Quantification of the Dynamic Interface Evolution in High-Efficiency Working Li Metal Batteries

In situ Gel Electrolytes for the Interfacial Regulation of Lithium Metal Batteries

With the popularity and development of electronic devices, the demand for lithium batteries is increasing, which also puts high demands on the energy density, cycle life and safety of lithium batteries. Gel electrolytes achieve both of these requirements by curing the electrolytes to reduce the interfacial side reactions of lithium metal batteries. The ionic conductivity of the gel electrolytes prepared by in situ curing reach 8.0×10-4  S cm-1 , and the ionic mobility number is 0.53. Meanwhile, the gel electrolytes maintain a stable electrochemical window of 1.0-5.0 V. Benefited with the interfacial regulation of PEGDA gel electrolytes, the gel lithium metal batteries show better cycling stability, and achieved 97 % capacity retention after 200 cycles (0.2 C) with a lower increasing rate of impedance.

Demystifying the Salt-Induced Li Loss: A Universal Procedure for the Electrolyte Design of Lithium-Metal Batteries

Lithium (Li) metal electrodes show significantly different reversibility in the electrolytes with different salts. However, the understanding on how the salts impact on the Li loss remains unclear. Herein, using the electrolytes with different salts (e.g., lithium hexafluorophosphate (LiPF6), lithium difluoro(oxalato)borate (LiDFOB), and lithium bis(fluorosulfonyl)amide (LiFSI)) as examples, we decouple the irreversible Li loss (SEI Li+ and "dead" Li) during cycling. It is found that the accumulation of both SEI Li+ and "dead" Li may be responsible to the irreversible Li loss for the Li metal in the electrolyte with LiPF6 salt. While for the electrolytes with LiDFOB and LiFSI salts, the accumulation of "dead" Li predominates the Li loss. We also demonstrate that lithium nitrate and fluoroethylene carbonate additives could, respectively, function as the "dead" Li and SEI Li+ inhibitors. Inspired by the above understandings, we propose a universal procedure for the electrolyte design of Li metal batteries (LMBs): (i) decouple and find the main reason for the irreversible Li loss; (ii) add the corresponding electrolyte additive. With such a Li-loss-targeted strategy, the Li reversibility was significantly enhanced in the electrolytes with 1,2-dimethoxyethane, triethyl phosphate, and tetrahydrofuran solvents. Our strategy may broaden the scope of electrolyte design toward practical LMBs.

A Three-Dimensional (3D) Framework of Freestanding Vanadium Nitride Nanowires for Dendrite-Free and Long Life-Span Lithium Metal Anodes

Lithium (Li) metal is a promising anode candidate for high-energy-density batteries owing to its high theoretical capacity and low electrochemical potential. However, uneven Li nucleation, uncontrollable dendritic growth, infinite voltage change and even safety issues hinder its commercial application. Herein, a three-dimensional (3D) framework of freestanding vanadium nitride nanowires (VN NWs) is established as Li host for dendrite-free Li metal anode. A lithiophilic Li3 N interlayer which in situ formed by the surface reaction between molten Li and VN NWs is utilized to guide a uniform Li nucleation and deposition within the skeleton, as well as avoid the dendrite formation. Meanwhile, VN NWs can decrease local current density, homogenize Li-ion flux and accommodate volume fluctuations of the anode due to its 3D structure with high electron conductivity. Thus, the corresponding composite Li metal anode delivers a long-life span of 500 cycles (1000 h) at a current density of 0.5 mA cm-2 , and exhibits lower nucleation over-potential and voltage hysteresis at different current densities from 0.5~5 mA cm-2 in carbonate electrolyte. In conclusion, this work provides a new type of scaffold with both high electronic conductivity and excellent lithiophilicity for stable Li anodes.

Ion modulation engineering toward stable lithium metal anodes

Homogeneous ion transport during Li+ plating/stripping plays a significant role in the stability of Li metal anodes (LMAs) and the electrochemical performance of Li metal batteries (LMBs). Controlled ion transport with uniform Li+ distribution is expected to suppress notorious Li dendrite growth while stabilizing the susceptible solid electrolyte interfacial (SEI) film and optimizing the electrochemical stability. Here, we are committed to rendering a comprehensive study of Li+ transport during the Li plating/stripping process related to the interactions between the Li dendrites and SEI film. Moreover, rational ion modulation strategies based on functional separators, artificial SEI films, solid-state electrolytes and structured anodes are introduced to homogenize Li+ flux and stabilize the lithium metal surface. Finally, the current issues and potential opportunities for ion transport regulation to boost the high energy density of LMBs are described.

Ultrathin Li-rich Li-Cu alloy anode capped with lithiophilic LiC6 headspace enabling stable cyclic performance

Li-rich dual-phase Li-Cu alloy is a promising candidate toward practical application of Li metal anode due to its in situ formed unique three-dimensional (3D) skeleton of electrochemical inert LiCu_x solid-solution phase. Since a thin layer of metallic Li phase appears on the surface of as-prepared Li-Cu alloy, the LiCu_x framework cannot regulate Li deposition efficiently in the first Li plating process. Herein, a lithiophilic LiC₆ headspace is capped on the upper surface of the Li-Cu alloy, which can not only offer free space to accommodate Li deposition and maintain dimensional stability of the anode, but also provide abundant lithiophilic sites and guide Li deposition effectively. This unique bilayer architecture is fabricated via a facile thermal infiltration method, where the Li-Cu alloy layer with an ultrathin thickness around 40 μm occupies the bottom of a carbon paper (CP) sheet, and the upper part of this 3D porous framework is reserved as the headspace for Li storage. Notably, the molten Li can quickly convert these carbon fibers of the CP into lithiophilic LiC₆ fibers while the CP is touched with the liquid Li. The synergetic effect between the LiC₆ fibers framework and LiCu_x nanowires scaffold can ensure a uniform local electric field and stable Li metal deposition during cycling. As a consequence, the CP capped ultrathin Li-Cu alloy anode demonstrates excellent cycling stability and rate capability.

Optimizing the Fermi Level of a 3D Current Collector with Ni3 S2 /Ni3 P Heterostructure for Dendrite-Free Sodium-Metal Batteries

Rechargeable sodium-metal batteries (RSMBs) with high energy density and low cost are attracting extensive attention as promising energy-storage technologies. However, the poor cyclability and safety issues caused by unstable solid electrolyte interphase (SEI) structure and dendrite issues limit their practical application. Herein, it is theoretically predicted that constructing the Ni₃ S₂ /Ni₃ P heterostructure with high work function can lower the Fermi energy level, and therefore effectively suppressing continuous electrolyte decomposition derived from the electron-tunneling effect after long-term sodiation process. Furthermore, the Ni₃ S₂ /Ni₃ P heterostructure on 3D porous nickel foam (Ni₃ S₂ /Ni₃ P@NF) is experimentally fabricated as an advanced Na-anode current collector. The seamless Ni₃ S₂ /Ni₃ P heterostructure not only offers abundant active sites to induce uniform Na⁺ deposition and enhance ion-transport kinetics, but also facilitates the formation of stable SEI for dendrite-free sodium anode, which are confirmed by cryogenic components transmission electron microscopy tests and in situ spectroscopy characterization. As a result, the Na-composite anode (Ni₃ S₂ /Ni₃ P@NF@Na) delivers stable plating/stripping process of 5000 h and high average Coulombic efficiency of 99.7% over 2500 cycles. More impressively, the assembled sodium-ion full cell displays ultralong cycle life of 10 000 cycles at 20 C. The strategy of stabilizing the sodium-metal anode gives fundamental insight into the potential construction of advanced RSMBs.

Influencing Factors on Li-ion Conductivity and Interfacial Stability of Solid Polymer Electrolytes, Exampled by Polycarbonates, Polyoxalates and Polymalonates

The application of solid polymer electrolytes (SPEs) in all-solid-state(ASS) batteries is hindered by lower Li⁺ -conductivity and narrower electrochemical window. Here, three families of ester-based F-modified SPEs of poly-carbonate (PCE), poly-oxalate (POE) and poly-malonate (PME) were investigated. The Li⁺ -conductivity of these SPEs prepared from pentanediol are all higher than the counterparts made of butanediol, owing to the enhanced asymmetry and flexibility. Because of stronger chelating coordination with Li⁺ , the Li⁺ -conductivity of PME and POE is around 10 and 5 times of PCE. The trifluoroacetyl-units are observed more effective than -O-CH₂ -CF₂ -CF₂ -CH₂ -O- during the in situ passivation of Li-metal. Using trifluoroacetyl terminated POE and PCE as SPE, the interfaces with Li-metal and high-voltage-cathode are stabilized simultaneously, endowing stable cycling of ASS Li/LiNi_0.6 Co_0.2 Mn_0.2 O₂ (NCM622) cells. Owing to an enol isomerization of malonate, the cycling stability of Li/PME/NCM622 is deteriorated, which is recovered with the introduce of dimethyl-group in malonate and the suppression of enol isomerization. The coordinating capability with Li⁺ , molecular asymmetry and existing modes of elemental F, are all critical for the molecular design of SPEs.

Dual-Functional Hosts for Polysulfides Conversion and Lithium Plating/Stripping towards Lithium-Sulfur Full Cells

The practical application of lithium-sulfur (Li-S) batteries is greatly hindered by the shuttle effect of dissolved polysulfides in the sulfur cathode and the severe dendritic growth in the lithium anode. Adopting one type of effective host with dual-functions including both inhibiting polysulfide dissolution and regulating Li plating/stripping, is recently an emerging research highlight in Li-S battery. This review focuses on such dual-functional hosts and systematically summarizes the recent research progress and application scenarios. Firstly, this review briefly describes the stubborn issues in Li-S battery operations and the sophisticated counter measurements over the challenges by dual-functional behaviors. Then, the latest advances on dual-functional hosts for both cathode and anode in Li-S full cells are catalogued as species, including metal chalcogenides, metal carbides, metal nitrides, heterostuctures, and the possible mechanisms during the process. Besides, we also outlined the theoretical calculation tools for the dual-functional host based on the first principles. Finally, several sound perspectives are also rationally proposed for fundamental research and practical development as guidelines.

Fluorinated Covalent Organic Framework-Based Nanofluidic Interface for Robust Lithium-Sulfur Batteries

To realize the practical application of lithium-sulfur (Li-S) batteries, there is a need to inhibit uncontrolled Li deposition by facilitating Li-ion migration, and suppress the irreversible consumption of cathodes by preventing polysulfide shuttling. However, a permselective artifical membrane or interlayer which features fast ion transport but low polysulfide crossover is elusive. Here, we report the design and synthesis of a fluorinated covalent organic framework (4F-COF)-based membrane with a high permselectivity and increased battery lifespan. Combining density functional theory calculation, molecular dynamic simulation, and in situ Raman analysis, we demonstrate that fluorinated COF eliminates polysulfides shutting and dendritic lithium formation. Consequently, Li symmetrical cells demonstrate Li plating/stripping behaviors for 2000 h under 1 mA cm^-2. More importantly, Li-S batteries based on the 4F-COF/PP separator achieve cycling retention of 82.3% over 1000 cycles at 2 C, rate performance of 568.0 mA h g^-1 at 10 C, and an areal capacity of 7.60 mA h cm^-2 with a high sulfur loading (∼9 mg cm^-2). This work demonstrates that functionalizing nanochannels in COFs can impart permselectivity for energy storage applications.

Anisotropically Hybridized Porous Crystalline Li-S Battery Separators

Anisotropically hybridized porous crystalline Li-S battery separators based on porous crystalline materials that can meet the multiple functionalities of both anodic and cathodic sides are much desired for Li-S battery yet still challenging in directional design. Here, an anisotropically hybridized separator (CPM) based on an ionic liquid-modified porphyrin-based covalent-organic framework (COF-366-OH-IL) and catalytically active metal-organic framework (Ni₃ (HITP)₂ ) that can integrate the lithium-polysulfides (LiPSs) adsorption/catalytic conversion and ion-conduction sites together to directionally meet the requirements of electrodes is reported. Remarkably, the-obtained separator exhibits an exceptional high Li⁺ transference-number (t_Li+  = 0.8), ultralow polarization-voltage (<30 mV), high initial specific-capacity (921.38 mAh g^-1 at 1 C), and stable cycling-performance, much superior to polypropylene and monolayer-modified separators. Moreover, theoretical calculations confirm the anisotropic effect of CPM on the anodic side (e.g., Li⁺ transfer, LiPSs adsorption, and anode-protection) and cathodic side (e.g., LiPSs adsorption/catalysis). This work might provide a new perspective for separator exploration.

Ultralight Porous Cu Nanowire Aerogels as Stable Hosts for High Li-Content Metal Anodes

Using porous copper (Cu) as the host is one of the most effective approaches to stabilize Li metal anodes. However, the most widely used porous Cu hosts usually account for the excessive mass proportion of composite anodes, which seriously decreases the energy density of Li metal batteries. Herein, an ultralight porous Cu nanowire aerogel (UP-Cu) is reported as the Li metal anode host to accommodate a high mass loading of Li content of 77 wt %. Specifically, the Li/UP-Cu electrode displays a satisfactory gravimetric capacity of 2715 mAh g^-1, which is higher than that of the most reported Li metal composite anodes. The UP-Cu host achieves a high Coulombic efficiency of ∼98.9% after 250 cycles in the half cell and exceptional electrochemical stability under high-current-density and deep-plating-stripping conditions in the symmetrical cell. The Li/UP-Cu|LiFePO₄ battery displays a specific capacity of 102 mAh g^-1 at 5 C for 5000 cycles. The Li/UP-Cu|LiFePO₄ pouch cell achieves a significantly high capacity of 146.3 mAh g^-1 with a high capacity retention of 95.83% for 360 cycles. This work provides a lightweight porous host to stabilize Li-metal anodes and maintain their high mass-specific capacity.

Induction/Inhibition Effect on Lithium Dendrite Growth by a Binary Modification Layer on a Separator

In lithium metal batteries (LMB), unrestricted growth of lithium dendrites will pierce the separator and cause an internal short circuit. Therefore, we designed modified separator with an InN thin layer, which could be in situ converted into a binary mixed-modified layer of Li-In alloy and Li₃N during the lithium plating/stripping process. Among them, Li-In alloy induces the lateral growth of lithium dendrites and prevents the separator from being pierced; Li₃N balances ion distribution at the lithium anode/separator interface, which is beneficial to inhibit the growth of lithium dendrites. Under the synergistic effect of the two phases, the performance of LMBs was obviously improved. In addition, the separator modification does not need to be carried out in a protective atmosphere and is suitable for large-scale roll-to-roll processing.

Ionic Liquid-Type Additive for Lithium Metal Batteries Operated in LiPF6 Based-Electrolyte Containing 2500 ppm H2O

The presence of trace amounts of moisture in the electrolyte can cause hydrolysis of LiPF₆ and deteriorate the stability of lithium metal batteries. Herein, we propose a multifunctional ionic liquid-type additive constituting a 1-methyl-1-butyl pyrrolidium cation (Py₁₄⁺) and an acetate anion (CH₃COO^-) (denoted as IL-AC in this study), which can effectively adsorb the trace moisture and thus prevent the hydrolysis of LiPF₆ via intermolecular interactions. The prepared IL-AC can also remove HF to suppress the dissolution of transition metal ions from cathode materials through the reaction CH₃COO^- + HF → CH₃COOH + F^-. Compared with the baseline electrolyte, the contents of HF and transition metal ions are significantly lower in the electrolyte with 0.5% IL-AC. Upon the addition of 0.5% IL-AC additive and 2500 ppm H₂O, the Li||NCM811 battery shows a capacity of 153.7 mAh g^-1 after 300 cycles, while the Li||LNMO battery possesses stable capacity retention of 93.22% after 500 cycles at 1C and a Coulombic efficiency greater than 99%. Thus, this work provides a convenient and effective method to absorb trace amounts of water and remove HF in the electrolyte and provides a new path for the expensive and tedious process of water removal from the electrolyte in industry.

Advances in the Emerging Gradient Designs of Li Metal Hosts

Developing host has been recognized a potential countermeasure to circumvent the intrinsic drawbacks of Li metal anode (LMA), such as uncontrolled dendrite growth, unstable solid electrolyte interface, and infinite volume fluctuations. To realize proper Li accommodation, particularly bottom-up deposition of Li metal, gradient designs of host materials including lithiophilicity and/or conductivity have attracted a great deal of attention in recent years. However, a critical and specialized review on this quickly evolving topic is still absent. In this review, we attempt to comprehensively summarize and update the related advances in guiding Li nucleation and deposition. First, the fundamentals regarding Li deposition are discussed, with particular attention to the gradient design principles of host materials. Correspondingly, the progress of creating different gradients in terms of lithiophilicity, conductivity, and their hybrid is systematically reviewed. Finally, future challenges and perspective on the gradient design of advanced hosts towards practical LMAs are provided, which would provide a useful guidance for future studies.

One-Pot Preparation of Lithium Compensation Layer, Lithiophilic Layer, and Artificial Solid Electrolyte Interphase for Lean-Lithium Metal Anode

Lithium metal is an ideal anode for high-energy-density batteries. However, the low Coulomb efficiency and the generation of dendrites pose a significant limitation to its practical application, while the excess lithium in the battery also generates serious safety concerns. Herein, a layer-by-layer optimized multilayer structure integrating an artificial solid electrolyte interphase (LiF) layer, a lithiophilic (Li_xAu alloy) layer, and a lithium compensation layer is reported for a lean-lithium metal battery, where each layer acts synergistically to stabilize the lithium deposition behaviors and enhances the cycling performance of the battery. The optimized anode could effectively induce homogeneous reversible lithium deposition under the synergistic effect of multilayer films and keep the integrity of the morphological structure unbroken during the deposition. The presence of the lithium compensation layer allows the half-cell to have a high initial CE of 158.9%, and the action of the LiF layer and lithiophilic layer maintains an average CE of 98.8% over 160 cycles, which further demonstrates the stability of the structure. As a result, when combined with LiFePO₄ cathode, an initial capacity of 148 mAh g^-1 and a retention rate of 97.5% over 130 cycles were achieved.

Microstructure of Lithium Dendrites Revealed by Room-Temperature Electron Microscopy

The uncontrolled deposition/dissolution process of lithium dendrites during electrochemical cycling in batteries limits the large-scale application of Li metal anodes. Investigating the microstructure of Li dendrites is a focal point. Currently, the only way to protect and observe sensitive Li dendrites is through low-temperature transmission electron microscopy (LT-TEM), whereas room-temperature characterization is still lacking. In this work, the room-temperature microstructure of Li dendrites was obtained by TEM using both vacuum- and inert-gas-transfer methods. Detailed comparison between LT- and room-temperature (RT-)TEM characterizations was provided to show the pros and cons of each method. Especially, RT-TEM shows the advantage of flexible incorporation with multifunctional characterizations, such as 3D tomography. By using RT-TEM, microstructural evolution of Li dendrites during the electrodeposition/dissolution process, including increase of the quantity of inorganic Li₂O compounds in the solid electrolyte interphase, lateral growth behavior, and two types of inactive Li, has been revealed, enriching the understanding of the structure-property relationship of Li dendrites.