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Safety-enhanced polymer electrolytes for sodium batteries: recent progresses and perspectives Jinfeng Yang, Huanrui Zhang, Qian Zhou, Hongtao Qu, Tiantian Dong, Min Zhang, Ben Tang, Jianjun Zhang, and Guanglei Cui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01239 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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ACS Applied Materials & Interfaces

Safety-enhanced Polymer Electrolytes for Sodium Batteries: Recent Progresses and Perspectives Jinfeng Yang § ,†, ‖ , Huanrui Zhang § ,†, Qian Zhou†, Hongtao Qu†, ‖ , Tiantian Dong†, Min Zhang†, Ben Tang†,‖, Jianjun Zhang*,†,‖and Guanglei Cui*,† †Qingdao

Industrial Energy Storage Research Institute, Qingdao Institute of Bioenergy and Bioprocess

Technology, Chinese Academy of Sciences, Qingdao 266101,P. R. China ‖ Center

of Materials Science and Optoelectronics Engineering, University of Chinese Academy of

Sciences, Beijing 100049, China Keywords: polymer electrolytes, sodium batteries, safety, natural merits, recent progresses ABSTRACT: Sodium batteries (SBs) have aroused increasing attentions due to the abundance and low cost of elemental sodium. In recent decades, intensive efforts have been under way to exploit advanced SBs for practical applications. However, conventional liquid electrolytes used in SBs suffer from serious safety hazards (high volatility, inflammability and leakage), severe side reactions between electrodes and electrolytes, and inevitable sodium dendrite problems, which is greatly detrimental to battery performance. Notably, polymer electrolytes are recognized as the optimal solution to resolve the abovementioned bottlenecks. Herein, we mainly summarize a series of polymer electrolytes based on polymers containing ethoxylated units, poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)), poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN), polyvinylpyrrolidone (PVP), single ion conductors, polysaccharides and so on. Notably, this review demonstrates the natural merits of polymer electrolytes for SBs (such as high safety, suppression of sodium dendrite formation and reduced electrolyte decomposition), presents the requirements for ideal polymer electrolytes for the first time, and provides concrete discussions into recent progresses of various polymer electrolytes as well. Furthermore, potential challenges and perspectives of polymer electrolytes for advanced SBs are also envisioned at the end of this review. Overall, we hope this discussion will make sense to resolve fundamental research and practical issues of polymer electrolytes for advanced SBs.

INTRODUCTION Since lithium ion batteries (LIBs) were firstly launched to consumer markets by Sony in 1991, they have been widely used in various fields (e.g. portable electronic devices and electric vehicles) owing to their advantages including high voltage, high energy density, long cycle life and low self-discharge property.1-4 Nevertheless, global reserves with economically feasible for extraction are only 14 million tons lithium carbonate equivalent (LCE).5 If 95 million annual vehicle sales are fully electrified, it is estimated to require 6.7 million tons per annum, which almost accounts for 50% of current global reserves. This is why the lithium salts have become one of the world's hottest commodity, and the price of lithium carbonate as the raw material for preparing various lithium compounds and lithium metal has increased from 126,000 RMB/ton to 171,000 RMB/ton since 2017.6 Consequently, the limited lithium reserves and increasingly costly lithium salts weaken the merits of lithium batteries (LBs), and force researchers to develop promising candidates. It is under this context that sodium batteries (SBs) have earned considerable attentions due to 1

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abundant reserves (the 4th most abundant element in the earth crust) and low cost of sodium salts, and suitable redox potential of sodium element (-2.71 V vs SHE, only 0.3 V above that of lithium).7-10 Moreover, SBs can offer a higher energy density (1165 Wh/kg) (Table 1), than aqueous batteries and have much lower cost than LBs, indicating great potential for practical applications. Currently, more than a dozen companies around the world are conducting the practical research and development of sodium ion batteries (SIBs). Very recently, Hu et al. completed the first demonstration application of SIBs powered low-speed electric vehicles with an energy density of 120 Wh/kg, marking the impending commercialization of SIBs. Table 1. The comparisons of physical properties of Na, Li.2,4,11-12 Parameters

Li

Na

Relative atomic mass

6.94

23.00

Shannon's ionic radii (Å)

0.76

1.02

E° (vs SHE) (V)

-3.04

-2.71

Melting point (°C)

180.5

97.7

Cost (for carbonate) ($/ton)

5000

150

Mohs’ scale of hardness

0.6

0.4

(g/cm3)

0.534

0.97

Thermal conductivity (W/(m·K))

84.8

142

Ionic volume(Å3)

1.84

4.44

Theoretical capacity (mAh/g)

3829

1165

0.0065

2.74

70% in South America

Everywhere

Density

Crustal content (%) Distribution (*)

Thus far, great efforts have been devoted to research technologies of SBs, in which many cathode and anode active materials with various working potential ranges and reversible capacities have been developed 13-17,

and the most investigated electrolytes are carbonate-based electrolytes, which possess high ionic

conductivities due to high dielectric constants and low viscosities.18-19 However, they suffer from high volatility, inflammability, poor thermal stability and leakage risk, and thus easily lead to safety hazards, hampering practical applications. In addition, we can take warnings from the burning accidents of LBs powered consumer devices (e.g. Samsung Galaxy Note 7 smartphones) owing to the use of liquid electrolytes as well. Aside from safety hazards, common liquid electrolyte-based SBs still encounter severe side reactions between electrodes and electrolytes, and inevitable sodium dendrite problems. (1) Safety hazards: Organic solvents in the liquid electrolyte such as PC, EC, and DMC possess low boiling points and flash points, delivering high volatility and inflammability. Thus, liquid electrolyte-based batteries encounter severe safety hazards, especially under overcharged, short-circuit, or elevated temperature conditions. In addition, continual decompositions of liquid electrolytes during charge-discharge processes produce a lot of gas resulting in the large volume expansion of batteries, which might further induce electrolyte leakages and even consequent explosions. Despite great endeavors have already been made to improve the safety of liquid electrolytes, such as the development of nonflammable electrolytes by adding flame-retarding additives such as ethoxy(pentafluoro)-cyclotriphosphazene (EFPN) and organic electrolyte-ionic liquid hybrid electrolytes, however, inherent safety issues are still inevitable, or certain aspects of battery performance will deteriorate.20-21 (2) Severe side reactions between electrodes and electrolytes: Liquid electrolytes can react with anodes and cathodes, and form solid interface layers passivating electrodes during the first few charge-discharge cycles. But they can hardly form a uniform and compact solid electrolyte interface (SEI) on the electrode 2


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surface,22 leading to large interface impedances, irreversible capacities and low Coulombic efficiencies. Therefore, film-forming additives including cathode film-forming additives (e.g. VC as an electrolyte additive for passivating a Na2MnSiO4 cathode)23 and anode film-forming additives (e.g. rubidium and cesium ions as electrolyte additives for passivating a hard carbon anode)24 have been employed to resolve these issues, and concentrated electrolytes have also been developed to stabilize hard carbon anode surface and achieve long-term cycling stability of SBs.25 However, successive decompositions of liquid electrolytes during cycling processes can happen inevitably, bringing about capacity fade and poor cycling stability of SBs. (3) Inevitable sodium dendrite problems in sodium metal batteries (SMBs): It is very difficult to obtain uniform deposition/dissolution on sodium metal anode surface for liquid electrolyte-based SMBs during cycling processes, which contributes to uncontrolled sodium dendrite growth followed by sodium dendrites penetrating the separator and posing battery short-circuit, and thus results in serious safety hazards. Therefore, approaches including applying porous Al current collector to achieve uniform sodium plating/stripping behaviors,26 fabricating a processable and moldable composite Na metal anode,27 and introducing buffer layers fabricated onto the sodium metal surface through chemical and physical modifications have been performed, in which buffer layers usually leads to inferior rate performance.28 Given these issues caused by liquid electrolytes, other kinds of electrolytes have also been explored for high performance SBs. For instance, inorganic solid electrolytes are invariably characterized by high ionic conductivities (> 10-4 S/cm) and superior thermal stabilities, and yet they are afflicted with poor processability and inferior interfacial compatibility with electrodes.29-30 In another example, ionic liquid electrolytes

with

the

use

of

N-butyl-N-methylpyrrolidiniumbis

(trifluoromethanesulfonyl)

imide

(PYR14TFSI), have been developed for SBs due to their wide electrochemical windows and non-flammability,31-36 and unfortunately, they still possess disadvantages such as high cost and low ionic conductivities at ambient temperature, severely hampering practical applications for SBs. Strikingly, polymer electrolytes exhibit remarkable characteristics, such as no liquid leakage, low flammability and excellent processability for various shapes and good flexibility, showing good potential for practical applications in SBs. Above-mentioned characteristics can allow SBs based on polymer electrolytes to deliver high energy density, long-term cycling stability and enhanced safety. So far, various polymer host materials (e.g. polymers containing ethoxylated units, poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)), poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN), polyvinylpyrrolidone (PVP), single ion conductors, polysaccharides and so on) have been developed for SBs. However, although polymer electrolytes possess outstanding electrochemical properties, comprehensive performances such as long-term cycling stability can hardly meet the requirements in practical applications of SBs. To our knowledge, a series of reviews on electrodes and non-aqueous liquid electrolytes for SBs have been reported,37-40 but reviews on polymer electrolytes for SBs are rare. In addition, despite several recent reviews of SBs have mentioned the properties and development of polymer electrolytes in brief,2,41-42 however, they have hardly debated polymer electrolytes from the perspectives of structure-performance relationships. Consequently, the ideal requirements, natural merits and recent progresses of polymer electrolytes for SBs are firstly presented in this discussion. Finally, potential challenges and future perspectives are also proposed to guide future development of safety-reinforced SBs.

2. POLYMER ELECTROLYTES FOR ADVANCED SBs: INHERENT ADVANTAGES AND IDEAL REQUIREMENTS 3



Polymer electrolytes stemmed from the pioneering research by Wright et al. in 1973 and have been widely explored in LIBs for several decades. Recently, a focus of polymer electrolytes has been turned onto the SBs, which can be explicitly noticed from plenty of recent research publications on sodium ion-conducting polymer electrolytes. Based on the addition of plasticizers, polymer electrolytes can be divided into two types: all-solid-state polymer electrolytes (APEs) and gel polymer electrolytes (GPEs). As for GPEs, many plasticizers such as organic solvents, ionic liquids, plastic crystals and inorganic fillers have been employed up to date. Compared with liquid electrolytes, polymer electrolytes possess several inherent advantages, as shown in Figure 1, which include high safety (e.g. non-leakage and non-flammability), suppression of sodium dendrite formation and reduced electrolyte decomposition, endowing polymer electrolyte-based SBs with superior safety properties and electrochemical performances. (1) High safety: The factor is an inherent merit of polymer electrolytes, which can effectively reduce and even avoid the hazards resulting from liquid electrolytes with inflammability and leakage. (2) Suppression of sodium dendrite formation: Polymer electrolytes with high mechanical strengths can enable uniform deposition/dissolution behaviors of sodium ions on the anode surface, and meanwhile suppress the generation of sodium dendrites. This factor facilitates high compatibility between polymer electrolytes and sodium anodes, suggesting that polymer electrolytes can match with sodium anodes to assemble high energy density SMBs. (3) Reduced electrolyte decomposition: In generally, polymer electrolytes can effectively avoid electrolyte decomposition during cycling processes by building a rigid barrier between liquid electrolytes and cathode active materials, endowing investigated SBs with high Coulombic efficiencies and long-term cycling stabilities.

Figure 1. Schematic illustration of the inherent advantages of polymer electrolytes for advanced SBs as compared with that of liquid electrolytes: (a) Non-leakage and flexible properties, (b) Suppressing the growth of sodium dendrites and (c) Less decompositions of electrolytes. Here, liquid electrolytes and polymer electrolytes are denoted as LE and PE, respectively. Based on the aforementioned merits of polymer electrolytes, here we firstly proposed that an ideal polymer electrolyte should possess following traits: (I) A high ionic conductivity (up to 10-3 S/cm) in a wide range of operating temperatures in order to obtain high cycle and rate performance. (II) A high sodium ion transference number (≈ 1) for preventing ionic concentration polarization and 4


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delivering high power performance. (III) A wide electrochemical window (0 ~ 5 V) in order to effectively avoid obvious side reactions with electrodes, enable the unicity of electrochemical reactions during the charge-discharge process, and cope with the high-voltage cathodes to achieve high energy density SBs. (IV) Superior thermal stability (-40 oC - 150 oC for SBs) and high safety.43 Polymer electrolytes with high dimensional stabilities can hardly deform at high temperatures, thus avoiding subsequent short-circuits and internal thermal runaway due to the deformation of electrolyte membranes within the batteries. (IV) A high mechanical strength for suppression of sodium dendrites and applications in wearable fields. According to the report, polymer electrolytes with a high mechanical strength can suppress the growth of sodium dendrites, especially for inorganic/polymer hybrid electrolytes.44 Besides, polymer electrolytes with high elasticity have the potential to be applied in wearable fields for artificial intelligence. (V) Relatively low-cost and non-toxicity for wide applications in SBs on a large scale. Overall, inherent advantages of polymer electrolytes indicate great potential for advanced SBs applications in the near future, and the ideal requirements of polymer electrolytes can provide a useful guide for researchers. Meanwhile, according to the different structural characteristics of polymer matrixes, advanced polymer electrolyte-based SBs can be mainly classified into diverse types as follows: polymers containing ethoxylated units, P(VDF-HFP), PMMA, PVP, PAN, single ion conductors, polysaccharides and others. 3. RECENT PROGRESSES ON POLYMER ELECTROLYTES FOR SODIUM BATTERY APPLICATIONS 3.1 POLYMERS CONTAINING ETHOXYLATED UNITS BASED ELECTROLYTES Polymers with repeating −CH2CH2O− (EO) units favor solvations and dissociations of sodium salts because non-shared electron pairs of EO chains can coordinate with Na+, and exhibit flexible backbones and good mechanical strengths, indicating great promise in applications for advanced SBs (Table 2 - Table 4). 3.1.1 PEO BASED POLYMER ELECTROLYTES Up to date, polyethylene oxide (PEO) based polymer electrolytes have been widely explored for SBs, and according to the use of plasticizers can be mainly classified into two types: PEO based APEs and GPEs. PEO/alkali metal salt based APEs originated from the seminal work of Wright and coworkers,45-49 and have been reported to exhibit low ion-conducting abilities at room temperature (on the order of 10-5 S/cm), since ionic conductivities of PEO based APEs were determined through segment motions of amorphous domains of PEO matrixes. Therefore, great efforts have been made to lower glass transition temperatures (Tg) of PEO based solid polymer electrolytes by optimizing types and concentrations of Na-salts, blending with other polymers, or adding inorganic nano-particles, in order to obtain high ion-conducting APEs at room temperature (Table 2). Table 2. Performance parameters of PEO based APEs for SBs. Year 2001

Polymer electrolyte

Ionic

ingredients

conductivity

PEO/NaClO3 (70:30, w/w)

2008

[(PEO)6:NaPO3:6wt.%

2.12 ×

10-4

S/cm at 105

3.4 ×

10-6

S/cm at 35

1.2 ×

10-6

S/cm at 72 oC

oC

Electrochemical

Battery

stabilities

configuration

__

(I2+C+electrolyte)/Na

Ref. 50

oC

__

__

58

__


56

BaTiO3] [EO]:[Na]=20:1 2013

[97(75PEO:25NaPO3):

1.07 × 10-5 S/cm 5



2014 2015

3PVP] (wt.%)

at room temperature

PEO/PVP/NaBr

1.9 × 10-6 S/cm

(62.5:22.5:15,w/w/w)

at room temperature

[PEO:NaClO4:5wt.%TiO2]

2.62 × 10-4 S/cm at 60 oC

__ __

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55

Na2/3Co2/3Mn1/3O2/Na

59

TiO2/Na at 60 oC

57

[EO]:[Na] =20:1 2015

PEO/NaClO4/Na-CMC

~ 10-3 S/cm at 80 oC

4.5 V

(82:9:9, w/w/w) 2016

[95(70PEO:30NaHCO3)

NaFePO4/Na at 60 2.04 ×

10-5

S/cm at 27

oC

oC

__


60

3.36 × 10-4 S/cm at 80 oC

4.87 V

NaCu1/9Ni2/9Fe1/3Mn1/3O2

51

:5wt.% SiO2] 2017

PEO/NaFNFSI

/Na

[EO]:[Na]=15:1 2017

16PEO/NaTCP (molar ratio)

> 10-3 S/cm at 70 oC

5V

__

54

2018

PEO/NaTFSI+50

6 × 10-5 S/cm at 30 °C

4.3 V

Na3V2(PO4)3/Na

61

wt %NASICON

2.8 × 10-3 S/cm at 80 °C

As essential parts for PEO/Na-salts systems, sodium salts with various solubilities and stabilities greatly affect cation concentrations and transference properties, and subsequently influence battery performance. So far, various Na-salts such as sodium chlorate (NaClO3), sodium perchlorate (NaClO4), sodium polyphosphate (NaPO3), sodium bromide (NaBr), sodium iodide (NaI), sodium bis(trifluoromethanesulfonyl) imide (NaTFSI), sodium bis(fluoro-sulfonyl)imide (NaFSI) and so on, have been employed for SBs. To improve ion-conducting abilities of PEO based electrolytes at room temperature, optimizing Na-salts concentrations in PEO based APEs can be an effective method by increasing the amorphous regions of PEO based APEs. For example, Chandrasekaran et al. prepared a poly(ethylene oxide)-sodium chlorate (PEO-NaClO3) APE via a facile solution-casting technique,50 and demonstrated that the highest ionic conductivity of 3.40 × 10-6 S/cm at 35 oC was obtained at the PEO/NaClO3 weight ratio of 70:30. In contrast with NaClO3, Na-salts with larger anions tend to exhibit more delocalized charge distributions, and hence possess reduced crystal lattice energies and enhanced solubilities in common organic solvents, which is in favor of ionic conductivities. Benefited from these merits, large-volume Na-salts like NaTFSI, NaFSI and sodium(fluorosulfonyl) (n-nonafluorobutanesulfonyl) imide (Na[(FSO2)(n-C4F9SO2)N], NaFNFSI) have been applied to PEO based polymer electrolytes (Figure 2).51-52 For instance, the NaFNFSI/PEO ([EO]:[Na]=15:1) solid polymer electrolyte possessed a superior ionic conductivity (3.36 × 10-4 S/cm at 80 oC),

sufficient thermal stability (> 300 oC) and a high anti-oxidative stability (4.87 V vs. Na+/Na). Here, the

investigated NaCu1/9Ni2/9Fe1/3Mn1/3O2/Na battery applying PEO-NaFNFSI electrolyte delivered a high initial reversible capacity (122.4 mAh/g at 0.1 C), good cycling performance (70% of capacity retention after 150 cycles at 1 C) and an excellent rate capability at 80 oC (Figure 3c-d). However, the resultant PEO-NaFNFSI electrolyte showed a low ionic conductivity at 55 oC because of the high crystallization degree of PEO.

6


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Figure 2. The optical photograph (a) and typical SEM image (b) of NaFNFSI/PEO ([EO]:[Na+]=15:1) blended

solid

polymer

electrolyte.

NaCu1/9Ni2/9Fe1/3Mn1/3O2/NaFNFSI/PEO/Na cells at 80

Electrochemical oC:

performances

of

(c) Cycling performance and Coulombic

efficiency at 1 C (after 5 cycles of activation at 0.1 C). (d) Rate capability at various C rates. Reproduced with permission from ref 51. Copyright 2017 Royal Society of Chemistry. Despite abovementioned Na-salts with large anions containing fluorine atoms exhibited good electrochemical properties, however, most of them exhibited poor anti-oxidative abilities and can corrode current collectors.53 Therefore, a series of fluorine-free Na-salts (e.g. sodium pentacyanopropenide (NaPCPI), sodium 2,3,4,5-tetracyanopirolate (NaTCP) and sodium 2,4,5-tricyanoimidazolate (NaTIM)), have been reported for solid polymer electrolytes.54 As for NaTCP, lower cation-anion (Na+-TCP-) interaction energies in NaTCP provide more effective charge carriers, and anions (TCP--TCP-) can form relatively larger tetrameric units via π-π stacking, which can easily plasticize PEO, and thus benefit the charge mobility. As a result, the investigated PEO16NaTCP exhibited a "liquid-like" ionic conductivity (over 10-3 S/cm at 70 °C), and a wide electrochemical window (5 V), indicating that these fluorine-free salts provide a new perspective to optimize PEO-based APE systems for advanced SBs. Aside from ion-conducting optimization of PEO based APEs by adjusting the types and concentrations of Na-salts, blending PEO with other polymers have been investigated so far to optimize ionic conductivities of PEO

based

electrolytes

by

suppressing

the

formation

of

crystallization

domains

in

PEO.

Polyvinylpyrrolidone (PVP) is a polymer with high amorphous content, which can permit faster ionic mobility as compared to other semi-crystalline polymers.55 In one study, Kumar et al. developed a PEO/PVP blend system by doping with various concentrations of NaBr,55 and demonstrated that the PEO/PVP/NaBr electrolyte displayed a lower Tg with increasing NaBr (58.49 oC for pure PEO/PVP electrolyte, 40.07 oC for PEO/PVP/15% NaBr electrolyte), and showed ionic conductivities of up to 1.90 × 10-6 S/cm under room temperature at the PEO/PVP/NaBr weight ratio of 62.5:22.5:15 (Figure 3a). Unfortunately, ionic conductivity values of PEO/NaBr (85:15) system were not presented in this study. In another example, 7



Chandra et al.56 also optimized the electrochemical properties of PVP-blended polymer electrolyte systems by using NaPO3 as the sodium salt and a hot-pressed technique, and demonstrated that the investigated APE delivered an ionic conductivity of 1.07 × 10-5 S/cm at room temperature, roughly an order of magnitude higher than that of PEO/PVP/NaBr systems41 mentioned above (Figure 3b). Considering importance of biomass utilization for environment protection, renewable cellulose-based composite APEs have also been investigated for SBs. In one study, Colò et al.57 developed an APE containing sodium carboxymethyl cellulose due to its low-cost, biodegradability, biocompatibility and non-toxic characteristics, and demonstrated that the resultant PEO/NaClO4/Na-CMC APE delivered an enhanced mechanical property without affecting the ionic mobility, compared with the bare electrolyte. Here, the PEO/NaClO4/Na-CMC electrolyte with the weight ratio of 82:9:9 delivered a maximum ionic conductivity of 10-3 S/cm at 80 oC, and showed a relative high electrochemical window of 4.5 V at 60 oC. Besides, the TiO2/Na-metal battery or NaFePO4/Na-metal battery employing PEO/NaClO4/Na-CMC electrolyte delivered a good cycling stability at 60 oC.

Figure 3. Arrhenius plots for (a) PEO/PVP blend films containing NaBr ratios of 0 wt.% (□), 5 wt.% (○), 10 wt.% (△) and 15 wt.% (▽). Reproduced with permission from ref 55. Copyright 2013 Elsevier. (b) Hot-pressed solid polymer electrolyte host (75PEO:25NaPO3) (□) and solid polymer electrolyte OCC [97 wt.% (75PEO:25NaPO3) + 3 wt.% PVP] (○). Reproduced with permission from ref 56. Copyright 2013 Chinese Chemical Society, Institute of Chemistry, Chinese Academy of Sciences and Springer-Verlag Berlin Heidelberg. Digital photographs of the PEO/NaClO4 ([EO]:[Na]=20:1) solid polymer electrolyte films (c) and (d) 5 wt.% TiO2-blended APE. Temperature-dependent conductivity plots (e) of PEO and PEO/NaClO4 polymer electrolyte films in various concentrations and (f) for PEO, PEO-NaClO4 and TiO2 blended nCPE. (g) Cycle performance of Na2/3Co2/3Mn1/3O2/Na half-cell using TiO2-blended APE and liquid electrolyte at 0.1 C. (h) Charge and discharge profiles of Na2/3Co2/3Mn1/3O2/Na half-cell using TiO2-blended APE under 0.1 C at 60 oC. Reproduced with permission from ref 59. Copyright 2014 Elsevier. It was also reported that electrochemical properties and mechanical strengths of polymer electrolytes can be improved through the use of inorganic particles, which opens up new possibilities for applications in safe SBs.56,58-61 Thus far, some inorganic particles (zirconium dioxide (ZrO2), barium titanate (BaTiO3), antimonous oxide (Sb2O3), aluminium oxide (Al2O3) or silica nano-particles (SiO2), etc.) have been added into PEO/Na-salts systems to form composite or nano-composite polymer electrolytes (nCPEs). For example, Bhide and Hariharan et al. developed a composite polymer electrolyte based on (PEO)6/NaPO3 plus submicrometre-sized BaTiO3,58 and reported that the investigated electrolyte with 6 wt.% BaTiO3 exhibited an ionic conductivity of 1.17 × 10-6 S/cm at 72 oC, two orders of magnitude larger than the bare electrolyte. 8


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Compared with abovementioned fillers with large sizes, inorganic nano-particles with higher surface areas can exhibit more efficient contact with PEO and sodium salts and shorten the Na+ diffusion pathways, thus delivering enhanced ionic conductivities. For example, Ni'mah et al. investigated a nanoscale TiO2-blended composite polymer electrolytes (PEO/NaClO4/TiO2 (3.4 nm)) for SBs (Figure 3c-d),59 and demonstrated that the ionic conductivity of such polymer electrolyte with the composition of PEO/NaClO4/TiO2 (TiO2=5 wt.%, [EO]:[Na]=20:1) was 2.62 × 10-4 S/cm at 60 oC (Figure 3e-f). Here, the resultant Na2/3Co2/3Mn1/3O2/Na battery using PEO/NaClO4/TiO2 electrolyte displayed a specific discharge capacity of only 45 mAh/g at 0.1 C, much lower than that (~110 mAh/g) of Na2/3Co2/3Mn1/3O2/Na battery employing liquid electrolytes, as well as good reversibility and long-term cycling stability with a capacity retention of 90% at 0.1 C after 25 cycles (Figure 3g-h). In addition to nano-sized TiO2, other nano-particles such as SiO2 was also employed to prepare composite polymer electrolytes. For example, Chandra et al. developed a PEO/NaHCO3/SiO2 nano-composite polymer electrolyte through a hot-press technique,60 and reported that the resultant electrolyte with 5 wt.% SiO2 delivered a maximum ionic conductivity of 2.04 × 10-5 S/cm at 27 oC. In this study, the authors attributed high ion-conducting ability of this electrolyte to the interactions between PEO backbones and SiO2 that may provide additional hopping sites and create more ion-conducting pathways for fast ionic conduction. Other endeavors62-63 were also made to prepare composite polymer electrolytes with nanoscale fillers, giving as-prepared APEs with high ionic conductivities, about two or three orders of magnitude higher than those of previously reported pristine PEO/Na-salts (e.g. PEO/NaClO4). In brief, PEO-based APEs have been reported to possess a maximum ionic conductivity on the order of 10-5 S/cm at room temperature, however, they can hardly satisfy application requirements. Besides, they also suffer from inferior anodic stabilities and poor mechanical properties under elevated temperatures, and can be further optimized by introducing some sulfonate or phosphate units and aromatic motifs onto polymer backbones, respectively. PEO based APEs with low ionic conductivities at room temperature exhibit inferior battery performance, which allow researchers to seek efficient strategies to resolve this ion-conducting issue. As a result, small molecular plasticizers have been employed to gelatinize PEO-based polymer electrolytes (i.e. GPEs), as shown in Table 3. For example, room-temperature ionic liquids have been adopted as the plasticizers for PEO based polymer electrolytes because of their appealing merits including non-flammability, wide electrochemical stability windows, high ionic conductivities and excellent thermal stabilities.64-67 In contrast with PEO based APEs, the ionic conductivities of PEO based GPEs using ionic liquids (10-4 S/cm vs. 10-6 ~ 10-8 S/cm) at room temperature increased by more than two orders of magnitude. In another study, 1-butyl-3-methylimidazoliummethylsulfate (BMIM-MS) was also added into PEO-sodium methylsulfate (NaMS) electrolytes for SBs,64 which delivered an ionic conductivity of up to 1.05 × 10-4 S/cm at 30 oC. Here, the authors attributed high ion-conducting properties of as-prepared GPE to the addition of BMIM-MS that can decrease the PEO crystallinity, and found that investigated GPEs with high BMIM-MS contents (e.g. PEO-10 wt.% NaMS-60 wt.% BMIM-MS) delivered decreased ionic conductivities owing to the formation of ion-pairs or higher order ionic aggregates. Similar GPE systems such as (PEO)9-NaTFSI-Pyr13TFSI and PEO-NaClO4-1-butyl-3-methylimidazolium thiocyanate electrolytes were also reported to show high ionic conductivities.66-67 Recently, inorganic nano-particles were also employed to further optimize the electrochemical properties of PEO/Na-salts/ionic liquids polymer electrolytes. For one study, Song et al. prepared

hybrid polymer

electrolytes containing SiO2 (5-15 nm) and 1-ethyl-3-methylimidazolium bis(uoro-sulfonyl) imide(Emim FSI) for Na-metal batteries,65 and reported that the resultant hybrid polymer electrolytes with the compositions of (PEO20-NaClO4-5 wt.% SiO2)-70 wt.% Emim FSI showed a room-temperature ionic 9



Page 10 of 31

conductivity of 1.3 × 10-3 S/cm and a high Na+ transference number of 0.61, and also displayed suitable mechanical strengths and wide electrochemical windows up to 4.2 V. In this study, the assembled hybrid cathode/Na cell using the composite electrolyte exhibited a 51% capacity retention after 100 cycles at 0.05 C at room temperature. These results indicated that it's an effective approach to add nanometer-sized inorganic oxides and ionic liquids plasticizers simultaneously to polymer matrixes for enhancing the electrochemical performance of polymer electrolytes. Table 3. Performance parameters of PEO based GPEs for SBs Year 2010

Polymer electrolyte

Ionic

Electrochemical

Battery

ingredients

conductivity

stabilities

configuration

10-4

__

__

69

5V

__

64

__

__

66

4.2 V

Hybrid cathode/Na

65

4.7 V

TiO2/Na

68

__

__

67

[(60 wt.% PEO : 40 wt.%

~1.1 ×

S/cm at 25 °C

Ref.

NaCF3SO3) + 50 wt.% SN] 2016 2016

[(PEO + 10 wt.% NaMS) +

~1.05 × 10-4 S/cm at 30 °C

60 wt.% BMIM-MS]

4×

10-4

[NaTFSI(PEO)9 + 20 wt.%

10-4

S/cm at 75 °C

S/cm at 20 °C

Pyr13TFSI] 2017 2017

[(PEO20/NaClO4) + 5 wt.%

1.3 × 10-3 S/cm

SiO2 +70 wt.% Emim FSI]

at room temperature

[(5 wt.% PEO : 50 wt.%

>1 × 10-3 S/cm at 25 °C

NaClO4:45 wt.% PC) + 4

1 × 10-4 S/cm at 0 °C

wt.% 4-methoxybenzophenone] 2017

[PEO/NaClO4([EO]:[Na]=2

~5 × 10-4 S/cm

0:1)

at room temperature

+

30

wt.%

1-butyl-3-methylimidazoliu m thiocyanate] Succinonitrile (SN), as a polar organic plasticizer (Tm ≈ 60 °C, the plasticity range: -35 °C to 60 °C), could dissolute a wide variety of ionic salts and enable SN-gelated plastic electrolytes to exhibit high ambient-temperature ionic conductivities of ~10−4 S/cm - 10−2 S/cm by adjusting salt types and concentrations. In one example, a SN-containing GPE was prepared by Bhattacharyya et al.,69 in which the as-formed PEO-NaCF3SO3-50 wt.% SN electrolyte system delivered high ionic conductivities, approximately 45 times higher than that of PEO-NaCF3SO3 electrolytes. Here, the authors proposed a sodium ion-conduction mechanism of this GPE via a vivid model (Scheme 1): the amorphous proportions of this GPE at room temperature is far below the percolation threshold (left hand side of Scheme 1), leading to a low ionic conductivity, while the amorphous proportions of the host polymer is equal to or higher than percolation threshold, resulting in a high ionic conductivity. A combination of both situations lead to non-percolation; plasticizers (liquid solvents or ionic liquids)-soaked polymer electrolytes could greatly enlarge the amorphous regions at room temperature (right hand side of Scheme 1), which is benefited for reaching percolation threshold and thus results in increased ionic conductivities.

10


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Scheme 1. Transformation of a soft matter solid electrolyte such as polymer electrolyte with a non-percolative arrangement of highly disordered regions to a percolative arrangement of disordered regions as shown in gel electrolytes. The percolative network of disordered regions provide fast ion transport pathways for the mobile ion. Reproduced with permission from ref 69. Copyright 2010 Elsevier. Overall, to solve low room-temperature ionic conductivities (10-5 S/cm ~ 10-8 S/cm) of PEO based APEs, some efforts have been made by adjusting types and concentrations of Na-salts, blending with polymers, or adding diverse plasticizers (e.g. organic solvents, ionic liquids and plastic crystals), delivering high ionic conductivities on the order of 10-3 S/cm at room temperature. It is worth noticing that PEO-based APEs exhibit poor mechanical strengths at the operation temperature of above the melting point of PEO (i.e. 60 oC), bringing about serious safety risks, and that PEO-based GPEs usually exhibit inferior anodic stabilities. Great efforts should be further devoted to develop and introduce high performance inorganic nanofillers or polymers for achieving PEO-based solid polymer electrolytes with high mechanical properties and anti-oxidative abilities. 3.1.2 PEG BASED POLYMER ELECTROLYTES Compared with PEO based electrolytes, polyethylene glycol (PEG) based polymer electrolytes possess relatively high ion-conducting abilities owing to lower molecular weights and more efficient segmental motions of polymer chains (Table 4). For example, the hybrid nanoparticles containing SiO2 nanoparticles grafted PEG (SiO2-PEG-anion) was synthesized, and the investigated hybrid polymer electrolyte prepared by immersing these nanoparticles into a matrix of PEO and polyethyleneglycol dimethylether (PEGDME).70 Here, the resultant polymer-SiO2 nano-dispersed electrolyte exhibited high room-temperature conductivities of >10-5 S/cm owing to the plasticizing effect of grafted PEG segments, indicating potential applications for APEs. Table 4. Performance parameters of PEG based polymer electrolytes for SBs Year 2013

Polymer electrolyte

Ionic

Electrochemical

Battery

ingredients

conductivity

stabilities

configuration

10-5

PEO/PEGDME-SiO2-PEG-

>2×

S/cm

RSO2N(-)SO3CF3 anion

at room temperature

Ref.

3.8 V

__

70

(EO/Na ≈ 20) 2017

CPMEA/NASICON

__

__

NaTi2(PO4)3/Na

71

2017

Polysulfonamide/ Poly

1.2 × 10-3 S/cm at 25 oC

4.7 V

__

72

4.5 × 10-6 S/cm at 30 oC

__

δ-NaxV2O5/Na

73

(ethylene glycol) divinyl ether/LiBF4/NaClO4/PC 2018

POSS-4PEG2K ([EO]: [Na]=16)

In comparison with linear PEG-based electrolytes mentioned above, cross-linked PEG-based electrolytes showed higher mechanical strengths, which can suppress unwanted sodium dendrite formation. For one 11



study,

hybrid

network-based

APE

was

octakis(3-glycidyloxypropyldimethylsiloxy)octasilsesquioxane

Page 12 of 31

synthesized (octa-POSS)

polyethylene glycol (PEG) using sodium perchlorate (NaClO4) as the sodium

by with

salt.73

polymerizing amine-terminated

The investigated APEs

in this study demonstrated excellent suppression of sodium dendrite growth in the plating and stripping experiments (5150 h (still running) at a current density of 0.1 mA/cm2 and 3550 h when the current density was increased to 0.5 mA/cm2), indicating potential applications for SMBs. The in-situ polymerization method has been considered as a good choice for preparation of high-performance polymer electrolytes with enhanced interfacial compatibility and battery performance. Recently, Cui et al. proposed an in-situ strategy to exploit polysulfonamide (PSA)-supported poly(ethylene glycol) divinyl ether based polymer electrolyte (here after abbreviated as "PPDE-CPE") (Figure 4a).72 In this study, the resultant PPDE-CPE exhibited a relatively high ionic conductivity of 1.2 × 10-3 S/cm at ambient temperature, a wide electrochemical window of 4.7 V, a favorable mechanical strength (25 MPa), and a superior cyclic stability with a capacity retention of 84% after 1000 cycles at 0.5 C in Na3V2(PO4)3/MoS2 battery (Figure 4b-d). In addition, there were no internal short-circuit failures in investigated SIBs with PPDE-CPE, even in the case of a bended and wrinkled state (Figure 4e-f), showing high safety characteristics. The cross-linked PEG-based polymer matrixes can also be employed together with inorganic electrolyte in order to obtain high performance organic-inorganic hybrid APEs. For example, Zhou et al. prepared a cross-linked polymer matrix through cross-linking polymerization of poly(ethylene glycol) methyl ether acrylate

(CPMEA),71

and

demonstrated

that

the

resultant

NaTi2(PO4)3/Na

cell

using

the

CPMEA/NASICON/CPMEA sandwich electrolyte displayed a stable capacity of around 102 mAh/g for 70 cycles with high Coulombic efficiencies of ~ 99.7% at 0.2 C and 65 °C. Here, they also found that the APEs with a stable sandwich structure can suppress the formation of sodium dendrites during cycing process.

Figure 4. (a), (b) and (c) Rate capability and charge/discharge profiles of Na3V2(PO4)3/MoS2 sodium-ion full battery at varied rates, (d) Long-term cycling performance and its corresponding Coulombic efficiency of the sodium-ion full cell at 0.5 C. Optical photographs for the soft package Na3V2(PO4)3/MoS2 sodium-ion full cell with PPDE-CPE that powers a red LED: bending (e) and (f) wrinkling. Reproduced with permission from ref 72. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA. Polyethylene glycol (PEG)-based polymer electrolytes possess relatively high ion-conducting abilities, in which cross-linked PEG-based electrolytes can suppress the growth of sodium dendrites, indicating high safety. However, there is a tradeoff between the ionic conductivity and mechanical strength for cross-linked 12


Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PEG-based electrolytes. The conflicting requirements for high ionic conductivities and excellent mechanical properties can be addressed in cross-linked PEG-based electrolyte systems by building semi-interpenetrating polymer networks with optimal lengths and amounts of ion-conducting side chains. Comprehensively, polymers containing ethoxylated units based electrolytes (including PEO and PEG based polymer electrolytes) exhibit high ionic conductivities compared with other APEs, indicating potential candidates to replace organic solvent-based electrolytes. This is especially true for cross-linked PEG based polymer electrolytes that can suppress the growth of sodium dendrites, endowing the resultant SBs with superior cycling stabilities and safeties. However, they still suffer from dissatisfactory ion-conducting abilities and inferior anodic stabilities for applications, and can be further optimized by copolymerizing with monomers with high anti-oxidative units such as cyano and sulfone groups and blending with polymers with low Tg like polysiloxanes, respectively. 3.2 P(VDF-HFP) BASED POLYMER ELECTROLYTES Poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) was developed as a polymer host for LIBs by Telcordia Technologies in 1994,74 and was composed of a crystalline phase and an amorphous phase. The crystalline phase (-VDF) on P(VDF-HFP) endows P(VDF-HFP) with enhanced mechanical properties while the amorphous phase (-HFP) enables P(VDF-HFP) reduced crystallinities and Tg. Besides, owing to plenty of -C-F bonds with high polarity, P(VDF-HFP) possesses a relatively high dielectric constant (ɛ ≈ 8.4), superior electrochemical stability and non-inflammability, indicating that P(VDF-HFP) based electrolytes can be good candidates for high safety SBs. As a rule, P(VDF-HFP) based membranes are soaked in liquid electrolyte (Na-salts/organic solvents) to form gel-state polymer electrolytes for SBs. For example, Wu et al. prepared P(VDF-HFP) based GPE membranes through a simple phase separation process using EC/DMC/DEC

(1/1/1,

w/w/w)

as

organic

plasticizers.75

Here,

the

resultant

P(VDF-HFP)/NaClO4/EC/DMC/DEC polymer electrolyte displayed a high ionic conductivity of 6.0 × 10-4 S/cm at ambient temperature and a wide electrochemical window of 4.6 V. Based on this, with the use of an electrospinning technique, enhanced electrolyte uptakes of P(VDF-HFP) based GPE membranes have been achieved, endowing investigated P(VDF-HFP)/NaClO4/EC/DEC systems with high ion-conducting abilities (up to 1.13 × 10-3 S/cm at room temperature).76 In addition, the resultant GPE in this study showed a broad electrochemical window (4.8 V), indicating great potential to be applied for high voltage SBs. In this study, however, the GPE-assembled NaNi0.5Mn0.5O2/Na cells delivered inferior cyclabilities at 0.2 C, limiting SB applications. One reason for poor cycling stabilities of the GPEs-based cells may result from poor compatibilities between the GPE and sodium anode, as shown in Table 5. Table 5. Performance parameters of P(VDF-HFP) based polymer electrolytes of SBs Year

2009

Polymer electrolyte

Ionic

Electrochemical

Battery

Ingredients

conductivity

stabilities

configuration

[P(VDF-HFP)/PEMA(20:10,w/w) +

5.69 × 10-4 S/cm

NaCF3SO3/DEC/EC(5:27.5:27.5,

at ambient temperature

Ref.

__

__

87

5.74 × 10-3 S/cm at 27 oC

2.5 V

__

84

5.4 × 10-3 S/cm at 25 oC

4.8 V

Na2MnFe(CN)6/Na

83

3.8 × 10-3 S/cm at 25 oC

__

Hard carbon/Na

81

w/w/w) + 10 wt.%Sb2O3] 2010

[P(VDF-HFP) + 0.5 M EMITf/NaCF3SO3]

2015

[GF/P(VDF-HFP)/PDA + 1 M NaClO4/PC]

2015

[(GF/P(VDF-HFP) + 1 M NaClO4

13



Page 14 of 31

/EC/PC(1:1, w/w)] 7 wt.% DI (before drying) 2015 2016

2017

[P(VDF-HFP)/1 M NaClO4

6.0 × 10-4 S/cm

4.6 V

__

75

/EC/DMC/DEC (1/1/1, w/w/w)]


[(P(VDF-HFP) + 0.5 M

6.3 - 6.8 × 10-3 S/cm

__

__

85

NaTf/EMITf) + 5 wt.%Al2O3]

at room temperature

[(P(VDF-HFP) + 0.5 M

5.5 - 6.5 × 10-3 S/cm

NaTf/EMITf) + 5 wt.% NaAlO2]

at room temperature

[P(VDF-HFP)/nonwoven+NaClO4/

8.2 × 10-4 S/cm at 25 oC

4.8 V

__

80

[P(VDF-HFP)/NaN(CF3SO2)2]

2.2 × 10-4 S/cm

5.0 V

__

86

/Monocationic ionic liquids

at room temperature

EC/DMC/EMC] 2017

(70:30, w/w) 2017

P(VDF-HFP) + 1 M

—

__

NaClO4/PC/2 vol% FEC

PDMS/rGO sponge

77

/VOPO4/PDMS/rGO sponge/hard carbon

2018

10-4

[P(VDF-HFP) + 1 M NaClO4/

4.2 ×

S/cm

PC/FEC(95:5, v/v)]

at room temperature

[P(VDF-HFP) + 1 M NaClO4

1.13 × 10-3 S/cm

/EC/DEC(1:1, v/v)]

at room temperature

[P(VDF-HFP) + 1 M NaClO4/

~5.0 × 10-4 S/cm

EC/PC/5 % FEC]

at room temperature

[GF/P(VDF-HFP) + 1 M NaClO4

4.1 × 10-3 S/cm

/EC/PC(1:1,v/v)]

at room temperature

__

Na3V2(PO4)2O2F/Na

78

Na3V2(PO4)2O2F/ Cotton cloth

2018 2018 2018

4.8 V

NaNi0.5Mn0.5O2/Na

76

4.8 V

PNTCDA/Na

79

Hard/Na

82

Na0.67Ni0.23Mg0.1Mn0.67O2

88

__

3 % methanol-4 % DI water (before drying) 2018

P(VDF-HFP)/PMMA/ Na3Zr2Si2PO12

2.78 × 10-3 S/cm at 30

oC

~ 4.9 V

/Na/C

+ 1 M NaPF6 EC/PC (1:1,v/v)

Na0.67Ni0.23Mg0.1Mn0.67O2 /Na

To solve the inferior cycling stability of SBs based on P(VDF-HFP) polymer electrolyte, film-forming additives in the solid electrolyte interface (e.g. fluorinated ethylene carbonate (FEC) were introduced to create a stable interface film between electrodes and electrolytes.77-79 For one study, Li et al.77 prepared an all-stretchable-component sodium-ion full battery using a P(VDF-HFP)-based GPE soaked with 1.0 M NaClO4/PC/2 vol% FEC electrolyte, in which the GPE-assembled cell delivered a remarkable reversible capacity retention (~ 85% for poly(dimethylsiloxane)(PDMS)/reduced graphene oxide (rGO) sponge/VOPO4 and nearly no decay for PDMS/rGO (reduced graphene oxide) sponge/hard carbon half cells after 300 cycles). Additionally, it was worth noting that the obtained sodium-ion full battery showed a tensile strength of up to 0.85 MPa and a tensile strain of 60%, indicating possible applications for wearable fields. By using P(VDF-HFP)/NaClO4/PC/FEC (95:5, v/v) electrolyte systems, the resultant Na3V2(PO4)2O2F/Na and Na3V2(PO4)2O2F/cotton cloth cells exhibited outstanding capacity retentions of 94.6% after 1500 cycles at 1 C and 90% after 500 cycles at 10 C, respectively.78 In addition, organic SBs based on a PI cathode with P(VDF-HFP)/NaClO4/EC/PC/5% FEC electrolyte showed a superior cyclability (80% after 1000 cycles) as 14


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well.79 Other than the stable interfacial property, such excellent cycling stability of investigated SBs was ascribed to the porous structure of P(VDF-HFP)-based membrane benefitting the storage of liquid electrolyte. Aside from inferior compatibility with sodium anodes, P(VDF-HFP)-based GPE still suffers from low ionic conductivities owing to low uptake of liquid electrolytes (less than 200 wt%). Therefore, supporting materials such as glass fiber (GF) and nonwoven (NW) have been employed to composite P(VDF-HFP) for high uptake of liquid electrolytes. For one study, the PP nonwoven was used as a reinforcement material in P(VDF-HFP) based polymer electrolytes (NW/P(VDF-HFP)/NaClO4/EC/DMC/EMC).80 Such composite polymer electrolyte achieved a high ionic conductivity of 8.2 × 10-4 S/cm at room temperature due to the porous structure of NW/P(VDF-HFP). As shown in Figure 5c, there were many well-organized interconnecting cavities surrounded PP fibers in the inner structure of the NW/P(VDF-HFP), which assured a high porosity (66%) and enhanced uptake of liquid electrolytes (151 wt.%). In this study, the tensile strength of NW/P(VDF-HFP) was 29.0 MPa, much higher than that of P(VDF-HFP) (4.8 MPa).

Figure 5. Typical SEM images of (a) Celgard 2730 separator, (b) NW nonwoven and (c) NW/P(VDF-HFP) composite membrane. (d) The cross-section SEM image of NW/P(VDF-HFP) composite membrane. Reproduced with permission from ref 80. Copyright 2016 Elsevier. In another study, Kim et al.81 developed a GF/P(VDF-HFP) composite membrane with a controllable pore structure

using

a

nonsolvent

induced

phase

separation

(NIPS)

method,

and

the

resultant

P(VDF-HFP)/GF/NaClO4/EC/PC electrolyte exhibited a high electrolyte uptake (69.0%) and a high ionic conductivity

(3.8

×

10-3

S/cm).

Here,

the

investigated

hard

carbon/Na

cells

using

P(VDF-HFP)/GF/NaClO4/EC/PC electrolyte showed a high discharge capacity retention of 91.0% after 100 cycles at 0.5 C, evidently outperforming that of pristine GF (only 27.9%). Based on this, Kim et al. further developed a GF/P(VDF-HFP) composite polymer electrolyte via a binary non-solvent induced separation method by using methanol and deionized water as positioner and pore inducer, respectively.82 Here, they concluded that well-organized pores inside of GF enhanced polymer membrane electrolyte uptake and the thin P(VDF-HFP) layer on both sides of the GF increased the interfacial adhesion to electrodes, thereby promoting a uniform ionic flow between the cathode and anode and improving ionic transport between the electrolyte and electrode. On the basis of using GF as reinforcement, surface-coating techniques are also employed to improve the hydrophobic nature of P(VDF-HFP)-based membranes. Gao et al. reported a novel GF/P(VDF-HFP) composite polymer electrolyte with a polydopamine (PDA) coating (Figure 6a),83 and reported that the investigated GF/P(VDF-HFP)/PDA membrane exhibited enhanced entrapment of 1.0 M NaClO4/PC (269.7 wt.% vs. 245.3 wt.% for GF/P(VDF-HFP)), as well as a suitable mechanical strength (21.6 MPa) and a superior thermal stability (at 200 oC for 0.5 h in air) (Figure 6b). In this study, the obtained GPE delivered a high ionic conductivity of 5.4 × 10-3 S/cm at 25 oC, and the GPE-assembled 15



Na2MnFe(CN)6/Na cells showed suitable rate performance (Figure 6c-d), a discharge capacity retention of 89.4% after 100 cycles at 1 C, and high Coulombic efficiencies of about 99.7% after 100 cycles (Figure 6e-f). Such high Coulombic efficiencies of investigated cells may be ascribed to that the GPE can prevent the decomposition of the liquid electrolytes at electrode interfaces during the charging processes.

Figure 6. (a) Chemical structure of polydopamine. (b) Photographs of various membranes at room temperature (top) and at 200 °C for 30 min (bottom). Cycling performance of the Na2MnFe(CN)6/GPE/Na cells: (c) and (d) at various current densities, (e) and (f) at 1 C. Reproduced with permission from ref 83. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA. There is a consensus that carbonate-based organic solvents as common plasticizers encounter high flammability and leakage risks, leading to poor safeties of SBs using these plasticizers. As good candidates for carbonate-based organic solvents, ionic liquids are typically composed of organic cations and inorganic anions, and can possess distinguished characteristics, such as non-volatilities, non-flammabilities, wide electrochemical windows, high ionic conductivities and excellent thermal and chemical stabilities. These merits of ionic liquids endow ionic liquids-containing polymer electrolytes with improved electrochemical properties.84-86 For example, a P(VDF-HFP) based GPEs by introducing an ionic liquid (EMI-triflate or EMITf) was studied,84 such GPE with the composition of P(VDF-HFP)/0.5 M EMITf/NaCF3SO3 provided a high ionic conductivity of 5.74 × 10-3 S/cm at 27 °C. Besides, inorganic nano-particles were also introduced to further improve the comprehensive performance of P(VDF-HFP)/ionic liquid systems. For example, Hashmi et al. investigated the effects of active fillers (NaAlO2 particles) and passive fillers (Al2O3 particles) on ionic liquid (1-ethyl 3-methyl imidazolium trifluoromethane sulfonate (EMITf))/P(VDF-HFP) systems.85 The optimum ionic conductivities of the electrolyte systems were 6.8 × 10-3 S/cm for Al2O3-dispersed GPEs and 6.5 × 10-3 S/cm for NaAlO2-dispersed GPEs at room temperature, respectively. In this study, they also found that NaAlO2 particles-based GPEs exhibited slightly improved ionic conductivities compared to GPEs with Al2O3 particles, since that Al2O3 particles directly involved in the ion-conducting process in the composite GPEs. Aside from strategies mentioned above, blending with other polymer hosts was reported as another effective approach to optimize the cycle performance of cells using P(VDF-HFP)-based electrolytes.87-88 For one study, given that poly(methyl methacrylate) (PMMA)-based GPEs tend to exhibit high ionic conductivities and wide potential windows, Yi et al. firstly reported a poly(methyl methacrylate)-filled composite electrolyte (named as GHSE) through in-situ polymerization of methyl methacrylate in the porous Na3Zr2Si2PO12-P(VDF-HFP) composite membrane.88 Here, the GHSE membrane exhibited a high ionic conductivity (2.78 × 10-3 S/cm at room temperature), a wide electrochemical window (~ 4.9 V), a high tNa+ (~ 0.63) and a good thermal stability (up to 120

oC).

16


Moreover, the capacity of the

Page 16 of 31

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Na3Zr2Si2PO12/GHSE/Na/C cell did not reveal any decay (192 mA/g after 600 cycles), while the Na3Zr2Si2PO12/liquid electrolyte/Na/C cell only showed a low capacity retention of 32.6%. Above-mentioned experimental results suggest that GHSE has potential for SB applications. Overall, P(VDF-HFP) based GPEs exhibit several merits such as superior electrochemical stabilities and non-inflammabilities, and can deliver enhanced compatibilities with sodium anodes and safeties, and endow investigated cells with enhanced ionic conductivities as well as superior cycling stabilities through introduction of film-forming additives, supporting materials, ionic liquids, blended polymers and nanofillers, indicating a great possibility for practical applications in SBs. However, P(VDF-HFP) based GPEs still face insufficient stabilities with sodium anodes, but the defect can be resolved through introduction of atomic protective layers onto the sodium anodes. 3.3 PMMA BASED POLYMER ELECTROLYTES Owing to abundant ester groups, poly(methyl methacrylate) (PMMA) is compatible with various polar organic solvents and can absorb a large amount of liquid electrolytes, and hence, PMMA-based GPEs possess high room-temperature ionic conductivities. In addition, GPEs based on PMMA exhibit relatively good compatibility with cathodes.89 However, PMMA suffers from a brittle property, and can hardly meet the requirements for practical applications. To address this, PMMA based polymer electrolytes have been optimized by introducing inorganic particles, as shown in Table 6. For example, Kumar and Hashmi et al. prepared a PMMA based composite polymer electrolyte by mixing PMMA with silica (SiO2) nano-particles (Figure 7a),90 and demonstrated that ionic conductivities of investigated electrolytes showed a hump-like feature with the increase of SiO2 particles concentration in the curves of Figure 7b. Here, PMMA/NaClO4/EC/PC-4 wt.% SiO2 electrolyte delivered a maximum ionic conductivity of 3.4 × 10-3 S/cm at 20 oC due to more amorphous domains in the electrolyte systems and defects in the space charge regions generated around SiO2 nanoparticles in the polymer matrix, and showed an electrochemical window of 5 V, indicating that this electrolyte has potential for practical applications in high-voltage SBs. Aside from inorganic particles, blending with other polymers can be also an effective way to improve mechanical properties of PMMA based GPEs. In one study, the PMMA/polycarbonate blend polymer based electrolyte doped with sodium tetrafluoroborate (NaBF4) have been presented by Xue and Quesnel et al.,91 and displayed an ionic conductivity of up to 5.67 × 10-4 S/cm at room temperature. And unfortunately, the authors did not measure mechanical strengths of composite polymer electrolytes.

Figure 7. (a) The digital photograph of nano-composite GPE film (EC-PC-NaClO4 + PMMA dispersed with 10 wt.% SiO2). (b) Variation of room temperature electrical conductivities of GPE nano-composite films as function of nano-sized SiO2 contents. Reproduced with permission from ref 90. Copyright 2010 Elsevier. Table 6．Performance parameters of PMMA based polymer electrolytes for SBs Year

Polymer electrolyte

Ionic

Electrochemical

Battery

ingredients

conductivity

stabilities

configuration

17


Ref.


2010

[(PMMA/1 M NaClO4/EC/PC

Page 18 of 31

3.4 × 10-3 S/cm at 20 oC

5V

__

90

5.67 × 10-4 S/cm

5V

__

91

4.9 V

Na3V2(PO4)3/Na

92

(1:3, w/w))/SiO2] 2016 2018

[PMMA/polycarbonate

(1:1,

w/w) + PC/EC/25 wt.%NaBF4

at room temperature

Poly(MATEPP/MMA/TFMA)

6.29 × 10-3 S/cm

+ liquid electrolyte


SnS2/Na

Besides, mechanical and electrochemical properties of PMMA-based GPEs can be improved through a cross linking strategy as well. Very recently, Zheng et al. synthesized porous cross-linked GPEs containing 6.8 wt% MATEPP, 1.2 wt% MMA, 2.0 wt% TFMA and 90 wt% liquid electrolytes through in-situ thermal polymerization.92 As a result, the obtained GPE showed an ionic conductivity of up to 6.29 × 10-3 S/cm at ambient temperature and a high potential stability of up to 4.9 V vs. Na+/Na, and the GPE-assembled Na3V2(PO4)3/Na cell displayed a superior long-term cycling stability with capacity retentions of 81.8% and 69.2% after 4500 and 10000 cycles at 5 C, respectively. In this study, high electrochemical properties of the GPEs resulted from high electrolyte uptake, and suppression of the continual decompositions of the liquid electrolyte during the long-term cycling process. Although PMMA based polymer electrolytes for SBs delivered high ionic conductivities (up to 6.29 × 10-3 S/cm for GPEs) at room temperature, however, they also encounter inferior cyclabilities. Further optimizations should be focused on the use of in-situ polymerization methods and development of MMA-based polymers with semi-interpenetrating networks. 3.4 PAN BASED POLYMER ELECTROLYTES Polyacrylonitrile (PAN) based polymer electrolytes have been diffusely explored as polymer matrixes, since PAN based GPEs possess high ionic conductivities and anti-oxidative abilities (above 5.1 V vs. Na+) as well as superior thermal stabilities.96 For example, Jyothi et al. synthesized a PAN based GPE using ethylene carbonate (EC) and dimethyl formamide (DMF) as plasticizing solvents through a facile solution-casting technique.97 The variation of ionic conductivities with NaI concentrations ranging from 10 wt.% to 40 wt.% was studied and finally the sample containing 30 wt.% of NaI exhibited ionic conductivities of up to 2.35 × 10-4 S/cm at 30 oC and 1 × 10-3 S/cm at 100 oC. Similar electrolyte systems such as PAN/NaClO4/EC/PC and PAN/NaClO4/EC/PC/DME electrolyte systems were also developed,94-95 delivering high ionic conductivities of up to 4.5 × 10-3 S/cm at room temperature . Although PAN based GPEs exhibited attractive electrochemical properties such as high anodic stabilities, they still face severe challenges such as inferior mechanical strengths, and can be further optimized by copolymerizing acrylonitrile with cross-linking agents. 3.5 PVP BASED POLYMER ELECTROLYTES Polyvinylpyrrolidone (PVP) with numerous amide groups exhibit good biocompatibilities and high film-forming abilities, and can solvate sodium salts and facilitate sodium ion dissociation.41 For example, Chandrasekaran et al.98 prepared a GPE composed of polyethylene glycol (PEG), propylene carbonate (PC) and dimethyl formamide (DMF), in which an optimal ionic conductivity (6.71 × 10-5 S/cm at 35 oC) of the PVP/PEG/NaClO3 electrolyte was achieved by adding 10 wt.% PEG. After that, Chen and Raja et al.99-100 developed PVP-based APEs using different sodium salts, and demonstrated that both APEs exhibited ionic conductivities on the order of ~10-6 S/cm at ambient temperature. Despite PVP-based polymer electrolytes can facilitate lithium ion dissociation, however, they also exhibit poor ion-conducting abilities. Hence, further efforts can be devoted to blend PVP with polymers or inorganic 18


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electrolytes with high ionic conductivities, or to prepare copolymers of N-vinylpyrrolidone and monomers containing ion-conducting segments, in order to resolve the issue of PVP-based polymer electrolytes. 3.6 SINGLE-ION SONDUCTING POLYMER ELECTROLYTES There is a consensus that electrolytes with high tNa+ can benefit SB performance, since high currents bring about anion concentration gradients within the SBs, resulting in high internal resistances that can impede the SB cycle performance, rate performance and energy density. Consequently, high tNa+is one of requirements for ideal polymer electrolytes. To enhance tNa+ of polymer electrolytes, single-ion conducting polymer electrolytes by immobilizing anions covalently bonded to polymer backbones have been developed for SBs, as summarized in Table 7. Table 7. Performance parameters of single-ion conducting polymer electrolytes for SBs Year 2013 2014

Polymer electrolyte

Ionic

Electrochemical

Battery

ingredients

conductivity

stabilities

configuration

4.5 V

Na0.44MnO2/Na

101

__

Na3V2(PO4)3/Na

103

4.10 × 10-4 S/cm at 80 oC

6.0 V

Na0.44MnO2/Na

102

7.9 × 10-9 S/cm at 100 oC

__

PFSA-Na/EC-PC Nafion 115 membranes /EC-PC

2017

PVDF-HFP blended

~10-3 S/cm at 60 oC 3.52 ×

10-4

S/cm

Ref.

at room temperature

NaPA membrane /EC-DMC 2015

Na[PSTFSI-blend-5EA]

__

Figure 8. (a) Synthesis of sodium 4-styrenesulfonyl (trifluoromethylsulfonyl) imide, (NaSTFSI). (b) Synthesis of the homopolymer sodium poly(4-styrenesulfonyl (trifluoromethylsulfonyl) imide)Na[PSTFSI]. (c) Copolymerization of Na[STFSI] and ethyl acrylate (EA) in different ratios. (d) Temperature dependent conductivities of solid state polymer electrolytes and blend electrolytes. Reproduced with permission from 19


104


ref 104. Copyright 2015 Elsevier. It is worth mentioning that single-ion conductors with strong electron-withdrawing perfluorinated alkyl (RF–) groups covalently attached to sodium sulfonate units can possess low lattice energies, which can increase the delocalized degree of anions, thus promoting the dissociations of Na ions. For example, Li et al.101 firstly developed a perfluorinated sulfonic membrane in the Na-form (PFSA-Na)-based single-ion-conductor type GPE swollen with organic solvents (EC/PC), in which the resultant GPE film delivered an ionic conductivity of ~10-3 S/cm at 60 oC. Here, the assembled Na0.44MnO2/ PFSA-Na-based GPE/Na metal cell delivered a superior reversible capacity (116.1 vs. 89.5 mAh/g at 0.05 C) and a higher cycling stability (97.6% vs. 67.6% capacity retention for 50 cycles at 0.1 C) than SIBs with conventional liquid electrolytes. Based on this, Cao et al. developed another kind of PFSA-Na based single ion-conducting polymer electrolyte (Nafion-Na membrane swollen with EC:PC (1:1, v/v)).103 Here, they found that the Nafion-Na/EC/PC electrolyte-assembled Na3V2(PO4)3/Na cell delivered a long-term cycling stability retention of 92.5% after 50 cycles compared with that (67.6% after 50 cycles) using conventional liquid electrolytes (1 mol/L NaClO4 in EC:PC=1:1 (v/v)). Aside from perfluorinated alkyl (RF–) groups, phenyl groups are also used to covalently immobilize sulfonate anions, promoting the dissociation of soidum ions. For one study, Pan et al. fabricated a single ion conductor polymer electrolyte consisted of P(VDF-HFP) blended with sodium ion exchanged poly(bis(4-carbonyl benzene sulfonyl)imide-co-2.5-diamino benzesulfonic acid) macromolecule (named P(VDF-HFP)/NaPA).102 The ionic conductivity of P(VDF-HFP)/NaPA GPE swollen with EC/DMC was 9.1 × 10-5 S/cm at 20 oC. Strikingly, P(VDF-HFP)/NaPA possessed an excellent thermal stability (weight losses of 3 % until 400 °C) and a suitable tensile strength of 14.44 MPa. For another study, Li et al.104 also developed a poly[4-styrenesulfonyl(trifluoromethylsulfonyl)imide-co-ethylacrylate] (Na[PSTFSI]-co-EA) based single-ion conducting APE, in which Na[PSTFSI]-co-EA delivered an ionic conductivity of ~10-3 S/cm at 60 oC (Figure 8a-c). In this study, they demonstrated that Na[PSTFSI-blend-5EA] with more EA units exhibited higher ionic conductivities (7.9 × 10-9 S/cm at 100 oC) than that of Na[PSTFSI]-co-EA at different temperatures (Figure 8d), which may result from the increased interactions between Na+ with EA units that can facilitate the cation dissociation. Overall, single-ion conducting polymer electrolytes exhibit high tNa+, but very low ionic conductivities at room temperature. The reason for low ion-conducting properties of these electrolyte systems may result from tight ion pairings and ionic interactions at room temperature. The ionic conductivities of these polymer electrolytes can be further optimized through the use of more ion-ionducting side chains on the polymeric backbones, or introduction of silicon tripod motifs with single-ion conducting units. 3.7 POLYSACCHARIDE-BASED POLYMER ELECTROLYTES Thus far, polysaccharides including hydroxypropyl methylcellulose (HPMC), chitosan and cyclodextrin derivatives have been developed as polymer matrixes for SBs, owing to biodegradable and non-toxic characteristics, as well as the merits of abundant and renewable resources. For example, Sannappa et al. prepared a sodium iodide (NaI)-doped HPMC polymer electrolyte,105 and demonstrated that the composition of HPMC/NaI (5:4, w/w) exhibited the lowest crystallinity and the highest ionic conductivity (1.126 × 10-6 S/cm at 40 oC). In another study, Aziz et al.106 reported chitosan (CS) based APEs with various NaTf salt concentrations, and revealed that the investigated APE delivered a maximum ionic conductivity of 2.41 × 10−4 S/cm at room temperature for the sample containing 40 wt.% of NaTf, and confirmed the occurrence of ion association for the sample with 50 wt.% of NaTf. Given that branch-structured or star-like polymer-based electrolytes tend to exhibit high ionic conductivities owing to their intrinsically amorphous 20


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characteristics favoring segmental motions of polymers,107 Ma et al.93 studied a star-like hyperbranched β-cyclodextrin-based

APE

by

grafting

β-cyclodextrin

with

multiple

oligo(methyl

methacrylate)-block-oligo(ethylene glycol) methyl ether methacrylate short chains through atom transfer radical polymerization. And they demonstrated that this APE with NaSO3CF3 as the sodium salt can form a free-standing, transparent, and flexible film, and delivered an excellent ionic conductivity of 1.3 × 10−4 S/cm at 60 oC, a wide potential window of 5.2 V (vs. Na+/Na) and a superior thermal stability. Here, the authors also revealed that the resultant APE can form a stable and protective interfacial layer onto the Na electrode surface, endowing investigated Na/NaNFM cells using this APE to deliver high cycling stabilities (a capacity retention of 87.8% after 80 cycles) at 60 °C. These above results indicated that polysaccharides-based polymer electrolytes are good candidates for SMBs, and more efforts should be directed towards the development of novel polysaccharide derivatives-based polymer electrolytes. 3.8 OTHERS Aside from polymer electrolyte systems mentioned above, other polymer electrolyte systems such as poly(vinyl alcohol) (PVA), polyvinylchloride (PVC), poly(trimethylene carbonate) (PTMC) and poly(ionic liquids)-based polymer electrolytes have been developed for SBs as well. PVC contains abundant C-Cl bonds and can interact with Na-salts to form polymer/Na-salts complex. PVC is commercially available, relatively inexpensive and compatible with a large number of plasticizers. In one study, Reddy et al.108 developed PVC-based electrolytes with NaClO4 as the sodium salt for SMBs, in which the as-prepared APE (PVC/NaClO4 = 90:10) delivered a high ionic conductivity of 10-3 S/cm at 25 oC. In this study, high ionic conductivities of PVC-based electrolytes may result from the porous structure of the electrolyte membrane. Interestingly, the ionic conductivities of investigated APEs did not show any abrupt jump with temperatures, indicating that these electrolytes exhibited a completely amorphous structure. As a similar structure as PVC, polyvinylalcohol (PVA) is a polymer with plenty of hydroxyl groups (-OH) covalently attached to carbon chains. The high polarity of -OH groups contributes to good mechanical strengths of PVA-based APEs. In 2007, poly(vinyl alcohol) (PVA)-based polymer electrolyte using NaBr as the sodium salt for solid-state SBs was synthesized by Rao et al.,52 and delivered a maximum ionic conductivity of 1.12 × 10-6 S/cm at 30 oC. In consideration of successful applications of aliphatic polycarbonates for LBs, Mindemark et al. firstly implemented a novel polymer electrolyte comprised of poly(trimethylene carbonate) (PTMC)/NaTFSI (called PTMC3NaTFSI) in solid-state SIBs,109 in which the as-prepared APE exhibited an ionic conductivity of ~10-8 S/cm at room temperature (~10-6 S/cm at 60

oC).

Here, the resultant prussian

blue/PTMC3NaTFSI/Na half battery delivered a high specific discharge capacity and a good cycling stability (from 110 mAh/g to 105 mAh/g after 8 cycles at 0.1 C and 60 oC), indicating superior electrode-electrolyte interfacial properties and limited polarization within the cell. Given high anodic stabilities of ionic liquids, Zhou et al. developed a hierarchical poly(ionic liquid) (PIL)-based solid electrolyte (HPILSE) through an in-situ synthesis method, and used the nesting doll-like models vividly describing the final HPILSE in Figure 9a.110 In this study. HPILSE showed a high ionic conductivity of 1.15 × 10-3 S/cm at 25 °C (Figure 9b), and HPILSE-assembled Na0.9[Cu0.22Fe0.30Mn0.48]O2/Na battery exhibited superior rate performance (101.0 mAh/g at 0.05 C and 59.9 mAh/g at 1 C, respectively), a long-term cycling stability (85.6 mAh/g at 0.1 C after 100 cycles with a capacity retention of 85.5% and high Coulombic efficiencies near 100 %) (Figure 9c-e). PVA, PVC, PTMC and poly(ionic liquids)-based polymer electrolytes delivered good electrochemical properties like high ion-conducting abilities, but they all suffer from inferior stabilities with sodium, and can 21



be further optimized by blending them with polymers that exhibit high compatibilities with sodium anodes.

Figure 9. (a) Schematic illustration for the in-situ synthesis route of nesting doll-like HPILSE. (b) The ionic conductivities of PDDATFSI, polymerized C1-4TFSI, Li-EMITFSI-based electrolyte, Na-EMITFSI-based electrolyte, Li-HPILSE and Na-HPILSE as a function of temperature. (c) Typical charge and discharge curves, (d) rate performance (e) cycling performance (0.1 C) of Na0.9[Cu0.22Fe0.30Mn0.48]O2/Na-HPILSE/Na cell using Na-HPILSE. Reproduced with permission from ref 110. Copyright 2017 Elsevier. 4. CONCLUSION AND PERSPECTIVES Since safety accidents of liquid electrolyte based LIBs in commercial electronic vehicles occur frequently, polymer electrolytes have been increasingly accepted as good candidates to replace liquid electrolytes for battery applications due to their non-leakage properties, good flexibilities and superior electrochemical properties. Strikingly, polymer electrolytes possess several natural merits compared with liquid electrolytes, such as high safety, suppression of sodium dendrite formation and reduced electrolyte decomposition. Such merits of polymer electrolytes enable investigated SBs with excellent long-term cycling stabilities and safety properties. Then, we present the requirements for ideal polymer electrolytes for the first time and focus on the recent progresses of sodium ion-conducting polymer electrolytes based on various polymer hosts (i.e. polymers containing ethoxylated units, P(VDF-HFP), PMMA, PAN, PVP, single ion conductors, polysaccharides and so on). Notably, we demonstrate the natural merits and structure-performance relationships of these polymer hosts. In addition, various approaches to optimize electrochemical properties of these polymer electrolytes have been proposed as well. With increasing attentions and efforts, we believe that the path to fulfill commercial applications of polymer electrolytes-based SBs with high safety is not far away from us. Here, we propose current challenges and future prospects concerning these polymer electrolytes from the following several major aspects: 1) APEs still suffer from low room-temperature ionic conductivities, leading to SBs with inferior cycling performance and low rate capabilities. Developing various new polymer hosts with low Tg and constructing organic-inorganic composite polymer electrolytes can be effective strategies to resolve the ion-conducting issue. 2) GPEs for SBs usually face inferior heat resistances, high flammabilities, and low mechanical strengths. Hence, new organic solvent systems with high boiling points and excellent compatibilities with electrodes need to be further developed as plasticizers for GPEs. Moreover, flame-retardant GPEs should be widely 22


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exploited to further enhance the thermal safeties of SBs. Finally, the exploration of GPEs with rigid and flexible characteristics could be an attractive field, since that they tend to possess high ionic conductivities and can suppress sodium dendrites owing to high uptakes of liquid electrolytes and superior mechanical strengths.111-112 3) Novel sodium salts with high safety, excellent electrochemical performance and low cost should be further developed. Nowadays, Na-salts including NaClO4, NaPF6, NaTFSI, NaFSI, NaBF4 and NaCF3SO3 have been reported: Among them, NaClO4 possesses good electrochemical performance and cost-effective advantages, but it suffers from potential explosion hazards;4 electrolytes using NaPF6 possess low viscosities and outstanding electrochemical performance such as superior electronic conductivities; electrolytes with NaBF4 or NaCF3SO3 as the Na-salts encounter poor compatibilities with electrodes; NaTFSI and NaFSI possess several disadvantages such as high price, low anti-oxidative abilities and corrosion of aluminum foil. Despite studies have reported that suitable electrochemical properties can be obtained by adjusting Na-salts concentration at various temperatures, however, electrolytes using excess Na-salts in polymer electrolytes are cost-prohibitive or exhibit inferior electrochemical properties such as low ionic conductivities. Therefore, the development of new, green and high performance Na-salts could be an effective method towards advanced polymer electrolytes. 4) Interface issues between polymer electrolytes and electrodes need to be further studied at a deeper and more rigorous level by theoretical calculations and multi-dimensional characterizations, in order to clearly explain internal action mechanisms. A stable interface layer with good compatibility with the electrode not only compensates the volume changes of electrodes but also inhibits the occurrence of side reactions, enabling SBs high long-cycling stabilities. Although good efforts have been made to explain interface issues between polymer electrolytes and electrodes,113-114 however, they have barely unraveled the internal action mechanisms. So further studies should focus on the action mechanisms on interface issues between polymer electrolytes and electrodes during operation.115 5) When it talked to the application of polymer electrolytes, they are expected to be applied in some fields (such as large-scale grid energy storage, wind power generation energy storage and low-speed electric vehicles) in order to improve safety. In addition, compared with nonpolymer based solid-state electrolytes, polymer electrolytes with light weight and mechanical processibility, which can be used to prepare flexible electronic devices combined with flexible electrodes, and further applied in the artificial intelligence wearable field.77 In addition, nontoxic, well-biocompatible polymer electrolytes also can be applied in biomedical field. 6) Electrolyte takes the lion’s share in the system cost. We compared the prices of conventional liquid electrolytes and PEO-based APEs according to the market price in 2019, and demonstrated that the cost of 1 M PC-NaClO4 electrolyte and PEO-NaClO4 are 0.54 $/g and 0.74 $/g, respectively. Obviously, the cost of APEs is slightly higher than that of the liquid electrolyte, and the cost for gel polymer electrolyte is even higher in consideration of the addition of various plasticizers. Therefore, it is of significant importance to further reduce the cost and meanwhile obtain excellent performance. To further realize cost reduction, simple and facile preparation of polymer electrolytes like in-situ polymerization of monomers within cells and novel polymer matrixes with a cost-effective characteristic should be further developed. In conclusion, among polymer electrolytes mentioned above, polymers based on PEG with cross-linked networks show more potential for APEs owing to effective suppression of sodium dendrites, and further studies should be focus on the development of novel polymer electrolytes with semi-interpenetrating networks that exhibit both high mechanical strengths and excellent ion-conducting abilities; And moreover, cross-linked polycarbonates that may possess high ionic conductivities with no compromise in compatibility 23



with sodium anodes should be well investigated; For GPE systems, P(VDF-HFP), PMMA and PAN based GPEs that exhibit high anodic stabilities, are good alternatives for high voltage SBs. Success for future applications lies in the modification on the anode surface and the development of copolymers with cross-linked networks containing those building blocks; The cost of single-ion conducting polymer electrolytes should be further reduced for practical applications. Choosing cost-effective intermediates and designing facile steps to synthesize cheap single-ion conducting polymer electrolytes become more significantly important; Polysaccharides, as biodegradable, non-toxic and abundant natural products, need to be well modified by introducing ion-conducting units for polymer electrolytes, which would possess great potential for high performance solid-state SBs.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions §These

authors contributed equally to this work.

All authors have given approval to the final version of the manuscript. Notes We thank Qingdao Key Lab of Solar Energy Utilization and Energy Storage Technology, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P. R. China for fruitful help. The authors declare no competing financial interest. ACKNOWLEDGMENT This original research was financially supported by the National Natural Science Foundation of China (Grant No. 51703236, U1706229), the National Science Fund for Distinguished Young Scholars (Grant No. 51625204), the National Key Research and Development Program of China (2018YFB0104300), and Think-Tank Mutual Fund of Qingdao Energy Storage Industry Scientific Research, Key Scientific and Technological Innovation Project of Shandong (Grant No. 2017CXZC0505).

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