Reductive Competition Effect-Derived Solid Electrolyte Interphase with Evenly Scattered Inorganics Enabling Ultrahigh Rate and Long-Life Span Sodium Metal Batteries

Sulfite-Based Electrolyte Chemistry with Ion-Dipole Interactions and Robust Interphase Achieves Wide-Temperature Sodium-Ion Batteries

Currently, ether- and carbonate-based electrolytes have been extensively studied for applications in harsh conditions; however, it is difficult to develop a suitable electrolyte system that is compatible with both high and low temperatures. Herein, for the first time, a cyclic sulfite-based electrolyte is formulated to successfully achieve the wide-temperature operation of sodium-ion batteries (SIBs) from -60 to 60 °C. By precisely modulating ion-dipole interactions, the dominant ion coordination states are screened directionally to accelerate the desolvation process and simultaneously maintain sufficient electrostatic constraints, laying the foundation for high- and low-temperature compatibility. And the coordinated anions and additives synergistically decompose to enable inorganic-rich interphases with robustness and favorable ion diffusion, extending the voltage window and temperature range. As a result, Na3V2(PO4)2O2F demonstrates 58 mA h g-1 at -50 °C while stably cycling at 60 °C for 300 cycles with 80% capacity retention. Additionally, Na3V2(PO4)3 and NaFe1/3Ni1/3Mn1/3O2 cathodes also exhibit discharge specific capacities of 50 and 65 mA h g-1 at -60 °C. Eventually, the Ah-class pouch cell displays 0.64 A h with 56% capacity retention at -40 °C. In short, the introduced electrolyte formulation enhances the wide temperature operation of SIBs, shedding light on the development of all-weather systems.

Computational approaches to electrolyte design for advanced lithium-ion batteries

Theoretical calculations have shown great potential as an instructional, reliable, and robust tool for designing and optimizing electrolyte formulations for lithium-ion batteries. However, there is still a lack of clear understanding of the design principles and synergistic effects between each component of electrolytes, including lithium salts, solvents, additives, etc., especially on how to optimize each part of electrolytes from the atomic scale and molecular scale. In this review, we cover the quantum chemistry in lithium salt selection, functional additive design, solid electrolyte interphase film study, and reaction mechanism speculation; molecular dynamics simulations in solvation structures, interphase simulations, and dendrite growth studies; and high throughput simulations in functional electrolyte screening. Meanwhile, the limitations of each type of simulation are discussed. Finally, conclusions and an outlook regarding theoretical calculations for the electrolyte design of lithium-ion batteries are presented.

Constructing Dissolution-Resistant Interphases for Long-Life Sodium-Ion Batteries at Elevated Temperatures

Rechargeable sodium-ion batteries (SIBs) utilizing NaPF6-carbonate electrolytes consistently exhibit unsatisfactory cycle life at elevated temperatures, posing a significant challenge for their large-scale commercialization. This is mainly caused by the instability of interphase layers at elevated temperatures, especially the high solubility of interphase components (especially NaF) in carbonate solvents. In this study, a novel additive of sodium difluorobis(oxalato) phosphate (NaDFBOP) is synthesized and introduced into NaPF6-carbonate electrolytes to enhance the cycle life of commercial SIBs composed of NaNi1/3Fe1/3Mn1/3O2 (NFM) cathode and hard carbon (HC) anode, particularly at 50 °C. Specifically, the NaDFBOP enables NFM/HC SIBs to retain 85.45% of initial capacity after 1000 cycles at 30 °C and 90.76% after 500 cycles at 50 °C. Theoretical calculations reveal that DFBOP⁻ anions enter the first solvation shell of Na+, and NaDFBOP exhibits a strong propensity for decomposition. Characterizations suggest that NaDFBOP favors the formation of dissolution-resistant robust interphase layers enriched of dissolution-resistant oxalate-containing species and inorganic NaF, which have strong mutual binding energy. This work underscores the critical importance of designing functional additives and constructing dissolution-resistant robust interphases to enhance the elevated temperature cycle life of SIBs.

Synergy of Strongly Coordinating Salts and Weakly Coordinating Solvents Enables Stable and Fast-Kinetics Zinc Metal Batteries

The development of Zn metal batteries is hindered by Zn dendrites, notorious side reactions, and performance decay in harsh temperatures. Despite the efficacy of strongly coordinating organic solvents in addressing these issues, challenges persist regarding low ionic conductivity, high viscosity, and high desolvation barrier, particularly at low temperatures. Additionally, the strongly coordinating solvents around Zn2+ diminish anions participating in the first solvation shell, leading to the formation of an organic-rich interphase. To achieve balanced physicochemical properties, an electrolyte system combining chaotropic Zn(ClO4)2 salts with weakly coordinating solvents (MeOH) and highly coordinating salts (Zn(OAc)2) is proposed. Experimental and simulation results reveal that this system creates an anion-rich solvation shell with low desolvation barriers, inhibiting water decomposition and promoting the formation of an inorganic-organic-rich solid electrolyte interphase. OAc- also assists in the dense vertical zinc deposition along the (101) crystal plane. The reconstructed weak hydrogen bonds between MeOH and H2O break the highly ordered structure of water at low temperatures, enabling a higher ionic conductivity. Consequently, the battery employing the designed system yields superior electrochemical performance across a wide temperature range (-80 °C-40 °C). The proposed strategy facilitates the electrolyte design for wide-temperature Zn metal batteries with fast reaction kinetics.

Photogenerated Holes Induced Deep Sodium Storage of TiO2/CdSe/NFPP Cathode for High-Efficiency Photorechargeable Sodium Batteries

Resource-friendly photorechargeable sodium batteries (PRSBs) integrate energy storage devices with solar cells, offering a promising path for sustainable energy. Herein, a novel TiO2/CdSe/Na3Fe2(PO4)P2O7 (NFPP) cathode was prepared layer-by-layer utilizing resource-abundant commercialized NFPP and photoactive CdSe. The aligned energy levels with type II band structure ensure effective transfer of photogenerated holes from CdSe (-5.71 eV) to higher valence band of NFPP (-5.10 eV). Experimental results reveal that, during charging, the induced holes in NFPP accelerate the transition of Fe2+ to Fe3+ with a change of O-Fe hybrid orbitals. The calculations of bond valence sum and energy distribution reveal that NFPP-holes possesses broad Na+ transport path with reduced transport barrier (from 0.512 to 0.428 eV), improving Na+ extraction efficiency. Additionally, photogenerated holes could regulate surface charge distribution on NFPP and thus form a film-forming agent fluoroethylene carbonate (FEC)-dominated electric double layer. Finally, it converts to a thinner (9.75 nm illumination) NaF-rich cathode interphase layer, avoiding subsequent excessive electrolyte decomposition. As a result, the NFPP under illumination delivers high capacity of 119.1 mAh g-1 at 1 C, showing 41.11% improvement compared to dark conditions.

Realizing a 3 C Fast-Charging Practical Sodium Pouch Cell

Sodium-ion batteries (SIBs), endowed with relatively small Stokes radius and low desolvation energy of Na+, are reckoned as a promising candidate for fast-charging endeavors. However, the C-rate charging capability of practical energy-dense sodium-ion pouch cells is currently limited to ≤1 C, due to the high propensity for detrimental metallic Na plating on the hard carbon (HC) anode at elevated rates. Here, an ampere-hour-level sodium-ion pouch cell capable of 3 C charging is successfully developed via phosphorus (P)-sulfur (S) interphase chemistry. By rational electrolyte regulation, desired P-S constituents, namely, Na3PO4 and Na2SO4, are generated in the solid-electrolyte interphase with favorable Na+ interface kinetics. Specifically, Na+ desolvation energy barrier has been greatly lowered by the weak ion-solvent coordination near the inner Helmholtz plane on Na3PO4 interphase, while Na2SO4 expedites charge carrier mobility due to its intrinsically high ionic conductivity. Consequently, an energy-dense (126 Wh kg-1) O3-Na(Ni1/3Fe1/3Mn1/3)O2||HC pouch cell capable of 3 C charging (100 % state of charge) without Na plating can be achieved, with a great capacity retention of 91.5 % over 200 cycles. Further, the assembled power-type Na3V2(PO4)3||HC pouch cell displays an impressive fast-charging capability of 50 C, which surpasses that of previously reported high-power SIBs. This work serves as an enlightenment for developing fast-charging SIBs.

Advanced Interphases Layers for Dendrite-Free Sodium Metal Anodes

Sodium (Na) metal anode is considered the cornerstone of next-generation energy storage technology, owing to its high theoretical capacity and cost-effectiveness. However, the development of Na metal batteries is hindered by the instability and nonuniformity of the solid electrolyte interphase (SEI) and notorious formation of Na dendrites. Recently, various advanced artificial interphase designs have been developed to control notorious dendrite growth and stabilize the SEI layer. In this Review, we provide a comprehensive overview of artificial interphase designs, focusing on inorganic interphase layer, organic interphase layer, and hybrid inorganic/organic interphase layer, all aimed at inhibiting the notorious Na dendrites growth. Finally, future interphase engineering strategies are also envisioned to offer new insights into the optimization of Na anodes.

Advances in Anion Chemistry in the Electrolyte Design for Better Lithium Batteries

Electrolytes are crucial components in electrochemical energy storage devices, sparking considerable research interest. However, the significance of anions in the electrolytes is often underestimated. In fact, the anions have significant impacts on the performance and stability of lithium batteries. Therefore, comprehensively understanding anion chemistry in electrolytes is of crucial importance. Herein, in-depth comprehension of anion chemistry and its positive effects on the interface, solvation structure of Li-ions, as well as the electrochemical performance of the batteries have been emphasized and summarized. This review aims to present a full scope of anion chemistry and furnish systematic cognition for the rational design of advanced electrolytes for better lithium batteries with high energy density, lifespan, and safety. Furthermore, insightful analysis and perspectives based on the current research are proposed. We hope that this review sheds light on new perspectives on understanding anion chemistry in electrolytes.

Anion-Tailored EDL Induced Triple-Layer SEI on High-Capacity Anodes Enabling Fast-Charging and Durable Sodium-Storage

High-capacity electrodes face a great challenge of cycling stability due to particle fragmentation induced conductive network failure and accompanied by sustained electrolyte decomposition for repeatedly build solid electrolyte interphase (SEI). Herein, Se-solubility induced Sex 2- as self-adjustment electrolyte additive to regulate electric double layer (EDL) for constructing novel triple-layer SEI (inner layer: Se; mediate layer: inorganic; outer layer: organic) on high-capacity FeS2 anode as an example for achieving stable and fast sodium storage. In detail, Sex 2- in situ generated at 1.30 V (vs. Na+/Na) and was preferentially adsorbed onto EDL of anode, then converted to Se0 as inner layer of SEI. In addition, the Sex 2- causes anion-enhanced Na+ solvation structure could produce more inorganic (Se0, NaF) and less organic SEI components. The unique triple-layer SEI with layer-by-layer dense structure alleviate the excessive electrolyte consumption with less gas evolution. As a result, the anode delivered long-lifespan at 10 A g-1 (383.7 mAh g-1, 6000 cycles, 93.1 %, 5 min/cycle). The Se-induced triple-layer SEI could be also be formed on high-capacity SnS2 anode. This work provides a novel SEI model by anion-tailored EDL towards stable sodium-storage of high-capacity anode for fast-charging.

Synergistic Modulation of Solid- and Cathode-Electrolyte Interphase via a Lithium Salt Additive toward Stable Sodium Metal Batteries

Constructing feasible sodium metal batteries (SMBs) faces complex challenges in stabilizing cathodes and sodium metal anodes. It is imperative, but often underemphasized, to simultaneously regulate the solid-electrolyte interphase (SEI) to counter dendrite growth and the cathode-electrolyte interphase (CEI) to mitigate cathode deterioration. Herein, we introduce lithium 2-trifluoromethyl-4,5-dicyanoimidazolide (LiTDI) as an efficacious additive in a carbonate-based electrolyte to extend cycle lifespan of full SMBs: the capacity retention reaches 77.8% after 8000 cycles at room temperature and 74.3% after 5000 cycles at 50 °C. Cryogenic transmission electron microscopy characterization reveals that LiTDI promotes formation of inorganics-condensed SEI and CEI. The former inhibits continuous electrolyte decomposition and ensures homogeneous sodium plating, while the latter shields cathode from transition metal dissolution. This study highlights the crucial role of LiTDI in stabilizing both anodes and cathodes in SMBs, and it provides insights into designing functional additives for synergistic modulation of SEI and CEI.

Designing a Refined Multi-Structural Polymer Electrolyte Framework for Highly Stable Lithium-Metal Batteries

Rational structural designs of solid polymer electrolytes featuring rich interface-phase morphologies can improve electrolyte connection and rapid ion transport. However, these rigid interfacial structures commonly result in diminished or entirely inert ionic conductivity within their bulk phase, compromising overall electrolyte performance. Herein, a multi-component ion-conductive electrolyte was successfully designed based on a refined multi-structural polymer electrolyte (RMSPE) framework with uniform Li+ solvation chemistry and rapid Li+ transporting kinetics. The RMSPE framework is constructed via polymerization-induced phase separation based on a rational combination of lithiophilic components and rigid/flexible chain units with significant hydrophobic/hydrophilic contrasts. Further refined by coating a robust polymer network, this all-organic design endows a homogeneous micro-nano porous structure, providing a novel framework favorable for rapid ion transport in both its soft interfacial and bulk phases. The RMSPE exhibited excellent ion conductivity of 1.91 mS cm-1 at room temperature and a high Li+ transference number of 0.7. Assembled symmetrical Li cells realized stable cycling for over 2400 h at 3.0 mA cm-2. LiFePO4 full batteries demonstrated a long lifespan of 3300 cycles with a capacity retention of 93.5 % and stable cycling performance at -35 °C. This innovative design concept offers a promising perspective for achieving high-performance polymer-based Li metal batteries.

Modulating the Inner Helmholtz Plane towards Stable Solid Electrolyte Interphase by Anion-π Interactions for High-Performance Anode-Free Lithium Metal Batteries

Anode-free lithium (Li) metal batteries (AFLBs) featured high energy density are viewed as the viable future energy storage technology. However, the irregular Li deposition and unstable solid electrolyte interphase (SEI) on anode current collectors reduce their cycling performance. Here, we propose a concept of anion-recognition electrodes enabled by anion-π interactions to regulate the inner Helmholtz plane (IHP) and electrolyte solvation chemistry for high-performance AFLBs. By engineering the electrodes with electron-deficient aromatic-π systems that possess high permanent quadrupole moment (Qzz), the anion-π interactions can be generated to concentrate the anions on the electrode surface and tune the IHP structure to construct a stable anion-derived SEI layer, thus achieving highly reversible Li plating/stripping process. Through designing various current collectors with different Qzz values, the intimate correlations among the surface charge of the electrode, competitive adsorption of the IHP, and SEI structures are demonstrated. Particularly, the modified carbon cloth current collector with a high Qzz value (+35.1) delivers a high average Li stripping/plating Coulombic efficiency of 99.1 % over 230 cycles in the carbonated electrolyte, enabling a long lifespan and high capacity retention of LiNi0.8Co0.1Mn0.1O2-based AFLBs with a commercial-level areal capacity (4.1 mAh cm-2).

Building Robust Manganese Hexacyanoferrate Cathode for Long-Cycle-Life Sodium-Ion Batteries

Manganese Hexacyanoferrate (Mn─HCF) is a preferred cathode material for sodium-ion batteries used in large-scale energy storage. However, the inherent vacancies and the presence of H2O within the imperfect crystal structure of Mn─HCF lead to material failure and interface failure when used as a cathode. Addressing the challenge of constructing a stable cathode is an urgent scientific problem that needs to be solved to enhance the performance and lifespan of these batteries. In this review, the crystal structure of Mn─HCF is first introduced, explaining the formation mechanism of vacancies and exploring the various ways in which H2O molecules can be present within the crystal structure. Then comprehensively summarize the mechanisms of material and interfacial failure in Mn─HCF, highlighting the key factors contributing to these issues. Additionally, eight modification strategies designed to address these failure mechanisms are encapsulated, including vacancy regulation, transition metal substitution, high entropy, the pillar effect, interstitial H2O removal, surface coating, surface vacancy repair, and cathode electrolyte interphase reinforcement. This comprehensive review of the current research advances on Mn─HCF aims to provide valuable guidance and direction for addressing the existing challenges in their application within SIBs.

Weakly Solvating Electrolytes for Safe and Fast-Charging Sodium Metal Batteries

Electrolytes for high-performance sodium metal batteries (SMBs) are expected to have high electrode compatibility, low solvation energy, and nonflammability. However, conventional flammable carbonate ester electrolytes show high Na+ desolvation energy and poor compatibility with sodium metal anodes, leading to slow Faradaic reactions and significant degradation of SMBs. Herein, we report a weakly solvating electrolytes (WSEs) design developed by an ionized ether-induced solvent molecule polarization strategy. The steric hindrance and electron-withdrawing effect of the pyrrolidine cation weaken the solvation ability of the ionized ether and enable carbonate ester with low solvation energy through intermolecular polarization interactions. It enables WSEs with fast Na+ migration kinetics and electric-field-reinforced cationic electrode/electrolyte interface, thereby promoting the stability and reversibility of SMBs even under high-charge-rate conditions. The Na||Na3V2(PO4)3 battery with ionized ether-based WSEs exhibits a capacity retention of 83.5% with an average Coulombic efficiency (CE) of 99.69% after 500 cycles at 10C. Furthermore, the Na||Na2Fe2(SO4)3 cells maintained 92.8% capacity retention after 1000 cycles at 5C with an average CE of 99.77% at a cutoff voltage of 4.5 V. The ionized ether also eliminates the fire and safety risks associated with WSEs. This work offers valuable insights into the design of WSEs for safe and high-performance sodium metal batteries.

A Gel Polymer Electrolyte with High Uniform Na+ Flux and its Constructed Hybrid Interface Synergistically to Facilitate High-Performance Sodium Batteries

Sodium metal batteries (SMBs) can be developed on a large scale to achieve low-cost and high-capacity energy storage systems. Gel polymer electrolyte (GPE) can relieve volatilization of liquid electrolyte, adapt to volume changes in electrodes, and better satisfy the requirements of long-term SMBs. Herein, a dense polyurethane-based GPE modified with polyacrylonitrile is synthesized by rapidly swelling two-component polyurethane/polyacrylonitrile electrospun fiber film. Compared to traditional porous GPEs obtained by swelling porous matrixes, the fiber film provides uniform high Na+ flux inside GPE due to its partial solubility property and ability to dissociate salts. Therefore, it can reduce the polarization effect and induce uniform metal deposition under high current in conjunction with its constructed hybrid N/F-containing solid electrolyte interface (SEI) that possesses low ionic diffusion barrier. The study demonstrates that GPE has an ionic conductivity of 1.816 mS cm-1 at 20 °C and an ion transference number of 0.53. The full battery (NVP/GPE/Na) assembled with this GPE and Na3V2(PO4)3 (NVP) cathode shows 90.8% capacity retention rate after 1000 cycles at 10 C. Considering the convenient preparation and outstanding electrochemical performances of the obtained GPE, it can also be matched with other electrodes in the future to expand the application of sodium-based batteries.

Co3O4-Induced Na2CO3-Rich SEI Film on an FeNCN Electrode with Promoted Loading and High-Rate Na-Storage Performance

Iron carbodiimide (FeNCN) often suffers from unstable interfacial structure with an unexpected failure of Na-ion storage performance. In this work, Co3O4 particles were deposited on the surface of FeNCN. This Co3O4 nanolayer led to the formation of a Na2CO3-rich inorganic component SEI film to enhance the stability of a promoted-loading FeNCN electrode interface with fast Na+ migration pathway. Benefitting from this strategy, the FeNCN electrode could present a capacity retention rate of 99.95% per cycle after 1500 cycles at 1 A g-1. The design of interfacial structure in a promoted-loading electrode could be a reference for stable and high-rate performance of carbodiimide-based materials.

In-Depth Understanding of Interfacial Na+ Behaviors in Sodium Metal Anode: Migration, Desolvation, and Deposition

Interfacial Na+ behaviors of sodium (Na) anode severely threaten the stability of sodium-metal batteries (SMBs). This review systematically and in-depth discusses the current fundamental understanding of interfacial Na+ behaviors in SMBs including Na+ migration, desolvation, diffusion, nucleation, and deposition. The key influencing factors and optimization strategies of these behaviors are further summarized and discussed. More importantly, the high-energy-density anode-free sodium metal batteries (AFSMBs) are highlighted by addressing key issues in the areas of limited Na sources and irreversible Na loss. Simultaneously, recent advanced characterization techniques for deeper insights into interfacial Na+ deposition behavior and composition information of SEI film are spotlighted to provide guidance for the advancement of SMBs and AFSMBs. Finally, the prominent perspectives are presented to guide and promote the development of SMBs and AFSMBs.

Carbonate Ester-Based Sodium Metal Battery with High-Capacity Retention at -50 °C Enabled by Weak Solvents and Electrodeposited Anode

Sodium metal batteries (SMBs) have received increasing attention due to the abundant sodium resources and high energy density, but suffered from the sluggish interfacial kinetic and unstable plating/stripping of sodium anode at low temperature, especially when matched with ester electrolytes. Here, we develop a stable ultra-low-temperature SMBs with high-capacity retention at -50 °C in a weak solvated carbonate ester-based electrolyte, combined with an electrodeposited Na (Cu/Na) anode. The Cu/Na anode with electrochemically activated "deposited sodium" and stable inorganic-rich solid electrolyte interphase (SEI) is favor for the fast Na+ migration, therefore accelerating the interfacial kinetic process. As a result, the Cu/Na||NaCrO2 battery exhibited the highest capacity retention (compared to room-temperature capacity) in carbonate ester-based SMBs (98.05 % at -25 °C, 91.3 % at -40 °C, 87.9 % at -50 °C, respectively). The cyclic stability of 350 cycles at -25 °C with a high energy efficiency of 96.15 % and 70 cycles at -50 °C can be achieved. Even in chill atmospheric environment with the fluctuant temperature, the battery can still operate over one month. This work provides a new opportunity for the development of low-temperature carbonate ester-based SMBs.

Cyclic Ether Derived Stable Solid Electrolyte Interphase on Bismuth Anodes for Ultrahigh-Rate Sodium-Ion Storage

The bismuth anode has garnered significant attention due to its high theoretical Na-storage capacity (386 mAh g-1). There have been numerous research reports on the stable solid electrolyte interphase (SEI) facilitated by electrolytes utilizing ether solvents. In this contribution, cyclic tetrahydrofuran (THF) and 2-methyltetrahydrofuran (MeTHF) ethers are employed as solvents to investigate the sodium-ion storage properties of bismuth anodes. A series of detailed characterizations are utilized to analyze the impact of electrolyte solvation structure and SEI chemical composition on the kinetics of sodium-ion storage. The findings reveal that bismuth anodes in both THF and MeTHF-based electrolytes exhibit exceptional rate performance at low current densities, but in THF-based electrolytes, the reversible capacity is higher at high current densities (316.7 mAh g-1 in THF compared to 9.7 mAh g-1 in MeTHF at 50 A g-1). This stark difference is attributed to the formation of an inorganic-rich, thin, and uniform SEI derived from THF-based electrolyte. Although the SEI derived from MeTHF-based electrolyte also consists predominantly of inorganic components, it is thicker and contains more organic species compared to the THF-derived SEI, impeding charge transfer and ion diffusion. This study offers valuable insights into the utilization of cyclic ether electrolytes for Na-ion batteries.

Lithiation: A Pathway to Strengthen Sodiophilic Metal Interlayer of 3D Metallic Skeleton for High-Performance Sodium Metal Anodes

Constructing a three-dimensional (3D) skeleton with a sodiophilic-modified layer (SML) has been proven to be an effective strategy to alleviate excessive volumetric deformation and continuous dendrite growth for sodium (Na) metal anodes. However, the weak binding force and violent reaction between the SML and the 3D skeleton lead to numerous cracks/defects and even pulverization of the SML during repeated Na plating/stripping. Herein, a lithiation pathway is presented to construct a sodiophilic Li-Sn alloy layer onto a 3D copper mesh to strengthen the SML for stable Na metal anodes. The lithiation 3D skeleton exhibits superior sodiophilicity, higher charge-transfer efficiency, and lower ion-diffusion barrier, contributing to the homogenization of ion-electronic flux and Na deposition. Simultaneously, the dense Li-Sn alloy is more stable than the monometal Sn-layer, which effectively prevents damage to the SML and enhances the stability of the SML. As a result, the asymmetrical cell exhibits great performance with a negligible nucleation overpotential and a high average Coulombic efficiency of 99.4%. Moreover, the full cell assembled with Na3V2(PO4)3 cathode delivers superior capacity retention of 91.3% after 1000 cycles at a current of 3C.

Nonflammable Succinonitrile-Based Deep Eutectic Electrolyte for Intrinsically Safe High-Voltage Sodium-Ion Batteries

Intrinsically safe sodium-ion batteries are considered as a promising candidate for large-scale energy storage systems. However, the high flammability of conventional electrolytes may pose serious safety threats and even explosions. Herein, a strategy of constructing a deep eutectic electrolyte is proposed to boost the safety and electrochemical performance of succinonitrile (SN)-based electrolyte. The strong hydrogen bond between S═O of 1,3,2-dioxathiolane-2,2-dioxide (DTD) and the α-H of SN endows the enhanced safety and compatibility of SN with Lewis bases. Meanwhile, the DTD participates in the inner Na+ sheath and weakens the coordination number of SN. The unique solvation configuration promotes the formation of robust gradient inorganic-rich electrode-electrolyte interphase, and merits stable cycling of half-cells in a wide temperature range, with a capacity retention of 82.8% after 800 cycles (25 °C) and 86.3% after 100 cycles (60 °C). Correspondingly, the full cells deliver tremendous improvement in cycling stability and rate performance.

Chlorination Design for Highly Stable Electrolyte toward High Mass Loading and Long Cycle Life Sodium-Based Dual-Ion Battery

Sodium-based dual ion batteries (SDIBs) have garnered significant attention as novel energy storage devices offering the advantages of high-voltage and low-cost. Nonetheless, conventional electrolytes exhibit low resistance to oxidation and poor compatibility with electrode materials, resulting in rapid battery failure. In this study, for the first time, a chlorination design of electrolytes for SDIB, is proposed. Using ethyl methyl carbonate (EMC) as a representative, chlorine (Cl)-substituted EMC not only demonstrates increased oxidative stability ascribed to the electron-withdrawing characteristics of chlorine atom, electrolyte compatibility with both the cathode and anode is also greatly improved by forming Cl-containing interface layers. Consequently, a discharge capacity of 104.6 mAh g-1 within a voltage range of 3.0-5.0 V is achieved for Na||graphite SDIB that employs a high graphite cathode mass loading of 5.0 mg cm-2, along with almost no capacity decay after 900 cycles. Notably, the Na||graphite SDIB can be revived for an additional 900 cycles through the replacement of a fresh Na anode. As the mass loading of graphite cathode increased to 10 mg cm-2, Na||graphite SDIB is still capable of sustaining over 700 times with ≈100% capacity retention. These results mark the best outcome among reported SDIBs. This study corroborates the effectiveness of chlorination design in developing high-voltage electrolytes and attaining enduring cycle stability of Na-based energy storage devices.

Sustainable Electrolytes: Design Principles and Recent Advances

Today, rechargeable batteries are omnipresent and essential for our existence. In order to improve the electrochemical performance of electric fields, the introduction of electrolytes with fluorine (F)-based inorganic elemental compositions is a direction of exploration. However, most fluorocarbons have a high global warming potential and ozone depletion potential, which do not meet the sustainability requirements of the battery industry. Therefore, developing sustainable electrolytes is a viable option for future battery development. Although researchers have made much progress in electrolyte optimization, little attention has been paid to developing low-toxic and safe electrolytes. This review aims to elucidate the design principles and recent advances in this direction for solvents and salts. It concludes with a summary and outlook on future research directions for the molecular design of green electrolytes for practical high-voltage rechargeable batteries.

Electrolytes Design for Extending the Temperature Adaptability of Lithium-Ion Batteries: from Fundamentals to Strategies

With the continuously growing demand for wide-range applications, lithium-ion batteries (LIBs) are increasingly required to work under conditions that deviate from room temperature (RT). However, commercial electrolytes exhibit low thermal stability at high temperatures (HT) and poor dynamic properties at low temperatures (LT), hindering the operation of LIBs under extreme conditions. The bottleneck restricting the practical applications of LIBs has promoted researchers to pay more attention to developing a series of innovative electrolytes. This review primarily covers the design of electrolytes for LIBs from a temperature adaptability perspective. First, the fundamentals of electrolytes concerning temperature, including donor number (DN), dielectric constant, viscosity, conductivity, ionic transport, and theoretical calculations are elaborated. Second, prototypical examples, such as lithium salts, solvent structures, additives, and interfacial layers in both liquid and solid electrolytes, are presented to explain how these factors can affect the electrochemical behavior of LIBs at high or low temperatures. Meanwhile, the principles and limitations of electrolyte design are discussed under the corresponding temperature conditions. Finally, a summary and outlook regarding electrolytes design to extend the temperature adaptability of LIBs are proposed.

Interface-Compatible Gel-Polymer Electrolyte Enabled by NaF-Solubility-Regulation toward All-Climate Solid-State Sodium Batteries

Gel-polymer electrolyte (GPE) is a pragmatic choice for high-safety sodium batteries but still plagued by interfacial compatibility with both cathode and anode simultaneously. Here, salt-in-polymer fibers with NaF salt inlaid in polylactide (PLA) fiber network was fabricated via electrospinning and subsequent in situ forming gel-polymer electrolyte in liquid electrolytes. The obtained PLA-NaF GPE achieves a high ion conductivity (2.50×10-3 S cm-1) and large Na+ transference number (0.75) at ambient temperature. Notably, the dissolution of NaF salt occupies solvents leading to concentrated-electrolyte environment, which facilitates aggregates with increased anionic coordination (anion/Na+ >1). Aggregates with higher HOMO realize the preferential oxidation on the cathode so that inorganic-rich and stable CEI covers cathode' surface, preventing particles' breakage and showing good compatibility with different cathodes (Na3V2(PO4)3, Na2+2xFe2-x(SO4)3, Na0.72Ni0.32Mn0.68O2, NaTi2(PO4)3). While, passivated Na anode induced by the lower LUMO of aggregates, and the lower surface tension between Na anode and PLA-NaF GPE interface, leading to the dendrites-free Na anode. As a result, the assembled Na || Na3V2(PO4)3 cells display excellent electrochemical performance at all-climate conditions.

Building Stable Anodes for High-Rate Na-Metal Batteries

Due to low cost and high energy density, sodium metal batteries (SMBs) have attracted growing interest, with great potential to power future electric vehicles (EVs) and mobile electronics, which require rapid charge/discharge capability. However, the development of high-rate SMBs has been impeded by the sluggish Na+ ion kinetics, particularly at the sodium metal anode (SMA). The high-rate operation severely threatens the SMA stability, due to the unstable solid-electrolyte interface (SEI), the Na dendrite growth, and large volume changes during Na plating-stripping cycles, leading to rapid electrochemical performance degradations. This review surveys key challenges faced by high-rate SMAs, and highlights representative stabilization strategies, including the general modification of SMB components (including the host, Na metal surface, electrolyte, separator, and cathode), and emerging solutions with the development of solid-state SMBs and liquid metal anodes; the working principle, performance, and application of these strategies are elaborated, to reduce the Na nucleation energy barriers and promote Na+ ion transfer kinetics for stable high-rate Na metal anodes. This review will inspire further efforts to stabilize SMAs and other metal (e.g., Li, K, Mg, Zn) anodes, promoting high-rate applications of high-energy metal batteries towards a more sustainable society.

Unprotected Organic Cations─The Dilemma of Highly Li-Concentrated Ionic Liquid Electrolytes

Highly Li-concentrated electrolytes have been widely studied to harness their uniquely varying bulk and interface properties that arise from their distinctive physicochemical properties and coordination structures. Similar strategies have been applied in the realm of ionic liquid electrolytes to exploit their improved functionalities. Despite these prospects, the impact of organic cation behavior on interfacial processes remains largely underexplored compared to the widely studied anion behavior. The present study demonstrates that the weakened interactions between cations and anions engender "unprotected" organic cations in highly Li-concentrated ionic liquid electrolytes, leading to the decomposition of electrolytes during the initial charge. This decomposition behavior is manifested by the substantial irreversible capacities and inferior initial Coulombic efficiencies observed during the initial charging of graphite negative electrodes, resulting in considerable electrolyte consumption and diminished energy densities in full-cell configurations. The innate cation behavior is ascertained by examining the coordination environment of ionic liquid electrolytes with varied Li concentrations, where intricate ionic interactions between organic cations and anions are unveiled. In addition, anionic species with high Lewis basicity were introduced to reinforce the ionic interactions involving organic cations and improve the initial Coulombic efficiency. This study verifies the role of unprotected organic cations while highlighting the significance of the coordination environment in the performance of ionic liquid electrolytes.