An in-depth understanding of (de)lithiation induced phase transition in electrode materials is crucial to grasp their structure-property relationships and provide guidance to the design of more desirable electrodes. By operando synchrotron XRD (SXRD) measurement, a reversible first-order phase transition is discovered for the first time during (de)lithiation of CeO₂ nanoparticles. The Li_xCeO₂ compound phase is identified to possess the same fluorite crystal structure with FM3M space group as that of the pristine CeO₂ nanoparticles. The SXRD determined lattice constant of the Li_xCeO₂ compound phase is 0.551 nm, larger than that of 0.541 nm of the pristine CeO₂ phase. The X-ray absorption spectroscopy analysis further reveals that the Li induced redistribution of electrons causes the inc...[ Read more ]

An in-depth understanding of (de)lithiation induced phase transition in electrode materials is crucial to grasp their structure-property relationships and provide guidance to the design of more desirable electrodes. By operando synchrotron XRD (SXRD) measurement, a reversible first-order phase transition is discovered for the first time during (de)lithiation of CeO₂ nanoparticles. The Li_xCeO₂ compound phase is identified to possess the same fluorite crystal structure with FM3M space group as that of the pristine CeO₂ nanoparticles. The SXRD determined lattice constant of the Li_xCeO₂ compound phase is 0.551 nm, larger than that of 0.541 nm of the pristine CeO₂ phase. The X-ray absorption spectroscopy analysis further reveals that the Li induced redistribution of electrons causes the increase in the Ce-O covalent bonding, the shuffling of Ce and O atoms, and the jump expansion of lattice constant, thereby resulting in the first-order phase transition.

Intrinsic understanding of the phase transitions that occur at nanoscale is crucial for perfecting the phase transition fundamentals and more efficient energy conversion. Using an operando Raman spectroscopy reveals the clear size-dependent trends emerging in (de)lithiation of TiO₂ nanocrystals. Meanwhile, the lithiation and delithiation induced phase transitions are asymmetrical regarding the size-dependence. The delithiation induced phase transition of 84nm nanocrystal is close to be a classic nucleation-growth process under the current density of 0.1C. Decreasing size or increasing rate deviates the delithiation induced phase transition from the classic one, and a continuous metastable solid solution phase pathway is preferred. However, the obvious deviation from the classic phase transition mechanism is already shown in the lithiation induced transition of 84nm nanocrystals. And the deviation trend is more significant when the nanocrystal size decreases. Decreasing size enhancing the Li⁺ diffusion kinetics may contribute to the size-dependent phenomenon. Moreover, a Li surface-segregation-stress model is proposed to explain the size effect, rate effect and the asymmetrical behavior between lithiation and delithiation. It shows these trends to be a consequence of surface lithium segregation induced stress effect, which is enhanced by decreasing size. The coupling behavior between the surface lithium concentration and the stress finally engenders the size-dependence.

Furtherly, ultrafast charging and long-life LIB anodes consisting of TiO₂-B/anatase dual-phase nanowires are reported. The anodes exhibit remarkable electrochemical performance with reversible capacities of ∼225, 172, and 140 mAh/g at current rates of 1C, 10C and 60C, respectively. They also present high capacity retention of >126 mAh/g and 93 mAh/g after 1000 cycles at 60C and 100C, respectively, potentially useful properties for high power applications. The electrochemical tests and operando Raman spectra present fast electrochemical kinetics in both Li⁺ and electron transports in the TiO₂ dual-phase nanowires than in anatase nanoparticles due to the enhanced Li⁺ diffusion coefficient and electronic conductivity of nanowires, and absence of a phase transition in the TiO₂-B phase.