Study Of Nano Lithium Fluoride And Nanocarbon As Artificial Cathode Electrolyte Interface On Nickel-Rich Ternary Materials For Lithium-Ion Batteries

Nickel-rich ternary cathode material has become one of the most popular cathode materials for lithium-ion battery due to its high capacity.Among many high nickel ternary materials,Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）is the one with the most potential and has been widely used.Compared with other ternary materials,the increase of nickel content helps to improve the energy density,so that the battery has the advantage of higher specific capacity and reduces the cost while increasing the battery life.However,it also brings the problem of poor interface stability.When the battery is in charge-discharge cycle,strong oxidizing Ni^3+/4+redox couple will be generated,which makes the electrolyte be reduced on the surface of the positive electrode by side reaction.Apart from consuming lithium ions in the electrolyte,these side reactions also decay the NCM811electrode structure,which further increase the internal impedance and polarization of the battery,and significantly attenuated the charge-discharge performance,especially at high current rate.To improve the problems caused by side reactions,nanometer lithium fluoride（LiF）was used as an artificial cathode electrolyte interface（CEI）material to coat the surface of NCM811 particles,which not only prevents the direct contact between the cathode active material and the electrolyte,but also enhances the structural stability.As a component of naturally formed CEI,LiF has good Li⁺conductivity and chemical stability,which can play a good role in protecting the electrode.However,the conductivity of LiF is poor,so nano-carbon materials are added into the positive electrode slurry to improve the conductivity of the electrode.Multi-walled carbon nanotubes（MWCNTs）and graphene（GR）have attracted much attention in recent years,due to their excellent chemical stability,good electrical conductivity,large specific surface area and other characteristics,they have been used in many fields such as photoelectric materials,microelectronic devices,as well as excellent conductive additives.In this paper,these two materials are used to modify the electrode respectively to improve the electrical conductivity of NCM811 material coated with artificial CEI.The electrochemical performance of the lithium-ion half battery was characterized by assembling the modified cathode materials,which reflected the role of artificial CEI and nano-carbon materials in the cycle.The research results obtained in this paper are as follows:1)Nano-LiF particles were prepared by liquid phase method and were added in the preparation of electrode slurry.Finally,a layer of artificial CEI was formed on the surface of NCM811 material particles.The interface layer can protect the active electrode,reducing the occurrence of side reactions between electrode and electrolyte,and inhibit the attenuation of battery performance.After the 0.1C-2C multi-rate cycles,the specific discharge capacity of NCM811 battery coated with artificial CEI decreased from 193.5 m Ah·g^-1 in the first cycle to 154.1 m Ah·g^-1,and the capacity retention rate was 79.10%.However,the Pristine NCM811 decayed from 182.1 m Ah·g^-1 to 138.6m Ah·g^-1 in the first loop,and the capacity retention rate was 75.59%.2)In order to improve the increase of internal resistance and polarization during the cycles,MWCNTs and GR were used to modify NCM811 respectively to maintain good electron and ionic conductivity during the cycle.Due to the difficulty in dispersing nano-carbon materials in the organic solvent N-Methyl pyrrolidone（NMP）,the methods of spray drying and commercial nano-carbon dispersion were used to reduce the serious agglomeration and precipitation in the slurry,which affected the electrical conductivity and modification effect of the nano-carbon materials.NCM811 modified by GR showed good magnification performance at high rates.After multi-magnification 2C cycle,the discharge capacity retention rate of NCM811 modified by GR reached66.22%,while that of the Pristine sample was only 62.55%.However,the high nickel NCM811material was in contact with water during the dispersion process,which affected the performance of the electrode material itself and leaded to poor performance of the battery in low rate cycling.However,the samples modified by commercial battery-grade MWCNTs dispersion showed good cycling performance at various rates,and the capacity retention rate reached 82.41%after multi-rate 2C cycling,which was significantly improved compared with the 75.59%of the Pristine samples.Therefore,commercial battery-grade MWCNTs dispersion was selected as the addition and modification method of nano-carbon materials.3)Because of the poor conductivity of LiF itself,MWCNTs were also added in the formation of artificial CEI to improve the conductivity of the electrode and reduce the generation of polarization in the cycles.The specific discharge capacity of the MWCNTs modified artificial CEI coated sample decreased from 195.9 m Ah·g^-1 to 163 m Ah·g^-1 and the capacity retention rate was81.92%after the 0.1C-2C multi-rate cycles.However,the Pristine NCM811 attenuated from 182.1m Ah·g^-1 in the first cycle to 138.6 m Ah·g^-1,and the capacity retention rate was only 75.59%.The pulse discharge measurements were carried out on the electrochemical workstation,and showed that the adding of MWCNTs can improve the increase of polarization caused due to the increasing charge and discharge current and bad LiF conductivity.At the same time,the modified artificial CEI coated samples after 50 cycles at 1C also kept high lithium ion diffusion coefficient,which was 0.647×10^-9 cm² s^-1,higher than 1.87×10^-10 cm² s^-1 of Pristine sample.