Probing thermally induced decomposition of delithiated Li _1.2-xNi_0.15Mn_0.55Co_0.1O₂ by in situ high-energy x-ray diffraction

Safety of lithium-ion batteries has been a major barrier to large-scale applications. For better understanding the failure mechanism of battery materials under thermal abuse, the decomposition of a delithiated high energy cathode material, Li_1.2-xNi_0.15Mn_0.55Co _0.1O₂, in the stainless-steel high pressure capsules was investigated by in situ high energy X-ray diffraction. The data revealed that the thermally induced decomposition of the delithiated transition metal (TM) oxide was strongly influenced by the presence of electrolyte components. When there was no electrolyte, the layered structure for the delithiated TM oxide was changed to a disordered Li_1-xM₂O₄-type spinel, which started at ca. 266 °C. The disordered Li_1-xM ₂O₄-type spinel was decomposed to a disordered M ₃O₄-type spinel phase, which started at ca. 327 °C. In the presence of organic solvent, the layered structure was decomposed to a disordered M₃O₄-type spinel phase, and the onset temperature of the decomposition was ca. 216 °C. When the LiPF₆ salt was also present, the onset temperature of the decomposition was changed to ca. 249 °C with the formation of MnF₂ phase. The results suggest that a proper optimization of the electrolyte component, that is, the organic solvent and the lithium salt, can alter the decomposition pathway of delithiated cathodes, leading to improved safety of lithium-ion batteries. © 2014 American Chemical Society.