First-principles Studies On High Voltage Cathode Material LiNi_0.5Mn_1.5O₄ For Lithium-ion Batteries

Towards a higher energy density for Li-ion battery,high voltage cathode materials have attracted much attention,among which spinel LiNi_0.5Mn_1.5O₄ is renowned for its high voltage?4.7 V vs.Li⁺/Li?,high specific energy,rate capability,and low cost.In-depth understanding of intrinsic properties is of great importance for the gradually improvement of LiNi_0.5Mn_1.5O₄ cathode materials.Density functional theory?DFT?based first principles calculations have been widely used in predicting the structural and electronic properties of materials,which reveals the underlying mechanism behind the electrochemical observations of lithium ion batteries.In this paper,employing DFT calculations,we investigated the bulk and surface properties of spinel LiNi_0.5Mn_1.5O₄ as cathode material for Li-ion batteries,i.e.,crystal/electronic structure,arrangement/migration of transition metal,oxygen defect/stability and cation doping/substitution.Specific contents are as follows:Ni/Mn ordered P4₃32 and disordered Fd3m phase of spinel Li Ni_0.5Mn_1.5O₄ show distinct electrochemical performances.Understanding the formation mechanism of P4₃32 and Fd3m phase could be a guidance for preparation of the specific Ni/Mn arrangement phase.Origin of Ni/Mn arrangement?order/disorder?in high-voltage spinel LiNi_0.5Mn_1.5O₄ is addressed,based on oxygen vacancies and cation doping.DFT calculations show that formation of 1:3 ordered Ni²⁺and Mn⁴⁺ions is energetically favorable compared to the disordered Ni³⁺and Mn³⁺ions caused by Ni aggregation in the stoichiometric P4₃32 phase.However,in oxygen deficientLiNi_0.5Mn_1.5O_4-?,the oxygen vacancies tend to diminish the valence discrepancy between the Ni aggregated and the ordered P4₃32 phases,making the former energetically competitive and consequently resulting in the disordered Ni/Mn distribution.Understanding the origin of Ni/Mn disorder also sheds light on the cation doping effect.Calculations show that Mg²⁺ion tends to replace Ni²⁺ion in ordered P4₃32 phase,and maintaining the Ni/Mn order.By contrast,Al doping promotes the Ni/Mn disorder,as Al³⁺ion prefers to substitute for Ni³⁺and Mn³⁺ions emerged in Ni/Mn disordered structure.Our findings rationalize the experimental observations,and further reveal that Ni/Mn arrangement could be controlled by adjusting the electronic structure of spinel LiNi_0.5Mn_1.5O₄ system.The high-voltage LiNi_0.5Mn_1.5O₄ cathode material suffers from severe problems during electrochemical cycling,such as surface structure distortion,oxygen loss, transition metal dissolution and electrolyte decomposition.Impact of high valence state cation Ti/Ta surface doping on the stabilization of spinel LiNi_0.5Mn_1.5O₄ cathode materials is systematically discussed basing on DFT calculations.Results suggest that Ti/Ta doping promotes Ni/Mn migration toward the formation of the rocksalt phase.Integrated net spin suggests that the valence state of transition metal ions,especially Ni,around Ti/Ta are slightly reduced in the fully charged state,resulting in a weakened surface oxidative property toward electrolyte.Both Ti/Ta doping and the consequent formation of the rocksalt phase not only stabilizes the oxygen frame of LiNi_0.5Mn_1.5O₄ by forming stronger metal-O bonds but also suppresses the oxygen evolution during cycling.The stabilized anion frame of oxygen further mitigates the dissolution of Mn ions from the lattice into the electrolyte.