Irreversible Made Reversible: Increasing the Electrochemical Capacity by Understanding the Structural Transformations of Na_xCo_0.5Ti_0.5O₂

Abstract

Two new structural forms of Na x Co0.5Ti0.5O2, the layered O3- and P3-forms, were synthesized and comprehensively characterized. Both materials show electrochemical activity as electrodes in Na-ion batteries. During cell charging (desodiation of the Na x Co0.5Ti0.5O2 cathode), we observed a structural phase transformation of O3-Na0.95Co0.5Ti0.5O2 into P3-Na x Co0.5Ti0.5O2, whereas no changes other than conventional unit cell volume shrinkage were detected for P3-Na0.65Co0.5Ti0.5O2. During Na insertion (cell discharging), the reconversion of the P3-form into O3-Na x Co0.5Ti0.5O2 was impeded for both materials and occurs well below 1 V versus Na+/Na only. The reconversion is hindered by the charge and spin transfers of Co (LS-Co3+ → HS-Co2+) and by a significant unit cell volume expansion at the P3 → O3 transformation, as revealed from the magnetization, crystallographic, and spectroscopic studies. As the kinetics of such transformations depend on numerous parameters such as time, temperature, and particle size, a large cell overpotential ensues. An extended cutoff voltage at 0.2 V versus Na+/Na during discharging allows to complete the P3 → O3 transformation and increases the specific discharging capacity to 200 mA h g–1. Moreover, a quasi-symmetrical full cell, based on the O3- and P3-forms, was designed, eliminating safety concerns associated with sodium anodes and delivering a discharge capacity of 130 mA h g–1.

Redox-Driven Spin Transition in a Layered Battery Cathode Material

“…This phenomenon is not caused by sluggish conversion/phase segregation because ex situ X-ray diffraction experiments confirmed a commonly observed smooth reversible transformation between the O3 and P3 phases upon Na-ion (de)­intercalation (Figures b and S4). Indeed, a similar potential hysteresis for Na x Ti 0.5 Co 0.5 O 2 was also reported very recently by Maletti et al Therefore, we attribute this large voltage hysteresis to specific energetics of the electron configuration.…”

Operando Studies on the NaNi0.5Ti0.5O2 Cathode for Na-Ion Batteries: Elucidating Titanium as a Structure Stabilizer

“…Interestingly, the high-voltage region above 4 V has already been associated with side reactions of different origins, as for example for Na x Ni 0.3 Mn 0.7 O 2 and Na x Co 0.5 Ti 0.5 O 2 . 12,19 Therein, a stagnation of lattice parameters and a higher degree of desodiation according to GCPL compared to the refined occupancies were measured. In our case, the lattice parameters keep changing and GCPL delivers a lower desodiation state than the refined occupancies, pointing at a considerably different situation.…”

Evidence of a Pseudo‐Capacitive Behavior Combined with an Insertion/Extraction Reaction Upon Cycling of the Positive Electrode Material P2‐NaxCo0.9Ti0.1O2 for Sodium‐ion Batteries

“…Despite this, a continuous change in the E=f(x) curve is observed (see Figure b), suggesting that other contribution is in concomitance with the intercalation/deintercalation mechanism. The sluggish evolution of unit cell parameters can be due to many reasons, magnetic spin of Co as reported by Maletti et al., zero strain effect like in the case of Li 4 Ti 5 O 12 , Another possibility could be contribution from surface. In the last case, the arising phenomenon could be a pseudocapacitive behavior in the voltage range 2–4.2 V. The contribution of pseudocapacitive mechanism rise with the voltage increasing, so in the voltage diapason from 3.37 V in desodiation process till 3.79 V in sodiation, we expect predominantly pseudocapacitor region.…”