The Mixed-Conductor Matrix Concept

In order to achieve appreciable macroscopic current densities while maintaining low local microscopic charge and particle flux densities, many battery electrodes that are used in conjunction with liquid electrolytes are produced with porous micro-structures containing very flne particles of the solid reactant materials. This porous structure of high reactant surface area is permeated with the electrolyte. 

A quite different approach was introduced in the early 1980s [48-50], in which a dense solid electrode is fabricated which has a composite microstructure in which particles of the reactant phase are finely dispersed within a solid, electronically conducting matrix in which the electroactive species is also mobile. There is thus a large internal reactant/mixed-conductor matrix interfacial area. The electroactive species is transported through the solid matrix to this interfacial region, where it undergoes the chemical part of the electrode reaction. Since the matrix material is also an electronic conductor, it can also act as the electrode s current collector. The electrochemical part of the reaction takes place on the outer surface of the composite electrode. 

When such an electrode is discharged by removal of the electroactive species, the residual particles of the reactant phase remain as relics in the microstructure. This provides fixed permanent locations for the reaction to take place during subsequent cycles, when the electroactive species again enters the structure. Thus this type of configuration can provide a mechanism for the achievement of true micros tructural reversibility. 

In order for this concept to be applicable, the matrix and the reactant phase must be thermodynamically stable in contact with each other. One can evaluate this possibility if one has information about the relevant phase diagram - which typically involves a ternary system - as well as the titration curves of the component binary systems. In a ternary system, the two materials must lie at corners of the same constant-potential tie-triangle in the relevant isothermal ternary phase diagram in order to not interact. The potential of the tie-triangle determines the electrode reaction potential, of course. 

An additional requirement is that the reactant material must have two phases present in the tie-triangle, but the matrix phase only one. This is another way of saying that the stabiUty window of the matrix phase must span the reaction potential, but that the binary titration curve of the reactant material must have a plateau at the tie-triangle potential. It has been shown that one can evaluate the possibihty that these conditions are met from knowledge of the binary titration curves, without having to perform a large number of ternary experiments. 

The kinetic requirements for a successful application of this concept are readily understandable. The primary issue is the rate at which the electroactive species can reach the matrix/reactant interfaces. The critical parameter is the chemical diffusion coefficient of the electroactive species in the matrix phase. This can be determined by various techniques, as discussed above. 

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Intermetallic and multi element alloys have also been investigated, where at least one element does not alloy with Li (e.g., Cu) and is used to buffer the strain/deformation induced by Li alloying with the other element(s). This is often called the mixed conductor matrix concept [155], designed to improve the cycla-bility of the electrode. The subject has recently been reviewed in detail by Zhang [156] and will not be developed further here. Figure 12 (from Ref. [156]) summarizes the main results from the literature for Si-based, Sn-based and Sb-based anodes. An example already quoted above is the use of an Alo.gCuo.2 multilayer which improves the cyclability of Al-based anodes [135]. One of the drawbacks of this approach is the reduction of electrode capacity due to the addition of the inactive buffering element. 

The Mixed-Conductor Matrix Concept 423 Table 14.5 Chemical diffusion data for lithium-tin phases at 25 °C. 

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