High-Capacity and Long-Cycle Life Aqueous Rechargeable Lithium-Ion Battery with the FePO₄ Anode

Aqueous lithium-ion batteries are emerging as strong candidates for a great variety of energy storage applications because of their low cost, high-rate capability, and high safety. Exciting progress has been made in the search for anode materials with high capacity, low toxicity, and high conductivity; yet, most of the anode materials, because of their low equilibrium voltages, facilitate hydrogen evolution. Here, we show the application of olivine FePO₄ and amorphous FePO₄·2H₂O as anode materials for aqueous lithium-ion batteries. Their capacities reached 163 and 82 mA h/g at a current rate of 0.2 C, respectively. The full cell with an amorphous FePO₄·2H₂O anode maintained 92% capacity after 500 cycles at a current rate of 0.2 C. The acidic aqueous electrolyte in the full cells prevented cathodic oxygen evolution, while the higher equilibrium voltage of FePO₄ avoided hydrogen evolution as well, making them highly stable. A combination of in situ X-ray diffraction analyses and computational studies revealed that olivine FePO₄ still has the biphase reaction in the aqueous electrolyte and that the intercalation pathways in FePO₄·2H₂O form a 2-D mesh. The low cost, high safety, and outstanding electrochemical performance make the full cells with olivine or amorphous hydrated FePO₄ anodes commercially viable configurations for aqueous lithium-ion batteries.