Garnet-based all-solid-state batteries
(ASBs) with high energy
density require composite cathodes with high areal loading and high-capacity
cathode active materials. While all ceramic cathodes can typically
be manufactured via cosintering, the elevated temperatures necessary
for this process pose challenges with respect to material compatibility.
High-capacity cathode active materials like Ni-rich LiNi
x
Co
y
Mn1–x–y
O2 (NCM) show
insufficient material compatibility toward the solid electrolyte Li6.45Al0.05La3Zr1.6Ta0.4O12 (LLZO:Ta) during cosintering, leading to the formation
of highly resistive interphases. We investigated this secondary phase
formation both experimentally and via density functional theory calculation
to get a mechanistic understanding of the cosintering behavior of
LLZO:Ta with NCM111 and Ni-rich NCM811. Furthermore, we employed B
doping of both NCM materials in order to assess its impact on the
cation interchange and subsequent secondary phase formation. While
secondary phases were formed for all four NCM materials, their onset
temperature, nature, and amount strongly depend on the NCM composition
and doping. Surprisingly, Ni-rich NCM811 turned out to be the most
promising cathode active material for the combination with garnet-type
LLZO:Ta. As proof of concept, fully inorganic, ceramic all-solid-state
lithium batteries featuring only a Li-metal anode, an LLZO:Ta separator,
and a composite cathode, consisting of LLZO:Ta, Li3BO3, and NCM811, were prepared by conventional sintering. The
purely inorganic full cells delivered a high specific areal discharge
capacity of 0.7 mA h cm–2 in the initial cycle.
LLZO is a promising solid-state electrolyte for Li-metal batteries. It is known that Al stabilizes the high conductivity cubic phase. In this study, the effect of Al concentration on the microstructure and electrochemical behavior was investigated.
Garnet-based Li7La3Zr2O12 (LLZO) is considered one of the most promising oxide-ceramic solid electrolyte materials for inorganic all-solid-state batteries. Dopants and substituents like Al, Ta, Nb, Ga, and W were shown to have a high impact on the total ionic conductivity, increasing it from 10−6 S/cm up to 10−3 S/cm. However, natural zirconium sources always contain a small amount of hafnium which could also act as dopant, but the separation of these two elements is complicated and expensive. In this work, we investigate the influence of various Hf-impurity concentrations on the performance of tantalum-doped LLZO. We synthesised Li6.45Al0.05La3Zr1.6−xHfxTa0.4O12 (LLZHO with x = 0 – 1.6) via conventional solid-state synthesis and have demonstrated that up to x = 0.1, hafnium impurities do not have a significant impact on the performance of the material. Above this concentration, the Li-ion conductivity is steadily reduced to around 70% when zirconium is fully substituted by hafnium resulting in Li6.45Al0.05La3Hf1.6Ta0.4O12. As the purity of Zr precursors has a great impact on their price, these findings can help to reduce the price of LLZO in general, as lower grade zirconium can be used in industrial scale applications.
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