Key factors influencing the plateau region in N-doped hard carbon for sodium storage

Abstract

Hard carbon (HC) is a promising anode material for the commercialization of sodium-ion (Na⁺) batteries (SIBs) due to its abundant resources, low voltage plateau, and high reversible capacity. The Na⁺ storage mechanism in HC can be categorized into adsorption, intercalation, and filling models. However, establishing a direct correlation between Na⁺ storage and HC architecture remains challenging due to the extremely difficult control of the HC structure during synthesis. To better understand the Na⁺ storage mechanism within HC, key factors such as specific surface area, heteroatom doping, and pore architecture must be carefully investigated. In this study, we synthesize nitrogen and oxygen co-doped HC nanosheets with negligible micro- and mesoporous structures and a low specific surface area, approaching the ideal “house of cards” model. This model sample allows for gaining insights into the Na⁺ electrochemical behavior. By combining in situ X-ray diffraction, ex situ Raman spectroscopy, and wide/small-angle X-ray scattering, we demonstrate a clear correlation between defect sites and electrochemical behavior in the slope region, and an expanded plateau region. We propose a dominant layer-filling mechanism to explain Na⁺ storage in the plateau region, attributing it to the filling of disordered carbon layers rather than the filling of nano-/micro-pores. Additionally, we demonstrate that the defect-adsorption behavior is highly influenced by the compactness of the carbon layer structure. This work offers a fresh perspective on Na⁺ storage and provides valuable insights for the rational design of high-performance HC anodes in SIBs.