A cellulose-derived supramolecule for fast ion transport

Abstract

Supramolecular frameworks have been widely synthesized for ion transport applications. However, conventional approaches of constructing ion transport pathways in supramolecular frameworks typically require complex processes and display poor scalability, high cost, and limited sustainability. Here, we report the scalable and cost-effective synthesis of an ion-conducting (e.g., Na⁺) cellulose-derived supramolecule (Na-CS) that features a three-dimensional, hierarchical, and crystalline structure composed of massively aligned, one-dimensional, and ångström-scale open channels. Using wood-based Na-CS as a model material, we achieve high ionic conductivities (e.g., 0.23 S/cm in 20 wt% NaOH at 25 °C) even with a highly dense microstructure, in stark contrast to conventional membranes that typically rely on large pores (e.g., submicrometers to a few micrometers) to obtain comparable ionic conductivities. This synthesis approach can be universally applied to a variety of cellulose materials beyond wood, including cotton textiles, fibers, paper, and ink, which suggests excellent potential for a number of applications such as ion-conductive membranes, ionic cables, and ionotronic devices.

Fig. 1.. Molecular engineering of cellulose toward the highly ordered Na-CS.

(A) Schematic diagrams of the hierarchical structure of cellulose in wood at different scales. The wood cells are typically in the range of ~10 to 100 μm in diameter (left), which contains cellulose nanofibers with diameters of ~2 to 100 nm (middle). The smallest unit of cellulose is composed of cellulose molecular chains at the subnanometer to nanometer scale (right). (B) Schematic image of the vastly aligned 1D channels formed by the cellulose molecular chains (ligands) and copper ions (metal nodes), which allow for fast ion transport. The structure is generated from crystal structure modeling based on XRD results. The channels have an available space of ~10 Å in diameter for ion conduction. The water molecules and Na⁺ ions are omitted in the drawing to clearly show the structure of the open channels.

Fig. 2.. Fabrication and structural characterization of Na-CS.

(A) Digital images showing the process of transforming delignified wood (white) as a model cellulose material to Na-CS (blue), which demonstrates a gradual color change over the course of ~1-week immersion in Cu²⁺-saturated NaOH solution. (B) Schematics showing the transformation at the molecular level, including the fast deprotonation and opening of the cellulose molecular chains upon the formation of Na-cellulose complex, and the slower coordination of the cellulose molecular chains to the copper ions. (C) Cu K-edge XANES and EXAFS spectra of Na-CS, Cu foil, CuO, and Cu₂O. (D) Measured and calculated XRD patterns of Na-CS, showing excellent agreement. (E) Na cellulose II crystal. (F) Na-CS crystal. The unit cells are labeled in (E) and (F). (E and F) Both viewed along the axial direction of the cellulose molecular chains.

Fig. 3.. Structure of Na-CS for ion conduction.

(A) EDS mapping of Na-CS before washing, showing signals of both copper and sodium. (B) EDS mapping of Na-CS after washing, in which the sodium signal is barely detectable, while the copper signal remains nearly unchanged. (C) Schematic (generated from our model) showing the massively aligned 1D channels, containing Na⁺ ions and the water molecules used to solvate them. (D) Schematic of the massively aligned 1D channels where water molecules and alkali ions are removed, while the copper ions remain at the same positions to hold the framework together. (E) In 20 wt% NaOH solution, the ionic conductivity of the wood-based Na-CS increases with reduced membrane thickness due to pressing. The nonpressed condition corresponds to the cross-sectional microstructure shown in fig. S17A. The pressed condition corresponds to the cross-sectional microstructure shown in fig. S17B. (F) SEM with a perspective view of the pressed wood-based Na-CS, showing that the nanofibers are densely packed. (G) Ionic conductivity of the wood-based Na-CS soaked with NaOH solutions at various concentrations. The inset shows the wood-based Na-CS used for the ionic conductivity measurement. The arrow indicates the channel direction.

Fig. 4.. Synthesizing other forms of Na-CS in cellulose materials and their application.

(A) Cotton fiber (top) and Na-CS fiber (bottom). (B) Transparent cellulose film (top) and Na-CS film (bottom). (C) Filter paper (top) and Na-CS paper (bottom). (D) Digital images of a Na-CS paper before and after soaking in 20 wt% NaOH solution for 3 months. No change of morphology or color was observed, indicating Na-CS’s good stability. (E) Comparison of the discharge and charge profiles as well as (F) the rate performance of aqueous sodium-ion batteries fabricated using filter paper-based Na-CS and glass fiber as membranes. The aqueous sodium-ion battery featured Ni(OH)₂ as the cathode and PAQS as the anode. 1 C corresponds to 0.2 A/g_PAQS.