Scalable Synthesis of Self-Assembled Metal-Oxide/Carbon Nanosheets for High Performance Lithium-Ion Battery Anodes

The assembly of uniform nanoparticles has attracted much attention since well-aligned ensembles of nanoparticles often show interesting properties that are different from those displayed by individual nanoparticles and bulk materials. Various methods were developed for periodically ordered arrays of the synthesized nanoparticles including “evaporation-driven” methods and “destabilization-driven” approaches. Herein, a simple and scalable method for the synthesis of self-assembled metal-oxide/carbon nanosheets is reported.

The carbon nanosheets embedded with self-assembled uniform metal-oxide nanoparticles were synthesized simultaneously through a solventless heating procedure using a single precursor for both metal-oxide and carbon. Two-dimensional nanostructured materials were got by using salt powder as a sacrificial template. The size and shape of the metal oxide nanoparticles embedded in carbon could be controlled by varying the experimental conditions. The nanostructured materials were characterized by using scanning electron microscopy, transmission electron microscopy and X-ray diffraction.

As a demonstration of the metal-oxide/carbon nanosheets as lithium-ion battery anode materials, electrochemical tests were carried out in a coin type cell assembly. Galvanostatic discharge/charge voltage profiles were got at a current density of 100 mA g⁻¹. The rate capability of the electrodes is also studied. At a high rate of 5000 mA g⁻¹, 81.5% of original capacity was retained for the manganese-ferrite/carbon nanosheets, whereas 68.7%, 56.8%, and 41.6% were retained for 16 nm iron-oxide/carbon nanosheets, 30 nm iron-oxide/carbon nanosheets, and 3- dimensional aggregated nanocomposites, respectively.

Comparing the iron-oxide/carbon nanosheets and three-dimensional aggregated nanocomposites, well-ordered iron-oxide/carbon nanosheets have several advantages in terms of resistance to mechanical deformation. The self-assembled nanostructure seems to induce the regularly spaced carbon walls between the nanoparticles, providing tensile strength in the vertical direction of the carbon sheet. In addition, iron-oxide/carbon nanosheets have enough room to expand in this direction than three-dimensional aggregated nanocomposites, which can provide a buffer for the volume change. As a result, these structural factors can contribute to improved cycle performance of the nanosheets. TEM measurements were made on the nanosheet electrodes after 10th charge/discharge cycling at a rate of 100 mA g⁻¹ to investigate the mechanical deformation during the electrochemical cycling. The iron-oxide nanosheet structure was sustained without peeling off. The encapsulating carbon shells seem to help to maintain the structure of iron-oxide/carbon nanosheet electrodes. The rate capacity of the electrodes is affected by the diffusion of lithium ions from the electrolyte to the active material.

The as-prepared hybrid nanosheets showed excellent cycling stability and rate performance as anode materials for lithium-ion batteries derived from two-dimensional nanostructural characteristics. The superior electrochemical performance of the nanosheets is due to their short diffusion path and large accessible surface for the effective insertion of lithium ions. Moreover, the size of the metal oxide nanoparticles strongly affected the electrochemical performance at high charging/discharging rates. In addition, the large Coulombic capacity at high discharge rates seems to have resulted from the uniform-sized ferrite nanoparticles.

The synthetic procedure is very simple, inexpensive, and scalable for mass production, and the highly ordered two-dimensional hybrid nanosheets have great potentials for various future applications.