Rational Hierarchical Structural Design Of Nanowires-based Anode Structures For High Performance Si-loaded Lithium Ion Battery Application

Exploiting a high capacity,high rate and stable anode material to replace graphite has become one of the most challenging and demanding tasks in the development of the next-generation high performance lithium ion batteries（LIBs）.Silicon（Si）has been well-known as an outstanding candidate,with the highest lithium storage capacity of～4200 mAh g^-1 which is more than ten times higher than that of graphite anode.However,the process of lithium ions（Li-ion）insertion/extraction comes with a huge volume change up to～400%that will result in a radical pulverization and fracture in the Si host,and thus cause rapid capacity fading due to the loss of electrical contact.In addition,silicon has a lower conductivity than graphite material.These factors have limited the practical application of Si-based anode in LIBs.A main strategy to counter these drawbacks is to explore various nanostructured Si materials and composited materials,which can help to accommodate the large volumetric change and shorten the charge transport path during charge/discharge cycling process,and thus improve the cycling stability and rate performance of Si anode.So far,the main challenge in this filed is to design and develop durable silicon anode material with high mass-loading.Meantime,improved electrical proprieties and rate performance of Si host has also become an important research part of further developing high performance LIBs.So,a rational design and fabrication of Si-based nanostructure will help to achieve both an ultra-high cycle stability and an excellent rate performance,addressing the major limitations for a Si-based high performance LIB application.In order to handle all these issues of Si anode structure,we have proposed and demonstrated a new 3D hierarchical structure composed of highly cross-interconnected core-shell nanowires or hollow nanotubes,which can help to boost simultaneously the cycling life-time,rate performance,areal capacity and mass load of Si-based anode structure.Furthermore,an alloy nanoparticle doping strategy has been proposed and testified to boost the conductivity and the rate performance of the Si hosting medium,enabling a rapid discharging/charging Si based lithium-ion battery.In this dissertation,the electrochemical cycling performance of several Si-based nanostructures have been investigated,including c-Si/SnO₂ nanowires（NWs）hierarchical structure,highly cross-linked Cu/a-Si NWs core shell and connected Cu-Si alloy nanotubes.The major innovations in this work can be summarized as follow:1.A 3D hierarchical nanowire structure for high capacity and stable Li-ion batteries:A hierarchical structure of tiny c-SiNWs have been grafted upon ultra-long tin dioxide（SnO₂）NW trunks,where the latter frame up a large,conductive and stable architecture to guarantee a good electric contact and fast charging process.The Si NWs branches are produced via a low-temperature vapor-liquid-solid（VLS）growth mediated by Sn catalyst droplets produced by a simple H₂ plasma treatment upon the SnO₂ trunks in a plasma enhanced chemical vapor deposition（PECVD）system.Compared to traditional one dimensional Si based anodes structures,with different nanostructured storage mediums,the c-Si/SnO₂ NWs hierarchical structure demonstrate an outstanding performance with a high areal capacity of～（1.8-4.4）mAh cm^-2,a high mass-load of～（1.5-3.3）mg cm^-2 and a lasting stability over 100 cycles,without the use of any additional polymer,conductive agent or binders.2.Cross-linked Cu/a-Si core-shell NWs exhibiting long cycling stability and good rate performance:a high quality a-Si thin film is conformally coated over a dense and mutual-crossing matrix of CuO NWs fabricated from Cu foam substrate via a simple thermal oxidation process.The a-Si coating layer plays a unique role of high capacity storage medium,as well as a binder layer that merges the individual CuO NWs into a highly cross-linked core-shell network,supported by highly conductive Cu cores.These results suggest that the a-Si/Cu core-shells have enabled a high rate charging at current density of～4.2A g^-1 for 1000 cycles,with still an excellent capacity retention rate and final capacity of～67%and～650 mAh g^-1,respectively.Under different charging/discharging conditions,the a-Si/Cu core-shell LIBs can sustain a high current density beyond～48.0 A g^-1,with still a reasonable capacity recover rate more than～78%.3.A novel highly connected Cu-Si alloy nanotubes demonstrating a long cycling lifetime,ultra-high cycle stability and an excellent rate performance:this has been fabricated through a novel alloy-forming approach to transform an amorphous Si（a-Si）coated copper-oxide（CuO）core-shell nanowires into a hollow and highly-connected Si-Cu alloy（mixture）nanotubes in a higher temperature reductive H₂ annealing.In addition,a hollow nanotube by itself is an advantageous geometry that will alleviate the volume expansion/contraction of the silicon medium during discharge/charge process which can help to improve the conductivity and rate performance.Assembled as anode structures in LIBs,a high final specific capacity of～1010 mAh/g has been achieved after 1000 cycles at～3.4 A g^-1,with a capacity retention of～84%and a full charging/discharging time of～30 min.Remarkably,these LIBs can even survive from～1.8A g^-1 to～70.0 A g^-1,and then back to～1.8A g^-1 for 320 runs,with still a reasonable capacity recover rate more than～88%.4.During the higher temperature reduction process,the crossing CuO nanowire（NW）cores can be reduced into Cu,and then diffuse into and alloy with the a-Si:H shell to produce a continuous self-sustainable 3D nanotube network,which can serve simultaneously as a fast discharging/charging anode materials.This outstanding rate performance can be attributed to the unique hollow Cu-Si nanotube anode structure with yet embedded Cu₃Si alloy nanoparticle inclusions that improve the electrical conductivity within the a-Si storage medium,which is a critical aspect to dissipate Li-ion insertion strain during a high rate charging and discharging operation.Taking this Si-Cu nanotubes as self-sustainable anode structure,we have achieved a final specific capacity of～780 mA g^-1 at～20 A g^-1 with a high retention ratio of～88%for 1000 cycles.This implicates indeed an ultrafast charging operation that allows a full battery charging process in less than 3 min with yet 2 times higher capacity than that of graphite LIBs（-372 mAh g^-1）.These results provide an important experimental foundation for constructing high capacity and fast discharging/charging Si based lithium-ion battery and its application in the field of new energy vehicles.All these results highlight the potential of a synergetic hierarchitecture design,while indicating a new direction and strategy in seeking a breakthrough to deal with the challenges of low mass load,poor rate performance and instability in one dimensional Si-based anode structures.Therefore,it represents an important progress in the development of high performance and commercially competitive Si-based lithium battery.