Study On The NiCoMn Cathode/Electrolyte Interface Control Of Lithium Ion Battery

The characteristics of cathode/electrolyte interface have an important effect on the output energy and working life of lithium-ion batteries（LIBs）.The formula design of electrolyte,the surface modification and protection of cathode materials can change the interface structure,improve the interface compatibility,and finally improve the comprehensive performance of LIBs.This dissertation mainly focuses on the effect of succinonitrile（SN）-based electrolyte,surface structure reconstruction,and the doping and coating synergy on cathode/electrolyte interface and electrochemical properties at high voltage.The research results include:（1）The nitrile electrolyte,that’s 1.0 mol/L LiBF₄dissolved in the mixed solvent of succinonitrile（SN）and fluoroethylene carbonate（FEC）,was prepared.Different contents of FEC have an influence on the physicochemical and electrochemical performances of the electrolytes.According to DFT calculation,it is predicted that the prepared SN/FEC electrolyte can preferentially form an interface layer on the cathode surface to protect its main structure.The results demonstrate that 4.7 V Li Ni_0.5Co_0.2Mn_0.3O₂（NCM）/Li cell has an extraordinary capacity retention up to 73.6%after 100 stable cycles,much higher than that of commercial electrolyte counterpart（35.5%）.This is mainly attributed to a thin,uniform,N-containing interphase layer on the surface of NCM,which can inhibit the side reaction between electrolyte and electrode,the dissolution of transition metal ions and the collapse of the cathode structure,resulting in excellent interface properties,and ultimately improve the electrochemical performance of LIBs under high voltage.（2）A facile water treatment procedure was applied to modify NCM material.According to the results,a surface reconstruction from layered structure to disordered layer and rock salt coherent region together with a uniform Li₂CO₃-dominant coating layer is firstly in situ constructed on the single-crystal NCM cathode.The unique surface structure is elucidated by Ar-sputtering assisted X-ray photoelectron spectroscopy（XPS）and transmission electron microscopy（Spherical aberration corrected-STEM,HRTEM,and TEM）.Meanwhile,neutron powder diffraction（NPD）indicates that the antisite defect concentration is mitigated in the treated materials.The modified samples display superior cycle stability with a capacity retention up to 87.5%at 1 C after 300 cycles,an elevated temperature（45^oC）cycling property with 80%capacity retention（4.5 V）and an improved full cell performance with 91%after 250 cycles at 1 C.（3）A modified NCM cathode was firstly prepared by the hydrolysis of Na PF₆.the modification effect on the surface structure was analyzed.The results show that a triple coupling involving concentration-gradient Na⁺,F^-co-doping and surface Na F coating are exploited on NCM.NFNCM-2 with 0.2 wt.%Na PF₆has outstanding capacity retentions of 91.3%at 25 ^oC and 85%at 45 ^oC after 500 cycles at 5 C cycling rate.Meanwhile,the synergistic enrichment effect enables the full cell a capacity retention of 92.4%at 1 C after 300 cycles.These prominent outcomes are mainly ascribed to the thin cathode electrode interface（CEI）film,high content of lithium fluoride,and the low solubility of transition metal ions as demonstrated by the time-of-flight secondary ion mass spectroscopy（TOF-SIMS）,XPS,HRTEM,XRD and scanning electron microscope（SEM）.