Many attempts have been made to increase the energy density of Li-ion batteries using positive electrode materials such as LiMO₂ (LiCoO₂, LiNiO₂, LiNi_1/3Mn_1/3Co_1/3O₂, etc.), which have high charging voltages (typically ≥4.4 V). Various studies to understand the associated degradation mechanism and improve the capacity reversibility during high-voltage charge/discharge have been reported. Generally, the inter-terminal voltage of Li-ion batteries can be practically controlled by changing the electric potential of the positive electrode. Thus, the positive electrodes used for high-voltage charge/discharge are exposed to a high electric potential as well as a large change in the electric potential. In this study, we examined the influence of the operating voltage range on the charge/discharge characteristics, and found that the discharge cutoff voltage greatly influences the capacity reversibility during high-voltage charge/discharge. Additionally, we discuss the mechanisms that determine the capacity reversibility based on the results of electrochemical impedance analysis and soft X-ray absorption spectroscopy (soft XAS).

Experimental

 LiNi_1/3Co_1/3Mn_1/3O₂ powder, with a secondary particle diameter of 10 μm (Toda Kogyo Corp.), was used as the active material. Positive electrodes were fabricated from a mixture of 90 wt% LiNi_1/3Co_1/3Mn_1/3O₂, 5 wt% acetylene black, and 5 wt% polyvinylidene fluoride. The electrochemical characteristics of the samples were examined in coin cells with a Li-metal counter electrode. A 1.0 mol dm^-3 solution of LiPF₆ in ethylene carbonate + diethyl carbonate was used as the electrolyte. The cells were cycled at discharge-charge cutoff voltages of 2.5–4.6, 3.0–4.6, 3.8–4.6, and 4.2–4.6 V, at a current rate of 1 C. The Li-ion transfer characteristics were measured by alternating current impedance spectroscopy. The electronic structure of the LiNi_1/3Co_1/3Mn_1/3O₂was investigated using soft XAS at the beam line BL11 of Ritsumeikan University SR Center (Shiga, JAPAN).

Results and Discussion

 Figures 1a and b show the discharge capacity and discharge capacity retention versus cycle number for LiNi_1/3Co_1/3Mn_1/3O₂ cycled with different voltage ranges. As the discharge cutoff voltage was increased, the initial discharge capacity decreased, accompanied by an improvement in the discharge capacity retention. The retentions at the 143^rd cycle for LiNi_1/3Co_1/3Mn_1/3O₂cycled with 2.5–4.6, 3.0–4.6, 3.8–4.6, and 4.2–4.6 V, were 8, 37, 56, and 81%, respectively.

 Figure 2a shows the Nyquist plots for LiNi_1/3Co_1/3Mn_1/3O₂ cycled with different voltage ranges at an open circuit voltage of ~4.2 V after 3 cycles. The charge transfer resistances (R_ct) calculated from the semicircles in the lower frequency region were almost equal (5–8 Ω) regardless of the discharge cutoff voltage. From the Nyquist plots after 143 cycles (Figure 2b), the R_ct for LiNi_1/3Co_1/3Mn_1/3O₂ cycled with 2.5–4.6, 3.0–4.6, 3.8–4.6, and 4.2–4.6 V, were obtained as 3800, 300, 65, and 17 Ω, respectively. The increase of R_ct with the number of cycles was significantly suppressed as the discharge cutoff voltage was increased, resulting in the higher capacity retention observed in Fig. 1b. These results suggest that a stable interface is retained between the electrode and electrolyte when the charge/discharge voltage is limited to the high-voltage region only. The interface structure and the capacity reversibility mechanism will be discussed along with the electronic state of LiNi_1/3Co_1/3Mn_1/3O₂analyzed by soft XAS.

Acknowledgements This work is financially supported by the RISINGII project of the NEDO and METI, Japan.