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

Abstract

The interface stability of cathode/electrolyte for Na-ion layered oxides is tightly related to the oxidized species formed during the electrochemical process. Herein, we for the first time decipher the coexistence of (O₂)^<i>n</i>- and trapped molecular O₂ in the (de)sodiation process of P2-Na_0.66[Li_0.22Mn_0.78]O₂ by using advanced electron paramagnetic resonance (EPR) spectroscopy. An unstable interface of cathode/electrolyte can thus be envisaged with conventional carbonate electrolyte due to the high reactivity of the oxidized O species. We therefore introduce a highly fluorinated electrolyte to tentatively construct a stable and protective interface between P2-Na_0.66[Li_0.22Mn_0.78]O₂ and the electrolyte. As expected, an even and robust NaF-rich cathode-electrolyte interphase (CEI) film is formed in the highly fluorinated electrolyte, in sharp contrast to the nonuniform and friable organic-rich CEI formed in the conventional lowly fluorinated electrolyte. The <i>in situ</i> formed fluorinated CEI film can significantly mitigate the local structural degeneration of P2-Na_0.66[Li_0.22Mn_0.78]O₂ by refraining the irreversible Li/Mn dissolutions and O₂ release, endowing a highly reversible oxygen redox reaction. Resultantly, P2-Na_0.66[Li_0.22Mn_0.78]O₂ in highly fluorinated electrolyte achieves a high Coulombic efficiency (CE) of >99% and an impressive cycling stability in the voltage range of 2.0-4.5 V (vs Na⁺/Na) under room temperature (147.6 mAh g^-1, 100 cycles) and at 45 °C (142.5 mAh g^-1, 100 cycles). This study highlights the profound impact of oxidized oxygen species on the interfacial stability of cathode/electrolyte and carves a new path for building stable interface and enabling highly stable oxygen redox reaction.