Exploring Hollandite-Type K<i>_y</i>V<i>_x</i>Ti_8–<i>x</i>O₁₆ (0.25 ≤ <i>x</i> ≤ 2) as Electrode Materials in Potassium-Ion Batteries (KIBs)

Hollandite-type oxides, K<i>_y</i>V<i>_x</i>Ti_8-<i>x</i>O₁₆, <i>x</i> = 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 <i>I</i>4/<i>m</i> structure, uniform K content (1.4 ≤ <i>y</i> ≤ 1.6), and high potassium isotropic displacement parameter (B_iso). This decreases significantly for <i>x</i> ≥ 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 <i>c</i> axis with disorder between tunnels. Magnetic studies reveal paramagnetic behavior down to 2 K, with antiferromagnetic interactions at low temperature except for <i>x</i> = 0.25 composition, which exhibits ferromagnetic interactions. The experimental magnetic moment suggests a low Ti³⁺ content, with notable deviations at <i>x</i> = 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_0.17TiO₂. At C/5, K_1.5V_0.75Ti_7.25O₁₆ demonstrates a reversible de/insertion of 1 K⁺/f.u., highlighting its potential for rechargeable KIBs.