Entropy change characteristics of LiMn_0.67Fe_0.33PO₄ and Li₄Ti₅O₁₂ electrode materials

The combination of LiMn_0.67Fe_0.33PO₄ positive and Li₄Ti₅O₁₂ negative electrode is studied in terms of its entropy change behavior, which affects the reversible heat generation of a lithium-ion cell. This electrode combination is especially interesting for large applications, as it is proposed to be a very safe choice having still an adequate energy density. The entropy change of LiMn_0.67Fe_0.33PO₄ and Li₄Ti₅O₁₂ electrode materials is measured at different states of charge using a potentiometric method. The results are compared with conventional electrode materials, LiFePO₄ and artificial graphite. The entropy change of LiMn_0.67Fe_0.33PO₄ is found to follow the distinct plateaus of Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ redox couples and to be clearly different from LiFePO₄. This difference is suggested to be due to single-phase solid solution regions, originating from effects of substituting Mn for Fe. For Li₄Ti₅O₁₂, mostly a constant entropy change typical for a two-phase reaction is observed, except for the region near 0% state of charge. The data from individual electrodes is used to simulate and compare the entropy change behavior and thus the reversible heat generation rate of different electrode combinations.