Etsuro Iwama

Nonflammable Hydrofluoroether for Lithium-Ion Batteries: Enhanced Rate Capability, Cyclability, and Low-Temperature Performance

A hydrofluoroether, 2-trifluoromethyl-3-methoxyperfluoropentane (TMMP), was investigated as a nonflammable electrolyte for lithium-ion batteries. This paper reports on the psychochemical properties of the TMMP-mixed electrolyte [1 M lithium bis(pentafluoroethylsulfonyl) imide (LiBETI)/ethylene carbonate+diethyl carbonate (EC + DEC) + TMMP (5:45:50)] in relation with the battery performances. The TMMP-mixed electrolyte showed effective nonflammability with prolonged ignition time (150 s). No differences in oxidative and reductive decomposition currents were observed in the operation voltage range of lithium-ion batteries (0-4.2 V vs Li/Li + ) for the electrolytes with and without TMMP. At a high rate of 12 C, 75% of the capacity (obtained at 0.2 C) was obtained for the TMMP-mixed electrolyte, while less than 40% was obtained for the electrolytes without TMMP [1 M LiBETI/EC + DEC (50:50) and EC + DEC (5:95)]. The low-temperature performance and cyclability were also significantly improved for the TMMP-mixed electrolyte.

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).

Discharge Behavior and Rate Performances of Lithium-Ion Batteries in Nonflammable Hydrofluoroethers(II)

Hydrofluoroethers (HFEs) of 2-trifluoromethyl-3-methoxyperfluoropentane (TMMP) and 2-(trifluoro-2-fluoro-3-difluoropropoxy)-3-difluoro-4-fluoro-5-trifluoropentane (TPTP) were investigated as cosolvents of ethylene carbonate (EC) +diethyl carbonate (DEC). Both lithium bis(pentafluoroethylsulfonyl)imide and lithium hexafluorophosphate (LiPF 6 ) are soluble in EC + DEC + TPTP (5:45:50 in volume), while LiPF 6 is not soluble in EC + DEC + TMMP (5:45:50). The enhanced discharge properties of Li-ion batteries (typically graphite/LiCoO 2 ) in those electrolyte systems were evaluated and compared with the conventional carbonate mixed ones. The TPTP-mixed electrolyte [1 M LiPF 6 /EC + DEC + TPTP (5:45:50)] gave a high capacity retention of 80% at an extremely high rate of 12C. An EC + DEC-based electrolyte normally exhibited poor retention of less than 40%. Such a rate-enhancing effect of HFEs (TPTP and TMMP) was manifested by observation over a wide temperature range from - 20 to + 25°C. The striking enhancement was observed at -20°C and at the first 10% dosage and level off at 50%. The activation energy, as substantiated by microvoltammetry, for the Li + desolvation process decreases from 45-100 kJ mol -1 (over the Li + concentration range of 0.01-2 M, respectively) to 25 kJ mol -1 by adding TMMP or TPTP. This implies that in the presence of HFEs, the kinetics for the rate-determining intercalation process is enhanced by an accelerated Li + transport with a reduced solvation number.

Ultrafast Nanocrystalline‐TiO2(B)/Carbon Nanotube Hyperdispersion Prepared via Combined Ultracentrifugation and Hydrothermal Treatments for Hybrid Supercapacitors

Anisotropically grown (b-axis short) single-nano TiO2 (B), uniformly hyper-dispersed on the surface of multiwalled carbon nanotubes (MWCNT), was successfully synthesized via an in situ ultracentrifugation (UC) process coupled with a follow-up hydrothermal treatment. The uc-TiO2 (B)/MWCNT composite materials enable ultrafast Li(+) intercalation especially along the b-axis, resulting in a capacity of 235 mA h g(-1) per TiO2 (B) even at 300C (1C = 335 mA g(-1) ).

Progress of the conversion reaction of Mn3O4 particles as a function of the depth of discharge

A comprehensive investigation of the morphological and interfacial changes of Mn3O4 particles at different lithiation stages was performed in order to improve our understanding of the mechanism of the irreversible conversion reaction of Mn3O4. The micronization of Mn3O4 into a Mn-Li2O nanocomposite microstructure and the formation of a solid electrolyte interphase (SEI) on the Mn3O4 surface were carefully observed and characterized by combining high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and in situ X-ray absorption fine structure (XAFS) measurements. Accumulation of a thin SEI film of 2-5 nm thickness on the surfaces of the Mn3O4 particles due to their catalytic decomposition was observed at a depth of discharge (DOD) of 0%. As the DOD increases from 25% to 75%, the SEI layer composed of Li2CO3 and LiF continues to grow to 20-30 nm, and Li2O nanoparticles are clearly observed. At 100% DOD, the Mn-Li2O particles with diameters of 2-5 nm become totally encapsulated within a huge organic-inorganic coating structure, while the overall starting shape of the particles remains.

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.