Negative electrode materials with a large capacity and long cycle life are required for practical large‐scale sodium secondary batteries. Copper phosphide‐carbon composites containing various carbon contents were investigated for this purpose using an ionic liquid (IL) electrolyte. The composites were prepared by a two‐step high energy ball‐milling process: preparation of copper phosphide (CuP2) and composite formation between CuP2 and acetylene black (AB) at different ratios. A CuP2 to Cu3P phase transition was observed at AB contents of 30 and 40 wt%. The sample containing 30 wt% AB gave an optimal electrochemical performance (98.9 % capacity retention, 350 cycles, reversible capacity=358 mAh g−1 at 8000 mA g−1 and 90 °C), and upon cycling at 25 °C, a high capacity retention of 111 % was achieved (350 cycles). Ex situ XRD revealed the reversible conversion of Cu3P and the formation of Na3P during charging. The high performance was attributed to the P−O−C bond and the robust interfacial layer formed in the IL.
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Kaushik et al. (2020) studied this question.