Study On Structural Regulation And Electrochemical Properties Of High Voltage Lithium Nickel Manganate Positive Material

Environmental pollution is becoming more serious while non-renewable resources continue to be depleted with each passing day.It is an urgent matter to develop renewable resources to solve the problems of energy shortage and ecological security.As an energy storage device,lithium-ion batteries（LIBs）are widely utilized because of their high safety,low price,high-energy density,long cycle life and low self-discharge.In recent years,frequent research and development has taken place regarding high energy density and high-voltage LIBs for electric vehicles and hybrid electric vehicles.The Li Ni_0.5Mn_1.5O₄（LNMO）positive material has great advantages in voltage（4.7 V）and energy density(650Wh kg^-1).However,its structure is unstable at high voltage,resulting in occurring serious side reactions on its surface and transition metal dissolution.Moreover,LNMO occurs two-phase reaction during the charging and discharging process,resulting in lattice mismatch,mechanical stress and large changes in cell volume.In addition,the electrochemical performance of LNMO is poor at high C-rates.Therefore,it is necessary to regulate the structure of LNMO to improve the electrochemical performance of the material.The LNMO positive material is optimized using a surface phase control strategy.The effects of surface phase control on the structure and electrochemical properties of LNMO are studied.In addition,the effects of control temperature on the structure and electrochemical properties of LNMO are also studied.By regulating the surface phase of LNMO,phase transition occurs at the surface of LNMO.The spinel phase is transformed into the layered phase and rock-salt like phase,with the latter being induced by Li/Mn exchange.Theoretical calculations show that the diffusion energy barrier of Mn ions becomes high when Li/Mn exchange exists in LNMO,which inhibits the dissolution of Mn ions from the material.At the same time,the rock-salt like phase induced by Li/Mn exchange is an inert material that protects LNMO from corrosion due to the electrolyte.However,compared with the parent material,the electrochemical activity of the rock-salt like phase is relatively poor,which is not conducive to charge transfer.Nevertheless,the layered phase has a good charge transfer ability,which compensates for the negative impact of the rock-salt like phase on LNMO.Through process optimization,it is found that the amount of Mn ions in the Li layer on LNMO first increased and then decreased with the increase in control temperature.By regulating the layered phase and rock-salt like phase at the surface of LNMO,LNMO is able to achieve excellent electrochemical performance.At 0.2 C and 25℃,the first specific discharge capacity is 129.3 m Ah g^-1.After 500 cycles at 1 C and 25℃,the capacity retention rate is 83.4%.LNMO is doped with Co³⁺ and coated with TiO₂ either separately or simultaneously via coprecipitation.The effects of Co³⁺doping and TiO₂ coating on the structure and electrochemical properties of LNMO are studied.Co³⁺is doped into the octahedral interstitial 16c sites of LNMO.High-temperature calcination causes TiO₂ to react with the surface of the positive material and a small amount of Ti⁴⁺is doped into the octahedral interstitial 16c sites of the LNMO surface layer.The structure of LNMO changes due to the regulation of 16c sites,which inhibits the phase transition behavior of LNMO during the charging and discharging process.The two-phase reaction transforms into a solid solution reaction.It prevents the lattice mismatch and mechanical stress in LNMO particles from occurring and reduces the change in cell volume for LNMO during the charging and discharging process.This is conducive to the diffusion of lithium ions in LNMO.The TiO₂ coating prevents direct contact of the LNMO surface and the electrolyte,which slows down the effect of corrosion from the electrolyte on the positive material.Co³⁺doping and TiO₂ coating successfully regulate the interstitial 16c sites of LNMO,inhibit the dissolution of Mn ions,stabilize the structure of LNMO and improve the electrochemical performance of LNMO.The capacity retention rate is 89.9%after 500 cycles at 1 C and 25℃.The effects of Cu²⁺ doping and different doping amounts on the structure and electrochemical properties of LNMO and the effect of the TiO₂ coating on Cu²⁺-doped materials are studied.It is proven with FIB-SEM that Cu²⁺can be doped into the LNMO bulk phase using coprecipitation.Cu²⁺occupies the octahedral16c sites of LNMO.Because the interstitial 16c sites and the 16c sites which coincide with the vertical direction of the 16d sites are occupied by Cu²⁺,the structure of LNMO is more complete.Therefore,the two-phase reaction that takes place during charging and discharging is inhibited and the change of cell volume decreases,improving the structural stability of LNMO.Theoretical calculations show that Cu²⁺doping increases the energy required for the dissolution of Mn ions,which is conducive to inhibiting the dissolution of Mn ions and resisting to structural decline.Doping Cu²⁺improves the electronic conductivity of LNMO,which is beneficial for improving the rate performance and cycling performance of LNMO.The specific discharge capacity is 108.4 m Ah g^-1 at 20 C.The capacity retention rate is 90.3%after 500 cycles at 1 C and 25℃and the capacity retention rate is 96.8%after 100 cycles at 1 C and 55℃.