| In the pursuit of electrical energy storage with high energy density and high safety,using solid-state electrolyte matching with lithium metal anode with high specific capacity to fabricate solid-state battery has become the focus of both science and industry communities.As the key material of solid-state battery,solid-state electrolyte is the key factor to determine its commercialization.Solid polymer electrolytes composed of poly（ethylene oxide）（PEO）and lithium salt possess high ionic conductivity at high temperature,good contact with electrode,easy processing and low cost,having become one of the most promising solid electrolyte materials at present.However,ionic conductivity of solid polymer electrolytes at room temperature is still low,which cannot meet the requirement of room-temperature all-solid-state lithium metal batteries in application of electric transportation,grid storage,intelligent and wearable electronic devices.In order to realize the fast ion transport in solid polymer electrolytes and the practical application in all-solid-state lithium metal battery at room temperature,this thesis carry out structural design and performance improvement of solid polymer electrolytes,based on brush-like graft polymers with short PEO side chains.On the one hand,we explored the new methods towards electrolyte materials with high room-temperature ionic conductivity and high mechanical strength,including mixed graft block copolymers and inorganic particle crosslinked polymer-brush electrolytes.On the other hand,through the synthesis and characterization of series of samples,the structure-property relationship between polymer composition and electrolyte properties was revealed,which provided theoretical guidance for performance optimization of solid polymer electrolyte.Finally,through facile synthetic process,solid-state electrolyte film with excellent performance was prepared,and the assembled all-solid-state lithium metal battery can work well at room temperature.1.Effect of side chain architectures on ionic conductivity of polymer-brush electrolytes（PBE）The variation of ionic conductivity of PBE with side chain length from 300 Da to2000 Da and the corresponding mechanism were studied.The results show that the room-temperature ionic conductivity of PBE exhibits a non-monotonic variation with the PEO side chain length increasing.In range of low side chain length（<750 Da）,the proportion of effective Li+solvation sites and high-mobility segmets on PEO side chain increase with the side chain length,and so the ionic conductivity increases with the side chain length.In range of higher side chain length（>750 Da）,crystallization and entanglement of polymer chains begin to form and increase.Therefore,further increase of side chain length would decrease the segmental mobility of the whole side chains,and thus decreasing the ionic conductivity.The optimal room-temperature ionic conductivity(5.3×10-5 S·cm-1)was achieved at side chain length of around 750 Da.The variation of ionic conductivity of PBE with side chain graft density from 100%to 20%and its corresponding mechanism were studied.The results show that,for PBE with short amorphous side chain（<750 Da）,the decrease of grafting density would cause decreased connectivity of Li+solvation sites,resulting in the decrease of ionic conductivity.PBE with 100%graft density showed the highest ionic conductivity.For PBE with long side chains with crystallization（2000 Da）,the decrease of graft density resulted in looser local environment and more free volume,and thus reduced the crystallization and entanglement of PEO side chains,resulting in a slightly increased ionic conductivity with the graft density decreasing from 100%to 60%.Further decrease of graft density would also result in the decrease of ionic conductivity.Above studies provided a comprehensive mechanism diagram for ion transport behavior of graft polymer brush electrolyte,and determined the boundary conditions for optimal room-temperature ionic conductivity（750 Da side chain length and 100%graft density）,providing theoretical reference for the structural design and performance improvement of solid polymer electrolytes.2.Efficient room-temperature solid-state lithium-ion conductors enabled by mixed-graft block copolymer（mGBCP）Based on the 750 Da PEO side chain,we utilized the high thermodynamic incompatibility between PEO and PDMS and the unique structure of mGBCP to achieve microphase-separation of ultra-short PEO and PDMS chains.The results show that the microphase-separation of ultra-short PEO（750 Da）and PDMS（1000 Da）in block copolymers depends on not only the high thermodynamic incompatibility between PEO and PDMS,but also the interface stabilizing effect and inhibition of mixing entropy of the backbone of mGBCP architecture.The ring-opening metathesis polymerization and random copolymerization of linear macromolecular monomer（LMM）can not only regulate the PEO side chain length by adjusting the molecular weight of LMM,but also regulate the phase composition of mGBCP more simply and efficiently by adjusting the ratio of PEO to PDMS side chains by changing the feeding ratio of PEO to PDMS LMM.mGBCP solid-state electrolyte with double gyroidal,lamellar phase and hexagonal packing cylinderal microphase-separation morphology was obtained by mixing mGBCP samples at different volume fractions of PEO(f PEO)with lithium salts.The results show that the PEO chains complexed with lithium ions has greater thermodynamic incompatibility with PDMS,and thus promotes the microphase-separation behavior of mGBCP electrolytes.The nano-ordered structure formed by microphase-separation of mGBCP electrolytes overcomes the intrinsic softness of polymer brush electrolytes,without the sacrifice of high ionic conductivity of PEO750 side chains.At room temperature,the ionic conductivity of mGBCP-2(f PEO=0.63)is 1.8×10-5 S·cm-1,while its storage modulus can achieve to 104–105 Pa,which is 5 orders of magnitude higher than that of PEO750 homopolymer brush h-PEO without microphase-separation nano-ordered structure.Based on the nano-ordered structure-induced high mechanical strength,releasing partial PEO side chains covalently fixed on backbones into PEO free chain can obtain higher ionic conductivity without the sacrifice of mechanical strength,only if keeping the same microphase-separation nano-ordered structure.The storage modulus of mGBCP-7-type solid-state electrolyte obtained by mixing mGBCP-2 with 29 wt%PEO free chain remained storage modulus in range of 104–105 Pa,and achieved room-temperature ionic conductivity as high as 2.1×10-4 S·cm-1.3.Room-temperature all-solid-state lithium metal battery enabled by inorganic particle crosslinked polymer brush electrolytes（CPBE）Using hard polyhedral oligomeric polysiloxane（POSS）as mechanical reinforcement and crosslinking core,we combined the graft brush structure and crosslinking network structure to fabricate CPBE self-standing solid-state electrolyte films with higher mechanical strength.The results show that by casting the reactant solution of POSS crosslinker,PEO brush macromonomers,free PEO chains,and lithium salt onto a substrate surface and following simple step photopolymerization,we can not only obtain self-standing CPBE film from inert substrate surface,but also prepare in situ polymerized CPBE film on surface of electrode materials to capture better electrolyte/electrode interfacial contact.Through a solvent pre-evaporation strategy,we can not only effectively avoid the solvent removing process and residue problem in most preparative routes of widely-studied crosslinked polymer electrolyte,but also readily produce membranes with controllable thickness by regulating the solution concentration,providing a facile approach to ultrathin solid-state electrolyte films.The ionic conductivity and mechanical properties of CPBE films are affected by the side chain length of PEO brush,the contents of POSS crosslinking core and PEO free chain.The effect of side chain length is similar to that of PBE.Moderate side chain length has the highest room-temperature ionic conductivity.The increase of POSS crosslinking core content has negative effect on ionic conductivity,but is beneficial to improve mechanical strength.The increase of PEO free chain content can increase the ionic conductivity,but decrease the mechanical strength.CPBE750 films with PEO side chain length of 750 Da,PEO molecular brush,POSS crosslinking core,and PEO free chain contents of 45 wt%,15 wt%,and 40 wt%can achieve high room-temperature ionic conductivity of 2.3×10-4 S·cm-1 and high Young’s modulus of 57 MPa.Meanwhile,due to the good thermal stability of PEO free chain and the role of POSS material in thermostability and flame retardance,CPBE750 exhibits great thermal stability and fire resistance,which effectively reduces the risk of battery safety accidents.CPBE750 films exhibit stable Li electrodeposition and dendrite resistance in Li/Li symmetric batteries,owing to the high segmental mobility of graft brush structure and the robust inorganic particle crosslinking network.At 30°C and current density of 0.1m A·cm-2,Li/Li symmetric battery equipped with CPBE750 film exhibits stable stripping/plating cycling for over 500 cycles without internal short circuit.The mechanical strength and dendrite resistance based on crosslinking network can remain at elevated temperature.At 60°C,the Li/Li cell can also cycle stably for over 500 times at current density of 0.2 m A·cm-2.The all-solid-state lithium metal battery assembled from Li Fe PO4 cathode and lithium metal anode with in situ polymerized CPBE750 ultra-thin films（20μm）can work well at 30°C,with the first discharge specific capacity up to 161m Ah·g-1 and 147 m Ah·g-1 at C/10 and C/2,respectively.At C/10,the retention ratio of discharge specific capacity after 50 cycles is up to 88%. |
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