Constructing Polymer Ionic Conductor And Mixed Conductor For Solid-state Batteries

Lithium-ion batteries are highly demanded with the continuous growth of the electric vehicle and energy storage market.The solid-state lithium metal batteries(SLMB)using solid electrolyte with high mechanical strength and a metal lithium anode with excellent specific capacity(3860 mAh g-1)greatly improves the safety and energy density of the battery simultaneously,and are considered to be a strong candidate of next-generation secondary battery and has been received extensive attention.The low lithium ion conductivity and transference number of lithium ions(tLi+)of solid electrolytes impair the cycle stability and coulombic efficiency of the batteries.Meanwhile,the poor electrochemical stability also greatly affect the energy density and rate performance of the batteries;The nucleation and growth of lithium dendrites on the surface of the lithium anode greatly affect the stability of the electrode/electrolyte interface and reduce the power density and safety of the battery.The instability of the multi-phase solid-solid interface in the composite cathode of SLMB can easily induce a large charge diffusion barrier,resulting in the hindrance of lithium ion and electron transport inside the electrode,which in turn leads to a rapid capacity decline of the battery.In this thesis,based on lithium monooxalate borate grafted polyvinyl formal polymer(LiPVFM),solid polymer electrolytes(SPE)and ion/electronic dual-conductive polymers(DCP)with excellent performance are prepared by macromolecular structure design and the molecules interaction control,which greatly improve the interfacial problems and thus boost electrochemical performance of solid-state batteries.Firstly,LiPVFM/lithium difluorooxalate(LiODFB)single-ion conductor solid polymer electrolyte with excellent performance was successfully designed and prepared.The strong electronegative 1,3-dioxane ring in LiPVFM promotes the dissociation of lithium difluorooxalate borate(LiODFB)and coordinates with lithium ions to form O-Li+;In the meantime,the hydroxyl group in LiPVFM can interact with ODFB-to generate hydrogen bonds and restrict the movement of the anions.Furthermore,existence of ODFB-between the molecular chains of LiPVFM induce electrostatic effects.Based on the above characteristics,the carrier lithium ion concentration of the LiPVFM/LiODFB electrolyte is further increased,the lithium ion migration number is increased to 0.63,the lithium ion conductivity reaches 3.82 ×10-4 S cm-1,the electroche1ical stability window is about 5 V,and the mechanical strength is significantly increased to 60 MPa with good interface stability to lithium metal anode.Solid state Li metal batteries with LiPVFM/LiODFB SPE and LiFePO4 cathode exhibits 77.7%capacity retention after 150 cycles at 2.5-4.25 V and 0.3 C.Secondly,Poly 3,4-ethylenedioxythiophene/polystyrene sulfonate(PEDOT:PSS)and LiODFB molecules are assembled on the LiPVFM polymer chain segment through covalent bonding,coordination and hydrogen bond interactions,and therefore electron/lithium-ion dual conductive polymer(DCP)with high carrier concentration and independent cation and electron diffusion channels was prepared.DCP through reasonable design and regulation delivers a lithium ion conductivity of 2.76 × 10-4 S cm-1 and an electronic conductivity of 68.9 S cm-1.At room temperature.In addition,the combination of rigid and flexible segments enables DCP with an improved flexibility and elastic modulus(about 6.8 GPa).Besides,DCP can form a stable and uniform coating layer on the surface of the active material particles during the slurry preparation process,which further promotes the short-range/long-range lithium ion/electron transport,alleviates the interfacial side reactions and maintains the integrity of the cathodes.These excellent characteristics enable DCP-based LiFePO4 composite cathode with a high load of 11.7 mg cm-2 and a high active material ratio of 89 wt%.The assembled solid state Li metal batteries has a capacity retention rate of 71.2%after 500 cycles of 0.3 C with coulombic efficiency above 99.7%.Finally,in order to improve the oxidation stability of the polymer electrolyte,the polymethyl vinyl ether/maleic anhydride copolymer(PME)is subjected to a partial ring-opening reaction to generate carboxylic acid groups.The "Polymer in Salt" solid composite polymer electrolyte(PISE)was prepared through supramolecular interactions between carboxylic acid groups and the hydroxyl groups in the LiPVFM segment as well as coordination effect of-OH and maleic anhydride group.PME with abundant C=O and-OCH3 groups can further promote the dissociation of LiTFSI,achieving an improved ionic conductivity of 3.57 x 10-4 S cm-1 and a tLi+of 0.62 at room temperature.In the meantime,PISE has a wide electrochemical window exceed 5.0 V,a high modulus of 3.7 GPa,and excellent electrode interface compatibility.The assembled solid-state lithium metal battery based on PISE and DCP based LiCoO2 cathode delivers a capacity retention of 89.2%after 225 cycles at 0.5 C and 3.00-4.45 V;Furthermore,the batteries can be operated at 2.8-4.5 V and 0.1 C with excellent initial discharge capacity of 196.3 mAh g-1;Besides,discharge capacity of 2.58 mAh cm-2 can be achieved with 17.4 mg cm-2 cathode loading.In addition,batteries with PISE and DCP-based LiNi0.7Mn0.2Co0.1O2 cathode have a specific discharge capacity of 160.7 mAh g-1 after 80 cycles at 0.5 C and 2.8-4.3 V with excellent battery safety.