Janus Solid-Liquid Interface Enabling Ultrahigh Charging and Discharging Rate for Advanced Lithium-Ion Batteries

LiFePO₄ has long been held as one of the most promising battery cathode for its high energy storage capacity. Meanwhile, although extensive studies have been conducted on the interfacial chemistries in Li-ion batteries,1-3 little is known on the atomic level about the solid-liquid interface of LiFePO₄/electrolyte. Here, we report battery cathode consisted with nanosized LiFePO₄ particles in aqueous electrolyte with an high charging and discharging rate of 600 C (3600/600 = 6 s charge time, 1 C = 170 mAh g^-1) reaching 72 mAh g^-1 energy storage (42% of the theoretical capacity). By contrast, the accessible capacity sharply decreases to 20 mAh g^-1 at 200 C in organic electrolyte. After a comprehensive electrochemistry tests and ab initio calculations of the LiFePO₄-H₂O and LiFePO₄-EC (ethylene carbonate) systems, we identified the transient formation of a Janus hydrated interface in the LiFePO₄-H₂O system, where the truncated symmetry of solid LiFePO₄ surface is compensated by the chemisorbed H₂O molecules, forming a half-solid (LiFePO₄) and half-liquid (H₂O) amphiphilic coordination environment that eases the Li desolvation process near the surface, which makes a fast Li-ion transport across the solid/liquid interfaces possible.