the 
art for projecting or depositing metals or their alloys in liquid or 
gaseous form. 
Compatible metals for making such a deposit are chosen from the group 
constituted by: lead, tin, gold, bismuth, zinc, cadmium, and aluminum, 
with aluminum being particularly well adapted and well mastered for making 
metal deposits in vacuo. 
This porous covering situated at minimal distance from the anode reduces 
the purely ohmic drop of the system. Further, it increases the initial 
number of reaction sites, thereby reducing the polarization of the 
electrochemical system. Since the parameters determining the impedance of 
the electrochemical cell are its ohmic drop and its polarization, the 
impedance is greatly reduced and the cell can reach its stationary state 
almost immediately.

MORE DETAILED DESCRIPTION 
Test have been performed on cells of the type shown in FIG. 1 which shows a 
button cell having a diameter of 11.5 mm and thickness of 5.4 mm. 
The envelope of the cell is composed of a negative cup 2 and a positive cup 
1, both of which are made of metal, e.g. stainless steel. The 
lithium-based negative electrode 4 is lodged in the cup 2. A positive mass 
5 lodged in the cup 1 is held in place by a metal ring 6. It is separated 
from the negative electrode 4 by a separator constituted by several layers 
7 of cellulose felt and a microporous sheet 8. This separator is 
impregnated with an organic liquid electrolyte. The assembly is sealed by 
means of a polypropylene seal 3. The positive mass 5 is a compressed 
mixture of lead powder and of lead bismuthate powder in which the lead 
powder constitutes a reducing agent which participates in the 
electrochemical reaction. The positive mass 5 weighs slightly more than 1 
gram and is in the form of a pellet which is 8.7 mm in diameter and 1.35 
mm thick. 
In accordance with the invention the face of the positive mass 5 which is 
turned towards the separator 7 is provided with a porous covering 12 of 
aluminum. The thickness of this covering is a few microns and it is 
deposited by in vacuo metallization. This covering is in contact with the 
metal ring 6, which is itself in contact with the positive cup 1 which 
serves as a terminal of the cell. 
For a covering which is two microns thick, the mass of aluminum used in 
about 0.9 mg, i.e. less than 0.1% of the total positive active mass. 
Thirty prior art cells were made, i.e. without the covering 12, and thirty 
cells in accordance with the invention were also made. Table I shows the 
values of the impedance of the cells as measured prior to discharge. 
(These values and their variations are arithmetic means). 
TABLE 1 
______________________________________ 
Initial impedance 
Dispersion 
______________________________________ 
Prior Art Cells 
512 ohms .+-.132 ohms 
Cells in Accordance 
210 ohms .+-.42 ohms 
with the Invention 
______________________________________ 
Reference is now made to FIG. 2 which shows the variation in the voltage V 
and the impedance Z (in ohms) as a function of time t in minutes: 
for prior art cells (voltage curve A and impedance curve A'); and 
for cells in accordance with the invention (voltage curve B and impedance 
curve B'). 
The discharge was performed through a 5 K.OMEGA. resistance. 
An examination of Table I and FIG. 2 clearly shows the advantage of the 
covering 12 which gives the cells initial characteristics which are 
greatly improved and which are practically identical to the 
characteristics of a cell in the stationary state. 
It is also essential to point out that the existence of a covering in 
accordance with the invention has no ill effects on the performance per 
unit volume of the electrochemical system concerned. This point 
constitutes the second essential advantage of the invention. In 
practically all lithium anode systems having an organic or an inorganic 
electrolyte and based on oxides, sulfides or oxidized salts of chromium, 
molybdenum, etc . . . , the electrochemical reactions caused by the system 
discharging take place via an accumulation of discharge products in the 
cathode. This phenomenon gives rise to a considerable increase in the 
volume of the cathode even if its initial porosity is high, which increase 
may be as much as 100% of the initial volume depending on the system used. 
This thermodynamic increase is unavoidable and any limitation thereon has 
an initial effect of excessively polarizing the system (due to a reduction 
in the number of active sites) and a subsequent effect of a premature end 
to discharge due to the reaction becoming blocked. 
In accordance with the invention, although the deposited metal covering is 
sufficient to ensure optimal starting of the system, it does not 
constitute a brake on the expansion of the cathode during discharge of the 
electrochemical cell. 
FIG. 3 demonstrates the above explanation. In this figure voltage curves V 
(D, E, C) and impedance curves Z (D', E', C') are obtained by discharging 
cells identical to those described above. Time t is expressed in days. 
The curves D, D' show the characteristics obtained on prior art cells, the 
curves E, E' show the characteristics obtained on prior art cells in which 
the cathode is covered with a fine woven metal grid; and curves C, C' show 
the characteristics obtained using cells in accordance with the invention. 
It can be seen that a mechanical artifice (such as a grid or a thin 
perforated metal sheet) also makes it possible to improve the initial 
characteristics of the cell, but does not enable optimal efficiency to be 
obtained because of the mechanical disturbance which it applies to the 
system. No substantial loss is observed in cells in accordance with the 
invention. 
Naturally, the invention is in no way limited to the example which has been 
described. Any means could be replaced by equivalent means without going 
beyond the scope of the invention.