A Healable Quasi-Solid Polymer Electrolyte with Balanced Toughness and Ionic Conductivity.

<p indent="0mm">Since the commercialization of lithium-ion batteries in the 1990s, lithium-ion batteries have been successfully applied in portable electronics, electric vehicles, and grid energy storage. Although current organic liquid electrolytes have high ionic conductivities, they are inherently flammable, volatile, and prone to leakage. Moreover, severe side reactions and dendrite growth on the surface of the lithium anode during the charge-discharge process can cause safety hazards, which greatly impede their applications in lithium metal batteries. Solid electrolytes, including inorganic solid electrolytes and polymer electrolytes, are regarded as effective alternatives to organic liquid electrolytes for the construction of lithium metal batteries with high energy density and safety. Among them, solid polymer electrolytes offer excellent flexibility, processability, and interfacial compatibility over inorganic solid electrolytes, and they are extraordinarily promising for lithium metal batteries with high energy density and safety. Ideal solid polymer electrolytes should have following features: (1) High ionic conductivity (&gt; <sc>10 ^–4 S cm ^–1) </sc> at room temperature; (2) high lithium ion transference number (~1) to reduce the concentration polarization and improve the rate performance of batteries; (3) intimate contact at the electrode/electrolyte interfaces; (4) wide electrochemical window <sc>(&gt;4.5 V</sc> vs. Li/Li ⁺) to match high-voltage cathodes and improve the energy density of batteries; (5) good mechanical stability to resist processing, buffer electrode volume change and inhibit dendrite growth; (6) good thermal stability to withstand environmental changes. Generally, the ionic conductivity of pure solid polymer electrolytes at room temperature is low <sc>(~10 ^–6 S cm ^–1). </sc> Researchers have tried to improve the ionic conductivities by adjusting the lithium salt concentration, such as developing “polymer-in-salt” solid electrolytes. However, increasing the concentration of lithium salt leads to the deterioration of the mechanical strength. Strategies such as developing novel lithium salts, modifying polymer matrix, and incorporating inorganic fillers into solid polymer electrolytes are proposed to promote ionic conductivities of solid polymer electrolytes. In particular, composite polymer electrolytes, fabricated by dispersing a certain amount of inorganic fillers into solid polymer electrolytes, have improved ionic conductivities without sacrificing their mechanical performances. Poor interfacial property between electrodes and electrolytes is also a critical issue for solid polymer electrolytes. On one hand, poor and uneven solid/solid contacts at the electrode/electrolyte interfaces lead to high resistance and sluggish ionic transport kinetics. Furthermore, the volume change of the positive and negative electrodes in the charge/discharge process deteriorates the interfacial contacts, blocks the ion and electron transport through the interfaces, and greatly reduces the electrochemical reaction kinetics. On the other hand, the electrochemical windows of solid polymer electrolytes are usually narrow <sc>(＜4.5 V).</sc> During cycling, redox reactions are prone to occur at the electrode/electrolyte interfaces, causing battery failure. Solid polymer electrolytes have also poor thermal and mechanical stabilities. Therefore, design and synthesis of polymer-based solid electrolytes with excellent comprehensive performances and construction of fast and stable ion transport channels at the electrolyte/electrode interfaces are of great significance for the successful development of solid-state lithium metal batteries. This paper presents a brief review of the research progress in solid polymer electrolytes from two aspects: Improving the ionic conductivities of solid polymer electrolytes and enhancing the interfacial performance at electrolyte/electrode interfaces. First, targeted optimization strategies on ionic conductivities of solid polymer electrolytes, including constructing continuously aligned ionic transport paths and shortening the ionic transport distance, are summarized. Second, interface optimization strategies, including constructing wetting interfaces and synthesizing asymmetric electrolytes, are presented to reduce the interface resistance and improve the interfacial contact. Finally, perspectives on the development of solid polymer electrolytes and high-performance solid-state lithium metal batteries are discussed, and key research directions and advanced test methods are proposed. This review may provide a comprehensive understanding and further guidance for not only the material design of solid polymer electrolytes, but also the structural design of lithium metal batteries with favorable electrochemical and interfacial performances.

In-situ formation of quasi-solid polymer electrolyte for wide-temperature applicable Li-metal batteries

Solid electrolytes are being considered as an effective solution to replace liquid ones for building safer batteries, but they suffer either low ionic conductivity or large contact ohmic impedance. Here, a highly conductive and thermomechanically stable quasi‐solid electrolytes (QSE) by self‐polymerization of a commercially available liquid electrolytes (1,3‐dioxolane (DOL), 1,2‐dimethoxyethane (DME), lithium trifluoromethane sulfonimide (LiTFSI) and LiPF6) at room temperature in a porous polyimide nanofiber film are reported. LiPF6 initiates the ring‐opening reaction of DOL and LiTFSI promotes the self‐polymerization to form poly‐DOL (PDOL), while the plasticizer of DME solidifies the PDOL. On the other hand, polyimide has a great affinity with PDOL that facilitates to form stable QSE network. As a result, the QSE film shows a high room‐temperature ionic conductivity of 2.9 × 10−3 S cm−1, a high mechanical strength of 31 MPa, and high heat resistance to 160 °C. The LiFePO4||QSE||Li batteries with a high cathode loading of ≈18.7 mg cm−2 show great cycling performance and stable electrode/electrolyte interfaces at a high current rate of 0.5 C with a capacity retention of 91.8% over 200 cycles at room temperature.

The practical application of solid polymer electrolytes (SPEs) is limited by their low ionic conductivity and poor interfacial compatibility with silicon (Si) anodes in high-energy-density, safe lithium-ion batteries (LIBs). Here, we introduce a lithiated covalent organic framework (COF) into a poly(vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP)-based quasi-solid polymer electrolyte (QSPE) containing plasticizers to increase its electrochemical performance. The resulting COF-based QSPE has a high ionic conductivity of 2.99 mS cm-1 and enables Si anodes to deliver a remarkable specific capacity of 1722 mAh g-1 after 500 cycles, along with an excellent rate capability of up to 10 A g-1. Furthermore, we provide new insights into the role of lithiated COFs in modulating the Li+ solvation structure, facilitating ionic migration and promoting desolvation in plasticizer-containing solid electrolytes at the molecular scale. These findings offer valuable guidelines for the rational design of stable Si anodes in solid-state batteries.

The insufficient ionic conductivity, limited lithium-ion transference number (tLi +), and high interfacial impedance severely hinder the practical application of quasi-solid polymer electrolytes (QSPEs). Here, a sandwich-structured polyacrylonitrile (PAN) based QSPE is constructedin which MXene-SiO2 nanosheets act as a functional filler to facilitate the rapid transfer of lithium-ion in the QSPE, and a polymer and plastic crystalline electrolyte (PPCE) interface modification layer is coated on the surface of the PAN-based QSPE of 3wt.% MXene-SiO2 (SS-PPCE/PAN-3%) to reduce interfacial impedance. Consequently, the synthesized SS-PPCE/PAN-3% QSPE delivers a promising ionic conductivity of ≈1.7mScm-1 at 30°C, a satisfactory tLi + of 0.51, and a low interfacial impedance. As expected, the assembled Li symmetric battery with SS-PPCE/PAN-3% QSPE can stably cycle more than 1550h at 0.2 mA cm-2 . The Li||LiFePO4 quasi-solid-state lithium metal battery (QSSLMB) of this QSPE exhibits a high capacity retention of 81.5% after 300 cycles at 1.0C and at RT. Even under the high-loading cathode (LiFePO4 ≈10.0mgcm-2 ) and RT, the QSSLMB achieves a superior area capacity and good cycling performance. Besides, the assembled high voltage Li||NMC811(loading≈7.1 mgcm-2 ) QSSLMB has potential applications in high-energy fields.

Currently, electrolytes used in lithium-ion batteries are flammable and leaky, causing safety problems when used under high temperatures or extreme conditions. Solid electrolytes can fundamentally avoid the occurrence of such issues, at the same time, it can significantly increase the energy density of solid state lithium batteries. Solid electrolytes are classified into inorganic solid electrolytes and solid polymer electrolytes. Although inorganic solid electrolytes have the high mechanical strength and room temperature ionic conductivity, it has poor flexibility and complicated preparation process. In contrast, solid polymer electrolyte has good elasticity and good interface contact, which can overcome many disadvantages of the inorganic solid electrolyte. Among them, polyethylene oxide (PEO) is first proposed as a polymer electrolyte. Due to its excellent mechanical properties, electrochemical stability and thermal stability, it has been a hot topic for polymer solid electrolytes for decades. However, its low room temperature ionic conductivity limits practical application. Given the problems existing in PEO-based polymer electrolytes, several methods for improving ionic conductivity have been proposed. The primary methods include plasticization, the organic-inorganic composite, and polymer blending. Comparing these methods, it can be seen that adding plasticizers to the polymer can effectively increase the conductivity of the electrolyte, but at the same time it will cause a decrease in the mechanical properties, and the safety issues limit its application in lithium-ion batteries. Inorganic fillers can balance the problems of ionic conductivity and mechanical properties, but the construction of a good filler-polymer dispersion system is complicated in preparing such composite electrolytes, design a new type of multi-dimensional the ion transport network also face tremendous challenges. Undoubtedly, in the future research, inorganic fillers will be the dominant means in the modification of composite electrolytes. Compared with the former two modification methods, polymer blending is easy to operate and friendly to industry, but low ionic conductivity at room temperature still exist. To give full play to the advantages of polymer blending, using it as an auxiliary means to improve the performance of composite electrolytes will be the primary development direction of such modification methods. PEO is the most widely studied polymer electrolyte. Even though a large number of modification methods have emerged in its development history, the ionic conductivity at room temperature is still limited to a relatively low level. To meet the needs of the application in lithium-ion batteries, a high operating temperature (50−70℃) is still needed, the biggest problem remains exist. To solve the problems in polymer solid electrolytes, the research directions in the future may mainly focus on: Expanding the modification method of PEO to other polymer electrolyte host materials, broadening the scope of research, and finding the available polymer electrolytes at room temperature. Concerned about the preparation of ultra-thin polymer electrolytes, reducing the ion transmission path to increase the transmission efficiency of lithium ions between positive and negative electrodes. Effectively combines inorganic solid electrolytes and solid polymer electrolytes to give full play to their advantages. In addition, exploring the effect of inorganic filler morphology on the conductivity of the composite electrolyte will also deserve attention. Many one-dimensional, two-dimensional materials can build ion transport networks so that lithium ions can travel rapidly along the path. With the deepening of research, it is expected that PEO-based polymer electrolytes may replace liquid electrolytes and become an essential cornerstone for building the next generation of safe and reliable lithium-ion batteries.

An electrolyte is one of the essential components in diverse electrochemical devices including lithium secondary batteries and dye-sensitized solar cells (DSSCs). Traditional liquid electrolytes having a high ion conductivity suffer from a problem of lowering the durability of the electrochemical devices because they have high flowability and volatility. Therefore, to improve the reliability of the electrochemical devices, it is preferable to replace the liquid electrolyte with a quasi-solid electrolyte. However, quasi-solid electrolytes are usually difficult to inject the devices and have a lower ion conductivity due to a higher viscosity than liquid electrolyte. In this study, we have developed a quasi-solid electrolyte which is based on a low molecular weight gelator (LMWG) with high conductivity and stability through proper physical crosslinking. In addition, the combination of various polymer materials including special functional groups with LMWG has been investigated for the optimization of the electrolyte performances. Especially, the prepared quasi-solid electrolytes have been systematically analyzed by various electrochemical methods and applied to both lithium battery and DSSC for evaluating their performances. The detailed results will be presented and discussed at the conference. This work was supported by a grant (No.2017000140002/ RE201702218) from the Environmental Industry Advancement Technology Development Project of Korea Environmental Industry & Technology funded by Korea Ministry of Environment.

Quasi solid state polymer electrolyte with binary iodide salts for photo-electrochemical solar cells

Hybrid graphene nanoplatelet/manganese oxide electrodes for solid-state supercapacitors and application to carbon fiber composite multifunctional materials

Micropores-in-macroporous polymer membranes containing an immobilized-liquid electrolyte enable dendrite-free alkali metal batteries.

Three-dimensional (3D) battery architectures have been envisioned to enable high energy density electrodes without the associated power drop experienced by planar cells. However, the development of 3D cells is hampered by difficulties producing conformal solid-state electrolytes (SSE), solid polymer electrolytes (SPE) and gel polymer electrolytes (GPE) that are pinhole-free and have adequate ionic conductivities. Fortunately, electrolytes in 3D cells are often utilized at lower thickness, which may compensate the decreased ionic conductivity. Here, we comprehensively review potential 3D SSE, SPE and GPE electrolyte materials by compiling their thickness and room temperature ionic conductivity. We use area specific resistance (ASR) as a metric to compare 3D electrolytes with one another and conventional electrolytes. We find that certain process-material combinations, such as atomic layer deposition of SSEs, electrodeposition of SPEs and GPEs, and initiated chemical vapor deposition of SPEs demonstrate ASRs beneath the interfacial impedances of Li-based systems and approach state-of-the-art electrolytes. We also comment on additional factors, such as electrochemical stability, that should be evaluated when determining 3D electrolyte suitability. Future research should focus on adapting known materials chemistries for conformal deposition techniques to further improve the ionic conductivity, as these techniques are capable of producing the necessary thicknesses and conformality.

A highly flexible and adhesive polymer electrolyte with fast ion conduction prepared via UV-induced in-situ polymerization for lithium metal batteries

Low room temperature ionic conductivity and interfacial incompatibility are the key factors that hinder the practical application of solid polymer electrolyte (SPEs) in lithium metal batteries. Increasing the ability of the SPEs to dissolve and dissociate lithium salt is helpful to enhance ion transport capacity of the SPEs. Herein, ketone groups with high solubility and dissociation ability of lithium salt are introduced into the structural design of SPE, an aliphatic ketone solid polymer electrolyte (KT@SPE) with crosslinking structure is prepared by ultraviolet (UV) polymerization. The prepared KT@SPE shows excellent viscoelastic and possess room temperature ionic conductivity of 10 −4 S cm −1 with 200 wt% lithium bis((trifluoromethyl)sulfonyl)azanide (LiTFSI). Thanks to the contribution of high ion transport capacity, construction of multi‐hydrogen bonds network structure of KT@SPE and a wettability of controlling residual dimethyl sulfoxide (DMSO) solvent to the interface, the assembled symmetrical Li cell realizes stable cycling for over 2000 h at 0.15 mA cm −2 . Moreover, LiFePO 4 cell achieves stable long cycle at 5C and enable Li/KT@SPE 3 /LiFe 0.6 Mn 0.4 PO 4 cell operates at 4.4 V. This work not only provides a design strategy for preparing novel solid polymer electrolytes, but also exhibits the excellent application potential of aliphatic ketone‐based polymer electrolyte in solid‐state lithium batteries at high current density and high voltage.

ABSTRACTPoly(1-vinylpyrrolidone-co-vinyl acetate)-based quasi-solid polymer electrolytes doped with potassium iodide are prepared. The highest room temperature ionic conductivity of 1.72 mS cm−1 is obtained. The dielectric studies are performed to understand the ion conduction mechanism. Fourier transform infrared study has been performed to understand complexation of the poly(1-vinylpyrrolidone-co-vinyl acetate)-based quasi-solid polymer electrolytes. The best power energy conversion efficiency (η) obtained is 2.94% with short circuit current density (Jsc) of 8.05 mA cm−2, open circuit voltage (Voc) of 0.58 V, and fill factor of 63% under standard light intensity of 100 mW cm−2 irradiation.

Flexible, nonflammable, highly conductive and high-safety double cross-linked poly(ionic liquid) as quasi-solid electrolyte for high performance lithium-ion batteries