Surface phosphorylated Li5FeO4 prelithiation additive synergistically improves the air-stability and lithium-ion conductivity

Because of the higher delithiation capacity, Li5FeO4 (LFO) can be used as sacrificial prelithiation additive to compensate the initial capacity loss caused by the formation of solid electrolyte interphase in lithium-ion batteries (LIBs). However, the extremely poor air stability and severe phase transition after Li+ deintercalation make it difficult for LFO to be applied in prelithiation engineering practice. Herein, the residual alkali layer generated on the LFO due to air exposure is directly converted into a Li3PO4 layer by a one-pot solid–liquid reaction. The as-obtained surface phosphorylated LFO (LP-LFO) ensures high air stability and effective prelithiation capability. Introducing only 3 wt% LP-LFO to the LiFePO4/Li half-cell can maximally replenish the active Li+ loss and the initial coulombic efficiency, displaying a 9.9 % increase in the initial charge capacity. Moreover, the Li3PO4 coating enables LP-LFO to be a fast ionic conductor, not only inhibiting the phase transition of LFO, but also improving the electronic and ionic conductivity of the electrode, which is confirmed by the complementary tests of galvanostatic intermittent titration technique and electrochemical impedance spectroscopy, as well as the structure and morphology characterization. As a result, the LiFePO4/Li half-cell with LP-LFO additive exhibits a higher rate capability (144 mAh g−1 at 2 C) and cycle stability (157 mAh g−1 after 100 cycles at 0.1 C). And a 17.4 % increase in the discharge capacity and a 12.7 % increase in energy density after 200 cycles for the 3 wt% LP-LFO-added LiFePO4||graphite full cell is also achieved. This work provides a facile and efficient conversion strategy for the air-instable electrode additives, and furtherly broadens their industrial application prospect in high energy density LIBs.