Surface Modification And Performance Study Of NCM 523 High-Voltage Cathode For Lithium-Ion Batteries

Lithium-ion batteries（LIBs）have been widely used in electric vehicles（EVs）,portable electronic devices,and energy storage grid due to their high energy density and long service life.However,LIBs still face some critical problems limiting their large-scale application.For example,further energy density increase is demanded to eliminate driving anxiety of the electric vehicles powered by the LIBs.Lifting working voltage is a promising way to increase energy density of lithium-ion batteries.Cyclic and rate performance are deteriorated due to excessive electrolyte decomposition and uncontrolled formation of cathode electrolyte interface（CEI）layer at high voltage.New concepts are proposed in this thesis to construct high voltage stable electrode-electrolyte interface.It is imminent to construct high-voltage stable electrode/electrolyte interface to enhance the electrochemical performance.Surface modification of single-crystal Li Ni_0.5Co_0.2Mn_0.3O₂（NCM 523）cathodes is reported in this thesis.Constructing a stable electrode/electrolyte interface at high voltages to improve the electrochemical performance of NCM523 cathodes for Li-ion batteries at high voltages.1.Modification of single crystal NCM 523 cathode is carried out using a novel binder.Elastomeric poly（dimethyl siloxane）（PDMS）binder is incorporated into the electrode to modify the Li Ni_0.5Co_0.2Mn_0.3O₂（NCM 523）particle surface via in situ cross-linking reaction between hydroxy terminated PDMS and methyl trimethoxy silane promoted by moisture at ambient condition（MPDMS）.MPDMS exercises multiple effects including cathode surface passivation,solvation structure tuning,electrolyte uptake enhancement,and mechanical stress relief.The work provides an inspiring route to realize high voltage application of the lithium-ion battery technology.Improved electrochemical performance is achieved with the MPDMS binder in terms of reversible capacity(201 m Ah·g^-1vs.185 m Ah·g^-1at 0.2 C),capacity retention（80%vs.68%,after 300 cycles at 1 C）,and rate performance（55.6%increase at 5 C）,as demonstrated by the NCM 523//Li half-cell at 4.6V.The NCM523//graphite full-cell also shows improved performance at 4.6 V(147 m Ah·g^-1vs.128 m Ah·g^-1,82%vs.76%,after 200 cycles at 1 C).2.Al₂O₃coating was deposited on NCM 523 electrode for surface modification by ALD technique.Amorphous alumina films grown by ALD play a key role in passivating the cathode surface,relieving mechanical stress,reducing side reactions,and building stable interfaces.The results show that the electrochemical performance of the NCM 523 cathode at 4.6 V voltage is effectively improved in the NCM 523||Li half-cell.After 200 cycles at a rate of 1C(1 C=170 m A·g^-1),the capacity retention rate is increased from 63%to 83%,the cycle stability is enhanced,and the capacity is significantly improved at a large rate（28%increase at 5 C）.