Development of the Electrode for All-Solid-State Lithium Rechargeable Batteries Using MoS2 by Liquid Phase Mixing
Authors: Kazuto Fujiwara , Higuchi Eiji , Chiku Masanobu , Inoue Hiroshi 
Published: December 2024

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Purpose

Rechargeable batteries using inorganic solid electrolytes are currently attracting attention from many quarters as next-generation rechargeable batteries because they can use lithium metal (3860 mAh g-1), which has a large theoretical capacity, as the negative electrode due to its ability to suppress dendrites, and can operate at high temperatures. MoS2, a transition metal dichalcogenite, has a layered structure consisting of sheets of MoS2, which has been studied as a cathode active material because of its extremely high theoretical capacity (670 mAh g-1 , 3390 mAh cm-3) and relatively low cost. In the case of liquid electrolytes, electrodes with MoS2 crystals grown on graphene by a hydrothermal method have been developed to prevent re-lamination of MoS2 and to ensure conductivity(1), but there were problems such as a small bulk density of active material and a very complicated electrode fabrication method. In contrast, we thought that the issues of preventing re-lamination and ensuring conductivity could be solved by using an inorganic solid electrolyte with a structure in which solid electrolyte and conductivity aids are sandwiched between single to several layers of MoS2 and MoS2 particles. In this method, the bulk density of the active material can be freely selected, and electrode fabrication becomes relatively easy. Therefore, in this study, electrodes were fabricated using single~several layers of MoS2 solvated in a solvent in order to fabricate electrodes while preventing re-lamination. The solid electrolyte (Li3PS4) used for mixing was also synthesized by the liquid-phase method, and the electrodes were prepared only by liquid-phase mixing, which is advantageous for industrialization.

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experiment

MoS2 (1.0 g) and butyl lithium-hexane solution (2.0 M, 25 mL) were mixed, and MoS2@Li powder was obtained by inserting lithium atoms between layers of MoS2 by irradiating with bath-type ultrasound for 3 hours. MoS2@Li was then exfoliated and dispersed by ultrasonic homogenization in butyl acetate solvent for 20 minutes to prepare MoS2 dispersion.
For electrode preparation, MoS2 dispersion, conductive auxiliary material (vapor-grown carbon fiber), solid electrolyte (Li3PS4 dispersion solution), and zirconia balls (φ4 mm, 20 g) were mixed by shaking at 1500 rpm for 10 minutes. The solvent was then removed by centrifugation and decantation, vacuum dried for 48 hours, and annealed at 100 °C for 2 hours. The Li3PS4 dispersion solution was synthesized in advance by a liquid-phase method using butyl acetate as solvent. The cell was fabricated using amorphous Li3PS4 synthesized by a mechanochemical method as the electrolyte and a Li/In alloy as the reference electrode and anode, and combined with the above cathode to form a two-pole cell.

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Results and discussion

Transmission electron microscope images of the MoS2 dispersion used to fabricate the electrode are shown in Fig. 1. As a result, it was confirmed that MoS2 of about several hundred nm was present in the dispersion solution in a single to several layer state, and that single to several layers of MoS2 were dispersed in the solvent.
The charge-discharge curves of electrodes with exfoliated and untreated MoS2 are shown in Fig. 2. A charge plateau was observed at approximately 2.2 V for both electrodes. Multiple plateaus were observed during the discharge process, confirming that several types of reactions were occurring in a multi-step manner. No reduction reaction was observed at 2.5 to 4.5 V. The electrode made of untreated MoS2 had a discharge capacity of about 500 mAh g-1, whereas the electrode made of exfoliated MoS2 showed a very high discharge capacity of 1105 mAh g-1, more than twice the maximum. The Cyclic voltammograms of electrodes with exfoliated and untreated MoS2 are shown in Fig. 3. The CV measurements have peaks at similar positions for these electrodes, and the electrodes with exfoliated MoS2 showed higher current values overall. These results suggest that the reaction mechanism is almost the same for these electrodes, and that the utilization of MoS2 was improved by using stripped MoS2.
(References)
(1) Liu , K.Jia , J.Yang , S.He , Z.Liu , X.Wang , J.Qiu Chem.Eng.J 475,146181(2023)

Figure 1