Phosphonium Bromides Regulating Solid Electrolyte Interphase Components and Optimizing Solvation Sheath Structure for Suppressing Lithium Dendrite Growth

Electrolyte additives play important roles in suppressing lithium dendrite growth and improving the electrochemical performance of long-life lithium metal batteries (LMBs), however, it is still challenging to design individual additive for adjusting the solid electrolyte interphase (SEI) components and changing lithium ion solvation sheath in the electrolyte at the same time for optimizing electrochemical performance. Herein, alkyl-triphenyl-phosphonium bromides (alkyl-TPPB) are designed as the electrolyte additive to enhance the stability of metallic Li anode under the guidance of multi-factor principle for electrolyte additive molecule design (EDMD). Both alkyl-TPP cations and Br⁻ anions produce positive influences on suppressing Li dendrite growth and stabilizing the unstable interphase between metallic Li anode/electrolyte. As expected, the optimized solvation sheath structure, and the stable SEI suppress Li dendrite growth. As a result, the Li||Li₄Ti₅O₁₂ cell reveals a long stable life over 1000 cycles with high Coulombic efficiency (99.9%). This work provides an insight on stabilizing SEI and optimizing solvation sheath structure with novel approach to develop long-term stability and safety LMBs.     

Dendrite Suppression by a Polymer Coating: A Coarse‐Grained Molecular Study

A major hurdle to the successful deployment of high‐energy‐density lithium metal based batteries is dendrite growth during battery cycling, which raises safety and cycle life concerns. Coating the Li metal anode with a soft polymer layer has been previously shown to be effective in suppressing dendrite growth, leading to uniform lithium deposition even at high current densities. A 3D coarse‐grained molecular model to study the mechanism of dendrite suppression is presented. It is found that the most effective coatings delay or even prevent dendrites from penetrating the polymer layer during deposition. The optimal deposition can be achieved by jointly tuning the polymer stiffness and relaxation time. Higher polymer dielectric permittivity and coating thickness are also effective, but the deposition rate and, therefore, the charging current density is reduced. These findings provide the basis for rational design of soft polymer coatings for stable lithium deposition.     

Functional Separators Regulating Ion Transport Enabled by Metal-Organic Frameworks for Dendrite-Free Lithium Metal Anodes

Developing high energy density lithium secondary batteries is pivotal for satisfying the increasing demand in advanced energy storage systems. Lithium metal batteries (LMBs) have attracted growing attention due to their high theoretical capacity, but the lithium dendrites issue severely fetter their real-world applications. It is found that reducing anion migration near lithium metal prolongs the nucleation time of dendrites, meanwhile, promoting homogeneous lithium deposition suppresses the dendritic growth. Thus, regulating ion transport in LMBs is a feasible and effective strategy for addressing the issues. Based on this, a functional separator is developed to regulate ion transport by utilizing a well-designed metal-organic frameworks (MOFs) coating to functionalize polypropylene (PP) separator. The well-defined intrinsic nanochannels in MOFs and the negatively charged gap channels both restricts the free migration of anions, contributing to a high Li⁺ transference number of 0.68. Meanwhile, the MOFs coating with uniform porous structure promotes homogeneous lithium deposition. Consequently, a highly-stable Li plating/stripping cycling for over 150 h is achieved. Furthermore, implementation of the separator enables LMBs with high discharge capacity, prominent rate performance and good capacity retention. This work is anticipated to aid developement of dendrite-free LMBs by utilizing advanced separators with ion transport management.     

Dendrite‐Free Lithium Plating Induced by In Situ Transferring Protection Layer from Separator

Lithium (Li) metal anodes are regarded as a promising pathway to meet the rapidly growing requirements on high energy density cells, owing to their highest gravimetric capacity (3840 mAh g^?1) and their lowest redox potential. The application of Li metal anodes, however, is still hindered by undesired dendrites formation and endless consumption of liquid electrolyte due to a continuous reaction on interface of electrolyte/Li‐metal without a stable solid–electrolyte–interface (SEI) layer. A stable protection layer is formed on Li metal anode by in situ transferring the coating layer from polymer separator. The Li anode protection strategy is developed with an in situ formed protection layer transferred through the reduction of a coating layer on polymer separator. A PbZr_0.52Ti_0.48O₃ (PZT) coating layer on polypropylene (PP) separator is reduced by Li metal anode to produce a Pb metal containing composite layer, which could form Pb–Li alloy and adhere to the surface of Li metal anode after the reaction and improves the Li plating/stripping efficiency owing to the formation of a more homogenized electric field. Both the Li/Li symmetric cells and LiFePO₄/Li cells with this PZT precoated PP separators exhibit significantly improved Coulombic efficiency and cycling life.     

Low-Cost Regulating Lithium Deposition Behaviors by Transition Metal Oxide Coating on Separator

The application of lithium metal anode, despite being of the highest capacity, is hindered by low Coulombic efficiency (CE) and serious lithium dendrites formation. A strategy of transition metal oxides (TMOs) particles coated porous polypropylene (PP) separator is developed to regulate lithium deposition behaviors through in situ forming artificial solid electrolyte interface (SEI) passivating layers. By virtue of quite low solubilities of TMOs in the electrolyte, the concentration of TMOs in the electrolyte can be maintained at a constant and the dissolved TMOs can be reduced to produce Li₂O and Mn particles, which not only function as lithium nucleating seeds but are also involved in the formation of the SEI layer. The sustainably existed trace of TMOs ensures the artificial SEI layer can be re-healed once damaged by the volume expansion of lithium. With the help of one typical TMO of MnO coating on PP, an interesting dendrite-free dual layer Li deposition is observed, which significantly improves the CE of Li||Cu cells and cycling life of Li||Li cells. Using MnO coated PP, ultra-thin lithium films are deposited on copper foils with an in situ constructed SEI passivating layer, which exhibits a much improved cycling performance in liquid ether electrolyte and even better performance in gel polymer electrolyte.     

Mechanisms of the Planar Growth of Lithium Metal Enabled by the 2D Lattice Confinement from a Ti3C2Tx MXene Intermediate Layer

The propensity of Li to form irregular and nonplanar electrodeposits has become a fundamental barrier for fabricating Li metal batteries. Here, a planar, dendrite-free Li metal growth on 2D Ti₃C₂T_x MXene is reported. Ab initio calculations suggest that Li forms a hexagonal close-packed (hcp) layer on the surface of Ti₃C₂T_x via ionic bonding and the lattice confinement. The ionic bonding weakens gradually after a few monolayers, resulting in a nanometers-thin transition region of hcp-Li. Above this transition region, the deposition is dominated by plating of body-centered cubic (bcc) Li via metallic bonding. Formation of a dense and planar Li metal anode with preferential growth along the (110) facet is explained by the lattice matching between Ti₃C₂T_x and hcp-Li and then with bcc-Li, as well as preferred thermodynamic factors including the large dendrite formation energy and small migration barrier for Li. The prepared Li metal anode shows stable cycling in a wide current density range from 0.5 to 10.0 mA cm^–2. The LiFePO₄‖Li full cell fabricated with this Li metal anode exhibits only 9.5% capacity fading after 500 charge–discharge cycles at 1 C rate.     

Prestoring Lithium into Stable 3D Nickel Foam Host as Dendrite‐Free Lithium Metal Anode

Lithium metal is considered a “Holy Grail” of anode materials for high‐energy‐density batteries. However, both dendritic lithium deposition and infinity dimension change during long‐term cycling have extremely restricted its practical applications for energy storage devices. Here, a thermal infusion strategy for prestoring lithium into a stable nickel foam host is demonstrated and a composite anode is achieved. In comparison with the bare lithium, the composite anode exhibits stable voltage profiles (200 mV at 5.0 mA cm^?2) with a small hysteresis beyond 100 cycles in carbonate‐based electrolyte, as well as high rate capability, significantly reduced interfacial resistance, and small polarization in a full‐cell battery with Li₄Ti₅O₁₂ or LiFePO₄ as counter electrode. More importantly, in addition to the fact that lithium is successfully confined in the metallic nickel foam host, uniform lithium plating/stripping is achieved with a low dimension change (merely ≈3.1%) and effective inhibition of dendrite formation. The mechanism for uniform lithium stripping/plating behavior is explained based on a surface energy model.     

Lithiophilic MXene-Guided Lithium Metal Nucleation and Growth Behavior

The positive effects of a lithiophilic substrate on the electrochemical performance of lithium metal anodes are confirmed in several reports, while the understanding of lithiophilic substrate-guided lithium metal nucleation and growth behavior is still insufficient. In this study, the effect of a lithiophilic surface on lithium metal nucleation and growth behaviors is investigated using a large-area Ti₃C₂T_x MXene substrate with a large number of oxygen and fluorine dual heteroatoms. The use of the MXene substrate results in a high lithium-ion concentration as well as the formation of uniform solid–electrolyte-interface (SEI) layers on the lithiophilic surface. The solid–solid interface (MXene-SEI layer) significantly affects the surface tension of the deposited lithium metal nuclei as well as the nucleation overpotential, resulting in the formation of uniformly dispersed lithium nanoparticles ( ≈ 10–20 nm in diameter) over the entire MXene surface. The primary lithium nanoparticles preferentially coalesce and agglomerate into larger secondary particles while retaining their primary particle shapes. Subsequently, they form close-packed structures, resulting in a dense metal layer composed of particle-by-particle microstructures. This distinctive lithium metal deposition behavior leads to highly reversible cycling performance with high Columbic efficiencies >  99.0% and long cycle lives of over 1000 cycles.     

Eliminating Tip Dendrite Growth by Lorentz Force for Stable Lithium Metal Anodes

Lithium metal anodes are deemed as the “Holy Grail” for next generation high energy density batteries, due to the reported highest specific capacity (3860 mAh g^?1) and the lowest negative electrochemical potential (?3.04 V vs the standard hydrogen electrode). However, the notorious tip‐induced dendrite growth leads to low Coulombic efficiency, restricted lifespan, and even catastrophic short‐circuits, blocking the roadmap of their commercialization. Here, a magnetic field is introduced into the lithium plating process. The Li⁺ concentrated around the tips by the uneven electric field distribution can be taken off the hotpots by the Lorentz force and the tip dendrite growth can be eliminated. The relationship between current density and magnetic flux intensity is established by monitoring the deposited lithium morphology as well as the electrochemical performance, which is confirmed by mathematic modeling and COMSOL Multiphysics simulation. It is also demonstrated that the Lorentz force–induced tip dendrite elimination can be utilized practically by assembling permanent magnet‐containing prototype coin cell. It is anticipated that this physical approach can be applied to other high energy density systems as well.     

Lithium Induced Nano-Sized Copper with Exposed Lithiophilic Surfaces to Achieve Dense Lithium Deposition for Lithium Metal Anode

Li metal batteries have attracted extensive research attention because of their extremely high theoretical capacity. However, the commercialization of the Li metal batteries is hindered, as uncontrolled Li dendrites growth leads to safety concerns and a low coulombic efficiency. To suppress Li dendrites growth and achieve dense Li deposition, a lithiophilic 3D Cu host is designed for Li metal anode, in which the nano-sized Cu is in situ formed with the aid of infused Li metal. The fabricated Li metal anode exhibit a superior electrochemical stability than raw Li metal anode, and compact Li is maintained during cycling. The experimental results and density functional theory calculations demonstrate that the nano-sized Cu formed on the surface of the skeleton host shows highly exposed Cu (100) and Cu (110) surfaces, which exhibits a strong affinity toward Li, and effectively eliminates the formation of Li dendrites, leading to a dense Li deposition. With the strategy of adjusting exposed surfaces of Cu host, the optimized Li metal anode enhances the electrochemical performance of full cells, and concomitantly demonstrates their potential for future designs of next-generation Li metal anodes or Li-free anodes for Li metal batteries.     

Nanoflake Arrays of Lithiophilic Metal Oxides for the Ultra‐Stable Anodes of Lithium‐Metal Batteries

A molten lithium infusion strategy has been proposed to prepare stable Li‐metal anodes to overcome the serious issues associated with dendrite formation and infinite volume change during cycling of lithium‐metal batteries. Stable host materials with superior wettability of molten Li are the prerequisite. Here, it is demonstrated that a series of strong oxidizing metal oxides, including MnO₂, Co₃O₄, and SnO₂, show superior lithiophilicity due to their high chemical reactivity with Li. Composite lithium‐metal anodes fabricated via melt infusion of lithium into graphene foams decorated by these metal oxide nanoflake arrays successfully control the formation and growth of Li dendrites and alleviate volume change during cycling. A resulting Li‐Mn/graphene composite anode demonstrates a super‐long and stable lifetime for repeated Li plating/stripping of 800 cycles at 1 mA cm^?2 without voltage fluctuation, which is eight times longer than the normal lifespan of a bare Li foil under the same conditions. Furthermore, excellent rate capability and cyclability are realized in full‐cell batteries with Li‐Mn/graphene composite anodes and LiCoO₂ cathodes. These results show a major advancement in developing a stable Li anode for lithium‐metal batteries.     

Controlling Li Ion Flux through Materials Innovation for Dendrite‐Free Lithium Metal Anodes

Lithium (Li) metal with high theoretical capacity and the lowest electrochemical potential has been proposed as the ideal anode for high‐energy‐density rechargeable battery systems. However, the practical commercialization of Li metal anodes is precluded by a short lifespan and safety problems caused by their intrinsically high reductivity, infinite volume change, and uncontrollable dendrite growth during deposition and dissolution processes. Plenty of strategies have been introduced to solve the above‐mentioned problems. Among these, controlling Li⁺ flux plays a vital role to directly influence the plating and stripping process. In this work, the fundamental effect of Li⁺ flux distribution on Li nucleation and early dendrite growth is discussed. Then, recent strategies of controlling Li⁺ flux to suppress dendrite formation and growth through materials design are summarized, including homogenizing Li⁺ flux, localizing Li⁺ flux, and guiding gradient Li⁺ distribution. Finally, underexplored materials are proposed and explored to control Li⁺ flux and further directions for dendrite‐free Li anodes. It is expected that this progress report will help to deepen the understanding of Li⁺ flow tuning and morphology control of Li anodes and eventually facilitate the practical application of Li metal batteries.     

Rational Design of Mixed Electronic-Ionic Conducting Ti-Doping Li7La3Zr2O12 for Lithium Dendrites Suppression

Garnet structured ceramic electrolyte Li₇La₃Zr₂O₁₂ (LLZO) attracts much attention in solid-state lithium batteries for its high ionic conductivity, wide electrochemical window, and lack of reducible element. However, the application of LLZO has been hindered by severe dendrite penetration. The theoretical investigations on the mechanisms of lithium dendrite evolution are carried out, aiming at quantifying the promotion effects of overpotential and the limitation counterpart of bulk modulus. Since dendrites preferentially propagate along connected defects, while intrinsic defects are difficult to be compeletely eliminated, manipulation of overpotential should be a more feasible way for dendrites suppression. The mixed electronic-ionic conducting interphase, which in situ forms by introducing a Ti-doping Li₅₆La₂₄Zr₁₅TiO₉₆ (T-LLZO) interlayer between Li and LLZO, is suggested based on the proposed mechanisms, which effectively facilitates to alleviate the overpotential thus suppress the lithium dendrites theoretically. This strategy is verified experimentally by obviously improved stability of Li/Li symmetric cell using T-LLZO ceramic pellet electrolyte.     

Multifunctional Electrode Design Consisting of 3D Porous Separator Modulated with Patterned Anode for High‐Performance Dual‐Ion Batteries

Searching for low‐cost and high‐capacity electrode materials such as metal anodes is of important significance for the development of new generation rechargeable batteries. However, metal anodes always suffer from severe volume expansion/contraction during a repeated electrochemical alloying/dealloying process. In this study, a novel concept about modifying metal‐anodes‐based battery construction with a multifunctional electrode (ME) design is provided. The ME consists of a 3D porous separator that is modulated with a patterned aluminum anode, which simultaneously works as a current collector, anode material, and separator in a dual‐ion battery (DIB). The 3D porous separator not only enables the ME to possess significantly improved electrolyte uptake and retention capabilities, but also acts as a protecting layer to restrict the surface pulverization of the Al anode. The ME‐DIB displays remarkably enhanced cell performances, including excellent cycling stability with 92.4% capacity retention after 1000 cycles at a current density of 2 C, and superior rate performance with 80.7% capacity retention at 10 C.     

Anode‐Free Rechargeable Lithium Metal Batteries

Anode‐free rechargeable lithium (Li) batteries (AFLBs) are phenomenal energy storage systems due to their significantly increased energy density and reduced cost relative to Li‐ion batteries, as well as ease of assembly because of the absence of an active (reactive) anode material. However, significant challenges, including Li dendrite growth and low cycling Coulombic efficiency (CE), have prevented their practical implementation. Here, an anode‐free rechargeable lithium battery based on a Cu||LiFePO₄ cell structure with an extremely high CE (>99.8%) is reported for the first time. This results from the utilization of both an exceptionally stable electrolyte and optimized charge/discharge protocols, which minimize the corrosion of the in situly formed Li metal anode.     

A Self-Limited Free-Standing Sulfide Electrolyte Thin Film for All-Solid-State Lithium Metal Batteries

All-solid-state (ASS) lithium metal batteries (LMBs) are considered the most promising next-generation batteries due to their superior safety and high projected energy density. To access the practically desired high energy density of ASS LMBs, an ultrathin solid-state electrolyte (SSE) film with fast ion-transport capability presents as an irreplaceable component to reduce the proportion of inactive materials in ASS batteries. In this contribution, an ultrathin (60  µ m), flexible, and free-standing argyrodite (Li₆PS₅Cl) SSE film is designed through a self-limited strategy. A chemically compatible cellulose membrane is employed as the self-limiting skeleton that not only defined the thinness of the sulfide SSE film but also strengthened its mechanical properties. The ionic conductivity of the SSE film reaches up to 6.3 × 10⁻³ S cm⁻¹ at room temperature, enabling rapid lithium-ion transportation. The self-limited SSE thin films are evaluated in various ASS LMBs with different types of cathode (sulfur and lithium titanate) and anode materials (lithium and lithium-indium alloy) at both mold-cell and pouch-cell levels, demonstrating a stable performance and high-rate capability. This study provides a general strategy for the rational design of an SSE thin film towards high-energy-density ASS batteries.     

A Pressure Self-Adaptable Route for Uniform Lithium Plating and Stripping in Composite Anode

Lithium (Li) metal anode confronts impressive challenges to revolutionize the current rechargeable batteries due to the intractably unstable interface. The composite Li anode is proposed to relieve volume fluctuations and suppress Li dendrites apparently. However, the inner space of composite anodes still affords feasibility for the continuous growth of unconstrained Li dendrites, leading to a low utilization of deposited Li and even safety hazards. Herein, an emerging and rational strategy to design composite anodes is proposed to regulate the inner Li plating/stripping. The self-adaptable pressure is generated by the filled elastic polymer inside conductive hosts, surpassing the yield strength of Li and confining Li to form a smooth morphology with a high utilization owing to the persistent electronic pathways under pressure. The pressure self-adaptable composite anode renders 160 cycles with a capacity retention of 80% in comparison to 60 cycles with a planar Li under practical conditions. Moreover, a 1.0 Ah pouch cell undergoes 68 cycles impressively. This work not only presents a fresh perspective on regulation of inner Li plating/stripping by introducing a self-adaptable pressure into the composite anode, but also demonstrates the avenue of exploring multifunctional composite anodes for practical Li metal batteries.     

Long Cycle Life Lithium Metal Batteries Enabled with Upright Lithium Anode

The lithium metal anode is the holy grail of the battery field due to its lowest reduction potential and high specific capacity; however, its application is hindered by severe safety hazards and inferior cyclic stability due to dendrites and unstable solid electrolyte interphase (SEI). Aiming at these problems, a coiled Li anode with a unique upright structure is proposed. The upright structure endows coiled Li anode with abundant inner reaction interface/space/mass for lithium deposit/storage/transport, which can induce the inner growth of Li dendrites and SEI. The Li⁺ transport/deposit behavior and mechanism of coiled Li anode are clarified via in situ observation and numerical simulation. Benefiting from the small volume expansion and sufficient Li⁺ transport, the coiled Li anodes combined with Li₄Ti₅O₁₂ cathodes achieve a long life of over 2000 cycles at 5C with a reversible capacity of 129 mAh g^?1 and 100% Coulombic efficiency.     

Interlamellar Lithium-Ion Conductor Reformed Interface for High Performance Lithium Metal Anode

Lithium metal anodes are promising for application in new-type secondary batteries. Unfortunately, low cycle life and safety peril induced by uncontrollable dendrites growth and weak solid electrolyte interface (SEI) have blocked their utilization. In this work, an interlamellar lithium-ion conductor of lithium-montmorillonite (Li-MMT) is applied to enhance the SEI properties, inhibit dendrites-germination, and thus significantly enhance electrochemical performance. Such a well-designed Li-MMT SEI not only possesses inherent fast lithium-ion channels, but also works as a reservoir to supply adequate lithium-ions in the interlaminations and periphery of Li-MMT nanosheets, offering fast lithium-ion transfer in interlaminations and sheet-to-sheet. Furthermore, the strong trend of lithium-ion absorption of Li-MMT is confirmed by density functional theory calculations and stable lithium deposition under Li-MMT SEI layer at 10 mA cm⁻² is verified via finite element modeling. As a result, a steady lithium deposition process without dendrites is achieved. Coulombic efficiency of the half-cell accomplishes a mean value of 99.1% over 400 cycles at 1 mA cm⁻², while Li-LiFePO₄ full cells show a stable capacity up to 120 mAh g⁻¹ and steady circulation over 400 loops at 1C. This work offers a novel strategy to design a high-performance SEI layer and suppress dendrite growth.