A Thermally Conductive Separator for Stable Li Metal Anodes

2D Hybrid Nanocomposite Materials (h-BN/G/MoS2) as a High-Performance Supercapacitor Electrode

The nanocomposites of hexagonal boron nitride, molybdenum disulfide, and graphene (h-BN/G/MoS2) are promising energy storage materials. The originality of the current work is the first-ever synthesis of 2D-layered ternary nanocomposites of boron nitrate, graphene, and molybdenum disulfide (h-BN/G/MoS2) using ball milling and the sonication method and the investigation of their applicability for supercapacitor applications. The morphological investigation confirms the well-dispersed composite material production, and the ternary composite appears to be made of h-BN and MoS2 wrapping graphene. The electrochemical characterization of the prepared samples is evaluated by cyclic voltammetry and galvanostatic charge/discharge tests. With a high specific capacitance of 392 F g-1 at a current density of 1 A g-1 and an outstanding cycling stability with around 96.4% capacitance retention after 10,000 cycles, the ideal 5% BN_G@MoS2_90@10 composite demonstrates exceptional capabilities. Furthermore, a symmetric supercapacitor (5% BN_G@MoS2_90@10 composite) exhibits a 94.1% capacitance retention rate even after 10,000 cycles, an energy density of 16.4 W h kg-1, and a power density of 501 W kg-1. The findings show that the preparation procedure is safe for the environment, manageable, and suitable for mass production, which is crucial for advancing the electrode materials used in supercapacitors.

Two-dimensional materials for high density, safe and robust metal anodes batteries

With a high specific capacity and low electrochemical potentials, metal anode batteries that use lithium, sodium and zinc metal anodes, have gained great research interest in recent years, as a potential candidate for high-energy-density storage systems. However, the uncontainable dendrite growth during the repeated charging process, deteriorates the battery performance, reduces the battery life and more importantly, raises safety concerns. With their unique properties, two-dimensional (2D) materials, can be used to modify various components in metal batteries, eventually mitigating the dendrite growth, enhancing the cycling stability and rate capability, thus leading to safe and robust metal anodes. In this paper, we review the recent advances of 2D materials and summarize current research progress of using 2D materials in the applications of (i) anode design, (ii) separator engineering, and (iii) electrolyte modifications by guiding metal ion nucleation, increasing ion conductivity, homogenizing the electric field and ion flux, and enhancing the mechanical strength for safe metal anodes. The 2D material modifications provide the ultimate solution for obtaining dendrite-free metal anodes, realizes the high energy storage application, and indicates the importance of 2D materials development. Finally, in-depth understandings of subsequent metal growth are lacking due to research limitations, while more advanced characterizations are welcome for investigating the metal deposition mechanism. The more facile and simplified preparation of 2D materials possess great prospects in high energy density metal anode batteries, and thus fulfils the development of EVs.

Enhancing Crystallization in Hybrid Perovskite Solar Cells Using Thermally Conductive 2D Boron Nitride Nanosheet Additive

Controlling crystallization and grain growth is crucial for realizing highly efficient hybrid perovskite solar cells (PSCs). In this work, enhanced PSC photovoltaic performance and stability by accelerating perovskite crystallization and grain growth via 2D hexagonal boron nitride (hBN) nanosheet additives incorporated into the active perovskite layer are demonstrated. In situ X-ray scattering and infrared thermal imaging during the perovskite annealing process revealed the highly thermally conductive hBN nanosheets promoted the phase conversion and grain growth in the perovskite layer by facilitating a more rapid and spatially uniform temperature rise within the perovskite film. Complementary structural, physicochemical, and electrical characterizations further showed that the hBN nanosheets formed a physical barrier at the perovskite grain boundaries and the interfaces with charge transport layers, passivating defects, and retarding ion migration. As a result, the power conversion efficiency of the PSC is improved from 17.4% to 19.8%, along with enhanced device stability, retaining ≈90% of the initial efficiency even after 500 h ambient air storage. The results not only highlight 2D hBN as an effective additive for PSCs but also suggest enhanced thermal transport as one of the pathways for improved PSC performance by 2D material additives in general.

Regulation of Li+ Diffusion via an Engineered Separator to Realize a Homogeneous Lithium Microstructure in Advanced Li-Metal Batteries

Lithium metal, the most promising anode material, is receiving increasing interest owing to its high theoretical capacity (3860 mA h g^-1) and low negative potential (-3.04 V vs. standard hydrogen electrode). However, the uneven Li dissolution/deposition behavior causes a degraded cycle stability and safety issues, thus seriously restricting the application of Li-metal batteries (LMBs). Separator modification is one of the most versatile and feasible approaches to overcome this problem. In this study, polypropylene (PP) separators are prepared and coated with an inert hexagonal boron nitride (h-BN) layer, which can provide sufficient ion transport channels and physical protection. The h-BN@PP separator exhibits a remarkable effect on the regulation of the diffusion and nucleation of Li⁺ to realize a homogeneous Li microstructure, thereby reducing the voltage polarization and improving the cycle performance of the battery. All LMBs equipped with the modified separators exhibit excellent cycling stabilities. The Li|Li symmetric cell exhibits a stable cycling for over 2300 h with a polarization voltage of 13 mV. In conclusion, the modified h-BN@PP separator has significant potential for stabilizing various Li metal anodes, which strongly promotes the applications of advanced LMBs.

Direct Ink Printing of PVdF Composite Polymer Electrolytes with Aligned BN Nanosheets for Lithium-Metal Batteries

The use of polymer electrolytes is of great interest for lithium-metal batteries (LMBs) due to their stability with lithium metal. However, the low thermal conductivity of polymer electrolytes poses a significant barrier to minimizing the formation of local hot spots during electrochemical reactions in lithium batteries that may lead to dendritic plating of Li or thermal runaway events. Electrolyte nanocomposites with proper distribution of thermally conductive nanomaterials offer an opportunity to address this shortcoming. Utilizing a custom-designed direct ink writing (DIW) process, we show that highly aligned boron nitride (BN) nanosheets can be embedded in poly(vinylidene fluoride-hexafluoropropylene) (PVdF) polymer composite electrolytes (CPE-BN), enabling novel architectural designs for safe Li-metal batteries. It is observed that the CPE-BN electrolytes possess a 400% increase in their in-plane thermal conductivity, which enables faster heat distribution in the CPE-BN electrolyte compared to the polymer electrolytes without BN nanosheets. The CPE-BN containing symmetric lithium cell exhibits stable Li plating/stripping for over 2000 cycles without short-circuiting due to the suppression of dendritic lithium. The lithium-ion half-cells made with the CPE-BN show stable cycling performance at 1C charge-discharge rate for 250 cycles with 90% capacity retention. This reported DIW-printed PVdF composite polymer electrolyte could be used as a model for developing new architectures for other electrolytes or electrodes, thus enabling new chemistry and improved performances in energy-storage devices.

Emerging Flexible Thermally Conductive Films: Mechanism, Fabrication, Application

Effective thermal management is quite urgent for electronics owing to their ever-growing integration degree, operation frequency and power density, and the main strategy of thermal management is to remove excess energy from electronics to outside by thermal conductive materials. Compared to the conventional thermal management materials, flexible thermally conductive films with high in-plane thermal conductivity, as emerging candidates, have aroused greater interest in the last decade, which show great potential in thermal management applications of next-generation devices. However, a comprehensive review of flexible thermally conductive films is rarely reported. Thus, we review recent advances of both intrinsic polymer films and polymer-based composite films with ultrahigh in-plane thermal conductivity, with deep understandings of heat transfer mechanism, processing methods to enhance thermal conductivity, optimization strategies to reduce interface thermal resistance and their potential applications. Lastly, challenges and opportunities for the future development of flexible thermally conductive films are also discussed.

Two Birds with One Stone: Using Indium Oxide Surficial Modification to Tune Inner Helmholtz Plane and Regulate Nucleation for Dendrite-free Lithium Anode

Lithium metal has been considered as the most promising anode material due to its distinguished specific capacity of 3860 mAh g^-1 and the lowest reduction potential of -3.04 V versus the Standard Hydrogen Electrode. However, the practicalization of Li-metal batteries (LMBs) is still challenged by the dendritic growth of Li during cycling, which is governed by the surface properties of the electrodepositing substrate. Herein, a surface modification with indium oxide on the copper current collector via magnetron sputtering, which can be spontaneously lithiated to form a composite of lithium indium oxide and Li-In alloy, is proposed. Thus, the growth of Li dendrites is effectively suppressed via regulating the inner Helmholtz plane modified with LiInO₂ to foster the desolvation of Li-ion and induce the nucleation of Li-metal in two-dimensions through electro-crystallization with Li-In alloy. Using the In₂ O₃ modification, the Li-metal anode exhibits outstanding cyclic stability, and LMBs with lithium cobalt oxide cathode present excellent capacity retention (above 80% over 600 cycles). Enlightening, the scalable magnetron sputtering method reported here paves a novel way to accelerate the practical application of the Li anode in LMBs to pursue higher energy density.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal-Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal-sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li-S and the emerging Na-S and K-S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li-S are relatively mature, less progress has been made with Na-S and K-S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal-selenium and the metal-selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal-sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li-S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure-battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

Lithium-Sulfur Batteries Meet Electrospinning: Recent Advances and the Key Parameters for High Gravimetric and Volume Energy Density

Lithium-sulfur (Li-S) batteries have been regarded as a promising next-generation energy storage technology for their ultrahigh theoretical energy density compared with those of the traditional lithium-ion batteries. However, the practical applications of Li-S batteries are still blocked by notorious problems such as the shuttle effect and the uncontrollable growth of lithium dendrites. Recently, the rapid development of electrospinning technology provides reliable methods in preparing flexible nanofibers materials and is widely applied to Li-S batteries serving as hosts, interlayers, and separators, which are considered as a promising strategy to achieve high energy density flexible Li-S batteries. In this review, a fundamental introduction of electrospinning technology and multifarious electrospinning-based nanofibers used in flexible Li-S batteries are presented. More importantly, crucial parameters of specific capacity, electrolyte/sulfur (E/S) ratio, sulfur loading, and cathode tap density are emphasized based on the proposed mathematic model, in which the electrospinning-based nanofibers are used as important components in Li-S batteries to achieve high gravimetric (WG ) and volume (WV ) energy density of 500 Wh kg-1 and 700 Wh L-1 , respectively. These systematic summaries not only provide the principles in nanofiber-based electrode design but also propose enlightening directions for the commercialized Li-S batteries with high WG and WV .

Multifunctional Separator Allows Stable Cycling of Potassium Metal Anodes and of Potassium Metal Batteries

This is the first report of a multifunctional separator for potassium-metal batteries (KMBs). Double-coated tape-cast microscale AlF₃ on polypropylene (AlF₃ @PP) yields state-of-the-art electrochemical performance: symmetric cells are stable after 1000 cycles (2000 h) at 0.5 mA cm^-2 and 0.5 mAh cm^-2 , with 0.042 V overpotential. Stability is maintained at 5.0 mA cm^-2 for 600 cycles (240 h), with 0.138 V overpotential. Postcycled plated surface is dendrite-free, while stripped surface contains smooth solid electrolyte interphase (SEI). Conventional PP cells fail rapidly, with dendrites at plating, and "dead metal" and SEI clumps at stripping. Potassium hexacyanoferrate(III) cathode KMBs with AlF₃ @PP display enhanced capacity retention (91% at 100 cycles vs 58%). AlF₃ partially reacts with K to form an artificial SEI containing KF, AlF₃ , and Al₂ O₃ phases. The AlF₃ @PP promotes complete electrolyte wetting and enhances uptake, improves ion conductivity, and increases ion transference number. The higher of K⁺ transference number is ascribed to the strong interaction between AlF₃ and FSI^- anions, as revealed through ¹⁹ F NMR. The enhancement in wetting and performance is general, being demonstrated with ester- and ether-based solvents, with K-, Na-, or Li- salts, and with different commercial separators. In full batteries, AlF₃ prevents Fe crossover and cycling-induced cathode pulverization.

Highly stable lithium anodes from recycled hemp textile

Three-dimensional lithium (Li) hosts have been shown to suppress the growth of Li dendrites for next generation Li metal batteries. Here, we report a cost-effective and scalable approach to produce highly stable Li composite anodes from industrial hemp textile waste. The hemp@Li composite anodes demonstrate stable cycling both in half and full cells.

Synergistic Effect of Hexagonal Boron Nitride-Coated Separators and Multi-Walled Carbon Nanotube Anodes for Thermally Stable Lithium-Ion Batteries

In this work, we report the development of separators coated with hexagonal boron nitride (hBN) to improve the thermal stability of Li-ion batteries (LIBs). Aiming to achieve a synergistic effect of separators and anodes on thermal stability and electrochemical performance, multiwalled carbon nanotubes (MWCNTs) were prepared via plasma-enhanced chemical vapor deposition (PECVD) method and used as potential anode materials for LIBs. The grown MWCNTs were well characterized by using various techniques which confirmed the formation of MWCNTs. The prepared MWCNTs showed a crystalline structure and smooth surface with a diameter of ~9–12 nm and a length of ~10 μm, respectively. Raman spectra showed the characteristic peaks of MWCNTs and BN, and the sharpness of the peaks showed the highly crystalline nature of the grown MWCNTs. The electrochemical studies were performed on the fabricated coin cell with a MWCNT anode using a pristine andBN-coated separators. The results show that the cell with the BN-coated separator in a conventional organic carbonate-based electrolyte and MWCNTs as the anode resulted in a discharge capacity (at 65 °C) of ~567 mAhg−1 at a current density of 100 mAg−1 for the first cycle, and delivered a capacity of ~471 mAhg−1 for 200 cycles. The columbic efficiency was found to be higher (~84%), which showed excellent reversible charge–discharge behavior as compared with the pristine separator (69%) after 200 cycles. The improved thermal performance of the LIBs with the BN-coated separator and MWCNT anode might be due to the greater homogeneous thermal distribution resulting from the BN coating, and the additional electron pathway provided by the MWCNTs. Thus, the fabricated cell showed promising results in achieving the stable operation of the LIBs even at higher temperatures, which will open a pathway to solve the practical concerns over the use of LIBs at higher temperatures without compromising the performance.

Gel polymer electrolytes with high performance based on a polyvinylidene fluoride composite with eco-friendly lignocellulose for lithium-ion batteries

A gel polymer electrolyte composed of polyvinylidene fluoride and lignocellulose regulates the transference of lithium ions.

Revealing initial nucleation of hexagonal boron nitride on Ru(0001) and Rh(111) surfaces by density functional theory simulations

The nucleation of h-BN on Ru(0001) and Rh(111) surfaces via an energy-driven process is systematically studied by density functional theory simulations.

Boron Nitride Nanotube-Based Separator for High-Performance Lithium-Sulfur Batteries

To prevent global warming, ESS development is in progress along with the development of electric vehicles and renewable energy. However, the state-of-the-art technology, i.e., lithium-ion batteries, has reached its limitation, and thus the need for high-performance batteries with improved energy and power density is increasing. Lithium-sulfur batteries (LSBs) are attracting enormous attention because of their high theoretical energy density. However, there are technical barriers to its commercialization such as the formation of dendrites on the anode and the shuttle effect of the cathode. To resolve these issues, a boron nitride nanotube (BNNT)-based separator is developed. The BNNT is physically purified so that the purified BNNT (p−BNNT) has a homogeneous pore structure because of random stacking and partial charge on the surface due to the difference of electronegativity between B and N. Compared to the conventional polypropylene (PP) separator, the p−BNNT loaded PP separator prevents the dendrite formation on the Li metal anode, facilitates the ion transfer through the separator, and alleviates the shuttle effect at the cathode. With these effects, the p−BNNT loaded PP separators enable the LSB cells to achieve a specific capacity of 1429 mAh/g, and long-term stability over 200 cycles.

Two-dimensional materials towards separator functionalization in advanced Li-S batteries

Functional separators have played important roles in improving the electrochemical performance of lithium-sulfur (Li-S) batteries by addressing the key issues of both the sulfur cathode and lithium anode. Compared with other materials that are used for separator functionalization, two-dimensional (2D) materials with atomic layer thickness and infinite lateral dimensions feature several advantages of ultra-thin laminate structure, remarkable physical properties and tunable surface chemistry, which show potential applications in separator functionalization towards addressing the issues of both the shuttle effect and formation of Li dendrites in Li-S batteries. In this review, the unique advantages of 2D materials for separator functionalization in Li-S batteries and their common construction methods are introduced. Then, recent progress and advances in the construction of 2D materials functional separators are summarized in detail towards inhibiting the shuttle effect of polysulfides and suppressing Li dendrite growth in Li-S batteries. Finally, some opportunities and challenges of 2D materials for constructing high-performance functional separators are proposed. We anticipate that this review will provide new insights into separator functionalization for developing advanced Li-S batteries.

Stabilized Solid Electrolyte Interphase Induced by Ultrathin Boron Nitride Membranes for Safe Lithium Metal Batteries

Lithium-ion batteries (LIBs) are still facing safety problems, mainly due to dendrite growth on the anode that leads to combustion and explosion. Forming a stable solid electrolyte interface (SEI) layer is an effective way to suppress this. To induce the formation of stable SEI using simple methods at a low cost, we report an ultrathin and large-scale hexagonal boron nitride (h-BN)/polyimide (PI) layer that was coated on a commercial polypropylene (PP) separator. The formation of a stabilized SEI component induced by the h-BN coating layer is proposed, as suggested by theoretical calculations and confirmed by electrochemical analysis and spectroscopy. It effectively suppresses Li dendrite growth and reduces the consumption of active lithium. The separator also has good electrolyte wettability, excellent mechanical strength and thermal conductivity, and high thermal stability. When using the h-BN modified separator in a full cell, the capacity is extremely stable after long cycling and high temperature.

Thermal-Responsive and Fire-Resistant Materials for High-Safety Lithium-Ion Batteries

As one of the most efficient electrochemical energy storage devices, the energy density of lithium-ion batteries (LIBs) has been extensively improved in the past several decades. However, with increased energy density, the safety risk of LIBs becomes higher too. The frequently occurred battery accidents worldwide remind us that safeness is a crucial requirement for LIBs, especially in environments with high safety concerns like airplanes and military platforms. It is generally recognized that the catastrophic thermal runaway (TR) event is the major cause of LIBs related accidents. Tremendous efforts have been devoted to coping with the TR concerns in LIBs, and thus enhance battery safety. This review first gives an introduction to the fundamentals of LIBs and the origins of safety issues. Then, the authors summarize the recent advances to improve the safety of LIBs with a unique focus on thermal-responsive and fire-resistant materials. Finally, a perspective is proposed to guide future research directions in this field. It is anticipated this review will stimulate inspiration and arouse extensive studies on further improvement in battery safety.

Functional polyethylene separator with impurity entrapment and faster Li+ ions transfer for superior lithium-ion batteries

An organic/inorganic hybrid coating consisting of molecular sieve (MS)/sulfonated melamine formaldehyde condensate (SMF) is fabricated on the polyethylene (PE) separator by a simple dip-coating process. The MS/SMF coating with high polarity enhances the electrolyte uptake of PE separator, and therefore favors higher ionic conductivity and Li⁺ transference number of separators. By regulating the ratio of MS/SMF, much higher Li⁺ transference number up to 0.5 can be obtained compared with the original PE separator (0.25). The PE separator possesses highest effective Li⁺ ionic conductivity when the ratio of MS/SMF is 3:1, which promotes more uniform Li⁺ deposition on the lithium metal surface for excellent lithium plating/stripping cycling stability up to 1000 h without any signs of short-circuit. Moreover, the functional PE separator possesses excellent H₂O and HF capturing ability due to strong adsorption of MS and SMF for H₂O and the scavenging of SMF for HF. The MS-SMF@PE separator-employed LiCoO₂/Li unit cell shows superior C-rates capability and cycling performance, and no obvious lithium dendrite growth is found on the surface of lithium metal anode after cycling.

Surface-Functionalized Separator for Stable and Reliable Lithium Metal Batteries: A Review

Metallic Li has caught the attention of researchers studying future anodes for next-generation batteries, owing to its attractive properties: high theoretical capacity, highly negative standard potential, and very low density. However, inevitable issues, such as inhomogeneous Li deposition/dissolution and poor Coulombic efficiency, hinder the pragmatic use of Li anodes for commercial rechargeable batteries. As one of viable strategies, the surface functionalization of polymer separators has recently drawn significant attention from industries and academics to tackle the inherent issues of metallic Li anodes. In this article, separator-coating materials are classified into five or six categories to give a general guideline for fabricating functional separators compatible with post-lithium-ion batteries. The overall research trends and outlook for surface-functionalized separators are reviewed.

Dendrite-Free and Stable Lithium Metal Battery Achieved by a Model of Stepwise Lithium Deposition and Stripping

A facile method is adopted to obtain cucumber-like lithiophilic composite skeleton. Massive lithiophilic sites in cucumber-like lithiophilic composite skeleton can promote and guide uniform Li depositions. A unique model of stepwise Li deposition and stripping is determined. The uncontrolled formation of lithium (Li) dendrites and the unnecessary consumption of electrolyte during the Li plating/stripping process have been major obstacles in developing safe and stable Li metal batteries. Herein, we report a cucumber-like lithiophilic composite skeleton (CLCS) fabricated through a facile oxidation-immersion-reduction method. The stepwise Li deposition and stripping, determined using in situ Raman spectra during the galvanostatic Li charging/discharging process, promote the formation of a dendrite-free Li metal anode. Furthermore, numerous pyridinic N, pyrrolic N, and Cu_xN sites with excellent lithiophilicity work synergistically to distribute Li ions and suppress the formation of Li dendrites. Owing to these advantages, cells based on CLCS exhibit a high Coulombic efficiency of 97.3% for 700 cycles and an improved lifespan of 2000 h for symmetric cells. The full cells assembled with LiFePO₄ (LFP), SeS₂ cathodes and CLCS@Li anodes demonstrate high capacities of 110.1 mAh g^-1 after 600 cycles at 0.2 A g^-1 in CLCS@Li|LFP and 491.8 mAh g^-1 after 500 cycles at 1 A g^-1 in CLCS@Li|SeS₂. The unique design of CLCS may accelerate the application of Li metal anodes in commercial Li metal batteries.

Constructing nitrided interfaces for stabilizing Li metal electrodes in liquid electrolytes

Traditional Li ion batteries based on intercalation-type anodes have been approaching their theoretical limitations in energy density. Replacing the traditional anode with metallic Li has been regarded as the ultimate strategy to develop next-generation high-energy-density Li batteries. Unfortunately, the practical application of Li metal batteries has been hindered by Li dendrite growth, unstable Li/electrolyte interfaces, and Li pulverization during battery cycling. Interfacial modification can effectively solve these challenges and nitrided interfaces stand out among other functional layers because of their impressive effects on regulating Li⁺ flux distribution, facilitating Li⁺ diffusion through the solid-electrolyte interphase, and passivating the active surface of Li metal electrodes. Although various designs for nitrided interfaces have been put forward in the last few years, there is no paper that specialized in reviewing these advances and discussing prospects. In consideration of this, we make a systematic summary and give our comments based on our understanding. In addition, a comprehensive perspective on the future development of nitrided interfaces and rational Li/electrolyte interface design for Li metal electrodes is included.

In this perspective, we make a systematic summary and give out our comments on constructing nitrided interfaces for stabilizing Li metal electrodes.

Sodium borohydride (NaBH4) as a high-capacity material for next-generation sodium-ion capacitors

Energy storage is an integral part of the modern world. One of the newest and most interesting concepts is the internal hybridization achieved in metal-ion capacitors. In this study, for the first time we used sodium borohydride (NaBH4) as a sacrificial material for the preparation of next-generation sodium-ion capacitors (NICs). NaBH4 is a material with large irreversible capacity of ca. 700 mA h g−1 at very low extraction potential close to 2.4 vs Na+/Na0. An assembled NIC cell with the composite-positive electrode (activated carbon/NaBH4) and hard carbon as the negative one operates in the voltage range from 2.2 to 3.8 V for 5,000 cycles and retains 92% of its initial capacitance. The presented NIC has good efficiency >98% and energy density of ca. 18 W h kg−1 at power 2 kW kg−1 which is more than the energy (7 W h kg−1 at 2 kW kg−1) of an electrical double-layer capacitor (EDLC) operating at voltage 2.7 V with the equivalent components as in NIC. Tin phosphide (Sn4P3) as a negative electrode allowed the reaching of higher values of the specific energy density 33 W h kg−1 (ca. four times higher than EDLC) at the power density of 2 kW kg−1, with only 1% of capacity loss upon 5,000 cycles and efficiency >99%.

Design of Robust, Lithiophilic, and Flexible Inorganic-Polymer Protective Layer by Separator Engineering Enables Dendrite-Free Lithium Metal Batteries with LiNi0.8 Mn0.1 Co0.1 O2 Cathode

As a promising candidate for the high energy density cells, the practical application of lithium-metal batteries (LMBs) is still extremely hindered by the uncontrolled growth of lithium (Li) dendrites. Herein, a facile strategy is developed that enables dendrite-free Li deposition by coating highly-lithiophilic amorphous SiO microparticles combined with high-binding polyacrylate acid (SiO@PAA) on polyethylene separators. A lithiated SiO and PAA (lithiated-SiO/PAA) protective layer with synergistic flexible and robust features is formed on the Li metal anode via the in situ reaction to offer outstanding interfacial stability during long-term cycles. By suppressing the formation of dead Li and random Li deposition, reducing the side reaction, and buffering the volume changes during the lithium deposition and dissolution, such a protective layer realizes a dendrite-free morphology of Li metal anode. Furthermore, sufficient ionic conductivity, uniform lithium-ion flux, and interface adaptability is guaranteed by the lithiated-SiO and Li polyacrylate acid. As a result, Li metal anodes display significantly enhanced cycling stability and coulombic efficiency in Li||Li and Cu||Li cells. When the composite separator is applied in a full cell with a carbonate-based electrolyte and LiNi_0.8 Mn_0.1 Co_0.1 O₂ cathode, it exhibits three times longer lifespan than control cell at current density of 5 C.

Thermoregulating Separators Based on Phase-Change Materials for Safe Lithium-Ion Batteries

Safety issues in lithium-ion batteries (LIBs) have aroused great interest owing to their wide applications, from miniaturized devices to large-scale storage plants. Separators are a vital component to ensure the safety of LIBs; they prevent direct electric contact between the cathode and anode while allowing ion transport. In this study, the first design is reported for a thermoregulating separator that responds to heat stimuli. The separator with a phase-change material (PCM) of paraffin wax encapsulated in hollow polyacrylonitrile nanofibers renders a wide range of enthalpy (0-135.3 J g^-1 ), capable of alleviating the internal temperature rise of LIBs in a timely manner. Under abuse conditions, the generated heat in batteries stimulates the melting of the encapsulated PCM, which absorbs large amounts of heat without creating a significant rise in temperature. Experimental simulation of the inner short-circuit in prototype pouch cells through nail penetration demonstrates that the PCM-based separator can effectively suppress the temperature rise due to cell failure. Meanwhile, a cell penetrated by a nail promptly cools down to room temperature within 35 s, benefiting from the latent heat-storage of the unique PCM separator. The present design of separators featuring latent heat-storage provides effective strategies for overheat protection and enhanced safety of LIBs.

Electroless Formation of a Fluorinated Li/Na Hybrid Interphase for Robust Lithium Anodes

Engineering a stable solid electrolyte interphase (SEI) is one of the critical maneuvers in improving the performance of a lithium anode for high-energy-density rechargeable lithium batteries. Herein, we build a fluorinated lithium/sodium hybrid interphase via a facile electroless electrolyte-soaking approach to stabilize the repeated plating/stripping of lithium metal. Jointed experimental and computational characterizations reveal that the fluorinated hybrid SEI mainly consisting of NaF, LiF, Li_xPO_yF_z, and organic components features a mosaic polycrystalline structure with enriched grain boundaries and superior interfacial properties toward Li. This LiF/NaF hybrid SEI exhibits improved ionic conductivity and mechanical strength in comparison to the SEI without NaF. Remarkably, the fluorinated hybrid SEI enables an extended dendrite-free cycling of metallic Li over 1300 h at a high areal capacity of 10 mAh cm^-2 in symmetrical cells. Furthermore, full cells based on the LiFePO₄ cathode and hybrid SEI-protected Li anode sustain long-term stability and good capacity retention (96.70% after 200 cycles) at 0.5 C. This work could provide a new avenue for designing robust multifunctional SEI to upgrade the metallic lithium anode.

Building Better Li Metal Anodes in Liquid Electrolyte: Challenges and Progress

Li metal has been widely recognized as a promising anode candidate for high-energy-density batteries. However, the inherent limitations of Li metal, that is, the low Coulombic efficiency and dendrite issues, make it still far from practical applications. In short, the low Coulombic efficiency shortens the cycle life of Li metal batteries, while the dendrite issue raises safety concerns. Thanks to the great efforts of the research community, prolific fundamental understanding as well as approaches for mitigating Li metal anode safety have been extensively explored. In this Review, Li electrochemical deposition behaviors have been systematically summarized, and recent progress in electrode design and electrolyte system optimization is reviewed. Finally, we discuss the future directions, opportunities, and challenges of Li metal anodes.

Functionalized Boron Nitride-Based Modification Layer as Ion Regulator Toward Stable Lithium Anode at High Current Densities

It is difficult to achieve higher energy density with the existing system of lithium (Li)-ion batteries. As a powerful candidate, Li metal batteries are in the renaissance. Unfortunately, the uncontrolled growth process of Li dendrites has limited their actual application. Hence, inhibiting the formation and spread of Li dendrites has become an enormous challenge. Herein, a novel composite separator is developed with functionalized boron nitride nanosheet modification layer as a Li-ion regulator to regulate Li-ion fluxes. The composite separator contains abundant polar groups and nanoscale channels and could achieve uniform electrochemical deposition via the lithiophilic effect and shunting action. Under the synergy influence of the lithiophilic effect and shunting action, Li dendrites are effectively suppressed. As proof, the Li||Li symmetrical cells with composite separators can circulate steadily for a long time under high current densities (10 mA cm^-2, 800 h). Moreover, the LiFePO₄||Li full cells display excellent long cycling performance (82% retention after 800 cycles).

Porous Materials Applied in Nonaqueous Li-O2 Batteries: Status and Perspectives

Porous materials possessing high surface area, large pore volume, tunable pore structure, superior tailorability, and dimensional effect have been widely applied as components of lithium-oxygen (Li-O₂ ) batteries. Herein, the theoretical foundation of the porous materials applied in Li-O₂ batteries is provided, based on the present understanding of the battery mechanism and the challenges and advantageous qualities of porous materials. Furthermore, recent progress in porous materials applied as the cathode, anode, separator, and electrolyte in Li-O₂ batteries is summarized, together with corresponding approaches to address the critical issues that remain at present. Particular emphasis is placed on the importance of the correlation between the function-orientated design of porous materials and key challenges of Li-O₂ batteries in accelerating oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) kinetics, improving the electrode stability, controlling lithium deposition, suppressing the shuttle effect of the dissolved redox mediators, and alleviating electrolyte decomposition. Finally, the rational design and innovative directions of porous materials are provided for their development and application in Li-O₂ battery systems.

A Review of Functional Separators for Lithium Metal Battery Applications

Lithium metal batteries are considered “rough diamonds” in electrochemical energy storage systems. Li-metal anodes have the versatile advantages of high theoretical capacity, low density, and low reaction potential, making them feasible candidates for next-generation battery applications. However, unsolved problems, such as dendritic growths, high reactivity of Li-metal, low Coulombic efficiency, and safety hazards, still exist and hamper the improvement of cell performance and reliability. The use of functional separators is one of the technologies that can contribute to solving these problems. Recently, functional separators have been actively studied and developed. In this paper, we summarize trends in the research on separators and predict future prospects.

Thermal Conductive 2D Boron Nitride for High-Performance All-Solid-State Lithium-Sulfur Batteries

Polymer-based solid-state electrolytes are shown to be highly promising for realizing low-cost, high-capacity, and safe Li batteries. One major challenge for polymer solid-state batteries is the relatively high operating temperature (60-80 °C), which means operating such batteries will require significant ramp up time due to heating. On the other hand, as polymer electrolytes are poor thermal conductors, thermal variation across the polymer electrolyte can lead to nonuniformity in ionic conductivity. This can be highly detrimental to lithium deposition and may result in dendrite formation. Here, a polyethylene oxide-based electrolyte with improved thermal responses is developed by incorporating 2D boron nitride (BN) nanoflakes. The results show that the BN additive also enhances ionic and mechanical properties of the electrolyte. More uniform Li stripping/deposition and reversible cathode reactions are achieved, which in turn enable all-solid-state lithium-sulfur cells with superior performances.

Nanopile Interlocking Separator Coating toward Uniform Li Deposition of the Li Metal Anodes

Uncontrollable growth of lithium (Li) dendrite has severely hindered the development of Li metal anodes, while separator modification is regarded as a simple and effective way to mitigate the growth of Li dendrite. However, the "drop-dregs" phenomenon of coating layer desquamated from polyolefin separator due to their different Young's modulus would induce a nonuniform Li ionic flux, finally resulting in deteriorative electrochemical performance and even thermal runaway of the battery. Herein, we introduce a novel nanopile mechanical interlocking strategy to create delamination-free separator modification, which could stably generate a homogeneous Li ionic flux to guide long-term uniform Li deposition. Both experimental and simulation results demonstrate a strong bonding strength between the coating layer and membrane matrix based on this physical interlocking mechanism. Consequently, with a nearly dendrite-free Li deposition and a largely reduced interface impedance, 1000 h stable cycling of Li/Li half cells enrolled this modified separator is successfully achieved. Also, a significant improvement in Li/LiFePO₄ full cells in long-term cycling stability to 500 cycles further indicates its promising practical potential. Moreover, this presented approach without any binding agents or surface activation procedures could be facilely scaled up, providing an applicable and durable separator modification solution toward stable Li metal anodes.

Colossal Granular Lithium Deposits Enabled by the Grain-Coarsening Effect for High-Efficiency Lithium Metal Full Batteries

The low Coulombic efficiency of the lithium metal anode is recognized as the real bottleneck to practical high-efficiency lithium metal batteries with limited Li excess. The grain size and microstructure of deposited lithium strongly influences the lithium plating/stripping efficiency. Here, a solubilizer-mediated carbonate electrolyte that can realize grain coarsening of lithium deposits (>20 µm in width) with oriented columnar morphology, which is in sharp contrast with conventional nanoscale dendrite-like lithium deposits in carbonate electrolytes, is reported. It exhibits improved Li Coulombic efficiency to 98.14% at a high capacity of 3 mAh cm^-2 over 150 cycles, because the colossal lithium deposition with minimal tortuosity can maintain the bulk Li with continuous electron conducting pathway during the stripping process, thus enabling efficient Li utilization. Li/NMC811 full batteries, composed of thin Li anode (45 µm) and a high-capacity NMC811 cathode (16.7 mg cm^-2 ), can achieve at least 12 times longer lifespan (200 cycles).

Realizing Dendrite-Free Lithium Deposition with a Composite Separator

A dendrite-free Li deposition strategy is developed with a composite separator of MnCO₃ coated porous polypropylene. Mn²⁺ ions are preferentially reduced to form Mn nanoparticles on Li anodes, which helped to reduce the nucleation overpotential and achieve a dendrite-free deposition of Li bulky grains. When MnCO₃ contacts the Li metal anode directly, an in situ artificial solid electrolyte interphase passivating layer was created from the reaction of Li metal, MnCO₃ and liquid electrolyte. Li metal anodes show significantly improved stability for more than 2000 h of plating/stripping in Li||Li symmetric cells. The homemade ultrathin Li films on Cu foils (Li@Cu), obtained by electrochemical Li deposition with PP/MnCO₃ separators, give enhanced cycling stability in LFP||Li@Cu cells. Combined with gel polymer electrolyte, the cycling stability of quasi-solid-state LFP||Li@Cu was further improved. This strategy for dendrite-free deposition via a composite separator provides a low-cost but efficient choice for alkaline metal batteries.

Two-Dimensional Materials to Address the Lithium Battery Challenges

Despite the ever-growing demand in safe and high power/energy density of Li⁺ ion and Li metal rechargeable batteries (LIBs), materials-related challenges are responsible for the majority of performance degradation in such batteries. These challenges include electrochemically induced phase transformations, repeated volume expansion and stress concentrations at interfaces, poor electrical and mechanical properties, low ionic conductivity, dendritic growth of Li, oxygen release and transition metal dissolution of cathodes, polysulfide shuttling in Li-sulfur batteries, and poor reversibility of lithium peroxide/superoxide products in Li-O₂ batteries. Owing to compelling physicochemical and structural properties, in recent years two-dimensional (2D) materials have emerged as promising candidates to address the challenges in LIBs. This Review highlights the cutting-edge advances of LIBs by using 2D materials as cathodes, anodes, separators, catalysts, current collectors, and electrolytes. It is shown that 2D materials can protect the electrode materials from pulverization, improve the synergy of Li⁺ ion deposition, facilitate Li⁺ ion flux through electrolyte and electrode/electrolyte interfaces, enhance thermal stability, block the lithium polysulfide species, and facilitate the formation/decomposition of Li-O₂ discharge products. This work facilitates the design of safe Li batteries with high energy and power density by using 2D materials.

Toward Stable Lithium Plating/Stripping by Successive Desolvation and Exclusive Transport of Li Ions

Li has been regarded as the most attractive anode for next-generation high-energy-density batteries due to its high specific capacity and low electrochemical potential. However, its low electrochemical potential leads to the side reaction of Li with the solvent of the electrolyte (the solvation of Li ions exacerbates the reaction). This adverse side reaction results in uneven Li distribution and deposition, low Coulombic efficiency, and the formation of Li dendrites. Herein, we demonstrate an efficient method for achieving successive desolvation and homogeneous distribution of Li ions by using a double-layer membrane. The first layer is designed to enable the desolvation of Li ions. The second layer with controllable and ordered nanopores is expected to facilitate the homogeneous and exclusive transport of Li ions. The efficiency of the double-layer membrane on desolvation and exclusive transport of Li ions is confirmed by theoretical calculations, the significantly enhanced Li-ion transference number, improved Coulombic efficiency, and the inhibition of Li dendrites. These results will deepen our understanding of the modulation of ions and pave a way to the next-generation high-energy-density Li-metal batteries.