A Thermally Conductive Separator for Stable Li Metal Anodes

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.

Phase-Inversion Polymer Composite Separators Based on Hexagonal Boron Nitride Nanosheets for High-Temperature Lithium-Ion Batteries

By preventing electrical contact between anode and cathode electrodes while promoting ionic transport, separators are critical components in the safe operation of rechargeable battery technologies. However, traditional polymer-based separators have limited thermal stability, which has contributed to catastrophic thermal runaway failure modes that have conspicuously plagued lithium-ion batteries. Here, we describe the development of phase-inversion composite separators based on carbon-coated hexagonal boron nitride (hBN) nanosheets and poly(vinylidene fluoride) (PVDF) polymers that possess high porosity, electrolyte wettability, and thermal stability. The carbon-coated hBN nanosheets are obtained through a scalable liquid-phase shear exfoliation method using ethyl cellulose as a polymer stabilizer and source of the carbon coating following thermal pyrolysis. When incorporated within the PVDF matrix, the carbon-coated hBN nanosheets promote favorable interfacial interactions during the phase-inversion process, resulting in porous, flexible, free-standing composite separators. The unique chemical composition of these carbon-coated hBN separators implies high wettability for a wide range of liquid electrolytes. This combination of high porosity and electrolyte wettability enables enhanced ionic conductivity and lithium-ion battery electrochemical performance that exceeds incumbent polyolefin separators over a wide range of operating conditions. The hBN nanosheets also impart high thermal stability, providing safe lithium-ion battery operation up to 120 °C.

Reduced-Graphene-Oxide-Guided Directional Growth of Planar Lithium Layers

Rechargeable lithium (Li) metal batteries hold great promise for revolutionizing current energy-storage technologies. However, the uncontrollable growth of lithium dendrites impedes the service of Li anodes in high energy and safety batteries. There are numerous studies on Li anodes, yet little attention has been paid to the intrinsic electrocrystallization characteristics of Li metal and their underlying mechanisms. Herein, a guided growth of planar Li layers, instead of random Li dendrites, is achieved on self-assembled reduced graphene oxide (rGO). In situ optical observation is performed to monitor the morphology evolution of such a planar Li layer. Moreover, the underlying mechanism during electrodeposition/stripping is revealed using ab initio molecular dynamics simulations. The combined experiment and simulation results show that when Li atoms are deposited on rGO, each layer of Li atoms grows along (110) crystallographic plane of the Li crystals because of the fine in-plane lattice matching between Li and the rGO substrate, resulting in planar Li deposition. With this specific topographic characteristic, a highly flexible lithium-sulfur (Li-S) full cell with rGO-guided planar Li layers as the anode exhibits stable cycling performance and high specific energy and power densities. This work enriches the fundamental understanding of Li electrocrystallization without dendrites and provides guidance for practical applications.

Mechanistics of Lithium-Metal Battery Performance by Separator Architecture Design

Lithium (Li)-metal anode has attracted renewed research interest due to its high specific capacity and the lowest negative potential. However, Li-metal batteries have safety issues and severe capacity fading. In this study, we demonstrate a facile and effective technique by adding an anodic aluminum oxide nanostructured interlayer onto the commercial polypropylene separator (PP) to create a novel architecture (AP). It is found that AP-based symmetric Li-Li cells and Li-NCM523 cells exhibit enhanced cycling performance and delayed capacity decay. Furthermore, compared with the cells with PP, the cells with AP show reduced overpotentials and improved cycle stability at low temperatures and various current densities, implying the wide applications of the designed architecture. The superior performance of AP is ascribed to its high electrolyte retention, high mechanical strength, and precisely ordered architecture, which contribute to uniform Li nucleation and growth. This unique separator architecture provides mechanistic insights into the design of rechargeable lithium-metal batteries, which are aimed at high energy density and cycling stability.

Towards practical lithium-metal anodes

Lithium-ion batteries have had a tremendous impact on several sectors of our society; however, the intrinsic limitations of Li-ion chemistry limits their ability to meet the increasing demands of developing more advanced portable electronics, electric vehicles, and grid-scale energy storage systems. Therefore, battery chemistries beyond Li ions are being intensively investigated and need urgent breakthroughs toward commercial applications, wherein the use of metallic Li is one of the most intuitive choices. Despite several decades of oblivion due to safety concerns regarding the growth of Li dendrites, Li-metal anodes are now poised to be revived because of the advances in investigative tools and globally invested efforts. In this review, we first summarize the existing issues with regard to Li anodes and their underlying reasons and then highlight the recent progress made in the development of high-performance Li anodes. Finally, we propose the persisting challenges and opportunities toward the exploration of practical Li-metal anodes.

Guidelines and trends for next-generation rechargeable lithium and lithium-ion batteries

This review article summarizes the current trends and provides guidelines towards next-generation rechargeable lithium and lithium-ion battery chemistries.

Regulating electrodeposition morphology of lithium: towards commercially relevant secondary Li metal batteries

Rational approaches for achieving fine control of the electrodeposition morphology of Li are required to create commercially-relevant rechargeable Li metal batteries.

Nanoscaled Lithium Powders with Protection of Ionic Liquid for Highly Stable Rechargeable Lithium Metal Batteries

To suppress the dendrite formation and alleviate volume expansion upon striping/platting is a key challenge for developing practical lithium metal anodes. Lithium metal in powder form possesses great potential to address this issue due to large specific surface area. However, the fabrication of powdery metallic lithium is largely restricted because of its unique softness, stickiness, and high reactivity. Here, a safe and readily accessible cryomilling process toward lithium powders is reported. Nanoscaled lithium powders (<500 nm) are successfully prepared from lithium foils with the assistance of a high-melting-point ionic liquid under cryogenic temperature. The prepared lithium powder anode exhibits superior electrochemical properties in symmetric cells, including extraordinarily low yet stable overpotential (≈50 mV), ultrahigh area capacity (30 mAh cm-2), and good long-term cyclability (1200 h) even cycling at high current density (10 mA cm-2). The powdery form of lithium also functions as a favorable prelithiation reagent for lithium-free anodes (e.g., Si, SiO, and SnO2). The findings open up a new avenue for the real-world application of lithium metal anodes for next-generation lithium batteries.

Energy storage: The future enabled by nanomaterials

Lithium-ion batteries, which power portable electronics, electric vehicles, and stationary storage, have been recognized with the 2019 Nobel Prize in chemistry. The development of nanomaterials and their related processing into electrodes and devices can improve the performance and/or development of the existing energy storage systems. We provide a perspective on recent progress in the application of nanomaterials in energy storage devices, such as supercapacitors and batteries. The versatility of nanomaterials can lead to power sources for portable, flexible, foldable, and distributable electronics; electric transportation; and grid-scale storage, as well as integration in living environments and biomedical systems. To overcome limitations of nanomaterials related to high reactivity and chemical instability caused by their high surface area, nanoparticles with different functionalities should be combined in smart architectures on nano- and microscales. The integration of nanomaterials into functional architectures and devices requires the development of advanced manufacturing approaches. We discuss successful strategies and outline a roadmap for the exploitation of nanomaterials for enabling future energy storage applications, such as powering distributed sensor networks and flexible and wearable electronics.

Gradient-Distributed Nucleation Seeds on Conductive Host for a Dendrite-Free and High-Rate Lithium Metal Anode

Much attention is paid to metal lithium as a hopeful negative material for reversible batteries with a high specific capacity. Although applying 3D hosts can relieve the dendrite growth to some extent, gradient-distributed lithium ion in 3D uniform hosts still induces uncontrolled lithium dendrites growth, especially at high lithium capacity and high current density. Herein, a 3D conductive carbon nanofiber framework with gradient-distributed ZnO particles as nucleation seeds (G-CNF) to regulate lithium deposition is proposed. Based on such a unique structure, the G-CNF electrode exhibits a high average Coulombic efficiency (CE) of 98.1% for 700 cycles at 0.5 mA cm^-2 . Even at 5 mA cm^-2 , the G-CNF electrode performs a stable cycling process and high CE of 96.0% for over 200 cycles. When the lithium-deposited G-CNF (G-CNF-Li) anode is applied in a full cell with a commercial LiFePO₄ cathode, it exhibits a stable capacity of 115 mAh g^-1 and high retention of 95.7% after 300 cycles. Through inducing the gradient-distributed nucleation seeds to counter the existing Li-ion concentration polarization, a uniform and stable lithium deposition process in the 3D host is achieved even under the condition of high current density.

Water-Dispersed Poly(p-Phenylene Terephthamide) Boosting Nano-Al2O3-Coated Polyethylene Separator with Enhanced Thermal Stability and Ion Diffusion for Lithium-Ion Batteries

Polyethylene (PE) membranes coated with nano-Al2O3 have been improved with water-dispersed poly(p-phenylene terephthamide) (PPTA). From the scanning electron microscope (SEM) images, it can be seen that a layer with a honeycombed porous structure is formed on the membrane. The thus-formed composite separator imbibed with the electrolyte solution has an ionic conductivity of 0.474 mS/cm with an electrolyte uptake of 335%. At 175 °C, the assembled battery from the synthesized composite separator explodes at 3200 s, which is five times longer than the battery assembled from an Al2O3-coated polyethylene (PE) membrane. The open circuit voltage of the assembled battery using a composite separator drops to zero at 600 s at an operating temperature of 185 °C, while the explosion of the battery with Al2O3-coated PE occurs at 250 s. More importantly, the interface resistance of the cell assembled from the composite separator decreases to 65 Ω. Hence, as the discharge rate increases from 0.2 to 1.0 C, the discharge capacity of the battery using composite separator retains 93.5%. Under 0.5 C, the discharge capacity retention remains 99.4% of its initial discharge capacity after 50 charge-discharge cycles. The results described here demonstrate that Al2O3/PPTA-coated polyethylene membranes have superior thermal stability and ion diffusion.

Synthesis of sub-millimeter single-crystal grains of aligned hexagonal boron nitride on an epitaxial Ni film

Hexagonal boron nitride (h-BN), an insulating two-dimensional (2D) layered material, has attracted increasing interest due to its electrical screening effect, high-temperature-resistant gas barrier properties, and other unique applications. However, the presence of grain boundaries (GBs) in h-BN is a hindrance to obtain these properties. Here, we demonstrate the epitaxial growth of monolayer h-BN by chemical vapor deposition (CVD) on Ni(111) thin films deposited on c-plane sapphire. The Ni(111) films showed higher thermal stability than Cu(111) and Cu-Ni(111) alloy films, allowing us to perform CVD growth at a high temperature of 1100 °C. This resulted in an increase of the h-BN grain sizes to up to 0.5 millimeter, among the highest reported so far, and in a well-defined triangular grain shape. Low-energy electron microscopy (LEEM) revealed the epitaxial relationship between h-BN and the underlying Ni(111) lattice, leading to a preferential alignment of the h-BN grains. Both the large grain size and the alignment are expected to facilitate the synthesis of h-BN with a low density of GBs. We also found that the addition of N2 gas during the CVD improves the crystalline shape of the h-BN grains, changing from an irregular, truncated to a sharp triangle. The growth behavior of monolayer h-BN is further discussed in terms of the dependences on growth temperature and pressure, as well as on the structural evolution of the Ni metal catalyst. Our findings not only help understand the h-BN growth mechanism but also offer a new route to grow high-quality, monolayer h-BN films.

Delamination-Free Multifunctional Separator for Long-Term Stability of Lithium-Ion Batteries

Next-generation lithium-ion batteries (LIBs) that satisfy the requirements for an electric vehicle energy source should demonstrate high reliability and safety for long-term high-energy-density operation. This inevitably calls for a novel approach to advance major components such as the separator. Herein, a separator is designed and fabricated via application of multilayer functional coating on both sides of a polyethylene separator. The multilayer-coated separator (MCS) has a porous structure that does not interfere with lithium ion diffusion and exhibits superior heat resistance, high electrolyte uptake, and persistent adhesion with the electrode. More importantly, it enables high capacity retention and reduced impedance build up during cycling when used in a coin or pouch cell. These imply its promising application in energy sources requiring long-term stability. Fabrication of the MCS without the use of organic solvents is not only environmentally beneficial but also effective at cost reduction. This approach paves the way for the separator, which has long been considered an inactive major component of LIBs, to become an active contributor to the energy density toward achieving longer cycle stability.

Surface Functionalization of a Conventional Polypropylene Separator with an Aluminum Nitride Layer toward Ultrastable and High-Rate Lithium Metal Anodes

Lithium (Li) metal as a next-generation anode has received great interest from industry and academic institutes due to its attractive benefits of a high theoretical capacity (3860 mAh g^-1) and the lowest negative potential (-3.04 V vs SHE) among the anode candidates. However, major issues associated with dendritic Li growth, infinite volume expansion of Li, and low Coulombic efficiency cause severely degraded cycle stabilities and fatal safety issues (such as short-circuit). Herein, we first designed a functional membrane, comprising an aluminum nitride (AlN) layer and a polypropylene (PP) separator, in order to curb the sharp Li dendrite growth, restrain the propagation of dendritic Li toward the PP separator, and consequently improve the electrochemical stabilities of Li metal batteries. When the designed membrane was introduced in either the Li/Cu half-cell or the Li/LCO full-cell, Li dendrite growth was significantly suppressed and side reactions associated with electrode degradation was effectively prevented by the material benefits of the AlN layer, thus leading to the significantly enhanced cycle performances. Low temperature stability tests further demonstrated the optimiztic potentiality of the designed membrane for enabling the stable operation of Li metal batteries under harsh conditions. Our approach of adopting a metal nitride layer to the PP separator can be a compelling strategy to improve the long-term electrochemical stability of the Li metal electrode.

Harnessing the unique properties of 2D materials for advanced lithium-sulfur batteries

In the past decade, lithium-sulfur batteries have attracted tremendous attention owing to their high theoretical energy densities. The electrochemical performances of lithium-sulfur batteries are strongly dependent on the electrode materials. Among all the electrode material candidates, the application of 2D materials in lithium-sulfur batteries including a sulfur cathode, a lithium anode, a separator and/or an electrolyte has gained great success in enhancing their electrochemical performance by overcoming their intrinsic obstacles. Thus, it is necessary to summarize the relationships between the unique features of 2D materials and the electrochemical performances of lithium-sulfur batteries, guiding the development of next-generation lithium-sulfur batteries. In this review, we focus on recent advances in harnessing the unique properties of 2D materials, including their high surface area, 2D feature, high mechanical strength, plentiful active sites and functional groups to improve the electrochemical properties of sulfur cathodes, lithium anodes, electrolytes and/or separators, respectively. Finally, we propose possible directions and strategies for harnessing various properties of 2D materials to promote the development and applications of lithium-sulfur batteries.

A 3D free-standing lithiophilic silver nanowire aerogel for lithium metal batteries without lithium dendrites and volume expansion: in operando X-ray diffraction

A 3D free-standing lithiophilic silver nanowire aerogel (AgNWA) can stop the dendritic growth of lithium metal at the initial nucleation process. The 3D structure can also suppress the infinite volume expansion of lithium during cycling. The active AgNWA scaffold can serve as a Li reservoir to compensate for the irreversible consumption of Li. The lithiated AgNWA anode was coupled with a lithium iron phosphate (LFP) cathode in a full-cell configuration providing much higher performance than the Li//LFP cell.

Li2O-Reinforced Solid Electrolyte Interphase on Three-Dimensional Sponges for Dendrite-Free Lithium Deposition

Lithium (Li) metal, with ultra-high theoretical capacity and low electrochemical potential, is the ultimate anode for next-generation Li metal batteries. However, the undesirable Li dendrite growth usually results in severe safety hazards and low Coulombic efficiency. In this work, we design a three-dimensional CuO@Cu submicron wire sponge current collector with high mechanical strength SEI layer dominated by Li₂O during electrochemical reaction process. The 3D CuO@Cu current collector realizes an enhanced CE of above 91% for an ultrahigh current of 10 mA cm^-2 after 100 cycles, and yields decent cycle stability at 5 C for the full cell. The exceptional performances of CuO@Cu submicron wire sponge current collector hold promise for further development of the next-generation metal-based batteries.

In Situ Li3 PS4 Solid-State Electrolyte Protection Layers for Superior Long-Life and High-Rate Lithium-Metal Anodes

A thin and adjustable Li₃ PS₄ (LPS) solid-state electrolyte protection layer on the surface of Li is proposed to address the dynamic plating/stripping process of Li metal. The LPS interlayer is formed by an in situ and self-limiting reaction between P₄ S₁₆ and Li in N-methyl-2-pyrrolidone. By increasing the concentration of P₄ S₁₆ , the thickness of the LPS layer can be adjusted up to 60 nm. Due to the high ionic conductivity and low electrochemical activity of Li₃ PS₄ , the intimate protection layer of LPS can not only prevent the formation of Li dendrites, but also reduces parasitic side reactions and improves the electrochemical performance. As a result, symmetric cells with the LPS protection layer can deliver stable Li plating/stripping for 2000 h. Full cells assembled with the LPS-protected Li exhibit two times higher capacity retention in Li-S batteries (≈800 mAh g^-1 ) at 5 A g^-1 for over 400 cycles compared to their bare Li counterparts. Furthermore, high rate performances can be achieved with Li-LPS/LiCoO₂ cells, which are capable of cycling at rates as high as 20 C. This innovative and scalable approach to stabilizing the Li anode can serve as a basis for the development of next-generation high-performance lithium-metal batteries.

An ion redistributor for dendrite-free lithium metal anodes

Lithium (Li) metal anodes have attracted considerable interest due to their ultrahigh theoretical gravimetric capacity and very low redox potential. However, the issues of nonuniform lithium deposits (dendritic Li) during cycling are hindering the practical applications of Li metal batteries. Herein, we propose a concept of ion redistributors to eliminate dendrites by redistributing Li ions with Al-doped Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO) coated polypropylene (PP) separators. The LLZTO with three-dimensional ion channels can act as a redistributor to regulate the movement of Li ions, delivering a uniform Li ion distribution for dendrite-free Li deposition. The standard deviation of ion concentration beneath the LLZTO composite separator is 13 times less than that beneath the routine PP separator. A Coulombic efficiency larger than 98% over 450 cycles is achieved in a Li | Cu cell with the LLZTO-coated separator. This approach enables a high specific capacity of 140 mAh g^-1 for LiFePO₄ | Li pouch cells and prolonged cycle life span of 800 hours for Li | Li pouch cells, respectively. This strategy is facile and efficient in regulating Li-ion deposition by separator modifications and is a universal method to protect alkali metal anodes in rechargeable batteries.

Nanostructured Li-Rich Fluoride Coated by Ionic Liquid as High Ion-Conductivity Solid Electrolyte Additive to Suppress Dendrite Growth at Li Metal Anode

Blending additive with electrolyte is a facile and effective method to suppress anode dendrite growth in Li metal batteries (LMBs), especially when a LiF-rich solid electrolyte interface (SEI) is formed as a consequence of additive decomposition or deposition. However LiF still suffers from poor bulk ion conductivity as well as the difficult access to tailored nanostructure. Exploring new Li fluoride of high Li-ion conductivity as SEI component is still a big challenge in view of the lacking of desired structure prototype or mineral phase. Here, we propose a Li-rich Li₃AlF₆ derivative from cryolite phase as solid electrolyte additive, which is characterized by textured nanoporous morphology and ionic liquid coating. Its room temperature ion conductivity is as high as ∼10^-5 S/cm with a low activation energy of 0.29 eV, the best level among fluoride-based solid electrolytes. These features guarantee a homogenization of Li⁺ fluxing through bulk and grain boundary of Li₃AlF₆-rich SEI and reinforce the effect on Li dendrite suppression. Li₃AlF₆ additive enables a stable cyclability of Li∥Li symmetric cells for at least 100 cycles even under a high areal capacity of 3 mA h/cm² and a significant improvement on capacity retention for various LMBs based on LiFePO₄, FeS₂, and S cathodes.

Fabrication of Lithiophilic Copper Foam with Interfacial Modulation toward High-Rate Lithium Metal Anodes

Although metallic lithium is regarded as an ideal anode material for high-energy-density batteries, the low cycling efficiency and safety issues hinder its practical application. In this study, a three-dimensional (3D) lithium composite anode was developed through infusing molten lithium inside the Cu foam anchored by ZnO nanoparticles. The introduced ZnO layer provides the driving force for infusion, leading to the spontaneous wetting of molten lithium. Benefiting from well-confined preloaded lithium in the Cu network, the anode displays ultralow internal resistance and stabilized interface. The fabricated anode for the symmetric cell shows extraordinarily low overpotential at high current densities (15, 33, and 50 mV at 3, 5, and 8 mA cm^-2 after 100 cycles, respectively). When paired with Li₄Ti₅O₁₂ electrode, the half-type cell demonstrates superior rate capability and long-term cycling stability after 1000 cycles at an ultrahigh rate of 10C. To the best of our knowledge, this anode shows the lowest overpotential and the highest rate capacity ever reported for 3D design anodes, confirming their great potential as lithium metal anodes.

High-Performance All-Solid-State Polymer Electrolyte with Controllable Conductivity Pathway Formed by Self-Assembly of Reactive Discogen and Immobilized via a Facile Photopolymerization for a Lithium-Ion Battery

All-solid-state polymer electrolytes (SPEs) have aroused great interests as one of the most promising alternatives for liquid electrolyte in the next-generation high-safety, and flexible lithium-ion batteries. However, some disadvantages of SPEs such as inefficient ion transmission capacity and poor interface stability result in unsatisfactory cyclic performance of the assembled batteries. Especially, the solid cell is hard to be run at room temperature. Herein, a novel and flexible discotic liquid-crystal (DLC)-based cross-linked solid polymer electrolyte (DLCCSPE) with controlled ion-conducting channels is fabricated via a one-pot photopolymerization of oriented reactive discogen, poly(ethylene glycol)diacrylate, and lithium salt. The experimental results indicate that the macroscopic alignment of self-assembled columns in the DLCCSPEs is successfully obtained under annealing and effectively immobilized via the UV photopolymerization. Because of the existence of unique oriented structure in the electrolytes, the prepared DLCCSPE films exhibit higher ionic conductivities and better comprehensive electrochemical properties than the DLCCSPEs without controlled ion-conductive pathways. Especially, the assembled LiFePO₄/Li cells with oriented electrolyte show an initial discharge capacity of 164 mA h g^-1 at 0.1 C and average specific discharge capacities of 143, 135, and 149 mA h g^-1 at the C-rates of 0.5, 1, and 0.2 C, respectively. In addition, the solid cell also shows the first discharge capacity of 124 mA h g^-1 (0.2 C) at room temperature. The outstanding cell performance of the oriented DLCCSPE should be originated from the macroscopically oriented and self-assembled DLC, which can form ion-conducting channels. Thus, combining the excellent performance of DLCCSPE and the simple one-pot fabricating process of the DLC-based all-solid-state electrolyte, it is believed that the DLC-based electrolyte can be one of the most promising electrolyte materials for the next-generation high-safety solid lithium-ion batteries.

Incorporating Ionic Paths into 3D Conducting Scaffolds for High Volumetric and Areal Capacity, High Rate Lithium-Metal Anodes

Lithium-metal batteries can fulfill the ever-growing demand of the high-energy-density requirement of electronics and electric vehicles. However, lithium-metal anodes have many challenges, especially their inhomogeneous dendritic formation and infinite dimensional change during cycling. 3D scaffold design can mitigate electrode thickness fluctuation and regulate the deposition morphology. However, in an insulating or ion-conducting matrix, Li as the exclusive electron conductor can become disconnected, whereas in an electron-conducting matrix, the rate performance is restrained by the sluggish Li+ diffusion. Herein, the advantages of both ion- and electron-conducting paths are integrated into a mixed scaffold. In the mixed ion- and electron-conducting network, the charge diffusion and distribution are facilitated leading to significantly improved electrochemical performance. By incorporating Li6.4 La3 Zr2 Al0.2 O12 nanoparticles into 3D carbon nanofibers scaffold, the Li metal anodes can deliver areal capacity of 16 mAh cm-2 , volumetric capacity of 1600 mAh cm-3 , and remain stable over 1000 h under current density of 5 mA cm-2 . The volumetric and areal capacities as well as the rate capability are among the highest values reported. It is anticipated that the 3D mixed scaffold could be combined with further electrolytes and cathodes to develop high-performance energy systems.

Three-dimensional sp2 carbon networks prepared by ultrahigh temperature treatment for ultrafast lithium-sulfur batteries

The current challenge in the development of high-performance lithium-sulfur (Li-S) batteries is to facilitate the redox kinetics of sulfur species as well as to suppress the shuttle effect of polysulfides, especially at high current rates. Herein, aiming the application of Li-S at high current rates, we coupled a sp2 carbon configuration consisting of 3D carbon nanotubes/graphene prepared by ultrahigh temperature treatment (2850 °C) with S (2850CNTs-Gra-S) for application in Li-S batteries. The 2850CNTs-Gra as the host material exhibits a nearly perfect sp2 hybridized structure because ultrahigh temperature treatment not only repairs the raw defects in CNTs and graphene, but it also forms new sp2 C-C bonds between them. The 3D sp2 carbon network ensures ultrafast ion/electron transfer and efficient heat dissipation to protect the integrity of the separator when the Li-S battery is running at an ultrahigh rate. Based on these unique advantages, the 2850CNTs-Gra-S cathode shows a high current rate performance. Critically, it still delivers a considerable specific capacity after 1500 cycles even at a current rate of 15C and exhibits an extremely low capacity degradation rate of 0.0087% per cycle.

Three-Dimensional, Solid-State Mixed Electron-Ion Conductive Framework for Lithium Metal Anode

Solid-state electrolytes (SSEs) have been widely considered as enabling materials for the practical application of lithium metal anodes. However, many problems inhibit the widespread application of solid state batteries, including the growth of lithium dendrites, high interfacial resistance, and the inability to operate at high current density. In this study, we report a three-dimensional (3D) mixed electron/ion conducting framework (3D-MCF) based on a porous-dense-porous trilayer garnet electrolyte structure created via tape casting to facilitate the use of a 3D solid state lithium metal anode. The 3D-MCF was achieved by a conformal coating of carbon nanotubes (CNTs) on the porous garnet structure, creating a composite mixed electron/ion conductor that acts as a 3D host for the lithium metal. The lithium metal was introduced into the 3D-MCF via slow electrochemical deposition, forming a 3D lithium metal anode. The slow lithiation leads to improved contact between the lithium metal anode and garnet electrolyte, resulting in a low resistance of 25 Ω cm². Additionally, due to the continuous CNT coating and its seamless contact with the garnet we observed highly uniform lithium deposition behavior in the porous garnet structure. With the same local current density, the high surface area of the porous garnet framework leads to a higher overall areal current density for stable lithium deposition. An elevated current density of 1 mA/cm² based on the geometric area of the cell was demonstrated for continuous lithium cycling in symmetric lithium cells. For battery operation of the trilayer structure, the lithium can be cycled between the 3D-MCF on one side and the cathode infused into the porous structure on the opposite side. The 3D-MCF created by the porous garnet structure and conformal CNT coating provides a promising direction toward new designs in solid-state lithium metal batteries.

A Scalable Approach to Dendrite-Free Lithium Anodes via Spontaneous Reduction of Spray-Coated Graphene Oxide Layers

Li-metal batteries (LiMBs) are experiencing a renaissance; however, achieving scalable production of dendrite-free Li anodes for practical application is still a formidable challenge. Herein, a facile and universal method is developed to directly reduce graphene oxide (GO) using alkali metals (e.g., Li, Na, and K) in moderate conditions. Based on this innovation, a spontaneously reduced graphene coating can be designed and modulated on a Li surface (SR-G-Li). The symmetrical SR-G-Li|SR-G-Li cell can run up to 1000 cycles at a high practical current density of 5 mA cm-2 without a short circuit, demonstrating one of the longest lifespans reported with LiPF6 -based carbonate electrolytes. More significantly, a practically scalable paradigm is established to fabricate dendrite-free Li anodes by spraying a GO layer on the Li anode surface for large-scale production of LiFePO4 /Li pouch cells, reflected by the continuous manufacturing of the SR-G-Li anodes based on the roll-to-roll technology. The strategy provides new commercial opportunities to both LiMBs and graphene.

In Situ Plating of Porous Mg Network Layer to Reinforce Anode Dendrite Suppression in Li-Metal Batteries

Li dendrite suppression enables a highly reversible Li-metal battery. However the strategy to smooth Li surface, especially under long-term cycling and high current density, is still a big challenge. Here, we propose a facile additive strategy to reinforce Li dendrite inhibition in a range of ether and carbonate electrolytes. Dissoluble Mg(TFSI)₂ is employed as a degradable electrolyte additive, leading to in situ plating of porous Mg network when contacting reductive Li atoms. Mg adatoms with extremely low diffusion energy barrier as lithiophilic sites enable a smooth or flaky morphology of Li surface even under a high current density of 2 mA/cm² and high capacity of 6 mAh/cm². Mg-salt additive significantly extends the cycling life of Li||Cu asymmetric cells up to 240 and 200 cycles for carbonate and ether electrolytes, respectively. Under a current density as high as 5 mA/cm², the Li||Cu cell based on ether system can still survive up to 140 cycles with a small voltage hysteresis close to 60 mV. The hysteresis can be reduced to below 25 mV for lasting 200 cycles at 1 mA/cm². This additive strategy provides a facile solution to in situ construction of conductive anode-electrolyte interface with low interface resistance for alleviating uneven Li nucleation.

Suppressing Dendritic Lithium Formation Using Porous Media in Lithium Metal-Based Batteries

Because of its ultrahigh specific capacity, lithium metal holds great promise for revolutionizing current rechargeable battery technologies. Nevertheless, the unavoidable formation of dendritic Li, as well as the resulting safety hazards and poor cycling stability, have significantly hindered its practical applications. A mainstream strategy to solve this problem is introducing porous media, such as solid electrolytes, modified separators, or artificial protection layers, to block Li dendrite penetration. However, the scientific foundation of this strategy has not yet been elucidated. Herein, using experiments and simulation we analyze the role of the porous media in suppressing dendritic Li growth and probe the underlying fundamental mechanisms. It is found that the tortuous pores of the porous media, which drastically reduce the local flux of Li⁺ moving toward the anode and effectively extend the physical path of dendrite growth, are the key to achieving the nondendritic Li growth. On the basis of the theoretical exploration, we synthesize a novel porous silicon nitride submicron-wire membrane and incorporate it in both half-cell and full-cell configurations. The operation time of the battery cells is significantly extended without a short circuit. The findings lay the foundation to use a porous medium for achieving nondendritic Li growth in Li metal-based batteries.

Design Principles of Functional Polymer Separators for High-Energy, Metal-Based Batteries

Next-generation rechargeable batteries that offer high energy density, efficiency, and reversibility rely on cell configurations that enable synergistic operations of individual components. They must also address multiple emerging challenges,which include electrochemical stability, transport efficiency, safety, and active material loss. The perspective of this Review is that rational design of the polymeric separator, which is used widely in rechargeable batteries, provides a rich set of opportunities for new innovations that should enable batteries to meet many of these needs. This perspective is different from the conventional view of the polymer separator as an inert/passive unit in a battery, which has the sole function to prevent direct contact between electrically conductivecomponents that form the battery anode and cathode. Polymer separators, which serve as the core component in a battery, bridge the electrodes and the electrolyte with a large surface contact that can be utilized to apply desirable functions. This Review focuses specifically on recent advances in polymer separator systems, with a detailed analysis of several embedded functional agents that are incorporated to improve mechanical robustness, regulate ion and mass transport, and retard flammability. The discussion is also extended to new composite separator concepts that are designated traditionally as polymer/gel electrolytes.

Redox-Active Separators for Lithium-Ion Batteries

A bilayered cellulose-based separator design is presented that can enhance the electrochemical performance of lithium-ion batteries (LIBs) via the inclusion of a porous redox-active layer. The proposed flexible redox-active separator consists of a mesoporous, insulating nanocellulose fiber layer that provides the necessary insulation between the electrodes and a porous, conductive, and redox-active polypyrrole-nanocellulose layer. The latter layer provides mechanical support to the nanocellulose layer and adds extra capacity to the LIBs. The redox-active separator is mechanically flexible, and no internal short circuits are observed during the operation of the LIBs, even when the redox-active layer is in direct contact with both electrodes in a symmetric lithium-lithium cell. By replacing a conventional polyethylene separator with a redox-active separator, the capacity of the proof-of-concept LIB battery containing a LiFePO₄ cathode and a Li metal anode can be increased from 0.16 to 0.276 mA h due to the capacity contribution from the redox-active separator. As the presented redox-active separator concept can be used to increase the capacities of electrochemical energy storage systems, this approach may pave the way for new types of functional separators.

Directing lateral growth of lithium dendrites in micro-compartmented anode arrays for safe lithium metal batteries

Uncontrolled growth of lithium dendrites during cycling has remained a challenging issue for lithium metal batteries. Thus far, various approaches have been proposed to delay or suppress dendrite growth, yet little attention has been paid to the solutions that can make batteries keep working when lithium dendrites are already extensively present. Here we develop an industry-adoptable technology to laterally direct the growth of lithium dendrites, where all dendrites are retained inside the compartmented copper current collector in a given limited cycling capacity. This featured electrode layout renders superior cycling stability (e.g., smoothly running for over 150 cycles at 0.5 mA cm⁻²). Numerical simulations indicate that reduced dendritic stress and damage to the separator are achieved when the battery is abusively running over the ceiling capacity to generate protrusions. This study may contribute to a deeper comprehension of metal dendrites and provide a significant step towards ultimate safe batteries.

The formation of lithium dendrites remains a great challenge to lithium metal batteries. Here the authors show an anode design to laterally direct the dendrite growth inside the compartments, providing a feasible post-mortem solution to batteries with lithium dendrites already present.

Trapping Lithium into Hollow Silica Microspheres with a Carbon Nanotube Core for Dendrite-Free Lithium Metal Anodes

Li metal anodes, which have attracted much attention for their high specific capacity and low redox potential, face a great challenge in realizing their practical application. The fatal issue of dendrite formation gives rise to internal short circuit and safety hazards and needs to be addressed. Here we propose a rational strategy of trapping Li within microcages to confine the deposition morphology and suppress dendrite growth. Microcages with a carbon nanotube core and porous silica sheath were prepared and proved to be effective for controlling the electrodeposition behavior. In addition, the insulative coating layer prevents concentrated electron flow and decreases the possibility of "hot spots" formation. Because of the Li trapper and uniform electron distribution, the electrode with delicate structure exhibits a dendrite-free morphology after plating 2 mA h cm^-2 of Li. As the dendrite growth is suppressed, the as-obtained electrode maintains a high plating/stripping efficiency of 99% over 200 cycles. This work delivers new insights into the design of rational Li metal anodes and hastens the practical application of Li metal batteries.

A bidirectional growth mechanism for a stable lithium anode by a platinum nanolayer sputtered on a polypropylene separator

The issue of uncontrollable Li dendrite growth, caused by irregular lithium deposition, restricts the wide applications of Li metal based high energy batteries. In this paper, a polypropylene separator with a sputtered platinum nanolayer has been developed to improve the stability of the Li metal anodes. It was found that cells using the modified separators resulted in a smooth Li surface and a stable “electrode–electrolyte” interface. On the one hand, platinum nanolayers can enhance the mechanical properties and micro-structures of commercial polypropylene separators. On the other hand, platinum nanolayers provide stable Li deposition during repeated charging/discharging by a bidirectional growth mechanism. After long-time cycling, the dendrites from opposite directions and dead Li are integrated into a flat and dense new-formed Li anode, decreasing the risk of low Coulombic efficiency and cycling instability that may end in cell failure. This design may provide new ideas in next-generation energy storage systems for advanced stable metallic battery technologies.

Stable Li nucleation and deposition were achieved by applying a platinum-modified-separator with a bidirectional lithium growth mechanism.

Smart Electrochemical Energy Storage Devices with Self-Protection and Self-Adaptation Abilities

Currently, with booming development and worldwide usage of rechargeable electrochemical energy storage devices, their safety issues, operation stability, service life, and user experience are garnering special attention. Smart and intelligent energy storage devices with self-protection and self-adaptation abilities aiming to address these challenges are being developed with great urgency. In this Progress Report, we highlight recent achievements in the field of smart energy storage systems that could early-detect incoming internal short circuits and self-protect against thermal runaway. Moreover, intelligent devices that are able to take actions and self-adapt in response to external mechanical disruption or deformation, i.e., exhibiting self-healing or shape-memory behaviors, are discussed. Finally, insights into the future development of smart rechargeable energy storage devices are provided.

Transient Behavior of the Metal Interface in Lithium Metal-Garnet Batteries

The interface between solid electrolytes and Li metal is a primary issue for solid-state batteries. Introducing a metal interlayer to conformally coat solid electrolytes can improve the interface wettability of Li metal and reduce the interfacial resistance, but the mechanism of the metal interlayer is unknown. In this work, we used magnesium (Mg) as a model to investigate the effect of a metal coating on the interfacial resistance of a solid electrolyte and Li metal anode. The Li-Mg alloy has low overpotential, leading to a lower interfacial resistance. Our motivation is to understand how the metal interlayer behaves at the interface to promote increased Li-metal wettability of the solid electrolyte surface and reduce interfacial resistance. Surprisingly, we found that the metal coating dissolved in the molten piece of Li and diffused into the bulk Li metal, leading to a small and stable interfacial resistance between the garnet solid electrolyte and the Li metal. We also found that the interfacial resistance did not change with increase in the thickness of the metal coating (5, 10, and 100 nm), due to the transient behavior of the metal interface layer.

Protected Lithium-Metal Anodes in Batteries: From Liquid to Solid

High-energy lithium-metal batteries are among the most promising candidates for next-generation energy storage systems. With a high specific capacity and a low reduction potential, the Li-metal anode has attracted extensive interest for decades. Dendritic Li formation, uncontrolled interfacial reactions, and huge volume effect are major hurdles to the commercial application of Li-metal anodes. Recent studies have shown that the performance and safety of Li-metal anodes can be significantly improved via organic electrolyte modification, Li-metal interface protection, Li-electrode framework design, separator coating, and so on. Superior to the liquid electrolytes, solid-state electrolytes are considered able to inhibit problematic Li dendrites and build safe solid Li-metal batteries. Inspired by the bright prospects of solid Li-metal batteries, increasing efforts have been devoted to overcoming the obstacles of solid Li-metal batteries, such as low ionic conductivity of the electrolyte and Li-electrolyte interfacial problems. Here, the approaches to protect Li-metal anodes from liquid batteries to solid-state batteries are outlined and analyzed in detail. Perspectives regarding the strategies for developing Li-metal anodes are discussed to facilitate the practical application of Li-metal batteries.

Reviving Lithium-Metal Anodes for Next-Generation High-Energy Batteries

Lithium-metal batteries (LMBs), as one of the most promising next-generation high-energy-density storage devices, are able to meet the rigid demands of new industries. However, the direct utilization of metallic lithium can induce harsh safety issues, inferior rate and cycle performance, or anode pulverization inside the cells. These drawbacks severely hinder the commercialization of LMBs. Here, an up-to-date review of the behavior of lithium ions upon deposition/dissolution, and the failure mechanisms of lithium-metal anodes is presented. It has been shown that the primary causes consist of the growth of lithium dendrites due to large polarization and a strong electric field at the vicinity of the anode, the hyperactivity of metallic lithium, and hostless infinite volume changes upon cycling. The recent advances in liquid organic electrolyte (LOE) systems through modulating the local current density, anion depletion, lithium flux, the anode-electrolyte interface, or the mechanical strength of the interlayers are highlighted. Concrete strategies including tailoring the anode structures, optimizing the electrolytes, building artificial anode-electrolyte interfaces, and functionalizing the protective interlayers are summarized in detail. Furthermore, the challenges remaining in LOE systems are outlined, and the future perspectives of introducing solid-state electrolytes to radically address safety issues are presented.