Sodium-ion batteries: present and future

Selenium-induced anion vacancy and active site migration stimulating remarkable sulfide Na-Ion storage

Heterojunctions and controllable anionic vacancies are perceived to be powerful means of ameliorating the performance of sodium-ion batteries assignable to their unique physical and chemical properties. However, the mechanism by which heterojunction and vacancy structures affect sodium-ion battery storage remains to be systemically explored. In this study, the Se doped CoS2@CoS1.035@Carbon (Se-CoS2@CoS1.035@C) heterostructure with anion vacancy was synthesized by a one-step calcination. These heterostructures with lower metal oxidation states and anionic vacancies exhibit exceptional Na+ storage performance (554.3 mA h g-1 after 1500 cycles at 5.0 A g-1). Both electrochemical tests and theoretical calculations demonstrate excellent pseudocapacitive behavior and enhanced Na+ adsorption during discharge because of anionic vacancies and Se doping. Additionally, introducing weaker Co-Se bonds and extending Co-S and Co-Se bonds reduce binding energies, which effectively accelerates the conversion reaction. Our findings provide a feasible way to rationally design and facilely prepare heterostructured anode materials with rich anionic vacancies for sodium-ion batteries.

Towards metal selenides: a promising anode for sodium-ion batteries

Metal selenides have garnered significant attention as promising anode materials for sodium-ion batteries, thanks to their high theoretical capacity, excellent conductivity, and natural abundance. However, their potential is hampered by disappointing capacity retention and unsatisfactory lifespan, primarily attributed to volume expansion and unwanted structural collapse resulting from the insertion and extraction of relatively large Na+ ions during the charge and discharge processes. This feature article provides a brief overview of our endeavors to address the challenges associated with metal selenide-based anode materials, aiming to achieve high-performance electrode materials for sodium-ion batteries. Our strategy encompasses nanostructure design, materials composite engineering, heteroatoms doping, and topography and interface engineering. Additionally, future research directions are also outlined.

Trace Y Doping Regulated Bulk/Interfacial Reactions of P2-Layered Oxides for Ultrahigh-Rate Sodium-Ion Batteries

P2-phase layered cathodes play a pivotal role in sodium-ion batteries due to their efficient Na+ intercalation chemistry. However, limited by crystal disintegration and interfacial instability, bulk and interfacial failure plague their electrochemical performance. To address these challenges, a structural enhancement combined with surface modification is achieved through trace Y doping. Based on a synergistic combination of experimental results and density functional theory (DFT) calculations, the introduction of partial Y ions at the Na site (2d) acts as a stabilizing pillar, mitigating the electrostatic repulsions between adjacent TMO2 slabs and thereby relieving internal structural stress. Furthermore, the presence of Y effectively optimizes the Ni 3d-O 2p hybridization, resulting in enhanced electronic conductivity and a notable rapid charging ability, with a capacity of 77.3 mA h g-1 at 40 C. Concurrently, the introduction of Y also induces the formation of perovskite nano-islands, which serve to minimize side reactions and modulate interfacial diffusion. As a result, the refined P2-Na0.65 Y0.025[Ni0.33Mn0.67]O2 cathode material exhibits an exceptionally low volume variation (≈1.99%), an impressive capacity retention of 83.3% even at -40 °C after1500 cycles at 1 C.

Mechanical Activation of Graphite for Na-Ion Battery Anodes: Unexpected Reversible Reaction on Solid Electrolyte Interphase via X-Ray Analysis

Although sodium-ion batteries (SIBs) offer promising low-cost alternatives to lithium-ion batteries (LIBs), several challenges need to be overcome for their widespread adoption. A primary concern is the optimization of carbon anodes. Graphite, vital to the commercial viability of LIBs, has a limited capacity for sodium ions. Numerous alternatives to graphite are explored, particularly focusing on disordered carbons, including hard carbon. However, compared with graphite, most of these materials underperform in LIBs. Furthermore, the reaction mechanism between carbon and sodium ions remains ambiguous owing to the structural diversity of disordered carbon. A straightforward mechanical approach is introduced to enhance the sodium ion storage capacity of graphite, supported by comprehensive analytical techniques. Mechanically activated graphite delivers a notable reversible capacity of 290.5 mAh·g-1 at a current density of 10 mA·g-1. Moreover, it maintains a capacity of 157.7 mAh·g-1 even at a current density of 1 A·g-1, benefiting from the defect-rich structure achieved by mechanical activation. Soft X-ray analysis revealed that this defect-rich carbon employs a sodium-ion storage mechanism distinct from that of hard carbon. This leads to an unexpected reversible reaction on the solid electrolyte surface. These insights pave the way for innovative design approaches for carbon electrodes in SIB anodes.

Na2.5Cr0.5Zr0.5Cl6: A New Halide-Based Fast Sodium-Ion Conductor

All-solid-state batteries employing solid electrolytes (SEs) have received widespread attention due to their high safety. Recently, lithium halides are intensively investigated as promising SEs while their sodium counterparts are less studied. Herein, a new sodium-ion conductor with a chemical formula of Na2.5Cr0.5Zr0.5Cl6 is reported, which exhibits high room temperature ionic conductivity of 0.1 mS cm-1 with low migration energy barrier of ≈0.41 eV. Na2.5Cr0.5Zr0.5Cl6 has a Fm-3m structure with 41.67 mol.% of cationic vacancies owing to the occupation of Cr (8.33 mol.%) and Zr (8.33 mol.%) ions at Na sites. Supercell calculations show that the lowest columbic energy configuration has Cr/Zr/V (where V is the vacancy) clusters in the structure. Nonetheless, the clusters have mixed effects on the sodium ion conduction pathway, based on the Bond Valence Energy Landscape calculation. A global 3D Na-ion transport percolation network can be revealed in the lowest energy supercell. Effective pathways are connected through the NaCl6 and VCl6 nodes. Besides, Raman spectroscopy and 23Na solid-state nuclear magnetic resonance spectroscopy further prove the tunable structure of the SEs with different Cr to Zr ratios. The optimization between the concentration of Na+ and vacancies is crucial to create an improved network of Na+ diffusion channels.

Concept and investigation of Selective Forward Osmosis (SFO) for Salt-Salt Separation as a Pretreatment of Seawater for Resource Utilization

Seawater utilization is crucial for the sustainable human development. Despite growing interest in forward osmosis (FO) due to its unique properties, conventional FO membranes with salt-water selectivity have limitations in applying to specific salt-salt separation processes, which hinders their application in resource utilization. In this work, a new concept, "selective forward osmosis (SFO)", was proposed, which ingeniously employed an SFO membrane consisting of an ion-selective layer on a denser substrate. The denser substrate is designed to control water flux so as to alleviate the solution dilution and improve the salt-salt separation. Moreover, the sucrose and pure water were used separately as feed solution to provide different water flux to influence the various salt fluxes, showing that pure water feed could enhance the salt-salt separation efficiency, although it could dilute the draw solution to some extent. Therefore, pure water was selected as feed in the subsequent experiments. The optimized SFO membrane achieved high Na2SO4/NaCl selectivity (∼54.8) and MgCl2/NaCl selectivity (∼9.2) in single-salt draw solutions. With mixed-salt and heavy-metal-mixed-salt draw solutions, the Mg2+/Na+ selectivity was enhanced to ∼14.5, and further to 29.3. In real seawater tests, the SFO system effectively permeated monovalent elements (such as Na flux of ∼68.6 g m-2 h-1) while maintaining a higher rejection for bivalent elements (such as Mg flux of ∼0.08 g m-2 h-1), showing high selectivities for Mg/Na, U/Na, Sr/Na, Ni/Na, and Ca/Na. These results demonstrate the potential of SFO for resource utilization, especially in complex saline environments. This work contributes a new route for salt-salt separation in the pretreatment of seawater resources.

Self-Adaptive Graphdiyne/Sn Interface for High-Performance Sodium Storage

Efficiently reconciling the substantial volume strain with maintaining the stabilities of both interfacial protection and three-dimensional (3D) conductive networks is a scientific and technical challenge in developing tin-based anodes for sodium ion storage. To address this issue, a proof-of-concept self-adaptive protection for the Sn anode is designed, taking advantage of the arbitrary substrate growth of graphdiyne. This protective layer, employing a flexible chain doping strategy, combines the benefits of 2D graphdiyne and linear chain structures to achieve 2D mechanical stability, electronic and ion conductions, ion selectivity, adequate elongation, and flexibility. It establishes close contact with the Sn particles and can adapt to dynamic size changes while effectively facilitating both electronic and ion transports. It successfully mitigates the detrimental effects of particle pulverization and coarsening induced by large-volume changes. The as-obtained Sn electrodes demonstrate exceptional stability, enduring 1800 cycles at a high current density of 2.5 A g-1. This strategy promises to address the general issues associated with large-strain electrodes in next-generation of high-energy-density batteries.

Layered Cathode with Ultralow Strain Empowers Rapid-Charging and Slow-Discharging Capability in Sodium Ion Battery

The development of the electric vehicle industry has spurred demand for secondary batteries capable of rapid-charging and slow-discharging. Among them, sodium-ion batteries (SIBs) with layered oxide as the cathode exhibit competitive advantages due to their comprehensive electrochemical performance. However, to meet the requirements of rapid-charging and slow-discharging scenarios, it is necessary to further enhance the rate performance of the cathode material to achieve symmetrical capacity at different rates. Simultaneously, minimizing lattice strain during asymmetric electrochemical processes is also significant in alleviating strain accumulation. In this study, the ordered distribution of transition metal layers and the diffusion pathway of sodium ions are optimized through targeted K-doping of sodium layers, leading to a reduction of the diffusion barrier and endowment of prominent rate performance. At a 20C rate, the capacity of the cathode can reach 94% of that at a 0.1C rate. Additionally, the rivet effect of the sodium layers resulted in a global volume strain of only 0.03% for the modified cathode during charging at a 10C rate and discharging at a 1C rate. In summary, high-performance SIBs, with promising prospects for rapid-charging and slow-discharging capability, are obtained through the regulation of sodium layers, opening up new avenues for commercial applications.

Unexpected Elevated Working Voltage by Na+/Vacancy Ordering and Stabilized Sodium-Ion Storage by Transition-Metal Honeycomb Ordering

Na+/vacancy ordering in sodium-ion layered oxide cathodes is widely believed to deteriorate the structural stability and retard the Na+ diffusion kinetics, but its unexplored potential advantages remain elusive. Herein, we prepared a P2-Na0.8Cu0.22Li0.08Mn0.67O2 (NCLMO-12 h) material featuring moderate Na+/vacancy and transition-metal (TM) honeycomb orderings. The appropriate Na+/vacancy ordering significantly enhances the operating voltage and the TM honeycomb ordering effectively strengthens the layered framework. Compared with the disordered material, the well-balanced dual-ordering NCLMO-12 h cathode affords a boosted working voltage from 2.85 to 3.51 V, a remarkable ~20 % enhancement in energy density, and a superior cycling stability (capacity retention of 86.5 % after 500 cycles). The solid-solution reaction with a nearly "zero-strain" character, the charge compensation mechanisms, and the reversible inter-layer Li migration upon sodiation/desodiation are unraveled by systematic in situ/ex situ characterizations. This study breaks the stereotype surrounding Na+/vacancy ordering and provides a new avenue for developing high-energy and long-durability sodium layered oxide cathodes.

Architecting O3/P2 layered oxides by gradient Mn doping for sodium-ion batteries

O3 phase layered oxides are highly attractive cathode materials for sodium-ion batteries because of their high capacity and decent initial Coulombic efficiency. However, their rate capability and long cycling life are unsatisfactory due to the narrow Na+ transfer channel and irreversible phase transitions of O3 phase during sodiation/desodiation process. Constructing O3/P2 multiphase structures has been proven to be an effective strategy to overcome these challenges. In this study, we synthesized bi-phasic structured O3/P2 Na(Ni2/9Fe1/3Cu1/9Mn1/3)1-xMnxO2 (x = 0.01, 0.02, 0.03, 0.04, 0.05) materials through Mn doping during sodiation process. Benefiting from surface P2 phase layer with the enhanced Na+ transfer dynamics and high structural stability, the Na(Ni2/9Fe1/3Cu1/9Mn1/3)0.98Mn0.02O2 (NFCM-M2) cathode delivers a reversible capacity of 139.1 mA h g-1 at 0.1 C, and retains 71.4 % of its original capacity after 300 cycles at 1 C. Our work provides useful guidance for designing multiphase cathodes and offers new insights into the structure-performance correlation for sodium-ion cathode materials.

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.

Metallic CoSb and Janus Co2AsSb monolayers as promising anode materials for metal-ion batteries

Structural symmetry breaking plays a pivotal role in fine-tuning the properties of nano-layered materials. Here, based on the first-principles approaches we propose a Janus monolayer of metallic CoSb by breaking the out-of-plane structural symmetry. Specifically, within the CoSb monolayer by replacing the top-layer 'Sb' with 'As' atoms entirely, the Janus Co2AsSb monolayer can be formed, whose structure is confirmed via structural optimization and ab initio molecular dynamics simulations. Notably, the Janus Co2AsSb monolayer demonstrates stability at an elevated temperature of 1200 K, surpassing the stability of the CoSb monolayer, which remains stable only up to 900 K. We propose that both the CoSb and Janus Co2AsSb monolayers could serve as capable anode materials for power-driven metal-ion batteries, owing to their substantial theoretical capacity and robust binding strength. The theoretical specific capacities for Li/Na reach up to 1038.28/1186.60 mA h g-1 for CoSb, while Janus Co2AsSb demonstrates a marked improvement in electrochemical storage capacity of 3578.69/2215.38 mA h g-1 for Li/Na, representing a significant leap forward in this domain. The symmetry-breaking effect upgrades the CoSb monolayer, as a more viable contender for power-driven metal-ion batteries. Furthermore, electronic structure calculations indicate a notable charge transfer that augments the metallic nature, which would boost electrical conductivity. These simulations demonstrate that the CoSb and Janus Co2AsSb monolayers have immense potential for application in the design of metal-ion battery technologies.

Unraveling the modified mechanism of ruthenium substitution on Na3V2(PO4)3 with superior rate capability and ultralong cyclic performance

Na3V2(PO4)3(NVP) is an ideal cathode material for sodium ion battery due to its stable three-dimensional frame structure and high operating voltage. However, the low intrinsic conductivity and serious structural collapse limit its further application. In this work, a simultaneous optimized Na3V1.96Ru0.04(PO4)3/C@CNTs cathode material is synthesized by a simple sol-gel method. Specifically, the ionic radius of Ru3+ is slightly larger than that of V3+ (0.68 Å vs 0.64 Å), which not only ensures the feasibility of Ru3+ replacing V3+ site, but also appropriately expands the migration channel of sodium ions in NVP and stabilizes the structure, effectively improving the diffusion efficiency of sodium ions. Moreover, CNTs construct a three-dimensional conductive network between the grains, reducing the impedance at the interface and effectively improving the electronic conductivity. Ex-situ XRD analysis at different SOC were performed to determine the change in the crystal structure of Ru3+doped Na3V2(PO4)3, and the refinement results simultaneously demonstrate the relatively low volume shrinkage value of less than 3 % during the de-intercalation process, further verifying the stabilized crystal construction after Ru3+ substitution. Furthermore, the ex-situ XRD/SEM/CV/EIS after cycling indicate the significantly improved kinetic characteristics and enhanced structural stability. Notably, the modified Na3V1.96Ru0.04(PO4)3/C@CNTs reveals superior rate capability and ultralong cyclic performance. It submits high capacities of 82.3/80.9 mAh g-1 at 80/120C and maintains 71.3/59.6 mAh g-1 after 14800/6250 cycles, indicating excellent retention ratios of 86.6 % and 73.6 %, respectively. This work provides a multi-modification strategy for the realization of high-performance cathode materials, which can be widely applied in the optimization of various materials.

Emerging high voltage V4+/V5+ redox reactions in Na3V2(PO4)3-based cathodes for sodium-ion batteries

Na3V2(PO4)3 (NVP) cathode materials with the advantages of long cycle life and superior thermal stability have been considered promising cathode candidates for SIBs. However, the unsatisfactory energy density derived from low theoretical capacity and operating voltage (3.35 V vs. Na+/Na, based on the V3+/V4+ redox couple) inevitably limits their practical application. Therefore, the activation of the V4+/V5+ redox couple (∼4.0 V vs. Na+/Na) in NVP-based cathode materials to boost the energy density of SIBs has attracted extensive attention. Herein, we first analyze the challenges of activation of the V4+/V5+ redox couple in NVP-based cathode materials. Subsequently, the recent achievement of NVP-based cathode materials with activated V4+/V5+ redox reactions for SIBs is overviewed. Finally, further research directions of high voltage V4+/V5+ redox reactions in NVP-based cathodes are proposed. This review provides valuable guidance for developing high energy density NVP-based cathode materials for SIBs.

Achieving High Initial Coulombic Efficiency and Capacity in a Surface Chemical Grafting Layer of Plateau-type Sodium Titanate

The plateau-type sodium titanate with suitable sodiation potential is a promising anode candidate for high safe and high energy density of sodium-ion batteries (SIBs). However, the poor initial Coulombic efficiency (ICE) and cyclic instability of sodium titanate are attributed to the unstable interfacial structure along with the decomposition of electrolytes, resulting in the continuous formation of solid electrolyte interface (SEI) film. To address this issue, a chemical grafting method is developed to fabricate a highly stable interface layer of inert Al2O3 on the sodium titanate anode, rendering the high ICE and excellent cycling stability. Based on theoretical calculations, NaPF6 are more likely adsorption on the Al2O3 surface and produce sodium fluoride. The formation of a thin and dense SEI film with rich sodium fluoride achieves the low interfacial resistances and charge-transfer resistances. Benefitting from our design, the obtained sodium titanate exhibits a high ICE from 67.7 % to 79.4 % and an enhanced reversible capacity from 151 mAh g-1 to 181 mAh g-1 at 20 mA g-1, along with an increase in capacity retention from 56.5 % to 80.6 % after 500 cycles. This work heralds a promising paradigm for rational regulation of interfacial stability to achieve high-performance anodes for SIBs.

Low-Surface-Area Nitrogen-Doped Carbon Submicrospheres as High-Coulombic-Efficiency and High-Capacity Anodes for Practical Sodium-Ion Batteries

Nitrogen-doped carbon submicrospheres (NCSMs) are synthesized via an efficient and environmentally friendly one-pot polymerization reaction at room temperature, in which dopamine hydrochloride serves as the source for both carbon and nitrogen. Through leverage of its distinctive structure characterized by minimal surface area, fewer oxygen-containing functional groups, and a heightened presence of active nitrogen-doping sites, the synthesized NCSM showcases a noteworthy initial Coulombic efficiency (ICE) of 84.8%, a remarkable sodium storage capacity of 384 mAh g-1, an impressive rate capability of 215 mAh g-1 at 10 A g-1, and a superior cyclic performance, maintaining 83.0% of its capacity after 2000 cycles. The submicron spherical structure, with its limited surface area and scarce oxygen-containing moieties, effectively curtails the irreversible sodium-ion loss in solid-electrolyte interphase film formation, resulting in heightened ICE. The abundant nitrogen doping can expand carbon-layer spacing as well as improve the electron/ion-transport dynamics, guaranteeing a high sodium storage capacity and a strong rate capability. Crucially, the synthesis method presented here is straightforward, effective, and amenable to scaling, offering a novel avenue for the commercialization of sodium-ion batteries.

Tailoring of High-Valent Sn-Doped Porous Na3V2(PO4)3/C Nanoarchitechtonics: An Ultra High-Rate Cathode for Sodium-Ion Batteries

NASICON structured Na3V2(PO4)3 (NVP) has captured enormous attention as a potential cathode for next-generation sodium-ion batteries (SIBs), owing to its sturdy crystal structure and high theoretical capacity. Nonetheless, its poor intrinsic electronic conductivity has led to inferior electrochemical performance in terms of rate capability and long cycling performance. To address this problem, a combined strategy is adopted, such as (1) carbon coating and (2) high valent Sn4+ ion doping in the lattice site of vanadium in the NVP cathode. Carbon coating can effectively enhance the surface electronic conductivity, wherein high-valent Sn4+ improves the bulk intrinsic electronic conductivity of the materials. Moreover, Sn is a well-known alloying/dealloying type anode for SIBs; thus, doping of such metal in cathode materials will assume the role of structure stabilizing pillars and establishing high-performing cathode materials. Herein, Na3V2-xSnx(PO4)3/C (denoted as Sn(x)-NVP/C, where x = 0.00, 0.03, 0.05, 0.07, 0.1) were synthesized via sol-gel route, followed by calcination at 800 °C. XRD, Raman, XPS, and electron microscopy data confirmed the high purity of the synthesized cathode. The optimized Sn(0.07)-NVP/C exhibited excellent electrochemical performance in terms of high rate capability and long cycling performance, a high appreciable capacity of 98 mAh g-1 with capacity retention of 85% after 500 cycles. Similarly, at a high current of 20C, it is still able to deliver a stable capacity of 76 mAh g-1 with 85% capacity retention after 3000 cycles. The rate capability study indicates the high current tolerance of Sn(0.07)-NVP/C up to 70 C with a capacity delivery of 55 mAh g-1. It is worth mentioning that CV and EIS analysis for Sn(0.07)-NVP/C cathode displayed minimum voltage polarization and enhanced diffusion coefficient. Moreover, DFT calculation also proved that the electronic and ionic conductivity of NVP is promoted by Sn doping. Hence, the present results demonstrated that Sn(0.07)-NVP/C is considered a promising cathode for sodium-ion battery application.

Advanced Insights on MXenes: Categories, Properties, Synthesis, and Applications in Alkali Metal Ion Batteries

The development and optimization of promising anode material for next-generation alkali metal ion batteries are significant for clean energy evolution. 2D MXenes have drawn extensive attention in electrochemical energy storage applications, due to their multiple advantages including excellent conductivity, robust mechanical properties, hydrophilicity of its functional terminations, and outstanding electrochemical storage capability. In this review, the categories, properties, and synthesis methods of MXenes are first outlined. Furthermore, the latest research and progress of MXenes and their composites in alkali metal ion storage are also summarized comprehensively. A special emphasis is placed on MXenes and their hybrids, ranging from material design and fabrication to fundamental understanding of the alkali ion storage mechanisms to battery performance optimization strategies. Lastly, the challenges and personal perspectives of the future research of MXenes and their composites for energy storage are presented.

Boosting sodium-ion battery performance by anion doping in NASICON Na4MnCr(PO4)3 cathode

Na superionic conductor (NASICON)-structured Na4MnCr(PO4)3 (NMCP) possessing unique three-electron transfer process renders admirable energy density for sodium ion batteries (SIBs). However, the current issues like its sluggish Na+ diffusion kinetics, deficient intrinsic conductivity, and unsatisfactory structural stability, hinder its practical application. Herein, a selective replacement of O elements in PO4 group by Cl anions in the NMCP system was developed to significantly enhance its electrochemical performance. The results affirm that the enhanced performance of Cl doped samples can be attributed to the enlargement of cell size, the creation of Na vacancies and the weakness of Na2O bond after Cl doping. The as-prepared Na3.85□0.15MnCr(PO3.95Cl0.05)3/C (NMCPC - 15/C) cathode delivers a high capacity (128.0 mAh/g at 50 mA g-1) and excellent rate performance (73.0 mAh/g at 1000 mA g-1) in contrast to NMCP/C that merely provides 105.2 mAh/g at 50 mA g-1 and reduces to 47.4 mAh/g at 1000 mA g-1. Meanwhile, NMCPC - 15/C shows a capacity retention of 60.7 % at 1000 mA g-1 after 500 cycles, while only 37.1 % for NMCP/C in the same test conditions. Moreover, the satisfactory performance and energy density of NMCPC - 15/C||hard carbon (HC) full cell confirm the potential practicality of NMCPC - 15. Therefore, chloride ions doping into NMCP has practical application prospects in the preparation of high-performance cathode materials and our work also offers new inspiration to apply anion doping strategies in promoting the performance of the other NASICON-structured cathodes for SIBs.

Electrochemical Investigations of Sulfur-Decorated Organic Materials as Cathodes for Alkali Batteries

Alkali metal-sulfur batteries (particularly, lithium/sodium- sulfur (Li/Na-S)) have attracted much attention because of their high energy density, the natural abundance of sulfur, and environmental friendliness. However, Li/Na-S batteries still face big challenges, such as limited cycle life, poor conductivity, large volume changes, and the "shuttle effect" caused by the high solubility of Li/Na-polysulfides. Herein, novel organosulfur-containing materials, i.e., bis(4-hydroxy-2,2,6,6-tetramethylpiperidin-1-yl)disulfide (BiTEMPS-OH) and 2,4-thiophene/arene copolymer (TAC) are proposed as cathode materials for Li and Na batteries. BiTEMPS-OH shows an initial discharge/charge capacity of 353/192 mAh g-1 and a capacity of 62 mAh g-1 after 200 cycles at 100 mA g-1 in ether-based Li-ion electrolyte. Meanwhile, TAC has an initial discharge/charge capacity of 270/248 mAh g-1 and better cycling performance (106 mAh g-1 after 200 cycles) than BiTEMPS-OH in the same electrolyte. However, the rate capability of TAC is limited by the slow diffusion of Li-ions. Both materials show inferior electrochemical performances in Na battery cells compared to the Li analogs. X-ray powder diffraction reveals that BiTEMPS-OH loses its crystalline structure permanently upon cycling in Li battery cells. X-ray photoelectron spectroscopy demonstrates the cleavage and partially reversible formation of S-S bonds in BiTEMPS-OH and the formation/decomposition of thick solid electrolyte interphase on the electrode surface of TAC.

Deciphering the Performance Enhancement, Cell Failure Mechanism, and Amelioration Strategy of Sodium Storage in Metal Chalcogenides-Based Andes

Transition metal chalcogenides (TMCs) emerge as promising anode materials for sodium-ion batteries (SIBs), heralding a new era of energy storage solutions. Despite their potential, the mechanisms underlying their performance enhancement and susceptibility to failure in ether-based electrolytes remain elusive. This study delves into these aspects, employing CoS2 electrodes as a case in point to elucidate the phenomena. The investigation reveals that CoS2 undergoes a unique irreversible and progressive solid-liquid-solid phase transition from its native state to sodium polysulfides (NaPSs), and ultimately to a Cu1.8S/Co composite, accompanied by a gradual morphological transformation from microspheres to a stable 3D porous architecture. This reconstructed 3D porous structure is pivotal for its exceptional Na+ diffusion kinetics and resilience to cycling-induced stress, being the main reason for ultrastable cycling and ultrahigh rate capability. Nonetheless, the CoS2 electrode suffers from an inevitable cycle life termination due to the microshort-circuit induced by Na metal corrosion and separator degradation. Through a comparative analysis of various TMCs, a predictive framework linking electrode longevity is established to electrode potential and Gibbs free energy. Finally, the cell failure issue is significantly mitigated at a material level (graphene encapsulation) and cell level (polypropylene membrane incorporation) by alleviating the NaPSs shuttling and microshort-circuit.

Fast Na+ Kinetics and Suppressed Voltage Hysteresis Enabled by a High-Entropy Strategy for Sodium Oxide Cathodes

O3-type layered transition metal cathodes are promising energy storage materials due to their sufficient sodium reservoir. However, sluggish sodium ions kinetics and large voltage hysteresis, which are generally associated with Na+ diffusion properties and electrochemical phase transition reversibility, drastically minimize energy density, reduce energy efficiency, and hinder further commercialization of sodium-ion batteries (SIBs). Here, this work proposes a high-entropy tailoring strategy through manipulating the electronic local environment within transition metal slabs to circumvent these issues. Experimental analysis combined with theoretical calculations verify that high-entropy metal ion mixing contributes to the improved reversibility of redox reaction and O3-P3-O3 phase transition behaviors as well as the enhanced Na+ diffusivity. Consequently, the designed O3-Na0.9Ni0.2Fe0.2Co0.2Mn0.2Ti0.15Cu0.05O2 material with high-entropy characteristic could display a negligible voltage hysteresis (<0.09 V), impressive rate capability (98.6 mAh g-1 at 10 C) and long-term cycling stability (79.4% capacity retention over 2000 cycles at 5 C). This work provides insightful guidance in mitigating the voltage hysteresis and facilitating Na+ diffusion of layered oxide cathode materials to realize high-rate and high-energy SIBs.

Interface engineering of metal sulfides-based composites enables high-performance anode materials for sodium-ion batteries

Metal sulfides (MSs) have attracted much attention as anode materials for sodium-ion batteries (SIBs) due to their high sodium storage capacity. However, the unsatisfactory electrochemical performance induced by the huge volume change and sluggish kinetics hampered the practical application of SIBs. Herein, guided by the heterostructure interface engineering, novel multicomponent metal sulfide-based anodes, including SnS, FeS, and Fe3N embedded in N-doped carbon nanosheets (SnS/FeS/Fe3N/NC NSs), have been synthesized for high-performance SIBs. The as-prepared SnS/FeS/Fe3N/NC NSs with abundant heterointerfaces and high conductivity of N-doped carbon nanosheet matrix can shorten the Na+ diffusion path and promote reaction kinetics during the sodiation/desodiation process. Moreover, the presence of Fe3N can promote the reversible conversion of SnS and FeS during the cycling process. As a consequence, when evaluated as anode materials for SIBs, the SnS/FeS/Fe3N/NC NSs can maintain a high sodium storage capacity of 473.6 mAh g-1 after 600 cycles at 2.0 A g-1 and can still provide a high reversible capacity of 537.4 mAh g-1 even at 5.0 A g-1 This discovery offers a novel strategy for constructing metal sulfide-based anode materials for high-performance SIBs.

Electrolyte and Additive Engineering for Zn Anode Interfacial Regulation in Aqueous Zinc Batteries

Aqueous Zn-metal batteries (AZMBs) have gained great interest due to their low cost, eco-friendliness, and inherent safety, which serve as a promising complement to the existing metal-based batteries, e.g., lithium-metal batteries and sodium-metal batteries. Although the utilization of aqueous electrolytes and Zn metal anode in AZMBs ensures their improved safety over other metal batteries meanwhile guaranteeing their decent energy density at the cell level, plenty of challenges involved with metallic Zn anode still await to be addressed, including dendrite growth, hydrogen evolution reaction, and zinc corrosion and passivation. In the past years, several attempts have been adopted to address these problems, among which engineering the aqueous electrolytes and additives is regarded as a facile and promising approach. In this review, a comprehensive summary of aqueous electrolytes and electrolyte additives will be given based on the recent literature, aiming at providing a fundamental understanding of the challenges associated with the metallic Zn anode in aqueous electrolytes, meanwhile offering a guideline for the electrolytes and additives engineering strategies toward stable AZMBs in the future.

Dual-Site Doping in Transition Metal Oxide Cathode Enables High-Voltage Stability of Na-Ion Batteries

Designing cathode materials that effectively enhancing structural stability under high voltage is paramount for rationally enhancing energy density and safety of Na-ion batteries. This study introduces a novel P2-Na0.73K0.03Ni0.23Li0.1Mn0.67O2 (KLi-NaNMO) cathode through dual-site synergistic doping of K and Li in Na and transition metal (TM) layers. Combining theoretical and experimental studies, this study discovers that Li doping significantly strengthens the orbital overlap of Ni (3d) and O (2p) near the Fermi level, thereby regulates the phase transition and charge compensation processes with synchronized Ni and O redox. The introduction of K further adjusts the ratio of Nae and Naf sites at Na layer with enhanced structural stability and extended lattice space distance, enabling the suppression of TM dissolution, achieving a single-phase transition reaction even at a high voltage of 4.4 V, and improving reaction kinetics. Consequently, KLi-NaNMO exhibits a high capacity (105 and 120 mAh g-1 in the voltage of 2-4.2 V and 2-4.4 V at 0.1 C, respectively) and outstanding cycling performance over 300 cycles under 4.2 and 4.4 V. This work provides a dual-site doping strategy to employ synchronized TM and O redox with improved capacity and high structural stability via electronic and crystal structure modulation.

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.

A Comprehensive Review on Strategies for Enhancing the Performance of Polyanionic-Based Sodium-Ion Battery Cathodes

The significant consumption of fossil fuels and the increasing pollution have spurred the development of energy-storage devices like batteries. Due to their high cost and limited resources, widely used lithium-ion batteries have become unsuitable for large-scale energy production. Sodium is considered to be one of the most promising substitutes for lithium due to its wide availability and similar physiochemical properties. Designing a suitable cathode material for sodium-ion batteries is essential, as the overall electrochemical performance and the cost of battery depend on the cathode material. Among different types of cathode materials, polyanionic material has emerged as a great option due to its higher redox potential, stable crystal structure, and open three-dimensional framework. However, the poor electronic and ionic conductivity limits their applicability. This review briefly discusses the strategies to deal with the challenges of transition-metal oxides and Prussian blue analogue, recent developments in polyanionic compounds, and strategies to improve electrochemical performance of polyanionic material by nanostructuring, surface coating, morphology control, and heteroatom doping, which is expected to accelerate the future design of sodium-ion battery cathodes.

Heterogeneous MoS2 Nanosheets on Porous TiO2 Nanofibers toward Fast and Reversible Sodium-Ion Storage

2D layered molybdenum disulfide (MoS2) has garnered considerable attention as an attractive electrode material in sodium-ion batteries (SIBs), but sluggish mass transfer kinetic and capacity fading make it suffer from inferior cycle capability. Herein, hierarchical MoS2 nanosheets decorated porous TiO2 nanofibers (MoS2 NSs@TiO2 NFs) with rich oxygen vacancies are engineered by microemulsion electrospinning method and subsequent hydrothermal/heat treatment. The MoS2 NSs@TiO2 NFs improves ion/electron transport kinetic and long-term cycling performance through distinctive porous structure and heterogeneous component. Consequently, the electrode exhibits excellent long-term Na storage capacity (298.4 mAh g-1 at 5 A g-1 over 1100 cycles and 235.6 mAh g-1 at 10 A g-1 over 7200 cycles). Employing Na3V2(PO4)3 as cathode, the full cell maintains a desirable capacity of 269.6 mAh g-1 over 700 cycles at 1.0 A g-1. The stepwise intercalation-conversion and insertion/extraction endows outstanding Na+ storage performance, which yields valuable insight into the advancement of fast-charging and long-cycle life SIBs anode materials.

High-Entropy Configuration Strategy to Build High Performance Na-Ion Layered Oxide Cathodes Derived from Simple Techniques

Layered transition metal oxides are commonly used as the cathode materials in sodium-ion batteries due to their low cost and easy manufacturing. However, the application is hindered by poor rate performance and complex phase transitions. To address these challenges, a new seven-component high-entropy layered oxide cathode material, O3-NaNi0.25Fe0.15Mn0.3Ti0.1Sn0.05Co0.05Li0.1O2 (HEO) has been developed. The entropy stabilization effect plays a crucial role in improving the performance of electrochemical systems and the stability of structures. The HEO exhibits a specific discharge capacity of 154.1 mA h g-1 at 0.1 C and 94.5 mA h g-1 at 7 C. In-situ and ex-situ XRD results demonstrate that the HEO effectively retards complex phase transitions. This work provides a high-entropy design for the storage materials with a high energy density. Meanwhile, it eliminates industry doubts about the performance of sodium ion layered oxide cathode materials.

A Review of Macrocycles Applied in Electrochemical Energy Storge and Conversion

Macrocycles composed of diverse aromatic or nonaromatic structures, such as cyclodextrins (CDs), calixarenes (CAs), cucurbiturils (CBs), and pillararenes (PAs), have garnered significant attention due to their inherent advantages of possessing cavity structures, unique functional groups, and facile modification. Due to these distinctive features enabling them to facilitate ion insertion and extraction, form crosslinked porous structures, offer multiple redox-active sites, and engage in host-guest interactions, macrocycles have made huge contributions to electrochemical energy storage and conversion (EES/EEC). Here, we have summarized the recent advancements and challenges in the utilization of CDs, CAs, CBs, and PAs as well as other novel macrocycles applied in EES/EEC devices. The molecular structure, properties, and modification strategies are discussed along with the corresponding energy density, specific capacity, and cycling life properties in detail. Finally, crucial limitations and future research directions pertaining to these macrocycles in electrochemical energy storage and conversion are addressed. It is hoped that this review is able to inspire interest and enthusiasm in researchers to investigate macrocycles and promote their applications in EES/EEC.

Modulating Valence Electrons and Na Occupancy in Layered Cathodes for High-Performance Na-Ion Batteries

Na-ion batteries (NIBs) hold promise as a leading option for large-scale energy storage. However, their development faces challenges due to the lack of high-performance cathode materials. P2-type layered oxides are seen as potential cathode materials for NIBs due to higher structure stability, yet their commercialization is hindered by limited capacity and subpar phase transitions during Na extraction and insertion at high voltages. In this study, we introduce a new P2-type cathode material, Na0.76Ni0.23Li0.1Ti0.02Mn0.65O1.998F0.02 (NLTMOF), synthesized with ternary Li/Ti/F substitution. This modification of ternary Li/Ti/F substitution significantly tailors the electronic structures, increasing the number of valence electrons near the Fermi energy level. This facilitates the electronic conductivity and their involvement in charge compensation, thereby enhancing reversible capacity. Additionally, ternary doping synergistically adjusts the Na occupancy at the Na layer for favorable Na extraction without P2-O2 phase transitions even under a high voltage of 4.4 V, boosting cycling stability. As a result, NLTMOF demonstrates a reversible capacity of 110.0 and 132.2 mAh g-1 at 2-4.2 and 2-4.4 V, respectively, and maintains greatly enhanced cycling stability over long cycles. This study sheds light on the design of transition metal oxides for advanced cathode materials through the modulation of electronic structure and Na occupancy in cathode materials, thus promoting the development of NIBs.

Improved Compatibility of α-NaMnO2 Cathodes at the Interface with Ionic Liquid Electrolytes

The behaviour and compatibility of monoclinic sodium manganite, α-NaMnO2, cathodes at the interface with electrolytes based on the 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIFSI) and N-trimethyl-N-butylammonium bis(fluorosulfonyl)imide (N1114FSI) ionic liquids is presented and discussed. The Na+ insertion process was analysed through cyclic voltammetry tests combined with impedance spectroscopy measurements and the cell performance was tested by charge-discharge cycles. XPS and FIB-SEM measurements allowed analysis of the surface composition and the morphology of post-mortem cathodes. Overall, the α-NaMnO2 cathode showed high reversibility in N1114FSI-based electrolyte, delivering 60 % of the initial capacity after 1200 cycles in conjunction with a Coulombic efficiency above 99 %. To our knowledge, these very promising results are the best result obtained till now for monolithic α-NaMnO2 cathodes, are ascribable to the formation of a stable passive layer onto the electrode surface, as confirmed by spectroscopic analysis.

Optimizing electrochemical performance of Na0.67Ni0.17Co0.17Mn0.66O2 with P2 structure via preparing concentration-gradient particles for sodium-ion batteries

P2-type layered oxides for rechargeable sodium-ion batteries have drawn a lot of attention because of their excellent electrochemical performance. However, these types of cathodes usually suffer from poor cyclic stability. To overcome this disadvantage, in this work, novel ball-shaped concentration-gradient oxide Na0.67Ni0.17Co0.17Mn0.66O2 with P2 structure modified by Mn-rich surface is successfully prepared using co-precipitation method. The concentration of Mn increased from the inner core to the surface, endowing the material with an excellent cyclic stability. The cathode exhibits enhanced electrochemical properties than that of the sample synthesized by solid-state method and concentration-constant material. It shows 143.2 mAh/g initial discharge capacity and retains 131 mAh/g between 2 V and 4.5 V after 100 rounds. The significant improvement in the electrochemical properties of the sample benefits from the unique concentration-gradient structure, and the Mn-rich surface that effectively stabilizes the basic P2 structure. The relatively higher Ni content in the core leads to a slight improvement in the discharge capacity of the sample. This strategy may provide new insights for preparing layered cathodes for sodium-ion batteries with high electrochemical performance.

Ion Dynamics at the Intermediate Charging State of the Sodium Vanadium Fluorophosphate Cathode

Na super ionic conductor (NASICON)-type polyanionic vanadium fluorophosphate Na3V2O2(PO4)2F (NVOPF) is a promising cathode material for high-energy sodium-ion batteries. The dynamic diffusion and exchange of sodium ions in the lattice of NVOPF are crucial for its electrochemical performance. However, standard characterizations are mostly focused on the as-synthesized material without cycling, which is different from the actual battery operation conditions. In this work, we investigated the hopping processes of sodium in NVOPF at the intermediate charging state with 23Na solid-state nuclear magnetic resonance (ssNMR) and density functional theory (DFT) calculations. Our experimental characterizations revealed six distinct sodium coordination sites in the intermediate structure and determined the exchange rates among these sites at variable temperatures. The theoretical calculations showed that these dynamic processes correspond to different ion transport pathways in the crystalline lattice. Our combined experimental and theoretical study uncovered the underlying mechanisms of the ion transport in cycled NVOPF and these understandings may help the optimization of cathode materials for sodium-ion batteries.

Anodic voltage performance of conducting polymer-functionalized boron nitride nanosheets: a DFT assessment

The search for low-diffusion barriers and high-capacity anode materials is considered a key step in boosting the efficiency of metal-ion batteries. Herein, we investigate the impact of a series of conducting polymers (CPs), namely, polyacetylene (PA), polypyrrole (PP), poly-p-phenylene (PPPh), and polythiophene (PT), on enhancing the material design and anodic performance of boron nitride nanosheet (BNNS)-based Li-ion and Na-ion batteries. For this purpose, first principle DFT simulations, utilizing both clustered and periodic models, are systematically performed to assess the stability of such nanostructures and their electronic behavior as potential anodic materials. It is revealed that frontier molecular orbitals calculated for BNNSs are stabilized upon association with the series of CPs, resulting in a reduction in the energy gaps of CP-BNNSs by nearly 50%, which in turn improves the charge transfer properties and cell reaction kinetics. A remarkable improvement in the cell voltage is predicted for PP and PT functionalized BNNSs, reaching approximately 3.5 V for Li+ and 3.0 V for Na+ ions. The outcome of the study emphasizes the influence of the size of metal ions, whether mono- or di-valent, and the nature of adsorbed conducting polymers. Manipulating the electronic features of boron nitride nanostructured surfaces through non-covalent functionalization with conducting polymers could pave the way for the design of highly efficient energy storage anodic CP-BNNS-based systems.

Electrolyte Design with Dual -C≡N Groups Containing Additives to Enable High-Voltage Na3V2(PO4)2F3-Based Sodium-Ion Batteries

Na3V2(PO4)2F3 is recognized as a promising cathode for high energy density sodium-ion batteries due to its high average potential of ∼3.95 V (vs Na/Na+). A high-voltage-resistant electrolyte is of high importance due to the long duration of 4.2 V (vs Na/Na+) when improving cyclability. Herein, a targeted electrolyte containing additives with two -C≡N groups like succinonitrile has been designed. In this design, one -C≡N group is accessible to the solvation sheath and enables the other -C≡N in dinitrile being exposed and subsequently squeezed into the electric double layer. Then, the squeezed -C≡N group is prone to a preferential adsorption on the electrode surface prior to the exposed -CH2/-CH3 in Na+-solvent and oxidized to construct a stable and electrically insulating interface enriched CN-/NCO-/Na3N. The Na3V2(PO4)2F3-based sodium-ion batteries within a high-voltage of 2-4.3 V (vs Na/Na+) can accordingly achieve an excellent cycling stability (e.g., 95.07% reversible capacity at 1 C for 1,5-dicyanopentane and 98.4% at 2 C and 93.0% reversible capacity at 5 C for succinonitrile after 1000 cycles). This work proposes a new way to design high-voltage electrolytes for high energy density sodium-ion batteries.

Machine-Learning-Driven High-Throughput Screening for High-Energy Density and Stable NASICON Cathodes

The Na super ionic conductor (NASICON), which has outstanding structural stability and a high operating voltage, is an appealing material for overcoming the limits of low specific energy and larger volume distortion of sodium-ion batteries. In this study, to discover ideal NASICON cathode materials, a screening platform based on density functional theory (DFT) calculations and machine learning (ML) is developed. A training database was generated utilizing the previous 124 545 electrode databases, and a test set of 3126 potential NASICON structures [NaxMyM'1-y(PO4)3] with 27 dopants at the metal site and 6 dopants at the polyanion central site was constructed. The developed ML surrogate model identifies 796 materials that satisfy the following criteria: formation energy of <0.0 eV/atom, energy above hull of ≤0.025 eV/atom, volume change of ≤4%, and theoretical capacity of ≥50 mAh/g. The thermodynamically stable configurations of doped NASICON structures were then selected using machine learning interatomic potential (MLIP), enabling rapid consideration of various dopant site configurations. DFT calculations are followed on 796 screened materials to obtain energy density, average voltage, and volume change. Finally, 50 candidates with an average voltage of ≥3.5 V are identified. The suggested platform accelerates the exploration for optimal NASICON materials by narrowing the focus on materials with desired properties, saving considerable resources.

Hard Carbon Derived From Different Precursors for Sodium Storage

Due to the almost unlimited resource and acceptable performance, Sodium-ion batteries (SIBs) have been regarded as a promising alternative for lithium-ion batteries (LIBs) for grid-scale energy storage. As the key material of SIBs, hard carbon (HC) plays a decisive role in determining the batteries' performance. Nevertheless, the micro-structure of HCs is quite complex and the random organization of turbostratically stacked graphene layers, closed pores, and defects make the structure-performance relationship insufficiently revealed. On the other hand, the impending large-scale deployment of SIBs leads to producing HCs with low-cost and abundant precursors actively pursued. In this work, the recent progress of preparing HCs from different precursors including biomass, polymers, and fossil fuels is summarized with close attention to the influences of precursors on the structural evolution of HCs. After a brief introduction of the structural features of HCs, the recent understanding of the structure-performance relationship of HCs for sodium storage is summarized. Then, the main focus is concentrated on the progress of producing HCs from distinct precursors. After that, the pros and cons of HCs derived from different precursors are comprehensively compared to conclude the selection rules of precursors. Finally, the further directions of HCs are deeply discussed to end this review.

Hard carbon for sodium-ion batteries: progress, strategies and future perspective

Because of its abundant resources, low cost and high reversible specific capacity, hard carbon (HC) is considered as the most likely commercial anode material for sodium-ion batteries (SIBs). Therefore, reasonable design and effective strategies to regulate the structure of HCs play a crucial role in promoting the development of SIBs. Herein, the progress in the preparation approaches for HC anode materials is systematically overviewed, with a special focus on the comparison between traditional fabrication methods and advanced strategies emerged in recent years in terms of their influence on performance, including preparation efficiency, initial coulombic efficiency (ICE), specific capacity and rate capability. Furthermore, the advanced strategies are categorized into two groups: those exhibiting potential for large-scale production to replace traditional methods and those presenting guidelines for achieving high-performance HC anodes from top-level design. Finally, challenges and future development prospects to achieve high-performance HC anodes are also proposed. We believe that this review will provide beneficial guidance to actualize the truly rational design of advanced HC anodes, facilitating the industrialization of SIBs and assisting in formulating design rules for developing high-end advanced electrode materials for energy storage devices.

Classification of battery compounds using structure-free Mendeleev encodings

Machine learning is a valuable tool that can accelerate the discovery and design of materials occupying combinatorial chemical spaces. However, the prerequisite need for vast amounts of training data can be prohibitive when significant resources are needed to characterize or simulate candidate structures. Recent results have shown that structure-free encoding of complex materials, based entirely on chemical compositions, can overcome this impediment and perform well in unsupervised learning tasks. In this study, we extend this exploration to supervised classification, and show how structure-free encoding can accurately predict classes of material compounds for battery applications without time consuming measurement of bonding networks, lattices or densities. SCIENTIFIC CONTRIBUTION: The comprehensive evaluation of structure-free encodings of complex materials in classification tasks, including binary and multi-class separation, inclusive of three classifiers based on different logic function, is measured four metrics and learning curves. The encoding is applied to two data sets from computational and experimental sources, and the outcomes visualised using 5 approaches to confirms the suitability and superiority of Mendeleev encoding. These methods are general and accessible using source software, to provide simple, intuitive and interpretable materials informatics outcomes to accelerate materials design.

A dual strategy of Na+/vacancy disorder and high Na to construct a P2-type cathode for high-stability sodium-ion batteries

P2-type layered oxides are widely regarded as highly promising contenders for cathode materials in sodium-ion batteries. However, the occurrence of severe reactive phase transitions hinders satisfactory cycling stability and rate performance, thereby imposing limitations on their practical application. Here we prepared P2-type Na0.75Ni0.23Mg0.1Mn0.67O2 cathode materials using the agar gel approach. The use of agar reduces the synthesis time significantly, and the high Na content enhances the stability of the structure and contributes to its capacity. Meanwhile, the introduction of electrochemically inactive Mg ions into sodium layers not only disrupts the Na+/vacancy ordering, but also increases the spacing between sodium layers, thus reducing the diffusion barrier for sodium ions. The dual modification strategy led to excellent stability of Na0.75Ni0.23Mg0.1Mn0.67O2 with 94% capacity retention after 100 cycles at 1C. This work provides new insights into the design of sodium-ion cathode materials.

Progress and perspectives on iron-based electrode materials for alkali metal-ion batteries: a critical review

Iron-based materials with significant physicochemical properties, including high theoretical capacity, low cost and mechanical and thermal stability, have attracted research attention as electrode materials for alkali metal-ion batteries (AMIBs). However, practical implementation of some iron-based materials is impeded by their poor conductivity, large volume change, and irreversible phase transition during electrochemical reactions. In this review we critically assess advances in the chemical synthesis and structural design, together with modification strategies, of iron-based compounds for AMIBs, to obviate these issues. We assess and categorize structural and compositional regulation and its effects on the working mechanisms and electrochemical performances of AMIBs. We establish insight into their applications and determine practical challenges in their development. We provide perspectives on future directions and likely outcomes. We conclude that for boosted electrochemical performance there is a need for better design of structures and compositions to increase ionic/electronic conductivity and the contact area between active materials and electrolytes and to obviate the large volume change and low conductivity. Findings will be of interest and benefit to researchers and manufacturers for sustainable development of advanced rechargeable ion batteries using iron-based electrode materials.

Routes to high-performance layered oxide cathodes for sodium-ion batteries

Sodium-ion batteries (SIBs) are experiencing a large-scale renaissance to supplement or replace expensive lithium-ion batteries (LIBs) and low energy density lead-acid batteries in electrical energy storage systems and other applications. In this case, layered oxide materials have become one of the most popular cathode candidates for SIBs because of their low cost and comparatively facile synthesis method. However, the intrinsic shortcomings of layered oxide cathodes, which severely limit their commercialization process, urgently need to be addressed. In this review, inherent challenges associated with layered oxide cathodes for SIBs, such as their irreversible multiphase transition, poor air stability, and low energy density, are systematically summarized and discussed, together with strategies to overcome these dilemmas through bulk phase modulation, surface/interface modification, functional structure manipulation, and cationic and anionic redox optimization. Emphasis is placed on investigating variations in the chemical composition and structural configuration of layered oxide cathodes and how they affect the electrochemical behavior of the cathodes to illustrate how these issues can be addressed. The summary of failure mechanisms and corresponding modification strategies of layered oxide cathodes presented herein provides a valuable reference for scientific and practical issues related to the development of SIBs.

Optimizing cell voltage dependence on size of carbon nanotube-based electrodes in Na-ion and K-ion batteries

Sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs) are gaining extensive attention as promising alternatives to lithium-ion batteries owing to their superior energy density and cost-effectiveness. However, the larger ionic radius of Na+ and K+ ions in comparison to Li+ ions poses a challenge in designing anode materials characterized by enduring structures and elevated voltage to facilitate the efficacy of high-performance NIBs and KIBs. Carbon nanomaterials, particularly carbon nanotubes (CNTs), have emerged as a potential candidate in anode materials. Herein, we used density functional theory calculations to study the cell voltage of CNTs in relation to Na-ion and K-ion storage as a function of CNT size. The adsorption energy profiles of both Na+@CNT and K+@CNT systems exhibit a descending trend concomitant with the increase in the CNT diameter, where Na+/K+ ion primarily prefers to adsorb in the interior wall of CNT. Conversely, the cell voltage for the Na and K system gradually increases with the increasing diameter of CNT, which can be attributed to the stronger electrostatic interaction validated by energy decomposition calculation. The voltage of Na-ion adsorbed on the inter wall of (10,10) CNT attains 1.29 V, close to the previously theoretical voltage of Li-ion on the same CNT (1.35 V), while the much lower voltage pertaining to K-ion adsorption on the inter wall of (10,10) CNT just stands at 0.59 V, suggesting the viability of CNT-based electrode for NIBs but not for KIBs. These findings lay a solid foundation for delineating the interrelationship between the voltage properties of CNT as prospective anode material and their structural characteristics, thereby expanding the application of CNT-based optoelectronic devices.

Efficient sulfur atom-doped three-dimensional porous MXene-assisted sodium ion batteries

Recently, it has been reported that MXene is a promising pseudocapacitive material for energy storage, primarily due to its intercalation mechanism. However, Ti3C2Tx MXenes face challenges, such as inadequate layer spacing and low specific capacity, which greatly hinder their potential as anode materials for sodium storage. In this study, MXene was doped with sulfur to create a three-dimensional porous structure that resulted in an increased layer spacing. The sulfur-doped porous MXene (SPM) demonstrated exceptional performance as sodium ion battery anodes, with a capacity of 335.2 mA h g-1 after 490 cycles at 2 A g-1 and a long-term cycling performance of 256.1 mA h g-1 even after 2480 cycles at 5 A g-1. It is worth noting that the porous structure formed after sulfur-doping exhibits superior sodium storage performance compared to previously reported MXene-based electrodes. This highlights the feasibility of the structural construction strategy, offering an effective solution for energy storage and conversion applications.

Regulating the Pore Structure of Activated Carbon by Pitch for High-Performance Sodium-Ion Storage

The pore structure of carbon anodes plays a crucial role in enhancing the sodium storage capacity. Designing more confined pores in carbon anodes is accepted as an effective strategy. However, current design strategies for confined pores in carbon anodes fail to achieve both high capacity and initial Coulombic efficiency (ICE) simultaneously. Herein, we develop a strategy for utilizing the repeated impregnation and precarbonization method of liquid pitch to regulate the pore structure of the activated carbon (AC) material. Driven by capillary coalescence, the pitch is impregnated into the pores of AC, which reduces the specific surface area of the material. During the carbonization process, numerous pores with diameters less than 1 nm are formed, resulting in a high capacity and improved ICE of the carbon anode. Moreover, the ordered carbon layers derived from the liquid pitch also enhance the electrical conductivity, thereby improving the rate capability of as-obtained carbon anodes. This enables the fabricated material (XA-4T-1300) to have a high ICE of 91.1% and a capacity of 383.0 mA h g-1 at 30 mA g-1. The capacity retention is 95.5% after 300 cycles at 1 A g-1. This study proposes a practical approach to adjust the microcrystalline and pore structures to enhance the performance of sodium-ion storage in materials.

Electron paramagnetic resonance as a tool to determine the sodium charge storage mechanism of hard carbon

Hard carbon is a promising negative electrode material for rechargeable sodium-ion batteries due to the ready availability of their precursors and high reversible charge storage. The reaction mechanisms that drive the sodiation properties in hard carbons and subsequent electrochemical performance are strictly linked to the characteristic slope and plateau regions observed in the voltage profile of these materials. This work shows that electron paramagnetic resonance (EPR) spectroscopy is a powerful and fast diagnostic tool to predict the extent of the charge stored in the slope and plateau regions during galvanostatic tests in hard carbon materials. EPR lineshape simulation and temperature-dependent measurements help to separate the nature of the spins in mechanochemically modified hard carbon materials synthesised at different temperatures. This proves relationships between structure modification and electrochemical signatures in the galvanostatic curves to obtain information on their sodium storage mechanism. Furthermore, through ex situ EPR studies we study the evolution of these EPR signals at different states of charge to further elucidate the storage mechanisms in these carbons. Finally, we discuss the interrelationship between EPR spectroscopy data of the hard carbon samples studied and their corresponding charging storage mechanism.

Regulating Ion-Dipole Interactions in Weakly Solvating Electrolyte towards Ultra-Low Temperature Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are recognized as promising energy storage devices. However, they suffer from rapid capacity decay at ultra-low temperatures due to high Na+ desolvation energy barrier and unstable solid electrolyte interphase (SEI). Herein, a weakly solvating electrolyte (WSE) with decreased ion-dipole interactions is designed for stable sodium storage in hard carbon (HC) anode at ultra-low temperatures. 2-methyltetrahydrofuran with low solvating power is incorporated into tetrahydrofuran to regulate the interactions between Na+ and solvents. The reduced Na+-dipole interactions facilitate more anionic coordination in the first solvation sheath, which consistently maintains anion-enhanced solvation structures from room to low temperatures to promote inorganic-rich SEI formation. These enable WSE with a low freezing point of -83.3 °C and faster Na+ desolvation kinetics. The HC anode thus affords reversible capacities of 243.2 and 205.4 mAh g-1 at 50 mA g-1 at -40 and -60 °C, respectively, and the full cell of HC||Na3V2(PO4)3 yields an extended lifespan over 250 cycles with high capacity retention of ~100 % at -40 °C. This work sheds new lights on the ion-dipole regulation for ultra-low temperature SIBs.

Microporous Sulfur-Carbon Materials with Extended Sodium Storage Window

Developing high-performance carbonaceous anode materials for sodium-ion batteries (SIBs) is still a grand quest for a more sustainable future of energy storage. Introducing sulfur within a carbon framework is one of the most promising attempts toward the development of highly efficient anode materials. Herein, a microporous sulfur-rich carbon anode obtained from a liquid sulfur-containing oligomer is introduced. The sodium storage mechanism shifts from surface-controlled to diffusion-controlled at higher synthesis temperatures. The different storage mechanisms and electrode performances are found to be independent of the bare electrode material's interplanar spacing. Therefore, these differences are attributed to an increased microporosity and a thiophene-rich chemical environment. The combination of these properties enables extending the plateau region to higher potential and achieving reversible overpotential sodium storage. Moreover, in-operando small-angle X-ray scattering (SAXS) reveals reversible electron density variations within the pore structure, in good agreement with the pore-filling sodium storage mechanism occurring in hard carbons (HCs). Eventually, the depicted framework will enable the design of high-performance anode materials for sodium-ion batteries with competitive energy density.

Dynamic Behavior of Spatially Confined Sn Clusters and Its Application in Highly Efficient Sodium Storage with High Initial Coulombic Efficiency

Advanced battery electrodes require a cautious design of microscale particles with built-in nanoscale features to exploit the advantages of both micro- and nano-particles relative to their performance attributes. Herein, the dynamic behavior of nanosized Sn clusters and their host pores in carbon nanofiber) during sodiation and desodiation is revealed using a state-of-the-art 3D electron microscopic reconstruction technique. For the first time, the anomalous expansion of Sn clusters after desodiation is observed owing to the aggregation of clusters/single atoms. Pore connectivity is retained despite the anomalous expansion, suggesting inhibition of solid electrolyte interface formation in the sub-2-nm pores. Taking advantage of the built-in nanoconfinement feature, the CNF film with nanometer-sized interconnected pores hosting Sn clusters (≈2 nm) enables high utilization (95% at a high rate of 1 A g-1) of Sn active sites while maintaining an improved initial Coulombic efficiency of 87%. The findings provide insights into electrochemical reactions in a confined space and a guiding principle in electrode design for battery applications.