Yunhua Xu

Sulfur-Impregnated Disordered Carbon Nanotubes Cathode for Lithium–Sulfur Batteries

The commercialization of lithium-sulfur batteries is hindered by low cycle stability and low efficiency, which are induced by sulfur active material loss and polysulfide shuttle reaction through dissolution into electrolyte. In this study, sulfur-impregnated disordered carbon nanotubes are synthesized as cathode material for the lithium-sulfur battery. The obtained sulfur-carbon tube cathodes demonstrate superior cyclability and Coulombic efficiency. More importantly, the electrochemical characterization indicates a new stabilization mechanism of sulfur in carbon induced by heat treatment.

Expanded graphite as superior anode for sodium-ion batteries

Graphite, as the most common anode for commercial Li-ion batteries, has been reported to have a very low capacity when used as a Na-ion battery anode. It is well known that electrochemical insertion of Na(+) into graphite is significantly hindered by the insufficient interlayer spacing. Here we report expanded graphite as a Na-ion battery anode. Prepared through a process of oxidation and partial reduction on graphite, expanded graphite has an enlarged interlayer lattice distance of 4.3 Å yet retains an analogous long-range-ordered layered structure to graphite. In situ transmission electron microscopy has demonstrated that the Na-ion can be reversibly inserted into and extracted from expanded graphite. Galvanostatic studies show that expanded graphite can deliver a high reversible capacity of 284 mAh g(-1) at a current density of 20 mA g(-1), maintain a capacity of 184 mAh g(-1) at 100 mA g(-1), and retain 73.92% of its capacity after 2,000 cycles.

Electrospun Sb/C Fibers for a Stable and Fast Sodium-Ion Battery Anode

Sodium-ion batteries (SIBs) are considered a top alternative to lithium-ion batteries (LIBs) for large-scale renewable energy storage units due to their low cost and the abundance of sodium-bearing precursors in the earths mineral deposits. However, the development of anode materials for SIBs to date has been mainly limited to carbonaceous materials with minimal research devoted to high capacity alloy-based materials. In this study, an antimony (Sb)/carbon (C) electrode with ~30 nm Sb nanoparticles (NPs) uniformly encapsulated in interconnecting one-dimensional (1D) 400 nm carbon fibers (denoted as SbNP@C) was fabricated using a simple and scalable electrospinning method. This binder-free, current collector-free SbNP@C electrode demonstrated high capacity and stable long-term cycling performance at various current densities. The SbNP@C electrode showed an initial total capacity of 422 mAh/gelectrode and retained 350 mAh/gelectrode after 300 deep charge-discharge cycles under 100 mA/gSb. Moreover, because of the efficient 1D sodium-ion transport pathway and the highly conductive network of SbNP@C, the electrode preserved high overall capacities even when cycled at high currents, extending its usability to high power applications.

Uniform Nano-Sn/C Composite Anodes for Lithium Ion Batteries

Nano-Sn/C composites are ideal anode materials for high energy and power density Li-ion batteries. However, because of the low melting point of Sn and the tendency of grain growth, especially during high temperature carbonization, it has been a significant challenge to create well-dispersed ultrasmall Sn nanoparticles within a carbon matrix. In this paper, we demonstrate an aerosol spray pyrolysis technique, as a facile and scalable method, to synthesize a nano-Sn/C composite with uniformly dispersed 10 nm nano-Sn within a spherical carbon matrix. The discharge capacity of nano-Sn/C composite sphere anodes maintains the initial capacity of 710 mAh/g after 130 cycles at 0.25 C. The nano-Sn/C composite sphere anodes can provide ~600 mAh/g even at a high rate of 20 C. To the best of our knowledge, such high rate performance for Sn anodes has not been reported previously. The exceptional performance of the nano-Sn/C composite is attributed to the unique nano-Sn/C structure: (1) carbon matrix offers mechanical support to accommodate the stress associated with the large volume change of nano-Sn, thus alleviating pulverization; (2) the carbon matrix prevents Sn nanoparticle agglomeration upon prolonged cycling; and (3) carbon network provides continuous path for Li ions and electrons inside the nano-Sn/C composite spheres.

Selenium@Mesoporous Carbon Composite with Superior Lithium and Sodium Storage Capacity

Selenium-impregnated carbon composites were synthesized by infusing Se into mesoporous carbon at a temperature of 600 °C under vacuum. Ring-structured Se8 was produced and confined in the mesoporous carbon, which acts as an electronic conductive matrix. During the electrochemical process in low-cost LiPF6/EC/DEC electrolyte, low-order polyselenide intermediates formed and were stabilized by mesoporous carbon, which avoided the shuttle reaction of polyselenides. Exceptional electrochemical performance of Se/mesoporous carbon composites was demonstrated in both Li-ion and Na-ion batteries. In lithium-ion batteries, Se8/mesoporous carbon composite cathodes delivered a reversible capacity of 480 mAh g(-1) for 1000 charge/discharge cycles without any capacity loss, while in Na-ion batteries, it provided initial capacity of 485 mAh g(-1) and retained 340 mAh g(-1) after 380 cycles. The Se8/mesoporous carbon composites also showed excellent rate capability. As the current density increased from 0.1 to 5 C, the capacity retained about 46% in Li-ion batteries and 34% in Na-ion batteries.

Tin-Coated Viral Nanoforests as Sodium-Ion Battery Anodes

Designed as a high-capacity alloy host for Na-ion chemistry, a forest of Sn nanorods with a unique core-shell structure was synthesized on viral scaffolds, which were genetically engineered to ensure a nearly vertical alignment upon self-assembly onto a metal substrate. The interdigital spaces thus formed between individual rods effectively accommodated the volume expansion and contraction of the alloy upon sodiation/desodiation, while additional carbon-coating engineered over these nanorods further suppressed Sn aggregation during extended electrochemical cycling. Due to the unique nanohierarchy of multiple functional layers, the resultant 3D nanoforest of C/Sn/Ni/TMV1cys, binder-free composite electrode already and evenly assembled on a stainless steel current collector, exhibited supreme capacity utilization and cycling stability toward Na-ion storage and release. An initial capacity of 722 mA·h (g Sn)(-1) along with 405 mA·h (g Sn)(-1) retained after 150 deep cycles demonstrates the longest-cycling nano-Sn anode material for Na-ion batteries reported in the literature to date and marks a significant performance improvement for neat Sn material as alloy host for Na-ion chemistry.

Porous amorphous FePO4 nanoparticles connected by single-wall carbon nanotubes for sodium ion battery cathodes.

Sodium ion batteries (SIBs) are promising candidates for the applications of large-scale energy storage due to their cost-effective and environmental-friendly characteristics. Nevertheless, it remains a practical challenge to find a cathode material of SIBs showing ideal performance (capacity, reversibility, etc.). We report here a nanocomposite material of amorphous, porous FePO(4) nanoparticles electrically wired by single-wall carbon nanotubes as a potential cathode material for SIBs. The hydrothermally synthesized nanocomposite shows excellent cell performance with unprecedented cycling stability and reversibility. The discharge capacity of as high as 120 mAh/g is delivered at a 0.1 C rate (10 mA/g). The capacity retentions are about 70 mAh/g, 60 mAh/g, and 55 mAh/g at higher currents of 20 mA/g, 40 mA/g, and 60 mA/g, respectively. Even at a 1 C rate (100 mA/g), a capacity of about 50 mAh/g is still retained after 300 cycles. With a simple synthetic procedure, cost-effective chemicals, and desirable cell performance, this method offers a highly promising candidate for commercialized cathode materials of SIBs.

In Situ Formed Lithium Sulfide/Microporous Carbon Cathodes for Lithium-Ion Batteries

Highly stable sulfur/microporous carbon (S/MC) composites are prepared by vacuum infusion of sulfur vapor into microporous carbon at 600 °C, and lithium sulfide/microporous carbon (Li2S/MC) cathodes are fabricated via a novel and facile in situ lithiation strategy, i.e., spraying commercial stabilized lithium metal powder (SLMP) onto a prepared S/MC film cathode prior to the routine compressing process in cell assembly. The in situ formed Li2S/MC film cathode shows high Coulombic efficiency and long cycling stability in a conventional commercial Li-ion battery electrolyte (1.0 M LiPF6 + EC/DEC (1:1 v/v)). The reversible capacities of Li2S/MC cathodes remain about 650 mAh/g even after 900 charge/discharge cycles, and the Coulombic efficiency is close to 100% at a current density of 0.1C, which demonstrates the best electrochemical performance of Li2S/MC cathodes reported to date. Furthermore, this Li2S/MC film cathode fabricated via our in situ lithiation strategy can be coupled with a Li-free anode, such as graphite, carbon/tin alloys, or Si nanowires to form a rechargeable Li-ion cell. As the Li2S/MC cathode is paired with a commercial graphite anode, the full cell of Li2S/MC-graphite (Li2S-G) shows a stable capacity of around 600 mAh/g in 150 cycles. The Li2S/MC cathodes prepared by high-temperate sulfur infusion and SLMP prelithiation before cell assembly are ready to fit into current Li-ion batteries manufacturing processes and will pave the way to commercialize low-cost Li2S-G Li-ion batteries.

In Situ Transmission Electron Microscopy Study of Electrochemical Sodiation and Potassiation of Carbon Nanofibers

Carbonaceous materials have great potential for applications as anodes of alkali-metal ion batteries, such as Na-ion batteries and K-ion batteries (NIB and KIBs). We conduct an in situ study of the electrochemically driven sodiation and potassiation of individual carbon nanofibers (CNFs) by transmission electron microscopy (TEM). The CNFs are hollow and consist of a bilayer wall with an outer layer of disordered-carbon (d-C) enclosing an inner layer of crystalline-carbon (c-C). The d-C exhibits about three times volume expansion of the c-C after full sodiation or potassiation, thus suggesting a much higher storage capacity of Na or K ions in d-C than c-C. For the bilayer CNF-based electrode, a steady sodium capacity of 245 mAh/g is measured with a Coulombic efficiency approaching 98% after a few initial cycles. The in situ TEM experiments also reveal the mechanical degradation of CNFs through formation of longitudinal cracks near the c-C/d-C interface during sodiation and potassiation. Geometrical changes of the tube are explained by a chemomechanical model using the anisotropic sodiation/potassiation strains in c-C and d-C. Our results provide mechanistic insights into the electrochemical reaction, microstructure evolution and mechanical degradation of carbon-based anodes during sodiation and potassiation, shedding light onto the development of carbon-based electrodes for NIBs and KIBs.

Sponge-like porous carbon/tin composite anode materials for lithium ion batteries

A novel sponge-like porous C/Sn composite is synthesized by dispersing SnO2 nanoparticles into a soft-template polymer matrix followed by carbonization. The mesoporous C/Sn anodes can deliver a capacity as high as 1300 mAh g−1 after 450 charge/discharge cycles, and provide a capacity of 180 mAh g−1 even at 4000 mA g−1 charge/discharge current density. An extra reversible capacity over the theoretical value of the porous C/Sn anode is observed, which is attributed to the reversible formation/decomposition of the gel-like polymers formed on the mesoporous C/Sn composite due to the catalytic effect of the Sn nanoparticles. The high capacity, long cycle life, high power, ∼100% Coulombic efficiency, and inexpensive production method make the sponge-like porous C/Sn composite an attractive anode material in Li-ion batteries for electric vehicles and renewable energy storage.