Metal silicide electrode in lithium cells

The negative electrode of an electrochemical cell with lithium as the electroactive species includes a metal silicide. This metal silicide can be an alloy (that is, a multimetallic silicide) that reacts with lithium and acts as a reversible lithium reservoir during cell operation. Electrochemical cells in accordance with the invention have excellent kinetics and higher theoretical specific energy that the Li-Si binary alloys presently used in some thermal batteries. Magnesium silicide, calcium silicide and molybdenum silicide are particularly preferred materials for these negative electrodes due to their thermodynamic properties.

FIELD OF INVENTION 
This invention relates generally to cells, such as electrochemical cells, 
in which lithium is the electroactive species, and more particularly to 
electrochemical cells having an improved negative electrode. 
BACKGROUND OF THE INVENTION 
There is a great deal of interest in better methods for energy storage. 
This is especially important for applications such as electric vehicles 
and the large scale storage of electric energy to level the load of 
stationary power plants. 
Reversible lithium batteries with high energy (and power) density as well 
as high specific energy (and power) have been investigated for a number of 
uses, such as power sources for electric vehicles and energy storage 
devices However, problems associated with negative electrode materials 
often limit the cycling performance of such electrochemical cells, 
especially at lower temperatures. 
When solid lithium in primary or secondary batteries is heated above its 
melting point, then the lithium can become liquid. This can lead to 
uncontrollable reactions and may result in safety concerns. Lithium alloys 
hold the potential of higher melting points and thus can reduce the 
potential for safety problems. 
The use of elemental lithium as negative electrodes in batteries operated 
at elevated temperatures, where lithium is a liquid, presents serious 
problems, such as corrosion and difficulty in containment. Also, 
dissolution of lithium in a molten salt electrolyte can give rise to 
electronic conduction, which leads to severe self-discharge. On the other 
hand, when used at temperatures below its melting point, there can be the 
tendency for dendritic and filamentary growth of lithium upon recharging. 
This in turn may lead to electrical shorting between the electrodes and 
may also isolate the active electrode materials from electrochemical 
reaction, resulting in a loss of capacity. 
Thus, one of the developments currently being pursued involves a 
lithium-based cell, in which the negative electrode is a lithium alloy 
(typically either lithium-aluminum or lithium-silicon). 
Although the reversibility and cycle life of electrochemical cells can 
generally be increased by the use of such alloys, there is an accompanying 
reduction in the specific energy (and power) and energy (and power) 
density. These are a consequence of the reduced activity of the 
electroactive species in the negative electrode, which implies reduced 
cell voltage, as well as increased weight. There is also the problem of 
mechanical instability of the electrode structure as a result of volume 
and shape changes involved in the electrode reaction. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to avoid the safety problems 
involved with pure lithium as negative electrode and to provide 
electrochemical cells that can be recharged, if desired, where lithium is 
the electroactive species. 
In one aspect of the present invention, the negative electrode of an 
electrochemical cell with lithium as the electroactive species includes a 
metal silicide. This metal silicide reacts with lithium and acts as a 
reversible lithium reservoir during cell operation. Electrochemical cells, 
or batteries, in accordance with the invention have excellent kinetics and 
higher theoretical specific energy than the Li-Si binary alloys presently 
used in some thermal batteries, and are useful as primary batteries as 
well as secondary, or rechargeable batteries, for a wide variety of 
applications.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The electrode potentials as well as their compositional capacities are 
important parameters if an electrochemical cell is to be useful as an 
energy storage device, while the power is related to the kinetics of 
electrode reactions. 
Electrode reactions can generally be divided into two different types: 
insertion reactions, in which the composition of a solid solution changes; 
and, reconstitution reactions, in which the identity and amounts of the 
phases present can change. In an insertion reaction, the electroactive 
species enters, but does not change, the framework structure of the 
reactant; such reactions are thus often described as topotactic. The Gibbs 
phase rule requires that the chemical potential of the electroactive 
species, and thus the electric potential of the electrode reactant, varies 
with composition. The advantage of this type of reaction is that the 
electrode can be easily reversible if the chemical diffusion coefficient 
is high, since no phase change is involved. However, it has the 
disadvantage that the electrode potential is composition-dependent and 
there may be a limited compositional range for the electroactive species. 
Reconstitutional reactions can involve the formation of a new phase. This 
may lead to a composition-independent value of the electric potential 
determined by the Gibbs free energies of formation of the phases present. 
If the number of phases present is equal to the number of components under 
constant temperature and pressure conditions, the Gibbs phase rule 
specifies that all intensive thermodynamic variables are fixed and 
independent of the overall composition. Chemical potentials, and thus 
electrode potentials under equilibrium conditions, are therefore 
independent of composition. The length of such a constant potential 
plateau is determined, in the case of a binary system, by the extent of 
the two-phase equilibrium, and, in a ternary case, by the extent of the 
three-phase equilibrium. The advantage of a reconstitution reaction is 
that it can lead to a constant equilibrium potential over a useful 
compositional range. Such reactions, however, have the disadvantage that 
their reversibility can be quite slow, since the relative amounts of the 
reactant and product phases change with the extent of reaction. 
The performance of electrochemical cells employing lithium as the 
electroactive species can be improved by the use of ternary system alloys 
as the negative electrode. If information on a lithium-based binary 
system, Li-Si, is available, one can proceed to understand the behavior of 
a Li-Si based ternary system by the addition of a third component M. The 
addition of M to a binary alloy Li.sub..alpha. Si can result in a 
significant change in either or both of the electrode potential and 
capacity, even when M does not itself react with lithium. First consider 
the case where M does not react with lithium. The reaction of lithium with 
Si to form the alloy Li.sub..alpha. Si can be represented by the equation 
EQU Si+.alpha.Li.revreaction.Li.sub.60 Si (1) 
and the voltage is given by 
EQU E.sub.1 =-.DELTA.G.sub.f (Li.sub..alpha. Si).alpha.F (2) 
where .alpha. is the number of moles of lithium involved in the reaction, 
and F is Faraday's constant. 
Small additions of M do not change the voltage of the electrode. However, 
on addition of sufficient M, the voltage of the electrode versus pure 
lithium changes such that 
EQU E.sub.2 =-.DELTA.G.sub.r /.alpha.F&lt;E.sub.1 (3) 
Where .DELTA. G.sub.r is the Gibbs free energy change of the reaction 
EQU 1/.beta.Si.sub..beta. M+.alpha.Li.revreaction.Li.sub..alpha. 
Si+1/.beta.M(4) 
The consequence of the addition of M is then to make the electrode 
potential more negative, and thus to increase the voltage of any cell 
which uses M-Si as the negative electrode. However, one needs to minimize 
the amount of M added in order to take advantage of the reduced electrode 
potential without adding unnecessary weight. 
Where M reacts with lithium, the potential of the electrode versus lithium 
is given by the reaction 
EQU 1/.beta.Si.sub..beta. 
M=(.alpha.+.gamma./.beta.)Li.revreaction.Li.sub..alpha. 
Si+1/.beta.Li.sub..gamma. M (5) 
and 
E.sub.3 =-.DELTA.G.sub.r /(.alpha.+.gamma./.beta.)F&lt;E.sub.1(6) 
Again, the addition of M makes the electrode potential more negative and 
thus increases the cell voltage. The electrode capacity expressed in terms 
of the number of moles of lithium per mole of Si, also increases. 
A ternary phase diagram which involves the binary system in question might 
increase both the cell voltage and storage capacity of the electrode. The 
consequence could be an increase in the specific energy or energy density 
if the third component is optimally maximized. In some cases, the 
existence of an Li-M phase may not be necessary if there exists a Li-Si-M 
ternary phase which gives rise to the same geometry in the lower part of 
the diagram 
The present invention provides negative electrodes for electrochemical 
cells with lithium as the electroactive species that are improved with 
respect to the lithium-based cell in which the negative electrode is a 
binary, lithium-silicon material. Electrochemical cells of the invention 
are improved by including a metal silicide negative electrode The metal 
silicide reacts with lithium and acts as a reversible lithium reservoir 
during cell operation This addition may involve either an insertion 
reaction or a reconstitution reaction. In some cases, there might be at 
least three phases in the negative electrode. 
Light weight materials and those that form high density alloys with Si are 
desirable as the third component. In this way, one enhances either the 
specific energy or energy density. 
The prior art Li-Si system is well characterized in terms of the alloy 
potential and lithium capacities. For example, in an equilibrium titration 
curve (EMF versus composition), there are four plateau regions, with 
potentials 332, 288, 158, and 44 mV positive of pure lithium. Of 
particular interest is the third plateau, which has a voltage of 158 mV 
versus lithium and a capacity of about 0.92 moles of lithium per mole of 
silicon. This converts to 1.81 * 10.sup.-2 moles of lithium per gram total 
weight. 
The corresponding maximum theoretical specific energy (MTSE), when a 
material on this plateau is used as the negative electrode in a 
hypothetical cell with a 60 g/Li cathode which is 2.0 volts positive of 
pure lithium, is 428 watt hr/kg. This is comparable to a 44 wt.% Li-Si 
alloy and can be viewed as a baseline against which to compare the 
potentials and capacities of ternary systems of the invention. 
The relevant information needed for the prediction of Li-Si-M ternary phase 
diagrams is the Gibbs free energy of all the phases, both binary and 
ternary, that are known to exist. Values for the Li-Si binary system have 
been calculated from the above-discussed titration curve. Ternary 
additions found suitable for negative electrodes of the invention include 
Ca, Mg, Mn, Mo, and V. These components all form alloys with Si, and thus 
are described as "metal silicides". Such metal silicides are either 
commercially available or can be readily prepared by heating finely 
divided metals in appropriate proportions. For example, magnesium silicide 
may be prepared by heating finely powdered Mg and Si in a proportion of 20 
to 6. The magnesium silicide (Mg,Si), calcium silicide (CaSi) and 
molybdenum silicide (MoSi.sub.2) are particularly preferred due to their 
thermodynamic properties. However, experiments with other metal silicides, 
such as NbSi.sub.2 and CrSi.sub.2, have indicated such to be also useful 
as the third component in the lithium-based cells. 
Table 1 includes the electrode potentials, as well as capacities, for 
illustrative three-phase equilibria of negative electrodes in accordance 
with the invention. Also included in Table 1 are the values of maximum 
theoretical specific energy (MTSE) for the respective ternary reaction 
plateaus. They were obtained by using the equation 
EQU MTSE=26.805*10.sup.3 *(Xe.sub.av. /W) (7) 
where X is the number of moles of lithium transferred during the cell 
reaction, E.sub.av. is the difference between the positive and negative 
electrode potentials in volts, and W is the total weight in grams. For 
this purpose, a material with a capacity of 60 grams per lithium and a 
potential 2.0 volts positive of pure lithium was used as the reference 
positive electrode. 
TABLE 1 
__________________________________________________________________________ 
Electrode 
Electrode Capacity 
Starting Phases in Potential 
Li/Si 
Li/gram 
MTSE 
System 
Composition 
Equilibrium (mV vs. Li) 
(.times.10.sub.3) 
watt.hr/kg 
__________________________________________________________________________ 
(prior art): 
Li.sub.7 Si.sub.3 
Li.sub.7 Si.sub.3 --Li.sub.13 Si.sub.4 
158 0.92 
18.1 428 
Mg.sub.2 Si 
Mg.sub.2 Si--Mg--Li.sub.13 Si.sub.4 
60 3.25 
32.7 574 
Mn.sub.3 Si 
Mn.sub.3 Si--Mn--Li.sub.22 Si.sub.5 
43 4.40 
19.7 474 
MoSi MoSi--Mo.sub.5 Si.sub.3 --Li.sub.13 Si.sub.4 
120 2.275 
24.8 502 
VSi.sub.2 
VSi.sub.2 --V.sub.5 Si.sub.3 --Li.sub.7 Si.sub.3 
191 1.63 
25.2 486 
CaSi CaSi--Ca.sub.2 Si--Li.sub.22 Si.sub.5 
13 2.20 
26.4 544 
__________________________________________________________________________ 
As may be seen by the data of Table I, the three systems (Li-Si-Mg, 
Li-Si-Ca, and Li-Si-Mo) are particularly preferred, based on their 
excellent thermodynamic properties and improved MTSE with respect to the 
prior art Li-Si binary system. 
Positive electrodes for cells in accordance with the invention may be 
formed of a wide variety of materials. Substantially any materials that 
react with lithium and have a potential greater than the negative 
electrode may be used for the positive electrode. For example, suitable 
materials for the positive electrode include manganese, vanadium or 
molybdenum oxides, titanium, iron or molybdenum sulfides. 
Suitable electrolytes for lithium-based cells in accordance with the 
invention will be selected with a view toward the temperature at which the 
cell is desired to operate. For example, at elevated temperatures (on the 
order of about 400.degree. C.) the electrolyte must have high ionic 
conductivity for lithium, be stable at the elevated temperatures and in 
the environments imposed by the two electrodes, by the container and 
accessory apparatus (such as current collectors, seals and the like). 
Suitable electrolytes for such elevated temperature operation include 
halides such as LiX-KX eutectics (where X is a halide ion) Preferred 
electrolytes for the elevated temperature operation have a low electronic 
conductivity to reduce or eliminate problems with self-discharge. 
Electrolytes for intermediate temperature operations (such as an order of 
about 100.degree.-120.degree. C.) include alkali metal salt ammoniates, 
such as LiClO.sub.4 .multidot.xNH.sub.3, and organometallic salts, such as 
LiAlEt.sub.4 .multidot.(where Et is ethyl). Suitable electrolytes for 
about room temperatures or below operation of electrochemical cells in 
accordance with the invention include SO.sub.2 containing LiAlCl.sub.4. 
The metal silicide negative electrodes of the invention preferably are 
formed to be reasonably porous so that they have a large internal surface 
area, and they may include a binding agent, such as for example a 
tetrafluoroethylene polymer (for lower temperature operation) or elemental 
molybdenum, nickel or iron. Such binding agent may be in powdered form, 
admixed with a selected metal silicide powder, and the admixture pressed 
into the desired size and shape. 
Aspects of the present invention will now be illustrated with reference to 
the particularly preferred negative electrode embodiment; however, the 
following is not intended to limit the scope of the invention as defined 
in the appended claims. 
EXPERIMENTAL 
The starting material for Li-Si-Mg system experiments, Mg.sub.2 Si(98%), 
was obtained from Morton Thiokol, Inc. (Alfa products). The LiCl-KCl 
eutectic used as electrolyte was from Lithium Corporation of America, 
while the 44% Li-Si alloy was from Sandia National Laboratory. Additional 
purification of the LiCl-KCl eutectic was required to remove any residual 
trace of water since this would react with lithium alloys. The 
purification process was by equilibration with Li, followed by 
introduction of Al to remove excess Li by formation of Li-Al alloy. 
Electrode samples were made by cold pressing the appropriate powder into 
pellets. 
The three electrode galvanic cell arrangement used in the experiments can 
be represented by 
(-) 44% Li-Si alloy/LiCI-KCl eutectic/Li.sub.x Mg.sub.2 Si (+) with the 44% 
Li-Si alloy (e.g. Li at 44 wt.% and Si at 56 wt.% for a Li:Si ratio=3.18) 
as the reference electrode. The operating temperature was held at about 
440 degrees centigrade. Both dynamic and equilibrium open circuit 
measurements were controlled using a potentiostat/galvanostat (, model 
173) coupled with a digital coulometer (, model 179). Potentials were 
monitored through a digital multimeter (Keithley, model 172) and chart 
recorder arrangement. All experiments and materials preparations were 
carried out in a heliumfilled glove box. 
FIG. 2 shows the performance of the electrode sample upon repeated cycles, 
in terms of the cell voltage versus capacity utilization at different 
current densities. Both the flatness of these discharge/charge curves and 
their relatively small sensitivity to current indicate fast kinetics in 
the sample electrode. 
At the onset of a discharge or charge process, a small voltage spike is 
observed. This is normally associated with the nucleation of a new phase 
as the addition or removal of the electroactive specie moves the 
equilibria from one three-phase triangle in the ternary phase diagram to 
another. This voltage spike is readily avoidable if one prepares the 
starting material in such a way as to include a minute amount of the 
nucleated phase, and the discharge and charge cycles are terminated before 
reaching the ends of the three-phase equilibria. In this way, all three 
phases will always be present in the reactant sample. 
The deviation of the dynamic cell voltage from both the predicted and 
measured equilibrium potential is shown in FIG. 3 as a function of the 
current density. The polarization is shown for a 50% state of utilization. 
These numbers, of the order of 0.40 and 0.57 mV/mA-cm.sup.-2 of 
macroscopic surface area for the charge and discharge cycles respectively, 
are considered small at these current densities and temperature, and 
become only slightly larger near the end of the discharge and charge 
cycles. 
The lithium-rich limit of the three phase equilibrium represented by the 
region marked "I" of FIG. 1 was determined by adding and deleting small 
amounts of charge to a sample of composition corresponding to the 
intersection of the dotted line joining the Mg.sub.2 Si phase to the 
lithium corner and the Li.sub.13 Si.sub.4 -Mg tie-line. Two voltage 
plateau regions exist with voltages corresponding to the Mg.sub.2 
Si-Mg-Li.sub.13 Si.sub.4 and Li.sub.13 Si.sub.4 Li.sub.22 Si.sub.5 
equilibria. This in turn establishes the validity of the ternary phase 
diagram, FIG. 1, and thus confirms the capacity of the Mg.sub.2 
Si-Mg-Li.sub.13 Si.sub.4 three phase equilibrium as predicted from 
thermodynamic considerations, at 3.25 moles of lithium per mole of silicon 
(Li/Si). 
The theoretical specific energy of cells using preferred materials can be 
about 35% higher than that for a comparable cell using the Li-Si binary 
alloy presently being used in thermal batteries. In addition, metal 
silicide negative electrodes of the invention show evidence of fast 
kinetics. 
Thus, use of negative electrodes in accordance with the present invention 
can avoid the safety problems involved with pure lithium as negative 
electrode. When desired, the negative electrodes can be used in 
electrochemical cells that can be recharged. Alternatively, the inventive 
negative electrodes can be used in primary batteries. 
Although the present invention has been described with reference to 
specific examples, it should be understood that various modifications and 
variations can be easily made by those skilled in the art without 
departing from the spirit of the invention. Accordingly, the foregoing 
disclosure should be interpreted as illustrative only and not to be 
interpreted in a limiting sense. The present invention is limited only by 
the scope of the following claims.