In situ observation of macroscopic phase separation in cobalt hexacyanoferrate film

Lithium-ion secondary batteries (LIBs) store electric energy via Li(+) deintercalation from cathode materials. The Li(+) deintercalation frequently drives a first-order phase transition of the cathode material as a result of the Li-ordering or Li-concentration effect and causes a phase separation (PS) into the Li-rich and Li-poor phases. Here, we performed an in situ microscopic investigation of the PS dynamics in thin films of cobalt hexacyanoferrate, Li(x)Co[Fe(CN)(6)](0.9), against Li(+) deintercalation. The thick film (d = 1.5 μm) shows a characteristic macroscopic PS of several tens of μm into the green (Li(1.6)Co[Fe(CN)(6)](0.9)) and black (Li(.6)Co[Fe(CN)(6)](0.9)) phases in the x range of 1.0 < x < 1.6. Reflecting the substrate strain, the thin film (d = 0.5 μm) shows no trace of the PS in the entire x region. Our observation suggests that the macroscopic PS plays a significant role in the charge/discharge dynamics of the cathode.