Structural Origins of Voltage Hysteresis in the Na-Ion Cathode P2–Na_0.67[Mg_0.28Mn_0.72]O₂: A Combined Spectroscopic and Density Functional Theory Study

Abstract

P2-layered sodium-ion battery (NIB) cathodes are a promising class of Na-ion electrode materials with high Na + mobility and relatively high capacities. In this work, we report the structural changes that take place in P2− Na 0.67 [Mg 0.28 Mn 0.72 ]O 2 . Using ex situ X-ray diffraction, Mn K-edge extended X-ray absorption fine structure, and 23 Na NMR spectroscopy, we identify the bulk phase changes along the first electrochemical charge−discharge cycleincluding the formation of a high-voltage "Z phase", an intergrowth of the OP4 and O2 phases. Our ab initio transition state searches reveal that reversible Mg 2+ migration in the Z phase is both kinetically and thermodynamically favorable at high voltages. We propose that Mg 2+ migration is a significant contributor to the observed voltage hysteresis in Na 0.67 [Mg 0.28 Mn 0.72 ]O 2 and identify qualitative changes in the Na + ion mobility.

Coexistence of (O2)n− and Trapped Molecular O2 as the Oxidized Species in P2-Type Sodium 3d Layered Oxide and Stable Interface Enabled by Highly Fluorinated Electrolyte

“…Figure b presents the isotropic slices of 23 Na pj-MATPASS NMR spectra under different charge/discharge states during the first cycle with HPF as the electrolyte, and the series with EPF as the electrolyte is presented in Figure S14b. Of note, the 23 Na peak representing prismatically coordinated Na ions shows conspicuous intensity decline upon electrochemical reaction, which could be ascribed to the slower Na + ion hopping of the activated electrodes that results in faster T 2 (transverse) relaxation. , On charging, the 23 Na resonance from P2 phase presents a progressive skewing to lower frequencies, implying that the desodiation process follows a quasi-solid-solution reaction. Contrarily, the 4.5 V charged Na 0.66 [Li 0.22 Mn 0.78 ]­O 2 with EPF as the electrolyte shows no noticeable signal with the pj-MATPASS pulse sequence (Figure S14b), indicative of a dramatic increase in the local disorder of the residual Na ions. , Such disordered Na occupation is also consistent with the 23 Na MAS NMR spectrum acquired via Hahn echo pulse sequence (Figure S16).…”

“…Of note, the 23 Na peak representing prismatically coordinated Na ions shows conspicuous intensity decline upon electrochemical reaction, which could be ascribed to the slower Na + ion hopping of the activated electrodes that results in faster T 2 (transverse) relaxation. 75,76 On charging, the 23 Na resonance from P2 phase presents a progressive skewing to lower frequencies, implying that the desodiation process follows a quasi-solid-solution reaction. Contrarily, the 4.5 V charged Na 0.66 [Li 0.22 Mn 0.78 ]O 2 with EPF as the electrolyte shows no noticeable signal with the pj-MATPASS pulse sequence (Figure S14b), indicative of a dramatic increase in the local disorder of the residual Na ions.…”

Tailoring Anionic Redox Activity in a P2-Type Sodium Layered Oxide Cathode via Cu Substitution

“…On charging to 4.5 V, the 23 Na peak shows an ineluctable intensity decline, which could be ascribed to slower Na + ion hopping that results in faster T 2 relaxation. 52,53 Meanwhile, the 23 Na resonance peak drifts to lower frequencies due to O oxidization (about 1233 ppm), 18 implying a solid−solution reaction upon desodiation. Contrarily, the 4.5 V charged NLMO shows weaker and broader signals (Figure S7a), indicative of a pernicious increase in the local disorder of the residual Na ions.…”

“…In addition, a weak signal close to 0 ppm stems from Na-containing diamagnetic compounds such as Na 2 CO 3 and NaOH. On charging to 4.5 V, the 23 Na peak shows an ineluctable intensity decline, which could be ascribed to slower Na + ion hopping that results in faster T 2 relaxation. , Meanwhile, the 23 Na resonance peak drifts to lower frequencies due to O oxidization (about 1233 ppm), implying a solid–solution reaction upon desodiation. Contrarily, the 4.5 V charged NLMO shows weaker and broader signals (Figure S7a), indicative of a pernicious increase in the local disorder of the residual Na ions. , Such findings further explain that NLMCO has superior OR stability; thus, phase transition is absent even after charging to 4.5 V.…”

Achieving a long-life 165 Wh/kg sodium-ion pouch-cell via regulating intergrowth structure in layered oxide cathode with anionic redox

“…6,[18][19] On the aspect of TM migration, with the removal of Na-ions from the prismatic sites and the formation of O-stacking layers, TM would have access to the thermodynamically preferred octahedral sites in the AM layer, which induce the formation of vacancies in the TM layer, resulting in irreversible structural distortion and voltage decay. [20][21][22] Moreover, in order to meet the low-cost principle of SIBs, Fe/Mn-based TM oxides are the most desirable candidates, 4 but the Jahn-Teller effect of Fe and Mn redox would aggravate the TM irreversible migration. After doping Li in the TM layer, such irreversible LiTM migration and even the direct extraction of LiTM from the lattice due to overcharging would further affect the ARR reversibility.…”