In order to estimate a reliable reaction area (a_r) between cathode and liquid electrolyte in a Li-ion battery, we wish to propose a muSR experiment on a charged-state cathode material, LxiNiO2 to determine an intrinsic diffusion coefficient of Li ions (D_Li) as a function of x. Then, we plan to compare such D_Li with the diffusion coefficient estimated by electrochemical measurements in a liquid electrolyte. As a result, we could obtain the relationship between a_r and x, although a_r is assumed to be independent of x at present. The proposed experiment, thus, provides crucial information on the change in surface properties of cathode with the state of charge.