Hard Carbon Negative Electrode Research Articles

Structural and chemical analysis of hard carbon negative electrode for Na-ion battery with X-ray Raman scattering and solid-state NMR spectroscopy

This study explores the structural changes of hard carbon (HC) negative electrodes in sodium-ion batteries induced by insertion of Na ions during sodiation. X-ray Raman spectroscopy (XRS) was used to record both C and Na K-edge absorption spectra from bulk HC anodes carbonized at different temperatures and at several points during sodiation and desodiation. Comparing the π∗/σ∗ regions in the C K-edge spectra sp2/sp3 hybridization ratio of material was determined. Higher carbonization temperatures led to increased order in graphitic structure and shorter ▪ bond lengths. Sodiation caused a decrease in graphitic layer order due to inserted Na ions. Complementary operando solid state 23Na nuclear magnetic resonance (ssNMR) studies confirmed the structural changes, while showing pore filling mechanism, which is not observed in ex situ measurements, primarily at higher carbonization temperatures. XRS analysis of Na K-edge spectra revealed systematic variations in the solid electrolyte interface (SEI) composition during cycling. Changes in XRS spectra were attributed to both SEI composition alterations, accompanied by the insertion/adsorption of Na ions at defect sites within the carbon structure.

Unlocking the local structure of hard carbon to grasp sodium-ion diffusion behavior for advanced sodium-ion batteries

The proposal of dispersion region can complete the HC model, explain the anomalous diffusion in the low-voltage region, guide the microstructure design of high-performance HC materials, and provide new insights into the storage mechanism of SIBs.

Approaches to Control Air Stability, Capacity Retention, Gas Generation, and Impedance Growth of Layered Oxide Positive Electrode Materials for Sodium-Ion Batteries

Sodium-ion batteries made of earth-abundant elements could potentially become affordable, scalable, and sustainable energy storage solutions. Among different sodium-ion positive electrode chemistries, layered transition metal oxides are of special interest due to their promising energy densities. Specific energy densities close to LiFePO4-based lithium-ion batteries have been demonstrated. [1] Promising capacity retention has been demonstrated as well, with >4,000 cycles to 80% at 30°C. [1] However, the poor air-stability of layered sodium-ion positive electrode materials poses challenges in materials storage and processing in both laboratory and industry scales. Moreover, sodium layered oxides can go through complex phase transitions during electrochemical sodium intercalation, which can result in poor capacity retention unless mitigated by effective materials design. Other challenges are gas generation, impedance growth and sodium plating during cycling, which must all be addressed to make viable layered oxide-based sodium-ion batteries for large-scale storage of renewable energy from wind and solar.In this contribution, we first explore compositional modification as an approach to mitigate structural degradation in sodium layered oxides during air exposure and sodium intercalation. A combination of substituent elements is utilized to dramatically improve air stability as evidenced by X-ray diffraction (XRD) and scanning electron microscopy (SEM) experiments. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) are employed to understand the underlying mechanism that leads to improved air stability. Furthermore, coin cell results suggest that compositional modification enhances the capacity retention without sacrificing reversible capacity. [2]Secondly, we explore the performance of layered oxide positive electrodes in machine-made 250 mAh pouch cells with hard carbon negative electrodes. The capacity retention and impedance growth are studied in 40°C cycling tests with various electrolyte additives. High cycling stability with >99% capacity retention after 200 cycles can be attained with simple electrolyte additives. However, specialized additives need to be selected to suppress gas generation and impedance growth. Symmetric cell impedance spectroscopy is used to reveal whether impedance growth stems from the positive or negative electrode. [3]Gas generation is assessed by in-situ volume measurements using the Archimedes principle. [4] Interestingly, sodium-ion cells show distinct differences to lithium-ion batteries in terms of their gas generation during the formation cycle. Moreover, sodium plating can have a surprising effect on the gas generation during charge/discharge cycling. Using ultra-high precision coulometry, the critical conditions for sodium plating and its effect on cell performance are determined. In-situ volume measurements reveal a surprising mechanism that explains the interesting relationship between gas generation and cell voltage in sodium-ion batteries. [5]References[1] J. Barker, 242nd ECS Meeting, Atlanta (2022).[2] L. Zhang et al., manuscript in preparation.[3] H. Hijazi, Z. Ye, et al., manuscript in preparation.[4] C. Aiken et al., J. Electrochem. Soc. 161, A1548–A1554 (2014).[5] E. Alter et al., manuscript in preparation.Figure 1. Discharge capacity (a), discharge capacity normalized to cycle 5 (b), and voltage polarization vs cycle number (c) for Na-ion pouch cells cycling at C/5 and 40°C with various electrolyte additives in 1M NaPF6 EC:DEC (1:1 w/w). Figure 1

Hard-Carbon Negative Electrodes from Biomasses for Sodium-Ion Batteries.

With the development of high-performance electrode materials, sodium-ion batteries have been extensively studied and could potentially be applied in various fields to replace the lithium-ion cells, owing to the low cost and natural abundance. As the key anode materials of sodium-ion batteries, hard carbons still face problems, such as poor cycling performance and low initial Coulombic efficiency. Owning to the low synthesis cost and the natural presence of heteroatoms of biomasses, biomasses have positive implications for synthesizing the hard carbons for sodium-ion batteries. This minireview mainly explains the research progress of biomasses used as the precursors to prepare the hard-carbon materials. The storage mechanism of hard carbons, comparisons of the structural properties of hard carbons prepared from different biomasses, and the influence of the preparation conditions on the electrochemical properties of hard carbons are introduced. In addition, the effect of doping atoms is also summarized to provide an in-depth understanding and guidance for the design of high-performance hard carbons for sodium-ion batteries.

Electrochemical Investigation of Spherical Hard Carbon Negative Electrodes for Sodium Secondary Batteries

Hard carbon (HC) is receiving widespread attention as a negative electrode material for sodium secondary batteries. In this study, spherical HC with three different diameters (1 µm, 1.5 µm, and 5 µm) are investigated using ionic liquid (IL) and organic electrolytes. The HC with a diameter of 5 µm demonstrated the best performance with IL at 90 °C (capacities of 327 mAh g−1 at 20 mA g−1 and 77 mAh g−1 at 2000 mA g−1 and initial Coulombic efficiency of 78.9 %). The HCs showed stable cycling in ILs as compared to the organic electrolytes in the first few cycles.

Electrodeposition of Cu-Based Nanofoams and Sodiophilic Metals for Highly Stable Anode-Free Sodium Batteries

Sodium-ion batteries (SIBs) represent a cost-effective and sustainable alternative to lithium-ion battery (LIBs).1 One of the main issues of SIB technology is the higher ionic radius of sodium, compared to lithium, which limits the cyclability due to unfavourable intercalation of Na+ ions into carbon-based anodes.2 Anode-free batteries represents a promising approach to improve the cyclability, changing the working principle at the anode from intercalation of Na+ ions to electrodeposition metallic sodium.3 Moreover, thanks to the anode-free configuration the energy density of the overall device can be drastically increased. To efficiently control the electrodeposition of the metallic layer of sodium, the current collector must be carefully designed. Copper nanofoams were produced by dynamic hydrogen-bubble template (DHBT) as high-surface area template for the sodium electrodeposition, as an innovative alternative to the traditionally flat electrodes. The electrodeposition of sodiophilic materials, i.e. tin and antimony, onto the nanofoam, was also investigated to control the nucleation of the metallic layer at the anode.4 Sn and Sb baths were optimized to guarantee an homogeneous distribution of the sodiophilic metal on the nanofoam. The morphology and the composition of the nanofoams was characterized and electrochemical characterization was performed in half-cell configuration, showing coulombic efficiency higher than 99% and good cyclability for 100 cycles, for both Sn and Sb compositions compared to the bare Cu. This demonstrates the feasibility of the usage of sodiophilic nanofoams as current collectors in anode-free sodium batteries. Vaalma, C., Buchholz, D., Weil, M. & Passerini, S. A cost and resource analysis of sodium-ion batteries. Nat. Rev. Mater. 2018 34 3, 1–11 (2018).Irisarri, E., Ponrouch, A. & Palacin, M. R. Review—Hard Carbon Negative Electrode Materials for Sodium-Ion Batteries. J. Electrochem. Soc. 162, A2476–A2482 (2015).Cohn, A. P. et al. Rethinking sodium-ion anodes as nucleation layers for anode-free batteries. J. Mater. Chem. A 6, 23875–23884 (2018).Seh, Z. W., Sun, J., Sun, Y. & Cui, Y. A highly reversible room-temperature sodium metal anode. ACS Cent. Sci. 1, 449–455 (2015).

Improvement on Electrochemical Performance and Safety Using Mixtures of Ionic Liquid and Conventional Organic Electrolyte for Sodium-Ion Batteries

The success of commercializing sodium-ion batteries is inhibited by the implementation of suitable electrolytes that have economic feasibility and stable electrochemical performance. Organic electrolytes such as Na[ClO4]- or Na[PF6]-including organic solvents, EC, PC, DMC, and DEC, are most widely used to investigate the performance of sodium-ion batteries. These electrolytes are widely accepted because of the analogous studies in LIBs, but their flammability can cause reliability issues. Moreover, the SEI layer composed of sodium cations is known to be inhomogeneous and fragile, giving a higher solubility than that based on lithium cations.(1) In this regard, ionic liquid electrolytes are a drawing huge attention to constructing safe electrochemical devices owing to their unique properties such as low volatility and low flammability. Furthermore, recent studies report ionic liquids show similar electrochemical performance at room temperature to organic electrolytes and better interfacial properties.(2-4) Despite this bright improvement, the cost of ionic liquids drags down the advantage of economical sodium-ion batteries.In this study, ionic liquid and organic liquid electrolytes are mixed to adjust the balance of cost, performance, and safety. The [C3C1pyrr][FSA] (C3C1pyrr+ = N-methyl-N-propylpyrrolidinium) ionic liquid and PC organic solvent are used as mother electrolytes with Na[FSA] (FSA = bis(fluorosulfonyl)amide) and Na[ClO4] salts. The physical and electrochemical properties of the various mixing ratio of those mixture electrolytes are investigated for sodium-ion batteries. These mixture electrolytes improve not only flammability but also show better rate and cycling performance. The detailed electrochemical properties are evaluated by comparing reference electrolytes to clarify the dual salts effect and dual solvents (ionic liquids and organic solvent) effect. This performance will be discussed based on half cell tests using a Na2FeP2O7 positive electrode and full cell tests using a hard carbon negative electrode and a NaCrO2 positive electrode.

Origins of irreversible capacity loss in hard carbon negative electrodes for potassium-ion batteries.

Hard carbon (HC) is considered as a negative electrode material for potassium-ion batteries, but it suffers from significant irreversible capacity loss at the first discharge cycle. Here, we investigated the possible reasons of this capacity loss with a combination of in situ AFM and various ex situ TEM techniques (high resolution TEM and high angle annular dark field scanning TEM imaging, and STEM-EELS and STEM-EDX spectroscopic mapping) targeting the electrode/electrolyte interphase formation process in the carbonate-based electrolyte with and without vinylene carbonate (VC) as an additive. The studied HC consists of curved graphitic layers arranged into short packets and round cages, the latter acting as traps for K+ ions causing low Coulombic efficiency between cycling. Our comparative study of solid electrolyte interphase (SEI) formation in the carbonate-based electrolyte with and without the VC additive revealed that in the pristine electrolyte, the SEI consists mostly of inorganic components, whereas adding VC introduces a polymeric organic component to the SEI, increasing its elasticity and stability against fracturing upon HC expansion/contraction during electrochemical cycling. Additionally, significant K+ loss occurs due to Na+ for K+ exchange in Na-carboxymethyl cellulose used as a binder. These findings reflect the cumulative impact of the internal HC structure, SEI properties, and binder nature into the electrochemical functional properties of the HC-based anodes for K-ion batteries.

Reversible and High-rate Hard Carbon Negative Electrodes in a Fluorine-free Sodium-salt Electrolyte

Reversible and High-rate Hard Carbon Negative Electrodes in a Fluorine-free Sodium-salt Electrolyte

Effect of Prelithiation Process for Hard Carbon Negative Electrode on the Rate and Cycling Behaviors of Lithium-Ion Batteries

Two prelithiation processes (shallow Li-ion insertion, and thrice-repeated deep Li-ion insertion and extraction) were applied to the hard carbon (HC) negative electrode (NE) used in lithium-ion batteries (LIBs). LIB full-cells were assembled using Li(Ni0.5Co0.2Mn0.3)O2 positive electrodes (PEs) and the prelithiated HC NEs. The assembled full-cells were charged and discharged under a low current density, increasing current densities in a stepwise manner, and then constant under a high current density. The prelithiation process of shallow Li-ion insertion resulted in the high Coulombic efficiency (CE) of the full-cell at the initial charge-discharge cycles as well as in a superior rate capability. The prelithiation process of thrice-repeated Li-ion insertion and extraction attained an even higher CE and a high charge-discharge specific capacity under a low current density. However, both prelithiation processes decreased the capacity retention during charge-discharge cycling under a high current density, ascertaining a trade-off relationship between the increased CE and the cycling performance. Further elimination of the irreversible capacity of the HC NE was responsible for the higher utilization of both the PE and NE, attaining higher initial performances, but allowing the larger capacity to fade throughout charge-discharge cycling.

Improving the specific energy of Li-Ion capacitor laminate cell using hybrid activated Carbon/LiNi0.5Co0.2Mn0.3O2 as positive electrodes

In this work, we investigated the performance impact of LiNi0.5Co0.2Mn0.3O2 (NMC) as an additive to activated carbon (AC) electrodes within a high-performance Li-ion capacitor (LIC) fabricated with activated carbon positive electrodes (PEs) and hard carbon negative electrodes (NEs) having lithium thin films as Li sources loaded on the surface of the negative electrodes. The NMC additive impact in initial testing showed an increase in specific energy of the LIC of nearly 50.5% with a 32% maximum specific power loss. Contrary to its typical low rate battery decay at high rate cycling the cell having a hybrid PE is still able to maintain over 90% capacity at a 0.7C rate after 11,000 cycles at rate of 18C and an additional 9000 cycles at a rate of 36C. We conclude at high rate cycling minimal impacts occurs to the NMC properties which can be seen with low rate intercepts.

Unusual Passivation Ability of Superconcentrated Electrolytes toward Hard Carbon Negative Electrodes in Sodium-Ion Batteries.

The passivation of negative electrodes is key to achieving prolonged charge-discharge cycling with Na-ion batteries. Here, we report the unusual passivation ability of superconcentrated Na-salt electrolytes. For example, a 50 mol % sodium bis(fluorosulfonyl)amide (NaFSA)/succinonitrile (SN) electrolyte enables highly reversible Na+ insertion into a hard carbon negative electrode without any electrolyte additive, functional binder, or electrode pretreatment. Importantly, an anion-derived passivation film is formed via preferential reduction of the anion upon charging, which can effectively suppress further electrolyte reduction. As a structural characteristic of the electrolyte, most anions are coordinated to multiple Na+ cations at high concentration, which shifts the lowest unoccupied molecular orbitals of the anions downward, resulting in preferential anion reduction. The present work provides a new understanding of the passivation mechanism with respect to the coordination state of the anion.

Synthesis and characterization of d-glucose derived nanospheric hard carbon negative electrodes for lithium- and sodium-ion batteries

The electrochemical performance of glucose-derived hard carbon (GDHC) anode has been evaluated using Li- and Na-salts in ethylene carbonate and propylene carbonate electrolyte mixtures. The LiPF6/EC:PC (1:1) system exhibits high capacity at low current densities (400mAhg−1 at 25mAg−1) and also good power characteristics retaining 150mAhg−1 capacity at 2Ag−1 current density. The best overall performance was achieved with 1M NaPF6/EC:PC (1:1) electrolyte based system with capacities of 175mAhg−1 at 0.1V vs Na/Na+ and 330mAhg−1 at 1.5V vs Na/Na+. The electrode has been physically characterized ex-situ using SEM, Raman and TOF-SIMS methods TOF-SIMS analysis revealed that the solid electrolyte interphase is more inorganic on the negative electrode in the Na-cell than on the negative electrode the Li-cell. The positive ion-specific images established by TOF-SIMS analysis show the non-homogeneous distribution of various fragments from the pristine GDHC, which is caused by slightly inhomogeneous mixture of GDHC and conducting carbon black (Super P®) particles.

Performance of wide temperature range electrolytes for Li-Ion capacitor pouch cells

Abstract Four types of wide temperature-range electrolyte formulations based on carbonate and carboxylate esters were evaluated at various temperatures in lithium-ion capacitor (LIC) pouch cells consisting of both hard carbon (HC) and graphite negative electrodes (NEs) with thin lithium foil and an activated carbon (AC) positive electrodes (PEs). The electrolytes containing methyl butyrate (MB) with various additives enabled the LIC to operate at −40 °C, where all electrolytes based only on carbonates fail. MB-containing electrolyte with lithium Difluoro(oxalato)borate (LiDFOB) as additive showed the best cycling performance over 5000 cycles. Lithium plating also occurred on graphite NEs when charged at low temperatures starting at −20 °C, which resulted in the non-linear curves. When charged at 30 °C and discharged at −40 °C, graphite NE based LIC displayed regular linear charge-discharge curves without lithium plating. In comparison, HC NE based LICs showed better capacity retention at −40 °C and no signs of lithium plating. It could be concluded that low temperature performance of LIC was influenced by both electrolyte formulations and negative electrode material.

Optimizing the Electrolyte for Glucose-Derived Carbon Based Na-Ion Battery

Sodium-ion batteries have attracted great deal of attention due to sodium’s abundance and lower price compared to lithium and cobalt. A major obstacle that prevents sodium-ion batteries going to a mass-scale is the negative electrode (anode during discharge). Graphite is currently the safest negative electrode material for lithium-ion batteries, but does not intercalate sodium reversibly, because sodium ions prefer prismatic or octahedral coordination sites [1]. Our previous work [2] focused on the synthesis and electrochemical characterization of glucose-derived hard carbon negative electrodes for sodium-ion batteries. Unfortunately, the tested electrochemical half-cell showed high capacity fade during cycling. At the time, we thought it was due to material’s spherical morphology (Fig. 1a). But when we substituted the previously used electrolyte (1M NaClO4 in PC) with 1M NaPF6 in PC:EC (9:1) capacity fade was significantly reduced. The discharge capacity for the 100th discharge cycle was only 177 mAh g-1 when using NaClO4 in PC, but remained at 267 mAh g-1 when using NaPF6 in PC:EC (9:1) (Fig. 1d - e). The results of full cell testing with NVP-type cathode [3] will also be discussed. Acknowledgements This research was supported by the EU through the European Regional Development Fund TK141 “Advanced materials and high-technology devices for energy recuperation systems”, Estonian target research project IUT20–13 and personal research grant PUT55. Mr. Väli thanks Estonian Students Fund in USA and University of Tartu Foundation for financial support.

Influence of Stabilized Lithium Metal Powder Loadings on Negative Electrode to Cycle Life of Advanced Lithium-Ion Capacitors

The cycling performance of Li-ion capacitors (LICs) with various stabilized lithium metal powder (SLMP) loadings on hard carbon (HC) negative electrode was studied and compared. LICs were made with activated carbon (AC) positive electrode and HC/SLMP negative electrode. It was found that the optimal mass ratio of HC to SLMP should be from 5:1 to 8:1 for the LICs with 4.1 V as the maximum voltage to achieve stable cycle life. X-ray photoelectron spectroscopy (XPS) results showed that lithium fluoride (LiF) was observed on the surface of the HC electrode after cycling test. The generation of LiF was considered to be due to a reaction of hydrofluoric acid (HF) with the extra SLMP, lithium alkyl carbonate (semicarbonate, ROCOOLi) or lithium carbonate (Li2CO3) on the surface of the HC electrodes. The extra Li powders protect the solid electrolyte interphase (SEI) layers from reacting with HF so that the surface of the HC electrode is not continuously exposed to electrolyte directly. It was thus confirmed that enough SLMP loadings on HC negative electrode can realize 4.1 V LICs with excellent cycling performance.

Alkali-Metal Insertion Processes on Nanospheric Hard Carbon Electrodes: An Electrochemical Impedance Spectroscopy Study

In this study, we report the results of electrochemical impedance spectroscopy data modelling of various battery half-cells with different alkali metal (Li, Na, K) salts. Test results of electrochemical half-cells were evaluated for the D-glucose derived hard carbon negative electrode in 1.0 M LiPF6 + EC:DMC (1:1 volume ratio), 1.0 M NaPF6 + EC:DMC (1:1), 1.0 M NaClO4 + PC, 0.8 M KPF6 + EC:DEC (1:1) and 0.8 M KPF6 + EC:DMC (1:1) solutions at 0.5 mV s−1 potential scan rate measured within the potential region from 0.05 V to 1.2 V (vs Me/Me+) (where Me is Li, Na or K). Modelling of electrochemical impedance spectroscopy data was employed to characterize alkali metal insertion processes in/on D-glucose derived hard carbon anode. Detailed analysis of impedance data shows that Newman equivalent circuit modified with a constant phase element can be applied for calculation of impedance spectra and fitting of calculated data to experimental ones, using non-linear least square root fitting method. Equivalent circuit fit parameters depend strongly on electrolyte composition. Very slow processes have been observed for KPF6 + EC:DEC based half-cell. Comparatively quick metal-cation reduction and accumulation processes have been observed in NaClO4 + PC and LiPF6 +EC:DMC based half-cell anodes.

Electrochemical Impedance Spectroscopy Study of Alkali-Metal Insertion Processes on Hard Carbon Electrodes

Sodium-ion batteries have attracted great deal of attention due to sodium’s abundance and lower price compared to lithium and cobalt. A major obstacle that prevents sodium-ion batteries from going into mass-scale production is the negative electrode (anode during discharge). Graphite is currently the safest negative electrode material for lithium-ion batteries, but does not intercalate sodium reversibly, because sodium ions prefer prismatic or octahedral coordination sites [1]. Our previous work [2] focused on the synthesis and electrochemical characterization of glucose-derived hard carbon negative electrodes for sodium-ion batteries. The results inspired us to conduct a more detailed impedance study of the electrode processes. Hard carbons have shown to absorb/insert and desorb/de-insert sodium reversibly [2,3]. However, the exact mechanism is difficult to understand without detailed studies which apply different physical and electrochemical methods. Therefore, electrochemical impedance analysis with equivalent circuit fitting (Figure 1a, b) was conducted to analyse surface processes at various potentials. ICP-MS, SEM-EDS (Figure 1c) and TOF-SIMS confirmed alkali-metal absorption in the carbon structure. Acknowledgements This research was supported by the EU through the European Regional Development Fund TK141 “Advanced materials and high-technology devices for energy recuperation systems”, Estonian target research project IUT20–13 and personal research grant PUT55. The authors thank Dr. Kalle Kirsimäe for ICP-MS analysis.

Sodium Insertion/Extraction in Carbon-Coated Nanoscale Anatase TiO2

Rechargeable sodium batteries, as the replacement of rechargeable lithium batteries, are attracting much attention for large-scale applications such as electric vehicles and stationary energy storage due to the abundant resource of sodium compared to lithium. Up to now, there are only a few materials which have reasonable performances as negative electrodes of rechargeable sodium batteries, such as alloy materials [1], hard carbon [2], and sodium titanium oxides [3]. Recently, TiO2, an attractive alternative negative electrode material to graphite for rechargeable lithium batteries, was also investigated for rechargeable sodium batteries [4, 5]. However, the reported reversible capacities of TiO2 electrodes are less than 200 mAh g-1 and the Na insertion/extraction mechanism is still unclear. In the previous study, carbon-coated anatase TiO2 (TiO2/C) nanopowders were investigated as negative electrode material for rechargeable sodium battery with Na[FSA]-[C3C1pyrrr][FSA] (FSA = bis(fluorosulfonyl)amide; C3C1pyrr = N-methyl-N-propylpyrrolidinium) ionic liquid electrolyte [6]. In the present study, the electrochemical performance of TiO2/C electrode was further investigated in Na[FSA]-[C3C1pyrrr][FSA] ionic liquid electrolyte, and the Na insertion/extraction mechanism was explored. The TiO2/C electrode shows a high reversible discharge capacity of 275 mAh g-1 at 10 mA g-1 at 363 K. The observed capacity is higher than that reported for anatase TiO2 nanorods (193 mAh g-1 at 10 mA g-1) in organic electrolytes at room temperature [5]. Moreover, this capacity is comparable to that of a hard carbon negative electrode (260 mAh g-1) in Na[FSA]-[C3C1pyrrr][FSA] electrolyte [7]. Since the density of anatase TiO2 (ca. 3.9 g cm-3) is much higher than that of hard carbon (ca. 1.52 g cm-3), the TiO2/C nanopowder electrode is a promising negative electrode material for the rechargeable sodium batteries with high volumetric energy densities. The Na insertion/extraction mechanism was investigated by means of ex-situ X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy and electron diffraction. Ex-situ XRD analysis of the TiO2/C electrodes in the different charged/discharged states show that during the charge process, the anatase TiO2 crystal phase disappears and a new crystal phase is formed. During the discharge process, the newly formed crystal phase disappears and no anatase TiO2 phase appears again. According to the TEM, EDX and electron diffraction studies, the fully discharged TiO2/C electrode is suggested to consist of amorphous TiO2, amorphous Na2TiO3 and cubic TiO phases, and the crystal phase in the fully charged TiO2/C electrode is possibly a solid solution of cubic Na2TiO3 and cubic TiO phases. Thus, amorphous TiO2, amorphous Na2TiO3 and cubic TiO are regarded as active electrode materials, and reversibly react with Na ions to form Na2TiO3•TiO solid solution. The TiO2/C negative electrode also exhibits a high rate capability and good cycling performance. When the charge-discharge rate is increased, the shapes of the charge-discharge curves remain unchanged. The discharge capacities decrease gradually with an increase in the current rate. The TiO2/C electrode shows reversible discharge capacities of 174 and 93 mAh g-1 at current rates of 500 and 2000 mA g-1, respectively, corresponding to about 69 and 37% of the capacity at 20 mA g -1. The discharge capacity is 174 mAh g-1 after 400 cycles at a current rate of 200 mA g-1, exhibiting only 10% drop in capacity. After 1000 cycles, the capacity decreases by approximately 21%. Acknowledgements This study was partly supported by the Advanced Low Carbon Technology Research and Development Program (ALCA, No. 3428) of the Japan Science and Technology Agency (JST) and the “Elements Strategy Initiative to Form Core Research Center" program of the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT).

MgO-templated carbon as a negative electrode material for Na-ion capacitors

In this study, MgO-templated carbon with different pore structures was investigated as a negative electrode material for Na-ion capacitors. With increasing the Brunauer–Emmett–Teller surface area, the irreversible capacity increased, and the coulombic efficiency of the 1st cycle decreased because of the formation of solid electrolyte interface layers. MgO-templated carbon annealed at 1000°C exhibited the highest capacity and best rate performance, suggesting that an appropriate balance between surface area and crystallinity is imperative for fast Na-ion storage, attributed to the storage mechanism: combination of non-faradaic electric double-layer capacitance and faradaic Na intercalation in the carbon layers. Finally, a Na-ion capacitor cell using MgO-templated carbon and activated carbon as the negative and positive electrodes, respectively, exhibited an energy density at high power density significantly greater than that exhibited by the cell using a commercial hard carbon negative electrode.