Biomass-Derived Hard Carbon Anodes for Sodium-Ion Batteries

Abstract

Hard carbon (HC) is currently one of the most promising anode materials for sodium-ion batteries (SIBs). For commercial applications, HC anodes require a high initial Coulombic efficiency (ICE) and a high reversible capacity, which are closely related to a low open-pore volume and a high closed-pore volume in the HC structure. In parallel, HC requires production routes with lower energy consumption and smaller environmental footprints in order to support a circular economy. However, current HC production predominantly relies on conventional resistance-heated carbonization, in which heat is supplied externally and transferred indirectly to the material. This leads to high energy consumption and typically requires extreme carbonization temperatures to induce closed-pore formation. Therefore, additional chemical treatments or post-modification processes are often required, further increasing energy demand and environmental impact. These limitations collectively restrict the scalable and sustainable application of HC anodes.

This thesis aims to develop feasible and energy-efficient modification and carbonization processes for HC production to enhance material resource circularity. Accordingly, fundamental studies combining data-driven modeling, laboratory-scale experiments, and process simulations are conducted. The thesis is based on four different studies that together establish a systematic understanding of the process–structure–performance relationships in biomass-derived HC and propose an energy-efficient pore-engineering strategy enabled by bio-oil modification and induction heating carbonization (IC).

First, through comprehensive literature data analysis and machine-learning modeling, carbonization temperature and HC structure are identified as the dominant factors for HC electrochemical performance. Low open-pore structures and surface defect densities are found to be critical for achieving high ICE, while large interlayer spacing and closed-pore volumes are beneficial for a high plateau capacity. Importantly, the analysis reveals the limitation of conventional carbonization in simultaneously achieving low open porosity and promoting closed-pore formation.

To overcome these limitations, a sustainable bio-oil surface engineering strategy is developed to suppress open pores and surface defects. This approach reduces the specific surface area of HC from 28 to 9 m²/g and increases the ICE from 84.4% to 89.9%. By combining bio-oil surface engineering, a novel IC route is further developed. IC enables direct volumetric heating through eddy currents, which simultaneously minimizes open porosities and promotes closed-pore formation. As a result, optimal IC-derived HC exhibits open-pore volume below 0.003 cm³/g and closed-pore volume of up to 0.23 cm³/g, with an ultra-high ICE value exceeding 95%, and plateau capacity above 260 mAh/g.

Finally, energy analysis and life cycle assessment demonstrate that IC reduces the carbonization energy consumption by approximately 60% and lowers overall environmental impacts by approximately 35% compared with conventional routes. Overall, this thesis demonstrates that the combination of bio-oil modification and IC provides an energy-efficient and low-carbon pathway for producing high-performance biomass-derived HC anodes, supporting the sustainable development of next-generation sodium-ion batteries.

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