Solid-state lithium (Li) metal batteries promise both high energy density and safety. However, current solid ion conductors fail to satisfy the rigorous requirements in battery operations. Generally, polymer ion conductors that are Li-metal-stable usually provide good interfacial compatibility and mechanical tolerance but typically suffer from inferior ionic conductivity due to the coupling of the ion transport with the polymer chain motion. The natural abundant cellulose contains rich polar functionalities, which could solvate Li⁺ and aid in Li⁺ movement. However, the narrow spacing between the cellulose molecular chains does not naturally allow Li⁺ incorporation. Here, we present a general strategy for achieving high-performance solid polymer ion conductors via molecular channel engineering in cellulose.¹ Through Cu²⁺-coordination with one-dimensional (1D) cellulose nanofibrils, we demonstrate that the opening of molecular channels within the normally ion-insulating cellulose enables rapid Li⁺ transport along the polymer chains. In addition to a high Li⁺ conductivity (1.5×10⁻³ S cm⁻¹ at room temperature), the Cu²⁺-coordination cellulose ion conductor also exhibits a high transference number (0.78 vs. 0.2–0.5 in other polymers) and a wide electrochemical stability window (0–4.5 V) that can accommodate both the Li metal anode and high-voltage cathodes. This 1D ion conductor also allows ion percolation in thick LiFePO₄ solid-state cathodes for high energy density batteries. Furthermore, we have verified the universality of this molecular-channel engineering approach with other biopolymers and cations, similarly achieving high conductivities, with significance that could go far beyond safe, high-performance solid-state batteries.

Qisheng Wu

Appearances