67
M1 and M2 interstitial positions filled with sodium ions (Fig. 5) [20, 23, 24]. The
conduction of alkali cations through the NaSICON crystal structure is believed to be
based on the movement of ions from site to site through the crystal lattice via channels (bottlenecks) constricted by zirconia octahedra and phosphate or silica
tetrahedra.
Compositions range from the “P 3 ” composition (x = 0) to the “Si 3 ” composition
(x = 3), though the highest selective conductivity compositions are generally
reported near x = 2 [20]. Substitutions on the NaSICON lattice can be used to vary
the charge distribution, defect chemistry, and critically, the size of channels and
bottlenecks in the crystal lattice, and this approach has been used to improve the
conductivity of select NaSICON compounds [25]. NaSICON can be prepared using
a number of approaches, ranging from traditional solid-phase ceramic syntheses to
sol-gel and microwave syntheses [26].
Among the challenges to synthesizing quality NaSICON is controlling the formation of secondary phases, including zirconia, silicate, and glassy inclusions as
well as a poorly defined grain boundary phase [22]. These secondary phases can
impact the ionic conductivity as well as the chemical stability and mechanical properties of the solid electrolyte. Highly dense NaSICON with proper composition and
minimal secondary phases present, however, can be expected to be highly conductive with values >1 × 10
−3
S cm
−1
at room temperature and stable against molten
sodium. Although there has been some historical debate about the stability of
NaSICON exposed to sodium at high temperatures (>300 °C) [27–31], recent
reports have shown that NaSICON appears to be “quite stable” at moderate temperatures [16, 22].
Fig. 5 Crystal structure of NaSICON showing M1 and M2 interstitial positions. Reprinted from
Ref. [23], Copyright 1997, with permission from Elsevier
Molten Sodium Batteries
M1 and M2 interstitial positions filled with sodium ions (Fig. 5) [20, 23, 24]. The
conduction of alkali cations through the NaSICON crystal structure is believed to be
based on the movement of ions from site to site through the crystal lattice via channels (bottlenecks) constricted by zirconia octahedra and phosphate or silica
tetrahedra.
Compositions range from the “P 3 ” composition (x = 0) to the “Si 3 ” composition
(x = 3), though the highest selective conductivity compositions are generally
reported near x = 2 [20]. Substitutions on the NaSICON lattice can be used to vary
the charge distribution, defect chemistry, and critically, the size of channels and
bottlenecks in the crystal lattice, and this approach has been used to improve the
conductivity of select NaSICON compounds [25]. NaSICON can be prepared using
a number of approaches, ranging from traditional solid-phase ceramic syntheses to
sol-gel and microwave syntheses [26].
Among the challenges to synthesizing quality NaSICON is controlling the formation of secondary phases, including zirconia, silicate, and glassy inclusions as
well as a poorly defined grain boundary phase [22]. These secondary phases can
impact the ionic conductivity as well as the chemical stability and mechanical properties of the solid electrolyte. Highly dense NaSICON with proper composition and
minimal secondary phases present, however, can be expected to be highly conductive with values >1 × 10
−3
S cm
−1
at room temperature and stable against molten
sodium. Although there has been some historical debate about the stability of
NaSICON exposed to sodium at high temperatures (>300 °C) [27–31], recent
reports have shown that NaSICON appears to be “quite stable” at moderate temperatures [16, 22].
Fig. 5 Crystal structure of NaSICON showing M1 and M2 interstitial positions. Reprinted from
Ref. [23], Copyright 1997, with permission from Elsevier
Molten Sodium Batteries