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.
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