Role of mesoporosity in cellulose fibers for paper-based fast electrochemical energy storage

Abstract

Paper, a low-cost and flexible substrate made from cellulose fiber, is explored in this study as a platform for fast electrochemical energy storage devices. Conductivity and Li-storage capabilities are introduced to the paper by functionalization with carbon nanotubes (CNTs) and V₂O₅, respectively. The Li-storage paper cathodes present a remarkably high rate performance due to the high conductivity of CNTs, short Li⁺ diffusion length in V₂O₅ nanocrystals, and more importantly the hierarchical porosity in paper for Li⁺ transport. The specific capacity of V₂O₅ is as high as 410 mA h g⁻¹ at 1 C rate, and retains 116 mA h g⁻¹ at a high rate of 100 C in the voltage range of 4.0–2.1 V. To understand the role of mesoporosity in individual cellulose fibers, we created a control structure by intentionally blocking the mesopores in paper with a 20 nm Al₂O₃ coating applied via atomic layer deposition (ALD). We found that the V₂O₅ capacity decreases by about 30% at high rates of 5–100 C after blocking, which serves to be the first confirmative evidence of the critical role of mesoporosity in paper fibers for high-rate electrochemical devices.