Synergetic effect of aqueous electrolyte and ultra‐thick millimeter‐scale LiFePO4 cathode in aqueous lithium‐ion batteries

Summary 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 LiFePO4 (LFP) electrodes containing Li2SO4 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−2. 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. With aqueous electrolyte, millimeter‐scale thick electrode and high rate capability