Solid-State Battery Fabricated By Li3.5Ge0.5V0.5O4 Electrolyte
Authors: Okumura Toyoki , Takeuchi Tomonari , Kobayashi Hironori 
Published: February 2020

Oxides are generally non-flammable, durable and non-toxic materials; high safety and reliability in a battery system is assured by the use of oxide as an electrolyte instead of the highly-reactive non-aqueous liquid. To develop an oxide-based solid-state all-lithium-ion battery (ASS-LIB) for large-scale applications such as next-generated wireless IoT devices, the thicknesses of stacked electrode and electrolyte layers need to be curtailed to several ten to hundred μm order. Although the powder technology is an easy formula for obtaining such thick electrode layers, some sort of powder interfacial design is required for the fast-ionic transfer at the electrode/electrolyte hetero-interface. Present-progressive oxide-based electrolytes as perovskites or garnets have high conductivities of ≈ 10-3 S cm-1, but were generally difficult to design the ionic-transferable interface since most electrode materials easily engage in thermochemical reactions with the aforementioned electrolytes.
In present study, γ-Li3PO4-type Li3.5Ge0.5V0.5O4 (LGVO), which was one of the Lithium Super-Ionic Conductor (LISICON) families, was focused on as electrolyte of the SS-LIB.1 The LGVO was synthesized via a conventional solid-state reaction, in which pre-prepared Li4GeO4 and pre-prepared Li3VO3 were used as the starting materials. The required amount of each material was ground and cold-pressed into a pellet, and then heated at 700 °C for 12 h. A single γ-Li3PO4-type phase was identified by a powder XRD pattern of the mortar-milled pellet. The prepared LGVO powder was put in a 80 mL ZrO2 cap with five of 20 mm ZrO2 disks, and was further-crushed by disk mill for obtaining sub-micron-sized fine powder before sintering processes. Samples for AC impedance measurement were prepared by two sintering process; conventional furnace sintering (CFS) of uniaxial pressing powder and SPS of Au/ LGVO powder/Au. (Both sides were polished and coated with Au after CFS process.) The sintering temperature and time at CFS and SPS were 700 °C, 2 h and 650 °C, 1 min, respectively. Total Li-ion conductivities for LGVO SPS pellet was 9.5 × 10-5 S cm-1 at 25 °C, which was slightly higher than that of CFS pellet: 8.5 × 10-5 S cm-1 because of the minimalizing grain-boundary resistance. The conductivity was enhanced due to reducing the high-resistive layer near the grain boundary by a short period of SPS process, which has been confirmed in case of γ-Li3PO4-type Li3.5Ge0.75S0.25O4.2

No impurity peak was observed in the powder XRD pattern of the LiNi1/3Mn1/3Co1/3O2 (NMC)-LGVO co-sintered composite produced by either CFS or SPS. This result indicates that the LGVO is thermally-stable during fabricating SS-LIB by powder process. Composite electrode powder was prepared from a mixture of 50 wt% NMC and 50 wt% LGVO. A layer-stacking compact (Au | composite electrode powder | LGVO powder as separator) was co-sintered by SPS process at 500 °C for 1 min. A lithium foil was used as a reference/counter electrode. A poly-(ethylene oxide)-based dry polymer electrolyte film was inserted between the lithium foil and the LGVO electrolyte to reduce the interfacial resistance with adhesion. Electrochemical charge-discharge test was performed at a constant current of 50 μA cm-2 at 60 °C. The SS-LIB shows the reversible charge/discharge capacities of over 120 mAh g-1 thanks to using LGVO electrolyte, which possess the relatively high conductivity of ≈ 10-4 S cm-1 and high thermally-stability even under co-sintering with NMC electrode.

Acknowledgements

This work was financially supported by the Advanced Low Carbon Technology Research and Development Program of the Japan Science and Technology Agency for Specially Promoted Research for Innovative Next Generation Batteries (JST-ALCA SPRING).

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[1] J. Kuwano and A. R. West, Mat. Res. Bull., 15 (1980) 1661.
[2] T. Okumura, S. Taminato, T. Takeuchi and H. Kobayashi, ACS Appl. Energy Mater., 1 (2018) 6303.