Research Progress in Silicon Alloy Anode Materials for Lithium-Ion Batteries
Lithium-ion batteries(LIBs),as a type of rechargeable battery,are widely used in the fields such as 3C electronic prod-ucts,digital weapons,aerospace,commercial energy storage power stations,and new energy power vehicles due to their high average output voltage,high energy density,low self-discharge,and no memory effect.At present,since graphite anode materials in LIBs have been developed to the theoretical specific capacity limit(372 mAh·g-1),semiconductor Si is considered one of the most promis-ing anode materials due to its high specific capacity,suitable lithium removal/insertion potential,and abundant reserves.However,there are two challenges for silicon-based anodes.Firstly,when silicon is lithiated,it forms Li-Si compounds to achieve the effect of lithium storage.Whereas Li-ion cannot be completely removed when delithiation occurs,resulting in the formation of"dead lithium".This not only leads to the generation of irreversible capacity,but also increases the Si volume expansion which causes internal stress and fragmentation.The detachment of active material from the collector results in complete deactivation of the electrode and thus af-fects,the electrode cycling life;Secondly,silicon is an intrinsic semiconductor with low conductivity,which makes it difficult to con-duct charging and discharging tests at a high current density.Therefore,unmodified silicon anodes generally exhibit poor rate perfor-mance.The above two challenges hinder the further commercial application of silicon-based anodes.In order to overcome the above-mentioned problems,silicon alloying can improve Si conductivity and result in the formation of second-phase acting as a buffer phase to suppress the volume expansion of silicon.This article divided silicon alloys into binary,ternary and multiple types based on the number of components,and reviewed their research progress as anode materials for LIBs.Emphasis was placed on analyzing the im-pact of silicon alloying strategies on electrochemical performance and mechanism research.The remark and summary were given:(1)Due to the small number of elements,the cost of binary silicon alloys as anodes was relatively low,and the diverse synthesis processes brought about advantages including particles various morphologies such as nanowires and thin films.These morphologies also had dif-ferent advantages and disadvantages on electrochemical performance,such as the excellent cycling performance of nano silicon parti-cles but an increase in specific surface area,leading to a decrease in Coulombic efficiency for the first time.When silicon was com-bined with lithium inert metals(such as Fe,Ti,Cu,Ni neither of which could store lithium),metal silicide such as FeSi2,TiSi2,and Cu3Si would be formed to buffer volume expansion and extend the cycling life of the electrode.However,if there were too many inert phases,the specific capacity would severely decrease,leading to the loss of the silicon anodes advantages.When silicon was com-bined with lithium active metals(such as Mg and Ge,which could store lithium or form metal silicide for lithium storage),the initial specific capacity was very high.Due to the lack of hard buffer phase,its reversible specific capacity was low,and subsequent stability was poor.The mechanism of binary silicon alloy was not complicated,but the electrochemical performance of simple binary silicon al-loy anode was very slow to improve,so it should be paid attention to the application of lithium inert metal elements,and look for the appropriate proportion of the components in the binary system as well as the morphological structure.The relationship between equilib-rium cycle and capacity was one of the future work directions.(2)The composition selectivity of ternary silicon alloys was wide.In ad-dition to metallic elements,non-metallic elements could also be doped to form multiphase composites and further enhance conductivi-ty.In addition,there was a highly expandable cationic disordered Zn(Cu)-Si-P composite material,which had a special structure that brought more ion channels and stronger conductivity than cationic ordered materials.Moreover,the cation-disordered material had strong scalability and high research value.The ternary system was more applicable,but how to introduce elements beneficial to the comprehensive electrochemical performance enhancement and the controllable preparation of the ternary silicon alloy matrix of the composite phase still needed further in-depth study.(3)The expansion rate of the multisilicon alloy anode was very low,and the elec-trochemical performance was excellent.Especially for high entropy alloys,their large configuration entropy could stabilize the phase structure.However,multicomponent silicon alloys faced the problem of complex phase structures caused by multi-components and dif-ficulty in accurately controlling component content,and the operating mechanism was still unclear,these were required for further re-search.In addition,this review also provided prospects for the development of silicon-based alloy anodes and technologies,with the expectation of accelerating the commercial application of silicon alloy anodes in high-energy density LIBs.
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