Ryoichi Tatara

One-step, template-free synthesis of highly porous nitrogen/sulfur-codoped carbons from a single protic salt and their application to CO2 capture

Traditional methods for preparing highly porous, nitrogen/sulfur-codoped carbons require either template-based tedious, time-consuming procedures or harsh chemical/physical activation processes. In this paper, we report a very facile method to prepare such carbons via the direct carbonization of a single protic salt, prop-2-en-1-aminium hydrogensulfate, in the absence of any hard/soft template or activation agent. Depending on the carbonization temperatures, the obtained carbons exhibited tunable structures and properties, in terms of their morphologies, surface areas, pore structures, and chemical compositions. Particularly, the carbon material obtained at 1000 °C had a very large surface area (1149 m2 g−1), even comparable to that of traditionally activated carbon. Among all the samples, the carbon obtained at 900 °C, exclusively having a narrowly distributed microporous structure, exhibited a significant CO2 uptake of 2.58 mmol g−1 at 298 K and 1 atm. This carbon could also be easily regenerated and reused without any evident loss of CO2-adsorption capacity.

Stability of Glyme Solvate Ionic Liquid as an Electrolyte for Rechargeable Li−O2 Batteries

A solvate ionic liquid (SIL) was compared with a conventional organic solvent for the electrolyte of the Li-O2 battery. An equimolar mixture of triglyme (G3) and lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]), and a G3/Li[TFSA] mixture containing excess glyme were chosen as the SIL and the conventional electrolyte, respectively. Charge behavior and accompanying gas evolution of the two electrolytes was investigated by electrochemical mass spectrometry (ECMS). From the linear sweep voltammetry performed on an as-prepared cell, we demonstrate that the SIL has a higher oxidative stability than the conventional electrolyte and, furthermore, offers the advantage of lower volatility, which would benefit an open-type lithium-O2 cell design. Moreover, CO2 evolution during galvanostatic charge was less in the SIL, which implies less side reaction. However, O2 evolution during charge did not reach the theoretical value in either of the two electrolytes. Several mass spectral fragments were generated during the charge process, which provided evidence for side reactions of glyme-based electrolytes. We further relate the difference in observed discharge product morphology for these electrolytes to the solubility of the superoxide intermediate, determined by rotating ring disk electrode (RRDE) measurements.

Three-Dimensionally Hierarchical Ni/Ni3S2/S Cathode for Lithium–Sulfur Battery

Lithium-sulfur (Li-S) batteries have attracted interest as a promising energy-storage technology due to their overwhelming advantages such as high energy density and low cost. However, their commercial success is impeded by deterioration of sulfur utilization, significant capacity fade, and poor cycle life, which are principally originated from the severe shuttle effect in relation to the dissolution and migration of lithium polysulfides. Herein, we proposed an effective and facile strategy to anchor the polysulfides and improve sulfur loading by constructing a three-dimensionally hierarchical Ni/Ni3S2/S cathode. This self-supported hybrid architecture is sequentially fabricated by the partial sulfurization of Ni foam by a mild hydrothermal process, followed by physical loading of elemental sulfur. The incorporation of Ni3S2, with high electronic conductivity and strong polysulfide adsorption capability, can not only empower the cathode to alleviate the shuttle effect, but also afford a favorable electrochemical environment with lower interfacial resistance, which could facilitate the redox kinetics of the anchored polysulfides. Consequently, the obtained Ni/Ni3S2/S cathode with a sulfur loading of ∼4.0 mg/cm2 demonstrated excellent electrochemical characteristics. For example, at high current density of 4 mA/cm2, this thick cathode demonstrated a discharge capacity of 441 mAh/g at the 150th cycle.

One-pot pyrolysis of lithium sulfate and graphene nanoplatelet aggregates

Lithium sulfide (Li2S) as a cathodic material in Li-S batteries can not only deliver a high theoretical specific capacity of 1166 mA h g(-1), but also is essential for batteries using Li-free anode materials such as silicon and graphite. Various efforts have been made to synthesize a highly efficient Li2S-carbon composite; however, the electronically and ionically insulating nature and high melting point of Li2S strongly complicate the synthetic procedures, making it difficult to realize the expected capacity. Herein, a very simple method is proposed to prepare Li2S/graphene composites by one-pot pyrolysis of a mixture of graphene nanoplatelet aggregates (GNAs) and low-cost lithium sulfate (Li2SO4). For the first time, the entire pyrolysis process is clarified by thermogravimetry-mass spectrometry, wherein GNAs were found to partly serve as a carbon source to reduce Li2SO4 to Li2S, while the remaining GNAs formed thin graphene sheets as a result of this in situ etching, as a highly conductive host can immobilize the generated Li2S by intimate contact. Consequently, the obtained Li2S/graphene composite, combined with a Li2Sx-insoluble (x = 4-8) electrolyte developed by our group, exhibits excellent electrochemical behavior for Li-S batteries.