Thermal evolution of LiCoO2 structure and Raman spectra below 400 °C.

Lithium cobalt oxide is a convenient model material for the vast family of cathode materials with a layered structure and still retains some commercial perspectives for microbatteries and some other applications. In this work, we have used ab initio calculations, x-ray diffraction, Raman spectroscopy, and a theoretical physical model, based on quasi-harmonic approximation with anharmonic contributions of the three-phonon and four-phonon processes, to study a temperature-induced change of Raman spectra for LiCoO2. The obtained values of shift and broadening for Eg and A1g bands can be used for quantitative characterization of temperature change, for example, due to laser-induced heating during Raman spectra measurements. The theoretical analysis of the experimental results lets us conclude that Raman spectra changes for LiCoO2 can be explained by the combination of thermal expansion of the crystal lattice and phonon damping by anharmonic coupling with comparable contributions of the three-phonon and four-phonon processes. The obtained results can be further used to develop Raman-based quality control tools.

Spatial Resolution of Micro-Raman Spectroscopy for Particulate Lithium Iron Phosphate (LiFePO4).

The fast-growing lithium battery industry needs quality control tools. Micro-Raman spectroscopy is a popular technique for structural characterization and can be used for impurity revealing. The problem is that the method resolution can be appropriately quantified for a sample with a simple planar geometry, like a single crystal. Much less studied are powders consisting of particles of irregular shape and sizes close to the wavelengths of the probing laser irradiation. In this work, we have examined a series of single particles of transparent lithium iron phosphate (LiFePO4) on a Si substrate. This model experiment revealed the significant spread of local optical properties, blocking properties of pores, and abnormal enhancement of Raman response from a bottom Si layer under some of particles. As the result, we can conclude that vertical resolution of micro-Raman spectroscopy for particulate systems with inhomogeneity of shape and structure should be described not quantitative, but qualitative, and the Raman probing of powder samples can be both multilayer and superficial.

Defects in Li4Ti5O12 induced by carbon deposition: an analysis of unidentified bands in Raman spectra.

Lithium titanate (Li4Ti5O12, LTO) has already occupied its niche as an anode material for high-power and long-lifespan lithium batteries, but some novel directions for basic and applied research are still open. One of the most promising approaches in improving its properties, e.g., electronic conductivity and rate capability, is based on controllable defect engineering. The "defects" may be intentionally introduced into LTO via doping, surface modifications, and the synergy between them. However, the defects, which have significant effects to the electrical and electrochemical properties, are usually extremely dilute. Reliable material characterizations are essential and challenging, but the instrumental tools for revealing dilute defects are still insufficient. Herein, detailed analyses on the surface or subsurface defects of carbon-coated LTO were performed using various material characterization methods. Raman spectroscopy has been identified as a unique tool for the probing of structural defects.