Mechanochemical activation and sodium intercalation in the NaTi2(PO4)3 NASICON structure

Abstract

NaTi₂(PO₄)₃, a NASICON-type sodium titanium phosphate (NTP), is a promising candidate for sodium-ion energy storage due to its robust structural and electrochemical properties. This study explores the impact of mechanochemical activation (MCA) on NTP synthesized via high-temperature methods, with a focus on sodium intercalation behaviour under various milling conditions. Sodiation experiments were performed in suspension using different sodium sources and the resulting structural and compositional changes after activation and the mean sodium content (〈x〉 defined as the average number of sodium ions per formula unit in Na_xTi₂(PO₄)₃) were determined by powder X-ray diffraction (PXRD) combined with Rietveld refinement as well as scanning electron microscopy (SEM), physisorption isotherm measurements, inductively coupled plasma mass spectrometry (ICP-MS), Raman spectroscopy and solid-state NMR. While MCA did not induce a phase transformation, extended milling times led to reduced crystallite sizes and increased structural disorder. The extent of sodium incorporation was strongly influenced by both the milling duration and the chemical nature of the sodium source, particularly its ability to reduce titanium. The highest sodium content of 3.5 approaching the theoretical maximum of 4.0 (Na₄Ti₂(PO₄)₃) was achieved using sodium naphthalene as the sodiation reagent.