Yujing Sha

Trapping sulfur in hierarchically porous, hollow indented carbon spheres: a high-performance cathode for lithium–sulfur batteries

Hierarchically porous hollow carbon spheres with an indented void structure have been designed as hosts for high-performance cathode materials for lithium–sulfur batteries. With a diameter of approximately 100 nm and a pore volume of 3.72 cm3 g−1, the hosts can retain sulfur within the porous structures, including the external cone-like cavities, the porous carbon shells, and the inner linings. The exquisite indented structure provides excellent electron and Li-ion pathways while the symmetrically indented voids evenly alleviate the stress induced by the volume change during cycling. The oxygen functional groups further relieve the shuttle effect of polysulfide. A composite electrode with 52% sulfur loading demonstrates a remarkable initial discharge capacity of 1478 mA h g−1 at 1/10C (1C = 1675 mA g−1), corresponding to 88% sulfur utilization. Even when the sulfur/carbon (S/C) ratio of the composite is increased threefold from 1 : 1 to 3 : 1 (75% sulfur loading), a very high capacity retention is still maintained, achieving an ultraslow rate of capacity fading, ∼0.047% per cycle over 1200 cycles at 1/2C.

Synthesis of well-crystallized Li4Ti5O12 nanoplates for lithium-ion batteries with outstanding rate capability and cycling stability

As a lithium-intercalation material, high crystallinity is important for Li4Ti5O12 to achieve good capacity and cycling stability, while a large surface area and a short lithium diffusion distance are critical to increase rate capacity. In this study, well-crystallized Li4Ti5O12 nanoplates with outstanding electrochemical performance were facially prepared through a two-step hydrothermal preparation with benzyl alcohol–NH3·H2O (BN) as the solvent and a subsequent intermediate-temperature calcination at 500 °C for 2 h in air. To support the superiority of benzyl alcohol–NH3·H2O (BN) for hydrothermal synthesis, ethanol–NH3·H2O (EN) was also comparatively studied as solvent. In addition, different hydrothermal reaction times were tried to locate the optimal reaction time. The nature of as-prepared Li4Ti5O12–BN (LTO–BN) and Li4Ti5O12–EN (LTO–EN) was characterized by XRD, N2 adsorption/desorption tests, SEM, TEM and TGA-DSC. Compared with EN, the BN hydrothermal solvent facilitated the formation of nanosheet-Li4Ti5O12 with wall thicknesses of 8–15 nm and better crystallization. After a 6 h hydrothermal reaction at 180 °C and subsequent calcination, well-crystallized Li4Ti5O12–BN nanoplates were produced, which demonstrate a superior discharge capacity of 160 mA h g−1, even at 40 C, maintaining a capacity of 88.8% compared with that at 1 C. The nanoplates also exhibited excellent cycling stability, retaining a discharge capacity of 153 mA h g−1 after 1000 charge–discharge cycles at 10 C.

The solid-state chelation synthesis of LiNi1/3Co1/3Mn1/3O2 as a cathode material for lithium-ion batteries

A facile solid-state chelation method using citric acid as the solid chelant was investigated for the synthesis of layered LiNi1/3Co1/3Mn1/3O2 as a cathode material for rechargeable lithium-ion batteries. The reaction was promoted by high-energy ball milling. During the synthesis, PVP was used as an additive. For comparison, LiNi1/3Co1/3Mn1/3O2 was also synthesized by a conventional sol–gel method using citric acid as the chelant. The as-prepared samples were characterized by TG-DSC, XRD, FESEM, BET specific surface area and galvanostatic charge–discharge tests. Based on the XPS, TEM and ED results, the sample synthesized by the solid-state chelation method with the PVP as an additive and subsequent calcination at 900 °C for 12 h in air was well indexed to a pure-phase hexagonal α-NaFeO2 structure with the highest crystallinity. The resulting sample showed an initial discharge capacity of 173 mA h g−1 in the potential range of 2.6–4.5 V and at a rate of 0.1 C, higher than that of the sample prepared by the same method without the use of a PVP additive during the synthesis (146 mA h g−1). Moreover, the electrochemical results at different current rates and the cycle performance for 100 cycles at 0.5 C indicated that the sample prepared by the solid-state chelation method exhibited better rate capability and cyclic stability than that prepared by the conventional sol–gel method. This phenomenon promises solid-state chelation as a new universal method for the preparation of functional materials.

Modified template synthesis and electrochemical performance of a Co3O4/mesoporous cathode for lithium–oxygen batteries

Rechargeable lithium–oxygen batteries (LOBs) with much higher energy density than conventional lithium-ion batteries are supposed to be the next generation of electrochemical energy storage devices. The oxygen electrode is the key component that determines the capacity and cycling performance of this type of battery. In this study, a Co3O4/mesoporous carbon composite (Co3O4/C) with a carbon content of 42 wt%, a rich mesoporous pore content and a homogeneous distribution of Co3O4 nanoparticles over the carbon surface was prepared using a facile silica template method with sucrose as the carbon source, and H3BO3 as an agent for expanding the space between the silica and carbon to impregnate and accommodate the Co3O4 precursor. This composite was used directly as the oxygen electrode in LOBs without additional conductive carbon additives. Galvano charge–discharge tests showed that the capacity of the composite electrode based on the mass of the mesoporous carbon reached approximately 4500 mA h gcarbon−1 at a current density of 123 mA gcarbon−1. The cell was further successfully run for over 200 discharge–charge cycles at a fixed current density of 246 mA gcarbon−1 and a trapped capacity of 740 mA h gcarbon−1, which indicated the superior electrochemical performance of the electrode. Different materials have been comparatively tested as oxygen electrodes, including Super P (SP), SP + nano Co3O4, carbon derived from KIT-6 (KIT-6-C), and the as-prepared Co3O4/C, among which the current Co3O4/C composite showed the best performance. The proposed Co3O4/C material has significant potential as an electrode material for rechargeable Li–O2 batteries.

3D amorphous carbon and graphene co-modified LiFePO4 composite derived from polyol process as electrode for high power lithium-ion batteries

Abstract Amorphous carbon and graphene co-modified LiFePO4 nanocomposite has been synthesized via a facile polyol process in connection with a following thermal treatment. Various characterization techniques, including XRD, Mossbauer spectra, Raman spectra, SEM, TEM, BET, O2-TPO, galvano charge-discharge, CV and EIS were applied to investigate the phase composition, carbon content, morphological structure and electrochemical performance of the synthesized samples. The effect of introducing way of carbon sources on the properties and performance of LiFePO4/C/graphene composite was paid special attention. Under optimized synthetic conditions, highly crystalized olivine-type LiFePO4 was successfully obtained with electron conductive Fe2P and FeP as the main impurity phases. SEM and TEM analyses demonstrated the graphene sheets were randomly distributed inside the sample to create an open structured LiFePO4 with respect to graphene, while the glucose-derived carbon mainly coated over LiFePO4 particles which effectively connected the graphene sheets and LiFePO4 particles to result in a more efficient charge transfer process. As a result, favorable electrochemical performance was achieved. The performance of the amorphous carbon-graphene co-modified LiFePO4 was further progressively improved upon cycling in the first 200 cycles to reach a reversible specific capacity as high as 97 mAh·g−1 at 10 C rate.

Facile Conversion of Commercial Coarse-Type LiCoO2 to Nanocomposite-Separated Nanolayer Architectures as a Way for Electrode Performance Enhancement

Coarse-type LiCoO2 is the state-of-the-art cathode material in small-scale lithium-ion batteries (LIBs); however, poor rate performance and cycling stability limit its large-scale applications. Here we report the modification of coarse-type LiCoO2 (LCO) with nanosized lithium lanthanum titanate (Li3xLa2/3-xTiO3, LLTO) through a facile sol-gel process, the electrochemical performance of commercial LiCoO2 is improved effectively, in particular at high rates. The crystalline structure of pristine LiCoO2 is not affected by the introduction of the LLTO phase, while nanosized LLTO particles are likely incorporated into the space of the LiCoO2 layers to form a LCO-LLTO nanocomposite, which separate the LCO layers with the increase of layer spacing to ∼100 nm. The LLTO incorporation through the facile post-treatment effectively reduces the charge-transfer resistance and increases the electrode reactions; consequently, the LLTO-incorporated LCO electrode shows higher capacity than LiCoO2 at a higher rate and prolonging cycling stability in both potential ranges of 2.7-4.2 V and 2.7-4.5 V, making it also suitable for high-rate operation. This novel concept is general, which may also be applicable to other electrode materials. It thus introduces a new way for the development of high rate-performance electrodes for LIBs for large scale applications such as electric vehicles and electrochemical energy storage for smart grids.

Free-standing nitrogen doped V-O-C nanofiber film as promising electrode for flexible lithium-ion batteries

Flexible vanadium and nitrogen co-modified amorphous carbon nonwoven films are successfully synthesized and applied as free-standing film anodes for lithium-ion batteries (LIBs). The developed films exhibit outstanding capacity with an initial Coulombic efficiency of 63.3%. Furthermore, favorable rate capability and good cycling stability are achieved. Cycling at a current density of 0.1 A g−1, the specific capacity of the nitrogen doped V-O-C nanofiber film reaches as high as 1380 mA h g−1, showing considerable promise for use as an electrode for flexible lithium-ion batteries.