X. P. Gao

Enhancement of long stability of sulfur cathode by encapsulating sulfur into micropores of carbon spheres

To enhance the long stability of sulfur cathode for a high energy lithium–sulfur battery system, a sulfur–carbon sphere composite was prepared by encapsulating sulfur into micropores of carbon spheres by thermal treatment of a mixture of sublimed sulfur and carbon spheres. The elemental sulfur exists as a highly dispersed state inside the micropores of carbon spheres with a large surface area and a narrow pore distribution, based on the analyses of the X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), thermogravimetry (TG) and local element line-scanning. It is demonstrated from galvanostatic discharge–charge process, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) that the sulfur–carbon sphere composite has a large reversible capacity and an excellent high rate discharge capability as cathode materials. In particular, the sulfur–carbon sphere composite with 42 wt% sulfur presents a long electrochemical stability up to 500 cycles, based on the constrained electrochemical reaction inside the narrow micropores of carbon spheres due to strong adsorption. Therefore, the electrochemical reaction constrained inside the micropores proposed here would be the dominant factor for the enhanced long stability of the sulfur cathode. The knowledge acquired in this study is important not only for the design of efficient new electrode materials, but also for understanding the effect of the micropores on the electrochemical cycle stability.

Highly Pt-like electrocatalytic activity of transition metal nitrides for dye-sensitized solar cells

Pt-like electrocatalytic activity of MoN, WN, and Fe2N for dye-sensitized solar cells (DSSCs) is demonstrated in this work. Among the transition metal nitrides, MoN has superior electrocatalytic activity and a higher photovoltaic performance. This work presents a new approach for developing low-cost and highly-efficient counter electrodes for DSSCs.

Aluminum storage behavior of anatase TiO2 nanotube arrays in aqueous solution for aluminum ion batteries

The electrochemical aluminum storage of anatase TiO2 nanotube arrays in AlCl3 aqueous solution is investigated. It is firstly demonstrated that aluminum ions can be reversibly inserted/extracted into/from anatase TiO2 nanotube arrays in AlCl3 aqueous solution due to the small radius steric effect of aluminum ions, indicating a potential application in aluminum ion batteries.

Surface nitridation of Li-rich layered Li(Li0.17Ni0.25Mn0.58)O2 oxide as cathode material for lithium-ion battery

A Li-rich layered Li(Li0.17Ni0.25Mn0.58)O2 oxide is prepared by a combination of co-precipitation and solid-state reaction. The surface nitridation is introduced into a Li-rich layered Li(Li0.17Ni0.25Mn0.58)O2 for the first time via heating at 400 °C in the ammonia atmosphere. The microstructure and morphology of the two samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It is demonstrated that the nitrogen exists with a trace amount in the surface layer of the Li-rich layered Li(Li0.17Ni0.25Mn0.58)O2 oxide after the nitridation treatment. Electrochemical performances of the electrodes are measured by galvanostatic charge–discharge tests, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). As expected, the discharge capacity, high-rate capability and cycle stability of the nitrided sample are improved dramatically as compared with the as-prepared sample, which is further confirmed by the high electrocatalytic activity and accelerated lithium diffusion process. Apparently, the existence of nitrogen in the surface layer is responsible for the improvement of the reaction kinetics and electrochemical performance of the nitrided sample.

Surface modification of Li-rich layered Li(Li0.17Ni0.25Mn0.58)O2 oxide with Li–Mn–PO4 as the cathode for lithium-ion batteries

Enhancement of the discharge capacity, high-rate capability, and cycle stability of the Li-rich layered Li(Li0.17Ni0.25Mn0.58)O2 oxide with a large specific capacity is highly significant for high energy lithium-ion batteries. In this work, the Li-rich layered Li(Li0.17Ni0.25Mn0.58)O2 oxide is prepared by a spray-drying method. The surface modification with the Li–Mn–PO4 is introduced onto Li-rich layered Li(Li0.17Ni0.25Mn0.58)O2 oxide for the first time. It is demonstrated that the surface of Li(Li0.17Ni0.25Mn0.58)O2 grains is coated with the thin amorphous Li–Mn–PO4 layer (5 wt%). With increasing calcination temperature after the surface coating, a strong interaction can be induced on the interface between the amorphous Li–Mn–PO4 layer and the top surface of Li(Li0.17Ni0.25Mn0.58)O2 grains. As anticipated, the discharge capacity and high-rate capability are obviously improved for the Li–Mn–PO4-coated sample after calcination at 400 °C, while excellent cycle stability is obtained for the Li–Mn–PO4-coated sample after calcination at 500 °C as compared with the as-prepared Li(Li0.17Ni0.25Mn0.58)O2 oxide during cycling. Apparently, the interface interaction between the amorphous Li–Mn–PO4 layer and the top surface of Li(Li0.17Ni0.25Mn0.58)O2 grains is responsible for the improvement of the reaction kinetics and the electrochemical cycle stability of Li–Mn–PO4-coated samples.

PO43− polyanion-doping for stabilizing Li-rich layered oxides as cathode materials for advanced lithium-ion batteries

Advanced Li-ion batteries, with Li-rich layered oxides as cathode materials and Si-based composites as anode materials, are considered as high energy battery systems for the next generation of smart communications and electric vehicles (EVs). At the current stage, it is significant to develop Li-rich layered oxides with stable output energy density. However, the gradual capacity degradation and potential decay during cycling lead to the continual decrease in the energy density of Li-rich layered oxides. Therefore, a new strategy should be introduced to block the migration of transition metal cations and maintain the parent layered structure during cycling, in order to stabilize the energy density of oxides. In this work, large tetrahedral PO43− polyanions with high electronegativity with respect to O2− anions are doped into oxides for minimizing the local structure change during cycling. When doping with PO43− polyanions, the parent layered structure is retained during long cycling, due to the strong bonding of PO43− polyanions to transition metal cations (Ni especially). Correspondingly, PO43− polyanion-doped oxides present excellent energy density retention during long cycling, integrated with the discharge capacity and midpoint potential. These results suggest that polyanion-doping can meet the performance requirement of stabilizing the energy density of Li-rich layered oxides for advanced lithium ion batteries.

Copper hexacyanoferrate nanoparticles as cathode material for aqueous Al-ion batteries

Copper hexacyanoferrate (CuHCF) nanoparticles with Prussian blue structure are prepared via a simple co-precipitation method, which present the ability to insert Al ions reversibly in aqueous solution. CuHCF is verified to be a promising cathode material for aqueous Al-ion batteries.

Morphology dependence of molybdenum disulfide transparent counter electrode in dye-sensitized solar cells

Molybdenum disulfide attracts additional attention due to its layered structure which allows transformation into a two-dimensional morphology, like graphene. In this paper, three kinds of molybdenum disulfides with distinguishable morphologies, i.e. multilayers, a few layers and nanoparticles, are prepared and used as counter electrode materials for dye-sensitized solar cells (DSSCs). The characterization results from X-ray diffraction (XRD) and transmission electron microscopy (TEM) demonstrate that the molybdenum disulfides have an obviously different edge area to basal-plane ratio, with the order: synthesized MoS2 nanoparticles (MoS2-NPs) > multilayered MoS2 (ML-MoS2) > few-layered MoS2 (FL-MoS2). It is interesting that the MoS2 counter electrodes show the same order as above in the energy conversion efficiency measurements of the corresponding DSSCs. Electrochemical impedance spectra (EIS) show that the MoS2-NPs electrode has the minimum charge-transfer resistance, while the FL-MoS2 electrode provides the maximum. Combined with the results from triiodine ion adsorption experiences and N2-adsorption measurements, it is proposed that the catalytically active sites of molybdenum disulfide lie on the edges of the typical layered material, but not on the basal planes. In addition, the transparency of the FL-MoS2 electrode is obviously higher than that of the other MoS2 and Pt electrodes.

Preparation of Li4Ti5O12 Nanorods as Anode Materials for Lithium-Ion Batteries

Li 4 Ti 5 O 12 nanorods are fabricated after calcination of the hydrated lithium titanate precursor, which is prepared from hydrothermal treatment of titanate nanorods in aqueous LiOH based on titanate nanorod reactivity. The morphology, composition, and phase transformation of the calcined samples at different temperatures were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Titanate nanorods as starting materials exhibit higher chemical reactivity, regarded as a structure template for retaining the one-dimensional structure of final products after calcination. The formation of Li 4 Ti 5 O 12 nanorods is related to ion-exchange reaction and Ostwald ripening process due to high chemical reactivity of titanate nanorods. The galvanostatic charge/discharge tests were conducted to measure the electrochemical performance of the Li 4 Ti 5 O 12 nanorods. It is demonstrated that the Li 4 Ti 5 O 12 nanorods calcined at 800°C have excellent high rate discharge capability and good cycle stability during insertion and extraction processes, owing to the good crystallinity, unique structure, and the short diffusion distances originated from one-dimensional morphology.

Electrochemical Performance of Anatase Nanotubes Converted from Protonated Titanate Hydrate Nanotubes

Titanium oxides with one-dimensional nanostructure are of significant interest for the electrochemical lithium insertion and extraction because of their large specific surface area and numerous surface defects. Nanotubes with 10-15 nm outer diameters and 200 ∼ 400 nm long were prepared by a reaction between rutile and caustic soda under hydrothermal conditions. These nanotubes are protonated titanate and can be converted into the anatase nanotubes after calcination at 500°C in argon atmosphere. The anatase nanotubes exhibited a high reversible discharge capacity, excellent high-rate discharge capability, and good cycle stability under the large current density.