To the best of our knowledge, the o

To the best of our knowledge, the only other Si material that has
similar scalability and cost is metallurgical-grade Si. It is made up of
micron-sized powders with ,98% purity produced by carbothermic
reduction from quartzite at ,2000uC, as shown in Fig. 1a. The particle
size are big due to the melting of Si during the high temperature
process. We compared its battery performance with the nano-Si
recovered from RHs (Fig. 4c). After only 20 cycles, the capacity of
the metallurgical-grade Si drops below 500 mA h g21, while RHderived
nano-Si still delivers 2200 mA h g21 after 100 cycles.
Therefore, the nano-Si synthesized by our method has superior battery
performance together with scalable production and low cost. In
addition, the nano-Si anodes derived from RHs, without any coating,
have better cycling stability and longer cycle life than those derived
from other silica sources, such as diatomaceous earth43 and synthesized
nanostructured silica40,44,45, with either carbon or silver coating
(Fig. 1a). The small size, high porosity and interconnectedness of the
nano-Si derived from RHs are the key to achieve such high performance.
The unique morphology is inherited by the natural silica nanoparticles
in RHs and the successful preservation of the nanostructure
throughout the recovery process.