A Flexible Solid Electrolyte Interphase Layer for Long‐Life Lithium Metal Anodes

High-performance all-solid-state batteries enabled by salt bonding to perovskite in poly(ethylene oxide)

Flexible and low-cost poly(ethylene oxide) (PEO)-based electrolytes are promising for all-solid-state Li-metal batteries because of their compatibility with a metallic lithium anode. However, the low room-temperature Li-ion conductivity of PEO solid electrolytes and severe lithium-dendrite growth limit their application in high-energy Li-metal batteries. Here we prepared a PEO/perovskite Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃ composite electrolyte with a Li-ion conductivity of 5.4 × 10^-5 and 3.5 × 10^-4 S cm^-1 at 25 and 45 °C, respectively; the strong interaction between the F^- of TFSI^- (bis-trifluoromethanesulfonimide) and the surface Ta⁵⁺ of the perovskite improves the Li-ion transport at the PEO/perovskite interface. A symmetric Li/composite electrolyte/Li cell shows an excellent cyclability at a high current density up to 0.6 mA cm^-2 A solid electrolyte interphase layer formed in situ between the metallic lithium anode and the composite electrolyte suppresses lithium-dendrite formation and growth. All-solid-state Li|LiFePO₄ and high-voltage Li|LiNi_0.8Mn_0.1Co_0.1O₂ batteries with the composite electrolyte have an impressive performance with high Coulombic efficiencies, small overpotentials, and good cycling stability.

Facile Generation of Polymer-Alloy Hybrid Layers for Dendrite-Free Lithium-Metal Anodes with Improved Moisture Stability

Lithium-metal anodes are recognized as the most promising next-generation anodes for high-energy-storage batteries. However, lithium dendrites lead to irreversible capacity decay in lithium-metal batteries (LMBs). Besides, the strict assembly-environment conditions of LMBs are regarded as a challenge for practical applications. In this study, a workable lithium-metal anode with an artificial hybrid layer composed of a polymer and an alloy was designed and prepared by a simple chemical-modification strategy. Treated lithium anodes remained dendrite-free for over 1000 h in a Li-Li symmetric cell and exhibited outstanding cycle performance in high-areal-loading Li-S and Li-LiFePO₄ full cells. Moreover, the treated lithium showed improved moisture stability that benefits from the hydrophobicity of the polymer, thus retaining good electrochemical performance after exposure to humid air.

Interlayered Dendrite-Free Lithium Plating for High-Performance Lithium-Metal Batteries

For its high theoretical capacity and low redox potential, Li metal is considered to be one of the most promising anode materials for next-generation batteries. However, practical application of a Li-metal anode is impeded by Li dendrites, which are generated during the cycling of Li plating/stripping, leading to safety issues. Researchers attempt to solve this problem by spatially confining the Li plating. Yet, the effective directing of Li deposition into the confined space is challenging. Here, an interlayer is constructed between a graphitic carbon nitrite layer (g-C₃ N₄ ) and carbon cloth (CC), enabling site-directed dendrite-free Li plating. The g-C₃ N₄ /CC as an anode scaffold enables extraordinary cycling stability for over 1500 h with a small overpotential of ≈80 mV at 2 mA cm^-2 . Furthermore, prominent battery performance is also demonstrated in a full cell (Li/g-C₃ N₄ /CC as anode and LiCoO₂ as cathode) with high Coulombic efficiency of 99.4% over 300 cycles.

Review of Recent Development of In Situ/Operando Characterization Techniques for Lithium Battery Research

The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions, and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X-ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high-energy-density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.

An Autotransferable g-C3 N4 Li+ -Modulating Layer toward Stable Lithium Anodes

Commercial deployment of lithium anodes has been severely impeded by the poor battery safety, unsatisfying cycling lifespan, and efficiency. Recently, building artificial interfacial layers over a lithium anode was regarded as an effective strategy to stabilize the electrode. However, the fabrications reported so far have mostly been conducted directly upon lithium foil, often requiring stringent reaction conditions with indispensable inert environment protection and highly specialized reagents due to the high reactivity of metallic lithium. Besides, the uneven lithium-ion flux across the lithium surface should be more powerfully tailored via mighty interfacial layer materials. Herein, g-C₃ N₄ is employed as a Li⁺ -modulating material and a brand-new autotransferable strategy to fabricate this interfacial layer for Li anodes without any inert atmosphere protection and limitation of chemical regents is developed. The g-C₃ N₄ film is filtrated on the separator in air using a common alcohol solution and then perfectly autotransferred to the lithium surface by electrolyte wetting during normal cell assembly. The abundant nitrogen species within g-C₃ N₄ nanosheets can form transient LiN bonds to powerfully stabilize the lithium-ion flux and thus enable a CE over 99% for 900 cycles and smooth deposition at high current densities and capacities, surpassing most previous works.

Nanowire Array-Coated Flexible Substrate to Accommodate Lithium Plating for Stable Lithium-Metal Anodes and Flexible Lithium-Organic Batteries

Although lithium metal is an ideal anode with high theoretical capacity, Li dendrite formation and volume change have limited its application. We report a vertical polyaniline nanowire-coated carbon nanotube (CNT/PANI) composite flexible electrode on which Li could be homogeneously deposited to obtain a CNT/PANI@Li anode. In the composite, CNT/PANI acted as a host matrix with well-distributed Li ion flux attributed to high electroactive surface area, thereby effectively suppressing the Li dendrite. Compared with the pure CNT electrode, the CNT/PANI electrode presented low overpotential and stable long-term cycling with much less fluctuant stripping/plating profiles. The potential application of CNT/PANI@Li in all-flexible full cells was demonstrated by combining flexible organic poly(2,5-dihydroxyl-1,4-benzoquinonyl sulfide)/carbon nanotube (PDHBQS/CNT) composite films, in which the cathode achieves an eminent performance of 120 mA h g^-1 at 50 mA g^-1. Furthermore, pouch batteries with good flexibility were tested successfully, which demonstrated a promising future for all-flexible and high-performance Li-metal batteries.

Puncture-Resistant Hydrogel: Placing Molecular Complexes Along Phase Boundaries

Trendy advances in electric cars and wearable electronics triggered growing awareness in device lethality/survivability from accidents. A divergent design in protection calls for high stress resistance, large ductility, as well as efficient energy dissipation, all from the device itself, while keeping the weight-specific device performance to its premium. Unfortunately, the polymer electrolyte or the ductile elastomer lacks a mechanistic design to resist puncture or tear at a high stress level. Here, we designed molecular complexes along phase boundaries to mitigate the damages by placing these mechanically strong complexes along the phase boundaries or between two immiscible polymers. This puncture-resistant gel, dubbed as gel-nacre, is able to survive a few challenging incidents, including a 400 MPa puncture from a sharp nail, a 1 cm steel ball traveling at 540 km/h, and attempted rupture on stitched samples.

Nitriding-Interface-Regulated Lithium Plating Enables Flame-Retardant Electrolytes for High-Voltage Lithium Metal Batteries

Safety concerns are impeding the applications of lithium metal batteries. Flame-retardant electrolytes, such as organic phosphates electrolytes (OPEs), could intrinsically eliminate fire hazards and improve battery safety. However, OPEs show poor compatibility with Li metal though the exact reason has yet to be identified. Here, the lithium plating process in OPEs and Li/OPEs interface chemistry were investigated through ex situ and in situ techniques, and the cause for this incompatibility was revealed to be the highly resistive and inhomogeneous interfaces. Further, a nitriding interface strategy was proposed to ameliorate this issue and a Li metal anode with an improved Li cycling stability (300 h) and dendrite-free morphology is achieved. Meanwhile, the full batteries coupled with nickel-rich cathodes, such as LiNi_0.8 Co_0.1 Mn_0.1 O₂ , show excellent cycling stability and outstanding safety (passed the nail penetration test). This successful nitriding-interface strategy paves a new way to handle the incompatibility between electrode and electrolyte.

Dual-Phase Single-Ion Pathway Interfaces for Robust Lithium Metal in Working Batteries

The lithium (Li) metal anode is confronted by severe interfacial issues that strongly hinder its practical deployment. The unstable interfaces directly induce unfavorable low cycling efficiency, dendritic Li deposition, and even strong safety concerns. An advanced artificial protective layer with single-ion pathways holds great promise for enabling a spatially homogeneous ionic and electric field distribution over Li metal surface, therefore well protecting the Li metal anode during long-term working conditions. Herein, a robust dual-phase artificial interface is constructed, where not only the single-ion-conducting nature, but also high mechanical rigidity and considerable deformability can be fulfilled simultaneously by the rational integration of a garnet Al-doped Li_6.75 La₃ Zr_1.75 Ta_0.25 O₁₂ -based bottom layer and a lithiated Nafion top layer. The as-constructed artificial solid electrolyte interphase is demonstrated to significantly stabilize the repeated cell charging/discharging process via regulating a facile Li-ion transport and a compact Li plating behavior, hence contributing to a higher coulombic efficiency and a considerably enhanced cyclability of lithium metal batteries. This work highlights the significance of rational manipulation of the interfacial properties of a working Li metal anode and affords fresh insights into achieving dendrite-free Li deposition behavior in a working battery.

Locally Concentrated LiPF6 in a Carbonate-Based Electrolyte with Fluoroethylene Carbonate as a Diluent for Anode-Free Lithium Metal Batteries

Currently, concentrated electrolyte solutions are attracting special attention because of their unique characteristics such as unusually improved oxidative stability on both the cathode and anode sides, the absence of free solvent, the presence of more anion content, and the improved availability of Li⁺ ions. Most of the concentrated electrolytes reported are lithium bis(fluorosulfonyl)imide (LiFSI) salt with ether-based solvents because of the high solubility of salts in ether-based solvents. However, their poor anti-oxidation capability hindered their application especially with high potential cathode materials (>4.0 V). In addition, the salt is very costly, so it is not feasible from the cost analysis point of view. Therefore, here we report a locally concentrated electrolyte, 2 M LiPF₆, in ethylene carbonate/diethyl carbonate (1:1 v/v ratio) diluted with fluoroethylene carbonate (FEC), which is stable within a wide potential range (2.5-4.5 V). It shows significant improvement in cycling stability of lithium with an average Coulombic efficiency (ACE) of ∼98% and small voltage hysteresis (∼30 mV) with a current density of 0.2 mA/cm² for over 1066 h in Li||Cu cells. Furthermore, we ascertained the compatibility of the electrolyte for anode-free Li-metal batteries (AFLMBs) using Cu||LiNi_1/3Mn_1/3Co_1/3O₂ (NMC, ∼2 mA h/cm²) with a current density of 0.2 mA/cm². It shows stable cyclic performance with ACE of 97.8 and 40% retention capacity at the 50th cycle, which is the best result reported for carbonate-based solvents with AFLMBs. However, the commercial carbonate-based electrolyte has <90% ACE and even cannot proceed more than 15 cycles with retention capacity >40%. The enhanced cycle life and well retained in capacity of the locally concentrated electrolyte is mainly because of the synergetic effect of FEC as the diluent to increase the ionic conductivity and form stable anion-derived solid electrolyte interphase. The locally concentrated electrolyte also shows high robustness to the effect of upper limit cutoff voltage.

A Scalable Approach for Dendrite-Free Alkali Metal Anodes via Room-Temperature Facile Surface Fluorination

Alkali metals are attractive anode materials for advanced high-energy-density battery systems because of their high theoretical specific capacities as well as low electrochemical potential. However, severe dendrite growth as well as high chemical reactivity restrict their practical application in energy storage technologies. Herein, we propose a facile scalable solution-based approach to stabilize Li and Na anodes via the facile process of immersing the Li/Na metal in a nonhazardous ionic liquid 1-butyl-2,3-dimethylimidazolium tetrafluoroborate for several minutes at room temperature before battery assembly. This produces a dense and robust artificial fluoride layer, formed in situ by the reaction of the ionic liquid and Li/Na metal. As a demonstration, a homogeneous and compact LiF coating on the Li metal anode was fabricated via our method and it can effectively suppress the growth of Li dendrites and the continuous decomposition of electrolytes during cycling. As a result, the LiF-coated metallic Li anode achieves an enhanced cycling lifespan of over 700 h with low overpotential (∼22 mV) at 1 mA cm^-2, as well as a very high Coulombic efficiency of up to 98.1% for 200 cycles at 1 mA cm^-2. Furthermore, the successful achievements of the dendrite-free Na deposition show the versatility of room-temperature surface fluorination for potential battery applications.

Lithiophilicity chemistry of heteroatom-doped carbon to guide uniform lithium nucleation in lithium metal anodes

The uncontrollable growth of lithium (Li) dendrites seriously impedes practical applications of Li metal batteries. Various lithiophilic conductive frameworks, especially carbon hosts, are used to guide uniform Li nucleation and thus deliver a dendrite-free composite anode. However, the lithiophilic nature of these carbon hosts is poorly understood. Herein, the lithiophilicity chemistry of heteroatom-doped carbon is investigated through both first principles calculations and experimental verifications to guide uniform Li nucleation. The electronegativity, local dipole, and charge transfer are proposed to reveal the lithiophilicity of doping sites. Li bond chemistry further deepens the understanding of lithiophilicity. The O-doped and O/B-co-doped carbons exhibit the best lithiophilicity among single-doped and co-doped carbons, respectively. The excellent lithiophilicity achieved by O-doping carbon is further validated by Li nucleation overpotential measurement. This work uncovers the lithiophilicity chemistry of heteroatom-doped carbons and affords a mechanistic guidance to Li metal anode frameworks for safe rechargeable batteries.

Lithiophilic Faceted Cu(100) Surfaces: High Utilization of Host Surface and Cavities for Lithium Metal Anodes

Lithium metal anodes suffer from poor cycling stability and potential safety hazards. To alleviate these problems, Li thin-film anodes prepared on current collectors (CCs) and Li-free types of anodes that involve direct Li plating on CCs have received increasing attention. In this study, the atomic-scale design of Cu-CC surface lithiophilicity based on surface lattice matching of the bcc Li(110) and fcc Cu(100) faces as well as electrochemical achievement of Cu(100)-preferred surfaces for smooth Li deposition with a low nucleation barrier is reported. Additionally, a purposely designed solid-electrolyte interphase is created for Li anodes prepared on CCs. Not only is a smooth planar Li thin film prepared, but a uniform Li plating/stripping on the skeleton of 3D CCs is achieved as well by high utilization of the surface and cavities of the 3D CCs. This work demonstrates surface electrochemistry approaches to construct stable Li metal-electrolyte interphases towards practical applications of Li anodes prepared on CCs.

Flexible Artificial Solid Electrolyte Interphase Formed by 1,3-Dioxolane Oxidation and Polymerization for Metallic Lithium Anodes

Lithium-tin (Li-Sn) alloys are perfect substrate materials for anodes in high-energy density lithium metal secondary batteries. A new approach is proposed to further prevent the Li deposit on Li-Sn alloy substrates from reaction with electrolytes using an artificial solid electrolyte interphase (ASEI) based on electrochemical oxidation and polymerization of 1,3-dioxolane precursor with LiTFSI additive. This ASEI layer is flexible, stable, ion conductive, and electrically insulating, which can provide very stable cycling of Li-Sn alloy substrate anodes for Li deposition/stripping with an average Coulombic efficiency of 98.4% at a current density of 1 mA cm^-2. The Li-Sn alloy substrate is kept uniform and smooth without any dendrites and cracks after cycles. When the Li-Sn alloy substrate protected by ASEI is used as the anode of lithium-sulfur full cell, the cell shows much higher discharge capacity and better cycleability. This innovative and facile strategy of ASEI formation demonstrates a new and promising approach to the solution of the tough problems of Li dendrites in Li metal batteries.

In Situ Formed Shields Enabling Li2CO3-Free Solid Electrolytes: A New Route to Uncover the Intrinsic Lithiophilicity of Garnet Electrolytes for Dendrite-Free Li-Metal Batteries

Introduction of inorganic solid electrolytes is believed to be an ultimate strategy to dismiss dendritic Li in high-energy Li-metal batteries (LMBs), and garnet-type Li₇La₃Zr₂O₁₂ (LLZO) electrolytes are impressive candidates. However, the current density for stable Li plating/stripping in LLZO is still quite limited. Here, we create in situ formed Li-deficient shields by the high-temperature calcination at 900 °C. By this novel process, the formation of Li₂CO₃ on LLZO is restrained, and then we successfully obtain Li₂CO₃-free LLZO after removing the Li-deficient compounds. Without any surface modification, Li₂CO₃-free LLZO shows an intrinsic "lithiophilicity" characteristic. The contact angles of metallic Li on LLZO garnets are assessed by the first-principle calculation to confirm the lithiophilicity characteristic of LLZO electrolytes. The wetting of metallic Li on the Li₂CO₃-free LLZO surface leads to a continuous and tight Li/LLZO interface, resulting in an ultralow interfacial resistance of 49 Ω cm² and a homogeneous current distribution in the charge/discharge processes of LMBs. Consequently, the current density for the stable Li plating/stripping in LLZO increases to 900 μA cm^-2 at 60 °C, one of the highest current density for LMBs based on garnet-type LLZO electrolytes. Our findings not only offer insight into the lithiophilicity characteristics of LLZO electrolytes to suppress dendritic Li at high current densities but also expand the avenue toward high-performance, safe, and long-life energy-storage systems.

Triboelectric Nanogenerator-Enabled Dendrite-Free Lithium Metal Batteries

Lithium metal batteries (LMBs) are prominent among next-generation energy-storage systems because of their high energy density. Unfortunately, the commercial application of LMBs is hindered by the dendrite growth issue during the charging process. Herein, we report that the triboelectric nanogenerator (TENG)-based pulse output with a novel waveform and frequency has restrained the formation of dendrites in LMBs. The waveform and operation frequency of TENG can be regulated by TENG-designed and smart power management circuits. By regulating the waveform and frequency of the TENG-based pulse output, the pulse duration becomes shorter than the lithium dendrite formation time at any current of pulse waveform, and lithium ions can replenish in the entire electrode surface during rest periods, eliminating concentration polarization. Therefore, the optimized TENG-based charging strategy can improve the Coulombic efficiency of lithium plating/stripping and realize homogeneous lithium plating in LMBs. This TENG-based charging technology provides an innovative strategy to address the Li dendrite growth issues in LMBs, and accelerates the application of TENG-based energy collection systems.

Favorable Lithium Nucleation on Lithiophilic Framework Porphyrin for Dendrite-Free Lithium Metal Anodes

Lithium metal constitutes promising anode materials but suffers from dendrite growth. Lithiophilic host materials are highly considered for achieving uniform lithium deposition. Precise construction of lithiophilic sites with desired structure and homogeneous distribution significantly promotes the lithiophilicity of lithium hosts but remains a great challenge. In this contribution, a framework porphyrin (POF) material with precisely constructed lithiophilic sites in regard to chemical structure and geometric position is employed as the lithium host to address the above issues for dendrite-free lithium metal anodes. The extraordinary lithiophilicity of POF even beyond lithium nuclei validated by DFT simulations and lithium nucleation overpotentials affords a novel mechanism of favorable lithium nucleation to facilitate uniform nucleation and inhibit dendrite growth. Consequently, POF-based anodes demonstrate superior electrochemical performances with high Coulombic efficiency over 98%, reduced average voltage hysteresis, and excellent stability for 300 cycles at 1.0 mA cm⁻², 1.0 mAh cm⁻² superior to both Cu and graphene anodes. The favorable lithium nucleation mechanism on POF materials inspires further investigation of lithiophilic electrochemistry and development of lithium metal batteries.

In Situ Solid Electrolyte Interphase from Spray Quenching on Molten Li: A New Way to Construct High-Performance Lithium-Metal Anodes

Uncontrollable growth of Li dendrites and low utilization of active Li severely hinder its practical application. Construction of an artificial solid electrolyte interphase (SEI) on Li is demonstrated as one of the most effective ways to circumvent the above problems. Herein, a novel spray quenching method is developed in situ to fabricate an organic-inorganic composite SEI on Li metal. By spray quenching molten Li in a modified ether-based solution, a homogeneous and dense SEI consisting of organic matrix embedded with inorganic LiF and Li₃ N nanocrystallines (denoted as OIFN) is constructed on Li metal. Arising from high ionic conductivity and strong mechanical stability, the OIFN can not only effectively minimize the corrosion reaction of Li, but also greatly suppresses the dendrite growth. Accordingly, the OIFN-Li anode presents prominent electrochemical performance with an enhanced Coulombic efficiency of 98.15% for 200 cycles and a small hysteresis of <450 mV even at ultrahigh current density up to 10 mA cm^-2 . More importantly, during the full cell test with limited Li source, a high utilization of Li up to 40.5% is achieved for the OIFN-Li anode. The work provides a brand-new route to fabricate advanced SEI on alkali metal for high-performance alkali-metal batteries.

An artificial solid interphase with polymers of intrinsic microporosity for highly stable Li metal anodes

Polymers of intrinsic microporosity (PIM-1) have an appropriate pore size to reduce the solvation number of Li ions in electrolytes. This unique pore structure of PIM-1 as a solid interphase can suppress transport of solvent and consequently unwanted chemical reactions at the interface of anodes, thereby extending the cycle life of Li metal anodes.

Asymmetric behaviour of Li/Li symmetric cells for Li metal batteries

The asymmetric behaviour of Li metal electrodes in Li/Li symmetric cells is demonstrated in terms of electrochemical performance and changes in the morphology of Li metal.

Mixed Lithium Oxynitride/Oxysulfide as an Interphase Protective Layer To Stabilize Lithium Anodes for High-Performance Lithium-Sulfur Batteries

Lithium metal is strongly recognized as a promising anode material for next-generation high-energy-density systems. However, unstable solid electrolyte interphase and uncontrolled lithium dendrites growth induce severe capacity decay and short cycle life accompanied by high security risks. Here, we propose a simple method for constructing an artificial solid electrolyte interphase layer on the surface of lithium metal through spontaneous reaction, where ammonium persulfate and lithium nitrate are exploited as oxidants. The satisfactory artificial protective layer with uniform and dense morphology is composed of mixed lithium compounds, mainly including Li _xSO _y and Li _xNO _y species, which could effectively stabilize the interphase between electrolyte and lithium metal anode and restrain the "shuttle effect" of polysulfides. By employing the premodified lithium metal as anodes for lithium-sulfur batteries, the resulting cells exhibit excellent cycle stability (capacity decay of 0.09% per cycle over 300 cycles at 1 C and Coulombic efficiency of over 98%) and outstanding rate capability (850.8 mAh g^-1 even at 4 C). Hence, introducing a stable artificial protective layer to protect lithium anode delivers a new strategy for solving the issues related to lithium-metal batteries.

Li3N-Modified Garnet Electrolyte for All-Solid-State Lithium Metal Batteries Operated at 40 °C

Lithium carbonate on the surface of garnet blocks Li⁺ conduction and causes a huge interfacial resistance between the garnet and electrode. To solve this problem, this study presents an effective strategy to reduce significantly the interfacial resistance by replacing Li₂CO₃ with Li ion conducting Li₃N. Compared to the surface Li₂CO₃ on garnet, Li₃N is not only a good Li⁺ conductor but also offers a good wettability with both the garnet surface and a lithium metal anode. In addition, the introduction of a Li₃N layer not only enables a stable contact between the Li anode and garnet electrolyte but also prevents the direct reduction of garnet by Li metal over a long cycle life. As a result, a symmetric lithium cell with this Li₃N-modified garnet exhibits an ultralow overpotential and stable plating/stripping cyclability without lithium dendrite growth at room temperature. Moreover, an all-solid-state Li/LiFePO₄ battery with a Li₃N-modified garnet also displays high cycling efficiency and stability over 300 cycles even at a temperature of 40 °C.

A Polysulfide-Immobilizing Polymer Retards the Shuttling of Polysulfide Intermediates in Lithium-Sulfur Batteries

Lithium-sulfur batteries are regarded as one of the most promising candidates for next-generation rechargeable batteries. However, the practical application of lithium-sulfur (Li-S) batteries is seriously impeded by the notorious shuttling of soluble polysulfide intermediates, inducing a low utilization of active materials, severe self-discharge, and thus a poor cycling life, which is particularly severe in high-sulfur-loading cathodes. Herein, a polysulfide-immobilizing polymer is reported to address the shuttling issues. A natural polymer of Gum Arabic (GA) with precise oxygen-containing functional groups that can induce a strong binding interaction toward lithium polysulfides is deposited onto a conductive support of a carbon nanofiber (CNF) film as a polysulfide shielding interlayer. The as-obtained CNF-GA composite interlayer can achieve an outstanding performance of a high specific capacity of 880 mA h g^-1 and a maintained specific capacity of 827 mA h g^-1 after 250 cycles under a sulfur loading of 1.1 mg cm^-2 . More importantly, high reversible areal capacities of 4.77 and 10.8 mA h cm^-2 can be obtained at high sulfur loadings of 6 and even 12 mg cm^-2 , respectively. The results offer a facile and promising approach to develop viable lithium-sulfur batteries with high sulfur loading and high reversible capacities.

Functional Scanning Force Microscopy for Energy Nanodevices

Energy nanodevices, including energy conversion and energy storage devices, have become a major cross-disciplinary field in recent years. These devices feature long-range electron and ion transport coupled with chemical transformation, which call for novel characterization tools to understand device operation mechanisms. In this context, recent developments in functional scanning force microscopy techniques and their application in thin-film photovoltaic devices and lithium batteries are reviewed. The advantages of scanning force microscopy, such as high spatial resolution, multimodal imaging, and the possibility of in situ and in operando imaging, are emphasized. The survey indicates that functional scanning force microscopy is making significant contributions in understanding materials and interfaces in energy nanodevices.

MXene Aerogel Scaffolds for High-Rate Lithium Metal Anodes

Li metal is considered to be an ultimate anode for metal batteries owing to its extremely high theoretical capacity and lowest potential. However, numerous issues such as short lifespan and infinite volume expansion caused by the dendrite growth during Li plating/stripping hinder its practical usage. These challenges become more grievous under high current densities. Herein, 3D porous MXene aerogels are proposed as scaffolds for high-rate Li metal anodes using Ti₃ C₂ as an example. With high metallic electron conductivity, fast Li ion transport capability, and abundant Li nucleation sites, such scaffolds could deliver high cycling stability and low overpotential at current density up to 10 mA cm^-2 . High rate performance is also demonstrated in full cells with LiFePO₄ as cathodes. This work provides a new type of scaffolds for Li metal anodes and paves the way for the application of non-graphene 2D materials toward high energy density Li metal batteries.

PVDF/Palygorskite Nanowire Composite Electrolyte for 4 V Rechargeable Lithium Batteries with High Energy Density

Solid electrolytes are crucial for the development of solid state batteries. Among different types of solid electrolytes, poly(ethylene oxide) (PEO)-based polymer electrolytes have attracted extensive attention owing to their excellent flexibility and easiness for processing. However, their relatively low ionic conductivities and electrochemical instability above 4 V limit their applications in batteries with high energy density. Herein, we prepared poly(vinylidene fluoride) (PVDF) polymer electrolytes with an organic plasticizer, which possesses compatibility with 4 V cathode and high ionic conductivity (1.2 × 10^-4 S/cm) at room temperature. We also revealed the importance of plasticizer content to the ionic conductivity. To address weak mechanical strength of the PVDF electrolyte with plasticizer, we introduced palygorskite ((Mg,Al)₂Si₄O₁₀(OH)) nanowires as a new ceramic filler to form composite solid electrolytes (CPE), which greatly enhances both stiffness and toughness of PVDF-based polymer electrolyte. With 5 wt % of palygorskite nanowires, not only does the elastic modulus of PVDF CPE increase from 9.0 to 96 MPa but also its yield stress is enhanced by 200%. Moreover, numerical modeling uncovers that the strong nanowire-polymer interaction and cross-linking network of nanowires are responsible for such significant enhancement in mechanically robustness. The addition of 5% palygorskite nanowires also enhances transference number of Li⁺ from 0.21 to 0.54 due to interaction between palygorskite and ClO₄^- ions. We further demonstrate full cells based on Li(Ni_1/3Mn_1/3Co_1/3)O₂ (NMC111) cathode, PVDF/palygorskite CPE, and lithium anode, which can be cycled over 200 times at 0.3 C, with 97% capacity retention. Moreover, the PVDF matrix is much less flammable than PEO electrolytes. Our work illustrates that the PVDF/palygorskite CPE is a promising electrolyte for solid state batteries.

An Alternative to Lithium Metal Anodes: Non-dendritic and Highly Reversible Sodium Metal Anodes for Li-Na Hybrid Batteries

Highly reversible, stable, and non-dendritic metal anode (Li, Na etc.) is a crucial requirement for next-generation high-energy batteries. Herein, we have built a Li-Na hybrid battery (LNHB) based on Na plating/stripping, which features a high and stable coulombic efficiency of 99.2 % after 100 cycles, low voltage hysteresis (42 mV at 2 mA cm^-2 ), and fast charge transfer. As a result of the Li⁺ electrostatic shield layer, the Na deposition showed cubic morphology rather than dendritic, even at high current density of 5 mA cm^-2 . The solvation/desolvation of Li⁺ and Na⁺ were modelled by density functional theory calculations, demonstrating the fast desolvation kinetics of Na⁺ . Owing to the superior performance of the Na metal anode, the LNHB coupled with LiFePO₄ cathode exhibited low voltage hysteresis and stable cycling performance that demonstrates its feasibility in practical applications.

Ultrahigh-Capacity and Long-Life Lithium-Metal Batteries Enabled by Engineering Carbon Nanofiber-Stabilized Graphene Aerogel Film Host

A safe, high-capacity, and long-life Li metal anode is highly desired due to recent developments in high-energy-density Li-metal batteries. However, there are still rigorous challenges associated with the undesirable formation of Li dendrites, lack of suitable host materials, and unstable chemical interfaces. Herein, a carbon nanofiber-stabilized graphene aerogel film (G-CNF film), inspired by constructional engineering, is constructed. As the host material for Li deposition, the G-CNF film features a large surface area, porous structure, and a robust skeleton that can render low local current density. This allows for dendrite-free Li deposition and mitigation of problems associated with large volume change. Importantly, the G-CNF film can keep high Li plating/stripping efficiency at nearly 99% for over 700 h with an areal capacity of 10 mA h cm^-2 (the specific capacity up to 2588 mA h g^-1 based on the total mass of carbon host and Li metal). The symmetric cells can stably run for more than 1000 h with low voltage hysteresis. The full cell with the LiFePO₄ cathode also delivers enhanced capacity and lowered overpotential. As two-in-one host materials for both cathodes and anodes in Li-O₂ batteries, the battery exhibits a capacity of 1.2 mA h cm^-2 .

A high performance lithium-ion-sulfur battery with a free-standing carbon matrix supported Li-rich alloy anode

A high-performance lithium-ion–sulfur battery has been built by using a carbon supported Li-rich alloy anode and sulfurized polyacrylonitrile (S@pPAN) cathode.

Although the lithium–sulfur battery exhibits high capacity and energy density, the cycling performance is severely retarded by dendrite formation and side-reactions of the lithium metal anode and the shuttle effect of polysulfides. Therefore, exploring lithium rich-alloy (or compound) anodes and suppressing the shuttling of polysulfides have become practical technical challenges for the commercialization of lithium–sulfur batteries. Here, a lithium ion sulfur full battery system combining a lithium-rich Li–Si alloy anode and sulfurized polyacrylonitrile (S@pPAN) cathode has been proposed. The free-standing CNF matrix supported Li–Si alloy anode is prepared by a simple and effective method, which is practical for scale-up production. The obtained Li–Si alloy anode demonstrates high cycling stability without dendrite growth, while the use of the S@pPAN cathode avoids the shuttle effect in carbonate electrolytes. The constructed Li–Si/S@pPAN battery could be cycled more than 1000 times at 1C and 3000 times at 3C, with a capacity fading rate of 0.01% and 0.03% per cycle. The exceptional performance should originate from the stable integrated anode structure and the excellent compatibility of the S@pPAN cathode and Li–Si alloy anode with carbonate electrolytes.

Dendrite-Free Epitaxial Growth of Lithium Metal during Charging in Li-O2 Batteries

Lithium (Li) dendrite formation is one of the major hurdles limiting the development of Li-metal batteries, including Li-O₂ batteries. Herein, we report the first observation of the dendrite-free epitaxial growth of a Li metal up to 10-μm thick during charging (plating) in the LiBr-LiNO₃ dual anion electrolyte under O₂ atmosphere. This phenomenon is due to the formation of an ultrathin and homogeneous Li₂ O-rich solid-electrolyte interphase (SEI) layer in the preceding discharge (stripping) process, where the corrosive nature of Br^- seems to give rise to remove the original incompact passivation layer and NO₃^- oxidizes (passivates) the freshly formed Li surface to prevent further reactions with the electrolyte. Such reactions keep the SEI thin (<100 nm) and facilitates the electropolishing effect and gets ready for the epitaxial electroplating of Li in the following charge process.

A Dual-Salt Gel Polymer Electrolyte with 3D Cross-Linked Polymer Network for Dendrite-Free Lithium Metal Batteries

Lithium metal batteries show great potential in energy storage because of their high energy density. Nevertheless, building a stable solid electrolyte interphase (SEI) and restraining the dendrite growth are difficult to realize with traditional liquid electrolytes. Solid and gel electrolytes are considered promising candidates to restrain the dendrites growth, while they are still limited by low ionic conductivity and incompatible interphases. Herein, a dual-salt (LiTFSI-LiPF6) gel polymer electrolyte (GPE) with 3D cross-linked polymer network is designed to address these issues. By introducing a dual salt in 3D structure fabricated using an in situ polymerization method, the 3D-GPE exhibits a high ionic conductivity (0.56 mS cm-1 at room temperature) and builds a robust and conductive SEI on the lithium metal surface. Consequently, the Li metal batteries using 3D-GPE can markedly reduce the dendrite growth and achieve 87.93% capacity retention after cycling for 300 cycles. This work demonstrates a promising method to design electrolytes for lithium metal batteries.

Flexible, Flame-Resistant, and Dendrite-Impermeable Gel-Polymer Electrolyte for Li-O2 /Air Batteries Workable Under Hurdle Conditions

Gel-polymer electrolytes are considered as a promising candidate for replacing the liquid electrolytes to address the safety concerns in Li-O₂ /air batteries. In this work, by taking advantage of the hydrogen bond between thermoplastic polyurethane and aerogel SiO₂ in gel polymer, a highly crosslinked quasi-solid electrolyte (FST-GPE) with multifeatures of high ionic conductivity, high mechanical flexibility, favorable flame resistance, and excellent Li dendrite impermeability is developed. The resulting gel-polymer Li-O₂ /air batteries possess high reaction kinetics and stabilities due to the unique electrode-electrolyte interface and fast O₂ diffusion in cathode, which can achieve up to 250 discharge-charge cycles (over 1000 h) in oxygen gas. Under ambient air atmosphere, excellent performances are observed for coin-type cells over 20 days and for prototype cells working under extreme bending conditions. Moreover, the FST-GPE electrolyte also exhibits durability to protect against fire, dendritic Li, and H₂ O attack, demonstrating great potential for the design of practical Li-O₂ /air batteries.

Electrochemically Controlled Solid Electrolyte Interphase Layers Enable Superior Li-S Batteries

Lithium-sulfur (Li-S) batteries suffer from shuttle reactions during electrochemical cycling, which cause the loss of active material sulfur from sulfur-carbon cathodes, and simultaneously incur the corrosion and degradation of the lithium metal anode by forming passivation layers on its surface. These unwanted reactions therefore lead to the fast failure of batteries. The preservation of the highly reactive lithium metal anode in sulfur-containing electrolytes has been one of the main challenges for Li-S batteries. In this study, we systematically controlled and optimized the formation of a smooth and uniform solid electrolyte interphase (SEI) layer through electrochemical pretreatment of the Li metal anode under controlled current densities. A distinct improvement of battery performance in terms of specific capacity and power capability was achieved in charge-discharge cycling for Li-S cells with pretreated Li anodes compared to pristine untreated ones. Importantly, at a higher power density (1 C rate, 3 mA cm^-2), the Li-S cells with pretreated Li anodes protected by a controlled elastomer (Li-Protected-by-Elastomer, LPE)) show the suppression of the Li dendrite growth and exhibit 3-4 times higher specific capacity than the untreated ones after 100 electrochemical cycles. The formation of such a controlled uniform SEI was confirmed, and its surface chemistry, morphology, and electrochemical properties were characterized by X-ray photoelectron spectroscopy, focused-ion beam cross sectioning, and scanning electron microscopy. Adequate pretreatment current density and time are critical in order to form a continuous and uniform SEI, along with good Li-ion transport property.

Structural Design of Lithium–Sulfur Batteries: From Fundamental Research to Practical Application

Abstract

Lithium–sulfur (Li–S) batteries have been considered as one of the most promising energy storage devices that have the potential to deliver energy densities that supersede that of state-of-the-art lithium ion batteries. Due to their high theoretical energy density and cost-effectiveness, Li–S batteries have received great attention and have made great progress in the last few years. However, the insurmountable gap between fundamental research and practical application is still a major stumbling block that has hindered the commercialization of Li–S batteries. This review provides insight from an engineering point of view to discuss the reasonable structural design and parameters for the application of Li–S batteries. Firstly, a systematic analysis of various parameters (sulfur loading, electrolyte/sulfur (E/S) ratio, discharge capacity, discharge voltage, Li excess percentage, sulfur content, etc.) that influence the gravimetric energy density, volumetric energy density and cost is investigated. Through comparing and analyzing the statistical information collected from recent Li–S publications to find the shortcomings of Li–S technology, we supply potential strategies aimed at addressing the major issues that are still needed to be overcome. Finally, potential future directions and prospects in the engineering of Li–S batteries are discussed.

Graphical Abstract

Stabilization of Lithium-Metal Batteries Based on the in Situ Formation of a Stable Solid Electrolyte Interphase Layer

Lithium (Li) metals have been considered most promising candidates as an anode to increase the energy density of Li-ion batteries because of their ultrahigh specific capacity (3860 mA h g^-1) and lowest redox potential (-3.040 V vs standard hydrogen electrode). However, unstable dendritic electrodeposition, low Coulombic efficiency, and infinite volume changes severely hinder their practical uses. Herein, we report that ethyl methyl carbonate (EMC)- and fluoroethylene carbonate (FEC)-based electrolytes significantly enhance the energy density and cycling stability of Li-metal batteries (LMBs). In LMBs, using commercialized Ni-rich Li[Ni_0.6Co_0.2Mn_0.2]O₂ (NCM622) and 1 M LiPF₆ in EMC/FEC = 3:1 electrolyte exhibits a high initial capacity of 1.8 mA h cm^-2 with superior cycling stability and high Coulombic efficiency above 99.8% for 500 cycles while delivering a unprecedented energy density. The present work also highlights a significant improvement in scaled-up pouch-type Li/NCM622 cells. Moreover, the postmortem characterization of the cycled cathodes, separators, and Li-metal anodes collected from the pouch-type Li/NCM622 cells helped identifying the improvement or degradation mechanisms behind the observed electrochemical cycling.

Designable ultra-smooth ultra-thin solid-electrolyte interphases of three alkali metal anodes

Dendrite growth of alkali metal anodes limited their lifetime for charge/discharge cycling. Here, we report near-perfect anodes of lithium, sodium, and potassium metals achieved by electrochemical polishing, which removes microscopic defects and creates ultra-smooth ultra-thin solid-electrolyte interphase layers at metal surfaces for providing a homogeneous environment. Precise characterizations by AFM force probing with corroborative in-depth XPS profile analysis reveal that the ultra-smooth ultra-thin solid-electrolyte interphase can be designed to have alternating inorganic-rich and organic-rich/mixed multi-layered structure, which offers mechanical property of coupled rigidity and elasticity. The polished metal anodes exhibit significantly enhanced cycling stability, specifically the lithium anodes can cycle for over 200 times at a real current density of 2 mA cm^-2 with 100% depth of discharge. Our work illustrates that an ultra-smooth ultra-thin solid-electrolyte interphase may be robust enough to suppress dendrite growth and thus serve as an initial layer for further improved protection of alkali metal anodes.