Performance Investigations Of Electrolyte Containing Cyclotriphosphazene Flame Retardant For Lithium-ion Batteries

The security issue has become an important reason for the restriction of high energy lithium-ion battery.Electrolyte of lithium-ion batteries contains organic carbonates,which is very easy to burn.It is important to improve the thermal stability of electrolyte.Sorts of researches have verified that adding small amount of flame retardant additive can suppress the combustion of electrolyte,improving the safety performance of lithium-ion batteries.In this paper,ethoxy-（pentafluoro）-cyclotriphosphazene（PFPN）is studied intensively,the flammability of PFPN and that with electrolyte are estimated by self-extinguishing time,oxygen index and self-propagating rate measurements.The electrochemical performances of LiCoO₂（cathode materials）and graphite（anode materials）containing flame retardants are investigated by galvanostatic charge-discharge test and cyclic voltammetry.The interfacial properties of the electrode materials are studied by impedance spectroscopy.The surface morphology of the electrode is characterized by scanning electron microscopy.The flammability tests show that the electrolyte becomes difficult to burn if increasing the PFPN content.When the flame retardant content is greater than 3 vol.%,the electrolyte combustion flame cannot spread.5 vol.%content PFPN of the electrolyte is nonflammable.The efficiency of PFPN is high due to the synergistic flame retardance of fluorine,phosphorus and nitrogen elements in PFPN.A small amount of PFPN can inhibit the combustion of electrolyte,reducing the risk of battery combustion.The results of electrochemical measurements show that an excellent electrochemical compatibility of PFPN with LiCoO₂ electrode,and confirm that 5 vol.%content of the additive can improve cycling performance of the electrode.Cyclic voltammetry tests indicate that F elements in PFPN can weaken the intermolecular force and improve the ionic conductivity in the electrolyte,so that ions can move faster in the electrolyte.PFPN also helps increase the specific discharge capacities of LiCoO₂ electrodes.The surface morphology of LiCoO₂ after cycling is observed by scanning electron microscope.The surface of the electrode with PFPN after cycling is smooth and complete,and SEI film is uniform and compact,which demonstrates that PFPN contributes to the formation of stable SEI films.The electrochemical performances at low temperature indicate that PFPN is also conducive to the battery charge transfer,the battery low temperature performance has improved.PFPN is also favorable for reducing the charge transfer impedance,accelerate the lithium ion through the SEI film and improve the low temperature performance of the cell.The electrochemical measurements results show that an excellent electrochemical compatibility of PFPN with graphite electrode.The specific capacity of the cell with 5%PFPN is increased by 6%after cycles.PFPN can reduce the thickness of SEI film to help form a better uniform SEI during the cycles,further reducing the SEI impedance.Moreover,the addition of PFPN reduces the charge transfer impedance in the cell and increases the ionic conductivity of the electrolyte.The high rate measurements indicate that the discharge capacity of the cell with PFPN also increased.In summary,the addition of PFPN can improve the thermal stability of the electrolyte,and have an excellent electrochemical compatibility with electrodes.The additive not only has no negative impact on the electrochemical performance,but also improves the low temperature performance of the lithium-ion battery.Therefore,PFPN can be used as additives for the commercial production of lithium-ion batteries.