Rechargeable Sodium-ion Batteries Research Articles

Electrolyte-Salts Regulated Hydrogen-Bonding Configuration and Interphase Formation Achieving Highly Stable Anode for Rechargeable Aqueous Sodium-Ion Batteries.

Hydrogen evolution reactions that cause the alkalization of aqueous electrolytes generally frustrate the structural stability and cycling performance of NaTi2(PO4)3/C anode material for rechargeable aqueous sodium-ion batteries (ASIBs). Herein, a novel highly concentrated electrolyte with a large hydrogen-evolution overpotential and hydroxide-capture ability is rationally established by incorporating a bifunctional Mg(Ac)2 additive into a concentrated NaAc aqueous solution. The highly concentrated electrolyte salts (4m NaAc+3m Mg(Ac)2) favor regulation on hydrogen-bonding configurations and kinetically shift the hydrogen evolution potential to a lower value of -1.37 V (vs Ag/AgCl). The Mg(Ac)2 additive plays particular roles in spontaneously capturing hydroxide ions generated during hydrogen evolution reactions on anode surfaces and simultaneously forming a protective Mg(OH)2-like interphase. As a result, the unique electrolyte significantly improves the structural stability and cycling performance of NaTi2(PO4)3/C anode (94.8% capacity retention after 100 cycles at 100 mA·g-1). The effect of salt concentration on hydrogen bonding configurations of aqueous electrolytes is investigated with Raman spectroscopy and FTIR spectroscopy. The interphase is identified by coupling EDS mapping, X-ray photoelectron spectroscopy, and X-ray diffraction. This work provides a new strategy for improving the cycling stability of aqueous sodium-ion batteries.

Organometallic Polymer Constructed by Active Fe-C12N8 Centers for Boosting Sodium-Ion Storage.

Organic-metal coordination materials with rich structural diversity are considered as promising electrode materials for rechargeable sodium-ion batteries. However, the electrochemical performance can be constrained by the limited number of active sites and structural instability under the discharge/charge process. Herein, organometallic polymer microspheres (Fe-PDA-220) with a unique d-π conjugated structure was designed and successfully constructed through a simple synchronous polymerization and coordination reactions. Polymerization of phenylenediamine was initiated by Fe3+ and Fe2+ ions generated synchronously during the polymerization integrated with poly-aminoquinone chains to form Fe-C12N8 active centers. Used as electrode materials for sodium-ion batteries, the distinctive Fe-C bond significantly boosts the structural stability, and the π-d conjugation system could facilitate electron transfer. A high reversible capacity of 345 mAh g-1 was delivered at 0.1 A g-1 and a capacity of 106 mAh g-1 was maintained even after discharged/charged at 1.0 A g-1 for 5000 cycles, outperforming most reported coordination materials. Spectroscopic and electronic analyses revealed that a two-electron reaction occurred per active unit, accompanied by the reversible redox evolution of the C=N groups and Fe ions during the sodiation/desodiation. This work provides a promising and efficient strategy for boosting the electrochemical performance of organic electrode materials by the design of organometallic polymers.

Operando Stress Evolution in Hard Carbon Anodes – Insight into the Mechanical Degradation Induced Failure Mode in Rechargeable Sodium Ion Batteries

Microstructural instabilities in electrodes arising from cycling induced stress is a primary failure mode in rechargeable batteries. Therefore, a quantitative knowledge of the cycling induced stress evolution in rechargeable battery electrodes is important for understanding the electrode microstructural instabilities during cycling that can potentially inform the electrode design to prolong battery cycle life. The cycling induced stress in electrodes typically arises from the volume changes associated with insertion/extraction of the electroactive species in the electrode matrix. For example, operando measurement of lithiation-delithiation induced stress in graphite electrodes that typically show 10% volume change has previously resulted in useful information that correlates well with the lithiation-delithiation mechanism in graphite anodes1. The size of the shuttling-electroactive species can be taken as an indicator of the cycling induced stress magnitude that drives the electrode microstructural instabilities. In this regard, quantification of the cycling induced stress in rechargeable Na-ion battery electrodes is warranted for understanding mechanical degradation induced failure modes owing to the larger size of the electroactive species, i.e., Na ions (vs. Li-ion).Hard carbons as potential anodes in rechargeable sodium ion batteries has gained considerable attention in the recent past due to their superior Na-ion storage capability (vs. graphite that is a canonical anode in rechargeable lithium-ion batteries). However, hard carbon anodes typically show significant capacity fade that can potentially arise from cycling induced mechanical degradation of the anodes. Operando stress measurements coupled with comprehensive electrode microstructural characterization will provide valuable insights in this context. Here we report on the operando stress evolution in hard carbon anodes in rechargeable sodium ion batteries as probed by Multiple Optical-beam Sensing (MOS) technique that basically involves monitoring the spacings among a group of laser spot (Fig. 1). Preliminary measurements have shown that the stress correlates with the potential with a maximum around 12 MPa in compression during sodiation and tensile stress of ~ 2MPa during desodiation. Operando stress measurements are performed in hard carbon anodes in two different electrode configurations – typical composite anodes (with binder and conductive agents) and hard carbon thin film anodes (without binder and conductive agents). The thin film configuration is considered to evaluate intrinsic stress response in hard carbon anodes in absence of any secondary phase. A comparison of operando stress response in these two types of hard carbon anodes along with comprehensive electrode microstructural characterization will be presented. Additionally, attempts will be made to shed light on the current debate in the mechanism of sodium storage in hard carbon anodes by comparing operando stress response of hard carbons from multiple sources along with their structural characterization (Raman, XPS, surface area etc.). V. A. Sethuraman, N. Van Winkle, D. P. Abraham, A. F. Bower, and P. R. Guduru, Journal of Power Sources, 206, 334–342 (2012). Figure 1

Mechanistic Insight into Ion Transport in Layered Materials for Sodium-Ion Rechargeable Batteries

As a feasible alternative to Li-ion batteries (LIBs), Na-ion batteries (SIBs) are an emerging energy storage solution for electric vehicles, electronic devices, and large grid level energy storage systems. The TMD (transition metal dichalcogenides) component MoS2 and the NASICON structured-type NaxTi2(PO4)3 are two materials of interest as next-generation SIB materials. MoS2 is a 2D layered material with a graphene-like structure that can be used as a high-capacity anode or Na-ion host material as well as an electrocatalyst. Our experiments on the Al-doped 1T phase of MoS2 on reduced graphene oxide (rGO) nanosheets support demonstrated good electrochemical performance, such as 450, 400, etc. mAh/g at current densities of 0.05, 0.1 A/g. Experiments further revealed that Na-ion intercalation supports increased interlayer distance in 1T Al-MoS2@rGO, resulting in a stable phase structure. Density functional theory (DFT)-based simulations are utilized to study the intercalation properties of Na+ ions, in addition to GITT, EIS, and CV analysis. In this study, the Na-ion diffusion mechanism for 1T Al-MoS2@rGO interface and Al-MoS2-MoS2 interlayer host structure is explored. According to the experiments, the host structure is doped with 5% Al at a specific stable site, and Na-ions are introduced to the interface and interlayer (an interlayer distance of 9.3 Å is chosen, as estimated by the experiments). The Na-ion is hopped in both structures, and activation energies (Eact) are calculated to be 57.9 kJ/mol and 40.3 kJ/mol in the 1T Al-MoS2@rGO interface at migration distances of 0-2 Å and 2-4 Å, respectively. The related Na-ion diffusion coefficients are computed using an Arrhenius type fit to be in the order of 10-13 cm2/s and 10-10 cm2/s. Similarly, the Na-ion activation energy barriers in the 1T Al-MoS2-MoS2 interlayer are estimated to be between 49.8 and 59.6 kJ/mol, with related diffusivity ranging between 10-11 and 10-13 cm2/s. The outcomes of the DFT simulations are consistent with those of our experiments. The Na-ion transport properties in the NaTi2(PO4)3 bulk structure via the Na-ion vacancy assisted mechanism are investigated using MD simulations and DFT. The self-diffusion coefficients (DNa+) of bulk are examined, and then the lattice strain effect is proposed to alter its transport properties by introducing the lattice strain in uniaxial (x, y, z) and biaxial (xy, xz, yz) directions, and the changes are compared for strained and unstrained structure. In comparison to the unstrained structure at 323K, a significant improvement by 1-2 order change in DNa+ values (10-8 cm2/s - 10-10 cm2/s) is measured. DFT is also used to investigate the diffusion mechanisms along the various planes (line [001], [100], and [010]) of the bulk NaTi2(PO4)3 structure and Eact is determined along those directions, respectively. The values obtained are consistent with the experimental value ranges obtained for similar materials. In addition, a mixed ion-system is represented by inserting Li-ions at the surface of a NaTi2(PO4)3 structure, yielding the formula LixNay-xTi2(PO4)3. The sluggish kinetics of Li-ion in the NaTi2(PO4)3 structure were proposed by MD analysis. This corresponds well with experimental data that the NaTi2(PO4)3 framework exhibits preferred Na-ion insertion when cycling in mixed-ion electrolytes of varied concentrations, implying slow diffusion of Li-ions in the bulk Li+/Na+ mixed-ion crystal.

Ethylene Carbonate-based Concentrated Electrolyte Composed of Asymmetric Amide Anion for Rechargeable Sodium-ion Batteries

Ethylene Carbonate-based Concentrated Electrolyte Composed of Asymmetric Amide Anion for Rechargeable Sodium-ion Batteries

Ab Initio Prediction of Two-Dimensional GeSiBi2 Monolayer as Potential Anode Materials for Sodium-Ion Batteries.

The conceptualization and deployment of electrode materials for rechargeable sodium-ion batteries are key concerns for next-generation energy storage systems. In this contribution, the configuration stability of single-layer GeSiBi2 is systematically discussed based on first-principles calculations, and its potential as an anode material is further investigated. It is demonstrated that the phonon spectrum confirms the dynamic stability and the adsorption energy identifies a strong interaction between Na atoms and the substrate material. The electronic bands indicative of inherent metallicity contribute to the enhancement of electronic conductivity after Na adsorption. The multilayer adsorption of Na provides a theoretical capacity of 361.7 mAh/g, which is comparable to that of other representative two-dimensional anode materials. Moreover, the low diffusion barriers of 0.19 and 0.15 eV further guarantee the fast diffusion kinetics. These contributions signal that GeSiBi2 can be a compatible candidate for sodium-ion batteries anodes.

First-principles study on the electrochemical properties of Na-ion-intercalatable heterostructures formed by transitional metal dichalcogenides and blue phosphorus

Extensive first-principles calculations have been performed to examine the electrochemical properties of Na-ion-intercalatable heterostructures formed by transitional metal dichalcogenides (MS2, where M = Ti, V, Nb and Mo) and blue phosphorus (BlueP), which have been reported as potential anode materials for rechargeable sodium-ion batteries. Upon formation of heterostructures, much improved structural stabilities have observed compared with the pristine MS2 and BlueP. Metallic T-TiS2, T-MoS2, H(T)-VS2 and H(T)-NbS2 would retain the conductive character after formation of heterostructures with BlueP, however, H-TiS2/BlueP and H-MoS2/BlueP would undergo a semiconductor to metallic transition accompanied by Na intercalation. Moreover, the presence of relatively low diffusion barriers ranging from 0.04 eV to 0.08 eV, coupled with the suitable average open-circuit voltage spanning from 0.12 eV to 0.89 eV, guarantee exceptional charge-discharge rates and ensure the safety of battery performance. Among these heterostructures, H(T)-NbS2/BlueP and T-TiS2/BlueP exhibit best Na adsorption ability of up to 4 layers, corresponding to theoretical capacities of 570.2 and 746.7 mAh/g, respectively. These encouraging properties indicate that T-TiS2/BlueP and H(T)-NbS2/BlueP could serve as suitable anode materials for high-performance sodium-ion batteries.

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.

Identifying the plateau sodium storage behavior of hard carbon through the spin state

Hard carbon has emerged as a promising anode material for rechargeable sodium-ion batteries, owing to its abundant precursor sources and remarkable reversible charge storage capacity. The key to achieving an extended sodium storage capacity lies in the plateau capacity largely associated with the closed-pore structure, yet a comprehensive understanding in this area remains incomplete. In our study, we utilized electron paramagnetic resonance (EPR) spectra to discern the spin state of unpaired electrons within hard carbon materials, enabling the identification of sodium storage plateau regions. Hard carbon anodes with plateau capacity predominantly exhibit delocalized spins, evident in the EPR spectrum by limited spin density and a linewidth exceeding 100 G. These spins originate from the extended aromatic structure within small-sized but curved nano-graphite and the closed-pore walls, which are crucial for plateau capacity. Conversely, hard carbon anodes exclusively possessing slope capacity are characterized by localized spins, presenting a typical narrow and intense Lorentzian EPR signal. These localized spins arise from the edge defects of the nano-graphite structure, serving as favourable sites for sodium adsorption. Our development of a methodology to discern hard carbon plateau capacity behavior based on EPR spectra presents a facile and rapid approach for advancing high-performance hard carbon materials.

Theoretical prediction of two-dimensional CrSi2N4 as a potential anode material for Na-ion batteries

Efficient anode materials are critical for high-performance rechargeable lithium-ion batteries (LIBs) and sodium-ion batteries. This paper systematically investigates the potential of the CrSi2N4 monolayer as anode material for LIBs and sodium-ion batteries by first-principles density functional theory calculations. It was found that CrSi2N4 exhibits outstanding performance in sodium-ion batteries, with a low diffusion energy barrier of 0.10 eV and a high theoretical specific capacity of 490 mAh g−1. Meanwhile, the average open circuit voltage is 0.47 V, comparable to the typical anode materials. In addition, a small lattice constant change of 0.3%–3.1% ensures the cycling stability of CrSi2N4 in sodium-ion batteries. This work suggests a promising candidate anode material for sodium-ion batteries.

Fully-Printed Flexible Aqueous Rechargeable Sodium-Ion Batteries.

The flexible aqueous rechargeable sodium-ion batteries (ARSIBs) are a promising portable energy storage system that can meet the flexibility and safety requirements of wearable electronic devices. However, it faces huge challenges in mechanical stability and facile manufacturing processes. Herein, the first fully-printed flexible ARSIBs with appealing mechanical performance by screen-printing technique is prepared, which utilizes Na3V2(PO4)2F3/C (NVPF/C) as the cathode and 2% nitrogenous carbon-loaded Na3MnTi(PO4)3/C (NMTP/C/NC) as the anode. In particular, the organic co-solvent ethylene glycol (EG) is cleverly added to 17m (molkg-1) NaClO4 electrolyte to prepare a 17 m NaClO4-EG mixed electrolyte. This mixed electrolyte can withstand low temperatures of -20°C in practical applications. Encouragingly, the fully-printed flexible ARSIBs (NMTP/C/NC//NVPF/C) exhibit a discharge capacity of 40.1mAhg-1, an energy density of 40.1Whkg-1, and outstanding cycle performance. Moreover, these batteries with various shapes can be used as an energy wristband for an electronic watch in the bending states. The fully-printed flexible ARSIBs in this work are expected to shed light on the development of energy for wearable electronics.

Perylenetetracarboxylic Diimide Composite Electrodes as Organic Cathode Materials for Rechargeable Sodium-Ion Batteries: A Joint Experimental and Theoretical Study.

The organic semiconductor 3,4,9,10-perylenetetracarboxylic diimide (PTCDI), a widely used industrial pigment, has been identified as a diffusion-less Na-ion storage material, allowing for exceptionally fast charging/discharging rates. The elimination of diffusion effects in electrochemical measurements enables the assessment of interaction energies from simple cyclic voltammetry experiments through the theoretical work of Laviron and Tokuda. In this work, the two N-substituted perylenes, N,N'-dimethyl-3,4,9,10-perylenetetracarboxylic diimide (Me2PTCDI) and N,N'-diphenyl-3,4,9,10-perylenetetracarboxylic diimide (Ph2PTCDI), as well as the parent molecule 3,4,9,10-perylenetetracarboxylic diimide (H2PTCDI) are investigated as thin-film composite electrodes on carbon fibers for sodium-ion batteries. The composite electrodes are analyzed with Raman spectroscopy. Interaction parameters are extracted from cyclic voltammetry measurements. The stability and rate capability of the three PTCDI derivatives are examined through galvanostatic measurements in sodium-ion half-cell batteries and the influence of the interactions on those parameters is evaluated. In addition, self-consistent charge density function tight binding calculations of the different PTCDI systems interacting with graphite have been carried out. The results show that the binding motif displays notable deviations from an ideal ABA stacking, especially for the neutral state. In addition, data obtained for the electron-transfer integrals show that the difference in performance between different PTCDI thin-film batteries cannot be solely explained by the electron-transfer properties and other factors such as H-bonding have to be considered.

Fe doping mechanism of Na0.44MnO2 tunnel phase cathode electrode in sodium-ion batteries

Due to its stability and low cost, the tunnel-style sodium-manganese oxide (Na0.44MnO2) material is deemed a popular cathode choice for sodium-ion rechargeable batteries. However, the Jahn-Teller effect caused by Mn3+ in the material results in poor capacity and cycling stability. The purpose of this experimental study is to partially replace Mn3+ with Fe3+, in order to reduce the Jahn-Teller effect of the material during charging and discharging process. The results of Raman spectroscopy and X-ray photoelectron spectroscopy confirmed that the content of Mn3+ decreased after Fe3+ doping. Electrochemical studies show that the Na0.44Mn0.994Fe0.006O2 cathode has better rate performance (exhibits a reversible capacity of 87.9 mAh/g at 2 C) and cycle stability in sodium-ion batteries. The diffusion coefficient of sodium ions increases by Fe3+ doping. The excellent rate performance and capacity improvement are verified by density functional theory (DFT) calculation. After doping, the band gap decreases significantly, and the results show that the state density of O 2p increases near the Fermi level, which promotes the oxidation–reduction of oxygen. This work provides a straightforward approach to enhance the performance of Na0.44MnO2 nanorods, and this performance improvement has guiding significance for the design of other materials in the energy storage domain.

Tuning the electrochemical performance of Cr2B2 MBene anodes for Li and Na-ion batteries through F and Cl-functionalization: A DFT and AIMD study

In an era in which sustainable energy sources and electric vehicles are prevalent, the development of efficient anode materials is paramount to address the pressing demand for improved energy storage technologies. After the graphene revolution, many other 2D materials such as graphene, MXene (atomically thin layer of Transition Metal Carbide/Nitride/Carbonitride), TMDC (Transition Metal Dichalcogenide), and MBene (atomically thin layer of Transition Metal Borides) have emerged as potential high-performance anode materials. A recent study demonstrated the possibility of synthesizing chromium-based MBenes for the first time, and the electrochemical performance of Cr2B2 as a potential anode material has also been studied theoretically. Inspired by this, the current study utilized density functional theory and ab initio molecular dynamics simulations to probe the role of Cl and F-functionalization of Cr2B2 MBenes as anode materials for rechargeable lithium-ion batteries (LIB) and sodium-ion batteries (NIB). The electronic structure, adsorption, diffusion, and storage capacity of Li (Lithium) and Na (Sodium) were systematically investigated. This study revealed that Cr2B2F2 and Cr2B2Cl2 exhibit excellent conductivity due to their intrinsic metallic properties. In addition, they have low diffusion energy barriers for Li and Na ions (0.13/0.14 and 0.20/0.20 eV for Cr2B2F2 and Cr2B2Cl2, respectively) facilitating rapid charge/discharge rates. The calculations further unveiled a notable Li-ion storage capacity of 818 mAh g-1 for 2D Cr2B2Cl2, accompanied by a moderate open-circuit voltage (OCV) range of 0.38 V to 0.98 V, illustrating its potential as a superior anodic material for LIBs. Similarly, Cr2B2F2 demonstrated a substantial Na-ion storage capacity of 655 mAh g-1, coupled with a reasonably ranged average OCV of 0.20 V to 0.71 V, positioning it as a compelling candidate for NIBs. In all, this investigation has identified two superior electrode materials for alkali-ion batteries, which can spark additional interest in exploring MBene as a promising candidate for advancing next-generation anode materials.

Critical review of the recent progress and challenges of polyanion Na3V2(PO4)3 cathode materials in rechargeable sodium-ion batteries

With the increasing demand for sustainable energy sources, sodium-ion batteries have emerged as a rational substitute for large energy storage grid applications in contrast to current lithium-ion batteries owing to the low cost of sodium precursors.

Sodium layered oxide cathodes: properties, practicality and prospects.

Rechargeable sodium-ion batteries (SIBs) have emerged as an advanced electrochemical energy storage technology with potential to alleviate the dependence on lithium resources. Similar to Li-ion batteries, the cathode materials play a decisive role in the cost and energy output of SIBs. Among various cathode materials, Na layered transition-metal (TM) oxides have become an appealing choice owing to their facile synthesis, high Na storage capacity/voltage that are suitable for use in high-energy SIBs, and high adaptivity to the large-scale manufacture of Li layered oxide analogues. However, going from the lab to the market, the practical use of Na layered oxide cathodes is limited by the ambiguous understanding of the fundamental structure-performance correlation of cathode materials and lack of customized material design strategies to meet the diverse demands in practical storage applications. In this review, we attempt to clarify the fundamental misunderstandings by elaborating the correlations between the electron configuration of the critical capacity-contributing elements (e.g., TM cations and oxygen anion) in oxides and their influence on the Na (de)intercalation (electro)chemistry and storage properties of the cathode. Subsequently, we discuss the issues that hinder the practical use of layered oxide cathodes, their origins and the corresponding strategies to address their issues and accelerate the target-oriented research and development of cathode materials. Finally, we discuss several new Na layered cathode materials that show prospects for next-generation SIBs, including layered oxides with anion redox and high entropy and highlight the use of layered oxides as cathodes for solid-state SIBs with higher energy and safety. In summary, we aim to offer insights into the rational design of high-performance Na layered oxide cathode materials towards the practical realization of sustainable electrochemical energy storage at a low cost.

A highly stable high-energy layered oxide cathode for rechargeable sodium ion batteries

A Li/Ti-dual doped nickel/manganese oxide is designed as a new cathode material for sodium ion batteries, which shows excellent electrochemical performance with a high capacity of 153 mA h g−1 and an impressive retention of 83% after 100 cycles.

Synthesis and electrochemical studies of NaCoPO4 as an efficient cathode material using natural deep eutectic solvents for aqueous rechargeable sodium-ion batteries.

In this work, sodium cobalt phosphate (NaCoPO4) was successfully prepared by a cost-effective ionothermal method using a deep eutectic solvent (DES) for the first time. The synthesized NaCoPO4 was used to fabricate a cathode material for aqueous rechargeable sodium-ion batteries. The surface morphology of the prepared materials and its compositional analysis were done by using field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray (EDX) analysis, respectively. The X-ray diffraction (XRD), SEM, and EDX studies revealed that the material has orthorhombic-shaped particle morphology with uniform distribution and is in nanoscale (approximately 50nm). The nature of the cation inserted (Na+ ion insertion) was confirmed by recording CV profiles at different concentrations of the Na2SO4 electrolyte. The reversibility of the electrode redox reaction was studied by varying the scan rate in CV studies, and it was found that the electrode exhibits a reversible behavior with a resistive behavior. In GCPL studies, the cell TiO2/2MNa2SO4/NaCoPO4 showed significant reversibility with a prominent discharge capacity of 85mAh g-1 at 0.1°C and 88% of capacity retention after 100 cycles. Thus, the prepared materials could be used as an effective futuristic alternative battery material for rechargeable batteries.

Electrochemical Storage Behavior of a High-Capacity Mg-Doped P2-Type Na2/3Fe1-yMnyO2 Cathode Material Synthesized by a Sol-Gel Method.

Grid-scale energy storage applications can benefit from rechargeable sodium-ion batteries. As a potential material for making non-cobalt, nickel-free, cost-effective cathodes, earth-abundant Na2/3Fe1/2Mn1/2O2 is of particular interest. However, Mn3+ ions are particularly susceptible to the Jahn-Teller effect, which can lead to an unstable structure and continuous capacity degradation. Modifying the crystal structure by aliovalent doping is considered an effective strategy to alleviate the Jahn-Teller effect. Using a sol-gel synthesis route followed by heat treatment, we succeeded in preparing an Mg-doped Na2/3Fe1-yMnyO2 cathode. Its electrochemical properties and charge compensation mechanism were then studied using synchrotron-based X-ray absorption spectroscopy and in situ X-ray diffraction techniques. The results revealed that Mg doping reduced the number of Mn3+ Jahn-Teller centers and alleviated high voltage phase transition. However, Mg doping was unable to suppress the P2-P'2 phase transition at a low voltage discharge. An initial discharge capacity of about 196 mAh g-1 was obtained at a current density of 20 mAh g-1, and 60% of rate capability was maintained at a current density of 200 mAh g-1 in a voltage range of 1.5-4.3 V. This study will greatly contribute to the ongoing search for advanced and efficient cathodes from earth-abundant elements for rechargeable sodium-ion batteries operable at room temperature.

Tuning the Interfacial Chemistry for Stable and High Energy Density Aqueous Sodium-Ion/Sulfur Batteries

The environmental-related issues arising from the fossil fuel assorted industrial revolution and worldwide development have prompted the quest for rechargeable batteries. In these predicaments, lithium-ion batteries (LIBs) took ownership to reshape our lives. However, the limited abundance, non-uniform geographical distribution and severe flammability of organic electrolytes, increase the uncertainty over their large-scale application. Recently, aqueous rechargeable sodium-ion batteries (ARSIBs) have gained considerable curiosity for large-scale energy storage due to their much-assured safety, environment friendliness, high-rate capacity, and low cost. However, the prospects of ARSIBs seeing commercial success remained remote due to the narrow water stability window (1.23 V), which translates into low cell voltage (< 1.6 V), low energy density (< 70 Wh Kg-1), and compromised cycling stability. The aforesaid dilemmas can be resolved by generating a protective layer known as a solid electrolyte interface (SEI) like in organic electrolytes. However, the SEI concept in aqueous electrolytes is relatively unexplored, as water dissociation leads to O2 and H2, and will enhance the parasitic reactions. The SEI formed in WiSE due to salt reduction is often inhomogeneous with a porous mosaic structure and is susceptible to mechanical cracking, and increase the overall cost. Hence, high-capacity electrodes and high voltage electrolytes capable of forming a stable SEI are urgently required to fulfill the dream of the large-scale application of ARSIBs.Recently low cost, highly abundant sulfur-based electrode material has attracted significant research attention due to its high theoretical capacity (1675 mA h g-1) and energy density. However, sluggish sulfur redox kinetics, acute polysulfide shutting, dendritic growth on the metal-based anode and low conductivity of sulfur and its discharge products proves to be a major roadblock for its commercialization. Utilizing abundant sulfur in an aqueous electrolyte along with abundant Na+ can resolve the kinetics and conductivity anxieties and leads to a new greener and safer Na-ion/S batteries chemistry. However lower order polysulfide dissolution in water is more feasible, leading to the rapid capacity decay and active sulfur loss due to H2S formation. Polysulfide dissolution is an interfacial mechanism occurring at the electrode-electrolyte interface and depends on both electrode and electrolyte merits. Therefore, an effective approach will be to couple efficient sulfur host with an electrolyte capable of generating a stable SEI on the electrode surface to prevent the direct attack of water on polysulfide.Herein we have explored urchin-like CoWO4 as a sulfur host coupled with Na-W-U-D electrolyte. The CoWO4 exhibits high conductivity, strong chemical interaction for sulfur and its discharge products, and urchin-like morphology having exposed edges facilitate the charge transport results in excellent polysulfide redox kinetics. The high voltage Na-W-U-D electrolyte was prepared by mixing NaClO4, urea, and N, N-dimethylformamide DMF in 1:2:1 ratio in water. Each component in the electrolyte plays an important role. Urea has very high water solubility and tends to form stable SEI, while DMF has a high dielectric constant, good solvation ability, and develop stabilize SEI. As a result, Na-W-U-D shows a stability window close to the 3.1 V regime due to reduced water activity resulting from complex ion solvent interaction and stable and uniform SEI formation, consisting of Na2CO3, polyurea, and reduction products of DMF. Despite the addition of DMF, non-flammable features of aqueous electrolytes remain well maintained. Herein for the first time, the SEI concept was successfully used for the aq. Na-ion/S battery. We discovered that the lower water activity of Na-W-U-D electrolyte hindered polysulfide dissolution and stable SEI prevent the direct attack of water on polysulfide and results in extended cycling stability. At the same time, the urchin-like CoWO4 host enhances the sluggish polysulfide redox kinetics and provides an abundant anchoring site for polysulfide adsorption. We investigate the effect of time, C-rate, depth of discharge, and dissolved oxygen on polysulfide dissolution and self-discharge of the negative electrode. The high electrode capacity combined with the safety and stable SEI of Na-W-U-D electrolyte translated into a record high initial capacity of 834 mA h g-1 w.r.t sulfur, with remarkable cycling stability up to 500 cycles @ 0.5 C. Post analysis by SEM and XPS, evident that the stable SEI consists of Na2CO3, polyurea, and reduced products of DMF (CO and NHMe2), which also prevent the negative electrode from self-discharge by mitigating the parasitic reaction of dissolved oxygen in the electrolyte. Moreover, a full cell assembled by integrating S@CoWO4 anode and Na0.44MnO2 cathode showed remarkable stability and a high energy density of 119 Wh kg-1, making it a promising candidate for a future energy storage system. Figure 1