The construction and lithium storage performance of silicon-based hollow structured anode materials
With the rapid developments of consumer electronics,electric vehicles,and stationary energy storage,there is an urgent need to develop new-generation lithium-ion batteries(LIBs)with higher energy density,longer cycle life,and better safety.Elemental silicon(Si)has been considered as one of the most promising anode materials for the next-generation LIBs,due to its high theoretical capacity(4200 mAh g-1),rich abundance,easy synthesis,and environmental friendliness.However,the Si-based anode materials suffer from low intrinsic electrical conductivity and large volume variation(300％-400％)during lithiation/de-lithiation processes.The large volume fluctuation would lead to pulverization of Si particles,detachment of the Si particles from the current collector(electrode disintegration),and repeated formation and rupture of solid electrolyte interphase(SEI)films,eventually resulting in severe capacity fading.With unique merits such as high specific surface area,abundant active sites,full electrode/electrolyte contact,short charge(especially lithium ion)transfer paths,and volume buffering effect,hollow nanostructures are promising anode materials for LIBs.When compared to the solid counterpart,the hollow structures are able to provide adequate empty space for the volume expansion of Si,leading to significantly enhanced cycling stability.In this review,we summarize the recent advances of Si-based hollow structured anode materials for lithium storage.The Si-based hollow structured anode materials can be generally categorized into four families:(i)Si-based hollow spheres/cages,especially double-shelled and multi-shelled hollow spheres;(ii)Si-based yolk-shell structures,especially that with a nanostructured Si yolk and a highly conductive carbon shell;(iii)one-dimensional Si-based nanotubes;and(iv)other Si-based intricate hollow structures,including pomegranate-like structure,necklace-like structure,honeycomb structure.The synthesis methods,electrochemical performances,as well as their structural merits and demerits have been discussed in detail.Finally,the future perspectives on silicon-based hollow structured anode materials for LIBs have been discussed.(i)Both conventional and newly emerging methodologies for synthesizing Si-based hollow structures suffer from cumbersome synthesis processes.In this regard,more simple and scalable methods should be developed for constructing Si-based hollow micro/nanostructures.(ii)Optimizing the cavity size is of key importance for the practical application of Si-based hollow structures.Introducing too much cavity in the material would sacrifice the tap density and volumetric capacity,which is detrimental to the energy density of LIBs.(iii)Considering the low conductivity of Si-based materials,it is essential to composite the Si with highly conductive materials such as carbon.However,introducing too much carbon in the composite may compromise the capacity of the silicon-based anode materials due to the low lithium storage capacity of carbon.(iv)The initial Coulombic efficiency of Si-based materials is relatively low,and the introduction of hollow cavity would increase the surface area and thus further lower the initial Coulombic efficiency.Thus,it is essential to develop unique pre-lithiation technologies for the Si-based hollow structures.With the development of new synthetic methods and unceasing endeavor from the scientists,we strongly believe that the Si-based hollow structures would finally find real applications in next-generation LIBs.
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