Yuanfu Deng

Review on recent advances in nitrogen-doped carbons: preparations and applications in supercapacitors

It is of great interest to develop new carbon-based materials as electrodes for supercapacitors because the conventional electrodes of activated carbons in supercapacitors cannot meet the ever-increasing demands for high energy and power densities for electronic devices. Due to their high electronic conductivity and improved hydrophilic properties, together with their easy syntheses and functionalization, N-doped carbons have shown a great potential in energy storage and conversion applications. In this review, after a brief introduction of electrochemical capacitors, we summarize the advances, in the recent six years, in the preparation methods of N-doped carbons for applications in supercapacitors. We also discuss and predict futuristic research trends towards the design and syntheses of N-doped carbons with unique properties for electrochemical energy storage.

Controllable synthesis of spinel nano-ZnMn2O4via a single source precursor route and its high capacity retention as anode material for lithium ion batteries

Agglomerated pure spinel ZnMn2O4 nanoparticles with flake-shaped structure have been synthesized viacalcination of an agglomerated Zn–Mn citrate complex precursor, which was prepared with high yield by a convenient, environmentally benign and low temperature route. The composition, morphology and thermal decomposition of the Zn–Mn citrate complex were studied by C&H elemental analysis (EA), Fourier transform infrared spectroscopy (FTIR), energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The resulting ZnMn2O4 nanoparticles obtained from the precursor calcination at 700 °C were systematically characterized by XRD, FTIR, N2Adsorption/Desorption, SEM, TEM, HRTEM and selected area electron diffraction (SAED). The results show that the ZnMn2O4 material was agglomerated to form a porous texture in pure phase. The electrochemical properties of the agglomerated ZnMn2O4 material were investigated to determine its reversible capacity, rate and cycling performance as the anode material for lithium ion batteries (LIBs). This ZnMn2O4 material exhibited promising capacity retention of over 200 cycles at varying discharge rates. The electrode also exhibited attractive rate capabilities yielding capacity of 330 mAh g−1 after more than 35 cycles at 600 mA g−1.The ameliorated electrochemical performance can be ascribed to the high crystallinity and porous texture of the ZnMn2O4 material which provided short diffusion paths for lithium ions. Ex situXRD analysis of the electrodes after discharging and charging to the selected voltage was conducted and the possible lithium insertion mechanisms are discussed. This study suggests that the ZnMn2O4 material synthesized via the single source precursor route is a promising anode material for LIBs.

Recent advances in Mn-based oxides as anode materials for lithium ion batteries

The development of new electrode materials for lithium-ion batteries (LIBs) is of great interest because available electrode materials may not meet the high-energy demands for electronic devices, especially the demands for good cyclic and rate performance. Mn-based oxides have received substantial attention as promising anode materials for LIBs due to their high theoretical specific capacities, low charge potential vs. Li/Li+, environmental benignity and natural abundance. Herein, the preparation of Mn-based oxide nanomaterials with various nanostructures and chemical compositions along with their applications as negative electrodes for LIBs are reviewed. The review covers MnO, Mn3O4, Mn2O3, MnO2, CoMn2O4, ZnMn2O4 and their carbonaceous composite/oxide supports with different morphologies and compositions. The aim of this review is to provide an in-depth and rational understanding of the relationships among the chemical compositions, morphologies and electrochemical properties of Mn-based anode materials and, to understand how electrochemical performance can be improved using materials engineering strategies. Special attention has been paid to the discussion of challenges in the practical applications of Mn-based oxides in LIB full cells.

Graphene-encapsulated sulfur (GES) composites with a core–shell structure as superior cathode materials for lithium–sulfur batteries

Relatively uniform sized graphene-encapsulated sulphur (GES) composites with a core (S)–shell (graphene) structure were synthesized in one pot based on a solution-chemical reaction–deposition method. These novel GES particles were characterized by XRD, Raman spectrometry, SEM, TGA, EDS and TEM. The electrochemical tests showed that the present GES composites exhibit high specific capacity, good discharge capacity retention and superior rate capability when they were employed as cathodes in rechargeable Li–S cells. A high sulphur content (83.3 wt%) was obtained in the GES composites. Stable discharge capacities of about 900, 650, 540 and 480 mA h g−1 were achieved at 0.75, 2.0, 3.0 and 6.0 C, respectively. The good electrochemical performance is attributed to the high electrical conductivity of the graphene, the reasonable particle size of sulphur particles, and the core–shell structures that have synergistic effects on facilitating good transport of electrons from the poorly conducting sulphur, preserving fast transport of lithium ions to the encapsulated sulphur particles, and alleviating the polysulfide shuttle phenomenon. The present finding may provide a significant contribution to the enhancement of cathodes for the lithium–sulphur battery technology.

Improving the electrochemical performance of the LiNi0.5Mn1.5O4 spinel by polypyrrole coating as a cathode material for the lithium-ion battery

Conductive polypyrrole (PPy)-coated LiNi0.5Mn1.5O4 (LNMO) composites are applied as cathode materials in Li-ion batteries, and their electrochemical properties are explored at both room and elevated temperature. The morphology, phase evolution, and chemical properties of the as-prepared samples are analyzed by means of X-ray powder diffraction, thermogravimetric analysis, Raman spectroscopy, X-ray photoelectron spectroscopy and scanning and transmission electron microscopy techniques. The composite with 5 wt% polypyrrole coating shows a discharge capacity retention of 92% after 300 cycles and better rate capability than the bare LNMO electrode in the potential range of 3.5–4.9 V vs. Li/Li+ at room temperature. At the elevated temperature, the cycling performance of the electrode made from LNMO–5 wt% PPy is also remarkably stable (∼91% capacity retention after 100 cycles). It is revealed that the polypyrrole coating can suppress the dissolution of manganese in the electrolyte which occurs during cycling. The charge transfer resistance of the composite electrode is much lower than that of the bare LNMO electrode after cycling, indicating that the polypyrrole coating significantly increases the electrical conductivity of the LNMO electrode. Polypyrrole can also work as an effective protective layer to suppress the electrolyte decomposition arising from undesirable reactions between the cathode electrode and electrolyte on the surface of the active material at elevated temperature, leading to high coulombic efficiency.

Rh(III)-Catalyzed [4 + 2] Annulation of Indoles with Diazo Compounds: Access to Pyrimido[1,6-a]indole-1(2H)-ones

The Rh(III)-catalyzed regioselective C2-H bond carbenoid insertion/cyclization of N-amidoindoles with α-acyl diazo compounds has been developed. This method provides a novel approach to 2H-pyrimido[1,6-a]indol-1-ones with a broad range of functional group tolerance. The synthetic utilities of the approach are demonstrated by versatile chemical transformations.

Porous LiMn2O4 microspheres as durable high power cathode materials for lithium ion batteries

Porous LiMn2O4 microspheres, which are constructed with nanometer-sized primary particles, have been synthesized by a facile method using porous MnCO3 microspheres as a self-supporting template. The LiMn2O4 microspheres were characterized by XRD, SEM and HR-TEM. The as-synthesized porous LiMn2O4 microspheres exhibit high rate capability and long-term cyclability as cathode materials for lithium ion batteries, with the specific discharge capacity of 119, 107 and 98 mA h g−1 and the corresponding capacity retention of 82, 91 and 80% for up to 500 cycles at 2, 10 and 20 C, respectively. The high rate performance and good cyclability are believed to result from the porous structure, reasonable primary particle size and high crystallinity of the obtained material, which favor fast Li intercalation/deintercalation kinetics by allowing electrolyte insertion through the nanoparticles and high structural stability during the reversible electrochemical process. The high level of Mn4+ concentration on the surface of the sample can alleviate the Jahn–Teller transition, which was triggered normally by the equal amounts of Mn4+/Mn3+ concentration on the surface of the LiMn2O4 cathode material. This good example offering extended cycle life at 20 C rate for the LiMn2O4 microspheres indicates their promising application as cathode materials for high performance LIBs.

The enhanced rate performance of LiFe0.5Mn0.5PO4/C cathode material via synergistic strategies of surfactant-assisted solid state method and carbon coating

The rate performance of LiMnPO4-based materials is further improved via synergistic strategies including a surfactant-assisted solid state method, Fe-substitution and carbon-coating. The surfactant-assisted solid state strategy effectively decreases the primary particle size of the cathode material, which can greatly shorten the diffusion distance of lithium ions. The Fe-substitution improves the effectiveness of Li+ insertion/extraction reactions in the solid phase. The uniform carbon coating layer and the conductive networks provided by the carbon between the nanoparticles ensure the continuous conductivity by the nanoparticles. As a consequence of the synergistic effects, the as prepared LiFe0.5Mn0.5PO4 sample with 6.10 wt% carbon exhibits high specific capacities and superior rate performance with discharge capacities of 155.0, 140.9 and 121 mA h g−1 at 0.1, 1 and 5 C (1 C = 170 mA g−1), respectively. Meanwhile, it shows stable cycling stability at both room temperature (25 °C, 94.8% and 90.8% capacity retention after 500 cycles at 1 and 5 C rates, respectively) and elevated temperature (55 °C, 89.2% capacity retention after 300 cycles at 5 C rate). This material may have great potential application in advanced Li-ion batteries.

Graphene Oxide-Immobilized NH2-Terminated Silicon Nanoparticles by Cross-Linked Interactions for Highly Stable Silicon Negative Electrodes

There is a great interest in the utilization of silicon-based anodes for lithium-ion batteries. However, its poor cycling stability, which is caused by a dramatic volume change during lithium-ion intercalation, and intrinsic low electric conductivity hamper its industrial applications. A facile strategy is reported here to fabricate graphene oxide-immobilized NH2-terminated silicon nanoparticles (NPs) negative electrode (Si@NH2/GO) directed by hydrogen bonding and cross-linked interactions to enhance the capacity retention of the anode. The NH2-modified Si NPs first form strong hydrogen bonds and covalent bonds with GO. The Si@NH2/GO composite further forms hydrogen bonds and covalent bonds with sodium alginate, which acts as a binder, to yield a stable composite negative electrode. These two chemical cross-linked/hydrogen bonding interactions-one between NH2-modified Si NPs and GO, and another between the GO and sodium alginate-along with highly mechanically flexible graphene oxide, produced a robust network in the negative electrode system to stabilize the electrode during discharge and charge cycles. The as-prepared Si@NH2/GO electrode exhibits an outstanding capacity retention capability and good rate performance, delivering a reversible capacity of 1000 mAh g(-1) after 400 cycles at a current of 420 mA g(-1) with almost 100% capacity retention. The results indicated the importance of system-level strategy for fabricating stable electrodes with improved electrochemical performance.

A [4 + 1] Cyclative Capture Access to Indolizines via Cobalt(III)-Catalyzed Csp2–H Bond Functionalization

A Co(III)-catalyzed [4 + 1] cycloaddition of 2-arylpyridines or 2-alkenylpyridines with aldehydes through Csp(2)-H bond activation has been developed. This protocol provides a facile approach to structurally diverse indolizines including benzoindolizines with a broad range of functional group tolerance.