Marriage of an Ether-Based Electrolyte with Hard Carbon Anodes Creates Superior Sodium-Ion Batteries with High Mass Loading

Consummating ion desolvation in hard carbon anodes for reversible sodium storage

Hard carbons are emerging as the most viable anodes to support the commercialization of sodium-ion (Na-ion) batteries due to their competitive performance. However, the hard carbon anode suffers from low initial Coulombic efficiency (ICE), and the ambiguous Na-ion (Na+) storage mechanism and interfacial chemistry fail to give a reasonable interpretation. Here, we have identified the time-dependent ion pre-desolvation on the nanopore of hard carbons, which significantly affects the Na+ storage efficiency by altering the solvation structure of electrolytes. Consummating the pre-desolvation by extending the aging time, generates a highly aggregated electrolyte configuration inside the nanopore, resulting in negligible reductive decomposition of electrolytes. When applying the above insights, the hard carbon anodes achieve a high average ICE of 98.21% in the absence of any Na supplementation techniques. Therefore, the negative-to-positive capacity ratio can be reduced to 1.02 for full cells, which enables an improved energy density. The insight into hard carbons and related interphases may be extended to other battery systems and support the continued development of battery technology.

Facilitating Rapid Na+ Storage through MoWSe/C Heterostructure Construction and Synergistic Electrolyte Matching Strategy

The realization of sodium-ion devices with high-power density and long-cycle capability is challenging due to the difficulties of carrier diffusion and electrode fragmentation in transition metal selenide anodes. Herein, a Mo/W-based metal-organic framework is constructed by a one-step method through rational selection, after which MoWSe/C heterostructures with large angles are synthesized by a facile selenization/carbonization strategy. Through physical characterization and theoretical calculations, the synthesized MoWSe/C electrode delivers obvious structural advantages and excellent electrochemical performance in an ethylene glycol dimethyl ether electrolyte. Furthermore, the electrochemical vehicle mechanism of ions in the electrolyte is systematically revealed through comparative analyses. Resultantly, ether-based electrolytes advantageously construct stable solid electrolyte interfaces and avoid electrolyte decomposition. Based on the above benefits, the Na half-cell assembled with MoWSe/C electrodes demonstrated excellent rate capability and a high specific capacity of 347.3 mA h g-1 even after cycling 2000 cycles at 10 A g-1. Meanwhile, the constructed sodium-ion capacitor maintains ∼80% capacity retention after 11,000 ultralong cycles at a high-power density of 3800 W kg-1. The findings can broaden the mechanistic understanding of conversion anodes in different electrolytes and provide a reference for the structural design of anodes with high capacity, fast kinetics, and long-cycle stability.

Recent Progress in Improving Rate Performance of Cellulose-Derived Carbon Materials for Sodium-Ion Batteries

Cellulose-derived carbon is regarded as one of the most promising candidates for high-performance anode materials in sodium-ion batteries; however, its poor rate performance at higher current density remains a challenge to achieve high power density sodium-ion batteries. The present review comprehensively elucidates the structural characteristics of cellulose-based materials and cellulose-derived carbon materials, explores the limitations in enhancing rate performance arising from ion diffusion and electronic transfer at the level of cellulose-derived carbon materials, and proposes corresponding strategies to improve rate performance targeted at various precursors of cellulose-based materials. This review also presents an update on recent progress in cellulose-based materials and cellulose-derived carbon materials, with particular focuses on their molecular, crystalline, and aggregation structures. Furthermore, the relationship between storage sodium and rate performance the carbon materials is elucidated through theoretical calculations and characterization analyses. Finally, future perspectives regarding challenges and opportunities in the research field of cellulose-derived carbon anodes are briefly highlighted.

BF4- Tailoring Solvation Chemistry of Ether-Based Electrolytes to Construct Stable Electrode/Electrolyte Interfaces for Sodium-Ion Full Batteries

The ether-based electrolytes show excellent performance on anodes in sodium-ion batteries (SIBs), but they still show poor compatibility with the cathodes. Here, ether electrolytes with NaBF4 as the main salt or additive were applied in NFM//HC full cells and showed enhanced performance than the electrolyte with NaPF6. Then, BF4- was found to have a stronger interaction with Na+, which could reduce the solvation of Na+ with the solvent, thus inducing the formation of the cathode electrolyte interface (CEI) and solid electrolyte interface (SEI) layers rich in inorganic species. Moreover, the morphology, structure, composition, and solubility of CEI and SEI were explored, concluding that NaBF4 could induce more stable CEI and SEI layers rich in B-containing species and inorganics. This work proposes using NaBF4 as the main salt or additive to improve the performance of ether electrolytes in NFM//HC full cells, which provides a strategy to improve the compatibility of ether-based electrolytes and cathodes.

Research progress of organic liquid electrolyte for sodium ion battery

Electrochemical energy storage technology has attracted widespread attention due to its low cost and high energy efficiency in recent years. Among the electrochemical energy storage technologies, sodium ion batteries have been widely focused due to the advantages of abundant sodium resources, low price and similar properties to lithium. In the basic structure of sodium ion battery, the electrolyte determines the electrochemical window and electrochemical performance of the battery, controls the properties of the electrode/electrolyte interface, and affects the safety of sodium ion batteries. Organic liquid electrolytes are widely used because of their low viscosity, high dielectric constant, and compatibility with common cathodes and anodes. However, there are problems such as low oxidation potential, high flammability and safety hazards. Therefore, the development of novel, low-cost, high-performance organic liquid electrolytes is essential for the commercial application of sodium ion batteries. In this paper, the basic requirements and main classifications of organic liquid electrolytes for sodium ion batteries have been introduced. The current research status of organic liquid electrolytes for sodium ion batteries has been highlighted, including compatibility with various types of electrodes and electrochemical properties such as multiplicative performance and cycling performance of electrode materials in electrolytes. The composition, formation mechanism and regulation strategies of interfacial films have been explained. Finally, the development trends of sodium ion battery electrolytes in terms of compatibility with materials, safety and stable interfacial film formation are pointed out in the future.

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.

Recent progress and strategic perspectives of inorganic solid electrolytes: fundamentals, modifications, and applications in sodium metal batteries

Solid-state electrolytes (SEs) have attracted overwhelming attention as a promising alternative to traditional organic liquid electrolytes (OLEs) for high-energy-density sodium-metal batteries (SMBs), owing to their intrinsic incombustibility, wider electrochemical stability window (ESW), and better thermal stability. Among various kinds of SEs, inorganic solid-state electrolytes (ISEs) stand out because of their high ionic conductivity, excellent oxidative stability, and good mechanical strength, rendering potential utilization in safe and dendrite-free SMBs at room temperature. However, the development of Na-ion ISEs still remains challenging, that a perfect solution has yet to be achieved. Herein, we provide a comprehensive and in-depth inspection of the state-of-the-art ISEs, aiming at revealing the underlying Na⁺ conduction mechanisms at different length scales, and interpreting their compatibility with the Na metal anode from multiple aspects. A thorough material screening will include nearly all ISEs developed to date, i.e., oxides, chalcogenides, halides, antiperovskites, and borohydrides, followed by an overview of the modification strategies for enhancing their ionic conductivity and interfacial compatibility with Na metal, including synthesis, doping and interfacial engineering. By discussing the remaining challenges in ISE research, we propose rational and strategic perspectives that can serve as guidelines for future development of desirable ISEs and practical implementation of high-performance SMBs.

Electrolyte Engineering on Performance Enhancement of NiCo2 S4 Anode for Sodium Storage

NiCo₂ S₄ is an attractive anode for sodium-ion batteries (SIBs) due to its high capacity and excellent redox reversibility. Practical deployment of NiCo₂ S₄ electrode in SIBs, however, is still hindered by the inferior capacity and unsatisfactory cycling performance, which result from the mismatch between the electrolyte chemistry and electrode. Herein, a functional electrolyte containing 1.0 m NaCF₃ SO₃ in diethylene glycol dimethyl ether (DEGDME) (1.0 m NaCF₃ SO₃ -DEGDME) is developed, which can be readily used for NiCo₂ S₄ anode with high initial coulomb efficiency (96.2%), enhanced cycling performance, and boosted capacities (341.7 mA h g^-1 after 250 continuous cycles at the current density of 200 mA g^-1 ). The electrochemical tests and related phase characterization combined with density functional theory (DFT) calculation indicate the ether-based electrolyte is more suitable for the NiCo₂ S₄ anode in SIBs due to the formation of a stable electrode-electrolyte interface. Additionally, the importance of the voltage window is also demonstrated to further optimize the electrochemical performance of the NiCo₂ S₄ electrode. The formation of sulfide intermediates during charging and discharging is predicted by combining DFT and verified by in situ XRD and HRTEM. The findings indicate that electrolyte engineering would be an effective way of performance enhancement for sulfides in practical SIBs.

Step-by-step desolvation enables high-rate and ultra-stable sodium storage in hard carbon anodes

Hard carbon is regarded as the most promising anode material for sodium-ion (Na-ion) batteries, owing to its advantages of high abundance, low cost, and low operating potential. However, the rate capability and cycle life span of hard carbon anodes are far from satisfactory, severely hindering its industrial applications. Here, we demonstrate that the desolvation process defines the Na-ion diffusion kinetics and the formation of a solid electrolyte interface (SEI). The 3A zeolite molecular sieve film on the hard carbon is proposed to develop a step-by-step desolvation pathway that effectively reduces the high activation energy of the direct desolvation process. Moreover, step-by-step desolvation yields a thin and inorganic-dominated SEI with a lower activation energy for Na⁺ transport. As a result, it contributes to greatly improved power density and cycling stability for both ester and ether electrolytes. When the above insights are applied, the hard carbon anode achieves the longest life span and minimum capacity fading rate at all evaluated current densities. Moreover, with the increase in current densities, an improved plateau capacity ratio is observed. This step-by-step desolvation strategy comprehensively enhances various properties of hard carbon anodes, which provides the possibility of building practical Na-ion batteries with high power density, high energy density, and durability.

Tailoring the surface chemistry of hard carbon towards high-efficiency sodium ion storage

Hard carbon (HC) is most likely to be a commercialized anode material for sodium-ion batteries (SIBs). However, its low initial coulombic efficiency (ICE) impedes its further large-scale industrialization. Since the ICE is greatly related to the side reactions of the electrolyte on the HC surface, herein, we focus on tailoring the surface chemistry of HC via a facile low-temperature oxygen plasma (LTOP) treatment technique. The modified HC after a suitable treatment time possesses a highly ordered and low defect surface without a negligible change in layer spacing, thus facilitating Na⁺ deinsertion/insertion and reducing the HC/electrolyte side reactions. Moreover, LTOP treatment also brings oxygen functional groups (CO) to the HC surface to enrich Na⁺ storage active sites. Consequently, the modified HC reveals a higher ICE of 80.9% compared to 60.6% in the bare HC. Also, the modified HC delivers an ultrahigh specific capacity of 331.0 mA h g^-1 at 0.1 A g^-1 and exhibits superior rate performance with a high specific capacity of 211.0 mA h g^-1 at 5 A g^-1. This work provides a feasible strategy to tailor the surface chemistry of HC for high-efficiency Na-storage and provides a novel avenue to construct high-efficiency SIBs.

Ether-based electrolytes for sodium ion batteries

Sodium-ion batteries (SIBs) are considered to be strong candidates for large-scale energy storage with the benefits of cost-effectiveness and sodium abundance. Reliable electrolytes, as ionic conductors that regulate the electrochemical reaction behavior and the nature of the interface and electrode, are indispensable in the development of advanced SIBs with high Coulombic efficiency, stable cycling performance and high rate capability. Conventional carbonate-based electrolytes encounter numerous obstacles for their wide application in SIBs due to the formation of a dissolvable, continuous-thickening solid electrolyte interface (SEI) layer and inferior stability with electrodes. Comparatively, ether-based electrolytes (EBEs) are emerging in the secondary battery field with fascinating properties to improve the performance of batteries, especially SIBs. Their stable solvation structure enables highly reversible solvent-co-intercalation reactions and the formation of a thin and stable SEI. However, although EBEs can provide more stable cycling and rapid sodiation kinetics in electrodes, benefitting from their favorable electrolyte/electrode interactions such as chemical compatibility and good wettability, their special chemistry is still being investigated and puzzling. In this review, we provide a thorough and comprehensive overview on the developmental history, fundamental characteristics, superiorities and mechanisms of EBEs, together with their advances in other battery systems. Notably, the relation among electrolyte science, interfacial chemistry and electrochemical performance is highlighted, which is of great significance for the in-depth understanding of battery chemistry. Finally, future perspectives and potential directions are proposed to navigate the design and optimization of electrolytes and electrolyte/electrode interfaces for advanced batteries.

Anisotropic electrene T'-Ca2P with electron gas magnetic coupling as anode material for Na/K ion batteries

There is an urgent need for high-performance rechargeable electrical storage devices as a supplement or a substitution for lithium ion batteries (LIBs) due to the shortage of lithium in nature. Herein we propose a stable 2D electrene T'-Ca₂P as an anode material for Na/K ion batteries developed using first principles calculations. Our calculated results show that the T'-Ca₂P monolayer is an antiferromagnetic semiconducting electrene with a spin-polarized electron gas. It exhibits suitable adsorption for both Na and K atoms, and its anisotropic migration energy barriers are 0.050/0.101 eV and 0.037/0.091 eV in the b/a direction, respectively. The theoretical capacities for Na and K are both 482 MA h g^-1, whereas the average working voltage platforms are 0.171-0.226 V and 0.013-0.267 V, respectively. All the results reveal that the T'-Ca₂P monolayer has promising prospects for application as an anode material for Na/K ion batteries.

Modulating the Graphitic Domains of Hard Carbons Derived from Mixed Pitch and Resin to Achieve High Rate and Stable Sodium Storage

Resin derived hard carbons (HCs) generally demonstrate remarkable electrochemical performance for both sodium ion batteries (SIBs) and potassium-ion batteries (KIBs), but their practical applications are hindered by their high price and high temperature pyrolysis (≈1500 °C). Herein, low-cost pitch is coated on the resin surface to compromise the cost, and meanwhile manipulate the microstructure at a relatively low pyrolysis temperature (1000 °C). HC-0.2P-1000 has a large number of short graphitic layer structures and a relatively large interlayer spacing of 0.3743 nm, as well as ≈1 nm sized nanopores suitable for sodium storage. Consequently, the as produced material demonstrates a superior reversible capacity (349.9 mAh g^-1 for SIBs and 321.9 mAh g^-1 for KIBs) and excellent rate performance (145.1 mAh g^-1 at 20 A g^-1 for SIBs, 48.5 mAh g^-1 at 20 A g^-1 for KIBs). Furthermore, when coupled with Na₃ V₂ (PO₄ )₃ as cathode, the full cell exhibits a high energy density of 251.1 Wh kg^-1 and excellent stability with a capacity retention of 73.3% after 450 cycles at 1 A g^-1 .

Constructing a Stable Si-N-Enriched Interface Boosts Lithium Storage Kinetics in a Silicon-Based Anode

The stable operation of a SiOx anode largely depends on the intrinsic chemistry of the electrode/electrolyte interface; however, an unstable interface structure and undesirable parasitic reactions with the electrolyte of the SiOx anode often result in the formation of a fragile solid-electrolyte interphase (SEI) and serious capacity decay during the lithiation/delithiation process. Herein, a Si-N-enriched N-doped carbon coating is constructed on the surface of SiOx yolk-shell nanospheres (abbreviated as SiOx@NC) to optimize the SEI film. The two-dimensional covalently bound Si-N interface, on one hand, can suppress the interfacial reactivity of the SiOx anode to enable the formation of a thin SEI film with accelerated diffusion kinetics of ions and, on the other hand, acts as a Li+ conductor during the delithiation process, allowing Li+ to diffuse rapidly in the SiOx matrix, thereby improving the long-term cycling stability and rapid charge/discharge capability of the SiOx anode. A series of characterizations show that the interface charge-transfer barrier and the Li+ diffusion energy barrier through the SEI film are the main factors that determine the interfacial electrochemical behavior and lithium storage performance. This work clarifies the relationship between the SEI characteristics and the interfacial transfer dynamics and aims to offer a more basic basis for the screening of other electrode materials.

Impact of Electrolyte Salts on Na Storage Performance for High-Surface-Area Carbon Anodes

High-surface-area carbon (HSAC) has been regarded as one of the most promising anode materials for sodium-ion batteries. However, it generally suffers from low initial Coulombic efficiency (ICE), which is closely related to the formation process of a solid electrolyte interface (SEI). Herein, the impact of different electrolyte salts on the electrochemical performance and SEI formation of a commercial HSAC anode is studied. It is found that the use of NaCF₃SO₃ enables much higher ICE (69.28%) and reversible capacity (283 mA h g^-1) of the HSAC anode compared with the NaPF₆ electrolyte (59.65%, 243 mA h g^-1). Through comprehensive characterizations, the improvement in electrochemical performance facilitated by NaCF₃SO₃ could be attributed to the reduced amount of Na_xC and the thinner SEI formed on the surface of HSAC during the initial cycle, which not only provides extra active sites for Na⁺ storage but also contributes to the promoted ICE. This work not only provides a deeper understanding of the role of electrolyte salt in SEI formation in the HSAC anode but also proposes a new method to further promote the ICE of the HSAC anode in sodium-ion batteries.

Boosting Sodium Storage Performance of Hard Carbon Anodes by Pore Architecture Engineering

Hard carbon (HC) displays great potential for high-performance sodium-ion batteries (SIBs) due to its cost-effective, simple fabrication and most likely to be commercialized. However, the complicated microstructures of HC lead to difficulties in deeply understanding the structure-performance correlation. Particularly, evaluation of influence of pore structure on Na storage performances is still causing disputes and rational strategies of designing pore architecture of HC are still necessary. In this work, the skillful and controllable phase-inversion method is applied to construct porous HC with abundantly interconnected and permeable tunnel-like pores, which can promote ionic diffusion and improve electrode-electrolyte interfacial affinity. Structure-performance investigation reveals that porous HC with cross-coupled macropore architecture can boost Na storage performances comprehensively. Compared to pristine HC with negligible pores, well-regulated porous HC anodes show an obvious enhancement on initial Coulombic efficiency (ICE) of 68.3% (only 51.5% for pristine HC), reversible capacity of 332.7 mAh g^-1 at 0.05 A g^-1, rate performance with 67.4% capacity retention at 2 A g^-1 (46.5% for pristine HC), and cycling stability with 95% capacity maintained for 90 cycles (86.4% for pristine HC). Additionally, the ICE can be optimized up to 76% by using sodium carboxymethyl cellulose as a binder. This work provides an important view of optimizing Na storage performances of HC anodes by pore engineering, which can be broadened into other electrode materials.

Tremella-like Hydrated Vanadium Oxide Cathode with an Architectural Design Strategy toward Ultralong Lifespan Aqueous Zinc-Ion Batteries

Rechargeable aqueous zinc-ion batteries (ZIBs) are promising systems for energy storage due to their operational safety, low cost, and environmental friendliness. However, the development of suitable cathode materials is plagued by the sluggish dynamics of Zn²⁺ with strong electrostatic interaction. Herein, an Al³⁺-doped tremella-like layered Al_0.15V₂O₅·1.01H₂O (A-VOH) cathode material with a large pore diameter and high specific surface area is demonstrated to greatly boost electrochemical performance as ZIB cathodes. Resultant ZIBs with a 3 M Zn(CF₃SO₃)₂ electrolyte deliver a high specific discharge capacity of 510.5 mAh g^-1 (0.05 A g^-1), and an excellent energy storage performance is well maintained with a specific capacity of 144 mAh g^-1 (10 A g^-1) even after ultralong 10,000 cycles. The decent electrochemical performance roots in the novel tremella-like structure and the interlayer of Al³⁺ ions and water molecules, which could improve the electrochemical reaction kinetics and structural long cycle stability. Furthermore, the assembled coin-type cells could power a light-emitting diode (LED) lamp for 2 days. We believed that the design philosophy of unique morphology with abundant active sites for Zn²⁺ storage will boost the development of competitive cathodes for high-performance aqueous batteries.

Elucidating the Mechanism of Fast Na Storage Kinetics in Ether Electrolytes for Hard Carbon Anodes

The sodium storage performance of a hard carbon (HC) anode in ether electrolytes exhibits a higher initial Coulombic efficiency (ICE) and better rate performance compared to conventional ester electrolytes. However, the mechanism behind faster Na storage kinetics for HC in ether electrolytes remains unclear. Herein, a unique solvated Na⁺ and Na⁺ co-intercalation mechanism in ether electrolytes is reported using designed monodispersed HC nanospheres. In addition, a thin solid electrolyte interphase film with a high inorganic proportion formed in an ether electrolyte is visualized by cryo transmission electron microscopy and depth-profiling X-ray photoelectron spectroscopy, which facilitates Na⁺ transportation, and results in a high ICE. Furthermore, the fast solvated Na⁺ diffusion kinetics in ether electrolytes are also revealed via molecular dynamics simulation. Owing to the contribution of the ether electrolytes, an excellent rate performance (214 mAh g^-1 at 10 A g^-1 with an ultrahigh plateau capacity of 120 mAh g^-1 ) and a high ICE (84.93% at 1 A g^-1 ) are observed in a half cell; in a full cell, an attractive specific capacity of 110.3 mAh g^-1 is achieved after 1000 cycles at 1 A g^-1 .

Revisit Electrolyte Chemistry of Hard Carbon in Ether for Na Storage

Hard carbons (HCs) as an anode material in sodium ion batteries present enhanced electrochemical performances in ether-based electrolytes, giving them potential for use in practical applications. However, the underlying mechanism behind the excellent performances is still in question. Here, ex situ nuclear magnetic resonance, gas chromatography-mass spectrometry, and high-resolution transmission electron microscopy were used to clarify the insightful chemistry of ether- and ester-based electrolytes in terms of the solid-electrolyte interphase (SEI) on hard carbons. The results confirm the marked electrolyte decomposition and the formation of a SEI film in EC/DEC but no SEI film in the case of diglyme. In situ electrochemical quartz crystal microbalance and molecular dynamics support that ether molecules have likely been co-intercalated into hard carbons. To our knowledge, these results are reported for the first time. It might be very useful for the rational design of advanced electrode materials based on HCs in the future.

Two-Dimensional Organic/Inorganic Hybrid Nanosheet Electrodes for Enhanced Electrical Conductivity toward Stable and High-Performance Sodium-Ion Batteries

To investigate the effect of electrical conductivity on the energy-storage characteristics of anode materials in sodium-ion batteries, covalent organic nanosheets (CONs) are hybridized with highly conductive graphene nanosheets (GNs) via two different optimized synthesis routes, that is, reflux and solvothermal methods. The reflux-synthesized hybrid shows a well-overlapped 2D structure, whereas the solvothermally prepared hybrid forms a segregated phase in which the contact area between the CONs and GNs is reduced. These two hybrids synthesized by facile methods are fully characterized, and the results reveal that their energy-storage properties can be significantly improved by enhancing the electrical conductivity via the formation of a well-overlapped structure between CONs and GNs. The discharge capacity and rate capability of the reflux-synthesized hybrid was considerably larger than that of the bare CONs, highlighting that the improvement in the charge-carrier transport properties can improve the accessibility of Na ions to the surface of the hybrids. This synthetic methodology can be extended to the fabrication of high-performance anodes for Na-ion batteries.

Insights on Electrochemical Behaviors of Sodium Peroxide as a Sacrificial Cathode Additive for Boosting Energy Density of Na-Ion Battery

The development of Na-ion full cells (NIFCs) suffers from the issue that the solid electrolyte interphase formation on the carbon anode consumes the limited sodium from cathode and thus incurs the decreased energy density and poor cyclic stability. To address these issues, we herein report that Na₂O₂ could be used as a sacrificial Na source through spraying its slurry on the surface of cathode, and investigate its stability as well as electrochemical behavior toward NIFCs. The results show that Na₂O₂ has good chemical and storage stability under a dry atmosphere and has no negative effect on the electrochemical performance of the cathode. Compared with the pristine cathode, the Na₂O₂-decorated cathode exhibits higher discharge capacity, superior capacity retention, and rate capability in a full cell with a carbon anode. Our cathode Na compensation strategy provides an effective avenue to make up for the irreversible Na⁺ loss cause by the formation of solid electrolyte interphase on the anode, thereby promoting the electrochemical performance and energy density of NIFCs toward the large-scale application.

Surface Charge Engineering for Covalently Assembling Three-Dimensional MXene Network for All-Climate Sodium Ion Batteries

MXenes, as excellent candidate anode materials for sodium ion batteries (SIBs), suffer from sluggish ion-diffusion kinetics resulting from the anchoring effect of the negatively charged functional groups on their surface on sodium ions. Herein, we introduce positively charged conductive polyaniline (PANI) to induce self-assembly of Ti₃C₂T_x MXenes into a three-dimensional PANI/Ti₃C₂T_x network. In this PANI/Ti₃C₂T_x network, PANI not only intercalates into Ti₃C₂T_x nanosheets to enlarge the interlayer spacing, but also promotes negative-to-positive transition of the surface charges of the Ti₃C₂T_x nanosheets, significantly improving ion-diffusion kinetics. Electrochemical test results further confirm the superb ion-diffusion kinetics of the PANI/Ti₃C₂T_x network. Meanwhile, a covalent interaction (Ti-N) between PANI and Ti₃C₂T_x, proved by X-ray photoelectron spectra (XPS) and X-ray absorption near-edge structure (XANES) tests, plays a key role in stabilizing this network structure. Therefore, PANI/Ti₃C₂T_x exhibits excellent sodium storage performances with a high specific capacity, superior rate performance and ultralong lifespan at high current density. More importantly, when operated at rigorous temperatures from +50 to -30 °C, PANI/Ti₃C₂T_x also exhibits good electrochemical performances. The present work presents a simple strategy for designing 3D porous MXene-based materials to realize high rate performance and all-climate energy storage device.

Engineering Surface Oxygenated Functionalities on Commercial Carbon toward Ultrafast Sodium Storage in Ether-Based Electrolytes

The pursuit of a high-capacity anode material has been urgently required for commercializing sodium-ion batteries with a high energy density and an improved working safety. In the absence of thermodynamically stable sodium intercalated compounds with graphite, constructing nanostructures with expanded interlayer distances is still the mainstream option for developing high-performance carbonaceous anodes. In this regard, a surface-functionalized and pore-forming strategy through a facile CO₂ thermal etching route was rationally adopted to engineer negligible oxygenated functionalities on commercial carbon for boosting the sodium storage process. Benefitted from the abundant ionic/electronic pathways and more active reaction sites in the microporous structure with noticeable pseudocapacitive behaviors, the functionalized porous carbon could achieve a highly reversible capacity of 505 mA h g^-1 at 50 mA g^-1, an excellent rate performance of 181 mA h g^-1 at 16,000 mA g^-1, and an exceptional rate cycle stability of 176 mA h g^-1 at 3200 mA g^-1 over 1000 cycles. These outstanding electrochemical properties should be ascribed to a synergistic mechanism, fully utilizing the graphitic and amorphous structures for synchronous intercalations of sodium ions and solvated sodium ion compounds, respectively. Additionally, the controllable generation and evolution of a robust but thin solid electrolyte interphase film with the emergence of obvious capacitive reactions on the defective surface, favoring the rapid migration of sodium ions and solvated species, also contribute to a remarkable electrochemical performance of this porous carbon black.