Lithium electric cell

A lithium electric cell. Said electric cell includes a positive electrode (1) and a negative electrode (2) in contact with a liquid electrolyte impregnates in a separator (3). The positive electrode comprising an active compound whose general formula is Li.sub.x Fe.sub.2 S.sub.z with x lying between 0 and 2, and z being not less than 3.

The present invention relates to a lithium cell with a liquid organic 
electrolyte. 
A number of surveys have been made during the past three years concerning 
the electrochemical insertion, or, rather, "intercalation" of lithium in 
two-dimensional inorganic compounds; there should be cited in particular 
the work of M. S. Wittingham, disclosed in U.S. Pat. No. 4,049,879, as 
well as the surveys made by R. Brec and F. Le Mehaute, disclosed in French 
Pat. No. 77 045 18. 
These authors describe in particular compounds such as TiS.sub.2 and 
NiPS.sub.3 installed in the charged state in an electric cell as well as 
compounds of the Li.sub.x TiS.sub.2 or Ni.sub.x PS.sub.3 type used in the 
discharged state. 
However, it has been observed that such compounds, as well as a number of 
inorganic compounds cannot be directly synthesised by the dry method, 
since at their formation temperature, these compounds are not stable, but 
in contrast, they prove to be stable at ambient temperature. 
Due to these considerations, research workers of Bell Telephone 
Laboratories have envisaged in particular stabilizing quadrivalent 
vanadium by producing LiVS.sub.2 initially, followed by electrochemical 
oxidation to remove the lithium. 
Such work has also been disclosed in the article "Cathodes for non-aqueous 
VS.sub.2 based lithium batteries" on pages 825-850 of Vol 12 (1977) of the 
Material Research Bulletin. 
The applicant has sought to produce new substances of high capacity and 
which remain stable even after a high number of charge/discharge cycles. 
The present invention provides an electric cell which includes a positive 
electrode and a negative electrode in contact with a liquid electrolyte, 
said electric cell being characterized in that said positive electrode 
includes an active compound whose general formula is Li.sub.x Fe.sub.2 
S.sub.z with x lying between 0 and 2 and z being not less than 3.

The applicant has developed a new positive electrode for an electric cell 
with a lithium negative electrode. 
For that purpose, the applicant has produced improved ternary compounds, in 
particular of lithium, iron and sulphur in which the iron has a value 
which lies between 2 and 3, it being possible for such a compound to 
acquire, by limited electrochemical oxidation, an oxido-reduction state 
greater by about 1 than the preceding state and having a stable or 
metastable state. 
Such a compound is, in particular, Li.sub.2 Fe.sub.2 S.sub.3. 
The preparation method is as follows: 
An intimate mixture of Li.sub.2 CO.sub.3 and Fe.sub.2 O.sub.3 is formed in 
stoichiometric proportions and is placed in an alumina boat which is 
placed in a quartz tube disposed in a furnace. The mixture is heated to a 
temperature of 800.degree. C. while allowing argon which has bubbled 
through carbon bisulphide CS.sub.2 to pass through the tube for about 6 
hours. 
The mixture of CO.sub.3 Li.sub.2 and Fe.sub.2 O.sub.3 is sulphurized by 
nascent sulphur which results from the decomposition of the carbon 
bisulphide and as shown in the following reactions: 
EQU 2CS.sub.2 .revreaction.2C+4S 
EQU Li.sub.2 CO.sub.3 .fwdarw.Li.sub.2 O+CO.sub.2 
EQU Li.sub.2 O+Fe.sub.2 O.sub.3 +2C+4S.fwdarw.Li.sub.2 Fe.sub.2 S.sub.3 
+2CO.sub.2 +S 
The compound Li.sub.2 Fe.sub.2 S.sub.3 can be determined quantitively by a 
spectro-photometric absorption method and its presence in the form of a 
single phase is revealed by its X-ray spectrum which is set out in the 
table hereinbelow: 
______________________________________ 
d (A) 6.210 3.370 3.120 
3.010 
2.952 
2.669 
2.296 
2.050 
1.951 
in- 
ten- 1 1 m 1 vl m m H 1 
sity 
d (A) 1.899 1.773 1.741 
1.627 
1.520 
1.339 
1.114 
1.062 
in- 
ten- 
sity 1 m m vl vl m m m 
______________________________________ 
In the table, 
1 represents a low intensity 
vl represents a very low intensity 
m represents a medium intensity 
H represents a high intensity. 
Further, the spectrum allows the complete lack of the sulphides Li.sub.2 S, 
FeS, FeS.sub.2 to be detected. In other words, Li.sub.2 Fe.sub.2 S.sub.3 
actually corresponds to a specific single phase. 
It will further be observed that if the operating conditions--in particular 
the temperature--are varied, it is possible to obtain different compounds 
with a general formula of Li.sub.2 Fe.sub.2 S.sub.z 
where 3.ltoreq.z&lt;4. 
For example, for a temperature of 650.degree., the compound Li.sub.2 
Fe.sub.2 S.sub.3.5 is obtained. The temperature may be varied generally 
between 200.degree. and 1000.degree. C. As the applicant has observed, 
such compounds are not reducible in the charged state to FeS or FeS.sub.2. 
These substances are already used in electric cells. 
In general, by a method similar to the one described above, it is possible 
to produce compounds whose general formula, as has been seen, is: 
EQU Li.sub.x Fe.sub.2 S.sub.z 
These previously described compounds can therefore be used in electric 
cells with an alkaline negative electrode, as will be described 
hereinafter. 
The positive electrode 
The positive electrode includes said compound Li.sub.x Fe.sub.2 S.sub.z and 
in particular Li.sub.2 Fe.sub.2 S.sub.z. Further, it may include other 
materials necessary to ensure good electronic conductivity or good contact 
with the collector, namely, carbon, graphite, copper, nickel, iron or a 
transition element. 
The negative electrode 
The negative electrode includes an alkali metal and in particular lithium. 
The collector must be made of metal which corrodes only slightly at the 
potential of the electrode. 
By way of example, it would be possible to use an element in columns IVb, 
Vb, VIb, VIIb or VIII of the periodic table as well as copper, silver, 
zinc, aluminium or alloys thereof. Further, carbides, nitrides or borides 
of these compounds can be used. 
The electrolyte 
The electrolyte includes an organic solvent which is stable with respect to 
the positive electrode and the negative electrode and in which the salt of 
an alkali metal, in particular lithium, is dissolved. 
More precisely, said solvent can be chosen from among propylene carbonate, 
dioxolane, dimethoxyethane, nitromethane, tetrahydrofurane and generally 
cyclic esters. 
Said salt can be chosen from among perchlorates, hexafluoroborates, 
hexfluoroarseniates, nitrates, sulphates and methylchlorosulfonates. 
With reference to FIG. 1, a practical embodiment will now be given of a 
button type electric cell in accordance with the invention. 
FIG. 1 shows the positive active mass 1, the negative active mass 2 and a 
porous separator 3 impregnated with electrolyte. References 4 and 5 
denote, respectively, the positive collector in the form of a cup and the 
negative collector in the form of a cap. 
The positive active mass 1 (in the case, Li.sub.2 Fe.sub.2 S.sub.3) is 
compressed at a pressure of 800 kg in the cup 4 in a dry nitrogen 
atmosphere. 
The negative active mass 2 (in this case, lithium) is compressed in argon 
in the cap 5. The separator 3 is of the cellulose type and is impregnated 
with electrolyte formed by 1 M propylene carbonate in which lithium 
perchlorate is dissolved. After crimping, the electric cell thus 
constituted is charged for 80 hours at 200 .mu.A/cm.sup.2. 
Since the weight of the active material is 66 mg, its capacity is about 8 
mAh. 
FIG. 2 which is a graph of the electromotive force E in volts as a function 
of the discharged capacity Q in grams equivalent, shows, by means of 
curves A, B and C, the chargedischarge cycles of the above-described 
electric cell, at various currents, namely, 100 .mu.A for cycle A, 780 
.mu.A for cycle B and 100 .mu.A for cycle C. 
In accordance with another embodiment, the positive mass 1 is Li.sub.2 
Fe.sub.2 S.sub.3.64 and the negative mass 2 is lithium, the electrolyte 
being the same as previously. The cell is charged at 200 .mu.A/cm.sup.2. 
FIG. 3, which shows the same parameters as FIG. 2, gives the charge curve D 
and the discharge curve F of Li.sub.2 Fe.sub.2 S.sub.3.64 and the charge 
curve E and the discharge curve G of Li.sub.2 Fe.sub.2 S.sub.3. 
In accordance with yet another embodiment, the positive active mass 1 
includes 66% of Li.sub.2 Fe.sub.2 S.sub.3 (or Li.sub.2 Fe.sub.2 
S.sub.3.64) and 33% of acetylene black, the negative electrode being 
lithium. 
Contrary to the previous examples, the positive mass 1 is not compressed in 
the cup 4, but simply packed. In that case, less polarization is observed 
than in the preceding cases, the discharge voltage being stabilized at 2 
volts at 200 .mu.A/cm.sup.2. 
Analogous results are obtained by forming a latex with the positive active 
mass by adding a binding agent such as polytetrafluoroethylene.