Xiaohui Ning

Enhanced conversion reaction kinetics in low crystallinity SnO2/CNT anodes for Na-ion batteries

The specific capacities of SnO2 anodes in sodium ion batteries (SIBs) are far below the values expected from theory. Herein, we propose that the kinetically-controlled, reversible ‘conversion reaction’ between Na ions and SnO2 is responsible for Na ion storage in SnO2 anodes where the ion diffusion rate is the limiting factor. This revelation is contrary to the current understanding of the ‘alloying reaction’ as the major reaction process. Aiming to fully utilize the theoretical capacity from the conversion reaction, a composite electrode consisting of carbon nanotubes coated with a mainly amorphous SnO2 phase together with crystalline nanoparticles is synthesized. The SnO2/CNT anodes deliver a superior specific capacity of 630.4 mA h g−1 at 0.1 A g−1 and 324.1 mA h g−1 at a high rate of 1.6 A g−1 due to the enhanced kinetics. The volume expansion of the composite is accommodated by the CNT substrate, giving rise to an excellent 69% capacity retention after 300 cycles. The aforementioned findings give new insight into the fundamental understanding of the electrochemical kinetics of SnO2 electrodes and offer a potential solution to the low capacity and poor cyclic stability of other metal oxide anodes based on conversion reactions.

Electrochemical dissolution behavior of conductive TiC x O 1-x solid solutions*

Conductive TiCxO1–x solid solutions were prepared by carbothermic reduction of titanium dioxide. Studies were focused on the possibility of electrochemically dissolving TiCxO1–x in NaCl–KCl molten salt. The tail-gas from the anode was monitored during the electrolysis. It was discovered that carbon monoxide (CO) or carbon dioxide (CO2) gases were generated, with the process being dependent upon the consumption of the TiCxO1–x solid solution anode materials. Furthermore, a series of electrochemical methods was used to investigate the valence state of titanium ions dissolved into molten salt when electrolyzing TiCxO1–x solid solutions. A significant result was that titanium ion species dissolved from the TiCxO1–x solid solutions, and this is changed between Ti2+ and Ti3+ depending on the electrochemically dissolving potentials. The significant result discovered in this paper will be potentially beneficial in the preparation of high-purity titanium by electrorefining TiCxO1–x solid solutions.

Thermal treatment-induced ductile-to-brittle transition of submicron-sized Si pillars fabricated by focused ion beam

Si pillars fabricated by focused ion beam (FIB) had been reported to have a critical size of 310–400 nm, below which their deformation behavior would experience a brittle-to-ductile transition at room temperature. Here, we demonstrated that the size-dependent transition was actually stemmed from the amorphous Si (a-Si) shell introduced during the FIB fabrication process. Once the a-Si shell was crystallized, Si pillars would behave brittle again with their modulus comparable to their bulk counterpart. The analytical model we developed has been proved to be valid in deriving the moduli of crystalline Si core and a-Si shell.

In situ transmission electron microscopy study of the electrochemical sodiation process for a single CuO nanowire electrode

In this work, an in situ transmission electron microscopy (TEM) study of the electrochemically driven sodiation and desodiation of a CuO nanowire (NW) was performed. Upon sodiation, Na ions first reacted with CuO, yielding a mixture of Cu, Cu2O, and Na2O, and then some Cu2O was subsequently reduced into nanocrystal Cu. The final sodiation product was nanocrystalline Cu mixed with Na2O and Cu2O. Upon extraction of Na+, the nanocrystalline Cu first oxidized into Cu2O and finally transformed back to nanocrystalline CuO. The volume of the CuO NW was found to expand markedly during the first sodiation, but the morphology of the NW remained unchanged during the following cycles. The mechanism by which high-performance CuO NW anodes are used for rechargeable Na ion batteries was analyzed by carrying out an in situ TEM technique based on our results, and this study may be beneficial for designing the optimal structure of the CuO anode, improving its cycle performance, and making CuO more feasible for Na ion batteries.

In situ TEM observing structural transitions of MoS2 upon sodium insertion and extraction

In this study, the sodiation and desodiation processes of MoS2 were characterized by using an in situ TEM technique. The structural evolution of MoS2 and its performance in a coin-type cell are recognized. Our findings provide a fundamental understanding of the reaction mechanism of MoS2 as anode for Na ion batteries.

Reduced expansion and improved full-cell cycling of a SnOx#C embedded structure for lithium-ion batteries

SnOx exhibits a much larger theoretical capacity compared to graphite as an anode material in lithium-ion batteries (LIBs). However, the cycling stability and initial coulombic efficiency (ICE) of SnOx based electrodes need to be improved. In this study, by coating carbon on a dried SnOx electrode film using one-step chemical vapor deposition, a SnOx#C composite is obtained, wherein ∼70 nm sized SnOx nanoparticles are uniformly dispersed and embedded in a carbon matrix. Owing to its relatively small electrochemical surface area and mechanical robustness, the ICE is largely improved from ∼40% to >65%. Besides, a Li-matched full cell with 36% excess LiCoO2 cathode material can run stably for more than 100 cycles at 0.1 A g−1, delivering a gravimetric and a volumetric capacity of 456 mA h g−1 and 644 mA h cm−3, respectively, which are superior to graphite. The lithiation/delithiation process of SnOx#C observed using an in situ transmission electron microscope technique reveals that the embedded structure expands by only ∼5%. Besides, the thickness increment of the electrode film after 100 cycles is measured to be 32%, which is much smaller than the acceptable 50% in the LIB industry, illustrating the good stability of the solid-electrolyte interphase (SEI) skeleton.