Ferrous Pyrophosphate Hollow Nanorods Enable Lithium Metal Batteries Under Low Negative/Positive Capacity Ratios

Lithium (Li) metal batteries (LMBs) exhibit a promising paradigm for their high energy density. However, their realization is still hindered by the inherent instability and excess of Li metal. Herein, we rationally design and synthesize ferrous pyrophosphate hollow nanorods encapsulated by an N-doped carbon shell (Fe₂P₂O₇@NC) as a Li host for Li metal anodes (LMAs). The Fe₂P₂O₇@NC-casted Cu electrode (Fe₂P₂O₇@NC-Cu) guides uniform Li deposition and suppresses volume expansion, enabling uniform Li plating and enhanced Coulombic efficiency. Time-of-flight secondary ion mass spectrometry and in-depth X-ray photoelectron spectroscopy analyses reveal that the existence of Fe₂P₂O₇ promotes the generation of LiF and Li₂O in LMAs, thereby promoting uniform Li plating. The Fe₂P₂O₇@NC-Li electrode presents stable Li deposition/stripping with small voltage polarization for 1400 h at 1 mA cm^-2 and 1 mAh cm^-2. Furthermore, a full cell coupled with a LiFePO₄ cathode, demonstrates consistent cycling stability, maintaining a capacity retention of 80.6% over 136 cycles at a low negative/positive capacity ratio of 1.5, highlighting the great potential of Fe₂P₂O₇@NC for practical high-energy-density LMBs.