Copper sulfate as additive for Fe₃O₄ electrodes of rechargeable Ni-Fe batteries

CuSO4 center dot 5H(2)O was used as an additive for paste Fe3O4 electrode by a simple physical mixing method. The results of cyclic voltammetry and energy dispersive spectroscopy tests confirm that during the initial charge, copper sulfate could be converted to metallic Cu with uniform distribution in the electrode and stable during normal charge-discharge cycles. Compared with the electrode without the additive, the discharge capacity of the one with 5-wt% CuSO4 center dot 5H(2)O increases from 483 to 513 mAh center dot g(-1) at a current density of 90 mAh center dot g(-1) and from 147 to 221 mAh center dot g(-1) at a current density of 900 mAh center dot g(-1), respectively. After 50 cycles at a current density of 225 mAh center dot g(-1), its capacity retention rate is 85.7%, but it is only 42.4% for the electrode without the additive. Combined with the results of powder X-ray diffraction and scanning electron microscopy tests, the improvement in high-power and cycle performance of the Fe3O4 electrodes might be due to the obvious depassivation caused by metallic Cu.