Self‐Supporting Solid Electrolyte Based on Supramolecular Interaction for Stable Li Metal Batteries

<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.

Solid composite polymer electrolytes are the optimal candidate for all solid-state lithium batteries, because of their enhanced ionic conductivities, long-life cycle ability and compatibility to lithium anode. Herein, we reported a kind of solid composite polymer electrolyte comprised of poly(ethylene oxide), graphitic-like carbon nitride and lithium perchlorate, which was prepared by a facile solution blending method. Microstructure of the solid composite polymer electrolyte was regulated by thermal annealing and interaction among components and was characterized by XRD, DSC, FTIR-ATR, and ROM. The obtained solid composite polymer electrolyte achieved an ionic conductivity as high as 1.76 × 10−5 S cm−1 at 25°C. And the electrochemical stable window and the lithium ion transference number, t+, were also obviously enhanced. LiFePO4/Li solid-state batteries with the annealed PEO-LiClO4-g-C3N4 solid polymer electrolyte presented a high initial discharge capacity of 161.2 mAh g−1 and superior cycle stability with a capacity retention ratio of 81% after 200 cycles at 1C at 80°C. The above results indicates that the thermal annealing treatment and g-C3N4 as a novel structure modifier is crucial for obtaining the high-performance solid composite polymer electrolytes used in the all solid-state lithium battery.

Covalent organic frameworks (COFs)/poly(ethylene oxide) (PEO) solid electrolytes offer bright application prospects in all-solid-state lithium–metal batteries (ASSLBs), while they struggle with low ionic conductivity due to poor interfacial Li+ transport kinetics. This work employs acetamide (EA) as a shuttle to transfer Li+ at COF/PEO interfaces via the synergistic EA-PEO, EA-COF, and EA-Li+ interactions, thereby constructing continuous channels for fast Li+ transport. The resulting EA-modified COF/PEO solid electrolyte delivers an ionic conductivity of 1.01 × 10–4 S·cm–1 at 30 °C. In addition, the synergistic interactions induced by EA promote the dissociation of lithium salt, leading to a LiF- and Li3N-rich solid electrolyte interface (SEI) layer and thus enhanced interfacial stability. As a result, the LiFePO4-based ASSLB assembled with the PEO/LiTFSI-COF/EA solid electrolyte demonstrates exceptional cycling stability with 70.34% capacity retention over 1200 cycles at 1 C and 60 °C. Our work provides a powerful strategy to facilitate interfacial ion transport and reinforce long-term stability in solid polymer electrolytes.

Polyethylene oxide (PEO) based-solid polymer electrolytes were prepared with low weight polymers bearing carboxylic acid groups added onto the polymer backbone, and the variation of the conductivity and performance of the resulting Li ion battery system was examined. The composite solid polymer electrolytes (CSPEs) were composed of PEO, LiClO4, PAA (polyacrylic acid), PMAA (polymethacrylic acid), and Al2O3. The addition of additives to the PEO matrix enhanced the ionic conductivities of the electrolyte. The composite electrolyte composed of PEO:LiClO4:PAA/PMAA/Li0.3 exhibited a low polarization resistance of 881.5 ohms in its impedance spectra, while the PEO:LiClO4 film showed a high value of 4,592 ohms. The highest ionic conductivity of 9.87 × 10−4 S cm−1 was attained for the electrolyte composed of PEO:LiClO4:PAA/PMAA/Li0.3 at 20 °C. The cyclic voltammogram of Li+ recorded for the cell consisting of the PEO:LiClO4:PAA/PMAA/Li0.3:Al2O3 composite electrolyte exhibited the same diffusion process as that obtained with an ultra-microelectrode. Based on this electrolyte, the applicability of the solid polymer electrolytes to lithium batteries was examined for an Li/SPE/LiNi0.5Co0.5O2 cell.

Solid state lithium metal batteries are the most promising next‐generation power sources owing to their high energy density and safety. Solid polymer electrolytes (SPE) have gained wide attention due to the excellent flexibility, manufacturability, lightweight, and low‐cost processing. However, fatal drawbacks of the SPE such as the insufficient ionic conductivity and Li+ transference number at room temperature restrict their practical application. Here vertically aligned 2D sheets are demonstrated as an advanced filler for SPE with enhanced ionic conductivity, Li+ transference number, mechanical modulus, and electrochemical stability, using vermiculite nanosheets as an example. The vertically aligned vermiculite sheets (VAVS), prepared by the temperature gradient freezing, provide aligned, continuous, run‐through polymer‐filler interfaces after infiltrating with polyethylene oxide (PEO)‐based SPE. As a result, ionic conductivity as high as 1.89 × 10−4 S cm−1 at 25 °C is achieved with Li+ transference number close to 0.5. Along with their enhanced mechanical strength, Li|Li symmetric cells using VAVS–CSPE are stable over 1300 h with a low overpotential. LiFePO4 in all‐solid‐state lithium metal batteries with VAVS–CSPE could deliver a specific capacity of 167 mAh g−1 at 0.1 C at 35 °C and 82% capacity retention after 200 cycles at 0.5 C.

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.

Lithium-metal batteries (LMBs) with high energy densities are highly desirable for energy storage, but generally suffer from dendrite growth and side reactions in liquid electrolytes; thus the need for solid electrolytes with high mechanical strength, ionic conductivity, and compatible interface arises. Herein, a thiol-branched solid polymer electrolyte (SPE) is introduced featuring high Li+ conductivity (2.26 × 10-4 S cm-1 at room temperature) and good mechanical strength (9.4 MPa)/toughness (≈500%), thus unblocking the tradeoff between ionic conductivity and mechanical robustness in polymer electrolytes. The SPE (denoted as M-S-PEGDA) is fabricated by covalently cross-linking metal-organic frameworks (MOFs), tetrakis (3-mercaptopropionic acid) pentaerythritol (PETMP), and poly(ethylene glycol) diacrylate (PEGDA) via multiple CSC bonds. The SPE also exhibits a high electrochemical window (>5.4 V), low interfacial impedance (<550 Ω), and impressive Li+ transference number (tLi+ = 0.44). As a result, Li||Li symmetrical cells with the thiol-branched SPE displayed a high stability in a >1300 h cycling test. Moreover, a Li|M-S-PEGDA|LiFePO4 full cell demonstrates discharge capacity of 143.7 mAh g-1 and maintains 85.6% after 500 cycles at 0.5 C, displaying one of the most outstanding performances for SPEs to date.

The development of metallic lithium anode is restrained by lithium dendrite growth during cycling. The solid polymer electrolyte with high mechanical strength and lithium ion conductivity could be applied to inhibit lithium dendrite growth. To prepare the high‐performance solid polymer electrolyte, the environment‐friendly and cheap bacterial cellulose (BC) is used as filler incorporating with PEO‐based electrolyte owing to good mechanical properties and Li salts compatibility. PEO/LiTFSI/BC composite solid polymer electrolytes (CSPE) are prepared easily by aqueous mixing in water. The lithium ion transference number of PEO/LiTFSI/BC CSPE is 0.57, which is higher than PEO/LiTFSI solid polymer electrolyte (SPE) (0.409). The PEO/LiTFSI/BC CSPE exhibits larger tensile strength (4.43 MPa) than PEO/LiTFSI SPE (1.34 MPa). The electrochemical window of composite electrolyte is widened 1.43 V by adding BC. Density functional theory calculations indicate that flex of PEO chains around Li atoms is suppressed, suggesting the enhanced lithium ion conductivity. Frontier molecular orbitals results suggest that an unfavorable intermolecular charge transfer lead to achieve higher potential for BC composite electrolyte. All solid‐state Li metal battery with PEO/LiTFSI/BC CSPE delivers longer cycle life for 600 cycles than PEO/LiTFSI SPE battery (50 cycles). Li symmetrical battery using PEO/LiTFSI/BC CSPE could be stable for 1160 h.

An advanced solid polymer electrolyte composed of poly(propylene carbonate) and mesoporous silica nanoparticles for use in all-solid-state lithium-ion batteries

Composite solid polymer electrolytes with silicate nanophases

Graphitic carbon nitride assisted PVDF-HFP based solid electrolyte to realize high performance solid-state lithium metal batteries

Experimental and computational techniques have been applied to investigate the influence of Li0.5La0.5TiO3 nanoparticles on the ionic conductivity of the poly(vinylidene fluoride) (PVDF)/LiClO4 nanocomposite solid polymer electrolyte. The theoretical evidence facilitated to suggest a plausible mechanism for Li-ion conduction across the PVDF/LiClO4/Li0.5La0.5TiO3 based solid polymer electrolytes. The solid composite polymer electrolyte with 30wt% of Li0.5La0.5TiO3 (LLTO) nanofiller exhibited an unprecedented ionic conductivity of 2.3687 × 10−3 S cm−1 at room temperature. The addition of LLTO nanoparticles to the polymer matrix enhanced its ionic conductivity by two orders of magnitude. The activation energy (Ea) and total transference number (t) were estimated to be 0.29 eV and 0.853, respectively. The interaction between the filler and polymer matrix has been inferred by the density functional theory (DFT)-IR analysis. The DFT calculations have been performed on the above system using the basis set of B3LYP-LANL2DZ. The calculated IR spectra were compared with the experimental FTIR data, which allowed us to propose accurate vibrational assignments and to clarify the complex IR vibration of the samples. All-solid-state Li2FeSiO4/CPVDF/LiClO4/LLTO graphite lithium cell has been fabricated using the highest Li-ion conducting PVDF/LiClO4/LLTO composite polymer electrolyte. The all-solid-state cell exhibits an excellent initial specific capacity of 87.13 and 73.24 mAh g−1 after 30 cycles, demonstrating higher capacity retention. The findings provide an avenue for exploring the simple all-solid-state lithium batteries, which are potential candidates for next-generation energy storage technology.

Solid-state lithium-metal (Li0) batteries are gaining traction for electric vehicle applications because they replace flammable liquid electrolytes with a safer, solid-form electrolyte that also offers higher energy density and better resistance against Li dendrite formation. Solid polymer electrolytes (SPEs) are highly promising candidates because of their tuneable mechanical properties and easy manufacturability; however, their electrochemical instability against lithium-metal (Li0), mediocre conductivity and poorly understood Li0/SPE interphases have prevented extensive application in real batteries. In particular, the origin of the low Coulombic efficiency (CE) associated with SPEs remains elusive, as the debate continues as to whether it originates from unfavoured interfacial reactions or lithium dendritic growth and dead lithium formation. In this work, we use state-of-the-art cryo-EM imaging and spectroscopic techniques to characterize the structure and chemistry of the interface between Li0 and a polyacrylate-based SPE. Contradicting the conventional knowledge, we find that no protective interphase forms, owing to the sustained reactions between deposited Li dendrites and polyacrylic backbones and succinonitrile plasticizer. Due to the reaction-induced volume change, large amounts of cracks form inside the Li dendrites with a stress-corrosion-cracking behaviour, indicating that Li0 cannot be passivated in this SPE system. On the basis of this observation, we then introduce additive engineering, leveraging from knowledge of liquid electrolytes, and demonstrate that the Li0 surface can be effectively protected against corrosion using fluoroethylene carbonate, leading to densely packed Li0 domes with conformal and stable solid-electrolyte interphase films. Owing to the high room-temperature ionic conductivity of 1.01 mS cm-1, the high transference number of 0.57 and the stabilized lithium-electrolyte interface, this improved SPE delivers an excellent lithium plating/stripping CE of 99% and 1,800 hours of stable cycling in Li||Li symmetric cells (0.2 mA cm-2, 1 mAh cm-2). This improved cathodic stability, along with the high anodic stability, enables a record high cycle life of >2,000 cycles for Li||LiFePO4 and >400 cycles for Li||LiCoO2 full cells.

Fibre-reinforced multifunctional composite solid electrolytes for structural batteries

Solid-state sodium ion batteries are frequently referred to as the most promising technology for next-generation energy storage applications. However, developing a suitable solid electrolyte with high ionic conductivity, excellent electrolyte–electrode interfaces, and a wide electrochemical stability window, remains a major challenge. Although solid-polymer electrolytes have attracted great interest due to their low cost, low density and very good processability, they generally have significantly lower ionic conductivity and poor mechanical strength. Here, we report on the development of a low-cost composite solid polymer electrolyte comprised of poly(ethylene oxide), poly(vinylpyrrolidone) and sodium hexafluorophosphate, mixed with indium arsenide nanowires. We show that the addition of 1.0% by weight of indium arsenide nanowires increases the sodium ion conductivity in the polymer to 1.50 × 10−4 Scm−1 at 40 °C. In order to explain this remarkable characteristic, we propose a new transport model in which sodium ions hop between close-spaced defect sites present on the surface of the nanowires, forming an effective complex conductive percolation network. Our work represents a significant advance in the development of novel solid polymer electrolytes with embedded engineered ultrafast 1D percolation networks for near-future generations of low-cost, high-performance batteries with excellent energy storage capabilities.