3.1. Linear Polymer
The simplest polymer structure is a linear chain, and initial investigations of SPEs started with a linear PEO. Wright et al. first discovered the ionic conductivity of PEO/alkali salts in 1973 [43
]. Five years later, M. Armand proposed the use of this complex as an electrolyte in secondary lithium batteries [10
]. In the past 40 years, it has been demonstrated that PEO, which is the most frequently studied polymer host, has good ionic conductivity in an amorphous state and electrochemical stability. It also has an excellent compatibility with other organic or inorganic materials, which is crucial for a polymer to be regarded as the best candidate for SPEs. The advantages and disadvantages of PEO-based electrolytes have already been reported by certain articles [23
]; hence, these are not repeated here.
In addition to PEO, polymers that contain oxygen or nitrogen atoms (e.g., polyethers, polycarbonates, polyesters, and polynitriles) as solvating hosts of lithium salts have been investigated. However, the degradation of polycarbonates and polyesters [44
] and the low ionic conductivity of lithium cation in polynitriles [45
] limit their further application.
Although other polyethers, such as poly(propylene oxide) [46
], polytetrahydrofuran [48
], and poly(1,3-dioxolane) [49
], have also exhibited physical and electrochemical properties similar to those of PEO, these have not been widely investigated because of the dearth of products with high molecular weights.
At present, as the demands of the battery market continue to increase, there remain three main problems in PEO-based SPEs that should be resolved. These include low ionic conductivity at low temperatures, low transference number, and relatively narrow electrochemical window when a high-voltage cathode is used. To overcome the foregoing, various strategies have been conceived, including the addition of organic plasticizers [51
], inorganic fillers [52
], natural fibers [53
], and ceramic particles [54
] to SPEs. The use of PEO-based polymer blends and copolymers has also been explored, and a recent review has summarized the benefits of employing PEO-based block copolymer electrolytes for lithium batteries [55
]. The incorporation of different polymer segments can enhance not only the ionic conductivity but also the mechanical properties of PEO. Furthermore, copolymers with a single-lithium ion block have been observed to exhibit a higher transference number.
Another effective approach that affects the electrochemical properties of PEO-based electrolytes is modifying the polymer host architecture. Three structures are discussed in the succeeding paragraphs.
3.2. Comb-Like Polymer
Typically, the operational temperature of linear PEO-based SPEs is approximately 80 °C, which exceeds the PEO melting temperature. Under this condition, the PEO’s mechanical resistance is considerably inadequate to alleviate the growth of lithium metal dendrites during cycling [56
]. Based on this, many comb-like polymers with side oligo(ethylene oxide) chains, acting as lithium cation coordinating sites, were synthesized, and the influence of the main polymer chain structure and grafting degree on ionic conductivity and mechanical strength has been investigated. Table 1
summarizes the common main polymer backbone and some of the electrochemical results.
] and polysiloxane are generally used as the main polymer chains for oxyethylene-grafted polymer electrolytes. The benefits afforded by these main chains result from their thermal stability, high dielectric constant, and flexibility. Oxyethylene-grafted polyphosphazenes (MEEPs) are usually synthesized with alkoxide-end oligo(ethylene oxide) and poly(chlorophosphazene) (Figure 4
). In literature, however, only extremely short grafted side chains are reported [59
]. The MEEPs are amorphous with a Tg
less than −60 °C [61
], indicating good segmental mobility. Consequently, MEEP-based SPEs exhibit a considerably higher conductivity than PEOs at room temperature [62
]. However, MEEPs undergo viscous flow under pressure because of the lack of dimensional stability and mechanical resistance.
Polymers, being considerably similar to MEEPs (with polysiloxane as backbone and side PEO chains), are also amorphous with a low Tg
value of less than −60 °C. In view of this, polymers also have a good ionic conductivity at room temperature but insufficient mechanical strength [63
]; hence, polysiloxane-based SPEs usually have a crosslinking structure. Their mechanical and electrochemical properties are discussed in the section on crosslinked polymers. Borodin et al. presented a report on the mechanism of lithium cation transport in an all-ethylene oxide (EO) comb-like polymer in which a considerably flexible main chain was used [64
]. Their study clearly illustrates the occurrence of ion transport in the main and side chains—Li+
cations have the highest and lowest probabilities of being coordinated by EOs near the polyepoxide ether polymer backbone and at the side chain ends, respectively. The most mobile Li+
cations hop from a side chain to another without being complexed by the backbone.
Compared with flexible backbones, a methacrylate polymer main chain is considerably stronger. Poly(ethylene glycol) methacrylate (PEGMA) is the most widely used monomer in the preparation of comb-like polymers with ethylene oxide-containing side groups. The PEGMA homopolymer is amorphous and has a good mechanical resistance. Recently, Yao et al. reported a high-conductivity SPE based on poly(ethylene glycol) methyl ether methacrylate (PEGMEM) [65
]. Three comb-like polymer-bearing side poly(ethylene glycol) (PEG) chains with different molecular weights are prepared by ultraviolet (UV) polymerization. Optimal electrochemical results are obtained for the SPE with LiTFSI salt and a 950 g∙mol−1
PEGMEM. The SPE exhibits a high conductivity of 1.44 × 10−4
at 30 °C with EO/Li+
= 18:1. In a previous report, Zhang et al. studied the physicochemical and electrochemical behaviors of linear PEO in the presence of lithium bis(fluorosulfonyl)imide (LiFSI) and LiTFSI salts [66
]. The SPE membranes prepared by solution casting with PEO/LiFSI and PEO/LiTFSI at an EO/Li+
= 20:1 exhibit crystallinities of 44% and 47%, respectively, resulting in a decrease in the conductivity of SPEs at 60 °C. A comparison of ionic conductivity as a function of temperature between linear and comb-like polymers is shown in Figure 5
. Ignoring the slight difference in Li+
concentration, the conductivities in both cases are similar, i.e., above 60 °C. The SPE with a comb-like PEO host, however, exhibits a better conductivity at a lower temperature. Moreover, they all show an electrochemical window (>5 V) and a low transference number (<0.3). In another investigation based on a PEG-grafted SPE, Thelakkat et al. revealed the influence of PEG content on the ionic conductivity [67
]. With an EO/Li+
= 16:1, the ionic conductivity increases with the side chain length because of the increasing amount of dissolved Li+
; this observation is similar to the results reported by Yao. However, the anodic breakdown potential is found to be ∼3.5 V vs. Li/Li+
because of the electrolyte decomposition by oxidation; this value is lower than that described above. This is probably because of the difference in experimental conditions under which they have been obtained.
In addition to the methacrylate polymer backbone, other rigid polymers are also used as the main chain. Song et al. reported the effects of grafting degree and side chain lengths on the ionic conductivity using an acrylonitrile-butadiene copolymer backbone with low molecular weight side PEO chains [68
]. They found that its degree of crystallinity is significantly reduced, and the mechanical strength is significantly improved compared with those of a linear PEO. The conductivity varies with the side chain length and grafting degree. The maximum conductivity exceeds 10−5
at room temperature. Using ring-opening metathesis polymerization, a PEO-grafted polynorbornene with different side chain lengths is synthesized [67
]. It exhibits a lower conductivity than a methacrylate backbone polymer because its non-conductive hydrocarbon content in the main chain is higher; similar results are obtained for comb-like PEO−mimetic polypeptoids [69
]. In a poly(ether ether ketone)-based comb-like copolymer with short side PEO chains [70
], it is also observed that the non-conductive backbone only affects its mechanical resistance, rather than significantly improving its electrochemical properties. In the foregoing examples, only short side chains are grafted. Zardalidis et al. studied the physicochemical and electrochemical properties of poly(hydroxylstyrene) with long side PEO chains [71
]. The copolymer exhibits a similar electrochemical behavior as the linear PEO because of the crystallization of PEO chains but a better mechanical strength. Because of the incorporation of rigid main chain, the highest storage modulus (G
’) obtained at 80 °C reaches 108
Pa, much better than that of linear PEO (Mw
= 22 kg·mol−1
) at the same temperature [72
]. It is also found that G
’ decreases with the salt concentration. In addition to side PEO chains, Aldalur et al. successfully grafted other ethylene oxide-containing polymers, such as Jeffamine®
-oxypropylene)), to a main poly(ethylene-alt
-maleic anhydride) chain [73
]. The degree of crystallinity of this polymer decreases with the side chain length similar to that of the side PEO chain copolymer. The ionic conductivity (4.5 × 10−5
) of SPE with Jeffamine-based matrix at room temperature is considerably better than that with a linear PEO.
In conclusion, PEO-based comb-like polymers usually comprised of short PEO chains and a hard backbone, which enhances mechanical strength. The SPEs that constitute such a polymer demonstrate a higher ionic conductivity and a better mechanical resistance compared with linear PEO-based SPEs; however, they exhibit similar transference numbers (< 0.5). The electrochemical stability of these materials is overestimated because it is usually measured by cyclic voltammetry using a symmetric cell with smooth metallic electrodes, which is different from real electrode composition. On the other hand, many poly(ethylene oxide) methacrylate (PEOMA)-based block copolymers with an amorphous block, such as polystyrene [76
], poly(n-butyl methacrylate) [77
], and poly(methyl methacrylate) [78
], are synthesized. The derived results are similar to those of the above mentioned comb-like polymers. In contrast, combining a single-ion transport block with a PEOMA block can improve not only its mechanical properties but also increase its transference number. The single-ion conducting copolymer, i.e., lithium poly[(4-styrenesulfonyl) (trifluoromethanesulfonyl)imide-co
-methoxy-polyethylene glycol acrylate], reported by Zhou et al. [79
] exhibits the highest ionic electrolyte conductivities of 7.6 × 10−6
at 25 °C and 10−4
at 60 °C with an EO/Li+
= 20.5/1; the transference number (>0.9) approaches unity. In another report [80
], a single ion-conducting copolymer composed of poly(lithium 1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethylsulfonyl)imide) and poly(ethylene glycol) methyl ether methacrylate blocks exhibits similar conductivities of 2.3 × 10−6
at 25 °C and 1.2 × 10−5
at 55 °C with an EO/Li+
= 36/1; the transference number is 0.83.
3.3. Hyper-Branched Polymer
Different from two-dimensional block copolymers or comb-like copolymer electrolytes, hyper-branched polymers have a globular structure with mobile polymer branches. This type of structure can also suppress PEO crystallization and improve the ionic conductivity and mechanical strength of SPEs. In 2009, Niitani et al. designed a star-shaped copolymer electrolyte with a hyper-branched polystyrene core and poly(poly(ethylene glycol) methyl ether methacrylate) (PPEGMA) arms [81
]. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) show that this copolymer forms a well-ordered spherical micro-phase separation structure, in which the star polymers are systematically ordered to form the PPEGMA continuous phase. As a result of the polymer’s unique morphology, the SPEs that contain lithium bis(pentafluoroethanesulfonyl)imide salts exhibit high ionic conductivities, i.e., 10−4
at 30 °C and 10−5
at 5 °C with EO/Li+
= 33/1. Another star-shaped copolymer based on PS and PPEGMA is synthesized by atom transfer radical polymerization [82
]. The ionic conductivity increases with the chain length of PEGMA and reaches 8 × 10−5
at 25 °C with EO/Li+
Recently, Zhang et al. synthesized a similar core–shell hyper-branched copolymer [83
] with hyper-branched polystyrene (HBPS) as core and poly(methyl methacrylate)-block
-poly(poly(ethylene glycol) methyl ether methacrylate) (PMMA-b
-PPEGMA) as arms (Figure 6
). The ionic conductivities of the polymer electrolyte composed of HBPS-(PMMA-b
and LiTFSI are 8.3 × 10−5
at 30 °C (PPEGMA content: 83.7%) and 2.0 × 10−4
at 80 °C (PPEGMA content: 51.6%) with a transference number of up to 0.31. Furthermore, the electrolyte exhibited good interfacial compatibility in a symmetric Li/SPE/Li cell. Subsequently, the authors studied the electrochemical behaviors of the composite polymer electrolyte with HBPS-(PMMA-b
(PPEGMA content: 69.56%), ionic liquid, and LiTFSI [84
]. In the presence of ionic liquid, the electrolytes exhibited better ionic conductivities but a considerably lower transference number.
In contrast to the hard core–soft arm structure, Yao et al. reported a hyper-branched polymer structure with the PEO as star core and the linear polystyrene (PS) as arms (hyper-branched PPEGMAm
)), as illustrated in Figure 7
]. The authors found that the self-assembly of rigid PS arms during phase separation increases the copolymer’s mechanical strength. The highest storage modulus reaches 1.4 MPa for hbPPEGMA25
at 60 °C. The PEGMA chain length affects its segmental motion in the core. The room-temperature ionic conductivities of hbPPEGMAm
SPEs are 5−16 times higher than those of PEO20k
SPEs and 2−7 times that of the linear PPEGMA50
SPE. An all-solid-state Li/LiFePO4 battery based on the hyper-branched SPE achieves a stable capacity of 142 mAh∙g−1
at 0.2 C.
The branched PEO chains of comb-like and hyper-branched polymer electrolytes are both mobile, and it is this similarity that contributes to the ion transport. In a network structure, crosslinked polymer chains can effectively suppress PEO crystallization, increase thermal and mechanical stabilities, and mitigate the growth of lithium dendrites.
3.4. Crosslinked Polymer
In network electrolytes formed by linear PEO chains or corresponding block copolymers without a liquid additive, the ionic conductivity depends on the crosslinking degree and chain length between adjacent junction points [86
]. Moreover, the ionic conduction mechanism also depends on the mesh size of polymer networks [88
]. When this size is small enough, the Li+
transport follows Arrhenius model rather than VFT model. In this case, ions jump to the nearest vacant sites. In contrast, when comb-like copolymers with side PEO chains are crosslinked, the PEO chain mobility is better than that of the linear counterpart, resulting in a better ion transport. The network’s mechanical strength varies with the backbone structure and crosslinking degree. H. Kawakami et al. found that linear PEG-based crosslinked network exhibits a much higher stress modulus (6.84 MPa) than PEO (0.55 MPa) with similar molecular mass to the network precursor at ambient temperature [86
]. As mentioned, when polysiloxane or polyphosphazene is used as the backbone with the side PEO chain, the network is more flexible than the methacrylate or styrene backbone. The ionic conductivity is mainly tuned by the PEO segment content and mobility as well as the amount of liquid additives introduced. The crosslinking structure is usually obtained through physical means (e.g., micro-phase separation, H-bonding and chain entanglement) or chemical techniques (e.g., copolymerization with a difunctional or multifunctional crosslinker).
Numerous PEO-based block copolymers can form an ion-conducting pathway during the micro-phase separation [89
]. Depending on the nature and length ratio of blocks, micro-phases with different morphologies are formed by the copolymer self-assembly. Balsara et al. studied the relationship between ion transport and morphology and the effect of nanoparticle addition on the micro-phase transition [93
]. Figure 8
shows the dependence of the ideal morphology factor, fideal
, on the conductive phase morphology; a bicontinuous morphology provides the best ion transport. The authors also observed that the addition of 2 wt% polyethylene glycol polyhedral oligomeric silsesquioxane (PEO-POSS) nanoparticles into the PS-b
-PEO copolymer causes a lamellar-to-bicontinuous phase transition, leading to a remarkable increase in the ionic conductivity. This type of phase transition that induces the increase in ionic conductivity is also observed in a PEO/PS/PS-b
-PEO ternary system [94
]. The total homopolymer volume fraction varies from 0 to 0.70, and the ratio, r = [Li+
]/[EO], is maintained at 0.06. It is found that the increase in conductivity through the order−disorder transition is most probably caused by the elimination of grain boundaries. In either the disordered or ordered state, the conductivity decreases as the total amount of homopolymer increases.
Using the same approach, Chopade et al. synthesized a PEO-b
-PS-based polymer electrolyte through polymerization-induced micro-phase separation (Figure 9
]. In this material, the non-conductive phase is chemically crosslinked, thus yielding a higher mechanical strength. The addition of succinonitrile plasticizer allows the electrolyte to reach an excellent conductivity that even exceeds 10−4
at room temperature.
Recently, Wu et al. reported a self-healing solid polymer electrolyte (SHSPE) that is crosslinked by the dynamic intermolecular hydrogen bond between urea and ester groups located in the PEO chains [96
]. The authors indicated that the inter-chain hydrogen bond and attractive Coulombic forces among the ions in the lithium salt facilitate fast self-healing. This means that the self-healing rate is considerably affected by the lithium salt concentration. As a result of the fast self-healing capability, a high electrolyte mechanical stability is enabled after damage. The optimized SHSPE with high ionic conductivity (1.9 × 10−4
), low electronic conductivity (1.87 × 10−8
), and high Li-ion transference number (0.44) at room temperature is stable up to 5 V (vs. Li+
/Li). Moreover, using the SHSPE, full batteries with a lithium metal anode and an LFP or NCM cathode exhibit good performance. The lithium storage capacities in both cases reach 147.9 mAh∙g−1
(Li/SHSPE/LFP battery) and 170.6 mAh∙g−1
(Li/SHSPE/NCM battery) at 0.1 C. The extremely low polarization between the charge and discharge plateaus of the Li/SHSPE/LFP battery after 20 cycles at 0.1 C indicate the excellent compatibility and stability between the SHSPE and electrodes.
Poly(ethylene glycol) dimethacrylate (PEGDMA) and diacrylate (PEGDA) are the most widely used monomers for the chemical crosslinking of polymer electrolytes. The networks are typically prepared by free radical photo-polymerization or thiol-ene reaction. Zhang et al. demonstrated a facile approach by UV irradiation to prepare a flexible PEO-based crosslinked electrolyte (PTT) consisting of linear PEO, tetraglyme (TEGDME), and tetraethylene glycol dimethacrylate (TEGDMA) (Figure 10
]. The SPE exhibits superior electrochemical properties with high ionic conductivity (2.7 × 10−4
at 24 °C), high transference number (0.56), wide electrochemical stability window that exceeds 5 V (Li+
/Li), and low interfacial resistance; similar SPEs have also been reported [98
]. In the work of Choudhury et al. [98
], the crosslinked polymer matrix comprised of PEGDMA and bis(2-methoxyethyl) ether (diglyme), and the Tg
and mechanical strength were found to increase with the PEGDMA content as shown in Figure 11
. The authors observed that the materials behaved as single-phase soft solids at a critical PEGDMA content of 40%. At this point, the barrier to ionic transport in the oligoether is sufficiently low to produce high ionic conductivity; however, the oligoether is also prevented from exhibiting large-scale convective motions by interacting with network chain segments. At the same time, the authors indicated that the membranes with high crosslinking degree (>40%) are able to completely suppress electroconvective instability. In another report [100
], Khurana et al. synthesized a series of cross-linked polyethylene/poly(ethylene oxide) SPEs with high ionic conductivity (>1.0 × 10−4
S/cm) at 25 °C and a relatively low storage modulus (G′ ≈ 1.0 × 105
Pa) at 90 °C. In lithium dendrite tests, using symmetric Li/SPE/Li cell, they found that low-modulus cross-linked SPEs exhibit remarkable dendrite growth resistance compared with a linear PEO standard (Mw
= 900 kg·mol−1
). This result suggests that a high-modulus SPE is not a requirement for dendrite nucleation and growth. Furthermore, when single-ion segments are incorporated into an SPE network, the transference number improves. Nevertheless, the use of a liquid plasticizer remains advantageous for ion conduction and improving the flexibility of the SPE membrane [101
In addition to the linear PEO-based networks, crosslinked comb-like polymer electrolytes have also been studied; PEO-grafted polysiloxane is one of the most frequently investigated polymers because of the excellent mechanical and thermal stabilities of siloxane backbone. To prepare this type of network, the polymer chains are commonly crosslinked among themselves or through the addition of a crosslinker. Jiang et al. prepared a comb-like polysiloxane network by photo-polymerization (Figure 12
]. They found that the network chain structure has a significant effect on the ionic conductivity and mechanical properties of the electrolyte, and rich PEO side chains enhance conductivity and flexibility. The SPEs exhibit maximum conductivity and tensile strength of 1.01 × 10−4
and 0.66 MPa at 30 °C, respectively.
Oh et al. synthesized a crosslinked mono-comb-type poly(siloxane-g
-ethylene oxide) electrolyte using PEGDMA as a crosslinker [104
]. A maximum ionic conductivity of 1.33 × 10−4
at 25 °C is obtained, indicating that the crosslinking method has no significant effect on the properties of this type of SPEs.
An epoxy group is also employed to prepare a network structure. Daigle et al. prepared a crosslinked SPE composed of a poly(poly(ethylene glycol) methacrylate-co
-glycidyl methacrylate) copolymer [105
]; the polymer chains were crosslinked through amino-epoxy polymerization. The obtained SPE exhibited excellent electric stability and mechanical resistance. Grewal et al. synthesized free-standing polydimethylsiloxane-based crosslinked network solid polymer electrolytes via in situ thiol-epoxy polymerization [106
]. In the presence of lithium salt and base catalyst, poly(ethylene glycol) diglycidyl ether, mercapto-terminated polydimethylsiloxane, and pentaerythritol tetrakis (3-mercaptopropionate) were copolymerized at the stoichiometric ratio. Without a liquid plasticizer, the crosslinked SPEs exhibited the following: Ionic conductivities between 1.5 × 10−6
at room temperature and 1.6 × 10−4
at 90 °C; lithium ion transference number of 0.15–0.20; electrochemical window of up to 5.3 V (vs. Li+
/Li); thermal resistance of approximately 254.5 °C; maximum tensile strength of approximately 1.6 MPa.
A number of SPEs that employ different node groups also exhibit similar electrochemical and mechanical properties. In conclusion, the crosslinking structure affords a stronger mechanical support, and the lithium cation transport in the PEO-based network mainly depends on chain segmental motion and hopping between adjacent coordinating sites.