Herein, we applied a novel superconcentrated aqueous electrolyte in calcium-ion batteries for the first time to improve electrochemical performance by decreasing the hydration number and radius of calcium ions. Their charge/discharge capacities with the superconcentrated electrolyte were ca. 13% higher than that with the dilute electrolyte. The cycling performance improved quite remarkably in the superconcentrated electrolyte, because of the suppression of structural collapse of copper hexacyanoferrate used as the electrode. The results have been confirmed through X-ray diffraction results.Keywords: Calcium-ion battery | Superconcentrated aqueous electrolyte | CuHCF electrodePolyvalent ion batteries (PIBs) have been suggested as one of the alternatives for lithium ion batteries, because the former are expected to have high energy density. 17 However, polyvalent cations with large charge densities, such as Mg 2+ , Ca 2+, and Al 3+ , are much slowly diffused in an electrode than lithium because they are seriously affected by cationcation repulsive and cationanion attractive forces. 510 This results in poor electrochemical performances such as low charge/ discharge capacities, poor rate capability, and cycling ability. Therefore, the first step to develop PIBs is to resolve this problem.Recently, it has been demonstrated that the problem of the slow diffusion of polyvalent cations can be resolved by using aqueous electrolytes. 11,12 Since polyvalent cations are hydrated by water molecules in an aqueous electrolyte, their charge density is weakened by the shielding effects of water, which enables rapid diffusion of polyvalent cations in an electrode during coinsertion/extraction reactions. 11,12 In such a case, an active material with large channels that allow the passage of hydrated polyvalent cations is necessary. Wang et al. 13,14 observed that various polyvalent cations could rapidly diffuse into Prussian Blue analogue (PBA) electrodes, which contain large open interstitial sites (4.6 ¡ in diameter) and ©100ª channels (3.2 ¡ in diameter) in polyvalent cation-based aqueous electrolytes. However, the low charge/discharge capacities and capacity reduction during the initial cycles must be improved.Therefore, to improve the electrochemical performance, such as charge/discharge capacities and cycle ability, in a PBA electrode and polyvalent cation-based aqueous electrolyte system, we applied a novel superconcentrated aqueous electrolyte in this study. The hydration number and radius of polyvalent cations are changed by applying a superconcentrated electrolyte, which, in turn, have a positive influence on the electrochemical performance.Calcium-ion batteries, which are a type of PIBs, have attracted much attention because they are expected to have high energy density (low reduction potential, similar to that of the lithium ion, and high theoretical capacities) and low cost (abundant resource). 1519 In this research, therefore, we studied a calcium-based aqueous electrolyte as the electrolyte for PIBs.The electrochem...

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Thin-film electrodes are considered to be desirable for understanding the detailed surface characteristics of active materials for rechargeable batteries. This study attempts to elucidate the effects of a solid solution outer layer (SSOL) of TiO2 on the surface and electrochemical properties of LiNi0.6Co0.2Mn0.2O2 (NCM622) cathodes by using thin-film electrodes synthesized by a spin-coating technique. The SSOL phase is induced on the NCM622 thin-film surface by a post-annealing process after the TiO2 coating using atomic layer deposition. Structural and morphological analyses revealed that the bare NCM622 thin-film electrode without a thin SSOL has a spinel-like derivative phase induced by an oxygen vacancy at the surface, which is considered to be the crucial factor for the poor electrochemical properties of Ni-rich NCM. In particular, additional measurements including in situ Raman spectroscopy revealed that the spinel-like derivative phase rapidly makes the surface structure become corrupt and change to the amorphous state during electrochemical reactions. In contrast, the oxygen vacancy can be eliminated by forming a SSOL phase at the surface of the NCM622 thin-film through the rapid migration of Ni, Ti, and O atoms during the post-annealing process, significantly enhancing the structural stability, which ultimately improves the electrochemical performance, including cyclability and Coulombic efficiency.

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This study investigated the fundamental mechanisms of the loss of capacity of LiNiO2 (LNO) electrodes for Li+ insertion/deinsertion with a special focus on the origin of this deterioration in an aqueous system. In situ Raman spectra revealed that the intercalation of H+ ions formed a NiOOH x film at the surface of LNO during the initial electrochemical cycles; this NiOOH x film was also confirmed by X-ray photoelectron spectroscopy and transmission electron microscopy analysis. The formation of an electrochemically inactive spinel-like phase (Ni3O4) at the subsurface was triggered by the absence of Li in the NiOOH x film at the surface. These structural changes of LNO, accelerated by the intercalation of H+ ions, were considered to be the fundamental cause of the greater loss of capacity in the aqueous system.

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