Effects Of Ions Doping On The Structure And Electrochemical Properties Of Li-rich Oxides

Adjustment of energy structure is one of the important tasks for developing China’s energy.At present,an innovative and improved energy storage material is greatly needed to cope with the limited supply of fossil fuels and effectively utilize renewable energy.Li-rich oxide cathode materials have received extensive attention due to their high energy density,but disadvantages such as the irreversible loss of lattice oxygen,capacity fading caused by spinel phase transition,and poor cycling stability during charging and discharging processes restrict their applications.In this paper,Na⁺or Fe³⁺doped nano-layered Li-rich oxides are synthesized by sol-gel method and high-temperature calcination.The microstructure and electrochemical properties such as cycle stability and rate capability are investigated by XRD,RAMAN,SEM,TEM,XPS,GCD,CV,EIS and So on.The correlations between irreversible reactions,redox reactions and the material structure,phase stability during the electrochemical process are also analyzed.（1）Na⁺replaces lithium sites of the layered ternary cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂and the cobalt-free layered lithium-rich oxide Li_1.2Mn_0.54Ni_0.26O₂,improving their electrochemical performances such as cycle performance,rate capability.The large radius Na⁺doping can effectively increase the lithium interlayer spacing and lithium ion diffusion coefficient,improve the layered structure and phase stability of the material,affect the chemical environment of local transition metal elements,slow down decomposition of Li₂Mn O₃ and irreversible oxygen loss during the activation process,reduce side reactions such as Mn dissolution and electrode polarization.1)The initial discharge capacities of Li_1.18Na_0.02Mn_0.54Ni_0.13Co_0.13O₂are 265.2m Ah g^-1（0.1C）and 223.0m Ah g^-1（0.5C）,respectively.Moreover,the discharge capacity of the material after 100 cycles of charging and discharging at a current density of 0.5C still keeps 204.4m Ah g^-1,and the capacity retention rate is 91.6%.The lithium ion diffusion coefficient is1.6367×10^-15cm²s^-1,which is an order of magnitude higher than that of the undoped material.2)The initial discharge capacities of Li_1.18Na_0.02Mn_0.54Ni_0.26O₂ are202.8m Ah g^-1（0.1C）and 150.5m Ah g^-1（2C）,respectively.Furthermore,the discharge capacity of the material after 100 cycles of charging and discharging at a current density of 2C still keeps 141.6m Ah g^-1,and the capacity retention rate is 94.1%.While it is only 33.9m Ah g^-1 for the undoped material after cycling under the same condition.The lithium ion diffusion coefficient of the doping material is 1.2152×10^-15cm²s^-1,which is also an order of magnitude higher than that of the undoped one.（2）Fe replaces transition metal ion sites of co-free layered lithium-rich oxide Li_1.2Mn_0.54Ni_0.26O₂,showing better rate capability,higher initial discharge capacity,and better electrochemical cycling performance compared to undoped oxides.The high Fe-O bond dissociation energy stabilizes the microstructure of the cathode material.The decreased charge transfer resistance and electrode polarization can enhance the electrochemical capacity and cycling performance of the electrode at high current density.The initial discharge capacities of Li_1.2Mn_0.54Ni_0.21Fe_0.05O₂are 236.7m Ah g^-1（0.1C）,216.8m Ah g^-1（0.5C）,193.3m Ah g^-1（1C）and190.7m Ah g^-1（2C）,respectively.And the Li_1.2Mn_0.54Ni_0.21Fe_0.05O₂shows excellent high current density tolerance.The lithium ion diffusion coefficient of the doping material is 5.8163×10^-16cm²s^-1.The increase of the charge transfer resistance value of the doping material after cycling is only 74.6Ω,which is much smaller than that of the undoped one（201.5Ω）.