G. R. Li

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.

A 3D hierarchical porous α-Ni(OH)2/graphite nanosheet composite as an electrode material for supercapacitors

Supercapacitors are the most promising energy storage devices by virtue of high power density, long cycle life, short charging time and environmental benignity. In order to enhance the energy density, rate capability and cycle stability for supercapacitors, a α-Ni(OH)2/graphite nanosheet composite is prepared via a homogeneous precipitation method. The morphology and microstructure of the as-prepared composite are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. It is demonstrated that after introducing the graphene oxide nanosheets into α-Ni(OH)2, a 3D hierarchical porous structure of fine α-Ni(OH)2 nanocrystals as building blocks is formed directly on the matrix of graphite nanosheets in the presence of urea as a mild reducing agent. The electrochemical performance of the as-prepared α-Ni(OH)2 and α-Ni(OH)2/graphite nanosheet composites as electro-active materials for supercapacitors is investigated by a galvanostatic charge–discharge method. As expected, the as-prepared α-Ni(OH)2/graphite nanosheet composite exhibits large specific capacitance, good rate capability and long cycle stability as compared to the pure α-Ni(OH)2. Apparently, the unique structure of fine α-Ni(OH)2 nanocrystals fabricated on the matrix of graphite nanosheets is responsible for the improvement of the reaction kinetics and subsequent electrochemical performance of the composite.

Protected lithium anode with porous Al2O3 layer for lithium–sulfur battery

The performance of the metallic lithium anode is one of the major factors that affect the cycle stability of a lithium–sulfur battery. The protection of the lithium anode is extremely essential, especially for lithium–sulfur full-cells. Here, a porous Al2O3 layer is fabricated on the surface of a metallic lithium anode by using a spin-coating method as protective layer for a lithium–sulfur battery. The porous Al2O3 protective layer acts as a stable interlayer and suppresses the side reactions between soluble lithium polysulfides and lithium anode by direct contact during the charge–discharge process. In addition, the inhomogeneous dissolution–deposition reaction, and the formation of serious cracks on the protected lithium anode are suppressed to a certain extent, which is beneficial to ensure the good and stable electrochemical activity of the lithium anode. Correspondingly, the sulfur cathode with the protected lithium anode exhibits improved electrochemical performance, accompanied simultaneously with relatively homogeneous lithium deposition on the anode surface due to the even distribution of Li ion flux via the Al2O3 protective layer.

Sn-stabilized Li-rich layered Li(Li0.17Ni0.25Mn0.58)O2 oxide as a cathode for advanced lithium-ion batteries

Li-rich layered oxides have been intensively investigated as cathodes for high energy lithium-ion batteries. However, oxygen loss from the lattice during the initial charge and gradual structural transformation during cycling can lead to capacity degradation and potential decay of the cathode materials. In this work, Sn4+ is used to partially substitute Mn4+ to prepare a series of Li(Li0.17Ni0.25Mn0.58−xSnx)O2 (x = 0, 0.01, 0.03, and 0.05) samples through a spray-drying method. Structural characterization reveals that the Sn4+ substituted samples with a suitable amount show low cation mixing, indicating an enhanced ordered layer structure. Moreover, the metal–oxygen (M–O) covalency is gradually decreased with increasing Sn4+ amount. It is shown from the initial charge–discharge curves that Sn4+ substituted samples present a shorter charging potential plateau at 4.5 V (vs. Li/Li+), implying that oxidation of the O2− ion to O2 is suppressed by Sn4+ substitution and leads to a minor structural change. Among the Sn4+ substituted samples, the Li(Li0.17Ni0.25Mn0.55Sn0.03)O2 sample exhibits a higher capacity retention of 86% after 400 cycles at 0.1C rate and 92% after 200 cycles at 1C rate, showing excellent cycle stability and high-rate capability as compared with the as-prepared sample. The electrochemical performance improvement can be attributed to the influences of Sn such as enlarging the Li ion diffusion channel due to the large ionic radius of Sn4+ substitution with respect to Mn4+, a higher bonding energy of Sn–O than Mn–O, and weakening the M–O covalency. All the influences are favorable for stabilization of the host lattice in Li-rich layered oxides.

Solar rechargeable redox flow battery based on Li2WO4/LiI couples in dual-phase electrolytes

A solar rechargeable redox flow battery is fabricated with Li2WO4 as anode in aqueous electrolyte, LiI as cathode in organic electrolyte, and LISICON film as membrane to separate liquid anode/cathode-active species. The as-fabricated battery presents feasible solar rechargeable capability.

Synergistic effect of molybdenum nitride and carbon nanotubes on electrocatalysis for dye-sensitized solar cells

As a counter electrode for dye-sensitized solar cells (DSSCs), MoN presents a high intrinsic electrocatalytic activity for the reduction of triiodide ions. However, the photovoltaic performance of DSSCs with a MoN counter electrode is hindered by the large diffusion impedance of the MoN electrode. In response to this problem, a MoN–carbon nanotube (CNT) composite is prepared by nitridation of the precursor MoO2–CNTs, fabricated via a hydrothermal reaction of ammonium molybdate and carboxyl-functionalized CNTs. In the composite, MoN nanoparticles are well and stably dispersed on the surface of the CNTs, with a particle size of several tens of nanometers. Employing the composite as a counter electrode, the DSSC shows an energy conversion efficiency of 6.74%, which is much higher than that (5.57%) of the DSSC using pure MoN nanoparticles. The improvement is mainly attributed to a synergistic effect between the MoN nanoparticles and CNTs on ion diffusion and electrocatalysis. Electrochemical impedance spectra (EIS) indicate that the MoN–CNTs electrode has a lower ion diffusion impedance. It is believed that the smaller size of the MoN nanoparticles and the abundant porous structure in the MoN–CNTs composite are able to shorten the ion diffusion path and improve ion diffusion flux.

Encapsulating sulfur into a hybrid porous carbon/CNT substrate as a cathode for lithium–sulfur batteries

A hybrid carbon substrate as a sulfur immobilizer is obtained via simple processes to fabricate cathode materials for lithium–sulfur batteries. The microstructure and morphology of the sulfur/carbon composites are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is demonstrated that commercial carbon black and multi-walled carbon nanotubes (CNTs) in the hybrid substrate cooperate well with each other in an appropriate mass ratio. In particular, a large sulfur content of 81.7 wt% can be loaded into the hybrid carbon substrate forming the sulfur/carbon composite. When the mass ratio of carbon black and CNTs is 1 : 1, the composite delivers a high initial capacity of 837.3 and 685.9 mA h g−1(composite) at the current densities of 80 and 160 mA g−1(composite) when used as a cathode-active material. The discharge capacity remains at 554.4 mA h g−1(composite) at a current density of 160 mA g−1(composite) after 150 cycles, indicating a low capacity fading of about 0.12% per cycle. Besides, the composite offers a high Coulombic efficiency of about 100%. The significant improvements in the electrochemical performance are associated with the desirable combination of carbon black and CNTs in the hybrid carbon substrate. Therefore, this work proposes a low-cost and effortless approach to prepare sulfur/carbon composites with high performance as cathodes for lithium–sulfur batteries.

High performance LiMnPO4/C prepared by a crystallite size control method

A carbon matrix, for restricting growth of LiMnPO4 crystallites, is built on the small Li3PO4 crystallites precipitated from aqueous solutions, by the pyrolysis of sucrose. LiMnPO4 is prepared using the carbon coated Li3PO4 as one of the reactants (the other reactant is MnSO4) and the nuclei by a solvothermal method. Smaller-crystallite-size (8–12 nm) LiMnPO4 is successfully obtained on the carbon matrix by a crystallite size control method. The as-prepared LiMnPO4/C sample presents the desired electrochemical performance, including higher discharge potential plateau, larger discharge capacity, excellent high-rate capability, and good cycle stability. It is also confirmed that the smaller LiMnPO4 crystallites on the carbon matrix are beneficial for shortening the lithium ion diffusion path and increasing the electrical conductivity of the LiMnPO4/C sample, contributing to an improvement in electrochemical performance. The methodology described in this work could be helpful in the development of LiMnPO4/C cathode materials for lithium ion batteries with high energy density.

Tailoring atomic distribution in micron-sized and spherical Li-rich layered oxides as cathode materials for advanced lithium-ion batteries

Li-rich layered oxides with large capacity are considered as one of the most promising cathode materials for the next generation lithium-ion batteries (LIBs). However, Li-rich layered oxides usually deliver unsatisfactory volumetric energy density, poor cycle life and inferior thermal stability. Here, a concentration-gradient doping strategy is introduced for the first time to meet the above challenges. Surprisingly, the atomic distribution in micron-sized and spherical Li-rich layered oxides is tailored after concentration-gradient PO43− polyanion doping, in which Ni and Co atoms decrease continually and Mn atoms increase gradually from the center to the surface in a single particle. As expected, the concentration-gradient PO43− doped oxides exhibit a high initial volumetric energy density of 2027 W h L−1, long cycle life with a capacity retention of 88.2% within 400 cycles, and enhanced thermal stability. These improved performances are believed to be attributed to the formation of the stable Mn-rich and PO43−-rich shell layer, which is beneficial to mitigate the interreaction between Ni4+/Co4+ and the electrolyte in the highly delithiated state and suppress the aggregation of primary grains during cycles. These results demonstrate the feasibility of manipulating atomic distribution by the innovative concentration-gradient doping means, which also provides new insights into desired cathode for LIBs.