In solid-state ionic devices, such as all-solid-state lithium ion batteries (LIBs) and solid oxide fuel cells (SOFCs), mechanical strain might be induced at the electrode/electrolyte hetrointerfaces during the cell fabrication process and the operation because of differences in the crystal structures and the chemical/thermal expansion coefficients. Such mechanical strain at the heterointerfaces might affect thermodynamic and kinetic properties of the electrodes and the electrolyte, resulting in deterioration or improvement of the devices’ performance.

In this work, we investigated the influence of mechanical strain on lithium chemical potential in cathode materials for LIBs. Dense thin film electrodes, such as LiCoO₂ and LiFePO₄, were fabricated on a solid-state lithium ion conductor, such as Li_0.29La_0.57TiO₃ (LLT) and Li₇La₃Zr₂O₁₂ (LLZ). The change in the lithium chemical potential in the electrode was evaluated by measuring electromotive force (EMF) between the electrode under compressively- or tensely-strained condition and one under stress-free condition.

A clear EMF was generated when a mechanical strain was applied to the electrode. Typically, a positive and negative EMF was observed for a compressive and tensile stress, respectively. When the mechanical strain was released, EMF returned to zero. EMF seemed to increase proportionally with increasing the amount of mechanical strain. Such an EMF accompanied with the mechanical strain was discussed from thermodynamic points of view and, as results, could be related to the lithium partial molar volume of the electrode material.

References

[1] K. Funayama, T. Nakamura, N. Kuwata, J. Kawamura, T. Kawada, K. Amezawa, Electrochemistry, 83(10), 894-897 (2015).

[2] K. Funayama, T. Nakamura, N. Kuwata, J. Kawamura, T. Kawada, K. Amezawa, Solid State Ionics, in press.