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Poly(Ethylene Oxide)-Based Solid Polymer Electrolytes for Solid-State Sodium Ion Batteries

Zhao Lanqing , Hou Minjie , Zhang Da , Zhou Yingjie , Xie Zhipeng , Liang Feng

Progress in Chemistry ›› 2023, Vol. 35 ›› Issue (11) : 1625 -1637.

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Progress in Chemistry ›› 2023, Vol. 35 ›› Issue (11) : 1625-1637 DOI: 10.7536/PC230324
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Poly(Ethylene Oxide)-Based Solid Polymer Electrolytes for Solid-State Sodium Ion Batteries
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Abstract

One of the most promising candidates for large-scale energy storage applications is the solid-state sodium ion battery, which replaces conventional organic liquid electrolytes with solid electrolytes and has the advantages of high safety, high energy density, and extended cycle life. Among many solid electrolyte materials, Poly(ethylene oxide) (PEO)-based polymer solid electrolytes are considered promising solid electrolyte materials because of their high safety, easy manufacturing, low cost, high energy density, favorable electrochemical stability, and excellent solubility in sodium salts. However, the high crystallinity of the ethylene oxide (EO) chain segment results in low ionic conductivity at room temperature, which is unable to meet the requirements of practical application. To overcome the aforementioned limitations, researchers have used a variety of strategies to lessen the crystallinity of PEO-based polymer electrolyte and hence increase its ionic conductivity. Common techniques include polymer block copolymerization, blending, crosslinking, adding plasticizers, and adding inorganic fillers. In the review, the physical and chemical properties, preparation methods, and modification techniques of PEO-based polymer electrolytes are evaluated, and the most recent advancements on PEO-based polymer electrolytes are reviewed.

Contents

1 Introduction

2 PEO-based polymer solid electrolyte

2.1 Physicochemical properties of PEO

2.2 PEO polymer solid electrolyte

2.3 Ion transport mechanism

3 Preparation method of PEO-based polymer solid electrolyte

3.1 Solution casting

3.2 Hot pressing

3.3 Other methods

4 Modification strategy

4.1 Polymer block copolymerization,blending and crosslinking

4.2 Adding plasticizers

4.3 Adding inorganic fillers

5 Conclusion and outlook

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Keywords

solid-state sodium ion batteries / solid-state electrolytes / poly(ethylene oxide) (PEO) / preparation methods / modification strategy

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Zhao Lanqing, Hou Minjie, Zhang Da, Zhou Yingjie, Xie Zhipeng, Liang Feng. Poly(Ethylene Oxide)-Based Solid Polymer Electrolytes for Solid-State Sodium Ion Batteries. Progress in Chemistry, 2023, 35(11): 1625-1637 DOI:10.7536/PC230324

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1 Introduction

Under the background of low-carbon energy, electrification of transportation and intelligent equipment, the development of renewable energy not only provides security for human sustainable development, but also alleviates climate warming, environmental pollution and other issues. Since the commercialization of lithium-ion batteries, with the continuous improvement of energy density, output power and other indicators, they have been able to meet people's demand for high energy density and high power, and have been widely used in electric vehicles and smart grids. However, the reserves of lithium resources are low and the geographical distribution is uneven. With the electrification of human society, the supply of lithium resources is gradually in short supply. Therefore, sodium-ion batteries have become the most ideal alternative to lithium-ion batteries because of their abundant sodium resources, low cost, similar structure and working principle to lithium-ion batteries, and compatibility with equipment.
In the past 10 years, the research and development of sodium-ion batteries have made rapid progress, but there are still some safety problems similar to lithium-ion batteries, such as easy leakage, volatility, flammability, explosiveness and poor thermal stability, which limit their development. Solid-state sodium-ion batteries using solid electrolytes instead of liquid electrolytes are expected to fundamentally solve the safety problems of batteries[1⇓⇓⇓⇓~6]. As a key material for solid-state sodium-ion batteries, solid electrolytes should have high ionic conductivity (>10-4S·cm-1 at room temperature), electronic insulation, high chemical/electrochemical stability, good electrode/electrolyte interfacial contact, high mechanical strength, good flexibility, and thermal stability[7,8]. At present, solid electrolytes are mainly divided into three categories: inorganic solid electrolyte (ISE), solid polymer electrolyte (SPE) and composite polymer solid electrolyte (CPE). Among them, polymer solid electrolytes have the advantages of high safety, good flexibility, easy processing into films, and excellent interfacial contact. Commonly used polymer matrices are polyethylene oxide (PEO), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinyl pyrrolidone (PVP) and polymethyl methacrylate (PMMA), and the comparison of polymer matrices is shown in Table 1[7]. PEO has been widely studied in solid electrolytes because of its good film-forming property, low glass transition temperature (Tg), and the ability to dissolve many sodium salts[9⇓⇓⇓~13]. However, the single solid polymer electrolyte still has some problems, such as low ionic conductivity, dendrite formation and interface instability. Researchers often use copolymerization, blending, crosslinking and adding inorganic fillers to solve the above problems. Among them, the composite solid electrolyte obtained by adding inorganic fillers into the polymer matrix can improve the mechanical properties and electrochemical properties, and realize the optimization of the single solid electrolyte system, which is regarded as a very promising solid electrolyte material.
In this paper, the physical and chemical properties of PEO and its mechanism are introduced from the PEO-based polymer solid electrolyte. The preparation process and modification strategy of PEO-based polymer solid electrolyte, as well as the current important research progress were emphatically discussed. Finally, the existing problems in the research of PEO-based polymer solid electrolyte at present are put forward and its development prospect is prospected.

2 PEO-based polymer solid electrolyte

2.1 Physicochemical properties of PEO

PEO is a crystalline thermoplastic polyether, which is formed by heterogeneous catalytic ring-opening polymerization of ethylene oxide and has a linear and regular helical structure, and its segmental molecular structure is shown in Figure 1A[14,15][16]. As the earliest and most widely studied polymer matrix, it has many advantages, such as light weight, good viscoelasticity, easy film formation, wide electrochemical window, good chemical stability and so on, so it is widely used in industrial manufacturing, medicine and other fields. PEO has a strong ability to dissolve sodium salt and can complex with sodium salt to form polymer solid electrolyte[17⇓~19]. Its ethylene oxide (EO) unit has a high Na+ donor number and molecular chain flexibility, which is of great significance for promoting ion transport. In addition, PEO has a high dielectric constant, which allows the PEO polymer matrix to rapidly dissociate sodium salts and produce mobile Na+. However, as a semi-crystalline polymer, PEO is easy to crystallize at room temperature (RT), and the slow polymer chain dynamics in the crystalline region leads to its low ionic conductivity (Figure 1B), while only the amorphous phase in the activated chain segment (above Tg) is conducive to ion transport. Therefore, the design guideline for PEO-based electrolytes is to suppress PEO crystallization to increase the percentage of PEO amorphous phase for ion transport.

2.2 PEO polymer solid electrolyte

The polymer phase (fig. 2b) of the polymer solid electrolyte for solid-state sodium-ion batteries (fig. 2a) includes a polymer matrix and a sodium salt. The concentration of Na+ and its transference number determine the ionic conductivity of the electrolyte, while the ability of sodium salt to dissociate into free ions in PEO polymer is related to the solubility of sodium salt in PEO polymer, the higher the solubility, the more the Na+. In 1973, Wright et al. First discovered that PEO complexed with alkali metal sodium salts could show ionic conductivity under the action of an applied electric field[20]. PEO and its derivatives are considered to be an ideal choice for polymer solid electrolyte matrix due to their good sodium salt dissolving ability[21,22]. However, the low ionic conductivity at room temperature (about 10-9S·cm-1) and the small oxidation potential window of PEO-based polymer solid electrolyte hinder its application in batteries, while sodium salt can reduce the crystallinity of PEO, optimize the carrier concentration of PEO, and then improve its ionic conductivity (about 10-7S·cm-1)[23]. Different kinds of sodium salts have different effects on the microstructure and ionic conductivity of solid polymer electrolytes. Sodium perchlorate (NaClO4), sodium hexafluorophosphate (NaPF6), sodium trifluoromethanesulfonate (NaTf), sodium difluorosulfonimide (NaFSI) and sodium bis (trifluorosulfonimide) (NaTFSI) are common sodium salts in solid polymer electrolytes[24][25][26][27][27]. Among them, NaTf, NaFSI and NaTFSI have larger anions, which are easy to dissociate in PEO matrix and provide more Na+ to obtain electrolytes with higher ionic conductivity.

2.3 Ion transport mechanism

Polymer solid electrolytes usually contain polar groups (— O —, C = O, C ≡ N), and the solvation of salts is achieved by the coordination of lone electron pairs on the groups with cations. At present, the generally accepted ion transfer mechanism in solid polymer electrolytes is that ion transfer occurs in the amorphous region, and when the temperature is above Tg, the polymer molecular chain segment vibrates, and the cation and polymer chain segment are continuously complexed and dissociated, thus realizing ion transfer[28,29]. As shown in Fig. 3, the Na+ ion generated after the dissociation of sodium salt first occupies the position coordinated with the polar group (— O — in PEO) of the polymer, and under the action of the electric field, some chain segments of the polymer molecule move, so that the Na+ migrates from one coordination site to another to realize the transmission of Na+[28].
Sodium ion conductivity is one of the important indexes to evaluate solid electrolyte. In general, the ionic conductivity can be referred to the Arrhenius or Vogel-Tammann-Fulcher model. Below the Tg of solid polymer electrolyte (crystalline polymer), the ionic conductivity follows the Arrhenius equation; Above Tg, the ionic conductivity follows the Vogel-Tammann-Fulcher (VTF) model[30,31].

3 PEO-based polymer solid electrolyte preparation method

3.1 Solution casting

The solution casting method comprises the following steps of: mixing solid PEO and sodium salt according to a certain proportion, dissolving in an organic solvent, stirring to form a solution, casting the solution on a polytetrafluoroethylene (PTFE) mold, and drying and demoulding to obtain the PEO-based polymer solid electrolyte. This method is the most commonly used preparation method of polymer solid electrolyte at present. The advantages of solution casting are that the membrane structure is uniform and the membrane thickness is easy to control, but the disadvantages are that it takes a long time and is easy to cause pollution. When the organic solvent remains,The solvent can cause the decomposition of solvent molecules and the side reaction with the electrode at the interface during the cycle of the battery, which leads to the increase of electrode/electrolyte interface impedance, polarization, short cycle life and low coulombic efficiency of the battery[10]. West et al. First reported PEO-based polymer solid electrolyte, which was prepared by solution casting method with NaClO4 as sodium salt. When the molar ratio of ether oxygen bond (EO) to Na+ was 12 ∶ 1, the ionic conductivity of P(EO)12/NaClO4 was 3.1×10-6S·cm-1 at 60 ℃[32]. Niu et al. Prepared PEO/NZSP(Na3Zr2Si2PO12) composite solid electrolyte by solution casting method, and the all-solid-state Na0.67Ni0.33Mn0.67O2‖Na batteries based on this electrolyte all showed excellent electrochemical performance, with a capacity retention of 98.4% after 100 cycles at 0.5 C and 55 ° C[33]. Guo et al. Injected PEO-based electrolyte solution into the fiber skeleton, and the three-dimensional continuous channel provided a fast Na+ transport path, thereby improving the ionic conductivity[34].

3.2 Hot pressing method

The process of preparing PEO-based polymer solid electrolyte by hot pressing method is to mix the polymer, salt and filler in proportion after vacuum drying, and then hot press them at appropriate temperature and pressure to obtain the solid electrolyte. The hot pressing method can effectively remove the organic solvent to prepare the polymer solid electrolyte with excellent performance, which has the advantages of rapidity and low cost, and was first realized by Gray et al[35,36][37]. Appetecchi et al. Studied the effects of temperature, PEO molecular weight, EO/Li molar ratio, different fillers and their contents on the ionic conductivity of polymer solid electrolyte prepared by hot-pressing method, and found that the conductivity increased with the increase of temperature, and was not affected by PEO molecular weight and EO/Li molar ratio above 70 ℃[38]. With the decrease of PEO molecular weight, the viscosity decreases gradually, which leads to the increase of cation fluidity and conductivity. When the filler content is higher than 5%, the effect on the conductivity is unchanged. Pandey et al. Used SiO2 as filler to prepare nanocomposite polymer solid electrolyte (NCPE) membrane, and compared with the NCPE membrane prepared by traditional solution casting method, the crystallinity of the polymer solid electrolyte prepared by hot pressing method was smaller, but the conductivity of the polymer electrolyte prepared by the two methods was similar[39].

3.3 Other methods

In addition to solution casting and hot pressing, electrospinning and in situ polymerization have also been used to prepare PEO-based polymer solid electrolytes.
Electrospinning is often used to produce micro-nanofibers because of its simple equipment, convenient operation, mild preparation conditions, high specific surface area, porosity, good biocompatibility and high absorption rate of electrolyte. The method comprises the following steps of: placing a polymer melt or solution in a high-voltage electrostatic field, so that the polymer melt or solution is charged and forms a voltage difference with a collecting plate; forming a jet flow at the tail end of a spray nozzle along the direction of the force of the electric field; and finally depositing on the collecting plate after solvent volatilization or melt cooling and fiber bundle solidification to form polymer fibers with the diameter of nano[40]. However, it is greatly affected by environmental factors such as temperature, humidity and environmental gas composition. And that prepare polymer fiber can provide a continuous ion transmission channel and obviously enhance the conductivity of the Na+[41]. Freitag et al. Studied the ionic conductivity of NaBF4@PEO electrolyte membrane prepared by electrospinning, and found that when the molar ratio of PEO∶NaBF4 increased from 36:1 to 18:1, the ionic conductivity increased significantly due to the rise of sodium ion concentration[42]. In addition, the crystalline phase of PEO in the electrospun membrane is reduced, thereby making the ionic conductivity of the NaBF4@PEO electrolyte membrane much higher than that of the solution-cast electrolyte membrane of similar composition. Liu et al. Used PEO as the matrix to construct a three-dimensional Na3Zr2Si2PO12/PAN-[PEO-NaTFSI] composite solid electrolyte by electrospinning. Compared with the pure PEO-NaTFSI solid electrolyte, the introduction of the composite fiber network reduced the crystallinity of PEO, thereby improving the ionic conductivity.The NVP/SPE/Na battery assembled with the composite polymer solid electrolyte has an initial charge-discharge capacity of 109.7 mAh·g-1,200 at 0.1 C rate and a coulombic efficiency of nearly 100% after cycles, and shows excellent cycling stability[43].
The in-situ polymerization preparation is to dissolve polymer monomer and initiator in organic solvent, cast the obtained precursor solution on a PTFE mold, and obtain the polymer solid electrolyte by in-situ polymerization under ultraviolet light or at a certain temperature. After the liquid electrolyte is solidified by in-situ polymerization, the interface between the solid electrolyte and the electrode can be closely contacted, thereby effectively reducing the interface resistance[44]. However, due to the limitation of the types of monomers polymerized in situ, the electrolyte is difficult to match well with the high-voltage cathode and high-capacity anode, and the system after polymerization will remain liquid polymer monomers, thus affecting the battery performance. Yao et al. Prepared PEO-based polymer solid electrolyte by in situ UV polymerization using poly (ethylene glycol) dimethacrylate (PEGDMA) as the polymer monomer, and the in situ polymerized PEGDMA-NaFSI-SPE had high ionic conductivity at room temperature (about 10-5S·cm-1), reduced interfacial resistance between electrolyte and electrode, and improved interfacial compatibility[45]. The polymerized NVP @ C | PEGDMA-NaFSI-SPE | Na cell reaches a high capacity of 105 mAh·g-1 at 1 C rate, and the soft pack cell has no capacity degradation after 740 cycles at 0.5 C rate.
Preparation method is an important step in the development of solid electrolytes. Among the several preparation methods of PEO-based polymer solid electrolyte, the solution casting method is the most commonly used preparation method at present, which is simple and easy to implement and has a uniform electrolyte structure; The hot pressing method can effectively eliminate the influence of organic solvents and has low cost; Polymer fibers prepared by electrospinning can provide continuous ion transport channels and improve the mechanical strength of electrolytes. In-situ polymerization can improve the compatibility of electrode/electrolyte interface and reduce the interfacial resistance. However, in order to realize the large-scale preparation of solid electrolyte, it is necessary not only to improve its performance and reduce its cost, but also to solve the problems of organic solvent in solution casting and electrospinning and initiator residue in in-situ polymerization. In general, it can be solved by controlling the amount of solvent added and prolonging the drying time.

4 Modification strategy

4.1 Polymer blending, block copolymerization, crosslinking

Conventional PEO-based polymer solid electrolyte has low ionic conductivity at room temperature, which is difficult to meet the requirements of ideal solid electrolyte (>10-4S·cm-1). To this end, researchers have done a lot of modification work. The design criterion is usually to reduce the crystallinity of PEO to increase the proportion of amorphous phase of PEO for ion transport. The common methods include polymer block copolymerization, blending and crosslinking to introduce high dielectric constant chain segments to enhance the degree of salt dissociation. The addition of plasticizer reduces the crystallinity of PEO and improves the chain segment motion ability. The composite polymer solid electrolyte prepared by adding the filler improves the mechanical strength and the thermal stability, widens the electrochemical window, and further improves the ionic conductivity of the electrolyte[46,47].
Polymer blending can not only increase the amorphous phase of polymer solid electrolyte, but also combine a variety of polymers to improve the performance of the electrolyte. Chandra prepared PEO/PVP-NaPO3 polymer solid electrolyte by blending PEO and PVP polymer using solvent-free hot pressing method. When 3% (mass fraction) PVP was added, the conductivity of PEO-NaPO3 polymer solid electrolyte increased by 2 orders of magnitude (about 1.07×10-5S·cm-1) due to the increase of amorphous region of the polymer after blending with PVP, which led to the enhancement of chain mobility[48]. Su et al. Designed a novel polymer composed of β-cyclodextrin (β-CD) and poly (trifluoroethyl methacrylate) (PTFEMA), as shown in Figure 4A[49]. Attributed to the strong electron-withdrawing effect of fluorine atoms, the fluorine-containing acrylate monomer TFEMA has the lowest HOMO energy level (− 7.82 eV), indicating a high antioxidant capacity (Fig. 4 B). PEO-based all-solid-state polymer electrolyte (PEO-ASPE) was prepared by blending the polymer with PEO. The novel polymer can form a semi-interpenetrating network with PEO, resulting in high ionic conductivity. The polymer is used in a Na|PEO-ASPE-Na|Na3V2(PO4)3(NVP) battery, and the battery is stably cycled for more than 400 cycles at the temperature of 80 deg C. Ganta et al. Used TiO2 as filler to blend PVDF with PEO to improve the flexibility of PVDF and increase the amorphous region of PEO. When the ratio of raw materials was 8.75×10-5S·cm-1 (mass fraction), the crystallinity of the polymer was the lowest, and the ionic conductivity of the electrolyte system was as high as 8.75×10-5S·cm-1 at room temperature (see Table 2)[50]. Matios et al. Found that metal Na, as a reactant, could spontaneously transform amorphous poly (propylene carbonate) (PPC) from a long-chain polymer to a short-chain segment, and used this effect to incorporate PPC and NASICON particles into the PEO matrix, which increased the amorphous region of the composite SPE, thereby improving the ionic conductivity[51]. The doped PPC reacted with the Na metal electrode during battery assembly to form a conductive wetting layer, thereby reducing the interfacial impedance (Fig. 4C). The ionic conductivity of the prepared solid electrolyte was increased to 1.2×10-5S·cm-1 (see Table 2), and the capacity retention of the solid-state battery using the solid electrolyte was 87% after 500 cycles at 1 C rate.
Similar to blending, the copolymer formed by copolymerization of different monomers can reduce the crystallinity of the polymer through intermolecular interaction, improve the chain segment motion ability, and play different block functions at the same time, thereby enhancing the ionic transference number of the polymer solid electrolyte[52]. Wang et al. Designed a solvent-free SPE based on perfluoropolyether (PFPE) -terminated PEO-based block copolymer. Compared with the traditional PEO-based SPE, the block copolymer design can form self-assembled nanostructures, thereby upgrading its mechanical properties and enhancing the storage modulus. PEO can also provide sodium ion transport channels even at high salt concentration (EO, Na+ molar ratio of 8:2)[53]. At the same time, the addition of perfluoropolyether segments at 80 ℃ increased the sodium transference number of the electrolyte to 0. 46, and maintained the glass transition and melt characteristics of the block copolymer, thus achieving good electrode/electrolyte contact. In addition, the symmetric cell composed of this solid electrolyte was cycled stably at high current density (0.5 mA·cm-2) for more than 1000 H. The assembled Na|SPE|Na3V2(PO4)3 all-solid-state Na-metal battery can be cycled 900 times at 2 C (about 0.2 mA·cm-2) rate and maintain a coulombic efficiency of 99.91% with a capacity retention rate as high as 97.5%, and it also shows excellent charge-discharge performance using a high loading NaFePO4 cathode, which can be charged and discharged for 1000 H.
By constructing the polymer solid electrolyte with a crosslinked network structure, the crystallization of the polymer matrix can be inhibited, the mechanical properties of the polymer solid electrolyte can be significantly improved, and a stable amorphous polymer can be formed[54]. Col Colò et al. Proposed that a pyranose ring based ionic skeleton (sodium carboxymethyl cellulose, Na-CMC) was added to a PEO-based polymer matrix containing a sodium salt to construct a three-dimensional crosslinked structure (Fig. 4D), which improved the mechanical properties of the solid electrolyte, resulting in a stable electrode/electrolyte interface and an electrochemical window of 5 V(vs Na/Na+)[55]. Hou et al. Crosslinked low molecular weight PEO (PEG) with polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) to form a semi-interpenetrating network, which increased the loading of NASICON active filler, promoted the dissociation of sodium salt, increased the concentration of Na+, and the SPE room temperature ionic conductivity was as high as 2.4×10-4S·cm-1 (see Table 2)[56]. The assembled Na|SPE|Na3V2(PO4)3 cell has an initial discharge specific capacity of 100.3 mAh·g-1 at 1 C rate and a high first-turn coulombic efficiency of 99.4% (Figure 4E).
Polymer blended, block copolymerized, crosslinked PEO-based SPEs are summarized and discussed in this section. Polymer block copolymerization, blending and crosslinking change the physical properties of PEO by introducing other polymer segments, such as blending multiple polymers to enhance the flexibility and mobility of the polymer segments, and introducing higher dielectric constant segments to improve the solubility of sodium salts; Different monomers copolymerize to form a new type of copolymer to play different block functions, crosslink to form a network structure, and enhance the mechanical properties of electrolytes. Polymer blending, block copolymerization and crosslinking can be designed by selecting polymer chain segments with different functions to solve the problems of low ionic conductivity, dendrite formation and interface instability of single polymer solid electrolyte. However, if the degree of polymer mixing is poor, there will be macroscopic phase separation, which can not achieve the purpose of blending, so we can choose the polymer with better compatibility to mix. Compared with blending, the process of copolymerization and crosslinking is complex, and the problem of residual initiator and crosslinking agent needs to be paid attention to, which can be solved by drying, centrifugation or adding reducing agent.

4.2 Additive plasticizer

The addition of low molecular weight polyethylene glycol (PEG) as a plasticizer to PEO-based polymer solid electrolyte can reduce the crystallinity of PEO through plasticization, enhance the chain segment movement, increase the number of ion transference in the electrolyte, and thus improve the ionic conductivity of the electrolyte[57]. Bhide et al. Studied the conductive characteristics of PEG-plasticized PEO-based polymer solid electrolyte PEO/NaPO3[58]. Due to the addition of PEG plasticizer, the amorphous phase of the polymer increases and the glass transition temperature decreases (Figure 5A), thereby increasing the flexibility of the polymer chain. The sodium ion transference number increases from 0.23 for the pure PEO electrolyte to 0.42 for the PEO/NaPO3 electrolyte. The ionic conductivity of the PEO/PEG/NaClO3 (mass ratio 30:60:10) electrolyte reported by Chandrasekaran et al. Reached 3.07×10-5S·cm-1 at 35 ° C (see Table 3)[59].
Organic solvents such as ethylene carbonate (EC) and propylene carbonate (PC) have high relative dielectric constants, thereby promoting the dissociation of salt ion pairs. EC or PC is a commonly used liquid electrolyte in lithium metal or lithium ion batteries and is also widely used as a PEO-based electrolyte plasticizer. Its high dielectric constant increases the charge storage capacity of the electrolyte, and its low viscosity makes the polymer chain easy to bend and provides a large free volume for ion movement[60⇓⇓~63]. Dave et al. Studied the effect of EC-PC plasticizer on the electrochemical performance of sodium trifluoride (NaCF3SO3), PEO-PAM blend electrolyte system[64]. The dielectric constant of PC is higher than that of EC, while the viscosity of EC is lower than that of PC. When both liquids are used as plasticizers for PEO, the properties of both liquids can be complemented. As the content of EC-PC mixed plasticizer increases, the crystallinity of the film decreases and the conductivity increases, as shown in Figure 5 B, and the conductivity reaches the optimum value (5.74×10-4S·cm-1 (see Table 3)) at 15 wt% (EC + PC). Menisha et al. Prepared a polymer solid electrolyte using EC and PC as plasticizers (Figure 5C), and the room temperature ionic conductivity of the optimized PEO/NaClO4/EC-PC (mass ratio 7:13:40:40) electrolyte was 9.5×10-3S·cm-1 (see Table 3)[65].
Succinonitrile (SN) is a highly polar and nonionic solid organic plastic material, which can dissolve sodium salts at room temperature to form a solution, and is often used as a plasticizer to synthesize SPE. Wang et al. Reported a novel asymmetric bilayer solid electrolyte PEO-SN-NaClO4|NZSP-NSO (Figure 5D), the addition of SN reduces the PEO melting temperature, thereby reducing the crystallinity, meanwhile, the presence of SN weakens the coupling between Na+ and oxygen, and promotes the migration rate of Na+ on the PEO segment[66]. When the ratio of PEO: SN is 1.25: 1, the ionic conductivity of SPE increases to 3.55×10-5S·cm-1.
Ionic liquids (ILs), as a kind of low melting point molten salt, are often used as wetting agents in solid polymer electrolytes because of their high ionic conductivity, good thermal and chemical stability[67,68]. The ILs themselves can act as charge carriers while enhancing the polymer chain dynamics through the plasticization effect, allowing the SPE to exhibit high conductivity. Shen et al. Added a trace amount of ionic liquid [Py13]+[NTf2]- into a polymer solid electrolyte with 80% (mass fraction) Na3.4Zr1.9Zn0.1Si2.2P0.8O12 as filler, and achieved a flexible, non-flammable, and highly ionically conductive CPE (Figure 5E)[69]. The NZP-PEO @ IL CPE has an ionic conductivity of 1.48×10-4S·cm-1 at 25 ° C (see Table 3), and the assembled Na‖Na3V2(PO4)3 solid-state sodium battery has a capacity retention of 90.0% after 150 cycles at 0.5 C, 60 ° C, showing good cycling stability. Chen et al. Added Pyr13FSI into PEO/NaClO4 polymer solid electrolyte, as shown in Fig. 5F, the induced interaction between FSI- anions and PEO made the polymer have good chain flexibility, and when the IL content was 40%, the CPE conductivity at room temperature reached 6.8×10-5S·cm-1 (see Table 3), and the transference number of sodium ion reached 0.44[70]. Boschin et al. Used Pyr13TFSI and Pyr13FSI to prepare solid electrolytes with NaX(PEO)n-Pyr13X(X as TFSI and FSI, respectively, and the addition of Pyr13FSI to NaFSI(PEO)n with high salt concentration (n = 6 and 9) was more effective in reducing the crystallinity of PEO than the TFSI ternary system[71]. The results show that although the conduction mechanism of Na+ is independent of ILs, the addition of ILs reduces the crystallinity of PEO, especially for FSI- anion, and is also beneficial to enhance the polymer chain dynamics and plasticize the ternary system. However, the conductivity of FSI-based electrolyte is still lower than that of TFSI-based system.
The modification of PEO-based SPEs by adding plasticizer is summarized and discussed in this section. Plasticizing is a common method to reduce the crystallinity and increase the amorphous phase of PEO-based electrolytes. Commonly used plasticizers such as aprotic organic solvents and low molecular weight PEGs can reduce the crystallinity, increase the amorphous region, improve the movement of polymer chain segments, increase the mobility of ions in the electrolyte, and improve the conductivity of the electrolyte, but the flammability and explosion of liquid solvents will lead to serious safety problems. Therefore, nonflammable novel plasticizers such as plastic crystals (e.g., SN) and ionic liquids have received much attention and have been applied in polymer electrolytes.

4.3 Composite polymer electrolyte

The preparation of composite polymer solid electrolyte by adding inorganic fillers into polymer is an effective strategy to improve the conductivity of electrolyte[72]. There are mainly two kinds of fillers, one is inert filler without ionic conductivity, which does not contain Na+, such as Al2O3, SiO2, TiO2, etc[73]. Shenbagavalli et al. Used Al2O3 as filler to prepare Na+ ion-conducting composite polymer solid electrolyte by solution casting method (Fig. 6a), and the ionic conductivity at room temperature increased from 10-7S·cm-1 to 10-5S·cm-1 when the content of Al2O3 was 4% (mass fraction) due to the complexation among polymer, salt and filler[74]. Chandra et al. Used SiO2 as filler to prepare PEO-NaClO4-SiO2 polymer solid electrolyte by hot pressing method. When the content of SiO2 was 7%, the ionic conductivity of the electrolyte at room temperature was the highest, which was 7.6×10-6S·cm-1 (see Table 4), and the transference number of sodium ion was as high as 0.92[75]. The other is an active filler which is an ionic conductor itself, such as NASICON solid electrolyte, Na-β-Al2O3, etc[76⇓~78]. The addition of filler can inhibit the agglomeration of polymer chain, reduce its recrystallization rate, increase the amorphous phase of PEO, and then weaken the interaction between polymer chain and Na+, promote the dissociation of sodium salt, and produce more free Na+. The ionic transport channel formed by the particles at the interface can improve the ionic conductivity of the polymer solid electrolyte. The characteristics of high mechanical strength and high thermal stability of the filler itself can significantly improve the mechanical strength and thermal stability of the polymer solid electrolyte, and broaden the electrochemical window. In addition, the high ionic conductivity of the active filler particles can also enhance the ionic transport rate of the polymer solid electrolyte[79⇓⇓⇓~83]. The addition of CPE,NaAl5O8 prepared by Bublil et al. Using PEO, active ceramic filler NaAl5O8 alleviated the reaction activity of PEO with Na, and the ionic conductivity of CPE at 70 ° C was 3.6×10-6S·cm-1 (see Table 4), which was two orders of magnitude higher than that of PEO[84]. Peta et al. Studied that the addition of 4% (mass fraction) TiO2 nanowires in NaPF6/P(EO)16(TiO2)0.04 electrolyte to prepare CPE,TiO2 ceramic filler improved the bulk conductivity between CPE and metal sodium anode, while removing the interfacial by-product (HF) to reduce the interfacial impedance, and the transference number of sodium ions was as high as 0.68. The CPE applied to NaTi2(PO4)3 cathode had excellent stability at 70 ° C, and the initial capacity was 110 mAh·g-1[85]. As shown in Fig. 6 B, Xu et al. Synthesized Na3PS4-PEO-NaClO4CPE with Na3PS4(NPS) as filler, in which the NPS particles were wrapped by PEO-NaClO4 to reduce the interfacial resistance, ease the reaction between NPS and sodium metal negative electrode, and improve the interfacial compatibility between electrolyte and electrode, and the ionic conductivity of this solid electrolyte was up to 9.5×10-4S·cm-1 (see Table 4)[86].
Wu et al. Prepared flexible PEO/NZTO CPE by dispersing gallium-doped Na2Zn2TeO6(NZTO) in PEO, and the NZTO particles were uniformly dispersed in the polymer matrix (Fig. 6 C), which inhibited the crystallization of PEO[87]. Compared with PEO-NaTFSI SPE without filler, the ionic conductivity of PEO/NZTO CPE increased from 1×10-6S·cm-1 to 4×10-5S·cm-1 at 30 ° C and up to 1×10-3S·cm-1 at 80 ° C with 50% NZTO addition (see Table 4). The all-solid-state sodium battery using the electrolyte has a capacity retention of 93% after 100 cycles at 0.2 C and 80 ℃ (Fig. 6d), and the modified CPE improves the ion transport kinetics, finally achieving an all-solid-state sodium battery with high ionic conductivity. Yu et al. Reported PEO-NaClO4-NASICON CPE with PEO as matrix and NASICON as filler, as shown in Fig. 6e[88]. The composite electrolyte can enhance the ionic conductivity of the Na+, inhibit the dendrite growth, and has superior elasticity. The ionic conductivity reached 5.6×10-4S·cm-1 at 60 ° C (see Table 4), and the assembled Na | SPE | Na symmetric cell showed stable cycling performance, and the coulombic efficiency of the cell with Na2MnFe(CN)6 cathode was close to 100% after 300 cycles at 0.5 C rate. The above findings demonstrate that polymer/ceramic composites are a promising strategy for the development of next-generation all-solid-state sodium batteries.
In this section, CPEs composed of polymers and inorganic fillers are summarized and discussed. The effect of inert filler on PEO matrix is mainly reflected in two aspects: (1) reducing the crystallinity of PEO, increasing the amorphous region and improving its ionic conductivity; And (2) that anion of the sodium salt is fix through the action of Lewis acid and base, so that more free Na+ is generated, and the ionic conductivity is improve. In addition to the above two functions, the active filler itself can provide Na+ for ion transport, and the ion transport channels formed by particles at the interface can promote ion transport. Polymer-ceramic composites combine the advantages of SPE and ISE. Compared with SPE, CPE has higher mechanical strength, electrochemical window, thermal stability, ionic conductivity and sodium dendrite inhibition, and reduces the electrode/electrolyte interface resistance, which is an ideal choice for solid-state sodium-ion batteries. However, the aggregation of filler particles and the poor interfacial compatibility between filler particles and polymer matrix greatly limit their practical applications. These obstacles can be eliminated by constructing a three-dimensional framework to maintain continuous ion transport and surface modification to increase the adhesion of filler particles to the polymer matrix.

5 Conclusion and prospect

In recent years, PEO has been widely concerned because of its excellent thermal stability and mechanical properties. However, the low ionic conductivity of PEO at room temperature will lead to a decline in the electrochemical performance of the battery, so the development of PEO-based solid electrolytes with high ionic conductivity at room temperature and good mechanical properties is of great significance for the development of all-solid-state sodium-ion batteries.
The development of large-scale electrolyte preparation process and new high room temperature ionic conductivity polymer solid electrolyte has been the focus of research. At present, among the commonly used preparation methods of PEO-based polymer solid electrolyte, the solution casting process is mature and the prepared electrolyte structure is uniform. The hot pressing method can eliminate the interference of organic solvents, and has the advantages of rapid preparation process and low cost; The integration of three-dimensional structure construction by electrospinning and in-situ polymerization can achieve rapid ion transport and interface compatibility. However, there are still some problems such as high cost and excessive solvent. Therefore, the combination of various preparation methods is an effective strategy to improve the ionic conductivity, mechanical strength and interfacial stability of solid polymer electrolytes.
The modification strategies currently used are as follows: (1) Polymer block copolymerization, blending and crosslinking can effectively control the polymer structure, and the modified polymer can obtain good mechanical properties and high dielectric constant, improve the dissociation degree of sodium salt in the polymer, and then improve the overall ionic conductivity. (2) The addition of plasticizer can effectively reduce the crystallinity of PEO and enhance its segmental motion ability. At the same time, the organic solvent plasticizer has a high dielectric constant, which can promote the dissociation of sodium salt. However, liquid solvents are expensive, flammable and explosive. Therefore, it is of great significance to explore low-cost and high-safety plasticizers for the development of advanced solid electrolytes. In addition, effective plasticizers are expected to alleviate the interfacial side reactions between polymers and sodium, build a strong interfacial layer and achieve high-performance solid-state sodium-ion batteries. (3) The mechanical strength and thermal stability of the composite polymer solid electrolyte synthesized by adding fillers can be significantly improved, and the electrochemical window of the composite polymer solid electrolyte can be widened. Although the addition of filler results in a slight increase in polymer solid electrolyte impedance, it can provide additional ion transport channels, accelerate ion transport, and stabilize the electrode/electrolyte interface. Therefore, composite polymer solid electrolyte is also an effective strategy to develop all-solid-state sodium batteries.
With the development of PEO-based polymer solid electrolytes, some systems have achieved high ionic conductivity (10-4~10-3S·cm-1) comparable to commercial separator/electrolyte systems, and have shown potential for practical application with a variety of electrode materials for sodium-ion batteries. At present, the application research trends of PEO-based polymer solid electrolyte in sodium-ion battery are as follows: (1) to reduce the crystallinity of PEO and improve the stability of electrolyte by modification; (2) The interaction between polymer chains, small molecules and anions/cations in the system is clarified by theoretical calculation and electrochemical analysis, and the sodium ion conduction mechanism in PEO-based polymer solid electrolyte is explained; (3) Solve the complex interfacial side reactions between PEO-based polymer solid electrolyte and sodium metal anode by interfacial modification. To sum up, there are still many challenges in the application of PEO-based polymer solid electrolytes in solid-state batteries, which need to be explored continuously. However, based on its good safety, flexibility and excellent interfacial contact, it is believed that the solid-state sodium-ion battery with PEO-based polymer as solid electrolyte will have broad application prospects.

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Funding

National Natural Science Foundation of China(12175089)

National Natural Science Foundation of China(12205127)

Key Research and Development Program of Yunnan Province(202103AF140006)

Applied Basic Research Programs of Yunnan Provincial Science and Technology Department(202001AW070004)

Applied Basic Research Programs of Yunnan Provincial Science and Technology Department(202301AS070051)

Applied Basic Research Programs of Yunnan Provincial Science and Technology Department(202301AU070064)

Yunnan Major Scientific and Technological Projects(202202AG050003)

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