Reactive surface area of the Lix(Co1/3Ni1/3Mn1/3)O2 electrode determined by μ+SR and electrochemical measurements

Abstract

The self-diffusion coefficient of Li⁺ ions (D_Li) in the positive electrode material Li_x(Co_1/3Ni_1/3Mn_1/3)O₂ has been estimated by muon-spin relaxation (μ⁺SR) using powder samples with x = 1–0.49, which were prepared by an electrochemical reaction in a Li-ion battery. Here, since the implanted muons sense a slight change in the internal magnetic field due to Li-diffusion, μ⁺SR provides an intrinsic D_Li through the temperature dependence of the nuclear field fluctuation rate (ν) [Sugiyama et al., Phys. Rev. Lett., 2009, 103, 147601]. Both D_Li at 300 K and activation energy (E_a) were estimated to be ∼2.9 × 10⁻¹² cm² s⁻¹ and 0.074 eV for the x = 1 sample, ∼11.0 × 10⁻¹² cm² s⁻¹ and 0.097 eV for x = 0.70, and ∼8.9 × 10⁻¹² cm² s⁻¹ and 0.062 eV for x = 0.49, assuming that the diffusing Li⁺ ions mainly jump from a regular occupied site to a regular vacant site. The estimated D_Li was smaller by roughly one order of magnitude than those for Li_xCoO₂ in the whole x range measured. Furthermore, by making comparison with D_Li obtained by electrochemical measurements, the reactive surface area of the Li_x(Co_1/3Ni_1/3Mn_1/3)O₂ electrode in a liquid electrolyte was found to strongly depend on x particularly at x > 0.8.