Sodium-ion Battery System Research Articles

Research on the Electrochemical Impedance Spectroscopy Evolution of Sodium-Ion Batteries in Different States.

Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs) due to the abundant availability of sodium, lower costs, and comparable electrochemical performance characteristics. A thorough understanding of their performance features is essential for the widespread adoption and application of SIBs. Therefore, in this study, we investigate the output characteristics and electrochemical impedance spectroscopy (EIS) features of sodium-ion batteries (SIBs) under various states. The research results show that, unlike conventional lithium iron phosphate (LFP) batteries, SIBs exhibit a strong linear relationship between state of charge (SOC) and open-circuit voltage (OCV) across various SOC and temperature conditions. Additionally, the discharge capacity of the battery remains relatively stable within a temperature range of 15 °C to 35 °C; when the temperatures are outside this range, the available capacity of the sodium-ion battery reduces significantly. Moreover, the EIS profiles in the high-frequency region are predominantly influenced by the ohmic internal resistance, which remains largely unaffected by SOC variations. In contrast, the low-frequency region demonstrates a significant correlation between SOC and impedance, with higher SOC values resulting in reduced impedance, indicated by smaller semicircle radii in the EIS curves. This finds highlights that EIS profiling can effectively monitor SOC and state of health (SOH) in SIBs, offering a clear correlation between impedance parameters and the battery's operational state. The research not only advances our understanding of the electrochemical properties of SIBs but also provides a valuable reference for the design and application of sodium-ion battery systems in various scenarios.

Performance Investigations on All-Solid-State Polymer-Ceramic Sodium-Ion Batteries through a Spatially Resolved Electrochemical Model

Rechargeable batteries are crucial in modern energy storage, with lithium-ion batteries dominating the market. However, the scarcity and environmental concerns associated with lithium have spurred interest in alternative battery chemistries, particularly sodium-ion batteries (SIBs), which utilize abundant sodium resources. Despite extensive experimental research on all-solid-state SIBs (ASSSIBs), theoretical investigations have primarily focused on molecular-level analyses, overlooking the impact of cell composition on overall performance. This paper aims to address this gap by developing a physical model for simulating ASSSIBs at the particle scale. Our methodology involves integrating experimental data with simulation results to identify key factors influencing battery performance. The study reveals slow sodium ion transport as a significant bottleneck, attributed to factors such as low porosity of the half-cell and limited electrolyte ionic conductivity. Simulation outcomes emphasize the importance of advancing fast-ion-conducting solid electrolytes to enhance ASSSIB performance. Moreover, the results suggest that electrodes with high electrolyte active filler content and reduced thickness are necessary for achieving optimal battery capacity utilization. Overall, this research underscores the intricate relationship between electrode microstructure and battery performance, offering valuable insights for the design and optimization of sustainable sodium-ion battery systems suitable for stationary and mobile applications.

Structural engineering of MoSe2 via interfacial effect and phosphorus doping incorporation for high energy density sodium ion storage

Sodium-ion batteries (SIBs) are gaining increasing attention for the development of electrode materials due to their potential to alleviate the safety concerns and resource scarcity associated with lithium-ion batteries (LIBs). Based on this, the present work, built upon the electrospinning preparation of hollow carbon nanofibers (HCNFs), facilitated the in-situ growth of MoSe2 nanosheets on their surface. Subsequently, an in-situ phosphorization treatment was applied, resulting in phosphorus-doped MoSe2 (P-MoSe2)@HCNFs. The dual-channel inner layer of HCNFs in this structure promotes ion transport and directly interfaces with MoSe2, leading to abundant interfacial effects. The outer layer of MoSe2 serves as the primary redox-active unit for sodium ions, retaining numerous active sites. P doping further optimizes the overall electronic structure of the material. P-MoSe2@HCNFs serves as the anode material in SIBs, demonstrating exceptional reversible specific capacity and rate capabilities. It achieves a remarkable 479.17 mAh g−1 over 500 cycles at 1 A g−1 and 397.01 mAh g−1 over 950 cycles at 10 A g−1, showcasing its outstanding performance. These values significantly surpass those of MoSe2@HCNFs and pure MoSe2, validating the pronounced advantages of the structure we have constructed. Leveraging the outstanding kinetic performance of P-MoSe2@HCNFs, when assembled with Na3V2(PO4)3 as the cathode, the full cell exhibits remarkable sodium storage capability. It achieves a high energy density of up to 194.75 Wh kg−1, providing a feasible solution for the application of transition metal selenides in sodium-ion battery systems.

A dual aliovalent ion doped NASICON ceramic filler embedded in the PEO–NaTFSI polymer matrix for high-performance solid-state sodium-ion batteries

Demonstrating a high-performance all-solid-state sodium-ion battery system, with a maximum discharge capacity of ∼105 mA h g−1 at 0.1C, achieved for NVP symmetric cells enabled via polymer–ceramic composite electrodes and electrolytes.

High-performance ε-VOPO4 cathode materials for sodium ion battery applications

The nanoscale ε-VOPO4 materials were successfully prepared by hydrothermal synthesis of H2VOPO4 precursors and further calcination. XRD and AFM characterization showed that the prepared nano-ε-VOPO4 belonged to monoclinic crystalline system with high purity of physical phase and an average particle size of 150 nm. FT-IR spectroscopy showed that the characteristic peak of the H2VOPO4 precursor (at 797 cm−1) disappeared after calcination and a characteristic peak representing ε-VOPO4 was captured at 914 cm−1. Furthermore, the XPS results showed that the V in the prepared ε-VOPO4 was mainly +5 valence. In order to further evaluate the potential of ε-VOPO4 as an energy storage material, it was applied as cathode material in sodium ion battery system. Electrochemical tests have demonstrated that ε-VOPO4 has satisfactory electrochemical performance as cathode material for sodium ion batteries. The initial capacities of ε-VOPO4 electrode materials reached 162.3, 135.1, 86.7, 51.4, and 29.7 mAh g−1 at 0.05, 0.2, 1, 2, and 5 C, respectively, and the long-cycle performance results showed that the electrochemical stability was still superior after 200 cycles at a current density of 0.5 C. Meanwhile, the density functional theory calculation shows that the optimized lattice constants (a = 7.279 Å, b = 6.896 Å and c = 7.268 Å) are in good agreement with the experimental data. Notably, the experimental ε-VOPO4 material has a higher voltage plateau than the theoretical calculation, which shows the potential advantage of ε-VOPO4 as a new generation electrode material for high energy storage systems.

Hybridization of Layered Titanium Oxides and Covalent Organic Nanosheets into Hollow Spheres for High-Performance Sodium-Ion Batteries with Boosted Electrical/Ionic Conductivity and Ultralong Cycle Life.

While development of a sodium-ion battery (SIB) cathode has been approached by various routes, research on compatible anodes for advanced SIB systems has not been sufficiently addressed. The anode materials based on titanium oxide typically show low electrical performances in SIB systems primarily due to their low electrical/ionic conductivity. Thus, in this work, layered titanium oxides were hybridized with covalent organic nanosheets (CONs), which exhibited excellent electrical conductivity, to be used as anodes in SIBs. Moreover, to enlarge the accessible areas for sodium ions, the morphology of the hybrid was formulated in the form of a hollow sphere (HS), leading to the highly enhanced ionic conductivity. This synthesis method was based on the expectation of synergetic effects since titanium oxide provides direct electrostatic sodiation sites that shield organic components and CON supports high electrical and ionic conductivity with polarizable sodiation sites. Therefore, the hybrid shows enhanced and stable electrochemical performances as an anode for up to 2600 charge/discharge cycles compared to the HS without CONs. Furthermore, the best reversible capacities obtained from the hybrid were 426.2 and 108.5 mAh/g at current densities of 100 and 6000 mA/g, which are noteworthy results for the TiO2-based material.

Sodium/Lithium-Ion Transfer Reaction at the Interface between Low-Crystallized Carbon Nanosphere Electrodes and Organic Electrolytes.

Carbon nanosphere (CNS) electrodes are the candidate of sodium-ion battery (SIB) negative electrodes with small internal resistances due to their small particle sizes. Electrochemical properties of low-crystallized CNS electrodes in dilute and concentrated sodium bis(trifluoromethanesulfonyl) amide/ethylene carbonate + dimethyl carbonate (NaTFSA/EC + DMC) were first investigated. From the cyclic voltammograms, both lithium ion and sodium ion can reversibly insert into/from CNSs in all of the electrolytes used here. The cycling stability of CNSs in concentrated electrolytes was better than that in dilute electrolytes for the SIB system. The interfacial charge-transfer resistances at the interface between CNSs and organic electrolytes were evaluated using electrochemical impedance spectroscopy. In the Nyquist plots, the semicircles at the middle-frequency region were assigned to the parallel circuits of charge-transfer resistances and capacitances. The interfacial sodium-ion transfer resistances in concentrated organic electrolytes were much smaller than those in dilute electrolytes, and the rate capability of CNS electrodes in sodium salt-concentrated electrolytes might be better than in dilute electrolytes, suggesting that CNSs with concentrated electrolytes are the candidate of SIB negative electrode materials with high rate capability. The calculated activation energies of interfacial sodium-ion transfer were dependent on electrolyte compositions and similar to those of interfacial lithium-ion transfer.

4.7 V Operation of the Cr4+/Cr3+ Redox Couple in Na3Cr2(PO4)2F3

Increasing the energy density of sodium-ion battery systems requires high-voltage cathode materials to compensate for the inherently higher redox potential of the Na/Na+ couple compared to the Li/L...

Effects of Cr doping on structural and electrochemical properties of Li4Ti5O12 nanostructure for sodium-ion battery anode

The appropriate Cr doping to Li 4 Ti 5 O 12 (LTO) greatly enhances electrochemical properties of pristine LTO in terms of high capacity, rate performance, and long-term cyclability for sodium-ion batteries anode. Sodium-ion batteries are considered as promising alternatives to lithium-ion batteries, owing to their low cost and abundant raw materials. Among the several candidate materials for the anode, spinel-type Li 4 Ti 5 O 12 (LTO) has potential owing to its superior safety originating from an appropriate operating voltage and the reversible Na + intercalation properties. However, a low diffusion coefficient for Na + and the insulating nature of LTO remains challenging for practical sodium-ion battery systems. Herein, we present a strategy for integrating physical and chemical approaches to achieve superior electrochemical properties in LTO. We demonstrate that carefully controlling the amount of Cr doping is crucial to enhance the electrochemical properties of nanostructured LTO. Optimized Cr doped LTO shows a superior reversible capacity of 110 mAh g −1 after 400 cycles at 1C, with a three-fold higher capacity (75 mAh g −1 ) at 10C compared with undoped LTO material. This suggests that appropriately Cr doped nanostructured LTO is a promising anode material for sodium-ion batteries.

Hybrid Anodes of Lithium Titanium Oxide and Carbon Onions for Lithium‐Ion and Sodium‐Ion Energy Storage

This study demonstrates the hybridization of Li4Ti5O12 (LTO) with different types of carbon onions synthesized from nanodiamonds. The carbon onions mixed with a Li4Ti5Ox precursor for sol–gel synthesis. These hybrid materials are tested as anodes for both lithium‐ion battery (LIB) and sodium‐ion battery (SIB). Electrochemical characterization for LIB application is carried out using 1 m LiPF6 in a 1:1 (by volume) ethylene carbonate and dimethyl carbonate as the electrolyte. For lithium‐ion intercalation, LTO hybridized with carbon onions from the inert‐gas route achieves an excellent electrochemical performance of 188 mAh g−1 at 10 mA g−1, which maintains 100 mAh g−1 at 1 A g−1 and has a cycling stability of 96% of initial capacity after 400 cycles, thereby outperforming both neat LTO and LTO with onions obtained via vacuum treatment. The performance of the best‐performing hybrid material (LTO with carbon onions from argon annealing) in an SIB is tested, using 1 m NaClO4 in ethylene/dimethyl/fluoroethylene carbonate (19:19:2 by mass) as the electrolyte. A maximum capacity of 102 mAh g−1 for the SIB system is obtained, with a capacity retention of 96% after 500 cycles.

Eutectic Synthesis of the P2-Type NaxFe1/2Mn1/2O2 Cathode with Improved Cell Design for Sodium-Ion Batteries

An engaging area of research in sodium-ion batteries (SIBs) has been focusing on discovery, design and synthesis of high-capacity cathode materials in order to boost energy density to levels close enough to the state-of-the-art lithium-ion batteries. Of particular interest, P2-type layered oxide, Na2/3Fe1/2Mn1/2O2, has been researched as a potential cathode in SIBs based on its high theoretical capacity of 260 mAh/g and use of non-critical materials. However, the reported synthesis methods are not only complex and energy-demanding, but also often yield inhomogeneous and impure materials with capacities less than 200 mAh/g under impractical test conditions. Here, we report a novel synthesis route using low temperature eutectic reaction to produce highly homogeneous and crystalline, and impurity-free P2-NaxFe1/2Mn1/2O2 with enhanced Na-ion diffusivity and kinetics. The overall electrochemical performances of the Na-ion cells have been improved by pairing the P2-cathode with pre-sodiated hard carbon anodes, leading to reversible capacities in the range of 180 mAh/g. This new approach is a contribution towards the simplification of synthesis and scalability of sodium-based cathodes with high crystallinity and fine-tuned morphology, and the realization of a sodium-ion battery system with lower cost and improved electrochemical performance.

A simple approach to minimize the first cycle irreversible loss of sodium titanate anode towards the development of sodium-ion battery

First cycle irreversible loss is a major problem of any kind of anode materials in sodium-ion battery systems (SIBs). Here, we present a stable (4000 cycles), high capacity (237 mAh g−1) and high rate capable hydrogenated metal-doped sodium titanium oxide (Na2Ti3O7) anode material for SIBs and that suffers from high 1st cycle irreversible loss (53.86%) initially. Later, we adopted a simple chemical shorting approach where, sodium metal was kept immediate tight contact with anode material in presence of electrolyte medium to reduce the irreversible loss. It was found that 60 min chemical shorting is enough to minimize the irreversible loss to 4.11% from 53.6% and enhance the Coulombic efficiency to 84% from 36.22% respectively without compromising the specific capacity and cycling stability. This approach is adopted in full-cell (sodium vanadium phosphate vs. hydrogenated metal-doped Na2Ti3O7) formation and the study also provided similar improved electrochemical performance. The current understanding and the approach provide a new direction to overcome the 1st cycle irreversible loss of sodium titanate anode and can be utilized further to develop sustainable rechargeable sodium-ion battery.

Microwave-assisted synthesis of platelet-like cobalt metal-organic framework, its transformation to porous layered cobalt-carbon nanocomposite discs and their utilization as anode materials in sodium-ion batteries

Abstract In this work a facile microwave-assisted synthesis of a platelet-like cobalt-based metal-organic framework (MOF) material is presented. This material was synthesized from cobalt(II) acetylacetonate and biphenyl-4,4′-dicarboxylic acid (Bpdc) in N,N’-dimethylformamide at 160°C. As-prepared Co-Bpdc MOF product with a platelet-like disc architecture was transformed by heat treatment in a nitrogen atmosphere at 800°C to porous cobalt-carbon nanocomposite discs. It is demonstrated that this synthetic strategy allows for obtaining magnetic microporous carbon layered discs with homogeneously incorporated metallic cobalt nanoparticles with a size of ca. 4 nm. The Co-C nanocomposite material was characterized by a variety of physico-chemical methods. It is shown that both Co-Bpdc MOF and Co-C nanocomposite were electrochemically active in sodium battery system as a material for the negative electrode. The high capacity retention over 80% and capacities over 200 mAh g–1 in the sodium-ion battery systems have been achieved.

(Invited) Na-Ion Batteries: Challenges and Opportunities

With the ever-increasing demands of modern societies, energy generation, storage, and distribution are becoming increasingly important research fields. Currently, one of the most promising areas of research and development is Sodium ion Battery (SIB) technology, which has a range of potential applications but remains particularly suited for use in stationary systems. [1-5]. Here we will discuss SIB systems in terms of what may be considered its three most significant components: anodes, electrolytes, and cathodes. SIB anodes are mainly based on hard carbon materials, due to their attractive combination of low cost and high energy density. However, there has also been interest in other systems, such as intermetallic alloying materials and metal oxides, as well as exploiting specific electrolyte co-solvation effects so as to enable the use of graphite. In general, the SIB research community uses organic electrolytes which are analogous to already existing Lithium ion batteries (LIB). However, recently there has been growing interest in developing new electrolytes which are specifically tailored for use in SIBs, such as optimized liquid and solid electrolytes. At the present time, cathodes are one of the most explored SIB components - with a plethora of options to choose from, including Prussian blue and organic materials. However, the most promising are polyanionic and layered materials, with their good combinations of electrochemical performance, low cost, stability and available constituents. Although interest in SIB technology has only relatively new, when compared to LIBs, it has been already developed at the prototyping level. A general overview of the most interesting electrode and electrolyte materials for Na-ion batteries paying special attention to those related to the current prototypes will be presented. By examining this topic in detail, it will be shown that there exists a strong drive to exploit this technology and that there are a wide range of opportunities to develop new and improved SIB technologies.

ZnV2O4: A potential anode material for sodium-ion batteries

A template-free solvothermal method was employed to successfully obtain ZnV2O4 spinel oxide and its electrochemical properties as anode materials for sodium ion battery system were investigated for the first time. The structural, morphological, elemental composition, electrochemical properties and theoretical calculations of the as-prepared ZnV2O4 were carried out. XRD revealed the presence of ZnO and VO2 impurities when synthesized for 1 day, while complete formation of ZnV2O4 was attained when the synthesis procedure was increased to 3 days. When cycled at 50 mA/g, it delivered an initial capacity of as high as 537 mAh/g at a potential window of 0.01–3.0 V. Meanwhile, a reversible capacity of ∼ 113 mAh/g was obtained when cycled at 100 mA/g for 30 cycles. These results indicate the potential applications of ZnV2O4 as anode materials for sodium ion battery systems.

Multi-layer graphene assembled fibers with porous structure as anode materials for highly reversible lithium and sodium storage

Multi-layer graphene assembled fibers (MLG fibers) are synthesized by template-assistant decomposition of ethanol. Morphology and structure characterization results indicate that the synthesized fibers possessed a porous structure with diameters ranged from 1 to 1.5 μm and a length of 8–15 μm. The electrochemical properties of the fibers as anode materials for lithium-ion batteries and sodium-ion batteries are evaluated. The results indicate that MLG fiber electrode could deliver much better energy storage performance than that of chemical vapor deposited carbon fibers and other reported graphene-containing one dimensional materials. Benefited from their structure advantages, the MLG fiber electrode exhibits a high reversible capacity of 837 mAhh g−1 after 150 cycles at 250 mA g−1 in lithium-ion battery system, and a good capacity of 392 mAhh g−1 after 100 cycles at 100 mA g−1 in sodium-ion battery system. The attractive lithium and sodium storage properties can be ascribed to the porous fiber structure built by graphene sheets, which provide advantages including large surface area, easy pathways for electrolyte diffusion and low charge transfer resistance.

Reaction Mechanism of Alluaudite Sodium Iron Sulphate As High Energy Density Cathode Material for Na-Ion Battery

The sodium-ion battery system is now growing as a potential alternative to the lithium-ion battery for large energy storage systems. The ubiquitous element Na allows us to realize stable production of large scale storages for broader applications. Electrode materials for the Na system is under intensive development by many battery researchers to make properties of the Na-ion battery comparable to those of Li-ion battery. In such pursuit we developed an ideal cathode material with abundant constituents sodium, iron and sulfate ions.1 Alluaidite-type sodium iron sulfate Na2+2x Fe2–x (SO4)3 exhibits high energy density due to high working potential of 3.8 V vs. Na/Na+. Large tunnel structure in the alluaudite framework derives high Na-ion conductivity. Thus Na2+2x Fe2–x (SO4)3 electrode allows high-rate battery operation.In this poster, we present detailed reaction mechanisms of Na2+2x Fe2–x (SO4)3 for Na batteries by means of crystal structural characterizations. As mentioned in our first report of this system, Na2+2x Fe2–x (SO4)3 (NFS) electrode undergoes irreversible change during first charing of NFS/Na half cells. Subsequent discharging and charging cycles are reversible. We carried out ex situ structural analysis to understand about the irreversible and reversible changes during electrochemical reaction. Crystal structure of NFS powder (x ~ 0.28) was successfully analyzed by the alluaudite structure model with C2/c symmetry.2 Ex situ structural analysis of NFS electrodes were carried out by powder X-ray diffraction using synchrotron radiation at Photon Factory in KEK, Japan. Electrochemically reacted samples were also successfully analyzed by using a monoclinic lattice with C2/c symmetry, which is isomorphic with the pristine lattice. Volume change is less than 5% in whole reaction range. The refined structure shows that the desodiation/resodiation reactions are essentially topochemical, while partially irreversible cation rearrangement is observed. One of the Na sites, Na3, is preferentially extracted due to higher site-potential than the other two sites. This model is consistent with computational evaluation of relative defect formation energy at the Na sites.1 Subsequent extraction sequence shows complex behaviors involving Fe migrations. [1] P. Barpanda, G. Oyama, S. Nishimura, S.-C. Chung, and A. Yamada, Nat. Commun., 5:4358, 2014. [2] G. Oyama, S. Nishimura, Y. Suzuki, M. Okubo, and A. Yamada. ChemElectroChem, 2(7):1019–1023, 2015. [3] G. Oyama, O. Pecher, K. J. Griffith, S. Nishimura, R. Pigliapochi, C. P. Grey and A. Yamada, in preparation. Figure 1

Thermal Stability Studies in Charged Layered Sodium Transition Metal Oxide Cathode Materials for Na-Ion Batteries

Layered alkali transition metal oxides (i.e., LiMO2 and NaMO2, M=transition metal) are the premier class of cathode materials in lithium- and sodium-ion battery system. The safety characteristics of LiMO2 cathode based LIBs are one of the most critical barriers to be overcome for the large scale application such as EV. One of the main reasons that might cause safety hazards of the LIB is associated with the thermal instability of charged Li1-xMO2 cathode materials, which is related to the occurrence of exothermic reactions between flammable electrolyte and liberating oxygen from charged Li1-xMO2 at high temperature. Based on the structural homology between the layered LiMO2 and NaMO2, we expect that the investigation of thermal stability of charged Na1-xMO2cathode materials is also important for the practical use of SIBs. However, few attentions have been paid on safety issue of SIBs. In this study, the thermal stability of charged Na1-xMO2 cathode materials is investigated by using combined in situ time-resolved X-ray diffraction and mass spectroscopy (TR-XRD/MS), which allows simultaneous observation of the structural changes and gas species that are evolved during thermal decomposition of charged cathode materials, especially O2 gas in our interest. In addition, the ex situ/in situ X-ray absorption spectroscopy (XAS) has been also utilized to look at the local and electronic structural changes occurring during thermal decomposition in an elemental selective way. By utilizing combined X-ray techniques, we are able to get better understanding of structural and electronic structure changes in charged cathode materials during thermal decomposition. In this presentation, the thermal decomposition behavior of charged Na1-xMO2(M=Co, Cr) cathodes will be covered. Acknowledgement The work done at Brookhaven National Lab. was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. DOE under Contract No. DE-SC0012704.

Enhanced Electrochemical Properties of Carbon Coated TiO2 anode for Sodium-Ion Battery

Many types of commercial batteries use carbonaceous materials materials for the anode, such as meso-carbon microbeads (MCMB) or hard carbon. However, MCMB cannot be used as an anode material for sodium batteries because of the high potential of the Na/Na+(~0.3V higher than Li/Li+) redox reaction and the large ionic radii of the Na+(102 pm).1,2 Therefore, many researchers have been efforts to find a suitable anode material for the sodium – ion battery system. In the case of insertion anodes, various types of non-graphitic carbon materials have been considered and tested.3,4 Here, we confirmed that the carbon coated anatase-type micro-sphere TiO2 anode material has several advantages for the sodium ion battery system, including excellent cycle retention, high efficiency, good rate capability, and high energy density. The carbon coating, introduced by sucrose addition, had an effect of suppressing the growth of the TiO2 primary particles during calcination. The carbon coated TiO2 (sucrose 20 wt.% coated) electrode exhibited excellent cycle retention during 50 cycles (100%) and superior rate capability up to 30 C-rate at room temperature. This cell delivered a high discharge capacity of 155 mAh g-1 at 0.1 C, 149 mAh g-1 at 1 C, and 82.7 mAh g-1 at 10 C-rate, respectively. (1) Stevens, D. A.; Dahn, J. R. High Capapcity Anode Materials for Rechargeable Sodium-Ion Batteries. J. Electrochem. Soc. 2000, 147, 1271−1273. (2) Wenzel, S.; Hara, T.; Janek, J.; Adelhelm, P. Room-Temperature Sodium-Ion Batteries: Improving the Rate Capability of Carbon Anode Materials by Templating Strategies. Energy Environ. Sci. 2011, 4, 3342−3345. (3) Stevens, D. A.; Dahn, J. R. The Mechanisms of Lithium and Sodium Insertion in Carbon Materials. J. Electrochem. Soc. 2001, 148, A803−A811. (4) Thomas, P.; Ghanbaja, J.; Billaud, D. Electrochemical Insertion of Sodium in Pitch-Based Carbon Fibers in Comparison with Graphite in NaClO4-Ethylene Carbonate Electrolyte. Electrochim. Acta 1999, 45, 423−430.

Ti-substituted tunnel-type Na₀.₄₄MnO₂ oxide as a negative electrode for aqueous sodium-ion batteries.

The aqueous sodium-ion battery system is a safe and low-cost solution for large-scale energy storage, because of the abundance of sodium and inexpensive aqueous electrolytes. Although several positive electrode materials, for example, Na₀.₄₄MnO₂, were proposed, few negative electrode materials, for example, activated carbon and NaTi₂(PO₄)₃, are available. Here we show that Ti-substituted Na₀.₄₄MnO₂ (Na₀.₄₄[Mn₁-xTix]O₂) with tunnel structure can be used as a negative electrode material for aqueous sodium-ion batteries. This material exhibits superior cyclability even without the special treatment of oxygen removal from the aqueous solution. Atomic-scale characterizations based on spherical aberration-corrected electron microscopy and ab initio calculations are utilized to accurately identify the Ti substitution sites and sodium storage mechanism. Ti substitution tunes the charge ordering property and reaction pathway, significantly smoothing the discharge/charge profiles and lowering the storage voltage. Both the fundamental understanding and practical demonstrations suggest that Na₀.₄₄[Mn₁-xTix]O₂ is a promising negative electrode material for aqueous sodium-ion batteries.