Sodium Storage Behavior Research Articles

Sodium storage and capacity retention behavior derived from high-spin/low-spin Fe redox reaction in monoclinic Prussian blue based on operando Mössbauer characterization

Owing to the high theoretical specific capacity, long cycle life, abundant resources and environmental benignity, iron-based Prussian blue analogues (PBAs) as cathode materials for sodium-ion batteries (SIBs) have been investigated widely in recent years. Although major efforts have been concentrated on exploitation of high performance’s PBAs cathode materials, there is still deficiency of a deep understanding of the relevance between the reaction processes and capacity degradation mechanism with the active high-spin (HS)/ low-spin (LS) iron sites, which is of great significance. In this work, well-crystallized and shape-controlled monoclinic Prussian blue (M-PB) was successfully prepared, with remarkable electrochemical performance, especially in cycling performance (88% capacity retention after 500 cycles at 120 mAh g−1, and 81% after 980 cycles). More significantly, with state-of-the-art operando Mössbauer spectroscopy, ex-situ 77 K Mössbauer spectroscopy, ex-situ X-ray absorption spectroscopy and operando X-ray diffraction, the reaction and capacity degradation mechanism were investigated thoroughly and the detailed reaction process was figured out for the first time. The result showed clearly that the HS Fe in M-PB reacts completely and contributes to most capacity, while only part of LS Fe reacts. Therefore, the capacity enhancement should be achieved by activating LS Fe. Furthermore, the LS Fe (Fe-CN) results in more severe crystal structure change than HS Fe (Fe-NC) of the same amount in the electrochemical reaction process, thus the partial reaction of LS Fe in M-PB could be the reason for its excellent cycling performance. This work not only investigated the reaction and capacity degradation mechanism, but also shed light on the design of high-performance PBAs-based cathodes for SIBs.

Embedding antimony nanoparticles into metal–organic framework derived TiO2@carbon nanotablets for high-performance sodium storage

Titanium dioxide (TiO2) has been widely investigated as a candidate for anode materials of sodium-ion batteries (SIBs) due to its low cost and high abundance. However, the intrinsic sluggish ion/electron transfer rate hinders its practical applications for high energy density storage devices. In contrast, antimony (Sb) shows high specific theoretical capacity (660 mAh/g) as well as excellent electron conductivity, but the large volume variation upon cycling usually leads to severe capacity fading. Herein, with the objective of achieving high-performance sodium storage anode materials, TiO2@C-Sb nanotablets with a small amount of Sb content (6.4 wt%) are developed through calcination Ti-metal–organic framework (MIL-125) derived TiO2@C/SbCl3 mixture under reductive atmosphere. Benefitting from the synergetic effect of well-dispersed Sb nanoparticles as well as robust porous TiO2@C substrate, the TiO2@C-Sb shows enhanced electron/ion transfer rate and predominantly pseudocapacitive sodium storage behavior, delivering a reversible capacity of 219 mAh/g at 0.5 A/g even after 1000 cycles. More significantly, this method may be commonly used to incorporate other alloy-based high-theoretical materials into MIL-125-derived TiO2@C, which is promising for developing high-energy-density TiO2-based energy storage devices.

Noncrystalline Carbon Anodes for Advanced Sodium-Ion Storage.

Developing an anode with excellent rate performance, long-cycle stability, high coulombic efficiency, and high specific capacity is one of the key research directions of sodium-ion batteries. Among all the anode materials, noncrystalline carbon (NCC) has great possibilities according to its supreme performance and low cost, but with the complexity and variability of the structure. With the in-depth study of the sodium storage behaviors of NCC in recent years, three modes of interlayer intercalation, clustering into micropores, and adsorption are reported and summarized. Although the storage mechanism has gradually become more evident, the complex behavior of the ions at different voltage regions, especially in the low-voltage (plateau) region, still remains controversial. It is essential to understand further the relationship between ions and NCC structure during energy storage processes. Based on the summary of previous works, this article has reviewed the storage mechanism of sodium ions in NCC and evaluated the structure-behavior relationship between sodium-ion storage and the carbon structure.

Sodium storage behavior and long cycle stability of boron-doped carbon nanofibers for sodium-ion battery anodes

A double heteroatom doping strategy is proposed to synthesize boron- and nitrogen-doped heteroatom carbon nanofibers (BNC NFs) as anode materials for sodium-ion batteries (SIBs). The specific capacity and rate performance of the BNC NF anode are higher than those of the NC NF anode. Particularly, the composite containing 5 wt% boric acid provides the highest reversible capacity of 249 mAh g−1 at a current density of 0.02 A g−1 and excellent cyclic stability of 144 mAh g−1 at 2 A g−1 after 3000 cycles. The excellent cyclic performance of the BNC NFs can be attributed to the defect-rich nanostructure derived by optimally doping boron under annealing, which is conducive to accelerating ion transport and introducing additional Na-ion storage active sites. The excellent capacity and long cycle stability of the full cell SIBs comprising the optimized BNC NFs anode and Na3V2(PO4)3 cathode suggest the promising potential of B-doped C NFs as anodes for rechargeable Na-ion batteries.

Performance investigation of Ti x N x−1O2 MXene (x = 2, 3, 4) as anode materials for Na-ion batteries by first-principles calculation

The number of MXene layers plays a crucial role in their performance when they are used as anode materials for sodium-ion batteries. Herein, Ti-based nitride MXenes with different layers, Ti x N x−1O2 MXene (x = 2, 3, 4) structures, were constructed to calculate the structural stability of their precursor, electronic properties after etching, and sodium storage behavior compared with the common Ti2CO2 and Ti3C2O2 MXene. First-principles calculations indicate that nitride MXenes possess a better rate capability than carbide MXenes of the same thickness. Moreover, the barrier for Na diffusion on the Ti2NO2 MXene surface (0.114 eV) is lowest. Meanwhile, comparing the properties of three nitride MXenes with different thicknesses, Ti2NO2 MXene performs relatively well with a high theoretical capacity with 756 mAh g−1 and a lower open circuit voltage of 1.1 V. In conclusion, the performance improvement of nitride MXene is not linear with thickness, because that of Ti3N2O2 MXene is relatively weaker. This work lays the foundation for the feasibility of Ti3N2T x experimental preparation and provides corresponding evidence on the choice of MXene thickness. More attention should be paid to the etching method for Ti2NT x MXene.

Towards an atomistic understanding of hard carbon electrode materials and sodium behaviors

Hard carbons are the primary candidate for the anode of next-generation sodium-ion batteries for large-scale energy storage, but the atomic scale cognition of materials and sodium storage behavior is still unclear. Combined annealing-MD simulations and Arrhenius framework, a series of large-scale molecular models for hard carbon anode were constructed to reproduce the main experimental characteristics at different annealing temperatures. From the simulations, we obtained the representative structural features and the detailed transformation information of each annealing stage at the atomic scale. Based on these reliable models, we used ReaxFF-MD to examine sodium behavior in hard carbon anodes with varying nanostructures. It was found that hard carbon will yield the highest ion diffusion rates of 1.99 × 10−10 m2/s and the minimum volume swelling of 7.4 % at low heat treatment temperature (1500 K). Finally, several optimization design strategies were proposed. These findings provide atomic insight into the future design and development of hard carbons for SIBs.

Molten Salts Etching Route Driven Universal Construction of MXene/Transition Metal Sulfides Heterostructures with Interfacial Electronic Coupling for Superior Sodium Storage

AbstractThe MXene‐based heterostructures have recently attracted great interest as anode materials for sodium‐ion batteries (SIBs). Nonetheless, the complicated and harsh preparation process impedes their further commercialization. Herein, a novel, safe, low‐destructive, and universal strategy for rationally fabricating Ti3C2Tx MXene/transition metal sulfides (MSy) heterostructures is presented via Lewis acidic molten salts etching and subsequent in situ sulfurization treatment. Benefiting from the interfacial electronic coupling between highly conductive Ti3C2Tx MXene (Tx = O and Cl) and MSy (M = Fe, Co and Ni), the heterostructures possess remarkably improved electronic conductivity, promoted Na+ migration kinetics, and robust architectures. As a proof‐of‐concept demonstration, the Ti3C2Tx/FeS2 heterostructure demonstrates outstanding rate performance (456.6 mAh g−1 at 10 A g−1) and long‐term cyclic stability (474.9 mAh g−1 after 600 cycles at 5 A g−1) when serving as SIB anodes. Impressively, a sodium‐ion full battery with Ti3C2Tx/FeS2 anode delivers an excellent reversible capacity of 431.6 mAh g−1 after 1000 cycles at 3 A g−1. Moreover, the dual sodium storage behavior of Ti3C2Tx/FeS2 heterostructure and underlying mechanism toward exceptional electrochemical performance are revealed by comprehensive characterizations and theoretical calculations. Based on the full utilization of molten salt etching products, the present work offers new insight into the fabrication of MXene‐based heterostructures.

Insight into the Structural Variation and Sodium Storage Behavior of Polyoxometalates Encapsulated within Single-Walled Carbon Nanotubes.

The host-guest interaction can remarkably alter the physiochemical properties of composite materials. It is crucial to clarify the mechanism by revealing the influence of the host on the electronic structure of the guest molecules. Herein, we study the structural variation of polyoxometalates (POMs) after being confined in single-walled carbon nanotubes (SWNT). What we found is that in addition to the reported charge transfer from SWNT to POM, an intramolecular electron transfer within a single POM cluster can be observed in the POM@SWNT composites. Moreover, the charge density on the bridged oxygen of POMs is prominently enhanced. The structural change and electron reconfiguration of POMs upon encapsulation in SWNT significantly speed up electron and ion transport, leading to the improved electrochemical performance for sodium ions storage.

Engineering carbon nanosheets with hexagonal ordered conical macropores as high-performance sodium-ion battery anodes

As one of the most promising candidates for sodium-ion battery anodes, hard carbons suffer from inferior rate performance owing to limited ion transfer rate and sluggish electrochemical kinetics. In this work, novel carbon nanosheets (CNS) with hexagonal ordered conical macropores are prepared. The CNS has a very thin thickness of approximately 370 nm, and the conical pores are penetrated through the whole nanosheet, forming well-connected ion transport freeway. In addition, the carbon microcrystal structure and interlayer spacing can be well tailored by adjusting the carbonization temperature, thereby controlling the sodium storage behavior of carbon electrodes. These structural merits endow CNS with accelerated ion transfer, minimized ion diffusion distance and fast electrochemical kinetics. Consequently, the CNS presents superior electrochemical performance. It delivers a high reversible capacity of 298 mAh g−1 at 0.1 A g−1; and after repeated charge/discharge for 500 times at 1 A g−1, its capacity remains 195 mA h g−1, with no rapid capacity loss. More importantly, CNS exhibits outstanding rate capability. Even under a very high current density of 2 A g−1, it still displays a large capacity of 210 mAh g−1, higher than most of state-of-the-art carbon anodes.

Engineering homotype heterojunctions in hard carbon to induce stable solid electrolyte interfaces for sodium‐ion batteries

AbstractDeveloping effective strategies to improve the initial Coulombic efficiency (ICE) and cycling stability of hard carbon (HC) anodes for sodium‐ion batteries is the key to promoting the commercial application of HC. In this paper, homotype heterojunctions are designed on HC to induce the generation of stable solid electrolyte interfaces, which can effectively increase the ICE of HC from 64.7% to 81.1%. The results show that using a simple surface engineering strategy to construct a homotypic amorphous Al2O3 layer on the HC could shield the active sites, and further inhibit electrolyte decomposition and side effects occurrence. Particularly, due to the suppression of continuous decomposition of NaPF6 in ester‐based electrolytes, the accumulation of NaF could be reduced, leading to the formation of thinner and denser solid electrolyte interface films and a decrease in the interface resistance. The HC anode can not only improve the ICE but elevate its sodium storage performance based on this homotype heterojunction composed of HC and Al2O3. The optimized HC anode exhibits an outstanding reversible capacity of 321.5 mAh g−1 at 50 mA g−1. The cycling stability is also improved effectively, and the capacity retention rate is 86.9% after 2000 cycles at 1 A g−1 while that of the untreated HC is only 52.6%. More importantly, the improved sodium storage behaviors are explained by electrochemical kinetic analysis.

Microporous Carbon Nanospheres with Fast Sodium Storage Capability Enabled by Dominant Capacitive Behavior.

Hard carbon is considered one of the most promising anode candidates for sodium ion batteries but suffers from a moderate rate performance. Here, we design microporous carbon nanospheres using a novel hybrid monomer that simultaneously involves an organic moiety and an inorganic moiety as the starting unit. The inorganic moiety forms a continuous network, which serves as a 3D scaffold and a nanometer-scale template, then supports the off-collapse of the carbon skeleton and creates a well-developed microporous structure. In addition, the graphite microcrystal structure can be tailored by adjusting the heating treatment temperatures. The electrochemical study demonstrates that the microporous carbon nanospheres show dominant capacitive sodium storage behavior, thus presenting an outstanding rate performance. Even if a very high current density of 10 A g-1 is applied, the hard carbon anode can deliver a large capacity of 127 mAh g-1 with a considerable plateau capacity of 53 mAh g-1, which has rarely been obtained in previous publications. Besides, the carbon anode has a good cycling stability, and the capacity reached 210 mAh g-1 after 1000 cycles with a current density of 1 A g-1, showing no dramatic capacity loss.

Insight into Sodium Storage Behaviors in Hard Carbon by ReaxFF Molecular Dynamics Simulation

Insight into Sodium Storage Behaviors in Hard Carbon by ReaxFF Molecular Dynamics Simulation

Replacing “Alkyl” with “Aryl” for inducing accessible channels to closed pores as plateau‐dominated sodium‐ion battery anode

AbstractHard carbons are promising anodes for sodium‐ion batteries. However, there is still considerable controversy regarding the sodium storage behaviors in hard carbons, which are mainly attributed to the varied precursors, confused pyrolysis mechanism, and different characterization methods. Herein, benefiting from the flexible molecular structure of polymers, a series of hard carbons with carefully tuned microstructures are fabricated by adjusting the ratio of aryl and alkyl groups in the epoxy resins. The results of dynamic mechanical analysis, in‐situ Fourier transform infrared spectra, and synchronous thermal gravimetric‐infrared spectrum‐gas chromatography/mass spectrometry reveal that replacing the alkyl with aryl groups in the resin can enhance the crosslink density, inhibit the degradation and rearrangement process, and further lead to a more disordered microstructure. In addition, it is suggested that accessible channels provided by sufficiently wide interlayer spacing are necessary for closed pore filling. The optimized anode delivers a high capacity of 375 mAh/g in half cell with an initial Coulombic efficiency of 80.61%, and an energy density of 252 Wh/kg is attained in full cell. Finally, a reliable relationship among precursor–pyrolysis mechanism–structure–performance is established, and the sodium storage mechanism of “adsorption–insertion–pore filling” is well proved.

An Overall Understanding of Sodium Storage Behaviors in Hard Carbons by an “Adsorption‐Intercalation/Filling” Hybrid Mechanism

AbstractHard carbon has the potential to serve as a high‐capacity anode material for sodium‐ion batteries (SIBs), however, its Na+ storage mechanism, particularly on the low potential plateau, remains controversial. To overcome this issue, two types of hard carbons with different microstructures are employed and the relationship between the microstructures and Na+ storage behaviors is evaluated. By the combination of operando X‐ray diffraction, ex situ Raman spectroscopy, NMR, and theoretical calculation, it is found that the sodium storage capacities of the hard carbons in the low potential plateau region contain the concurrent contributions from both interlayer intercalation and micropores filling, and the ratio of the two contributors greatly depends on the microstructure of hard carbon materials. Moreover, an electrochemical pointer (potential inflection point at the end of the discharge curve) is found to distinguish the dominance of interlayer intercalation and the micropores filling processes of sodium ions in the low potential plateau region. Based on this new finding, a microstructure‐dependent mechanism (“adsorption‐intercalation/filling” hybrid mechanism) is proposed to achieve an overall understanding of the sodium storage behaviors in different hard carbon materials, which may provide deep insight into the rational design of hard carbon structures as high‐performance anode materials for advanced SIBs.

Hard‐Carbon Anodes for Sodium‐Ion Batteries: Recent Status and Challenging Perspectives

Sodium‐ion batteries (SIBs) hold great potential in the application of large‐scale energy storage. With the coming commercialization of SIBs, developing advanced anode of particularly hard carbon is becoming increasingly urgent yet challenging. Hard carbon still suffers from unclear sodium storage mechanism, unsatisfactory performance, and low initial Coulombic efficiency (ICE). Herein, the current state‐of‐the‐art advances in designing hard carbon anodes for high‐performance SIBs is summarized. First, the formation process of hard carbon and typical sodium storage models of “insertion–adsorption,” “adsorption–insertion,” “adsorption–pore filling,” and “adsorption–insertion–pore filling” are introduced systematically. Then, the key strategies including morphological engineering, heteroatom doping, and graphitic structure regulation are presented to enhance the capacity of hard carbon based on the in‐depth understanding of sodium storage behaviors. Subsequently, to promote the practical application of hard carbon, more attention is paid to the methods of ICE improvement, including electrolyte optimization, defect and surface engineering, and presodiation. Whereafter, hard‐carbon‐based SIBs and their intriguing applications are briefly sketched. Finally, future directions and challenging perspectives of hard‐carbon anodes for SIBs are proposed from the viewpoints of storage mechanisms, electrode structures, and presodiation techniques.

Na3V2-xFex(PO4)2O2F: An advanced cathode material with ultra-high stability for superior sodium storage

Searching a suitable electrode material is one of the key issues of Sodium-ion batteries (SIBs) for the large-scale applications. Sodium-rich Na3V2(PO4)O2F (NVPOF) with a 3D tunnel structure is considered to be the promising cathode material for SIBs owning to the advantage of its high operating voltage, andstable structure, and good thermal stability. Nevertheless, the inferior reaction kinetics and unsatisfied sodium storage behavior resulting from its low electron conductivity hinder its further practical application. Here, Na3V2-xFex(PO4)2O2F microcuboids were synthesized via iron partially replacing V atom sites of Na3V2(PO4)2O2F by a simple hydrothermal method. Benefiting from the intrinsically improved electronic conductivity and enhanced charge transfer kinetics, Na3V1.85Fe0.15(PO4)2O2F (NVFPOF) with 15 wt% doping concentration shows an initial capacity of 137.2 mAh g−1 at 1C, which is about 31% higher than that of NVPOF. In addition, a full cell based on NVFPOF cathode and Hard carbon anode showed 82.3% capacity retention even after 100 cycles at 0.4C. Moreover, iron atoms can be used as the pillar of the host structure to buffer deformation and effectively alleviate the volume change (less than 1.05%) upon cycling, showing 86.5% capacity retention at 20C after 1000 cycles. The results provide an effective and simple way to construct advanced cathodes for SIBs.

Revealing the sodium storage behavior of biomass-derived hard carbon by using pure lignin and cellulose as model precursors

Lignin and cellulose are dominant components in biomass and hold the key for preparing hard carbons. Identifying the sodium storage behaviors of sole lignin/cellulose-derived hard carbons is significant for choosing optimal biomass precursors. Herein, milled-wood lignin and microcrystalline cellulose are used as model precursors to prepare hard carbons and the corresponding sodium storage performances are investigated to understand the contribution of each biomass component. Compared with lignin-derived carbon, cellulose-derived carbon enables a larger initial Coulombic efficiency of 87.1%, a higher reversible capacity of 343.3 mA h g−1 at 0.02 A g−1 and a good rate capability of 49.2 mA h g−1 at 1 A g−1 owing to larger La, lower ID/IG values and higher sp2C, CO contents with the benefit of enhancing the conductivity, plateau capacity, and the rapid diffusion of sodium ions. The excellent performance of cellulose-derived carbon provides guidance on the choice of biomass precursors for high-performance sodium-ion batteries.

Achieving in-situ hybridization of NaTi2(PO4)3 and N-doped carbon through a one-pot solid state reaction for high performance sodium-ion batteries

The development of green, sustainable and low-cost energy storage materials through simple and environmental friendly routes is of great significance for futural large-scale energy storage systems. Due to its unique sodium super ionic conductor (NASICON) structure, polyanion-type NaTi2(PO4)3 (NTP) is regarded as one of the promising potential cathode materials for sodium-ion batteries (SIBs). However, its practical application has been impeded by its intrinsic poor conductivity for unmodified NTP. To resolve this issue, this work hybridizes the NTP with a N-doped carbon matrix (NC) through a facile one-pot solid state reaction, taking an organophosphonic acid (ethylenediamine tetramethylene phosphonic acid, EDTMPA) as source material providing carbon, nitrogen and phosphorous element at the same time. The as-prepared NTP@NC composite not only possesses improved electrical conductivity, but also mitigates particle agglomeration during prolonged charge-discharge cycles. As a result, the NTP@NC composite exhibits an outstanding sodium storage behavior, in terms of a high specific charge capacity (105.6 ​mA ​h ​g−1 at 0.2 ​C with 87.3% initial coulombic efficiency), an excellent cycle performance (101.2 ​mA ​h ​g−1 charge capacity after 100 cycles at 0.2 ​C with a charge capacity retention of 95.8%) and outstanding rate capabilities (103.6 ​mA ​h ​g−1 charge capacity at 1 ​C and 71.5 ​mA ​h ​g−1 at 5 ​C, respectively).

In-situ conversion growth of carbon-coated MoS2/N-doped carbon nanotubes as anodes with superior capacity retention for sodium-ion batteries

• A new carbon-coated MoS 2 /N-doped carbon nanotubes composite has been designed. • The in-situ conversion growth strategy proposed is simple, and high-efficient. • The C-MoS 2 /NCNTs electrode prepared delivers a remarkable sodium storage behavior. • The unique structural merits contribute to the superior electrochemical performance. Layered structure MoS 2 nanosheets have shown great potential for energy storage applications. However, the methodology for elaborately controllable growth of MoS 2 onto carbonaceous matrix for promoting the electrochemical performance is highly desirable. Herein, a high-effective, all-in-one in-situ conversion growth strategy has been proposed to construct a stable sandwich-type nanostructure. The formation of the optimized C-MoS 2 /NCNTs product undergoes a dissolution-recrystallization process, in which ultrathin carbon layer-coated MoS 2 nanosheets densely assembled onto the surface of polyimide (PI) derived N-doped carbon nanotubes (CNTs). Theoretical simulation reveals that MoS 2 nanosheets possessing an expanded interlayer spacing of 0.92 nm can greatly reduce the barrier energy of Na ions mitigation. Accordingly, the as-made C-MoS 2 /NCNTs anode delivers superior cycling stability (82% capacity retention after 400 cycles at 1 A g −1 ) and rate performance (348 mAh g −1 at 2 A g −1 ). The results demonstrate that the expanded MoS 2 interlayer distance, ultrathin outer carbon coating, and N-doped CNTs matrix together accounts for the outstanding sodium storage capability for the C-MoS 2 /NCNTs electrode.

TiO2 bunchy hierarchical structure with effective enhancement in sodium storage behaviors

AbstractBronze phase TiO2 [TiO2(B)] has great research potential for sodium storage since it has a higher theoretical capacity and ion mobility compared with other phases of TiO2. In this case, preparing porous TiO2(B) nanosheets, which can provide abundant sodium insertion channels, is the most effective way to improve transport kinetics. Here, we use the strong one‐dimensional TiO2 nanowires as the matrix for stringing these nanosheets together through a simple solvothermal method to build a bunchy hierarchical structure [TiO2(B)‐BH], which has fast pseudocapacitance behavior, high structural stability, and effective ion/electron transport. With the superiorities of this structure design, TiO2(B)‐BH has a higher capacity (131 vs. 70 mAh g−1 [TiO2‐NWs] at 0.5 C). And it is worth mentioning that the reversible capacity of up to 500 cycles can still be maintained at 85 mAh g−1 at a high rate of 10 C. Meanwhile, we also further analyzed the sodium storage mechanism through the ex‐situ X‐ray powder diffraction test, which proved the high structural stability of TiO2(B)‐BH in the process of sodiumization/de‐sodiumization. This strategy of uniformly integrating nanosheets into a matrix can also be extended to preparing electrode material structures of other energy devices.