Operando Studies on the NaNi_0.5Ti_0.5O₂ Cathode for Na-Ion Batteries: Elucidating Titanium as a Structure Stabilizer

Abstract

O3-type layered NaNi0.5Ti0.5O2, which has been reported previously as a promising cathode material for Naion batteries, has been characterized using comprehensive operando techniques combined with electrochemical and magnetization measurements. Synchrotron diffraction revealed a reversible O3-P3 transformation during charge and discharge without any intermediate phases, which stands in contrast to NaNiO2 and NaNi0.5Mn0.5O2. X-ray absorption studies showed that the electrochemical process in the potential window of 1.5-4.2 V vs. Na + /Na is sustained exclusively by Ni oxidation and reduction while Ti remains inactive. These findings are further supported by ex situ magnetization measurements, yielding a lower paramagnetic moment in the charged state. On the basis of these insights, we elaborate on the beneficial stabilizing effect of Ti. However, a strong Crate dependence for NaNi0.5Ti0.5O2 and NaNi0.5Mn0.5O2 during cycling known from the literature points at a rather high influence of the original structure stacking and the associated Na migration paths. ASSOCIATED CONTENT Supporting Information. Structural parameters of O3 and P3 phases, OCV evolution, XAS edge shifts, Forward Fourier Transformed XAS spectra, EXAFS fit parameters, inverse susceptibility plots, XRD in cycled state and cycling performance. This material is available free of charge via the Internet at http://pubs.acs.org.

Direct Observation of Alternating Octahedral and Prismatic Sodium Layers in O3‐Type Transition Metal Oxides

“…[44] We demonstrate here that this JT activity is crucial for forming OP2. Note that the P3 phase, rather than OP2, is observed above 4 V in various Fe-free Ni-containing NaTMO2, [13,22,45] suggesting that OP2 stabilization at high state of charge by Ni 3+ is less likely. This is consistent with the fact that the Ni 3+ JT distortion is considerably weaker than for Mn 3+ and Fe 4+ and the fact that much of Ni 3+ is oxidized to the non-JT-active Ni 4+ .…”

Oxide cathodes for sodium‐ion batteries: Designs, challenges, and perspectives

“…O3‐type NaTi 0.5 Ni 0.5 O 2 delivered 121 mAh g −1 reversible capacity with an average potential of 3.1 V at a current of 0.2 C 137 . Reversible O3−P3 transformation was found, and Ti remained inactive in the potential window of 1.5−4.2 V 138 . A similar O3−P3 phase transition was also observed in NaNi 0.5 Co 0.5 O 2 and NaNi 0.5 Fe 0.5 O 2 139 .…”

Probing the Effect of Titanium Substitution on the Sodium Storage in Na3Ni2BiO6 Honeycomb-Type Structure

“…Energies 2020, 13, x FOR PEER REVIEW 2 of 14 it unsuitable for battery applications. Improvements of the cycling stability and rate capability [4,5] of NaNiO2 were achieved by substitution of 50% nickel with inactive Ti that led to phase transition suppression and only one remaining transformation (O3-P3) upon cycling [6]. These phases are distinguished according to the classification for layered oxides [7,8], primarily in terms of the oxygen coordination of sodium-ions, which is prismatic (P3) or octahedral (O3).…”

“…By inserting Ti into the honeycomb structure, The first sodiation step of Na 3 Ni 2 BiO 6 leads to an additional O'3 phase, which results in the O3-O'3-P3-O1 transformation [9,[11][12][13]. The multistep phase transformation causes a lattice structure reorganization, which results in low cycling stability [5,6]. There are two ways to suppress the phase transition of the honeycomb layered Na 3 Ni 2 BiO 6 structure: firstly, nickel-ion substitution, secondly-inactive bismuth substitution.…”

“…Despite the fact that the presence of Ti on the 3d metal positions in the structure of layered oxides is beneficial for the cycling stability [7], there is no information in the literature about the influence of a Ti additive on the properties of the honeycomb structure. Therefore, our work is based on mixing two state-of-the-art systems: O3-NaNi 0.5 Ti 0.5 O 2 and O3-Na 3 Ni 2 BiO 6 with honeycomb structure [6,9]. The present work includes the synthesis of several compositions obtained by the replacement of Bi 5+ (0.76 Å) and Ni 2+ (0.69 Å) by the appropriate stoichiometric amount of Ti 4+ (0.61 Å).…”