A binder-free high silicon content flexible anode for Li-ion batteries

Despite the high theoretical capacity of Si anodes, their huge volume change and poor electrical connectivity must be overcome by decreasing the feature sizes of the Si particles to the nanoscale and compositing them with highly conductive carbon materials. To ensure the mechanical integrity of the electrodes with uniform dispersion and good electrical contact of the nano-silicon, the industry has to use a relatively low amount of Si (o15% by mass) in commercial anodes. Such a low Si content severely weakens the capacity advantage of silicon at the electrode level. Here, we designed a cellulose-based topological microscroll by the self-rolling of cellulose nanosheets to form a binder-free, flexible, and free-standing electrode comprising an unprecedented 92% silicon content. In the microscroll, carboncoated silicon nanoparticles are anchored on conductive carbon nanotubes and subsequently confined in cellulose carbon rolls with enough internal voids to accommodate the volume expansion of silicon; thus, a uniform dispersion of silicon with high reactivity was achieved. This structure shows an ultrahigh electrode-specific capacity of 2700 mA h g1, excellent cycling stability under a high silicon content of 85% (42000 mA h g1 after 300 cycles) and a commercial-level areal capacity (5.58 mA h cm2). This strategy offers a new way to design electrodes with a high active material content for high performance batteries. Broader context To satisfy the growing energy demands for electric vehicles and portable electronics, silicon has been extensively developed as a promising anode material for next-generation Li-ion batteries. Tremendous efforts have been devoted to designing and modifying the material nanostructures for improving the electrochemical performance, and thus demonstrating the bright prospects of silicon anodes. However, energy storage technologies using silicon anodes still have some big obstacles to overcome on their way to commercialization. There is a big gap between the lab condition results and industrial requirements, especially in terms of the scale-up ability, nanostructure robustness, and actual available capacity at the electrode level. In this work, we designed a robust silicon electrode with an unprecedented 92% silicon content and unique microstructure by a simple, green, low-cost and scalable method, which aims to solve the above issues towards commercial application of silicon. Owing to the self-rolling of cellulose nanosheets in the synthetic process, Si nanoparticles are wrapped into binder-free flexible electrodes with controllable Si content. The designed microscroll electrodes with well-engineered and robust structure show high specific capacity and good cycling stability. The method of cellulose-based topological roll design can also be used for other materials and systems with a high active material content.