Surface‐Engineered Black Niobium Oxide@Graphene Nanosheets for High‐Performance Sodium‐/Potassium‐Ion Full Batteries

Nanoscale surface‐engineering plays an important role in improving the performance of battery electrodes. Nb₂O₅ is one typical model anode material with promising high‐rate lithium storage. However, its modest reaction kinetics and low electrical conductivity obstruct the efficient storage of larger ions of sodium or potassium. In this work, partially surface‐amorphized and defect‐rich black niobium oxide@graphene (black Nb₂O_5?x@rGO) nanosheets are designed to overcome the above Na/K storage problems. The black Nb₂O_5?x@rGO nanosheets electrodes deliver a high‐rate Na and K storage capacity (123 and 73 mAh g^?1, respectively at 3 A g^?1) with long‐term cycling stability. Besides, both Na‐ion and K‐ion full batteries based on black Nb₂O_5?x@rGO nanosheets anodes and vanadate‐based cathodes (Na_0.33V₂O₅ and K_0.5V₂O₅ for Na‐ion and K‐ion full batteries, respectively) demonstrate promising rate and cycling performance. Notably, the K‐ion full battery delivers higher energy and power densities (172 Wh Kg^?1 and 430 W Kg^?1), comparable to those reported in state‐of‐the‐art K‐ion full batteries, accompanying with a capacity retention of ≈81.3% over 270 cycles. This result on Na‐/K‐ion batteries may pave the way to next‐generation post‐lithium batteries.     

Progressively Exposing Active Facets of 2D Nanosheets toward Enhanced Pseudocapacitive Response and High‐Rate Sodium Storage

Sodium‐ion batteries are gradually regarded as a prospective alternative to lithium‐ion batteries due to the cost consideration. Here, three kinds of tin (IV) sulfide nanosheets are controllably designed with progressively exposed active facets, leading to beneficial influences on the Na⁺ storage kinetics, resulting in gradient improvements of pseudocapacitive response and rate performance. Interestingly, different forms of kinetics results are generated accompanying with the morphology and structure evolution of the three nanosheets. Finally, detailed density functional theory simulations are also applied to analyze the above experimental achievements, proving that different exposed facets of crystalline anodes possess dissimilar Na⁺ storage kinetics. The investigation experiences and conclusions shown in this work are meaningful to explore many other proper structure design routes toward the high‐rate and stable metal‐ions storage.     

O3‐Type Layered Ni‐Rich Oxide: A High‐Capacity and Superior‐Rate Cathode for Sodium‐Ion Batteries

Inspired by its high‐active and open layered framework for fast Li⁺ extraction/insertion reactions, layered Ni‐rich oxide is proposed as an outstanding Na‐intercalated cathode for high‐performance sodium‐ion batteries. An O3‐type Na_0.75Ni_0.82Co_0.12Mn_0.06O₂ is achieved through a facile electrochemical ion‐exchange strategy in which Li⁺ ions are first extracted from the LiNi_0.82Co_0.12Mn_0.06O₂ cathode and Na⁺ ions are then inserted into a layered oxide framework. Furthermore, the reaction mechanism of layered Ni‐rich oxide during Na⁺ extraction/insertion is investigated in detail by combining ex situ X‐ray diffraction, X‐ray photoelectron spectroscopy, and electron energy loss spectroscopy. As an excellent cathode for Na‐ion batteries, O3‐type Na_0.75Ni_0.82Co_0.12Mn_0.06O₂ delivers a high reversible capacity of 171 mAh g^?1 and a remarkably stable discharge voltage of 2.8 V during long‐term cycling. In addition, the fast Na⁺ transport in the cathode enables high rate capability with 89 mAh g^?1 at 9 C. The as‐prepared Ni‐rich oxide cathode is expected to significantly break through the limited performance of current sodium‐ion batteries.     

Pyrolytic Carbon Nanosheets for Ultrafast and Ultrastable Sodium‐Ion Storage

Na‐ion cointercalation in the graphite host structure in a glyme‐based electrolyte represents a new possibility for using carbon‐based materials (CMs) as anodes for Na‐ion storage. However, local microstructures and nanoscale morphological features in CMs affect their electrochemical performances; they require intensive studies to achieve high levels of Na‐ion storage performances. Here, pyrolytic carbon nanosheets (PCNs) composed of multitudinous graphitic nanocrystals are prepared from renewable bioresources by heating. In particular, PCN‐2800 prepared by heating at 2800 °C has a distinctive sp² carbon bonding nature, crystalline domain size of ≈44.2 Å, and high electrical conductivity of ≈320 S cm^?1, presenting significantly high rate capability at 600 C (60 A g^?1) and stable cycling behaviors over 40 000 cycles as an anode for Na‐ion storage. The results of this study show the unusual graphitization behaviors of a char‐type carbon precursor and exceptionally high rate and cycling performances of the resulting graphitic material, PCN‐2800, even surpassing those of supercapacitors.     

High‐Safety Nonaqueous Electrolytes and Interphases for Sodium‐Ion Batteries

Rapidly developed Na‐ion batteries are highly attractive for grid energy storage. Nevertheless, the safety issues of Na‐ion batteries are still a bottleneck for large‐scale applications. Similar to Li‐ion batteries (LIBs), the safety of Na‐ion batteries is considered to be tightly associated with the electrolyte and electrode/electrolyte interphase. Although the knowledge obtained from LIBs is helpful, designing safe electrolytes and obtaining stable interphases in Na‐ion batteries is still a huge challenge. Therefore, it is of significance to investigate the key factors and develop new strategies for the development of high‐safety Na‐ion batteries. This comprehensive review introduces the recent efforts from nonaqueous electrolytes and interphase aspects of Na‐ion batteries, proposes their design strategies and requirements for improving safety characteristics, and discusses the potential issues for practical applications. The insight to formulate safe electrolytes and design the stable interphase for Na‐ion batteries with high safety is intended to be provided herein.     

A Functionalized Carbon Surface for High‐Performance Sodium‐Ion Storage

Sodium‐ion batteries (SIBs) are promising for large‐scale energy storage systems and carbon materials are the most likely candidates for their electrodes. The existence of defects in carbon materials is crucial for increasing the sodium storage ability. However, both the reversible capacity and efficiency need to be further improved. Functionalization is a direct and feasible approach to address this issue. Based on the structural changes in carbon materials produced by surface functionalization, three basic categories are defined: heteroatom doping, grafting of functional groups, and the shielding of defects. Heteroatom doping can improve the electrochemical reactivity, and the grafting of functional groups can promote both the diffusion‐controlled bulk process and surface‐confined capacitive process. The shielding of defects can further increase the efficiency and cyclic stability without sacrificing reversible capacity. In this Review, recent progresses in the ways to produce surface functionalization are presented and the related impact on the physical and chemical properties of carbon materials is discussed. Moreover, the critical issues, challenges, and possibilities for future research are summarized.     

Scalable 2D Mesoporous Silicon Nanosheets for High‐Performance Lithium‐Ion Battery Anode

Constructing unique mesoporous 2D Si nanostructures to shorten the lithium‐ion diffusion pathway, facilitate interfacial charge transfer, and enlarge the electrode–electrolyte interface offers exciting opportunities in future high‐performance lithium‐ion batteries. However, simultaneous realization of 2D and mesoporous structures for Si material is quite difficult due to its non‐van der Waals structure. Here, the coexistence of both mesoporous and 2D ultrathin nanosheets in the Si anodes and considerably high surface area (381.6 m² g^?1) are successfully achieved by a scalable and cost‐efficient method. After being encapsulated with the homogeneous carbon layer, the Si/C nanocomposite anodes achieve outstanding reversible capacity, high cycle stability, and excellent rate capability. In particular, the reversible capacity reaches 1072.2 mA h g^?1 at 4 A g^?1 even after 500 cycles. The obvious enhancements can be attributed to the synergistic effect between the unique 2D mesoporous nanostructure and carbon capsulation. Furthermore, full‐cell evaluations indicate that the unique Si/C nanostructures have a great potential in the next‐generation lithium‐ion battery. These findings not only greatly improve the electrochemical performances of Si anode, but also shine some light on designing the unique nanomaterials for various energy devices.     

High‐Energy/Power and Low‐Temperature Cathode for Sodium‐Ion Batteries: In Situ XRD Study and Superior Full‐Cell Performance

Sodium‐ion batteries (SIBs) are still confronted with several major challenges, including low energy and power densities, short‐term cycle life, and poor low‐temperature performance, which severely hinder their practical applications. Here, a high‐voltage cathode composed of Na₃V₂(PO₄)₂O₂F nano‐tetraprisms (NVPF‐NTP) is proposed to enhance the energy density of SIBs. The prepared NVPF‐NTP exhibits two high working plateaux at about 4.01 and 3.60 V versus the Na⁺/Na with a specific capacity of 127.8 mA h g^?1. The energy density of NVPF‐NTP reaches up to 486 W h kg^?1, which is higher than the majority of other cathode materials previously reported for SIBs. Moreover, due to the low strain (≈2.56% volumetric variation) and superior Na transport kinetics in Na intercalation/extraction processes, as demonstrated by in situ X‐ray diffraction, galvanostatic intermittent titration technique, and cyclic voltammetry at varied scan rates, the NVPF‐NTP shows long‐term cycle life, superior low‐temperature performance, and outstanding high‐rate capabilities. The comparison of Ragone plots further discloses that NVPF‐NTP presents the best power performance among the state‐of‐the‐art cathode materials for SIBs. More importantly, when coupled with an Sb‐based anode, the fabricated sodium‐ion full‐cells also exhibit excellent rate and cycling performances, thus providing a preview of their practical application.     

A Heterostructure Coupling of Bioinspired,Adhesive Polydopamine,and Porous Prussian Blue Nanocubics as Cathode for High‐Performance Sodium‐Ion Battery

Prussian blue (PB) and its analogues are recognized as promising cathodes for rechargeable batteries intended for application in low‐cost and large‐scale electric energy storage. With respect to PB cathodes, however, their intrinsic crystal regularity, vacancies, and coordinated water will lead to low specific capacity and poor rate performance, impeding their application. Herein, nanocubic porous Na_xFeFe(CN)₆ coated with polydopamine (PDA) as a coupling layer to improve its electrochemical performance is reported, inspired by the excellent adhesive property of PDA. As a cathode for sodium‐ion batteries, the Na_xFeFe(CN)₆ electrode coupled with PDA delivers a reversible capacity of 93.8 mA h g^?1 after 500 cycles at 0.2 A g^?1, and a discharge capacity of 72.6 mA h g^?1 at 5.0 A g^?1. The sodium storage mechanism of this Na_xFeFe(CN)₆ coupled with PDA is revealed via in situ Raman spectroscopy. The first‐principles computational results indicate that Fe^II sites in PB prefer to couple with the robust PDA layer to stabilize the PB structure. Moreover, the sodium‐ion migration in the PB structure is enhanced after coating with PDA, thus improving the sodium storage properties. Both experiments and computational simulations present guidelines for the rational design of nanomaterials as electrodes for energy storage devices.     

Few‐Layered Tin Sulfide Nanosheets Supported on Reduced Graphene Oxide as a High‐Performance Anode for Potassium‐Ion Batteries

Anodes involving conversion and alloying reaction mechanisms are attractive for potassium‐ion batteries (PIBs) due to their high theoretical capacities. However, serious volume change and metal aggregation upon potassiation/depotassiation usually cause poor electrochemical performance. Herein, few‐layered SnS₂ nanosheets supported on reduced graphene oxide (SnS₂@rGO) are fabricated and investigated as anode material for PIBs, showing high specific capacity (448 mAh g^?1 at 0.05 A g^?1), high rate capability (247 mAh g^?1 at 1 A g^?1), and improved cycle performance (73% capacity retention after 300 cycles). In this composite electrode, SnS₂ nanosheets undergo sequential conversion (SnS₂ to Sn) and alloying (Sn to K₄Sn₂₃, KSn) reactions during potassiation/depotassiation, giving rise to a high specific capacity. Meanwhile, the hybrid ultrathin nanosheets enable fast K storage kinetics and excellent structure integrity because of fast electron/ionic transportation, surface capacitive‐dominated charge storage mechanism, and effective accommodation for volume variation. This work demonstrates that K storage performance of alloy and conversion‐based anodes can be remarkably promoted by subtle structure engineering.     

N‐Doped Carbon Nanofibers with Interweaved Nanochannels for High‐Performance Sodium‐Ion Storage

Although graphite materials have been applied as commercial anodes in lithium‐ion batteries (LIBs), there still remain abundant spaces in the development of carbon‐based anode materials for sodium‐ion batteries (SIBs). Herein, an electrospinning route is reported to fabricate nitrogen‐doped carbon nanofibers with interweaved nanochannels (NCNFs‐IWNC) that contain robust interconnected 1D porous channels, produced by removal of a Te nanowire template that is coelectrospun within carbon nanofibers during the electrospinning process. The NCNFs‐IWNC features favorable properties, including a conductive 1D interconnected porous structure, a large specific surface area, expanded interlayer graphite‐like spacing, enriched N‐doped defects and active sites, toward rapid access and transport of electrolyte and electron/sodium ions. Systematic electrochemical studies indicate that the NCNFs‐IWNC exhibits an impressively high rate capability, delivering a capacity of 148 mA h g^?1 at current density of as high as 10 A g^?1, and has an attractively stable performance over 5000 cycles. The practical application of the as‐designed NCNFs‐IWNC for a full SIBs cell is further verified by coupling the NCNFs‐IWNC anode with a FeFe(CN)₆ cathode, which displays a desirable cycle performance, maintaining acapacity of 97 mA h g^?1 over 100 cycles.     

Ultrathin Nanosheet Assembled Sn0.91Co0.19S2 Nanocages with Exposed (100) Facets for High‐Performance Lithium‐Ion Batteries

Ultrathin 2D inorganic nanomaterials are good candidates for lithium‐ion batteries, as well as the micro/nanocage structures with unique and tunable morphologies. Meanwhile, as a cost‐effective method, chemical doping plays a vital role in manipulating physical and chemical properties of metal oxides and sulfides. Thus, the design of ultrathin, hollow, and chemical doped metal sulfides shows great promise for the application of Li‐ion batteries by shortening the diffusion pathway of Li ions as well as minimizing the electrode volume change. Herein, ultrathin nanosheet assembled Sn_0.91Co_0.19S₂ nanocages with exposed (100) facets are first synthesized. The as‐prepared electrode delivers an excellent discharge capacity of 809 mA h g^?1 at a current density of 100 mA g^?1 with a 91% retention after 60 discharge–charge cycles. The electrochemical performance reveals that the Li‐ion batteries prepared by Sn_0.91Co_0.19S₂ nanocages have high capacity and great cycling stability.     

An Advanced MoS2/Carbon Anode for High‐Performance Sodium‐Ion Batteries

Molybdenum disulfide (MoS₂) is a promising anode for high performance sodium‐ion batteries due to high specific capacity, abundance, and low cost. However, poor cycling stability, low rate capability and unclear electrochemical reaction mechanism are the main challenges for MoS₂ anode in Na‐ion batteries. In this study, molybdenum disulfide/carbon (MoS₂/C) nanospheres are fabricated and used for Na‐ion battery anodes. MoS₂/C nanospheres deliver a reversible capacity of 520 mAh g^?1 at 0.1 C and maintain at 400 mAh g^?1 for 300 cycles at a high current density of 1 C, demonstrating the best cycling performance of MoS₂ for Na‐ion batteries to date. The high capacity is attributed to the short ion and electron diffusion pathway, which enables fast charge transfer and low concentration polarization. The stable cycling performance and high coulombic efficiency (～100%) of MoS₂/C nanospheres are ascribed to (1) highly reversible conversion reaction of MoS₂ during sodiation/desodiation as evidenced by ex‐situ X‐ray diffraction (XRD) and (2) the formation of a stable solid electrolyte interface (SEI) layer in fluoroethylene carbonate (FEC) based electrolyte as demonstrated by fourier transform infrared spectroscopy (FTIR) measurements.     

Formation of Hierarchical Cu‐Doped CoSe2 Microboxes via Sequential Ion Exchange for High‐Performance Sodium‐Ion Batteries

Electrode materials based on electrochemical conversion reactions have received considerable interest for high capacity anodes of sodium‐ion batteries. However, their practical application is greatly hindered by the poor rate capability and rapid capacity fading. Tuning the structure at nanoscale and increasing the conductivity of these anode materials are two effective strategies to address these issues. Herein, a two‐step ion‐exchange method is developed to synthesize hierarchical Cu‐doped CoSe₂ microboxes assembled by ultrathin nanosheets using Co–Co Prussian blue analogue microcubes as the starting material. Benefitting from the structural and compositional advantages, these Cu‐doped CoSe₂ microboxes with improved conductivity exhibit enhanced sodium storage properties in terms of good rate capability and excellent cycling performance.     

Controllable Interlayer Spacing of Sulfur‐Doped Graphitic Carbon Nanosheets for Fast Sodium‐Ion Batteries

The electrochemical behaviors of current graphitic carbons are seriously restricted by its low surface area and insufficient interlayer spacing for sodium‐ion batteries. Here, sulfur‐doped graphitic carbon nanosheets are reported by utilizing sodium dodecyl sulfate as sulfur resource and graphitization additive, showing a controllable interlayer spacing range from 0.38 to 0.41 nm and a high specific surface area up to 898.8 m² g^?1. The obtained carbon exhibits an extraordinary electrochemical activity for sodium‐ion storage with a large reversible capacity of 321.8 mAh g^?1 at 100 mA g^?1, which can be mainly attributed to the expanded interlayer spacing of the carbon materials resulted from the S‐doping. Impressively, superior rate capability of 161.8 mAh g^?1 is reserved at a high current density of 5 A g^?1 within 5000 cycles, which should be ascribed to the fast surface‐induced capacitive behavior derived from its high surface area. Furthermore, the storage processes are also quantitatively evaluated, confirming a mixed storage mechanism of diffusion‐controlled intercalation behavior and surface‐induced capacitive behavior. This study provides a novel route for rationally designing various carbon‐based anodes with enhanced rate capability.