On the Electrochemical Phase Evolution of Anti-PbO-Type CoSe in Alkali Ion Batteries.

The reaction mechanism of anti-PbO type CoSe in Li, Na, and K ion half cells is studied. Ex situ X-ray diffraction data is analyzed with the Rietveld method, in conjunction with discharge profiles and extended cycling data. These indicate that intercalation followed by a conversion reaction occur in all systems. For the case of Na, the intercalation reaction was associated with a contraction in the stacking axis lattice parameter, whereas Li and K exhibited expansion. Magnetic susceptibility versus temperature measurements of Li- and Na-intercalated CoSe samples produce unusual results, and several explanations are proposed, including the formation of a superconductive phase. Extended cycling experiments are also performed, and high initial capacities of 937, 657, and 972 mAh/g are observed for Li, Na, and K, respectively. However, all systems exhibit significantly lower second discharge capacities of 796, 530, and 515 mAh/g. The capacities continue to decline during extended cycling, with the systems exhibiting tenth cycle capacity fades of 52, 85, and 95% and Li half cells exhibit capacities over 150 mAh/g at 15 mA/g after 50 cycles. The capacity fade is likely attributable to volume changes and irreversibility associated with conversion and intercalation reactions. This work correlates electrochemical features to the structural evolution, magnetic properties, and reaction mechanisms.