Preparation Of Solid-State Polymer Composite Electrolytes And Their Applications In Lithium Metal Batteries

Lithium metal anode is considered as one of the most promising anode materials for lithium-ion batteries due to its ultra-high energy density(3860 mAh g-1)and extremely low redox potential(-3.04 V vs standard hydrogen electrode).However,the traditional lithium metal batteries(LMBs)use organic liquid electrolyte as the working medium.Due to the very high activity of lithium metal,the side reaction of Li and liquid electrolyte will occur.The liquid LMB s still have many problems to be solved,such as the safety problems caused by the growth of lithium dendrite and organic electrolyte solvent flammable.Compared with the traditional liquid electrolyte,the reaction activity of the solid-state electrolyte with lithium metal is greatly reduced,and the solid-state electrolyte can inhibit the growth of lithium dendrites.The solid-state polymer composite electrolytes(CPEs),formed by the combination of organic and inorganic phase,have the advantages of polymer with light weight and flexibility and inorganic filler with high strength and stability.The CPEs are the most potential solid-state electrolyte,and have the great potentials application in LMBs.Therefore,the use of CPEs instead of liquid electrolyte to realize all-solid-state battery is one of the effective strategies to inhibit the Li dendrites growth in LMBs,which is expected to solve the safety problems of LMBs.For all-solid-state LMBs,the development of CPEs with enhanced lithium ion conductivity,excellent flexibility and high mechanical strength is urgent.In addition,the design and optimization of the interface in CPEs is also a key factor affecting battery performance.To this end,this thesis aims to develop CPEs with excellent performance,optimize the electrolyte/electrode interface,and explore their potential applications in solid-state LMBs.The detailed research work and results of this paper are as follows:(1)We fabricate Li0.35La0.55TiO3(LLTO)nanofiber-enabled poly(vinylidene fluoride)(PVDF)-based CPEs with enhanced mechanical property and wide electrochemical window.The results show that 15 wt.%of LLTO nano fibers synergize with PVDF,giving a flexible electrolyte membrane with significantly improved performance,such as high ionic conductivity(5.3×10-4 S cm-1),wide electrochemical window(5.1 V),high mechanical strength(stress 9.5 MPa,strain 341%),and good thermal stability(thermal degradation 410?).In addition,an all-solid-state Li-metal battery of sandwich-type LiFePO4/PVDF-CPE(15 wt.%of LLTO)/Li delivers satisfactory cycling stability and outstanding rate performance.A reversible capacity of 121 mA h g-1 is achieved at 1 C after 100 cycles.This work exemplifies that the introduction of LLTO nanofibers can improve the electrochemical performances of PVDF-based CPEs used as electrolytes for all-solid-state LMBs.(2)To further improve the mechanical strength and flexibility of PVDF-CPEs,inspired by the dragonfly wings that are super-lightweight and ultrathin but have excellent stability when subject to bending and twisting during flapping,we have constructed a PVDF based solid electrolyte filled with LLTO nanofibers with the structure of dragonfly wings(PVDF-CPEs).In our design,the PVDF is used to construct the ultra-thin membrane,the 1D LLTO nanofibers are acted as the veins to give rise to a high mechanical strength of 10 MPa.The special design creates cellular surface of PVDF-CPEs,which guarantees the excellent flexibility as well as a good interface contact between CPE and Li anode.When evaluated as electrolyte for LiFePO4|Li battery,the PVDF-CPEs can suppress Li dendrites growth,and thus presents an excellent cycling stability of 140 mAh g-1 after 200 cycles.Moreover,the flexible LiFePO4|PVDF-CPEs|Li4Ti5O12 pouch cell delivers a satisfactory rate performance and cycle stability under folding and bending states.The excellent performances are attributed to the unique cellular structure and super mechanical strength of biomimetic PVDF-CPEs.(3)An ultrathin(9.6 ?m),flexible,mechanically strong,and sandwiched structure solid-state electrolytes(SSEs)are fabricated by a tape casting process,involving the 75 wt.%LLTO/PVDF-CPE(LLTO-75)layer designed as intermediate layer sandwiched by two 15 wt.%LLTO/PVDFCPE(LLTO-15)layers.The LLTO-15 is a soft,stable layer on upper and lower sides that could create excellent interfacial contact with electrodes,and LLTO-75 possesses physical rigidity that could inhibit the Li dendrites growth.The balancing between the excellent interfacial contact of LLTO-15 and the inhibition of lithium dendrites growth of LLTO-75 gives sandwiched SSE a high ionic conductivity(4.7×10-4 S cm-1)at room temperature,an excellent mechanical strength(7.2 MPa),and uniform Li plating/stripping.When coupled with LiFePO4 and Li metal,the battery assembled with sandwiched SSE maintains 91.7%of its capacity and 99.7%of the coulombic efficiency after 1000 cycles.(4)Due to the strong oxidizing action of high voltage cathode and strong reducing action of Li metal anode,the heterogeneous multilayer structure CPEs(SCPE)are further prepared by a layer-by-layer coating process.Oxidation resistant polyacrylonitrile(PAN)and reduction resistant polyethylene oxide(PEO)are used to contact with cathode and anode respectively to protect "polymer-in-ceramic"(PIC)-CPE in SCPE.In which,PAN-CPE contacts with LiNi0.8Mn0.1Co0.1O2(NCM811)to form a stable interface,and PEO-CPE contacts with Li to avoid reduction,thus ensuring high-voltage tolerance.More importantly,the synergistic effect of different CPEs in SCPE significantly facilitates high Li ion conductivity(2.81×10-4 S cm-1),wide electrochemical window(4.92 V),strong mechanical property and stable electrolyte/electrode interface.As a result,the Li symmetric cell using SCPE can be cycled over 500 h without voltage hysteresis.The NCM81 1/Li battery assembled with SCPE exhibits excellent cycle stability(88%after 500 cycles),and high Coulombic efficiency(over 99.2%after 500 cycles).(5)A simple and scalable method for an artificial LiF-rich solid electrolyte interphase(SEI)has been developed by using self-driven chemical reaction between Li3xLa2/3-xTiO3/polyvinylidene fluoride/dimethylacetamide(LLTO/PVDF/DMAc)solution and Li metal at room temperature.After coating LLTO/PVDF/DMAc solution on Li foil,the PVDF will react with Li spontaneously to form LiF.Simultaneously,Ti4+ ions(in LLTO)are reduced to Ti3+ions to form a mixed ionic and electronic conductor LixLLTO.The artificial SEI has the combined capacity to redistribute the Li ion transport,guide the uniform Li deposition,and suppress the formation of Li dendrites.When coupled with LiFePO4,NCM811 and S cathodes,the cells have demonstrated excellent capacity retention and cycling stability.More importantly,a high energy density of 181 mAh g-1 with 83%capacity retention after 200 cycles,has been achieved by using an integrated NCM811/LixLLTO-Li all-solid-state LMBs.This work provides a new avenue for the large-scale preparation of safe Li anode for the next-generation high-energy-density solid-state LMBs.