Modification And Application Of High Voltage Cathode Material LiNi_0.5Mn_1.5O₄

LiNi_0.5Mn_1.5O₄?LNMO?is a promising cathode material in the field of high energy EV battery due to its high voltage of 4.7V?vs Li+/Li?,which is much higher than that of commercial cathodes??4.35V?.However,there are two major problems hindering the practical application of LiNi_0.5Mn_1.5O₄,especially its cycle stability,including solvent decomposition at high voltage and the dissolution of Mn and Ni.In this thesis,the cycle performance of LiNi_0.5Mn_1.5O₄ had been improved via different methods as follows:1)Li₃PO₄ coating cathode,2)4-TB addictive in electrolyte,3)fluorine-containing solvent in electrolyte,4)LNMO and graphite anodes.Spherical LiMn_1.5Ni_0.5O₄ particles with different Li₃P04-coating content were prepared by a co-precipitation method.The morphology,structure and electrochemical performance of samples were characterized and tested.XRD and FT-IR results showed that the-pristine sample and coated samples belong to spinel structure.The pristine sample has P4332 structure,while the Li₃PO₄-coating materials contain two kinds of structure,P4332 and Fd-3m,and the P4332 structure is dominant.SEM showed that the coated materials have smooth surface,withcertain degree of aggregation.The small particle size?200 nm?reduces the lithium ion diffusion path.The discharge specific capacity and capacity retention of the Li₃PO₄ coated LiNi_0.5Mn_1.5O₄ are significantly improved,and the rate discharge performance are increased too.The electrochemical performance of the LiNi_0.5Mn_1.5O₄ electrode materials is the best when coated with 5wt.%Li₃PO₄.Li₃PO₄-coating can prevent the direct contact between cathode materials and electrolyte,and form stable structure to significantly improve electrochemical performance of LiNi_0.5Mn_1.5O₄.CV results showed that the redox potential difference and degree of polarization of coated Li₃PO₄ electrode decrease.EIS showed that the charge transfer impedance decreases,and the electronic conductivity increases,which is helpful for the reversible insertion/extraction of lithium ion.In particularly,the LiNi_0.5Mn_1.5O₄ electrode material coated with 5wt.%Li₃PO₄ has excellent electrochemical performance due to its higher lithium ion diffusion coefficient and electrical conductivity.P-trifluoromethylbenzonitrile?4-TB?as the electrolyte additives can improve the high rate performance and cycling performance of the high voltage LiNi_0.5Mn_1.5O₄ cathode materials.The battery with 1wt.%4-TB has the best rate discharge performance and cycling performance.The discharge specific capacity of the battery with 1wt.%4-TB is up to 109.1 mAh·g^-1 at _0.5C rate after 100 cycles,and the capacity retention is 89.7%,superior to that without 4-TB additives?68.5%?.At the same time,its discharge specific capacity is 120.3 mAh·g^-1,and the capacity retention is 98.1%after 50 rate cycles at the different rate,much better than the battery without 4-TB additives?89.8%?.The effect of the different proportion of FEC/DD cosolvent electrolyte on the electrochemical performance of high voltage LiNi_0.5Mn_1.5O₄ cathode material was studied.Linear cyclic voltammetric curves showed that the FEC/DD cosolvent can broaden the electrochemical window of the electrolyte,combining with SEM morphology and XRD analysis of LNMO electrode before and after cycling,the structure stability of LNMO electrode under high voltage is improved significantly.The results of the rate discharge and cycling performance showed that the battery with FEC/DD ratio of 1:3?S2 sample?has the best rate discharge and cycling performance.The discharge specific capacity of the S2 battery is up to 113.76 mAh/g at 1C rate after 100 cycles,and the capacity retention is 91.75%,while the S0 battery without FEC is only 73.18%.Even at high rate of 3C,S2 battery can still deliver capacity of 101.14 mAh·^-1.The above results show that the FEC/DD cosolvent electrolyte can improve the structure stability of LNMO under high voltage obviously.The effect of different additives on the performances of full cell based on LNMO/C was studied.The results indicated that the cell using 4-TB and FEC as the additives has the best capacity retention of 85.38%after 300 cycles.XRD results indicated that the additives of 4-TB and FEC can prevent the dissolusion of Mn and improve the structure stability of LNMO.