A study of the electrochemical kinetics of sodium intercalation in P2/O1/O3-NaNi1/3Mn1/3Co1/3O2

Structural and electrochemical progress of O3-type layered oxide cathodes for Na-ion batteries

Sodium-ion batteries (SIBs) have attracted great attention being the most promising sustainable energy technology owing to their competitive energy density, great safety and considerable low-cost merits. Nevertheless, the commercialization process of SIBs is still sluggish because of the difficulty in developing high-performance battery materials, especially the cathode materials. The discovery of layered transition metal oxides as the cathode materials of SIBs brings infinite possibilities for practical battery production. Thereinto, the O3-type layered transition metal oxides exhibit attractive advantages in terms of energy density benefiting from their higher sodium content compared to other kinds of layered transition metal oxides. Enormous research studies have largely put forward their progress and explored a wide range of performance improvement approaches from the morphology, coating, doping, phase structure and redox aspects. However, the progress is scattered and has not logically evolved, which is not beneficial for the further development of more advanced cathode materials. Therefore, our work aims to comprehensively review, classify and highlight the most recent advances in O3-type layered transition metal oxides for SIBs, so as to scientifically cognize their progress and remaining challenges and provide reasonable improvement ideas and routes for next-generation high-performance cathode materials.

Na2ZrFe(PO4)3─A Rhombohedral NASICON-Structured Material: Synthesis, Structure and Na-Intercalation Behavior

A NASICON-structured earth-abundant mixed transition metal (T_M) containing Na-T_M-phosphate, viz., Na₂ZrFe(PO₄)₃, has been prepared via a sol-gel route using a low-cost Fe³⁺-based precursor. The as-prepared material crystallizes in the desired rhombohedral NASICON structure (space group: R3̅c) at room temperature. Synchrotron X-ray diffraction (XRD), transmission electron microscopy, X-ray absorption spectroscopy, etc., have been performed to determine the crystal structure, associated details, composition, and electronic structures. In light of the structural features, as one of the possible functionalities of Na₂FeZr(PO₄)₃, Na-intercalation/deintercalation has been examined, which indicates the occurrence of reversible electrochemical Na-insertion/extraction via Fe²⁺/Fe³⁺ redox at an average potential of ∼2.5 V. The electrochemical data and direct evidences from operando synchrotron XRD indicate that the rhombohedral structure is preserved during Na-insertion/extraction, albeit within a certain range of Na-content (i.e., ∼2-3 p.f.u.), beyond which rhombohedral → monoclinic transformation takes place. Within this range, Na-insertion/extraction takes place via solid-solution pathway, resulting in outstanding cyclic stability, higher Na-diffusivity, and good rate-capability. To the best of the authors' knowledge, this represents the first in-depth structural, compositional, and electrochemical studies with Na₂ZrFe(PO₄)₃, along with the interplay between those, which provide insights into the design of similar low-cost materials for various applications, including sustainable electrochemical energy storage systems.

Electrochemical Properties and Ex Situ Study of Sodium Intercalation Cathode P2/P3-NaNi1/3Mn1/3Co1/3O2

In recent work, P2/P3-NaNi1/3Mn1/3Co1/3O2 (NaNMC) was obtained by the sol-gel process followed by calcination of the precursor at 900°C for 12 h. The electrochemical properties of NaNMC were investigated in the voltage range of 2.0–4.0 V. The material exhibited an initial discharge capacity of 107 mAh·g−1 and good capacity retention of 82.2% after 100 cycles. Ex situ XRD performance showed that the P3-phase transformed from the P3- to O1-phase and vice versa, while the P2-phase remained stable during the sodium intercalation. The kinetic of sodium intercalation of NaNMC upon reversible Na+ insertion/deinsertion was evaluated via a Galvanostatic Intermittence Titration Technique (GITT) and Electrochemical impedance spectroscopy (EIS). The diffusion coefficients of Na+ ion deduced from the GITT curve have a broad distribution ranging from 10−10 to 10−11 cm2·s−1 for the charging/discharging process. Besides, the evolution of diffusion coefficient and charge transfer resistance is consistent with the complex phase transition generally observed in sodium layered oxides.

Synergistic Effect of Polymorphs in Doped NaNi0.5Mn0.5O2 Cathode Material for Improving Electrochemical Performances in Na-Batteries

Layered NaNi0.5Mn0.5O2, employed as cathode materials in sodium ion batteries, is attracting interest due to its high working potential and high-capacity values, thanks to the big sodium amount hosted in the lattice. Many issues are, however, related to their use, particularly, the complex phase transitions occurring during sodium intercalation/deintercalation, detrimental for the structure stability, and the possible Mn dissolution into the electrolyte. In this paper, the doping with Ti, V, and Cu ions (10% atoms with respect to Ni/Mn amount) was used to stabilize different polymorphs or mixtures of them with the aim to improve the capacity values and cells cyclability. The phases were identified and quantified by means of X-ray powder diffraction with Rietveld structural refinements. Complex voltammograms with broad peaks, due to multiple structural transitions, were disclosed for most of the samples. Ti-doped sample has, in general, the best performances with the highest capacity values (120 mAh/g at C/10), however, at higher currents (1C), Cu-substituted sample also has stable and comparable capacity values.

Effect of 3D Metal on Electrochemical Properties of Sodium Intercalation Cathode P2-NaxMe1/3Mn2/3O2 (M = Co, Ni, or Fe)

This research aims to evaluate the influence of different 3D metals (Fe, Co, and Ni) substituted to Mn on the electrochemical performance of P2-NaxMe1/3Mn2/3O2 material, which was synthesized by the coprecipitation process followed by calcination at high temperature. X-ray diffraction (XRD) results revealed that the synthesized Mn-rich materials possessed a P2-type structure with a negligible amount of oxide impurities. The materials possessed their typical cyclic voltammogram and charge-discharge profiles; indeed, a high reversible redox reaction was obtained by NaxCo1/3Mn2/3O2 sample. Both NaxCo1/3Mn2/3O2 and NaxFe1/3Mn2/3O2 provided a high specific capacity of above 140 mAh·g−1; however, the former showed better cycling performance with 83% capacity retention after 50 cycles at C/10 and high rate capability. Meanwhile, the Ni-sub NaxNi1/3Mn2/3O2 exhibited excellent cycling stability but a low specific capacity of 110 mAh·g−1 and inferior rate capability. The diffusion coefficient of Na+ ions into the structure tended to decrease with a depth of discharge; those values were in the range of 10−10–10−9 cm2·s−1 and 10−11–10−10 cm2·s−1 in the solid solution region and biphasic region, respectively.