1 Introduction
With the shortage of fossil resources and the pollution to the environment, the state has intensified the development of sustainable and green new energy sources, but these new energy sources (such as wind energy, solar energy, geothermal energy, tidal energy, biomass energy, etc.)Due to the limitation of natural environment, it has the characteristics of unstable and non-continuous output, so a high specific energy energy storage device is needed to store electric energy to achieve stable output of electric energy. Compared with other chemical power sources, lithium secondary batteries have the advantages of high energy density and low self-discharge, and become the first choice of high specific energy storage devices
[1,2]. However, at present, most of the commercialized lithium secondary batteries use organic liquid electrolyte, which is prone to electrolyte leakage and volatilization, and even explosion in the case of improper use, short circuit or local overheating, thus limiting the commercial application of lithium secondary batteries with large capacity, high energy and power density
[3,4]. Therefore, there is an urgent need for safer and more reliable electrolyte systems, and the design and development of high-performance electrolytes are necessary to improve the safety and electrochemical performance of lithium secondary batteries.
In recent years, polymer electrolytes (PEs) have attracted much attention from the scientific research and industry because of their good safety, thermal stability, flexibility and excellent interface contact, which can greatly improve the safety performance of existing lithium-ion batteries
[5⇓~7]. As early as 1973, Fenton et al. First reported that polyethylene oxide (PEO) could be complexed with alkali metal salts to obtain electrolytes with ionic conductivity, and found that they had potential applications in lithium-ion batteries (LIBs)
[8]. Until 1979, Armand et al. Formally proposed the use of polymer electrolytes in lithium-ion batteries to improve the energy density and efficiency of batteries
[9]. Starting from this, the application of polymer electrolytes in lithium secondary batteries has been widely studied by researchers at home and abroad. With the development of research, there are more and more kinds of polymer electrolytes, which are further subdivided into Gel polymer electrolyte (GPE) and Solid polymer electrolyte (SPE). However, the goal of developing a commercially available polymer electrolyte and applying it to high-performance lithium secondary batteries is still a long way to go. At present, most SPEs have low ionic conductivity at room temperature (10
-8~10
-6S·cm
-1) and generally require high temperature (60 ~ 80 ℃) for normal operation, while GPE achieves the improvement of ionic conductivity at room temperature at the expense of its mechanical properties. In addition, the existing polymer electrolyte preparation process does not match the battery production line, which also limits the practical application of polymer electrolyte in lithium secondary batteries to a certain extent. At present, the research on the performance of polymer electrolytes for lithium secondary batteries is more focused on the modification of polymer matrix or the addition of inorganic nanoparticles to improve the performance of electrolytes, as well as the optimization of the preparation process of polymer electrolytes, in order to realize the commercial application of polymer electrolytes for lithium secondary batteries
[10⇓~12].
Among the currently used polymer lithium secondary battery systems, the traditional ex-situ polymerization methods (phase inversion method, Bellcore method, casting method, tape casting method, etc.) Are the most commonly used methods to prepare polymer electrolytes, but these methods have complicated steps and require a large amount of organic solvents, which are easy to cause environmental pollution in the later recovery process of solvents
[13][14][15][16]. At the same time, due to the poor compatibility of the electrolyte/electrode interface, the polymer electrolyte prepared by this method has a large interface impedance, which seriously affects the overall performance of the battery
[17,18]. Based on the above reasons, many researchers have focused on the development of in-situ polymer electrolytes for high-performance lithium secondary batteries, which can conduct lithium ions by in-situ polymerization of monomer molecules initiated by precursor solutions under certain conditions
[19]. The in-situ polymerization method can not only enhance the interfacial electrical contact performance between the electrolyte and the electrode material, but also greatly simplify the assembly process of the battery and improve the electrochemical performance of the battery
[20,21].
In this paper, the in-situ preparation of gel polymer electrolytes and in-situ preparation of solid polymer electrolytes are described respectively. Based on the characteristics of common in-situ polymer electrolyte substrates (Table 1), the research progress of in-situ reaction preparation of polymer electrolytes for lithium secondary batteries is briefly described, and the challenges and developments are prospected.
2 In Situ Preparation of Gel Polymer Electrolyte
Gel polymer electrolyte (GPE) is a kind of gel film with a certain amount of organic electrolyte wrapped in a network structure constructed by polymer materials, and its structure is between liquid and solid.Therefore, it has both the fast ionic conductivity of liquid electrolyte and the thermal stability and dendrite inhibition of solid polymer electrolyte, so that it has not only the safety performance of solid polymer electrolyte, but also the room temperature ionic conductivity close to liquid electrolyte
[26]. Based on this, this section will detail the research status of in-situ preparation of PEO, PMMA, PVDF-HPF and other GPE.
2.1 In-situ preparation of PEO-based GPE
Polyethylene oxide (PEO) can dissociate lithium salts, and the oxygen atoms on its molecular chain form a complex with lithium ions, which realizes the transport of ions through the peristalsis of the molecular chain, thus possessing ionic conductivity
[27]. At the same time, this material has good electrochemical stability to lithium metal, and is one of the most commonly used polymer electrolyte substrates.
In order to solve the problem of capacity loss of Li/S batteries during use, Li et al. Prepared a PEO-based GPE in situ by dissolving PEO-based precursor in LiTFSI/DME/DOL/LiNO
3 electrolyte, which can reduce the solubility of lithium polysulfide in lithium batteries
[28]. After 100 cycles at 835 mA·g
-1(0.5 C) current density, the assembled lithium-sulfur battery showed good cycle performance with a capacity retention of 72%, which was higher than that of the lithium-sulfur battery with saturated liquid electrolyte, and showed strong self-discharge inhibition ability.
However, due to the strong crystallinity of PEO, when it is used as the polymer matrix of GPE, there is a problem of poor mechanical properties. Based on this, the researchers used in-situ crosslinking method to balance the mechanical properties and ionic conductivity in the preparation of PEO-based GPE. For example, Li et al. Used PEO as polymer matrix, lithium hexafluorophosphate (LiPF
6) electrolyte as gelling agent, added trimethylolpropane ethoxy triacrylate (TMPPOTA) as ionomer, and polymerized a high-performance PEO-based GPE in situ under UV irradiation (Figure 1)
[29]. The ionic conductivity of the GPE is about 3.3×10
-3S·cm
-1 at room temperature, the ionic transference number is as high as 0.76, and the GPE has good mechanical properties. The LFP/Li battery assembled with the electrolyte exhibits a capacity retention of 81% after more than 500 cycles at a current density of 0.5 C, which is comparable to that of a commercial separator.
In addition, previous studies have shown that nano-fillers can further reduce the crystallinity of PEO, thereby improving the ionic conductivity, and the uniform dispersion of nano-fillers in the polymer matrix is the premise of performance enhancement
[30]. Therefore, in 2019, based on the initiator-free in-situ crosslinking synthesis strategy of ring-opening polymerization, Zhu et al prepared a new PEO-based 3D-crosslinked network gel composite polymer electrolyte (3D-GCPE) with nano-filler SiO
2 as crosslinking agent, which realized the uniform dispersion of nano-filler in the polymer matrix
[31]. The results show that the 3D-GCPE exhibits excellent electrochemical properties (room temperature ionic conductivity up to 4.65×10
-3S·cm
-1), high mechanical properties (Young's modulus ~ 8.9 MPa, strain at break 181%) and interfacial stability. The assembled LFP/Li battery retained more than 90% of its initial capacity (up to 160 mAh·g
-1) after 500 cycles at 1 C current density. In addition, the cycle performance of the new GCPE is still better than that of the traditional electrolyte when it is used with high-voltage cathode materials LiNi
0.8Co
0.1Mn
0.1O
2 and LiNi
0.5Mn
1.5O
4.
2.2 In-Situ Preparation of GPE Based on PMMA
Polymethyl methacrylate (PMMA), commonly known as plexiglass, is obtained by polymerization of methyl methacrylate (MMA) monomer. Because the PMMA molecular chain contains a carbonyl side chain, it can have a strong interaction with the oxygen atoms in the carbonate solvent and absorb a large number of electrolytes, so that the prepared PMMA-based GPE has high ionic conductivity, and its characteristics of easy synthesis and low cost have attracted a lot of research
[32].
For example, based on MMA and acrylic anhydride, Ma et al. Prepared a high-voltage-resistant gel polymer electrolyte (Figure 2) by in-situ polymerization and crosslinking, with an oxidation onset potential of 5 V(vs.Li/Li
+) and an ionic conductivity of 6.79×10
-4S·cm
-1 at room temperature
[33]. In order to further improve the ionic conductivity of PMMA-based GPE, Hosseinioun et al. Prepared a PMMA-based elastic gel polymer electrolyte by in-situ crosslinking method, which can reach the ionic conductivity close to that of liquid electrolyte (about 3.9×10
-3S·cm
-1) at 20 ℃.Lithium-ion batteries assembled by in-situ polymerization of gel polymer electrolyte in NCM111/graphite batteries still have 90% capacity retention after 400 cycles at room temperature, showing excellent cycle performance
[34].
However, PMMA itself is easy to dissolve in organic electrolytes, has a low glass transition temperature (only 105 ℃), and has a certain degree of brittleness, which is not conducive to the wide application of PMMA-based polymer electrolytes. Therefore, researchers have further modified or transformed the PMMA molecular chain to reduce the glass transition temperature and enhance its mechanical properties. For example, Shao et al. explored the effect of in-situ polymerization gel electrolyte on battery performance under two different battery assembly processes, and found that the GPE prepared by in-situ polymerization of two monomers, polyethylene glycol diacrylate (PEGDA) and ethoxytrimethylolpropane triacrylate (ETPTA), inside the battery had more excellent electrochemical and mechanical properties
[35]. The GPE has an ionic conductivity of 8.7×10
-4S·cm
-1 at room temperature and exhibits a lithium ion transference number as high as 0.72. At the same time, Zhang et al. Reported a simple method to realize the interface modification of in-situ UV-curable gel polymer electrolyte, which prepared polyethylene glycol diacrylate (PEGDA) -based GPE electrolyte in situ, and polymerized the GPE with lithium iron phosphate (LFP) in situ to form GPE @ LFP cathode
[36]. The assembled GPE @ LFP/CGPE/Li battery has smaller interface resistance, lower polarizability, better high-rate performance and excellent cycle stability.
2.3 In-situ preparation of GPE based on PVDF-HPF
Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) has more amorphous regions due to its copolymer structure effect, and these amorphous regions can absorb more electrolyte; At the same time, the crystalline region can provide sufficient mechanical properties, which in turn facilitates independent film formation
[37]. Therefore, GPE based on PVDF-HFP has many advantages, such as high mechanical strength, good thermal stability, high dielectric constant, easy to form thin films and high ionic conductivity, which has attracted much attention of researchers
[38,39].
In general, traditional organic carbonate solutions are prone to decomposition at high voltages, resulting in unsatisfactory high-voltage cycle stability of current commercial lithium-ion batteries. Therefore, exploring PVDF-HFP-based GPE with good electrochemical stability under high pressure is one of the strategies to improve its cycle stability. Chen et al. modified the high-performance flexible dual-ion battery of PVDF-HFP-based GPE, using polyvinylidene fluoride-hexafluoropropylene as the base material, and co-doped with polyethylene oxide and graphene oxide through weak bond interaction to prepare GPE with three-dimensional porous network in situ (Figure 3), which significantly improved the ionic conductivity (up to 2.1×10
-3S·cm
-1)
[40]. At the same time, the capacity retention rate of the assembled battery was 92% after 2000 cycles at a current density of 5 C, showing good cycle stability. In addition, the battery has good flexibility and thermal stability (up to 90 ℃), and has broad application prospects in flexible wearable electronic devices.
Because the commonly used PVDF-HPF-based GPE contains a certain amount of flammable organic liquid electrolyte, in order to further improve its safety and electrochemical performance, researchers reduce the content of organic liquid electrolyte by adding a certain amount of inorganic fillers to GPE. Khoon et al. prepared a nanocomposite polymer electrolyte (NCPE) by dispersing zirconia nanofillers uniformly in the PVDF-HFP/PMMA matrix using an in-situ sol-gel method, and obtained a composite electrolyte membrane with the best ionic conductivity of 2.39×10
-3S·cm
-1(6 wt% zirconia), which showed very good safety characteristics
[41]. Similarly, Xu et al. used garnet-type solid electrolyte (Li
6.4Ga
0.2La
3Zr
2O
12) as initiator and ionic conductive filler, and PVDF-HFP as polymer matrix, crosslinking reaction occurred in the mixed solution of triethyl phosphate/fluoroethylene carbonate (TEP/FEC).The ion conductivity of the GPE reaches 1.83×10
-3S·cm
-1 at 20 deg C, the oxidation starting potential reaches 4.75 V(vs.Li/Li
+), and the gel polymer electrolyte has certain fireproof self-extinguishing function
[42]. The soft-pack battery assembled by the GPE showed excellent safety performance in the cutting and combustion tests, which provided a new research idea for the preparation of fireproof GPE with excellent ionic conductivity and high safety performance.
2.4 In-situ preparation of other GPE
In addition to the above polymers and their derivatives as the matrix have been widely used in the preparation of GPE, researchers have also developed a series of other types of GPE prepared by in-situ polymerization, such as poly (vinylene carbonate) (PVCA) -based GPE, 1,3-dioxolane (DOL) -based GPE and so on, which further expands the research field of GPE.
For example, Cui et al. prepared poly (vinylene carbonate) (PVCA) -based GPE, and obtained high oxidation onset potential (4.8 V(vs.Li/Li
+) and high room temperature ionic conductivity (5.59×10
-4S·cm
-1)
[43]. Qiu et al prepared gel polymer electrolyte by using the strategy of surface active polyethylene (PE) support, and found that the polymer electrolyte has high ion transport capacity, good electrolyte retention capacity and mechanical properties, and has the function of stabilizing metal lithium anode
[44]. Liu et al. generated a series of 1,3-dioxolane/trioxane (DOL/TOM) -based polymer electrolytes in situ inside the battery by cationic ring-opening polymerization. They used lithium salt LiBF
4 as a source of initiator boron trifluoride (BF
3) to initiate monomer solvent polymerization without introducing additional non-electrolyte catalysts or initiators
[45].
3 In Situ Preparation of Solid Polymer Electrolyte
Solid polymer electrolyte is composed of polymer materials and lithium salts, which has the advantages of high safety, high film-forming and viscoelasticity
[46]. Because the electrolyte does not contain liquid components, the transport of ions is realized by the movement of polymer chain segments, so the ionic conductivity is low. Solid polymer electrolytes can be divided into ether-based solid polymer electrolytes, ester-based solid polymer electrolytes and polyionic liquid-based solid polymer electrolytes.The following content mainly describes the methods and properties of these three types of solid polymer electrolyte prepared by in-situ reaction.
3.1 In-situ Synthesis of Ether-based SPE
3.1.1 In-situ preparation of PEO-based SPE
In recent years, researchers have obtained PEO-based solid polymer electrolytes by in-situ polymerization, and have made a series of achievements in improving ionic conductivity and battery safety.
The network structure between polymer molecules constructed by crosslinking can endow polymers with excellent mechanical stability and improve their safety performance while maintaining high ionic conductivity. Based on this, Zhang et al. Prepared a high-performance flexible solid polymer electrolyte (PTT-SPE) composed of PEO matrix, triethylene glycol dimethyl ether (TEGDME) crosslinking agent and tetraethylene glycol dimethacrylate (TEGDMA) monomer by in-situ UV irradiation
[47]. Because the crystallinity of PEO is reduced by the crosslinking reaction, the mechanical strength of SPE can be enhanced without sacrificing the ionic conductivity by forming a linear oligomer through the self-polymerization of TEGDMA, and the room-temperature ionic conductivity as high as 2.7×10
-4S·cm
-1 and good mechanical strength are obtained.
In addition, some researchers have reduced the crystallinity of PEO by filling nano-fillers in the electrolyte, thus enhancing the segmental movement of PEO molecular chains and improving the ionic conductivity, mechanical properties and electrochemical properties of the electrolyte. For example, Lin et al. proposed to prepare a ceramic-solid polymer electrolyte by in-situ hydrolysis between monodisperse SiO
2 nanospheres with a diameter of 12 nm and polyethylene oxide (PEO) chains (Figure 4), which obtained an oxidation onset potential as high as 5.5 V(vs.Li/Li
+), breaking through the disadvantage of low electrochemical window of traditional PEO-based solid electrolyte, but its ionic conductivity at 30 ℃ was only 4.4×10
-5S·cm
-1[48]. Chen et al. polymerized the polymer matrix PEO and the nano-additive Li
3PS
4 in situ, and the resulting solid polymer electrolyte had an oxidation onset potential of 5.1 V(vs.Li/Li
+), but its ionic conductivity was only 3.5×10
-5S·cm
-1 at 30 ℃
[49]. In order to obtain higher ionic conductivity at room temperature, Tan et al. Recently reported the preparation of nanometer-sized gadolinium-based composite solid polymer electrolytes by in-situ sol-gel synthesis method. The ionic conductivity at 30 ℃ was increased to 1.1×10
-4S·cm
-1, and the oxidation onset potential also reached 5 V(vs.Li/Li
+)
[50].
3.1.2 In-situ preparation of poly (1,3-dioxolane-based) SPE
1,3-Dioxolane (DOL) is an organic solvent widely used in the electrolyte of lithium ion batteries. It is a five-membered cyclic ether with low dielectric constant, low viscosity and other good characteristics. It often undergoes cationic ring-opening polymerization under the catalysis of Lewis acid
[18].
In the process of charging and discharging of lithium-sulfur batteries, the shuttle of polysulfides often leads to the capacity fading of batteries. Therefore, as early as 2016, Zhong et al. First proposed the in-situ polymerization of polyDOL-based SPE by electrochemically initiated polymerization and applied it to lithium-sulfur batteries
[51]. They found that the unique structure of this electrolyte can achieve rapid ion conduction (1.6×10
-3S·cm
-1) at room temperature and inhibit the dissolution/diffusion of polysulfides, which greatly reduces the self-discharge performance of lithium-sulfur batteries and improves the cycle stability. Subsequently, in 2019, Zhao et al. Reported a method of in-situ generation of SPE by cation-initiated ring-opening polymerization of DOL inside the battery
[52]. The in-situ formed SPE exhibits excellent mechanical properties and electrochemical stability, maintains good interfacial contact with the battery electrode, and has an ionic conductivity greater than 1×10
-3S·cm
-1 at room temperature. Li/S, LFP/Li, and LiNi
0.6Mn
0.2Co
0.2O
2/Li batteries assembled with this electrolyte exhibit high coulombic efficiency (> 99%) and excellent cycle performance.
In addition, Zhou et al. Developed a temperature-responsive polymer electrolyte to solve the problem of thermal runaway of batteries by using its sudden drop in ionic conductivity at a specific temperature (Fig. 5)
[23]. They dissolved lithium hexafluorophosphate (LiPF
6) salt and diallyl disulfide (DADS) monomer in DOL liquid containing trifluoromethaneimine (LiTFSI), and catalyzed cationic polymerization by Lewis acid generated by LiPF
6 in the cell.A thermosensitive solid polymer electrolyte (TSPE) based on PDOL and poly (lithium allyl sulfide) (PLAS) was prepared, which can effectively block the ionic conduction between the electrodes when the temperature of the battery exceeds 70 ℃, prevent the electrochemical reaction of the battery from continuing, and thus eliminate the possibility of battery combustion and explosion. Meanwhile, TSPE shows high ionic conductivity (2×10
-4S·cm
-1 at 30 ℃) and excellent ability to inhibit dendrite growth, and the LFP/Li battery prepared in situ based on TSPE also has good cycle performance (> 400 cycles) and good rate capability (5 C).
3.1.3 In-situ preparation of polytetrahydrofuran-based SPE
Tetrahydrofuran (THF) is a kind of cyclic ether organic compound with strong polarity, which is often used as a moderately polar solvent or organic raw material
[53]. In recent years, researchers began to try to use THF as a polymer monomer to prepare solid polymer electrolytes.
Cross-linked PTHF polymers were prepared as in Mackanic et al. And high performance SPEs were obtained (fig. 6)
[24]. They believe that due to the low spatial concentration of oxygen atoms in the main chain of xPTHF, the interaction of O-Li
+ coordination is weakened, thus enhancing the fast transport ability of Li
+. Therefore, the ionic conductivity of the electrolyte is as high as 1.2×10
-4S·cm
-1 at room temperature, and the ion mobility is as high as 0.53. In addition, the study shows that the SPE has good electrochemical stability. In 2019, Huang et al. For the first time prepared an all-solid-state polymer lithium battery using pure THF as the polymer monomer, and used boron trifluoride etherate cation as the initiator of ring-opening polymerization to realize the in-situ preparation of solid polymer electrolyte (PTSPE), with room temperature ionic conductivity of (2.3×10
-4S·cm
-1)
[54]. Meanwhile, the boron trifluoride can coordinate with the anion in the lithium salt to increase the lithium ion transference number, and is beneficial to forming a stable SEI layer on the lithium metal anode, thereby effectively inhibiting the growth of lithium dendrite of the negative electrode, and the assembled LFP/Li battery also shows cycle characteristics far superior to those of a battery prepared by a non-in-situ method.
3.2 In Situ Preparation of Ester Based SPE
3.2.1 In Situ Synthesis of Poly (Vinylene Carbonate) Based SPE
Vinylene carbonate (VC) is a typical additive to regulate the growth of solid electrolyte interface (SEI) layer on the electrode surface, which can improve the compatibility of electrode-electrolyte interface and the cycle performance of batteries
[55]. Because it is easy to polymerize and contains polar oxygen atoms in the chain, researchers have begun to try to use it as a polymer monomer to prepare and study solid polymer electrolytes.
For example, Cui et al. Have made some achievements in PVCA-based SPE. They used VC as a polymer monomer to form PVCA/LiDFOB solid polymer electrolyte (PVCA-SPE) in situ in lithium metal batteries, but its low room temperature ionic conductivity (2.23×10
-5S·cm
-1) hinders its further application
[24,56]. In order to further improve the ionic conductivity at room temperature, they dispersed the inorganic filler lithium tin phosphorus sulfur (LSnPS) powder uniformly into the VC liquid monomer. PVCA-LSnPS-based SPE with good room temperature conductivity (2×10
-4S·cm
-1) and high oxidation onset potential (greater than 4.5 V(vs.Li/Li
+)) was obtained by in-situ polymerization (fig. 7). The compatible solid-solid interface between the solid electrode and PVCA-LSnPS was successfully designed by in-situ polymerization, and the interface impedance of the Li/Li symmetric battery was reduced from 1292Ω·cm
2 to 213Ω·cm
2. At the same time, the whole battery assembled with the solid electrolyte also achieved stable cycling at room temperature, and the capacity retention rate was 88% after 140 cycles at 0. 5 C current density.
3.2.2 In-situ preparation of polyethylene glycol diacrylate-based SPE
As a common polyacrylate, polyethylene glycol diacrylate (PEGDA) is one of the representative matrix materials of polymer electrolyte.
For example, Xu et al. used poly (pentaerythritol tetrakis (3-mercaptopropionate)) (PETMP) and poly (pentaerythritol tetrakis (3-mercaptopropionate)) (PEGDA) as polymer monomers and lithium (trifluoromethane) sulfonamide (LiTFSI) as lithium salt to form a solid polymer electrolyte with a double-network structure through in-situ crosslinking by adjusting the chain length in the system, which had good ionic conductivity (1.29×10
-4S·cm
-1 at 60 ℃) while maintaining sufficient mechanical strength
[57]. On the basis of the above research, Wang et al. Synthesized a series of metal-organic frameworks (MOFs) with spatial structure, which were connected with PEGDA through PETMP with branched structure.Under the action of ultraviolet light or thermal initiation, a thiol-branched SPE with a three-dimensional interpenetrating network structure is formed in situ to improve the ionic conductivity (2.26×10
-4S·cm
-1 at room temperature) and the mechanical strength (9.4 MPa), thereby solving the balance problem between the ionic conductivity and the mechanical strength of the polymer electrolyte (Figure 8)
[58]. In addition, the SPE also exhibits a high oxidation onset potential (5.4 V(vs.Li/Li
+), a low interfacial impedance (< 550 Ω), and a high lithium ion transference number (0.44). The discharge capacity of the lithium metal battery thus assembled was 143.7 mAh·g
-1 and retained 85.6% of the discharge capacity after 500 cycles at a current density of 0.5 C.
3.3 In Situ Synthesis of Polyionic Liquids Based on SPE
Polyionic liquids (PILs) are novel conductive polymers derived from ionic liquid monomers, in which the cations or anions or both of the monomers are incorporated into the polymer backbone, and have the excellent properties of both ionic liquids and polymers.Such as high electrical conductivity (up to 10
-3S·cm
-1), good thermal stability (300 ℃ or higher) and electrochemical stability (greater than 5 V(vs.Li/Li
+)), excellent mechanical properties and good processability, have attracted wide attention of researchers in recent years
[59,60].
For example, Zhou et al. employed a multifunctional in situ synthesis technique to insert the radical polymerized 1,4-bis [3- (2-acryloyloxyethyl) imidazol-1-yl] butane bis [bis (trifluoromethanesulfonyl) imide] (C1-4TFSI) skeleton into 1-ethyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide (EMITFSI) -based electrolyte,And fil in a porous membrane network of poly (diallyldimethylammonium) bis (trifluoromethanesulfonimide) imide (PDDATFSI),Hierarchical polyionic liquid solid polymer electrolytes (HPILSE) with high room-temperature ionic conductivity (>10
-3S·cm
-1), safety, thermal/electrochemical stability, and good mechanical properties have been prepared, but their low oxidation onset potential (4.4 V(vs.Li/Li
+) limits their application
[61].
In order to develop a polyionic liquid-like SPE that is stable to the high voltage cathode interface of high energy density solid-state batteries, Zhang et al. prepared a polymer ionic liquid-like SPE by using the easy polymerization of ionic liquid (IL) monomer and polyethylene glycol diacrylate monomer to maintain the in-situ plasticized polymer chain (Fig. 9)
[62]. The ionic conductivity of the SPE is 1.4×10
-4S·cm
-1 at 30 ℃, and it has a high oxidation onset potential (5 V(vs.Li/Li
+). Li et al. Used a one-step synthesis method to polymerize the ionic liquid monomer 1-vinyl-3-methylimidazolium bis (trifluoromethanesulfonimide) imide (EMITFSI) in situ in the PEO matrix. Due to the limitation of the interpenetrating network of polyionic liquids,The crystallinity of the prepared SPE is 15.5% lower than that of pure PEO solid electrolyte, so the prepared SPE has higher ionic conductivity (0.42×10
-4S·cm
-1 and 6.12×10
-4S·cm
-1 at 25 ℃ and 55 ℃, respectively), higher oxidation onset potential (5.44 V(vs.Li/Li
+) and higher thermal decomposition temperature (280 ℃)
[63]. Similarly, Liu et al. prepared a polyionic liquid-like solid polymer electrolyte (PIL-SN-PCE) with high voltage resistance by in-situ polymerization based on succinonitrile plastic crystal electrolyte and polyionic liquid. Its room temperature ionic conductivity can reach 6.5×10
-4S·cm
-1, and its oxidation onset potential is also as high as 5.4 V(vs.Li/Li
+)
[64].
4 Conclusion and prospect
As a novel and convenient preparation method of polymer electrolyte, that in-situ polymerization method has great potential application prospect in the lithium secondary battery with high safety and high performance, not only can simplify the process and improve the economic benefit,But also can obtain electrochemical performance superior to that prepared by the non-in-situ polymerization method, such as greatly improving the ionic conductivity, effectively improving the electrode/electrolyte interface compatibility, obtaining a higher electrochemical stability window, and simultaneously realizing the improvement of thermal stability and safety performance. However, it is worth noting that the preparation of polymer electrolytes by in-situ reaction has not yet been commercialized, and more is in the stage of scientific research
[65]. Therefore, in order to realize the engineering of polymer electrolytes prepared by in-situ reaction, the following challenges need to be overcome:
(1) Limitation of Polymer Electrolyte Substrat
Limited by the inherent defects of the existing polymer electrolyte substrate (narrow electrochemical stability window of PEO substrate, poor mechanical properties of PMMA substrate, etc.), most of the prepared polymer electrolytes have defects in electrochemical or mechanical properties.For example, the ionic conductivity of solid polymer electrolytes is low, and the safety performance of gel polymer electrolytes is not improved enough, which limits the development of in-situ reaction technology in the preparation of polymer electrolytes to a certain extent. Therefore, in addition to actively exploring the development of new polymer electrolytes, it is particularly important to make better use of in-situ generation technology to prepare high-performance polymer electrolytes with suitable polymer substrates.
(2) Negative Effect of Initiator
It should be noted that the initiator, while contributing to the formation of the polymer electrolyte, inevitably has some negative effects. Firstly, the unreacted initiator in the in-situ reaction is difficult to remove, which affects the ionic conductivity and stability of the polymer electrolyte. Secondly, the added initiator may have adverse effects on the reaction with the electrode. For example, the electron-rich -CN group on azobisisobutyronitrile (AIBN) is likely to react with the metal anode. Finally, initiators such as benzamide peroxide (BPO) can thermally decompose to produce gas, form bubbles in the gel electrolyte and evaporate the solvent, deteriorating the battery performance. In order to solve these problems, firstly, it is necessary to select the type of initiator carefully and control the amount of initiator. Second, the development of new systems of initiators and the exploration of relatively mild initiation conditions; Third, a process for generating the polymer electrolyte in situ without an initiator can also be considered. The negative effect of that initiator on the for polymer electrolyte is reduced or eliminate as much as possible.
(3) Limitation of external environment
In the process of preparing polymer electrolytes by in-situ reaction, the external environmental factors (such as temperature, light, etc.) May have a great impact on the performance of polymer electrolytes. This requires researchers to set up appropriate control models in their work, establish the relationship between external environmental factors and electrolyte and battery performance, and form a relatively complete system.
The unique advantages of polymer electrolytes prepared by in-situ reaction will further improve the comprehensive performance of lithium secondary batteries, especially the urgent market demand for high-performance polymer lithium secondary batteries, which will attract a large number of researchers to conduct extensive and in-depth research and accelerate the commercialization process.
National Natural Science Foundation of China(21805113)
Guangdong Basic and Applied Basic Research Foundation(2019A1515011656)
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