Na-doped LiNiO2 layered cathode material for Li -ion batteries

Abstract

With growing needs for renewable energy storage systems, Li-ion batteries (LIBs) have become the most widely used as a power source for portable electronic devices such as mobile phones, laptops and Electric vehicles (EV). Most widely commercialized cathode material is LiCoO2 which has R-3m layered structure. Although it shows good structural stability, due to structural instability in high operating voltage (over 4.2V), its reversible capacity is limited to about 150mAh g-1 which does not undergo severe structural change. As increasing needs for high capacity battery materials, Nickel-rich cathode materials are promising candidates for alternatives to commercialized cathode materials. LiNiO2 is intensively studied for several decades because of its structural similarity and higher reversible capacity than LiCoO2. Although LiNiO2 is able to perform high reversible capacity, reversibility of the material is critical problem which comes from cation mixing, transition metal dissolution, residual lithium on surface and phase transition during cycling. Those problems are closely related to instability of Ni3+ ion in octahedral site of R-3m structure. According to crystal field stabilization energy (CFSE), lone pair electron in eg is unstable so Ni3+ ion is easily reduced into Ni2+ ion. This instability induces the non-stoichiometric structure which causes collapse of local structure. Transition metal, Ni2+ ion in transition metal layer, migrates into Li layer and this phenomenon is called cation mixing. Cation mixing causes decrease in rate capability during cycling. Migrated Ni ions easily make spinel and rock-salt structure on near-surface region of the particle which hinders Li ion diffusion. Furthermore, unstable Ni3+ ions on near-surface region which is easily attacked by H2O or CO2 are reduced into Ni2+ and make LiOH or Li2CO3 on surface. LiOH on the surface is transformed to Li2CO3 accompanying the formation of H2O. Water accelerates decomposition of LiPF6 and produces HF and LiF on the surface which is known as insulating layer. There have been many tries to solve various problems of Ni-rich materials. One effective way to enhance the performance and stability of Ni-rich material is coating them with stable materials such as metal fluorides (AlF3, LaF3 and MgF2), electrochemically inactive metal oxides (ZrO2, V2O5, MgO) and metal phosphates (AlPO4, CoPO4). These coating layers protect active materials by inhibiting direct contact with electrolyte, water or HF attack. Substituting elements such as Co, Al, Ti, Ga, Mn, Y and Fe with Ni have effects on enhancing structure stability. In my research, I tried to enhance electrochemical property of LiNiO2 by substituting Li with Na ion. We confirmed that substitution of Na with Li enhanced cycle performance of LiNiO2. Substitution of Na with Li on LiNiO2 reduced cation mixing during electrochemical cycling giving structural stability by Rietveld refinement. Also, it reduced nickel ion dissolution into electrolyte during cycling. Not only enhancing electrochemical property, storage characteristic is also enhanced by Na doping on LiNiO2.