Flexible Conductive Anodes Based on 3D Hierarchical Sn/NS-CNFs@rGO Network for Sodium-Ion Batteries

Metallic Sn has provoked tremendous progress as an anode material for sodium-ion batteries(SIBs).However,Sn anodes suffer from a dramatic capacity fading,owing to pulverization induced by drastic volume expansion during cycling.Herein,a flexible three-dimensional(3D)hierarchical conductive network electrode is designed by constructing Sn quantum dots(QDs)encapsulated in one-dimensional N,S codoped carbon nanofibers(NS-CNFs)sheathed within two-dimensional(2D)reduced graphene oxide(rGO)scrolls.In this ingenious strategy,1D NS-CNFs are regarded as building blocks to prevent the aggregation and pulverization of Sn QDs during sodiation/desodiation,2D rGO acts as electrical roads and“bridges”among NS-CNFs to improve the conductivity of the electrode and enlarge the contact area with electrolyte.Because of the unique structural merits,the flexible 3D hierarchical conductive network was directly used as binder-and current collectorfree anode for SIBs,exhibiting ultra-long cycling life(373 mAh g?1 after 5000 cycles at 1 A g?1),and excellent high-rate capability(189 mAh g?1 at 10 A g?1).This work provides a facile and efficient engineering method to construct 3D hierarchical conductive electrodes for other flexible energy storage devices.

Enhanced sodium storage performance in flexible free- standing multichannel carbon nanofibers with enlarged interlayer spacing

A flexible and free-standing multichannel carbon nanofiber （MCNF） film electrode was fabricated through electrospinning and carbonization. After high-temperature treatment of MCNFs in vacuum, the obtained fibers （MCNFs-V） had a dilated interlayer spacing of graphene sheets （0.398 nm） and an ultra-low specific surface area （15.3 m2/g）. When used as an anode for sodium-ion batteries, the MCNFs-V showed a discharge plateau below 0.1 V, and sodium was intercalated into the stacked graphene sheets layers during the sodiation process. The MCNFs-V exhibited a reversible and high specific capacity of 222 mAh/g at a current density of 0.1 A/g after 100 cycles and excellent long-term cycling stability, which was superior to that of MCNFs. The improved sodium storage performance was attributed to the unique microstructure of the MCNFs-V with an enlarged interlayer spacing of graphene sheets for sodium intercalation. The MCNFs-V electrode holds great promise as an anode material for commercial sodium-ion batteries.

Flexible and free-standing La_(0.33)Ti_(2)(PO_(4))_(3)/C nanofibers film as a novel high-performance anode for sodium-and potassium-ion batteries

Na superionic conductor(NASICON)-type La_(0.33)Ti_(2)(PO_(4))_(3)(LaTP) is firstly proposed as sodium/potassium storage materials.The density functional theory(DFT) calculations show that LaTP has good electronic character and low Na^(+)/K^(+)migration barriers.The flexible La_(0.33)Ti_(2)(PO_(4))_(3)/C nanofiber film is synthesized via electrostatic spinning and investigated as free-standing electrode applied to sodium-ion batteries(SIBs) and potassiumion batteries(PIBs) in this work.The low band gap and Na^(+)/K^(+) migration barriers of LaTP,unique morphology,and complete conductive carbon net allow the La_(0.33)Ti_(2)(PO_(4))_(3)/C nanofibers film to deliver high capacity(296.3 mAh·g^(-1) for SIBs and 235.8 mAh·g^(-1) for PIBs),excellent rate performance(142.5 mAh·g^(-1) for SIBs and50.5 mAh·g^(-1) for PIBs at 1.00 A·g^(-1)),and superior cyclability above 1000 cycles.The full-cell tests show that the material has a good application prospect,indicating a promising flexible anode material for SIBs and PIBs.

Flexible polytriphenylamine-based cathodes with reinforced energystorage capacity for high-performance sodium-ion batteries

Owing to the excellent redox reversibility and structural diversity,polytriphenylamine(PTPAn)has been regarded as one of the promising cathode candidates for sodium-ion batteries.However,it still suffers from the bulk aggregation and low operating capacity in practical applications.Assisted by the in-situ polymerization,leaf-like PTPAn nanosheets are uniformly introduced on the surface of carbon nanofibers(CNFs)to form the free-standing CNF@PTPAn composite electrodes.Interestingly,the formation mechanism of the leaf-on-branch structure of CNF@PTPAn composites is systematically explored,confirming that the controlling of oxidation rate and growth degree plays crucial roles in tuning the morphology and active material content of the composite electrodes.Supported by the capacity-cutting analysis,the effective coupling between the active PTPAn and conductive CNFs can provide fast electron/ion-shuttling channels and deepen the electrochemical reaction process.At 50mAg^(-1),the capacity of the optimized CNF@PTPAn composite can reach 105mAh g^(-1),with a stable rate capability of 78mAh g^(-1)even at 400mAg^(-1)after 500 cycles in a half cell.The detailed kinetic analysis confirms that the ion-storage behaviors in the lowvoltage region can be tailored for the improved capacitive contribution and diffusion coefficients.Meanwhile,the flexible CNF-based full cell with CNF@PTPAn as the cathode and CNFs as the anode exhibits a high energy density of 60Wh kg^(-1)at 938Wkg^(-1).Based on this,the rational design is expected to provide more possibilities to obtain advanced freestanding electrode systems.

In-situ conversion growth of carbon-coated MoS_(2)/N-doped carbon nanotubes as anodes with superior capacity retention for sodium-ion batteries

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 ultra-thin 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.Ac-cordingly,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.

High N-doped hierarchical porous carbon networks with expanded interlayers for efficient sodium storage

Sodium-ion batteries (SIBs) have been attracting considerable attention as a promising candidate for large-scale energy storage because of the abundance and low-cost of sodium resources. However, lack of appropriate anode materials impedes further applications. Herein, a novel self-template strategy is designed to synthesize uniform flowerlike N-doped hierarchical porous carbon networks (NHPCN) with high content of N (15.31 at.%) assembled by ultrathin nanosheets via a self-synthesized single precursor and subsequent thermal annealing. Relying on the synergetic coordination of benzimidazole and 2-methylimidazole with metal ions to produce a flowerlike network, a self-formed single precursor can be harvested. Due to the structural and compositional advantages, including the high N doping, the expanded interlayer spacing, the ultrathin two-dimensional nano-sized subunits, and the three-dimensional porous network structure, these unique NHPCN flowers deliver ultrahigh reversible capacities of 453.7 mAh·g^−1 at 0.1 A·g^−1 and 242.5 mAh·g^−1 at 1 A·g^−1 for 2,500 cycles with exceptional rate capability of 5 A·g^−1 with reversible capacities of 201.2 mAh·g^−1. The greatly improved sodium storage performance of NHPCN confirms the importance of reasonable engineering and synthesis of hierarchical carbon with unique structures.

MoS2-intercalated carbon hetero-layers bonded on graphene as electrode materials for enhanced sodium/potassium ion storage

MoS2 is considered as an ideal electrode material in the field of energy storage due to high theoretical specific capacity and unique layered structure.However,limited interlayer distance and poor intrinsic electrical conductivity restrict its potential realworld application.Herein,an alternately intercalated structure of MoS2 monolayer and N-doped porous carbon(NC)layer is grown on reduced graphene oxide(rGO)via a chemical intercalated strategy.The expanded interlayer distance of MoS2(0.96 nm),enlarged by the intercalation of N-doped porous carbon layers,can enhance ion diffusion mobility,provide additional reactive sites for ion storage and maintain the stability of electrode structure.In addition,the hierarchical structures between rGO substrate and intercalated N-doped carbon layers construct a three-dimensional(3D)conductive network,which can significantly improve the electrical conductivity and the structural stability.As a result,the rGO-supported MoS2/NC electrode exhibits an ultrahigh reversible capacity and remarkable long cycling stability for sodium-ion batteries(SIBs)and potassium-ion(PIBs).Meanwhile,the as-obtained MoS2/NC@rGO electrode also delivers a superior cycle performance of 250 mAh·g−1 after 160 cycles at 0.5 A·g−1 when employed as an anode for sodium-ion full cells.

Assembly of flower-like VS_(2)/N-doped porous carbon with expanded(001)plane on rGO for superior Na-ion and K-ion storage

VS2 with natural layered structure and metallic conductivity is a prospective candidate for sodium-ion batteries(SIBs)and potassium-ion batteries(PIBs).However,due to large radius of Na+and K+,the limited interlayer spacing(0.57 nm)of VS2 generally determines high ion diffusion barrier and large volume variation,resulting in unsatisfactory electrochemical performance of SIBs and PIBs.In this work,flower-like VS_(2)/N-doped carbon(VS_(2)/N-C)with expanded(001)plane is grown on reduced graphene oxide(rGO)via a solvothermal and subsequently carbonization strategy.In the VS_(2)/N-C@rGO nanohybrids,the ultrathin VS2"petals"are alternately intercalated by the N-doped porous carbon monolayers to achieve an expanded interlayer spacing(1.02 nm),which can effectively reduce ions diffusion barrier,expose abundant active sites for Na+/K+intercalation,and tolerate large volume variation.The N-C and rGO carbonous materials can significantly promote the electrical conductivity and structural stability.Benefited from the synergistic effect,the VS2/N-C@rGO electrode exhibits large reversible capacity(Na+:407 mAh·g^(-1) at 1 A·g^(-1);K^(+):334 mAh·g^(-1) at 0.2 A·g^(-1)),high rate capacity(Na+:273 mAh·g^(-1) at 8 A·g^(-1);K+:186 mAh·g^(-1) at 5 A·g^(-1)),and remarkable cycling stability(Na+:316 mAh·g^(-1) at 2 A·g^(-1) after 1,400 cycles;K^(+):216 mAh·g^(-1) at 1 A·g^(-1) after 500 cycles).