Crystallographic Aspects and the Possibility of Selective Equilibrium

If we look at the mechanistic and crystallographic aspects of the operation of polycomponent 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 microstructure is reconstituted. 

In the simple case of a reconstitution reaction in which the incorporation of ad- 

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 [29]. 

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 local) composition of the solid solution, rather than the formation of additional phases. 

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