The Preparation And Structure-activity Relationship Of Precursors For Cathode Material LiNi_0.8Co_0.1Mn_0.1O₂

As a convenient and portable chemical power source,lithium-ion batteries（LIBs）have been used in multifarious mobile devices such as laptops,mobile phones and electric vehicles due to its high discharge capacity,stable cycle performance and lacking memory effect.To meet the demands of electric vehicles for high energy density and rapid charge/discharge capabilities,it is necessary to develop advanced LIBs cathode materials,and the transition metal layered oxides Li Nix Coy Mn1-x-y O2 are generally considered as one of the most promising candidates for LIBs cathodes,due to their high energy density and low cost.Such excellent performances of the ternary cathode materials could be attributed to the synergistic effect of nickel,cobalt and manganese,in which nickel provides higher specific capacities,cobalt improves the conductivity of the cathodes,and manganese enhances the structural stability.In order to further increase energy density and reduce cost,the NCM811 ternary material is selected as the cathode material,and the contents of nickel element are greatly increased accompanied with the decrease of other two ingredients,which brings higher reactivity with lower thermal and structural stability.Some strategies including element doping and surface coating have been adopted to address these issues,based on the mechanisms that inhibited phase transition and increased corrosion resistance during charge and discharge processes.Although the above-mentioned methods have achieved some results,the loss in capacity occurs to pay for the modification.In addition,the pulverization of secondary microspheres caused by intergranular stress is not could be improved by the traditional modification tactics.Previous researchers have identified that the reasonable design of configuration and structure of secondary particles of cathodes is beneficial to promote the structural stability and fully release the potential of materials.Currently,the large-scale preparation route of the NCM811 type cathode material includes synthesized precursors by a co-precipitation method and the followed high-temperature lithiation.In contrast to harsh reaction conditions during high-temperature solid-phase reaction,the facile conditions of co-precipitation make it easy to control the reaction process,thus adjusting the reaction conditions in co-precipitation process allows effective control of the precursor configuration and structure,and these features would pass to the final cathode materials.Therefore,the material design purposes are achieved through the above-mentioned steps.Understanding how the configuration and electrochemical performances of the NCM811 type cathode is affected by its precursors is essential to design ideal precursors through co-precipitation method.In the first part of the study,it was confirmed that many constructive features of precursors including grain size,grain thickness and arrangements pass to the synthesized cathodes through high-temperature lithiation,thus eventually affecting their electrochemical properties.Furthermore,the activated surfaces were confirmed for primary single grains to be（012）and（104）facets,which provide fast transport channels for lithium ions,and the cathode material formed by primary grains exposed（104）and（012）facets and orderly arranged along[001]direction could exhibit excellent electrochemical performances(the specific discharge capacity at 1 C could reach 184.5 m Ah g^-1,and the capacity retention rate is 89.3%after100 charge-discharge cycles).Such a special structure is formed originally from the orderly arrangement of precursor nanosheets with greater thicknesses.Therefore,promoting the layered growth and the thickness increase of precursor primary nanosheets is beneficial to synthesize high-performance cathode materials.The effects of the main reaction conditions in co-precipitation on precursor characteristics was examined.First,a thermodynamic modeling of TM（NH3）n²⁺-NH3-OH^--H2O reaction system was performed based on the related reaction equilibriums and the general conservation of mass equation to analyze the synergic effects of p H value and ammonia concentration on the metal-ammonia complex ions.Next,the synergic effects of p H and ammonia on the precursors was examined through designing orthogonal experiments.Then,the single factor experiments of precursor preparation were carried out to explore the effects of the p H,ammonia concentration,impeller rotational speed and feeding speed on particle size distribution,specific surface area,grain growth and grain arrangement,and the electrochemical properties of cathodes synthesized by above precursors were also tested.Finally,these results confirm the key role of both p H value and ammonia concentration in co-precipitation process.However,the influence mechanisms of p H and ammonia concentration on the precursor growth are not clear,and the crystal growth process needs to be studied in detail.To exhaustively investigate the mechanism of precursor growth affected by p H value and ammonia concentration,the crystal structure and surface morphology of precursors were characterized separately by XRD and SEM as a function of reaction time at different p H and ammonia concentration.The first-principles calculations were used to analyze the surface properties of primary grains and adsorption behaviors of metal ammonia complex ions on crystal surface.The results indicate that the growth of primary grains is anisotropic,and the（001）facets with low surface energy could grow slower and determine the crystal shape forming hexagonal nanosheets.The regulation of grain growth by p H and ammonia concentration mainly depends on changing the charged properties of crystal faces and the concentration and composition of metal ammonia complex ions.In addition,it has been proved that the formation process of precursor secondary particles mainly includes the formation of crystal nucleus,anisotropic growth of grains,aggregation and growth of flaky grains,self-assembly and growth of flaky grains,etc.Among them,the study on the influence mechanism of p H and ammonia concentration on layered growth and ordered layered stacking of flaky grains has important guiding significance for the synthesis of spherical precursors.Based on a deep understanding of the precursor growth mechanism,the primary grain growth process is controlled by adding anionic surfactant SPA,which would specifically adsorb on the unsaturated metal atoms exposed on the（010）facet,hindering the adsorption of metal ammonia complex ions on the（010）facet and crystal growth along[010]direction.In addition,SPA keeps away from the（001）crystal facet due to the electrostatic interaction,which provides favorable conditions for the adsorption of metal ammonia complex ions on the（001）facets and the crystal growth.Using SPA to control precursor growth can improve the efficiency of coprecipitation reaction and obtain cathode materials with relatively good electrochemical properties(the specific discharge capacity at 1 C is 176.8 m Ah g^-1 with the first coulombic efficiency of 90.62%,and the capacity retention rates are 93.0%and 84.3%after 100and 300 cycles,respectively).