Evaluation of the Electrochemical Properties of Copper Hydroxide and Related Copper Oxides in Sodium-Ion Batteries

Recently, CuO shows high charge and discharge capacity in Na-ion battery. The number of electrode materials for Na-ion battery is much less than that for Li-ion battery.  Copper oxide and related materials are candidate for the high potential electrode materials for Na-ion battery. The theoretical capacity of CuO is 674 mAh g^-1. When the charge-discharge reaction evaluated the range between 0 and 3.0 V, the discharge capacity shows 600 mAh g^-1 at 1st.  However, the discharge capacity in 2nd cycle is decreased to near 300 mAh g^-1. The low cycle performance is one of the week points.  In order to improve the cycle performance, we evaluate the properties in various voltage ranges and research the best performance condition. The charge-discharge reaction evaluated using a 2032 type coin cells.  The working electrode was prepared by mixing the samples, acetylene black (AB) and plytetrafluoroethylene (PTFE) in a 70:20:10 weight ratio. Na metal was used as a counter electrode. The 1M NaClO₄ in EC:DMC(1:1) solution was used as the electrolyte. The current density was 10 mA g^-1. When we set it between 1.0 and 4.0 V, which is 1.0 V higher than before, CuO shows reversible charge-discharge reaction up to 20 cycles.  The maximum discharge capacity is nearly 150 mAh g^-1. Cu₂O was also evaluated the properties, however the material did not show reversible reaction. The maximum discharge capacity is below 10 mAh g^-1. Cu(OH)₂ is a novel material as an electrode of Na-ion battery. The charge-discharge reaction was also evaluated. Figure 1 shows the charge-discharge profiles up to 10 cycles. The maximum discharge capacity is 247 mAh g^-1.  There is a plateau around 1.3 V, which is corresponded to the redox reaction of Cu²⁺ to Cu⁺ (vs Na⁺/Na).    When the voltage range expands to 0 V, the redox reaction of Cu⁺ to Cu would increase the capacities.  However, the redox reaction makes disadvantage effect of the cycle performance.  Figure 2 shows the cycling performances of Cu(OH)₂ in the voltage range of 1.0 to 4.0 V.  The reversible charge-discharge reaction is observed, although the discharge capacities above 200  mAh g^-1 show only first 4 cycles.  The cycling performance is not good.  We are investigating the best matching of the measurement ranges and the electrolytes.  Furthermore, we develop an in-situ measurement of X-ray diffraction and charge-discharge reaction.  It will clear the mechanism of the reaction from the view of changing crystal structures.