Synthesis Of High-power Manganese Cathode Materials And Construction Of Compatible Electrolyte System

With the continuous expansion of the application field of lithium ion batteries,the requirements of relevant industries for their power density are also increasing.For the reason that the performance of the battery is largely determined by that of the cathode material,so the key to improve the power density of the Li-ion battery is to develop a high-power cathode material with a high charge-discharge voltage platform and capable of achieving cycling at high current.Spinel-type manganese-based cathode materials,such as LiMn₂O₄ and LiNi_0.5Mn_1.5O₄,have a three-dimensional large tunnel structure suitable for lithium ion diffusion,and display the characteristics of low cost of law materials,high working voltage,good safety and environmental protection,which can be the first choice of power-type cathode materials for lithium ion battery.By choosing appropriate methods,it is of great practical significance to further improve their power performances.In this study,methods of improving the power performance of manganese-based cathode materials were firstly summarized.Developing new technological processes which are simple and easy to be industrialized is the starting points.The effects on the electrochemical properties of morphology,configuration of LiMn₂O₄ and LiNi_0.5Mn_1.5O₄ were studied.Meanwhile,the other aim was to resolve the problems of transition metal dissolution in LiNi_0.5Mn_1.5O₄ caused by oxidation decomposition of its electrolytes.Firstly,five kinds of submicron LiMn₂O₄ particles with the average particle size of 200-400 nm were prepared by a one-step solid phase method.The experimental results indicated that the LiMn₂O₄ product S_0.5.5 prepared by the molar ratio of C₂H₂O₄·2H₂O and Mn?CH₃COO?₂·4H₂O with 0.5:1,could balance the contradiction between diffusion distance and particle aggregation,had the smallest mixed row of Li and Mn,and had the preferred crystal growth orientation of?400?.Since?400?plane was the most stable thermodynamic and the internal stress to the intercalation and deintercalation of Li⁺ion was least,S_0.5 sample exhibited the most cycling and rate performances:the specific discharge capacity of 0.2 C rate in the first cycle was 125.7mAh·g^-1 with the retention of 91.4%after 100 cycles,and the specific discharge capacity of 10 C high rate could still reach 80.8 mAh·g^-1.Additionally,LiMn₂O₄material of porous microsphere structure with the diameter of 2.5?m and average pore size of 50 nm constituted by particles with the size of around 150 nm was further prepared by the co-precepitation method.The results showed that electrochemical performance of the microsphere structure material was better than that of submicron LiMn₂O₄ particles:the capacity retention of the porous LiMn₂O₄ increased to 97.0%after 100 cycles,and the specific discharge capacity of 10 C rate made up about70.9%of that of 0.2 C rate(the capacity retention of submicron LiMn₂O₄ particles S_0.5.5 was 63.3%).It was also cited that the porous microsphere structure displayed larger specific surface area,multiple active sites of Li⁺,smaller impedance,rapid migration rate of Li⁺and could suffer from the stress originated from the change of volume in the charge and discharge process as reason.Therefore,the design of porous microsphere structure is more benefial to improve the power performance of the Li-ion battery.LiNi_0.5Mn_1.5O₄ material has become one of the best candidates as power-type lithium ion battery due to its higher working voltage?⁴.7 V,vs.Li/Li⁺?.Herein,The LiNi_0.5Mn_1.5O₄ micro crystals with different particle sizes and different morphologies as spherical,polyhedral,octahedral and truncated octahedral were synthesized by improved solid phase method at different calcination temperatures.The results indicated that the material calcinated at 850 ^oC displays a truncated octahedral structure and exhibits the most excellent electrochemical performance.The reason is that this configuration contained?111?planes which is stable and?100?planes which is conductive to fast transmittion of Li⁺,but not contained?110?planes which caused the dissolution of manganese.Subsequently,the method of graphite assisted combustion was used to optimize the ratio of?111?planes with?100?planes of the truncated octahedral LiNi_0.5Mn_1.5O₄ material.By the means of XRD,SEM,TEM and TG-DSC detection method,the growth mechanism of different lattice planes in the LiNi_0.5Mn_1.5O₄ material and the reason why truncated octahedral configuration was formed were preliminary deduced.The electrochemical tests demonstrated that the LiNi_0.5Mn_1.5O₄ material with a moderate ratio of?111?planes with?100?planes displayed the best performance,in which the specific discharge capacity of 1 C,5 C,10 C and 20 C frequency was 123.6 mAh·g^-1,104.1 mAh·g^-1,89.5 mAh·g^-1 and 84.8mAh·g^-1,respectively,and the retention of that of 0.5 C frequency was 91.3%after100 cycles.This analysis suggested that LiNi_0.5Mn_1.5O₄ can form a solid electrolyte interface film with the electrolyte in the pre-cycle process.The dissolution of manganese could not be restricted effectively in low ratio of?111?planes with?100?planes,which formed a thick and loose SEI film,inducing the weak multiplying power and cyclic performance.While the materials with high ratio of?111?planes with?100?planes can not provide the amount of Li⁺effectively,resulting in the decrease of rate performance.Therefore,truncating the moderate ratio of?100?planes at the top of regular octahedron consist of?111?planes in the LiNi_0.5Mn_1.5O₄ material can be of great benefit to improve the power performance of Li-ion battery.Finally,to solve the problem that the continuous oxidation decomposition of electrolyte reduced the comprehensive performance of battery in a 5V high-voltage battery,the compatible electrolyte for power-type LiNi_0.5Mn_1.5O₄ cathode material was optimized.1wt.%LiPO₂F₂ in the LiPF₆-based electrolyte was added to improve the cycling performance of truncated octahedral LiNi_0.5Mn_1.5O₄.The capacity retention at the rate of 1C after 100 cycles was promoted from 91.3%to 96.5%.Further film-forming mechanism on the lithium nickel manganese oxide surface was analyzed by DFT calculation.It was shown that the Gibbs free energy change of decomposition reaction for LiPO₂F₂ was lower,whose hydrolysis reaction would prior to happen on the surface of LiNi_0.5Mn_1.5O₄ to produce a thin and compact CEI film with the thickness of 50 nm.It could not only reduce the interface impedance,but mitigate the continuous decomposition of electrolyte effectively.XPS tests were furtherly applied to discuss the film composition,the conclusion that the CEI film derived from LiPO₂F₂-adding electrolyte was consisted of organic carbonate and inorganic lithium phosphate which geatedly facilitated the transmission of Li⁺were drawn.Li₃PO₄ produced by the continuous hydrolysis of LiPO₂F₂ can effectively relieved the decomposition of LiPF₆ on the surface due to common-ion effect,which contributed to improving the stability of interface film and enhancing the cycling performance of LiNi_0.5Mn_1.5O₄ battery.Optimizing suitable film-forming additive for high voltage system provided a new solution to improve the power performance of manganese cathode materials.In conclusion,two aspects of the study about the modification and relevant mechanism of cathode materials and electrolyte were carried out respectively to constantly optimize the composition of high-power system.The optimized battery system could significantly improve the rate performance as well as ensure the enhancement of cycle stability.This paper can provide a new research approach and a theoretical guidance on design,analysis and development of the high power battery system.