Abstract
All-solid-state lithium-ion batteries using Li+-ion conducting ceramic electrolytes have been focused on as attractive future batteries for electric vehicles and renewable energy conversion systems because high safety can be realized due to non-flammability of ceramic electrolytes. In addition, a higher volumetric energy density than that of current lithium-ion batteries is expected since the all-solid-state lithium-ion batteries can be made in bipolar cell configurations. However, the special ideas and techniques based on ceramic processing are required to construct the electrochemical interface for all-solid-state lithium-ion batteries since the battery development has been done so far based on liquid electrolyte system over 100 years. As one of promising approaches to develop practical all-solid-state batteries, we have been focusing on three-dimensionally (3D) structured cell configurations such as an interdigitated combination of 3D pillars of cathode and anode, which can be realized by using solid electrolyte membranes with hole-array structures. The application of such kinds of 3D structures effectively increases the interface between solid electrode and solid electrolyte per unit volume, lowering the internal resistance of all-solid-state lithium-ion batteries. In this study, Li6.25Al0.25La3Zr2O12 (LLZAl), which is a Al-doped Li7La3Zr2O12 (LLZ) with Li+-ion conductivity of ~10–4 S cm–1 at room temperature and high stability against lithium-metal, was used as a solid electrolyte, and its pellets with 700 um depth holes in 700 x 700 um2 area were fabricated to construct 3D-structured all-solid-state batteries with LiCoO2 / LLZAl / lithium-metal configuration. It is expected that the LiCoO2-LLZAl interface is formed by point to point contact even when the LLZAl pellet with 3D hole-array structure is applied. Therefore, the application of mechanically soft Li3BO3 with a low melting point at around 700 °C was also performed as a supporting Li+-ion conductor to improve the LiCoO2-LLZAl interface.
Highlights
In recent years, the application of lithium-ion batteries has been expanded from portable electronic devices to large devices such as electric vehicles and energy storage systems on an industrial scale
If lithiummetal is used as an anode in all-solid-state lithium-ion batteries to realize high energy density, a Li+-conducting protective layer such as poly(methyl methacrylate) (PMMA) gel-electrolyte has to be formed as a buffer layer to prevent the direct contact of those solid electrolytes with Li-metal (Hoshina et al, 2005)
The X-ray diffraction (XRD) pattern of the 3D-structured LLZAl pellet shown in Figure 5 is attributed to the cubic crystal
Summary
The application of lithium-ion batteries has been expanded from portable electronic devices to large devices such as electric vehicles and energy storage systems on an industrial scale. The replacement of liquid electrolytes with solid electrolytes is an important issue to contribute the improvement of both safety and energy density of lithium-ion batteries These advantages can be confirmed in polymer lithium-ion batteries, in which gel-type electrolytes including liquid electrolytes have been still used owing to low conductivity of true polymer electrolytes. LLT is a perovskite-type oxide Li+-ion conductor with a high bulk ionic conductivity of 1 × 10–3 S cm−1 at 25°C, but its total ionic conductivity is as low as 7.5 × 10–5 S cm−1 (Inaguma et al, 1993) These are not stable against Li-metal due to the reduction of Ti4+ to Ti3+ at 1.8 V vs Li/Li+, resulting in the appearance of electronic conduction (Knauth, 2009). If lithiummetal is used as an anode in all-solid-state lithium-ion batteries to realize high energy density, a Li+-conducting protective layer such as poly(methyl methacrylate) (PMMA) gel-electrolyte has to be formed as a buffer layer to prevent the direct contact of those solid electrolytes with Li-metal (Hoshina et al, 2005)
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