Modulating the open pore structure of hard carbons derived from wood for sodium-ion battery anodes

Abstract

Hard carbon (HC) derived from renewable biomass is a promising anode material for sodium-ion batteries (SIBs). However, controlling the structure of hard carbon so that it has a high energy density, favorable rate performance, and cycling stability is still a challenge. We propose a strategy to control the open pore structure of hard carbon derived from wood for sodium-ion storage by the addition of sodium carbonate under carbonization at 1100 °C. The resulting HC has an increased interlayer spacing, and a more uniform open pore distribution (2–3 nm) with a high slope capacity, thereby enabling efficient sodium-ion transport and storage. The HC anode has a reversible capacity of 326 mAh g−1 at a current density of 30 mA g−1, and maintains a reversible capacity of 270 mAh g−1 at 1 A g−1 and a capacity of 68 mAh g−1 even at 10 A g−1 during rate performance tests. After 300 cycles, it retains 76.7% (207 mAh g−1) of its capacity at 1.0 A g−1. In situ Raman spectroscopy and the galvanostatic intermittent titration testing results reveal an adsorption-intercalation-filling sodium storage mechanism. This work provides a strategy to optimize the open pore structure of biomass derived hard carbon for high performance sodium ion storage.

1. Introduction

Due to a significant complement of sodium-ion batteries (SIBs) to lithium-ion batteries (LIBs), various materials are currently being employed as anode materials for SIBs, including carbon materials[1], alloys[2], transition metal oxides[3], sulphides[4] and others. Carbon-based materials, with the economic, stable and mature manufacture advantages, have attracted considerable attention[5]. Although the operating mechanism is fundamentally consistent with that of LIBs, the larger radius of Na+ limits the application of graphite in SIBs[6], where hard carbon is widely used. Biomass serves as a cost-effective precursor for preparation of hard carbons because of its high carbon content, affordability and scalability, making it a better option compared to costly polymer resins and non-renewable fossil fuels[6–8].

Generally, the storage pore types of hard carbons for sodium ions encompass open and closed pores (2–5 nm), wherein open pores comprise micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm)[9−10]. To optimize the pore structure of hard carbons, several strategies have been developed, including the regulation of biomass precursor components[11], the optimization of pyrolysis conditions[12], the templating methods[13], and surface coating techniques[14-15]. It should be noted that the recent research has mainly focused on the modulation of closed pores, which is closely related to the plateau capacity of hard carbons, where sodium ions fill the closed pores or even form quasi-metallic sodium clusters to provide the primary reversible capacity[16–18]. However, an excessive bias towards closed-pore formation will yield extremely high plateau capacity and high energy density, at the same time will suffer from insufficient ion transport pathways, poor rate performance, limited electrolyte wettability, and weak buffering capacity against volume changes during cycling[19]. The open pore structures, on the other hand, are believed to not only offer adsorption sites for sodium ions and contribute to the slope capacity, but also serve as an electrolyte reservoir, which facilitate rapid ion transport and improves rate performance[20-22]. Previous studies have demonstrated that carbon dioxide in carbonization can facilitate the formation of numerous closed pores in hard carbons[23]. During the CO2 etching process, open pores are initially created within the carbon matrix, which are subsequently reorganized into closed pores through in-situ reconstruction during high-temperature carbonization. This CO2-assisted pore modulation strategy effectively enhances both the diameter and the volumetric capacity of closed pores in the carbon material.