Clarification of the mechanism of hysteretic phenomena between charging and discharging in LiNi 0.5 Mn 1.5 O 4-σ under a closedcircuit voltage is crucial for achieving a high energy efficiency. We performed in situ X-ray diffraction to understand the relationship between the overpotential and the kinetically preferred state of LiNi 0.5 Mn 1.5 O 4-σ during charging and discharging. Very different crystal structures were observed for the cathode during charging and discharging, especially in the low state-of-charge single-phase reaction region. We consider that charging can be a lithium extraction process through the large lattice spacing, and lattice shrinkage occurs just after lithium diffusion, which enables a continuous reaction. Furthermore, we consider that discharging corresponds to the continuous insertion of lithium, requiring successive "forcible" lattice expansion for the reaction to occur. A kinetic reaction model for the cathode was proposed based on these results to explain the hysteresis for charging and discharging under a closed-circuit Spinel-type cathode materials containing Ni ions (LiNi 0.5 Mn 1.5 O 4−σ (σ ≥ 0): LNMO) are attractive candidates for lithiumion batteries because they exhibit a high theoretical energy density and high operation voltage, and they are cost-effective and nontoxic.1-4 Disordered LiNi 0.5 Mn 1.5 O 4−σ (d-LNMO) is known to have a superior rate performance, 5 and to react via a single phase, mixed/unresolved phases, and two structural phase processes with different lattice parameters according to the lithium concentrations. [6][7][8] Thermodynamically, a clear phase separation may be undesirable in this material because of the coherency strain, which causes defect formation. 9-11Kim et al. reported a large hysteresis for charging and discharging in d-LNMO and ordered LNMO (o-LNMO) under a closed-circuit voltage (CCV).5 This hysteresis depends on the deviation of the overpotential from the open-circuit voltage (OCV), especially during high rate operation, 5 which significantly reduces the energy efficiency. However, the hysteresis in d-LNMO is much less than that in o-LNMO. This hysteresis depends on the deviation of the overpotential from the open-circuit voltage (OCV). The deviation is high during discharging.Singer et al. employed X-ray diffraction (XRD) to investigate the hysteretic phenomena for the phase transition of d-LNMO.9 They directly observed the hysteretic phenomena in crystal structure at the single nanoparticle level. Their study certainly provides new avenues for understanding the fundamental reaction mechanism. Meanwhile, the data indicate that most of the phase transition proceeds during the last stage of charging and discharging, even though the operation rate is extremely small (1/4 C). They discussed the large discrepancy between the electrochemical data and XRD patterns due to the surface-dependent model based on the fact that electrochemistry is surface dependent and XRD patterns are volume-dependent. However, their explanation does not explain the...

In-situ X-ray diffraction measurements were performed on LiFePO 4 during charging and discharging using synchrotron radiation to elucidate the origin of the hysteresis in the kinetic mechanisms of the charging and discharging processes. The experimental results clearly showed that fewer intermediate phases were formed during the charging process than the discharging process, in keeping with previous studies. Further, during the discharging process, a Li-containing Li-lean phase and a Li-defective Li-rich phase with large lattice spacing were generated kinetically in the initial stage. Based on these results, we propose a new model for the hysteresis between the charging and discharging processes. Lithium iron phosphate [Li 1-x FePO 4 (0 ≤ x ≤ 1)] is a promising cathode material for Li-ion batteries. The charge/discharge processes of this material are well known to involve a two-phase Li insertion/extraction reaction between the structurally very similar LiFePO 4 and FePO 4 phases, both of which belong to the Pnma space group.1-3 Previously, material engineering approaches to improving the LiFePO 4 performances have been proposed, resulting in significant improvements. [4][5][6] There have been several studies focusing on the nonequilibrium intermediate phase. [7][8][9][10] We believe that elucidating this kinetic-state transition will further improve the electrochemical performance of LiFePO 4 and lead to the development of batteries based on LiFePO 4 cathodes, because the kinetically preferred states determine battery performance. Understanding the hysteretic phenomena between charging and discharging are especially important for using, controlling, and designing LiFePO 4 -based lithium ion batteries.Recently, Takahashi et al. clearly showed that the metastable intermediate phase is formed only during the discharge process based on the results of in-situ X-ray diffraction (XRD) measurements.11 However, the kinetics of the formation process of the intermediate phase remain unclear; the discussion did not focus on the formation process of this intermediate phase. Michael et al. also performed in-situ XRD measurements to determine the asymmetry of the constituent intermediate phases and firstly cleared the differences between the kinetic formation processes that occur during charging and discharging. 12Their data are of great value for understanding the differences. Phase transformation in the lateral direction of the electrode from outside the X-ray beam spot to the center of the spot dominated the reaction in the paper, as they explained. The average electrode voltage changes due to the progression of charging or discharging. It is considered about their cell structure that the reaction at the beam spot progresses depends on the potential difference as the driving-force. Even when the operation rate was very small (0.1 C), the reaction proceeded rapidly in the last stage of charging process. This indicates that their study focused on a system dominated kinetically not only by the materials' inherent kinetic stat...