Solvent-Free Method Prepared a Sandwich-like Nanofibrous Membrane-Reinforced Polymer Electrolyte for High-Performance All-Solid-State Lithium Batteries

Phase-Transition-Promoted Interfacial Anchoring of Sulfide Solid Electrolyte Membranes for High-Performance All-Solid-State Lithium Battery

Solvent-free manufacturing is crucial for fabricating high-performance sulfide-electrolyte-based all-solid-state lithium batteries (ASSLBs), with advantages including side reaction inhibition, less contamination, and practical scalability. However, the fabricated sulfide electrolytes commonly suffer from brittleness, limited ion transport, and unsatisfactory interfacial stability due to the uncontrolled dispersion of the sulfide particles within the polymer binder matrix. Herein, a "solid-to-liquid" phase transition strategy is reported to fabricate flexible Li6PS5Cl (LPSCl) electrolytes. The polycaprolactone (PCL)-based binder (PLI) with phase-transition characteristics fills the gap of LPSCl particles and tightly grafts on the particle surface via ion-dipole interaction, bringing a thin and compact electrolyte membrane (80 µm). The simultaneously high Li-ion conducting and electron insulating nature of PLI binder facilitates Li-ion transport and ensures good interfacial stability between electrolyte and anode. Consequently, the sulfide electrolyte membrane exhibits high ionic conductivity (8.5 × 10-4 S cm-1), enabling symmetric and full cells with 10 and 2.5 times longer cycling life compared with that of the cells with pristine LPSCl electrolyte, respectively. The demonstrated strategy is versatile and can be extended to ethylene vinyl acetate copolymer (EVA) that also brings enhanced electrochemical performance. The thin sulfide electrolyte with high interfacial stability potentially facilitates dendrite-free ASSLBs with high energy density.

Building Flame-Retardant Polymer Electrolytes via Microcapsule Technology for Stable Lithium Batteries

Flame retardants could improve the safety properties of lithium batteries (LBs) with the sacrifice of electrochemical performance due to parasitic reactions. To concur with this, we designed thermal-response clothes for hexachlorophosphazene (HCP) additives by the microcapsule technique with urea-formaldehyde (UF) resin as the shell. HCP@UF combines with polyacrylonitrile (PAN) by hydrogen bonds successfully to form PAN-HCP@UF as the flame-retardant solid polymer electrolyte. The hydrogen bonds ensure excellent mechanical properties of the polymer electrolyte. The multiscale free radical-annihilating agent HCP effectively eliminates hydrogen free radicals of electrolytes under high temperature, showing excellent flame retardation. During the operation of the battery, functional groups on the UF resin act as active sites to promote the migration of lithium ions, while the internal HCP is protected from electrochemical reaction. With 25% HCP@UF addition, the limiting oxygen index of the PAN-HCP@UF increases to 28% and the Li+ transfer number up to 0.80. By UF protection, the initial capacity retention rate of the Li||LFP battery that assembles with PAN-HCP@UF is 88.8% after 500 cycles at 0.5 C. Thus, the microcapsule-encapsulated approach is deemed to provide an innovative strategy to prepare high-safety solid-state LB with a stable long cycle life.

Polyaspartate Polyurea-Based Solid Polymer Electrolyte with High Ionic Conductivity for the All-Solid-State Lithium-Ion Battery

The existing in situ preparation methods of solid polymer electrolytes (SPEs) often require the use of a solvent, which would lead to a complicated process and potential safety hazards. Therefore, it is urgent to develop a solvent-free in situ method to produce SPEs with good processability and excellent compatibility. Herein, a series of polyaspartate polyurea-based SPEs (PAEPU-based SPEs) with abundant (PO)_x(EO)_y(PO)_z segments and cross-linked structures were developed by systematically regulating the molar ratios of isophorone diisocyanate (IPDI) and isophorone diisocyanate trimer (tri-IPDI) in the polymer backbone and LiTFSI concentrations via an in situ polymerization method, which gave rise to good interfacial compatibility. Furthermore, the in situ-prepared PAEPU-SPE@D₁₅ based on the IPDI/tri-IPDI molar ratio of 2:1 and 15 wt % LiTFSI exhibits an improved ionic conductivity of 6.80 × 10^-5 S/cm at 30 °C and could reach 10^-4 orders of magnitude when the temperature was above 40 °C. The Li|LiFePO₄ battery based on PAEPU-SPE@D₁₅ had a wide electrochemical stability window of 5.18 V, demonstrating a superior interface compatibility toward LiFePO₄ and the lithium metal anode, exhibited a high discharge capacity of 145.7 mAh g^-1 at the 100th cycle and a capacity retention of 96.8%, and retained a coulombic efficiency of above 98.0%. These results showed that the PAEPU-SPE@D₁₅ system displayed a stable cycle performance, excellent rate performance, and high safety compared with PEO systems, indicating that the PAEPU-based SPE system may play a crucial role in the future.

Interface Engineering of All-Solid-State Batteries Based on Inorganic Solid Electrolytes

All-solid-state batteries (ASSBs) based on inorganic solid electrolytes (SEs) are one of the most promising strategies for next-generation energy storage systems and electronic devices due to the higher energy density and intrinsic safety. However, the poor solid-solid contact and restricted chemical/electrochemical stability of inorganic SEs both in cathode and anode SE interfaces cause contact failure and the degeneration of SEs during prolonged charge-discharge processes. As a result, the increasing interface resistance significantly affects the coulombic efficiency and cycling performance of ASSBs. Herein, we present a fundamental understanding of physical contact and chemical/electrochemical features of ASSB interfaces based on mainstream inorganic SEs and summarize the recent work on interface modification. SE doping, optimizing morphology, introducing interlayer/coating layer, and utilizing compatible electrode materials are the key methods to prevent side reactions, which are discussed separately in cathode/anode-SE interface. We also highlight the constant extra stack pressure applied during ASSB cycling, which is important to the electrochemical performance. Finally, our perspectives on interface modification for practical high-performance ASSBs are put forward.

Defects-Abundant Ga2 O3 Nanobricks Enabled Multifunctional Solid Polymer Electrolyte for Superior Lithium-Metal Batteries

Polyethylene oxide (PEO)-based polymer electrolytes with good flexibility and viscoelasticity, low interfacial resistance, and fabricating cost have caught worldwide attention, but their practical application is still hampered by the instability at high voltages and the low ionic conductivity (10^-8 to 10^-6  S cm^-1 ). Herein, we rationally designed defects-abundant Ga₂ O₃ nanobricks as multifunctional fillers and constructed a PEO-based organic-inorganic electrolyte for lithium metal batteries. Due to the abundant O-defects feature of Ga₂ O₃ filler, this PEO-based composite electrolyte not only broadens electrochemical stability window (over 5.3 V versus Li/Li⁺ ) but also in situ forms a Li-Ga alloy and solid electrolyte interphase (SEI) film during the cycling process causing a rapid diffusion of Li⁺ ions. The as-prepared electrolyte has good interface compatibility with Li metal (without short-circuiting over 500 h at 0.2 mA cm^-2 ) and possesses superior high ionic conductivity. The assembled all-solid-state LiFePO₄ //Li cells attained an excellent cycling performance of 146 mAh g^-1 over 100 cycles at 0.5 C. The XPS analysis reveals that Ga₂ O₃ nanobricks can form in situ a Li-Ga alloy layer at the polymer/anode interface. This work shed a light on designing high ionic conductivity lithium alloys in the composite electrolyte, which can improve the electrochemical properties of PEO-based polymer electrolytes.

Advances in Nanofibrous Materials for Stable Lithium-Metal Anodes

Lithium metal is regarded as the most potential anode material for improving the energy density of batteries due to its high specific capacity and low electrode potential. However, the practical application of lithium-metal anodes (LMAs) still faces severe challenges such as uncontrollable dendrites growth and large volume expansion. The development of functional nanomaterials has brought opportunities for the revival of LMAs. Among them, nanofibrous materials show great application potential for LMAs protection due to their distinct functional and structural features. Here, the latest research progress in nanofibrous materials for LMAs is systematically outlined. First, the problems existing in the practical application of LMAs are analyzed. Then, prospective strategies and recent research progress toward stable LMAs based on nanofibrous materials are summarized from the aspects of artificial protective layers, three-dimensional frameworks, separators, and solid-state electrolytes. Finally, the future development of nanofibrous materials for the protection of lithium-metal batteries is summarized and prospected. This review establishes a close connection between nanofibrous materials and LMA modification and provides insight for the development of high-safety lithium-metal batteries.

Thermal Stability and the Effect of Water on Hydrogen Fluoride Generation in Lithium-Ion Battery Electrolytes Containing LiPF6

Lithium-ion batteries (LIBs) have been used as electrochemical energy storage devices in various fields, ranging from mobile phones to electric vehicles. LIBs are composed of a positive electrode, a negative electrode, an electrolyte, and a binder. Among them, electrolytes consist of organic solvents and lithium ion conducting salts. The electrolytes used in LIBs are mostly linear and cyclic alkyl carbonates. These electrolytes are usually based on their combinations to allow the use of Li as the anodic active component, resulting in the high power and energy density of batteries. However, these organic electrolytes have high volatility and flammability that pose a serious safety issue when exposed to extreme conditions such as elevated temperatures. At that time, these electrolytes can react with active electrode materials and release a considerable amount of heat and gas. In this study, a simultaneous thermal analysis-mass spectrometry analysis was performed on six different organic solvents to examine the effect of water on hydrogen fluoride (HF) generation temperature in the electrolyte of a LIB. The electrolytes used in the experiment were anhydrous diethyl carbonate, 1,2-dimethoxyethane, ethylene carbonate, 1,3-dioxolane, tetrahydrofurfuryl alcohol, and 2-methyl-tetrahydrofuran, each containing LiPF6. The HF formation temperature was observed and compared with that when water entered the electrolyte exposed to high-temperature conditions such as fire.

Synergistically reinforced poly(ethylene oxide)-based composite electrolyte for high-temperature lithium metal batteries

Traditional liquid lithium-ion batteries are not applicable for extreme temperatures, due to the shrinkage of separators and volatility of electrolytes. It is necessary to develop advanced electrolytes with desirable characteristics in terms of thermal stability, electrochemical stability and mechanical properties. Solid-state electrolytes, such as polyethylene oxide (PEO), outperform other types and bring the opportunity to realize the high-temperature lithium-ion batteries. However, the softness of PEO at elevated temperatures leads to battery failure. In this work, a three-dimensional fiber-network-reinforced PEO-based composite polymer electrolyte is prepared. The introduced polyimide (PI) framework and trimethyl phosphate (TMP) plasticizer decrease the crystallinity of PEO and increase the ionic conductivity at 30 °C from 8.79 × 10^-6 S cm^-1 to 4.70 × 10^-5 S cm^-1. In addition, the PEO bonds tightly with PI fiber network, improving both the mechanical strength and thermal stability of the prepared electrolyte. With the above strategies, the working temperature range of the PEO-based electrolytes is greatly expanded. The LiFePO₄/Li cell assembled with the PI-PEO-TMP electrolyte stably performs over 300 cycles at 120 °C. Even at 140 °C, the cell still survives 80 cycles. These excellent performances demonstrate the potential application of the PI-PEO-TMP electrolyte in developing safe and high-temperature lithium batteries.

In Situ Construction a Stable Protective Layer in Polymer Electrolyte for Ultralong Lifespan Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries (SLMBs) are attracting enormous attention due to their enhanced safety and high theoretical energy density. However, the alkali lithium with high reducibility can react with the solid-state electrolytes resulting in the inferior cycle lifespan. Herein, inspired by the idea of interface design, the 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide as an initiator to generate an artificial protective layer in polymer electrolyte is selected. Time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy reveal the stable solid electrolyte interface (SEI) is in situ formed between the electrolyte/Li interface. Scanning electron microscopy (SEM) images demonstrate that the constructed SEI can promote homogeneous Li deposition. As a result, the Li/Li symmetrical cells enable stable cycle ultralong-term for over 4500 h. Moreover, the as-prepared LiFePO4 /Li SLMBs exhibit an impressive ultra-long cycle lifespan over 1300 cycles at 1 C, as well as 1600 cycles at 0.5 C with a capacity retention ratio over 80%. This work offers an effective strategy for the construction of the stable electrolyte/Li interface, paving the way for the rapid development of long lifespan SLMBs.

Single-Ion-Conducting "Polymer-in-Ceramic" Hybrid Electrolyte with an Intertwined NASICON-Type Nanofiber Skeleton

The fast Li⁺ transportation of "polymer-in-ceramic" electrolytes is highly dependent on the long-range Li⁺ migration pathways, which are determined by the structure and chemistry of the electrolytes. Besides, Li dendrite growth may be promoted in the soft polymer region due to the inhomogeneous electric field caused by the commonly low Li⁺ transference number of the polymer. Herein, a single-ion-conducting polymer electrolyte is infiltrated into intertwined Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) nanofibers to construct free-standing electrolyte membranes. The composite electrolyte possesses a large electrochemical window exceeding 5 V, a high ionic conductivity of 0.31 mS cm^-1 at ambient temperature, and an extraordinary Li⁺ transference number of 0.94. The hybrid electrolyte in the lithium symmetric cell shows stable Li plating/stripping up to 2000 h under 0.1 mA cm^-2 without dendrite formation. The Li|hybrid electrolyte|LiFePO₄ battery exhibits enhanced rate capability up to 1 C and a stable cycling performance with an initial discharge capacity of 131.8 mA h g^-1 and a retention capacity of 122.7 mA h g^-1 after 500 cycles at 0.5 C at ambient temperature. The improved electrochemical performance is attributed to the synergistic effects of the LATP nanofibers and the single-ion-conducting polymer. The fibrous fast ion conductors provide continuous ion transport channels, and the polymer improves the interfacial contact with the electrodes and helps to suppress the Li dendrites.

A novel polymer electrolyte with high elasticity and high performance for lithium metal batteries

A polymer electrolyte with high elasticity and high performance is prepared by IN SITU polymerization. The polymer electrolyte is amorphous and has a high ionic conductivity of 7.9 × 10^-4 S cm^-1 and good elasticity. The discharge capacity of Li/LiFePO₄ in the 100th cycle is 133.90 mA h g^-1 (0.5C, 25 °C).

Lithium-Rich Anti-perovskite Li2OHBr-Based Polymer Electrolytes Enabling an Improved Interfacial Stability with a Three-Dimensional-Structured Lithium Metal Anode in All-Solid-State Batteries

All-solid-state lithium-metal batteries, with their high energy density and high-level safety, are promising next-generation energy storage devices. Their current performance is however compromised by lithium dendrite formation. Although using 3D-structured metal-based electrode materials as hosts to store lithium metal has the potential to suppress the lithium dendrite growth by providing a high surface area with lithiophilic sites, their rigid and ragged interface with solid-state electrolytes is detrimental to the battery performance. Herein, we show that Li₂OHBr-containing poly(ethylene oxide) (PEO) polymer electrolytes can be used as a flexible solid-state electrolyte to mitigate the interfacial issues of 3D-structured metal-based electrodes and suppress the lithium dendrite formation. The presence of Li₂OHBr in a PEO matrix can simultaneously improve the mechanical strength and lithium ion conductivity of the polymer electrolyte. It is confirmed that Li₂OHBr does not only induce the PEO transformation of a crystalline phase to an amorphous phase but also serves as an anti-perovskite superionic conductor providing additional lithium ion transport pathways and hence improves the lithium ion conductivity. The good interfacial contact and high lithium ion conductivity provide sufficient lithium deposition sites and uniform lithium ion flux to regulate the lithium deposition without the formation of lithium dendrites. Consequently, the Li₂OHBr-containing PEO polymer electrolyte in a lithium-metal battery with a 3D-structured lithium/copper mesh composite anode is able to improve the cycle stability and rate performance. The results of this study provide the experimental proof of the beneficial effects of the Li₂OHBr-containing PEO polymer electrolyte on the 3D-structured lithium metal anode.

Constructing Li-Rich Artificial SEI Layer in Alloy-Polymer Composite Electrolyte to Achieve High Ionic Conductivity for All-Solid-State Lithium Metal Batteries

To achieve high ionic conductivity for solid electrolyte, an artificial Li-rich interface layer of about 60 nm thick has been constructed in polymer-based poly(ethylene oxide)-lithium bis(trifluoromethanesulfonyl)imide composite solid electrolyte (briefly noted as PEO_m ) by adding Li-based alloys. As revealed by high-resolution transmission electron microscopy and electron energy loss spectroscopy, an artificial interface layer of amorphous feature is created around the Li-based alloy particles with the gradient distribution of Li across it. Electrochemical analysis and theoretical modeling demonstrate that the interface layer provides fast ion transport path and plays a key role in achieving high and stable ionic conductivity for PEO_m -Li₂₁ Si₅ composite solid electrolyte. The PEO_m -5%Li₂₁ Si₅ composite electrolyte exhibits an ionic conductivity of 3.9 × 10^-5  S cm^-1 at 30 °C and 5.6 × 10^-4  S cm^-1 at 45 °C. The LiFePO₄ | PEO_m -5%Li₂₁ Si₅ | Li all-solid-state batteries could maintain a stable capacity of 129.2 mA h g^-1 at 0.2 C and 30 °C after 100 cycles, and 111.3 mA h g^-1 after 200 cycles at 0.5 C and 45 °C, demonstrating excellent cycling stability and high-rate capability.

Three-Dimensional Porous Alginate Fiber Membrane Reinforced PEO-Based Solid Polymer Electrolyte for Safe and High-Performance Lithium Ion Batteries

The rational design and optimization of solid polymer electrolytes (SPEs) are critical for the application of safety and high efficiency lithium ion batteries (LIBs). Herein, we synthesized a novel poly(ethylene oxide) (PEO)-based SPE (PEO@AF SPE) with a cross-linking network by the introduction of alginate fiber (AF) membranes. Depending on the high-strength supporting AF skeleton and the cross-linking network formed by hydrogen bonds between the PEO matrix and AF skeleton, the obtained PEO@AF SPE exhibits an excellent tensile strength of 3.71 MPa, favorable heat resistance (close to 120 °C), and wide electrochemical stability window (5.2 V vs Li/Li⁺). Meanwhile, the abundant oxygen-containing groups in alginate macromolecular and the three-dimensional (3D) porous structure of the AF membrane can greatly increase Li⁺ anchor points and provide more Li⁺ migration pathways, leading to the enhancement of Li⁺ conduction and interfacial stability between the SPE and Li anode. Furthermore, the assembled LiFePO₄/PEO@AF SPE/Li cells also exhibit satisfactory electrochemical performance. These results reveal that PEO incorporating with AFs can boost the mechanical strength, thermostability, and electrochemical properties of the SPE simultaneously. Furthermore, one will expect that the newly designed PEO@AF SPE with cross-linked networks thus provides the possibility for future applications of safety and high-performance LIBs.