Preparation and Electrochemical Performance of Novel Structural Si-based Anode Materials

Author:Fu Ru Sheng

Supervisor:liu zhao ping xia yong gao


Degree Year:2018





Nowdays,new energy vehicle is considered to be one of the most appealing alternatives to replace the traditional petrol automobiles.While,it is still a big challenge because the energy density of advanced Li-ion batteries(LIBs)is insufficient to support the long driving distance.Anode material is one of the decisive parameters to enhance the energy density of LIBs.Comparing to the commercial graphite anodes,Si-based anodes exhibit impressive theoretical capacity.Nevertheless,massive insertion/extraction of lithium in the galvanostatic processes results in huge volume expansion and contraction,which triggers active particles pulverization,electronic contact loss and SEI damage and regeneration.Consequently,Si-based anodes display low Coulombic efficiency and rapid capacity decay.Previous researches confirmed that low-dimensional Si anode and silicon suboxides are feasible strategies to keep the structural integrity and realize long lifespan.Here,novel structral Si and SiOx materials are prepared and applied as the anode materials for LIBs.We mainly exploit two-dimensional(2D)silicon anode and 2D silicon suboxides(SiOx)anode,tailor oxygen content in SiOx anode and prepare Si nanowires interconnected micro-SiOx composites as modified anode materials.Then XRD,FTIR,SAED and Raman;ss-NMR,XPS and XRF SEM and TEM are performed to characterize the structure,component and morphology,respectively.Meanwhile,galvanostatic charge-discharge,GITT and EIS are employed to adequately investigate lithiation behaviors of active materials.Here we give the conclusion as following.(1)Sn4+redox assisted solvothermal method is applied and optimized to obtain2D sheet-like Si by delaminating layer CaSi2.Then electrochemical properities as anode for LIBs is measured.Preliminary experimental results show that Ca atoms are successfully extracted from CaSi2 by SnCl4.The size of received sheet-like Si is micrometer in 2D plane and hundreds nanometers in thickness.If combined with ultrasonic treatment,thinner Si sheet will be obtained.After coating 30wt%carbon,the sheet-like Si anode delivers reversible capacity of520 mAh/g and initial Coulombic efficiency(ICE)of71.5%with no apparent capacity decay after 200cycles.(2)Carbon coated sheet-like SiO1.1 nanocomposites formed by Si-nanodomains confined inside amorphous SiO2(nano-Si/a-SiO2)is designed,synthesized and characterized.As a proof-of-concept,we achieve sheet-like SiOx nanocomposites via in-situ transformation of delaminated siloxene.In particular,self-prepared siloxene with oxygen-inserted Si6 rings terminated with H and OH ligands is prepared by delamination of CaSi2 in dilute HCl.Importantly,the resulting carbon coated nano-Si/a-SiO2 material shows enhanced reaction kinetics and structural stability leading to 946 mAh/g capacity and ICE of 72.5%at 0.15 A/g,as well capacity retention of 86%after 200 cycles.Intriguingly,38.0%(360 mAh/g)of the maximum capacity is maintained even at 7.5 A/g,corresponding to a remarkable less than 3minutes charge/discharge time(20C).Finally,the electrode shows merely 24%of volume expansion and minor cracks with capacity retention of 92%after 300 cycles at7.5 A/g.(3)By tuning the oxidation degree of siloxene,oxygen content in silicon suboxides are successfully tailored.Meanwhile,we unveil the oxidiation mechanism of siloxene and the effect of oxygen content on electrochemical performance of silicon suboxides.As-prepared siloxene could be further oxidized in H2O or at ambient atmosphere with different times.Subsequently,siloxene is used to prepare SiOx through high temperature heat treatment.Here O/Si ratio in the SiOx increases as the time is extended.The O/Si ratio could be controlled between 1.0 and 2.0.Among various SiOx(O/Si=1.01,1.25,1.47 and 1.78)anodes,SiO1.47@C shows the best electrochemical performance with reversible capacity of 774.7mAh/g with ICE of67.2%at 0.1C and capacity of 685mAh/g at 0.5C.Among following 350 cycles,the capacity remains almost 100%.Additionally,GITT measurement demonstrates the lithium-ions diffusion coefficient is 10-1210-10 cm2/s.Full cell tests are carried out when pairing with LiNi0.8Co0.15Al0.05O2(NCA)cathode.At a relatively high mass loading of 1.77 and 2.25 mg/cm2,the full cells exhibit 1.27 and 1.81 mAh/cm2 with capacity retention of 95.4%and 85.9%after 200 cycles at 0.2C,respectively.Theoretical calculation shows siloxene could be oxidized spontaneously when exposing in H2O or O2,SiO1.5H and SiO2.0H display two possible atomic structures with lower energy in meta-structure.In addition,the lattice of siloxene increases step by step during oxidization process.(4)Due to high theoretical specific capacity and moderate volume expansion,SiOx have been considered as alternative anode materials for next-generation Li-ion batteries.However,the electronic isolate effect originated from the huge volume variation during the repeated cycling process results in poor cycling performance and low reversible capacity,as well as inferior rate performance.Here,we report a facile and catalyst-free synthetic approach to prepare carbon coated Si nanowires connected SiOx anode materials to address these problems.Micrometer-sized Si nanowires are in-situ grown on the surface of micro-SiO at optimal temperature without catalyst assistance,following a CVD process for carbon coating.The carbon coated Si nanowires not only act as electroactive materials,but also efficiently accommodate the electronic isolated effect of micro-SiO particles,thus remarkably improve the electrochemical performance.As a result,the carbon coated Si nanowires connected SiOx anode exhibits high reversible capacity of750 and 617.5 mAh/g at 0.5C and 1C(1C=1500 mA/g),respectively.Meanwhile,a ultrahigh capacity retention of>92%after 300 cycles is achieved at a high area mass loading of 1.8 mg/cm2.