The electrochemical dynamic behavior of the solid electrolyte interface (SEI) formed on LiCoO 2 (LCO) by lithium bis(oxalato)borate (LiBOB) is investigated at various cutoff voltages. Particularly, for layered cathode active materials, various cutoff voltages are used to control the delithiation states; however, systematic investigations of the voltage and SEI are lacking. To increase the practical energy density of the LCO, a high cutoff voltage is pursued to utilize a state of high delithiation. However, this high cutoff voltage causes the electrolyte to undergo side reactions and the crystalline structure changes irreversibly, limiting the cycle life. In a lowvoltage environment (<4.7 V), LiBOB improves the initial Coulombic efficiency and cycling performance by forming an effective SEI, which suppresses side reactions. At higher voltage levels (4.7−4.9 V), LiBOB no longer effectively protects the surface, causing the electrochemical performance to decrease rapidly. The main cause of this phenomenon is the decomposition of LiBOB-SEI at a high voltage, as shown by systematic surface and electrochemical analyses comprising linear sweep voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy. In conclusion, LiBOB can suppress side reactions of the electrolyte by SEI formation, but the SEI decomposes at voltage levels higher than 4.7 V.

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Aqueous rechargeable lithium-ion batteries (ARLBs) have garnered substantial attention for their desirable properties of safety, environmental compatibility, and rapid ion-transport in the electrolyte compared with organic lithium-ion batteries. However, the low energy density of ARLBs and poor electrochemical stability of the aqueous electrolyte are critical limitations. A proposed means of improving the energy density of ARLBs is the fabrication of thick electrodes, wherein the fraction of active material is increased and that of inactive material decreased without modifying the structure of the electrode. This study describes the construction of ultra-thick millimeter-scale LiFePO 4 (LFP) electrodes containing Li 2 SO 4 at various concentrations to control the conductivity of the aqueous electrolyte. The enhanced electrolyte conductivity mitigates sluggish electrochemical behavior by providing high lithium-ion flux even in 2.0 mm-thick LFP electrodes having a capacity of 52 mAh cm À2 . A straightforward method of electrode preparation is described. The operation of mm-thick LFP electrodes evaluated with a homemade three-electrode cell shows improved cycle performance and rate capability that are not attainable with organic electrolytes.