In situ observation of macroscopic phase separation in cobalt hexacyanoferrate film

Masamitsu Takachi,Yutaka Moritomo

doi:10.1038/srep42694

Abstract

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, LixCo[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 (Li1.6Co[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.

Highlights

The optical battery cell has a structure of Li1.6Co[Fe(CN)6]0.9 film on an ITO glass/Teflon sheet with a square hole/anode
To increase the signal-to-noise ratio, the chromaticity was averaged within 8 × 8 pixels (2 × 2 μm[2] area) around each position
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Summary

In Situ Observation of PS Dynamics

The lattice contraction causes a significant strain at the phase boundary In such a region, the Li+ deintercalation and subsequent phase transformation into the black phase is much easier than nucleation of a new micro-domain of Li0.6Co3+ [Fe2+ (CN)6]0.9 in another part of the green region. The Li+ deintercalation and subsequent phase transformation into the black phase is much easier than nucleation of a new micro-domain of Li0.6Co3+ [Fe2+ (CN)6]0.9 in another part of the green region This scenario is essentially the same as the ‘domino-cascade model’ of LixFePO414. This is because parts of Fe2+, which is the final state of the optical transition, are oxidized to Fe3+ with decrease in x

Normalized Absorption Intensity In Against x

Discussion

Additional Information