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

Heteroatom doping-induced formation of closed pores for high-performance sodium storage hard carbon anodes

A resin-based hard carbon with rich closed pores is prepared by the in situ reconstruction of cavities formed after heteroatoms evaporated during a high-temperature carbonization process. Various characterization results confirm that rich defect sites and micropores and enlarged layer spacing in hard carbon promote Na+ transport and facilitate high-performance Na+ storage.

3D-Printed Hierarchically Microgrid Frameworks of Sodiophilic Co3O4@C/rGO Nanosheets for Ultralong Cyclic Sodium Metal Batteries

Herein, hierarchically structured microgrid frameworks of Co3O4 and carbon composite deposited on reduced graphene oxide (Co3O4@C/rGO) are demonstrated through the three-dimensioinal (3D) printing method, where the porous structure is controllable and the height and width are scalable, for dendrite-free Na metal deposition. The sodiophilicity, facile Na metal deposition kinetics, and NaF-rich solid electrolyte interphase (SEI) formation of cubic Co3O4 phase are confirmed by combined spectroscopic and computational analyses. Moreover, the uniform and reversible Na plating/stripping process on 3D-printed Co3O4@C/rGO host is monitored in real time using in situ transmission electron and optical microscopies. In symmetric cells, the 3D printed Co3O4@C/rGO electrode achieves a long-term stability over 3950 at 1 mA cm-2 and 1 mAh cm-2 with a superior Coulombic efficiency (CE) of 99.87% as well as 120 h even at 20 mA cm-2 and 20 mAh cm-2, far exceeding the previously reported carbon-based hosts for Na metal anodes. Consequently, the full cells of 3D-printed Na@Co3O4@C/rGO anode with 3D-printed Na3V2(PO4)3@C-rGO cathode (≈15.7 mg cm-2) deliver the high specific capacity of 97.97 mAh g-1 after 500 cycles with a high CE of 99.89% at 0.5 C, demonstrating the real operation of flexible Na metal batteries.

Superior sodiophilicity and molecule crowding of crown ether boost the electrochemical performance of all-climate sodium-ion batteries

Sodium-ion batteries (SIBs) as one of the promising alternatives to lithium-ion batteries have achieved remarkable progress in the past. However, the all-climate performance is still very challenging for SIBs. Herein, 15-Crown-5 (15-C-5) is screened as an electrolyte additive from a number of ether molecules theoretically. The good sodiophilicity, high molecule rigidity, and bulky size enable it to reshape the solvation sheath and promote the anion engagement in the solvated structures by molecule crowding. This change also enhances Na-ion transfer, inhibits side reactions, and leads to a thin and robust solid-electrolyte interphase. Furthermore, the electrochemical stability and operating temperature windows of the electrolyte are extended. These profits improve the electrochemical performance of SIBs in all climates, much better than the case without 15-C-5. This improvement is also adopted to μ-Sn, μ-Bi, hard carbon, and MoS2. This work opens a door to prioritize the potential molecules in theory for advanced electrolytes.

Challenges and Breakthroughs in Enhancing Temperature Tolerance of Sodium-Ion Batteries

Lithium-based batteries (LBBs) have been highly researched and recognized as a mature electrochemical energy storage (EES) system in recent years. However, their stability and effectiveness are primarily confined to room temperature conditions. At temperatures significantly below 0 °C or above 60 °C, LBBs experience substantial performance degradation. Under such challenging extreme contexts, sodium-ion batteries (SIBs) emerge as a promising complementary technology, distinguished by their fast dynamics at low-temperature regions and superior safety under elevated temperatures. Notably, developing SIBs suitable for wide-temperature usage still presents significant challenges, particularly for specific applications such as electric vehicles, renewable energy storage, and deep-space/polar explorations, which requires a thorough understanding of how SIBs perform under different temperature conditions. By reviewing the development of wide-temperature SIBs, the influence of temperature on the parameters related to battery performance, such as reaction constant, charge transfer resistance, etc., is systematically and comprehensively analyzed. The review emphasizes challenges encountered by SIBs in both low and high temperatures while exploring recent advancements in SIB materials, specifically focusing on strategies to enhance battery performance across diverse temperature ranges. Overall, insights gained from these studies will drive the development of SIBs that can handle the challenges posed by diverse and harsh climates.

Fast-Charging Anode Materials for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have undergone rapid development as a complementary technology to lithium-ion batteries due to abundant sodium resources. However, the extended charging time and low energy density pose a significant challenge to the widespread use of SIBs in electric vehicles. To overcome this hurdle, there is considerable focus on developing fast-charging anode materials with rapid Na⁺ diffusion and superior reaction kinetics. Here, the key factors that limit the fast charging of anode materials are examined, which provides a comprehensive overview of the major advances and fast-charging characteristics across various anode materials. Specifically, it systematically dissects considerations to enhance the rate performance of anode materials, encompassing aspects such as porous engineering, electrolyte desolvation strategies, electrode/electrolyte interphase, electronic conductivity/ion diffusivity, and pseudocapacitive ion storage. Finally, the direction and prospects for developing fast-charging anode materials of SIBs are also proposed, aiming to provide a valuable reference for the further advancement of high-power SIBs.

The Enormous Potential of Sodium/Potassium-Ion Batteries as the Mainstream Energy Storage Technology for Large-Scale Commercial Applications

Cost-effectiveness plays a decisive role in sustainable operating of rechargeable batteries. As such, the low cost-consumption of sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) provides a promising direction for "how do SIBs/PIBs replace Li-ion batteries (LIBs) counterparts" based on their resource abundance and advanced electrochemical performance. To rationalize the SIBs/PIBs technologies as alternatives to LIBs from the unit energy cost perspective, this review gives the specific criteria for their energy density at possible electrode-price grades and various battery-longevity levels. The cost ($ kWh-1 cycle-1) advantage of SIBs/PIBs is ascertained by the cheap raw-material compensation for the cycle performance deficiency and the energy density gap with LIBs. Furthermore, the cost comparison between SIBs and PIBs, especially on cost per kWh and per cycle, is also involved. This review explicitly manifests the practicability and cost-effectiveness toward SIBs are superior to PIBs whose commercialization has so far been hindered by low energy density. Even so, the huge potential on sustainability of PIBs, to outperform SIBs, as the mainstream energy storage technology is revealed as long as PIBs achieve long cycle life or enhanced energy density, the related outlook of which is proceeded as the next development directions for commercial applications.

Carboxylate ester-based electrolytes for Na-ion batteries

Sodium-ion batteries (SIBs) is a promising technology for next-generation energy storage. However, their performance is limited at low temperatures due to the inferior bulk and interfacial resistance of current electrolytes. Here we present a systematic study to evaluate carboxylate ester-based electrolytes for SIB applications, due to their favorable properties (i.e., low melting point, low viscosity and high dielectric constant). The effects of salt, concentration and solvent molecular structure were systematically examined and compared with those of carbonate-based electrolytes. By combining electrochemical tests with spectroscopic characterization, the performance of selective carboxylate ester-based electrolytes in hard carbon/Na and Na3V2(PO4)3/Na half-cells was evaluated. We found carboxylates enable high electrolyte conductivities, especially at low temperatures. However, carboxylates alone are inadequate to form a stable interphase due to their high reactivity, which can be addressed by choosing a suitable anion and facilitating anion-rich Na+ solvation by increasing salt concentration. Fundamental knowledge on the chemistry-property-performance correlation of this new family of electrolytes was obtained, and their benefits and pitfalls were thoroughly discussed. These discoveries and knowledge will shed light on the potential of carboxylate ester-based electrolytes and provide the foundation for further electrolyte engineering.

Sulfur-bridged bonds enabled structure modulation and space confinement of MnS for superior sodium-ion capacitors

Manganese sulfide (MnS) is a promising converion-type anode for sodium storage, owing to the virtues of high theoretical capacity, coupled with it crustal abundance and cost-effectiveness. Nevertheless, MnS suffers from inadequate electronic conductivity, sluggish Na+ reaction kinetics and considerable volume variation during discharge/charge process, thereby impeding its rate capability and capacity retention. Herein, a novel lamellar heterostructured composite of Fe-doped MnS nanoparticles/positively charged reduced graphene oxide (Fe-MnS/PG) was synthesized to overcome these issues. The Fe-doping can accelerate the ion/electron transfer, endowing fast electrochemical kinetics of MnS. Meanwhile, the graphene space confinement with strong MnSC bond interactions can facilite the interfacial electron transfer, hamper volume expansion and aggregation of MnS nanoparticles, stabilizing the structural integrity, thus improving the Na+ storage reversibility and cyclic stability. Combining the synergistic effect of Fe-doping and space confinement with strong MnSC bond interactions, the as-produced Fe-MnS/PG anode presents a remarkable capacity of 567 mAh/g at 0.1 A/g and outstanding rate performance (192 mAh/g at 10 A/g). Meanwhile, the as-assembled sodium-ion capacitor (SIC) can yield a high energy density of 119 Wh kg-1 and a maximum power density of 17500 W kg-1, with capacity retention of 77 % at 1 A/g after 5000 cycles. This work offers a promising strategy to develop MnS-based practical SICs with high energy and long lifespan, and paves the way for fabricating advanced anode materials.

Recent Advances in Aqueous Non-Metallic Ion Batteries with Organic Electrodes

Aqueous non-metallic ion batteries have attracted much attention in recent years owing to their fast kinetics, long cycle life, and low manufacture cost. Organic compounds with flexible structural designability are promising electrode materials for aqueous non-metallic ion batteries. In this review, the recent progress of organic electrode materials is systematically summarized for aqueous non-metallic ion batteries with the focus on the interaction between non-metallic ion charge carriers and organic electrode host materials. Both the cations (proton, ammonium ion, and methyl viologen ions) and anions (chloridion, sulfate ion, perchlorate ion, trifluoromethanesulfonate and trifluoromethanesulfonimide ion) storage are discussed. Moreover, the design strategies toward improving the comprehensive performance of organic electrode materials in aqueous non-metallic ion batteries will be summarized. More organic electrode materials with new reaction mechanisms need to be explored to meet the diverse demands of aqueous non-metallic ion batteries with different charge carriers in the future. This review provides insights into developing high-performance organic electrodes for aqueous non-metallic ion batteries.

Binary Solvent Induced Stable Interphase Layer for Ultra-Long Life Sodium Metal Batteries

Sodium foil, promising for high-energy-density batteries, faces reversibility challenges due to its inherent reactivity and unstable solid electrolyte interphase (SEI) layer. In this study, a stable sodium metal battery (SMB) is achieved by tuning the electrolyte solvation structure through the addition of co-solvent 2-methyl tetrahydrofuran (MTHF) to diglyme (Dig). The introduction of cyclic ether-based MTHF results in increased anion incorporation in the solvation structure, even at lower salt concentrations. Specifically, the anion stabilization capabilities of the environmentally sustainable MTHF co-solvent lead to a contact-ion pair-based solvation structure. Time-of-flight mass spectroscopy analysis reveals that a shift toward an anion-dominated solvation structure promotes the formation of a thin and uniform SEI layer. Consequently, employing a NaPF6-based electrolyte with a Dig:MTHF ratio of 50% (v/v) binary solvent yields an average Coulombic efficiency of 99.72% for 300 cycles in Cu||Na cell cycling. Remarkably, at a C/2 cycling rate, Na||Na symmetric cell cycling demonstrates ultra-long-term stability exceeding 7000 h, and full cells with Na0.44MnO2 as a cathode retain 80% of their capacity after 500 cycles. This study systematically examines solvation structure, SEI layer composition, and electrochemical cycling, emphasizing the significance of MTHF-based binary solvent mixtures for high-performance SMBs.

Presodiation Architected Robust Surface Enables Packaging Optimal Performance of Sodium-Ion Batteries

Presodiation has shown great promise in compensating sodium storage losses. In the absence of a mechanistic understanding of how presodiation affects the surface of an electrode material, packaging optimization is restricted. Focusing on interfaces, we illustrate the working principle of presodiation in virtue of short-circuiting internal circuits. The presodiated carbon nanotubes (PS-CNTs) provide a thin, denser, and more robust solid electrolyte interfacial layer, enabling a high initial Coulombic efficiency (ICE), high power density, and cycling stability with the merits of uniformly distributed NaF. As a result, our assembled sodium-ion battery (SIB) full cell with PS-CNT has an ICE of 91.6% and an energy density of 226 Wh kg-1, which was superior to the pristine CNT control electrode (ICE of 42.9% and energy density of 163 Wh kg-1). The gained insights can be practically applied to directly promote the commercial uses of carbon-based materials in sodium-ion batteries.

Emerging Chemistry for Wide-Temperature Sodium-Ion Batteries

The shortage of resources such as lithium and cobalt has promoted the development of novel battery systems with low cost, abundance, high performance, and efficient environmental adaptability. Due to the abundance and low cost of sodium, sodium-ion battery chemistry has drawn worldwide attention in energy storage systems. It is widely considered that wide-temperature tolerance sodium-ion batteries (WT-SIBs) can be rapidly developed due to their unique electrochemical and chemical properties. However, WT-SIBs, especially for their electrode materials and electrolyte systems, still face various challenges in harsh-temperature conditions. In this review, we focus on the achievements, failure mechanisms, fundamental chemistry, and scientific challenges of WT-SIBs. The insights of their design principles, current research, and safety issues are presented. Moreover, the possible future research directions on the battery materials for WT-SIBs are deeply discussed. Progress toward a comprehensive understanding of the emerging chemistry for WT-SIBs comprehensively discussed in this review will accelerate the practical applications of wide-temperature tolerance rechargeable batteries.

Restructuring Electrolyte Solvation by a Versatile Diluent Toward Beyond 99.9% Coulombic Efficiency of Sodium Plating/Stripping at Ultralow Temperatures

The reversible and durable operation of sodium metal batteries at low temperatures (LT) is essential for cold-climate applications but is plagued by dendritic Na plating and unstable solid-electrolyte interphase (SEI). Current Coulombic efficiencies of sodium plating/stripping at LT fall far below 99.9%, representing a significant performance gap yet to be filled. Here, the solvation structure of the conventional 1 m NaPF6 in diglyme electrolyte by facile cyclic ether (1,3-dioxolane, DOL) dilution is efficiently reconfigured. DOL diluents help shield the Na+-PF6 - Coulombic interaction and intermolecular forces of diglyme, leading to anomalously high Na+-ion conductivity. Besides, DOL participates in the solvation sheath and weakens the chelation of Na+ by diglyme for facilitated desolvation. More importantly, it promotes concentrated electron cloud distribution around PF6 - in the solvates and promotes their preferential decomposition. A desired inorganic-rich SEI is generated with compositional uniformity, high ionic conductivity, and high Young's modulus. Consequently, a record-high Coulombic efficiency over 99.9% is achieved at an ultralow temperature of -55 °C, and a 1 Ah capacity pouch cell of initial anode-free sodium metal battery retains 95% of the first discharge capacity over 100 cycles at -25 °C. This study thus provides new insights for formulating electrolytes toward increased Na reversibility at LT.

Molecular Engineering Enabling High Initial Coulombic Efficiency and Rubost Solid Electrolyte Interphase for Hard Carbon in Sodium-Ion Batteries

Hard carbon (HC) as a potential candidate anode for sodium-ion batteries (SIBs) suffers from unstable solid electrolyte interphase (SEI) and low initial Coulombic efficiency (ICE), which limits its commercial applications and urgently requires the emergence of a new strategy. Herein, an organic molecule with two sodium ions, disodium phthalate (DP), was successfully engineered on the HC surface (DP-HC) to replenish the sodium loss from solid electrolyte interphase (SEI) formation. A stabilized and ultrathin (≈7.4 nm) SEI was constructed on the DP-HC surface, which proved to be simultaneously suitable in both ester and ether electrolytes. Compared to pure HC (60.8 %), the as-designed DP-HC exhibited a high ICE of >96.3 % in NaPF6 in diglyme (G2) electrolyte, and is capable of servicing consistently for >1600 cycles at 0.5 A g-1 . The Na3 V2 (PO4 )3 (NVP)|DP-HC full-cell with a 98.3 % exceptional ICE can be cycled stably for 450 cycles, demonstrating the tremendous practical application potential of DP-HC. This work provides a molecular design strategy to improve the ICE of HC, which will inspire more researchers to concentrate on the commercialization progress of HC.

Unraveling the underlying mechanism of good electrochemical performance of hard carbon in PC/EC-Based electrolyte

Although hard carbon in propylene carbonate / ethylene carbonate (PC/EC)-based electrolytes possesses favorable electrochemical characteristics in rechargeable sodium-ion batteries, the underlying mechanism is still vague. Numerous hypotheses have been proposed to solve the puzzle, but none of them have satisfactorily unraveled the reason at the molecular-level. In this study, we firstly attempted to address this mystery through a profound insight into the disparity of the ion solvation/desolvation behavior in electrolyte. Combining the results of density functional theory (DFT) calculations and experiments, the work explains that compared to the sole PC-based electrolyte, Na+-EC4 molecules in the PC/EC-based electrolyte preferentially undergo reduction and contribute to the emergence of a more stable protective film on the surface of hard carbon, leading to the preferable durability and rate capability of the cell. Nevertheless, applying the ion solvation/desolvation model, it also reveals that Na+-(solvent)n molecules in the PC/EC-based electrolyte can achieve faster Na+ desolvation processes than in the PC-based electrolyte alone, contributing to the enhancement of charge transfer kinetics. This research holds great importance in uncovering the possible mechanism of the remarkable electrochemical- properties of hard carbon in PC/EC-based electrolytes, and advancing its practical utilization in future sodium-ion batteries.

Non-aqueous Liquid Electrolyte Additives for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have been recognized as one of the most promising new energy storage devices for their rich sodium resources, low cost and high safety. The electrolyte, as a bridge connecting the cathode and anode electrodes, plays a vital role in determining the performance of SIBs, such as coulombic efficiency, energy density and cycle life. Therefore, the overall performance of SIBs could be significantly improved by adjusting the electrolyte composition or adding a small number of functional additives. In this review, the fundamentals of SIB electrolytes including electrode-electrolyte interface and solvation structure are introduced. Then, the mechanisms of electrolyte additive action on SIBs are discussed, with a focus on film-forming additives, flame-retardant additives and overcharge protection additives. Finally, the future research of electrolytes is prospected from the perspective of scientific concepts and practical applications.

Realizing Low-Temperature Graphite-based Rechargeable Potassium-Ion Full Battery

Graphite (Gr) has been considered as the most promising anode material for potassium-ion batteries (PIBs) commercialization due to its high theoretical specific capacity and low cost. However, Gr-based PIBs remain unfeasible at low temperature (LT), suffering from either poor kinetics based on conventional carbonate electrolytes or K+ -solvent co-intercalation issue based on typical ether electrolytes. Herein, a high-performance Gr-based LT rechargeable PIB is realized for the first time by electrolyte chemistry. Applying unidentate-ether-based molecule as the solvent dramatically weakens the K+ -solvent interactions and lowers corresponding K+ de-solvation kinetic barrier. Meanwhile, introduction of steric hindrance suppresses co-intercalation of K+ -solvent into Gr, greatly elevating operating voltage and cyclability of the full battery. Consequently, the as-prepared Gr||prepotassiated 3,4,9,10-perylene-tetracarboxylicacid-dianhydride (KPTCDA) full PIB can reversibly charge/discharge between -30 and 45 °C with a considerable energy density up to 197 Wh kgcathode -1 at -20 °C, hopefully facilitating the development of LT PIBs.

Highly Stable ZnS Anodes for Sodium-Ion Batteries Enabled by Structure and Electrolyte Engineering

Zinc sulfide is a promising high-capacity anode for practical sodium-ion batteries, considering its high capacity and the low cost of zinc and sulfur sources. However, the pulverization of particulate zinc sulfide causes active mass collapse and penetration-induced short circuits of batteries. Herein, a zinc sulfide encapsulated in a nitrogen-doped carbon shell (ZnS@NC) was developed for high-performance anodes. The confinement effect of nitrogen-doped carbon stabilizes the active mass structure during cycling thanks to the robust chemically and electronically bonded connections between nitrogen-doped carbon and zinc sulfide nanoparticles. Furthermore, the cycling stability of the ZnS@NC anode is boosted by the robust inorganic-rich solid electrolyte interphase (SEI) formed in cyclic and linear ether-based electrolytes. The ZnS@NC anode displayed a reversible specific capacity of 584 mAh g-1, an excellent rate capability of 327 mAh g-1 at 70 A g-1, and a highly stable cycling performance over 10000 cycles. This work provides a practical and promising approach to designing stable conversion anodes for high-performance sodium-ion batteries.

Weakly Solvating Few-Layer-Carbon Interface toward High Initial Coulombic Efficiency and Cyclability Hard Carbon Anodes

The carbonaceous anodes in sodium ion batteries suffer from low initial Coulombic efficiency (ICE) and poor cyclability due to rampant solid electrolyte interface (SEI) growth. The concept of the weakly solvating electrolyte (WSE) has been popularized for SEI regulation on the anode by adjusting the cation solvation structure. Nevertheless, the effects on the solvation sheath from the electrode/electrolyte interface are ignored in most WSE applications. In this work, we extend the WSE from the bulk electrolyte to the electrolyte/carbon interface. By recycling asphalt wastes into sp2 C enriched few-layer carbon on hard carbon, a weakly solvating interface is fabricated with lower adsorption energy to electrolyte solvent molecules than a pristine anode (-0.89 vs -1.08 eV for Na/diglyme). Accordingly, more anionic groups are attracted into the solvent-weakened solvation sheath during sodiation (2.30 vs 1.96 coordination number for PF6-). The anion-mediated contact ion pairs facilitate a thin, inorganic-rich SEI layer with a homogeneous distribution, which confers a high ICE of 97.9% and a high capacity of 335.6 mA h g-1 at 1 C (89.5% retention, 1000 cycles). The full battery also manifests an energy density of 209 W h kg-1. This interfacial design is applicable in both ether- and ester-based electrolytes, which is promising in cost-effective modification for carbonaceous electrodes.

Enhancing Sodium-Ion Energy Storage of Commercial Activated Carbon by Constructing Closed Pores via Ball Milling

Mechanical ball milling is a prevalent technology for material preparation and also serves as a post-treatment method to modify electrode materials, thus enhancing electrochemical performances. This study explores the microstructure modification of commercial activated carbon through mechanical ball milling, proving its efficacy in increasing sodium-ion energy storage. The evolution of activated carbon's physical and chemical properties during ball milling was systematically examined. It was observed that the quantity of closed pores and the graphitization degree in activated carbon increased with extended ball milling duration. The sodium storage mechanism in activated carbon transitions to an insertion-pore filling process, significantly elevating platform capacity. Additionally, ball-milled activated carbon demonstrates remarkable long-term cycling stability (92% capacity retention over 200 cycles at 200 mA g-1) and rate performance. This research offers a novel approach to developing advanced anode materials for sodium-ion batteries.

Potassium-Based Dual-Ion Batteries Operating at -60 °C Enabled By Co-Intercalation Anode Chemistry

Battery performance at subzero is restricted by sluggish interfacial kinetics. To resolve this issue, potassium-based dual-ion batteries (K-DIBs) based on the polytriphenylamine (PTPAn) cathode with anion storage chemistry and the hydrogen titanate (HTO) anode with K+ /solvent co-intercalation mechanism are constructed. Both the PTPAn cathode and the HTO anode do not undergo the desolvation process, which can effectively accelerate the interfacial kinetics at subzero. As revealed by theoretical calculations and experimental analysis, the strong K+ /solvent binding energy in the dilute electrolyte, the charge shielding effect of the crystal water, and the uniform SEI layer with high content of the flexible organic species synergically promote HTO to undergo K+ /solvent co-intercalation behavior. The special co-intercalation mechanism and anion storage chemistry enable HTO||PTPAn K-DIBs with superior rate performance and cycle durability, maintaining a capacity retention of 94.1% after 6000 cycles at -40 °C and 91% after 1000 cycles at -60 °C. These results provide a step forward for achieving high-performance energy storage devices at low temperatures.

Refined Electrolyte and Interfacial Chemistry toward Realization of High-Energy Anode-Free Rechargeable Sodium Batteries

Anode-free rechargeable sodium batteries represent one of the ultimate choices for the 'beyond-lithium' electrochemical storage technology with high energy. Operated based on the sole use of active Na ions from the cathode, the anode-free battery is usually reported with quite a limited cycle life due to unstable electrolyte chemistry that hinders efficient Na plating/stripping at the anode and high-voltage operation of the layered oxide cathode. A rational design of the electrolyte toward improving its compatibility with the electrodes is key to realize the battery. Here, we show that by refining the volume ratio of two conventional linear ether solvents, a binary electrolyte forms a cation solvation structure that facilitates flat, dendrite-free, planar growth of Na metal on the anode current collector and that is adaptive to high-voltage Na (de)intercalation of P2-/O3-type layered oxide cathodes and oxidative decomposition of the Na2C2O4 supplement. Inorganic fluorides, such as NaF, show a major influence on the electroplating pattern of Na metal and effective passivation of plated metal at the anode-electrolyte interface. Anode-free batteries based on the refined electrolyte have demonstrated high coulombic efficiency, long cycle life, and the ability to claim a cell-level specific energy of >300 Wh/kg.

Boosted Redox Kinetics Enabling Na3V2(PO4)3 with Excellent Performance at Low Temperature through Cation Substitution and Multiwalled Carbon Nanotube Cross-Linking

The open NASICON framework and high reversible capacity enable Na3V2(PO4)3 (NVP) to be a highly promising cathode candidate for sodium-ion batteries (SIBs). Nevertheless, the unsatisfied cyclic stability and degraded rate capability at low temperatures due to sluggish ionic migration and poor conductivity become the main challenges. Herein, excellent sodium storage performance for the NVP cathode can be received by partial potassium (K) substitution and multiwalled carbon nanotube (MWCNT) cross-linking to modify the ionic diffusion and electronic conductivity. Consequently, the as-fabricated Na3-xKxV2(PO4)3@C/MWCNT can maintain a capacity retention of 79.4% after 2000 cycles at 20 C. Moreover, the electrochemical tests at -20 °C manifest that the designed electrode can deliver 89.7, 73.5, and 64.8% charge of states, respectively, at 1, 2, and 3 C, accompanied with a capacity retention of 84.3% after 500 cycles at 20 C. Generally, the improved electronic conductivity and modified ionic diffusion kinetics resulting from K doping and MWCNT interconnecting endows the resultant Na3-xKxV2(PO4)3@C/MWCNT with modified electrochemical polarization and improved redox reversibility, contributing to superior performance at low temperatures. Generally, this study highlights the potential of alien substitution and carbon hybridization to improve the NASICON-type cathodes toward high-performance SIBs, especially at low temperatures.

Significantly Improving the Initial Coulombic Efficiency of TiO2 Anode for Sodium-Ion Batteries

Titanium dioxide (TiO2) can serve as a candidate anode material for sodium-ion batteries (SIBs) with the merits of their low cost, abundance, and environment friendliness. However, its low initial Coulombic efficiency (ICE) and sluggish sodium-ion diffusion greatly limit its further practical applications. Herein, we report a one-step prepotassiation strategy to modify commercial TiO2 by a spontaneous chemical reaction using potassium naphthalene (K-Nt). Prepotassiation effectively compensates for the irreversible Na loss and induces a homogeneous, dense, and robust artificial solid electrolyte interphase (SEI) on its surface. The well-distributed artificial SEI suppresses the excessive electrolyte decomposition, contributing to rapid interfacial kinetics and stable Na+ insertion/extraction. Therefore, such modified commercial TiO2 anodes demonstrate significantly improved ICE (72.4%) and outstanding rate performance (176.4 mAh g-1 at 5 A g-1). This simple and efficient method for promoting ICEs and interfacial chemistry also demonstrates universality and practical value for other anodes in SIBs.

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.

Toward High Performance Anodes for Sodium-Ion Batteries: From Hard Carbons to Anode-Free Systems

Sodium-ion batteries (SIBs) have been deemed to be a promising energy storage technology in terms of cost-effectiveness and sustainability. However, the electrodes often operate at potentials beyond their thermodynamic equilibrium, thus requiring the formation of interphases for kinetic stabilization. The interfaces of the anode such as typical hard carbons and sodium metals are particularly unstable because of its much lower chemical potential than the electrolyte. This creates more severe challenges for both anode and cathode interfaces when building anode-free cells to achieve higher energy densities. Manipulating the desolvation process through the nanoconfining strategy has been emphasized as an effective strategy to stabilize the interface and has attracted widespread attention. This Outlook provides a comprehensive understanding about the nanopore-based solvation structure regulation strategy and its role in building practical SIBs and anode-free batteries. Finally, guidelines for the design of better electrolytes and suggestions for constructing stable interphases are proposed from the perspective of desolvation or predesolvation.

Bioinspired design of Na-ion conduction channels in covalent organic frameworks for quasi-solid-state sodium batteries

Solid polymer electrolytes are considered among the most promising candidates for developing practical solid-state sodium batteries. However, moderate ionic conductivity and narrow electrochemical windows hinder their further application. Herein, inspired by the Na⁺/K⁺ conduction in biological membranes, we report a (-COO^-)-modified covalent organic framework (COF) as a Na-ion quasi-solid-state electrolyte with sub-nanometre-sized Na⁺ transport zones (6.7-11.6 Å) created by adjacent -COO^- groups and COF inwalls. The quasi-solid-state electrolyte enables selective Na⁺ transport along specific areas that are electronegative with sub-nanometre dimensions, resulting in a Na⁺ conductivity of 1.30×10^-4 S cm^-1 and oxidative stability of up to 5.32 V (versus Na⁺/Na) at 25 ± 1 °C. Testing the quasi-solid-state electrolyte in Na||Na₃V₂(PO₄)₃ coin cell configuration demonstrates fast reaction dynamics, low polarization voltages, and a stable cycling performance over 1000 cycles at 60 mA g^-1 and 25 ± 1 °C with a 0.0048% capacity decay per cycle and a final discharge capacity of 83.5 mAh g^-1.

High-Voltage Cyclic Ether-Based Electrolytes for Low-Temperature Sodium-Ion Batteries

Cyclic ethers are promising solvents for low-temperature electrolytes, but they still suffer from intrinsic poor antioxidant abilities. Until now, ether-based electrolytes have been rarely reported for high-voltage sodium-ion batteries (SIBs) operated under a low-temperature range. Herein, a novel ether-based electrolyte consisting of tetrahydrofuran as the main solvent is proposed and it could be utilized for a high-voltage Na2/3Mn2/3Ni1/3O2 (MN) cathode in a wide-temperature range from -40 to 25 °C. Meanwhile, a thin and robust inorganic component-rich cathode electrolyte interface layer is elaborately introduced on the MN cathode by this tailored electrolyte, resulting in excellent cycle life of MN cathode. Specifically, a capacity retention of 97.2% after 140 cycles could be delivered by MN at 0.3 C at room temperature (RT). Especially at an ultra-low temperature of -40 °C, the initial discharge capacity of MN could still approach 89.3% of that at RT, and the capacity retention is 94.1% at 0.2 C after 100 cycles. This work provides a new insight into the rational design of ether-based electrolytes for high-voltage and stable SIBs operated in a wide-temperature range.

Defect Engineering of Disordered Carbon Anodes with Ultra-High Heteroatom Doping Through a Supermolecule-Mediated Strategy for Potassium-Ion Hybrid Capacitors

Amorphous carbons are promising anodes for high-rate potassium-ion batteries. Most low-temperature annealed amorphous carbons display unsatisfactory capacities. Heteroatom-induced defect engineering of amorphous carbons could enhance their reversible capacities. Nevertheless, most lignocellulose biomasses lack heteroatoms, making it a challenge to design highly heteroatom-doped carbons (> 10 at%). Herein, we report a new preparation strategy for amorphous carbon anodes. Nitrogen/sulfur co-doped lignin-derived porous carbons (NSLPC) with ultra-high nitrogen doping levels (21.6 at% of N and 0.8 at% of S) from renewable lignin biomacromolecule precursors were prepared through a supramolecule-mediated pyrolysis strategy. This supermolecule/lignin composite decomposes forming a covalently bonded graphitic carbon/amorphous carbon intermediate product, which induces the formation of high heteroatom doping in the obtained NSLPC. This unique pyrolysis chemistry and high heteroatom doping of NSLPC enable abundant defective active sites for the adsorption of K+ and improved kinetics. The NSLPC anode delivered a high reversible capacity of 419 mAh g‒1 and superior cycling stability (capacity retention of 96.6% at 1 A g‒1 for 1000 cycles). Potassium-ion hybrid capacitors assembled by NSLPC anode exhibited excellent cycling stability (91% capacity retention for 2000 cycles) and a high energy density of 71 Wh kg-1 at a power density of 92 W kg-1.

Interfacial Chemistry Modulation via Amphoteric Glycine for a Highly Reversible Zinc Anode

Zn metal is thermodynamically unstable in aqueous electrolytes, which induces dendrite growth and ongoing parasitic reactions at the interface during the plating process and even during shelf time, resulting in rapid battery failure and hindering the practical application of aqueous Zn ion batteries. In this work, glycine, a common multifunctional additive, is utilized to modulate the solvation shell structure and enhance the interfacial stability to guard the reversibility and stability of the Zn anode. Apart from partially replacing the original SO₄^2- in the contact ion pair of Zn²⁺[H₂O]₅·OSO₃^2- complexes to suppress the formation of Zn₄(OH)₆SO₄·xH₂O byproducts at the interface, glycine molecules can also form a water-poor electrical double layer on the zinc metal surface during resting and be further reduced to build in situ a ZnS-rich solid electrolyte interphase (SEI) layer during cycling, which further suppresses side reactions and the random growth of Zn dendrites in the whole process. As expected, the cycle life of the symmetrical cells reaches over 3200 h in glycine-containing electrolytes. In addition, the Zn//NVO full cell shows exceptional cycling stability for 3000 cycles at 5 A g^-1. Given the low-cost superiority of glycine, the proposed strategy for interfacial chemistry modulation shows considerable potential in promoting the commercialization progress of aqueous batteries.

Balanced Interfacial Ion Concentration and Migration Steric Hindrance Promoting High-Efficiency Deposition/Dissolution Battery Chemistry

The solid-liquid transition reaction lays the foundation of electrochemical energy storage systems with high capacity, but realizing high efficiency remains a challenge. Herein, in terms of thermodynamics and dynamics, this work demonstrates the significant role of both interfacial H⁺ concentration and Mn²⁺ migration steric hindrance for the high-efficiency deposition/dissolution chemistry of zinc-manganese batteries. Specially, the introduction of formate anions can buffer the generated interfacial H⁺ to stabilize interfacial potential according to the Nernst equation, which stimulates high capacity. Compared with acetate and propionate anions, the formate anion also provides high adsorption density on the cathode surface to shield the electrostatic repulsion due to the small spatial hindrance. Particularly for the solvated Mn²⁺ , the formate-anion-induced lower energy barrier of the rate-determining step during the step-by-step desolvation process results in lower polarization and higher electrochemical reversibility. In situ tests and theoretical calculations verify that the electrolyte with formate anions achieve a good balance between ion concentration and ion-migration steric hindrance. It exhibits both the high energy density of 531.26 W h kg^-1  and long cycle life of more than 300 cycles without obvious decay.

Significance of Antisolvents on Solvation Structures Enhancing Interfacial Chemistry in Localized High-Concentration Electrolytes

Localized high-concentration electrolytes (LHCEs) provide a new way to expand multifunctional electrolytes because of their unique physicochemical properties. LHCEs are generated when high-concentration electrolytes (HCEs) are diluted by antisolvents, while the effect of antisolvents on the lithium-ion solvation structure is negligible. Herein, using one-dimensional infrared spectroscopy and theoretical calculations, we explore the significance of antisolvents in the model electrolyte lithium bis(fluorosulfonyl)imide/dimethyl carbonate (LiFSI/DMC) with hydrofluoroether. We clarify that the role of antisolvent is more than dilution; it is also the formation of a low-dielectric environment and intensification of the inductive effect on the C=O moiety of DMC caused by the antisolvent, which decrease the binding energy of the Li⁺···solvent and Li⁺···anion interactions. It also has beneficial effects on interfacial ion desolvation and Li⁺ transport. Furthermore, antisolvents also favor reducing the lowest unoccupied molecular orbital (LUMO) energy level of the solvated clusters, and FSI^- anions show a decreased reduction stability. Consequently, the influence of antisolvents on the interfacial chemical and electrochemical activities of solvation structures cannot be ignored. This finding introduces a new way to improve battery performance.

Recent advances for SEI of hard carbon anode in sodium-ion batteries: A mini review

The commercialization of sodium-ion batteries has been hampered by the anode’s performance. Carbon-based anodes have always had great application prospects, but traditional graphite anodes have great application limitations due to the inability of reversible insertion/de-insertion of sodium ions in them, while hard carbon materials have the high theoretical capacity, low reaction potential has received extensive attention in recent years. Nevertheless, the low first cycle Coulomb efficiency and rapid capacity decline of hard carbon materials limited its application. SEI has always played a crucial role in the electrochemical process. By controlling the formation of SEI, researchers have increased the efficiency of sodium-ion battery anodes, although the composition of SEI and how it evolved are still unknown. This paper briefly summarizes the research progress of hard carbon anode surface SEI in sodium-ion batteries in recent years. From the perspectives of characterization methods, structural composition, and regulation strategies is reviewed, and the future development directions of these three directions are suggested. The reference opinions are provided for the reference researchers.

Tailoring Nitrogen Terminals on MXene Enables Fast Charging and Stable Cycling Na-Ion Batteries at Low Temperature

Sodium-ion batteries stand a chance of enabling fast charging ability and long lifespan while operating at low temperature (low-T). However, sluggish kinetics and aggravated dendrites present two major challenges for anodes to achieve the goal at low-T. Herein, we propose an interlayer confined strategy for tailoring nitrogen terminals on Ti₃C₂ MXene (Ti₃C₂-N_funct) to address these issues. The introduction of nitrogen terminals endows Ti₃C₂-N_funct with large interlayer space and charge redistribution, improved conductivity and sufficient adsorption sites for Na⁺, which improves the possibility of Ti₃C₂ for accommodating more Na atoms, further enhancing the Na⁺ storage capability of Ti₃C₂. As revealed, Ti₃C₂-N_funct not only possesses a lower Na-ion diffusion energy barrier and charge transfer activation energy, but also exhibits Na⁺-solvent co-intercalation behavior to circumvent a high de-solvation energy barrier at low-T. Besides, the solid electrolyte interface dominated by inorganic compounds is more beneficial for the Na⁺ transfer at the electrode/electrolyte interface. Compared with of the unmodified sample, Ti₃C₂-N_funct exhibits a twofold capacity (201 mAh g^-1), fast-charging ability (18 min at 80% capacity retention), and great superiority in cycle life (80.9%@5000 cycles) at - 25 °C. When coupling with Na₃V₂(PO₄)₂F₃ cathode, the Ti₃C₂-N_funct//NVPF exhibits high energy density and cycle stability at - 25 °C.