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| Fig. 1: Cross section through a zinc-air button battery. (Source: Wikimedia Commons) |
Wood, a widely available renewable resource, is increasingly significant in sustainable technology. Recent advances in wood technology have expanded its use in areas such as energy management, environmental remedies, and battery development. [1] A particularly interesting case is wood engineering for battery applications.
Wood consists of hollow cells composed of lignin and cellulose. Lignin serves to bind cellulose fibers, providing wood its structural rigidity. The composition of wood typically includes about 20-30%wt of lignin and 70-80%wt of various types of celluloses. The specific ratios of lignin and the different celluloses result in a range of structural properties across various wood types. [1] These hollow cells create an interconnected network, facilitating the multiphase transport of nutrients, electrons, ions, and gases. [2] Advances in wood engineering have enabled the manipulation of these pathways for use in batteries. The following sections provide examples of how wood has been integrated into standard battery frameworks.
Zinc-air batteries, known for high specific energy density, are commonly used in hearing aid devices (Fig. 1). Typically, they have an energy density of ~350 Wh/kg, an operational voltage of ~1.2 V, and a specific capacity of ~290 mAh/g. [2] Zhong et. al. utilized wood as cathodes in rechargeable zinc-air batteries. [3] The wood was pretreated with FeCl3 acid and carbonized, introducing Fe-N ions and creating microchannels for better conductivity. These batteries showed an energy density of approximately 880 Wh/kg, an operational voltage of about 1.2 V, and a specific capacity of around 730 mAh/g. Despite being stable for 200 hours of charge/discharge cycling, the efficiency was only 60%, which is low for commercial rechargeable battery applications. For context, lithium-ion batteries typically have efficiencies between 80% to 95%. [4]
Lithium-air batteries are attractive due to their high theoretical energy density of 11,114 Wh/kg. [5] However, they are not yet mass-produced or commercially available. Experimental models haven't exceeded 400 Wh/kg, and these are values usually reported for very few cycles, far from the theoretical limit. Another approach to boost conductivity involves embedding conductive materials into wood's microstructure. [1] In a study, Chen et al. used balsa wood as the cathode in a Li-air battery. [6] The lignin in the balsa was partially removed and then infused with conductive carbon nanotubes and ruthenium catalyst nanoparticles. This modified battery exhibited a specific energy density of about 1200 Wh/kg, an operational voltage of around 2.5 V, and a specific capacity of approximately 480 mAh/g. These are in the range of typical Li-ion batteries. Although it achieved an efficiency of 82%, its stability was tested only up to 220 cycles. To compete with commercially available lithium-ion batteries, it should perform between 400-1200 cycles. [5]
In summary, the innovative application of wood in battery technology presents a interesting avenue for sustainable energy solutions. The hollow cell microstructure of wood have been harnessed to produce more sustainable of zinc-air and Li-air batteries. However, significant challenges persist, particularly in achieving the high efficiency and cycle stability essential for commercial viability. Moreover, these wood-based batteries have yet to demonstrate the groundbreaking performance necessary to transition from academic exploration to being of tangible value in the industry. [7]
© Carlos Kometter. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. He et al., "Engineered Wood: Sustainable Technologies and Applications," Annu. Rev. Mater. Res. 53, 195 (2023).
[2] X.-W. Lv et al., "Rechargeable Zinc-Air Batteries: Advances, Challenges, and Prospects," Small 2023, e2306396 (2023).
[3] L. Zhong et al., "Wood Carbon Based Single-Atom Catalyst For Rechargeable Zn-Air Batteries," ACS Energy Lett. 6, 3624 (2021).
[4] S. Randau et al., "Benchmarking the Performance of All-Solid-State Lithium Batteries," Nat. Energy 5, 259 (2020).
[5] S. Matsuda et al., "Criteria For Evaluating Lithium-Air Batteries in Academia to Correctly Predict Their Practical Performance in Industry," Mater. Horiz. 9, 856 (2022).
[6] C. Chen et al., "Nature-Inspired Tri-Pathway Design Enabling High-Performance Flexible Li-O2 Batteries," Adv. Energy Mater. 9, 1802964 (2019).
[7] Z. Lin et al., "Aligning Academia and Industry For Unified Battery Performance Metrics," Nat. Commun. 9, 5262 (2018).
