Sodiation Kinetics Research Articles

Boosting the Sodiation Kinetics of Sn Anode Using a Yolk-Shell Nanohybrid Structure for High-Rate and Ultrastable Sodium-Ion Batteries.

Metallic Sn (Tin) is a promising anode material for Na-ion batteries owing to its high theoretical capacity of 870mAhg-1. However, its large volumetric changes, interfacial instability, and sluggish sodiation kinetics limit its practical applications. Herein, a hierarchical yolk-shell nanohybrid composed of an Sn yolk and a Carbon/Silicon oxycarbide (C/SiOC) bilayer shell is prepared via the simple pyrolysis of a silicone oil dispersion containing an Sn precursor. The multifunctional bilayer helps boost sodiation kinetics by providing conductive pathways, enhancing the reversible capacity through surface capacitive reactions, and stabilizing the electrode/electrolyte interface. Abundant void interspaces inside the yolk-shell structure accommodate large volume changes of the Sn yolk. The Sn@C/SiOC nanohybrid demonstrates high specific capacity (≈500mAhg-1 at 1Ag-1), remarkable rate performance up to 10Ag-1, and ultrastable cyclability (91.1% retention after 1500 cycles at 5Ag-1). This yolk-shell nanohybrid structuring can guide the development of various high-capacity anodes for energy storage applications.

In Situ Lattice-Resolution Revelation of the Origins of Unexplored Anisotropic Sodiation Kinetics and Phase Transition in the Niobium Sulfide Anode.

Layered transition metal dichalcogenides (TMDs) have exhibited huge potential as anode materials for sodium-ion batteries. Most of them usually store sodium via an intercalation-conversion mechanism, but niobium sulfide (NbS2) may be an exception. Herein, through in situ transmission electron microscopy, we carefully investigated the insertion behaviors of Na ions in NbS2 and directly visualized anisotropic sodiation kinetics. Lattice-resolution imaging coupled with density functional theory calculations reveals the preferential diffusion of Na ions within layers of NbS2, accompanied by observable interlayer lattice expansion. Impressively, the Na-inserted layers can still withstand in situ mechanical testing. Further in situ observation vertical to the a/b plane of NbS2 tracked the illusive conversion reaction, which could result from interlayer gliding or wrinkling associated with stress accumulation. In situ electron diffraction measurements ruled out the possibility of such a conversion mechanism and identified a phase transition from pristine 3R-NbS2 to 2H-NaNbS2. Therefore, the NbS2 anode stores Na ions via only the intercalation mechanism, which conceptually differs from the well-known intercalation-conversion mechanism of typical TMDs. These findings not only decipher the whole sodiation process of the NbS2 anode but also provide valuable reference for unraveling the precise sodium storage mechanism in other TMDs.

Regulating Pseudo-Graphitic Domain and Closed Pores to Facilitate Plateau Sodium Storage Capacity and Kinetics for Hard Carbon.

Hard carbon anode demonstrates exceptional potential in sodium-ion batteries due to their cost-effectivenss and superior plateau capacity. However, the proximity of the plateau capacity to the cut-off voltage of battery operation and the premature cut-off voltage response caused by polarization at high rates greatly limit the exploitation of plateau capacities, raising big concerns about inferior rate performance of high-plateau-capacity hard carbon. In this work, a facile pre-oxidation strategy is proposed for fabricating lignin-derived hard carbon. Both high-plateau capacity and sodiation kinetics are significantly enhanced due to the introduction of expanded pseudo-graphitic domains and high-speed closed pores. Impressively, the optimized hard carbon exhibits an increased reversible capacity from 252.1 to 302.0 mAh g-1, alongside superior rate performance (174.7 mAh g-1 at 5 C) and stable cyclability over 500 cycles. This study paves a low-cost and effective pathway to modulate the microstructure of biomass-derived hard carbon materials for facilitating plateau sodium storage kinetics.

Topological proton regulation of interlayered local structure in sodium titanite for wide‐temperature sodium storage

AbstractDeveloping high‐capacity and high‐rate anodes is significant to engineering sodium‐ion batteries with high energy density and high power density. Layered Na2Ti3O7 (NTO), with an open crystal structure, large theoretical capacity, and low working potential, is recognized as one of the prospective anodes for sodium storage. Nevertheless, it suffers from sluggish sodiation kinetics and low (micro)structure stability triggered by a high Na+ diffusion barrier and weak adhesion of [Ti3O7] slabs. Herein, the interlayered local structure of NTO is regulated to solve the above issues, in which parts of interlayered Na+ sites are substituted by H+ (Na2−xHxTi3O7 [NHTO]). Theoretical calculations prove that the NHTO offers lower activation energy for Na+ transports and low interlayer spacings with alleviated Na–Na repulsion and relatively flexible [Ti3O7] slabs to reduce fractural stress. In situ and ex situ characterizations of (micro)structure evolution reveal that NHTO goes through transformation between H‐rich and Na‐rich phases, resulting in high structure stability and microstructure integrity. The optimal NHTO anode delivers a high capacity of 190.6 mA h g−1 at 0.5 C after 300 cycles and a superior high‐rate stability of 90.6 mA h g−1 at 50 C over 10,000 cycles at room temperature. Besides, it offers a capacity of 50.3 mA h g−1 after 1800 cycles at a low temperature of −20°C and 195.7 mA h g−1 after 500 cycles at a high temperature of 40°C at 0.5 C. The developed topologically interlayered local structure regulation strategy would raise the prospect of designing high‐performance layered anodes.

Dual-Functionalized Ca Enables High Sodiation Kinetics for Hard Carbon in Sodium-Ion Batteries.

Hard carbons (HCs), while a leading candidate for sodium-ion battery (SIB) anode materials, face challenges in their unfavorable sodiation kinetics since the intricate microstructure of HCs complicates the Na+ diffusion channel. Herein, a Hovenia dulcis-derived HC realizes a markedly enhanced high-rate performance in virtue of dual-functionalized Ca. The interlayer doped Ca2+ effectively enlarges the interlayer spacing, while the in situ-formed CaSe templates induce the formation of hierarchical pore structures and intrinsic defects, significantly providing fast Na+ diffusion channels and abundant active sites and thus enhancing the sodium storage kinetics. Achieved by the synergistic effect of regulation of intrinsic microcrystalline and pore structures, the optimized HC shows remarkable performance enhancements, including a high reversible capacity of 350.3 mA h g-1 after 50 cycles at 50 mA g-1, a high-capacity retention rate of 95.3% after 1000 cycles, and excellent rate performance (108.4 mA h g-1 at 2 A g-1). This work sheds light on valuable insight into the structural adjustment of high-rate HCs, facilitating the widespread utilization of SIBs.

Insights into the Sodiation Kinetics of Si and Ge Anodes for Sodium-Ion Batteries

Group IVA elements exhibit interesting Na storage capabilities due to the success of their Li alloy analogues. However, beyond hard carbon, they remain poorly understood as anodes for sodium-ion batteries (SIBs). Here, kinetic investigations of the electrochemical sodiation of Si and Ge are conducted using liquid electrolytes and half-cell configurations. Sodiation of Ge is found to be kinetically limited rather than thermodynamically limited. Either increasing temperature or decreasing sodiation rate can facilitate easier transformations from Ge to Na-Ge phases. A critical temperature seems to exist between 50 °C and 60 °C, beyond which a higher sodiation capacity is evident. The phase transformations are analyzed using Kolmogorov–Johnson–Mehl–Avrami theory. Following a one-dimensional growth, the Ge to NaGe4 is determined to be diffusion limited whereas NaGe4 to Na1+x Ge is controlled by reaction speed. Moreover, the Arrhenius equation is employed to investigate the temperature dependence on both phase transformations, giving activation energies of ∼50 kJ·mol−1 and ∼70 kJ·mol−1, respectively. Schematic models are proposed to elucidate the sodiation mechanisms, potentially influencing sought-after advancements in cell formats and classifications. Not only does this work lay the foundation for efforts on the Ge-based anodes, but also provides analogous kinetic information to Si/Sn-based ones for SIBs.

Porous NiS2 nanosheets anchored on reduced graphene oxide as high-rate and long-life anode materials for sodium-ion batteries

In this work, graphene oxide is used as a substrate for the nucleation and growth of Ni(OH)(OCH3) during solvothermal synthesis. As a result, 2D Ni(OH)(OCH3) nanosheets are vertically anchored on 2D reduced graphene oxide (rGO) nanosheets, forming a Ni(OH)(OCH3)@rGO composite. Afterward, a NiS2@rGO hybrid is created by sulfurizing the Ni(OH)(OCH3)@rGO intermediate, which presents the same morphology as the intermediate but a porous structure of NiS2 nanosheets. The characteristic 2D porous structure of NiS2 nanosheets offers abundant active sites for charge storage, accelerates ion/electron transport, and buffers volume expansion. Meanwhile, rGO nanosheets can suppress the shuttle of polysulfides while also enhancing electrical conductivity. These factors in conjunction with the thin and robust ether-derived SEI are favorable to mechanical stability and rapid sodiation kinetics in the NiS2@rGO electrode in the ether-based electrolyte, thereby endowing it with exceptional rate capability (324 mA h g-1 at 5 A g-1) and a long lifespan (378 mA h g-1 over 1800 cycles at 1 A g-1). However, in the ester-based electrolyte, NiS2@rGO shows poor rate capability and cyclability. The kinetic analysis unveils the mechanism underlying the different electrochemical behavior in NiS2@rGO in the two electrolytes. More importantly, the NiS2@rGO electrode outperforms most NiS2-based materials reported in terms of rate and cycling performance owing to its unique hierarchical structure, making it a viable contender for anode materials in sodium-ion batteries. We also propose a novel M(OH)(OCH3)@rGO intermediate for the fabrication of transition-metal oxides/sulfides@rGO composites for energy storage and catalysis.

Intercalative Motifs‐Induced Space Confinement and Bonding Covalency Enhancement Enable Ultrafast and Large Sodium Storage

AbstractAlloying‐type metal sulfides with high theoretical capacities are promising anodes for sodium‐ion batteries, but suffer from sluggish sodiation kinetics and huge volume expansion. Introducing intercalative motifs into alloying‐type metal sulfides is an efficient strategy to solve the above issues. Herein, robust intercalative InS motifs are grafted to high‐capacity layered Bi2S3 to form a cation‐disordered (BiIn)2S3, synergistically realizing high‐rate and large‐capacity sodium storage. The InS motif with strong bonding serves as a space‐confinement unit to buffer the volume expansion, maintaining superior structural stability. Moreover, the grafted high‐metallicity Indium increases the bonding covalency of BiS, realizing controllable reconstruction of BiS bond during cycling to effectively prevent the migration and aggregation of atomic Bi. The novel (BiIn)2S3 anode delivers a high capacity of 537 mAh g−1 at 0.4 C and a superior high‐rate stability of 247 mAh g−1 at 40 C over 10000 cycles. Further in situ and ex situ characterizations reveal the in‐depth reaction mechanism and the breakage and formation of reversible BiS bonds. The proposed space confinement and bonding covalency enhancement strategy via grafting intercalative motifs can be conducive to developing novel high‐rate and large‐capacity anodes.

Multi‐Level Modifications Enabling Chemomechanically Stable Ni‐Rich O3‐Layered Cathode toward Wide‐Temperature‐Tolerance Quasi‐Solid‐State Na‐Ion Batteries

AbstractThe O3‐type Ni‐rich NaNixCoyMn1−x−yO2 (x ≥ 0.6) oxides are regarded as one of the most promising cathodes for high‐capacity Na‐ion batteries (NIBs), however, they still suffer from severe structural/morphological degradation induced by complicated phase transitions, as well as sluggish de‐/sodiation kinetics. For this, a multi‐level structural/compositional modification strategy, including “core–shell” design, bulk heteroatom doping, and surface coating, is purposefully explored to construct an advanced NaNi0.6Co0.2Mn0.2O2 cathode (denoted as T‐CSN6@A). The Ni‐rich core guarantees the high capacity, and the Mn‐rich surface region coupled with bulk Ti doping and surface Al2O3 coating reinforces the structural stability. This well‐designed architecture not only effectively inhibits the bulk and sur‐/interface structural fractures caused by repeated lattice volume variations upon cycling, but also dramatically boosts the de‐/sodiated kinetics, thus resulting in high chemomechanical stability and improved electronic/ionic transport for efficient sodium storage. When utilized as a competitive cathode, the T‐CSN6@A‐based quasi‐solid‐state NIBs are endowed with remarkable wide‐temperature‐tolerance Na‐storage behaviors within practicable working temperatures from −20 to 50 °C, along with an attractive material‐level energy density of ≈255 Wh Kg−1 at 25 °C. The feasible modification here provides a new avenue for advanced O3‐type Ni‐rich cathodes toward large‐scale industrialization of next‐generation NIBs.

Defective Bi2S3 Anchored on CuS/C as an Ultrafast and Long-Life Anode for Sodium-Ion Storage.

Due to the high electrical conductivity and abundant redox active sites, bimetal sulfides are highly competitive anode materials for sodium storage with long-life and high-rate. Herein, a heterostructured metal sulfide (Bi2S3-CuS) with a carbon-based support is prepared by calcination and ion exchange methods. The synergistic effects of the heterostructure and defective structure provide facile diffusion channels, fast Na+ migration, and plentiful active sites for Na+, which reflect in the impressive electrochemical performance with a high reversible capacity of 592.2 mA h g-1 after 1000 cycles at 8 A g-1. Furthermore, the Na-ion full batteries exhibit an ultra-long cycling performance with a value of 216 mA h g-1 after 4000 cycles at 10 A g-1. Interestingly, the defective structure of Bi2S3 remains after cycling. Kinetic analyses and density functional theoretical calculations clarified that the heterointerfacial structure, especially on the interface containing sulfur defects in Bi2S3 of Bi2S3-CuS, could induce feasible ion adsorption and promote ion transfer, which lays the foundation for achieving ultrafast sodiation kinetics.

Bundled carbon nanofiber arrays grown on Cu-Ni tube textile boosting superior sodium-ion storage kinetics

Achieving high energy and power density in micro sodium-ion storage device is imperative and challenging due to the large radius of sodium ions and induced sluggish sodiation kinetics. Herein, we develop a vertically aligned carbon nanofiber arrays with a graphite dome (G-CNFAs/Textile) using the unique Cu-Ni solid solution textile as catalyst and thin substrate. As a flexible electrode with a maximum thickness of 4.0 µm, the G-CNFAs/Textile combines the merits of the thinner intertwined/ interconnected carbon nanofiber bundles, high porosity and abundant defects, thereby deriving enhanced sodium-ion storage kinetics. As a result, the flexible micrometer-scale G-CNFAs/Textile not only delivers an impressive gravimetric performance of 332.6 mA h g−1 at 1000 mA g−1 after 500 cycle numbers, but also exhibits outstanding volumetric capacity of ca. 1387.6 mA h cm−3 in half-type SIBs. Meanwhile, as the submicron-scale self-supporting symmetric electrode of SICs, the G-CNFAs//G-CNFAs capacitor also presents an impressive gravimetric capacitance of 66.5 F g−1 at 0.1 A g−1, superior energy-power density (80.9 W h kg−1 and 7771.8 W kg−1), as well as outstanding cyclic stability. Thus, we hope that this flexible micro-submicron-scale electrode structures could provide inspirations for thin, light-weight and miniaturized sodium-ion storage devices.

Tailoring Ultrafast and High-Capacity Sodium Storage via Binding-Energy-Driven Atomic Scissors.

Controllably tailoring alloying anode materials to achieve fast charging and enhanced structural stability is crucial for sodium-ion batteries with high rate and high capacity performance, yet remains a significant challenge owing to the huge volume change and sluggish sodiation kinetics. Here, a chemical tailoring tool is proposed and developed by atomically dispersing high-capacity Ge metal into the rigid and conductive sulfide framework for controllable reconstruction of GeS bonds to synergistically realize high capacity and high rate performance for sodium storage. The integrated GeTiS3 material with stable Ti-S framework and weak GeS bonding delivers high specific capacities of 678 mA h g-1 at 0.3 C over 100 cycles and 209 mA h g-1 at 32 C over 10000 cycles, outperforming most of the reported alloying type anode materials for sodium storage. Interestingly, in situ Raman, X-ray diffraction (XRD), and ex situ transmission electron microscopy (TEM) characterizations reveal the formation of well-dispersed Nax Ge confined in the rigid Ti-S matrix with suppressed volume change after discharge. The synergistically coupled alloying-conversion and surface-dominated redox reactions with enhanced capacitive contribution and high reaction reversibility by a binding-energy-driven atomic scissors method would break new ground on designing a high-rate and high-capacity sodium-ion batteries.

Plasma-promoted surface regulation of a novel integrative carbon network for boosting the long-cycle capability of sodium-ion storage

In the process of Na+-storage in hard carbon materials, the sluggish (de)sodiation kinetics and irreversible reactions of surface groups with electrolytes are bottlenecks that limit the rate performance and long-term cycle stability. We successfully prepared a novel integrative carbon network material (ICN) on a large scale, and the carbon structure and surface oxygen groups were regulated to increase the rate and cycle capability using a plasma-assisted catalyst-free carbonization process. ICNs typically have ultrathin carbon nanofiber units (∼10 nm) interconnected by sp2 hybrid covalent bonds, which form an interface-free integrative conductive network and ultra-short ion/electron transport pathways. The order degree of surface carbon stacking, interlayer spacing, and the content of –CO groups can be regulated by surface plasma treating to further boost kinetics and enhance reversible reactions. Thus, the ICN exhibits all-round improvements in terms of a large reversible capacity (389.5 mAh g−1 after 100 cycles at 50 mA g−1), a superior rate capability (285 mAh g−1 at 2000 mA g−1 after 3000 cycles), and good cycling stability (retain 72.2% after 10,000 cycles at 2000 mA g−1). According to the analysis of the dynamics, the capacitive mechanism by which Na+ quickly inserts/adsorbs with the surface/subsurface atoms is primarily responsible for the excellent electrochemical performance. A full cell made up of an ICN anode and a Na3V2(PO4)3 cathode successfully lit the diode, demonstrating the ultra-high rate capability of ICN in practical application. It has a reversible capacity of 101.9 mAh g−1 at 1000 mA g−1 and maintains 76% after 2000 cycles, demonstrating the ultra-high rate capability of ICN in practical application. These results suggest a new perspective on the structure design of carbon-based materials for fast and long-life sodium-storage anodes.

Ultrathin CuSe nanosheets as the anode for sodium ion battery with high rate performance and long cycle life

Sodium-ion batteries (SIBs) have quickly developed to be important alternative energy storage devices other than lithium-ion batteries in the explosively growing market of large-scale grid storage and low-speed electric vehicles, thanks to their low cost and the abundance of sodium resources. However, the sluggish sodiation kinetics remains major concerns in design of competent anode materials for SIBs. Herein, by using a one-step, facile hydrothermal synthesis, well-defined ultrathin CuSe nanosheets (thickness of ∼5 nm) were fabricated, leading to a narrow bandgap, flexible Cu–Se bonding and abundant electrochemically active sites. As a result, the product presents high rate performance: a high capacity of 404 mAh g–1 is achieved at a current density of 0.1 A g–1 after 100 operation cycles, and the capacity can be maintained to 276 mAh g–1 even at a high current density of 20 A g–1. According to kinetics analysis, surface capacitance contributes dominatly in the electrochemical process, facilitating fast sodiation. Moreover, coordinated the ultrathin thickness and two-dimensional morphology of the nanosheets with their three-dimensional open framework, the volume expansion-related issues during charge/discharge processes have been well addressed, resulting in ultrahigh cycling stability (with 100% capacity maintenance after 500 cycles at 0.5 A g–1) together with ultralong cycle life (up to 10,000 working cycles) at 20 A g–1.

Flexible SnS2/CNTs/porous Cu tube textile anode for enhanced sodium-ion batteries

Achieving fast charging and stability of metal sulfide anode materials is essential but challenging for advanced sodium-ion batteries (SIBs) because of their sluggish sodiation kinetics and poor intrinsic conductivity. Herein, the tin sulfide (SnS2) nanoparticles with tunable sizes were successfully synthesized on the carbon nanotubes via hydrothermal method, which grown on porous copper tube textiles via chemical vapor deposition to form free-standing electrodes. Obviously, the uniform carbon nanotubes supported on textiles could construct integrated conductive network and improve the electronic conductivity. The smaller SnS2 particles enable to expose more abundant grain boundaries and surface atoms, generating enhanced surface capacitive behaviors and effective solid phase diffusions of sodium ions. Therefore, the optimum SnS2/CNTs/textile electrode delivers a high reversible capacity of 565.5 mAh g−1 at the current density of 200 mA g−1 after 100 cycles and superior rate performance of 383.5 mAh g−1 at 1000 mA g−1 after 500 cycles. Based on above analysis, the flexible integrated electrode design is expected to provide useful instructions for advanced sodium ion batteries.

Synergistic voltage and electrolyte mediation improves sodiation kinetics in µ-Sn alloy-anodes

Alloying electrodes, such as tin (Sn), are promising candidates for sodium-ion batteries because of their high specific capacity, electronic conductivity, and low sodium insertion voltage. However, sizeable volumetric change and electrode-electrolyte interface evolution in Sn preclude prolonged performance. The electrochemical potential window, compounded by the choice of electrolyte and additive combination, plays a critical role in the interface instability, which yet remains unresolved. This study, based on a comprehensive set of electrochemical, microscopy, and spectroscopic analyses, sheds light into the interface instability and reveals that the use of fluoroethylene carbonate additives in carbonate-based electrolytes can dramatically improve the interface stability of such alloying anodes. Electrochemical and morphological analyses show that without the additive, a higher end-of-charge voltage can cause breakdown and reformation of an unstable passivating layer, leading to rapid electrochemical performance decay. A novel three-electrode-based analytics reveals that superior interphase stability with higher microstructural integrity of the Sn electrode can alleviate the detriments from the upper cut-off voltage restrictions. Addressing the hitherto unresolved role of the electrochemical potential window, this study comprehensively examines and elucidates the causality of interfacial instability and the underpinnings of electrochemical complexations in sodium-alloying anodes.

Quantifying Sodiation Kinetics in Alloying Tin Electrodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are promising next-generation energy storage devices because of the elemental abundance and low sodium cost. However, the lower storage capacity and short lifespan of SIBs necessitate the need for a fundamental understanding of the sodiation/de-sodiation kinetics complexation due to the inherent electrode materials and electrolyte interactions. This study comprehensively studied the kinetics of the sodium alloying and de-alloying mechanism in tin (Sn) electrodes, a promising anode, relying on GITT-based (galvanostatic intermittent titration technique) analytics. This study includes a limited combinatorial analysis of sodium salts, namely, NaPF6 and NaClO4, in conjunction with different carbonate solvents. This comprehensive analysis elicits a comparative paradigm of diffusivity, charge transfer resistance, intercalation rate constant, and exchange current density for the salt/solvent combinations. Overall, NaClO4 exhibits better kinetic and transport properties as compared to NaPF6. This study further elucidates the variation of ionic mobility and reaction rate with interfacial passivation due to excess fluorine donation from additives. The effect of active particle size reveals that nanoparticles exhibit reduced electrochemical (charge/discharge) hysteresis than microparticles. Overall, this study demonstrates a more considerable sensitivity of the charge transfer resistance, exchange current density, and reaction rate constants compared to the diffusivity.

Origin of anomalous high-rate Na-ion electrochemistry in layered bismuth telluride anodes

Summary van der Waals layered metal chalcogenide Bi2Te3 has shown exceptional capacity and rate capability in alkali-ion batteries but the underlying reaction mechanism with Li+, Na+, and K+ remains undiscovered. It is unexpected that Na+ electrochemistry outperforms Li+ and K+ at high current densities. Here, in situ transmission electron microscopy is used to uncover nanoscale transformations during lithiation, sodiation, and potassiation, which follows two-step conversion and alloying reactions with Li+ and Na+, and three-step intercalation-conversion-alloying reactions with K+. Counterintuitively, sodiation exhibits the highest reaction kinetics, and its origin can be elucidated by first-principles and finite-element simulations in two aspects. The lower interfacial strain accommodation energy between Bi2Te3 and its Na-conversion products allows more facile sodiation phase transformation than Li- and K-ion reactions. The higher electrochemo-mechanical stress concentration at the concave-shaped sodiation reaction front facilitates continued Na-ion diffusion and reaction propagation. These fundamental insights are essential for fast-charging alkali-ion batteries.

Confining Ultrathin 2D Superlattices in Mesoporous Hollow Spheres Renders Ultrafast and High‐Capacity Na‐Ion Storage

AbstractSodium‐ion batteries have attracted ever‐increasing attention in view of the natural abundance of sodium resources. Sluggish sodiation kinetics, nevertheless, remain a tough challenge, in terms of achieving high rate capability and high energy density. Herein, a sheet‐in‐sphere nanoconfiguration of 2D titania–carbon superlattices vertically aligned inside of mesoporous TiO2@C hollow nanospheres is constructed. In such a design, the ultrathin 2D superlattices consist of ordered alternating monolayers of titania and carbon, enabling interpenetrating pathways for rapid transport of electrons and Na+ ions as well as a 2D heterointerface for Na+ storage. Kinetics analysis discloses that the combination of 2D heterointerface and mesoporosity results an intercalation pseudocapacitive charge storage mechanism, which triggers ultrafast sodiation kinetics. In situ transmission electron microscope imaging and in situ synchrotron X‐ray diffraction techniques elucidate that the sheet‐in‐sphere architecture can maintain robust mechanical and crystallographic structural stability, resulting an extraordinary high rate capability, remarkable stable cycling with a low capacity fading ratio of 0.04% per cycle over 500 cycles at 0.2 C, and exceptionally long‐term cyclability up to 20 000 cycles at 50 C. This study offers a method for the realization of a high power density and long‐term cyclability battery by designing of a hierarchical nanoarchitecture.

Theoretical Study of the Charge Transport Mechanism in a Sodium Selenide (Na2Se) Cathode for Sodium-Selenium Batteries

Selenium (Se) is recognized as a promising cathode material for sodium-selenium (Na-Se) batteries because of its high electrical conductivity on the order of 10-3 S m-1 and facile sodiation kinetics. Although the terminal product, Na2Se, could limit the overall battery conductivity, fundamental understanding of the charge transport mechanism and the formation of final product remains largely unexplored. Based on density functional theory, we evaluate the influence of each type of defect and strain on conductivity in Na2Se by calculating the formation energy and mobility. We found that hole polaron (p+) can stably form on Se ion upon initial distortion. Our calculations also show that the negatively charged sodium vacancy (VNa -) could be the major charge transport carrier with its lowest formation energy and fast mobility. While diffusion barrier of p+ is considerably smaller than that of VNa -, the electronic transport related to p+ is suggested to be negligible due to its high formation energy. Once the vacancy-polaron (VNa - ˗ p+) complex forms, the charge transport could be hindered mainly due to its charge neutrality and the relatively large binding strength of the complex. Lastly, we will also discuss the influence of compression which could be induced by significant volume change of Na-Se alloys on the conductivity of Na2Se.