Localized High-Concentration Sulfone Electrolytes for High-Efficiency Lithium-Metal Batteries

Electrolyte Engineering for High-Voltage Lithium Metal Batteries

High-voltage lithium metal batteries (HVLMBs) have been arguably regarded as the most prospective solution to ultrahigh-density energy storage devices beyond the reach of current technologies. Electrolyte, the only component inside the HVLMBs in contact with both aggressive cathode and Li anode, is expected to maintain stable electrode/electrolyte interfaces (EEIs) and facilitate reversible Li⁺ transference. Unfortunately, traditional electrolytes with narrow electrochemical windows fail to compromise the catalysis of high-voltage cathodes and infamous reactivity of the Li metal anode, which serves as a major contributor to detrimental electrochemical performance fading and thus impedes their practical applications. Developing stable electrolytes is vital for the further development of HVLMBs. However, optimization principles, design strategies, and future perspectives for the electrolytes of the HVLMBs have not been summarized in detail. This review first gives a systematical overview of recent progress in the improvement of traditional electrolytes and the design of novel electrolytes for the HVLMBs. Different strategies of conventional electrolyte modification, including high concentration electrolytes and CEI and SEI formation with additives, are covered. Novel electrolytes including fluorinated, ionic-liquid, sulfone, nitrile, and solid-state electrolytes are also outlined. In addition, theoretical studies and advanced characterization methods based on the electrolytes of the HVLMBs are probed to study the internal mechanism for ultrahigh stability at an extreme potential. It also foresees future research directions and perspectives for further development of electrolytes in the HVLMBs.

Remedies to Avoid Failure Mechanisms of Lithium-Metal Anode in Li-Ion Batteries

Rechargeable lithium-metal batteries (LMBs), which have high power and energy density, are very attractive to solve the intermittence problem of the energy supplied either by wind mills or solar plants or to power electric vehicles. However, two failure modes limit the commercial use of LMBs, i.e., dendrite growth at the surface of Li metal and side reactions with the electrolyte. Substantial research is being accomplished to mitigate these drawbacks. This article reviews the different strategies for fabricating safe LMBs, aiming to outperform lithium-ion batteries (LIBs). They include modification of the electrolyte (salt and solvents) to obtain a highly conductive solid–electrolyte interphase (SEI) layer, protection of the Li anode by in situ and ex situ coatings, use of three-dimensional porous skeletons, and anchoring Li on 3D current collectors.

Antioxidation Mechanism of Highly Concentrated Electrolytes at High Voltage

It has been researched that highly concentrated electrolytes (HCEs) can solve the problem of the excessive decomposition of dilute electrolytes at a high voltage, but the mechanism is not clear. In this work, the antioxidation mechanism of HCE at a high voltage was investigated by in situ electrochemical tests and theoretical calculations from the perspective of the solvation structure and physicochemical property. The results indicate that compared with the dilute electrolyte, the change of solvation structures in HCE makes more PF₆^- anions easier to be oxidized prior to the dimethyl carbonate solvents, resulting in a more stable cathode-electrolyte interphase (CEI) film. First, the lower oxidation potential of the solvation structure with more PF₆^- anions inhibits the side effects of the electrolyte effectively. Second, the CEI film, consisted of LiF and Li_xPO_yF_z generated from the oxidation of PF₆^- and Li₃PO₄ generated from the hydrolysis of LiPF₆ via the soluble PO₂F₂^- intermediate, can reduce the interface impedance and improve the conductivity. Intriguingly, the high viscosity of HCEs and the hydrolysis of LiPF₆ are proven to play a positive role in enhancing the interfacial stability of the electrolyte/electrode at a high voltage. This study builds a deep understanding of the bulk and interface properties of HCEs and provides theoretical support for their large-scale application in high-voltage battery materials.

Ether-Based Electrolyte Chemistry Towards High-Voltage and Long-Life Na-Ion Full Batteries

Although ether-based electrolytes have been extensively applied in anode evaluation of batteries, anodic instability arising from solvent oxidability is always a tremendous obstacle to matching with high-voltage cathodes. Herein, by rational design for solvation configuration, the fully coordinated ether-based electrolyte with strong resistance against oxidation is reported, which remains anodically stable with high-voltage Na₃ V₂ (PO₄ )₂ O₂ F (NVPF) cathode under 4.5 V (versus Na⁺ /Na) protected by an effective interphase. The assembled graphite//NVPF full cells display superior rate performance and unprecedented cycling stability. Beyond that, the constructed full cells coupling the high-voltage NVPF cathode with hard carbon anode exhibit outstanding electrochemical performances in terms of high average output voltage up to 3.72 V, long-term cycle life (such as 95 % capacity retention after 700 cycles) and high energy density (247 Wh kg^-1 ). In short, the optimized ether-based electrolyte enriches systematic options, the ability to maintain oxidative stability and compatibility with various anodes, exhibiting attractive prospects for application.

Designing and Demystifying the Lithium Metal Interface toward Highly Reversible Batteries

Reversible lithium (Li) plating/stripping is essential for building practical high-energy-density batteries based on Li metal chemistry, which unfortunately remains a severe challenge. In this contribution, it is demonstrated that through the rational regulation of strong Li⁺ -anion coordination structures in a highly compatible low-polarity solvent, 2-methyl tetrahydrofuran, the Li plating/stripping assisted by a nucleation modulation procedure delivers a remarkably high average Coulombic efficiency under rather demanding conditions (99.7% and 99.5% under 1.0 mA cm^-2 , 3.0 mAh cm^-2 and 3.0 mA cm^-2 , 3.0 mAh cm^-2 , respectively). The exceedingly reversible cycling obtained herein is fundamentally correlated with the flattened Li deposition and minimized solid electrolyte interphase (SEI) generation/reconstruction in the customized condition, which notably restrains the growth rates of both dead Li⁰ (0.0120 mAh per cycle) and SEI-Li⁺ (0.0191 mAh per cycle) during consecutive cycles. Benefiting from the efficient Li plating/stripping manner, the assembled anode-free Cu|LiFePO₄ (2.7 mAh cm^-2 ) coin and pouch cells exhibit impressive capacity retention of 43.8% and 41.6% after 150 cycles, respectively, albeit with no optimization on the test conditions. This work provides guidelines into the targeted interfacial design of high-efficiency working Li anodes, aiming to pave the way for the practical deployment of high-energy-density Li metal batteries.

Hybrid Electrolyte with Dual-Anion-Aggregated Solvation Sheath for Stabilizing High-Voltage Lithium-Metal Batteries

Lithium (Li)-metal batteries (LMBs) with high-voltage cathodes and limited Li-metal anodes are crucial to realizing high-energy storage. However, functional electrolytes that are compatible with both high-voltage cathodes and Li anodes are required for their developments. In this study, the use of a moderate-concentration LiPF₆ and LiNO₃ dual-salt electrolyte composed of ester and ether co-solvents (fluoroethylene carbonate/dimethoxyethane, FEC/DME), which forms a unique Li⁺ solvation with aggregated dual anions, that is, PF₆ ^- and NO₃ ^- , is proposed to stabilize high-voltage LMBs. Mechanistic studies reveal that such a solvation sheath improves the Li plating/stripping kinetics and induces the generation of a solid electrolyte interphase (SEI) layer with gradient heterostructure and high Young's modulus on the anode, and a thin and robust cathode electrolyte interface (CEI) film. Therefore, this novel electrolyte enables colossal Li deposits with a high Coulombic efficiency (≈98.9%) for 450 cycles at 0.5 mA cm^-2 . The as-assembled LiǁLiNi_0.85 Co_0.10 Al_0.05 O₂ full batteries deliver an excellent lifespan and capacity retention at 4.3 V with a rigid negative-to-positive capacity ratio. This electrolyte system with a dual-anion-aggregated solvation structure provides insights into the interfacial chemistries through solvation regulation for high-voltage LMBs.

Formulating a New Electrolyte: Synergy between Low-Polar and Non-polar Solvents in Tailoring the Solid Electrolyte Interface for the Silicon Anode

Currently, lithium-ion batteries (LIBs) are assembled with polar electrolytes; thus, resulting SEI layers are dominated with organics. Herein, a low-polarity electrolyte is formulated with a low-polarity solvent (tetraethyl silicate, TEOS) and a non-polar inert shielding co-solvent (cyclohexane, CYH); solvation behaviors of lithium salt are investigated. The use of such a low-polarity solvent is found to improve the fraction of anions in the solvation sheath of Li⁺, and the presence of the non-polar co-solvent further shields the reductive decomposition of the solvent on the anode. The resulting SEI layer is relatively rich in LiF and has a 3D cross-linked Si-O network as a skeleton from the decomposition of TEOS molecules, which is more robust to tolerate the damage from the volume expansion of silicon. A Si-nanoparticle-based anode in such a low-polarity electrolyte delivers a capacity as high as 1491 mAh g^-1 after 200 cycles, outperforming those in the commercial polar electrolytes.

An Overcrowded Water-Ion Solvation Structure for a Robust Anode Interphase in Aqueous Lithium-Ion Batteries

The water-in-salt electrolyte (WISE) features intimate interactions between a cation and anion, which induces the formation of an anion-derived solid electrolyte interphase (SEI) and expands the aqueous electrolyte voltage window to >3.0 V. Although further increasing the salt concentration (even to >60 molality (m)) can gradually improve water stability, issues about cost and practical feasibility are concerned. An alternative approach is to intensify ion-solvent interactions in the inner solvation structure by shielding off outward electrostatic attractions from nearby ions. Here, we design an "overcrowded" electrolyte using the non-polar, hydrogen-bonding 1,4-dioxane (DX) as an overcrowding agent, thereby achieving a robust LiF-enriched SEI and wide electrolyte operation window (3.7 V) with a low salt concentration (<2 m). As a result, the electrochemical performance of aqueous Li₄Ti₅O₁₂/LiMn₂O₄ full cells can be substantially improved (88.5% capacity retention after 200 cycles, at 0.57 C). This study points out a promising strategy to develop low-cost and stable high-voltage aqueous batteries.

Enhancing the Reduction Kinetics of LiSF6 Batteries by Dispersed Cobalt Phthalocyanines on Porous Carbon

Reducing SF₆ (as gas cathode) in Li batteries is a promising concept for the double benefit of mildly converting greenhouse SF₆ and providing a high theoretical energy density of 3922 Wh kg^-1 . However, the reduction process is hampered by its sluggish kinetics. Here, cobalt phthalocyanine (CoPc) molecules immobilized on porous carbon matrix are, for the first time, introduced to the LiSF₆ chemistry to deliver an enhanced energy density. It is revealed that the high redox potential of Co(II)Pc/[Co(I)Pc]^- (≈2.85 V) facilitates the formation of Co(I)N₄ sites to catalyze the SF₆ electrochemical reduction. By using highly porous holey nitrogen-doped carbon nanocages as carbon matrix, the LiSF₆ cells deliver a high discharge voltage of 2.82 V at 50 mA g_C+CoPc ^-1 and an unprecedented areal capacity of 25 mAh cm^-2 at 0.1 mA cm^-2 , much superior to previous results. This work opens up new possibilities for high-efficiency conversion of SF₆ in lithium batteries.

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

Lithium metal is a promising anode material for lithium metal batteries (LMBs). However, dendrite growth and limited Coulombic efficiency (CE) during cycling have prevented its practical application in rechargeable batteries. Herein, a highly concentrated electrolyte composed of an ether solvent and lithium bis(fluorosulfonyl)imide (LiFSI) salt is introduced, which enables the cycling of a lithium metal anode at a high CE (up to ≈99%) without dendrite growth, even at high current densities. Using 3.85 m LiFSI in tetrahydrofuran (THF) as the electrolyte, a Li||Li symmetric cell can be cycled at 1.0 mA cm^-2 for more than 1000 h with stable polarization of ≈0.1 V, and Li||LFP cells can be cycled at 2 C (1 C = 170 mA g^-1 ) for more than 1000 cycles with a capacity retention of 94.5%. These excellent performances are observed to be attributed to the increased cation-anion associated complexes, such as contact ion pairs and aggregate in the highly concentrated electrolyte; revealed by Raman spectroscopy and theoretical calculations. These results demonstrate the benefits of a high-concentration LiFSI-THF electrolyte system, generating new possibilities for high-energy-density rechargeable LMBs.

Robust Solid Electrolyte Interphases in Localized High Concentration Electrolytes Boosting Black Phosphorus Anode for Potassium-Ion Batteries

Black phosphorus (BP) shows superior capacity toward K ion storage, yet it suffers from poor reversibility and fast capacity degradation. Herein, a BP-graphite (BP/G) composite with a high BP loading of 80 wt % is synthesized and stabilized via the utilization of a localized high concentration electrolyte (LHCE), i.e., potassium bis(fluorosulfonyl)imide in trimethyl phosphate with a fluorinated ether as the diluent. We reveal the benefits of high concentration electrolytes rely on the formation of an inorganic component rich solid electrolyte interphase (SEI), which effectively passivates the electrode from copious parasite reactions. Furthermore, the diluent increases the electrolyte's ionic conductivity for achieving attractive rate capability and homogenizes the elemental distribution in the SEI. The latter essentially improves the SEI's maximum elastic deformation energy for accommodating the volume change, resulting in excellent cyclic performance. This work promotes the application of advanced potassium-ion batteries by adopting high-capacity BP anodes, on the one hand. On the other hand, it unravels the beneficial roles of LHCE in building robust SEIs for stabilizing alloy anodes.

Stable Anion-Derived Solid Electrolyte Interphase in Lithium Metal Batteries

High-energy-density lithium (Li) metal batteries are severely hindered by the dendritic Li deposition dictated by non-uniform solid electrolyte interphase (SEI). Despite its unique advantages in improving the uniformity of Li deposition, the current anion-derived SEI is unsatisfactory under practical conditions. Herein regulating the electrolyte structure of anions by anion receptors was proposed to construct stable anion-derived SEI. Tris(pentafluorophenyl)borane (TPFPB) anion acceptors with electron-deficient boron atoms interact with bis(fluorosulfonyl)imide anions (FSI^- ) and decrease the reduction stability of FSI^- . Furthermore, the type of aggregate cluster of FSI^- in electrolyte changes, FSI^- interacting with more Li ions in the presence of TPFPB. Therefore, the decomposition of FSI^- to form Li₂ S is promoted, improving the stability of anion-derived SEI. In working Li | LiNi_0.5 Co_0.2 Mn_0.3 O₂ batteries under practical conditions, the anion-derived SEI with TPFPB undergoes 194 cycles compared with 98 cycles of routine anion-derived SEI. This work inspires a fresh ground to construct stable anion-derived SEI by manipulating the electrolyte structure of anions.

Solvate electrolytes for Li and Na batteries: structures, transport properties, and electrochemistry

Polar solvents dissolve Li and Na salts at high concentrations and are used as electrolyte solutions for batteries. The solvents interact strongly with the alkali metal cations to form complexes in the solution. The activity (concentration) of the uncoordinated solvent decreases as the salt concentration is increased. At extremely high salt concentrations, all the solvent molecules are involved in the coordination of the ions and form the solvates of the salts. In this article, we review the structures, transport properties, and electrochemistry of Li/Na salt solvates. In molten solvates, the activity of the uncoordinated solvent is negligible; this is the main origin of their peculiar characteristics, such as high thermal stability, wide electrochemical window, and unique ion transport. In addition, the solvent activity greatly influences the electrochemical reactions in Li/Na batteries. We highlight the attractive features of molten solvates as promising electrolytes for next-generation batteries.

Fire-Retardant, Stable-Cycling and High-Safety Sodium Ion Battery

The safety of energy storage equipment has always been a stumbling block to the development of battery, and sodium ion battery is no exception. However, as an ultimate solution, the use of non-flammable electrolyte is susceptible to the side effects, and its poor compatibility with electrode, causing failure of batteries. Here, we report a non-flammable electrolyte design to achieve high-performance sodium ion battery, which resolves the dilemma via regulating the solvation structure of electrolyte by hydrogen bonds and optimizing the electrode-electrolyte interphase. The reported non-flammable electrolyte allows stable charge-discharge cycling of both sodium vanadium phosphate@hard carbon and Prussian blue@hard carbon full pouch cell for more than 120 cycles with a capacity retention of >85 % and high cycling Coulombic efficiency (99.7 %).

Solvent-Assisted Li-Ion Transport and Structural Heterogeneity in Fluorinated Battery Electrolytes

The electrolytes diluted with fluorinated solvents show promising properties toward better battery technology than existing ones. The transport of Li ions in these fluorinated electrolytes is essential to access the performance of a battery. It is believed that the transport of the Li ion in these electrolytes occurs through polar solvents in the matrix of nonpolar solvent molecules. The atomistic details of this mechanism are yet to be proved using the dynamics of these mixtures. In this study, we performed classical molecular dynamics simulations at various temperatures to probe this mechanism through the structure and dynamics of electrolytes at the atomic level. Here, we have shown that the polar fluorinated solvents assist the Li-ion transport in a region of nonpolar solvent. Highly polar molecules also solvate the Li ion at a lower temperature. The nonpolar solvent solvates the Li ion weakly as compared to others. The calculated values of the ionic conductivity from the Green-Kubo relation provide a better match than that from an experimental conductivity meter. Furthermore, we probed the heterogeneity in the dynamics of the electrolytes by calculating the non-Gaussian parameter. We also show that the transport mechanism of the Li ion in diluted concentrated electrolytes is different than a few of the other reported electrolytes. We have also calculated the ion-pair and ion-cage lifetimes to see the most and least lived ion/ion-solvent pairs. The mechanism given from the present study may help to design the fluorinated electrolytes for Li-ion batteries.

High-voltage liquid electrolytes for Li batteries: progress and perspectives

Since the advent of the Li ion batteries (LIBs), the energy density has been tripled, mainly attributed to the increase of the electrode capacities. Now, the capacity of transition metal oxide cathodes is approaching the limit due to the stability limitation of the electrolytes. To further promote the energy density of LIBs, the most promising strategies are to enhance the cut-off voltage of the prevailing cathodes or explore novel high-capacity and high-voltage cathode materials, and also replacing the graphite anode with Si/Si-C or Li metal. However, the commercial ethylene carbonate (EC)-based electrolytes with relatively low anodic stability of ∼4.3 V vs. Li⁺/Li cannot sustain high-voltage cathodes. The bottleneck restricting the electrochemical performance in Li batteries has veered towards new electrolyte compositions catering for aggressive next-generation cathodes and Si/Si-C or Li metal anodes, since the oxidation-resistance of the electrolytes and the in situ formed cathode electrolyte interphase (CEI) layers at the high-voltage cathodes and solid electrolyte interphase (SEI) layers on anodes critically control the electrochemical performance of these high-voltage Li batteries. In this review, we present a comprehensive and in-depth overview on the recent advances, fundamental mechanisms, scientific challenges, and design strategies for the novel high-voltage electrolyte systems, especially focused on stability issues of the electrolytes, the compatibility and interactions between the electrolytes and the electrodes, and reaction mechanisms. Finally, novel insights, promising directions and potential solutions for high voltage electrolytes associated with effective SEI/CEI layers are proposed to motivate revolutionary next-generation high-voltage Li battery chemistries.

Electrolyte Design for Lithium Metal Anode-Based Batteries Toward Extreme Temperature Application

Lithium anode-based batteries (LBs) are highly demanded in society owing to the high theoretical capacity and low reduction potential of metallic lithium. They are expected to see increasing deployment in performance critical areas including electric vehicles, grid storage, space, and sea vehicle operations. Unfortunately, competitive performance cannot be achieved when LBs operating under extreme temperature conditions where the lithium-ion chemistry fail to perform optimally. In this review, a brief overview of the challenges in developing LBs for low temperature (<0 °C) and high temperature (>60 °C) operation are provided followed by electrolyte design strategies involving Li salt modification, solvation structure optimization, additive introduction, and solid-state electrolyte utilization for LBs are introduced. Specifically, the prospects of using lithium metal batteries (LMBs), lithium sulfur (Li-S) batteries, and lithium oxygen (Li-O2 ) batteries for performance under low and high temperature applications are evaluated. These three chemistries are presented as prototypical examples of how the conventional low temperature charge transfer resistances and high temperature side reactions can be overcome. This review also points out the research direction of extreme temperature electrolyte design toward practical applications.

Confronting the Challenges in Lithium Anodes for Lithium Metal Batteries

With the low redox potential of -3.04 V (vs SHE) and ultrahigh theoretical capacity of 3862 mAh g-1 , lithium metal has been considered as promising anode material. However, lithium metal battery has ever suffered a trough in the past few decades due to its safety issues. Over the years, the limited energy density of the lithium-ion battery cannot meet the growing demands of the advanced energy storage devices. Therefore, lithium metal anodes receive renewed attention, which have the potential to achieve high-energy batteries. In this review, the history of the lithium anode is reviewed first. Then the failure mechanism of the lithium anode is analyzed, including dendrite, dead lithium, corrosion, and volume expansion of the lithium anode. Further, the strategies to alleviate the lithium anode issues in recent years are discussed emphatically. Eventually, remaining challenges of these strategies and possible research directions of lithium-anode modification are presented to inspire innovation of lithium anode.

Flame-Retardant and Polysulfide-Suppressed Ether-Based Electrolytes for High-Temperature Li-S Batteries

Lithium-sulfur (Li-S) batteries are drawing huge attention as attractive chemical power sources. However, traditional ether-based solvents (DME/DOL) suffer from safety issues at high temperatures and serious parasitic reactions occur between the Li metal anodes and soluble lithium polysulfides (LiPSs). Herein, we propose a polysulfide-suppressed and flame-retardant electrolyte operated at high temperatures by introducing an inert diluent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl (TTE) into the high-concentration electrolyte (HCE). Li dendrites are also efficiently suppressed by the formed LiF-rich protective layer. Furthermore, the shuttle effect is mitigated by the decreased solubility of LiPSs. At 60 °C, Li-S batteries using this nonflammable ether-based electrolyte exhibit a high capacity of 666 mAh g^-1 over 100 cycles at a current rate of 0.2C, showing the greatly improved high-temperature performance compared to batteries with traditional ether-based electrolytes. The improved electrochemical performance across a range of temperatures and the enhanced safety suggest that the electrolyte has a great practical prospect for safe Li-S batteries.

Localized Water-In-Salt Electrolyte for Aqueous Lithium-Ion Batteries

Water-in-salt (WIS) electrolytes using super-concentrated organic lithium (Li) salts are of interest for aqueous Li-ion batteries. However, the high salt cost, high viscosity, poor wettability, and environmental hazards remain a great challenge. Herein, we present a localized water-in-salt (LWIS) electrolyte based on low-cost lithium nitrate (LiNO₃ ) salt and 1,5-pentanediol (PD) as inert diluent. The addition of PD maintains the solvation structure of the WIS electrolyte, improves the electrolyte stability via hydrogen-bonding interactions with water and NO₃ ^- molecules, and reduces the total salt concentration. By in situ gelling the LWIS electrolyte with tetraethylene glycol diacrylate (TEGDA) monomer, the electrolyte stability window can be further expanded to 3.0 V. The as-developed Mo₆ S₈ |LWIS gel electrolyte|LiMn₂ O₄ (LMO) batteries delivered outstanding cycling performance with an average Coulombic efficiency of 98.53 % after 250 cycles at 1 C.

Enhanced Li+ Transport in Ionic Liquid-Based Electrolytes Aided by Fluorinated Ethers for Highly Efficient Lithium Metal Batteries with Improved Rate Capability

FSI^- -based ionic liquids (ILs) are promising electrolyte candidates for long-life and safe lithium metal batteries (LMBs). However, their practical application is hindered by sluggish Li⁺ transport at room temperature. Herein, it is shown that additions of bis(2,2,2-trifluoroethyl) ether (BTFE) to LiFSI-Pyr₁₄ FSI ILs can effectively mitigate this shortcoming, while maintaining ILs' high compatibility with lithium metal. Raman spectroscopy and small-angle X-ray scattering indicate that the promoted Li⁺ transport in the optimized electrolyte, [LiFSI]₃ [Pyr₁₄ FSI]₄ [BTFE]₄ (Li₃ Py₄ BT₄ ), originates from the reduced solution viscosity and increased formation of Li⁺ -FSI^- complexes, which are associated with the low viscosity and non-coordinating character of BTFE. As a result, Li/LiFePO₄ (LFP) cells using Li₃ Py₄ BT₄ electrolyte reach 150 mAh g^-1 at 1 C rate (1 mA cm^-2 ) and a capacity retention of 94.6% after 400 cycles, revealing better characteristics with respect to the cells employing the LiFSI-Pyr₁₄ FSI (operate only a few cycles) and commercial carbonate (80% retention after only 218 cycles) electrolytes. A wide operating temperature (from -10 to 40 °C) of the Li/Li₃ Py₄ BT₄ /LFP cells and a good compatibility of Li₃ Py₄ BT₄ with LiNi_0.5 Mn_0.3 Co_0.2 O₂ (NMC532) are demonstrated also. The insight into the enhanced Li⁺ transport and solid electrolyte interphase characteristics suggests valuable information to develop IL-based electrolytes for LMBs.

Potentiometric Measurement to Probe Solvation Energy and Its Correlation to Lithium Battery Cyclability

The electrolyte plays a critical role in lithium-ion batteries, as it impacts almost every facet of a battery's performance. However, our understanding of the electrolyte, especially solvation of Li⁺, lags behind its significance. In this work, we introduce a potentiometric technique to probe the relative solvation energy of Li⁺ in battery electrolytes. By measuring open circuit potential in a cell with symmetric electrodes and asymmetric electrolytes, we quantitatively characterize the effects of concentration, anions, and solvents on solvation energy across varied electrolytes. Using the technique, we establish a correlation between cell potential (E_cell) and cyclability of high-performance electrolytes for lithium metal anodes, where we find that solvents with more negative cell potentials and positive solvation energies-those weakly binding to Li⁺-lead to improved cycling stability. Cryogenic electron microscopy reveals that weaker solvation leads to an anion-derived solid-electrolyte interphase that stabilizes cycling. Using the potentiometric measurement for characterizing electrolytes, we establish a correlation that can guide the engineering of effective electrolytes for the lithium metal anode.

Structural Effects of Solvents on Li-Ion-Hopping Conduction in Highly Concentrated LiBF4/Sulfone Solutions

Li-ion-hopping conduction is known to occur in certain highly concentrated electrolytes, and this conduction mode is effective for achieving lithium batteries with high rate capabilities. Herein, we investigated the effects of the solvent structure on the hopping conduction of Li ions in highly concentrated LiBF₄/sulfone electrolytes. Raman spectroscopy revealed that a Li⁺ ion forms complexes with sulfone and anions, and contact ion pairs and ionic aggregates are formed in the highly concentrated electrolytes. Li⁺ exchanges ligands (sulfone and BF₄^-) rapidly to produce unusual hopping conduction in highly concentrated electrolytes. The structure of the solvent significantly influences the hopping conduction process. We measured the self-diffusion coefficients of Li⁺ (D_Li), anions (D_anion), and sulfone solvents (D_sol) in electrolytes. The ratio of the self-diffusion coefficients (D_Li/D_sol) tended to be higher for cyclic sulfones (sulfolane and 3-methylsulfolane) than for acyclic sulfones, which suggests that cyclic sulfone molecules facilitate Li-ion hopping. The hopping conduction increases the Li⁺-transference number (tLi+abc) under anion-blocking conditions, and tLi+abc of [LiBF₄]/[cyclic sulfone] = 1/2 is as high as 0.8.

Non-Solvating and Low-Dielectricity Cosolvent for Anion-Derived Solid Electrolyte Interphases in Lithium Metal Batteries

Lithium (Li) metal anodes hold great promise for next-generation high-energy-density batteries, while the insufficient fundamental understanding of the complex solid electrolyte interphase (SEI) is the major obstacle for the full demonstration of their potential in working batteries. The characteristics of SEI highly depend on the inner solvation structure of lithium ions (Li⁺ ). Herein, we clarify the critical significance of cosolvent properties on both Li⁺ solvation structure and the SEI formation on working Li metal anodes. Non-solvating and low-dielectricity (NL) cosolvents intrinsically enhance the interaction between anion and Li⁺ by affording a low dielectric environment. The abundant positively charged anion-cation aggregates generated as the introduction of NL cosolvents are preferentially brought to the negatively charged Li anode surface, inducing an anion-derived inorganic-rich SEI. A solvent diagram is further built to illustrate that a solvent with both proper relative binding energy toward Li⁺ and dielectric constant is suitable as NL cosolvent.

Simultaneous Stabilization of the Solid/Cathode Electrolyte Interface in Lithium Metal Batteries by a New Weakly Solvating Electrolyte

So far, the practical application of Li metal batteries has been hindered by the undesirable formation of Li dendrites and low Coulombic efficiencies (CEs). Herein, 1,2-diethoxyethane (DEE) is proposed as a new electrolytic solvent for lithium metal batteries (LMBs), and the performances of 1.0 m LiFSI in DEE are evaluated. Because of the low dielectric constant and dipole moment of DEE, the majority of the FSI^- exists in associated states like contact ion pairs and aggregates, which is similar to the highly concentrated electrolytes. These associated complexes are involved in the reduction reaction on the Li metal anode, forming sound solid electrolyte interphase layers. Furthermore, free FSI^- ions in DEE are observed to participate in the formation of cathode electrolyte interphase layers. These passivation layers not only suppress dendrite growth on the Li anode but also prevent unwanted side-reactions on the LiFePO₄ cathode. The average CE of the Li||Cu cells in LiFSI-DEE is observed to be 98.0%. Moreover, LiFSI-DEE also plays an important role in enhancing the cycling stability of the Li||LiFP cell with a capacity retention of 93.5% after 200 cycles. These results demonstrate the benefits of LiFSI-DEE, which creates new possibilities for high-energy-density rechargeable LMBs.

Stabilized Cathode Interphase for Enhancing Electrochemical Performance of LiNi0.5Mn1.5O4-Based Lithium-Ion Battery via cis-1,2,3,6-Tetrahydrophthalic Anhydride

The continuous degradation of carbonate electrolytes and the dissolution of transition metal cations due to parasitic reactions on the cathode-electrolyte interphase (CEI) block the practical application of LiNi_0.5Mn_1.5O₄-based lithium-ion batteries (LNMO-based LIBs) at a high voltage. cis-1,2,3,6-Tetrahydrophthalic anhydride (CTA) has been used as a functional additive in a carbonate baseline electrolyte (BE) for constructing the CEI film to enhance the cyclic stability of LNMO-based LIBs. The LNMO/Li cell with CTA exhibits a preponderant capacity retention of 83.3% compared with those of propionic anhydride (PA) (46.5%) and BE (13.6%) after 500 cycles at the current density of 1 C from 3.5 to 4.9 V. Additionally, the LNMO/graphite full cell with CTA still has a higher capacity retention of 95.46% even after 300 cycles at 1 C. By characterizations, it is reasonably demonstrated that CTA was oxidated to participate in the construction of a CEI film. An unsaturated aromatic group was introduced into the composition of the CEI film along with CTA in the formation process of the CEI film, which further improved the antioxidative activity of the CEI film under the influence of field-effect. Specifically, the CEI film obtains appreciable stability because of its higher antioxidative activity under the influence of field-effect. The stabilized CEI can significantly suppress the parasitic reactions of electrolytes, decrease the consumption of active-Li⁺, and protect the LNMO cathode structure, thereby enhancing the cyclic compatibility of LNMO-based LIBs with the carbonate electrolytes from 3.5 to 4.9 V.

Li-Rich Li2 [Ni0.8 Co0.1 Mn0.1 ]O2 for Anode-Free Lithium Metal Batteries

Anode-free lithium metal batteries can maximize the energy density at the cell level. However, without the Li compensation from the anode side, it faces much more challenging to achieve a long cycling life with a competitive energy density than Li metal-based batteries. Here, we prolong the lifespan of an anode-free Li metal battery by introducing Li-rich Li₂ [Ni_0.8 Co_0.1 Mn_0.1 ]O₂ into the cathode as a Li-ions extender. The Li₂ [Ni_0.8 Co_0.1 Mn_0.1 ]O₂ can release a large amount of Li-ions during the first charging process to supplement the Li loss in the anode, then convert into NCM811, thus extending the lifespan of the battery without the introduction of inactive elements. By the benefit of Li-rich cathode and high reversibility of Li metal on Cu foil, the anode-free pouch cells enable to achieve 447 Wh kg^-1 energy density and 84 % capacity retention after 100 cycles in the condition of limited electrolyte addition (E/C ratio of 2 g Ah^-1 ).

Localized high concentration electrolytes decomposition under electron-rich environments

Localized high concentration electrolytes have been proposed as an effective route to construct stable solid-electrolyte interphase (SEI) layers near Li-metal anodes. However, there is still a limited understanding of the decomposition mechanisms of electrolyte components during SEI formation. In this work, we investigate reactivities of lithium bis(fluorosulfonyl)imide (LiFSI, salt), 1,2-dimethoxyethane (DME, solvent), and tris(2,2,2-trifluoroethyl)orthoformate (TFEO, diluent) species in DME + TFEO mixed solvents and 1M LiFSI/DME/TFEO solutions. By supplying an excess of electrons into the simulation cell, LiFSI is initially reduced via a four-electron charge transfer reaction yielding F^- and N(SO₂)₂ ^3-. The local solvation environment has little effect on the subsequent TFEO reaction, which typically requires 6 |e| to decompose into F^-, HCOO^-, CH₂CF^-, and ^-OCH₂CF₃. Besides, the TFEO dehydrogenation reaction mechanism under an attack of anions is also identified. Unlike salt and diluent, DME shows good stability with any excess of electrons. The energetics of most relevant reactions are characterized. Most reactions are thermodynamically favorable with low activation barriers.

Effects of fluorinated solvents on electrolyte solvation structures and electrode/electrolyte interphases for lithium metal batteries

Electrolyte is very critical to the performance of the high-voltage lithium (Li) metal battery (LMB), which is one of the most attractive candidates for the next-generation high-density energy-storage systems. Electrolyte formulation and structure determine the physical properties of the electrolytes and their interfacial chemistries on the electrode surfaces. Localized high-concentration electrolytes (LHCEs) outperform state-of-the-art carbonate electrolytes in many aspects in LMBs due to their unique solvation structures. Types of fluorinated cosolvents used in LHCEs are investigated here in searching for the most suitable diluent for high-concentration electrolytes (HCEs). Nonsolvating solvents (including fluorinated ethers, fluorinated borate, and fluorinated orthoformate) added in HCEs enable the formation of LHCEs with high-concentration solvation structures. However, low-solvating fluorinated carbonate will coordinate with Li+ ions and form a second solvation shell or a pseudo-LHCE which diminishes the benefits of LHCE. In addition, it is evident that the diluent has significant influence on the electrode/electrolyte interphases (EEIs) beyond retaining the high-concentration solvation structures. Diluent molecules surrounding the high-concentration clusters could accelerate or decelerate the anion decomposition through coparticipation of diluent decomposition in the EEI formation. The varied interphase features lead to significantly different battery performance. This study points out the importance of diluents and their synergetic effects with the conductive salt and the solvating solvent in designing LHCEs. These systematic comparisons and fundamental insights into LHCEs using different types of fluorinated solvents can guide further development of advanced electrolytes for high-voltage LMBs.

High-Efficacy and Polymeric Solid-Electrolyte Interphase for Closely Packed Li Electrodeposition

The industrial application of lithium metal anode requires less side reaction between active lithium and electrolyte, which demands the sustainability of the electrolyte-induced solid-electrolyte interface. Here, through a new diluted lithium difluoro(oxalato)borate-based (LiDFOB) high concentration electrolyte system, it is found that the oxidation behavior of aggregated LiDFOB salt has a great impact on solid-electrolyte interphase (SEI) formation and Li reversibility. Under the operation window of Cu/LiNi0.8Co0.1Mn0.1O2 full cells (rather than Li/Cu configuration), a polyether/coordinated borate containing solid-electrolyte interphase with inner Li2O crystalline can be observed with the increasing concentration of salt, which can be ascribed to the reaction between aggregated electron-deficient borate species and electron-rich alkoxide SEI components. The high Li reversibility (99.34%) and near-theoretical lithium deposition enable the stable cycling of LiNi0.8Co0.1Mn0.1O2/Li cells (N/P < 2, 350 Wh kg-1) under high cutoff voltage condition of 4.6 V and lean electrolyte condition (E/C ≈ 3.2 g Ah-1).

Electrolyte-Mediated Stabilization of High-Capacity Micro-Sized Antimony Anodes for Potassium-Ion Batteries

Alloying anodes exhibit very high capacity when used in potassium-ion batteries, but their severe capacity fading hinders their practical applications. The failure mechanism has traditionally been attributed to the large volumetric change and/or their fragile solid electrolyte interphase. Herein, it is reported that an antimony (Sb) alloying anode, even in bulk form, can be stabilized readily by electrolyte engineering. The Sb anode delivers an extremely high capacity of 628 and 305 mAh g^-1 at current densities of 100 and 3000 mA g^-1 , respectively, and remains stable for more than 200 cycles. Interestingly, there is no need to do nanostructural engineering and/or carbon modification to achieve this excellent performance. It is shown that the change in K⁺ solvation structure, which is tuned by electrolyte composition (i.e., anion, solvent, and concentration), is the main reason for achieving this excellent performance. Moreover, an interfacial model based on the K⁺ -solvent-anion complex behavior is presented. The electronegativity of the K⁺ -solvent-anion complex, which can be tuned by changing the solvent type and anion species, is used to predict and control electrode stability. The results shed new light on the failure mechanism of alloying anodes, and provide a new guideline for electrolyte design that stabilizes metal-ion batteries using alloying anodes.

Electrolytes and Interphases in Potassium Ion Batteries

Potassium ion batteries (PIBs) are recognized as one promising candidate for future energy storage devices due to their merits of cost-effectiveness, high-voltage, and high-power operation. Many efforts have been devoted to the development of electrode materials and the progress has been well summarized in recent review papers. However, in addition to electrode materials, electrolytes also play a key role in determining the cell performance. Here, the research progress of electrolytes in PIBs is summarized, including organic liquid electrolytes, ionic liquid electrolytes, solid-state electrolytes and aqueous electrolytes, and the engineering of the electrode/electrolyte interfaces is also thoroughly discussed. This Progress Report provides a comprehensive guidance on the design of electrolyte systems for development of high performance PIBs.

Three-Dimensional (3D) Nanostructured Skeleton Substrate Composed of Hollow Carbon Fiber/Carbon Nanosheet/ZnO for Stable Lithium Anode

The practical applications of Li metal batteries (LMBs) have long been limited by the obstacles of low Coulombic efficiency (CE) and formation of dendrites on Li metal electrode. Herein, we demonstrated the synthesis of a novel three-dimensional (3D) nanostructured skeleton substrate composed of nitrogen-doped hollow carbon fiber/carbon nanosheets/ZnO (NHCF/CN/ZnO) using 2-methylimidazole (2-MIZ)-coated 3D cloth as a scaffold. The mechanism of formation of this novel hierarchical structure was investigated. The multilayered hierarchical structure and abundant lithiophilic nucleation sites of the substrate provide a stable environment for the deposition and stripping of lithium metal, thus preventing the generation of lithium dendrites. Consequently, the lithium anode based on the NHCF/CN/ZnO current collector demonstrated an excellent Coulombic efficiency of 96.47% after 400 cycles at 0.5 mA cm^-2. The prepared NHCF/CN/ZnO/Li electrode also showed outstanding cycling performance of over 800 h and an ultralow voltage hysteresis of less than 30 mV in a symmetric cell at 5 mA cm^-2 and 5 mAh cm^-2. Even at a high loading of the cathode with 10.4 mg cm^-2, the full cell of NHCF/CN/ZnO/Li anode with LiFePO₄ can also work very well. Our work offers a path toward the facial preparation of 3D hierarchical structure for high-performance lithium metal batteries.

Building Better Li Metal Anodes in Liquid Electrolyte: Challenges and Progress

Li metal has been widely recognized as a promising anode candidate for high-energy-density batteries. However, the inherent limitations of Li metal, that is, the low Coulombic efficiency and dendrite issues, make it still far from practical applications. In short, the low Coulombic efficiency shortens the cycle life of Li metal batteries, while the dendrite issue raises safety concerns. Thanks to the great efforts of the research community, prolific fundamental understanding as well as approaches for mitigating Li metal anode safety have been extensively explored. In this Review, Li electrochemical deposition behaviors have been systematically summarized, and recent progress in electrode design and electrolyte system optimization is reviewed. Finally, we discuss the future directions, opportunities, and challenges of Li metal anodes.

Insights into the deposition chemistry of Li ions in nonaqueous electrolyte for stable Li anodes

Comprehensive understanding of the Li deposition chemistry from Li⁺ to Li atom is crucial for suppressing dendrite formation and growth.

Advanced liquid electrolytes enable practical applications of high-voltage lithium-metal full batteries

High-voltage lithium metal batteries (HVLMBs) have received widespread attention as next generation high-energy-density batteries to meet the urgent demands of modern life. However, the unstable interphase between electrolytes and highly reactive electrodes is still an important threshold for practical applications. In this feature article, we review the formation mechanism of the electrode-electrolyte interphase in terms of cathodes and the Li metal anode, respectively, and summarize the surface modification methods to stabilize the interphase of HVLMBs. Electrolyte regulation strategies especially those using electrolyte additives are introduced, and the relationship between liquid electrolyte formulation, interphase engineering and the electrochemical performance of HVLMBs is analyzed. Finally, an industry-level evaluation is carried out and the remaining challenges are discussed for advanced electrolytes to guarantee the practical applications and commercialization of HVLMBs.