Direct evidence for high Na+ mobility and high voltage structural processes in P2-Nax[LiyNizMn1−y−z]O2 (x, y, z ≤ 1) cathodes from solid-state NMR and DFT calculations

Abstract

Structural processes occurring upon electrochemical cycling in P2-Na_x[Li_yNi_zMn_1−y−z]O₂ (x, y, z ≤ 1) cathode materials are investigated using ²³Na and ⁷Li solid-state nuclear magnetic resonance (ssNMR). The interpretation of the complex paramagnetic NMR data obtained for various electrochemically-cycled Na_xNi_1/3Mn_2/3O₂ and Na_xLi_0.12Ni_0.22Mn_0.66O₂ samples is assisted by state-of-the-art hybrid Hartree–Fock/density functional theory calculations. Two Na crystallographic environments are present in P2-Na_x[Li_yNi_zMn_1−y−z]O₂ compounds, yet a single ²³Na NMR signal is observed with a shift in-between those computed for edge- and face-centered prismatic sites, indicating that Na-ion motion between sites in the P2 layers results in an average signal. This is the first time that experimental and theoretical evidence are provided for fast Na-ion motion (on the timescale of the NMR experiments) in the interlayer space in P2-type Na_xTMO₂ materials. A full assignment of the ⁷Li NMR data confirms that Li substitution delays the P2 to O2 phase transformation taking place in Na_xNi_1/3Mn_2/3O₂ over the range 1/3 ≥ x_Na ≥ 0. ²³Na ssNMR data demonstrate that Na_xNi_1/3Mn_2/3O₂ samples charged to ≥3.7 V are extremely moisture sensitive once they are removed from the cell, water molecules being readily intercalated within the P2 layers leading to an additional Na signal between 400 and 250 ppm. By contrast, the lithiated material Na_xLi_0.12Ni_0.22Mn_0.66O₂ shows no sign of hydration until it is charged to ≥4.4 V. Since both TMO₂ layer glides and water intercalation become increasingly favorable as more vacancies are present in the Na layers, the higher stability of the Li-doped P2 phase at high voltage can be accounted for by its higher Na content at all stages of cycling.