No abstract

In situ time-resolved X-ray absorption near-edge structure (XANES) and X-ray diffraction (XRD) measurements are applied to track the phase transition processes of Li x Ni 0.5 Mn 1.5 O 4 , which is one of the most promising positive electrode materials for lithium ion batteries with its high redox potential of 4.7 V vs. Li/Li + and good rate capability. Thanks to the high time resolution brought by a strong synchrotron X-ray beam, the XANES and XRD measurements separately capture the three phases involved in nearly the same potential region, namely LiNi 0.5 Mn 1.5 O 4 (Li1), Li 1/2 Ni 0.5 Mn 1.5 O 4 (Li1/2) and Ni 0.5 Mn 1.5 O 4 (Li0), and the phase transition kinetics under fast charging-discharging conditions is elucidated. The detailed kinetic analysis shows that the phase transitions are apparently expressed by the first-order kinetics and that the transition between the Li1 and Li1/2 phases is faster than that between the Li1/2 and Li0 phases, which leads to kinetically asymmetric behavior of the LiNi 0.5 Mn 1.5 O 4 electrode. The obtained rate constants can be used to characterize and optimize the rate capability of the electrode. † Electronic supplementary information (ESI) available: Neutron diffraction pattern of the sample, charge-discharge curves of the electrode at a rate of 0.1 C, XANES spectrum of Li 0.5 Ni 0.5 Mn 1.5 O 4 , current and nominal composition changes during potential step charging and discharging, nominal composition changes obtained by current and XANES spectrum responses during potential step charging and discharging, fraction changes of Li1, Li1/2 and Li0 phases obtained by XRD during potential step charging and discharging.‡ Present address:

Herein, we investigate the formation of a cathode electrolyte interphase (CEI) by electrolyte oxidation on a LiNi x M 1−x O 2 (x > 0.5; M, transition metal) layered oxide (Ni-rich) cathode and compare this phenomenon with a Li-rich layered oxide (Li-rich) cathode. Our investigations focused on two electrochemical properties, the potential and kinetics of electrolyte oxidation, studied using hard X-ray photoelectron spectroscopy (HAXPES), soft X-ray absorption spectroscopy, and density functional theory calculations. HAXPES revealed that a thicker CEI formed on the Ni-rich cathode compared to that on the Li-rich cathode, despite the operation potential of the Ni-rich cathode being lower than that of the Li-rich cathode. Thus, the Ni-rich cathode induces the CEI formation through active oxidation of the electrolyte during charge−discharge cycles. The electronic state of the Ni-rich cathode indicates that the antibonding hybrid orbital of the transition metal 3d−O 2p corresponds to the lowest unoccupied molecular orbital energy level, that is, the hole, and lies near the highest occupied molecular orbital energy level of the electrolyte. In addition, the hole concentration in the charged state was found to be significantly increased, in comparison to other active materials, which promotes oxidization of the electrolyte.

A mixture of metallic vanadium, tungsten and molybdenum powder was dissolved in a hydrogen peroxide solution, to yield a polyvanadate sol containing W and Mo. A thin film of V1-x-y W x Mo y O2 was fabricated on a fused silica substrate by spin-coating using such a sol followed by reduction in hydrogen and successive annealing in a nitrogen atmosphere. Thin films in the system V1-x-z W x Ti z O2 were similarly prepared using a sol containing W and Ti. All the films obtained in this study showed a clear thermochromic switching performance with a large transmittance change in the infrared region. For V1-x-y W x Mo y O2, the switching temperature (T t) was lowered by doping at approximately T t=T t0 (=67°C)-ax-by, where a and b are the decrease rates of each individual dopant (about 1920 and 870°C, respectively, for 0≦x, y≦0.03). Doping of Ti had little effect on T t, but markedly reduced hysteresis of the transition. We demonstrated that a double-doped film of V1-x-z W x Ti z O2 (x=0.018 and y=0.10) showed thermochromic switching almost without any hysteresis at a temperature as low as 45°C.