Fanxing Bu

Porous Fe2O3 Nanoframeworks Encapsulated within Three-Dimensional Graphene as High-Performance Flexible Anode for Lithium-Ion Battery

Integrating nanoscale porous metal oxides into three-dimensional graphene (3DG) with encapsulated structure is a promising route but remains challenging to develop high-performance electrodes for lithium-ion battery. Herein, we design 3DG/metal organic framework composite by an excessive metal-ion-induced combination and spatially confined Ostwald ripening strategy, which can be transformed into 3DG/Fe2O3 aerogel with porous Fe2O3 nanoframeworks well encapsulated within graphene. The hierarchical structure offers highly interpenetrated porous conductive network and intimate contact between graphene and porous Fe2O3 as well as abundant stress buffer nanospace for effective charge transport and robust structural stability during electrochemical processes. The obtained free-standing 3DG/Fe2O3 aerogel was directly used as highly flexible anode upon mechanical pressing for lithium-ion battery and showed an ultrahigh capacity of 1129 mAh/g at 0.2 A/g after 130 cycles and outstanding cycling stability with a capacity retention of 98% after 1200 cycles at 5 A/g, which is the best results that have been reported so far. This study offers a promising route to greatly enhance the electrochemical properties of metal oxides and provides suggestive insights for developing high-performance electrode materials for electrochemical energy storage.

Integration of Graphene, Nano Sulfur, and Conducting Polymer into Compact, Flexible Lithium–Sulfur Battery Cathodes with Ultrahigh Volumetric Capacity and Superior Cycling Stability for Foldable Devices

Lithium-sulfur batteries, as one of the most promising next-generation batteries, attract tremendous attentions due to their high energy density and low cost. However, their practical application is hindered by their short cycling life and low volumetric capacity. Herein, compact, flexible, and free-standing films with a sandwich structure are designed simply by vacuum filtration, in which nanosulfur is homogenously coated by graphene and poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). This unique hierarchical structure not only provides a highly conductive network and intimate contacts between nanosulfur and graphene/PEDOT:PSS for effective charge transportation, but also offers synergistic physical restriction and chemical confinement of dissoluble intermediate lithium polysulfides during electrochemical processes. Therefore, these conductive compact films, used directly as cathodes, show the highest reversible volumetric capacity of 1432 Ah L-1 at 0.1 C and 1038 Ah L-1 at 1 C, and excellent cycling stability with a minimal decay rate of 0.04% per cycle over 500 cycles at 1 C. Meanwhile, remarkable rate performance with a high capacity of 701 mAh g-1 at 4 C is also achieved. Soft-packaged batteries based on this flexible cathode are further fabricated and demonstrate excellent mechanical and electrochemical properties with little capacity decay under folded state, highlighting the practical application of our deliberately designed electrode in a flexible power system.

Integration of ultrathin graphene/polyaniline composite nanosheets with a robust 3D graphene framework for highly flexible all-solid-state supercapacitors with superior energy density and exceptional cycling stability

Here we report a unique hierarchical free-standing graphene/polyaniline (G/PANI) composite electrode with ultrathin G/PANI composite nanosheets embedded in the skeleton of a three-dimensional (3D) graphene framework. Due to the intrinsic structural advantage of ultrathin G/PANI composite nanosheets and their synergetic interaction with the 3D graphene network, the 3D-G/PANI composite electrode can deliver a high specific capacitance of 777 F g−1 and 990 F cm−3 at 1 A g−1 and an exceptional cycling stability with 85% capacitance retention after 60 000 deep cycles in a three-electrode cell configuration. The further assembled all-solid-state supercapacitor based on the 3D-G/PANI composite electrode can not only show extraordinary mechanical flexibility allowing bending, twisting and folding, but also demonstrate remarkable electrochemical performance under its folded state, including an ultrahigh specific capacitance of 665 F g−1 and 847 F cm−3 for the 3D-G/PANI composite electrode, excellent rate capability with a capacitance retention of 86% at 20 A g−1 and superior cycling stability with no capacitance decay after 10 000 cycles, as well as ultralow self-discharge characteristics. Furthermore, the entire ultrathin device (∼45 μm, much thinner than a commercial standard A4 paper) can deliver volumetric, gravimetric and areal energy densities up to 14.2 mW h cm−3, 10.9 W h kg−1 and 64 μW h cm−2, respectively, which are much higher than those of current high-level commercial supercapacitors (∼5 W h kg−1) and even lithium thin-film batteries (∼8 mW h cm−3, 4 V/500 μA h).

Solution Synthesis of Semiconducting Two-Dimensional Polymer via Trimerization of Carbonitrile

The synthesis of crystalline two-dimensional polymers (2DPs) with proper bandgaps and well-defined repeating units presents a great challenge to synthetic chemists. Here we report the first solution synthesis of a single-layer/few-layer triazine-based 2DP via trimerization of carbonitrile at the interface of dichloromethane and trifluoromethanesulfonic acid. The processable triazine-based 2DP can be assembled into mechanically strong layered free-standing films with a high specific surface area via filtration. Moreover, the highly crystalline triazine-based 2DP can function as the active semiconducting layer in a field-effect transistor via drop coating and exhibits slightly bipolar behavior with a high on/off ratio of 103 and a remarkable mobility of 0.15 cm2 V-1 s-1.

Graphene/polyaniline@carbon cloth composite as a high-performance flexible supercapacitor electrode prepared by a one-step electrochemical co-deposition method

Increasing numbers of flexible universal energy storage devices are required for wearable and portable products. A typical representation of energy storage devices are supercapacitors. However, their applications are greatly limited due to their low conductivity, which is strongly restricted by the effect of the binder. We designed and prepared a flexible and binder-free supercapacitor based on a graphene–polyaniline@carbon cloth composite by a one-step electrochemical co-deposition process. The composite exhibited excellent electrochemical performance with a high specific capacitance of 793 F g−1 at a current density of 0.5 A g−1 and favorable cycle stability (retention of 81% of initial specific capacitance after 10 000 cycles). The flexible symmetric device also showed satisfactory specific capacitance of 512 F g−1 at a current density of 0.5 A g−1, desirable cycling stability (retention of 85% specific capacitance over 10 000 cycles) and considerable energy density of 11.38 W h kg−1 at a power density of 199.80 W kg−1. We believe that this flexible and binder-free supercapacitor with excellent electrochemical properties will meet the demands in roll-up displays, electronic paper and wearable electronic products.

Dispersion–Assembly Approach to Synthesize Three-Dimensional Graphene/Polymer Composite Aerogel as a Powerful Organic Cathode for Rechargeable Li and Na Batteries

Polymer cathode materials are promising alternatives to inorganic counterparts for both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to their high theoretical capacity, adjustable molecular structure, and strong adaptability to different counterions in batteries, etc. However, they suffer from poor practical capacity and low rate capability because of their intrinsically poor conductivity. Herein, we report the synthesis of self-assembled graphene/poly(anthraquinonyl sufide) (PAQS) composite aerogel (GPA) with efficient integration of a three-dimensional (3D) graphene framework with electroactive PAQS particles via a novel dispersion-assembly strategy which can be used as a free-standing flexible cathode upon mechanical pressing. The entire GPA cathode can deliver the highest capacity of 156 mAh g-1 at 0.1 C (1 C = 225 mAh g-1) with an ultrahigh utilization (94.9%) of PAQS and exhibits an excellent rate performance with 102 mAh g-1 at 20 C in LIBs. Furthermore, the flexible GPA film was also tested as cathode for SIBs and demonstrated a high-rate capability with 72 mAh g-1 at 5 C and an ultralong cycling stability (71.4% capacity retention after 1000 cycles at 0.5 C) which has rarely been achieved before. Such excellent electrochemical performance of GPA as cathode for both LIBs and SIBs could be ascribed to the fast redox kinetics and electron transportation within GPA, resulting from the interconnected conductive framework of graphene and the intimate interaction between graphene and PAQS through an efficient wrapping structure. This approach opens a universal way to develop cathode materials for powerful batteries with different metal-based counter electrodes.

Rational design of three-dimensional graphene encapsulated core–shell FeS@carbon nanocomposite as a flexible high-performance anode for sodium-ion batteries

The development of high-performance electrochemical energy storage systems is highly dependent on the synergistical structural design of electrode materials and whole electrodes with appropriate compositions. Here we create a novel flexible three-dimensional graphene (3DG) hybrid electrode with a core–shell FeS@carbon (FeS@C) nanocomposite encapsulated within 3DG by one-step thermal transformation of the deliberately designed 3DG wrapped metal–organic framework (3DG/MOF) composite based on the newly disclosed spatially confined phase separation of the metal and organic moieties of MOFs and the following in situ composition transformation mechanism. Benefitting from effective ion/charge transport in the whole electrode and the robust structural stability of FeS during electrochemical processes guaranteed by the highly interpenetrated porous conductive network of 3DG and the carbon protective layer with N- and S-codoping, the free-standing 3DG/FeS@C electrode delivers an ultrahigh specific capacity of 632 mA h g−1 after 80 cycles at 100 mA g−1, and excellent rate capacities of 363.3 and 152.5 mA h g−1 at 1 and 6 A g−1 with unprecedented cycling stability with a capacity retention of 97.9% after 300 cycles at 1 A g−1, which is the best ever reported result for FeS-based anode materials for sodium ion batteries. This study opens up a new MOF-based phase separation avenue to construct sophisticated 3DG wrapped core@shell nanocomposites and represents an important step to the structural design of high-performance electrodes for electrochemical energy storage.

Fe7Se8@C core–shell nanoparticles encapsulated within a three-dimensional graphene composite as a high-performance flexible anode for lithium-ion batteries

Nanocomposites with tailored core–shell structure encapsulated within three-dimensional graphene (3DG) architectures are attractive for applications of electrochemical energy storage. Here, we design a hierarchical structure with nanoparticles consisting of the Fe7Se8-enriched inner core and the carbon-enriched outer shell encapsulated within three-dimensional graphene (denoted as 3DG/Fe7Se8@C) by using a 3DG/metal organic framework (MOF) aerogel as the template to react with Se powder at elevated temperatures. This unique structure provides an excellent conductive and mechanically robust network which benefits electronic transport and suppresses volume change effectively. When directly used as a flexible anode for lithium-ion batteries, the 3DG/Fe7Se8@C composite exhibited a high reversible capacity of 884.1 mA h g−1 at 0.1 A g−1 after 120 cycles and 815.2 mA h g−1 at 1 A g−1 after 250 cycles between 0.01 and 3.00 V, as well as an exceptional rate performance.

One Versatile Route to Three‐dimensional Graphene Wrapped Metal Cyanide Aerogels for Enhanced Sodium Ion Storage

Facile and controllable integration of metal cyanides (MCs) into three-dimensional graphene (3DG) with advantageous structures is of fundamental importance for the development of superior MC-based electrode materials for electrochemical energy storage and catalysis. Here a facile and versatile spatially-confined Ostwald ripening strategy was developed to synthesize a series of 3DG wrapped MC aerogels with different compositions, size, and structure based on the chemical instability of MC in the reaction system. Remarkably, the integration of Prussian blue (PB) into 3DG, with such unique architecture, largely improves the rate performance and long-term cycling stability of PB as a cathode material for sodium ion batteries.

Sub-5 nm Ultrasmall Metal–Organic Framework Nanocrystals for Highly Efficient Electrochemical Energy Storage

Synthesis of ultrasmall metal-organic framework (MOF) nanoparticles has been widely recognized as a promising route to greatly enhance their properties but remains a considerable challenge. Herein, we report one facile and effective spatially confined thermal pulverization strategy to successfully transform bulk Co-MOF particles into sub-5 nm nanocrystals encapsulated within N-doped carbon/graphene (NC/G) by using conducting polymer coated Co-MOFs/graphene oxide as precursors. This strategy involves a feasible mechanism: calcination of Co-MOFs at proper temperature in air induces the partial thermal collapse/distortion of the framework, while the uniform coating of a conducting polymer can significantly improve the decomposition temperature and maintain the component stability of Co-MOFs, thus leading to the pulverization of bulk Co-MOF particles into ultrasmall nanocrystals without oxidation. The pulverization of Co-MOFs significantly increases the contact area between Co-MOFs with electrolyte and shortens the electron and ion transport pathway. Therefore, the sub-5 nm ultrasmall MOF nanocrystals-based composites deliver an ultrahigh reversible capacity (1301 mAh g-1 at 0.1 A g-1), extraordinary rate performance (494 mAh g-1 at 40 A g-1), and outstanding cycling stability (98.6% capacity retention at 10 A g-1 after 2000 cycles), which is the best performance achieved in all reported MOF-based anodes for lithium-ion batteries.