Publication date: 10th April 2024
All-solid-state lithium-ion batteries are expected as one of the energy storage technologies of next generation. In contrast to sulfide-based solid electrolytes, which can be densified via simple pressing to achieve sufficient ionic conductance, application of oxide-based solid electrolytes generally requires high-temperature sintering for its densification. Considering cell fabrication via co-sintering of electrolyte and active material, high process temperature may lead to undesirable reaction between them and thereby degraded interfacial conductance. Thus low-temperature sintering of oxide electrolytes is an important processing technology.
In the preceding study on co-firing process for all-solid-state battery, it was found that a NASICON-type lithium-ion conducting electrolyte Li1.3Al0.3Ti1.7(PO4)3 (LATP) in contact with LiCoPO4 shows quite high sinterability. In order to understand the mechanism of the sinterability enhancement, the present authors have investigated the influence of cobalt-introduction on the sintering behavior of LATP as well as the phase equilibrium. It was suggested that 1~2 mol% of cobalt is allowed to substitute for the constituent cation in the NASICON structure, leaving no traces of Co-including second phases in the XRD patterns. The slightly Co-doped LATP phase shows a moderately high sinterability; the relative density approaches 80% after firing at 800°C, while a normal LATP undergoes almost no densification at the same temperature. Further increase in the cobalt content results in the excess of cobalt in reference to the solubility limit in the LATP phase, forming LiCoPO4 together with the other second phases. In the case of the nominal composition such that titanium in LATP is partly replaced by cobalt (Li1.3+2xAl0.3CoxTi1.7-x(PO4)3), a significant densification was observed; e.g., a relative density approximating 100% was attained after firing at 800°C for x=0.1. The significantly high sinterability is ascribed to a low melting-point compound LiPO3, which forms as the second phase together with LiCoPO4 and LiTiO(PO4).
This work was financially supported by Materials Processing Science project ("Materealize") of MEXT, Grant Number JPMXP0219207397, the Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING, grant number JPMJAL1301), JST Grant Number JPMJPF2016, and JSPS KAKENHI Grant Numbers JP17H01317, JP19K05020. The SEM/EDS and TEM/EDS analyses were carried out at National Institute for Materials Science (NIMS) Battery Research Platform.
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