Hysteretic Phenomena Between Charging and Discharging Process in LiNi 0.5 Mn 1.5 O 4

Introduction LiNi 0.5 Mn 1.5 O 4-σ (LNMO) cathode material for lithium ion batteries are promising because of the high-energy density of 686 Wh/kg. However, the large hysteresis between the charging and the discharging profiles induces the low energy efficiency. Andrej et . al . found a hysteresis in the phase transition of LiNi 0.5 Mn 1.5 O 4 during battery operation in a range; the solid solution and two phase coexisting reaction for low lithium concentration dominate the charging and discharging, respectively [1] . Meanwhile, the factors determining the extent of the overpotential during the charging/discharging have not been understood. In this study, we measured in-situ XAFS and XRD of LNMO and discussed the factors determining the over(under)potential. Experimental Commercial LNMO (Kojundo Chemical Laboratery Co., Ltd. Japan) was used in this study. The cathode was composed of LNMO, carbon black, and polyvinylidenefluoride with a weight ratio of 80:10:10. The cathode and lithium metal anode were separated by a polypropylene membrane separator. LiPF 6 in ethylene carbonate/diethyl carbonate (EC/DEC) was used as the electrolyte. The 2032-type coin cell with X-Ray windows was assembled in an argon-filled glove box. We first charged the cell to 4.85 V at a rate of 1.0 C and took 10 minutes for rest when the voltage got to 4.85 V. We subsequently discharged the cell at a rate of 1.0 C to 3.5 V. In-situ Ni K -edge XANES measurement was performed in a transmission mode at beamline 5S1 of Aichi Synchrotron Radiation Center. We took 80 sec. measurement with 40 sec. interval per scan and the patterns were analyzed by using “Athena” program [2] . In-situ XRD measurement was also conducted at beamline 5S1 of Aichi Synchrotron Radiation Center. Results and Discussion The charge/discharge curves show two reaction plateaus: ≃ 4.0 V and 4.7 V regions for LNMO as shown in figure 1. Previous studies reported that these plateaus are ascribed to Mn and Ni redox reactions, respectively [1] . The dashed line indicates the estimated OCV from 1/10C charge-discharge cycle performed separately. We found that both the gap between the CCV (closed circuit voltage) and estimated OCV (open circuit voltage) curve and the gradient of CCV during discharging are larger than that during charging. We anticipate that discharging process requires larger overpotential kinetically, since charging process is a lithium-extraction process through the large lattice spacing requiri