Kazuaki Kisu

The origin of anomalous large reversible capacity for SnO2 conversion reaction

Single-nanocrystalline SnO2 (2–4 nm ϕ) particles completely encapsulated within hollow-structured carbon black structures (Ketjen Black (KB), typically 40 nm ϕ) were prepared using our original in situ ultracentrifugation (UC treatment) materials processing technology. Ultracentrifugation at 75000g induces an in situ sol–gel reaction that brings about optimized linking between limited-size SnO2 nanocrystals and microcrystalline graphitic carbons of KB. Efficient entanglement and nanonesting have been accomplished by simultaneous nanofabrication and nanohybridization in the UC treatment, specifically at a ratio of SnO2/KB = 45/55. This composite exhibited a reversible capacity of 837 mA h g−1 per composite, equivalent to 1444 mA h g−1 (per pure SnO2 after subtracting the capacity attributed to KB in the composite) for remarkably many cycles, over 1200. Such high performance in regard to both capacity and cyclability has never been attained so far for SnO2 anode materials. The reversibility of changes in the Sn valence state (defined as “formal valence state” in the manuscript) from Sn(2.9+) to Sn(4.4−) was demonstrated by in situ XAFS measurements during the lithiation–delithiation process. Peculiar nanodots of typically 2–4 nm that look like single-crystal SnO2/carbon core–shell structures were found for the optimized dose ratio (45/55) in the HRTEM observation. After 10 cycles, all the materials showed complete encapsulation of the same-sized nanoparticles, which were covered and nested within the KB matrix and an electrolyte-derived polymeric film. These results indicate that the initially prepared SnO2/KB composites were transformed into a new species, represented as LixSnO1.45 (x: 0–7.3), which shows perfect reversibility and cyclability. This species can exchange a total of 7.3 electrons, including 2.9 electrons for the conversion reaction (1–2 V) and 4.4 electrons for the subsequent alloying process (0–1 V).

Ultrafast nano-spherical single-crystalline LiMn0.792Fe0.198Mg0.010PO4 solid-solution confined among unbundled interstices of SGCNTs

Spherical LiMn0.792Fe0.198Mg0.010PO4 nanocrystals, which are highly dispersed and encapsulated within the interstices of supergrowth (single-walled) carbon nanotubes (SGCNTs), were successfully synthesized by in situ material processing technology called “ultra-centrifuging (UC) treatment”. TEM images of these LiMn0.792Fe0.198Mg0.010PO4/SGCNT composites suggest the direct attachment of the LiMn0.792Fe0.198Mg0.010PO4 nanocrystals (10–40 nm) onto the surface of highly conductive SGCNTs. Mg-doping brought out 10% increase of Li+ capacity in Mn sites with 200% increase of Li+ diffusivity and 50% decrease of electrical resistance owing to such peculiar “nano–nano LiMn0.792Fe0.198Mg0.010PO4/SGCNT composites”. The synthesized LiMn0.792Fe0.198Mg0.010PO4/SGCNT composites overcome the inherent restrictions of one-dimensional diffusion and deliver a high electrochemical capacity density of ca. 54 mA h g−1 per composite (corresponding to 77 mA h g−1 per pure LiMn0.792Fe0.198Mg0.010PO4) at a high rate of 50 C, while showing excellent cycle life, retaining 84% of the initial capacity over 3000 cycles.