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The practical usage of sodium metal batteries is mainly hampered by their potential safety risks caused by conventional liquid-state electrolytes. Hence, solid-state sodium metal batteries, which employ inorganic solid electrolytes and/or solid-state polymer electrolytes, are considered an emerging technology for addressing the safety hazards. Unfortunately, these traditional inorganic/polymer solid electrolytes, most of which are prepared via ex-situ methods, frequently suffer from inadequate ionic conductivity and sluggish interfacial transportation. In light of this, in-situ polymerized solid-state polymer electrolytes are proposed to simplify their preparation process and simultaneously address these aforementioned challenges.
- solid-state polymer electrolyte
- in-situ polymerization
- sodium metal batteries
- high safety
- stable interfacial chemistry
1. Introduction
Sodium-ion batteries (SIBs) are one of the auspicious alternatives to state-of-the-art lithium-ion batteries (LIBs) because of the natural plenitude of sodium (Na) in the Earth’s crust. Additionally, SIBs can be discharged to 0 V without causing overdischarge issues, which greatly improves their safety properties for their deployment for electric vehicles. Moreover, SIBs could employ cheaper aluminum as current collectors instead of copper, which greatly reduces the cost of SIBs. In the last, SIBs usually possess a longer service life, higher rate performance, and wider operating temperature range compared with LIBs [1,2,3]. With the rapid growth of plug-in or hybrid electric vehicles and large-format green energy storage power stations, high-energy and high-safety SIBs are urgently desired [4,5,6]. To remarkably improve the energy density of SIBs, significant research efforts have been dedicated to finding and identifying suitable electrode materials and several promising types of cathodes (e.g., layered transition metal oxides, sodium fluorophosphates, etc.) and anode materials (carbonaceous materials, sodium alloys, and transition metal oxides/sulfides) have been identified [7,8,9,10]. In contrast, less attention has been paid to developing reliable electrolytes, which are also important in guaranteeing improved battery performance and safety property of SIBs [11,12,13]. It is also worth noting that the conventional liquid-state electrolytes (such as carbonate electrolytes, ether electrolytes, etc.) are plagued by their severe safety risks because of the potential electrolyte leakage and even combustion [14,15]. Consequently, developing new kinds of highly safe electrolytes is highly desirable for building high-energy and high-safety SIBs [16,17].
Solid-state electrolytes are believed to effectively relieve the unsafe factors brought by conventional liquid-state electrolytes. Solid-state electrolytes include inorganic solid-state electrolytes and solid-state polymer electrolytes. Even though the inorganic solid-state electrolytes possess ionic conductivity that is comparable to or even higher than that of liquid-state electrolytes at room temperature, its practical application is limited due to the high brittleness and poor interfacial contact. In contrast, solid-state polymer electrolytes exhibit better electrode–electrolyte interfacial compatibility, flexibility, and processability. From the material’s point of view, the solid-state polymer electrolytes (SPEs), which also possess superior thermal stability characteristics, are expected to be the ideal electrodes for building high-safety solid-state SIBs. In addition, their wide electrochemical window is also advantageous to enable a compatible and stable electrode–electrolyte interface [18,19]. Hence, the SPEs are recognized as one of the promising candidates to replace the traditional liquid-state electrolytes for SIBs.
2. SPEs Prepared Using Photoinduced In-Situ Polymerization
The earliest research on SMBs using photoinduced in-situ polymerized SPEs dates back to 2015. Federico et al. demonstrated a gel polymer electrolyte using di-methacrylate oligomer (i.e., bisphenol A ethoxylate dimethacrylate, BEMA) and poly (ethylene glycol) methyl ether methacrylate (PEGMA) polymer matrices and Irgacure 1173 photo-initiator (2 wt.%) [56]. It was confirmed using the Fourier transform infrared (FTIR) that the photoinduced polymerization reaction was accomplished in no more than 5 min with 100% conversion. The results imply that this in-situ polymerization process is much faster compared with the conventional preparation processes for fabricating polymer membranes (such as solvent casting and melt flow).
The prepared BEMA/PEGMA-based photopolymer electrolyte was characterized by the authors, and they found that it exhibited high ionic conductivity (5.1 mS cm−1 at 20 °C), excellent sodium ion transportability (sodium-ion transference number, 0.53 ± 0.05, 20 °C), stable interfacial characteristics, and a wider electrochemical stability window (4.8 V versus Na/Na). Considering that thermal stability is also one of the key factors affecting its successful application in SMBs, the authors also conducted thermogravimetric analysis (TGA), and the results implied that the prepared BEMA/PEGMA-based photopolymer electrolyte possessed good thermal stability for SMBs. The authors assembled TiO+2-based cells using the BEMA/PEGMA-based photopolymer electrolyte and conducted electrochemical cycling tests. The cycling test displayed that the studied cell delivered stable capacity exceeding 50 mAh g−1 after 250 cycles at 1 mA cm−2, proving the decent electrochemical cycling performance of the as-prepared BEMA/PEGMA-based photopolymer electrolyte.
In 2020, Yu et al. reported fabricating solid-state SMBs using flexible SPE, which was in-situ polymerized using poly(ethylene glycol) methyl ether methacrylate (PEGDMA) monomer and sodium bis(fluorosulfonyl)imide (NaFSI) sodium salt as well as the 2,2-dimethoxy-2-phenylacetophenone (DMPA, 1 wt%) photoinitiator (as-obtained PEGDMA-NaFSI-SPE) [57]. This report showcased the probability of building flexible wearable solid-state SMBs using the in-situ polymerization methods. The ionic conductivity of the designed PEGDMA-NaFSI-SPE was 1.1 × 10−4 S cm−1 at 30 °C. The as-obtained PEGDMA-NaFSI-SPE exhibited both high ionic conductivity and good electrolyte–electrode interface bonding. Moreover, the authors used density functional theory (DFT) calculations to study the Na migration barrier in Na+3V2(PO4)3 (NVP) and PEGDMA-NaFSI-SPE.
To further improve the Na ionic conductivity and extend the electrochemical window of the traditional SPEs, Wen et al. reported a new kind of ethoxylated trimethylolpropane triacrylate (ETPTA)-based SPE [58]. This work suggests that utilizing the in-situ polymerization strategy to manufacture SPEs is a new direction for developing high energy-density, flexible, and room-temperature solid-state SMBs. Nevertheless, it should be mentioned that the NVP loading is only 1 mg cm+−2, which may not be sufficient for practical usage. Hence, in the future, high mass-loading (≥20 mg cm−2) of cathodes is suggested to be used in building and testing SMBs.
These abovementioned works imply that the photoinduced in-situ polymerization is advantageous to construct advanced SPEs. However, in light of the fact that ultraviolet light cannot penetrate the battery shell, it is still a difficulty to directly build solid-state SMBs using photoinduced in-situ polymerization. In the future, other types of rays (e.g., gamma rays) with stronger penetration capability and more suitable precursors can be selected to realize the in-situ polymerization of SMBs.
3. SPEs Prepared by Thermally Induced In-Situ Free Radical Polymerization
This section will concentrate mainly on the research progress of the SPEs manufactured by thermally induced in-situ free radical polymerization, which is a widely used strategy for preparing SPEs. Several categories of such polymer electrolytes will be included: conventional polymer electrolytes, poly(ionic liquid)-based polymer electrolytes, and flame-retardant polymer electrolytes.
3.1. Conventional Polymer Electrolytes
Most of the vinyl monomers can be polymerized into high-mass polymers using the thermally induced in-situ free radical polymerization process. For example, vinylene carbonate (VC), an effective additive of lithium/sodium batteries, can be polymerized to high-molecular-mass polymer via free radical polymerization.
In light of this, Chen et al. prepared a poly(vinylene carbonate)-based composite polymer electrolyte (PVC-CPE) for ambient temperature solid-state batteries for the first time [62]. This PVC-CPE exhibited high ionic conductivity at 25 °C (0.12 mS cm−1), high Na transference number (0.60), and decent interfacial stability between the electrode and the electrolyte. It was reported that the solid-state NaNi+1/3Fe1/3Mn1/3O2/PVC-CPE/Na battery, which was built using a facile in-situ polymerization method, presented a discharge capacity retention of 86.8% after 250 cycles at 0.2 C and 71.8% after 200 cycles at 1 C, suggesting that the as-prepared PVC-CPE is favorable for building ambient temperature solid-state SMBs.
Goodenough et al. synthesized a cut-price gel-polymer electrolyte of cross-linked PMMA by in situ polymerization and tested in a sodium-ion full-cell Sb/Na3V2(PO4)3 [63]. On account of promoting the gelation of organic solvents and helping the solvation of electrolytic ions, the PMMA chains are actually crucial components of the gel-polymer electrolyte. Moreover, the sodium ion full-cell delivered a capacity of 106.8 mAh g−1 at 0.1 C and also showed a capacity of 86.3 mAh g−1 with the coulombic efficiency approaching 100% after 100 charge–discharge cycles. Furthermore, when tested at an elevated temperature of 60 °C, the cycling performance of the sodium-ion full-cell Sb/Na3V2(PO4)3 could be improved obviously by the presence of the gel-polymer electrolyte instead of liquid electrolyte.
Zhou et al. built high cathode mass loading SMBs by using trihydroxymethylpropyl triacrylate (TMPTA)-based electrolytes [64]. Intimate and conformal solid–solid contact between the electrodes and the SPE, which is conducive to the Na migration, was achieved via the in-situ polymerization.+
It has to be noted that these three abovementioned polymer electrolytes are flammable, which is unfavorable to the safety of SMBs. Hence, it is suggested that more efforts are needed to develop promising flame-retardant polymer electrolytes for safe SMBs.
3.2. Poly(ionic liquid)-Based Polymer Electrolytes
Besides these common organic polymers mentioned above, poly(ionic liquid)s can also be utilized to manufacture SPEs. Ionic liquids are suitable candidates for in-situ preparing high-performance SPEs due to their merits of low volatility, nonflammability, sufficient electrochemical stability window, and relatively high ionic conductivity.
In this regard, Zhou et al. reported a hierarchical poly (ionic liquid)-based SPE for SMBs [66]. This SPE was manufactured via in-situ polymerizing 1,4-bis[3-(2-acryloyloxyethyl)imidazolium-1-yl]butane bis[-bis(trifluoromethanesulfonyl)imide] (C1-4TFSI) monomer in the 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI)-based electrolyte (HPILSE). And the as-prepared polymer electrolyte was later filled in the poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (PDDATFSI) porous membrane. The elaborately designed SPE delivered high ionic conductivity (1.15 mS cm−1 at 25 °C), satisfied oxidation window (4.6 V vs. Na/Na), sufficient mechanical strength, and intrinsic flame retardancy. Additionally, the full cells built with this SPE exhibited high-capacity retention of 85.5% after 100 cycles and high average coulombic efficiency at 0.1 C.+
Each type of SPE matrix features its own advantages and disadvantages. The high cost and cumbersome preparation process of ionic liquids-based polymer electrolytes have greatly limited their further development. In this regard, advanced eutectogel polymer electrolytes of low-cost and flame-retardant properties may be a future development direction.
3.3. Flame-Retardant Polymer Electrolytes
In recent years, in-situ formed flame-retardant polymer electrolytes have attracted much interest in SMBs. These flame-retardant SPEs can be mainly categorized into two types: (1) Triethyl phosphate (TEP) or trimethyl phosphate (TMP)-based SPE [67,68] and (2) Flame retardant polymer electrolytes formed by polymerization of unsaturated phosphate monomers [69,70,71,72].
For the former type, Chen et al. demonstrated a flexible poly(ethylene glycol) methyl ether methacrylate (PEGMA)-based SPE [67]. The optimized flame-retardant electrolyte, PGT32-5% (ratio of PEGMA, TEP, and fluoroethylene carbonate (FEC) is 3:2:5%), exhibited high ionic conductivity and extended electrochemical window and excellent mechanical strength. In addition, the built full battery delivered a high-capacity retention. The introduction of FEC is beneficial to improve the electrode–electrolyte interface compatibility. In a similar way, Park et al. [68] also reported a nonflammable SPE prepared via in-situ cross-linking polycaprolactone triacrylate (PCL-TA) and TMP.
These two flame-retardant SPEs are compromised by the existence of small combustible molecules. Hence, Zheng et al. carried out a variety of systematic studies to design and fabricate the intrinsically flame-retardant SPEs [69,70,71,72]. In 2021, Zheng et al. reported stable di(2-methylacryloyldioxyethyl) methyl phosphonate (MADEMP)-based gel terpolymer electrolytes [72].
As stated above, the multifunctional phosphonate-containing polymer electrolytes provide a lot of opportunities for preparing flame-retardant SPEs for SMBs. However, the incompatibility between the phosphate ester and the sodium metal limits its further practical application, which will be the focus of future work.
4. SPEs Manufactured Using In-Situ Cationic Polymerization
The research progress of SPEs prepared via photoinduced in-situ polymerization and thermally induced in-situ free radical polymerization for SMBs is reviewed in the previous parts. As stated above, the thermally induced in-situ free radical polymerization is widely used for preparing in-situ formed polymer electrolytes for SMBs. However, this strategy usually requires additional initiators (e.g., AIBN) and high-temperature conditions (e.g., 80 °C), both of which are unfavorable to achieving large-scale production under mild conditions. In contrast, in-situ cationic polymerization employs sodium salts as an initiator, bypassing the potential heating and/or ultraviolet irradiation damage during the in-situ polymerization. So far, there are a few in-situ cationic polymerized SPE for SMBs. In this part, the research advance of the in-situ cationic polymerized SPE for SMBs from the perspective of poly(ethylene glycol) divinyl ether and 1,3-dioxolane (DOL), together with their corresponding performance improvement principles are summarized.
In 2017, Zhang et al. reported the polysulfonamide-supported poly(ethylene glycol) divinyl ether-based SPE (PPDE-CPE), which was prepared via a cost-effective cationic polymerization induced by a small amount of LiBF4. The PPDE-CPE possessed good flexibility and reliable mechanical stability for SMBs [77]. It was also found that the resultant PPDE-CPE exhibited an extended electrochemical stability window (4.7 V) and a relatively high ionic conductivity of 1.2 mS cm−1 at room temperature. To further investigate the performance of the PPDE-CPE in sodium battery, NVP/Na half cells and MoS2/Na half cells using PPDE-CPE were assembled and tested. The capacity retention of NVP/Na half-cell was 97.7% after 300 cycles at 3 C. The pouch-type cells consisting of MoS2 anode, NVP cathode, and PPDE-CPE exhibited stable charge–discharge curves during 20 cycles, indicating promising applicability of the as-prepared PPDE-CPE for SMBs.
Besides the linear monomer, DOL, which is a typical cyclic monomer, can also be used to prepare SPE for SMBs through in-situ cationic polymerization. Niu et al. prepared a cross-linked polyether network (GPE-CPN) [78]. During the GPE-CPN preparation, in-situ copolymerization of trimethylolpropane triglycidyl ether (TMPTGE) and DOL were initiated by sodium hexafluorophosphate (NaPF6) at room temperature. The obtained GPE-CPN possessed good thermal stability (maintained its gelation state from 20 °C to 80 °C), enhanced ionic conductivity at ambient temperature (0.82 mS cm−1), and a broad operating voltage window (4 V versus Na/Na). The built NVP/Na cell employing GPE-CPN exhibited satisfactory capacity retention. Moreover, they found that the surface of the collected sodium is smooth after cycling during post-mortem morphology analysis. Additionally, the built NVP/pretreated hard carbon full cell using GPE-CPN also delivered a high-capacity retention. These results suggest stable interface between GPE-CPN and electrodes.+
Although these results indicate that the GPE-CPN is very promising in building durable, safe, solid-state sodium batteries, its practical application in SMBs remains to be further studied (e.g., high-mass-loading sodium cathode coupled with high-mass-loading sodium anode). In the meantime, its electrochemical oxidation voltage is low to match high-voltage cathode materials (e.g., Na3V2(PO4)2O2F, two stable high-voltage plateaus at 4.01 and 3.60 V), which is unfavorable to achieve high-energy-density SMBs. Therefore, some effective modification strategies are desirable to further widen its electrochemical stability window.
5. SPEs Prepared through a Cross-Linking Reaction
Apart from the SMBs, Na-air batteries (one kind of SMBs, using Na anode and oxygen/air cathode) are also attracting great attention. However, the uncontrolled growth of Na dendrites, as well as the irreversible self-corrosion of Na, result in severe safety issues and rapid performance decay. As a result, controlling the Na electrodeposition and improving the structural stability of Na anode are essentially important for the further application of Na-air batteries.
To resolve the abovementioned issues, Liu et al. [80] demonstrated an NAB using an in-situ generated tetraethylene glycol dimethyl ether (G4)-based SPE initiated by Li ethylenediamine (LiEDA). This SPE was obtained within the assembled NAB via cross-linking LiEDA at the anode surface in G4 in the liquid electrolyte. The reaction process is as follows: The Li reacts firstly with ethylenediamine (EDA) to produce a layer, and secondly the formed LiEDA is cross-linked with G4 to form a gel polymer electrolyte at the Na anode surface. After this reaction, residual trace amount of Li, which remains on the Na surface, is found to alloy with Na, and the formed Li–Na alloy is favorable for guiding uniform Na electrodeposition.
The prepared gel polymer electrolyte can prevent O2 and H2O crossover, and hence, the Na anode corrosion can be inhibited. Moreover, the Na dendrites can be ameliorated by the well-known electrostatic shield effect of Li. As a result, the NAB using this gel electrolyte delivered stable cycling performance that surpasses previous reports. Apart from that, the in-situ formed SPE avoids liquid leakage during battery bending. This property is conducive to the development of wearable NABs. Overall, the in-situ formed G4-based SPEs initiated by Li ethylenediamine LiEDA can tremendously enhance the NAB performance.+
This entry is adapted from the peer-reviewed paper 10.3390/batteries9110532
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