All-solid-state lithium ion batteries containing solid electrolyte are believed a promising candidate for these demand for following reasons. First, solid electrolyte is generally considered to have a wide electrochemical window allowing the use of high-voltage cathode material (5 V vs. Li+/Li). Second, Solid electrolyte has no liquid junction. So, it is expected that series-connected cells in a package can be easily constructed.
Oxide electrolytes that are one of the kinds of solid electrolytes allow a reducing of a thickness of separate layer due to its high mechanical strength, so high capacities of the battery are expected. Furthermore, a layer-by-layer structure of oxide electrolyte / oxide active material such as multi-layer ceramic capacitors is a great merit to construct for a bi-polar cell for high capacity. However, low lithium ion conductivities of oxide electrolytes (e.g. 1 mScm-1 at R.T.) limited their use for all-solid-state battery.
The great breakthrough in oxide electrolyte was brought in 2007 by Prof. R. Murugan, who reported a garnet type oxide: Li7La3Zr2O12 (LLZO) having high lithium ion conductivity (0.1 mScm-1 at 25 oC) and this discover motivated many researchers into the research of the all-solid-state lithium ion battery using oxide electrolyte.
We investigated the electrochemical performance of a model battery using an as-sintered Nb doped LLZO (LLZO-Nb, 1 mScm-1 at 25 oC) bulk pellet and LiCoO2 (LCO : one of typical cathode active material), and found that the model battery exhibited good charge-discharge capacities (> 90 % vs. theoretical capacities) and low interfacial resistance between LCO and LLZO-Nb (~150 Wcm2 at 25 oC) comparable with that of lithium ion batteries with liquid organic electrolytes. Thus, LLZO-Nb is a promising candidate for a solid electrolyte for all-solid-state lithium ion battery.
Our next issue is a selection of a negative electrode for all-solid-state lithium ion battery using LLZO-Nb. Si is a one of a promising candidate of anode materials for high capacity. Generally, electrochemical performances of all-solid-state lithium ion battery would be strongly affected by interfacial contact condition. Therefore, the intrinsic electrochemical performance and interfacial resistance would not be clarified by a constructing all-solid-state lithium ion battery using Si powder and LLZO-Nb powder. In order to clarify the electrochemical performance and interfacial resistance between Si and LLZO-Nb, Si/LLZO-Nb/Li model cell was fabricated using sputtering process. Si layer which was deposited on the top face of a high dense (~ 99 %) LLZO-Nb bulk pellet was confirmed as amorphous by Raman spectra. Lithium metal was deposited on the bottom side of LLZO-Nb bulk pellet by vacuum evaporation deposition. This model cell exhibited a good charge - discharge capacities. The Interfacial resistance of Si/LLZO-Nb was decreased with Li alloying of Si, and the interfacial resistance of Li3Si/LLZ reached the same value as the interface resistance of Li/ LLZO-Nb. These results indicate that Si is a promising candidate as an anode material for all-solid-state lithium ion battery using LLZO-Nb.