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 followings: 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 and/or play 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.

The use of high capacity electrode materials based on alloying and dealloying reactions with Na is very effective for improving energy density of batteries. However, their application brings on electrical isolation such as detachment of the electrode mixture layer from a current collector, causing rapid capacity fading. We previously found that Cu electrochemically grows in sheet form by electroplating in a CuSO4-based aqueous solution with poly­(acrylic acid) (PAA). In the present study, our goal was to elucidate the formation mechanism of Cu sheets by characterization using scanning transmission electron microscopy (STEM), X-ray diffraction (XRD) analysis, and electron scatter diffraction patterns (EBSD) mapping. Then, the cycling performance of a Sn negative electrode for Na-ion batteries was significantly upgraded by the application of a roughened-Cu substrate with optimized sheet thickness. The STEM images and EBSD maps revealed that the Cu sheet was a single crystal, and the results obtained from XRD and the cathodic polarization behavior of Cu electrodeposition in PAA-containing solutions suggested that PAA molecules adsorbed onto Cu (100) to suppress the Cu growth on the plane, resulting in the formation of Cu sheets. Although the initial reversible capacities of flat-Cu/Sn and roughened-Cu/Sn electrodes were comparable, the developed Cu substrate (1.0 × 10–4 M PAA) delivered a noticeable increase in the reversible capacity by 210 mA h g–1 from the first to the second cycle, whereas the flat-Cu remained the increase by 100 mA h g–1. In addition, the roughened-Cu substrate suppressed the detachment of the active material layer to maintain a high capacity of 685 mA h g–1 with good capacity retention of more than 90% by the anchor effect. These results demonstrate that the roughened-Cu substrate prepared in the present work is a promising candidate as a current collector for rechargeable batteries.