Investigation on electrochemical reaction mechanisms of lithium-ion battery electrode materials by transmission electron microscopy

Abstract

Having high energy densities, lithium ion rechargeable batteries are widely used for portable electronic devices, and they are still the subject of intensive investigation because of the possibility on a wide range of future applications including battery-only electric vehicles. To identify the electrochemical reaction mechanisms of lithium-ion battery electrode materials and suggest the appropriate directions for the improvement of the performance, there have been numerous researches using various analysis methods. In particular, transmission electron microscopy (TEM) studies can provide significant information on the electrochemical reaction mechanisms of lithium-ion battery at nano-scale, but there have been a lot of limitations due to the requirements of special configuration and sampling techniques that are compatible to the lithium-ion battery system. Here, we have focused on the better approaches to TEM investigation of lithium-ion battery electrode materials that can well describe and explain the electrochemical reactions in the lithium-ion battery. We improved TEM investigation techniques and applied properly for specific purpose of studies on lithium-ion battery electrode materials. First, to reveal the origin of fast capacity fading shown in Ni-rich NCM cathode material, we directly compared the inside of secondary particles between NCM 1/1/1 and 8/1.5/0.5 materials by using FIB sampling technique. The observation on the cross-section of the same secondary particles before and after the electrochemical cycles clearly showed the micro-cracks between primary particles and the severe surface degradation in Ni-rich NCM material, which should be the major source of fast capacity fading. We revealed that the instability of Ni3+ ions in Ni-rich NCM material may result in the severe surface degradation by causing the structural transformation and transition metal-ion dissolution into the electrolyte from the surface of primary particles. Also, it was identified that the low-angle grain boundaries in a primary particle can also act as the surface of primary particles, which even lead to the development crack in the primary particle. Furthermore, we improved the ex situ TEM experimental method to track the phase evolution of SnO2 anode material and reveal the possibility of reverse conversion reaction and the origin of the additional capacity, which has been controversial. SnO2 particles dispersed on a carbon film-coated copper TEM grid were placed with the conventional SnO2 slurry electrode in a coin battery cell and electrochemically (dis)charged toward specific voltages to investigate the phase evolution of the same particles during the first full cycle by TEM. By this method, we demonstrated that reactions of Li2O phase contribute to the extra capacity and the reverse conversion reaction of SnO2 hardly occurs in the real battery system. Finally, we applied the in situ TEM experimental technique to identify the actual lithium-ion diffusion paths and the lithiation mechanism of tunnel-structured MnO2 nanowire. By setting up the special configuration using a carbon film-deposited copper TEM grid, we verified the facile diffusion through the unique tunnels in the α-MnO2 nanowire rather than through the side wall of the nanowire. In addition, we revealed the precise lithiation mechanism of α-MnO2 material up to the full-lithiation procedure. It was exposed that MnO intermediate phase appears during the conversion reaction of α-MnO2 material. It was inferred that the MnO phase is developed at the original position of Mn ions in the octahedral site of the oxygen framework with the partial collapse of the tunnel structure. Moreover, we revealed that lithium and oxygen ions well maintain the 1-D array along the tunnels even after the full lithiation. These studies suggest the robust TEM experimental methods which are widely applicable to the battery analysis. Furthermore, the studies provide the further profound information on the electrochemical reaction mechanisms of various promising lithium-ion battery electrode materials.