Synergistic Composite Solid Electrolyte Based on Multifunctional Polymer Networks for High-Performance Lithium Metal Batteries.

Poly(ethylene oxide) (PEO)-based solid composite electrolytes suffer from poor conductivity and lithium dendrite growth, especially toward the metallic lithium metal anode. In this study, succinonitrile (SN) is incorporated into a PEO composite electrolyte to fabricate an electrode-compatible electrolyte with good electrochemical performance. The SN-doped electrolyte successfully inhibits the lithium dendrite growth and facilitates the SEI layer formation, as determined by the operando nanofocus wide-angle X-ray scattering (nWAXS), meanwhile, stably cycled over 500 h in Li/SN-PEO/Li cell. Apart from the observation of lithium dendrite, the robust SEI layer formation mechanism in the first cycle is investigated in the SN-enhanced composite electrolyte by nWAXS. The inorganic electrochemical reaction products, LiF and Li3N, are found to initially deposit on the electrolyte side, progressively extending toward the lithium metal anode. This growth process effectively protected the metallic lithium, inhibited electron transfer, and facilitated Li⁺ transport. The study not only demonstrates a high-performance interfacial-stable lithium metal battery with composite electrolyte but also introduces a novel strategy for real-time visualizing dendrite formation and SEI growth directing at the interface area of electrolyte and metallic lithium.

Polymer electrolytes (PEs) with high flexibility, low cost, and excellent interface compatibility have been considered as an ideal substitute for traditional liquid electrolytes for high safety lithium metal batteries (LMBs). Nevertheless, the mechanical strength of PEs is generally poor to prevent the growth of lithium dendrites during the charge/discharge process, which seriously restricts their wide practical applications. Herein, a mechanical robust ZIF-8/epoxy composite electrolyte with unique pore structure was prepared, which effectively inhibited the growth of lithium dendrites. Meanwhile, the in situ growth of ZIF-8 in porous epoxy matrix can promote the uniform flux and fast transport of lithium ions. Ultimately, the optimal electrolyte shows high ionic conductivity (2.2 × 10−3 S cm−1), wide electrochemical window (5 V), and a large Li+ transference number (0.70) at room temperature. The Li||NCM811 cell using the optimal electrolyte exhibits high capacity and excellent cycling performance (83.2% capacity retention with 172.1 mA h g−1 capacity retained after 200 cycles at 0.2 C). These results indicate that the ZIF-8/epoxy composite electrolyte is of great promise for the application in LMBs.

<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.

Poly(ethylene oxide) (PEO)-based solid-state polymer electrolytes have emerged as one of the most promising candidates for high-energy-density lithium-metal batteries in energy storage applications. However, their notably low ionic conductivity has significantly hindered PEO-based polymer electrolytes' practical application and development, particularly at room temperature (RT). To address the critical challenge of low RT ionic conductivity in PEO-based solid-state electrolytes, this study develops a composite solid electrolyte (CSE) synergistically reinforced by high-concentration lithium salts and Al2O3 filler. The "polymer-in-salt" strategy disrupts PEO crystallinity via concentrated Li salt to form continuous ion channels, while Al2O3 further suppresses crystallization and bridges ionic cluster transport. This dual optimization achieves an ultrahigh ionic conductivity of 9.62 × 10-4 S cm-1 and a 4.8 V electrochemical stability window at RT. The Li symmetric cell exhibits stable cycling for 1000 h at 1 mA cm-2/1 mAh cm-2, and the all-solid-state Li//LiFePO4 coin cell retains 70.7% capacity after 300 cycles at 0.1C. Moreover, Li//LiFePO4 pouch cell maintains 96.4% capacity over 100 cycles at RT. This work establishes a new paradigm for material design and performance enhancement in practical solid-state lithium batteries.

Composite polymer electrolyte with three-dimensional ion transport channels constructed by NaCl template for solid-state lithium metal batteries

The lithium metal battery is strongly considered to be one of the most promising candidates for high-energy-density energy storage devices in our modern and technology-based society. However, uncontrollable lithium dendrite growth induces poor cycling efficiency and severe safety concerns, dragging lithium metal batteries out of practical applications. This review presents a comprehensive overview of the lithium metal anode and its dendritic lithium growth. First, the working principles and technical challenges of a lithium metal anode are underscored. Specific attention is paid to the mechanistic understandings and quantitative models for solid electrolyte interphase (SEI) formation, lithium dendrite nucleation, and growth. On the basis of previous theoretical understanding and analysis, recently proposed strategies to suppress dendrite growth of lithium metal anode and some other metal anodes are reviewed. A section dedicated to the potential of full-cell lithium metal batteries for practical applications is included. A general conclusion and a perspective on the current limitations and recommended future research directions of lithium metal batteries are presented. The review concludes with an attempt at summarizing the theoretical and experimental achievements in lithium metal anodes and endeavors to realize the practical applications of lithium metal batteries.

Polymer electrolytes (PEs) have emerged as alternatives to conventional liquid electrolytes in lithium metal batteries (LMBs) by addressing critical safety and performance challenges, including flame hazards, dendrite growth, and interfacial instability. While oxygen-based PEs, such as poly(ethylene oxide), have been widely investigated, their limited ionic conductivity and susceptibility to electrochemical degradation under high-voltage operation have prompted the exploration of heteroatom-functionalized PEs. Incorporating heteroatoms significantly enhances ion transport, electrochemical stability, and mechanical properties─all critical factors for robust and reliable LMB performance. This review focuses on fluorine-, nitrogen-, and boron-functionalized PEs, highlighting their distinctive features and design strategies for LMB applications. Recent developments and key design principles are summarized in the context of high-performance PEs for safer and more reliable LMBs for advanced energy storage applications.

Lithium metal batteries are widely considered to be a next-generation battery technology due to their high energy density, fast charging capability, and potential to power a wide range of applications, from portable electronics to electric vehicles.Compared to conventional lithium-ion batteries, which use graphite as the anode material, lithium metal batteries have the potential to offer significantly higher energy density. This is because lithium metal has a much higher theoretical capacity than graphite, meaning it can store more energy per unit weight or volume. As a result, lithium metal batteries could potentially offer longer run times and smaller form factors than lithium-ion batteries, which are already widely used in consumer electronics and electric vehicles.In addition to their high energy density, lithium metal batteries also offer fast charging capabilities. This is because the lithium metal anode can be charged more quickly than the graphite anode used in conventional lithium-ion batteries. This could enable faster charging times for electric vehicles and other applications, reducing the time required for recharging and improving their overall usability.However, one of the major challenges with lithium metal batteries is dendrite growth, which can occur when lithium metal ions deposit unevenly on the surface of the anode during charging. These dendrites are needle-like structures that can penetrate the separator and cause short-circuits, leading to reduced battery performance and safety issues. As a result, much of the research on lithium metal batteries has focused on developing strategies to prevent dendrite growth, such as using coatings or electrolytes with additives that can stabilize the lithium metal anode.Despite these challenges, lithium metal batteries continue to be a promising next-generation battery technology. Researchers are working on improving their energy density, charging speed, and safety, and they are exploring a range of applications, from portable electronics to electric vehicles and grid-scale energy storage. If these challenges can be overcome, lithium metal batteries could revolutionize the energy storage industry and enable a wide range of new and exciting applications.To address this issue, we have explored the use of various coatings on the lithium metal anode to prevent dendrite growth. In this study, ordered mesoporous silica with different pore sizes and pore structures, such as SBA-15 and KIT-6, were used as coatings between the lithium metal anode and separator.SBA-15 and KIT-6 are two types of mesoporous silica materials that are widely used in various applications due to their unique properties. The synthesis of these materials involves the use of templates to control the pore size and structure.They are synthesized using a combination of a surfactant(pluronic P123 or cetyltrimethylammonium bromide;CTAB) as the template and tetraethyl orthosilicate(TEOS) as the silica source. The synthesis involves mixing a solution of surfactant and TEOS in acidic conditions, followed by aging and calcination steps. The resulting materials have a highly ordered hexagonal pore structure(SBA-15) and body-centered cubic pore structure(KIT-6) with a pore size ranging from 5 to 20 nm.By coating the surface of the lithium metal with a uniform pore structure material such as ordered mesoporous silica, dendrite growth can be suppressed and lithium deposition can occur more uniformly.The uniform pore structure material acts as a physical barrier to lithium ion diffusion and helps to control the deposition of lithium ions onto the surface of the lithium metal. When lithium ions are deposited onto the surface of the lithium metal, they can form a layer of lithium that can grow and become thick over time. If the lithium layer is too thick, it can become unstable and form dendrites. However, the uniform pore structure material can help to limit the thickness of the lithium layer and promote the deposition of lithium ions in a more uniform manner, thus reducing the likelihood of dendrite formation.Additionally, the uniform pore structure material can help to reduce the concentration of lithium ions at the surface of the lithium metal, which is another factor that can contribute to dendrite growth. By limiting the concentration of lithium ions at the surface, the uniform pore structure material can help to prevent the formation of high-concentration regions that can promote dendrite growth.Overall, the mechanism by which a uniform pore structure material can suppress dendrite growth involves controlling the deposition of lithium ions onto the surface of the lithium metal, limiting the thickness of the lithium layer, and reducing the concentration of lithium ions at the surface. These factors help to promote more uniform lithium deposition and prevent the formation of dendrites, leading to improved battery performance and safety.

Poly(ethylene oxide)-based composite gel polymer electrolyte is widely used in lithium metal batteries to address dendrite growth and side reactions. However, the low oxidative decomposition potential (<4.0V) of poly(ethylene oxide) limits the cyclic stability with Ni-rich layered cathodes. What's more, poor interface compatibility between fillers and polymer severely deteriorates lithium-ion pathways, which cannot achieve lithium metal batteries with high-load cathode. Herein, polyether monomers coordinate with aluminum ethoxide nanowires via in situ ultraviolet curing, stabilizing the lone pair electrons of ethereal oxygen atoms and suppressing oxidative degradation. This coordination also forms abundant and tight interfaces as the predominant lithium-ion conduction pathways, contributing to ordered lithium-ion fluxes and dendrite-free deposition on the lithium anode. In addition, a robust solid electrolyte interphase containing aluminum-based species enhances the interfacial stability of lithium anode. Meanwhile, the good compatibility between the electrolyte and the cathode effectively suppresses side reactions and contributes to the structural stabilization of the cycled cathode. The delicate design allows the Li||LiNi0.6Co0.2Mn0.2O2 cells to present excellent cycling stability from -20°C to 60°C. Specially, cells with 8.8mg cm-2 cathode cycle stably for over 120 cycles. This molecular structure engineering will greatly promote the practical application of solid-state lithium metal batteries.

Insufficient ionic conductivity, limited Li+ transfer number (tLi+), and narrow electrochemical windows have heavily restricted the actual application of PEO （poly(ethylene) oxide）-based polymer electrolytes (PEs). Herein, a novel deep eutectic solvent (DES)-based PEO PE for stabilizing high voltage lithium metal battery (LMB) is designed. The DES reduces the crystallinity of PEO while promoting the dissociation of LiTFSI to release more free Li+, thus facilitating the transport of Li+ in the PEO matrix. In addition, the interaction between DES and the PEO, thereby improving the stability of the PEO-based PE under high voltage. Consequently, the PEO-DES-FEC (for short PDF) PE possesses satisfactory ionic conductivity, good mechanical properties, and high electrochemical stability. Meanwhile, PDF PE can build a robust/uniform LiF-rich solid electrolyte interface (SEI) to ensure electrode/electrolyte interface stability. As a concept proof, the Li symmetrical battery and Li||LiFePO4 LMBs of PDF PE exhibit good cycle stability. Applied to the high voltage Li||NCM811 LMBs, the PDF PE-based cell has excellent cycling performance at 4.3 and 4.5V. This tactic is one of the successful demonstrations of PEO-based electrolytes under 4.5V high voltage conditions, which breaks through the voltage constraint of conventional PEO-based PEs.

Discontinuous and uneven Li+ flux leads to inhomogeneous reactions, accelerating lithium (Li) dendrite growth and reducing the utilization of active materials, which severely impacts the performance of lithium metal batteries (LMBs). To address this challenge, we propose an effective homogeneous reaction design facilitated by an all-aligned nanofibrous architecture, which establishes continuous, uniform, and rapid Li+ pathways throughout the battery. This design enhances Li+ diffusion dynamics and ensures a uniform distribution of current density, hence promoting homogeneous Li nucleation at the anode and efficient Li+ insertion/extraction at the cathode. Moreover, the architecture exhibits superior mechanical strength and flexibility, maintaining structural stability during long-term cycling and suppressing dendrite growth, thereby minimizing the risk of short circuits. As a result, LMBs incorporating this homogeneous reaction design exhibit exceptional electrochemical performance. This work provides valuable insights into the design of homogeneous reactions for high-performance LMBs.

Compounding of suitable fillers with PEO-based polymers is the key to forming high-performance electrolytes with robust network structures and homogeneous Li+-transport channels. In this work, we innovatively and efficiently prepared Al2O3 nanofibers and deposited an aqueous dispersion of Al2O3 into a membrane via vacuum filtration to construct a nanofiber membrane with a three-dimensional (3D) network structure as the backbone of a PEO-based solid-state electrolyte. The supporting effect of the nanofiber network structure improved the mechanical properties of the reinforced composite solid-state electrolyte and its ability to inhibit the growth of Li dendrites. Meanwhile, interconnected nanofibers in the PEO-based electrolyte and the strong Lewis acid-base interactions between the chemical groups on the surface of the inorganic filler and the ionic species in the PEO matrix provided facilitated pathways for Li+ transport and regulated the uniform deposition of Li+. Moreover, the interaction between Al2O3 and lithium salts as well as the PEO polymer increased free Li+ concentration and maintained its stable electrochemical properties. Hence, assembled Li/Li symmetric cells achieved a cycle life of more than 2000 h. LFP/Li and NMC811/Li cells provided high discharge specific capacities of up to 146.9 mA h g-1 (0.5C and 50 °C) and 166.9 mA h g-1 (0.25C and 50 °C), respectively. The prepared flexible self-supporting 3D nanofiber network structure construction can provide a simple and efficient new strategy for the exploitation of high-performance solid-state electrolytes.

Cell failures of all-solid-state lithium metal batteries with inorganic solid electrolytes: Lithium dendrites

Fumaronitrile-fixed in-situ gel polymer electrolyte balancing high safety and superior electrochemical performance for Li metal batteries

Enhancing ionic conductivity and expanding the electrochemical window in polymer electrolytes via ferroelectric-metal-organic-frameworks to manipulate charge spatial distribution.