Study On Preparation And Performance Of Polymer Electrolytes For Low-Temperature Solid-State Batteries

In recent years,with the wide application of lithium batteries in portable electronic devices,electric vehicles and large-scale grid energy storage,people have higher requirements for electrochemical performance of lithium batteries.Solid-state lithium batteries,have become one of the most promising candidates for the next generation of high-safety and high-energy density batteries.This is because solid-state polymer electrolytes(SPE)are non-volatile,non-leaky,lightweight,easy to process and low cost.However,due to the low ionic conductivity,poor interfacial stability and lithium dendrite growth,solid-state lithium batteries usually need to be operated at high temperature,and face challenges,such as low capacity and poor cycling stability,which severely limit their application.This thesis focuses on the bulk ionic conductivity of SPE,the SPE-electrode interface,and the infiltrative growth of lithium dendrites in SPE,resulting in significantly improved electrochemical performance.The results provide a scientific insight into the design of high-safety solid-state lithium batteries with high energy density and long cycle life at low temperatures.The main findings are as follows:(1)We proposed to construct homogeneous and fast ionic channels from the microscopic scale of ion transport,which is conducive to achieve high performance SPE.Homogeneous SPE was formed by the molar ratio of succinonitrile and ethylene oxide(EO),which can suppress the crystallization of PEO and mitigate the affinity between PEO and lithium ions(Li+).Therefore,the homogeneous-inspired design of PEO-based solid-state electrolyte possesses a continuous Li+ binding site with a lower energy barrier,which increased its ionic conductivity by 100 times,and a high ionic conductivity of 1.9×10-4 S cm-1 was achieved at room temperature(25℃).A specific discharge capacity of 145.5 mAh g-1 and excellent cycling stability of 750 cycles at room temperature and 0.5 C were achieved.In addition,82%of the discharge capacity can be maintained even at 0℃.This work provides new ideas for the design of lowtemperature solid-state lithium batteries.(2)We developed a solidified localized high-concentration electrolyte(S-LHCE)with the decoupling of ion pairing and ion conduction by freeze-drying method.By decoupling electrolytes with a non-solvating solid framework(PVDF-HFP),the interfacial compatibility was further improved with stable lithium plating and stripping process and a wider electrochemical stability potential up to 5 V.Anion migration was limited with a high Li+transference number of 0.72,and the Li+conduction was enhanced(1.9×10-4 S cm-1 at 20 ℃)by the regulated solvation structure in an ultrahigh salt concentration regime.Based on the electrochemical properties,the symmetric lithium cell with S-LHCE exhibited a stable plating and stripping process over 4100 h at a current density of 0.2 mA cm-2.The S-LHCE strategy enabled solid-state lithium batteries with excellent electrochemical performance over a wide temperature range from-10 ℃ to 100 ℃,exhibiting excellent cycling stability of 800 cycles at 1C,and 83.3%(30 C)and 60.1%(50 C)capacity retention when cycled at an evaluated temperature.This work opens new avenues for designing and manufacturing of highperformance solid-state lithium batteries operating over a wide temperature range.(3)We discovered the rapid ion diffusion transport behavior in highly crystalline pure PEO,and quantified the ion transport speed of 0.463 μm min-1 by the steady-state measurement method based on electrochemical impedance spectroscopy.We also proposed to construct a solid-state electrolyte group with a Li+-poor interlayer to inhibit the lithium dendrites infiltration growth in solid-state electrolytes.The symmetric lithium cell with solid-state electrolyte group exhibited an improved cycling stability from 19 h to more than 2400 h at a current density of 0.2 mA cm-2.And the solid-state lithium battery with solid-state electrolyte group shows stable cycling up to 1400 cycles without short circuit.This phenomenon of ion diffusion transport behavior in PEO and its universality in various metal battery(such as Na+,K+,Zn2+ and Al3+)are preliminarily discussed.This work provides a new research direction for inhibiting the infiltrating growth of metal dendrites in solid-state electrolytes and realizing safer solidstate batteries.