The Study On SnO₂ Nanostructures Modified Polyolefin Separators For Lithium Secondary Battery Applications

Polyolefin separators have been widely used in lithium secondary batteries due to their excellent chemical stability,moderate thickness,good mechanical strength and low cost.However,the safety performance and electrochemical properties of lithium-ion secondary batteries are restricted by the poor thermal stability,low electrolyte wettability and affinity of polyolefin separators.In addition,in lithium-sulfur secondary battery system,polar and soluble polysulfides will form during the charge-discharge process.Due to the small size of polysulfide and its weak interaction between nonpolar polyolefin membranes,polysulfide ions can transfer between sulfur cathode and lithium anode,leading to serious“shuttle effect”,self-discharge and rapid decay of battery life.To solve the afore-mentioned problems,low-cost SnO₂ nanostructures with good thermal stability,strong polarity,high morphology tenability are used for the modification of polyolefin separators to improve the physical/chemical properties and electrochemical performance of lithium secondary batteries in this work.The main contents of the works are summarized as follows:1)SnO₂ hollow nanostructures are synthesized via a hydrothermal method,and then coated onto polyethylene?PE?separators with polyvinydene fluoride?PVDF?as the binder.The PE@SnO₂ separators are characterized with a series of physical and electrochemical measurements.The results show that the nano-SnO₂ particles coating improves the thermal dimensional stability and the mechanical properties of the separator.In addition,the wettability and affinity between the separator and polar electrolyte are enhanced,leading to a decreased interfacial impendence and improved ion conductivity of PE@SnO₂ separator.As a result of these favorable features,the rate performance of the LiCoO₂/Li half-cell is improved by 13.6%at 750 mA g^-1.More importantly,due to the good thermal stability of the separator and the increase of electrolyte retention,the battery assembled with PE@SnO₂ separator exhibits higher capacity retention,better columbic efficiency and excellent safety at harsh circumstances,such as at 60°C and 80°C.2)Polypropylene?PP?separator is activated with air plasma and immersed into the solution of SnCl₂ to generate SnO₂ grafted PP?PP-SnO₂?separators through hydrolysis and oxidization of SnCl₂.Subsequently,the composite separators are characterized with Scanning Electron Microscopy?SEM?,X-ray Diffraction?XRD?,Fourier Transform Infrared?FTIR?,which demonstrate SnO₂ nanoparticles are successfully grafted onto the PP separator.Compared with the blade coating method,the volume and mass of the separators remain almost unchanged after grafting,which is favorable for the maintenance of the energy density of the battery.Through the contact angle and interfacial resistance test,the improved compatibility between the modified separator,electrolyte and electrode is demonstrated.In addition,the ion conductivity and Li⁺transfer number of the PP-SnO₂ separator are also increased,which leads to the decrease of charge transfer resistance of the battery.The reversible capacity based on PP-SnO₂ separator is 458.7 mAhg^-11 at the current density of 3350mA g^-1,significantly higher than that for pristine PP separator(103.7 mAh g^-1).Furthermore,due to the strong interaction between polar polysulfides and SnO₂,which is immobilized on the surface and pores of the separator,the“shuttle effect”is effectively suppressed,thereby significantly improving the cyclic life of lithium-sulfur batteries.