Study On Structural Stability And Surface Modification Of Ni-rich Cathode Materials

Lithium-ion battery has been considered as the secondary energy storage component with the best comprehensive performance,while being applied in large-scale application.However,the change of application scenarios puts forward chanllenge on higher performance for lithium ion battery,especially with new energy vehicles rapidly occupy more automobile market.In lithium-ion batteries,the positive electrode is the key factor that determines the performance.Li Ni_1-x-yCo_yMn_zO₂（NCM）materials have the advantages of high capacity,good stability,low cost and industrialization simplicity,attracting attention in the research of lithium-ion battery cathode materials.The improvement of reversible capacity,however,with Ni content increasing usually,also brings many issues to cathode material,including cation disorder degree rise,interface reaction activity,thermal stability,easy to appear crack and extension and generation of residual lithium compounds on surface.The combition of these issues leads to the failure of the Ni-rich cathode material,which hinders practical application seriously.In terms of the issues,polysiloxane coating at molecular level was used to improve the surface stability of Ni-rich cathode materials,inhibit the emergence of residual lithium compounds and phase transformation on surface,Reduce the requirements for storage environment,reduce the difficulties in the application of high nickel positive electrode.Meanwhile,the strategy of concentration gradient bulk with concentration gradient doping was adopted to improve the cycle stability and rate performance of Ni-rich cathode.Concerning the issue that the Ni-rich cathode materials are prone to deterioration for its reactivity with H₂O and CO₂ in air,we use polydimethylsiloxane（PDMS）as coating layer to protect Ni-rich cathode by a facile method.The solvent and concentration of the coating solution were investigated.Compared with the different coating effects with acetone or n-hexane employed as solvent for PDMS coating,it was found out that the coating layer from PDMS/n-hexane solution is thin and uniform,with controllable thickness,while the process of acetone solution leads to thick coating on surface for polarity interference.The coating thickness of PDMS/n-hexane solution processed sample is 1.53 nm,only several layers of polysiloxane molecular thickness.Subsequently,we designed a storage experiment under extreme damp condition and exposed bare and coated samples in 50℃/50 RH%environment,in which the protection of PDMS would be disdinguished.As a result,the coated sample generated less lithium residues than the bare one and maintained the original layered structure on the surface,while rocksalt phase appeared in bare sample.What’s more,the reversible capacity of coated sample was 190.1 m Ah g^-1,while that of bare sample was only 167.9m Ah g^-1.To further improve the protective effect of polysiloxane coating and enhance the stability of Ni-rich materials in humid environment,the single crystal NCM811 was coated with polymethylhydrosiloxane（PHMS）and hydroxy-terminated polydimethylsiloxane（PDMS-OH）cross-linking.With the proportion of PDMS-OH increasing,three-dimensional cross-linking network formed on NCM811 surface,which brought about adverse effect to electrochemical performance.Therefore,the electrochemical performance of the coated one was almost unchanged with 1.0 wt.%PHMS+0.2 wt.%PDMS-OH ratio.The thickness of the coating layer is only 1.03 nm.According to the length of Si-O bond（1.64 A）,the coating layer is less than 3 layers of siloxane,but the contact Angle to water is more than 110°.The cross-linking coated sample was also exposed at 50℃/50 RH%for 5 and 10 days to investigate the protection of hydrophobic coating.The reversible capacity of bare samples for 10 days is 132.4 m Ah g^-1,and the coulomb efficiency of first cycle is 73.0%.As for coated sample,the specific discharge capacity is 185.5 m Ah g^-1 and the coulombic efficiency at first cycle is 80.8%.After exposed for 10 d,the reversible capacity of coated sample at5 C rate was 30.0 m Ah g^-1 higher than bare sample.In addition,in order to improve the bulk stability and electrochemical performance of Ni-rich material,combined with concentration gradient design and element doping method,we synthesized the Ni-rich cathode materials with concentration gradient of bulk and doped element（Zr）by coprecipitation method.The results of characterizations and electrochemical test indicate the doping of Zr reduce electrochemical impedance and Li/Ni mixed degree significantly.Besides,the strength of M-O bond and stability are improved.Concentration gradient doping brings a more stable bulk structure and surface.The capacity retention rate of 1.0 at%Zr（c（Zr）=1.0%c（Ni））gradient doped sample is 85.6%after 300 cycles,which is 14.9%higher than that of blank sample.The reversible capacity of 1.0 at%Zr gradient doped sample at 10 C rate is 154.7 m Ah g^-1,which is 24.3 m Ah g^-1 higher than bare sample.What’s more,with the 10 C rate current charging and discharging,the capacity retention of the gradient doped sample reaches71.9%after 300 cycles,while that of the bare sample is only 53.6%.Concentration gradient doping can effectively improve the stability and electrochemical performance of Ni-rich cathode,which provides a new inspiration for the design of new stable Ni-rich cathode materials in the future study or application.