Abstract
ABSTRACTDivalent ion-conducting solid electrolytes with NASICON-type three-dimensional network structures (MxHf1−x)4/(4−2x)Nb(PO4)3 (M = Ni, Mg, Ca, Sr) were successfully developed by introducing M2+ cations into HfNb(PO4)3 solids. The presence of high-valence cations such as Hf4+, Nb5+, and P5+ in the structures effectively reduced the electrostatic interaction between the conducting M2+ cations and the surrounding oxide anions, enabling the M2+ cations to migrate smoothly in the rigid crystal lattice. The relationship between the cation conductivity in NASICON-type solids and the ionic radius of the migrating divalent cation species was also clarified by taking into account the relative sizes of the conduction pathways in the structures.
Highlights
Solid electrolytes are considered promising candidates for use as key components of next-generation electrochemical devices such as all-solid-state batteries, fuel cells, and chemical sensors because of their high chemical/thermal stability and energy density [1,2,3,4]
In a recent study we reported successful development of various kinds of trivalent and tetravalent cation conductors with Na+ superionic conductor (NASICON)-type structures [14,15,16,17,18]; we showed that the presence of cations with a higher valence than that of the conducting cation is essential for achieving smooth ion migration, together with the selection of an appropriate crystal structure possessing large conduction pathways for the migration of bulky ions
We developed high-conductivity divalent cation solid electrolytes with a NASICON-type structure based on the HfNb(PO4)3 material that was identified as a tetravalent Hf4+ cation conductor in our previous work [19]
Summary
Solid electrolytes are considered promising candidates for use as key components of next-generation electrochemical devices such as all-solid-state batteries, fuel cells, and chemical sensors because of their high chemical/thermal stability and energy density [1,2,3,4]. In the case of the Ba-doped samples, no single NASICON-type phase was obtained for any composition, and the NbPO5 impurity phase always appeared when NbCl5 was dissolved in nitric acid in a stoichiometric ratio during the sample preparation process (Figure S4) This result suggests that the Ba2+ ion (0.149 nm, CN = 6 [22]) is too large to substitute the Hf4+ site If the ion conduction in a crystal lattice is influenced only by the relationship between the size
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