Structure Regulation And Properties Of Ni-rich Cathode Materials For Lithium-ion Batteries

Ni-rich(NCM811)ternary cathode material has been considered as the preferred cathode material for high energy density and low cost lithium-ion batteries due to its large discharge capacity and low raw material cost.To solve the problems of higher Ni/Li mixing degree,circulation,poor stability,serious secondary crystal cracking,poor thermal stability,NCM811 cathodes materials with different components and structures were prepared and the materials were doped and coated.The relationship was gradually studied among the Ni/Li mixing,the crystal structure of multiphase composition,the structure of secondary crystal cracking degree and the electrochemical properties.(1)The Ni_0.8Co_0.07Fe_0.03Mn_0.1(OH)₂ precursor was synthesized by using co-precipitation and Li Ni_0.8Co_0.07Fe_0.03Mn_0.1O₂(Fe3-NCM871)was sintered at high temperature with lithium source.The result shows that the Fe3-NCM871 cathode achieved an initial capacity of 207.5 m Ah g^-1 at the C/10 rate between 2.8-4.3V,which is higher than that of NCM811(188.7 m Ah g^-1).Its rate capacity is 145.8 m Ah g^–1 and25.7 m Ah g^–1 higher than the NCM811 at a rate of 5C.Furthermore,the healthy state(SOH=80%)of Fe3-NCM811 electrode material reached 421 cycles,while NCM811reached only 184 cycles.The better cycling performance of Fe3-NCM871 cathode material is mainly attributed to the reduction of Ni/Li mixing after Fe replacing part of Co and the phase transition with a small degree of damage in the cycle process,which inhibits the crystal growth and prevents the cracking of cathode particles and protects the damage degree of particles in the charge-discharge cycle,thus improving the capacity retention rate and cycle life.The results of density functional theory(DFT)calculations show that the interaction of Fe-O bond is weaker than that of Co-O bond,and the Ni/Li exchange energy(E_ex)decreases,which inhibits the Ni/Li mixing and improves the electrochemical performance,especially the cyclic performance and thermal stability.It further indicates that the linear superexchange plays a leading role in regulating the Ni/Li mixing between transition metals.(2)Coating Fe PO₄ on the surface of NCM811 cathode produces LFP3@NCM811cathode with yolk-shell structure via a sol-gel method.It shows that LFP3@NCM811cathode material obtains the initial discharge capacity of the LFP3@NCM811 cathode is 218.8 m Ah g^-1 at the rate of 0.1C,it is due to the reduction of cation mixing for a certain amount of Fe²⁺/Fe³⁺or PO₄^3-doped on the surface of NCM811.Further,the discharge capacity of LFP3@NCM811 cathode achieves to 151.4 m Ah g^-1 at 5C between 2.8-4.5V,which is 15 m Ah g^-1 higher than that of the NCM811 cathode,a yolk-shell structure can improve the electronic conductivity,accelerate the transmission of Li⁺,so it improves the rate performance.LFP3@NCM811 cathode material still retains 97%of the initial discharge capacity after 100 cycles between 2.7-4.5V,while NCM811 cathode material retains 78%of the initial discharge capacity.In addition,the discharge capacity retention rates of LFP3@NCM811 and NCM811 cathode materials after 400 cycles are 86%and 63%,respectively.This is due to the fact that the yell-shell structure inhibits the secondary cracking,maintains the structural integrity of the cathode material particles in the long cycle process,and it protects the particle interior from harmful electrolyte damage.(3)The coating PI3-NCM811 cathode material was synthesized with MWCNT/PI by two-step(co-precipitation-imide)method.The result shows that PI3-NCM811cathode material has the discharge capacity of 196.6 m Ah g^-1 at the rate of 0.1C between 2.8-4.3V,which is little different from uncoated MWCNT/PI composite electrode material(201.1m Ah g^-1).The discharge capacity retention rates of PI3-NCM811 and NCM811 cathode materials are 90.6%and 64.8%after 500 cycles at2.8-4.3V,respectively.Furthermore,the difference(21.1%)of capacity retention rate between PI3-NCM811 and NCM811 under the condition of 2.8-4.5V became smaller compared with the the difference(25.8%)under the condition of 2.8-4.3V.This is because PI has good elasticity and toughness,which inhibits the secondary crystal cracking,maintains the structural integrity of the cathode material particles in the long cycle process,and protects the particles from being harmed by electrolytes,thus improving the cycle life.