Bohang Song

Ultrasmall Fe3O4 Nanoparticle/MoS2 Nanosheet Composites with Superior Performances for Lithium Ion Batteries

A novel composite consisting of graphene-like MoS₂ nanosheets and ultrasmall Fe₃O₄ nanoparticles (≈3.5 nm) is synthesized as an anode for lithium ion battery application. In such composite anode, MoS₂ nanosheets provide flexible substrates for the nanoparticle decoration, accommodating the volume changes of Fe₃O₄ during cycling process; while Fe₃O₄ nanoparticles primarily act as spacers to stabilize the composite structure, making the active surfaces of MoS₂ nanosheets accessible for electrolyte penetration during charge/discharge processes. Owing to the high reversible capacity provided by the MoS₂ nanosheets and the superior high rate performance offered by ultrasmall Fe₃O₄ nanoparticles, superior cyclic and rate performances are achieved by FeFe₃O₄/MoS₂ anode during the subsequent electrochemical tests, delivering 1033 and 224 mAh g⁻¹ at current densities of 2000 and 10,000 mA g⁻¹, respectively.

High rate capability caused by surface cubic spinels in Li-rich layer-structured cathodes for Li-ion batteries.

Modified Li-rich layered cathode Li(Li0.2Mn0.54Ni0.13Co0.13)O2 has been synthesized by a simple strategy of using surface treatment with various amounts (0-30 wt.%) of Super P (carbon black). Based on detailed characterizations from X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS), it is suggested that the phase transformation from Li2MnO3-type of structure to spinel-like phase take place at the surface regions of particles during post annealing process at 350 °C, leading to increase in both first coulombic efficiency and rate capability, from 78% and 100 mAh · g(-1) (charge capacity at 2500 mA · g(-1)) of the pristine material to 93.4% and 200 mAh · g(-1). The evidences of spinel formation and the reasons for electrochemical enhancement are systematically investigated.

Graphene-based surface modification on layered Li-rich cathode for high-performance Li-ion batteries

A Li-rich cathode material Li(Li0.2Mn0.54Ni0.13Co0.13)O2 synthesized by a sol–gel method is further modified by wrapping with graphene oxide (GO) using a simple chemical approach and post thermal annealing. X-ray diffraction (XRD), Raman spectroscopy, high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) characterizations show that annealing of the GO-wrapped material leads to a transformation from the original layered structure to a spinel phase on the surfaces of the powder particles. The electrochemical performance, and in particular the rate capability, is observed to be significantly enhanced as a result of this unique surface-treated Li-rich cathode/graphene composite structure. The modified sample reveals a very high discharge capacity of 313 mA h g−1 at a current density of 12.5 mA g−1, and 201 mA h g−1 charge capacity at an extremely high current density of 2500 mA g−1. Such improvement is ascribed to the co-existence of reduced GO, the locally transformed spinel-like structure and the recrystallized particles on the surfaces of the primary particles.

One-step synthesis of hollow porous Fe3O4 beads–reduced graphene oxide composites with superior battery performance

We report the synthesis of a novel hollow porous Fe3O4 bead–rGO composite structure for lithium ion battery anode application via a facile solvothermal route. The formation of hollow porous Fe3O4 beads and reduction of graphene oxide (GO) into rGO were accomplished in one step by using ethylene glycol (EG) as a reducing agent. In this composite structure, the hollow porous Fe3O4 beads were either chemically attached or tightly wrapped with rGO sheets, leading to a strong synergistic effect between them. As a result, the obtained Fe3O4–rGO composite electrodes could deliver a reversible capacity of 1039 mA h g−1 after 170 cycles between 3 V and 50 mV at a current density of 100 mA g−1, with an increment of 30% compared to their initial reversible capacity, demonstrating their superior cycling stability.

Ultra-small Fe3O4 nanoparticle decorated graphene nanosheets with superior cyclic performance and rate capability

Advanced anode materials for next generation lithium ion batteries have attracted great interest due to the ever increasing demand for powerful, light-weight, and compact electrical devices. In this work, graphene nanosheets decorated with ultra-small Fe3O4 nanoparticles (USIO/G) were synthesized via a facile hydrothermal method. Compared with other reported Fe3O4-based anode composites, USIO/G demonstrated superior cyclic ability and excellent rate capability owing to its ultra-small size of active lithium storage sites, Fe3O4, with an average diameter less than 5 nm. Furthermore, graphene nanosheets played an important role in the overall electrochemical performance of the composite by enhancing the electrical conductivity, forming a flexible network, and providing extra lithium storage sites. The obtained composites were tested for electrochemical performance for a total number of 2120 cycles: a rate capability test with current densities ranged from 90 to 7200 mA g(-1) for 920 cycles, followed by a cycling test at 1800 mA g(-1) for 1200 cycles. For the rate capability test, steady reversible capacities were delivered under each current density with final reversible capacities of 1177, 1096, 833, 488, 242, and 146 mA h g(-1) at 90, 180, 900, 1800, 3600, and 7200 mA g(-1), respectively. The subsequent cyclic test demonstrated the superior cyclic stability of USIO/G and a reversible capacity of 437 mA h g(-1) at the 2120(th) cycle was delivered.

A study of the superior electrochemical performance of 3 nm SnO2 nanoparticles supported by graphene

Owing to the discovery of its new reaction mechanism towards lithium, SnO2 has recently gained the attention from scientific field as a promising potential anode material. A theoretical capacity of 1494 mA h g−1 can be reached, provided that the SnO2 particles are reduced to ultra-small sizes. In addition, two other important aspects, namely cyclic stability and power density, can also be greatly enhanced by applying SnO2 particles with the appropriate dimensions. Therefore, size controlling of SnO2 nanoparticles is critical for their applications in lithium ion batteries (LIBs). In this work, SnO2 nanoparticles with an average diameter of 3 nm are decorated on graphene nanosheets. Owing to the small sizes of SnO2 nanoparticles and the electronic conductive graphene network, an anode consisting of SnO2 and graphene (SnO2/G) delivers a high first reversible capacity of 1239 mA h g−1 at an initial current density of 0.1 A g−1, and surprisingly a 574 mA h g−1 charge capacity under an exceptionally high current density of 10 A g−1. Meanwhile, superior cyclability is also achieved in view of increasing the reversible capacity up to 1813 mA h g−1 after over 1000 cycles under a high current density of 2 A g−1. Such stunning performance is carefully studied with various characterization techniques, including electrochemical measurements, TEM, and ex situ XRD. The high reversible capacity of SnO2/G is attributed to the 3 nm sized SnO2 nanoparticles, which almost doubled the theoretical capacity of the oxide. Additionally, the excellent performance under high current densities is ascribed to the enhanced lithium and electron diffusion, resulting from the significantly shortened lithium diffusion length within each SnO2 particle and the conductive graphene network, respectively. In addition, the prominent increase in reversible capacity upon cycling is explained by the increasing number of active lithium storage sites and polymeric gel-like film formation during the prolonged charge–discharge process.

Role of carbon coating in improving electrochemical performance of Li-rich Li(Li0.2Mn0.54Ni0.13Co0.13)O2 cathode

Li-rich Li(Li0.2Mn0.54Ni0.13Co0.13)O2 cathode coated with carbon layer has been prepared by a hydrothermal approach followed by a post annealing process. The cathode after surface modification exhibits both enhanced cyclability and improved rate capability compared with the pristine one without coating. The carbon coating process causes a phase transformation from Li2MnO3-like domain to cubic-spinel domain with Fdm symmetry at surface regions of particles. As a consequence, the valence state of Mn on the surface accordingly varies. Such transformed surface spinels, as well as the wrapped carbon layers as a result of this coating strategy are believed to be responsible for the enhanced electrochemical performance.

Li-rich Thin Film Cathode Prepared by Pulsed Laser Deposition

Li-rich layer-structured cathode thin films are prepared by pulsed laser deposition. X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscopy (XPS) and electrochemical testing in half battery cells are used to characterize crystal structure, surface morphology, chemical valence states and electrochemical performance of these thin films, respectively. It is observed that partial layer to spinel transformation takes place during post annealing, and the layered structure further gradually transforms to spinel during electrochemical cycling based on the analysis of dQ/dV. Electrochemical measurement shows that the thin film electrode deposited at 350 mTorr and post-annealed at 800°C possesses the best performance.

ELECTROCHEMICAL PROPERTY OF LiMn2O4 IN OVER-DISCHARGED CONDITIONS

This work studies LiMn2O4 in an over-discharged condition. Electrochemical measurement shows that the LiMn2O4 electrode undergoes an irreversible electrochemical reduction process where its structure is permanently destroyed during over-discharge. Although LiMn2O4 shows a poor over-discharge durance with a reduction starting at 2.5 V vs. Li/Li+, the galvanostatic test indicates that LiMn2O4 can be considered to be used as a high capacity anode material for lithium ion batteries with an initial charge capacity of 814 mAh ⋅ g-1 and 452 mAh ⋅ g-1 at the 100th cycle.