As introduced in Section 2, ion flux on the electrodes must be controlled uniformly to avoid Li-dendrite formation [101]. Therefore, researchers have coated functional materials with high electrolyte wettability to separators because of their ordered pore size and polar functional groups. Here are some papers that have given these properties to separator through materials coatings.
Choi group improved the performance of a commercial separator using polydopamine [102]. When polydopamine is coated on the pe separator, it changes the surface of the PE separator to be hydrophilic, increasing the extent of liquid electrolyte absorption. This enables Li-ions to be transported homogeneously to the Li-metal surface and inhibits the growth of dendrites ( a). The catechol moieties group, which is part of polydopamine, has excellent adhesion to versatile substrates. Moreover, since the bonding strength is excellent even in a liquid electrolyte, the separator can adhere well to the electrode. Since the separator adheres well to Li-metal electrodes, the deformation of the substrate during charge/discharge cycles could be minimized. After polydopamine coating, electrolyte uptake increased from 15 ± 2.7% to 112 ± 3.1%, ionic conductivity increased from 0.04 × 10−3 to 0.3 × 10−3 S cm−1, and 80% of the initial capacity was maintained even after 150 cycles in a Li|LCO cell. Sun group reported on PP separators coated with PVDF and Li6.4La3Zr1.4Ta0.6O12 (LLZTO) coated to increase ion conduction [103]. The interaction between PVDF and LLZTO creates a three-dimensional high-speed Li-ion channel along the PVDF/LLZTO interface. This can effectively transport and distribute Li-ions to the anode electrode, inhibiting dendrite growth. In addition, this separator immobilizes negative ions to evenly disperse Li-ions in the Li-metal surface.
Open in a separate windowIn other studies, paper-type polymers were applied to increase ionic conductivity. Nyholm group prepared overoxidized polypyrrole (PPy) paper and cellulose composite films [104]. The overoxidizing process can electrically insulate PPy without structural damage ( b). In addition, both oxidized PPy and nanocellulose are hydrophilic properties, which increases the wettability of the electrolyte. Moreover, overoxidized PPy has higher thermal stability and ionic conductivity (1.1 mS cm−1) than commercial polyolefin-based separators. In Li|Li symmetric cell, this enables the cycle to operate for 600 h, thus proving to have a longer cyclic life than that of commercial separators.
There is a case of introducing ceramic material to increase ionic conductivity, taking advantage of entangled structure of polymer at the same time. Zhang and co-workers synthesized a separator using PAN and silica via centrifugal spinning. This cost-effective method developed separators with significant ionic conductivity and good wettability owing to the high porous fibril structure of PAN [107]. In this separator, PAN provided high ionic conductivity when the electrolyte was absorbed and had good thermal stability, with synergetic effects with SiO2. Electrolyte uptake was 310% and ionic conductivity was 3.6 × 10−3 S cm−1 in 12wt.% SiO2/PAN. They applied SiO2/PAN membranes to a Li|LFP full cell, which exhibited excellent rate performance with a capacity exceeding 160 mAh g−1.
The primary task of a separator is to prevent short circuits between the cathode and anode while maintaining ionic conductivity [82]. As described earlier, high mechanical strength is required to prevent dendrites from penetrating the separator [108]. Moreover, separators should have good electrolyte wettability and proper porosity [109]. In this section, high-modulus and porous materials coatings, which help in increasing the mechanical strength of separators, are discussed [61].
Ni group reported PVDF-HFP separator cross-linked with Al2O3 as the cross-linker [110]. The separator had a high ionic conductivity of 1.37 mS cm−1 in a Li|LFP half-cell. Because of the cross-linking and the presence of Al2O3, the mechanical strength was significantly increased to 30.4 MPa and thermal stability increased up to 180 °C. Kim group fabricated a high-strength separator using high-density polyethylene (HDPE) and ultra-high molecular weight polyethylene (UHMWPE) [105]. As the ratio of UHMWPE increased, the mechanical strength increased ( c). A film with 6wt.% of UHMWPE had a tensile strength of 1000 kg cm−2. In addition, it had uniform pores (0.1–0.12 µm) and excellent thermal stability that could withstand temperatures up to 160 °C.
Wang group fabricated an ultrastrong nanofiber membrane [106]. A nanoporous membrane was fabricated using a poly(p-phenylene benzobisoxazole) nanofiber (PBO-NF) through blade casting ( d). This separator was low cost and had a high strength of 525 MPa and Young’s modulus of 20 GPa. The membrane was stable up to 600 °C. In Li|Li symmetric cell, a pure Li-metal surface was observed after 700 cycles. It exhibited excellent performance in preventing dendrites growth. Kotov group synthesized aramid nanofibers (ANFs) [111]. In a layer by layer (LBL) structure, poly(ethylene oxide) (PEO) was applied in the ANFs as an ionic conductor. The tensile strength, Young’s modulus, and shear modulus were recorded as σICM = 170 ± 5 MPa, EICM = 5.0 ± 0.05 GPa, and GICM = 1.8 ± 0.06 GPa, respectively. The crystallization of PEO, which is known to be detrimental to ion transport, can be controlled by the presence of ANF networks. As a result, in Li|Li symmetric cell, ionic conductivity was 1.7 × 10−4 S cm−1, which was higher than that of conventional polyolefin-based separators.
For the commercial use of LMBs, thermal stability is a very important factor in separators. At high temperatures, the ionic conduction is very active, accelerating dendritic growth. Simultaneously, the separators lose their mechanical stability at high temperatures [112]. Polyolefin-based separators are not stable at high temperatures (130–160 °C) [48]. To complement this, a method of coating the separator with a ceramic-based substance has been developed. This can provide high thermal stability but has the disadvantage that ceramic materials can block the pores in the separator, complicating ionic transport and should use polymer binder such as PVDF-HFP [113] and PMMA [114]. In this section, we introduce studies that have improved thermal stability.
Lin group fabricated a sandwich-structured separator composed of PI/PVDF/PI using the electrospinning method [115]. This separator had a shutdown function ( ). Because PI has a high thermal stability of 500 °C and low shrinkage, it is thermally and mechanically stable. The PVDF between the PI layers melted in 10 min at high temperatures above 170 °C. This is approximately 40 °C higher than that of a PE membrane. In addition, the electrolyte uptake was recorded at 476%, the ionic conductivity was 3.46 mS cm−1 in a Li|LiMnO2 coin cell, and the porosity was measured at 83%. Because of this, the battery had a high thermal stability, good cyclic life, and 95.1% capacity retention.
Open in a separate windowLi group fabricated polystyrene-poly(butyl acrylate) and silica(PS-b-PBA@SiO2) core-shell structures and used PS-b-PBA@SiO2 as a thermal shutdown switch [116]. The reason for covering silica is that it increases the thermal stability. PS-b-PBA@SiO2 was coated on a commercial PP separator to prepare a self-shutdown separator. PS-b-PBA@SiO2 fundamentally blocks the possibility of a short circuit of Li-dendrite because the porosity decreases to 7.5% at 80 °C. The separator produced using this method had almost the same electrochemical performance as a commercial PP separator in a Li|LFP cell.
Thermal stability can be enhanced by coating a nonflammable polymer on the separator. Xiong group synthesized a PAN and ammonium polyphosphate (APP) separator (PAN@APP) using electrospinning and applied this film as a separator in LSBs [117]. When the temperature increased rapidly, the PAN@APP could cross-link while releasing liquid and gaseous ammonia to create an insulating polymer layer. The characteristics and shape of the PAN@APP collapsed at 430 °C. In LSBs, the separator reduced the shuttle effect. The very strong interaction between LiPS and the separator, caused by the rich amine groups of APP and the phosphoric acid radical, prevented the LiPS from passing through the separator, exhibiting capacity retention higher than 83% for 800 cycles.
Some methods to stabilize the Li-metal anode exist: (1) changing direction of Li dendritic growth by placing Li crystal seeds on the separator and (2) coating materials that react with Li-ions on the separator.
To address Li-dendrite formation, Liu group conducted a study on inhibiting dendritic growth by attaching polyacrylamide (PAM)-grafted GO to a commercial PP membrane (GO-g-PAM@PP) [91]. The GO-g-PAM@PP separator had high porosity that increased electrolyte uptake and provided Li-ion channels. Moreover, PAM has a strong lithiophilic property. Therefore, these properties guarantee rapid ionic conduction and low electrode/electrolyte interface resistance, resulting in stable cyclic performance. However, without the addition of GO, the PAM@PP film was brittle, breaking even at a small impact. By adding GO to PAM, the durability increased; it was no longer brittle even bent or folded conditions. As a result, a Li-metal electrode completed a 2600 h cycles at 2 mA cm−2 for a Li|Li symmetric cell, and a current density with high CE values (98%) for a Li|Cu cell was achieved.
Lee et al. used magnetron DC sputtering to coat a Cu thin film (CuTF) onto one side of a commercial PE separator [82]. The overall concept of this Janus-type separator is as follows: the side without CuTF is on the cathode side and remains as an insulator, while the CuTF side regulates Li dendritic growth at the anode side. The CuTF enables rapid electron conduction without interfering with ion transportation. Thus, the separator simultaneously enables facile electrochemical plating/stripping and inhibits the accumulation of dead Li. In addition, the deposited Li merges in the space between the CuTF and the Li-metal anode and expands along the surface of the anode; thus, internal short circuits are avoided ( a).
Open in a separate windowSong and co-workers introduced Mg nanoparticles on one side of a separator [81]. Lithiophilic Mg nanoparticles offer the sites for heterogeneous nucleation and produce a strong guiding effect to form fixed Li crystal seeds at the initial plating process, and consequently aid in retaining a dendritic-free and dense Li anode after the long cyclic process. Li nucleation occurs in separator-to-Cu direction rather than Cu-to-separator direction; this was confirmed using SEM after a Li|Cu half-cell test ( b). Cui group reported the silica nanoparticle sandwiched tri-layer separators by coating SiO2 nanoparticles between two commercial PE separators [118]. Previous studies focused on the use of SiO2 as a physical barrier because of its thermal stability and high wettability. This study emphasized the additional role of SiO2: it guides the growth direction of Li-dendrites due to the chemical reactions between SiO2 and Li. They conducted Li|Li symmetric cell tests after making pinholes on various types of separators to promote severe Li growth conditions to investigate the formation mechanisms. Four types of separators (bare, SiO2 coated, Si coated, PMMA coated) were used for the experiments. The SiO2-coated separator exhibited the longest lifespan (≈152 h) among the others ( c).
Yuan group fabricated a ZrO2/polyhedral oligomeric silsesquioxane multilayer-assembled PE separator, which was synthesized using a simple LBL self-assembly process [119]. This separator effectively reduces electrolyte polarization and protects Li-metal anodes from Li dendritic growth, and it exhibits excellent electrochemical performance and stability. Xie group reported an interesting strategy, which guided the direction of dendrite growth [101]. Their concept was to allow dendritic growth from both separator and Li-metal surfaces. These Li layers grew by facing each other, resulting in a fused and dense Li formation. This concept was realized by coating a conductive carbon layer on the separator surface, which faced the Li-metal anode. This structure enabled dendrites to spread widely in a direction parallel to the electrode. The Li-metal electrode exhibited a stable cyclic life with a capacity retention of 80% even after 800 cycles.
Using toxic materials during the process of fabricating separators can cause environmental problems. For this reason, several studies on using non-toxic water-based solvents to be eco-friendly and reduce cost have been conducted.
Lee group created a concept of a plasma-treated ceramic-coated separator (plasma CCS) [120]. In contrast to previous studies, in which toxic organic solvents were applied, they used cost-effective and eco-friendly water-based plasma treatment to modify the surface of PE separator, increase the pore size, and strongly fix the alumina layer. Additionally, a ceramic-coated separator was prepared using the surfactant technique (surfactant CCS), which was commonly used in previous studies. A Li|LiMn2O4 cell using plasma CCS exhibited better performance than other batteries in terms of discharge capacity, resistance, and cyclic performance.
Peng et al. introduced graphene to a separator. They coated cellular graphene frameworks onto a PP separator, suppressing the migration of LiPS [121]. However, used synthetic methods, chemical vapor deposition (CVD) with low yields and vacuum filtration processes are not cost-effective for mass production. Hence, they developed Janus-type porous-graphene (PG) modified separators, which were scalable and suitable for green fabrication [60]. First, they used fluidized-bed CVD, which had a yield of 5 g h−1. Additionally, an industrially compatible blade coating method was applied, and toxic organic solvents were replaced with water. An amphiphilic polymer with a high polarity, poly(vinyl pyrrolidone), was adopted as the aqueous binder because of its wettability to the PP separator and PG and polarity for LiPS interaction. Consequently, the PG separator induced a significantly low self-discharge rate (90% retention) at high sulfur use (86.5%) and increased the rate capability.
In addition, Lei et al. also fabricated separators using Al2O3 nanowires under mild conditions [122]. The existing methods to synthesize Al2O3 nanowires cannot be easily scaled up, since the process is complicated and requires harsh solvents. In their study, they introduced a facile extraction process to extract Li from Al or Mg alloys using alcohol solvents for the synthesis of alumina nanowires. The as-made separator exhibited increased thermal stability, ionic conductivity, and wettability compared with commercial PP and cellulose fiber separators. When applied to a graphite|LFP cell, it exhibited much better performance than other batteries in terms of capacity retention, rate capability, etc. This research created the possibility of the mass production and commercialization of ceramic separators in which Al2O3 nanowires were synthesized and applied as a separator under ambient conditions without a catalyst or external stimulus.
Wu et al. reported a functional separator by stacking a Prussian blue (PB) layer on an rGO film (PB/G) [123]. PB, a type of MOF, is stable, non-toxic, and scalable material and has an appropriate lattice size and open framework with large interstitial sites. Therefore, it can accommodate Li-ions while obstructing the migration of LiPS [124]. The PB barriers are evenly distributed at the anode side and alleviate the growth of Li-dendrites by maintaining a homogeneous Li-ion concentration. PB/G is on the cathode side, hindering the diffusion of LiPS and increasing the conductivity of a cell. With these characteristics, a Li|S full cell achieved a high capacity of 1481 mAh g−1 at 0.1 C and 744 mAh g−1 after 2000 cycles at 2 C.
Kim et al. have developed an eco-friendly coating process of GO to fabricate modified functional separators [125]. Through this method, surface of the separator can be fully covered by GO flakes, which provided the hydrophilic wetting nature owing to many hydrophilic functional groups existing on the GO, enabling eco-friendly water-based slurry method. They fabricated GO-SiO2 composite layer coated separator to confirm its applicability to LMBs. The SiO2 nanoparticles performed the function of suppressing Li dendritic growth, and exhibited more stable cycling performance compared to bare separator. Also, this study emphasized that this method can be used with not only SiO2 but also other 1- or 2-dimensional materials including CNT, graphene, TMD, etc.