Na₂CO₃ was introduced into the composite positive electrode of P2−Na_2/3Fe_1/2Mn_1/2O₂ as a sacrificial salt, supplying Na⁺ through the electrochemical oxidative decomposition of Na₂CO₃ during the first charging process, and its decomposition mechanism was investigated in detail.

Abstract

Owing to their high discharge capacities, P2-type transition metal layered oxides have attracted attention for use as positive electrode materials in Na-ion batteries. However, owing to the Na-deficient compositions of these oxides, additional Na⁺ must be supplied using a Na-metal negative electrode to attain a high capacity in a half-cell configuration. In this study, solid Na₂CO₃ powder was introduced into the P2−Na_2/3Fe_1/2Mn_1/2O₂ composite positive electrode as a sacrificial salt to compensate for the Na deficiency. Na⁺ was supplied through the electrochemical oxidative decomposition of Na₂CO₃ during the initial charging process; the decomposition mechanism responsible for this process was investigated in detail. Online electrochemical mass spectrometry confirmed that Na₂CO₃ was oxidatively decomposed in combination with the decomposition of the ethylene carbonate electrolyte. This reaction produced CO₂, wherein the carbon source was derived from both Na₂CO₃ and the electrolyte. Consequently, Na⁺ supplementation improved the reversible capacity of the Na-ion full cell. This study offers practical insights and a mechanistic understanding of the pre-doping technique for Na-free negative electrodes. This approach also compensates for the irreversible reductive capacity in a process that can be easily applied to practical sodium- and lithium-ion batteries and capacitors.