Anode Material For Na-ion Batteries Research Articles

Janus SnXY (X/Y = O, S, Se, Te, X ≠ Y) monolayers for sodium ion storage anodes: In view of first principles understanding

The exploration of two-dimensional (2D) anode materials with fascinating theoretical specific capacity and ion diffusion rate has been paid much attention in the domain of alkali ion batteries. In this present contribution, the potency of monolayer Janus SnXY (X/Y = O, S, Se, Te, X ≠ Y) to function as an anode material for Na-ion batteries (SIBs) is anticipated upon the ground of first-principles calculations. The outcomes indicate that the cohesion energies of all the SnXY are positive, signifying that they are energetically stabilized. The distribution of electrons in the SnXY structure is strongly dependent on the electronegativity of the atoms on both sides, and the higher the electronegativity, the more electrons are gathered. Meanwhile, the larger (smaller) charge transfer between Sn atoms and X/Y atoms enhances (weakens) the bond strength and leads to a decrease (increase) in the interlayer spacing. In service, Na atoms tend to be steadily adsorbed at the top of O, Sn, Se and Te, and a large number of electrons are transferred from Na atoms to SnXY. SnXY exhibits a metallic state after embedding, increasing its own electrical conductivity. Particularly, the SnSSe has a low diffusion barrier of 0.12 eV and a high Na storage capacity of 1380 mAh/g. These insights demonstrate that monolayer Janus SnXY is a prospective anode material for SIBs possessing high storage capacity and rapid charge/discharge rates.

Exploration of the two-dimensional transition metal phosphide MoP2 as anode for Na/K ion batteries

Transition metal phosphides are regarded to be potential anode materials for alkali metal ion batteries with abundant availability of the constituent elements. However, the volume changes and resulting structure deterioration during the charge-discharge process are challenges. Using evolutionary search combined with ab initio calculations, we discover a dynamically, thermally, and mechanically stable MoP2 monolayer, which turns out to be an excellent anode material for Na-ion batteries providing a high specific capacity of 339 mA h g−1, low diffusion barrier of 0.12 eV, and low open-circuit voltage of 0.48 V. The volume expansion (125%) is found to be decisively smaller than in the case of black phosphorus (443%), for example.

Tuning the porosity of hard carbons elaborated from sucrose

Hard carbons are among the most promising anode materials for Na-ion batteries (NIB) and are extensively studied. Nevertheless, it is still necessary to decipher the electrochemical intercalation behavior of sodium which is still under debate in the literature. Here, we present an innovating approach for elaborating hard carbon from simple sucrose precursor. It consists in a low-temperature pyrolysis at 500 °C under controlled argon pressure, followed by low-temperature carbonization. Using complementary characterization techniques (XRD, Raman, TEM, gas adsorption, SAXS) and galvanostatic electrochemical measurements, we show that this elaboration route allows the possibility to control and modify the porosity of the hard carbons without any modification of the structure of the material. This work leads us to assign the low potential plateau to the development of quasi-metal clustering of sodium into narrow porosity developed tuned thanks to the pyrolysis under pressure.

Theoretical prediction of two-dimensional CrSi2N4 as a potential anode material for Na-ion batteries

Efficient anode materials are critical for high-performance rechargeable lithium-ion batteries (LIBs) and sodium-ion batteries. This paper systematically investigates the potential of the CrSi2N4 monolayer as anode material for LIBs and sodium-ion batteries by first-principles density functional theory calculations. It was found that CrSi2N4 exhibits outstanding performance in sodium-ion batteries, with a low diffusion energy barrier of 0.10 eV and a high theoretical specific capacity of 490 mAh g−1. Meanwhile, the average open circuit voltage is 0.47 V, comparable to the typical anode materials. In addition, a small lattice constant change of 0.3%–3.1% ensures the cycling stability of CrSi2N4 in sodium-ion batteries. This work suggests a promising candidate anode material for sodium-ion batteries.

Unraveling the (De)sodiation Mechanisms of BiFeO3 at a High Rate with Operando XRD.

Development of new anode materials for Na-ion batteries strongly depends on a detailed understanding of their cycling mechanism. Due to instrumental limitations, the majority of mechanistic studies focus on operando materials' characterization at low cycling rates. In this work, we evaluate and compare the (de)sodiation mechanisms of BiFeO3 in Na-ion batteries at different current densities using operando X-ray diffraction (XRD) and ex situ X-ray absorption spectroscopy (XAS). BiFeO3 is a conversion-alloying anode material with a high initial sodiation capacity of ∼600 mAh g-1, when cycled at 0.1 A g-1. It does not change its performance or cycling mechanism, except for minor losses in capacity, when the current density is increased to 1 A g-1. In addition, operando XRD characterization carried out over multiple cycles shows that the Bi ⇋ NaBi (de)alloying reaction and the oxidation of Bi at the interface with the Na-Fe-O matrix are detrimental for cycling stability. The isolated NaBi ⇋ Na3Bi reaction is less damaging to the cycling stability of the material.

Coral-like CoSe2@N-doped carbon with a high initial coulombic efficiency as advanced anode materials for Na-ion batteries.

Na-ion batteries (NIBs) have attracted great interest as a possible technology for grid-scale energy storage for the past few years owing to the wide distribution, low cost and environmental friendliness of sodium resources and similar chemical mechanisms to those of established Li-ion batteries (LIBs). Nonetheless, the implementation of NIBs is seriously hindered because of their low rate capability and cycling stability. This is mainly because the large ionic size of Na+ can reduce the structural stability and cause sluggish reaction kinetics of electrode materials. Herein, three-dimensional nanoarchitectured coral-like CoSe2@N-doped carbon (CL-CoSe2@NC) was synthesized through solvothermal and selenizing techniques. As a result, CL-CoSe2@NC for NIBs at 2 A g-1 exhibits an ultrahigh specific capacity of 345.4 mA h g-1 after 2800 cycles and a superhigh initial coulombic efficiency (ICE) of 93.1%. Ex situ XRD, HRTEM, SAED and XPS were executed to study the crystal structure evolution between Na and CoSe2 during sodiation/de-sodiation processes. The aforementioned results indicate that the improved sodium storage property of CL-CoSe2@NC could be attributed to better electrode kinetics and a stable SEI film because of the 3D nanoarchitecture and the existence of the NC layer.

First principles study of a triazine-based covalent organic framework as a high-capacity anode material for Na/K-ion batteries.

The rational design of high-performance anode materials is crucial for the development of rechargeable Na-ion batteries (NIBs) and K-ion batteries (KIBs). In this study, based on density functional theory (DFT) calculations, we have systematically investigated the possibility of a bilayer triazine-based covalent organic framework (bilayer TCOF) as an anode for NIBs and KIBs. The calculation of the electronic band structure shows that the bilayer TCOF is a direct band gap semiconductor with a band gap of 2.01 eV. After the adsorption of Na/K at the most favorable sites, the bilayer TCOF transitions from a semiconductor to a metal state, guaranteeing good electronic conductivity. The low diffusion barriers of the bilayer TCOF are 0.45 and 0.26 eV, respectively, indicating a fast diffusion rate of Na/K ions. In addition, the bilayer TCOF has a theoretical storage capacity of up to 628 mA h g-1. Finally, it is found that the average voltage of the bilayer TCOF for NIBs and KIBs is 0.53 and 0.48 V, respectively. Based on these results, we can conclude that the bilayer TCOF may be a suitable anode material for NIBs and KIBs.

High-Rate Performance of Antimony Chalcogenides (Sb2X3) Studied By Operando X-Ray Diffraction (XRD)

Na-ion batteries (NIBs) represent an environmentally-friendly and sustainable alternative to Li-ion batteries (LIBs), owing to the higher abundance of Na compared to Li. [1] However, many challenges associated with the successful implementation of NIBs are yet to be addressed, which includes the optimization of the present set of materials. Among others, the development of high-performing anode materials as a replacement for conventionally used hard carbon is essential to deliver NIBs with high energy density. A class of materials promising for implementation in NIBs is based on the combined conversion/alloying reaction pathway as a main motive of their operating mechanism. [2] However, such conversion/alloying materials (CAMs) typically undergo a set of complex structural and chemical transformations during electrochemical cycling (formation of new phases, amorphization, etc). [3] Investigation of the (de)sodiation mechanism using advanced characterization techniques, such as operando X-ray diffraction (XRD), is therefore the key to revealing the true potential of these battery mechanisms.Among many CAMs, Sb-based chalcogenides (Sb2X3, where X = O, S, Se, Te) have demonstrated superior electrochemical performance in NIBs. In the present work, the research efforts have been focused on elucidating the (de)sodiation mechanism of Sb2Se3/Sb (Sb-rich Sb2Se3) materials using operando XRD. The study reveals the true complexity of the (de)sodiation mechanism for Sb2Se3/Sb (Figure 1), where the active material passes through a combination of multiple phases (Na2Se, Na2Se4, Sb, NaSb, hexagonal Na3Sb, cubic Na3Sb) during sodiation, where some of these phases are amorphous.One of the discovered benefits of such chalcogenide materials is the ability to operate at high cycling rates. Therefore, another objective of this study is to understand if and how the (de)sodiation mechanism change as a function of the C-rate. The preliminary operando XRD analysis suggests that kinetics plays a crucial role in the formation of certain phases during cycling leading to a rate-dependent cycling mechanism. Thus, a systematic study utilizing specific currents of 100 mA g-1, 1000 mA g-1, and 5000 mA g-1 have been applied to the Sb2Se3/Sb system to deepen the understanding of the (de)sodiation mechanism.This presentation will underpin the fundamentals of the cycling mechanism of Sb2Se3/Sb using operando XRD as a function of cycling rates. A better knowledge of the various mechanisms by which sodium can move in and out of this material is crucial for our understanding and potentially mitigate the limited cycling stability that limits their practical usefulness.References. Hasa, I., et al., Challenges of today for Na-based batteries of the future: From materials to cell metrics. Journal of Power Sources, 2021. 482: p. 228872. Skurtveit, A., et al., Benefits and Development Challenges for Conversion-Alloying Anode Materials in Na-Ion Batteries. Frontiers in Energy Research, 2022. 10. Brennhagen, A., et al., Understanding the (De)Sodiation Mechanisms in Na‐Based Batteries through Operando X‐Ray Methods. Batteries & Supercaps, 2021. 4(7): p. 1039-1063. Figure 1

Revealing the Cycling and Degradation Mechanism of Bi-Metallates As Anode Materials for Na-Ion Batteries

Na-ion batteries (NIBs) represent an emerging alternative to modern Li-ion batteries. To improve the overall performance of NIBs it is crucial to develop new types of anode materials with high capacity and long cycle life. Among the anode candidates, materials operating under combined conversion and alloying mechanisms (CAMs) are considered as promising. They are capable of delivering high capacity, however, despite theoretical predictions the cycling stability is still below expectations. Therefore, a comprehensive understanding of the cycling and degradation mechanisms of CAMs is crucial to improve their performance. However, these materials undergo multiple chemical transformations, which often involves formation of amorphous and nanocrystalline phases during cycling. Such chemical complexity substantially impedes their full characterization and understanding.Bismuth metallates, with a general formula of Bi-TM-O (TM = transition metal), is a group of ternary CAMs. Their general cycling mechanism consists of an irreversible conversion reaction, which leads to formation of Bi nanoparticles embedded in a Na-TM-O matrix during the first sodiation. This is followed by a reversible two-step alloying reaction leading to formation of Na3Bi with NaBi as an intermediate phase. In this work, we have used operando X-ray diffraction over the course of ∼30 cycles to study the cycling and degradation mechanisms of two CAMs - Bi2MoO6 and BiFeO3. With the help of ex situ X-ray absorption spectroscopy and pair distribution function (PDF) analysis, we have revealed new insights into these materials in addition to confirming the general cycling mechanism.Both materials form the cubic phase of Na3Bi (c-Na3Bi) in the sodiated state with some noticeable differences. While, the hexagonal Na3Bi (h-Na3Bi) phase starts to form after 10 cycles in Bi2MoO6, a combination of c-Na3Bi and h-Na3Bi can be observed after the first cycle in BiFeO3. The formation of c-Na3Bi is kinetically favorable in these systems, while h-Na3Bi is the more thermodynamically stable and, therefore, forms later in the cycling process. Both materials shows three distinct regions of capacity degradation, where the first is initiated by a growth in the crystallite sizes of Bi-particles. This is followed by a disappearance of the Bi-phase as the second step. The last degradation phase is related to the disappearance of the NaBi-phase and locking the system in the sodiated state. The present talk will discuss the mechanisms of these degradation steps in view of the future development of CAMs in greater details. Figure 1

Spatial Effect on the Performance of Carboxylate Anode Materials in Na-Ion Batteries.

Developing low-voltage carboxylate anode materials is critical for achieving low-cost, high-performance, and sustainable Na-ion batteries (NIBs). However, the structure design rationale and structure-performance correlation for organic carboxylates in NIBs remains elusive. Herein, the spatial effect on the performance of carboxylate anode materials is studied by introducing heteroatoms in the conjugation structure and manipulating the positions of carboxylate groups in the aromatic rings. Planar and twisted organic carboxylates are designed and synthesized to gain insight into the impact of geometric structures to the electrochemical performance of carboxylate anodes in NIBs. Among the carboxylates, disodium 2,2'-bipyridine-5,5'-dicarboxylate (2255-Na) with a planar structure outperforms the others in terms of highest specific capacity (210 mAh g-1), longest cycle life (2000 cycles), and best rate capability (up to 5 A g-1). The cyclic stability and redox mechanism of 2255-Na in NIBs are exploited by various characterization techniques. Moreover, high-temperature (up to 100°C) and all-organic batteries based on a 2255-Na anode, a polyaniline (PANI) cathode, and an ether-based electrolyte are achieved and exhibited exceptional electrochemical performance. Therefore, this work demonstrates that designing organic carboxylates with extended planar conjugation structures is an effective strategy to achieve high-performance and sustainable NIBs.

Effect of the Calcination Duration on the Electrochemical Properties of Na2Ti3O7 as Anode Material for Na-Ion Batteries

The growing interest in Na-ion batteries as a “beyond lithium” technologies for energy storage drives the research for high-performance and environment-friendly materials. Na2Ti3O7 (NTO) as an eco-friendly, low-cost anode material shows a very low working potential of 0.3 V vs. Na+/Na but suffers from poor cycling stability, which properties can be significantly influenced by materials synthesis and treatment. Thus, in this work, the influence of the calcination time on the electrochemical performance and the reaction mechanism during cycling were investigated. NTO heat-treated for 48 h at 800 °C (NTO-48h) demonstrated enhanced cycling performance in comparison to NTO heat-treated for only 8 h (NTO-8h). The pristine material was thoroughly characterized by X-ray diffraction, laser granulometry, X-ray photoelectron spectroscopy, and specific surface area measurements. The reaction mechanisms induced by sodiation/desodiation and cycling were investigated by operando XRD. Electrochemical impedance spectroscopy was used to evidence the evolution of the solid electrolyte interface layer (SEI) and modification of charge transfer resistances as well as the influence of cycling on capacity decay. The evolution of the crystallographic structure of NTO-48h revealed a more ordered structure and lower surface contamination compared to NTO-8h. Moreover, the residual Na4Ti3O7 phase detected after the sodium extraction step in NTO-8h seems correlated to the lower electrochemical performance of NTO-8h compared to NTO-48h.

Atomistic insights into the enhanced Na storage performance of CuP2 anchored on single vacancy graphene

CuP2 is considered as a promising anode material for sodium-ion batteries due to its considerable theoretical capacity (1280 mAh/g), but it suffers from rapid decline of capacity and its cyclic reversibility should be further improved. As far as possible, preventing reaction energy barrier from sudden change and holding back Cu diffusion at atomic level both are key strategies to solve these problems occurring on the electrochemical reaction between CuP2 and Na. Based on the surface anchoring effect and excellent electronic conductivity of single vacancy graphene (SVG), first principles calculations have been carried out to capture the key role of SVG in regulating the electrochemical properties of CuP2. Firstly, among various (CuP2)n (n = 1–6) clusters, haploid CuP2 cluster possesses the fairly stable structure due to its highest HOMO-LUMO gap (2.45 eV) and lowest vertical electronic affinity energy (0.64 eV). Most of all, CuP2/SVG composites exhibit much more stable reaction energy barrier than CuP2 during their Na storage. It is attributed to the strong C–P and Cu–C anchoring effect between CuP2 and SVG, slowing the break of Cu–P bond. This contributes to holding back volume mutations of CuP2/SVG during Na storage. Besides, after conversion reaction between CuP2/SVG and Na, Cu showsdiffusion energy barrier of 0.75 eV, exhibiting great difficulty for its migration in reaction products. Compared with CuP2, CuP2/SVG not only exhibits more stable voltage distribution, but also it has much higher theoretical capacity (1851 mAh/g vs.1280 mAh/g). That is to say. CuP2/SVG can be acted as anode materials for Na-ion batteries with high energy density. And our results also provide a new way to design long-life CuP2-based anode materials for Na-ion batteries.

Na-storage reactions of rutile TiNbO4

As a novel anode material for Na-ion battery, we evaluated rutile-type TiNbO4 synthesized by a sol−gel method. It was revealed for the first time that TiNbO4 showed reversible reactions of Na+-insertion and Na+-extraction in the potential ranges of 0.005–1.0 and 0.6–1.6 V vs. Na+/Na, respectively. TiNbO4 maintained its rutile structure even after charge−discharge cycling. The size of Na+ diffusion path along its c-axis was broadened with increasing Nb amount in TiNbO4, leading to the improvement in the reversible capacity. The TiNbO4 electrode with Nb 48 at.% exhibited a high capacity of 360 mA h g−1. The long-term cyclability and rate capability were also improved by increasing Nb amount. We consider that the wider space in the diffusion path increases the degree of freedom of Na+ occupancy position, which weakens the electrostatic repulsion between Na+ and Na+ to increase Na storage capacity. These results clearly demonstrate that TiNbO4 can be applied as a promising anode material.

Nitrogen doped soap-nut seeds derived hard carbon as an efficient anode material for Na-ion batteries

Biomass-derived hard carbon materials have received promising research interest because of their low cost, high abundance, and renewability. In the present study, high-performance hard carbon material is prepared from soap-nut seeds (SNS-0) using a simple pyrolysis process. Further, N (nitrogen) heteroatoms are introduced into the hard carbon matrix to modify its structure using melamine as the N source. High-resolution transmission electron microscopy images reveal the development of the localized graphite-like structure after the introduction of N atoms in the hard carbon, which are found to be beneficial for the storage and transport of the larger Na ions in the battery. The optimal doping of N (1:1 wt. ratio, SNS-1) in the hard carbon has improved the storage performance of the SNS-0 electrode, providing a reversible capacity of 353 mA h g−1 at a current density of 25 mA g−1. The improved electrochemical characteristics of the SNS-1 electrode over the other compositions are associated with its lesser specific surface area (282.87 m2 g−1) and higher interlayer spacing (0.38 nm). The present study not only reveals the useful conversion of a biomass waste, soap-nut seeds, into a low-cost and sustainable eco-friendly hard carbon material but also its potential use as an efficient anode for Na-ion battery applications.

Monolayer and bilayer AsC5 as promising anode materials for Na-ion batteries

Since the abundance of sodium resources is much higher than lithium, the global distribution is uniform, and the price is low and stable, making Na-ion batteries (NIBs) have broad application prospects in the field of next-generation renewable energy storage. However, a significant barrier to the realization of high-performance NIBs is the absence of adequate anode materials. In this work, we perform unbiased first-principles structure-search simulations to find a novel AsC5 monolayer with a variety of functionally advantageous characteristics. Based on theoretical simulations, the proposed AsC5 has been found to be energetically, mechanically, dynamically, and thermally stable, supporting the viability of experiment. Stable adsorption of Na on monolayer (bilayer) AsC5 with an adsorption energy from −2.42eV to −0.33eV is shown by our computational simulations. The metallic behavior of monolayer (bilayer) AsC5 is maintained when Na atoms are inserted, ensuring excellent electric transportation. Furthermore, the Na diffusion barrier on monolayer (bilayer) AsC5 is just 0.16 (0.09) eV, which indicates a rapid charging/discharging capacity. The monolayer (bilayer) AsC5 materials have higher specific capacity than some known 2D materials, reaching a maximum of 794.28 mA h/g (596.01 mA h/g). Small volume fluctuations (<1.2%) after AsC5 sodiation show strong cycle reversibility. Therefore, such crucial characteristics make AsC5 be a promising candidate for anode materials of NIBs.

Facile and Scalable Development of High-Performance Carbon-Free Tin-Based Anodes for Sodium-Ion Batteries.

Tin (Sn)-based anodes for sodium (Na)-ion batteries possess higher Na-storage capacity and better safety aspects compared to hard carbon -based anodes but suffer from poor cyclic stability due to volume expansion/contraction and concomitant loss in mechanical integrity during sodiation/desodiation. To address this, the usage of nanoscaled electrode-active particles and nanoscaled-carbon-based buffers has been explored, but with compromises with the tap density, accrued irreversible surface reactions, overall capacity (for "inactive" carbon), and adoption of non-scalable/complex preparation routes. Against this backdrop, anode-active "layered" bismuth (Bi) has been incorporated with Sn via a facile-cum-scalable mechanical-milling approach, leading to individual electrode-active particles being composed of well-dispersed Sn and Bi phases. The optimized carbon-free Sn-Bi compositions, benefiting from the combined effects of "buffering" action and faster Na transport of Bi, to go with the greater Na-storage capacity and lower operating potential of Sn, exhibit excellent cyclic stability (viz., ∼83-92% capacity retention after 200 cycles at 1C) and rate capability (viz., no capacity drop from C/5 to 2C, with only ∼25% drop at 5C), despite having fairly coarse particles (∼5-10 μm). As proven by operando synchrotron X-ray diffraction and stress measurements, the sequential sodiation/desodiation of the components and, concomitantly, stress build-ups at different potentials provide "buffering" action even for such "active-active" Sn-Bi compositions. Furthermore, the overall stress development upon sodiation of Bi has been found to be significantly lower than that of Sn (by a factor of ∼3.8), which renders Bi promising as a "buffer" material, in general. Dissemination of such complex interplay between electrode-active components during electrochemical cycling also paves the way for the development of high-performance, safe, and scalable "alloying-reaction"-based anode materials for Na-ion batteries and beyond, sans the need for ultrafine/nanoscaled electrode particles or "inactive" nanoscaled-carbon-based "buffer" materials.

NaNbV(PO4)3: Multielectron NASICON-Type Anode Material for Na-Ion Batteries with Excellent Rate Capability.

NASICON-type NaNbV(PO4)3 electrode material synthesized by the Pechini sol-gel technique undergoes a reversible three-electron reaction in a Na-ion cell which corresponds to the Nb5+/Nb4+, Nb4+/Nb3+, and V3+/V2+ redox processes and provides a reversible capacity of 180 mAh·g-1. The sodium insertion/extraction takes place in a narrow potential range at an average potential of 1.55 V versus Na+/Na. Structural characterization by operando and ex situ X-ray diffraction disclosed the reversible evolution of the NaNbV(PO4)3 polyhedron framework during cycling, while XANES measurements in the operando regime confirmed the multielectron transfer upon sodium intercalation/extraction into NaNbV(PO4)3. This electrode material demonstrates extended cycling stability and excellent rate capability maintaining the capacity value of 144 mAh·g-1 at 10 C current rates. It can be regarded as a superior anode material suitable for application in high-power and long-life sodium-ion batteries.

Tin-functionalized silicon oxycarbide as a stable, high-capacity anode material for Na-ion batteries

In this study tin-functionalized silicon oxycarbide, Sn/SiOC, composite anode materials are synthesized using a carbon-rich polysiloxane as the preceramic polymer and nano-sized SnO2, which converts to metallic tin via carbothermal reduction at approximately 700 °C. The in-situ Sn formation leads to a uniform distribution of tin particles within a carbon-rich SiOC matrix. Raman spectra show no significant changes despite the carbothermal reduction of SnO2. The composite material provides a stable reversible capacity of 234 mAh g-1. By adjusting the composition and pyrolysis temperature a reversible capacity of 131 mAh g-1 at a high current rate of 2380 mA g-1 is achieved.

Facile Synthesis of Nickel Phosphide @ N-Doped Carbon Nanorods with Exceptional Cycling Stability as Li-Ion and Na-Ion Battery Anode Material

Nickel phosphide (Ni2P), as an anode material for both lithium- and sodium-ion batteries, offers high theoretical specific and volumetric capacities. However, considerable challenges include its limited rate capability and low cycle stability arising from its volume change and degradation during cycling. To solve these issues, appropriate composite micro/nanoparticle designs can improve conductivity and provide confinement. Herein, we report a simple pyrolysis method to synthesize nitrogen-doped carbon-coated Ni2P nanorod arrays (Ni2P@NC) from nickel foam and an ionic resin as a source of carbon, nitrogen and phosphorus. The N-doped open-ended carbon shells provide Ni2P containment, good electrical conductivity, efficient electrolyte access and the buffering of bulk strain during cycling. Consequently, as a LIB anode material, Ni2P@NC has impressive specific capacity in long-term cycling (630 mAh g−1 for 150 cycles at 0.1 A g−1) and a high rate capability of 170 mAh g−1 for 6000 cycles at 5 A g−1. Similarly, as a SIB anode, Ni2P@NC retains a sizable 288 mAh g−1 over 300 cycles at 0.1 A g−1, and 150 mAh g−1 over 2000 cycles at 2 A g−1. Furthermore, due to a sizable portion of its capacity coinciding with adequately low voltage, the material shows promise for high volumetric energy storage in full-cell format. Lastly, the simple synthesis method has the potential to produce other carbon-coated metal phosphides for electrochemical applications.

SnCo Nanoparticles Loaded in Hollow Carbon Spheres Interlinked by N-Doped Carbon Fibers for High-Performance Sodium-Ion Batteries.

The development of Sn-based materials with electrochemically inactive matrices is a novel strategy to alleviate the volume expansion and giant structure strain/stress during the sodiation/desodiation process. In this work, a freestanding membrane based on the unique bean pod-like host composed by nitrogen-doped carbon fibers and hollow carbon spheres (HCSs) encapsulated with SnCo nanoparticles is synthesized by electrospinning (B-SnCo/NCFs). In this unique bean pod-like structure, Sn acts as a host for Na+ storage, while the Co plays the important role of an electrochemically inactive matrix that can not only buffer the volume variations but also inhibit aggregation and particle growth of the Sn phase during the electrochemical Na-Sn alloying process. Meanwhile, the introduction of hollow carbon spheres can not only provide enough sufficient void space to withstand the volume expansion during the (de)sodiation processes but also improve the conductivity of the anode along the carbon fibers. Furthermore, the B-SnCo/NCF freestanding membrane can increase the contact area between the active material and the electrolyte, which can provide more active sites during the cycling process. When used as an anode material for Na-ion batteries, the freestanding B-SnCo/NCF anode exhibits an outstanding rate capacity of 243.5 mA h g-1 at 1.6 A g-1and an excellent specific capacity of 351 mA h g-1 at 0.1 A g-1 for 300 cycles.