Design of a LiF-Rich Solid Electrolyte Interphase Layer through Highly Concentrated LiFSI-THF Electrolyte for Stable Lithium Metal Batteries

Highly Performing Sodium Metal Batteries Reinforced by a Self-Regulated Dual-Layered Solid Electrolyte Interphase via a Metal-Organic Framework

Sodium-metal batteries, heralded for high energy density and cost-effectiveness, are compromised by an unstable solid electrolyte interphase (SEI) and dendrite formation, which hinder practical applications. Herein, a zirconium-based metal-organic framework nanostructure coating (ZMOF-NSC) was constructed in a low-loss, flexible manner. Comprehensive studies show that ZMOF-NSC, with its periodically ordered nanochannels and organized pore structures, enhances ion transport and decreases the Na+ migration energy barrier, thus ensuring uniform ion flux and achieving uniform spherical deposition. Additionally, ZMOF-NSC facilitates partial desolvation, catalyzing the formation of an inorganic-rich, dual-layered SEI that effectively protects the anode and suppresses dendrite formation. Consequently, the ZMOF-NSC@Na symmetric battery exhibits an impressive lifespan of over 2500 h, demonstrating extended operational longevity. The Na3V2(PO4)3∥ZMOF-NSC@Na batteries demonstrate exceptional cycling stability with 81% capacity retention after 2000 cycles at 10 C, maintaining stability over 3000 cycles at 20 C. Moreover, the NVP∥ZMOF-NSC@Na battery achieves an energy density of 370 Wh kg-1 and a power density of 10,484 W kg-1, indicating superior durability and performance. This significant finding highlights the significant potential of structured MOFs to induce a dual-layered SEI, advancing the commercialization of durable, dendrite-free sodium metal batteries. The precise design of self-assembled pore structures and surface active sites in MOFs demonstrates significant potential in advancing the commercialization of durable, dendrite-free electrodes of metal-based rechargeable batteries.

Designing Temperature-Insensitive Solvated Electrolytes for Low-Temperature Lithium Metal Batteries

Lithium metal batteries face problems from sluggish charge transfer at interfaces, as well as parasitic reactions between lithium metal anodes and electrolytes, due to the strong electronegativity of oxygen donor solvents. These factors constrain the reversibility and kinetics of lithium metal batteries at low temperatures. Here, a nonsolvating cosolvent is applied to weaken the electronegativity of donor oxygen in ether solvents, enabling the participation of anionic donors in the solvation structure of Li+. This strategy significantly accelerates the desolvation process of Li+ and reduces the side effects of solvents on interfacial transport and stability. The designed anion-aggregated electrolyte has a unique temperature-insensitive solvation structure and enables lithium metal anodes to achieve a high average Coulombic efficiency at room temperature and -20 °C. A high-loading LiFePO4||Li cell exhibited high reversibility with a 100% capacity retention after 150 cycles at room temperature, -20, and -40 °C. The practical 1 Ah-level LiFePO4||Li pouch-cell delivered 81% and 61% of the capacity at room temperature when charged and discharged at -20 and -40 °C, respectively. This strategy of constructing temperature-insensitive solvation by electronegativity regulation offers a novel approach for developing electrolytes of low-temperature batteries.

Synergistic Effects of Bisalt Additives in High-Voltage Rechargeable Lithium Batteries

The stability of high-energy-density lithium metal batteries (LMBs) heavily relies on the composition of the solid electrolyte interphase (SEI) formed on lithium metal anodes. In this study, the inorganic-rich SEI layer was achieved by incorporating bisalts additives into carbonate-based electrolytes. Within this SEI layer, the presence of LiF, polythionate, and Li3N was observed, generated by combining 1.0 м lithium bis(trifluoromethanesulfonyl)imide in ethylene carbonate: ethyl methyl carbonate:dimethyl carbonate in a 1 : 1 : 1 volume ratio, with the addition of 2 wt% lithium difluorophosphate and 2 wt% lithium difluoro(oxalato)borate additives (EL-DO). Furthermore, this formulation effectively mitigated corrosion of aluminum current collectors. EL-DO exhibited outstanding performance, including an average coulombic efficiency of 98.2 % in Li||Cu cells and a stable discharge capacity of approximately 162 mAh g-1 after 200 cycles in a Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) configuration. Moreover, EL-DO displayed the potential to enhance the performance not only of LMBs but also of lithium-ion batteries. In the case of Gr||NCM811 cell using EL-DO, it consistently maintained high discharge capacities, even achieving around 135 mAh g-1 after the 100th cycle, surpassing the performance of other electrolytes. This study underscores the synergistic impact of bisalts additives in elevating the performance of lithium batteries.

Unveiling the Mystery of LiF within Solid Electrolyte Interphase in Lithium Batteries

Over the past decades, significant advances have been made in lithium-ion batteries. However, further requirement on the electrochemical performance is still a powerful motivator to improve battery technology. The solid electrolyte interphase (SEI) is considered as a key component on negative electrode, having been proven to be crucial for the performance, even in safety of batteries. Although numerous studies have focused on SEI in recent years, its specific properties, including structure and composition, remain largely unclear. Particularly, LiF, a common and important component in SEI, has sparked debates among researchers, resulting in divergent viewpoints. In this review, the recent research findings on SEI and delve into the characteristics of the LiF component is aim to consolidated. The cause of SEI formation and the evolution of SEI models is summarized. The distinctive properties of SEI generated on various negative electrodes is further discussed, the ongoing scholarly controversy surrounding the function of LiF within SEI, and the specific physicochemical properties about LiF and its synergistic effect in heterogeneous components. The objective is to facilitate better understanding of SEI and the role of the LiF component, ultimately contributing to the development of Li batteries with enhanced electrochemical performance and safety for battery communities.

Compositionally Sequenced Interfacial Layers for High-Energy Li-Metal Batteries

Electrolyte additives with multiple functions enable the interfacial engineering of Li-metal batteries (LMBs). Owing to their unique reduction behavior, additives exhibit a high potential for electrode surface modification that increases the reversibility of Li-metal anodes by enabling the development of a hierarchical solid electrolyte interphase (SEI). This study confirms that an adequately designed SEI facilitates the homogeneous supply of Li+, nonlocalized Li deposition, and low electrolyte degradation in LMBs while enduring the volume fluctuation of Li-metal anodes on cycling. An in-depth analysis of interfacial engineering mechanisms reveals that multilayered SEI structures comprising mechanically robust LiF-rich species, electron-rich P-O species, and elastic polymeric species enabled the stable charge and discharge of LMBs. The polymeric outer SEI layer in the as-fabricated multilayered SEI could accommodate the volume fluctuation of Li-metal anodes, significantly enhancing the cycling stability Li||LiNi0.8Co0.1Mn0.1O2 full cells with an electrolyte amount of 3.6 g Ah-1 and an areal capacity of 3.2 mAh cm-2. Therefore, this study confirms the ability of interfacial layers formed by electrolyte additives and fluorinated solvents to advance the performance of LMBs and can open new frontiers in the fabrication of high-performance LMBs through electrolyte-formulation engineering.

Wide Temperature Electrolytes for Lithium Batteries: Solvation Chemistry and Interfacial Reactions

Improving the wide-temperature operation of rechargeable batteries is crucial for boosting the adoption of electric vehicles and further advancing their application scope in harsh environments like deep ocean and space probes. Herein, recent advances in electrolyte solvation chemistry are critically summarized, aiming to address the long-standing challenge of notable energy diminution at sub-zero temperatures and rapid capacity degradation at elevated temperatures (>45°C). This review provides an in-depth analysis of the fundamental mechanisms governing the Li-ion transport process, illustrating how these insights have been effectively harnessed to synergize with high-capacity, high-rate electrodes. Another critical part highlights the interplay between solvation chemistry and interfacial reactions, as well as the stability of the resultant interphases, particularly in batteries employing ultrahigh-nickel layered oxides as cathodes and high-capacity Li/Si materials as anodes. The detailed examination reveals how these factors are pivotal in mitigating the rapid capacity fade, thereby ensuring a long cycle life, superior rate capability, and consistent high-/low-temperature performance. In the latter part, a comprehensive summary of in situ/operational analysis is presented. This holistic approach, encompassing innovative electrolyte design, interphase regulation, and advanced characterization, offers a comprehensive roadmap for advancing battery technology in extreme environmental conditions.

In Situ Engineering of Inorganic-Rich Solid Electrolyte Interphases via Anion Choice Enables Stable, Lithium Anodes

The discovery of liquid battery electrolytes that facilitate the formation of stable solid electrolyte interphases (SEIs) to mitigate dendrite formation is imperative to enable lithium anodes in next-generation energy-dense batteries. Compared to traditional electrolyte solvents, tetrahydrofuran (THF)-based electrolyte systems have demonstrated great success in enabling high-stability lithium anodes by encouraging the decomposition of anions (instead of organic solvent) and thus generating inorganic-rich SEIs. Herein, by employing a variety of different lithium salts (i.e., LiPF6, LiTFSI, LiFSI, and LiDFOB), it is demonstrated that electrolyte anions modulate the inorganic composition and resulting properties of the SEI. Through novel analytical time-of-flight secondary-ion mass spectrometry methods, such as hierarchical clustering of depth profiles and compositional analysis using integrated yields, the chemical composition and morphology of the SEIs generated from each electrolyte system are examined. Notably, the LiDFOB electrolyte provides an exceptionally stable system to enable lithium anodes, delivering >1500 cycles at a current density of 0.5 mAh g-1 and a capacity of 0.5 mAh g-1 in symmetrical cells. Furthermore, Li//LFP cells using this electrolyte demonstrate high-rate, reversible lithium storage, supplying 139 mAh g(LFP) -1 at C/2 (≈0.991 mAh cm-2 , @ 0.61 mA cm-2 ) with 87.5% capacity retention over 300 cycles (average Coulombic efficiency >99.86%).

In Situ Construction of a Polymer Coating Layer on the LiNi0.8Co0.1Mn0.1O2 Cathode for High-Performance Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are known for their high energy density but exhibit poor cyclic stability and safety risks due to side reactions between the electrode and electrolyte. To address these issues, a novel approach involving construction of a polymer coating layer (PCL) via in situ self-polymerization using 2,2,3,4,4,4-hexafluorobutyl methacrylate (HFBM) as an electrolyte additive on the cathode is proposed. The PCL endows the electrolyte with a high onset oxidation potential (4.78 V) and lithium-ion transference number (0.52). The uniform and robust in situ constructed PCL can effectively inhibit the severe irreversible side reactions and suppress harmful reactions, thus providing a protective barrier against degradation. The resulting Li||LiNi0.8Co0.1Mn0.1O2 batteries exhibit an improved discharge capacity retention of 80% at 1C over 100 cycles. These results demonstrate that the in situ self-polymerization strategy holds promising potential for enhancing LIB performance and long-term stability, especially when high-voltage cathode materials are used.

Phase regulation enabling dense polymer-based composite electrolytes for solid-state lithium metal batteries

Solid polymer electrolytes with large-scale processability and interfacial compatibility are promising candidates for solid-state lithium metal batteries. Among various systems, poly(vinylidene fluoride)-based polymer electrolytes with residual solvent are appealing for room-temperature battery operations. However, their porous structure and limited ionic conductivity hinder practical application. Herein, we propose a phase regulation strategy to disrupt the symmetry of poly(vinylidene fluoride) chains and obtain the dense composite electrolyte through the incorporation of MoSe2 sheets. The electrolyte with high dielectric constant can optimize the solvation structures to achieve high ionic conductivity and low activation energy. The in-situ reactions between MoSe2 and Li metal generate Li2Se fast conductor in solid electrolyte interphase, which improves the Coulombic efficiency and interfacial kinetics. The solid-state Li||Li cells achieve robust cycling at 1 mA cm-2, and the Li||LiNi0.8Co0.1Mn0.1O2 full cells show practical performance at high rate (3C), high loading (2.6 mAh cm-2) and in pouch cell.

Probing the Mechanically Stable Solid Electrolyte Interphase and the Implications in Design Strategies

The inevitable volume expansion of secondary battery anodes during cycling imposes forces on the solid electrolyte interphase (SEI). The battery performance is closely related to the capability of SEI to maintain intact under the cyclic loading conditions, which basically boils down to the mechanical properties of SEI. The volatile and complex nature of SEI as well as its nanoscale thickness and environmental sensitivity make the interpretation of its mechanical behavior many roadblocks. Widely varied approaches are adopted to investigate the mechanical properties of SEI, and diverse opinions are generated. The lack of consensus at both technical and theoretical levels has hindered the development of effective design strategies to maximize the mechanical stability of SEIs. Here, the essential and desirable mechanical properties of SEI, the available mechanical characterization methods, and important issues meriting attention for higher test accuracy are outlined. Previous attempts to optimize battery performance by tuning SEI mechanical properties are also scrutinized, inconsistencies in these efforts are elucidated, and the underlying causes are explored. Finally, a set of research protocols is proposed to accelerate the achievement of superior battery cycling performance by improving the mechanical stability of SEI.

Decreasing Water Activity Using the Tetrahydrofuran Electrolyte Additive for Highly Reversible Aqueous Zinc Metal Batteries

Aqueous zinc metal batteries show great promise in large-scale energy storage. However, the decomposition of water molecules leads to severe side reactions, resulting in the limited lifespan of Zn batteries. Here, the tetrahydrofuran (THF) additive was introduced into the zinc sulfate (ZnSO₄) electrolyte to reduce water activity by modulating the solvation structure of the Zn hydration layer. The THF molecule can play as a proton acceptor to form hydrogen bonds with water molecules, which can prevent water-induced undesired reactions. Thus, in an optimal 2 M ZnSO₄/THF (5% by volume) electrolyte, the hydrogen evolution reaction and byproduct precipitation can be suppressed, which greatly improves the cycling stability and Coulombic efficiency of reversible Zn plating/stripping. The Zn symmetrical cells exhibit ultralong working cycles with a wide range of current density and capacity. The THF additive also enables a high Coulombic efficiency in the Zn||Cu cell with an average value of 99.59% over 400 cycles and a high reversible capacity with a capacity retention of 97.56% after 250 cycles in the Zn||MnO₂ full cells. This work offers an effective strategy with high scalability and low cost for the protection of the Zn metal electrodes in aqueous rechargeable batteries.

Ion-Solvent Interplay in Concentrated Electrolytes Enables Subzero Temperature Li-Ion Battery Operations

Despite the essential role of ethylene carbonate (EC) in solid electrolyte interphase (SEI) formation, the high Li⁺ desolvation barrier and melting point (36 °C) of EC impede lithium-ion battery operation at low temperatures and induce sluggish Li⁺ reaction kinetics. Here, we demonstrate an EC-free high salt concentration electrolyte (HSCE) composed of lithium bis(fluorosulfonyl)imide salt and tetrahydrofuran solvent with enhanced subzero temperature operation originating from unusually rapid low-temperature Li⁺ transport. Experimental and theoretical characterizations reveal the dominance of intra-aggregate ion transport in the HSCE that enables efficient low-temperature transport by increasing the exchange rate of solvating counterions relative to that of solvent molecules. This electrolyte also produces a <5 nm thick anion-derived LiF-rich SEI layer with excellent graphite electrode compatibility and electrochemical performance at subzero temperature in half-cells. Full cells based on LiNi_0.6Co_0.2Mn_0.2O₂||graphite with tailored HSCE electrolytes outperform state-of-the-art cells comprising conventional EC electrolytes during charge-discharge operation at an extreme temperature of -40 °C. These results demonstrate the opportunities for creating intrinsically robust low-temperature Li⁺ technology.

Practical High-Voltage Lithium Metal Batteries Enabled by Tuning the Solvation Structure in Weakly Solvating Electrolyte

Li metal batteries (LMBs) are ideal candidates for future high-energy-density battery systems. To date, high-voltage LMBs suffer severe limitations because of electrolytes unstable against Li anodes and high-voltage cathodes. Although ether-based electrolytes exhibit good stability with Li metal, compared to carbonate-based electrolytes, they have been used only in ≤4.0 V LMBs because of their limited oxidation stability. Here, a high concentration electrolyte (HCE) comprising lithium bis(fluorosulfonyl)imide (LiFSI) and a weakly solvating solvent (1,2-diethoxyethane, DEE) is designed, which can regulate unique solvation structures with only associated complexes at relatively lower concentration compared to the reported HCEs. This effectively suppresses dendrites on the anode side, and preserves the structural integrity of the cathode side under high voltages by the formation of stable interfacial layers on a Li metal anode and LiNi_0.8 Mn_0.1 Co_0.1 O₂ (NMC811) cathode. Consequently, a 3.5 m LiFSI-DEE plays an important role in enhancing the stability of the Li||NMC811 cell with a capacity retention of ≈94% after 200 cycles under a high current density of 2.5 mA cm^-2 . In addition, the 3.5 m LiFSI-DEE electrolyte exhibits good performance with anode-free batteries. This study offers a promising approach to enable ether-based electrolytes for high-voltage LMBs applications.