Common Modification Strategies for Lithium Battery Separators

Introduction

Lithium battery has the advantages of high energy density, light weight, strong flexibility, slow self-discharge rate, high charging rate, and long battery life. It is a promising energy storage that integrates renewable resources and high-power applications. device [1]. Separator is one of the most important components in rechargeable lithium batteries. It can not only provide a physical barrier between the positive and negative electrodes to prevent short circuits, but also serve as the main transport path for lithium ions and electrolyte anions by adjusting their composition and structure, can achieve the purpose of improving the ion transport characteristics of the battery.

Although they have good chemical/electrochemical stability, excellent mechanical strength, and appropriate thermal shutdown properties, the most widely commercialized polyolefin-based (such as polyethylene (PE) (Product No.: P434353), polypropylene (PP) (Product No.: P301642) and PP/PE/PP sandwich composites) microporous separators have problems such as severe heat shrinkage, poor electrolyte wettability, low porosity, and flammability, which will inevitably cause safety hazards and affect lithium batteries. Electrochemical performance of the battery. [2-4]

Diaphragm Structure Optimization

When the separator is prepared by phase inversion method and electrospinning method, the structure of the separator can be optimized by adjusting the precursor and experimental parameters, and the purpose of improving the ion transmission rate of the separator can be achieved. The phase inversion method is to dissolve the polymer in a solvent, and through solvent exchange, the polymer is precipitated to form a microporous structure [4]. This method avoids the stretching process in dry and wet methods, and can effectively prevent the thermal shrinkage of the porous structure of the separator.

Wang et al. prepared a sponge-like poly sulfonamide (PSA)/SiO2 composite membrane by phase inversion method, and successfully applied it to the separator of lithium-ion batteries (LIBs) [5]. Compared with the commercial polypropylene (PP) separator, the sponge-like PSA/SiO2 composite exhibits better physical and electrochemical properties, such as higher porosity, ionic conductivity, thermal stability, and flame retardancy. The LiCoO2/Li half-cells using the sponge-like composite separator exhibit better rate capability and cycle performance than those using the commercial PP separator. In addition, the sponge-like composite separator can guarantee the normal operation of the LiCoO2/Li half -cell at extremely high temperature of 90°C.

Ma et al prepared 9 kinds of polyacrylonitrile (PAN) (project number: P303200) nanofiber membranes with different fiber diameters and membrane porosity by electrospinning-hot pressing method [6]. Subsequently, these films were explored as lithium- ion battery (LIB) separators. The effects of fiber diameter and membrane porosity on electrolyte uptake, Li-ion transport through the membrane, electrochemical oxidation potential, and performance as a Li-ion separator were investigated during cathode half-cell charge-discharge cycling and rate capability tests. The results show that the PAN -based separator has a smaller fiber diameter: 200-300 nm, and exhibits the best performance at high pressure (over 20 MPa), with the highest discharge capacity (89.5mAh/g-1 at C/2 rate) half-cell cycle life (97.7% capacity retention). This study reveals the prospect of hot-pressed electrospun PAN nanofibrous membranes, especially those composed of thin nanofibers, as high-performance Li-ion separation materials.

Figure 1: Initial charge-discharge voltage distribution of PAN-based separator at C/2 rate

Polar Group Grafting

lithium metal batteries has always been affected by lithium precipitation. Wu et al. developed a functionalized porous bilayer composite separator by simply coating polyacrylamide-grafted graphene oxide (Project No.: G405797) molecules onto commercial polypropylene membranes [7]. The composite separator combines the lithiophilic properties and fast electrolyte diffusion pathways of hairy polyacrylamide chains with the excellent mechanical strength of graphene oxide nanosheets, resulting in enhanced Li-ion flux at the electrode surface. The results show that at a high current density (2 mA cm−2), the Li metal anode achieves dendrite-free Li deposition with high Coulombic efficiency (98%) and ultra-long-term reversible Li plating / stripping (over 2600 h). Notably, the lithium metal anode exhibits a cycle stability of more than 1900 hours at an ultrahigh current density of 20 mA cm−2.

Figure 2: Schematic diagram of lithium deposition on battery electrodes assembled with PP separator and GO-g-PAM modified PP separator respectively

Inorganic Particle Coating

Separators play a vital role in the safety of Li-ion batteries. However, currently commercialized separators are mainly based on microporous polyolefin membranes, which pose serious safety risks, such as thermal stability. Although many efforts have been made to solve these problems, the safety of batteries cannot be fully ensured, especially in the scenario of large-scale application. Zhao et al. formed an integrally covered self-supporting film on the ceramic layer and pristine polyolefin film by a simple dip-coating process on polydopamine (PDA), so that the ceramic layer and the polyolefin film appeared as a single side, And further improved the film-forming properties of the separator [8]. At the same time, the separator also has good heat resistance properties.

Figure 3:  Schematic diagram of composite modified separator prepared by simple dip-coating process

Figure 4: (a) The thermal shrinkage ratio of the pristine PE separator, PE-SiO2 separator and PE-SiO2 @PDA separator before and after heat treatment at 170°C as a function of temperature; (b) Comparison of pristine PE separator before and after heat treatment; (c) Comparison of PE-SiO2 separator before and after heat treatment; (d) Comparison of PE-SiO2@PDA separator before and after heat treatment; (e) Comparison of PE-SiO2 @PDA separator before and after heat treatment at 220°C.

Polymer Modification

Cui et al modified environment-friendly cellulose microfibers with polydopamine-coated mussels as the carrier, and prepared cellulose/polydopamine (CPD) membranes by a simple and economical papermaking process [9]. The results show that the CPD film has a dense porous structure, excellent mechanical strength, and good thermal dimensional stability, and can be used as a separator for lithium- ion batteries. Further, the AC impedance of the battery using the CPD separator showed a slight change of 9 Ω after the 100th cycle, indicating that the battery has good interfacial stability. These excellent properties make CPD films have broad application prospects as high-performance Li-ion battery separators.

Figure 5: (a) Cycling performance of LiCoO2/graphite cells with separator, cellulose separator, and CPD separator, (b) rate performance and (c) Nyquist plots of the cells measured after the first cycle and (d)

Poor cycling stability and safety issues of lithium metal anodes are two major issues hindering the commercialization of high energy density lithium metal batteries. In this regard, Leif Nyholm proposed a new three-layer diaphragm design, which significantly improved the cycle stability and safety of lithium metal-based batteries [10]. The average pore size of the nanocellulose layer is about 20 nm, and the mesopore thickness is 2.5 μm, which provides a better channel for the flow of Li+ , thus stabilizing the lithium metal anode and improving the cycle stability. The separator is also relatively safe at high temperatures because the three-layer separator remains dimensionally stable even at 200°C when the inner PE layer melts and blocks ion transport through the separator. The nanocellulose-based three-layer separator is expected to greatly promote the realization of high energy density Li metal-based batteries.

Figure 6: Schematic diagram of the effect of the pore distribution of the separator on the morphology of the lithium electrode

References

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[2] SS Zhang, A review on the separators of liquid electrolyte Li-ion batteries J.Power Sources, 164(1) (2007) 351-364. https://doi.org/10.1016/j.jpowsour.2006.10.065

[3] JH Parka, JH Choa , W. Parka, D. Ryoob , SJ Yoonc , JH Kimc,YU . Jeongd, SY Lee, Close-packed SiO 2 /poly (methyl methacrylate) binary nanoparticles-coated polyethylene separators for lithium-ion batteries. J. Power Sources, 195 (24) (2010) 8306-8310. https://doi.org/10.1016/j.jpowsour.2010.06.112

[4] Yuan B, Wen K, Chen D, et al. Composite separators for robust high rate lithium ion batteries[J]. Advanced Functional Materials, 2021, 31(32): 2101420.https://doi.org/10.1002/adfm.202101420

[5] Wang X, Xu G, Wang Q, et al. A phase inversion-based sponge-like poly-sulfonamide/SiO 2 composite separator for high performance lithium-ion batteries[J]. Chinese journal of chemical engineering, 2018, 26(6): 1292-1299. https://doi.org/10.1016/j.cjche.2017.12.010

[6] Ma X, Kolla P, Yang R, et al. Electrospun polyacrylonitrile nanofibrous membranes with varied fiber diameters and different membrane porosities as lithium-ion battery separators[J]. Electrochimica Acta, 2017, 236: 417-423.https://doi.org/10.1016/j.electacta.2017.03.205

[7] Li C, Liu S, Shi C, et al. Two-dimensional molecular brush-functionalized porous bilayer composite separators toward ultra-stable high-current density lithium metal anodes[J]. Nature communications, 2019, 10(1) : 1363.https://www.nature.com/articles/s41467-019-09211-z

[8] Dai J, Shi C, Li C, et al. A rational design of separator with substantially enhanced thermal features for lithium-ion batteries by the polydopamine–ceramic composite modification of polyolefin membranes[J]. Energy & Environmental Science, 2016 , 9(10): 3252-3261. https://doi.org/10.1039/C6EE01219A

[9] Xu Q, Kong Q, Liu Z, et al. Polydopamine-coated cellulose micro fibrillated membrane as highly performance lithium-ion battery separator[J]. Rsc Advances, 2014, 4(16): 7845-7850. https: https://doi.org/10.1039/C3RA45879B

[10] Pan R, Xu X, Sun R, et al. Nanocellulose modified polyethylene separators for lithium metal batteries[J]. Small, 2018, 14 (21): 1704371.https://doi.org/10.1002/smll.201704371