In Situ TEM Investigation of Electron Irradiation Induced Metastable States in Lithium-Ion Battery Cathodes: Li2FeSiO4 versus LiFePO4

Energy storage and conversion in Li-ion batteries involve dynamic Li storage and transport through a series of electrochemical metastable states (or quasi-equilibrium configuration). Therefore, an investigation of these metastable states is helpful to fully understand the lithium storage mechanism. An accurate understanding of the storage mechanism is a key factor in the design and optimization of the next-generation high-performance batteries. Here, we report the results obtained from electron irradiation induced phase transitions in Li2FeSiO4 and LiFePO4 by electron microscopy. During prolonged irradiation, the crystalline Li2FeSiO4 particles experienced a transition from a monocrystalline structure to an amorphous phase, with a subsequent recrystallization process (monoclinic to orthorhombic phase). The fine structure of the electron energy loss (ELNES) spectra showed the electron beam-sensitive characteristic of Li2FeSiO4 that included the electron beam-induced mass loss (composition changes), formation of intermediate metastable states (Li2-x,FeSiO4), structural distortion/amorphization and valence state variation, all of which are much less prominent in LiFePO4 under the same flux of electron beam. These findings provide new insights into the structural stability of Li2FeSiO4 and LiFePO4 samples and is also important guidance in the characterization of electrode materials.