Jinkai Wang

Honeycomb-like carbon nanoflakes as a host for SnO2 nanoparticles allowing enhanced lithium storage performance

While possessing potential advantages as electrodes for lithium-ion batteries, SnO2@carbon composites have been suffering from one common drawback – aggregation of Sn particles during the repeated alloying–dealloying cycles and the resulting pulverization issue. We combat this issue through the fabrication of honeycomb-like SnO2@carbon nanoflakes (SnO2@CNFs) that are able to confine SnO2 nanoparticles within well-separated carbon cavities, so that the Li–Sn alloying–dealloying reaction occurs in the independent microreactors thus avoiding aggregation of Sn metal particles formed. The SnO2 particle size, loading amount and the coverage density are controlled by adjusting the weight ratio between the tin precursor and the CNF. Transmission electron microscopy confirms that the highly graphitic honeycomb-like CNF matrix efficiently buffers and accommodates volume changes of the Li–Sn alloy. Used as anode materials for lithium-ion batteries, the SnO2@CNFs with 66.0 wt% SnO2 display the highest lithium storage capacity, delivering a discharge capacity of 940 mA h g−1 after 150 cycles at 200 mA g−1. For the long-term and high-rate applications, the SnO2@CNFs with 41.5 wt% SnO2 show the best electrochemical performance, delivering a discharge capacity of 400 mA h g−1 at 1 A g−1 after 500 cycles.

Template synthesis of graphitic hollow carbon nanoballs as supports for SnOx nanoparticles towards enhanced lithium storage performance

To address the volume change-induced pulverization problem of tin-based anodes, a concept using hollow carbon nanoballs (HCNBs) as buffering supports is herein proposed. HCNBs with hollow interior, flexibility and graphitic crystallization are first prepared by a combined method of chemical vapor deposition (CVD) and template-synthesis using CH4 as the carbon source and CaCO3 as the conformal template. The ultrafine SnO2 nanoparticles are loaded onto the HCNBs (denoted as SnO2@HCNBs) via pyrolysis of tin(ii) 2-ethylhexanoate at 300 °C in air. On further annealing SnO2@HCNBs in Ar, SnO2 is partially reduced to SnOx by consuming a part of carbon of HCNBs as the reducing agent, and thus SnOx@HCNBs are obtained (note that SnOx represents a composite consisting of SnO2, SnO and Sn phases). When applied as anode materials for lithium ion batteries (LIBs), HCNBs deliver high reversible capacities of 841 mA h g-1 after 125 cycles at 200 mA g-1, and 726 mA h g-1 after 400 cycles even at 1000 mA g-1, while SnO2@HCNBs and SnOx@HCNBs exhibit discharge capacities of 1042 and 1299 mA h g-1 after 400 cycles at 200 mA g-1, respectively. Notably, all of them display gradually increased capacity with retention over 100% even after long-term cycling, which is attributed to the novel robust characteristic of the HCNBs as revealed by the ex situ TEM analysis. The flexible hollow HCNBs with high graphitic crystallization not only efficiently tolerate the volume changes of the Li-Sn alloying-dealloying but also facilitate the electrolyte/charge transfer owing to the hollow structure and high conductivity of the HCNBs.

Vapor-Infiltration Approach toward Selenium/Reduced Graphene Oxide Composites Enabling Stable and High-Capacity Sodium Storage

Emerging sodium-selenium batteries suffer from volume expansion of the selenium cathode and shuttling effects of soluble intermediates. Confining selenium within the carbon matrix is the most adopted strategy to address these two issues, which is generally realized via a melt-infusion method. Herein, we developed a vapor-infiltration method to fabricate selenium/carbon composites that are advantageous over the melt-infusion route in terms of several aspects: it relieves the requirement of intensive mechanical mixing and simplifies the ratio optimization between selenium and carbon; it avoids selenium aggregation and makes it possible to utilize all of the surface and pores of the carbon host. Utilizing this method, we fabricated a selenium/graphene composite from thermally reduced graphene oxide with a selenium loading equal to 71 wt %, thus approaching the record value. The obtained composite achieved the highest reported to date initial Coulombic efficiency of 88% among various selenium cathodes, with superior rate and cycle performance (410 and 367 mA h g-1 at 0.1 and 1 A g-1; capacity decay <10% after 800 cycles at 2 A g-1) enabled by the supporting graphene framework and the use of the ether electrolyte. In view of the distinct advantages of the vapor-infiltration method and the significant influence of the ether electrolyte on both initial Coulombic efficiency and cyclability of the batteries, we believe the introduced approach will be frequently adopted to incorporate selenium into various host materials, and the ether electrolyte will be widely considered for selenium-based electrodes.