UC Santa Barbara

²³ Na and ⁷ Li NMR on P2-Na _x [Li _y Ni _z Mn _1−y−z ]O ₂ sodium-ion battery cathodes provide evidence for fast Na-ion motion and structural stabilization of the Li-substituted materials upon Na electrochemical extraction.

Structural processes occurring upon electrochemical cycling in P2-Na _x [Li _y Ni _z Mn _1−y−z ]O ₂ ( x , y , z ≤ 1) cathode materials are investigated using ²³ Na and ⁷ Li solid-state nuclear magnetic resonance (ssNMR). The interpretation of the complex paramagnetic NMR data obtained for various electrochemically-cycled Na _x Ni _1/3 Mn _2/3 O ₂ and Na _x Li _0.12 Ni _0.22 Mn _0.66 O ₂ 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 _y Ni _z Mn _1−y−z ]O ₂ compounds, yet a single ²³ Na 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 _x TMO ₂ materials. A full assignment of the ⁷ Li NMR data confirms that Li substitution delays the P2 to O2 phase transformation taking place in Na _x Ni _1/3 Mn _2/3 O ₂ over the range 1/3 ≥ x _Na ≥ 0. ²³ Na ssNMR data demonstrate that Na _x Ni _1/3 Mn _2/3 O ₂ 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 _x Li _0.12 Ni _0.22 Mn _0.66 O ₂ shows no sign of hydration until it is charged to ≥4.4 V. Since both TMO ₂ 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.