Application In Sodium-ion Batteries Research Articles

Overview of coals as carbon anode materials for sodium-ion batteries

Abstract Energy storage is an important technology in achieving carbon-neutrality goals. Compared with lithium-ion batteries, the raw materials of sodium-ion batteries are abundant, low-cost, and highly safe. Furthermore, their costs are expected to be further reduced as large-scale applications take off, making them viable for energy storage applications. The primary anode material for sodium-ion batteries is hard carbon, which has a high sodium-ion storage capacity but is relatively expensive, limiting its applications in energy storage. In order to widen the applications of sodium-ion batteries in energy storage and other fields, it is particularly important to develop anode materials that have both high performance and low cost. Coals, with abundant reserves and worldwide availability, can serve as low-cost carbon sources for anode materials. Additionally, coals of different grades of metamorphism have different structural characteristics that can be tailored for the structural characteristics of coal-based anode materials for sodium-ion batteries. Recent research on tailoring coals as the anode materials for sodium-ion batteries is summarized and the recent progress made towards mitigating the existing issues is analysed in this review. Specifically, the impacts of different grades of metamorphism on the sodium-ion storage performance of coal-based anode materials prepared using direct carbonization are discussed in detail. Studies on improving the electrochemical performances of coal-based anode materials through pore and microcrystalline structure controls and surface as well as interface modifications are presented. Finally, the advantages and disadvantages of different preparation methods are identified. To make the industrial applications of coal-based anode materials for sodium-ion batteries more viable, the importance of the de-ashing process is introduced.

Designing Advanced Electrolytes for High-Safety and Long-Lifetime Sodium-Ion Batteries via Anion-Cation Interaction Modulation.

Safety hazards caused by flammable electrolytes have been major obstacles to the practical application of sodium-ion batteries (SIBs). The adoption of nonflammable all-phosphate electrolytes can effectively improve the safety of SIBs; however, traditional low-concentration phosphate electrolytes are not compatible with carbon-based anodes. Herein, we report an anion-cation interaction modulation strategy to design low-concentration phosphate electrolytes with superior physicochemical properties. Tris(2,2,2-trifluoroethyl) phosphate (TFEP) is introduced as a cosolvent to regulate the ion-solvent-coordinated (ISC) structure through enhancing the anion-cation interactions, forming the stable anion-induced ISC (AI-ISC) structure, even at a low salt concentration (1.22 M). Through spectroscopy analyses and theoretical calculations, we reveal the underlying mechanism responsible for the stabilization of these electrolytes. Impressively, both the hard carbon (HC) anode and Na4Fe2.91(PO4)2(P2O7) (NFPP) cathode work well with the developed electrolytes. The designed phosphate electrolyte enables Ah-level HC//NFPP pouch cells with an average Coulombic efficiency (CE) of over 99.9% and a capacity retention of 84.5% after 2000 cycles. In addition, the pouch cells can operate in a wide temperature range (-20 to 60 °C) and successfully pass rigorous safety testing. This work provides new insight into the design of the electrochemically compatibility electrolyte for high-safety and long-lifetime SIBs.

Design of the Particle Size and Morphology of Hard Carbon Anode Materials for Sodium-Ion Batteries Through Hydrothermal Carbonization

With sodium-ion batteries (SIBs) finding widespread application, the demand grows for hard carbon, the most popular anode material for SIBs. Hydrothermal carbonization facilitates the production of hard carbon with desired characteristics from various sources. Despite the considerable volume of literature addressing this subject, there is a notable absence of investigations elucidating the relationship between synthesis conditions and the electrochemical characteristics of the product. Here we study systematically the influence of hydrothermal carbonization parameters on hard carbon characteristics and emphasize the potential of hard carbon as an anode material for SIBs. The initial Coulombic efficiency (ICE) is significantly affected by the particle size of the glucose-derived hard carbon, which, in turn, depends on glucose concentration in the initial solution, pH, and stirring regime. By optimizing the hydrothermal carbonization parameters, the ICE up to 91% and a good reversible capacity of ∼300 mAh g−1 in a half cell are achieved. Full cells with Na3(VO)2(PO4)2F cathode material demonstrate ICE of about 80% and reversible capacity of up to 100 mAh g−1 cath. Considering the effective performance of pouch-cell SIB prototypes based on Na3(VO)2(PO4)2F and hard carbon, hydrothermal carbonization of glucose yields hard carbon with the necessary characteristics required for its successful application in SIBs.

High-efficacy multi-sodium carboxylate self-sacrificed additives for high energy density sodium-ion batteries

Sodium-ion batteries (SIBs) have gained significant interest in large-scale energy storage due to the abundance of sodium resources. However, interfacial side reactions lead to high irreversible Na+ loss and low Coulombic efficiency, which constrains the energy density of SIBs. Herein, two multi-sodium carboxylate cathode additives (sodium succinate and sodium malate) have been proposed for the first time to replenish the Na+ loss through in-situ decomposition. Breaking the double bond in the structure of disodium maleate lowered its decomposition voltage by 0.31 V; then, introducing a hydroxyl group further lowered its decomposition voltage by 0.2 V. With the addition of 10 wt% sodium succinate or sodium malate, the energy densities of the hard carbon/ /Na0.78Ni0.2Mn0.7Li0.05Ti0.05O2 full cells increases from 240 Wh kg−1 to 282 Wh kg−1 and 284 Wh kg−1, respectively. Moreover, the capacity retentions are improved from 62 % to 80 % and 79 % after 50 cycles with sodium succinate or sodium malate, respectively. This improvement occurs without sacrificing the rate and cycling performances, which is promising for large-scale application of SIBs.

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

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

Stable Structure and Fast Ion Diffusion: A Flexible MoO2@Carbon Hollow Nanofiber Film as a Binder-Free Anode for Sodium-Ion Batteries with Superior Kinetics and Excellent Rate Capability.

Designing innovative anode materials that exhibit excellent ion diffusion kinetics, enhanced structural stability, and superior electrical conductivity is imperative for advancing the rapid charge-discharge performance and widespread application of sodium-ion batteries. Hollow-structured materials have received significant attention in electrode design due to their rapid ion diffusion kinetics. Building upon this, we present a high-performance, free-standing MoO2@hollow carbon nanofiber (MoO2@HCNF) electrode, fabricated through facile coaxial electrospinning and subsequent heat treatment. In comparison to MoO2@carbon nanofibers (MoO2@CNFs), the MoO2@HCNF electrode demonstrates superior rate capability, attributed to its larger specific surface area, its higher pseudocapacitance contribution, and the enhanced diffusion kinetics of sodium ions. The discharge capacities of the MoO2@HCNF (MoO2@CNF) electrode at current densities of 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0 A g-1 are 195.55 (155.49), 180.98 (135.20), 163.81 (109.71), 144.05 (90.46), 121.16 (71.21) and 88.90 (44.68) mAh g-1, respectively. Additionally, the diffusion coefficients of sodium ions in the MoO2@HCNFs are 8.74 × 10-12 to 1.37 × 10-12 cm2 s-1, which surpass those of the MoO2@CNFs (6.49 × 10-12 to 9.30 × 10-13 cm2 s-1) during the discharging process. In addition, these prepared electrode materials exhibit outstanding flexibility, which is crucial to the power storage industry and smart wearable devices.

Mo6+ bifunctional substitution of P2-type manganese oxide for high performance sodium-ion batteries

Due to its cost-effectiveness and high specific capacity, P2-type manganese (Mn) oxide is considered a highly promising cathode material for sodium ion batteries (SIBs). However, the practical application of this material is hindered by poor cyclic stability caused by the phase transition of P2-P’2 due to the Jahn-Teller effect of Mn3+ at low voltage and significant volume changes at high voltage. In this study, we utilized Mo6+ dual-function P2-Na0.67Mn0.99Mo0.01O1.997(PO4)0.0008 (NMMPO-1) to achieve exceptional magnification performance and cycle stability. Advanced electron microscopy and X-ray diffraction analysis revealed that Mo6+ acts as a pillar in replacing interlayer Na+ sites within the P2 phase, resulting in extended layer spacing and a stable crystal structure. Additionally, Mo6+ substitution for Mn3+ weakens the Jahn-Teller distortion effect of Mn3+. Ex-situ X-ray diffraction experiments demonstrated that NMMPO-1 effectively inhibits the P2-P’2 phase transition at low voltage while minimizing volume changes during charge–discharge cycles. Overall, our modified NMMPO-1 exhibits excellent cycle stability with 144.55 mA h g−1 after 100 cycles at 0.5C, retaining 83 % capacity retention rate, along with impressive rate performance achieving capacity of 100.57 mA h g−1 at 5C. Furthermore, NMMPO-1 demonstrates superior air stability compared to unmodified samples when exposed to air over multiple cycles. Moreover, through detailed analysis using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic intermittent titration technique (GITT) curves; it is observed that NMMPO-1 possesses enhanced Na+ diffusion capacity and kinetic properties compared to other materials. This research provides novel insights into the role of substituting high valence cations in layered Mn oxide materials which can contribute towards designing improved performance for future sodium ion battery applications.

Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries

Sodium-ion batteries (SIBs) have attracted enormous attention as candidates in stationary energy storage systems, because of the decent electrochemical performance based on cheap and abundant Na-ion intercalation chemistry. Layered oxides, the workhorses of modern lithium-ion batteries, have regained interest for replicating their success in enabling SIBs. A unique feature of sodium layered oxides is their ability to crystallize into a thermodynamically stable P2-type layered structure with under-stoichiometric Na content. This structure provides highly open trigonal prismatic environments for Na ions, permitting high Na+ mobility and excellent structural stability. This review delves into the intrinsic characteristics and key challenges faced by P2-type cathodes and then comprehensively summarizes the up-to-date advances in modification strategies from compositional design, elemental doping, phase mixing, morphological control, and surface modification to sodium compensation. The updated understanding presented in this review is anticipated to guide and expedite the development of P2-type layered oxide cathodes for practical SIB applications.

Phosphorus‑carbon based hybrid electrodes for sodium ion batteries

In this study, commercial phosphorus powders were first subjected to mechanical ball-milling at varying times and reinforced with graphene oxide and multiwall carbon nanotubes to improve their electrochemical performances. All these processes were carried out by ball milling method to provide mechanical alloying of phosphorus. Subsequently, the crystallinity of the samples was analyzed by X-ray diffraction method, while the morphology of the samples was examined by field emission scanning electron microscopy. The main obstacles that inhibit the reversibility and the cyclic stability of red phosphorus are low electronic conductivity and huge volumetric expansion. Reinforcing amorphized phosphorous with multi-walled carbon nanotube and graphene oxide composites has presented similar initial capacity of 2100 mAh g−1. In addition, reversible capacity of multi-walled carbon nanotube and graphene oxide reinforced samples presented 370 mAh g−1 and 507 mAh g−1 after 250 cycles, respectively. Our results have shown that graphene oxide reinforced composites could be an alternative electrode material for sodium ion battery applications.

Scalable Synthesis of Layered Bismuth Germanate (Bi12GeO20) for Supercapacitors and Sodium ion Battery Applications

Scalable Synthesis of Layered Bismuth Germanate (Bi12GeO20) for Supercapacitors and Sodium ion Battery Applications

Investigating the effect of calcination temperature on the electrochemical properties of Na4MnV(PO4)3/NC@CNTs cathode materials for sodium ion batteries

Manganese-based NASICON-type Na4MnV(PO4)3 (NVMP) is used as an alternative material to the cathode materials (Na3V2(PO4)3) for sodium-ion battery (SIBs). By selecting the cheap and easily available and environmentally non-polluting Mn element to replace part of the highly toxic and expensive V element, it has the advantages of higher voltage platform, lower cost and weaker toxicity. However, the low intrinsic conductivity and poor cycling stability of NVMP severely limit its application in SIBs. In this paper, N-doped double carbon layer-coated NVMP composites (NVMP/NC@CNTs) are prepared by adding additional urea as a nitrogen source and CNTs as a carbon source using a simple and feasible sol-gel synthesis method, which improves their electronic conductivity and structural stability. On this basis, the effects of calcination temperature on the lattice structure, specific surface area, carbon disorder, Na+ diffusion coefficient (DNa+) and electrochemical properties of NVMP/NC@CNTs composites are mainly investigated. The results show that the optimal annealing temperature is 650 °C, and the NVMP/NC@CNTs material has the highest discharge capacity (105.6 mAh g−1 at 0.1C), the best rate performance (75.7 mAh g−1 at 5C), and the most stable cycling stability (79.4 %/89.3 % for 500 cycles at 1C / 5C, respectively). Even at high rate of 10C, an impressive cycling stability (77.5 % capacity retention over 1500 cycles). The sodium storage mechanism, electrochemical performance and pseudo-capacitance contribution of NVMP/NC@CNTs-650 are further investigated by kinetic analysis, GITT test, ex-situ XPS, XRD and EIS characterization. Finally, the assembled NVMP/NC@CNTs-650 || hard carbon (HC) full cell manifests a high capacity of 87.2 mAh g−1 with capacity retention of 92.5 % over 50 cycles at 1C. The results reveal that the appropriate annealing temperature can help to prepare electrochemically excellent and structurally stable NVMP composites, while the co-modification of N-doped dual-nanocarbon can produce a large number of defects and active sites, which is expected to facilitate the commercialization of SIBs.

Bi Nanospheres Embedded in N-Doped Carbon Nanowires Facilitate Ultrafast and Ultrastable Sodium Storage.

Sodium ion batteries (SIBs) are considered as the ideal candidates for the next generation of electrochemical energy storage devices. The major challenges of anode lie in poor cycling stability and the sluggish kinetics attributed to the inherent large Na+ size. In this work, Bi nanosphere encapsulated in N-doped carbon nanowires (Bi@N-C) is assembled by facile electrospinning and carbonization. N-doped carbon mitigates the structure stress/strain during alloying/dealloying, optimizes the ionic/electronic diffusion, and provides fast electron transfer and structural stability. Due to the excellent structure, Bi@N-C shows excellent Na storage performance in SIBs in terms of good cycling stability and rate capacity in half cells and full cells. The fundamental mechanism of the outstanding electrochemical performance of Bi@N-C has been demonstrated through synchrotron in-situ XRD, atomic force microscopy, ex-situ scanning electron microscopy (SEM) and density functional theory (DFT) calculation. Importantly, a deeper understanding of the underlying reasons of the performance improvement is elucidated, which is vital for providing the theoretical basis for application of SIBs.

Ti-doped O3-NaNi0.5Mn0.5O2 as high-performance cathode materials for sodium-ion batteries

With favorable electrochemical activity, NaNi0.5Mn0.5O2(NNMO) has been a key cathode material in sodium-ion batteries. However, the phase-transition irreversible and kinetics sluggish issues in NaNi0.5Mn0.5O2 greatly limit its application in sodium-ion batteries. Here, various Ti-doped O3-NaNi0.5Mn0.5-xTixO2 cathode materials were fabricated via solid-phase method. The extra Ti-doping treatment effectively stabilized the transition layer of metal and enlarged the Na layer spacing, as well as suppressed the Na+/vacancy ordering. In particular, the obtained O3-NaNi0.5Mn0.46Ti0.04O2 (NNMOTi-4) exhibits initial reversible capacity of 108.6 mAh g−1 at 0.1C and remarkable 65.2% capacity retention after 100 cycles at 1C. Accordingly, this work provides a facile Ti-doping strategy to regulate the structure and electrochemical performance of layered NaxTMO2 for sodium-ion batteries.

Insight into the effect of structural differences among pitch fractions on sodium storage performance of pitch-derived hard carbons

Pitch is an excellent precursor of hard carbons (HCs) with low-cost and high carbon content, enabling its commercial-scale application in sodium-ion batteries. Pre-oxidation is the key to obtaining pitch-derived HCs with disordered microstructures. However, pitch is a mixture of fractions with different molecular weights and structures. With the introduction of oxygen, the cross-linked structure established by each fraction could be different, the effect of which on the microstructure and sodium storage performance of HCs remains unclear. Here, we separate pitch into three fractions and further pre-oxidize and carbonize to produce HCs, in order to systematically investigate the effects of structural differences among pitch fractions on the cross-linking mechanism and microstructure of as-obtained HCs. It is found that tetrahydrofuran-insoluble (THFI) enables the enrichment of polar oxygen functional groups, thus enhancing its oxidative activity to obtain more oxygen for the construction of abundant three-dimensional cross-linked structure. It is conducive to preventing the rearrangement of carbon layers and promoting the development of micropores during high-temperature carbonization process, which facilitates the storage of sodium ions in the low-voltage plateau region. The resulting hard carbon (THFI-1400) has significantly improved overall sodium storage performances. This work provides a new strategy to prepare low-cost and high-performance HCs.

In-situ pre-sodiation of Prussian blue for the construction of high-performance sodium-ion batteries

Prussian blue and its analogs have attracted increasing attention owing to their stable structure and low cost, especially in the field of sodium-ion batteries. However, the insufficient sodium and high contents of water in the structure greatly hinder their practical applications. Herein, an in-situ pre-sodiation strategy is conducted to realize a one-step transformation of sodium-poor Prussian blue (NP-PB) to sodium-rich Prussian blue (NR-PB) without the introduction of other impurities. The introduced sodium ions take the place of coordination water, resulting in richer sodium ion transport channels and higher structural stability. Thanks to the high sodium content and low water content in the structure, the as prepared NR-PB presents a significantly higher initial coulombic efficiency of 95.18% compared to that of NP-PB (41.93%). When coupled with hard carbon (HC), the NP-PB//HC cell exhibits an ultra-high capacity retention of 93.71% after 300 cycles at a current density of 200 mAh/g. Our work provides guidance for the in-situ transformation of sodium-poor Prussian blue into sodium-rich Prussian blue, which is of great importance for the practical applications of sodium ion batteries.

Compositing pine pollen derived carbon matrix with Na4FeV(PO4)3 nanoparticle for cost-effective sodium-ion batteries cathode

Na super-ion conductor type material Na3V2(PO4)3 has been widely researched as the cathode of sodium-ion batteries (SIBs) in recent years, but the unsatisfying cost of Na3V2(PO4)3 impedes its wide application in SIBs. In this study, iron element is used to replace part of vanadium in Na3V2(PO4)3 to reduce its expense, and pine pollen is applied for the first time as a very effective carbon source to improve the performance of Na4FeV(PO4)3. The fabricated composite material achieves a capacity of 105 mA h g−1 under 0.2 C and fascinating cycling stability over 94 % under 2 C for 500 cycles and 98 % under 10 C for 1000 cycles. The excellent cycle performance is caused by the involvement of pine pollen that acts as a carbon matrix to enhance the electron conductivity and block the agglomeration of active material effectively, thus the well-dispersed nano sized Na4FeV(PO4)3 shortens the diffusion path of sodium ion and gains a remarkable rate capability. Moreover, the distinguished reversibility during the charge and discharge procedures is ascribed also to the robust structure of Na4FeV(PO4)3. This work provides an efficient route to realize the economic cathode material of SIBs with good performance.

Enhanced redox kinetics in hierarchical tubular FeSe2 by incorporating Se quantum dots towards high-performance sodium-ion batteries

Metal selenides have emerged as promising Na-storage anode materials owing to their substantial theoretical capacity and high cost-effectiveness. However, the application of metal selenides is hindered by inferior electronic conductivity, huge volume variation, and sluggish kinetics of ionic migration. In response to these challenges, herein, a hierarchical hollow tube consisting of FeSe2 nanosheets and Se quantum dots anchored within a carbon skeleton (HT-FeSe2/Se/C) is strategically engineered and synthesized. The most remarkable feature of HT-FeSe2/Se/C is the introduction of Se quantum dots, which could lead to high electron density near the Fermi level and significantly enhance the overall charge transfer capability of the electrode. Moreover, the distinctive hollow tubular structure enveloped by the carbon skeleton endows the HT-FeSe2/Se/C anode with robust structural stability and fast surface-controlled Na-storage kinetics. Consequently, the as-synthesized HT-FeSe2/Se/C demonstrates a reversible capacity of 253.5 mAh/g at a current density of 5 A/g and a high specific capacity of 343.9 mAh/g at 1 A/g after 100 cycles in sodium-ion batteries (SIBs). Furthermore, a full cell is assembled with HT-FeSe2/Se/C as the anode, and a vanadium-based cathode (Na3V2(PO4)2O2F), showcasing a high specific capacity of 118.1 mAh/g at 2 A/g. The excellent performance of HT-FeSe2/Se/C may hint at future material design strategies and advance the development and application of SIBs.

Iron‐Based Sulfate for Sodium‐Ion Batteries: Past, Present, and Future

Sodium‐ion batteries (SIBs) are crucial energy equipment that sustain low cost and better environmental benefit. Nevertheless, the practical energy density of SIBs is limited by cathode material. Over last decades, the iron‐based sulfate (IBS) has been extensively studied owing to its numerous advantages, including a large theoretical specific energy (over 100 Wh kg−1), high working potential (above 3.4 V), low cost, good structural stability, and environmental friendliness. Nevertheless, the application of IBS in SIBs is limited by its unsuitable electrolyte and low electronic/ionic conductivity. This review summarizes recently developed results on IBS materials for SIBs, ranging from the phase diagram–composition structure–electrochemical performance to modification research. A generalized summary of the future prospects of IBS‐based materials is also provided, with the hope of inspiring further advances in their application in SIBs.

Sb-Doped Biphasic P2/O3-Type Mn-Rich Layered Oxide Cathode Material for High-Performance Sodium-Ion Batteries.

Mn-rich P2-type layered oxide cathode materials suffer from severe capacity loss caused by detrimental phase transition and transition metal dissolution, making their implementation difficult in large-scale sodium-ion battery applications. Herein, we introduced a high-valent Sb5+ substitution, leading to a biphasic P2/O3 cathode that suppresses the P2-O2 phase transformation in the high-voltage condition attributed to the stronger Sb-O covalency that introduces extra electrons to the O atom, reducing oxygen loss from the lattices and improving structural stability, as confirmed by first-principle calculations. Besides, the enhanced Na+ diffusion kinetics and thermodynamics in the modified sample are associated with the enlarged lattice parameters. As a result, the proposed cathode delivers a discharge capacity of 142.6 mAh g-1 at 0.1C between 1.5 and 4.3 V and excellent performance at a high mass loading of 8 mg cm3 with a specific capacity of 131 mAh g-1 at 0.2C. Furthermore, it also possesses remarkable rate capability (90.3 mAh g-1 at 5C), specifying its practicality in high-energy-density sodium-ion batteries. Hence, this work provides insights into incorporating high-valent dopants for high-performance Mn-rich cathodes.

Density functional theory analysis of Na4Mn4Ti5O18 for sodium ion battery electrode

Density functional theory study has been performed on Na4Mn4Ti5O18 to evaluate its application feasibility as an electrode in Na ion battery. Two types of void in the structure of Na4Mn4Ti5O18, viz., Z shaped tunnel and polyhedral pentagonal void, has been observed that act as guest sites for reversible migration of Na+ ions. It is also noted that electrochemically active sites are placed in Z shaped tunnel that act as host for Na+ during anodic response while it acts as the Na+ donor during cathodic response. The Mn atom, located in the Z-shape tunnel, acts as redox active site for both cathodic and anodic response via formation of redox couple Mn+3/Mn+4 and Mn+3/Mn+2 respectively. On the other hand, Mn atom present at square pyramidal site remains inert during charge–discharge process. The redox action in Na4Mn4Ti5O18, occurring due to intercalation and de-intercalation of 2Na+ ions per formula unit, has been noted to exhibit voltage ∼2.63 V and 3.82 V respectively. The theoretical capacity ∼128 mAhg−1 has been estimated corresponding to migration of 2Na+ in Na4Mn4Ti5O18 structure. The present simulation study indicates the possibility of electrode action in Na4Mn4Ti5O18 for sodium ion battery application.