Electrolyte Tuning And Electrode Interface Optimization Of High Energy Density Secondary Li Batteries

High capacity and low-cost of cathode materials are the future trends for lithium secondary batteries.Spinel Li N_0.5M_1.5O₄displays high voltage platform of 4.7 V vs.Li/Li⁺and high lithium utilization of 99%.Instead of LiFePO₄,the batteries can achieve higher energy density.Besides,the layered Ni-rich cathodes,such as Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）,displays a specific capacity of≥200 m Ah·g^-1 at 4.3 V vs.Li/Li⁺.When coupled with lithium metal or Si-based anode,it can achieve ultra-high energy density of≥350 wh·kg^-1.However,the strong oxidizing of Ni⁴⁺along with high upper voltage will aggravate the decomposition of electrolyte components and deteriorate the cycling stability and safety of battery.Therefore,the optimization of high voltage electrode/electrolyte interface is the key to develop high energy density of lithium-ion batteries.To further improve the electrochemical performance and promote its practical application,this paper carried out a series of systematical research on electrolyte regulation and the electrode interface protection.Tris（trimethylsilyl）-based additives,including Tris（trimethylsilyl）borate（TMSB）and Tris（trimethylsilyl）phosphite（TMSPi）,are selected as the electrolyte additive for typical EC-LiPF₆ based electrolyte to develop practical Li Ni_0.5Mn_1.5O₄-based batteries.Combining theoretical calculations,physicochemical characterization and electrochemical measurements,we found that both TMSB and TMSPi can improve the Coulombic efficiency（CE）and cycling stability of 5 V Li Ni_0.5Mn_1.5O₄-based cells.Specifically,TMSPi with P（III）can be preferentially oxidized on the surface of charged Li Ni _0.5Mn_1.5O₄electrodes,resulting in a decent rate capacity and high discharge platform.In addition,TMSPi is conducive to the formation of robust SEI on graphite anode through nucleophilic attack,resulting in enhanced cycle performance.As a result,Graphite||Li Ni_0.5Mn_1.5O₄ pouch cells with 1wt.%TMSPi-containing electrolyte display a capacity retention of 88.9%after 100 cycles under 1C,superior to that in BE（60.5%）and 1 wt.%TMSB-containing（77.4%）electrolytes.Fluoroethylene carbonate（FEC）and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetra fluoropropyl ether（HFE）are selected as cosolvents to replace EC in conventional LiPF₆-based electrolyte,resulting in high electrochemical window of≥5 V vs.Li/Li⁺,reduced side reactions on the surface of Li Ni_0.5Mn_1.5O₄cathodes and improved CE.FEC has high dissociation ability for LiPF₆.Increasing its proportion can maintain the high Li⁺transport properties of electrolyte.However,there exists an inferior compatibility between FEC and LiPF₆.High proportion of FEC will decrease the thermal stability of LiPF₆-based electrolyte.Compared with FEC,HFE is more conducive to enhance the oxidation resistance of electrolyte.However,HFE can hardly dissolve the LiPF₆.Therefore,the ionic conductivity of electrolyte will decline sharply with high proportion of HFE,going against the high-rate property of Li Ni_0.5Mn_1.5O₄electrodes.Based on these results,20 vol.%FEC+20 vol.%HFE can balance the oxidation resistance,thermal stability and high Li⁺transport properties of electrolyte,and make Li||Li Ni_0.5Mn_1.5O₄ cells achieve a high coulomb efficiency of 99.6%under 1 C.With above optimized electrolyte,Graphite||Li Ni_0.5Mn_1.5O₄pouch cells display high capacity-retention rate of 75%after 350 cycles at 0.5 C and an average CE of≥99.7%.FEC can react with PF₅ from the decomposition product of LiPF₆ salt in the presence of trace amount of water,producing corrosive species such as HF,deteriorating the cycling life and safety of batteries.In this work,a non-nucleophilic organic base,ethoxy（pentafluoro）cyclotriphosphaz ene（PFN）,as a multifunctional additive is introduced to FEC-LiPF₆ based electrolytes to enhance its chemical stability and safety.The electron-donating-PF₂=N-group of PFN makes it able to catch the Lewis acid PF₅ and HF in the electrolyte,inhibiting its catalysis for FEC and LiPF₆,while its high steric hindrance from the resembles benzene structure prevents it from attacking the FEC.3 wt.%PFN can balance the physical properties of 1 M LiPF ₆ FEC/DMC（1:1,by volume）electrolyte and the electrochemical properties of Li||NCM811 cells.Furthermore,3 wt.%PFN combing with 2 wt.%Li DFOB can make Li dendrite-free growth in FEC-LiPF₆ electrolyte.With above optimal electrolyte,the 3 Ah Li||NCM811pouch cell with an energy density of about 400 Wh·kg^-1,consisting of ultrathin Li anode（35μm）and 6 m Ah·cm^-2 Li Ni_0.8Co_0.1Mn_0.1O₂ cathode,display stable cycling over 100 cycles at 0.3 CC/0.5 CD.Low-viscosity solvents of 1,2-Dimethoxyethane（DME）and HFE are adopted to optimize the high Li FSI concentration(3.2 mol·kg^-1)of ionic liquid（IL）.20 wt.%Pyr₁₃FSI instead by DME can greatly enhance the Li⁺transport properties in IL-based electrolyte and decrease its viscosity from 460 m Pa·s to85 m Pa·s at room temperature.HFE is introduced to dilute this hybrid IL electrolyte to further decrease its viscosity,improve ionic conductivity and wettability with PP separator.As the molar ratio of Li FSI:HFE is 1:2,the electrolyte viscosity significantly deceases to below 10.0 m Pa·s,displaying good fluidity and operability.Li deposition in this ionic liquid-based localized highly concentrated electrolytes（Li FSI-IL/DME-2HFE）is denser and a larger bulk,corresponding to the high CE of 99.5%.The anion-receptor,TPFBP,can complex with FSI^-anions,increasing the Li⁺transport number（an average value of 0.49）,making Li Ni_0.8Co_0.1Mn_0.1O₂electrodes obtain superior cycle stability at 2 C.Based on above optimization,pouch cell employing ultra-thin lithium metal（30μm）and 6.0 m Ah·cm^-2 NCM811 cathode display a high-capacity retention of 85.3%after 100 cycles at 0.2 C and 25℃with no flatulence and bulge.