Enhanced Electrochemical Delithiation of LiFePO4 in a Composite Aqueous Electrolyte for High-Performance Olivine FePO4

Development of delithiation methods with cost-efficiency and highly kinetic effectivity is of great importance for the preparation of diversified intercalation materials, mechanistic study, and lithium extraction from spent lithium-ion batteries. Electrochemical delithiation in aqueous electrolytes is low-cost, straightforward, and fast, but has been plagued by incomplete delithiation. To address this issue, we propose to add oxidation reagents, usually applied in chemical delithation, to traditional aqueous delithiation electrolyte. As a demonstration, herein, a Na2SO4 + Na2S2O8 composite aqueous electrolyte is used to obtain olivine FePO4 from the electrochemical delithiation of LiFePO4, and compared with the similar delithiation in Na2SO4 aqueous electrolyte. The delithiation goes completion in the composite electrolyte 79% faster than that in traditional electrolyte. The resulting olivine FePO4 exhibits integrity in terms of structure and electrochemical properties, with discharge specific capacity of 157.1 mAh g−1 at 0.1 C, constant voltage plateau of 3.37 V, and 98.8% capacity retention after 100 cycles, all comparable to the LiFePO4 starting material. Mechanistic studies show that Na2S2O8 promotes the electrochemical delithiation by providing acidic and chemically oxidative conditions.