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_xCo[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_1.6Co[Fe(CN)₆]_0.9) and black (Li_.6Co[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.