Less is more: a perspective on thinning lithium metal towards high-energy-density rechargeable lithium batteries

Facile Lithium Densification Kinetics by Hyperporous/Hybrid Conductor for High-Energy-Density Lithium Metal Batteries

Lithium metal anode (LMA) emerges as a promising candidate for lithium (Li)-based battery chemistries with high-energy-density. However, inhomogeneous charge distribution from the unbalanced ion/electron transport causes dendritic Li deposition, leading to "dead Li" and parasitic reactions, particularly at high Li utilization ratios (low negative/positive ratios in full cells). Herein, an innovative LMA structural model deploying a hyperporous/hybrid conductive architecture is proposed on single-walled carbon nanotube film (HCA/C), fabricated through a nonsolvent induced phase separation process. This design integrates ionic polymers with conductive carbon, offering a substantial improvement over traditional metal current collectors by reducing the weight of LMA and enabling high-energy-density batteries. The HCA/C promotes uniform lithium deposition even under rapid charging (up to 5 mA cm-2) owing to its efficient mixed ion/electron conduction pathways. Thus, the HCA/C demonstrates stable cycling for 200 cycles with a low negative/positive ratio of 1.0 when paired with a LiNi0.8Co0.1Mn0.1O2 cathode (areal capacity of 5.0 mAh cm-2). Furthermore, a stacked pouch-type full cell using HCA/C realizes a high energy density of 344 Wh kg-1 cell/951 Wh L-1 cell based on the total mass of the cell, exceeding previously reported pouch-type full cells. This work paves the way for LMA development in high-energy-density Li metal batteries.

Rapidly Prepared Lithophilic Frameworks Stabilizes Lithium Anodes via Altered Lithium Deposition Patterns

Lithium metal batteries are regarded as promising candidates for next-generation energy storage systems. However, their anodes are susceptible to interfacial instability due to significant volume changes, which significantly impacts the cycle life of lithium metal batteries. Here, a rapid method for the fabrication of 3D-hosts with interface modified layers is reported. A simple infiltration and heating process enables the transformation of copper foam into Zn-BDC-modified copper foam within 1 min, rendering it suitable for use as a current collector for lithium metal anodes. The Zn-BDC nanosheets with high lithiophilicity are uniformly distributed on the surface of the current collector, facilitating the uniform deposition of lithium and reducing the volume change. Consequently, the half cell exhibits a remarkably low overpotential (26 mV) at a current-density of 4 mA cm-2 and is cycled stably for 1000 h. Furthermore, it demonstrates a significant enhancement in performance in the LiFePO4 full cell. This study provides a crucial reference on the connection between the interfacial modification of the current collector and the lithium deposition behavior, which promotes the practicalization of lithium metal anodes.

High-Strength, Thin, and Lightweight Solid Polymer Electrolyte for Superior All-Solid-State Sodium Metal Batteries

The utilization of solid polymer electrolytes (SPEs) in all-solid-state sodium metal batteries has been extensively explored due to their excellent flexibility, processability adaptability to match roll-to-roll manufacturing processes, and good interfacial contact with a high-capacity Na anode; however, SPEs are still impeded by their inadequate mechanical strength, excessive thickness, and poor stability with Na anodes. Herein, a robust, thin, and cost-effective polyethylene (PE) film is employed as a skeleton for infiltrating poly(ethylene oxide)-sodium bis(trifluoromethanesulfonyl)imide (PEO/NaTFSI) to fabricate PE-PEO/NaTFSI SPE. The resulting SPE features a remarkable thickness of 25 μm, lightweight property (2.1 mg cm-2), superior mechanical strength (tensile strength = 100.3 MPa), and good flexibility. The SPE also shows an ionic conductivity of 9.4 × 10-5 S cm-1 at 60 °C and enhanced interfacial stability with a sodium metal anode. Benefiting from these advantages, the assembled Na-Na symmetric cells with PE-PEO/NaTFSI show a high critical current density (1 mA cm-2) and excellent long-term cycling stability (3000 h at 0.3 mA cm-2). The all-solid-state Na||PE-PEO/NaTFSI||Na3V2(PO4)3 coin cells exhibit a superior cycling performance, retaining 93% of the initial capacity for 190 cycles when matched with a 6 mg cm-2 cathode loading. Meanwhile, the pouch cell can work stably after abuse testing, proving its flexibility and safety. This work offers a promising strategy to simultaneously achieve thin, high-strength, and safe solid-state electrolytes for all-solid-state sodium metal batteries.

Li ions traffic controller on thin lithium metal anode: Regulating deposition, optimizing and protecting solid electrolyte interphase

The performance of thin lithium metal anodes is affected due to issues that weaken the electrode-electrolyte interphase. In this work, a coating layer serving as a Li+ traffic controller based on hexadecyl trimethyl ammonium bis(trifluoromethanesulphonyl)imide ([CTA][TFSI]) and poly (vinylidene difluoride co-hexafluoropropylene) (P(VDF-HFP)) is used to stabilize the thin lithium metal interface. The CTA+ ions in the coating layer can effectively regulate the distribution of Li+ concentration to promote uniform deposition of lithium. The anion of [CTA][TFSI] can optimize solid electrolyte interphase (SEI) with inorganic-rich components, which improve the ionic conductivity and reaction kinetics. Furthermore, the flexible polymer skeleton can fortify the fragile SEI, facilitating the consistent operation of the battery. Due to these improvements, a thin Li metal anode (4 mAh cm-2) with a coating layer in a Li||Li symmetric cell demonstrates a lifespan of 600 h at 1 mA cm-2 and 1 mAh cm-2. Notably, full cells with an ultra-low negative electrode/positive electrode = 1 (N/P = 1) demonstrate a stable performance over 200 cycles and 90 cycles at 0.5C and 1C (1C = 170 mA g-1), respectively.

Crystalline MnCO3@Amorphous MnOx Composite as Cathode Material for High-Performance Aqueous Zinc-Ion Batteries

Rechargeable aqueous zinc-ion batteries (RAZIBs) have received extensive attention because of their advantages of low cost, high safety, and nontoxicity. However, problems such as dissolution of the active cathode material, dendrites/passivation of the zinc anode, and slow reaction kinetics hindered their further applications. In this work, a crystalline/amorphous composite-type material composed of crystalline MnCO3 and amorphous MnOx was prepared and used as the cathode material for RAZIBs. The MnCO3@amorphous MnOx (MnCO3@A-MnOx) composite possesses the merits of both the pure crystalline phase of MnCO3 and the amorphous phase of MnOx, which can deliver better electrochemical performance than the corresponding single component in repeated cycles. In addition, crystalline MnCO3 undergoes a complex phase transition to the active MnO2 during the first charge process, providing the composite with a stable structure and additional electrochemical capacity. The electrochemical measurement results indicate that the MnCO3@A-MnOx electrode can display high reversible discharge capacity at 0.1 A g-1, excellent rate performance at 5.0 A g-1, and long cycling stability over 2000 cycles, showing great potential as a cathode material for high-performance RAZIBs.

Microstructures of layered Ni-rich cathodes for lithium-ion batteries

Millions of electric vehicles (EVs) on the road are powered by lithium-ion batteries (LIBs) based on nickel-rich layered oxide (NRLO) cathodes, and they suffer from a limited driving range and safety concerns. Increasing the Ni content is a key way to boost the energy densities of LIBs and alleviate the EV range anxiety, which are, however, compromised by the rapid performance fading. One unique challenge lies in the worsening of the microstructural stability with a rising Ni-content in the cathode. In this review, we focus on the latest advances in the understanding of NLRO microstructures, particularly the microstructural degradation mechanisms, state-of-the-art stabilization strategies, and advanced characterization methods. We first elaborate on the fundamental mechanisms underlying the microstructural failures of NRLOs, including anisotropic lattice evolution, microcracking, and surface degradation, as a result of which other degradation processes, such as electrolyte decomposition and transition metal dissolution, can be severely aggravated. Afterwards, we discuss representative stabilization strategies, including the surface treatment and construction of radial concentration gradients in polycrystalline secondary particles, the fabrication of rod-shaped primary particles, and the development of single-crystal NRLO cathodes. We then introduce emerging microstructural characterization techniques, especially for identification of the particle orientation, dynamic changes, and elemental distributions in NRLO microstructures. Finally, we provide perspectives on the remaining challenges and opportunities for the development of stable NRLO cathodes for the zero-carbon future.

Scalable Customization of Crystallographic Plane Controllable Lithium Metal Anodes for Ultralong-Lasting Lithium Metal Batteries

A formidable challenge to achieve the practical applications of rechargeable lithium (Li) metal batteries (RLMBs) is to suppress the uncontrollable growth of Li dendrites. One of the most effective solutions is to fabricate Li metal anodes with specific crystal plane, but still lack of a simple and high-efficient approach. Herein, a facile and controllable way for the scalable customization of polished Li metal anodes with highly preferred (110) and (200) crystallographic orientation (donating as polished Li(110) and polished Li(200), respectively) by regulating the times of accumulative roll bonding, is reported. According to the inherent characteristics of polished Li(110)/Li(200), the influence of Li atomic structure on the electrochemical performance of RLMBs is deeply elucidated by combining theoretical calculations with relative experimental proofs. In particular, a polished Li(110) crystal plane is demonstrated to induce Li+ uniform deposition, promoting the formation of flat and dense Li deposits. Impressively, the polished Li(110)||LiFePO4 full cells exhibit unprecedented cycling stability with 10 000 cycles at 10 C almost without capacity degradation, indicating the great potential application prospect of such textured Li metal. More valuably, this work provides an important reference for low-cost, continued, and large-scale production of Li metal anodes with highly preferred crystal orientation through roll-to-roll manufacturability.

An Ultrathin Composite Polymer Electrolyte Dual-Reinforced by a Polymer of Intrinsic Microporosity (PIM-1) and Poly(tetrafluoroethylene) (PTFE) Porous Membrane

The performances of solid-state polymer electrolytes are urgently required to be further improved for high energy density lithium metal batteries. Herein, a highly reinforced ultrathin composite polymer electrolyte (PLPP) is successfully fabricated in a large scale by densely filling the well-dispersed mixture of polyethylene oxide (PEO), Li-salt (LiTFSI) and a polymer of intrinsic microporosity (PIM-1) into porous poly(tetrafluoroethylene) (PTFE) matrix. Based on the macro-plus-micro synergistic enhancement of the PTFE with excellent mechanical properties and the soluble PIM-1 with suitable functional groups, the PLPP electrolyte exhibits excellent properties including mechanical stress, thermal stability, lithium-ion transference number, voltage window and ionic conductivity, which are all superior to the typical PEO/LiTFSI electrolytes. As a result, the Li/PLPP/Li symmetric cell can stably cycle for > 2000 h, and the LiFePO4/PLPP/Li full cell exhibits excellent rate performance (>10 C) and high cycling stability with an initial capacity of 158.8 mAh g-1 and a capacity retention of 78.8% after 300 cycles. In addition, the excellent mechanical properties as well as the wide voltage window reasonably result in the stable operation of full cells with either high-loading cathode up to 28.1 mg cm-2 or high voltage cathode with high energy density.

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.

Toward maximum energy density enabled by anode-free lithium metal batteries: Recent progress and perspective

Owing to the emergenceof energy storage and electric vehicles, the desire for safe high-energy-density energy storage devices has increased research interest in anode-free lithium metal batteries (AFLMBs). Unlike general lithium metal batteries (LMBs), in which excess Li exists to compensate for the irreversible loss of Li, only the current collector is employed as an anode and paired with a lithiated cathode in the fabrication of AFLMBs. Owing to their unique cell configuration, AFLMBs have attractive characteristics, including the highest energy density, safety, and cost-effectiveness. However, developing AFLMBs with extended cyclability remains an issue for practical applications because the high reactivity of Li with limited inventory causes severely low Coulombic efficiency (CE), poor cyclability, and dendrite growth. To address these issues, tremendous effort has been devoted to stabilizing Li metal anodes for AFLMBs. In this review, the importance and challenges of AFLMBs are highlighted. Then, diverse strategies, such as current collectors modification, advanced electrolytes, cathode engineering, and operation protocols are thoroughly reviewed. Finally, a future perspective on the strategy is provided for insight into the basis of future research. It is hoped that this review provides a comprehensive understanding by reviewing previous research and arousing more interest in this field.

An ultralight, pulverization-free integrated anode toward lithium-less lithium metal batteries

The high-capacity advantage of lithium metal anode was compromised by common use of copper as the collector. Furthermore, lithium pulverization associated with "dead" Li accumulation and electrode cracking deteriorates the long-term cyclability of lithium metal batteries, especially under realistic test conditions. Here, we report an ultralight, integrated anode of polyimide-Ag/Li with dual anti-pulverization functionality. The silver layer was initially chemically bonded to the polyimide surface and then spontaneously diffused in Li solid solution and self-evolved into a fully lithiophilic Li-Ag phase, mitigating dendrites growth or dead Li. Further, the strong van der Waals interaction between the bottommost Li-Ag and polyimide affords electrode structural integrity and electrical continuity, thus circumventing electrode pulverization. Compared to the cutting-edge anode-free cells, the batteries pairing LiNi0.8Mn0.1Co0.1O2 with polyimide-Ag/Li afford a nearly 10% increase in specific energy, with safer characteristics and better cycling stability under realistic conditions of 1× excess Li and high areal-loading cathode (4 milliampere hour per square centimeter).

Recent Progress of Advanced Functional Separators in Lithium Metal Batteries

As a representative in the post-lithium-ion batteries (LIBs) landscape, lithium metal batteries (LMBs) exhibit high-energy densities but suffer from low coulombic efficiencies and short cycling lifetimes due to dendrite formation and complex side reactions. Separator modification holds the most promise in overcoming these challenges because it utilizes the original elements of LMBs. In this review, separators designed to address critical issues in LMBs that are fatal to their destiny according to the target electrodes are focused on. On the lithium anode side, functional separators reduce dendrite propagation with a conductive lithiophilic layer and a uniform Li-ion channel or form a stable solid electrolyte interphase layer through the continuous release of active agents. The classification of functional separators solving the degradation stemming from the cathodes, which has often been overlooked, is summarized. Structural deterioration and the resulting leakage from cathode materials are suppressed by acidic impurity scavenging, transition metal ion capture, and polysulfide shuttle effect inhibition from functional separators. Furthermore, flame-retardant separators for preventing LMB safety issues and multifunctional separators are discussed. Further expansion of functional separators can be effectively utilized in other types of batteries, indicating that intensive and extensive research on functional separators is expected to continue in LIBs.

Overcoming Challenges: Extending Cycle Life of Aqueous Zinc-Ion Batteries at High Zinc Utilization through a Synergistic Strategy

Aqueous zinc-ion batteries (AZIBs) face challenges in achieving high energy density compared to conventional lithium-ion batteries (LIBs). The lower operating voltage and excessive Zn metal as anode pose constraints on the overall energy storage capacity of these batteries. An effective approach is to reduce the thickness of the Zn metal anode and control its mass appropriately. However, under the condition of using a thin Zn anode, the performance of AZIBs is often unsatisfactory. Through experiments and computational simulations, the electrode structural change and the formation of dead Zn as the primary reasons for the failure of batteries under a high Zn utilization rate are identified. Based on this understanding, a universal synergistic strategy that combines Cu foil current collectors and electrolyte additives to maintain the structural and thermodynamic stability of the Zn anode under a high Zn utilization rate (ZUR) is proposed. Specifically, the Cu current collectors can ensure that the Zn anode structure remains intact based on the spontaneous filling effect, while the additives can suppress parasitic side reactions at the interface. Ultimately, the symmetric cell demonstrates a cycling duration of 900 h at a 70% ZU, confirming the effectiveness of this strategy.

Multimodal Engineering of Catalytic Interfaces Confers Multi-Site Metal-Organic Framework for Internal Preconcentration and Accelerating Redox Kinetics in Lithium-Sulfur Batteries

The development of highly efficient catalysts to address the shuttle effect and sluggish redox kinetics of lithium polysulfides (LiPSs) in lithium-sulfur batteries (LSBs) remains a formidable challenge. In this study, a series of multi-site catalytic metal-organic frameworks (MSC-MOFs) were elaborated through multimodal molecular engineering to regulate both the reactant diffusion and catalysis processes. MSC-MOFs were crafted with nanocages featuring collaborative specific adsorption/catalytic interfaces formed by exposed mixed-valence metal sites and surrounding adsorption sites. This design facilitates internal preconcentration, a coadsorption mechanism, and continuous efficient catalytic conversion toward polysulfides concurrently. Leveraging these attributes, LSBs with an MSC-MOF-Ti catalytic interlayer demonstrated a 62 % improvement in discharge capacity and cycling stability. This resulted in achieving a high areal capacity (11.57 mAh cm-2 ) at a high sulfur loading (9.32 mg cm-2 ) under lean electrolyte conditions, along with a pouch cell exhibiting an ultra-high gravimetric energy density of 350.8 Wh kg-1 . Lastly, this work introduces a universal strategy for the development of a new class of efficient catalytic MOFs, promoting SRR and suppressing the shuttle effect at the molecular level. The findings shed light on the design of advanced porous catalytic materials for application in high-energy LSBs.

Crosslinked solubilizer enables nitrate-enriched carbonate polymer electrolytes for stable, high-voltage lithium metal batteries

High-voltage lithium metal batteries (LMBs) have been considered promising next-generation high-energy-density batteries. However, commercial carbonate electrolytes can scarcely be employed in LMBs owing to their poor compatibility with metallic lithium. N,N-dimethylacrylamide (DMAA)-a crosslinkable solubilizer with a high Gutmann donor number-is employed to facilitate the dissolution of insoluble lithium nitrate (LiNO3) in carbonate-based electrolytes and to form gel polymer electrolytes (GPEs) through in situ polymerization. The Li+ solvation structure of the GPEs is regulated using LiNO3 and DMAA, which suppresses the decomposition of LiPF6 and facilitates the formation of an inorganic-rich solid electrolyte interface. Consequently, the Coulombic efficiency (CE) of the Li||Cu cell assembled with a GPE increases to 98.5% at room temperature, and the high-voltage Li||NCM622 cell achieves a capacity retention of 80.1% with a high CE of 99.5% after 400 cycles. The bifunctional polymer electrolytes are anticipated to pave the way for next-generation high-voltage LMBs.

Recent progress in SEI engineering for boosting Li metal anodes

Lithium metal anodes (LMAs) are ideal anode candidates for achieving next-generation high-energy-density battery systems due to their high theoretical capacity (3680 mA h g-1) and low working potential (-3.04 V versus the standard hydrogen electrode). However, the non-ideal solid electrolyte interface (SEI) derived from electrolyte/electrode interfacial reactions plays a vital role in the lithium deposition/stripping process and battery cycling performance. The composition and morphology of a SEI, which is sensitive to the outside environment, make it difficult to characterize and understand. With the development of characterization techniques, the mechanism, composition, and structure of a SEI can be better understood. In this review, the mechanism formation, the structure model evolution, and the composition of a SEI are briefly presented. Moreover, the development of in situ characterization techniques in recent years is introduced to better understand a SEI followed by the properties of the SEI, which are beneficial to the battery performance. Furthermore, recent optimization strategies of the SEI including the improvement of intrinsic SEIs and construction of artificial SEIs are summarized. Finally, the current challenges and future perspectives of SEI research are summarized.

Electrochemically and Thermally Stable Inorganics-Rich Solid Electrolyte Interphase for Robust Lithium Metal Batteries

Severe dendrite growth and high-level activity of the lithium metal anode lead to a short life span and poor safety, seriously hindering the practical applications of lithium metal batteries. With a trisalt electrolyte design, an F-/N-containing inorganics-rich solid electrolyte interphase on a lithium anode is constructed, which is electrochemically and thermally stable over long-term cycles and safety abuse conditions. As a result, its Coulombic efficiency can be maintained over 98.98% for 400 cycles. An 85.0% capacity can be retained for coin-type full cells with a 3.14 mAh cm-2 LiNi0.5 Co0.2 Mn0.3 O2 cathode after 200 cycles and 1.0 Ah pouch-type full cells with a 4.0 mAh cm-2 cathode after 72 cycles. During the thermal runaway tests of a cycled 1.0 Ah pouch cell, the onset and triggering temperatures were increased from 70.8 °C and 117.4 °C to 100.6 °C and 153.1 °C, respectively, indicating a greatly enhanced safety performance. This work gives novel insights into electrolyte and interface design, potentially paving the way for high-energy-density, long-life-span, and thermally safe lithium metal batteries.

Ionic Layer Epitaxy Growth of Organic/Inorganic Composite Protective Layers for Large-Area Li and Zn Metal Anodes

Li and Zn metal batteries using organic and aqueous electrolytes, respectively, are desirable next-generation energy storage systems to replace the traditional Li-ion batteries. However, their cycle life and safety performance are severely constrained by a series of issues that are attributed to dendrite growth. To solve these issues, a nanothick ZnO-oleic acid (ZnO-OA) composite protective layer is developed by a facile ionic layer epitaxy method. The ZnO-OA layer provides strong lithophilic and zincophilic properties, which can effectively induce uniform ion deposition. As a result, the ZnO-OA protected Li and Zn metal anodes can cycle stably for over 600 and 1000 h under a large current density of 10 mA cm-2. Employing the ZnO-OA protected anodes, the Li||LiFePO4 cell can maintain a capacity retention of 99.5% after 600 cycles at a 1 C rate and the Zn||MnO2 cell can operate stably for 1000 cycles at 1 A g-1 current density.

Combining Solid Solution Strengthening and Second Phase Strengthening for Thinning Li Metal Foils

Thin lithium (Li) metal foils have been proved to be indispensable yet elusive for practical high-energy-density lithium batteries. Currently, the realization of such thin foils (<50 μm) is impeded by the inferior mechanical processability of metallic Li. In this work, we demonstrate that the combination of solid solution strengthening and second phase strengthening, achieved by the addition of silver fluoride (AgF) to Li metal, can substantially enhance both the strength and ductility of metallic Li. Benefiting from the improved machinability, we succeed in fabricating an ultrathin (down to 5 μm), freestanding, and mechanically robust Li-AgF composite foil. More interestingly, the in situ-formed LixAg-LiF skeleton in the composite facilitates Li diffusion kinetics and uniform Li deposition, where the thin Li-AgF electrode displays a prolonged cycle life over 500 h at 1 mA cm-2 and 1 mAh cm-2 in a carbonate electrolyte. Coupled with a commercial LiCoO2 cathode (3.4 mAh cm-2), the LiCoO2||Li-AgF cell delivers a notable capacity retention of ∼90% over 100 cycles at 0.5 C with a low negative/positive ratio of 2.5.