Preparation,Conducting Mechanism And Performance Optimization Of Polymer Solid Electrolytes For Solid-State Lithium Batteries

Li-ion batteries have been used globally in powering portable electronics and electric vehicles.However,liquid electrolytes in lithium-ion batteries bring safety risks of leakage and even combustion.Additionally,the development of the Li-ion batteries with a graphite anode almost reaches its theoretical density limit.Replacing liquid electrolytes with solid electrolytes is particularly promising in fundamentally solving the safety problems,because of the inherent safety characteristics of solid electrolytes.Moreover,using a lithium metal anode,solid-state batteries can deliver higher energy density.Solid polymer electrolytes possess many advantages,such as easy fabrication,low cost,scalability,and mechanical flexibility.However,applications of solid polymer electrolytes are limited by the low ionic conductivity and huge interfacial resistance.To solve these problems,we conduct the following works:(1)Composite solid electrolytes are synthesized by dispersing Li6.25Ga0.25La3Zr2O12(LLZO)nanoparticles in the polyethylene oxide(PEO)matrix.The PEO-LLZO composite shows a conductivity of 7.2×10-5 S/cm at 30?,about 4 orders of magnitude higher than the conductivity of PEO.The enhancement of the ionic conductivity is closely related to the space charge region(?3 nm)formed at the interface between the PEO matrix and the LLZO nanoparticles,which is observed by transmission electron microscope(TEM)and corroborated by the phase-field simulation.Additionally,we quantify the defects concentration and distribution in the space charge region to simulate the formation of the space charge region.Using the random resistor model,the lithium-ion transport in the composite polymer electrolyte is simulated by the Monte Carlo simulation,demonstrating that the enhanced ionic conductivity can be ascribed to the ionic conduction in the space charge regions and the percolation of the space charge regions.(2)Three-dimensional(3D)garnet network reinforced composite solid electrolytes(3D composite electrolytes)with continuous Li-ion transfer channels are prepared.The 3D garnet networks are fabricated via the polymeric sponge method,using the low-cost polyurethane foam as a template.Owning to interconnected 3D percolation networks,the 3D composite electrolytes show an ionic conductivity of 1.2 × 10-4 S/cm at 30?,about two times as high as that of PEO-LLZO composite electrolyte.The enhanced ionic conduction has verified the space charge layer theory.Additionally,3D composite electrolytes demonstrate enhanced Li-ion transference numbers,as well as thermal,mechanical and electrochemical stabilities.Symmetric Li/3D composite electrolytes/Li cells can be cycled more than 360 h without short-circuiting,suggesting an improved ability to suppress Li-dendrites.All-solid-state LiFePO4/3D composite electrolytes/Li batteries show stable cycle performances and high specific discharge capacities.(3)The ionic conductivity of composite solid electrolytes is further optimized by using LLZO nanofibers as fillers.LLZO nanofibers are prepared by electrospinning,and the average diameter of LLZO nanofibers is about 120 nm.Therefore,the ionic conductivity of the LLZO nanofibers reinforced composite electrolytes(nanofiber composite electrolyte)is 3.2×10-4 S/cm,which is about 4 times higher than that of the PEO-LLZO composite electrolyte,and the electrochemical window is 4.8 V.The Li/nanofiber composite electrolyte/Li symmetric cells can stably cycle over 1500 cycles at 0.45 mA/cm2,showing superior ability to suppress dendrite growth.Using PEO-LiTFSI-LLZO,the designed integrated interface maximizes the interfacial contacts between the cathode and the electrolyte,thus effectively mitigates the interface resistance.Therefore,LiFePO4/nanofiber composite electrolyte/Li all-solid-state batteries can achieve capacity retention of 91%after 800 cycles at 0.5 C.(4)Composite solid electrolytes are prepared by polymerizing liquid precursor containing LLZO nanoparticles.Polymerized composite electrolytes(PCEs)are electrochemically stable up to 6.5 V versus Li/Li+at room temperature.Additionally,the PCEs show an ionic conductivity of 1.8 × 10-4 S/cm and a Li-ion transference number of 0.58 at 30?.Li/PCE/Li symmetric cells maintain stable cycling for more than 1000 h at 0.5 mA/cm2,and the dendrite formation is effectively inhibited,showing excellent compatibility with lithium anodes.Using the in-situ polymerizing method,PCEs retain conformal interfacial contacts with all electrodes,and the interfacial resistance of the all-solid-state batteries is decreased by half.Coupled with LiNi0.6Co0.2Mn0.2O2(NCM622)and LiNi0.85Co0.05Al0.1O2(NCA)cathodes,all-solid-state Li metal batteries with PCEs exhibit excellent cycling performances,including superior specific capacity,high Columbic efficiency and long cycle lifetime at room temperature.(5)We report an in-situ strategy initiating polymerization of liquid electrolytes in electrochemical cells to produce quasi-solid electrolytes with cross-linked structure;with such a strategy,ultraconformal interfacial contacts between electrolyte/cathode and electrolyte/anode are created.The quasi solid electrolytes exhibit a high ionic conductivity of 4.3 × 10-3 S/cm at room temperature,a superior Li+transference number of 0.55 and a wide electrochemical window of 5.2 V.The robust LiF-rich solid electrolyte interphase(SEI)layer on the Li anode effectively prevents the dendrite growth and the pulverization of the Li anode.By applying quasi-solid electrolytes in NCA-Li,NCM622-Li solid-state batteries,high specific discharge capacities and long cycle lifetime are achieved at large current densities.The long-term cycling results of NCM622-Li solid-state batteries show that Coulombic efficiencies close to 100%,and stable cycling with 81%capacity retention(104 mAh/g)are achieved even after 400 cycles at 2.0 C.