Mixed Ionic and Electronic Conductor for Li-Metal Anode Protection

Energy Band Specific to Injected Li-Ion-Induced Formation of Lithium Dendrites in Garnet Solid Electrolytes

As one of the most significant challenges in solid-state batteries, thorough investigation is necessary on the formation process of lithium dendrites in solid-state electrolytes. Here, we reveal that the growth of lithium dendrites in solid electrolytes is a physical-electrochemical reaction process caused by injected lithium ions and electron carriers, which requires a low electrochemical potential. A unique energy band specific to injected Li ions is identified at the bottom of the conduction band, which can be occupied by electron carriers from low-potential electrodes, leading to dendrite formation. In this case, it is quantitatively determined that the employed anodes with higher working voltages (>0.2 V versus Li/Li+) can effectively prevent dendrite formation. Moreover, lithium dendrite formation exclusively occurs during the charging process (i.e., lithium plating), where lithium ions meet electrons at mixed conductive grain boundaries under highly reductive potentials. The proposed model has significant scientific significance and application value.

3D mixed ion/electron-conducting scaffolds for stable sodium metal anodes

Sodium (Na) metal batteries represent an optimal choice for the forthcoming generation of large-scale, cost-effective energy storage systems. However, Na metal anodes encounter several formidable challenges during the Na plating and stripping processes, which encompass the formation of an unstable solid electrolyte interface, uncontrollable dendrite growth, and infinite volume expansion. These issues result in a reduced Coulombic efficiency, shortened battery lifespan, and potential safety hazards, thereby constraining their commercial development. Therefore, addressing these challenges to ensure the cycling stability of Na metal anodes stands as a paramount requirement for practical applications. Among the reported strategies, three-dimensional conductive scaffolds possessing a high surface area and porous structure are acknowledged for their significant potential in stabilizing Na metal anodes. Compared with conventional electron-conducting scaffolds, emerging mixed ion/electron-conductive (MIEC) scaffolds provide rapid ion/electron transport pathways, which enable uniform Na+ flux and promote dendrite-free Na deposition, thus improving the cycle life of Na metal anodes, even at high current densities and large areal capacities. Therefore, this review primarily emphasizes the recent progress in applying MIEC scaffolds to Na metal anodes. It introduces diverse design methods, examines the electrochemical performance of MIEC scaffolds, and delves into their regulation mechanisms over Na deposition behaviour. Finally, the development prospects and research strategies for MIEC scaffolds from both fundamental research and practical application perspectives are discussed, suggesting directions for further designing high-performance Na metal batteries.

Control of metal oxides' electronic conductivity through visual intercalation chemical reactions

Cation intercalation is an effective method to optimize the electronic structures of metal oxides, but tuning intercalation structure and conductivity by manipulating ion movement is difficult. Here, we report a visual topochemical synthesis strategy to control intercalation pathways and structures and realize the rapid synthesis of flexible conductive metal oxide films in one minute at room temperature. Using flexible TiO2 nanofiber films as the prototype, we design three charge-driven models to intercalate preset Li+-ions into the TiO2 lattice slowly (µm/s), rapidly (mm/s), or ultrafast (cm/s). The Li+-intercalation causes real-time color changes of the TiO2 films from white to blue and then black, corresponding to the structures of LixTiO2 and LixTiO2-δ, and the enhanced conductivity from 0 to 1 and 40 S/m. This work realizes large-scale and rapid synthesis of flexible TiO2 nanofiber films with tunable conductivity and is expected to extend the synthesis to other conductive metal oxide films.

Solid-State Electrolytes in Lithium-Sulfur Batteries: Latest Progresses and Prospects

Solid-state lithium-sulfur batteries (SSLSBs) have attracted tremendous research interest due to their large theoretical energy density and high safety, which are highly important indicators for the development of next-generation energy storage devices. Particularly, safety and "shuttle effect" issues originating from volatile and flammable liquid organic electrolytes can be fully mitigated by switching to a solid-state configuration. However, their road to thecommercial application is still plagued with numerous challenges, most notably the intrinsic electrochemical instability of solid-state electrolytes (SSEs) materials and their interfacial compatibility with electrodes and electrolytes. In this review, a critical discussion on the key issues and problems of different types of SSEs as well as the corresponding optimization strategies are first highlighted. Then, the state-of-the-art preparation methods and properties of different kinds of SSE materials, and their manufacture, characterization and performance in SSLSBs are summarized in detail. Finally, a scientific outlook for the future development of SSEs and the avenue to commercial application of SSLSBs is also proposed.

Advances in Nanofibrous Materials for Stable Lithium-Metal Anodes

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

Thermal Percolation of Antiperovskite Superionic Conductor into Porous MXene Scaffold for High-Capacity and Stable Lithium Metal Battery

Lithium metal battery is considered an emerging energy storage technology due to its high theoretical capacity and low electrochemical potential. However, the practical exploitations of lithium metal batteries are not realized because of uncontrollable lithium deposition and severe dendrite formation. Herein, a thermal percolation strategy is developed to fabricate a dual-conductive framework using electronically conductive Ti₃ C₂ T_x MXene aerogels (MXAs) and Li₂ OHCl antiperovskite superionic conductor. By melting Li₂ OHCl at a low temperature, the molten antiperovskite phase can penetrate the MXA scaffold, resulting in percolative electron/ion pathways. Through density functional theory calculations and electrochemical characterizations, the hybridized lithiophilic (MXA)-lithiophobic (antiperovskite) interfaces can spatially guide the deposition of lithium metals and suppress the growth of lithium dendrites. The symmetric cell with MXA-antiperovskite electrodes exhibits superior cycling stability at high areal capacities of 4 mAh cm^-2 over 1000 h. Moreover, the full cell with MXA-antiperovskite anode and high-loading LiFePO₄ cathode demonstrates high energy and power densities (415.7 Wh kg_cell ^-1 and 231.0 W kg_cell ^-1 ) with ultralong lifespans. The thermal percolation of lithium superionic conductor into electronically conductive scaffolds promises an efficient strategy to fabricate dual-conductive electrodes, which benefits the development of dendrite-free lithium metal anodes with high energy/power densities.

Recent progresses and challenges in aqueous lithium–air batteries relating to the solid electrolyte separator: A mini-review

The lithium–air (Li–air) battery utilizes infinite oxygen in the air to store or release energy through a semi-open cathode structure and bears an ultra-high theoretical energy density of more than 1,000 Wh/kg. Therefore, it has been denoted as the candidate for next-generation energy storage in versatile fields such as electric vehicles, telecommunications, and special power supply. Among all types of Li–air batteries, an aqueous Li–air battery bears the advantages of a high theoretical energy density of more than 1,700 Wh/kg and does not have the critical pure oxygen atmosphere issues in a non-aqueous lithium–air battery system, which is more promising for the actual application. To date, great achievements have been made in materials’ design and cell configurations, but critical challenges still remain in the field of the solid electrolyte separator, its related lithium stripping/plating at the lithium anode, and catholyte design. In this mini-review, we summarized recent progress related to the solid electrolyte in aqueous Li–air batteries focusing on both material and battery device development. Moreover, we proposed a discussion and unique outlook on improving solid electrolyte compatibility and battery performance, thus designing an aqueous Li–air battery with higher energy density and better cycle performance in the future.

Adjustable Mixed Conductive Interphase for Dendrite-Free Lithium Metal Batteries

Lithium (Li) metal batteries with high energy density are of great promise for next-generation energy storage; however, they suffer from severe Li dendritic growth and an unstable solid electrolyte interphase. In this study, a mixed ionic and electronic conductive (MIEC) interphase layer with an adjustable ratio assembled by ZnO and Zn nanoparticles is developed. During the initial cycle, the in situ formed Li₂O with high ionic conductivity and a lithiophilic LiZn alloy with high electronic conductivity enable fast Li⁺ transportation in the interlayer and charge transfer at the ion/electron conductive junction, respectively. The optimized interface kinetics is achieved by balancing the ion migration and charge transfer in the MIEC Li₂O-LiZn interphase. As a result, the symmetric cell with MIEC interphase delivers superior cycling stability of over 1200 h. Also, Li||Zn-ZnO@PP||LFP (LFP = LiFePO₄) full cells exhibit long cyclic life for 2000 cycles with a very high capacity retention of 91.5% at a high rate of 5 C and stable cycling for 350 cycles at a high LFP loading mass of 13.27 mg cm^-2.

Engineered interfaces between perovskite La2/3xLi3xTiO3 electrolyte and Li metal for solid-state batteries

Perovskite La_2/3xLi_3xTiO₃ (LLTO) materials are promising solid-state electrolytes for lithium metal batteries (LMBs) due to their intrinsic fire-resistance, high bulk ionic conductivity, and wide electrochemical window. However, their commercialization is hampered by high interfacial resistance, dendrite formation, and instability against Li metal. To address these challenges, we first prepared highly dense LLTO pellets with enhanced microstructure and high bulk ionic conductivity of 2.1×10−4 S cm⁻¹ at room temperature. Then, the LLTO pellets were coated with three polymer-based interfacial layers, including pure (polyethylene oxide) (PEO), dry polymer electrolyte of PEO-LITFSI (lithium bis (trifluoromethanesulfonyl) imide) (PL), and gel PEO-LiTFSI-SN (succinonitrile) (PLS). It is found that each layer has impacted the interface differently; the soft PLS gel layer significantly reduced the total resistance of LLTO to a low value of 84.88 Ω cm⁻². Interestingly, PLS layer has shown excellent ionic conductivity but performs inferior in symmetric Li cells. On the other hand, the PL layer significantly reduces lithium nucleation overpotential and shows a stable voltage profile after 20 cycles without any sign of Li dendrite formation. This work demonstrates that LLTO electrolytes with denser microstructure could reduce the interfacial resistance and when combined with polymeric interfaces show improved chemical stability against Li metal.

Issues Concerning Interfaces with Inorganic Solid Electrolytes in All-Solid-State Lithium Metal Batteries

All-solid-state batteries have attracted wide attention for high-performance and safe batteries. The combination of solid electrolytes and lithium metal anodes makes high-energy batteries practical for next-generation high-performance devices. However, when a solid electrolyte replaces the liquid electrolyte, many different interface/interphase issues have arisen from the contact with electrodes. Poor wettability and unstable chemical/electrochemical reaction at the interfaces with lithium metal anodes will lead to poor lithium diffusion kinetics and combustion of fresh lithium and active materials in the electrolyte. Element cross-diffusion and charge layer formation at the interfaces with cathodes also impede the lithium ionic conductivity and increase the charge transfer resistance. The abovementioned interface issues hinder the electrochemical performance of all-solid-state lithium metal batteries. This review demonstrates the formation and mechanism of these interface issues between solid electrolytes and anodes/cathodes. Aiming to address the problems, we review and propose modification strategies to weaken interface resistance and improve the electrochemical performance of all-solid-state lithium metal batteries.

Construction of Mixed Ionic-Electronic Conducting Scaffolds in Zn Powder: A Scalable Route to Dendrite-Free and Flexible Zn Anodes

Zn powder (Zn-P)-based anodes are considered ideal candidates for Zn-based batteries because they enable a positive synergistic integration of safety and energy density. However, Zn-P-based anodes still experience easy corrosion, uncontrolled dendrite growth, and poor mechanical strength, which restrict their further application. Herein, a mixed ionic-electronic conducting scaffold is introduced into Zn-P to successfully fabricate anti-corrosive, flexible, and dendrite-free Zn anodes using a scalable tape-casting strategy. The as-established scaffold is characterized by robust flexibility, facile scale-up synthesis methodology, and exceptional anti-corrosive characteristics, and it can effectively homogenize the Zn²⁺ flux during Zn plating/stripping, thus allowing stable Zn cycling. Benefiting from these comprehensive attributes, the as-prepared Zn-P-based anode provides superior electrochemical performance, including long-life cycling stability and high rate capability in practical coin and flexible pouch cells; thus, it holds great potential for developing advanced Zn-ion batteries. The findings of this study provide insights for a promising scalable pathway to fabricate highly efficient and reliable Zn-based anodes and will aid in the realization of advanced flexible energy-storage devices.

Machine Learning enabled High-Throughput Screening of Inorganic Solid Electrolytes for Regulating Dendritic Growth in Lithium Metal Anodes

The Li-S secondary battery system has gained popularity owing to their advantage of higher specific energy compared to the Li ion battery. However, it suffers majorly due to the Li...

Strategies and challenges of carbon materials in the practical applications of lithium metal anode: a review

Lithium (Li) metal is strongly considered to be the ultimate anode for next-generation high-energy-density rechargeable batteries. Carbon materials and their composites with excellent structure tunability and properties have shown great potential applications in Li metal anodes.

Multifunctional Protection Layers via a Self-Driven Chemical Reaction To Stabilize Lithium Metal Anodes

The Li metal anode is considered one of the most potential anodes due to its highest theoretical specific capacity and the lowest redox potential. However, the scalable preparation of safe Li anodes remains a challenge. In the present study, a LiF-rich protection layer has been developed using self-driven chemical reactions between the Li_3xLa_2/3-xTiO₃/polyvinylidene fluoride/dimethylacetamide (LLTO/PVDF/DMAc) solution and the Li metal. After coating the LLTO/PVDF/DMAc solution to Li foil, PVDF reacted with Li spontaneously to form LiF, and the accompanying Ti⁴⁺ ions (in LLTO) were reduced to Ti³⁺ to form a mixed ionic and electronic conductor Li_xLLTO. The protective layer can redistribute the Li-ion transport, regulate the even Li deposition, and inhibit the Li dendrite growth. When paired with LiFePO₄, NCM811, and S cathodes, the batteries have demonstrated excellent capacity retention and cycling stability. More importantly, a volumetric energy density of 478 Wh L^-1 and 78% capacity retention after 310 cycles have been achieved by using a S/Li_xLLTO-Li pouch cell. This work provides a feasible avenue to provide large-scale preparation of safe Li anodes for the next-generation pouch-type Li-S batteries as ideal power sources for flexible electronic devices.

A Polar and Ordered-Channel Composite Separator Enables Antidendrite and Long-Cycle Lithium Metal Batteries

Lithium (Li) metal as an anode replacing the traditional graphite could largely enhance the specific energy density of Li batteries. However, the repeated formation of solid electrolyte interfaces on the surface of Li metal upon plating/stripping leads to a low Coulombic efficiency, and the growth of Li dendrites upon cycling probably causes the short circuit or even explosion of the batteries, both of which block the commercial application of Li metal in lithium metal batteries (LMBs). Herein, we report an antidendrite AAO@PVDF-HFP composite separator fabricated by a two-step method, which features the ordered pore channels and the polar groups in the channels. This novel composite separator has a good wettability to the electrolyte, high mechanical properties, and high ionic conductivity. Expectedly, the assembled batteries based on our novel composite separator show many impressive performances. In Li-Li cells, the cycling life up to 1600 h at an areal current density of 2 mA/cm² can be realized; in Li-Cu cells, the cycling life of more than 1000 h with a high Coulombic efficiency of 99.9% at 1 mA/cm² can be achieved. More interestingly, the Li/LiFePO₄ full batteries constructed by the novel AAO@PVDF-HFP composite separators show a high discharge capacity of 140 mAh/g and weak capacity decays even after 360 cycles. The novel design of the separator with ordered channels and polar groups presents an effective route for developing the next-generation LMBs.

Solid-State Lithium Metal Batteries with Extended Cycling Enabled by Dynamic Adaptive Solid-State Interfaces

Improving the long-term cycling stability of solid-state lithium (Li)-metal batteries (SSBs) is a severe challenge because of the notorious solid-solid interfacial contact loss originating from the repeated expansion and contraction of the Li anodes. Here, it is reported that high-performance SSBs are enabled by constructing brick-and-mortar electrolytes that can dynamically adapt to the interface changes during cycling. An electrolyte film with a high mechanical strain (250%) is fabricated by filling viscoelastic (600% strain) and piezoelectric block-copolymer electrolytes (mortar) into a mixed conductor Li_0.33 La_0.56 TiO_3-x nanofiber film (brick). During Li-plating, the electrolytes can homogenize the interfacial electric field and generate piezoelectricity to promote uniform Li-deposition, while the mortar can adhere to the Li-anode without interfacial disintegration in the reversed Li-stripping. As a result, the electrolytes show excellent compatibility with the electrodes, leading to a long electrochemical cyclability at room temperature. The symmetrical Li//Li cells run stably for 1880 h without forming dendrites, and the LiFePO₄ /Li full batteries deliver high coulombic efficiency (>99.5%) and capacity retention (>85%) over 550 cycles. More practically, the pouch cells exhibit excellent flexibility and safety for potential practical applications.

Dynamic Regulation of Lithium Dendrite Growth with Electromechanical Coupling Effect of Soft BaTiO3 Ceramic Nanofiber Films

Lithium (Li) metal batteries (LMBs) offer great opportunity for developing high-energy density energy-storage-systems, but the anodes suffer a severe problem of dendrite growth that hinders the practical applications of LMBs. Here, we report a soft BaTiO₃ ceramic nanofiber film with excellent ferroelectricity and piezoelectricity that enables one to transverse the dense deposition of Li metal. During Li plating, the strong ferroelectricity reduces the Li-ion concentration gradient near the anode and thus facilitating their uniform deposition. Once squeezed by these Li deposits, the BaTiO₃ film generates instantaneous piezo-effect to dynamically change the subsequent Li deposition from vertical to lateral. As a result, Li-Cu cells exhibit reversible plating-stripping processes for over 200 cycles with a high Coulombic efficiency of >98.3%. When pairing with high-voltage LiNi_0.8Co_0.15Al_0.05O₂ cathodes, the LMBs can retain >80% capacity in 300 cycles without forming dendrites even under challenging conditions including a high cathode loading of 7.2 mg/cm², a lean electrolyte amount of 7 μL/mg, and high current rates. The findings point to a promising electromechanical coupling strategy to dynamically adjust dendrite growth for designing Li metal anodes.

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

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

Nanometer-Thick Crystalline Carbon Films Having a Spinel Structure Grown on ZnO Substrates: Implications for New Ceramic-Carbon Composition

I developed a bottom-up process of crystal growth using a field emission (FE) electron beam without transfer of heat energy. In this study, highly crystalline single-walled carbon nanotubes were used as the FE electron source. Acetylene was irradiated with an electron beam of high-resolution energy emitted from the electron source. Then, zinc oxide (ZnO) was irradiated with the carbon-based ions dissociated from the acetylene and electron beam, which formed a nonequilibrium excitation reaction field. As a result, a crystalline carbon thin film with a spinel-like structure different from the structures of graphite and diamond was grown on the ZnO surface. It is considered that the carbon film can be formed on substrates with a periodic crystal structure, not only ZnO. I confirmed that a carbon film with a periodic crystal structure independent of the crystal structure of the underlying substrate was grown, which bridged with the substrate. Thus, I have established a technique of crystal bridging between a ceramic and carbon for the first time to the best of our knowledge.

A Self-Healing Amalgam Interface in Metal Batteries

Poor cyclability and safety concerns caused by the uncontrollable dendrite growth and large interfacial resistance severely restrict the practical applications of metal batteries. Herein, a facile, universal strategy to fabricate ceramic and glass phase compatible, and self-healing metal anodes is proposed. Various amalgam-metal anodes (Li, Na, Zn, Al, and Mg) show a long cycle life in symmetric cells. It has been found that liquid Li amalgam shows a complete wetting with the surface of lanthanum lithium titanate electrolyte and a glass-phase solid-state electrolyte. The interfacial compatibility between the lithium metal anode and solid-state electrolyte is dramatically improved by using an in situ regenerated amalgam interface with high electron/ion dual-conductivity, obviously decreasing the anode/electrolyte interfacial impedance. The lithium-amalgam interface between the metal anode and electrolyte undergoes a reversible isothermal phase transition between solid and liquid during the cycling process at room temperature, resulting in a self-healing surface of metal anodes.

Recent advances in research on anodes for safe and efficient lithium-metal batteries

The revival of lithium metal anodes (LMAs) makes it a potent influence on the battery research community in the recent years after the popularity of Li-ion batteries with graphite anodes. The main reason is due to the over ten-fold increase in the capacity of LMAs when compared with that obtained when using graphite, as well as the low redox potential of Li/Li⁺. However, the full potential of LMAs is heavily inhibited by several factors, such as dendrite growth, pulverization, side reactions, and volume changes. These adversities lower the cell's Coulombic efficiency dramatically if operated without massively excessive Li usage. In this review, we first introduce some of the most significant progresses made in the understandings of the charging/discharging processes at the anode. The importance of combining advanced characterization techniques with classical methods is highlighted. In particular, we aim to explore the hidden links between those studies for obtaining deeper insights. Two main categories of solutions to address common problems, namely, lithium-electrolyte interfacial engineering and three-dimensional hosting of Li, are subsequently illustrated, where each subsection takes a different methodological perspective to demonstrate the relevant state-of-the-art studies. Some interesting approaches to stop dendrites and a brief note on the practical aspects of lithium-metal batteries are provided, too. This review concludes with our essential discoveries from the current literature and valuable suggestions for future LMA research.

Hybrid solid electrolyte enabled dendrite-free Li anodes for high-performance quasi-solid-state lithium-oxygen batteries

The dendrite growth of Li anodes severely degrades the performance of lithium-oxygen (Li-O₂) batteries. Recently, hybrid solid electrolyte (HSE) has been regarded as one of the most promising routes to tackle this problem. However, before this is realized, the HSE needs to simultaneously satisfy contradictory requirements of high modulus and even, flexible contact with Li anode, while ensuring uniform Li⁺ distribution. To tackle this complex dilemma, here, an HSE with rigid Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) core@ultrathin flexible poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) shell interface has been developed. The introduced large amount of nanometer-sized LAGP cores can not only act as structural enhancer to achieve high Young's modulus but can also construct Li⁺ diffusion network to homogenize Li⁺ distribution. The ultrathin flexible PVDF-HFP shell provides soft and stable contact between the rigid core and Li metal without affecting the Li⁺ distribution, meanwhile suppressing the reduction of LAGP induced by direct contact with Li metal. Thanks to these advantages, this ingenious HSE with ultra-high Young's modulus of 25 GPa endows dendrite-free Li deposition even at a deposition capacity of 23.6 mAh. Moreover, with the successful inhibition of Li dendrites, the HSE-based quasi-solid-state Li-O₂ battery delivers a long cycling stability of 146 cycles, which is more than three times that of gel polymer electrolyte-based Li-O₂ battery. This new insight may serve as a starting point for further designing of HSE in Li-O₂ batteries, and can also be extended to various battery systems such as sodium-oxygen batteries.

Recent Advances in Lithiophilic Porous Framework toward Dendrite-Free Lithium Metal Anode

Rechargeable lithium metal anode (LMA) based batteries have attracted great attention as next-generation high-energy-density storage systems to fuel the extensive practical applications in portable electronics and electric vehicles. However, the formation of unstable solid-electrolyte- interphase (SEI) and growth of lithium dendrite during plating/stripping cycles stimulate safety concern, poor coulombic efficiency (CE), and short lifespan of the lithium metal batteries (LMBs). To address these issues, the rational design of micro/nanostructured Li hosts are widely adopted in LMBs. The high surface area of the interconnected conductive framework can homogenize the Li-ion flux distribution, lower the effective current density, and provides sufficient space for Li accommodation. However, the poor lithiophilicity of the micro/nanostructure host cannot govern the initial lithium nucleation, which leads to the non-uniform/dendritic Li deposition and unstable SEI formation. As a result, the nucleation overpotential and voltage hysteresis increases, which eventually leads to poor battery cycling performance. Thus, it is imperative to decorate a micro/nanostructured Li host with lithiophilic coatings or seeds for serving as a homogeneous nucleation site to guide the uniform lithium deposition. In this review, we summarize research progress on porous metal and non-metal based lithiophilic micro/nanostructured Li hosts. We present the synthesis, structural properties, and the significance of lithiophilic decorated micro/nanostructured Li host in the LMBs. Finally, the perspectives and critical challenges needed to address for the further improvement of LMBs are concluded.

Stable Lithium Metal Anode Enabled by a Lithiophilic and Electron/Ion Conductive Framework

Li metal anode has been considered as the ideal anode for next-generation batteries due to its ultrahigh capacity and lowest electrochemical potential. However, its practical application is still impeded by low Coulombic efficiency, huge volume change, and safety hazards arising from Li dendrite growth. In this work, a three-dimensional (3D) structured highly stable Li metal anode is designed and easily preapred. Benefiting from the in situ reaction between Li metal and AlN, highly Li⁺ conductive Li₃N and lithiophilic LiAl alloy have been simultaneously formed and homogeneously distributed in the framework, in which Li metal is finely dispersed and embedded. The outstanding electron/ion mixed conductivity of Li₃N/LiAl and 3D composite structure with enhanced interfacial area significantly improve the electrode kinetics and suppress the volume change on cycling, while a lithiophilic effect of LiAl alloy and uniform distribution of Li ion flux inside the electrode avoid dendritic Li deposition. As a result, the proposed Li metal electrode exhibits exceptional electrochemical reversibility in both carbonate and ether-based electrolytes. Paired with LiFePO₄ and sulfurized polyacrylonitrile (S@pPAN) cathodes, the full cells deliver highly stable and long-term cycling performance. Therefore, the proposed strategy to fabricate Li metal anodes could promote the practical application of Li metal batteries.

Understanding of Lithium Insertion into 3D Porous Carbon Scaffolds with Hybridized Lithiophobic and Lithiophilic Surfaces by In-Operando Study

In-operando study coupled with voltage/current profiles are presented in order to unveil lithium insertion processes into 3D porous carbon nanotube (CNT) structures whose surfaces were altered to have lithiophobic, lithiophilic, and hybridized lithiophobic/philic characteristics using graphitic surfaces with/without carboxyl/hydroxyl groups. We found the lithiophobic graphitic surfaces hindered lithium insertion into the scaffold despite the high conductivity of CNT. The lithiophilic surface caused another problem of lithium deposition on the outer surface of the electrode, clogging pores and engendering dendrites. Conversely, in the hybridized CNT, lithiophilic trenches partially created on the pristine CNT allowed for uniform lithium deposition into the pores by simultaneously improved lithium attraction and charge transfer, reaching a high areal capacity of 16 mAh cm^-2 even with a current density of 8 mA cm^-2 without noticeable dendrite formation and volume expansion. Our hybridization approach provides valuable insight to realize a high-energy-density anode by uniformly impregnating lithium into porous media.

A wet-processed, binder-free sulfur cathode integrated with a dual-functional separator for flexible Li-S batteries

Developing flexible, robust and lightweight sulfur cathodes by rationally designing their structures and configurations through a viable and scalable strategy is a critical enabler for fulfilling flexible lithium-sulfur (Li-S) batteries. However, besides the requirements for cathode flexibility, intrinsic limitations from the shuttling of lithium polysulfides and the growth of Li dendrites have restricted the widespread implementations of Li-S batteries. Here, we report a wet-processed strategy by dissolving and recrystallizing S in a suitable solvent to fabricate a flexible, binder-free S cathode. Integrating the resulting S cathode with a dual-functional separator has demonstrated to be able to suppress both the shuttle effect and growth of dendritic Li. The wet-processed strategy not only enables the fabrication of flexible and binder-free S-nanomat cathodes, but also facilitates the deposition of the cathodes on the separators. Meanwhile, a dual-functional separator is fabricated by vapor-phase polymerization of polypyrrole (PPy) coating on both surfaces of the commercial separator, which leads to the reduction of the shuttle effect and the suppression of the growth of dendritic Li simultaneously. As a result, by integrating the S-nanomat and the dual-functional separator, the cathode exhibits exceptional mechanical properties and electrochemical performance. Li-S pouch cells are further demonstrated to show stable cycling performance in the bending state, indicating the feasibility of the integrated S cathode for flexible Li-S batteries.

Solid electrolyte interphase formation between the Li0.29La0.57TiO3 solid-state electrolyte and a Li-metal anode: an ab initio molecular dynamics study

An ab initio molecular dynamics study of an electrochemical interface between a solid-state-electrolyte Li_0.29La_0.57TiO₃ and Li-metal is performed to analyze interphase formation and evolution when external electric fields of 0, 0.5, 1.0 and 2.0 V Å^-1 are applied. From this electrochemical stability analysis, it was concluded that lithium-oxide (Li₂O) and lanthanum-oxide (La₂O₃) phases were formed at the electrolyte/anode interphase. As the electric field increased, oxygen from the electrolyte diffused through the Li-metal anode, increasing the amount of O from deeper crystallographic planes of the electrolyte that reacted with Li and La. A strong reduction of Ti was expected from their Bader charge variation from +3.5 in the bulk to +2.5 at the interface. Due to the loss of Li atoms from the anode to form Li-oxide at the interphase, vacancies were created on the Li-metal, causing anode structure amorphization near the Li-oxide phase and keeping the rest of the anode structure as BCC. Therefore, the interface was unstable because of the continuous Li-oxide and La-oxide formation and growth, which were more pronounced when increasing the external electric field.

Developing high safety Li-metal anodes for future high-energy Li-metal batteries: strategies and perspectives

Developing high-safety Li-metal anodes (LMAs) are extremely important for the application of high-energy Li-metal batteries. The recently state-of-the-art technologies, strategies and perspectives for developing LMAs are comprehensively summarized in this review.

Self-Assembled Conductive Metal-Oxide Nanofiber Interface for Stable Li-Metal Anode

Li-metal anodes promise to build high-energy-storage systems, but they suffer from safety problems from severe dendrite growth. Here, we develop a thin and conformal hybrid ionic and electronic conducting metal-oxide nanofiber interface to stabilize Li-anodes without forming dendrites. The thin ionic-conductive Li_0.33La_0.56TiO₃ (LLTO) nanofiber film is first fabricated by electrospinning followed by pyrolysis. After connecting with the electrolytes-wetted Li-metal anodes, due to the self-driven chemical reactions, LLTO is reduced, and a hybrid conducting interface is developed. The interface can act as a reservoir to redistribute the nonuniform Li-ion flux above the anode surface and reduce the driving force of dendrite formation by leveling electric potential distribution, enabling a stable Li plating-stripping with a low overpotential of 80 mV over 800 h at a high current of 5 mA/cm². More practically, the Li-LiNi_0.8Co_0.15Al_0.05O₂ cells deliver a high capacity of 147 mA h/g at 1 C with a Coulombic efficiency of 99% over 150 cycles, offering prospects to achieve reliable Li-metal batteries.

Dendrites in Lithium Metal Anodes: Suppression, Regulation, and Elimination

With the increasing diversification of portable electronics and large-scale energy storage systems, conventional lithium-ion batteries (LIBs) with graphite anodes are now approaching their theoretical limits. Lithium metal, as the "Holy Grail" electrode for next-generation rechargeable batteries, is being revisited to meet the booming demand for high energy density electrodes due to its ultrahigh theoretical specific capacity and negative redox potential. Nevertheless, typical issues like notorious dendrite growth still hamper the bulk application of Li metal anodes. Dendrite growth renders increased surface area of the lithium metal, causing persistent depletion of the electrolyte and active materials, facilitating catastrophic failure of the battery, and even inducing fatal safety hazards. The consequences become more serious during operation at high current densities and over long cycling life. Therefore, it is urgent to suppress and even eliminate dendrite formation during the Li plating/stripping process. This Account highlights several innovative strategies for dendrite suppression, dendrite regulation, and dendrite elimination from the perspective of interface energy and bulk stresses. First, we review the fundamental mechanism of dendrite formation and growth in Li metal anodes. We show that the dendrite morphology could be substantially ameliorated, in theory, by homogenizing the electric field distribution, lowering the Li ion concentration gradient, and facilitating mechanical blocking. Next, we address the problem of dendrite suppression by applying two-dimensional (2D) materials to Li metal systems and preventing dendrite penetration through stress release and mechanical blocking. Graphene with a high specific area and vermiculite sheets (VSs) with a large physical rigidity were demonstrated to be efficacious in reinforcing Li anodes and polymer electrolytes separately. However, Li dendrite growth is a continuous process and remains inevitable with increasing current density and cycling life. Instead of suppressing dendrite growth, we focus on how to regulate homogeneous Li dendrite formation and growth. Dendrite regulation means to allow dendrite growth but take steps to transform it into Li with a smooth morphology. We introduce two main strategies to regulate Li growth: (i) guiding Li nucleation and (ii) controlling the Li growth pathways and directions. These processes greatly rely on the interface energy between the substrate and Li atoms. Elimination of the dendrites, which is the most formidable challenge for dendrite control, can also be achieved by dynamically engineering the force, such as deflecting the electric field by Lorentz force in a magnetic field, enhancing the integrated yield stress by the design of bulk nanostructured materials, and reducing the lateral Li diffusion barrier by a biomimetic co-deposition process. Solutions to the challenges of dendrite control in Li metal anodes can provide safe next-generation rechargeable lithium metal batteries that have a long cycling life. We also hope that our strategies presented in this Account can offer promise for other metal batteries.

Li0.35La0.55TiO3 Nanofibers Enhanced Poly(vinylidene fluoride)-Based Composite Polymer Electrolytes for All-Solid-State Batteries

Using polymer electrolytes with relatively high mechanical strength, enhanced safety, and excellent flexibility to replace the conventional liquid electrolytes is an effective strategy to curb the Li-dendrite growth in Li-metal batteries (LMBs). However, low ionic conductivity, unsatisfactory thermal stability, and narrow electrochemical window still hinder their applications. Here, we fabricate Li_0.35La_0.55TiO₃ (LLTO) nanofiber-enabled poly(vinylidene fluoride) (PVDF)-based composite polymer electrolytes (CPEs) with enhanced mechanical property and wide electrochemical window. The results show that 15 wt % of LLTO nanofibers synergize with PVDF, giving a flexible electrolyte membrane with significantly improved performance, such as high ionic conductivity (5.3 × 10^-4 S cm^-1), wide electrochemical window (5.1 V), high mechanical strength (stress 9.5 MPa, strain 341%), and good thermal stability (thermal degradation 410 °C). In addition, an all-solid-state Li-metal battery of sandwich-type LiFePO₄/PVDF-CPE (15 wt % of LLTO)/Li delivers satisfactory cycling stability and outstanding rate performance. A reversible capacity of 121 mA h g^-1 is delivered at 1 C after 100 cycles. This work exemplifies that the introduction of LLTO nanofibers can improve the electrochemical performances of PVDF-based CPEs used as electrolytes for all-solid-state LMBs.

Constructing Ionic Gradient and Lithiophilic Interphase for High-Rate Li-Metal Anode

Li metal is the optimal choice as an anode due to its high theoretical capacity, but it suffers from severe dendrite growth, especially at high current rates. Here, an ionic gradient and lithiophilic inter-phase film is developed, which promises to produce a durable and high-rate Li-metal anode. The film, containing an ionic-conductive Li_0.33 La_0.56 TiO₃ nanofiber (NF) layer on the top and a thin lithiophilic Al₂ O₃ NF layer on the bottom, is fabricated with a sol-gel electrospinning method followed by sintering. During cycling, the top layer forms a spatially homogenous ionic field distribution over the anode, while the bottom layer reduces the driving force of Li-dendrite formation by decreasing the nucleation barrier, enabling dendrite-free plating-stripping behavior over 1000 h at a high current density of 5 mA cm^-2 . Remarkably, full cells of Li//LiNi_0.8 Co_0.15 Al_0.05 O₂ exhibit a high capacity of 133.3 mA h g^-1 at 5 C over 150 cycles, contributing a step forward for high-rate Li-metal anodes.

Endowing the Lithium Metal Surface with Self-Healing Property via an in Situ Gas-Solid Reaction for High-Performance Lithium Metal Batteries

The property of the solid electrolyte interphase (SEI) layer is of prime importance for the performance of lithium metal anodes. Replacing the spontaneously formed inhomogeneous and unstable SEI layer with a high-performance artificial SEI is an effective strategy. Herein, a self-healing SEI layer with high lithium-ion conductivity and a stable framework to address the issues of poor performance of lithium metal anodes is achieved. C, Li₂S, and LiI are uniformly distributed on the lithium surface via a "sauna" reaction between CS₂-I₂ mixed steam and metal lithium, which has the potential to be applied to large-scale preparation. The obtained SEI layer possesses high mechanical strength and facilitated lithium-ion transport capability, which are inherited from the amorphous C and lithium compounds (Li₂S and LiI). Most importantly, the LiI component can migrate through the electrolyte and cover the exposed lithium caused by flaws and cracks, leading to a self-healing property. As a result, the C-Li₂S-LiI@Li electrode exhibits excellent electrochemical performance with low overpotential and long lifespan.

Bulk Nanostructured Materials Design for Fracture-Resistant Lithium Metal Anodes

Li metal is an ideal anode for next-generation batteries because of its high theoretical capacity and low potential. However, the unevenly distributed stress in Li metal anodes (LMAs) induced by volume fluctuation may cause the electrode to fracture easily, especially during high-rate plating/stripping processes. Here fracture-resistant LMAs using the concept of bulk nanostructured materials are designed via a metallurgical process. In bulk nanostructured Li (BNL), ionic conducting phases exist at grain boundaries, which promote Li⁺ transport. The refined Li grain size and precipitation hardening in BNL enhances the mechanical strength and fatigue endurance, alleviating the unevenly distributed stress and preventing electrode pulverization. Density functional theory is used to investigate the binding energy between Li and various kinds of oxides and SiO₂ is found to be optimal additive among screened oxides. BNL has 91% of the theoretical capacity of Li metal. In full cells with BNL anode, LiFePO₄ could deliver capacity of 90 mAh g^-1 at 10C, an order of magnitude higher than that in full cells with Li foil anode. This strategy is expected to pave the way for fracture-resistant LMAs in high-rate cycling with maximum capacity.

Dual Lithiophilic Structure for Uniform Li Deposition

The development of Li metal anodes is hindered by the Li dendrites arising from the random deposition of Li metal during cycles. Hence, uniform deposition of Li during repeated cycles is crucial for the development of Li metal batteries. However, it is difficult to regulate Li deposition because of convection in the electrolyte. Here, we employ a dual lithiophilic structure composed of a polar metal-organic framework (MOF) and highly conductive Ag nanoparticles, and we show that it brings about uniform lithium deposition. The binding energy for Li is increased by the abundant oxygen sites and large surface area of the MOF, and concomitantly, the uniform distribution of Li nuclei can be achieved with a low nucleation overpotential. When highly conductive lithiophilic Ag is incorporated into the MOF, the binding energy for Li is further increased and the nucleation overpotential is decreased to nearly zero. As a result, Li platting and stripping on the Ag@MOF (i.e., Ag@HKUST-1) substrate exhibit a Coulombic efficiency of 97% over 300 cycles and a high areal capacity of 5 mA h cm^-2 without dendrite formation.

An Armored Mixed Conductor Interphase on a Dendrite-Free Lithium-Metal Anode

Lithium-metal electrodes have undergone a comprehensive renaissance to meet the requirements of high-energy-density batteries due to their lowest electrode potential and the very high theoretical capacity. Unfortunately, the unstable interface between lithium and nonaqueous electrolyte induces dendritic Li and low Coulombic efficiency during repeated Li plating/stripping, which is one of the huge obstacles toward practical lithium-metal batteries. Here, a composite mixed ionic/electronic conductor interphase (MCI) is formed on the surface of Li by in situ chemical reactions of a copper-fluoride-based solution and Li metal at room temperature. The as-obtained MCI film acts like the armor of a soldier to protect the Li-metal anode by its prioritized lithium storage, high ionic conductivity, and high Young's modulus. The armored MCI can effectively suppress Li-dendrite growth and work effectively in LiNi_0.5 Co_0.2 Mn_0.3 O₂ /Li cells. The armored MCI presents fresh insights into the formation and regulation of the stable electrode-electrolyte interface and an effective strategy to protect Li-metal anodes in working Li-metal 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.

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.