Removal of hemicellulose from alkaline lignin improved electrochemical performance of hard carbon for sodium-ion battery application

A research team from Queensland University of Technology has developed an effective strategy to enhance sodium-ion batteries (SIBs) by using lignin, a natural polymer, as sustainable precursor for hard carbon anodes. Lignin, which is a by-product of processing biomass, has chemical treated to eliminate its hemicellulose. The purified lignin was used to make hard carbon (HC) with improved structural properties, like short-range graphitic layers, fewer defects, and a better pore structure that facilitates sodium storage. Their findings demonstrated that removing hemicellulose significantly boosts the initial Coulombic efficiency (ICE, 76.1%) and the reversible capacity of the hard carbon (277.5 mAh g^-1) , along with 86.1% capacity retention after 250 cycles. This study highlights that hemicellulose removal is a crucial first step in improving the electrochemical performance of lignin-derived HC.

As global energy demands soar, there is an urgent need for affordable, sustainable, and efficient battery technologies suitable for large-scale energy storage, such as powering homes or grid systems. Sodium-ion batteries (SIBs) are promising because sodium is abundant and cheap compared to lithium. However, the commercialization of SIBs is still limited by the lack of suitable anode materials offering high capacity, good rate performance and stable cycling. HC, a non-graphitizable carbon, has gained significant attention as anode material for SIBs. Its disordered microstructure with turbostratic domains and nanopores enables efficient sodium storage through adsorption and intercalation. Lignin, a renewable, aromatic-rich polymer and the byproduct of the pulp and paper industry, is very attractive as a precursor to synthesize HC. However, HC generated from alkaline lignin obtained from NaOH pretreatment often suffers from low ICE and insufficient reversible capacity, posing a major challenge for SIB application.

The Solution: The researchers reported a straightforward and effective strategy to remove hemicellulose from crude alkaline lignin (CAL) to enhance the electrochemical performance of HC anodes. The results of this work reveal that purified alkaline lignin (PAL) has higher carbon content, fewer oxygen-containing functional groups and more uniform aromatic structure compared to CAL, mainly due to the removal of hemicellulose (xylan and arabinan). PAL-based HC consistently outperformed CAL-based HC at every carbonization condition tested. The PAL-based HC sample that carbonized at optimum carbonization condition of 1300 °C and 4 °C min^-1 (PAL₄-13) achieved the best performance, delivering a reversible capacity if 277.5 mAh g^-1 and ICE of 76.1% and excellent capacity retention of 86.1% over 250 cycles. In contrast, the CAL-based HC showed its best performance, offering only 198.5 mAh g^-1 reversible capacity and 60.2% of ICE at 1400 °C at 4 °C min^-1 (CAL₄-14). The superior performance of PAL₄-13 is attributed to its favorable microstructure, including lower open porosity, higher closed pore volume, reduced defects and wide interlayer spacing. These results highlight hemicellulose removal as an effective and practical strategy for enhancing the electrochemical properties of alkaline lignin-based HC.

The Future: Future research will focus on further improving the ICE and reversible capacity through advanced modification approaches while investigating the underlying mechanism for the lower ICE. Despite the progress achieved, the relatively low ICE continues to be a key barrier to scaling up practical application. Future studies should aim to boost ICE while improving reversible capacity and electrochemical performance. Beyond purification, approaches such as heteroatom doping, structural modification and surface functional group regulation should be considered. Simultaneously, uncovering the underlying mechanism of SEI formation, its composition, stability and growth behavior will be vital for mitigating irreversible capacity loss and enhancing battery performance.

The Impact: This study provides a valuable foundation for the rational design of lignin-derived HC anodes, highlighting the critical role of the removal of hemicellulose and precise carbonization parameters. It contributes to advancing sustainable materials for next-generation energy storage systems.

The research has been recently published in the online edition of Materials Futures, a prominent international journal in the field of interdisciplinary materials science research.

Reference: Nethmi Kulanika Dayarathne, Eric Campbell, Mansi Goyal, Mu Xiao, Xueping Song, Cheng Yan, Hongxia Wang, Dawei Wang, Yu Lin Zhong, Zhanying Zhang. Removal of Hemicellulose from Alkaline Lignin Improved Electrochemical Performance of Hard Carbon for Sodium-Ion Battery Application[J]. Materials Futures. DOI: 10.1088/2752-5724/ae1522