A study on the improvement method of the electrochemical performance and stability of the layered structural Ni-rich cathode materials for lithium ion batteries

Abstract

Since 1991, when Sony succeeded in commercializing lithium-ion batteries using LiCoO2 cathode materials, studies on layered structural cathode materials have been actively conducted. Over the past two decades, the research flow was focused on compensating for the disadvantages of LiCoO2 cathode materials (low capacity, toxicity, high price) by partially replacing Co with Ni to increase the capacity and by adding Mn or Al to improve the structural stability and cycle life. By controlling the composition of Li[NixCoyMnz]O2 (NCM, x+y+z=1) or Li[NixCoyAlz]O2 (NCA, x+y+z=1) ternary compounds, changes in fundamental physicochemical properties, electrochemical properties, and thermal stability were observed and the studies were conducted to find the optimum composition. In order to overcome the lower capacity of cathode materials compared to that of graphite anode materials (~ 370 mAh g-1), developing Ni-rich NCM (or NCA) cathode materials (Ni ≥ 60%) has been required. However the poor cycle life and thermal stability of Ni-rich cathode materials was inevitable due to the limited Mn (or Al) content. In order to overcome the aforementioned problem, our research group developed the core-shell (CS) structured NCM cathode materials that can realize both high capacity and high stability at the same time by selectively positioning the transition metals to the core portion (Ni-rich) and the shell portion (Mn-rich) in the spherical particle. Furthermore, core-shell with gradient (CSG) cathode material, which gradually change the transition metal concentration in the shell, and full concentration gradient (FCG) cathode material, which gradually change the transition metal concentration throughout the particle, have been developed. In particular, the cross-section of FCG cathode materials has a unique morphology in which rod-shaped primary particles extending from the particle center to the surface are laminated. Further, the longitudinal direction of the primary particles coincides with the (003) plane in the crystal structure. The unique structure facilitates the intercalation-deintercalation of lithium ions, thereby having excellent rate capabilities. In this study, the research was conducted on the extension line of the development of concentration gradient cathode materials in our research group, and the various approaches were introduced to improve the electrochemical properties and structural and thermal stabilities of the layered cathode materials with extremely high nickel composition above 80%. First, the study was conducted on the physicochemical properties, electrochemical property, rate capability, and thermal stability of FCG Li[Ni0.65Co0.08Mn0.27]O2 cathode material. It was reconfirmed that FCG cathode materials are superior to the conventional Bulk cathode materials in the initial charge-discharge coulombic efficiency, rate capabilities, cycle life, and thermal stability. Next, modification of the FCG structure was tried to increase the Ni composition to 80%. TSFCG Li[Ni0.8Co0.06Mn0.14]O2 cathode material was developed to maintain the high manganese content of the surface and to increase the total nickel composition in the same time; by dividing the concentration gradient into two regions, the first concentration gradient in the particle core was made gentle and the second concentration gradient in the shell was made steep. Since the concentration gradient cathode was mainly developed using NCM materials, CSG Li[Ni0.865Co0.120Al0.015]O2 (CSG NCA) cathode material was synthesized and characterized to confirm the applicability of the concentration gradient structure to NCA cathode materials. As a results, it was confirmed that the cycle life and thermal stability of the CSG NCA was superior to the conventional NCA cathode materials as already confirmed using NCM cathode materials. Furthermore, the reduced H2-H3 phase transition of CSG NCA during charging above 4.1 V was relieved, which resulted in decrease of the lattice contraction along the c-axis and thereby suppressed the microcrack formation. Finally, in order to further improve the structural stability of the Ni-rich NCM cathode materials, boron doping was studied using Li[Ni0.90Co0.05Mn0.05]O2 cathode material. Density functional theory confirmed that boron doping at a level as low as 1 mol% alters the surface energies to produce a highly textured microstructure that can partially relieve the intrinsic internal strain generated during the deep charging of Li[Ni0.90Co0.05Mn0.05]O2. Therefore microcrack was suppressed and cycle life was highly improved especially at the elevated temperature. Likewise, in this study, the highly effective methods are proposed to improve the electrochemical properties and stabilities of the Ni-rich NCM and NCA (Ni ≥ 80%) cathode materials for lithium ion batteries.