Structure of amorphous materials in the NASICON system Na_(1+x)Ti_2Si_xP_(3-x)O_12

What is it about?

The paper gives a comprehensive account of the structure of the Na_(1+x)Ti_2Si_xP_(3-x)O_12 (x = 0.8 and x = 1.0) amorphous precursors for crystalline sodium super-ionic conducting (NASICON) systems prepared via the glass-ceramic route. The structural picture is built by combining the results from neutron and x-ray diffraction with those obtained from multinuclear solid-state NMR. The NASICON materials have received significant attention because of their large sodium-ion conductivity and structural stability in the solid state. They form part of the quest to find sodium-based solid electrolytes and electrode materials to replace those based on lithium for electrical energy storage devices, largely motivated by the low cost of sodium and its widespread abundance.

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Photo by Roberto Sorin on Unsplash

Photo by Roberto Sorin on Unsplash

Why is it important?

A strategy for increasing the ionic conductivity of a NASICON material such as NaTi_2(PO_4)_3 is to increase the sodium ion Na^+ content by replacing P^(5+) by Si^(4+) ions. In this substitution, additional Na^+ ions are required to ensure charge neutrality. Our results show a mismatch between the phosphorus and silicon coordination environments, which includes a difference of 8% between the P–O and Si–O bond lengths, that is likely to inhibit the incorporation of Si^(4+) ions into the P^(5+) sites of the NASICON crystal structure as the glass is annealed. Our work indicates that the replacement of phosphorus by silicon is not optimal for manipulating the NASICON structure to enhance its ionic conductivity, thus revealing an important design rule.

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