Controlling Nucleation in Lithium Metal Anodes

Dendrite Growth and Dead Lithium Formation in Lithium Metal Batteries and Mitigation Using a Protective Layer: A Phase-Field Study

Lithium metal batteries (LMBs) are considered one of the most promising next-generation rechargeable batteries due to their high specific capacity. However, severe dendrite growth and subsequent formation of dead lithium (Li) during the battery cycling process impede its practical application. Although extensive experimental studies have been conducted to investigate the cycling process, and several theoretical models were developed to simulate the Li dendrite growth, there are limited theoretical studies on the dead Li formation, as well as the entire cycling process. Herein, we developed a phase-field model to simulate both electroplating and stripping process in a bare Li anode and Li anode covered with a protective layer. A step function is introduced in the stripping model to capture the dynamics of dead Li. Our simulation clearly shows the growth of dendrites from a bare Li anode during charging. These dendrites detach from the bulk anode during discharging, forming dead Li. Dendrite growth becomes more severe in subsequent cycles due to enhanced surface roughness of the Li anode, resulting in an increasing amount of dead Li. In addition, it is revealed that dendrites with smaller base diameters detach faster at the base and produce more dead lithium. Meanwhile, the Li anode covered with a protective layer cycles smoothly without forming Li dendrite and dead Li. However, if the protective layer is fractured, Li metal preferentially grows into the crack due to enhanced Li-ion (Li+) flux and forms a dendrite structure after penetration through the protective layer, which accelerates the dead Li formation in the subsequent stripping process. Our work thus provides a fundamental understanding of the mechanism of dead Li formation during the charging/discharging process and sheds light on the importance of the protective layer in the prevention of dead Li in LMBs.

Development and Optimization of Thin-Lithium-Metal Anodes with a Lithium Lanthanum Titanate Stabilization Coating for Enhancement of Lithium-Sulfur Battery Performance

Lithium-ion batteries are dominating high-energy-density energy storage for 30 years. However, their development approaches theoretical limits, spurring the development of lithium-sulfur cells that achieve high energy densities through reversible electrochemical conversion reactions. Nevertheless, the commercialization of lithium-sulfur cells is hindered by practical challenges associated primarily with the use of thick-lithium anodes, low-loading sulfur cathodes, and high electrolyte-to-sulfur ratios, which prevent realization of the cells' full potential in terms of electrochemical and material performance. To solve these extrinsic and intrinsic problems, the effect of lithium-metal thickness on the electrochemical behavior of lithium-sulfur cells with high-loading sulfur cathodes in lean-electrolyte configurations is investigated. Specifically, lithium lanthanum titanate (LLTO), a solid electrolyte, is utilized to form an ionically/electronically conductive coating to stabilize lithium-metal anodes, thereby enhancing their lithium-ion pathways and interfacial charge transfer. Electrochemical analyses reveal that an LLTO coating significantly reduces excessive reactions between lithium metal and an electrolyte, thereby minimizing lithium consumption and electrolyte depletion. Further, LLTO-stabilized lithium anodes improve lithium-sulfur cell performance, and most importantly, allow the fabrication of thin-lithium, high-loading-sulfur cells that open a pathway toward high-energy-density batteries.

Interfacial chemistry in multivalent aqueous batteries: fundamentals, challenges, and advances

As one of the most promising electrochemical energy storage systems, aqueous batteries are attracting great interest due to their advantages of high safety, high sustainability, and low costs when compared with commercial lithium-ion batteries, showing great promise for grid-scale energy storage. This invited tutorial review aims to provide universal design principles to address the critical challenges at the electrode-electrolyte interfaces faced by various multivalent aqueous battery systems. Specifically, deposition regulation, ion flux homogenization, and solvation chemistry modulation are proposed as the key principles to tune the inter-component interactions in aqueous batteries, with corresponding interfacial design strategies and their underlying working mechanisms illustrated. In the end, we present a critical analysis on the remaining obstacles necessitated to overcome for the use of aqueous batteries under different practical conditions and provide future prospects towards further advancement of sustainable aqueous energy storage systems with high energy and long durability.

In Situ Visual Observation of Surface Energy-Controlled Heterogeneous Nucleation of Metal Nanocrystals

Electrochemical growth of metal nanocrystals is pivotal for material synthesis, processing, and resource recovery. Understanding the heterogeneous interface between electrolyte and electrode is crucial for nanocrystal nucleation, but the influence of this interaction is still poorly understood. This study employs advanced in situ measurements to investigate the heterogeneous nucleation of metals on solid surfaces. By observing the copper nanocrystal electrodeposition, an interphase interaction-induced nucleation mechanism highly dependent on substrate surface energy is uncovered. It shows that a high-energy (HE) electrode tended to form a polycrystalline structure, while a low-energy (LE) electrode induced a monocrystalline structure. Raman and electrochemical characterizations confirmed that HE interface enhances the interphase interaction, reducing the nucleation barrier for the sturdy nanostructures. This leads to a 30.92-52.21% reduction in the crystal layer thickness and a 19.18-31.78% increase in the charge transfer capability, promoting the formation of a uniform and compact film. The structural compactness of the early nucleated crystals enhances the deposit stability for long-duration electrodeposition. This research not only inspires comprehension of physicochemical processes correlated with heterogeneous nucleation, but also paves a new avenue for high-quality synthesis and efficient recovery of metallic nanomaterials.

Stable Zinc Electrode Reaction Enabled by Combined Cationic and Anionic Electrolyte Additives for Non-Flow Aqueous Zn─Br2 Batteries

Aqueous zinc-bromine batteries hold immense promise for large-scale energy storage systems due to their inherent safety and high energy density. However, achieving a reliable zinc metal electrode reaction is challenging because zinc metal in the aqueous electrolyte inevitably leads to dendrite growth and related side reactions, resulting in rapid capacity fading. Here, it is reported that combined cationic and anionic additives in the electrolytes using CeCl3 can simultaneously address the multiple chronic issues of the zinc metal electrode. Trivalent Ce3+ forms an electrostatic shielding layer to prevent Zn2+ from concentrating at zinc metal protrusions, while the high electron-donating nature of Cl- mitigates H2O decomposition on the zinc metal surface by reducing the interaction between Zn2+ and H2O. These combined cationic and anionic effects significantly enhance the reversibility of the zinc metal reaction, allowing the non-flow aqueous Zn─Br2 full-cell to reliably cycle with exceptionally high capacity (>400 mAh after 5000 cycles) even in a large-scale battery configuration of 15 × 15 cm2.

High-Throughput Combinatorial Analysis of the Spatiotemporal Dynamics of Nanoscale Lithium Metal Plating

The development of Li metal batteries requires a detailed understanding of complex nucleation and growth processes during electrodeposition. In situ techniques offer a framework to study these phenomena by visualizing structural dynamics that can inform the design of uniform plating morphologies. Herein, we combine scanning electrochemical cell microscopy (SECCM) with in situ interference reflection microscopy (IRM) for a comprehensive investigation of Li nucleation and growth on lithiophilic thin-film gold electrodes. This multimicroscopy approach enables nanoscale spatiotemporal monitoring of Li plating and stripping, along with high-throughput capabilities for screening experimental conditions. We reveal the accumulation of inactive Li nanoparticles in specific electrode regions, yet these regions remain functional in subsequent plating cycles, suggesting that growth does not preferentially occur from particle tips. Optical-electrochemical correlations enabled nanoscale mapping of Coulombic Efficiency (CE), showing that regions prone to inactive Li accumulation require more cycles to achieve higher CE. We demonstrate that electrochemical nucleation time (tnuc) is a lagging indicator of nucleation and introduce an optical method to determine tnuc at earlier stages with nanoscale resolution. Plating at higher current densities yielded smaller Li nanoparticles and increased areal density, and was not affected by heterogeneous topographical features, being potentially beneficial to achieve a more uniform plating at longer time scales. These results enhance the understanding of Li plating on lithiophilic surfaces and offer promising strategies for uniform nucleation and growth. Our multimicroscopy approach has broad applicability to study nanoscale metal plating and stripping phenomena, with relevance in the battery and electroplating fields.

Ultrauniform Plating of Lithium on 10-nm-Scale Ordered Carbon Grids for Long Lifespan Lithium Metal Batteries

Tailorable lithium (Li) nucleation and uniform early-stage plating is essential for long-lifespan Li metal batteries. Among factors influencing the early plating of Li anode, the substrate is critical, but a fine control of the substrate structure on a scale of ≈10 nm has been rarely achieved. Herein, a carbon consisting of ordered grids is prepared, as a model to investigate the effect of substrate structure on the Li nucleation. In contrast to the individual spherical Li nuclei formed on the flat graphene, an ultrauniform and nuclei-free Li plating is obtained on the ordered carbon with a grid size smaller than the thermodynamical critical radius of Li nucleation (≈26 nm). Simultaneously, an inorganic-rich solid-electrolyte-interphase is promoted by the cross-sectional carbon layers of such ordered grids which are exposed to the electrolyte. Consequently, the carbon grids with a grid size of ≈10 nm show a favorable cycling stability for more than 1100 cycles measured at 2 mA cm-2 in a half cell. With LiNi0.8Co0.1Mn0.1O2 as cathode, the assembled full cell with a cathode capacity of 3 mAh cm-2 and a negative/positive ratio of 1.67 demonstrates a stable cycling for over 130 cycles with a capacity retention of 88%.

Oxygen Vacancy and Bandgap Simultaneous Modulation to Achieve High Lithiophilicity and Mechanical Strength of Lithium Metal Anodes

Metal oxides with conversion and alloying mechanisms are more competitive in suppressing lithium dendrites. However, it is difficult to simultaneously regulate the conversion and alloying reactions. Herein, conversion and alloying reactions are regulated by modulation of the zinc oxide bandgap and oxygen vacancies. State-of-the-art advanced characterization techniques from a microcosmic to a macrocosmic viewpoint, including neutron diffraction, synchrotron X-ray absorption spectroscopy, synchrotron X-ray microtomography, nanoindentation, and ultrasonic C-scan demonstrated the electrochemical gain benefit from plentiful oxygen vacancies and low bandgaps due to doping strategies. In addition, high mechanical strength 3D morphology and abundant mesopores assist in the uniform distribution of lithium ions. Consequently, the best-performed ZnO-2 offers impressive electrochemical properties, including symmetric Li cells with 2000 h and full cells with 81% capacity retention after 600 cycles. In addition to providing a promising strategy for improving the lithiophilicity and mechanical strength of metal oxide anodes, this work also sheds light on lithium metal batteries for practical applications.

Manipulating the diffusion energy barrier at the lithium metal electrolyte interface for dendrite-free long-life batteries

Constructing an artificial solid electrolyte interphase (SEI) on lithium metal electrodes is a promising approach to address the rampant growth of dangerous lithium morphologies (dendritic and dead Li0) and low Coulombic efficiency that plague development of lithium metal batteries, but how Li+ transport behavior in the SEI is coupled with mechanical properties remains unknown. We demonstrate here a facile and scalable solution-processed approach to form a Li3N-rich SEI with a phase-pure crystalline structure that minimizes the diffusion energy barrier of Li+ across the SEI. Compared with a polycrystalline Li3N SEI obtained from conventional practice, the phase-pure/single crystalline Li3N-rich SEI constitutes an interphase of high mechanical strength and low Li+ diffusion barrier. We elucidate the correlation among Li+ transference number, diffusion behavior, concentration gradient, and the stability of the lithium metal electrode by integrating phase field simulations with experiments. We demonstrate improved reversibility and charge/discharge cycling behaviors for both symmetric cells and full lithium-metal batteries constructed with this Li3N-rich SEI. These studies may cast new insight into the design and engineering of an ideal artificial SEI for stable and high-performance lithium metal batteries.

Enhanced Capacity Retention of Li3V2(PO4)3-Cathode-Based Lithium Metal Battery Using SiO2-Scaffold-Confined Ionic Liquid as Hybrid Solid-State Electrolyte

Li₃V₂(PO₄)₃ (LVP) is one of the candidates for high-energy-density cathode materials matching lithium metal batteries due to its high operating voltage and theoretical capacity. However, the inevitable side reactions of LVP with a traditional liquid-state electrolyte under high voltage, as well as the uncontrollable growth of lithium dendrites, worsen the cycling performance. Herein, a hybrid solid-state electrolyte is prepared by the confinement of a lithium-containing ionic liquid with a mesoporous SiO₂ scaffold, and used for a LVP-cathode-based lithium metal battery. The solid-state electrolyte not only exhibits a high ionic conductivity of 3.14 × 10^-4 S cm^-1 at 30 °C and a wide electrochemical window of about 5 V, but also has good compatibility with the LVP cathode material. Moreover, the cell paired with a solid-state electrolyte exhibits good reversibility and can realize a stable operation at a voltage of up to 4.8 V, and the discharge capacity is well-maintained after 100 cycles, which demonstrates excellent capacity retention. As a contrast, the cell paired with a conventional liquid-state electrolyte shows only an 87.6% discharge capacity retention after 100 cycles. In addition, the effectiveness of a hybrid solid-state electrolyte in suppressing dendritic lithium is demonstrated. The work presents a possible choice for the use of a hybrid solid-state electrolyte compatible with high-performance cathode materials in lithium metal batteries.

Origin of Heterogeneous Stripping of Lithium in Liquid Electrolytes

Lithium metal batteries suffer from low cycle life. During discharge, parts of the lithium are not stripped reversibly and remain isolated from the current collector. This isolated lithium is trapped in the insulating remaining solid-electrolyte interphase (SEI) shell and contributes to the capacity loss. However, a fundamental understanding of why isolated lithium forms and how it can be mitigated is lacking. In this article, we perform a combined theoretical and experimental study to understand isolated lithium formation during stripping. We derive a thermodynamic consistent model of lithium dissolution and find that the interaction between lithium and SEI leads to locally preferred stripping and isolated lithium formation. Based on a cryogenic transmission electron microscopy (cryo TEM) setup, we reveal that these local effects are particularly pronounced at kinks of lithium whiskers. We find that lithium stripping can be heterogeneous both on a nanoscale and on a larger scale. Cryo TEM observations confirm our theoretical prediction that isolated lithium occurs less at higher stripping current densities. The origin of isolated lithium lies in local effects, such as heterogeneous SEI, stress fields, or the geometric shape of the deposits. We conclude that in order to mitigate isolated lithium, a uniform lithium morphology during plating and a homogeneous SEI are indispensable.

Progressive and instantaneous nature of lithium nucleation discovered by dynamic and operando imaging

The understanding of lithium (Li) nucleation and growth is important to design better electrodes for high-performance batteries. However, the study of Li nucleation process is still limited because of the lack of imaging tools that can provide information of the entire dynamic process. We developed and used an operando reflection interference microscope (RIM) that enables real-time imaging and tracking the Li nucleation dynamics at a single nanoparticle level. This dynamic and operando imaging platform provides us with critical capabilities to continuously monitor and study the Li nucleation process. We find that the formation of initial Li nuclei is not at the exact same time point, and Li nucleation process shows the properties of both progressive and instantaneous nucleation. In addition, the RIM allows us to track the individual Li nucleus's growth and extract spatially resolved overpotential map. The nonuniform overpotential map indicates that the localized electrochemical environments substantially influence the Li nucleation.

A Highly-Lithiophilic Mn3O4/ZnO-Modified Carbon Nanotube Film for Dendrite-Free Lithium Metal Anodes

Lithium metal anode is deemed as a potential candidate for high energy density batteries, which has attracted increasing attention. Unfortunately, Li metal anode suffers from issues such as dendrite grown and volume expansion during cycling, which hinders its commercialization. Herein, we designed a porous and flexible self-supporting film comprising of single-walled carbon nanotube (SWCNT) modified with a highly-lithiophilic heterostructure (Mn₃O₄/ZnO@SWCNT) as the host material for Li metal anodes. The p-n-type heterojunction constructed by Mn₃O₄ and ZnO generates a built-in electric field that facilitates electron transfer and Li⁺ migration. Additionally, the lithiophilic Mn₃O₄/ZnO particles serve as the pre-implanted nucleation sites, dramatically reducing the lithium nucleation barrier due to their strong binding energy with lithium atoms. Moreover, the interwoven SWCNT conductive network effectively lowers the local current density and alleviates the tremendous volume expansion during cycling. Thanks to the aforementioned synergy, the symmetric cell composed of Mn₃O₄/ZnO@SWCNT-Li can stably maintain a low potential for more than 2500 h at 1 mA cm^-2 and 1 mAh cm^-2. Furthermore, the Li-S full battery composed of Mn₃O₄/ZnO@SWCNT-Li also shows excellent cycle stability. These results demonstrate that Mn₃O₄/ZnO@SWCNT has great potential as a dendrite-free Li metal host material.

A Metal-Organic Framework Based Quasi-Solid-State Electrolyte Enabling Continuous Ion Transport for High-Safety and High-Energy-Density Lithium Metal Batteries

Solid-state lithium metal batteries are promising next-generation rechargeable energy storage systems. However, the poor compatibility of the electrode/electrolyte interface and the low lithium ion conductivity of solid-state electrolytes are key issues hindering the practicality of solid-state electrolytes. Herein, rational designed metal-organic frameworks (MOFs) with the incorporation of two types of ionic liquids (ILs) are fabricated as quasi-solid electrolytes. The obtained MOF-IL electrolytes offer continuous ion transport channels with the functional sulfonic acid groups serving as lithium ion hopping sites, which accelerate the Li⁺ transport both in the bulk and at the interfaces. The quasi-solid MOF-IL electrolytes exhibit competitive ionic conductivities of over 3.0 × 10^-4 S cm^-1 at room temperature, wide electrochemical windows over 5.2 V, and good interfacial compatibility, together with greatly enhanced Li⁺ transference numbers compared to the bare IL electrolyte. Consequently, the assembled quasi-solid Li metal batteries show either superior stability at low C rates or improved rate performance, related to the species of ILs. Overall, the quasi-solid MOF-IL electrolytes possess great application potential in high-safety and high-energy-density lithium metal batteries.

Nanoarray Architecture of Ultra-Lithiophilic Metal Nitrides for Stable Lithium Metal Anodes

Lithium metal anode (LMA) is puzzled by the serious issues corresponding to infinite volume change and notorious lithium dendrite during long-term stripping/plating process. Herein, the transition metal nitrides array with outstanding lithiophilicity, including CoN, VN, and Ni₃ N, are decorated onto carbon framework as "nests" to uniform Li nucleation and guide Li metal deposition. These transition metal nitrides with excellent conductivity can guarantee the fast electron transport, therefore maintain a stable interface for Li reduction. In addition, the designed multi-dimensional structure of metal nitride array decorated carbon framework can effectively regulate the growth of Li metal during the stripping/plating process. Of note, attributing to the lattice-matching between CoN and Li metal, the composite Li/CoN@CF anode exhibits ultra-stable cycling performance in symmetrical cells (over 4000 h@1 mA cm^-2 with 1 mAh cm^-2 and 1000h@20 mA cm^-2 with 20 mAh cm^-2 ). The assembled full cells based on Li/CoN@CF composite anode, LiFePO₄ or S as cathodes, deliver excellent cycling stability and rate capability. This strategy provides an effective approach to develop a stable lithium metal anode for lithium metal batteries.

Densely Packed Li-Metal Growth on Anodeless Electrodes by Li+ -Flux Control in Space-Confined Narrow Gap of Stratified Carbon Pack for High-Performance Li-Metal Batteries

Lithium (Li) is the "holy grail" for satisfying the increasing energy demand. This is because of its high theoretical capacity and low potential. Although Li is considered as a potential anode material, dendritic Li growth and the limited electrochemical properties continue to hinder its practical application. Structure-based self lithium ion (Li⁺ ) concentrating electrodes with high capacity and uniform Li⁺ -flux are recommended to overcome these shortcomings of Li. However, recent studies have been limited to structural perspectives. In addition, the electrokinetic principle of electrode materials remains a challenge. Herein, the space-confinement-based strategy is suggested for condensed Li⁺ -flux control in nanoscaled slit spaces that induce the dense Li growth on an anodeless electrode by using the stratified carbon pack (SCP). The micro/mesoporous slits of the SCP concentrate the electric field, which is strengthened by the space-confined electric field focusing, resulting in the accumulation of Li⁺ -flux in the host. The accumulated Li⁺ in host sites enables a uniform Li deposition with high capacity at high current density stably. Furthermore, SCPs have great compatibility with LiNi_0.8 Co_0.1 Mn_0.1 O₂ (NCM811) cathode, representing the outstanding full cell performance with Li deposited electrode which show the high specific of 115 mAh g^-1 at 4 C during 350 cycles.

Advanced Material Engineering to Tailor Nucleation and Growth towards Uniform Deposition for Anode-Less Lithium Metal Batteries

Anode-less lithium metal batteries (ALMBs), whether employing liquid or solid electrolytes, have significant advantages such as lowered costs and increased energy density over lithium metal batteries (LMBs). Among many issues, dendrite growth and non-uniform plating which results in poor coulombic efficiency are the key issues that viciously decrease the longevity of the ALMBs. As a result, lowering the nucleation barrier and facilitating lithium growth towards uniform plating is even more critical in ALMBs. While extensive reviews have focused to describe strategies to achieve high performance in LMBs and ALMBs, this review focuses on strategies designed to directly facilitate nucleation and growth of dendrite-free ALMBs. The review begins with a discussion of the primary components of ALMBs, followed by a brief theoretical analysis of the nucleation and growth mechanism for ALMBs. The review then emphasizes key examples for each strategy in order to highlight the mechanisms and rationale that facilitate lithium plating. By comparing the structure and mechanisms of key materials, the review discusses their benefits and drawbacks. Finally, major trends and key findings are summarized, as well as an outlook on the scientific and economic gaps in ALMBs.

CNT/PVDF Composite Coating Layer on Cu with a Synergy of Uniform Current Distribution and Stress Releasing for Improving Reversible Li Plating/Stripping

The uncontrollable formation of polymorphous Li deposits, e.g., whiskers, mosses, or dendrites resulting from nonuniform interfacial current distribution and internal stress release in the upward direction on the conventional current collector (e.g., Cu foil) of Li metal rechargeable batteries with a lithium-metal-free negatrode (LMFRBs), leads to rapid performance degradation or serious safety problems. The 3D carbon nanotubes (CNTs) skeleton has been proven to effectively reduce the current density and eliminate the internal accumulated stress. However, remarkable electrolyte decomposition, inherent Li source consumption due to repeated SEI formation, and Li⁺ intercalation in CNTs limit the application of 3D CNTs skeleton. Thus, it is necessary to avoid the side effects of the 3D CNTs skeleton and retain uniform interfacial current distribution and stress mitigation. In this work, we integrate the CNTs network with a soft functional polymer polyvinylidene fluoride (PVDF) to form a relatively dense coating layer on Cu foil, which can shield the contact between the internal surface of the 3D CNTs framework and the electrolyte. Simultaneously, the Li-F-rich SEI resulting from the partial reduction of PVDF with deposited Li and the soft nature of the coating layer release the accumulation of internal stress in the horizontal direction, resulting in mosses/whisker-free Li deposition. Thus, improved Li deposition/dissolution and stable cycling performance of the LMFRBs can be achieved.

LiI/Cu Mixed Conductive Interface via the Mechanical Rolling Approach for Stable Lithium Anodes in the Carbonate Electrolyte

The nonuniform ion/charge distribution and slow Li-ion diffusion at the Li metal/electrolyte interface lead to uncontrollable dendrites growth and inferior cycling stability. Herein, a simple mechanical rolling method is introduced to construct a mixed conductive protective layer composed of LiI and Cu on the Li metal surface through the replacement reaction between CuI nanoflake arrays and metallic Li. LiI can promote Li⁺ transportation across the interface, achieving homogeneous Li⁺ flux and suppressing the growth of Li dendrite, while the homogeneously dispersed Cu nanoparticles can offer abundant nucleation sites for Li deposition, resulting in a remarkably homogenized charge distribution. As expected, Li metal with the LiI/Cu protection layer (LiI/Cu@Li) exhibits a significantly prolonged lifespan over 350 h with slight polarization at a deposition capacity of 3 mAh cm^-2 in the carbonate electrolyte. Besides, when matched with high mass loading LiFePO₄ cathodes (20 mg cm^-2), the LiI/Cu@Li anodes exhibit much improved cycle stability and rate performance. Highly scalable preparation processes as well as the impressive electrochemical performances in half cells and full cells indicate the potential application of the LiI/Cu@Li anode.

Early Failure of Lithium-Sulfur Batteries at Practical Conditions: Crosstalk between Sulfur Cathode and Lithium Anode

Lithium-sulfur (Li-S) batteries are one of the most promising next-generation energy storage technologies due to their high theoretical energy and low cost. However, Li-S cells with practically high energy still suffer from a very limited cycle life with reasons which remain unclear. Here, through cell study under practical conditions, it is proved that an internal short circuit (ISC) is a root cause of early cell failure and is ascribed to the crosstalk between the S cathode and Li anode. The cathode topography affects S reactions through influencing the local resistance and electrolyte distribution, particularly under lean electrolyte conditions. The inhomogeneous reactions of S cathodes are easily mirrored by the Li anodes, resulting in exaggerated localized Li plating/stripping, Li filament formation, and eventually cell ISC. Manipulating cathode topography is proven effective to extend the cell cycle life under practical conditions. The findings of this work shed new light on the electrode design for extending cycle life of high-energy Li-S cells, which are also applicable for other rechargeable Li or metal batteries.

Confined Lithium Deposition Triggered by an Integrated Gradient Scaffold for a Lithium-Metal Anode

Constructing a composite lithium anode with a rational structure has been considered as an effective approach to regulate and relieve the tough problems of a sparkling Li anode. However, the potential short circuits risk that Li deposition at the surface of the framework has not yet been resolved. Here, we present a simple regulating-deposition strategy to guide the preferentially bottom-up deposition/growth of Li. The triple-gradient structure of modified porous copper with electrical passivation (top) and chemical activation (bottom) shows significant improvements in the morphological stability and electrochemical performance. Meanwhile, the in situ generation of Li₂Se can as an advanced artificial SEI layer be devoted to homogeneous Li plating/stripping. As a result, the composite anode exhibits a long-term cycling over 250 cycles with a high average CE of 98.2% at 1 mA cm^-2. Furthermore, a capacity retention of 94.4% in full cells can be achieved when pairing with LiFePO₄ as the cathode. These results ensure a bright direction for developing high-performance Li metal anodes.

Porous carbon architectures with different dimensionalities for lithium metal storage

Lithium metal batteries have recently gained tremendous attention owing to their high energy capacity compared to other rechargeable batteries. Nevertheless, lithium (Li) dendritic growth causes low Coulombic efficiency, thermal runaway, and safety issues, all of which hinder the practical application of Li metal as an anodic material. In this review, the failure mechanisms of Li metal anode are described according to its infinite volume changes, unstable solid electrolyte interphase, and Li dendritic growth. The fundamental models that describe the Li deposition and dendritic growth, such as the thermodynamic, electrodeposition kinetics, and internal stress models are summarized. From these considerations, porous carbon-based frameworks have emerged as a promising strategy to resolve these issues. Thus, the main principles of utilizing these materials as a Li metal host are discussed. Finally, we also focus on the recent progress on utilizing one-, two-, and three-dimensional carbon-based frameworks and their composites to highlight the future outlook of these materials.

Achieve Stable Lithium Metal Anode by Sulfurized-Polyacrylonitrile Modified Separator for High-Performance Lithium Batteries

To develop a high-energy-density lithium battery, there still are several severe challenges for Li metal anode: low Coulombic efficiency caused by its high chemical reactivity, Li dendrite formation, and "dead" Li accumulation during repeated plating/stripping processes. Especially, lithium dendrite growth imposes inferior cycling stability and serious safety issues. Herein, we propose a facile but effective strategy to suppress lithium dendrite growth through an artificial inorganic-polymer protective layer derived from sulfurized polyacrylonitrile on a polyethylene separator. Benefiting from the lithiated sulfurized polyacrylonitrile and poly(acrylic acid), the flexible and ion-conductive protective layer could regulate Li⁺ flux and facilitate dendrite-free lithium deposition. Consequently, lithium metal with the meritorious protective layer can achieve a long-term cycling with negligible overpotential rise in Li-Li symmetric cells, even at a high areal capacity of 5 mAh cm^-2. Remarkably, such a protective layer enables stable cycling performance of Li-S cell with a high areal capacity (∼9 mAh cm^-2). This work provides a valuable exploration strategy for potential industrial applications of high-performance lithium metal batteries.

Lithium reduction reaction for interfacial regulation of lithium metal anode

The lithium metal anode (LMA) is regarded as a very promising candidate for next-generation lithium batteries. The interfacial issue plays a pivotal role in affecting the lithium plating/stripping behavior, Coulombic efficiency and cycling lifespan of an LMA. The lithium reduction reaction (LRR) is an advanced regulating technique for optimizing the LMA interphase, which intelligently utilizes lithium metal itself as an interphase precursor. This strategy also possesses moderate operating conditions, high efficiency, great convenience and scalability. In this review, the latest developments of LRRs in interfacial regulation for LMAs are summarized, focusing on the interfacial regulation mechanism and the construction of various inorganic/organic interfaces in lithium metal liquid/solid batteries. The target interface properties and corresponding influence factors during LRRs are investigated in detail. Besides this, the superiority and insufficiency of LRRs are discussed and possible directions for LRRs are presented. This review highlights in situ modification characteristics for anode interface regulation during the LRR and can be extended to other metal anodes such as sodium, potassium and zinc.

Mitigating Interfacial Mismatch between Lithium Metal and Garnet-Type Solid Electrolyte by Depositing Metal Nitride Lithiophilic Interlayer

Solid-state lithium batteries are generally considered as the next-generation battery technology that benefits from inherent nonflammable solid electrolytes and safe harnessing of high-capacity lithium metal. Among various solid-electrolyte candidates, cubic garnet-type Li7La3Zr2O12 ceramics hold superiority due to their high ionic conductivity (10-3 to 10-4 S cm-1) and good chemical stability against lithium metal. However, practical deployment of solid-state batteries based on such garnet-type materials has been constrained by poor interfacing between lithium and garnet that displays high impedance and uneven current distribution. Herein, we propose a facile and effective strategy to significantly reduce this interfacial mismatch by modifying the surface of such garnet-type solid electrolyte with a thin layer of silicon nitride (Si3N4). This interfacial layer ensures an intimate contact with lithium due to its lithiophilic nature and formation of an intermediate lithium-metal alloy. The interfacial resistance experiences an exponential drop from 1197 to 84.5 Ω cm2. Lithium symmetrical cells with Si3N4-modified garnet exhibited low overpotential and long-term stable plating/stripping cycles at room temperature compared to bare garnet. Furthermore, a hybrid solid-state battery with Si3N4-modified garnet sandwiched between lithium metal anode and LiFePO4 cathode was demonstrated to operate with high cycling efficiency, excellent rate capability, and good electrochemical stability. This work represents a significant advancement toward use of garnet solid electrolytes in lithium metal batteries for the next-generation energy storage devices.

Interfacial studies on the effects of patterned anodes for guided lithium deposition in lithium metal batteries

The lifetime and health of lithium metal batteries are greatly hindered by nonuniform deposition and growth of lithium at the anode-electrolyte interface, which leads to dendrite formation, efficiency loss, and short circuiting. Lithium deposition is influenced by several factors including local current densities, overpotentials, surface heterogeneity, and lithium-ion concentrations. However, due to the embedded, dynamic nature of this interface, it is difficult to observe the complex physics operando. Here, we present a detailed model of the interface that implements Butler-Volmer kinetics to investigate the effects of overpotential and surface heterogeneities on dendrite growth. A high overpotential has been proposed as a contributing factor in increased nucleation and growth of dendrites. Using computational methods, we can isolate the aspects of the complex physics at the interface to gain better insight into how each component affects the overall system. In addition, studies have shown that mechanical modifications to the anode surface, such as micropatterning, are a potential way of controlling deposition and increasing Coulombic efficiency. Micropatterns on the anode surface are explored along with deformations in the solid-electrolyte interface layer to understand their effects on the dendritic growth rates and morphology. The study results show that at higher overpotentials, more dendritic growth and a more branched morphology are present in comparison to low overpotentials, where more uniform and denser growth is observed. In addition, the results suggest that there is a relationship between surface chemistries and anode geometries.

Formation sequence of solid electrolyte interphases and impacts on lithium deposition and dissolution on copper: an in situ atomic force microscopic study

Copper is the most widely used substrate for Li deposition and dissolution in lithium metal anodes, which is complicated by the formation of solid electrolyte interphases (SEIs), whose physical and chemical properties can affect Li deposition and dissolution significantly. However, initial Li nucleation and growth on bare Cu creates Li nuclei that only partially cover the Cu surface so that SEI formation could proceed not only on Li nuclei but also on the bare region of the Cu surface with different kinetics, which may affect the follow-up processes distinctively. In this paper, we employ in situ atomic force microscopy (AFM), together with X-ray photoelectron spectroscopy (XPS), to investigate how SEIs formed on a Cu surface, without Li participation, and on the surface of growing Li nuclei, with Li participation, affect the components and structures of the SEIs, and how the formation sequence of the two kinds of SEIs, along with Li deposition, affect subsequent dissolution and re-deposition processes in a pyrrolidinium-based ionic liquid electrolyte containing a small amount of water. Nanoscale in situ AFM observations show that sphere-like Li deposits may have differently conditioned SEI-shells, depending on whether Li nucleation is preceded by the formation of the SEI on Cu. Models of integrated-SEI shells and segmented-SEI shells are proposed to describe SEI shells formed on Li nuclei and SEI shells sequentially formed on Cu and then on Li nuclei, respectively. "Top-dissolution" is observed for both types of shelled Li deposits, but the integrated-SEI shells only show wrinkles, which can be recovered upon Li re-deposition, while the segmented-SEI shells are apparently top-opened due to mechanical stresses introduced at the junctions of the top regions and become "dead" SEIs, which forces subsequent Li nucleation and growth in the interstice of the dead SEIs. Our work provides insights into the impact mechanism of SEIs on the initial stage Li deposition and dissolution on foreign substrates, revealing that SEIs could be more influential on Li dissolution and that the spatial integration of SEI shells on Li deposits is important to improving the reversibility of deposition and dissolution cycling.

Flexible electrodes with high areal capacity based on electrospun fiber mats

The ever-growing portable, flexible, and wearable devices impose new requirements from power sources. In contrast to gravitational metrics, areal metrics are more reliable performance indicators of energy storage systems for portable and wearable devices. For energy storage devices with high areal metrics, a high mass loading of the active species is generally required, which imposes formidable challenges on the current electrode fabrication technology. In this regard, integrated electrodes made by electrospinning technology have attracted increasing attention due to their high controllability, excellent mechanical strength, and flexibility. In addition, electrospun electrodes avoid the use of current collectors, conductive additives, and polymer binders, which can essentially increase the content of the active species in the electrodes as well as reduce the unnecessary physically contacted interfaces. In this review, the electrospinning technology for fabricating flexible and high areal capacity electrodes is first highlighted by comparing with the typical methods for this purpose. Then, the principles of electrospinning technology and the recent progress of electrospun electrodes with high areal capacity and flexibility are elaborately discussed. Finally, we address the future perspectives for the construction of high areal capacity electrodes using electrospinning technology to meet the increasing demands of flexible energy storage systems.

Visualizing the Sensitive Lithium with Atomic Precision: Cryogenic Electron Microscopy for Batteries

Lithium (Li)-metal batteries are one of the most promising candidates for the next-generation energy storage devices due to their ultrahigh theoretical capacity. The realistic development of a Li metal battery is greatly impeded by the uncontrollable dendrite proliferation upon the chemically active metallic Li. To visualize the micromorphology or even the atomic structure of Li deposits is undoubtedly crucial, while imaging the sensitive Li still faces a huge challenge technically.Cryogenic electron microscopy (cryo-EM), an emerging imagery technology renowned for structural elucidation of biomaterials, is offering increased possibilities for analyzing sensitive battery materials reaching subangstrom resolution. Particularly for revealing metallic Li, cryo-EM exhibits remarkable superiority compared with the conventional electron imaging technique. On the one hand, cryo-EM could prevent the low melting-point Li metal from being damaged by the high electron dose induced thermal effect. On the other hand, the extremely low temperature immensely retards the rate of the side reaction where the Li reacts with the atmosphere or water vapor before the vacuum state. Consequently, the cryo-EM could acquire a high-resolution image of electron-beam sensitive Li in its native state at the nano- or even atomic scale, thus benefiting the fundamental perception and rational design of Li metal anodes.Thus, in this Account, we aim to highlight the significance of cryo-EM in analyzing metallic Li and developing a high-performance Li metal battery. We focus on how highly resolved cryo-EM realizes the breakthrough in detecting the crucial evolution during battery cycling, e.g., lattice ordering of Li, nanostructures of the solid electrolyte interphase (SEI), nucleation sites, and interface between the solid electrolyte and the Li anode. First, we briefly summarize the progress of Li metal imaging by cryo-EM in a timed sequence. In particular, the recent studies from our group are classified in order to systematically delineate the advantages that cryo-transmission electron microscopy (cryo-TEM) addressed on understanding and developing the Li metal battery. Second, the efforts of exhibiting the long-range ordering Li lattice are described to cognize the crystal orientation of both Li dendrites and uniform spheres. Subsequently, the nanostructures of SEI detected by cryo-TEM, maybe the most key information during Li plating/stripping, are systematically summarized. Benefitting from the subangstrom visualization on the newly formed and the particular inactive SEI after long-term cycling, we emphasize cryo-TEM's guidance in designing a robust, highly Li⁺ conductive, and Li-restoration facilitated SEI. We then propose the strategy of introducing a nucleation-site to enable uniform Li deposition by showing the evidence of Li nucleation atomically monitored through cryo-TEM. Moreover, the series of the work of atomic imagery and corresponding optimization of the interfaces between the polymer-based solid electrolyte and the Li anode are concluded. Finally, critical perspectives about the further step of cryo-TEM in the realistic development of high-energy density battery systems are also succinctly reviewed.

Strategy to Enhance the Cycling Stability of the Metallic Lithium Anode in Li-Metal Batteries

Based on the analysis of systematic research (density functional theory calculations, physical characterizations, and electrochemical performances), here, we report a novel mixture surface modification layer of LiC₆&LiF, which can enhance the lithium-ion diffusion and decrease the local current density. This is beneficial to the improvement of cycling stability. As a result, the Li@LiC₆&LiF-5/NCM half-cell possesses an excellent capacity retention of 94% after 100 cycles at 0.1C, with a capacity decay of only 0.06% per cycle. For comparison, the capacity retention of a pristine Li/NCM cell is only 9.3% after 100 cycles. Our study confirms that compositing the high ionic conductivity layer (e.g., LiC₆&LiF for the first time) is a promising avenue to stabilize lithium-metal anodes. From this perspective, we concisely review recent discoveries in this field and suggest possible new research directions for further development of Li-metal batteries.

Regulating Lithium Electrodeposition with Laser-Structured Current Collectors for Stable Lithium Metal Batteries

Lithium-metal batteries (LMBs) are promising electrochemical energy storage devices with high energy densities. However, the extreme reactivity of metallic lithium, the large volumetric change of the electrode during cycling, and the notorious dendrite formation issues lead to low cyclic stability and safety concerns, hindering the practical application of LMBs. In particular, the intrinsic tendency of uneven lithium deposition and the large internal electrode stress lead to the piecing of solid electrolyte interphases (SEIs), thereby resulting in fast decay of the anode. We develop a facile laser processing technique to fabricate laser-structured copper foils (LSCFs) that are able to regulate the lithium deposition kinetics and increase the cycle life of LMBs. By simply scribing commercial foils using a 355 nm laser, microstructural features with fish-scale patterns are obtained. The lithium deposition follows a drastically different mode on the LSCF compared with commercial planar copper foils which relieves the internal stress of lithium and prohibits the piecing of SEI. A high Coulombic efficiency of >96% of the lithium metal anode is maintained for over 100 cycles on the LSCF at a current density of 1 mA cm^-2 and an areal capacity of 1 mAh cm^-2 while the benchmark decayed to below 80% after 50 cycles. Full cells based on LiFePO₄ cathodes display a reasonable specific capacity of 125 mAh g^-1 over 300 cycles at a rate of 1 C. This work provides a fast yet effective laser-based approach to construct highly stable lithium metal anodes.

Review on Li Deposition in Working Batteries: From Nucleation to Early Growth

Lithium (Li) metal is one of the most promising alternative anode materials of next-generation high-energy-density batteries demanded for advanced energy storage in the coming fourth industrial revolution. Nevertheless, disordered Li deposition easily causes short lifespan and safety concerns and thus severely hinders the practical applications of Li metal batteries. Tremendous efforts are devoted to understanding the mechanism for Li deposition, while the final deposition morphology tightly relies on the Li nucleation and early growth. Here, the recent progress in insightful and influential models proposed to understand the process of Li deposition from nucleation to early growth, including the heterogeneous model, surface diffusion model, crystallography model, space charge model, and Li-SEI model, are highlighted. Inspired by the abovementioned understanding on Li nucleation and early growth, diverse anode-design strategies, which contribute to better batteries with superior electrochemical performance and dendrite-free deposition behavior, are also summarized. This work broadens the horizon for practical Li metal batteries and also sheds light on more understanding of other important metal-based batteries involving the metal deposition process.

Atomistic Insights into Lithium Storage Mechanisms in Anatase, Rutile, and Amorphous TiO2 Electrodes

Density functional theory calculations were used to investigate the phase transformations of Li_xTiO₂ (at 0 ≤ x ≤ 1), solid-state Li⁺ diffusion, and interfacial charge-transfer reactions in both crystalline and amorphous forms of TiO₂. It is shown that in contrast to crystalline TiO₂ polymorphs, the energy barrier to Li⁺ diffusion in amorphous TiO₂ decreases with increasing mole fraction of Li⁺ due to the changes of chemical species pair interactions following the progressive filling of low-energy Li⁺ trapping sites. Sites with longer Li-Ti and Li-O interactions exhibit lower Li⁺ insertion energies and higher migration energy barriers. Due to its disordered atomic arrangement and increasing Li⁺ diffusivity at higher mole fractions, amorphous TiO₂ exhibits both surface and bulk storage mechanisms. The results suggest that nanostructuring of crystalline TiO₂ can increase both the rate and capacity because the capacity dependence on the bulk storage mechanism is minimized and replaced with the surface storage mechanism. These insights into Li⁺ storage mechanisms in different forms of TiO₂ can guide the fabrication of TiO₂ electrodes to maximize the capacity and rate performance in the future.

Wetting Phenomena and their Effect on the Electrochemical Performance of Surface-Tailored Lithium Metal Electrodes in Contact with Cross-linked Polymeric Electrolytes

Li metal batteries (LMBs) containing cross-linked polymer electrolytes (PEs) are auspicious candidates for next-generation batteries. However, the wetting behavior of PEs on uneven Li metal surfaces has been neglected in most studies. Herein, it is shown that microscale defect sites with curved edges play an important role in a wettability-dependent electrodeposition. The wettability and the viscoelastic properties of PEs are correlated, and the impact of wettability on the nucleation and diffusion near the Li|PE interface is distinguished. It is found that the curvature of the edges is a key factor for the investigation of wetting phenomena. The appearance of microscale defects and phase separation are identified as main causes for erratic nucleation. It is emphasized that the implementation of stable and consistent long-term cycling performance of LMBs using PEs requires a deeper understanding of the "soft-solid"-solid contact between PEs and inherently rough Li metal surfaces.

Coupling a Sponge Metal Fibers Skeleton with In Situ Surface Engineering to Achieve Advanced Electrodes for Flexible Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) are regarded as promising next-generation energy storage systems, however, the uncontrollable dendrite formation and serious polysulfide shuttling severely hinder their commercial success. Herein, a powerful 3D sponge nickel (SN) skeleton plus in situ surface engineering strategy, to address these issues synergistically, is reported, and a high-performance flexible LSB device is constructed. Specifically, the rationally designed spray-quenched lithium metal on the SN matrix (solid electrolyte interface (SEI)@Li/SN), as dendrite inhibitor, combines the merits of the 3D lithiophilic SN skeleton and the in situ formed SEI layer derived from the spray-quenching process, and thereby exhibits a steady overpotential within 75 mV for 1500 h at 5 mA cm^-2 /10 mA h cm^-2 . Meanwhile, in situ surface sulfurization of the SN skeleton hybridizing with the carbon/sulfur composite (SC@Ni₃ S₂ /SN) serves as efficient lithium polysulfide adsorbent to catalyze the overall reaction kinetics. COMSOL Multiphysics simulations and density functional theory calculations are further conducted to explore the underlying mechanisms. As a proof of concept, the well-designed SEI@Li/SN||SC@Ni₃ S₂ /SN full cell shows excellent electrochemical performance with a negative/positive ratio in capacity of ≈2 and capacity retention of 99.82% at 1 C under mechanical deformation. The novel design principles of these materials and electrodes successfully shed new light on the development of flexible LSBs.