Cathode For Na-ion Batteries Research Articles

Mitigating the voltage fading and lattice cell variations of O3-NaNi0.2Fe0.35Mn0.45O2 for high performance Na-ion battery cathode by Zn doping

O3-type layered oxides have attracted considerable interest as cathode materials for sodium ion batteries. However, the oxides' poor rate capability, inferior cycling stability and voltage decay impede the use of these oxides in practical applications. In this study, a series of O3-type NaNi0.2Fe0.35Mn0.45-xZnxO2 (x = 0, 0.02, 0.05, 0.07 and 0.1) cathode materials were synthesized via a solid state reaction. With an optimized Zn2+ content of 0.05, NaNi0.2Fe0.35Mn0.4Zn0.05O2 cathode exhibits better electrochemical performance compared to the undoped-NaNi0.2Fe0.35Mn0.45O2 in terms of rate capability, cycling stability and suppressed voltage decay. The role of Zn has been elucidated. First, the substitution of Zn for Mn reduces the content of unfavourable Mn3+ and hence improves the cycling stability. Second, the introduction of Zn2+ into the TM-O layer stabilizes the crystal structure and mitigates the irreversible migration of Fe3+ into Na+ layer upon cycling, thereby alleviating the voltage fading during Na+ extraction/insertion. Third, Zn2+ doping promotes the O3-P3 reversible phase transformation. Moreover, Zn2+ doping reduces the lattice cell variations during Na+ extraction/insertion and improves the structure stability. The proposed insights into the role of Zn are also instructive for designing other high-performance cathode materials for sodium-ion batteries through lattice doping.

Highly Stable and High Rate-Performance Na-Ion Batteries Using Polyanionic Anthraquinone as the Organic Cathode.

Sodium 9,10-anthraquinone-2,6-disulfonate (Na2 AQ26DS), with polyanionic character and two O-Na ionic bonds, is found to be a highly stable organic cathode in Na-ion batteries, delivering capacities of approximately 120 mAh g-1 for 300 cycles (50 mA g-1 ) and around 99 mAh g-1 for 1000 cycles (1 A g-1 ). These results are the best performance reported to date for small-molecule, anthraquinone-based organic cathodes in Li-, Na-, or K-ion batteries.

Achieving long-life Prussian blue analogue cathode for Na-ion batteries via triple-cation lattice substitution and coordinated water capture

Prussian blue and its analogues have attracted tremendous interests as cathode materials for sodium ion batteries because of their low cost and high capacity. However, their commercial applications are hindered by low capacity utilization and insufficient cycle life. Here, we report on a MnCoNi-co-doped Prussian blue analogue with high purity and crystallinity by a citrate-assisted controlled crystallization process. Moreover, a novel strategy of in situ capturing for residual coordinated water during sodiation/desodiation process is proposed to achieve a high cyclability by using the aluminium chloride Lewis acid as electrolyte additive. The reversible cubic-rhombohedral phase transition is revealed by in situ X-ray diffraction, ex situ X-ray absorption spectroscopy and transient electrochemical impedance spectroscopy. The MnCoNi-co-doped composite delivers a capacity of 111 mAh g−1 even at 1C and retains a capacity retention of 78.7% after 1500 cycles for sodium ion batteries. This work provides important insights on eliminating the parasitical coordinated water in lattice and modulating the crystallinity to develop high-performance Prussian blue and its analogues.

Sodium Reactivity in Na-M-O Systems (with M = Mn, V), Cathodes for Na-Ion Batteries

Among the cathodes of the sodium ion batteries, the manganese and vanadium based oxide materials present many advantages due to their high energy density, low-cost and low-toxicity. In particular, numerous layered materials have been reported in the system Na-Mn-O1,2. These materials are interesting because they show weak interlayer interactions with free space allowing sodium diffusion. In search of new phases, we present two lamellar compounds of this system with interesting electrochemical properties as well as the first sodium extraction/insertion from Na2V3O7. The Kagome network of Na4Mn2O5 is made of layers of corner-sharing square-pyramids of MnO5 3. This material was synthesized by a conventional solid-state method starting from Mn2O3 and Na2O and it shows a first charge capacity of 380 mAh/g and a reversible capacity of 130 mAh/g. By mechanical alloying and from the same precursors, a nanostructured material shows a different structure, close to NaMnO2. The reversible capacity is then improved to 220 mAh/g. Here, we also report for the first time the electrochemical activity of another lamellar phase of this system: Na2Mn3O7. This material, obtained via a simple synthesis from cheap sodium and manganese salts, consists of Mn-vacancy-[Mn3O7]2- layers built up with edge-sharing MnO6 octahedra, separated by NaO6 and NaO5 polyhedra. Starting from this phase, a reversible capacity of 2 Na/f.u. (160 mAh/g) is obtained through a plateau at 2.1 V with a low polarization of 100 mV4. Thus, the electrochemical process allows a reduced phase Na4Mn3O7 to be obtained, which cans intercalate/de-intercalate two sodium per f.u., reversibly5. Interestingly, an additional reversible redox process, corresponding to the extraction of 1.5 Na+, is observed on oxidation at 4.1 V due to the oxygen redox activity, consistent with DFT calculations6. Based on the theoretical explanation given by Ceder et al.7, this oxygen redox activity is explained by the presence of ☐-O-Na axes due to the Mn vacancies in the [Mn3O7]2- layers8. Therefore, by cycling this material in the potential range 4.7-1.5 V, the reversible specific capacity reaches 200 mAh/g. In comparison, no oxygen activity has been observed for another Na2M3O7 phase: Na2V3O7. We report the sodium extraction from Na2V3O7 which is a tunnel type structure built of [V3O7]2- ∞ nanotubes hold by sodium ions9. In this case a reversible charge capacity of 80 mAh/g at 2.8 V vs Na+/Na is due to the V4+/V5+ redox activity. Moreover, no oxygen-redox reaction was observed in the vanadium (5+) oxide Na4V2O7. The mechanism of extraction as well as the structures of the as-prepared and oxidized phases will be discussed in this presentation. 1 X. Ma, H. Chen and G. Ceder, J. Electrochem. Soc., 2011, 158, A1307–A1312. 2 J. Billaud, R. J. Clément, A. R. Armstrong, J. Canales-Vázquez, P. Rozier, C. P. Grey and P. G. Bruce, J. Am. Chem. Soc., 2014, 136, 17243–17248. 3 G. Brachtel and R. Hoppe, Z. Für Anorg. Allg. Chem., 1980, 468, 130–136. 4 E. Adamczyk and V. Pralong, Chem. Mater., 2017, 29, 4645–4648. 5 E. Adamczyk, E. Anger, M. Freire and V. Pralong, Dalton Trans ., 2018, 47, 3112-3118. 6 Z. Zhang, D. Wu, X. Zhang, X. Zhao, H. Zhang, F. Ding, Z. Xie and Z. Zhou, J. Mater. Chem. A, 2017, 5, 12752. 7 D.-H. Seo, J. Lee, A. Urban, R. Malik, S. Kang and G. Ceder, Nat. Chem., 2016, 8, 692–697. 8 B. M. de Boisse, S. Nishimura, E. Watanabe, L. Lander, A. Tsuchimoto, J. Kikkawa, E. Kobayashi, D. Asakura, M. Okubo and A. Yamada, Adv. Energy Mater., 2018, 8, 1800409. 9 E. Adamczyk, M. Gnanavel and V. Pralong, Materials, 2018, 11, 1021.

Meso-Structure Controlled Synthesis of Sodium Iron-Manganese Oxides for Na-Ion Batteries

Increasing deployment of large scale electric storage applications would benefit from the development of inexpensive batteries. Na-ion batteries (NIBs) are of great interest since their cathodes can be made from inexpensive and abundant materials such as sodium, iron, and manganese. However, existing iron and manganese oxide cathodes lack the electrochemical performance, including capacity retention, required for these NIB applications and significant improvement is needed. Prior work on lithium ion battery cathodes demonstrates that meso-structured materials can improve electrochemical performance for batteries. Incorporating spherical meso-structured materials in lithium ion battery cathodes was shown to improve capacity retention. To emulate this approach in NIB cathode materials, a synthesis method for producing pure phase spherical meso-structured sodium iron manganese oxide is first needed. No literature has been found that describes a method for this synthesis. Key synthesis parameters to be controlled include the choice of method, material precursors, pH of reaction, calcination time, and cooling rate. In this work we demonstrate a modified co-precipitation method to synthesize spherical meso-structure pure phase P2-Na0.67Fe1/4Mn3/4O2 (see Figure 1) and we report initial characterization of the electrochemical performance of this material. The key parameter found to control the formation of a spherical meso-structure is the cooling rate applied during synthesis. The slow cooled material forms spherical meso-structures while the quenched material has no ordered meso-structure. Additionally, the spherical meso-structured material shows increased capacity, improved cyclability, and reduced polarization. The electrochemical performance enhancement is attributed to higher surface area, reduced surface concentration of sodium carbonate, and lower internal resistance during cycling. Further exploring the synthesis and mechanisms of meso-structured materials offers a path for improving Na-ion battery cathodes. Figure 1

Capacitance controlled, hierarchical porous 3D ultra-thin carbon networks reinforced prussian blue for high performance Na-ion battery cathode

3D ultra-thin carbon networks are ideal skeleton structures for loading active materials as energy storage and conversion devices. In this work, excellent cathode materials for sodium ion batteries were successfully prepared by homogeneously anchoring NaxKyMnFe(CN)6 (x + y ≤ 2, NaK-MnHCF) on hierarchical porous 3D N-doped ultra-thin carbon networks (3DNC). The compounds present a high reversible capacity, good rate performance, and superior cycling stability. Combined fully experimental analysis and first-principles calculation, the interfacial synergistic effect between 3DNC and NaK-MnHCF on the sodium storage capacity is revealed, contributing to the extra capacity and electrical conductivity. Furthermore, considerable content of capacitive-controlled sodium storage of NaK-MnHCF@3DNC conduces to the rate performance. These results reveal an efficient route for the fabrication of other cathode materials for sodium ion batteries as high-performance energy storage devices.

Meso-Structure Controlled Synthesis of Sodium Iron-Manganese Oxides Cathode for Low-Cost Na-Ion Batteries

A modified co-precipitation method is introduced to synthesize P2-type Na0.67Fe1/4Mn3/4O2 cathode for low-cost Na-ion batteries. The meso-structure of the obtained material is well controlled through regulated cooling after a high temperature calcination process. The material via slow-cooling consists of sphere-like secondary particles with a uniform dispersion while the quenched material exhibits a hexagonal plate-like primary particle without meso-structure. Meso-structure is found to have a distinct effect upon the electrochemical performance of P2-type layered cathode. The slow-cooled sample exhibits a larger capacity and improved cyclability compared with the quenched sample, which is attributed to the larger surface area, reduced surface contamination, and surprisingly higher transition metal redox activity. This work demonstrates that cooling rate plays the key role in controlling the formation of spherical meso-structure for sodium iron-manganese oxides with enhanced electrochemical performance.

Nature of the “Z”-phase in layered Na-ion battery cathodes

In this article, the nature of the “Z”-phase, which forms on charging many P2-type compounds to high voltages, is probed.

Anionic Redox Reaction-Induced High-Capacity and Low-Strain Cathode with Suppressed Phase Transition

Summary Layered metal oxides have attracted widespread attention as cathodes for Na-ion batteries (NIBs) because of easy synthesis, high specific capacity, and high energy density. However, most reported layered oxides suffer from complex phase transitions upon a large amount of Na deintercalation. Here we report a P2-type Na0.72[Li0.24Mn0.76]O2, exhibiting exceptionally high initial charge capacity of ∼210 mAh/g (0.72 Na) based on a pure anionic redox reaction (ARR). Surprisingly, global P2 structure can be maintained with minimal volume change (1.35%) upon complete removal of Na+ ions. This is due to the reduced Coulombic repulsion associated with ARR and consequent suppression of the phase transition as observed in other P2 materials. Here we reveal for the first time that ARR has the functionality of stabilizing the structure, in addition to its role in increasing its already known capacity. This would pave the way for the further improvement of high-energy-density NIBs.

Flowerlike Mesoporous FeF3·0.33H2O with 3D Hierarchical Nanostructure: Size-Controlled Green-Synthesis and Application as Cathodes for Na-Ion Batteries

A flowerlike mesoporous FeF3·0.33H2O with three-dimensional (3D) hierarchical nanostructure is successfully prepared for the first time through a facile PEG-assisted solvothermal route. By rationally regulating and controlling the amount of PEG-400 in solvent, a series of nanostructured FeF3·0.33H2O materials with various morphologies and sizes is obtained. The probable formation mechanism related to the role of the PEG is explored. Furthermore, the electrochemical performances of the as-prepared samples for Na-ion battery (NIB) are investigated and discussed. The flower-like mesoporous FeF3·0.33H2O can deliver the noticeable initial discharge capacity of 283 mAh g–1 and retain 190 mAh g–1 after 100 cycles within a potential range of 1.5–4.5 V at 0.1 C. Particularly, even at a high rate of 2.0 C, the material can still exhibit a high capacity of 155 mAh g–1. The excellent electrochemical performances can be attributed to the unique 3D hierarchical porous nanostructure and the small particle size, which ca...

High Energy Density CNT/NaI Composite Cathodes for Sodium‐Ion Batteries

AbstractThe Na‐iodine system has potential as a positive electrode in sodium‐ion batteries due to its fast reaction kinetics and high energy density. Here, the fabrication of a NaI‐loaded carbon nanotube (CNT) mat cathode is presented and the use of NaI as a Na‐ion battery cathode is reported for the first time. Freestanding electrodes show a capacity retention of 92% after 100 cycles at 100 mA h g−1 and a high specific energy density (≈473 W h kg−1, second discharge basis vs Na metal) and small hysteresis (0.03 V). Adding a CNT mat interlayer between the NaI composite cathode and the separator, and fluoroethylene carbonate to the electrolyte significantly suppresses shuttling of active materials during cycling. Using ex situ X‐ray photoelectron spectroscopy and Raman data, an understanding of the relevant processes occurring during cycling of the CNT/NaI cathode is developed.

Defect formations and pH-dependent kinetics in kröhnkite Na2Fe(SO4)2·2H2O based cathode for sodium-ion batteries: Resembling synthesis conditions through chemical potential landscape

Thermodynamics and kinetics of intrinsic point defects in Na2Fe(SO4)2·2H2O, a high-voltage cathode for Na-ion batteries, are studied by means of first-principles density functional theory. Electronic structures of charged defects are calculated to study their influences towards electronic and electrochemical properties as well as to probe hole polaron formation. As defect formation energy strongly depends on atomic chemical potentials, we initiate a systematic approach to determine their valid ranges for the pentrary Na-Fe-S-O-H compound under thermodynamic equilibria and correlate them with approximated pH parameters in solution-based synthesis. Given chemical potential landscape and formation energy, we find that FeNa1+,VNa1-,0andNaFe1-,0 are dominant and their concentrations could be manipulated through pH condition and oxygen content in the precursor solution. It is predicted that the channel blockage due to FeNa would appear under strong acidic growth condition but could be diminished under weak acidic condition (4.7 ≤ pH ≤ 5.6) where NaFe facilitates a faster migration between each diffusion channel. Our results do not only explain the origin of intercalation mechanism and improved electronic conduction, but also demonstrates the pH influence towards conductivities in the cathode material.

Waste Biomass as in Situ Carbon Source for Sodium Vanadium Fluorophosphate/C Cathodes for Na-Ion Batteries

This work presents a series of electrode materials constituted by a sodium vanadium fluorophosphate as the electroactive phase and an in situ generated carbon derived from waste biomasses as carbon source. The waste biomasses consist of grounded vine-shoots, eucalyptus wood, and sucrose as control sample. The materials were obtained by hydrothermal carbonization during the hydrothermal synthesis followed by a flash thermal treatment (FTT). This way, carbonaceous matter decomposes into hydrochar while the electroactive phase is formed under autogenous pressure at 170 or 200 °C. The electrochemical tests were performed by adjusting the electrode composition to a total carbon amount of 20 wt %, so that the suitability of the in situ carbon as conductive additive was evaluated. Electrochemical performance was significantly improved after an FTT treatment. Among the biomass-based materials, samples that had the highest amount of the in situ generated carbon, resulting from eucalyptus biomass, showed the best e...

VO2 (A)/graphene nanostructure: Stand up to Na ion intercalation/deintercalation for enhanced electrochemical performance as a Na-ion battery cathode

Intercalation/deintercalation of large size Na ion leads to serious electrode materials fragmentation that would be the main reason of Na-ion battery capacity fading. Herein, we reported a Na-ion battery cathode material composed of layer-structured VO2 (A) nanowires wrapped with graphene. The VO2 (A)/graphene nanostructure which effectively suppressed pulverization of VO2 (A) nanowires during cyclic charging and discharging exhibited prominent cycle stability and high specific capacity. A reversible capacity of 115 mA h g−1 was retained over 100 cycles at a high current density of 100 mA g−1. Comparing to the electrochemical performance of other vanadium oxides, VO2 (A)/graphene demonstrates a potential cathode material for Na-ion batteries.

Three-dimensional Na3V2(PO4)3/carbon nanofiber networks prepared by electrospinning as self-standing cathodes for high performance Na-ion batteries

A facile and simple electrospinning method was adopted to prepare a 3D network of Na3V2(PO4)3/C flexible fiber membrane as self-standing cathodes for sodium ion batteries. The physical and electrochemical performances are investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and electrochemical tests. The Na3V2(PO4)3/C nanorods displayed an excellent sodium storage performance. For example, it showed an initial capacity of 107 mA h g−1 at a current density of 0.1 C and retained almost 100% of the initial capacity at the 100th cycle. Moreover, the discharge capacity was up to 30 mA h g−1 at 30 C, suggesting a high rate capability.

Physical properties and electrochemical performance of Zn-substituted [formula omitted] nanostructures as cathode in Na-ion batteries

We report the synthesis, physical properties and electrochemical performance of Zn substituted Na0.44Mn1−xZnxO2 (x= 0 – 0.02) nanostructures as cathode in Na-ion batteries for energy storage applications. These samples stabilize in the orthorhombic structure and the morphology is found to be slab like with 100 – 200 nm width and few micrometer of length. The resistivity measurements show highly insulating nature for all the samples, where the activation energy decreases with increasing Zn concentration indicating more defect levels in the band gap. The cyclic voltammogram (CV) shows reversible oxidation and reduction peaks, which clearly shift towards higher/lower potentials with increasing Zn concentration up to 2%. We observed the specific capacity of about 100 mAh/g at current density of 4 mA/g and improved cycle life for Zn substituted x=0.005 sample. However, with further increasing the Zn concentration (x=0.02), the specific capacity decreases, which can be a manifestation of the decreased number of reduction peaks in the CV data.

Structural Characterization of O3-Structure Layered Cathode Material for Rechargeable Sodium Ion Batteries

Sodium ion batteries (SIBs) have emerged as suitable alternative energy storage systems to Li ion batteries (LIBs) due to the cost-effective and ample Na source. Sodium layered oxides have considerable attention as cathodes for Na-ion batteries due to easy synthesis and simple structure. O3-type NaTMO2 materials, where single transition metal is an oxidizable element such as Cr, Mn, Fe, Co, Ni [1], have the capability to reversible intercalation reaction of Na ions [2,3], which is different from their Li analogues system, where only nickel and cobalt do reversible intercalation of Li ions [4]. Furthermore, various mixed transition metals can be realized in the TM layer to create new oxide compounds [5,6]. Among them, O3-NaNi1/3Fe1/3Mn1/3O2 shows a capacity around 130 mAh/g with good capacity retention when cycled to 4.0 V [7]. In this study, layered sodium-ion battery cathode material, O3-type NaNi1/3Fe1/3Mn1/3O2, was systematically investigated by using synchrotron-based x-ray techniques to characterize the detailed redox mechanism during electrochemical process and to understand the role of each transition metal structural behavior in the ternary-element material. In Figure 1, reversible changes in the Ni, Fe, and Mn K-edge X-ray Absorption Near Edge Structure (XANES) spectra show that reversible electronic structural changes in the local level during Na+ deintercalation/intercalation in the voltage range of 2.0 – 4.0 V. Moreover, Ni and Fe elements are both active in Na1–xNi1/3Fe1/3Mn1/3O2 cathode and redox couples of Ni2+/Ni3+/Ni4+ and Fe3+/Fe4+ are responsible for the charge compensation mechanism. High Resolution Powder Diffraction (HRPD) results reveal that O3-type (R-3m) phase transforms into a P3 (R3m) structure coupled with Na+/vacancy ordering during charge and further elucidate the final P3-OP2 of phase transformation on over-sodiated state. Furthermore, in the structure with the small quantity of sodium, internal Fe ion migration occurs from octahedral site into tetrahedral site, which is possible due to formed vacant space by highly deintercalation of Na ions and this atomic level of movement causes the layered structure to have structural distortions. An in-depth analysis of the structural behavior and reaction mechanism for NaNi1/3Fe1/3Mn1/3O2 cathode material when the Na ions are used of the entire composition widens an electrochemical perspective and suggests a direction where we better understand the nature of structure which can be used as the cathode for the advanced rechargeable batteries with high energy density. From these experimental results, we will discuss structural evolution behavior of layered NaNi1/3Fe1/3Mn­1/3O2 cathode material. More detailed results and discussion including reaction process of Ni, Fe, and Mn in the material will be presented in the AiMES 2018 meeting. Figure 1. XANES spectra of (a, b) Ni K-edge, (c,d) Fe K-edge, and (e,f) Mn K-edge during the 1st cycle of Na+ deintercalation/intercalation process. Reference s : [1] C. Delmas, C. Fouassier, P. Hagenmuller, Phys. B + C 99B (1980) 81–85. [2] S. Komaba, C. Takei, T.Nakayama,A.Ogata,N. Yabuuchi, Electrochem. Commun. 12 (2010) 355–358. [3] N. Yabuuchi, H. Yoshida, S. Komaba, Electrochemistry 80 (2012) 716. [4] S.P. Ong, V.L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, et al., Energy Environ. Sci. 4 (2011) [5] I. Saadoune, A. Maazaz, M. Menetrier, C. Delmas, J. Solid State Chem. 117 (1996) 111–117. [6] M. Sathiya, K. Hemalatha, K. Ramesha, J.-M. Tarascon, A.S. Prakash, Chem. Mater. 24 (2012) 1846–1853. [7] D. Kim, E. Lee, M. Slater, W. Q. Lu, S. Rood, C. S. Johnson, Electrochem. Commun. (2012), 18, 66. Figure 1

Electrospun Hybrid Vanadium Oxide/Carbon Fiber Mats for Lithium- and Sodium-Ion Battery Electrodes

Vanadium oxide nanostructures are constantly being researched and developed for cathodes in lithium- and sodium-ion batteries. To improve the internal resistance and the discharge capacity, this study explores the synthesis and characterization of continuous one-dimensional hybrid nanostructures. Starting from a sol–gel synthesis, followed by electrospinning and controlled thermal treatment, we obtained hybrid fibers consisting of metal oxide crystals (orthorhombic V2O5 and monoclinic VO2) engulfed in conductive carbon. For use as Li-ion battery cathode, a higher amount of carbon yields a more stable performance and an improved capacity. Monoclinic VO2/C fibers present a specific capacity of 269 mAh·gVOx–1 and maintain 66% of the initial capacity at a rate of 0.5 A·g–1. Orthorhombic V2O5/C presents a higher specific capacity of 316 mAh·gVOx–1, but a more limited lithium diffusion, leading to a less favorable rate handling. Tested as cathodes for Na-ion batteries, we confirmed the importance of a conductiv...

Robust graphene layer modified Na2MnP2O7 as a durable high-rate and high energy cathode for Na-ion batteries

Na2MnP2O7 has been considered as a promising cathode candidate for advanced sodium-ion batteries due to its high potential, low cost and non-toxicity. However, the low initial Coulombic efficiency, poor high-rate and unsatisfactory cycling ability originated from the intrinsic inferior electronic conductivity and manganese dissolution severely hinder its practical application. Herein, we report an approach based on a feasible high energy vibrating activation process to fabricate a robust graphene layers (GL) modified Na2MnP2O7 material (noted as NMP@GL) for the first time. The as-prepared NMP@GL could exhibit an ultrahigh initial Coulombic efficiency of 90%, and a high energy density over 300 Wh kg-1. In addition, rate performance and cycling stability were also improved, with high capacity retention of 83% after 600 cycles at 2 C. These impressive progresses should be ascribed to the enhanced electron transportation with distinctive framework through graphene layer modifying, and structural stability of triclinic Na2MnP2O7 with spacious 3D ion migration channels. Ex-situ XRD and GITT demonstrate a consecutive multi-phase reaction mechanism with facile sodium diffusion. Our design makes Na2MnP2O7@GL to achieve its potential for practical application.

Na2Mn3+0.3Mn4+2.7O6.85: A cathode with simultaneous cationic and anionic redox in Na-ion battery

Here we report a high-capacity cathode, Na2Mn3+0.3Mn4+2.7O6.85, based on simultaneous cationic (Mn4+/Mn3+) and anionic (O2-/(O2)n-) redox process in Na-Mn-O system for Na-ion batteries (NIBs). An obvious voltage plateau at ~ 4.2 V vs. Na+/Na is reversibly presented during Na+ deinsertion/insertion demonstrated by XPS analysis of the participation of oxygen redox. This cathode delivers ~ 213 mA h g-1 in the voltage range of 1.5–4.5 V composed of two voltage plateaus from both cationic and anionic redox reaction. The two voltage plateaus exhibit the reversible phase transition upon Na+ insertion/deinsertion corresponding to a solid-solution region (high plateau) and a two-phase region (low plateau), respectively. The results indicate that anionic redox activity is available to develop high-capacity cathodes for NIBs.