Hironori Kobayashi

Addition of LiI and Li ₃ PS ₄ to hexagonal Li ₄ SnS ₄ enhances ionic conductivity without decreasing the air stability of Li ₄ SnS ₄ .

Lithium polysulfides (Li₂S_n) react with carbonate solvents, forming organic polysulfides (R–S_n–R) and sulfides (R–S–R); the concentrated electrolyte suppresses these reactions.

The electrode materials which exhibit high capacity have attracted much attention because the capacity of the materials directly commits to the energy density of LIBs and driving miles of EVs. So that, many researchers devote their efforts to find new electrode materials with high capacity such as lithium excess layered oxide, Ni-rich materials and silicon. Electrode manufacturing process, generally consists of (1) preparing slurry, (2) coating the slurry on current collector, (3) drying, (4) pressing steps, is another important issue to achieve LIBs with high energy density. Compared with numerous reports described with the electrode material development, however, only the limited numbers of papers related to the electrode construction process have been published [1-5]. To shed light the relationship between the electrode construction process and the battery performance is an essential research for obtaining the best performance from electrode materials and assembled LIB cells. In the present study, we focused our attention on the slurry preparation step among above-mentioned four steps. The slurry preparation is the first step in the electrode construction process, and we recognize the dispersion state of particles achieved during slurry preparation greatly influences the battery performance. Three different preparation processes were applied to achieve the different dispersion state of particles in slurries, and thus obtained slurries are characterized by rheological measurement. It is found that dispersion process with highly loaded slurry or under high shear flow reduces elastic moduli and that their combination achieved the most dispersed state of the particles among the slurries investigated. X-ray computed tomography unveiled that the electrodes manufactured from these slurries contain the aggregates and the size of aggregates in the electrodes was reduced with the decrease in elastic moduli of slurry. At the discharge current density of 10 mA g^–1, these electrodes exhibited 140 mAh g^–1 of discharge capacities. At 1000 mA g^–1, the electrode prepared from highly loaded slurry maintained 80% of the capacity that obtained at the current density of 10 mA g^–1. The capacity of the electrode prepared from the slurry under highly loaded and high shear flow, on the other hand, dropped below 10%, indicated that the electrode from the most dispersed slurry has the worst rate performance. [Ref.] 1) K. M. Kim et al., J. Power Sources, 83, 108 (1999). 2) E. Ligneel et al., J. Electrochem. Soc., 154, A235 (2007). 3) G.-W. Lee et al., J. Power Sources, 195, 6049 (2010). 4) B. Bitsch et al., J. Power Sources, 265, 81 (2014). 5) S. Lim et al., J. Power Sources, 299, 221 (2015).

Some basic but important guidelines for the development of sheet-type all-solid-state batteries using a practical slurry coating process are described in this paper. Li₃PS₄ glass powder that had been passed through a 25 μm sieve was prepared. Positive and negative electrode sheets with capacities of more than 1.5 mAh cm⁻² were developed. An all-solid-state full cell was constructed using the electrode sheets and the self-standing solid electrolyte sheets. The energy density of the cell was ca. 155 Wh kg⁻¹, where the weight of the current collectors and the exterior package was excluded from the weight for calculation.

A unique charge/discharge mechanism of amorphous TiS₄ is reported. Amorphous transition metal polysulfide electrodes exhibit anomalous charge/discharge performance and should have a unique charge/discharge mechanism: neither the typical intercalation/deintercalation mechanism nor the conversion-type one, but a mixture of the two. Analyzing the mechanism of such electrodes has been a challenge because fewer tools are available to examine the “amorphous” structure. It is revealed that the electrode undergoes two distinct structural changes: (i) the deformation and formation of S–S disulfide bonds and (ii) changes in the coordination number of titanium. These structural changes proceed continuously and concertedly for Li insertion/extraction. The results of this study provide a novel and unique model of amorphous electrode materials with significantly larger capacities.

Solid solution of Li₂TiS₃ (Li_8/3Ti_4/3S₄) and Li₃NbS₄ were prepared using mechanochemical syntheses. The materials with cubic rock-salt structure were obtained over the full range of compositions (Li_(8+x)/3Ti_(4−4x)/3Nb_xS₄; 0 ≦ x ≦ 1). Lithium, titanium, and niobium randomly share the cationic site. Each material showed electrode activity and had a large reversible capacity of more than 350 mAh g⁻¹ with more than 3 electron process per composition formulas of Li_(8+x)/3Ti_(4−4x)/3Nb_xS₄.

Cubic Li₂SnS₃ with NaCl structure and the solid solution with Li₃NbS₄ were developed by mechanochemical synthesis. Cubic Li₂SnS₃ is a metastable phase, and the previously reported monoclinic Li₂SnS₃ is a stable phase. Cubic Li₂SnS₃ acts as an ionic conductor, and the solid solution of Li₃NbS₄-Li₂SnS₃ can be used as an electrode active material in lithium secondary batteries. Li_2.933Nb_0.8Sn_0.267S₄ and Li_2.857Nb_0.571Sn_0.571S₄ shows large reversible capacities of 318 and 286 mAh g⁻¹, respectively. The capacity of these materials depends less on the niobium content than on the electrical conductivity. The reversible capacity in the 2-V region is mainly attributable to redox reactions involving rather sulfur than tin.