A high-energy conversion-type cathode activated by amorpholization for Li rechargeable batteries

Abstract

Although conversion-type electrodes deliver larger theoretical capacities than intercalation-type electrodes, their application as practical cathodes for Li rechargeable batteries is hindered by their intrinsically sluggish kinetics and low operating voltage. In this study, we demonstrate that the conversion-type electrochemical behaviors of Cu(PO₃)₂ in a Li-cell system are highly enhanced by amorpholization and carbon-mixing. In particular, the presence of the (PO₃)⁻ polyanion in the structure enables a much higher operation voltage of Cu(PO₃)₂ relative to that of other conversion-type metal-oxide electrodes, resulting from the inductive effect by phosphorus with high electronegativity. As a result, the amorphorized Cu(PO₃)₂/C composite delivers not only a large reversible capacity of ∼240 mA h g⁻¹ at 12 mA g⁻¹ but also an average operation voltage of ∼2.8 V (vs. Li⁺/Li). Even at a high current density of 1200 mA g⁻¹, up to ∼60% of the specific capacity at 12 mA g⁻¹ is retained. Furthermore, the capacity retention after 300 cycles at 480 mA g⁻¹ is ∼77% of the initial capacity. This outstanding power capability and cyclability of the amorphorized Cu(PO₃)₂/C composite differ markedly from the poor electrochemical properties of the well-crystallized Cu(PO₃)₂/C composites, indicating the enhanced kinetics of the conversion reaction in Cu(PO₃)₂ by amorpholization. In addition, the reversible conversion-reaction mechanism of Cu(PO₃)₂ in a Li-cell system is demonstrated through various experimental measurements.