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Lithium dendrites cause battery failures and safety hazards in liquid‐electrolyte lithium‐based batteries. To address this problem, each component of the battery, such as cathode, anode, electrolyte and separator, should be well matched and engineered for the integrated battery system. The separator plays an increasingly important role owing to its gradual transformation from an inert to an active component in the battery, especially for resolving Li dendrite issues in Li‐metal batteries. Armed with advanced nanotechnology solutions, separator engineering presents a formidable strategy to suppress dendrite growth dynamics by modifying the electrolyte chemistry, regulating/trapping unwanted ionic species and redirecting dendrite growth direction. This review summarizes recent advances in separator engineering for effective dendrite suppression of Li‐metal anodes. We first introduce the challenges that the Li‐metal anode faces and the irreplaceable role of separators in the battery system. The characterization parameters and design principles of separators are also discussed. Then, the mainstream techniques for separator functionalization and their impacts on dendrite suppression are scrutinized and highlighted. Lastly, the conclusion is drawn and the future outlook for separator engineering is presented.

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. Published version This work was financially supported by the National Natural Science Foundation of China (51902261 and 61935017), the National Key Research and Development Program of China (2020YFA0709900), the Joint Research Funds of the Department of Science & Technology of Shaanxi Province and NPU (2020GXLH-Z-024), the Natural Science Basic Research Program of Shaanxi (2021JQ-107), Guangdong Basic and Applied Basic Research Foundation (2020A1515110604), the Natural Science Foundation of Ningbo (202003N4053), and the Fundamental Research Funds for the Central Universities(31020180QD116 and G2021KY05106).

The oxidization of pure Li is rarely discussed, but substantially important in the design and optimization, especially the failure mechanism of Li metal anode for rechargeable batteries. In this context, the Li metal and inherent Li Li2O interface after oxidization are examined using first principles calculations. The results demonstrated that in Li itself, the Li ion exhibits a fluid-like transport feature, especially in the vacancy-assisted migration. The fresh Li surface can be oxidized quickly in air with a thick cover (mainly Li oxides), and an inherent Li Li2O interface is, therefore, introduced. With the lowest surface energy, the Li (111) plane is revealed with a strong capability of adsorbing O and N atoms to form the distorted XLi6 (X = O, N, etc.) octahedra in the outmost surface layers. Although the Li ion migrates fast in Li itself, the Li Li2O interface shows a sluggish Li diffusion kinetic in Li2O side. This ultimately leads to the Li diffusion prefers to go along the interface boundary in Li side, which loosens the Li Li2O interface structure to result in the exfoliation of surface metastable Li2O spontaneously during electrochemical cycles in Li-metal batteries.

Solid‐state batteries have been considered as promising next‐generation energy storage devices for potentially higher energy density and better safety compared with commercial lithium‐ion batteries that are based on organic liquid electrolytes. However, in terms of indispensable solid‐state electrolytes, there are remaining issues to be solved before entering the market. Most solid‐state electrolytes are air‐sensitive, which causes a complex and expensive cell assembly and impressible interface. Therefore, the solid‐state electrolytes are expected to be atmosphere‐stable, which will undoubtedly bring significant benefits to solid‐state battery manufacturing. This review covers air‐stability‐related issues of different types of inorganic solid‐state electrolytes and the corresponding strategies. First, we provide an overview of solid‐state electrolytes and solid‐state batteries, including their history and advantages/disadvantages. Then, different types of solid‐state electrolytes are selected as examples to illustrate the unfavorable interactions in air and the corresponding adverse effects. Next, according to recent advances, we summarize the effective strategies of constructing different types of air‐stable inorganic solid‐state electrolytes. Finally, perspectives on designing accessible air‐stable solid‐state electrolytes are provided, aiming to achieve the assembly of high‐performance solid‐state batteries in the atmosphere.

Lithium (Li) metal batteries (LMBs) provide superior energy densities far beyond current Li-ion batteries (LIBs) but practical applications are hindered by uncontrolled dendrite formation and the build-up of dead Li in “hostless” Li metal anodes. To circumvent these issues, we created a 3D framework of a carbon paper (CP) substrate decorated with lithiophilic nanowires (silicon (Si), germanium (Ge), and SiGe alloy NWs) that provides a robust host for efficient stripping/plating of Li metal. The lithiophilic Li22Si5, Li22(Si0.5Ge0.5)5, and Li22Ge5 formed during rapid Li melt infiltration prevented the forma?tion of dead Li and dendrites. Li22Ge5/Li covered CP hosts delivered the best performance, with the lowest overpotentials of 40 mV (three times lower than pristine Li) when cycled at 1 mA cm−2 /1 mAh cm−2 for 1000 h and at 3 mA cm−2 /3 mAh cm−2 for 500 h. Ex situ analysis confirmed the ability of the lithiophilic Li22Ge5 decorated samples to facilitate uniform Li deposi?tion. When paired with sulfur, LiFePO4, and NMC811 cathodes, the CP-LiGe/ Li anodes delivered 200 cycles with 82%, 93%, and 90% capacity retention, respectively. The discovery of the highly stable, lithiophilic NW decorated CP hosts is a promising route toward stable cycling LMBs and provides a new design motif for hosted Li metal anodes.

Li metal batteries have been widely regarded as the next stage of energy storage technology, which is enabled by the low electrochemical potential (-3.04 V vs. the standard hydrogen electrode) and high specific capacity (3860 mAh g-1) of the Li metal anode. However, the implementation of Li metal anodes has been hindered by several issues including parasitic side reactions with electrolyte, large volume fluctuations, and dendrite formation which can cause short-circuits and safety issues. This thesis will cover some novel Li anode stabilization strategies while using advanced characterization techniques to provide critical information on the working mechanisms of Li metal anodes and their modes of failure. The first part of this thesis covers the design of a conductive 3D copper nanowire host for Li metal. A simple and facile fabrication process yields a lithiophilic surface that enables molten Li infusion into the 3D host. The 3D structure is shown to lower the localized current density and minimize electrode volume change, thus allowing for high performance and long-life Li metal anodes. The second part of this thesis investigates the temperature-dependent behavior and failure mechanisms of Li metal anodes in carbonate electrolytes. Several electrochemical testing methodologies are used to study the cycling behavior of Li metal anodes cycled between 0-60 °C. Furthermore, advanced synchrotron characterization techniques including in-situ X-ray tomography and energy-dependent X-ray fluorescence mapping are used for the first time to study Li metal anode chemical and physical structure changes under different operating conditions. The third part of this thesis explores the application of molecular layer deposition (MLD) Zircone films on Li metal anodes. The nanoscale zircone coatings are able to improve the air stability and processability of the Li metal anode while enabling operation at higher current densities with improved stability. The application of the coating is shown in Li-O2 batteries which have a 10-fold improvement in cycle life. The fourth part of this thesis investigates the mechanism of atomic layer deposition (ALD) coating growth on Li metal anodes and compares it to Cu current collectors for anode-free applications. The chemical and physical structure of the coatings are explored and it is shown that the reactive Li surface results in non-ideal surface structures with unexpected chemical compositions that differ from other substrates with the same deposition parameters. The final part of this thesis studies three superionic halide solid-state electrolytes and their electrochemical reactivity towards Li metal anodes. The reactions are studied through advanced characterization techniques and their failure modes are explored.

Due to an ultrahigh theoretical specific capacity of 3860 mAh g−1, lithium (Li) is regarded as the ultimate anode for high‐energy‐density batteries. However, the practical application of Li metal anode is hindered by safety concerns and low Coulombic efficiency both of which are resulted fromunavoidable dendrite growth during electrodeposition. This study focuses on a critical parameter for electrodeposition, the exchange current density, which has attracted only little attention in research on Li metal batteries. A phase‐field model is presented to show the effect of exchange current density on electrodeposition behavior of Li. The results show that a uniform distribution of cathodic current density, hence uniform electrodeposition, on electrode is obtained with lower exchange current density. Furthermore, it is demonstrated that lower exchange current density contributes to form a larger critical radius of nucleation in the initial electrocrystallization that results in a dense deposition of Li, which is a foundation for improved Coulombic efficiency and dendrite‐free morphology. The findings not only pave the way to practical rechargeable Li metal batteries but can also be translated to the design of stable metal anodes, e.g., for sodium (Na), magnesium (Mg), and zinc (Zn) batteries., The effect of exchange current density on electrodeposition behavior of lithium (Li) is revealed by a phase‐field modeling and validated by experimental data. The results show that lower exchange current density contributes to uniform distribution of cathodic current density and formation of nuclei with larger critical radius, two factors that promote dense Li deposition, high Coulombic efficiency, and dendrite‐free morphology.

With the growing demand of advanced energy storage devices that have high energy density and high power density to power electric vehicles and electrical grid, scientists and engineers are exploring technologies beyond conventional Li-ion batteries which have transformed the industry in the past thirty years. Li-S batteries have much higher energy density than Li-ion batteries and are gaining momentum. However, the intrinsic issues of Li-S batteries require a comprehensive systematic study of the protection of Li metal anodes to put them into practical applications. In the first study of this dissertation, we investigated using conventional electrolyte of Li-S batteries that includes 1,3-dioxolane to electrochemically pretreat Li metal anodes. We concluded that the electrochemical pretreatment of Li metal anodes generated an organic-inorganic artificial solid electrolyte interface (ASEI) layer that greatly enhanced the battery performance of the Li-S batteries. The properties of this ASEI can be tuned by manipulating the current density and cycle number of the electrochemical pretreatment. In the second study, we studied the comprehensive development and surface protection of Li10GeP2S12 (LGPS) material as solid-state electrolyte, which has ionic conductivity comparable to liquid electrolytes, potentially for solid-state Li-S batteries. Lithium phosphorus oxynitride (LiPON) was coated onto LGPS pellets by atomic layer deposition (ALD). It demonstrated great compatibility with LGPS and extends the electrochemical stability window. The third study explored the potential of transferring this electrochemical pretreatment method to the protection of other metal anodes, particularly Mg. The study discovered the surprising catalytic capability of Mg2+ in the polymerization of solvent 1,3-dioxolane (DOL). A layer with poly-DOL component was also found to grow on the surface of Mg metal anodes as a result of the electrochemical pretreatment, and the overpotential of Mg-Mg symmetric cells cycling dropped with the growth of the layer. Future studies are required to test the effectiveness of this method in Mg batteries. Overall, these studies can help to understand the surface chemistry of the electrochemically pretreated Li metal anodes, provide guidelines on the improvement of Li-S batteries and contribute to the development of solid-state Li-S batteries and multivalent metal anode batteries.

We report on the electrochemical performance of V2O3 polycrystalline nanorods (poly-NRs) as a cathode material for Li-ion batteries. Poly-NRs are formed through the thermal treatment of V2O5 nanotubes in a N2 atmosphere. X-ray and electron diffraction techniques are used to confirm the thermal reduction. Through galvanostatic cycling, we demonstrate that poly-NRs offer excellent capacity retention over 750 cycles. The capacity retention from the 50th to the 750th cycle was an impressive 94 %, retaining a capacity of approximately 120 mAh g−1 after 750 cycles. The outstanding stability of the nanocrystal-containing V2O3 poly-NRs over many cycles demonstrates that vanadium(III) oxide (V2O3) performs very well as a cathode material. Full Li-ion cells with paired a V2O3 poly-NR cathode and a pre-charged Co3O4 inverse opal (IO) conversion mode anode demonstrated high initial capacities and retained a capacity of 153 mAh g−1 after 50 cycles. The capacities achieved with our V2O3 poly-NRs/Co3O4 IO full cells are comparable to the capacities obtained from the most commonly used cathode materials when cycled in a half-cell arrangement versus pure Li metal.

All-solid-state batteries are expected to enable batteries with high energy density with the use of lithium metal anodes. Although solid electrolytes are believed to be mechanically strong enough to prevent lithium dendrites from propagating, various reports today still show cell failure due to lithium dendritic growth at room temperature. While cell parameters such as current density, electrolyte porosity and interfacial properties have been investigated, mechanical properties of lithium metal and the role of applied stack pressure on the shorting behavior is still poorly understood. Here, we investigated failure mechanisms of lithium metal in all-solid-state batteries as a function of stack pressure, and conducted in situ characterization of the interfacial and morphological properties of the buried lithium in solid electrolytes. We found that a low stack pressure of 5 MPa allows reliable plating and stripping in a lithium symmetric cell for more than 1000 hours, and a Li | Li6PS5Cl | LiNi0.80Co0.15Al0.05O2 full cell, plating more than 4 um of lithium per charge, is able to cycle over 200 cycles at room temperature. These results suggest the possibility of enabling the lithium metal anode in all-solid-state batteries at reasonable stack pressures., 13 pages, 5 figures

Lithium metal has been considered the "holy grail" anode material for rechargeable batteries despite the fact that its dendritic growth and low Coulombic efficiency (CE) have crippled its practical use for decades. Its high chemical reactivity and low stability make it difficult to explore the intrinsic chemical and physical properties of the electrochemically deposited lithium (EDLi) and its accompanying solid electrolyte interphase (SEI). To prevent the dendritic growth and enhance the electrochemical reversibility, it is crucial to understand the nano- and mesostructures of EDLi. However, Li metal is very sensitive to beam damage and has low contrast for commonly used characterization techniques such as electron microscopy. Inspired by biological imaging techniques, this work demonstrates the power of cryogenic (cryo)-electron microscopy to reveal the detailed structure of EDLi and the SEI composition at the nanoscale while minimizing beam damage during imaging. Surprisingly, the results show that the nucleation-dominated EDLi (5 min at 0.5 mA cm-2) is amorphous, while there is some crystalline LiF present in the SEI. The EDLi grown from various electrolytes with different additives exhibits distinctive surface properties. Consequently, these results highlight the importance of the SEI and its relationship with the CE. Our findings not only illustrate the capabilities of cryogenic microscopy for beam (thermal)-sensitive materials but also yield crucial structural information on the EDLi evolution with and without electrolyte additives.

Yu‐Guo Guo, Qiang Zhang, and co‐workers review the use of micro/nanostructured lithium metal anodes to retard the formation of lithium dendrites in lithium metal batteries in article 1600445. With the unique surface, pore, and connecting structures of different nanomaterials, the lithium plating/stripping processes are significantly regulated.

Low ionic conductivity at room temperature and limited electrochemical window of poly(ethylene oxide) (PEO) are the bottlenecks restricting its further application in high‐energy density lithium metal battery. Herein, a differentiated salt designed multilayered PEO‐based solid polymer electrolyte (DSM‐SPE) is exploited to achieve excellent electrochemical performance toward both the high‐voltage LiCoO2 cathode and the lithium metal anode. The LiCoO2/Li metal battery with DSM‐SPE displays a capacity retention of 83.3% after 100 cycles at 60 °C with challenging voltage range of 2.5 to 4.3 V, which is the best cycling performance for high‐voltage (≥4.3 V) LiCoO2/Li metal battery with PEO‐based electrolytes up to now. Moreover, the Li/Li symmetrical cells present stable and low polarization plating/stripping behavior (less than 80 mV over 600 h) at current density of 0.25 mA cm−2 (0.25 mAh cm−2). Even under a high‐area capacity of 2 mAh cm−2, the profiles still maintain stable. The pouch cell with DSM‐SPE exhibits no volume expansion, voltage decline, ignition or explosion after being impaled and cut at a fully charged state, proving the excellent safety characteristic of the DSM‐SPE‐based lithium metal battery., A multilayered poly(ethylene oxide)‐based electrolyte is exploited to achieve excellent cycling performance for a high‐voltage LiCoO2 Li metal battery. A delicately designed salt combination and proportion is presented as the multilayer, boosting wide electrochemical working window, high ionic conductivity, and suppression of Li dendrites simultaneously.

Rechargeable Li metal batteries have attracted lots of attention because they can achieve high energy densities. However, the commercialization of rechargeable Li metal batteries is delayed because Li dendrites may be generated during the batteries’ electrochemical cycles, which may cause severe safety issues. In this research, a Li-B alloy is investigated as an anode for rechargeable batteries instead of Li metal. Results show that the Li-B alloy has better effects in suppressing the formation of dendritic lithium, reducing the interface impedance and improving the cycle performance. These effects may result from the unique structure of Li-B alloy, in which free lithium is embedded in the Li7B6 framework. These results suggest that Li-B alloy may be a promising anode material applicable in rechargeable lithium batteries.

Owning to their very high theoretical capacity, lithium metal anodes are expected to fuel the extensive practical applications in portable electronics and electric vehicles. However, unstable solid electrolyte interphase and lithium dendrite growth during lithium plating/stripping induce poor safety, low Coulombic efficiency, and short span life of lithium metal batteries. Lately, varies of micro/nanostructured lithium metal anodes are proposed to address these issues in lithium metal batteries. With the unique surface, pore, and connecting structures of different nanomaterials, lithium plating/stripping processes have been regulated. Thus the electrochemical properties and lithium morphologies have been significantly improved. These micro/nanostructured lithium metal anodes shed new light on the future applications for lithium metal batteries.

Capacity retention in lithium metal batteries needs to be improved if they are to be commercially viable, the low cycling stability and Li corrosion during storage of lithium metal batteries being even more problematic when there is no excess lithium in the cell. Herein, we develop in situ NMR metrology to study "anode-free" lithium metal batteries where lithium is plated directly onto a bare copper current collector from a LiFePO4 cathode. The methodology allows inactive or "dead lithium" formation during plating and stripping of lithium in a full-cell lithium metal battery to be tracked: dead lithium and SEI formation can be quantified by NMR and their relative rates of formation are here compared in carbonate and ether-electrolytes. Little-to-no dead Li was observed when FEC is used as an additive. The bulk magnetic susceptibility effects arising from the paramagnetic lithium metal were used to distinguish between different surface coverages of lithium deposits. The amount of lithium metal was monitored during rest periods, and lithium metal dissolution (corrosion) was observed in all electrolytes, even during the periods when the battery is not in use, i.e., when no current is flowing, demonstrating that dissolution of lithium remains a critical issue for lithium metal batteries. The high rate of corrosion is attributed to SEI formation on both lithium metal and copper (and Cu+, Cu2+ reduction). Strategies to mitigate the corrosion are explored, the work demonstrating that both polymer coatings and the modification of the copper surface chemistry help to stabilize the lithium metal surface.

Capacity retention in lithium metal batteries needs to be improved if they are to be commercially viable, the low cycling stability and Li corrosion during storage of lithium metal batteries being even more problematic when there is no excess lithium in the cell. Herein, we develop in situ NMR metrology to study "anode-free" lithium metal batteries where lithium is plated directly onto a bare copper current collector from a LiFePO4 cathode. The methodology allows inactive or "dead lithium" formation during plating and stripping of lithium in a full-cell lithium metal battery to be tracked: dead lithium and SEI formation can be quantified by NMR and their relative rates of formation are here compared in carbonate and ether-electrolytes. Little-to-no dead Li was observed when FEC is used as an additive. The bulk magnetic susceptibility effects arising from the paramagnetic lithium metal were used to distinguish between different surface coverages of lithium deposits. The amount of lithium metal was monitored during rest periods, and lithium metal dissolution (corrosion) was observed in all electrolytes, even during the periods when the battery is not in use, i.e., when no current is flowing, demonstrating that dissolution of lithium remains a critical issue for lithium metal batteries. The high rate of corrosion is attributed to SEI formation on both lithium metal and copper (and Cu+, Cu2+ reduction). Strategies to mitigate the corrosion are explored, the work demonstrating that both polymer coatings and the modification of the copper surface chemistry help to stabilize the lithium metal surface.