Preparation And Electrochemical Properties Of High Voltage Ternary Cathode Materials For Lithium-Ion Batteries

High-voltage medium-nickel low-cobalt NCM ternary materials are recognized as promising cathode materials for high energy density lithium-ion batteries due to their relatively high output capacity,low cost,and improved safety.However,the high voltage induced bulk structure degradation and electrode-electrolyte interfacial environment deterioration seriously restrict the cycle life of the battery.In this work,LiNi0.6Co0.05Mn0.35O2(NCM60535)high-voltage medium-nickel low-cobalt ternary cathode material with excellent lithium storage performance has been developed by using the strategies of material synthesis optimization,bulk phase doping,surface coating,and new-type electrolyte design.The main research contents and results are as follows:(1)The reaction mechanism for synthesizing of NCM ternary materials precursor by hydroxide coprecipitation-controlled crystallization method was analyzed,and the best synthesis conditions were optimized.In the nucleation stage,the salt solution gradually completes the nucleation reaction in the diffusion process after dropping into the reactor,and then precipitation and complexation reach a dynamic precipitation dissolution equilibrium.In the growth stage,the competitive relationship between precipitation and complexation has an important impact on the ordered growth of precursors.At any total ammonia concentration,the corresponding pH value is the best control condition for material synthesis when the concentration ratio of Me(NH3)n2+(Me=Ni/Co/Mn)and OH-is 3.4.The prediction formula of the optimal synthesis pH value corresponding to different total ammonia concentrations was obtained by fitting.(2)NCM60535 with good structure and electrochemical properties was prepared under the optimal synthesis conditions of pH 11.4 and lithium calcination temperature of 870℃.In addition,a new type of high-voltage electrolyte combining high lithium salt concentration,EC-free based fluorine-containing solvent system,and filmforming functional additives was designed.At 4.5 V high voltage,using the above electrolyte can achieve a capacity retention rate of 95.4%after 100 cycles at 0.5 C current density for the lithium-ion battery with NCM60535 as the cathode.The function is attributed to the high voltage decomposition resistance of the new composition electrolyte,which enhances the stability of the electrode-electrolyte interface.(3)NCM60535 with Ti uniformly doped in bulk phase was prepared by two steps of liquid phase mixing and solid phase roasting.At 4.5 V high voltage,using Ti doped materials as the cathode of lithium-ion batteries,the capacity retention rate is significantly improved compared to undoped Ti materials(95.7%vs.84.9%)after 100 cycles at 0.5 C current density,and the discharge specific capacity increases from 70 mAh g-1(undoped Ti)to 101 mAh g-1(doped Ti)at 5 C high current density.The above improvement effect is attributed to Ti doping to passivate the reactivity of the layered oxide cathode under high voltage,due to the element characteristics of strong Ti-O bond and no unpaired electrons for Ti4+.(4)NCM60535 coated with rare earth oxide was prepared by two steps of liquid phase mixing and solid phase roasting.It was found that all rare earth oxides have excellent coating modification effects.Taking Gd2O3 coating material as an example,the microstructure and lithium storage performance of the material were studied in detail.At 4.5 V high voltage,NCM60535 before and after Gd2O3 coating are used as the cathode for lithium-ion batteries,the capacity retention rate increases from 81.1%(before coating)to 95.3%(after coating)after 100 cycles at 0.5 C current density,and the output capacity increases from 51 mAh g-1(before coating)to 88 mAh g-1(after coating)at 5 C high current density.The improvement effect is attributed to the formation of a dense and thin coating layer on the surface of NCM60535 material,which plays a physical passivation role;In addition,due to the characteristics of rare earth elements with many extranuclear electrons,they can suppress harmful interface lattice oxygen loss and play a role in charge compensation.(5)Taking NCM613,NCM622 and NCM631 materials with different Co content as the research object,the effect of Co content on the lithium storage performance of materials at high voltage was studied.It is found that the output capacity and rate performance are decreased and the cycle performance is improved with the decrease of Co content.This result is attributed to the fact that the low cobalt material has more Ni2+,which not only increases the degree of Li+/Ni2+cations mixing and decreases the lithium layer spacing,but also reduces the redox reversibility and unstable Ni4+content.These factors are harmful to the capacity output and rate performance,but beneficial to the cycle performance.The above research results are conducive to the in-depth research and development of the low output capacity and rate capacity problems faced by the medium-nickel low-cobalt type NCM ternary materials under high voltage.