Nanocellulose toward Advanced Energy Storage Devices: Structure and Electrochemistry.

Cellulose is the most abundant biopolymer on Earth and has long been used as a sustainable building block of conventional paper. Note that nanocellulose accounts for nearly 40% of wood's weight and can be extracted using well-developed methods. Due to its appealing mechanical and electrochemical properties, including high specific modulus (∼100 GPa/(g/cm)), excellent stability in most solvents, and stability over a wide electrochemical window, nanocellulose has been widely used as a separator, electrolyte, binder, and substrate material for energy storage. Additionally, nanocellulose-derived carbon materials have also drawn increasing scientific interest in sustainable energy storage due to their low-cost and raw-material abundance, high conductivity, and rational electrochemical performance. The inexpensive and environmentally friendly nature of nanocellulose and its derivatives as well as simple fabrication techniques make nanocellulose-based energy storage devices promising candidates for the future of "green" and renewable electronics. For nanocellulose-based energy storage, structure engineering and design play a vital role in achieving desired electrochemical properties and performances. Thus, it is important to identify suitable structure and design engineering strategies and to better understand their relationship. In this Account, we review recent developments in nanocellulose-based energy storage. Due to the limited space, we will mainly focus on structure design and engineering strategies in macrofiber, paper, and three-dimensional (3D) structured electrochemical energy storage (EES) devices and highlight progress made in our group. We first present the structure and properties of nanocellulose, with a particular discussion of nanocellulose from wood materials. We then go on to discuss studies on nanocellulose-based macrofiber, paper, and 3D wood- and other aerogel-based EES devices. Within this discussion, we highlight the use of natural nanocellulose as a flexible substrate for a macrofiber supercapacitor and an excellent electrolyte reservoir for a breathable textile lithium-oxygen battery. Paper batteries and supercapacitors using nanocellulose as a green dispersant, nanocellulose-based paper as a flexible substrate, and nanocellulose as separator and electrolyte are also examined. We highlight recent progress on wood-based batteries and supercapacitors, focusing on the advantages of wood materials for energy storage, the structure design and engineering strategies, and their microstructure and electrochemical properties. We discuss the influence of structure (particularly pores) on the electrochemical performance of the energy storage devices. By taking advantage of the straight, nature-made channels in wood materials, ultrathick, highly loaded, and low-tortuosity energy storage devices are demonstrated. Finally, we offer concluding remarks on the challenges and directions of future research in the field of nanocellulose-based energy storage devices.