In July 2024, The research group of Mi Hongwei from Shenzhen University published a paper online in Nano letters (impact factor 9.6), “Unlocking 4.9V Quasi-Solid-State Lithium Metal Battery via Solvent Screening. “and Interfacial Manipulation”, this study effectively solved the problems of narrow electrochemical stability window and low ionic conductivity of polymer electrolytes through the efficient strategy of solvent screening and interface modification, and provided a new idea for the realization of high pressure solid lithium metal batteries.
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Research background
Solid lithium metal batteries are potential candidates for efficient storage and transmission of intermittent energy. However, the decomposition of the electrolyte and the destruction of the positive electrode structure at high potential are the reasons for the deterioration of the performance of solid lithium metal batteries, which greatly hinders their commercial application. A series of modification measures such as inorganic fillers and plasticizers have solved the problems of low intrinsic conductivity and poor electrochemical stability of polymer electrolyte. However, the general principle of solvent screening in polymer electrolytes and the microkinetic mechanism of electrolyte/electrode interface reaction have not been systematically studied.
Research questions
By DFT calculation, trimethyl phosphate (TMP), a medium polar solvent with flame retardancy, was selected as a liquid plasticizer for polymer electrolyte. The concept of medium polarity solvent effectively avoids the problem of low electrochemical stability window caused by the solvent-dominated sheath structure of polymer electrolyte. At the same time, the ionic conductivity of the TMP-based modified electrolyte (SPE-TMP) is 1.05× 10-4 S cm-1, which can meet the normal operation of quasi-solid LMBs at room temperature.
Figure 1. General strategies for solvent screening in polymer solid electrolytes
The Li|Li symmetric battery assembled on SPE-TMP has a cycle life of more than 2000 h at 0.1 mA cm-2 current density. With an electrochemical stability window of up to 5.16V, SPE-TMP is one of the batteries with the best oxidation resistance reported today. By increasing the cut-off voltage from 4.5V to 4.9V, the specific capacity is significantly increased by 27.7%, which fully reflects the prospect of commercial application.
Figure 2. Electrochemical performance test of symmetrical cells assembled with polymer electrolytes
In situ characterization techniques such as EIS and XRD combined with XPS deep etching analysis at different cut-off voltages were used to determine the ion transport kinetics at the positive electrode interface of NCM811. Results The solid electrolyte interfacial phase (CEI) with gradient distribution of LiF, LiPFxOy and organic components was the main reason for the complete positive electrode structure at 4.9V. At the same time, the DFT calculation and COMSOL simulation of electrodes and electrolytes provide theoretical feasibility for the stable performance of quasi-solid LMBs at 4.9V cutoff voltage.
Figure 4. Investigation of interface microscopic dynamics properties of 4.9V high voltage solid state lithium metal battery
Figure 5. In-depth analysis of interface components of 4.9V high voltage solid state lithium metal battery
Research summary
In this work, a novel design concept is proposed to construct a gradient-distributed positive electrolyte interface (CEI) by adjusting the intrinsic structure of the polymer electrolyte using the intermediate dielectric constant solvent TMP. This effective strategy not only improves the electroplating/stripping reversibility and stability of Li symmetric batteries, but also significantly widens the electrochemical stability window to meet the normal operation of quasi-solid lithium metal batteries at 4.9V. In addition, a variety of in-situ characterization combined with DFT theoretical analysis and COMSOL simulation demonstrated that the addition of TMP can produce LipoxFy-rich organic-inorganic gradient distribution of CEI, thus promoting uniform and rapid ion conduction and effectively inhibiting the structural destruction of NCM811. Finally, the assembled NCM811 | Li full battery operated stably at 2.8-4.9V for more than 400 cycles, with an average coulomb efficiency of 99.75%. This simple solvent screening and interface manipulation strategy is expected to provide a forward-looking perspective for the design of solid-state lithium metal batteries with high energy density.
The original link: https://doi.org/10.1002/acs.nanolett.4c01453
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