Rational Design And Electrochemical Performance Of Functional Separators For Rechargeable Lithium Batteries

Lithium-ion batteries(LIBs)have attracted much attention as one of the dominating energy storage systems due to their high energy density and long cycle life.However,the expansion of electric vehicles(EV)markets calls for higher energy density and safer lithium rechargeable batteries.The exploration of advanced battery systems brings great challenges.First,during the initial charge process,the formation of solid electrolyte interface(SEI)will consume the active Li⁺in cathode materials,which causes the irreversible capacity loss in lithium-ion battery.Second,the dendrite growth induced by uneven Li⁺deposition deteriorates battery performance and causes safety issues.Third,the shuttle effect caused by sulfur dissolution in lithium-sulfur(Li-S)batteries reduces active material utilization and shortens cycling life.As one of the key components of lithium rechargeable batteries,separator plays dual roles of transporting Li⁺and separating cathode from anode.Besides,separator directly contacts both cathode and anode,which demonstrates unique advantages towards addressing the issues of electrodes.Commercial polyolefin separators show poor wettability toward electrolyte and thermal stability,which limits the battery performance and causes safety problems.Aiming at the above issues,we have developed functional separators by designing novel inorganic separator,improving solid electrolyte and modifying commercial separator in order to improve energy density,cycling performance and safety at elevated temperature of rechargeable lithium batteries.The main research results are as following:(1)Li₂S/Coprelithiation nanomaterials were coated on polypropylene(PP)separator to compensate for the initial capacity loss of lithium-ion batteries.The fabrication procedure of the prelithiation separator is compatible with current production process,which reduces the cost,and is beneficial for commercialization.Before lithiation,the carbon coated CoS₂/Co₃S₄ composite shows high reaction kinetics(45 s)and lithiation capacity(993 mA h g^-1).Compared with PP separator,LiFePO₄|graphite full cells assembled with Li₂S/Coprelithiation separator exhibit an enhanced capacity,and the energy density improves from220 W h kg^-1 to 285 W h kg^-1,corresponding to 29.5%increase.(2)The introduction of lithium metal as the anode can effectively enhance energy density of rechargeable lithium batteries.However,uncontrollable dendrite growth will lead to poor cycle stability and safety issues.An inorganic hydroxyapatite(HAP)nanowires separator was synthesized by facile hydrothermal and filtration methods.The separator exhibits superior flexibility,thermal stability and flame-retardant property.When exposed to high temperature of 1000?,the separator maintains its structure integrity without any shrinkage.The excellent electrolyte wettability and high porosity of the HAP separator induce the uniform distribution and fast transfer of Li⁺,which gives rise to a flat deposition of lithium.Moreover,the porous framework of HAP separator can act as buffer layer to accommodate lithium plating,thus the dendrite growth can be further relieved.On cathode side,XPS analysis results indicate that Ca²⁺in HAP chemically bonds with F^-and reduce the HF contents in electrolyte.As a result,the dissolution of active Mn ion in LiMn₂O₄ cathode is effectively suppressed.At room temperature,the LiMn₂O₄|Licell assembled with HAP separator shows a high capacity of 77 mA h g^-1 at the current density of 20 C.When the temperature increases to 55?,the cell achieves a high initial capacity of 123 mA h g^-1 at0.5 C with a capacity retention of 71%after 100 cycles.The introduction of HAP separator has effectively improved the rate capacity and cycling performance of LiMn₂O₄|Libatteries.(3)The energy density can be further improved with sulfur as cathode.But the shuttle effect of polysulfides will cause a low coulombic efficiency and rapid capacity decay.A multifunctional inorganic composite separator(H@CM separator)for Li-S batteries was constructed by modifying the HAP separator with a CNT/Mn₃O₄ layer.The modified separator still maintains the excellent thermal stability and flame-retardant property,which keeps its structure integrity at a high temperature of 400?without shrinkage.H@CM separator saturated by electrolyte has a self-extinguish time of 22.8 s g^-1,far less than that of the PP separator(44 s g^-1).As for the CNT/Mn₃O₄ interlayer,on one hand,CNT plays the role of upper current collector to provide extra electron transfer channel for active sulfur,which accelerates the electrode reaction kinetics;on the other hand,the shuttle effects have been effectively suppressed by the strong chemical interaction between Mn₃O₄nanoparticles and polysulfides,thus improving the utilization of sulfur.Li-S batteries assembled with H@CM separator demonstrates superior electrochemical performance.At the high sulfur contents of 70 wt%,the cell shows a high initial capacity of 1319 mA h g^-1at 0.1 C with 1000 mA h g^-1 remaining after 150 cycles,corresponding to a 0.16%capacity decay per cycle.At a high rate of 4 C(8.7 A g^-1),the cell still achieves a reversible capacity of 607 mA h g^-1.(4)The substitution of liquid electrolyte with solid electrolyte can simultaneously suppress lithium dendrite growth and improve safety of lithium metal batteries and Li-S batteries.PEO-HAP composite electrolyte was designed by introducing PEO polymer into HAP framework.The HAP enhances the ion conductivity(2.71×10^-4 S cm^-1)and lithium-ion transference number(0.203)of PEO-HAP composite electrolyte.This consequently reduces the battery polarization and facilitates the uniform plating of lithium on anode.As a result,the battery achieves high reversible capacities and stable cycling performance.Lithium symmetric batteries paired with PEO-HAP composite electrolyte shows an ultralong stable cycling of 2300 h at the current density of 0.1 mA cm^-2 and capacity of 0.1mA h cm^-2.When the current density and capacity was increased to 0.2 mA cm^-2 and 0.2mA h cm^-2,respectively,the cells can still cycle for over 2200 h.The LiFePO₄|PEO-HAP|Libatteries achieves the capacity of 80 mA h^-1 at a high rate of 5 C,when the current density was set at 1 C,the cell exhibits an initial capacity of 134 mA h g^-1 with a capacity retention of 83%after 1000 cycles.The Li-S batteries paired with PEO-HAP composite also shows enhanced electrochemical performance of 1502 mA h g^-1 at 0.1 C,and a reversible capacity of 508 mA h g^-1 is still maintained at a high rate of 2 C.