In-situ Polymerization Of Solid-state Electrolytes For Lithium-ion Batteries

Organic electrolyte-based lithium-ion batteries are inadequate for meeting the high-performance demands of electric vehicles and power grids due to energy density and safety limitations.Lithium metal cathodes are a promising solution to increase the energy density of lithium-ion batteries.However,lithium dendrite growth,volume expansion,and interfacial side reactions during the cycle of lithium metal cathodes pose significant challenges to battery performance and safety.Polymer solid-state electrolytes are superior to organic electrolytes in suppressing lithium dendrites and improving interfacial stability and safety,but achieving a multifaceted balance of ion conduction,mechanical strength,and chemical stability remains a critical challenge for polymer electrolytes.In this thesis,we use in situ polymerization methods to develop new high-performance polymer electrolytes through various approaches,such as molecular module design,hybridized cross-linked backbone structures,and multi-component blending.Our goal is to improve the ionic conductivity,mechanical support,and interfacial stability of polymer electrolytes and construct stable polymer lithium-metal batteries.The primary research objectives are as follows:(1)The structural design of monomers is critical for the development of high-performance polymer electrolytes.In this study,we used the molecular modular design of polymer electrolyte materials to prepare a new polycarbonate electrolyte(p-MDE)via in situ polymerization.Alkenyl carbonate(MDE)monomers containing EC and DMC structural units were designed and synthesized.The study focused on the effect of chain segment motion of the terminal EC group of p-MDE on the ionic conductivity and elastic modulus of the polymer.The resulting polymeric solid electrolyte exhibited high room temperature ionic conductivity(1.3 mS cm-1)and Li+transference number(0.47),wide electrochemical window(>5 V),and high Young’s modulus(3 GPa).These properties make it well-suited for use in LiFePO4/Li and LiCoO2/Li cells.Our results demonstrate the efficacy of molecular modular design in the preparation of high-performance polymer electrolytes.(2)Molecular design to further enhance the mechanical stability of polycarbonate studies.Our previous work on p-MDE polycarbonate electrolytes demonstrated the need for small amounts of cross-linking agents to improve the mechanical strength of the polymer backbone.In this study,we prepared a new carbonate monomer,MMDE,which directly linked the polymerization site of EGDMA to DMC and EC units.This design eliminated the use of small molecule crosslinkers,thereby enhancing mechanical strength and cyclic stability.We synthesized a new polycarbonate electrolyte,pMMDE-ED,through heat-initiated in situ polymerization with the addition of 60%EC/DEC plasticizer.The polymer electrolyte exhibited a high Young’s modulus of 7.1 GPa,providing significant support to inhibit the growth of lithium dendrites and retard the volume change of the anode.The polymer electrolyte was stable during more than 3000 h of cycling at a current density of 0.25 mA cm-2 for Li/Li symmetric cells.Moreover,the SPAN/Li cell constructed using the electrolyte exhibited a capacity retention rate of 98%after 800 cycles of capacity.Our study demonstrates the efficacy of molecular design strategies in improving the mechanical stability and cyclic performance of polycarbonate electrolytes.(3)Design and study of polyether-based polymer electrolytes.In this study,we synthesized a polyether-based electrolyte using 1,3-dioxolane(DOL)as the polyether precursor and delta ether(BD)as the cross-linking agent.The polymerization reaction was initiated through Lewis acidic lithium salt.We found that the addition of BD could regulate the ionic conductivity and mechanical stability of the polyether and reduce its crystallization tendency.Through screening,we discovered that a balance between inhibiting lithium dendrite growth and maintaining ionic conductivity could be achieved by adding 1%BD to the pDOL-based electrolyte.The resulting electrolyte exhibited a stable room-temperature ionic conductivity of 1.2 mS/cm and a high Young’s modulus of 3.2 GPa.The electrolyte also demonstrated improved cycling stability compared to the control electrolyte without BD.Specifically,the LiFePO4/Li cells using the pDOL-based electrolyte achieved a stable cycle of 400 cycles at a current density of 0.5 C.Our findings highlight the effectiveness of incorporating BD as a cross-linking agent to improve the ionic conductivity and mechanical stability of polyether-based electrolytes.Our study provides a promising direction for developing high-performance electrolytes for lithium-ion batteries.(4)Study of diaphragmless composite polymer batteries.Polyether and polycarbonate precursors were mixed and then underwent UV light-initiated free radical polymerization and cation-initiated ring-opening polymerization successively to generate a composite polymer layer in situ between the positive and negative interfaces.The pre-polymerization of the polycarbonate component provides mechanical strength to the composite electrolyte,which eliminates the need for a diaphragm when making the battery.Additionally,stepwise polymerization achieves the co-blending of polycarbonate and polyether,which prevents crystallization of the system.The composite solidstate electrolyte(SPE)has a high electrochemical stability window(>4.5 V)and a high Li+transference number(0.60)with an ionic conductivity of 3.15 ×10-2 mS cm-1 at room temperature.The thickness of the polymer layer can be easily regulated,and the LiFePO4/Li cell constructed using this SPE achieves stable operation at room temperature.Moreover,the convenient preparation method enables the internal series connection of two LiFePO4/Li cells with an operating voltage of 6.9 V and a stable cycle of 100 cycles.