Regulating crystallinity to balance the electrochemical performance of cobalt-tin oxide composite anodes for sodium-ion batteries

As a promising anode candidate for sodium-ion batteries (SIBs), tin-based oxides suffer from rapid capacity fading, greatly limiting their practical applications. Herein, we designed and synthesized three cobalt-tin oxide composites (CSOs), with different degrees of crystallinity by controlling the annealing temperature, to understand the effect of amorphous and crystalline structures on the Na+ storage behavior of tin-based alloy anodes. Theoretical calculations suggest that the amorphous CSO (CSO-A) presents the lowest binding energy with Na+ and the lowest diffusion barriers of Na+ in comparison with that of crystallinity samples (CSO-AC and CSO-C), indicating that the amorphous CSO is the most energetically favorable for Na insertion. Similarly, the experimental results suggest that CSO-A delivers the highest initial specific capacity; however, it presents the worst cycling stability and reversibility. CSO-C displays the best cycling stability but the lowest specific capacity. Interestingly, the CSO-AC sample with both amorphous and crystalline domains achieves the best comprehensive electrochemical performance. Quantitative analysis of the electrochemical process reveals that controlled crystallinity significantly impacts the microstructure and band gap of CSO, which will further affect the reversibility of the conversion reaction and the percent of pseudocapacitance contribution. Our work suggests that, for the alloy anode, rational regulation of crystallinity is a substantial approach to improve capacity retention. For a cobalt tin oxide composite anode with an amorphous/crystal structure, a disordered structure increases the initial specific capacity and an ordered structure improves capacity retention.