Synthesis And Performance Optimization Of Ni-rich Layered Cathode Materials For Lithium-ion Batteries

With the foreseeable exhaustion of fossil fuel and the expectation for reduction greenhouse gas emissions,developing sustainable green energy technologies and storage systems are considered as a feasible solution.As a potential candidate for the power source of electronic vehicles and stationary energy storage devices,rechargeable lithium-ion batteries（LIBs）have attracted tremendous research interest in view of its high energy density,low cost as well as long cycling life.But the energy density of current commercial lithium ion battery is hard to meet consumer demand for electric cars range.In the whole system of lithium ion battery,cathode material with a disadvantage of low specific capacity(<200 m A h g^-1)is the limiting factor for the rapid increase of lithium ion battery energy density.Therefore,developing the next generation of cathode materials with high specific capacity,high working voltage,cycle performance and safety performance has become an important research direction for lithium ion battery.Ni-rich layered oxides Li Ni_xCo_yMn（Al）_1-x-yO₂（x≥0.8）,possessing a reversible capacity of above 200 m Ah g^-1 with high working voltage（3.8 V vs.Li/Li⁺）,are considered as the most promising high energy electrode materials and successfully used for electric vehicles with a single-charge driving range over 500km.However,there are still several fundamental challenges impeding its large-scale commercialization,such as structural evolution among long cycling,interfacial side reactions between the electrode and electrolyte,Li⁺/Ni²⁺cation mixing and poor safety performance.Thus,it is urgent to solve the above problem for the development of Ni-rich layered oxide cathode materials.In this paper,different modified strategies,such as surface doping,coating modification and single crystal design,have been adopted to optimize the performance of Ni-rich layered cathode materials,and greatly improved its structural stability and long terms cycle stability.The structure-performance relationship of the Ni-rich layered cathode materials have been thoroughly invesgated by tuning the structure and morphology with different synthesis methods.The specific research contents are as follows:（1）Nickel-rich and cobalt-poor cathode materials Li Ni_0.83Co_0.06Mn_0.11O₂ doped with different Si contents have been prepared by a simple solid state reaction,which exhibit more efficient lithium ion transport path and more stable surface interface structure.The structure,microstructure and chemical environment characteristics of the synthesized products are analyzed by XRD,SEM,TEM and XPS.The existence form of Si doping in NCM83 material has been analyzed,as well as its influence on the crystal structure,microstructure and electrochemical reaction of the material.Compared with the undoped NCM83blank sample,the Si-doped primary particles show a radially oriented rod shape.Besides,silicon-rich Ni-Si-O mixture layer with a thickness about 5 nm is formed on the deep surface of the NCM83material.The results of electrochemical test showed that Si-1.0%sample shows excellent cycle stability and rate performance.After 50 cycles at 0.3 C rate in coin cells tested at 45℃,Si-1.0%sample shows the highest discharge capacity retention rate of 97.2%.As tested in the pouch-type full cells,Si-1.0%sample still shows the highest capacity retention rate of 96.3%after 500 cycles,which increased by 12.2%compared with 85.8%of the undoped NCM83 sample.（2）The open-channel（011）facet exposed Ce O₂ shell coated nickel-rich layered cathode material Li Ni_0.8Co_0.1Mn_0.1O₂ has been synthesized by a wet chemistry method.The Ce O₂ coating can protect the cathode material from the erosion of electrolyte and reduce the dissolution of transition metal ions.Moreover,the exposed crystal surface（011）of Ce O₂shell also improves the diffusion kinetics of lithium ions.The effect of Ce O₂ coating on the surface and bulk phase structure is investigated by charge-discharge test and EIS analysis.Electrochemical test results show that 1.0%NCM811 sample with Ce O₂ coating amount of 1 wt.%shows the highest initial discharge capacity of 186.5 m Ah g^-1 with a coulomb efficiency of 91.6%.After 300 cycles,the capacity retention rate of 89.2%is significantly higher than that of 79.8%for NCM811 samples without coatings.In addition,as tested at the rate of 10 C,1.0%NCM811 samples still have a discharge specific capacity of 152.1 m Ah g^-1 with a capacity retention rate of 84.3%for the discharge capacity of 180.4 m Ah g^-1 at 0.5 C,exhibiting enhanced rate performance.（3）The ultrahigh-nickel single-crystal Li Ni_0.92Co_0.06Mn_0.02O₂ materials have been successfully synthesized by a novel Li OH-Li NO₃-H₃BO₃ ternary molten-salt method.The physical and chemical characteristics of the products under different synthesis conditions are analyzed by XRD,SEM,TEM and XPS.And the effects of calcination temperature and boric acid additives on the nucleation growth,morphology and crystal structure of the single crystal materials are thoroughly investigated.The failure mechanism of single crystal material and polycrystalline agglomerated material has been compared by analyzing the electrode after cycling.As tested in the pouch-type full cells,the single-crystal ultra-high nickel cathode material SC-780 shows a relatively excellent long cycle capacity retention rate of 86.3%after 300 cycles,while the agglomerated material only has a capacity retention rate of 71.5%.After cycling,the morphology analysis of single crystal and polycrystalline samples shows that the single-crystal material SC-780 has better mechanical structural integrity,which has reduced the risk of intergranular fracture and internal microcracks propagation in polycrystalline samples.The structure and cycling stability of nickel-rich layered cathode materials have been greatly improved through different performance optimization approaches,which provide the experimental and theoretical foundation for promoting their wide application in lithium-ion batteries.