Study of Transport Properties and Interfacial Kinetics of Na_2/3[Ni_1/3Mn_xTi_2/3-x]O₂(x = 0,1/3) as Electrodes for Na-Ion Batteries

We investigated the transport properties and interfacial kinetics of P2-Na 2/3 [Ni 1/3 Ti 2/3 ]O 2 (NT) and P2-Na 2/3 [Ni 1/3 Mn 1/3 Ti 1/3 ]O 2 (NMT) layered oxide materials, using three techniques: DC conductivity measurement, potentiostatic intermittent titration technique, and impedance spectroscopy. The measured electronic conductivity (NT: 3.96 × 10 −8 S/cm, NMT: 1.21 × 10 −7 S/cm at 110°C) was orders of magnitude lower than the ionic conductivity (NT: 4.89 × 10 −3 S/cm, NMT: 8.28 × 10 −3 S/cm at 110°C) in both materials. Manganese addition improved the charge carrier transport properties by a factor of 2–3. The potential-dependent diffusion coefficients of both materials were in the order of 10 −14 –10 −12 cm 2 /s. The charge transfer resistance was also found to have a strong potential dependency and the interfacial kinetics of NMT were considerably faster than NT. Due to its faster ionic/electronic transport in the pristine/intercalated states and faster interfacial kinetics, NMT was found to exhibit better rate performance than NT. Further performance improvements need to focus on boasting the intrinsic electronic conductivity of these materials.