By varying the synthetic temperature and time, Co3O4 with highly optimized electrochemical properties was obtained from the solid state reaction of CoCO3. As a result, Co3O4 showed a high capacity around 700mAh/g and stable capacity retention during cycling (93.4% of initial capacity was retained after 100 cycles). However, its initial irreversible capacity reached about 30% of capacity. Several phenomenological examinations in our previous results told us that the main causes of low initial coulombic efficiency, that is, large initial irreversible capacity, were solid electrolyte interphase (SEI) film formation on surface and incomplete decomposition of Li2O during the first discharge process. SEI film formation cannot be restrained without the development of a special electrolyte, and there has been little research on the proper electrolyte composition, whereas in our research, Ni had the catalytic activity to facilitate Li2O decomposition. Thus, in order to improve the low initial coulombic efficiency of Co3O4 (69%), Ni was added to Co3O4 using two methods like physical mixing and mechanical milling. When adding the same amount of Ni, the mechanical milling showed the improvement in initial coulombic efficiency, 79%, but physical mixing had no effect. Finally, when the charge–discharge mechanism of Co3O4 was considered and the morphologies of Ni–Co3O4 mixture obtained by physical mixing and Ni–Co3O4 composite prepared by mechanical milling were compared, it was revealed that the initial coulombic efficiency of Ni–Co3O4 composite depends on the contact area between the Ni and the Co3O4.