Page 439 - Handbook of Battery Materials

14.5 Thermodynamic Basis for Electrode Potentials and Capacities 411

electrodes in ternary systems [29–32]. This followed from the development of the
analysis methodology for the determination of the stability windows of electrolyte
phases in ternary systems [33]. In these cases, one uses isothermal sections
of ternary phase diagrams, the so-called Gibbs triangles, upon which to plot
compositions. In ternary systems, the Gibbs Phase Rule tells us that three-phase
equilibria will have composition-independent intensive properties, that is, activities
and potentials. Thus compositional ranges that span three-phase regions will lead
to potential plateaus at constant temperature and pressure.
Estimated data on a number of ternary lithium systems theoretically investigated
as extensions of the Li–Si binary system are included in Table 14.2. Also included
are comparable data for the binary Li–Si alloy that are currently being used in
commercial thermal batteries.
This thermodynamically based methodology provides predictions of the lithium
capacities in addition to the electrode potentials of the various three-phase equilibria
under conditions of complete equilibrium. This information is included as the last
column in Table 14.2, in terms of the number of moles of lithium per kilogram
total alloy weight.
From a practical standpoint, the most useful compositions would be those with
quite negative potentials (so as to give high cell voltages) that also have large
capacities for lithium. However, it must be recognized that the materials with the
most negative potentials, and thus the highest lithium activities, will be the most
reactive, and thus will be more difficult to handle than those whose potentials are
somewhat farther from that of pure lithium.
As recently pointed out [32], several of these ternary systems appear to have
potentials and capacities that should make them quite interesting for practical

◦
Table 14.2 Estimated data relating to lithium–silicon-based ternary systems at 400 C.
–1
System Starting composition Phases in equilibrium Voltage (mV) vs Li Li (mol kg )
Li–Si–Mo Mo 5 Si 3 Mo 5 Si 3 –Mo 3 Si–Li 22 Si 5 3 9.7
Li–Si–Ca CaSi CaSi–Ca 2 Si–Li 22 Si 5 13 26.4
Li–Si–Mn Mn 3 Si Mn 3 Si–Mn–Li 22 Si 5 43 19.7
Li–Si–Mn Mn 5 Si 3 Mn 5 Si 3 –Mn 3 Si–Li 13 Si 4 45 11.1
Li–Si–Mg Mg 2 Si Mg 2 Si–Mg–Li 13 Si 4 60 32.7
Li–Si–Mo MoSi 2 MoSi 2 –Mo 5 Si 3 –Li 13 Si 4 120 24.8
Li–Si–Cr Cr 5 Si 3 Cr 5 S 13 –Cr 3 Si–Li 13 Si 4 138 11.6
Li–Si Li 7 Si 3 Li 7 Si 3 –Li 13 Si 4 158 18.1
Li–Si–Mn MnSi MnSi–Mn 5 Si 3 –Li 7 Si 13 163 10.4
Li–Si–Ti TiSi TiSi–Ti 5 Si 3 –Li 7 Si 3 182 11.3
Li–Si–Nb NbSi 2 NbSi 2 –Nb 5 Si 3 –Li 7 Si 3 184 19.0
Li–Si–V VSi 2 VSi 2 –V 5 Si 3 –Li 7 Si 3 191 25.2
Li–Si–Cr CrSi CrSi–Cr 5 Si 3 –Li 7 Si 3 205 10.8
Li–Si–Ta TaSi 2 TaSi 2 –Ta 5 Si 3 –Li 7 Si 3 211 12.6
Li–Si–Cr CrSi 2 CrSi 2 –CrSi–Li 7 Si 3 223 18.8
Li–Si–Ni Ni 7 Si 13 Li 7 Si 13 –Nisi–Li 12 Si 7 316 12.1