Kechun Wen

From Metal–Organic Framework to Li2S@C–Co–N Nanoporous Architecture: A High-Capacity Cathode for Lithium–Sulfur Batteries

Owing to the high theoretical specific capacity (1166 mAh g-1), lithium sulfide (Li2S) has been considered as a promising cathode material for Li-S batteries. However, the polysulfide dissolution and low electronic conductivity of Li2S limit its further application in next-generation Li-S batteries. In this report, a nanoporous Li2S@C-Co-N cathode is synthesized by liquid infiltration-evaporation of ultrafine Li2S nanoparticles into graphitic carbon co-doped with cobalt and nitrogen (C-Co-N) derived from metal-organic frameworks. The obtained Li2S@C-Co-N architecture remarkably immobilizes Li2S within the cathode structure through physical and chemical molecular interactions. Owing to the synergistic interactions between C-Co-N and Li2S nanoparticles, the Li2S@C-Co-N composite delivers a reversible capacity of 1155.3 (99.1% of theoretical value) at the initial cycle and 929.6 mAh g-1 after 300 cycles, with nearly 100% Coulombic efficiency and a capacity fading of 0.06% per cycle. It exhibits excellent rate capacities of 950.6, 898.8, and 604.1 mAh g-1 at 1C, 2C, and 4C, respectively. Such a cathode structure is promising for practical applications in high-performance Li-S batteries.

Three-Dimensional Hierarchical Reduced Graphene Oxide/Tellurium Nanowires: A High-Performance Freestanding Cathode for Li–Te Batteries

Three-dimensional aerogel with ultrathin tellurium nanowires (TeNWs) wrapped homogeneously by reduced graphene oxide (rGO) is realized via a facile hydrothermal method. Featured with high conductivity and large flexibility, the rGO constructs a conductive three-dimensional (3D) backbone with rich porosity and leads to a free-standing, binder-free cathode for lithium-tellurium (Li-Te) batteries with excellent electrochemical performances. The cathode shows a high initial capacity of 2611 mAh cm(-3) at 0.2 C, a high retention of 88% after 200 cycles, and a high-rate capacity of 1083 mAh cm(-3) at 10 C. In particular, the 3D aerogel cathode delivers a capacity of 1685 mAh cm(-3) at 1 C after 500 cycles, showing pronounced long-cycle performance at high current density. The performances are attributed to the well-defined flexible 3D architecture with high porosity and conductivity network, which offers highly efficient channels for electron transfer and ionic diffusion while compromising volume expansion of Te in charge/discharge. Owing to such advantageous properties, the reported 3D rGO/tellurium nanowire (3DGT) aerogel presents promising application potentials as a high-performance cathode for Li-Te batteries.

Interfacial lattice-strain effects on improving the overall performance of micro-solid oxide fuel cells

Interfacial lattice-strain, typically capable of altering the energy states of electrical carriers associated with hetero-interfaces, has shown unprecedented efficiency for improving the performance of a variety of real-life devices involving heterostructure crystals, including fuel cells and batteries. In this review, we overview recent findings on interfacial lattice-strain effects on improving ionic conductivity, oxygen vacancy formation, and surface oxygen exchange kinetics at cathodes of micro-solid oxide fuel cells. Our review seeks to provide evidence of interfacial strain effects on the overall performance of solid oxide fuel cells, highlight the fundamental and technological relevance, and provide insightful guidelines to enable the operation of micro-solid oxide fuel cells at lower temperatures more efficiently by tailoring the lattice-strain.

Tellurium-Impregnated Porous Cobalt-Doped Carbon Polyhedra as Superior Cathodes for Lithium–Tellurium Batteries

Lithium-tellurium (Li-Te) batteries are attractive for energy storage owing to their high theoretical volumetric capacity of 2621 mAh cm-3. In this work, highly nanoporous cobalt and nitrogen codoped carbon polyhedra (C-Co-N) derived from a metal-organic framework (MOF) is synthesized and employed as tellurium host for Li-Te batteries. The Te@C-Co-N cathode with a high Te loading of 77.2 wt % exhibits record-breaking electrochemical performances including an ultrahigh initial capacity of 2615.2 mAh cm-3 approaching the theoretical capacity of Te (2621 mAh cm-3), a superior cycling stability with a high capacity retention of 93.6%, a ∼99% Columbic efficiency after 800 cycles as well as rate capacities of 2160, 1327.6, and 894.8 mAh cm-3 at 4, 10, and 20 C, respectively. The redox chemistry of tellurium is revealed by in operando Raman spectroscopic analysis and density functional theory simulations. The results illustrate that the performances are attributed to the highly conductive C-Co-N matrix with an advantageous structure of abundant micropores, which provides highly efficient channels for electron transfer and ionic diffusion as well as sufficient surface area to efficiently host tellurium while mitigating polytelluride dissolution and suppressing volume expansion.

Direct impregnation of SeS2 into a MOF-derived 3D nanoporous Co–N–C architecture towards superior rechargeable lithium batteries

Metal–organic framework (MOF) derived cobalt- and nitrogen-doped porous carbon (Co–N–C) polyhedra are employed, for the first time, as SeS2 immobilizers (Co–N–C/SeS2). As the cathode of lithium–sulfur (Li–S) batteries, the Co–N–C/SeS2 composite with a high loading (66.5 wt%) of SeS2 delivers a reversible capacity of 1165.1 mA h g−1 and an over 84.1% capacity retention of the initial capacity (970.2 mA h g−1) with a nearly 100% coulombic efficiency after 200 cycles. The Co–N–C/SeS2 cathode shows excellent rate performances with capacities of 760 mA h g−1, 604.1 mA h g−1, and 439.7 mA h g−1 at 1C, 2C, and 4C, respectively. The redox chemistry of SeS2 is revealed by in situ Raman spectroscopic analysis and density functional theory (DFT) simulations. The superior electrochemical performance of the cathode is attributed to the unique Co–N–C structure with abundant micropores and uniformly-embedded ultrafine Co nanoparticles, which provide abundant SeS2 absorption and catalytically active sites and efficiently prevent the dissolution of polysulfides and polyselenides. The conductive Co–N–C framework facilitates fast electron and ion transfer in electrochemical reactions. Our work facilitates the development of high-performance cathodes for Li–S batteries.

Overview of Graphene as Anode in Lithium-Ion Batteries

Graphene, as a fabulously new-emerging carbonaceous material with an ideal two-dimensional rigid honeycomb structure, has drawn extensive attention in the field of material science due to extraordinary properties, including mechanical robustness, large specific surface area, desirable flexibility, and high electronic conductivity. In particular, as an auxiliary material of electrode materials, it has the potential to improve the performance of lithium-ion batteries. However, wide utilization of graphene in lithium-ion batteries is not implemented since tremendous challenges and issues, such as quality, quantity, and cost concerns, hinder its commercialization. There remains a debate whether graphene can act as an impetus in the evolution of lithium-ion batteries. In this review, we summarize the desirable properties, several common synthesis methods as well as applications of graphene as the anode in lithium-ion batteries, seeking to provide insightful guidelines for further development of graphene-based lithium-ion batteries.

Initial-stage oriented-attachment one-dimensional assembly of nanocrystals: fundamental insight with a collision–recrystallization model

Oriented attachment (OA) growth has been a promising method for the synthesis of one dimensional (1D) anisotropic nanocrystals (NCs). An unresolved fundamental issue is to understand the growth mechanism at the initial stage of an OA nanorod (NR) growth. In this report, a collision–recrystallization model is proposed to investigate the initial OA growth of NRs. The repulsive electrical double layer (EDL) interaction and the attractive van der Waals (vdW) interaction at the initial OA stage are derived by the accurate surface element integration (SEI) technique and the classical Hamaker equation, respectively. Our results show that the self-recrystallization of nanochains increases the collision activation energy of NPs dramatically as their surface potentials and Hamaker constants increase. Under a specific electrolyte concentration, the collision activation energy reaches the maximum, indicating that the growth rate of OA can be controlled by adjusting the electrolyte concentration.

Oriented-Attachment Nanocrystals in Lithium Ion Batteries

The world has entered an era with an increasing demand for new energy, including wind, tide, solar, and geothermal energy, due to the prompt reduction of fossil-based energy. But these new energies are subject to the restrictions of space and time. To address these issues, renewable energy storage devices have been developed, such as the lithium ion batteries (LIBs).

Oriented-Attachment Nanocrystals in Supercapacitors

Supercapacitors are the energy storage devices with high power density (1–10 kW kg−1), long lifetime (500,000–1,000,000 cycles), fast charging (with seconds), a wide range of operation temperatures (−40 to 70 °C), and high safety.

Oriented-Attachment Nanocrystals in Solar Cells

Solar cells convert solar energy into electricity directly. The first-generation solar cells, which are silicon-based photovoltaic devices are efficient but costly. Although second-generation solar cells are cost-effective, the conversion efficiency is not desirable. Now, people are focusing on the third-generation solar cells with low cost, highly efficiency, and non-toxicity.