Both high ionic conductivity and stability of the interfacial film for a polymer electrolyte lithium-metal batteries

As people\’s passion for advanced high-power energy storage device, coupled with the energy density of conventional lithium-ion battery is nearly saturated, with high theoretical energy density of lithium-metal batteries widespread concern in the community. However, the high reactivity between the electrolyte and metallic lithium and lithium dendrite uncontrolled growth and other issues, will not only affect the electrochemical performance of the battery, and will bring a series of safety problems, which greatly limits the large-scale lithium-metal batteries application. By many researchers in-depth study of this issue, several strategies have been developed to inhibit the growth of lithium dendrites, to improve the cycle stability of the battery, improve the safety performance of the battery. Beijing Normal University, College of Chemistry, Professor Li Lin research group engaged in research of key materials such as polymer electrolyte membrane and lithium batteries, mainly focusing on the correlation between structure and performance aspects of functional polymers. The diaphragm is a lithium battery system, the most common polymer materials, in order to overcome the challenges faced by application of metallic lithium anode, Beijing Normal University, Professor Li Lin team and professor of Beijing University of Chemical Research Group Chow Wai-tung from the diaphragm, the use of lithium metal is highly lively , to design and develop a variety of functionality may be protected in situ and transferred to a separator metal lithium negative electrode, the redox reaction between the metal lithium on the diaphragm and the functional coating, the functional coating can be successfully transferred in situ on the membrane and binding the surface of the metallic lithium to form a protective layer. The protective layer is formed can be found in the cell operation, effectively control the morphology of deposited lithium, to obtain a low bulk specific surface area of ​​lithium is deposited, in order to impart excellent electrochemical battery performance and high safety. This part of the work respectively in 2019 and 2020 were published in \”Advanced Functional Materials\” (Adv.Funct. Mater., 2019, 30, 1907020) and \”Nano Letters\” (NanoLett., 2020, 20, 3798-3807) . In addition to the membrane, solid polymer electrolyte is a polymer material in another cell systems. The polymer electrolyte having ease of processing, the advantage of improving the growth of lithium dendrite and improve battery safety and the like, wherein the polyethylene oxide (PEO) polymer electrolyte is one of the most traditional and well studied polymeric body. However, large-scale application of the PEO-based polymer electrolyte is limited by its low room temperature ionic conductivity and lithium ion migration, therefore, to develop high-performance polymer electrolyte membraneIt is essential to promote the development of high energy density and high safety lithium metal batteries.

Preparation and properties of the polymer electrolyte membrane

Recently, Beijing Normal University professor Li Lin team polycondensation reaction i.e. by a two step synthesis (by polyethylene glycol 600 (PEG600) and fumaric acid to give an unsaturated polyester (FG600), then the double bond in the unsaturated polyester is opened by free radical polymerization), successfully obtained polymer electrolyte has a crosslinked structure. Studies have shown that the crosslinked polymer has good film-forming properties may be prepared directly as preferred flexible polymer electrolyte membrane (the PEM), and having a high thermal stability (~ 300 oC), battery safety can be improved sex. By pulsed gradient NMR tested lithium ion diffusion coefficient of the polymer electrolyte (3.0 × 10-13m2 s-1 at 40 oC) than FG600 liquid electrolyte (2.4 × 10-13m2 s-1) at room temperature prior to polymerization.

北师大李林教授团队《ACS AMI》:兼具高离子电导率和界面稳定性的聚合物电解质膜用于金属锂电池
FIG. 1. (a) FG600 a schematic view of the preparation and of the PEM; optical photograph (b) FG600 and PEM; and (c) at 25oC and 40 oC, FG600 liquid electrolyte and the PEM (c ) 7Li-NMR spectrum and (d) pulsed field gradient signal decay curve.

by the obtained and subjected to DSC PEM electrochemical properties tested PEM having a low glass transition temperature (Tg, -54.2 oC), high ionic conductivity (1.99 × 10 S -3 cm-1 at 30 oC) and high lithium ion transport number (0.58), the results indicate that cross-linking can significantly reduce the crystallinity of the monomer PEG600 obtain amorphous crosslinked polymer, combined with a low Tg to help to improve the lithium ion conductivity. Further, the low Tg PEM may be in close contact with the positive and negative, to effectively reduce the interfacial resistance between the electrode and the electrolyte.

北师大李林教授团队《ACS AMI》:兼具高离子电导率和界面稳定性的聚合物电解质膜用于金属锂电池
FIG. 2. (a) the temperature dependence of ionic conductivity of the PEM; (b) PEG600, FG600, FG600 PEM and the DSC curve after crosslinking; (c) cyclic dismantling before LiFePO4 | PEM | Li cells after cross-sectional view of the internal structure and element distribution.

PEM interface stability was evaluated by repeating the deposition of metallic lithium Li-Li symmetrical cell / lithium electrode and the protective release behavior. Studies have shown that, compared with the symmetrical liquid Li-Li batteries, at the same current density, the PEM cell overpotential exhibited stability and longer cycle times lower, which indicates a high ion conductivity and lithium ion transport number the PEM and the lithium electrode and good interfacial stability can be effectively suppressed the growth of lithium dendrites, improved cycle stability and safety of the battery.

北师大李林教授团队《ACS AMI》:兼具高离子电导率和界面稳定性的聚合物电解质膜用于金属锂电池
Figure 3. current density are (a) 0.25mA cm-2, (b) 0.5mA cm-2, and (c) 1.0mA cm-2, the deposition amounts are 1.0 mAh 2 cm-under, Li | LE | Li symmetrical cells (black line) and Li | PEM | Li battery symmetry (red) circular curve.

Application of the polymer electrolyte membrane in a metal lithium battery

with the PEM and the lithium electrode and LiFePO4 match, assembled into LiFePO4 | PEM | of Li batteries. Long cycle tests show that, given the PEM LiFePO4-Li batteries with excellent cycling stability (at 0.1 C, 25 oC, the ring 250 cycles, the capacity retention was 98.9%; at 0.5 C, 40 oC, cycle 500 laps , the capacity retention rate of 96.0%) and good rate performance.

北师大李林教授团队《ACS AMI》:兼具高离子电导率和界面稳定性的聚合物电解质膜用于金属锂电池
Fig 4. (a) at 0.1C, at 25 oC, LiFePO4 | PEM | Li cell cycle curve; (b) at 0.5C, 40 oC, LiFePO4 | LE | Li cells and LiFePO4 | PEM | cell cycle curve Li; (c) at 0.5C, 40 oC, LiFePO4 | PEM | Li battery charge and discharge curve; under (d) 40 oC, LiFePO4 | PEM | Li battery rate capability.

After the cell cycle LiFePO4-Li disassembled to observe the cross section and surface morphology of lithium. After circulating the liquid lithium anode cell surface LiFePO4-Li showed typical morphology of lithium dendrites, and a lithium sheet is severely corroded, presenting a cross-sectional chalking lithium and lithium loose deposition morphology. andPEM lithium anode cell presenting smooth surface, no obvious lithium dendritic morphology, which exhibits a uniform cross section lithium dense stacking. The results further indicate a good stability of the interface between the PEM and the metal lithium electrode, lithium facilitate uniform deposition / release, to improve the cycle stability and safety of the battery. LiFePO4 | PEM | Li battery pouch mechanical deformation, even after the cut portion of the LED light board can still guarantee a stable and bright, the results further indicate PEM produced had good flexibility, a tight contact between the electrodes and, can improve the safety of the battery.

北师大李林教授团队《ACS AMI》:兼具高离子电导率和界面稳定性的聚合物电解质膜用于金属锂电池
Figure 5. loop ring 50, LiFePO4 | (a) the surface topography of the negative electrode of lithium battery Li (b) a cross-sectional morphology and | LE; laps after 50 cycles, LiFePO4 | PEM | Li cell lithium negative electrode (c) a surface topography and (d) a cross-sectional morphology; (e) at the bending, LiFePO4 cropped | the PEM | Li battery pouch light LED light board.

by linear sweep voltammogram of the PEM prepared electrochemical stability characterization results show that the decomposition of the polymer electrolyte membrane voltage near 4.6 V, not only to satisfy the aforementioned garnet-type lithium iron phosphate batteries voltage requirements, and may try to match the positive electrode layer NCM three yuan a high voltage. By NCM532 | PEM | Li battery cycle test, the battery was found after 50 laps cycle capacity retention rate of 89.3%, an average coulombic efficiency of 99.4%, a stable damping capacity, but relatively rapid decay rate. Charge-discharge curve, the polarization cycle is increasing, indicating that the internal battery may not form a stable interfacial layer, NCM532 PEM may promote decomposition of the battery internal resistance results in increasing. Thus, despite having high decomposition voltages PEM, but poor compatibility with the positive electrode NCM, which may limit its application in the battery ternary positive electrode material in.

北师大李林教授团队《ACS AMI》:兼具高离子电导率和界面稳定性的聚合物电解质膜用于金属锂电池
Figure 6. 0.5 C, at 40oC, NCM532 | PEM | (a) Li cell cycle and curve (b) charge and discharge curve.

above related work published in \”ACS Applied Materials & Interfaces\” (ACSAppl.. Mater Interfaces, 2020, DOI: 10.1021 / acsami.9b21370) on. The first author of the paper is Dr. Liu Fengquan Beijing Normal University, corresponding author is a professor at Beijing Normal University, Li Lin, co-corresponding author is an associate professor of Beijing Normal University, Zhou Jianjun. Thanks to the support of National Natural Science Foundation project. Article link: [123 ]