Li-based layered nickel-tin oxide obtained through electrochemically-driven cation exchange

Abstract

The Li-based layered nickel-tin oxide Li_0.35Na_0.07Ni_0.5Sn_0.5O₂ has been synthesized via electrochemically-driven Li⁺ for Na⁺ exchange in O3-NaNi_0.5Sn_0.5O₂. The crystal structure of Li_0.35Na_0.07Ni_0.5Sn_0.5O₂ was Rietveld-refined from powder X-ray diffraction data (a = 3.03431(7) Å, c = 14.7491(8) Å, S. G. R3̄m). It preserves the O3 stacking sequence of the parent compound, but with ∼13% lower unit cell volume. Electron diffraction and atomic-resolution scanning transmission electron microscopy imaging revealed short-range Ni/Sn ordering in both the pristine and Li-exchanged materials that is similar to the "honeycomb" Li/M ordering in Li₂MO₃ oxides. As supported by bond-valence sum and density functional theory calculations, this ordering is driven by charge difference between Ni²⁺ and Sn⁴⁺ and the necessity to maintain balanced bonding for the oxygen anions. Li_0.35Na_0.07Ni_0.5Sn_0.5O₂ demonstrates reversible electrochemical (de)intercalation of ∼0.21 Li⁺ in the 2.8-4.3 V vs. Li/Li⁺ potential range. Limited electrochemical activity is attributed to a formation of the surface Li/Ni disordered rock-salt barrier layer as the Li⁺ for Na⁺ exchange drastically reduces the energy barrier for the Li/Ni antisite disorder.