Page 440 - Handbook of Battery Materials

412 14 Lithium Alloy Anodes

applications. Li–Si ternary systems with Mg, Ca, and Mo seem especially interesting
from the standpoint of their potentials and capacities. As an example, if one
assumes that a positive electrode is used that has a potential 2.0 V positive of
elemental lithium and a capacity of 1 mol of lithium per 60 g of active component,
these negative electrode materials provide a maximum theoretical specific energy
−1
of 574, 544, and 502 Wh kg , respectively, whereas the binary Li–Si alloy
−1
currently used in thermal batteries would have a maximum value of 428 Wh kg .
Confirmatory experimental information on the Li–Mg–Si system [34] was recently
presented [35].

14.6
Crystallographic Aspects and the Possibility of Selective Equilibrium

If we look at the mechanistic and crystallographic aspects of the operation of
poly-component electrodes, we see that the incorporation of electroactive species
such as lithium into a crystalline electrode can occur in two basic ways. In
the examples discussed above, and in which complete equilibrium is assumed,
the introduction of the guest species can either involve a simple change in the
composition of an existing phase by solid solution or it can result in the formation
of new phases with different crystal structures from that of the initial host material.
When the identity and/or amounts of phases present in the electrode change, the
process is described as a reconstitution reaction. That is, the micro-structure is
reconstituted.
In the simple case of a reconstitution reaction in which the incorporation of
additional electroactive species occurs by the nucleation and growth of a new phase,
the relative amount of this new phase with a higher solute content increases. If the
initial phase and the new phase are in local equilibrium, the respective compositions
at their joint interface do not change with the extent of the reaction. The amounts
of the phases, determined by the motion of the interfaces between these phases,
are related to the lengths of the two-phase constant-potential plateaus in binary
systems and the three-phase constant-potential plateaus in ternary systems, and
these, in turn, are determined by the extent of the corresponding regions in the
relevant phase diagrams.
In many systems, both single-phase and polyphase behaviors are found in
different composition ranges. Intermediate, as well as terminal, phases often have
been found to have quite wide ranges of composition. Examples are the broad Zintl
phases found in several of the binary lithium systems studied by Wen [23].
The second way in which an electroactive species such as lithium can be
incorporated into the structure of an electrode is by a topotactic insertion reaction.
In this case the guest species is relatively mobile and enters the crystal structure of
the host phase so that no significant change in the structural configuration of the
host lattice occurs.
Thus the result is the formation of a single-phase solid solution. The insertion
of additional guest species involves only a change in the overall (and thus also the