Design principles of heterointerfacial redox chemistry for highly reversible lithium metal anode.

High electrochemical reversibility is required for the application of high-energy-density lithium (Li) metal batteries; however, inactive Li formation and SEI (solid electrolyte interface)-instability-induced electrolyte consumption cause low Coulombic efficiency (CE). The prior interfacial chemical designs in terms of alloying kinetics have been used to enhance the CE of Li metal anode; however, the role of its redox chemistry at heterointerfaces remains a mystery. Herein, the relationship between heterointerfacial redox chemistry and electrochemical transformation reversibility is investigated. It is demonstrated that the lower redox potential at heterointerface contributes to higher CE, and this enhancement in CE is primarily due to the regulation of redox chemistry to Li deposition behavior rather than the formation of SEI films. Low oxidation potential facilitates the formation of the surface with the highly electrochemical binding feature after Li stripping, and low reduction potential can maintain binding ability well during subsequent Li plating, both of which homogenize Li deposition and thus optimize CE. In particular, Mg hetero-metal with ultra-low redox potential enables Li metal anode with significantly improved CE (99.6%) and stable cycle life for 700 cycles at 3.0 mA cm-2. This work provides insight into the heterointerfacial design principle of next-generation negative electrodes for highly reversible metal batteries.

Separators Modified with Ultrathin Montmorillonite/Polymer Nanocoatings Achieve Dendrite-Free Lithium Deposition at High Current Densities.

Fatal dendritic growth in lithium metal batteries is closely related to the composition and thickness of the modified separator. Herein, an ultrathin nanocoating composed of monolayer montmorillonite (MMT), poly(vinyl alcohol) (PVA) on a polypropylene separator is prepared. The MMT was exfoliated into monolayers (only 0.96 nm) by intercalating PVA under ultrasound, followed by cross-linking with glutaraldehyde. The thickness of the nanocoating on the polypropylene separator, as determined using the pull-up method, is only 200-500 nm with excellent properties. As a result, the lithium-symmetric battery composed of it has a low overpotential (only 40 mV) and a long lifespan of more than 7900 h at high current density, because ion transport is unimpeded and Li+ flows uniformly through the ordered ion channels between the MMT layers. Additionally, the separator exhibited excellent cycling stability in Li-S batteries. This study offers a new idea for fabricating ultrathin clay/polymer modified separators for metal anode stable cycling at high current densities.

Compressible and Elastic Reduced Graphene Oxide Sponge for Stable and Dendrite-Free Lithium Metal Anodes.

Dendritic Li deposition, an unstable solid-electrolyte interphase (SEI), and a nearly infinite relative volume change during cycling are three major obstacles to the practical application of Li metal batteries. Herein, we introduce a compressible and elastic reduced graphene oxide sponge (rGO-S) to simultaneously eliminate Li dendrite growth, stabilize the SEI, and accommodate the volume change. The volume change is contained by compressing and expanding the rGO-S anode, which effectively releases the Li plating-induced stress during cycling. The smooth and dense Li metal is deposited on rGO-S without dendrites, which preserves the SEI, reduces consumption of the electrolyte, and prevents the formation of Li debris. The half-cells employing rGO-S show a steady and high Coulombic efficiency. The Li@rGO-S symmetric cells demonstrate excellent cycling stability over 1200 cycles with a low overpotential. When paired with LiFePO4 (LFP), the Li@rGO-S||LFP full cells exhibit a high specific capacity (150.3 mAh g-1 at 1C), superior rate performance, and good capacity retention.

Trinitarian Design of Gradient Artificial Interphase Enables Colossal Granular Li Deposits for Stable Li-Metal Batteries.

The cycling lifespan of Li-metal batteries is compromised by the unstable solid electrolyte interphase (SEI) and the continuous Li dendrites, restricting their practical implementations. Given these challenges, establishing an artificial SEI holds promise. Herein, a trinitarian gradient interphase is innovatively designed through composite coatings of magnesium fluoride (MgF2), N-hexadecyltrimethylammonium chloride (CTAC), and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) on Li-metal anode (LMA). Specifically, the MgF2/CTAC/PVDF-HFP SEI spontaneously forms a lithium fluoride (LiF)-rich PVDF-HFP-based SEI, along with lithium-magnesium (Li-Mg) alloy substrate as lithiophilic electronic conductor and positively charged CTAC during plating. Noticeably, the Li-Mg alloy homogenizes the distribution of electric field and reduce the internal resistance, while the electronically insulated LiF/PVDF-HFP composite SEI offers fast ion-conducting and mechanical flexibility, accommodating the volumetric expansion and ensuring stable Li-ion flux. Additionally, CTAC at the dendritic tip is pivotal for mitigating dendrites through its electrostatic shield mechanism. Innovatively, this trinitarian synergistic mechanism, which facilitates colossal granular Li deposits, constructs a dendrite-free LMA, leading to stable cycling performances in practical Li||LFP, popular Li||NCM811, and promising Li||S full cells. This work demonstrates the design of multifunctional composite SEI for comprehensive Li protection, thereby inspiring further advancements in artificial SEI engineering for alkali-metal batteries.

Double-Salts Super Concentrated Carbonate Electrolyte Boosting Electrochemical Performance of Ni-Rich LiNi0.90Co0.05Mn0.05O2 Lithium Metal Batteries.

Current lithium-ion batteries cannot meet the requirement of higher energy density with further large-scale application of electrical vehicles. Lithium metal batteries combined with Ni-rich layered oxides cathode are expected as the one of promising solutions, while the poor electrode and electrolyte interface impedes the commercial development of lithium metal batteries. A new double-salts super concentrated (DSSC) carbonate electrolyte is proposed to improve the electrochemical performance of LiNi0.90Co0.05Mn0.05O2 (NCM9055)||Li metal battery which exhibits stable cycling performance with the capacity retention of 93.04% and reversible capacity of 173.8 mAh g-1 after 100 cycles at 1 C, while cells with conventional 1 m diluted electrolyte remains only 60.55% and capacity of 114.2 mAh g-1. The double salts synergistic effect in super concentrated electrolyte promotes the formation for more balanced stable cathode electrolyte interface (CEI) inorganic compounds of CFx, LiNOx, SOF2, Li2SO4, and less LiF by X-ray photoelectron spectroscopy (XPS) test, and the uniform 2-3 nm rock-salt phase protection layer on the cathode surface by transmission electron microscope (TEM) characterization, improving the cycling performance of the Ni-rich NCM9055 layered oxide cathode. The DSSC electrolyte also can relief the Li dendrite growth on Li metal anode, as well as exhibit better flame retardance, promoting the application of more safety Ni-rich NCM9055||Li metal batteries.

LiF-Rich Solid Electrolyte Interphase Formation by Establishing Sacrificial Layer on the Separator.

The formation of a stable solid electrolyte interphase (SEI) layer is crucial for enhancing the safety and lifespan of Li metal batteries. Fundamentally, a homogeneous Li+ behavior by controlling the chemical reaction at the anode/electrolyte interface is the key to establishing a stable SEI layer. However, due to the highly reactive nature of Li metal anodes (LMAs), controlling the movement of Li+ at the anode/electrolyte interface remains challenging. Here, an advanced approach is proposed for coating a sacrificial layer called fluorinated self-assembled monolayer (FSL) on a boehmite-coated polyethylene (BPE) separator to form a stable SEI layer. By leveraging the strong affinity between the fluorine functional group and Li+, the rapid formation of a LiF-rich SEI layer in the cell production and early cycling stage is facilitated. This initial stable SEI formation promotes the subsequent homogeneous Li+ flux, thereby improving the LMA stability and yielding an enhanced battery lifespan. Further, the mechanism behind the stable SEI layer generation by controlling the Li+ dynamics through the FSL-treated BPE separator is comprehensively verified. Overall, this research offers significant contributions to the energy storage field by addressing challenges associated with LMAs, thus highlighting the importance of interfacial control in achieving a stable SEI layer.

Dead Lithium in Lithium Metal Batteries: Formation, Characterization and Strategies.

Lithium (Li) metal anode (LMA) replacing graphite anode for developing Li metal batteries (LMB) with the higher energy density has attracted much attention. However, LMA faces many issues, e. g., Li dendrites, dead Li and the side reactions, which causes the safety hazards and low coulomb efficiency (CE) of battery, therefore, LMB still cannot replace the current Li ion battery for practical use. Among those issues, dead Li is one of the decisive factors affecting the CE of LMB. To better understand dead Li, we summarize the recent work about the generation of dead Li, its impact on batteries performance, and the strategies to reuse and eliminate dead Li. Finally, the prospect of the future LMA and resultant LMB is also put forward.

Structural Design of 3D Current Collectors for Lithium Metal Anodes: A Review.

Due to the ultrahigh theoretical specific capacity (3860 mAh g-1) and low redox potential (-3.04 V vs. standard hydrogen electrode), Lithium (Li) metal anode (LMA) received increasing attentions. However, notorious dendrite and volume expansion during the cycling process seriously hinder the development of high energy density Li metal batteries. Constructing three-dimensional (3D) current collectors for Li can fundamentally solve the intrinsic drawback of hostless for Li. Therefore, this review systematically introduces the design and synthesis engineering and the current development status of different 3D collectors in recent years (the current collectors are divided into two major parts: metal-based current collectors and carbon-based current collectors). In the end, some perspectives of the future promotion for LMA application are also presented.

High Rate and Low-Temperature Stable Lithium Metal Batteries Enabled by Lithiophilic 3D Cu-CuSn Porous Framework.

Lithium (Li) metal is regarded as the "Holy Grail" of anodes for high-energy rechargeable lithium batteries by virtue of its ultrahigh theoretical specific capacity and the lowest redox potential. However, the Li dendrite impedes the practical application of Li metal anodes. Herein, lithiophilic three-dimensional Cu-CuSn porous framework (3D Cu-CuSn) was fabricated by a vapor phase dealloying strategy via the difference in saturated vapor pressure between different metals and the Kirkendall effect. CuSn alloy sites were converted into LiSn alloy sites through the molten Li infusion method, and composite Li metal anodes (3D Cu-LiSn-Li) are achieved. Alloyed tin, as the bridge between the porous copper substrate and metallic Li, plays a critical role in optimizing Li nucleation and enhancing the fast lithium migration kinetics. This work demonstrates that lithiophilic binary copper alloys are an effective way to achieve room-temperature high rate performance and satisfied low-temperature cycling stability for Li metal batteries.

Building Solid-State Batteries: Insights from Swiss Research Labs.

This review article delves into the growing field of solid-state batteries as a compelling alternative to conventional lithium-ion batteries. The article surveys ongoing research efforts at renowned Swiss institutions such as ETH Zurich, Empa, Paul Scherrer Institute, and Berner Fachhochschule covering various aspects, from a fundamental understanding of battery interfaces to practical issues of solid-state battery fabrication, their design, and production. The article then outlines the prospects of solid-state batteries, emphasizing the imperative practical challenges that remain to be overcome and highlighting Swiss research groups' efforts and research directions in this field.

A 3D Hierarchical Host with Gradient-Distributed Dielectric Properties toward Dendrite-free Lithium Metal Anode.

The conventional conductive three-dimensional (3D) host fails to effectively stabilize lithium metal anodes (LMAs) due to the internal incongruity arising from nonuniform lithium-ion gradient and uniform electric fields. This results in undesirable Li "top-growth" behavior and dendritic Li growth, significantly impeding the practical application of LMAs. Herein, we construct a 3D hierarchical host with gradient-distributed dielectric properties (GDD-CH) that effectively regulate Li-ion diffusion and deposition behavior. It comprises a 3D carbon fiber host modified by layer-by-layer bottom-up attenuating Sb particles, which could promote Li-ion homogeneously distribution and reduce ion concentration gradient via unique gradient dielectric polarization. Sb transforms into superionic conductive Li3Sb alloy during cycling, facilitating Li-ion dredging and pumps towards the bottom, dominating a bottom-up deposition regime confirmed by COMSOL Multiphysics simulations and physicochemical characterizations. Consequently, a stable cycling performance of symmetrical cells over 2000 h under a high current density of 10 mA cm-2 is achieved. The GDD-CH-based lithium metal battery shows remarkable cycling stability and ultra-high energy density of 378 Wh kg-1 with a low N/P ratio (1.51). This strategy of dielectric gradient design broadens the perspective for regulating the Li deposition mechanism and paves the way for developing high-energy-density lithium metal anodes with long durability.

Large Organic Polar Molecules Tailored Electrode Interfaces for Stable Lithium Metal Battery.

Lithium (Li) metal batteries (LMBs) are deemed as ones of the most promising energy storage devices for next electrification applications. However, the uneven Li electroplating process caused by the diffusion-limited Li+ transportation at the Li metal surface inherently promotes the formation of dendritic morphology and instable Li interphase, while the sluggish Li+ transfer kinetic can also cause lithiation-induced stress on the cathode materials suffering from serious structural stability. Herein, a novel electrolyte designing strategy is proposed to accelerate the Li+ transfer by introducing a trace of large organic polar molecules of lithium phytate (LP) without significantly altering the electrolyte structure. The LP molecules can afford a competitive solvent attraction mechanism against the solvated Li+, enhancing both the bulk and interfacial Li+ transfer kinetic, and creating better anode/cathode interfaces to suppress the side reactions, resulting in much improved cycling efficiency of LMBs. Using LP-based electrolyte, the performance of LMB pouch cell with a practical capacity of ~1.5 Ah can be improved greatly. This strategy opens up a novel electrolyte designing route for reliable LMBs.

LiF-Rich Alloy-Doped SEI Enabling Ultra-Stable and High-Rate Li Metal Anode.

Li metal is regarded as the "Holy Grail" in the next generation of anode materials due to its high theoretical capacity and low redox potential. However, sluggish Li ions interfacial transport kinetics and uncontrollable Li dendrites growth limit practical application of the energy storage system in high-power device. Herein, separators are modified by the addition of a coating, which spontaneously grafts onto the Li anode interface for in situ lithiation. The resultant alloy possessing of strong electron-donating property promotes the decomposition of lithium bistrifluoromethane sulfonimide in the electrolyte to form a LiF-rich alloy-doped solid electrolyte interface (SEI) layer. High ionic alloy solid solution diffusivity and electric field dispersion modulation accelerate Li ions transport and uniform stripping/plating, resulting in a high-power dendrite-free Li metal anode interface. Surprisingly, the formulated SEI layer achieves an ultra-long cycle life of over 8000 h (20,000 cycles) for symmetric cells at a current density of 10 mA cm-2. It also ensures that the NCM(811)//PP@Au//Li full cell at ultra-high currents (40 C) completes the charging/discharging process in only 68 s to provide high capacity of 151 mAh g-1. The results confirm that this scalable strategy has great development potential in realizing high power dendrite-free Li metal anode.

The intrinsic behavior of lithium fluoride in solid electrolyte interphases on lithium.

Lithium is the most attractive anode material for high-energy density rechargeable batteries, but its cycling is plagued by morphological irreversibility and dendrite growth that arise in part from its heterogeneous "native" solid electrolyte interphase (SEI). Enriching the SEI with lithium fluoride (LiF) has recently gained popularity to improve Li cyclability. However, the intrinsic function of LiF-whether chemical, mechanical, or kinetic in nature-remains unknown. Herein, we investigated the stability of LiF in model LiF-enriched SEIs that are either artificially preformed or derived from fluorinated electrolytes, and thus, the effect of the LiF source on Li electrode behavior. We discovered that the mechanical integrity of LiF is easily compromised during plating, making it intrinsically unable to protect Li. The ensuing in situ repair of the interface by electrolyte, either regenerating LiF or forming an extra elastomeric "outer layer," is identified as the more critical determinant of Li electrode performance. Our findings present an updated and dynamic picture of the LiF-enriched SEI and demonstrate the need to carefully consider the combined role of ionic and electrolyte-derived layers in future design strategies.

Stabilizing Li-O2 Batteries with Multifunctional Fluorinated Graphene.

As a full cell system with attractive theoretical energy density, challenges faced by Li-O2 batteries (LOBs) are not only the deficient actual capacity and superoxide-derived parasitic reactions on the cathode side but also the stability of Li-metal anode. To solve simultaneously intrinsic issues, multifunctional fluorinated graphene (CFx, x = 1, F-Gr) was introduced into the ether-based electrolyte of LOBs. F-Gr can accelerate O2- transformation and O2--participated oxygen reduction reaction (ORR) process, resulting in enhanced discharge capacity and restrained O2--derived side reactions of LOBs, respectively. Moreover, F-Gr induced the F-rich and O-depleted solid electrolyte interphase (SEI) film formation, which have improved Li-metal stability. Therefore, energy storage capacity, efficiency, and cyclability of LOBs have been markedly enhanced. More importantly, the method developed in this work to disperse F-Gr into an ether-based electrolyte for improving LOBs' performances is convenient and significant from both scientific and engineering aspects.

Soybean Protein Fiber Enabled Controllable Li Deposition and a LiF-Nanocrystal-Enriched Interface for Stable Li Metal Batteries.

The proliferation of lithium (Li) dendrites stemming from uncontrollable Li deposition seriously limits the practical application of Li metal batteries. The regulation of uniform Li deposition is thus a prerequisite for promoting a stable Li metal anode. Herein, a commercial lithiophilic skeleton of soybean protein fiber (SPF) is introduced to homogenize the Li-ion flux and induce the biomimetic Li growth behavior. Especially, the SPF can promote the formation of a LiF-nanocrystal-enriched interface upon cycling, resulting in low interfacial impedance and rapid charge transfer kinetics. Finally, the SPF-mediated Li metal anode can achieve high Coulombic efficiency of 98.7% more than 550 cycles and a long-term lifespan over 3400 h (∼8500 cycles) in symmetric tests. Furthermore, the practical pouch cell modified with SPF can maintain superior electrochemical performance over 170 cycles under a low N/P ratio and high mass loading of the cathode.

Biomass-Derived Anion-Anchoring Nano-CaCO3 Coating for Regulating Ion Transport on Li Metal Surface.

The free transport of anions in a Li metal battery can cause multiple issues, including a high anion transference number, space charge, and concentration polarization, eventually leading to uncontrolled dendrite formation and decreased performance. Herein, we report an anion-anchoring nano-CaCO3 (NC) coating derived from eggshell biowaste for stabilizing Li metal anodes. As the adsorption of local TFSI- anions onto the NC adsorbent can undermine the anion concentration gradient and promote rapid Li-ion diffusion, it can effectively inhibit the proliferation of Li dendrites assisted by the NC coating. Consequently, Li/Cu cells with NC@Cu electrode can achieve a high Coulombic efficiency of ∼98.4% for more than 420 cycles and can even reach ∼99.1% at an ultrahigh areal capacity of 20 mAh cm-2. In particular, full cells with NC/Li@Cu electrodes show a stable lifespan of over 240 cycles with an average efficiency of ∼99.8% at a low N/P ratio of ∼3.3.

Stable All-Solid-State Lithium Metal Batteries Enabled by Machine Learning Simulation Designed Halide Electrolytes.

Solid electrolytes (SEs) with superionic conductivity and interfacial stability are highly desirable for stable all-solid-state Li-metal batteries (ASSLMBs). Here, we employ neural network potential to simulate materials composed of Li, Zr/Hf, and Cl using stochastic surface walking method and identify two potential unique layered halide SEs, named Li2ZrCl6 and Li2HfCl6, for stable ASSLMBs. The predicted halide SEs possess high Li+ conductivity and outstanding compatibility with Li metal anodes. We synthesize these SEs and demonstrate their superior stability against Li metal anodes with a record performance of 4000 h of steady lithium plating/stripping. We further fabricate the prototype stable ASSLMBs using these halide SEs without any interfacial modifications, showing small internal cathode/SE resistance (19.48 Ω cm2), high average Coulombic efficiency (∼99.48%), good rate capability (63 mAh g-1 at 1.5 C), and unprecedented cycling stability (87% capacity retention for 70 cycles at 0.5 C).

Perspective on design and technical challenges of Li-garnet solid-state batteries.

Solid-state Li-ion batteries based on Li-garnet Li7La3Zr2O12 (LLZO) electrolyte have seen rapid advances in recent years. These solid-state systems are poised to address the urgent need for safe, non-flammable, and temperature-tolerant energy storage batteries that concomitantly possess improved energy densities and the cycle life as compared to conventional liquid-electrolyte-based counterparts. In this vision article, we review present research pursuits and discuss the limitations in the employment of LLZO solid-state electrolyte (SSE) for solid-state Li-ion batteries. Particular emphasis is given to the discussion of pros and cons of current methodologies in the fabrication of solid-state cathodes, LLZO SSE, and Li metal anode layers. Furthermore, we discuss the contributions of the LLZO thickness, cathode areal capacity, and LLZO content in the solid-state cathode on the energy density of Li-garnet solid-state batteries, summarizing their required values for matching the energy densities of conventional Li-ion systems. Finally, we highlight challenges that must be addressed in the move towards eventual commercialization of Li-garnet solid-state batteries.

Thermodynamic Regulation of Dendrite-Free Li Plating on Li3Bi for Stable Lithium Metal Batteries.

Rechargeable batteries with metallic lithium (Li) anodes are attracting ever-increasing interests because of their high theoretical specific capacity and energy density. However, the dendrite growth of the Li anode during cycling leads to poor stability and severe safety issues. Here, Li3Bi alloy coated carbon cloth is rationally chosen as the substrate of the Li anode to suppress the dendrite growth from a thermodynamic aspect. The adsorption energy of a Li atom on Li3Bi is larger than the cohesive energy of bulk Li, enabling uniform Li nucleation and deposition, while the high diffusion barrier of the Li atom on Li3Bi blocks the migration of adatoms from adsorption sites to the regions of fast growth, which further ensures uniform Li deposition. With the dendrite-free Li deposition, the composite Li/Li3Bi anode enables over 250 cycles at an ultrahigh current density of 20 mA cm-2 in a symmetrical cell and delivers superior electrochemical performance in full batteries.