The New Electrolyte Is Used In The Research Of High-voltage Lithium Metal Batteries

Lithium-ion batteries have been widely used in portable electronic devices such as mobile phones in recent years due to their advantages of high power density,pollution-free,memory-free and long life.In order to meet the requirements of much higher energy density for batteries,lithium metal batteries have attracted broad attention.The metal lithium anode has the advantages of high theoretical specific capacity(3860 mAh g^-1),low operating voltage?-3.04 V?and low density,which can exhibit higher energy density after matching with high-voltage cathode.However,the conventional commercial electrolytes have limitations in their application in high-voltage lithium metal batteries due to their disadvantages of low flash point,easy combustion and incompatibility with the interface of lithium metal batteries.Therefore,it is extremely urgent to develop novel electrolyte systems to meet the compatibility of high-voltage lithium metal batteries.Based on this,we studied the mechanism of succinonitrile?SN?-based electrolytes on the cathode interface of high-voltage lithium metal battery.It was found that lithium difluoro?oxalato?borate?LiDFOB?had an excellent stability to the interface of high-voltage LiCoO₂ cathode.Therefore,we used lithium difluoro?oxalato?borate?LiDFOB?,lithium hexafluorophosphate?LiPF₆?and fluoroethylene carbonate?FEC?solvent to design a new functional dual-salt electrolyte?DSE?system,which was applied in high voltage lithium metal batteries.The details are as follows:?1?Succinonitrile and the two functional lithium salts?lithium bis?trifluoromethanesulfonimide??LiTFSI?and lithium difluorooxalate?were mixed to form a dual-anion deep eutectic solution?D-DES?at room temperature.Its mechanism on the interface of high-voltage LiCoO₂ was studied.By X-ray photoelectron diffraction?XPS?,transmission electron microscopy?TEM?,inductively coupled plasma mass spectrometry?ICP-MS?,infrared spectroscopy?IR?,nuclear magnetic resonance spectroscopy?NMR?tests,it was found that the electrolyte could form CEI film on the surface of LiCoO₂ cathode to protect the structure of LiCoO₂ and alleviate the decomposition reaction of the succinonitrile.?2?Based on the first experimental study,it was found that LiDFOB had a stable effect on the interface of high-voltage lithium metal batteries.Therefore,we combined difluoro?oxalato?borate?LiDFOB?,lithium hexafluorophosphate?LiPF₆?and fluoroethylene carbonate?FEC?solvent to form a new functional dual-salt electrolyte.The electrolyte exhibited a wide electrochemical window and good thermal stability.The Li|Li symmetrical battery was capable of achieving lithium plating/stripping performance for up to 3000 h.The LiCoO₂|Li battery assembled with this electrolyte achieved excellent high-voltage long-cycle performance:the battery capacity retention was 88.7%after 800 cycles under a voltage range of 3-4.5 V?rate:1 C,25 ^oC?;the battery capacity retention was about85%after 200 cycles under a voltage range of 3-4.6 V?rate:1 C,25 ^oC?.Its performance was far superior than that of traditional commercial electrolyte?1 M LiPF₆-EC/DMC?.In addition,the electrolyte was also applied in LiNi_0.5Mn_1.5O₄?LNMO?|Li battery under a voltage range of 3.5-5 V.The battery could cycle for 100 cycles?rate:1 C,25 ^oC?,and the capacity retention was 93.5%.