Comprehensive investigation of the lithium insertion mechanism of the Na2Ti6O13 anode material for Li-ion batteries

Abstract

Sodium hexatitanate Na₂Ti₆O₁₃ with a tunnel structure has been proposed to be an attractive anode material for lithium ion batteries because of its low insertion voltage, structural stability and good reversibility. In order to obtain a full understanding of the properties of this titanate, a combination of in situ synchrotron X-ray diffraction, neutron diffraction and ⁷Li MAS solid-state NMR spectroscopy is used in the present work. During the first insertion stage (centered at 1.3 V), lithium is allocated in square planar LiO₄ 2c (Li1) sites, minimizing electrostatic repulsion with Na ions. During the second lithium uptake (centered at 1.1 V), Li ions pass from 2c to empty 4i (Li2) sites of y/b = 0.5 planes, near Ti³⁺ cations. Distribution of Li⁺ and Na⁺ ions with respect to Ti³⁺ cations was deduced from Fourier map differences (Rietveld technique) and NMR quantitative analyses in Li_0.8Na₂Ti₆O₁₃ and Li_1.7Na₂Ti₆O₁₃ samples. ⁷Li MAS-NMR analysis showed that Li ions occupy three fourfold coordinated sites with reasonable Li⁺–O–Ti³⁺ bond distances, while Na cations remain at eightfold coordinated positions near Ti⁴⁺ cations as deduced from ²³Na MAS-NMR spectroscopy. ⁷Li MAS-NMR recorded at increasing temperatures suggests that Li ions move along sinusoidal paths to reduce Li–Na electrostatic interactions. Li mobility along the b-axis is favored by partial occupation of interstitial 4i sites (Li3) located at both sides of Na cations in y/b = 0 planes. In lithium inserted samples the most probable – Li1(2c) → Li3(4i) → Li1(2c) – conduction paths were deduced. However, formation of Li pairs at y/b = 0 planes (Li2 sites), where Li ions are located near Ti³⁺ cations, reduces the amount of mobile Li ions that participate in conduction processes. Proximity of lithium to Li and Na ions limits insertion to ca. 2 Li ions per structural formula.