Study Of The Effect Of Low/non-active Materials On The Electrochemical Performance Of LIBs And Their Working Mechanism

Over the past two decades,the application of lithium-ion batteries?LIBs?in consumer electronics has achieved a brilliant success and showed a great potential in the field of electric vehicles and large scale energy storage devices.To meet the ever-increasing demands of higher energy/power density,safety,lifetime and cost for electric cars and large grids,exploring and developing novel active materials?high-capacity anodes,high-capacity/voltage cathodes?and high performance system architectures?electrolyte,additives,separator and binders?has become a frontier research hotspots.As a very complex system,in addition to active materials,LIBs also contain several key auxiliary components?normally inactive or less active components?,such as binders,conductive additives,electrolyte and separator.It is well accepted that the electrochemical performance of LIBs is dominated by the synergistic effect of all components,not just active materials.Generally,these auxiliary components have little contribution to the energy density of LIBs,however,they play a critical role in determining the practical behavior of active materials,and further decide the power density,cycling performance and safety of batteries.Recently,increasing attention has been devoted to the advancement of auxiliary components,especially,electrolyte,binders and conductive additives.According to above discussions,in this thesis,we put emphasis on the preparation of highly crosslinked three-dimensional aqueous binders and in-situ growth of the graphene coating layer for LiFePO₄ cathode and Si-based anodes,respectively,aiming for the improvement of electrochemical performance of the above two kinds of active materials.The main work of this paper includes the following two aspects:?1?High-strength functional binders for Si-based anodesWe design a facile and self-assembly strategy to in-situ construct 3D highly cross-linked alginate network as a functional binder for high-performance silicon submicro particle?SiSMP,200 nm in diameter?anodes.The 3D polymeric network can greatly enhance the mechanical properties of SiSMP-based laminates through two ways:one by interconnecting alginate chains by a high concentration of Ca²⁺cations through robust coordinate bonds,and the other by forming strong hydrogen bonding between free alginate carboxylic groups and the hydroxylated Si surface.This highly cross-linked network can not only tolerate the severe volume change of Si upon deep galvanostatic cycling,but also restricts the volume expansion of laminates.All these help to maintain the electrical and mechanical integrity and significantly improve the electrochemical performance of the Si anode.As a result,SiSMPs with a 3D binder network exhibit a high reversible capacity of2522 mAhg^-1 after 500 cycles with a capacity retention of 76.5%and maintain a superior capacity of 1646 mAhg^-1at a high current density of 20C(84Ag^-1).We therefore anticipate that this innovative 3D polymeric network combining superb mechanical properties and strong interactions with Si can open a new approach to realize the industrial application of Si-based anodes in LIBs.When introducing N,N-methylenebisacrylamide?MBAA?as the cross-linking agent,under the catalysis of ammonium persulphate?APS?and tetramethylethy?TEMED?,covalently cross-linked 3D polyacrylamide?c-PAM?gel can be obtained through free radical polymerization and adopted as the high-strength functional binder for Si-based anodes.The adhesive property,stretchability and corresponding electrochemical performance of Si-based anodes using 3D c-polyacrylamide binder with different degrees of cross-linking were evaluated quantitatively.Moreover,the relationship between the mechanical property of the c-PAM binder and the electrochemical performance of the silicon anode was detailedly discussed.Our results demonstrated that even at a low cross-linking degree of 0.001,the obtained c-PAM gel delivered an outstanding draftability?6 times of its initial length?.The silicon anode using c-PAM binder with a cross-linking degree of 0.001 presented a reversible capacity of 2204.9 mAhg^-1after 100 cycles at a rate of 0.2C,which was in stark contrast to that of using traditional PVDF binder.?2?In-situ growth of graphene coatings on active materials'surfaceBased on the"dissolution and precipitation"mechanism,we developed a general strategy for the in-situ growth of 3D graphene coatings on micro/nano particles including lithium iron phosphate?LiFePO₄?,silver?Ag?,copper?Cu?and silicon?Si?with arbitrary shapes.To in-situ construct a nanometer-thick catalyst layer on the target materials surface,a trace amount of soluble transition metal salts?e.g.FeSO₄?were employed as catalyst precursors.Soluble solid saccharides?e.g.glucose?with strictly controlled content were chosen as carbon sources.A uniform and ultra-thin layer consisting of carbon sources and catalyst precursors was first deposited on the target materials surface through dissolution/recrystallization process.Growth of 3D graphene coating shell was then accomplished through a low-temperature solid-state reaction at around 600-800?.By precisely tuning the content and ratio of carbon sources to catalyst precursors,thickness and morphology of the graphene coating shell can be readily controlled.The growth mechanism of the 3D graphene shell was studied in detail and a growth model was also proposed.By in-situ constructing a nanometer-thick catalyst layer and introducing‘‘limited carbon sources'',our concept of growing 3D graphene coatings can be applied as a common route for synthesizing more functional graphene encapsulated composites for energy storage and conversion,material protections and spectrum studies.For instance,the obtained LiFePO₄@graphene nanocomposites exhibited much better rate capability and cycling stability than that of the LiFePO₄@amorphous carbon nanocomposites.At a rate of10C,50C and 100C,it was able to deliver a discharge capacity of 152.2,118.1 and 94.3mAhg^-1,respectively,and a reversible capacity of 152.2mAhg^-1 was maintained at a rate of1C until 300cycles with a capacity retention as high as 94.8%.