Electrode Preparation And Performance Optimization Of Silicon Based Anode For Lithium Ion Batteries

Silicon is known to be the most promising anode materials for lithium ion batteries due to their highest theoretical capacity(4200m Ah·g-1),low operation potential,environmental friendliness,and resource abundance.However,it’s still a great challenge for practical applications aroused from the huge volume change during lithiation and delithiation processes and the unstable solid-electrolyte interphase(SEI)films,which results in electrode pulverization and fast capacity fading.In this dissertation,the performance of silicon based anode electrode is optimized from three aspects: electrode structure,electrolyte system and binder category.The first is to prepare Si/C composite electrode which can accommodate the volume change and stabilize the SEI film.The second is using sulfur-containing additives which make the SEI film thin and compact.The last one is adopting a water-soluble polymer which is able to form hydrogen bonds with the Si particles.The main results are summarized as follows:(1)A unique hierarchical structure of Si@alginate microcapsule-graphite composite anode was designed,in which Si microcapsules are prepared by wrapping of calcium crosslinked alginate.The Si@alginate microcapsules are uniformly dispersed in graphite anode with the widely adopted CMC and SBR binder.The new geometry of electrode architecture has locally different binder systems,which enables both Si and graphite run in their optimized binder environment.The cross-linked alginate capsule effectively confines the volume change and prevents aggregation of the Si nano-particles,which contribute to a significant improvement of the electrochemical performances.At a weight ratio of 3:7 between Si and graphite,the composite electrode exhibits a reversible capacity of 916?m Ah·g-1 and a capacity retention of 92.4% after 200 cycles.Cycling performance of the composite electrode is remarkably enhanced.Based on the composite anode,the full cell prepared with NCM cathode also shows excellent cycling stability.The results reveal that the new architecture of electrode geometry is promising for large-scale applications in the future.(2)The effects of sulfur-containing electrolyte additives are investgated and the ethylene sulfite(DTD)is found quite effective contributing to an improvement of the electrochemical performance of silicon/graphite composite electrode.The structure of DTD is similar with ethylene carbonate(EC)and it is able to participate in the formation of a stable and uniform SEI film.The optimal proportion of DTD is 3wt.%.And the reversible capacity,cycling performance and rate performance of the composite electrode is significantly improved.The electrode shows a capacity of 1000 m Ah·g-1 and a capacity retention of 97% after 100 cycles.The SEM characterization confirms that the SEI film on the surface of the electrode is thin and uniform,the whole structure of the electrode remains intact.Meanwhile,CV,XPS and FTIR analysis results demonstrate that the SEI film include sulfur-containing components such as Li2SO3 and ROSO2 Li.(3)Polyacrylamide(PAM)is adopted as a new binder for silicon anode and the electrochemical performance of silicon anode was systematically investigated.PAM is a new type of water-soluble polymer binder with good mechanical properties and abundant amide groups,and the –NH2 group in it is known can easily form strong hydrogen bonding with the silicon surface.So that,it can tolerate the volume change of silicon and restrict the volume expansion of the laminates to a large degree,thus effectively maintaining the mechanical and electrical integrity of the electrode.At the concentration of PAM is 3%,the silicon anode exhibits a capacity retention of 80% after 300 cycles and 67% at 20 C,which demonstrate that the cycling performance and especially the rate capability is significantly improved.Meanwhile,the formation of hydrogen bonds are proved by FTIR and XPS analysis.These studies enhanced the electrochemical properties of silicon anode in different aspeacts including the first coulombic efficiency,rate capability and cycling performance.The results indicate that the commercialization of Si anode material needs not only the silicon material itself,but also the system optimizations.Only in this way,the development and application of silicon material in lithium ion batteries can be further promoted.