Tin Oxides as a Negative Electrode Material for Potassium-Ion Batteries

As one strategy for increasing energy density of K-ion batteries, electrochemical behavior of Sn oxides (SnO and SnO2) was studied as a negative electrode material. X-ray photoelectron spectroscopy and X-ray diffraction revealed the following: SnO underwent phase separation at the first charge (reduction) process to form metallic Sn and potassium oxide, and reversible alloying reactions between the resulting Sn and K proceeded up to a composition of KSn or more. In contrast, SnO2 showed little electrochemical reactivity to potassium. Interestingly, a reversible capacity obtained from SnO electrode at the initial cycle was comparable to that of Sn alone electrode. SnO electrode exhibited a reversible capacity of 183 mA h g–1 with a 80% capacity retention at the 30th cycle, whereas a capacity of Sn electrode rapidly decreased because of the electrode disintegration induced by the significant volume change during K–Sn alloying/dealloying reactions. No crack and peeling off of an active material layer were confirmed in SnO electrode. Scanning transmission electron microscope image of the SnO electrode after the first cycle displayed that Sn nanoparticles were dispersed in amorphous-like K2O matrices. The reason for the improved cycle stability of SnO electrode is probably that K2O suppressed Sn aggregation or played a role as a buffer to volumetric change in Sn. For achieving a further long cycle life of SnO electrode, the optimization of particle size and electrolyte solution, and elemental substitution of part of oxygen would be effective.