Tunable dynamic networks of cellulose nanofibrils (CNFs) are utilized to prepare high-performance polymer gel electrolytes. By swelling an anisotropically dewatered, but never dried, CNF gel in acidic salt solutions, a highly sparse network is constructed with a fraction of CNFs as low as 0.9%, taking advantage of the very high aspect ratio and the ultra-thin thickness of the CNFs (micrometers long and 2–4 nm thick). These CNF networks expose high interfacial areas and can accommodate massive amounts of the ionic conductive liquid polyethylene glycol-based electrolyte into strong homogeneous gel electrolytes. In addition to the reinforced mechanical properties, the presence of the CNFs simultaneously enhances the ionic conductivity due to their excellent strong water-binding capacity according to computational simulations. This strategy renders the electrolyte a room-temperature ionic conductivity of 0.61 ± 0.12 mS cm−1 which is one of the highest among polymer gel electrolytes. The electrolyte shows superior performances as a separator for lithium iron phosphate half-cells in high specific capacity (161 mAh g−1 at 0.1C), excellent rate capability (5C), and cycling stability (94% capacity retention after 300 cycles at 1C) at 60 °C, as well as stable room temperature cycling performance and considerably improved safety compared with commercial liquid electrolyte systems.

Long, slender cellulose nanofibrils (CNF) are unique with their high axial modulus, small diameter, high flexibility, and the possibility of chemical tailoring of, among other things, their surface charge density. The objective of this work has been to elucidate how the hydrogel network properties and how their related deformation mechanisms depend on CNF properties, concentration, and chemical environment. In addition, the influence of CNF characteristics on the formation of the structure and properties of isotropic hydrogels, anisotropic hydrogels, and aerogels has been studied. 

This was done by combining theoretical models describing the CNF network's topology and mechanics with high-resolution experiments to validate the theoretical models. Furthermore, the properties of the fibrils have been characterized in detail and linked to the material properties of materials formed from the fibrils. Finally, the CNF networks in this work have been functionalized in two different ways. In the first case, a flow channel was created within the hydrogel network at extremely low CNF concentrations, that could be surface treated with a Layer-by-layer (LbL) methodology with a consecutive addition of oppositely charged polyelectrolytes/nanoparticles to add new functionalities to the channels. Secondly, wet stable aerogels, prepared at higher concentrations of CNFs, were treated using the LbL methodology to adjust the aerogels' surface structure and surface energy, thereby controlling the liquid spreading rate properties of the formed networks.

The most important findings in this work are that CNF network topology and network mechanics can be described using theoretical, rather non-complicated, elastoplastic models. Furthermore, at lower concentrations of CNFs, the network structure is formed in a more organized way, meaning that the fibrils have the time and freedom to seek their optimal contact points during the network formation from a thermodynamic free energy point of view. It has also been shown that the low-density, wet fibrillar network structures formed by neutralizing the charges of the fibrils deform by sliding in fibril/fibril contacts upon straining the network structure above a critical stress. These fibril/fibril contacts are also shown to be re-established when the stress is released, provided that the networks have not been subjected to a macroscopic collapse. Finally, these cellulose networks show great potential for further functionalization using the LbL modification methodology.

Highly anisotropic cellulose nanofibrils can solidify liquid water, creating self-supporting structures by incorporating a tiny number of fibrils. These fibrillar hydrogels can contain as much as 99.99 wt% water. The structure and mechanical properties of fibrillar networks have so far not been completely understood, nor how they solidify the bulk water at such low particle concentrations. In this work, the mechanical properties of cellulose fibrillar hydrogels in the dilute regime from a wt% perspective have been studied, and an elastoplastic model describing the network structure and its mechanics is presented. A significant insight from this work is that the ability of the fibrils to solidify water is very dependent on particle stiffness and the number of contact points it can form in the network structure. The comparison between the experimental results and the theoretical model shows that the fibrillar networks in the dilute regime form via a non-stochastic process since the fibrils have the time and freedom to find contact points during network formation by translational and rotational diffusion. The formed, dilute fibrillar network deforms by sliding fibril contacts upon straining the network beyond its elastic limit. Our results also show that before macroscopic failure, the fibril contacts are restored once the load is released. The exceptional properties of this solidified water are exploited to host fluidic channels, allowing directed fluid transportation in water. Finally, the microfluidic channels formed in the hydrogels are tailored by the layer-by-layer technique to be interactive against external stimuli, a characteristic envisioned to be useful in biomedical applications.

Fibrillar hydrogels are remarkably stiff, low-density networks that can hold vast amounts of water. These hydrogels can easily be made anisotropic by orienting the fibrils using different methods. Unlike the detailed and established descriptions of polymer gels, there is no coherent theoretical framework describing the elastoplastic behavior of fibrillar gels, especially concerning anisotropy. In this work, the swelling pressures of anisotropic fibrillar hydrogels made from cellulose nanofibrils were measured in the direction perpendicular to the fibril alignment. This experimental data was used to develop a model comprising three mechanical elements representing the network and the osmotic pressure due to non-ionic and ionic surface groups on the fibrils. At low solidity, the stiffness of the hydrogels was dominated by the ionic swelling pressure governed by the osmotic ingress of water. Fibrils with different functionality show the influence of aspect ratio, chemical functionality, and the remaining amount of hemicelluloses. This general model describes physically crosslinked hydrogels comprising fibrils with high flexural rigidity - that is, with a persistence length larger than the mesh size. The experimental technique is a framework to study and understand the importance of fibrillar networks for the evolution of multicellular organisms, like plants, and the influence of different components in plant cell walls.