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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 3: 390 - 406

DOI: https://doi.org/10.7536/PC220913

Review

Application of Polyacrylonitrile in the Electrolytes of Lithium Metal Battery

  • Yu Xiaoyan ,
  • Li Meng ,
  • Wei Lei ,
  • Qiu Jingyi ,
  • Cao Gaoping ,
  • Wen Yuehua , *
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  • Research Institute of Chemical Defense, Beijing Key Laboratory of Advanced Chemical Energy Storage Technology and Materials,Beijing 100191, China
* Corresponding author e-mail:

Received date: 2022-09-15

  Revised date: 2023-01-14

  Online published: 2023-02-16

Supported by

National Natural Science Foundation of China(21975284)

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Abstract

With the rapid development of portable electronic devices, electric vehicles, and smart grids, there is an increasing interest in high-energy-density lithium metal batteries. Uneven Li stripping or deposition on the surface of lithium metal will lead to the growth of lithium dendrites, which can easily pierce the separator and cause the short circuit in the battery. Moreover, the highly reactive lithium metal will continue to react with the electrolyte, resulting in an unstable solid electrolyte. interfacial (SEI) film and irreversible capacity loss. Taking high-energy-density and high safety into account is a key scientific problem that needs to be solved urgently in the development and application of lithium metal batteries. The interaction of strong electron withdrawing group (C≡N) in polyacrylonitrile (PAN) polymer and C=O in carbonate solvent can form a more stable SEI film. As a lithium anode coating, PAN can also inhibit the growth of lithium dendrites. In addition, due to the low lowest unoccupied molecular orbital, high electrochemical stability and wide electrochemical window, PAN can be regard as polymer electrolytes for lithium metal batteries, and matched with a high-voltage cathode to achieve both high energy density and safety. Thus, PAN polymer has significant potential application in electrolytes for lithium metal batteries. This review mainly starts from the different states of electrolytes (liquid, gel, and solid state). Recent research development of PAN polymer as separators and lithium anode protective layers in liquid electrolytes, as well as its application in gel electrolytes and solid-state electrolytes are presented. Finally, the review prospects the development trend of PAN polymer in lithium metal battery electrolytes.

Key words： polyacrylonitrile; separator; lithium anode; gel electrolyte; solid-state electrolyte

Cite this article

Yu Xiaoyan , Li Meng , Wei Lei , Qiu Jingyi , Cao Gaoping , Wen Yuehua . Application of Polyacrylonitrile in the Electrolytes of Lithium Metal Battery[J]. Progress in Chemistry, 2023 , 35(3) : 390 -406 . DOI: 10.7536/PC220913

Contents

1 Introduction
2 The application of PAN in liquid state electrolytes
2.1 As separator
2.2 As lithium anode protective layers
3 The application of PAN in gel electrolytes
4 The application of PAN in solid-state electrolyte
4.1 Monolayer electrolytes containing PAN
4.2 Heterogeneous multilayer electrolytes containing PAN
4.3 PAN electrospinning fiber membrane
5 Conclusion and outlook

1 Introduction

With the rapid development of portable electronic devices, electric vehicles and smart grid, the demand for high-performance electrochemical memory devices is increasing[1⇓~3]. Among them, lithium-ion batteries have been playing an important role in the commercial market because of their high energy density, high working voltage, long cycle life and minimal memory effect. In terms of power batteries, the energy density of traditional lithium-ion batteries is close to the theoretical limit, but it still can not meet the needs of existing applications[4,5]. Therefore, lithium metal with high theoretical specific capacity (3860 mAh·g-1) and low redox potential (− 3.04 V vs standard hydrogen electrode) has gradually become a research hotspot, and when matched with a high specific capacity cathode, the mass energy density of the battery can be greater than that of 400 Wh·kg-1 (the volumetric energy density is greater than that of 1200 Wh·L-1), so lithium metal is regarded as the "holy grail" of the next generation energy storage system[6,7]. However, lithium metal batteries also have some problems. When liquid electrolyte is used, repeated lithium dissolution and deposition lead to lithium dendrite growth, which may pierce the diaphragm and easily lead to short circuit. There are safety problems such as thermal runaway and battery fire. On the other hand, due to the high reactivity of lithium, some components in the liquid electrolyte will react with lithium metal to form an unstable solid electrolyte interphase (SEI) film, resulting in reduced coulombic efficiency and irreversible capacity loss[8⇓~10]. To solve these problems, researchers have proposed solutions from different directions: on the one hand, on the basis of liquid electrolyte, the composition of electrolyte is adjusted, and additives are used to optimize the composition of electrolyte to help form a more stable SEI film; The separator is modified to inhibit the growth of lithium dendrite; And a lay of artificial SEI film is modified on that surface of the lithium metal to prevent the lithium metal from react with the electrolyte[11]. On the other hand, directly replacing the liquid electrolyte with gel electrolyte or solid electrolyte with higher safety can greatly reduce the use of flammable organic solvents, even without the use of organic solvents, which can effectively avoid the irreversible capacity loss of lithium batteries caused by the unstable SEI film generated by the continuous reaction between organic solvents and the surface of lithium metal[12⇓~14].
The distance of C ≡ N bond is very short, 1.1616Å, which is the sp hybridization of triple bond carbon, so C ≡ N is a strong polar electron-withdrawing group, and the dielectric constant of nitrile compounds with — C ≡ N group can be as high as 30[15]. Therefore, polyacrylonitrile (PAN) with C ≡ N groups has good thermal and electrochemical stability. PAN can be mixed with elemental sulfur and dehydrogenated under heating conditions to form a conductive chain with a structure similar to that of polyacetylene, and its C ≡ N group can be hybridized into a five-membered ring, thus playing a role in sulfur fixation.The resulting sulfurized polyacrylonitrile cathode exhibits ultrahigh coulombic efficiency and cycle stability, but the cathode material has the disadvantages of low discharge voltage (ca. 1.9 V) and limited sulfur content (typically less than 50 wt%)[16,17]. On the other hand, due to the dipole-dipole interaction and hydrogen bonding effect between the C ≡ N group and the surface of the active material, PAN can also be used as a binder for the active material, but its high glass transition temperature makes the electrode based on PAN binder rigid and easy to break up during battery assembly and cycling[18⇓⇓~21].
This paper focuses on the safety issues of lithium metal batteries, focusing on the application of PAN in electrolytes. PAN has a low lowest unoccupied molecular orbital, high electrochemical stability, and a wide electrochemical window, thereby being sufficient to match some high-voltage cathode materials and lithium metal anodes (such as LiCoO2, LiMnO2, LiNixCoyMnzO2, etc.) for high energy density batteries[22]. Electrospun PAN nanofiber membranes can be used as separators for lithium batteries due to their high ion transport, small diffusion resistance, good thermal/chemical/mechanical stability, long cycle life, high electrolyte uptake, and good compatibility with electrodes[23⇓⇓~26]. Furthermore, C = N in the polyacrylonitrile polymer can coordinate with Li+ to promote the dissolution of lithium salt, and can be applied to composite gel electrolyte and solid electrolyte to inhibit the growth of lithium dendrite while conducting lithium ions[22,27,28]. However, due to the poor mechanical strength of PAN film and the high reactivity of C ≡ N with lithium metal, its application in lithium metal batteries is still challenging. In this paper, the research progress of PAN polymer in lithium metal battery electrolyte in recent years is introduced, including the application of PAN polymer as lithium battery separator, negative electrode protective layer, gel electrolyte and solid electrolyte.

2 Application of polyacrylonitrile in liquid electrolytes

In liquid electrolytes, the role of the separator is to prevent the two electrodes from coming into physical contact while allowing Li+ transport through the liquid electrolyte filled in the porous structure of the membrane. Polyolefin-based (e.g., polyethylene-based, polypropylene-based) microporous membranes have considerable chemical stability and dominate the market for lithium battery separators. However, some inherent limitations of nonpolar polyolefin-based separators, such as low electrolyte absorption, poor adhesion to electrodes, low thermal stability, and low ion transport, greatly limit their ionic conductivity and cycling performance in lithium batteries[29,30]. PAN is a commercialized polymer, which meets the needs of high safety diaphragm materials because of its high melting point (more than 290 ℃), good affinity with electrolytes, high electronic insulation and low cost[31⇓~33]. In addition, PAN polymer with high dielectric constant contributes to more uniform lithium deposition, and C = N in PAN can also interact with Li+ in liquid electrolyte and C = O in solvent (propylene carbonate, ethylene carbonate), so PAN polymer can also be used as a protective layer of lithium metal anode in liquid electrolyte[34][35].

2.1 PAN as a diaphragm

The porosity of traditional non-polar polyolefin separator is low (40%), which is quite different from that of highly polar organic solvents, resulting in low electrolyte absorption and increased battery resistance. In contrast, the electrospun separator has higher porosity (60% – 90%), gas permeability, and ionic conductivity. Cho et al. Developed PAN electrospun membranes with thickness and pore size similar to those of traditional polyolefin microporous membranes, porosity and gas permeability similar to those of traditional electrospun membranes, and a fiber diameter of about 350 nm[26]. Due to the higher porosity in the PAN electrospun separator, its increased uptake of liquid electrolyte further enhances the ionic conductivity and the liquid retention of the separator between electrodes during cycling tests, so the LiCoO2‖Li cell based on this PAN separator exhibits better cycling performance than that based on conventional Celgard at 0.2 C. Table 1 lists the application status of PAN-based separators in lithium batteries. Due to the poor mechanical strength of PAN electrospun membrane, which can not withstand the large tension involved in the winding operation during the battery assembly process, Lee et al. Prepared a partially oxidized PAN separator with high mechanical strength by heat treatment (230 ℃, 30 min).The tensile strength of the PAN film without oxidation was 22. 6 MPa, while the tensile strength of the partially oxidized PAN film increased to 49. 6 MPa due to the cyclization and oxidation reactions during the heat treatment, which formed a ladder polymer structure, and its Young's modulus was 19 times higher than that of the PAN film without oxidation[36]. It should be noted that although the electrospun nanofiber membranes prepared by this method do not have through holes, tiny particles can still penetrate through these membranes through tortuous paths. Electrospun nanofiber membranes usually have a porosity of 85% to 90%, which will be reduced to about 75% after mechanical pressing, and the porosity can be further reduced to 50% or even lower with relatively high hot pressing[33]. By comparing the PAN membranes with different fiber diameters and different hot pressing pressures, the PAN membrane with smaller fiber diameter (200 ~ 300 nm) has higher porosity, higher electrolyte absorption rate, and higher ionic conductivity. After hot pressing at 60 ° C for 3 min, the specific discharge capacities at 0.5 C of the ‖ Li cell with LiMn2O4‖PAN separator at pressures of 5, 10, 20 MPa were 89.5, 80.4 and 72.6 mAh·g-1, respectively (based on the specific discharge capacity of 72.6 mAh·g-1 for PP separator).
表1 基于PAN聚合物的隔膜的物理性质

Table 1 The PAN-based separators and their physical performance

Component Thickness (μm) σ
(mS·
cm-1)		 Porosity (%) Electrolyte Uptake (%) Fracture strength
(MPa)/ Elongation
(%)		 Cathode ref
1 PAN 26 0.94 62.0 300 49.6/3.8 LiFePO4 36
2 PAN 24 1.06 54.7 336 - LiMn2O4 33
3 PDA@PAN 50 1.39 83.3 341 13.9/31.3 LiFePO4 37
4 PAN/cellulose/ nylon6/PVPK30 - - 55.7 225 71.24/33.7 - 38
5 PAN/CA/HAP 46 3.02 61 268 11.8/11.8 LiFePO4 39
6 PAN/ZSM-5 - 2.16 55.5 308 13/- LiFePO4 40
7 PAN/PEO/PAN 25 1.54 68 650 - LiFePO4 41
8 PAN/DOPO - 6.49 - 310 - LiFePO4 42
9 PAN/HPTCP - 0.95 46 162 40 NCM622 43
10 PAN-PEI 60 0.19 - - 19 S/NCM523 44
11 PAN-SiO2 115 1.98 85.3 - 9.6 NVP/LiFePO4 45
However, due to the strong reactivity between C ≡ N and Li metal, the lithium battery assembled with pure PAN electrospun membrane as a separator has a large polarization, which will induce lithium deposition into dendritic and porous structures, resulting in the formation of SEI layer by consuming a large amount of electrolyte. Branched polyethyleneimine (PEI) was immobilized on the electrospun PAN membrane by chemical grafting, and the amino groups provided by it could effectively regulate the uniform distribution of lithium ions and induce the formation of lithium-rich SEI layer, resulting in a 3D spherical surface and obtaining dendrite-free lithium deposition (Figure 1A)[44]. In the Li ‖ Cu battery, the initial coulombic efficiency of the pure PAN separator is only 91.0%, which indicates that a certain amount of lithium metal is sacrificed to form the SEI layer or dead lithium, and the relatively stable coulombic efficiency is 98.1%, but it tends to decay after the 80th cycle. In contrast, the ammoniated PAN separator showed a high initial coulombic efficiency of 98.8% and stabilized at 98% after 120 cycles. At 1.0 mA·cm-2, the assembled Li ‖ Li symmetric cell with lithium anode using Celgard separator showed a low polarization of about 15 mV at the initial cycle due to its low nucleation impedance, and after 200 H of stable cycling, the voltage curve showed a gradual increase to 300 mV, and after about 260 H of charge-discharge, the overpotential suddenly decreased, and the lithium symmetric cell appeared partial short circuit; However, the life of the battery based on PAN separator was extended to about 380 H, and the overpotential was only 30 mV. Furthermore, with the ammoniated PAN separator, the lithium battery can operate continuously for more than 400 H without short circuit, and the overpotential is stable at about 15 mV (Figure 1b). In the nucleation process, the limiting current density of lithium dendrite growth is inversely proportional to the anion transference number, that is, a higher anion transference number leads to a lower limiting current density, resulting in the growth of lithium dendrites. Therefore, reducing the anion transference number and promoting the transport of Li+ can alleviate the nucleation of lithium dendrite. Du et al. Compared the adsorption energy of metal sites with TFSI- in three Prussian blue analogs (FeFe-PB, NiFe-PBA, NiCo-PBA), and the adsorption energy of FeFe-PB and TFSI- on the molecular surface and inside the channel was − 0.60 eV and − 0.14 eV, respectively, which was significantly increased to − 1.07 eV and − 0.36 eV for NiFe-PBA, and further increased to − 1.73 eV and − 0.42 eV for NiCo-PBA (Fig. 2)[46]. NiCo-PBA (which has the strongest adsorption energy with TFSI-) was then blended with PAN and electrospun into a membrane, which was used as a lithium battery separator with a lithium ion transference number ( t L i +) of 0.78, compared with only 0.51 for the pure PAN membrane. The reason for the high t L i + of NiCo-PBA @ PAN membrane is that the migration of TFSI- is restricted by NiCo-PBA, and the pores of PBA can act as an effective ion sieve to cover TFSI-, but allow Li+ to pass through freely. The LiFePO4‖Li battery based on NiCo-PBA @ PAN separator showed a capacity retention of 85% and an average Coulombic efficiency of 99.6% after 400 cycles at 1.0 C. In contrast, the capacity of batteries using PP or PAN separators decayed to less than 50% after 103 and 202 cycles, respectively.
图1 基于(a)传统PP(Celgard)隔膜和(b)氨化PAN隔膜的Li-S电池示意图;(c)不同隔膜的对称电池的电压分布[44]

Fig. 1 Illustration of Li-S batteries with (a) conventional PP (Celgard) separator and (b) APANF separator; (c) Voltage profiles of symmetrical cells with different separators[44]. Copyright 2020, Elsevier

图2 (a)基于PBA@PAN隔膜的锂金属电池的优点;(b)FeFe-PB、(c)NiFe-PBA和(d)NiCo-PBA的SEM图像及晶体结构[46]

Fig. 2 (a) The merits of a PBA@PAN separator in Li metal batteries; The SEM images and crystalline structures of (b) FeFe-PB, (c) NiFe-PBA, and (d) NiCo-PBA[46]. Copyright 2022, American Chemical Society

Although PAN-based separator has excellent thermal stability and electrolyte absorption rate, it will dissolve in organic solvents (EC and PC, etc.), and its chemical stability is poor, which is a major obstacle as a separator. To solve this problem, Zhang et al. Reported a modified polyacrylonitrile/silica aerogel (M-PSA) composite separator[45]. The electrospun PAN membrane was first immersed in NaOH/H2O solution for nitrile group hydrolysis modification, and then a layer of SiO2 aerogel was grown in situ on the surface of the modified fiber. The prepared M-PSA membrane showed excellent chemical stability in diethylene glycol dimethyl ether, EC/PC and EC/DMC. Although the PAN film was stable in diethylene glycol dimethyl ether, it became transparent in EC/PC for 5 s and shrank severely in EC/DMC for 8 s. At the same time, in terms of thermal stability, the M-PSA separator can maintain its structural integrity at 250 ℃, and even at 350 ℃, it will not shrink seriously.
In order to control the thermal runaway of lithium batteries, a three-layer PP/PE/PP sandwich separator with thermal shutdown function near the melting temperature (130 ℃) has been widely used. In the sandwich structure, when the temperature approaches the melting temperature of PE, the PE layer melts and blocks the passage of ions, while the PP layer maintains its membrane integrity and prevents short circuits between the two electrodes. Therefore, such a sandwich structure can provide safety assurance while maintaining sufficient mechanical strength. However, the melting temperature of the outer PP layer is only 170 ℃, and the melting temperature difference between the outer PP layer and the inner PE layer is only 40 ℃, so such a small temperature difference can not provide enough buffer effect after thermal shutdown[47]. Therefore, Gong et al. Used electrospinning technology to prepare a sandwich structure separator with fast thermal shutdown performance and high thermal stability: polyacrylonitrile/polyethylene oxide/polyacrylonitrile (PAN/PEO/PAN),The fusible PEO fiber membrane in the inner layer imparts a rapid thermal shutdown function to the composite separator at 80 ° C, while the heat-resistant PAN membrane in the outer layer maintains thermal stability even at 200 ° C[41]. The large buffer temperature difference (120 ℃) between the thermal shutdown temperature and the thermal stability temperature of the composite separator is wider than that of most reported separators, which effectively improves the thermal safety of lithium batteries. Although the thermal stability of PAN reaches 200 ℃, when PAN polymer is exposed to heat for a long time, it will begin to degrade, producing flammable volatile compounds such as acrylonitrile, ammonia, other organic and inorganic nitriles, etc. In order to reduce the flammability of PAN-based separator, it is a convenient and effective solution to mix PAN with flame retardant and then electrospin the membrane. Yusuf et al. Electrospun a phosphorus-rich compound, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), with PAN, and the peak heat release rate of the obtained PAN composite membrane was reduced by 49%, indicating that the phosphorus in DOPO effectively quenches the free radicals released during combustion[42]. Similarly, Kang et al. electrospun a mixture of hexaphenoxy cyclotriphosphazene (HPCTP) flame retardant and PAN into a membrane. After heat treatment, the partially oxidized membrane had a high tensile strength (> 40 MPa) and a low regional thermal shrinkage (< 5% at 200 ℃ for 1 H)[43].

2.2 PAN as a protective layer for lithium metal anode

Since the 1970s, much research has been done on lithium metal anodes, but the commercialization of lithium metal secondary batteries (LMBs) is still hindered due to poor cycle life and serious safety hazards. The main challenges of lithium metal anode come from its high chemical reactivity, uneven lithium deposition and uncontrollable lithium dendrite growth[48⇓~50]. Zhang et al. Used acrylonitrile (AN) as a carbonate-based liquid electrolyte additive (0.5 wt%), and found by combining experimental and theoretical studies (Fig. 3) that AN was preferentially Electropolymerized on the surface of the lithium anode before the electrochemical decomposition of the electrolyte, and the resulting PAN artificial SEI made the nucleation and growth of lithium deposition more uniform, and the side reactions were significantly reduced[51]. A 0.4 Ah soft-pack cell assembled with a 50 μm thick lithium anode and AN NCM622 cathode with an areal capacity of 3 mAh·cm-2 was cycled at a current density of 0.3 mA·cm-2, and the specific energy could reach 245 Wh·kg-1,120 turns with a capacity retention rate of 84.7%, while the capacity of the soft-pack cell without AN additive in the electrolyte decreased rapidly after 20 charge-discharge cycles. Another strategy to improve Li metal anode is to construct a stable artificial SEI film to mitigate the side reaction of electrolyte with Li metal and achieve uniform Li deposition. PAN with a polar nitrile group in the side chain has a higher dielectric constant, which promotes more uniform lithium deposition. Xu et al. Coated the PAN-LiTFSI solution on the surface of lithium metal, and then heat-treated 5 min,TFSI- to nucleophilically attack the nitrile group of PAN at 120 ° C, resulting in linear polymerization of the nitrile group to produce a heterocyclic structure with conjugated double bonds, thus increasing the Young's modulus of PAN to 82.7 GPa[52]. The heat-treated PAN coating exhibits better compactability, higher mechanical strength, and improved morphology uniformity, and the coating achieves relatively high coulombic efficiency by homogenizing lithium deposition, reducing the side reaction of active lithium, and inducing uniform and dendrite-free lithium deposition.
图3 (a)在1 mV·s-1扫描速率下的阴极线性扫描曲线;(b)在5 M LiFSI 电解质中、1 mA·cm-2下,不含/含AN添加剂的恒电流锂沉积曲线;(c)代表性Li+溶剂化结构的计算还原电位;(d)Li+-AN、Li+-EC、Li+-DEC和Li+-FSI-对的计算还原电位[51]

Fig. 3 (a) Cathodic linear sweep at 1 mV·s-1 scan rate; (b) Galvanostatic Li deposition curves at 1 mA·cm-2 in 5 M LiFSI electrolytes without and with AN additive; (c) Calculated reduction potential of the representative Li+ solvation structures; (d) Calculated reduction potential of Li+-AN, Li+-EC, Li+-DEC, and Li+-FSI- pairs[51]. Copyright 2021, Elsevier

The carbonyl group (C = O) of commonly used carbonates (especially cyclic carbonates) often leads to solvolysis, forming a solvent-induced SEI layer, which shows chemical heterogeneity and is dominated by organic components.During the charge-discharge cycle of lithium battery, the SEI layer is often broken and dissolved, which loses the function of protecting lithium metal, and further consumes more electrolyte and lithium, resulting in low coulombic efficiency and eventual failure of the battery[53]. The PAN-rich C ≡ N groups can produce strong dipole-dipole interaction with C = O groups, which greatly reduces the side reaction between the electrode and the electrolyte, and is beneficial to promote the formation of inorganic components (such as Li3N) in the SEI layer[54]. Yu et al. Developed a polar polymer network on lithium metal, which reduces the amount of free carbonate solvent on lithium metal through the interaction of PAN-EC, which reduces the amount of organic components in the SEI layer, thus improving the chemical stability of the interface, and the polar polymer does not interact with PF 6 - or TFSI-, thus making more inorganic components in the SEI[55]. As can be seen from fig. 4C, the surface of the polymer-protected lithium sheet exhibits stable lithium deposition/exfoliation, while the bare lithium shows a sharp growth of lithium dendrites, ending with a large amount of dead lithium. The capacity retention of the Li-metal battery with LiNi1/3Co1/3Mn1/3O2(NCM) cathode and polar polymer protection is improved to 94.0% after 450 cycles at 1 C and 98.7% after 1000 cycles at 5 C, while the capacity retention of the bare Li-metal battery is 64% after 450 cycles at 1 C and only 7.6% after 1000 cycles at 5 C. Similarly, based on the strong interaction between the C ≡ N group of PAN and the C = O group of fluoroethylene carbonate, Zhang Qiang et al. Rolled the electrospun PAN (ELPAN) with lithium foil, and the FEC molecules in the electrolyte tended to be enriched near the ELPAN fiber, thus forming a LiF-rich SEI layer on the surface of lithium metal[56]. When the capacity is the fifth deposition of 2.5 mAh·cm-2, there is almost no lithium dendrite on the surface of Li/ELPAN, and the lithium deposition in the pores is dense and smooth, on the contrary, dendrites and protrusions are observed in the pores of Li/ELPS (electrospun polystyrene) (Figure 4E). The Li/NCM523 coin cell without treatment was cycled at 0.4 C for 60 cycles at 80% capacity retention, and the cycle life was extended to 90 cycles with the Li/ELPS cell and further extended to 145 cycles with the Li/ELPAN cell.
图4 (a)PAN和EC的静电势图;(b)PAN的C≡N基团和EC的C=O基团之间的偶极-偶极相互作用的示意图;(c)5 mA· cm-2下循环后的裸锂和具有极性聚合物网络涂层的锂片的截面及表面形貌[55];(d)两种聚合物和溶剂分子之间的结合能;(e)在锂锂对称电池中循环5次后的Li/ELPAN和Li/ELPS[56]

Fig. 4 (a) Electrostatic potential maps of PAN and EC; (b) Schematic illustration of the dipole-dipole interaction between the C≡N group of PAN and the C=O group of EC; (c) Cross-sectional and surface SEM morphologies of bare Li and Li sheets coated with polar polymer network after cycling under 5 mA·cm-2 [55]. Copyright 2019, Royal Society of Chemistry (d) Binding energy between the two polymers and solvent molecules; (e) Li/ELPAN and Li/ELPS after 5 cycles in a Li-Li symmetric battery[56]. Copyright 2022, Elsevier

Compared with a single polymer coating, Wang et al. Prepared a porous lithiophilic polymer coating blended with polyvinylidene fluoride-polyacrylonitrile (PVDF-PAN) by phase separation induction method for stabilizing lithium metal anode[57]. Among them, the binding energy between C ≡ N bond of PAN and Li+ is strong, and the (3.29 eV),Li+ tends to be uniformly concentrated near the surface of PVDF-PAN protective layer. On the other hand, the arrangement of F atoms in PVDF can enhance the contact between negatively charged C — F bonds and Li at the interface, thus achieving smooth Li deposition, and the higher Li+ migration on the PVDF surface can promote the diffusion of Li+ in the polymer coating, thus achieving a high rate of Li deposition/stripping. As a result, the assembled Li ‖ NCM811 cell exhibited excellent cycling stability with 80.1% capacity retention and 99.5% coulombic efficiency after 150 cycles under the conditions of lean electrolyte (7.5μL·mA·h-1), N/P = 2.4, and high areal capacity (4.0 mAh·cm-2).

3 Application of polyacrylonitrile in gel electrolyte

Organic solvents in liquid electrolytes are highly flammable, volatile and may cause leakage, which to some extent increases the safety problems of their use in electric vehicles. Gel electrolyte (GPE) uses the flexibility of polymer matrix, which can not only be used as a separator to prevent direct contact between positive and negative electrodes, but also absorb a limited amount of liquid electrolyte or conduct ions by its own chain movement, thus reducing the combustion probability of the battery. Therefore, GPE compromises safety and high ionic conductivity (about 10-3S·cm-1) and is a good alternative to liquid electrolytes[58⇓~60]. As one of the most frequently studied polymer materials, PAN has good thermal stability, wide electrochemical stability window and good electrolyte absorption. The ionic conductivity of PAN-based GPE can reach 3.5×10-3S·cm-1 at 30 ° C, and its lithium transference number is 0.6, which can significantly reduce the concentration polarization[61].
The pure PAN porous membrane prepared by non-solvent-induced phase separation method has a dense structure and is not suitable for ion conduction, which will lead to the failure of normal charging and discharging of lithium batteries[62]. Blending polymers is the most common method to improve the structure and properties of membranes[63]. He et al. Prepared a blend membrane of PAN and polyvinyl alcohol (PVA) polymer, which can provide high porosity, high electrolyte absorption rate and good affinity for liquid electrolyte[64]. The PAN/PVA blend membrane (PVA/PAN mass ratio of 10 ∶ 90) swollen with 1.0 mol·L LiPF6 carbonate liquid electrolyte was used as the GPE of the Li‖LiCoO2, and the electrolyte absorption rate reached 510%, and the capacity retention rate was 96% after 200 cycles at 1 C, while the battery based on pure PAN gel electrolyte failed completely after 50 cycles. Similarly, in order to enhance the ionic conductivity of PAN-based GPE, Yuan et al. Blended iron-nickel-cobalt trimetallic Prussian blue analogue with PAN and electrospun it into a film. After being impregnated with liquid electrolyte, the film was assembled into a Li‖LiFeO4 cell, which was observed to have a capacity retention of nearly 100% (144.9 mAh·g-1) at 0.2 C after 100 cycles, while the cell based on Celgard separator decayed to about 100 mAh·g-1[27]. The strong polar groups make the compatibility of PAN-based GPE with lithium metal anode poor under electrostatic interaction, and the interaction of C ≡ N groups on adjacent PAN molecular chains will lead to strong crystallinity of PAN, which is not conducive to the uniform migration of lithium ions in PAN-based GPE. Li et al. Blended PAN with hydroxypropyl methyl cellulose (HPMC). Because HPMC contains a large number of hydroxyl groups in the molecular chain, it can form hydrogen bonds with C ≡ N in PAN, which can increase the flexibility of the composite film. At the same time, it makes the polymer chain more chaotic and increases the amorphous region, which can increase the flexibility of the composite film and make the movement of lithium ions more smoothly[65]. More importantly, under the test condition of 1 mA·cm-2, the symmetric cell without HPMC addition shows a gradually increasing overpotential in the first 100 cycles of deposition/stripping cycles and an internal short circuit of the cell caused by lithium dendrite piercing the separator in the following cycles; However, the symmetric battery with composite GPE can be stably cycled for 996 times, which indicates that the composite GPE can improve the compatibility with lithium metal anode, which is essential for inhibiting the uneven deposition of lithium on the electrode. The assembled LiFeO4‖GPE‖Li had an initial capacity of 106 mAh·g-1,100 cycles at 2 C with a capacity retention of 94.2% and a coulombic efficiency of 98.9%. Similarly, in order to avoid the "passivation effect" of the lithium anode caused by the C ≡ N group, Wang et al. Used the hydrogen bonding between the electron-withdrawing hydroxyl group in PEO and the C ≡ N group in PAN to prepare an asymmetric bilayer electrolyte membrane, that is, the composite membrane of PAN, PEO and LATP corresponds to the lithium anode, and the composite membrane of PAN and LATP corresponds to the positive electrode side, thus alleviating the "passivation effect" caused by PAN[66]. Compared with the S @ C ‖ liquid electrolyte ‖ Li cell (32.1%) and the S @ C ‖ PAN + PEO + LATP ‖ Li cell (69.8%), the discharge capacity retention (from 903.5 to 714.1 mAh·g-1) at 0.1 C is much higher, which is about 79.0%, and the cell exhibits excellent coulombic efficiency (99.6% – 100.0%) and good cycling stability.
In the above work, the PAN-based GPE film was prepared ex situ, but in this case, only the surface of the electrode was in contact with the GPE, and to make full use of the active material, the porous surface of the electrode had to be wetted with an additional liquid electrolyte. However, this does not fully reflect all the advantages of GPE. In contrast, the GPE prepared in situ can simultaneously achieve excellent ionic conductivity and good contact with the electrode when using a small amount of electrolyte, which makes the interfacial transport between the electrode and the electrolyte more smooth[67,68]. Zhang et al. Constructed a three-dimensional crosslinked network of GPE by in situ polymerization of methyl methacrylate (MMA) and polyoxyethylene trimethyl propane triacrylate (ETPTA) monomers on PAN nanofibers prepared by electrospinning[69]. The initial capacity of the LiFePO4‖GPE‖Li cell prepared with it is 134.8 mAh·g-1 at 2 C room temperature and 105.9 mAh·g-1 after 600 cycles, while the liquid electrolyte has only 98.9 mAh·g-1 after 230 cycles. Wang et al. Used the direct amination reaction of the butyronitrile group of PAN and the amine group of polyethyleneimine (PEI) at high temperature (120 ℃) to electrospin PEI and PAN blends into membranes. After amination, the branched PEI with a flexible backbone was crosslinked with PAN, thus improving the mechanical properties of PAN nanofiber membranes[70]. The composite membrane was then placed in an organic carbonate-based liquid electrolyte, and GPE was prepared in situ using tripropylene glycol diacrylate as a crosslinking agent. The assembled high energy density full-cell LiNi0.8Co0.1Mn0.1O2‖ graphite battery has an initial discharge capacity of 175 mAh·g-1 at 0.5 C (with a base electrolyte of 171.8 mAh·g-1),200 and a capacity retention of 91.4% (with a base electrolyte of 75.0%) after cycles. In general, polymer electrolytes are characterized by a low lithium transference number ( t L i + < 0.6), which is easily affected by polarization phenomena, ultimately limiting the power transmission during battery discharge. Single-ion conducting polymer electrolyte (SIPE) can anchor charge-delocalized anions on the side chains of a cross-linked polymer matrix, thereby eliminating the severe concentration polarization effect in conventional dual-ion polymer electrolytes. Cheng et al. Combined the high ionic transference number of SIPE and the high mechanical strength of PAN electrospun membrane, lithium (4-styrenesulfonyl) (trifluoromethylsulfonyl) imide (LiSTFSI) monomer and crosslinking agent poly (ethylene glycol) diacrylate (PEGDA) were crosslinked with plasticizer PC on PAN electrospun membrane (SIPE-2.5-PAN, PC accounts for the mass ratio of LiSTFSI and PEGDA).The as-prepared GPE has a remarkable ionic conductivity of 8.09×10-4S·cm-1 and an excellent lithium ion transference number ( t L i + = 0.92) close to 1. At 0.2 C, the initial discharge capacity of SIPE-2.5 battery is 161 mAh·g-1, which is slightly higher than that of SIPE-2.5-PAN battery (155 mAh·g-1), due to the higher ionic conductivity of SIPE2.5, but the capacity begins to decrease after the 50th cycle and decreases to 125 mAh·g-1 after the 100th cycle (77.6% retention)[71]; In contrast, the discharge capacity of the SIPE-2.5-PAN cell remained at 158 mAh·g-1 after 100 cycles, and the coulombic efficiency was close to 100%, indicating its excellent cycling stability.

4 Application of PAN in Solid Electrolyte

Although the gel polymer electrolyte can solve the leakage and safety problems of batteries to a certain extent, the liquid components still remain in the gel polymer electrolyte, which can not fundamentally solve the safety problems of lithium batteries. Lithium metal batteries based on solid electrolyte (SPE) do not leak and are not flammable, so their safety is greatly improved. SPE can not only effectively inhibit the excessive formation of SEI film, but also improve the cycle performance of the battery. At the same time, the size of the battery is reduced, and the application range is further expanded[72⇓~74]. Among them, SPE can be divided into two categories: polymer materials and inorganic materials. Typical lithium ion conducting polymers include polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), and polyvinylidene fluoride (PVDF)[75⇓~77]. PAN is a promising solid polymer electrolyte matrix due to its high dielectric constant, oxidation potential, and coordination ability with lithium salts. However, its ionic conductivity, mechanical strength and compatibility with lithium anode still need to be further improved. Researchers mainly use PAN-containing monolayer electrolyte, asymmetric multilayer electrolyte, and electrospinning to improve the performance of PAN-based solid state electrolyte[78⇓~80].

4.1 Monolayer electrolyte containing PAN

PAN-based polymer electrolytes require high dielectric constant solvents such as N, N-dimethylformamide (DMF) to dissolve, and the residual DMF content leads to a difference of 3 orders of magnitude in their ionic conductivity. Liu et al. Systematically studied the effects of the preparation process of PAN (including solution mixing, casting and drying) on the morphology, ion transport, solvation structure, lithium and oxidation stability of PAN electrolyte when DMF was used as solvent[88]. In the case of high DMF content, the (>23 mol%),Li+ is mainly solvated by DMF through the N-(CH3)2 group, and PAN only acts as a host of DMF and does not participate in ion conduction. PAN electrolyte exhibits very low oxidation stability (2.8 V) and poor stability to lithium due to the low oxidation stability of free uncoordinated DMF; In the medium range of DMF content, the (5 mol%~11 mol%),Li+ is transported by the N-(CH3)2 with DMF and the coordination of — C = O, and a small amount of CN in PAN also contributes to the solvation of Li+, so the oxidation stability of PAN electrolyte with this content is 3. 5 V, and the stability to lithium is poor. The transport of solvated shell rich PAN,Li+ of (<4.8 mol%),Li+ in the low DMF content range is mainly dependent on the segmental mobility of PAN, thus low conductivity and high activation energy are observed. Due to the very low DMF content, the film shows good oxidation stability up to 4. 5 V, and the stability to lithium is also greatly improved. It is clear that the ion transport of PAN electrolyte is always assisted by DMF, but the presence of DMF has a negative effect on the electrochemical performance of PAN based electrolyte. In polymer electrolytes, the salt concentration is critical for the formation of an ionic conductive network conducive to Li+ transport, and when the PAN/LiCF3SO3 is close to that of the "polymer in salt" (70 wt% salt), the ionic conductivity can be increased by about 5 orders of magnitude[89]. The high concentration of lithium salt (CN: Li = 1:1) is helpful to form a stable interface layer on the lithium metal anode, which prevents the subsequent parasitic reaction and effectively inhibits the nucleation and growth of lithium dendrites, thus improving the safety of the battery[86]. However, with the passage of time, the conductivity of the "polymer-in-salt" electrolyte will decrease significantly due to the precipitation or separation of lithium salts from the polymer matrix. The addition of 0.9 wt% graphene oxide nanosheets to a mixture of 16 wt% PAN and 84 wt% LiTFSI by Wu et al. Could significantly alleviate this process (the conductivity only decreased from 7.36×10-5S·cm-1 to 3.40×10-5S·cm-1 after 60 d)[90].
From a practical point of view, the ionic conductivity of polymer electrolytes for rechargeable lithium batteries needs to be more than 10-3S·cm-1, but in fact, solid PAN-based polymers containing lithium salts have low ionic conductivity at room temperature, which can not meet the requirements of practical applications[91]. As shown in Table 2, the addition of inorganic fillers such as SiO2, Al2O3, TiO2, BaTiO3 and lithium titanate nanotubes is considered to be an excellent method to improve the ionic conductivity of polymer electrolyte, which can upgrade the conductivity of PAN-based polymer electrolyte to 10-4S·m-1[81,92,93]. It is believed that the addition of nanoparticles can reduce the crystallinity and glass transition temperature of the polymer, thereby increasing the segmental movement of the polymer, but the disadvantage of simply mixing nanoparticles with the polymer is that when the amount of nanoparticles reaches a threshold, they will agglomerate, which will further reduce the ionic conductivity. Yao et al. Constructed a network structure SPE exhibiting interconnected fast conducting Li+ by in situ hydrolysis of tetraethoxysilane in a PAN matrix[22]. The interconnected inorganic network formed in situ not only acts as a robust framework for the whole SPE, but also provides enough continuous surface and Lewis acidic sites to promote the dissociation of lithium salts, with a high Young's modulus of 8.627 GPa and an ionic conductivity of 3.5×10-4S·cm-1 (Fig. 5). The Li‖LiFePO4 battery showed excellent cycling stability in the range of 20 – 80 ° C, and the Li‖LiNi0.6Mn0.2Co0.2O2 battery showed a stable discharge capacity of 173.1 mAh·g-1 at room temperature, 0.1 C, with a capacity retention of 93.8% after 200 cycles.
表2 不同填料的PAN聚合物固态电解质

Table 2 Solid electrolyte based on blending PAN polymer with different fillers

Component σ(S·cm-1) Electrochemical window (V) t L i + Cathode ref
1 PAN/LiClO4/LLZO nanowires 1.31×104 (20 ℃) - 0.3 - 81
2 PVA/PAN/LATP/SN/LiTFSI 1.13×104 (25 ℃) 5.1 0.507 LiFePO4 82
3 PAN/LiClO4/LLTO nanotubes 3.6×104 (RT) 5 0.38 LiFePO4 83
4 PAN/LiClO4/LLZTO 2.2×104 (40 ℃) 4.9 0.3 LiFePO4 84
5 PAN/LiClO4/ graphene oxide 4×104 (30 ℃) 4.3 0.42 LiFePO4 85
6 PAN/LiTFSI/SiO2 1.8×104 (60 ℃) 4.8 0.47 LiFePO4 86
7 PAN/SiO2/LiTFSI/EMIMTFSI 3.5×104 (20 ℃) 5.2 0.52 LiFePO4/NCM622 22
8 SNE@SAG/PAN 7.45×104 (30 ℃) 5 0.7 NCM811 87
图5 (a)PAN内的原位水解路线;(b)带有SiO2网络的复合SPE示意图[22]

Fig. 5 ( a ) Synthetic routes of the PAN in situ; ( b ) Schematic illustration of the composite SPE with SiO2 networks[22]. Copyright 2022, Elsevier

In addition to being the host of polymer electrolyte, PAN polymer can also modify other solid electrolyte components. Garnet-type LLZO with high ionic conductivity is one of the most well-studied inorganic oxide electrolytes and exhibits excellent stability toward lithium metal, showing great potential for application in solid-state electrolytes. However, the inherently poor interfacial contact between the electrode and the electrolyte also contributes to the high interfacial impedance. The nitrile group shows a strong coordination effect on various metals or metal ions such as Cu, Pt, Co3+, and Ag+, so Yang et al. Studied the coordination of the butyronitrile group of succinonitrile (SCN) on the surface of garnet-type LLZTO ceramic electrolyte to alleviate the interfacial impedance problem between the electrode and the electrolyte[94]. They found that in LLZTO, La ions exposed on the surface coordinate with the nitrile group of SCN, which increases the charge density of the nitrile group, and then, the high charge density nitrile group acts as a nucleophilic structure to induce the connection of adjacent nitrile groups to form a conjugated C = N sequence, which initiates the polymerization of nitrile groups in SCN, resulting in a continuous decrease in its ionic conductivity. However, PAN contains a large number of more polar nitrile groups, and this stronger polarity causes competition between the nitrile groups of SCN and PAN, so that PAN preferentially adsorbs on the surface of LLZTO, thus avoiding SCN polymerization (Figure 6A).
图6 (a)用PAN修饰的SCN改性LLZTO电解质界面相的作用机理[94]。(b)PAN及不同LLZTO含量的PAN的1H NMR谱;(c)锂离子在复合电解质中粒子间传输的示意图[95]

Fig. 6 ( a ) The function mechanism of PAN-modified SCN electrolyte interphase on the surface of LLZTO electrolyte[94]. Copyright 2021, Wiley ( b )1H NMR spectra of PAN and PAN with different amounts of LLZTO; ( c ) Schematic illustration showing the interparticle Li+ transport in the bulk of the composite electrolyte[95]. Copyright 2021, American Chemical Society

Finally, the prepared electrolyte coordination interface layer of PAN-modified SCN has stable high ionic conductivity, enhanced chemical interaction, and improved film forming properties. The LLZTO-based solid-state battery assembled based on the modified interface maintains a discharge capacity of 99% after 250 cycles and 94% after 300 cycles at 25 ℃ and 0. 1 C. In contrast, the discharge capacity of the battery based on the SCN interface decreases rapidly after 90 cycles. In the same year, Chen et al also studied the interaction between PAN and LLZTO when dimethyl sulfoxide (DMSO) was used as solvent, and they believed that the coupling between DMSO and LLZTO led to the redistribution of electron density of DMSO molecules.Charges are gathered around the sulfoxide group, so that the Lewis basicity of the sulfoxide group is increased, the dehydronitrilation of the PAN on the garnet surface is promoted, and a conjugated structure is formed in a polymer phase[95]; Meanwhile, the electrostatic interaction among LLZTO, DMSO and PAN prevented the aggregation of LLZTO particles and promoted the uniform coverage of the polymer on the particle surface (Fig. 6 B). The dehydronitrilated PAN nanocoating helps to establish channels for interparticle lithium ion conduction in the ceramic phase, which improves the ionic conductivity of garnet, and the ionic conductivity of LLZTO @ PAN particles is 1.1×10-4S·cm-1 at 60 ° C, while that of cold pressed tablets with PAN-LiTFSI film as 6.53×10-7S·cm-1,LLZTO powder is only 1.97×10-7S·cm-1.

4.2 Heterogeneous multilayer electrolyte containing PAN

Compared with liquid electrolytes, solid electrolytes are more promising to achieve both high energy density and high safety, which requires solid electrolytes to meet both lithium stability and high voltage cathode adaptation. However, single-layer ceramic, polymer and composite electrolytes can not achieve the ideal fit between high voltage cathode and lithium metal anode, while heterogeneous multilayer solid state electrolytes can achieve this goal. PAN is often used as a suitable solid electrolyte polymer matrix material for high voltage cathode due to its high oxidation stability.
Guo Yuguo et al. Contacted the oxidation-resistant PAN polymer with a high-voltage positive electrode[96]; PEGDA prepared by in situ photopolymerization on the anode side is tightly bound to lithium metal; The interlayer is an organic-inorganic composite electrolyte with a Janus structure, which inhibits the free growth of lithium dendrites and forms a tight interface (Figure 7 a). This Janus composite electrolyte component is PAN@Li1.3Al0.3Ti(Ge) 1 . 7 (PO4)3(LAGP, one side enriched with PAN, in contact with PAN on the positive electrode; The other side enriched LAGP was in contact with PEGDA on the lithium metal side, avoiding the penetration of lithium dendrite. The electrochemical window of this multilayer electrolyte can be enlarged to 0 – 5 V with a total thickness of no more than 25 μm, and the symmetric Li battery can maintain low-voltage polarization as well as a smooth and dendrite-free surface after long-term Li stripping/deposition even at a current density of 2 mA·cm-2. When matched with NCM622 cathode, the lithium metal battery can achieve a high capacity retention of 81.5% and an average coulombic efficiency of 99.8% after 270 cycles at 0.5 C. In addition, when paired with NCM811 cathode, the discharge capacity of the battery is stable at about 170 mAh·g-1 at 0.5 C after 100 cycles, which is equivalent to 97.7% of the initial capacity. Similarly, in another work, in order to alleviate the contact problem between the solid-state electrolyte and the electrode interface, they coated PAN on the positive side of the LATP ceramic sheet to establish a soft contact between PAN and NCM622, and coated PEO on the negative side to protect LATP from reduction (Figure 7 B), which not only ensured the high voltage tolerance, but also improved the stability of the lithium metal anode on the LATP ceramic sheet[97]. In addition, the ceramic of LATP can adjust lithium ions and anions to guide their uniform distribution, thus inhibiting the formation of space charge layer. NCM622 cathode, polymer protected LATP ceramic electrolyte (DPCE) or LATP-free PAN-PEO bilayer electrolyte (PDPE) and lithium metal anode were assembled into batteries. It was found that the capacity retention rate of the battery with DPCE was as high as 89%(135.6 mAh·g-1) after 120 cycles at 60 ℃ and 0.5 C, and the coulombic efficiency was more than 99.5%. However, the cell with PDPE only showed a 68%(103.2 mAh·g-1) of the initial capacity, and the device coulombic efficiency was only 96%.
图7 (a)非均质多层固体电解质示意图[96];(b)具有原始LATP和DPCE的固体全电池示意图[97];(c)NCM622‖非均质双层电解质膜‖Li电池原理图[98];(d)SPE膜的制备工艺示意图[99];(e)双层UFF/ PEO/PAN/LiTFSI SPE膜的制备图[100]

Fig. 7 (a) Schematic diagram of the heterogeneous multilayered solid electrolyte[96]. Copyright 2019, Wiley (b) Illustrations of the solid full battery with pristine LATP and DPCE[97]. Copyright 2019, American Chemical Society. (c) Schematic diagram of the NCM622‖heterogeneous dual-layered electrolyte membrane‖Li battery[98]. Copyright 2021, Elsevier. (d) Schematic illustration of the preparation process for SPE membrane[99]. Copyright 2021, Elsevier. (e) The preparation diagram of the double-layer UFF/ PEO/PAN/LiTFSI SPE[100]. Copyright 2021, Wiley

In order to take advantage of the high voltage resistance of PAN and the lithium stability of PEO at the same time, and to avoid the lithium reducibility of LLTO while adding inorganic fillers to improve the ionic conductivity, Mu et al. Reported a heterobilayer electrolyte membrane that meets the compatibility of high voltage cathode and lithium anode at the same time: one layer is composed of PAN matrix and 10% LLTO nanowires[98]; One layer consists of PEO matrix and 40% LLZTO nanoparticles. Since both the PEO-based membrane and the PAN-based membrane are elastic, the PEO-LLZTO membrane can be easily attached to the PAN-LLTO membrane and can be in close contact with each other, and the ionic conductivity of the prepared heterobilayer electrolyte membrane is 1.7×10-4S·cm-1 at 30 ° C (Figure 7C). The discharge capacity of the battery matched with the high-voltage cathode NCM622 and the lithium metal cathode is 171 mAh·g-1 at 0.1 C and 30 deg C; While the LLZTO ‖ Li cell with a discharge capacity of 63 mAh·g-1,NCM622‖PEO-40% using a single PAN-10% LLTO electrolyte for solid-state Li cell cannot even cycle stably for more than 5 cycles at 30 ° C. To improve the energy density of batteries, reducing the thickness of solid electrolyte is the key, but if the thickness of solid electrolyte is too thin, it may be punctured by lithium dendrites. An effective strategy is to add rigid ceramic fillers to flexible lithium-ion conducting polymers, which can collectively improve the ionic conductivity, mechanical strength, and thermal and electrochemical stability of solid-state electrolytes. Li et al. Prepared a three-layer electrolyte by first coating an aluminum foil with oxidation-resistant PAN + 15% LLTO nanowires on the positive side, and then coating an intermediate layer of LLTO nanofibers with PVDF + 80 wt% on the positive side.The highly loaded LLTO nanofiber network can greatly improve the mechanical strength, improve the lateral transport of lithium ions, inhibit the growth of lithium dendrites, and finally coat the negative side of the anti-reduction PEO + 15% LLTO nanowire on the intermediate layer (Fig. 7d)[99]. The NCM811 ‖ Li cell was further assembled, and the cell with trilayer composite SPE showed an excellent capacity retention of 88% and a coulombic efficiency of more than 99.2% after 500 cycles at 0.5 C, while the cell assembled with PEO + 15% LLTO nanowire SPE and PAN + 15% LLTO nanowire SPE showed a capacity retention of only 30% and 28%, and a coulombic efficiency as low as 82% and 93%. He et al. Proposed a bilayer SPE of an ultrathin (4.2 μm) and lightweight (1.29 g·cm-3) 3D ultrathin framework (Figure 7E)[100]. The fireproof vermiculite is electrospun into a membrane, which can enhance the mechanical strength of the electrolyte while achieving ultrathin, and the vermiculite can promote the compact coagulation of the PEO/PAN polymer, thereby achieving continuous and rapid Li+ conduction; The upper layer of the electrospun vermiculite membrane is poured with PAN solution corresponding to the positive electrode, and the lower layer is poured with PEO solution corresponding to the negative electrode. This unique double-layer structure can stabilize the lithium negative electrode and the high-voltage positive electrode. The solid-state Li-metal battery with N/P of 1.1 and positive electrode of NCM811 exhibits stable cycling and long life of more than 3000 H at 50 ° C, achieving high energy density of 506 Wh·kg-1 and 1514 Wh·L-1.

4.3 PAN electrospun fiber membrane

The PAN film prepared simply by casting or phase inversion is often brittle, and it is easy to break during the battery packaging process, resulting in short circuit of the battery. PAN has good spinning properties and can produce flexible films with excellent mechanical strength. The PAN electrospun membrane can be used as a support for polymer membrane formation, especially for compounding with PEO-based polymers, which are difficult to form membranes at high lithium salt concentrations, which can not only improve the mechanical strength of the composite SPE, but also improve its cathode stability. Compared with PEO-LiTFSI electrolyte, the composite SPE with LATP/PAN as the support has good mechanical properties (tensile strength of 10.72 MPa) and reduced elongation[101]. In order to further reduce the crystallinity of PEO and increase the amorphous region in the electrolyte, Gao et al. Introduced polydimethylsiloxane containing siloxane (Si — O) in the main chain to promote the segmental movement of PEO polymer and improve the interfacial wetting ability of the composite electrolyte[102]. At the same time, in order to avoid the problems of short circuit and cycle performance in the battery, the PAN nanofiber membrane is used as the support to promote the interaction between adjacent polymer molecules, and the strong 3D framework structure can inhibit the growth of dendritic lithium dendrites, reduce the occurrence of short circuit in the battery, and ultimately help to improve the long-term stability of the battery. The Li ‖ Li symmetric cell equipped with the composite electrolyte had no significant change in the interfacial impedance after 15 d of continuous standing compared with the initial state, and the cell could maintain stable cycling for 1 200 H at 0.3 mA·cm-2 current. Shi et al. Injected PEO/LiTFSI electrolyte into the PAN electrospun membrane and micro-wetted the entire SPE by trace electrolyte (2 μL) vapor[103]. The 3D PAN network enhances the mechanical strength of SPE, and the adsorption energy of TFSI- with PAN and PEO is -0. 697 eV and -0. 053 eV, respectively, which indicates that PAN has a strong adsorption capacity for TFSI-, thus improving the conductivity of lithium ions and constructing efficient and uniform transport channels in SPE and at the interface.
When the cell temperature reaches 80 ° C, the trace electrolyte is converted into vapor and enters the Li/SPE interface, and LiPF6 is hydrolyzed with the trace water in the electrolyte solution to form POF3, and the further generated HPO2F2 reacts with the residual LiF to form LiPO2F2. While LiPO2F2 is a well-known electrolyte additive that improves the cycling stability and lifetime of lithium-ion batteries. The solid-state LFP ‖ SPE ‖ Li cell based on PAN support and electrolyte microwetting has a discharge capacity of 141.1 mAh·g-1, a capacity retention of 85.6% after 360 cycles at 0.1 C and 25 ° C, and a Coulombic efficiency of nearly 100% (the capacity of the cell based on SPE without PAN support is as low as 13.8 mAh·g-1; The LFP ‖ Li cell based on PAN support but without microwetting decreased to 8.7 mAh·g-1) after the 100th cycle (Fig. 8). The thickness of the SPE prepared by Shi et al. Using the PAN electrospun membrane as the support was 50 μm, and further, the thickness of the SPE membrane was 5 μm by calendering[104]. The combined technique of electrospinning and calendering is a facile, scalable, and easy to fabricate method for preparing polymer membranes, and in this work, they made the PAN membrane with higher volume percentage and modulus by calendering, thus ensuring ultrathin and dense SPE,And unlike the porous electrospun polymer film without calendering, the calendered dense PAN film greatly increases the interfacial contact area between the PAN fiber and the lithium electrode, so that the SPE/Li interface rich in Li3N and LiF can be formed, which is beneficial to the stable cycling of lithium batteries. More importantly, due to the high thermal stability of PAN, the assembled all-solid-state battery can work safely and stably at high temperatures up to 150 ° C (the specific capacity of the LiFeO4‖Li all-solid-state battery with PAN-PEO/LiTFSI electrolyte is maintained up to 73.8% after 500 cycles at 120 ° C and 0.5 C; It still shows high coulombic efficiency and good stability for 100 cycles at a higher temperature of 150 ℃ and a higher rate of 2 C).
图8 固态电池微润湿设计示意图,SPE是薄而高强度的注入PEO/LiTFSI电解质的PAN膜(PLN)。(a)电池组件示意图;(b)液体电解质的位置和蒸气产生过程;(c)在PLN内部和PAN/PEO界面形成快速离子传输通道的混合溶剂;(d)PAN网络对TFSI-离子的吸附;(e)在阳极/电解液界面处产生的电解液蒸气分解产物LiPO2 F 2 [103]

Fig. 8 Schematic illustrations of the micro-wetting design in a solid- state battery using thin and high-strength PAN network infused with PEO/LiTFSI electrolyte (PLN). (a) A schematic diagram of the battery assembly; (b) the position of the liquid electrolyte and the vapor generation process; (c) the mixed solvent forming fast-ion-transport channels at the internal PAN/PEO interface inside PLN; (d) the adsorption of TFSI- anions by the PAN network; (e) LiPO2F2, as the decomposition product of the electrolyte vapor, is generated at the external anode/electrolyte interface[103]. Copyright 2021, Royal Society of Chemistry

In addition to PEO polymers with poor mechanical strength, PAN electrospun membranes have also been used as film-forming supports for ester-based biodegradable polycaprolactone diol (PCL) polymers, while further improving the ionic conductivity of PCL electrolytes. Zhang et al. Utilized the strong chemical interaction between the ester group in polylactone diol and the PO 4 3 - anion in LAGP to pour the mixed solution of PCL, SCN, LiTFSI and LAGP onto the PAN electrospun membrane, and the prepared composite electrolyte showed good compatibility with lithium electrode and excellent lithium dendrite inhibition ability[105]. The LiFePO4‖Li cell exhibits high coulombic efficiency, superior cycling stability, and large rate capability over 10 C. Zhang et al. Used PVDF-HFP/PAN/PVDF-HFP electrospun membrane as support and poured PCL solution, in which trimethyl phosphate (TMP) with excellent thermal stability and higher boiling point (197 ℃) was added as flame retardant and plasticizer[106]. The prepared SPE exhibited a high ionic conductivity of 2.15×10-4S·cm-1 at room temperature and did not burn in the combustion test. The assembled LiFePO4‖Li cell was cycled at room temperature, 0.5 C, and the specific capacity reached a maximum of 120 mAh·g-1 after the initial 30 cycles and decreased slightly after 400 cycles, and the solid-state cell maintained a reversible discharge capacity of 98 mAh·g-1 with a Coulombic efficiency higher than 99.5%.

5 Conclusion and outlook

Lithium batteries based on lithium metal anode have high energy density, but they also bring a series of challenges due to the high activity of lithium anode. PAN polymer has high thermal stability and good electrolyte absorption rate, and can be used as a separator of a lithium battery; The strong electron-withdrawing group C ≡ N in the PAN polymer can also interact with the C = O bond in the electrolyte, thus protecting the lithium anode from the continuous consumption of the electrolyte. In order to further achieve both high safety and high energy density, replacing the liquid electrolyte with a gel electrolyte or a solid electrolyte is one of the most effective solutions. PAN polymer has lower lowest unoccupied molecular orbital, higher electrochemical stability and wider electrochemical window, which can be adapted to both high voltage cathode and lithium metal anode. However, there are still some problems to be solved in the application of PAN-based polymer in the electrolyte of lithium metal battery: (1) At present, most of the PAN-based separators reported in the literature are prepared by electrospinning, and the high porosity enables them to absorb more electrolyte, and the cycle and rate performance of the battery are more excellent. In addition, it has been reported that further modification of PAN electrospun membrane or blending of filler and PAN for electrospinning can improve the electrochemical performance and thermal stability of PAN separator. However, the yield of membranes prepared by electrospinning technology is low, and there are still some difficulties in large-scale membrane preparation by electrospinning. How to develop PAN fiber separator with high performance, low cost and simple preparation technology is one of the bottlenecks that need further research. (2) GPE or SPE based on PAN polymer can match the high-voltage cathode to achieve high energy density batteries, but the room temperature ionic conductivity of pure PAN polymer is low, so it needs to be blended, copolymerized or added with fillers to improve its room temperature ionic conductivity to meet the stable cycle of batteries at room temperature. However, the mechanical strength of the PAN polymer casting film or phase conversion film is too poor, and it is easy to break during the battery packaging process or cycling process, resulting in short circuit of the battery. To explore a new way of composite modification and combine it with electrode surface coating in order to make a breakthrough and promote its practical process. (3) There is still no unified explanation for the compatibility between PAN polymer and lithium anode. Some studies suggest that the strong polarity — C ≡ N of PAN can promote the uniform deposition of lithium, interact with — C = O bond, inhibit the decomposition of electrolyte, and react with Li to form a SEI layer containing Li3N. However, some studies suggest that the C ≡ N group has a strong reactivity with the lithium anode, which intensifies the surface side reaction and deteriorates the performance of the lithium anode, so it is necessary to design a multi-layer electrolyte to achieve a stable cycle of the lithium metal battery. Therefore, the compatibility of PAN polymer with lithium anode should be further investigated. In a word, in the development of lithium metal batteries, PAN polymer has a potential application in liquid electrolyte, gel electrolyte and solid electrolyte, which can not be ignored. How to apply and develop the advantages of PAN polymer and improve its shortcomings requires the joint efforts of researchers.

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