A Review: Enhanced Anodes of Li/Na-Ion Batteries Based on Yolk–Shell Structured Nanomaterials

Fossil fuels maintain the backbone for global energy supply. With the fast growth of energy consumption, it is urgent to exploit renewable energy [ 1 ]. Until now, solar and wind energy burst out a capability of relieving energy shortage [ 2 ]. Diverse energy conversion and storage devices are developed toward efficiently exerting solar and wind energy [ 3 , 4 , 5 , 6 , 7 ]. In this respect of energy storage from solar and wind energy to electrical energy, lithium-ion batteries (LIBs) account for a significant status due to its large capacity, long lifespan and high energy density [ 8 , 9 ]. Furthermore, LIBs play a key role in the development of portable electronic devices and electric vehicles (e.g., laptops and cell phones) [ 10 , 11 , 12 , 13 ]. Generally, the configuration of LIBs consists of an anode, a cathode and electrolyte. During the process of charging and discharging, Li ions intercalate and deintercalate between two electrodes via the electrolyte [ 14 ]. The composition, morphology and structure of cathodes and anodes, together with the diffusion kinetics in the electrolyte, are significant and have been widely studied to thoroughly exert these superior properties in LIBs [ 8 , 15 , 16 ]. Particularly, the performance of LIBs significantly depends on the active anodes, which are used to store and release Li ions during charging and discharging possess [ 17 , 18 ]. The most worldwide popular anode is graphite anode on account of its stable potential, low cost and long cycle life since its commercialization in LIBs by Sony Corporation [ 19 , 20 ]. However, the graphite anode has a limited theoretical specific capacity of 372 mAh g −1 and poor rate capacity, which cannot satisfy the development of the portable electronic devices and electric vehicles [ 21 , 22 , 23 ]. Additionally, the global storage of lithium cannot afford massive application [ 24 , 25 ]. Nevertheless, sodium enormously exists on earth [ 26 , 27 ]. Therefore, sodium-ion batteries (SIBs) with working principle identical to LIBs are expected to be an alternative strategy for decreasing the cost of LIBs, even though the energy density of SIB is tentatively inferior to LIBs [ 25 , 28 , 29 ].