TiP2O7-coated LiNi0.8Co0.15Al0.05O2 cathode materials with improved thermal stability and superior cycle life

摘要

The co-precipitation derived LiNi0.8Co0.15Al0.05O2 cathode material was modified by a coating layer of TiP2O7 through an ethanol-based process.The TiP2O7-coated LiNi0.8Co0.15Al0.05O2 is characterized by Xray diffraction analysis,scanning electron microscopy and transmission electron microscopy to investigate the microstructure and morphology.The differential scanning calorimetry was employed to confirm the improved thermal stability.The electrochemical properties were evaluated by the constant-current charge/discharge tests.The TiP2O7 coating layer is effectively suppressing the structural degradation and ameliorating the surface status of LiNi0.8Co0.15Al0.05O2 particles,and the intrinsic rhombohedral layered structure of TiP2O7-coated LiNi0.8Co0.15Al0.05O2 was well maintained during the long-term cycling process,while the surface structure of pristine LiNi0.8Co0.15Al0.05O2 was degraded from rhombohedral R3 m layered structure to cubic rock-salt structure.The charged state Ni4+ions will easily transform into Ni2+when the electrolytes oxidized at the interface of cathode/electrolytes and formed the cubic rock-salt NiO type structure,and the cubic rock-salt structure without electrochemical activity on the surface of LiNi0.8Co0.15Al0.05O2 particles will finally accelerate capacity fading.The thermal stability and cyclic performances of the LiNi0.8Co0.15Al0.05O2 electrode were remarkably improved by TiP2O7 coating,the total amount of heat release corresponding to the intensity of thermal runaway were 1075.5 and 964.6 J/g for pristine LiNi0.8Co0.15Al0.05O2 and TiP2O7-coated LiNi0.8Co0.15Al0.05O2 respectively,the pouch shaped full cells that employed TiP2O7-coated LiNi0.8Co0.15Al0.05O2 as cathode were able to perform more than 2200 cycles at 25℃and more than 1000 cycles at 45℃before the capacity retention fading to 80%.