Electrolytes, SEI Formation, and Binders: A Review of Nonelectrode Factors for Sodium-Ion Battery Anodes

Chloride-Ion-Enriched Solid Electrolyte Interphase with Rapid Na+ Migration toward High-Performance Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have emerged as potential alternatives to lithium-ion batteries (LIBs), particularly for large-scale applications. Alloy-type anode materials for sodium-ion batteries are esteemed as prospective candidate materials for sodium-ion anodes, owing to their elevated theoretical capacity, heightened utilization efficiency, and minimal production of insulating byproducts. However, the severe volume changes and sluggish ion diffusion kinetics can lead to irreversible particle fragmentation and reaggregation phenomena, ultimately resulting in electrode degradation. Additionally, repetitive volume changes can cause an unstable solid electrolyte interphase (SEI). This study presents the synthesis of chloride-ion-modulated bimetallic SnSb/C nanoparticle anode materials, highlighting the following advantages: (i) Designing a bimetallic SnSb alloy structure serves to buffer the structural stresses generated during sodium insertion/extraction processes, effectively mitigating particle fracture phenomena induced by electrode material expansion/contraction. (ii) Nanostructuring both alloy materials enables the full utilization of active materials and shortens diffusion pathways, thereby significantly enhancing the diffusion rate of sodium ions. (iii) Introducing a carbonaceous matrix serves to alleviate self-agglomeration phenomena of the material during charge/discharge cycles, enhancing the material's conductivity and structural stability. (iv) Utilizing chloride-ion interface modification to achieve a chloride-rich solid-electrolyte interphase (SEI) enhances battery performance.

Coupling Liquid Electrochemical TEM and Mass-Spectrometry to Investigate Electrochemical Reactions Occurring in a Na-Ion Battery Anode

A novel approach for investigating the formation of solid electrolyte interphase (SEI) in Na-ion batteries (NIB) through the coupling of in situ liquid electrochemical transmission electron microscopy (ec-TEM) and gas-chromatography mass-spectrometry (GC/MS) is proposed. To optimize this coupling, experiments are conducted on the sodiation of hard carbon materials (HC) using two setups: in situ ec-TEM holder and ex situ setup. Electrolyte (NP30) is intentionally degraded using cyclic voltammetry (CV), and the recovered liquid product is analyzed using GC/MS. Solid product (µ-chip) is analyzed using TEM techniques in a post-mortem analysis. The ex situ experiments served as a reference to for insertion of Na+ ions in the HC, SEI size (389 nm), SEI composition (P, Na, F, and O), and Na plating. The in situ TEM analysis reveals a cyclability limitation, this issue appears to be caused by the plating of Na in the form of a "foam" structure, resulting from the gas release during the reaction of Na with DMC/EC electrolyte. The foam structure, subsequently transformes into a second SEI, is electrochemically inactive and reduces the cyclability of the battery. Overall, the results demonstrate the powerful synergy achieved by coupling in situ ec-TEM and GC/MS techniques.

(De)sodiation Mechanism of Bi2MoO6 in Na-Ion Batteries Probed by Quasi-Simultaneous Operando PDF and XAS

Operando characterization can reveal degradation processes in battery materials and are essential for the development of battery chemistries. This study reports the first use of quasi-simultaneous operando pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) of a battery cell, providing a detailed, atomic-level understanding of the cycling mechanism of Bi2MoO6 as an anode material for Na-ion batteries. This material cycles via a combined conversion-alloying reaction, where electrochemically active, nanocrystalline Na x Bi particles embedded in an amorphous Na-Mo-O matrix are formed during the first sodiation. The combination of operando PDF and XAS revealed that Bi obtains a positive oxidation state at the end of desodiation, due to formation of Bi-O bonds at the interface between the Bi particles and the Na-Mo-O matrix. In addition, XAS confirmed that Mo has an average oxidation state of +6 throughout the (de)sodiation process and, thus, does not contribute to the capacity. However, the local environment of Mo6+ changes from tetrahedral coordination in the desodiated state to distorted octahedral in the sodiated state. These structural changes are linked to the poor cycling stability of Bi2MoO6, as flexibility of this matrix allows movement and coalescence of the Na x Bi particles, which is detrimental to the electrochemical stability.

Electrolytes for Sodium Ion Batteries: The Current Transition from Liquid to Solid and Hybrid systems

Sodium-ion batteries (NIBs) have recently garnered significant interest in being employed alongside conventional lithium-ion batteries, particularly in applications where cost and sustainability are particularly relevant. The rapid progress in NIBs will undoubtedly expedite the commercialization process. In this regard, tailoring and designing electrolyte formulation is a top priority, as they profoundly influence the overall electrochemical performance and thermal, mechanical, and dimensional stability. Moreover, electrolytes play a critical role in determining the system's safety level and overall lifespan. This review delves into recent electrolyte advancements from liquid (organic and ionic liquid) to solid and quasi-solid electrolyte (dry, hybrid, and single ion conducting electrolyte) for NIBs, encompassing comprehensive strategies for electrolyte design across various materials, systems, and their functional applications. The objective is to offer strategic direction for the systematic production of safe electrolytes and to investigate the potential applications of these designs in real-world scenarios while thoroughly assessing the current obstacles and forthcoming prospects within this rapidly evolving field.

Recent advances in rational design for high-performance potassium-ion batteries

The growing global energy demand necessitates the development of renewable energy solutions to mitigate greenhouse gas emissions and air pollution. To efficiently utilize renewable yet intermittent energy sources such as solar and wind power, there is a critical need for large-scale energy storage systems (EES) with high electrochemical performance. While lithium-ion batteries (LIBs) have been successfully used for EES, the surging demand and price, coupled with limited supply of crucial metals like lithium and cobalt, raised concerns about future sustainability. In this context, potassium-ion batteries (PIBs) have emerged as promising alternatives to commercial LIBs. Leveraging the low cost of potassium resources, abundant natural reserves, and the similar chemical properties of lithium and potassium, PIBs exhibit excellent potassium ion transport kinetics in electrolytes. This review starts from the fundamental principles and structural regulation of PIBs, offering a comprehensive overview of their current research status. It covers cathode materials, anode materials, electrolytes, binders, and separators, combining insights from full battery performance, degradation mechanisms, in situ/ex situ characterization, and theoretical calculations. We anticipate that this review will inspire greater interest in the development of high-efficiency PIBs and pave the way for their future commercial applications.

A Naphthalenetetracarboxdiimide-Containing Covalent Organic Polymer: Preparation, Single Crystal Structure and Battery Application

Inspired by dative boron-nitrogen (B←N) bonds proven to be the promising dynamic linkage for the construction of crystalline covalent organic polymers/frameworks (COPs/COFs), we employed 1,4-bis(benzodioxaborole) benzene (BACT) and N,N'-Di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxdiimide (DPNTCDI) as the corresponding building blocks to construct a functional COP (named as CityU-25), which had been employed as an anode in rechargeable lithium ion batteries. CityU-25 displayed an excellent reversible lithium storage capability of 455 mAh/g after 170 cycles at 0.1 A/g, and an impressive one of 673 mAh/g after 720 cycles at 0.5 A/g. These findings suggest that CityU-25 is a standout candidate for advanced battery technologies, highlighting the potential application of this type of materials.

Rapid preparation of binary mixtures of sodium carboxylates as anodes in sodium-ion batteries

Sodium-ion batteries are emerging as a sustainable solution to tackle the growing global energy demands. In this context, organic electrode materials complement such technologies as they are composed of earth-abundant elements. As organic anodes, sodium carboxylates exhibit promising applicability in a wide range of molecules. To harness the advantages of individual systems and to minimise their limitations, in this work, an approach to form binary mixtures of sodium carboxylates using one-pot, microwave-assisted synthesis is presented. The target mixtures were synthesised in 30 min with disodium naphthalene-2,6-dicarboxylate (Na-NDC) as a common constituent in all. Both components in all mixtures were shown to participate in the charge storage and had a considerable effect on the performance characteristics, such as specific capacity and working voltage, in half and full cell formats. This approach opens a new avenue for enabling organic materials to be considered as more competitive candidates in sodium-ion batteries and promote their use in other material classes to overcome their limitations.

Fluorine Chemistry in Rechargeable Batteries: Challenges, Progress, and Perspectives

The renewable energy industry demands rechargeable batteries that can be manufactured at low cost using abundant resources while offering high energy density, good safety, wide operating temperature windows, and long lifespans. Utilizing fluorine chemistry to redesign battery configurations/components is considered a critical strategy to fulfill these requirements due to the natural abundance, robust bond strength, and extraordinary electronegativity of fluorine and the high free energy of fluoride formation, which enables the fluorinated components with cost effectiveness, nonflammability, and intrinsic stability. In particular, fluorinated materials and electrode|electrolyte interphases have been demonstrated to significantly affect reaction reversibility/kinetics, safety, and temperature tolerance of rechargeable batteries. However, the underlining principles governing material design and the mechanistic insights of interphases at the atomic level have been largely overlooked. This review covers a wide range of topics from the exploration of fluorine-containing electrodes, fluorinated electrolyte constituents, and other fluorinated battery components for metal-ion shuttle batteries to constructing fluoride-ion batteries, dual-ion batteries, and other new chemistries. In doing so, this review aims to provide a comprehensive understanding of the structure-property interactions, the features of fluorinated interphases, and cutting-edge techniques for elucidating the role of fluorine chemistry in rechargeable batteries. Further, we present current challenges and promising strategies for employing fluorine chemistry, aiming to advance the electrochemical performance, wide temperature operation, and safety attributes of rechargeable batteries.

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.

Deciphering Electrolyte Dominated Na+ Storage Mechanisms in Hard Carbon Anodes for Sodium-Ion Batteries

Although hard carbon (HC) demonstrates superior initial Coulombic efficiency, cycling durability, and rate capability in ether-based electrolytes compared to ester-based electrolytes for sodium-ion batteries (SIBs), the underlying mechanisms responsible for these disparities remain largely unexplored. Herein, ex situ electron paramagnetic resonance (EPR) spectra and in situ Raman spectroscopy are combined to investigate the Na storage mechanism of HC under different electrolytes. Through deconvolving the EPR signals of Na in HC, quasi-metallic-Na is successfully differentiated from adsorbed-Na. By monitoring the evolution of different Na species during the charging/discharging process, it is found that the initial adsorbed-Na in HC with ether-based electrolytes can be effectively transformed into intercalated-Na in the plateau region. However, this transformation is obstructed in ester-based electrolytes, leading to the predominant storage of Na in HC as adsorbed-Na and pore-filled-Na. Furthermore, the intercalated-Na in HC within the ether-based electrolytes contributes to the formation of a uniform, dense, and stable solid-electrolyte interphase (SEI) film and eventually enhances the electrochemical performance of HC. This work successfully deciphers the electrolyte-dominated Na+ storage mechanisms in HC and provides fundamental insights into the industrialization of HC in SIBs.

Role of inorganic layers on polysulfide decomposition at sodium-metal anode surfaces for room temperature Na/S batteries

Sodium metal is a promising anode material for room-temperature sodium sulfur batteries. Due to its high reactivity, typical liquid electrolytes (e.g. carbonate-based solvents and a Na salt) can undergo reduction to form a solid electrolyte interphase (SEI) layer, with inorganic components such as Na2CO3, Na2O, and NaOH, covering the anode surface along with other SEI organic products. One of the challenges is to understand the effect of the SEI film on the decomposition of soluble sodium polysulfide molecules (e.g., Na2S8) upon shuttling from the cathode to anode during battery cycling. Here, we use ab initio molecular dynamics (AIMD) simulations to study the role of an inorganic SEI used as a model passivation layer in polysulfide decomposition. Compared to other film chemistries, it is found that the Na2CO3 film can suppress decomposition with the slowest reduction rate and the smallest amount of charge transfer towards Na2S8. The Na2CO3 film can maintain its structural properties during the simulations. In contrast, Na2O and NaOH allow some decomposed polysulfide fragments to be inserted into the SEI layer. Moreover, the decomposition of Na2S8 on both Na2O and NaOH SEI layers is more reactive with more charge transfer to Na2S8 when compared to that of Na2CO3. Thus, the ability of the SEI to suppress polysulfide decomposition is in the order: Na2CO3 > NaOH ∼ Na2O. Analyses of the density of states reveal that the Na2S8 molecule receives electrons from the Na metal directly in the presence of n-type semiconductor films of Na2CO3 and NaOH, while the charge migration behavior is different in a p-type semiconductor Na2O with the SEI film donating its electrons to the polysulfide solely. Thus, this work adds new insights into charge transfer behavior of inorganic thin film SEIs that could be present at the initial stages of SEI formation.

Breaking Barriers: Binder-Assisted NiS/NiS2 Heterostructure Anode with High Initial Coulombic Efficiency for Advanced Sodium-Ion Batteries

Developing sodium-ion batteries (SIBs) with high initial coulombic efficiency (ICE) and long-term cycling stability is crucial to meet energy storage device requirements. Designing anode materials that could exhibit high ICE is a promising strategy to realize enhanced energy density in SIBs. A trifunctional network binder substantially improves the electrochemical performance and ICE, providing excellent mechanical properties and strong adhesion strength. A rationally designed electrode material and binder can achieve high ICE, long cycling performance, and excellent specific capacity. Here, a NiS/NiS2 heterostructure as an anode material and a trifunctional network binder (SA-g-PAM) are designed for SIBs. Unprecedently, the anode comprising of an SA-g-PAM binder achieved the highest ICE of 90.7% and remarkable cycling stability for 19000 cycles at a current density of 10 A g-1 and maintained the specific capacity of 482.3 mAh g-1 even after 19000 cycles. This exciting work provides an alternate direction to the battery industry for developing high-performance electrode materials and binders with high ICE and excellent cycling stability for energy storage devices.

Three-dimensional heterogeneity in liquid electrolyte structures promotes Na ion transport and storage performance in Na-ion batteries

Unlike solid materials, the molecular structure and chemical distribution in electrolyte solutions have been considered in isotropic states. Herein, we reveal controllable regulation of solution structures in electrolytes by manipulating solvent interactions for Na-ion batteries. Low-solvation fluorocarbons as diluents in concentrated phosphate electrolytes induce adjustable heterogeneity in electrolyte structures through variable intermolecular forces between high-solvation phosphate and diluents. An optimal trifluorotoluene (PhCF3) diluent weakens the solvation strength around Na+ and spontaneously leads to a locally enlarged Na+ concentration and global 3D continuous Na+ transport path thanks to the appropriate electrolyte heterogeneity. Besides, strong correlations between the solvation structure and the Na+ storage performance and interphases are demonstrated. PhCF3 diluted concentrated electrolyte enables superior operations of Na-ion batteries at both room temperature and a high temperature of 60 °C. A hard carbon anode exhibits a reversible capacity of 300 mA h g-1 at 0.2C and excellent life over 1200 cycles without decay.

Azo-functionalised metal-organic framework for charge storage in sodium-ion batteries

Sodium-ion batteries (NIBs) are emerging as promising devices for energy storage applications. Porous solids, such as metal-organic frameworks (MOFs), are well suited as electrode materials for technologies involving bulkier charge carriers. However, only limited progress has been made using pristine MOFs, primarily due to lack of redox-active organic groups in the materials. In this work a azo-functional MOF, namely UiO-abdc, is presented as an electrode compound for sodium-ion insertion. The MOF delivers a stable capacity (∼100 mA h g^-1) over 150 cycles, and post-cycling characterisation validates the stability of the MOF and participation of the azo-group in charge storage. This study can accelerate the realisation of pristine solids, such as MOFs and other porous organic compounds, as battery materials.

Physical and numerical aspects of sodium ion solvation free energies via the cluster-continuum model

Sodium cation solvation Gibbs free energies (ΔG_solv(Na⁺)) have been obtained in water, dimethylformamide, dimethyl sulfoxide, ethanol, acetone, acetonitrile, and methanol through the "monomer cycle" cluster-continuum approach where a solvent reference state is described by infinitely separated molecules. The following steps are vital for obtaining reliable ΔG_solv(Na⁺) values: (a) a meticulous conformational search involving dispersion corrected density functional theory (DFT-D) and the continuum solvation model (CSM); (b) gas-phase DFT-D geometry optimization followed by single-point (SP) domain-based local pair natural orbital coupled clusters including single, double, and partly triple excitation (DLPNO-CCSD(T)) calculations in conjunction with the complete basis set extrapolation; (c) advanced statistical thermodynamic treatment of the low harmonic frequencies (<100 cm^-1) to obtain the robust gas-phase Gibbs free energy correction; (d) gas-phase and dielectric continuum SP with non-electrostatic contributions included in the CSM; (e) an evaluation of the relative thermodynamic stability of the Na⁺(S)_n clusters to identify the number of explicit solvent molecules n to be considered. Our refined computational protocol is promising with a Pearson correlation coefficient between the predicted and experimental data, ρ, of 0.82, and the mean signed and mean unsigned errors of 0.3 and 1.4 kcal mol^-1, respectively.

A Review of Nonaqueous Electrolytes, Binders, and Separators for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are the most important electrochemical energy storage devices due to their high energy density, long cycle life, and low cost. During the past decades, many review papers outlining the advantages of state-of-the-art LIBs have been published, and extensive efforts have been devoted to improving their specific energy density and cycle life performance. These papers are primarily focused on the design and development of various advanced cathode and anode electrode materials, with less attention given to the other important components of the battery. The “nonelectroconductive” components are of equal importance to electrode active materials and can significantly affect the performance of LIBs. They could directly impact the capacity, safety, charging time, and cycle life of batteries and thus affect their commercial application. This review summarizes the recent progress in the development of nonaqueous electrolytes, binders, and separators for LIBs and discusses their impact on the battery performance. In addition, the challenges and perspectives for future development of LIBs are discussed, and new avenues for state-of-the-art LIBs to reach their full potential for a wide range of practical applications are outlined.

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Nitrogen-Doped Hollow Carbon Spheres Based on Schiff Base Reaction as an Anode Material for High-Performance Lithium and Sodium Ion Batteries

Nitrogen-doped carbon has great potential in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), considering N-doping can not only improve the surface wettability of carbon materials, but also accelerate charge transfer by generating additional defects. However, designing carbon materials with a high nitrogen content and uniform distribution using conventional doping methods remains a challenge. In this study, a hollow carbon sphere with an ultrahigh nitrogen content of 9.58 wt % was successfully fabricated by rationally designing Schiff base chemistry (PTA-NHCS-700). Stable hierarchical pore structures, moderate defects, and large specific surface areas were formed during the carbonization process. Excellent electrochemical performance was observed in LIBs (204.2 mAh g^-1 after 7000 cycles at 5 A g^-1 ) and SIBs (154.2 mAh g^-1 after 10000 cycles at 5 A g^-1 ). This study not only promotes the development of efficient carbon anode materials for LIBs and SIBs, but also provides a novel idea for the doping of heteroatoms with special chemical structures.

Electrolyte Solvation Structure Design for Sodium Ion Batteries

Sodium ion batteries (SIBs) are considered the most promising battery technology in the post-lithium era due to the abundant sodium reserves. In the past two decades, exploring new electrolytes for SIBs has generally relied on the "solid electrolyte interphase (SEI)" theory to optimize the electrolyte components. However, many observed phenomena cannot be fully explained by the SEI theory. Therefore, electrolyte solvation structure and electrode-electrolyte interface behavior have recently received tremendous research interest to explain the improved performance. Considering there is currently no review paper focusing on the solvation structure of electrolytes in SIBs, a systematic survey on SIBs is provided, in which the specific solvation structure design guidelines and their consequent impact on the electrochemical performance are elucidated. The key driving force of solvation structure formation, and the recent advances in adjusting SIB solvation structures are discussed in detail. It is believed that this review can provide new insights into the electrolyte optimization strategies of high-performance SIBs and even other emerging battery systems.

Recent Progress and Challenges of Flexible Zn-Based Batteries with Polymer Electrolyte

Zn-based batteries have been identified as promising candidates for flexible and wearable batteries because of their merits of intrinsic safety, eco-efficiency, high capacity and cost-effectiveness. Polymer electrolytes, which feature high solubility of zinc salts and softness, are especially advantageous for flexible Zn-based batteries. However, many technical issues still need to be addressed in Zn-based batteries with polymer electrolytes for their future application in wearable electronics. Recent progress in advanced flexible Zn-based batteries based on polymer electrolytes, including functional hydrogel electrolytes and solid polymer electrolytes, as well as the interfacial interactions between polymer electrolytes and electrodes in battery devices, is comprehensively reviewed and discussed with a focus on their fabrication, performance validation, and intriguing affiliated functions. Moreover, relevant challenges and some potential strategies are also summarized and analyzed to help inform the future direction of polymer-electrolyte-based flexible Zn-based batteries with high practicability.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal-Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal-sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li-S and the emerging Na-S and K-S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li-S are relatively mature, less progress has been made with Na-S and K-S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal-selenium and the metal-selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal-sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li-S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure-battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

Operando XRD studies on Bi2MoO6as anode material for Na-ion batteries

Based on the same rocking-chair principle as rechargeable Li-ion batteries, Na-ion batteries are promising solutions for energy storage benefiting from low-cost materials comprised of abundant elements. However, despite the mechanistic similarities, Na-ion batteries require a different set of active materials than Li-ion batteries. Bismuth molybdate (Bi₂MoO₆) is a promising NIB anode material operating through a combined conversion/alloying mechanism. We report anoperandox-ray diffraction (XRD) investigation of Bi₂MoO₆-based anodes over 34 (de)sodiation cycles revealing both basic operating mechanisms and potential pathways for capacity degradation. Irreversible conversion of Bi₂MoO₆to Bi nanoparticles occurs through the first sodiation, allowing Bi to reversibly alloy with Na forming the cubic Na₃Bi phase. Preliminary electrochemical evaluation in half-cellsversusNa metal demonstrated specific capacities for Bi₂MoO₆to be close to 300 mAh g^-1during the initial 10 cycles, followed by a rapid capacity decay.OperandoXRD characterisation revealed that the increased irreversibility of the sodiation reactions and the formation of hexagonal Na₃Bi are the main causes of the capacity loss. This is initiated by an increase in crystallite sizes of the Bi particles accompanied by structural changes in the electronically insulating Na-Mo-O matrix leading to poor conductivity in the electrode. The poor electronic conductivity of the matrix deactivates the Na_xBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

Oxide Nanoclusters on Ti3 C2 MXenes to Deactivate Defects for Enhanced Lithium Ion Storage Performance

The commercialization of MXenes as anodes for lithium-ion batteries is largely impeded by low initial coulombic efficiency (ICE) and unfavorable cycling stability, which are closely associated with defects such as Ti vacancies (V_Ti ) in Ti₃ C₂ MXenes. Herein, an effective strategy is developed to deactivate V_Ti defects by in situ growing Al₂ O₃ nanoclusters on MXenes to alleviate the irreversible electrolyte decomposition and Li dendrites formation trend induced by defects, improving ICE and cycling stability. Furthermore, it is revealed that excessively lithiophilic V_Ti defects would impede Li ions diffusion due to their strong adsorption, leading to a locally nonuniform Li flux to these "hot spots," setting scene for the formation of Li dendrites. The Al₂ O₃ nanoclusters anchored on V_Ti sites can not only improve Li diffusion kinetics but also promote the homogeneous solid electrolyte interphase formation with small charge transfer resistance, achieving uniform Li deposition in a smaller overpotential without formation of Li dendrites. As expected, Ti₃ C₂ @Al₂ O₃ -11 electrode delivers a high ICE of 76.6% and an outstanding specific capacity of 285.5 mAh g^-1 after 500 cycles, which is much higher than that of pristine Ti₃ C₂ sample. This work sheds light on modulating defects for high-performance energy storage materials.

Copper nanowire-derived one-dimensional hollow copper sulfides as electrode materials for sodium-ion batteries

One-dimensional (1D) hollow CuxS nanotubes were obtained via a sacrificial template diffusion process by immersing 1D copper nanowires in thiourea solution. This structure exhibited excellent cycling stability when used as an electrode material for sodium-ion battery.

Development of advanced electrolytes in Na-ion batteries: application of the Red Moon method for molecular structure design of the SEI layer

This review aims to overview state-of-the-art progress in the collaborative work between theoretical and experimental scientists to develop advanced electrolytes for Na-ion batteries (NIBs). Recent investigations were summarized on NaPF6 salt and fluoroethylene carbonate (FEC) additives in propylene carbonate (PC)-based electrolyte solution, as one of the best electrolytes to effectively passivate the hard-carbon electrode with higher cycling performance for next-generation NIBs. The FEC additive showed high efficiency to significantly enhance the capacity and cyclability of NIBs, with an optimal performance that is sensitive at low concentration. Computationally, both microscopic effects, positive and negative, were revealed at low and high concentrations of FEC, respectively. In addition to the role of FEC decomposition to form a NaF-rich solid electrolyte interphase (SEI) film, intact FECs play a role in suppressing the dissolution to form a compact and stable SEI film. However, the increase in FEC concentration suppressed the organic dimer formation by reducing the collision frequency between the monomer products during the SEI film formation processes. In addition, this review introduces the Red Moon (RM) methodology, recent computational battery technology, which has shown a high efficiency to bridge the gap between the conventional theoretical results and experimental ones through a number of successful applications in NIBs.

Rapid Microwave-Assisted Synthesis and Electrode Optimization of Organic Anode Materials in Sodium-Ion Batteries

Sodium-ion batteries are commanding increasing attention owing to their promising electrochemical performance and sustainability. Organic electrode materials (OEMs) complement such technologies as they can be sourced from biomass and recycling them is environmentally friendly. Organic anodes based on sodium carboxylates have exhibited immense potential, except the limitation of current synthesis methods concerning upscaling and energy costs. In this work, a rapid and energy efficient microwave-assisted synthesis for organic anodes is presented using sodium naphthalene-2,6-dicarboxylate as a model compound. Optimizing the synthesis and electrode composition enables the compound to deliver a reversible initial capacity of ≈250 mAh g^-1 at a current density of 25 mA g^-1 with a high initial Coulombic efficiency (≈78%). The capacity is stable over 400 cycles and the compound also exhibits good rate performance. The successful demonstration of this rapid synthesis may facilitate the transition to preparing organic battery materials by scalable, efficient methods.

Binders for sodium-ion batteries: progress, challenges and strategies

Binders as a bridge in electrodes can bring various components together thus guaranteeing the integrity of electrodes and electronic contact during battery cycling. In this review, we summarize the recent progress of traditional binders and novel binders in the different electrodes of SIBs. The challenges faced by binders in terms of bond strength, wettability, thermal stability, conductivity, cost, and environment are also discussed in details. Correspondingly, the designing principle and advanced strategies of future research on SIB binders are also provided. Moreover, a general conclusion and perspective on the development of binder design for SIBs in the future are presented.

New Route to Battery Grade NaPF6 for Na-Ion Batteries: Expanding the Accessible Concentration

Sodium-ion batteries represent a promising alternative to lithium-ion systems. However, the rapid growth of sodium-ion battery technology requires a sustainable and scalable synthetic route to high-grade sodium hexafluorophosphate. This work demonstrates a new multi-gram scale synthesis of NaPF₆ in which the reaction of ammonium hexafluorophosphate with sodium metal in THF solvent generates the electrolyte salt with the absence of the impurities that are common in commercial material. The high purity of the electrolyte (absence of insoluble NaF) allows for concentrations up to 3 M to be obtained accurately in binary carbonate battery solvent. Electrochemical characterization shows that the degradation dynamics of sodium metal-electrolyte interface are different for more concentrated (>2 M) electrolytes, suggesting that the higher concentration regime (above the conventional 1 M concentration) may be beneficial to battery performance.

Superior Sodium Storage Properties in the Anode Material NiCr2 S4 for Sodium-Ion Batteries: An X-ray Diffraction, Pair Distribution Function, and X-ray Absorption Study Reveals a Conversion Mechanism via Nickel Extrusion

The pseudo-layered sulfide NiCr2 S4 exhibits outstanding electrochemical performance as anode material in sodium-ion batteries (SIBs). The Na storage mechanism is investigated by synchrotron-based X-ray scattering and absorption techniques as well as by electrochemical measurements. A very high reversible capacity in the 500th cycle of 489 mAh g-1 is observed at 2.0 A g-1 in the potential window 3.0-0.1 V. Full discharge includes irreversible generation of Ni0 and Cr0 nanoparticles embedded in nanocrystalline Na2 S yielding shortened diffusion lengths and predominantly surface-controlled charge storage. During charge, Ni0 and Cr0 are oxidized, Na2 S is consumed, and amorphous Ni and Cr sulfides are formed. Limiting the potential window to 3.0-0.3 V an unusual nickel extrusion sodium insertion mechanism occurs: Ni2+ is reduced to nanosized Ni0 domains, expelled from the host lattice, and is replaced by Na+ cations to form O3-type like NaCrS2 . Surprisingly, the discharge and charge processes comprise Na+ shuttling between highly crystalline NiCr2 S4 and NaCrS2 enabling a superior long-term stability for 3000 cycles. The results not only provide valuable insights for the electrochemistry of conversion materials but also extend the scope of layered electrode materials considering the reversible nickel extrusion sodium insertion reaction as new concept for SIBs.

Unraveling the Role of Fluorinated Alkyl Carbonate Additives in Improving Cathode Performance in Sodium-Ion Batteries

A key issue in the development of sustainable Na-ion batteries (NIBs) is the stability of the electrolyte solution and its ability to form effective passivation layers on both cathode and anode. In this regard, the use of fluorine-based additives is considered a promising direction for improving electrode performance. Fluoroethylene carbonate (FEC) and trans-difluoroethylene carbonate (DFEC) were demonstrated as additives or cosolvents that form effective passivating surface films in Li-ion batteries. Their effect is evaluated for the first time with cathodes in NIBs. By application of systematic electrochemical and postmortem investigations, the role of fluorinated additives in the good performance of Na_0.44MnO₂ (NMO) cathodes was deciphered. Despite the significant improvement in the performance of Li-ion cells enabled by the use of FEC and FEC + DFEC, the highest stability for NIBs was observed when only FEC was used as an additive. Mechanistic insights and analytical characterizations were carried out to shed light on the inferior effect of FEC + DFEC in NIBs, in contrast to its positive effect on the stability of Li-ion batteries.

Ship in bottle synthesis of yolk-shell MnS@hollow carbon spheres for sodium storage

Yolk-shell structure can effectively alleviate the volume change of electrodes during electrochemical charge/discharge. In this paper, we provide a new ship in bottle strategy to synthesize MnS@C sodium ion battery anode with yolk-shell nanostructure. The obtained yolk-shell structures were uniform spheres. The space between the carbon shell and MnS core allows the volume change of MnS without deforming the carbon shell or damaging the solid electrolyte interface film formed on the outer surface. The MnS@C yolk-shell structure showed stable cycle stability (336 mAh g^-1capacity after 200 cycles at 0.5 A g^-1current density).

Rational Design of Effective Binders for LiFePO4 Cathodes

Polymer binders are critical auxiliary additives to Li-ion batteries that provide adhesion and cohesion for electrodes to maintain conductive networks upon charge/discharge processes. Therefore, polymer binders become interconnected electrode structures affecting electrochemical performances, especially in LiFePO4 cathodes with one-dimensional Li+ channels. In this paper, recent improvements in the polymer binders used in the LiFePO4 cathodes of Li-ion batteries are reviewed in terms of structural design, synthetic methods, and working mechanisms. The polymer binders were classified into three types depending on their effects on the performances of LiFePO4 cathodes. The first consisted of PVDF and related composites, and the second relied on waterborne and conductive binders. Profound insights into the ability of binder structures to enhance cathode performance were discovered. Overcoming the bottleneck shortage originating from olivine structure LiFePO4 using efficient polymer structures is discussed. We forecast design principles for the polymer binders used in the high-performance LiFePO4 cathodes of Li-ion batteries. Finally, perspectives on the application of future binder designs for electrodes with poor conductivity are presented to provide possible design directions for chemical structures.

3D Ag@C Cloth for Stable Anode Free Sodium Metal Batteries

While sodium metal anodes (SMAs) feature many performance advantages in sodium ion batteries (SIBs), severe safety concerns remain for using bulk sodium electrodes. Herein, a 3D Ag@C natrophilic substrate prepared by a facile thermal evaporation deposition method, which can be employed as a much safer "anode-free" SMA, is reported. Initially, there is no bulk sodium on the Ag@C substrate in the assembled SIBs. Upon charging, sodium will be uniformly deposited onto the Ag@C substrate and afterwards functions as a real SMA, thus inheriting the intrinsic merits of SMA and enhancing safety simultaneously. While cycling, the as-synthesized substrate demonstrates superior sodium plating/stripping cycling stability at 1, 2 and 3 mA cm^-2 with a capacity of 2 mAh cm^-2 . Theoretical simulations reveal that Na ions prefer to bind with Ag and form a Na-Ag network, thus clearly revealing uniform sodium deposition on the Ag@C substrate. More importantly, a full battery based on Ag@C and Prussian white with impressive Coulomb efficiency (CE), high rate capability (from 0.1 C to 5 C) and long-term cycling life is illustrated for the first time, thus making Ag@C feasible for the establishment of "anode-free" SIBs with reduced cost, high gravimetric/volumetric energy density and enhanced safety.

Recent Advances in Silicon-Based Electrodes: From Fundamental Research toward Practical Applications

The increasing demand for higher-energy-density batteries driven by advancements in electric vehicles, hybrid electric vehicles, and portable electronic devices necessitates the development of alternative anode materials with a specific capacity beyond that of traditional graphite anodes. Here, the state-of-the-art developments made in the rational design of Si-based electrodes and their progression toward practical application are presented. First, a comprehensive overview of fundamental electrochemistry and selected critical challenges is given, including their large volume expansion, unstable solid electrolyte interface (SEI) growth, low initial Coulombic efficiency, low areal capacity, and safety issues. Second, the principles of potential solutions including nanoarchitectured construction, surface/interface engineering, novel binder and electrolyte design, and designing the whole electrode for stability are discussed in detail. Third, applications for Si-based anodes beyond LIBs are highlighted, specifically noting their promise in configurations of Li-S batteries and all-solid-state batteries. Fourth, the electrochemical reaction process, structural evolution, and degradation mechanisms are systematically investigated by advanced in situ and operando characterizations. Finally, the future trends and perspectives with an emphasis on commercialization of Si-based electrodes are provided. Si-based anode materials will be key in helping keep up with the demands for higher energy density in the coming decades.

Nitrogen-doped carbon encapsulated zinc vanadate polyhedron engineered from a metal-organic framework as a stable anode for alkali ion batteries

In this work, we fabricated vanadium/zinc metal-organic frameworks (V/Zn-MOFs) derived from self-assembled metal organic frameworks, to further disperse ultrasmall Zn₂VO₄ nanoparticles and encapsulate them in a nitrogen-doped nanocarbon network (ZVO/NC) under in situ pyrolysis. When employed as an anode for lithium-ion batteries, ZVO/NC delivers a high reversible capacity (807 mAh g^-1 at 0.5 A g^-1) and excellent rate performance (372 mAh g^-1 at 8.0 A g^-1). Meanwhile, when used in sodium-ion batteries, it exhibits long-term cycling stability (7000 cycles with 145 mAh g^-1 at 2.0 A g^-1). Additionally, when employed in potassium-ion batteries, it also shows outstanding electrochemical performance with reversible capacities of 264 mAh g^-1 at 0.1 A g^-1 and 140 mAh g^-1 at 0.5 A g^-1 for 1000 cycles. The mechanism by which the pseudocapacitive behaviour of ZVO/NC enhances battery performance under a suitable electrolyte was probed, which offers useful enlightenment for the potential development of anodes of alkali-ion batteries. The performance of Zn₂VO₄ as an anode for SIBs/PIBs was investigated for the first time. This work provides a new horizon in the design ZVO/NC as a promising anode material owing to the intrinsically synergic effects of mixed metal species and the multiple valence states of V.

High Stability and Long Cycle Life of Rechargeable Sodium-Ion Battery Using Manganese Oxide Cathode: A Combined Density Functional Theory (DFT) and Experimental Study

Sodium-ion batteries (SIBs) can develop cost-effective and safe energy storage technology for substantial energy storage demands. In this work, we have developed manganese oxide (α-MnO2) nanorods for SIB applications. The crystal structure, which is crucial for high-performance energy storage, is examined systematically for the metal oxide cathode. The intercalation of sodium into the α-MnO2 matrix was studied using the theoretical density functional theory (DFT) studies. The DFT studies predict Na ions' facile diffusion kinetics through the MnO2 lattice with an attractively low diffusion barrier (0.21 eV). When employed as a cathode material for SIBs, MnO2 showed a moderate capacity (109 mAh·g-1 at C/20 current rate) and superior life cyclability (58.6% after 800 cycles) in NaPF6/EC+DMC (5% FEC) electrolyte. It shows a much higher capacity of 181 mAh·g-1 (C/20 current rate) in NaClO4/PC (5% FEC) electrolyte, though it suffers fast capacity fading (11.5% after 800 cycles). Our findings show that high crystallinity and hierarchical nanorod morphology of the MnO2 are responsible for better cycling performance in conjunction with fast and sustained charge-discharge behaviors.

Strategies for Alleviating Electrode Expansion of Graphite Electrodes in Sodium-Ion Batteries Followed by In Situ Electrochemical Dilatometry

The electrochemical intercalation/deintercalation of solvated sodium ions into graphite is a highly reversible process, but leads to large, undesired electrode expansion/shrinkage ("breathing"). Herein, two strategies to mitigate the electrode expansion are studied. Starting with the standard configuration (-) sodium | diglyme (2G) electrolyte | graphite (poly(vinylidene difluoride) (PVDF) binder) (+), the PVDF binder is first replaced with a binder made of the sodium salt of carboxymethyl cellulose (CMC). Second, ethylenediamine (EN) is added to the electrolyte solution as a co-solvent. The electrode breathing is followed in situ (operando) through electrochemical dilatometry (ECD). It is found that replacing PVDF with CMC is only effective in reducing the electrode expansion during initial sodiation. During cycling, the electrode breathing for both binders is comparable. Much more effective is the addition of EN. The addition of 10 v/v EN to the diglyme electrolyte strongly reduces the electrode expansion during the initial sodiation (+100% with EN versus +175% without EN) as well as the breathing during cycling. A more detailed analysis of the ECD signals reveals that solvent co-intercalation temporarily leads to pillaring of the graphite lattice and that the addition of EN to 2G leads to a change in the sodium storage mechanism.

Strategies for Mitigating Dissolution of Solid Electrolyte Interphases in Sodium-Ion Batteries

The interfacial reactions in sodium-ion batteries (SIBs) are not well understood yet. The formation of a stable solid electrolyte interphase (SEI) in SIBs is still challenging due to the higher solubility of the SEI components compared to lithium analogues. This study therefore aims to shed light on the dissolution of SEI influenced by the electrolyte chemistry. By conducting electrochemical tests with extended open circuit pauses, and using surface spectroscopy, we determine the extent of self-discharge due to SEI dissolution. Instead of using a conventional separator, β-alumina was used as sodium-conductive membrane to avoid crosstalk between the working and sodium-metal counter electrode. The relative capacity loss after a pause of 50 hours in the tested electrolyte systems ranges up to 30 %. The solubility of typical inorganic SEI species like NaF and Na2 CO3 was determined. The electrolytes were then saturated by those SEI species in order to oppose ageing due to the dissolution of the SEI.

Impact of carbonate-based electrolytes on the electrochemical activity of carbon-coated Na3V2(PO4)2F3 cathode in full-cell assembly with hard carbon anode

An immense effort has been put into developing high-performance electrodes to commercialize sodium-ion batteries, but research on developing an efficient electrolyte is lacking. This study aims to find the best carbonate-based electrolyte systems by incorporating the existing ideas reported in this field. The sodium superionic conductor (NASICON) type Na₃V₂(PO₄)₂F₃-C (NVPF-C) was chosen as a cathode, and its compatibility with four different carbonate-based electrolyte solutions was studied in the half-cell assembly. Additionally, full-cell assembly with hard carbon as an anode is also explored. Binary and ternary combinations of the solvents ethylene carbonate, propylene carbonate, and dimethyl carbonate were employed with and without fluoroethylene carbonate as an additive. A systematic study was performed, including the in-situ impedance technique, and to determine the compatibility. Detailed galvanostatic studies for NVPF-C based half-cells, as well as hard carbon/NVPF-C full-cells, are performed, which shows that 1 M NaClO₄ in propylene carbonate:dimethyl carbonate + fluoroethylene carbonate is a better electrolyte composition for this assembly. Subsequently, a temperature study was carried out on this electrolyte to test its performance.

Sulfur-doped CoP@ Nitrogen-doped porous carbon hollow tube as an advanced anode with excellent cycling stability for sodium-ion batteries

Transition metal phosphides have attracted increasing attention as anode materials for sodium-ion batteries (SIBs). Cobalt phosphide (CoP) has been deemed as prospective anode materials owing to its high theoretical capacity. Nevertheless, the defects of cobalt phosphides are evident. Low conductivity, the non-negligible volume expansion and aggregation of particles during sodiation/desodiation process result in poor cycling performance and rapid capacity decay, which greatly limit their applications. Herein, we designed a hollow-nanotube structure of sulfur-doped cobalt phosphide (S-CoP) nanoparticles coated by nitrogen-doped porous carbon (S-CoP@NPC), which can be successfully synthesized via an ordinary hydrothermal process followed by the low-temperature phosphorization/sulfuration treatment. The doping of sulfur element provides more active sites, meanwhile, the carbon coating largely helps to avoid the agglomeration of nanoparticles, alleviate volume expansion and improve the conductivity of materials. The S-CoP@NPC composite presents stable cycling performance, showing a discharge specific capacity of 230 mAh g^-1 over 370 cycles at 0.2 A g^-1. In addition, it also exhibits good rate capability with a discharge specific capacity of 143 mAh g^-1 at 5 A g^-1, even when the current density returns to 0.2 A g^-1, the discharge specific capacity can recover 213 mAh g^-1. Furthermore, the kinetic analysis of S-CoP@NPC composite explains that the excellent cycling and rate performance benefit from the extrinsic pseudocapacitive behavior.