Investigation of nano-scale surface modification of Ni-rich cathode materials for lithium ion batteries

Abstract

Efforts have been devoted to the development of thermal stability and energy density in lithium ion batteries which have widely been applied as power sources for portable electronic devices, and electrical vehicles (EVs). Ni-rich systems (x > 0.6 mole in LiNixCoyMn1-x-yO2) have been considered as promising candidates as a cathode material for such applications, due to its high specific capacity and low cost. However, it has thermal instability and poor cycle life. Arising safety problem such as thermal instability will be inevitable by high Ni-contents for high capacity and power because of the occurrence of cell explosion with exothermic reactions in charged batteries at elevated temperature. Much research has been carried out to improve such structural and thermal stability of Ni-rich cathode materials via surface coating (Al2O3, AlPO4, ZnO etc.) and core-shell structure. Traditionally, research in the field has been reported that the coating can prevent their direct contact with the electrolyte solution, suppress the phase transitions, improve the structural stability, and decrease the disorder of cation in the crystal lattice. The surface structure of the electrode materials will play a more and more important role in their electrochemical performance due to directly influence electrical and physical character. Investigation of structural transformation at the surface of layered cathode materials in high temperature environment, such as Ni-rich system LixNi1-y-zCoyMnzO2 (that is, NCM), LiNi0.8Co0.15Al0.05O2 (that is, NCA) materials, have been accomplished by XAS (X-ray absorption spectroscopy), EELS(electron energy loss spectroscopy), atomic-scale STEM (Scanning transmission electron microscopy), in-situ TR-XRD (time resolved X-ray diffraction), during cycling. However, the lack of a thorough understanding of before and after surface treatment (coating or doping) still remains as an unsolved assignment. Structural degradation of Ni-rich cathode materials (LiNixM1-xO2; M = Mn, Co, and Al; x > 0.5) during cycling at both high voltage (>4.3 V) and high temperature (>50 °C) led to the continuous generation of microcracks in a secondary particle which consisted of aggregated micrometer-sized primary particles. These microcracks caused deterioration of the electrochemical properties by disconnecting the electrical pathway between the primary particles and creating thermal instability owing to oxygen evolution during phase transformation. To develop the safety and electrochemical properties, I suggested a new treatment concept and coating material. Firstly, We report a new concept to overcome those problems of the Ni-rich cathode material via nanoscale surface treatment of the primary particles. The resultant primary particles’ surfaces had a higher cobalt content and a cation-mixing phase (Fm3-m) with nanoscale thickness in the LiNi0.6Co0.2Mn0.2O2 cathode, leading to mitigation of the microcracks by suppressing the structural change from a layered to rock-salt phase. Furthermore, the higher oxidation state of Mn4+ at the surface minimized the oxygen evolution at high temperatures. This approach resulted in improved structural and thermal stability in the severe cycling-test environment at 60 °C between 3.0 and 4.45 V and at elevated temperatures, showing a rate capability that was comparable to that of the pristine sample. Secondly, introducing a glue-role thin nano-filler layer consisting of a middle-temperature spinel-like LiCoO2 phase between the grains, leads to significantly improved grain’s adhesion ability in the aggregated particle compared to pristine particle that exhibits severe pulverization of the grains under the pressure and cycling. One of the most striking performances is that the cathode treated with the glue-layer exhibits highly stable cycling performance at 60°C without using any electrolyte additives. Surprisingly, this performance is quite comparable to that at room temperature (89%). This unprecedented performances can be attributed to the increased binding energy between grain boundaries with the glue-layer.