Abstract
Hollandite-type oxides, K(y)V(x)Ti(8-x)O(16), x = 0.25, 0.5, 0.75, 1, 1.25, 1.5, and 2, are synthesized via the citrate method and evaluated as potential electrode materials for potassium-ion batteries (KIBs). Neutron powder diffraction (NPD) confirms an undistorted I4/m structure, uniform K content (1.4 ≤ y ≤ 1.6), and high potassium isotropic displacement parameter (B(iso)). This decreases significantly for x ≥ 1, correlating with tunnel narrowing and vanadium's stronger polarization. Transmission electron microscopy (TEM) techniques, including selected area electron diffraction (SAED), annular bright field (ABF), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) identify superstructure reflections assigned to potassium/vacancy short-range order along the c axis with disorder between tunnels. Magnetic studies reveal paramagnetic behavior down to 2 K, with antiferromagnetic interactions at low temperature except for x = 0.25 composition, which exhibits ferromagnetic interactions. The experimental magnetic moment suggests a low Ti(3+) content, with notable deviations at x = 1.25. The electrochemical performance is assessed via galvanostatic cycling using 2.5 M potassium bis(fluorosulfonyl)imide (KFSI) in triethyl phosphate (TEP) as electrolyte. At a rate of C/10, 2 K(+) are reversibly de/inserted per formula unit, comparable to K(0.17)TiO(2). At C/5, K(1.5)V(0.75)Ti(7.25)O(16) demonstrates a reversible de/insertion of 1 K(+)/f.u., highlighting its potential for rechargeable KIBs.