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

Highly Stable Lithium Metal Batteries Enabled by Tuning the Molecular Polarity of Diluents in Localized High-Concentration Electrolytes

Electrolyte plays a crucial role in ensuring stable operation of lithium metal batteries (LMBs). Localized high-concentration electrolytes (LHCEs) have the potential to form a robust solid-electrolyte interphase (SEI) and mitigate Li dendrite growth, making them a highly promising electrolyte option. However, the principles governing the selection of diluents, a crucial component in LHCE, have not been clearly determined, hampering the advancement of such a type of electrolyte systems. Herein, the diluents from the perspective of molecular polarity are rationally designed and developed. A moderately fluorinated solvent, 1-(1,1,2,2-tetrafluoroethoxy)propane (TNE), is employed as a diluent to create a novel LHCE. The unique molecular structure of TNE enhances the intrinsic dipole moment, thereby altering solvent interactions and the coordination environment of Li-ions in LHCE. The achieved solvation structure not only enhances the bulk properties of LHCE, but also facilitates the formation of more stable anion-derived SEIs featured with a higher proportion of inorganic species. Consequently, the corresponding full cells of both Li||LiFePO4 and Li||LiNi0.8Co0.1Mn0.1O2 cells utilizing Li thin-film anodes exhibit extended long-term stability with significantly improved average Coulombic efficiency. This work offers new insights into the functions of diluents in LHCEs and provides direction for further optimizing the LHCEs for LMBs.

High-efficiency Lithium Metal Stabilization and Polysulfide Suppression in Li-S Battery Enabled by Weakly Solvating Solvent

Lithium-sulfur batteries (LSBs) are considered a highly promising next-generation energy storage technology due to their exceptional energy density and cost-effectiveness. However, the practical use of current LSBs is hindered primarily by issues related to the "shuttle effect" of lithium polysulfide (LiPS) intermediates and the growth of lithium dendrites. In strongly solvating electrolytes, the solvent-derived solid electrolyte interphase (SEI) lacks mechanical strength due to organic components, leading to ineffective lithium dendrite suppression and severe LiPS dissolution and shuttling. In contrast, the weakly solvating electrolyte (WSE) can create an anion-derived SEI layer which can enhance compatibility with lithium metal anode, and restricting LiPS solubility. Herein, a WSE consisting of 0.4 м LiTFSI in the mixture of 1,4-dioxane (DX):dimethoxymethane (DMM) is designed to overcome the issues associated with LSB. Surface analyses confirmed the formation of a beneficial SEI layer rich in LiF, enabling homogeneous lithium deposition with an average Coulombic efficiency CE exceeding 99% over 100 cycles. Implementing the low-concentration WSE in Li||SPAN cells yielded an impressive initial specific capacity of 671 mAh g-1. This research highlights the advantages of WSE and offers the pathway for cost-effective electrolyte development, enabling the realization of high-performance LSBs.

Electrolyte-Induced Morphology Evolution to Boost Potassium Storage Performance of Perylene-3,4,9,10-tetracarboxylic Dianhydride

Organic materials have attracted extensive attention for potassium-ion batteries due to their flexible structure designability and environmental friendliness. However, organic materials generally suffer from unavoidable dissolution in aprotic electrolytes, causing an unsatisfactory electrochemical performance. Herein, we designed a weakly solvating electrolyte to boost the potassium storage performance of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). The electrolyte induces an in situ morphology evolution and achieves a nanowire structure. The weakly dissolving capability of ethylene glycol diethyl ether-based electrolyte and unique nanowire structure effectively avoid the dissolution of PTCDA. As a result, PTCDA shows excellent cycling stability (a capacity retention of 89.1% after 2000 cycles) and good rate performance (70.3 mAh g-1 at 50C). In addition, experimental detail discloses that the sulfonyl group plays a key role in inducing morphology evolution during the charge/discharge process. This work opens up new opportunities in electrolyte design for organic electrodes and illuminates further developments of potassium-ion batteries.

Co-Intercalation-Free Graphite Anode Enabled by an Additive Regulated Interphase in an Ether-Based Electrolyte for Low-Temperature Lithium-Ion Batteries

Graphite (Gr) anode, which is endowed with high electronic conductivity and low volume expansion after Li-ion intercalation, establishes the basis for the success of rocking-chair Li-ion batteries (LIBs). However, due to the high barrier of the Li-ion desolvation process, sluggish transport of Li ions through the solid electrolyte interphase (SEI) and the high freezing points of electrolytes, the Gr anode still suffers from great loss of capacity and severe polarization at low temperature. Here, 1,2-diethoxyethane (DEE) with an intrinsically wide liquid region and weak solvation ability is applied as an electrolyte solvent for LIBs. By rationally designing the additives of electrolytes, an intact SEI with fast Li-ion conductivity is constructed, enabling the co-intercalation-free Gr anode with long-term stability (91.8% after 500 cycles) and impressive low-temperature characteristics (82.6% capacity retention at -20 °C). Coupled with LiFePO4 and LiNi0.8Mn0.1Co0.1O2 cathodes, the optimized electrolyte also demonstrates low polarization under -20 °C. Our work offers a feasible approach to enable ether-based electrolytes for low-temperature LIBs.

Structure and Interface Engineering of Ultrahigh-Rate 3D Bismuth Anodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted tremendous attention as promising low-cost energy storage devices in future grid-scale energy management applications. Bismuth is a promising anode for SIBs due to its high theoretical capacity (386 mAh g-1 ). Nevertheless, the huge volume variation of Bi anode during (de)sodiation processes can cause the pulverization of Bi particulates and rupture of solid electrolyte interphase (SEI), resulting in quick capacity decay. It is demonstrated that rigid carbon framework and robust SEI are two essentials for stable Bi anodes. A lignin-derived carbonlayer wrapped tightly around the bismuth nanospheres provides a stable conductive pathway, while the delicate selection of linear and cyclic ether-based electrolytes enable robust and stable SEI films. These two merits enable the long-term cycling process of the LC-Bi anode. The LC-Bi composite delivers outstanding sodium-ion storage performance with an ultra-long cycle life of 10 000 cycles at a high current density of 5 A g-1 and an excellent rate capability of 94% capacity retention at an ultrahigh current density of 100 A g-1 . Herein, the underlying origins of performance improvement of Bi anode are elucidated, which provides a rational design strategy for Bi anodes in practical SIBs.

Solvent versus Anion Chemistry: Unveiling the Structure-Dependent Reactivity in Tailoring Electrochemical Interphases for Lithium-Metal Batteries

Electrolytes are critical for the reversibility of various electrochemical energy storage systems. The recent development of electrolytes for high-voltage Li-metal batteries has been counting on the salt anion chemistry for building stable interphases. Herein, we investigate the effect of the solvent structure on the interfacial reactivity and discover profound solvent chemistry of designed monofluoro-ether in anion-enriched solvation structures, which enables enhanced stabilization of both high-voltage cathodes and Li-metal anodes. Systematic comparison of different molecular derivatives provides an atomic-scale understanding of the unique solvent structure-dependent reactivity. The interaction between Li⁺ and the monofluoro (-CH₂F) group significantly influences the electrolyte solvation structure and promotes the monofluoro-ether-based interfacial reactions over the anion chemistry. With in-depth analyses of the compositions, charge transfer, and ion transport at interfaces, we demonstrated the essential role of the monofluoro-ether solvent chemistry in tailoring highly protective and conductive interphases (with enriched LiF at full depths) on both electrodes, as opposed to the anion-derived ones in typical concentrated electrolytes. As a result, the solvent-dominant electrolyte chemistry enables a high Li Coulombic efficiency (∼99.4%) and stable Li anode cycling at a high rate (10 mA cm^-2), together with greatly improved cycling stability of 4.7 V-class nickel-rich cathodes. This work illustrates the underlying mechanism of the competitive solvent and anion interfacial reaction schemes in Li-metal batteries and offers fundamental insights into the rational design of electrolytes for future high-energy batteries.

Improving performances of the electrode/electrolyte interface via the regulation of solvation complexes: a review and prospect

The electrode/electrolyte interface (EEI) is a research hotspot in lithium-ion batteries, while the electrolyte solvation complex can be regarded as a factor that cannot be ignored in determining the performance of the EEI. From the perspective of the electrolyte solvation complex, this review summarizes the effects of solvation complexes on the composition of an EEI film and the Li⁺ desolvation process, and further clarifies the internal mechanism of the electrolyte composition controlling solvation chemistry. Finally, combined with doubtful points that are not comprehensively considered in the regulation of solvated complexes, this review puts forward some cutting-edge views, which are of great significance for future guidance in improving the performance of lithium-ion batteries.

A "tug-of-war" effect tunes Li-ion transport and enhances the rate capability of lithium metal batteries

"Solvent-in-salt" electrolytes (high-concentration electrolytes (HCEs)) and diluted high-concentration electrolytes (DHCEs) show great promise for reviving secondary lithium metal batteries (LMBs). However, the inherently sluggish Li+ transport of such electrolytes limits the high-rate capability of LMBs for practical conditions. Here, we discovered a "tug-of-war" effect in a multilayer solvation sheath that promoted the rate capability of LMBs; the pulling force of solvent-nonsolvent interactions competed with the compressive force of Li+-nonsolvent interactions. By elaborately manipulating the pulling and compressive effects, the interaction between Li+ and the solvent was weakened, leading to a loosened solvation sheath. Thereby, the developed electrolytes enabled a high Li+ transference number (0.65) and a Li (50 μm)‖NCM712 (4 mA h cm-2) full cell exhibited long-term cycling stability (160 cycles; 80% capacity retention) at a high rate of 0.33C (1.32 mA cm-2). Notably, Li (50 μm)‖LiFePO4 (LFP; 17.4 mg cm-2) cells with a designed electrolyte reached a capacity retention of 80% after 1450 cycles at a rate of 0.66C. An 6 Ah Li‖LFP pouch cell (over 250 W h kg-1) showed excellent cycling stability (130 cycles, 96% capacity retention) under practical conditions. This design concept for an electrolyte provides a promising path to build high-energy-density and high-rate LMBs.

Electrolyte engineering via ether solvent fluorination for developing stable non-aqueous lithium metal batteries

Fluorination of ether solvents is an effective strategy to improve the electrochemical stability of non-aqueous electrolyte solutions in lithium metal batteries. However, excessive fluorination detrimentally impacts the ionic conductivity of the electrolyte, thus limiting the battery performance. Here, to maximize the electrolyte ionic conductivity and electrochemical stability, we introduce the targeted trifluoromethylation of 1,2-dimethoxyethane to produce 1,1,1-trifluoro-2,3-dimethoxypropane (TFDMP). TFDMP is used as a solvent to prepare a 2 M non-aqueous electrolyte solution comprising bis(fluorosulfonyl)imide salt. This electrolyte solution shows an ionic conductivity of 7.4 mS cm^-1 at 25 °C, an oxidation stability up to 4.8 V and an efficient suppression of Al corrosion. When tested in a coin cell configuration at 25 °C using a 20 μm Li metal negative electrode, a high mass loading LiNi_0.8Co_0.1Mn_0.1O₂-based positive electrode (20 mg cm^-2) with a negative/positive (N/P) capacity ratio of 1, discharge capacity retentions (calculated excluding the initial formation cycles) of 81% after 200 cycles at 0.1 A g^-1 and 88% after 142 cycles at 0.2 A g^-1 are achieved.

Electrolyte Design Strategies for Non-Aqueous High-Voltage Potassium-Based Batteries

High-voltage potassium-based batteries are promising alternatives for lithium-ion batteries as next-generation energy storage devices. The stability and reversibility of such systems depend largely on the properties of the corresponding electrolytes. This review first presents major challenges for high-voltage electrolytes, such as electrolyte decomposition, parasitic side reactions, and current collector corrosion. Then, the state-of-the-art modification strategies for traditional ester and ether-based organic electrolytes are scrutinized and discussed, including high concentration, localized high concentration/weakly solvating strategy, multi-ion strategy, and addition of high-voltage additives. Besides, research advances of other promising electrolyte systems, such as potassium-based ionic liquids and solid-state-electrolytes are also summarized. Finally, prospective future research directions are proposed to further enhance the oxidative stability and non-corrosiveness of electrolytes for high-voltage potassium batteries.

Tuning the Solvent Alkyl Chain to Tailor Electrolyte Solvation for Stable Li-Metal Batteries

1,2-Dimethoxyethane (DME) has been considered as the most promising electrolyte solvent for Li-metal batteries (LMBs). However, challenges arise from insufficient Li Coulombic efficiency (CE) and poor anodic stability associated with DME-based electrolytes. Here, we proposed a rational molecular design methodology to tailor electrolyte solvation for stable LMBs, where shortening the middle alkyl chain of the solvent could reduce the chelation ability, while increasing the terminal alkyl chain of the solvent could increase the steric hindrance, affording a diethoxymethane (DEM) solvent with ultra-weak solvation ability. When serving as a single solvent for electrolyte, a peculiar solvation structure dominated by contact ion pairs (CIPs) and aggregates (AGGs) was achieved even at a regular salt concentration of 1 m, which gives rise to anion-derived interfacial chemistry. This illustrates an unprecedentedly high Li||Cu CE of 99.1% for a single-salt single-solvent (non-fluorinated) electrolyte at ∼1 m. Moreover, this 1 m DEM-based electrolyte also remarkably suppresses the anodic dissolution of Al current collectors and significantly improves the cycling performance of high-voltage cathodes. This work opens up new frontiers in engineering electrolytes toward stable LMBs with high energy densities.

Chasing protons in lithium-ion batteries

Parasitic reactions between delithiated cathode materials and non-aqueous electrolytes have been a major barrier that limits the upper cutoff potential of cathode materials. It is of great importance to suppress such parasitic reactions to unleash the high-energy-density potential of high voltage cathode materials. Although major effort has been made to identify the chemical composition of the cathode electrolyte interface using various cutting edge characterization tools, the chemical nature of parasitic reactions remains a puzzle. This severely hinders the rational development of stable high voltage cathode/electrolyte pairs for high-energy density lithium-ion batteries. This feature article highlights our latest effort in understanding the chemical/electrochemical role of the cathode electrolyte interface using protons as a chemical tracer for parasitic reactions.

Weak Cation-Solvent Interactions in Ether-Based Electrolytes Stabilizing Potassium-ion Batteries

Conventional ether-based electrolytes exhibited a low polarization voltage in potassium-ion batteries, yet suffered from ion-solvent co-intercalation phenomena in a graphite anode, inferior potassium-metal performance, and limited oxidation stability. Here, we reveal that weakening the cation-solvent interactions could suppress the co-intercalation behaviour, enhance the potassium-metal performance, and improve the oxidation stability. Consequently, the graphite anode exhibits K⁺ intercalation behaviour (K||graphite cell operates 200 cycles with 86.6 % capacity retention), the potassium metal shows highly stable plating/stripping (K||Cu cell delivers 550 cycles with average Coulombic efficiency of 98.9 %) and dendrite-free (symmetric K||K cell operates over 1400 hours) properties, and the electrolyte exhibits high oxidation stability up to 4.4 V. The ion-solvent interaction tuning strategy provides a promising method to develop high-performance electrolytes and beyond.

Intrinsic Nonflammable Ether Electrolytes for Ultrahigh-Voltage Lithium Metal Batteries Enabled by Chlorine Functionality

The issues of inherent low anodic stability and high flammability hinder the deployment of the ether-based electrolytes in practical high-voltage lithium metal batteries. Here, we report a rationally designed ether-based electrolyte with chlorine functionality on ether molecular structure to address these critical challenges. The chloroether-based electrolyte demonstrates a high Li Coulombic efficiency of 99.2 % and a high capacity retention >88 % over 200 cycles for Ni-rich cathodes at an ultrahigh cut-off voltage of 4.6 V (stable even up to 4.7 V). The chloroether-based electrolyte not only greatly improves electrochemical stabilities of Ni-rich cathodes under ultrahigh voltages with interphases riched in LiF and LiCl, but possesses the intrinsic nonflammable safety feature owing to the flame-retarding ability of chlorine functional groups. This study offers a new approach to enable ether-based electrolytes for high energy density, long-life and safe Li metal batteries.

Electrode-Electrolyte Interfacial Chemistry Modulation for Ultra-High Rate Sodium-Ion Batteries

Sodium-ion batteries capable of operating at rate and temperature extremes are highly desirable, but elusive due to the dynamics and thermodynamics limitations. Herein, a strategy of electrode-electrolyte interfacial chemistry modulation is proposed. The commercial hard carbon demonstrates superior rate performance with 212 mAh g^-1 at an ultra-high current density of 5 A g^-1 in the electrolyte with weak ion solvation/desolvation, which is much higher than those in common electrolytes (nearly no capacity in carbonate-based electrolytes). Even at -20 °C, a high capacity of 175 mAh g^-1 (74 % of its room-temperature capacity) can be maintained at 2 A g^-1 . Such an electrode retains 90 % of its initial capacity after 1000 cycles. As proven, weak ion solvation/desolvation of tetrahydrofuran greatly facilitates fast-ion diffusion at the SEI/electrolyte interface and homogeneous SEI with well-distributed NaF and organic components ensures fast Na⁺ diffusion through the SEI layer and a stable interface.

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.

Recent Advances in Electrolytes for "Beyond Aqueous" Zinc-Ion Batteries

With the growing demands for large-scale energy storage, Zn-ion batteries (ZIBs) with distinct advantages, including resource abundance, low-cost, high-safety, and acceptable energy density, are considered as potential substitutes for Li-ion batteries. Although numerous efforts are devoted to design and develop high performance cathodes and aqueous electrolytes for ZIBs, many challenges, such as hydrogen evolution reaction, water evaporation, and liquid leakage, have greatly hindered the development of aqueous ZIBs. Developing "beyond aqueous" electrolytes can be able to avoid these issues due to the absence of water, which are beneficial for the achieving of highly efficient ZIBs. In this review, the recent development of the "beyond aqueous" electrolytes, including conventional organic electrolytes, ionic liquid, all-solid-state, quasi-solid-state electrolytes, and deep eutectic electrolytes are presented. The critical issues and the corresponding strategies of the designing of "beyond aqueous" electrolytes for ZIBs are also summarized.

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.

Steric Effect Tuned Ion Solvation Enabling Stable Cycling of High-Voltage Lithium Metal Battery

1,2-Dimethoxyethane (DME) is a common electrolyte solvent for lithium metal batteries. Various DME-based electrolyte designs have improved long-term cyclability of high-voltage full cells. However, insufficient Coulombic efficiency at the Li anode and poor high-voltage stability remain a challenge for DME electrolytes. Here, we report a molecular design principle that utilizes a steric hindrance effect to tune the solvation structures of Li⁺ ions. We hypothesized that by substituting the methoxy groups on DME with larger-sized ethoxy groups, the resulting 1,2-diethoxyethane (DEE) should have a weaker solvation ability and consequently more anion-rich inner solvation shells, both of which enhance interfacial stability at the cathode and anode. Experimental and computational evidence indicates such steric-effect-based design leads to an appreciable improvement in electrochemical stability of lithium bis(fluorosulfonyl)imide (LiFSI)/DEE electrolytes. Under stringent full-cell conditions of 4.8 mAh cm^-2 NMC811, 50 μm thin Li, and high cutoff voltage at 4.4 V, 4 M LiFSI/DEE enabled 182 cycles until 80% capacity retention while 4 M LiFSI/DME only achieved 94 cycles. This work points out a promising path toward the molecular design of non-fluorinated ether-based electrolyte solvents for practical high-voltage Li metal 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.