High-performance covalent organic framework solid-state electrolyte powers all-solid-state lithium battery technology

On October 27, 2022, Professor Xu Jijing’s team at Jilin University published a new study entitled “An integrated solid-state lithium-oxygen battery with highly stable anionic covalent organic frameworks electrolyte” in the journal Chem.

The research group reported a novel three-dimensional covalent organic framework (COF) solid electrolyte material for the construction of high-safety solid-state lithium-oxygen batteries and solid-state lithium-metal batteries, and revealed the transport mechanism of lithium ions in the material skeleton. Both solid-state lithium-oxygen batteries and solid-state lithium-metal batteries using COF solid electrolyte show better discharge capacity, rate performance and cycle life, demonstrating the huge application potential of COFs as new lithium-ion conductors in next-generation solid-state energy storage devices.

The corresponding author of the paper is Professor Xu Jijing of Jilin University; The first author is Dr. Wang Xiaoxue, a Dingxin scholar at Jilin University.

Due to the increasing demand for safe and efficient energy storage systems, lithium solid-state batteries have emerged as the next generation of energy storage devices to meet high energy density and safety requirements. Solid-state lithium batteries using solid-state electrolyte can not only effectively overcome safety problems, but also improve the energy density and cycle life of the battery, which is in line with the future development direction of high-safety and high-energy density lithium batteries. In particular, solid-state lithium-oxygen batteries have the highest theoretical energy density in the existing battery system and are expected to play an important role in the next generation of energy storage devices. As a key component of solid-state lithium-oxygen batteries, solid-state electrolytes must not only have high ionic conductivity, excellent stability to lithium metal, and good interface compatibility (Figure 1), but also have high stability to air components and excellent oxidation stability (resistance to attack by oxygen reduction intermediates). At present, the solid electrolyte used in solid-state lithium-oxygen batteries is mainly divided into two categories: solid polymer electrolyte and solid inorganic electrolyte. Although the polymer electrolyte has good flexibility and is easy to construct a solid-solid interface, its low ionic conductivity at room temperature leads to low rate performance and power density of the battery. Inorganic solid electrolytes (garnet, perovskite, sodium ion fast conductors, sulfides, molecular sieves, etc.) have high ionic conductivity, but their high rigidity makes the construction of solid-solid interfaces difficult. Therefore, there is an urgent need to find a new solid electrolyte with high ionic conductivity, high interface compatibility and high chemical stability for solid-state lithium-oxygen batteries. Covalent organic framework (COF) is a two-dimensional or three-dimensional framework material with regular pores formed by the connection of organic units with covalent bonds, which has the advantages of large specific surface area, adjustable function, good stability and low density, and the covalent bonds and functional groups in COF give it higher functional diversity. Chemical modification based on COF is beneficial to create excellent lithium-ion conduction channels and improve the durability of lithium-ion batteries. However, the preparation of COF usually requires harsh reaction conditions and the difficulty of compounding with cathode conductive materials limit its further application in solid-state lithium batteries.

In this work, Professor Xu Jijing’s research group based on covalent organic framework materials (COFs) with high porosity and periodic open pores, and realized a solid COF electrolyte with high ionic conductivity, electrochemically stable and air-stable through microwave-assisted synthesis methods, which exhibits excellent electrochemical properties in lithium batteries. At the same time, the two-dimensional exchange solid-state NMR technology was used to study the transport behavior of lithium ions in the pores in depth, which provided strong support for the experimental results.

Figure 1: Schematic diagram of solid-state electrolyte requirements for lithium-oxygen batteries.

(a) The composition of solid-state lithium-oxygen batteries and the corresponding requirements for their solid-state electrolytes; (b-d) Lithium ion transport mechanism and main performance evaluation of different solid electrolytes in solid-state lithium-oxygen batteries.

CD-COF-Li solid electrolyte with high ionic conductivity, low reaction activation energy and high chemical/electrochemical stability was successfully prepared by fast and environmentally friendly microwave-assisted method, and the stable cycle of solid-state lithium-air/lithium metal battery based on COF electrolyte was realized for the first time. The 7Li relaxation time test was performed by the reverse recovery method, and the saturation relaxation time of CD-COF-Li was calculated to be 0.452 s (Figure 2), indicating that a large number of movable lithium ions exist in the CD-COF-Li pore and can migrate rapidly in the directional pore of the COF.

Figure 2: Schematic diagram and characterization of COF solid electrolyte.

(a) Synthesis of CD-COF with condensation of γ-CD and B(OMe)3 with LiOH under microwaves to obtain CD-COF with different counter ions; (b) 13C CP-MAS NMR spectra for CD-COF-Li; (c) CD-COF-Li experiments and Pawley’s refined PXRD patterns and their differences; (d) SAED images with CD-COF-LI; (e) Saturation recovery plot of MAS 7Li NMR spectra.

The ionic conductivity of CD-COF-Li was evaluated using alternating current electrochemical impedance spectroscopy (EIS) over a temperature range of 25-70 °C. At room temperature, the ionic conductivity of CD-COF-Li can reach 2.7 mS cm−1, surpassing all currently reported ceramic electrolytes and filler-free polymer electrolytes for lithium-oxygen batteries (Figure 3). Further in-depth study of ion diffusion in COF pore by 7Li-7Li two-dimensional exchange solid NMR shows that the lithium ions on the CD-COF-Li skeleton have exchange behavior with the lithium ions dissociated by lithium salts in the pores. The high diffusivity of lithium ions through nanochannels may stem from the favorable lithium ion transport pathway provided by CD-COF-Li with regular open channels.

Figure 3: Lithium ion conduction behavior within the CD-COF-Li backbone.

(a) CD-COF-Li ionic conductivity (σ) at different temperatures, the corresponding oblique fit to the Arrhenius equation results; (b) Ionic conductivity comparison between CD-COF-Li and other solid electrolytes has been applied to lithium-oxygen batteries containing SPE and SIE; (c) Lithium ion transport mechanism and main performance evaluation of different solid electrolytes in solid-state lithium-oxygen batteries.

Symmetrical cells and finite element simulation (FEM) study the electrochemical stability of COF solid-state electrolytes. Symmetrical cells using COF solid electrolyte showed excellent cycle stability (Figure 4), indicating that COF solid electrolyte has the characteristics of regulating lithium ion flux, inducing uniform deposition of lithium, and its excellent mechanical strength effectively inhibits the formation of lithium dendrite. In order to explore how CD-COF-Li solid electrolyte inhibits the growth of lithium dendrite, FEM is used to simulate the evolution of Li+ concentration near the Li surface. Severe Li+ polarization and depletion occur in cells using liquid electrolytes, resulting in freely growing Li dendrides. In sharp contrast, the local concentration of Li+ near the solid electrolyte of CD-COF-Li increases significantly, and the current density is evenly distributed on the Li surface.

Figure 4: Lithium-symmetric battery cycle performance and interface stability.

(a) Lithium stripping/deposition voltage curves for symmetrical cells at 0.2 mA cm−2, illustrated with amplified voltage curves over 100-106 hours, 346-352 hours, 544-552 hours, and 746-752 hours; (b-d) Lithium ion concentration distribution and finite element simulation of lithium metal deposition in liquid electrolyte; (e-g) Finite element simulation of lithium ion concentration distribution and lithium metal deposition in CD-COF-Li solid electrolyte.

Thanks to the skeleton flexibility of COF electrolyte, CD-COF-Li/CNT solid air cathode was successfully prepared, and its abundant electron, ionic and gas diffusion channels can provide more product growth space. In order to verify the applicability of COF electrolyte, the solid-state lithium-oxygen battery was assembled with typical polymer electrolyte PEO and inorganic ceramic solid electrolyte LAGG as the contrast material, and various electrochemical tests were performed. The results show that the discharge capacity of solid-state lithium-oxygen batteries using CNT/CD-COF-LI is 9340 mAh g−1, which far exceeds the battery capacity using CNF/PEO and CNT/LAGP (2100 mAh g−1 and 5040 mAh g−1). In addition, solid-state lithium-oxygen batteries based on CNT/CD-COF-LI achieve 100 stable cycles, greatly surpassing CNT/PEO and CNT/LAGP-based batteries (38 and 50 cycles, respectively).

Figure 5: Construction and electrochemical performance of solid-state lithium-oxygen batteries.

(a) Electronic and ionic conductivity values for CD-COF-Li, LAGP, and PEO; (b-c) Finite element simulation of lithium ion concentration distribution and lithium metal deposition of lithium electrodes with LAGGP and PEO at selected simulation times; (d) SEM image of CD-COF-Li/CNT solid-state cathode; (e) Schematic diagram of the CD-COF-LI/CNT cathode with continuous lithium-ion and electron transfer pathways; (f) Capacity of solid-state lithium-oxygen batteries with integrated CD-COF-Li, LAGP and PEO at a current density of 100 mA g−1; (g) Curve of discharge voltage vs. number of cycles of a solid-state lithium-oxygen battery at a current density of 200 mA g−1 and a cut-off capacity of 500 mAh g−1.

The processability of COF solid electrolyte provides possibilities for its application in lithium metal battery systems. For this purpose, the CD-COF-Li solid electrolyte is assembled into a solid lithium metal battery by the traditional coating and pouring method. Lithium metal batteries with CD-COF-LI SCE can provide an initial discharge capacity of 149 mAh g−1 and stabilize to 135.8 mAh g−1 after 100 cycles at 0.1 C current density. In sharp contrast, lithium metal batteries with PEO and LAGG exhibit lower discharge capacity (50.1 and 75.8 mAh g−1) and poor capacity retention (62% and 75.4%). Based on these results, CD-COF-Li has proven to be an ideal solid-state electrolyte for high-performance solid-state lithium-oxygen and lithium-metal batteries.

Figure 6: Manufacturing and electrochemical performance of solid-state lithium metal batteries.

(a) Schematic diagram showing the construction of the CD-COF-Li electrolyte layer on the positive electrode; (b) EIS for lithium metal batteries with PEO, LAGP and CD-COF-LI SSE; (c) Constant current charge/discharge voltage curve at 0.5 C for lithium metal batteries with PEO, LAGP and CD-COF-LI SSE; (d) Lithium metal batteries with PEO, LAGP and CD-COF-LI SSEthe multiplier performance of the pool; (e) Cycling performance of lithium-metal batteries with PEO, LAGP and CD-COF-Li SSEs at 0.1 C; (f) Coulomb efficiency of lithium metal cells with PEO, LAGP and CD-COF-Li SSEs.

This work reveals the transport mechanism of lithium-ion in COF channels, provides new ideas for the diversified design of lithium-ion conductors, and opens up a new path for the selection and design of solid-state electrolyte materials for lithium-metal/air batteries. (Source: Web of Science)

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