Preparation Of Novel Piperidine Based Ionic Liquid Electrolytes For High Voltage And High Security Lithium Ion Batteries

With the in-depth application of lithium ion battery in many industry,people has higher requirement on the properties of lithium ion batteries.At present,safety issues and high energy density have become principal factors restricting its development.For larger power battery packs,the requirements are more stringent.One way to improve the energy density of Li-ion batteries is development of suitable high-voltage cathode materials for lithium-ion batteries.Currently,the market commercialized lithium-ion battery is mainly composed of the lithium salt LiPF₆ and carbonate-based organic solvent.First of all,LiPF₆ is very sensitive to water,decomposition can occur in case of traces of water,and the decomposition product HF will destroy the integrity of the high-voltage positive electrode structure,resulting in the decay of the battery capacity;Secondly,carbonate-based solvents that are volatile and flammable have serious safety hazards at high current densities.With its wide electrochemical windows,good thermal stability,and no vapor pressure,ionic liquids are widely favored by researchers.Based on the above ideas,this article focuses on high energy density lithium-rich cathode material(Li_1.1Ni_0.25Mn0.₆₅O₂)and 5 V high voltage cathode Li Ni_0.5Mn_1.5O₄ as the main research object.Therefore,the development of safe electrolyte has become a hot research topic,and designs and develops high safety electrolyte for high-voltage lithium ions battery and the results as following:?1?A novel piperidine based ionic liquid N-methyl-N-propyl piperidium difluorooxalatoborate(PP₁₃DFOB)was successfully synthesized by introducing a DFOB^-anion with a film-forming functionality.PP₁₃DFOB was used as a co-solvent in combination with dimethyl carbonate?DMC?and LiTFSI was used as the lithium salt.This mixed electrolyte system was applied to Al/Li corrosion cells.The mechanism of PP₁₃ DFOB's inhibition of aluminum corrosion was analyzed systematically by combining experimental testing and quantum chemistry.Through DFT calculation,it was found that the DFOB^-anion has the highest HOMO value,and during the first charging process,it can preferentially oxidize on the aluminum foil surface and participate in the formation of a passivation film. Moreover,when the content of ionic liquid PP₁₃DFOB reaches 20%or more,the passivation film can completely suppress aluminum corrosion caused by LiTFSI lithium salt at a high potential,and provides the possibility of applying this electrolyte system to high voltage lithium-ion batteries.?2?The novel electrolyte of LiTFSI-PP₁₃DFOB/DMC was further applied to high voltage Li/LiNi_0.5Mn_1.5O₄ lithium ion battery.It was found that B-O/B-F compound formed by DFOB^-oxidation and constitutes a stable CEI film on the LNMO cathode surface,which reduces the internal resistance of battery and inhibits dissolution of the transition metal,thereby improving the electrochemical performance of Li/LNMO battery.After 100 cycles at25oC and 0.5 C,Li/LNMO cell with 20%PP₁₃DFOB delivered a discharge capacity form112.3 mAh g^-1 with LiPF₆ based electrolyte to 121.2 mAh g^-1.In addition,this electrolyte system exhibits good compatibility with a lithium-rich cathode material(Li_1.1Ni_0.25Mn_0.65O₂)and a graphite anode,reducing battery resistance,and had a positive effect on improvement of the battery capacity.At 55oC,the discharge capacity of Li/RM cell with 50%PP₁₃DFOB based electrolyte is 190.2 mA h g^-1 after 100 cycles at 0.5 C and the capacity retention was improved from 58.86%to 81.67%.Above experimental results show that the LiTFSI-based electrolyte is suitable for high-voltage lithium-ion batteries when a suitable amount of PP₁₃DFOB ionic liquid as a co-solvent.?3?The N-methyl-N-propyl piperidium bistrifluoromethylsulfonylimide(PP₁₃TFSI)was selected in combination with DMC and fluorocarbon carbonate ethylene?FEC?as co-solvent and LiDFOB as a lithium salt.During the charging and discharging process,due to the synergistic effect of LiDFOB and FEC,a stable SEI film was formed on the positive and negative electrodes surface,through the oxidation oxidation?cathode?and reduction?anode?reactions,respectively.The high thermal stability and wide electrochemical window of the LiDFOB-based electrolyte system,as well as the positive and negative film formation characteristics,make the Li/RM,Li/Graphite,and RM/Graphite cells with LiDFOB-based electrolyte exhibited Out of excellent electrochemical performance at 40oC.The initial discharge specific capacity of the Li/RM half-cell using this electrolyte displaied a discapacity of 184.8 mAh g^-1 after 100 cycles,and the discapacity remained stable with almost no attenuation;the Li/Graphite half-cell had a capacity of 361.6 mAh g^-1 after 100 cycles at 0.5 C,close to theoretical capacity of graphite;When the electrolyte is applied to a RM/Graphite full cell,the initial discharge capacity is 214.7 mAh g^-1 at the current rate of 0.5 C and 40oC.The corresponding capacity retention is 87.54%,which is much higher than 58.97%of the commercial electrolyte;In addition,after the needle punching test was performed on 532 type and RM type soft-packaged batteries using LiDFOB-based electrolyte,the battery was not found to be ignited or swollen,and the safety was high.As a result,the LiDFOB+PP₁₃TFSI/DMC/FEC system is expected as a safety electrolyte for high-voltage lithium batteries.