Conversion Chemistry of Cobalt Oxalate for Sodium Storage

Conversion electrodes, which can realize high capacities by employing the wider valence states of transition metals, are investigated for sodium storage and applied for rechargeable sodium-ion batteries (SIBs). Importantly, this work is a first report for the sodium storage ability and related storage mechanism in oxalate compounds, specifically cobalt oxalate (CoC₂O₄) nanorods. The nanorods are intimately blended with acetylene black powders to achieve sufficient electrical conductivity (∼10^-3 S cm^-1). The resulting C-CoC₂O₄ electrode delivers an initial capacity of about 330 mA h (g-CoC₂O₄)^-1 at a rate of 0.2 C (60 mA g^-1) and preserves 75% of the initial capacity over 200 cycles. A high charge (oxidation) capacity, ∼111 mA h g^-1, was achieved even at 30 C (9000 mA g^-1). This remarkable electrode performance is reported for the first time for metal oxalate compounds tested for Na cells, to the best of our knowledge. X-ray diffraction, transmission electron microscopy, and time-of-flight secondary-ion mass spectroscopy analyses lead to the proposal of a new sodium storage mechanism. For this mechanism, CoC₂O₄ is converted into Co metal involving with the creation of Na₂C₂O₄ on discharge (reduction), and the Co metal is recovered to CoC₂O₄ on charge. The employed electroconducting carbon is likely to provide good electron conduction paths, which enables fast conversion on both discharge and charge. A full cell comprised of the C-CoC₂O₄ anode and carbon-coated NaCrO₂ cathode exhibits good retention capacity over prolonged cycling, with retention of about 84.7% of the first capacity [107 mA h (g-NaCrO₂)^-1] for 300 cycles, and is active at a rate of 5 C (550 mA g^-1), with a capacity of 79.5 mA h g^-1. This result demonstrates the potential of applying C-CoC₂O₄ as an anode material for rechargeable SIBs.