Material: Embedded nano Silicon in Graphene nanosheets Method: Plasma Assisted Milling Application: Li-ion Battery Anode material

Embedding nano-silicon in graphene nano-sheets by plasma assisted milling for high capacity anode materials in lithium ion batteries

Wei Sun, Renzong Hu, Hui Liu, Meiqin Zeng, Lichun Yang, Haihui Wang, Min Zhu

School of Materials Science and Engineering, South China University of TechnologyJournal of Power Sources 268 (2014) 610-618Antony Raj TM Sc StudentChemistry DepartmentLakehead UniversityGood Evening to one and all I am going to present the paper entitled 1Material: Embedded nano Silicon in Graphene nanosheetsMethod: Plasma Assisted MillingApplication: Li-ion Battery Anode materialSi(30% wt) + Graphite (70% wt) nano Si/GNHeat + Mechanical MillingSpecialties:Si act as nano miller and Cutting effect of Nano SiIn-situ conversion of Graphite to GrapheneBetter performance in LIB than Graphite and Silicon aloneGraphene Nanosheets act as buffers the Volume change issue of Si anodeCan produce the material BulkNano Si embedded in Graphene nano sheet using plasma assisted mechanical milling (so called p-milling) , Special attractions in this paper are Nano Si acts as miller to assist mechanical peeling off of graphite which is defined as cutintg effect of Si, In-Situ conversion Gr to GN, better anode, and ease method2Why Silicon in Graphene Nano sheets?1. Advantages Si & Graphite

Si - Can store Li reversibly, high Theoretical capacity (4200 mAh/g Li22Si5)

Graphite - Good conductivity, stable crystal structure, rate capability

2. Disadvantages of Si & Graphite

Silicon- High volume change (>300%) during Alloying De Alloying with Lithium and its a Semiconductor poor electronic conductivity causes Poor cyclability

Graphites theoretical capacity is less (372mAh/g LiC6), so limits the performance .

(Combination of Both gives better anode)

Si, and Graphite alone as anode has both adv & dis adv. Combination gives better & promising anode is the out come of this paper3P-Milling - SynthesisElectromotorElastic jointVibration exciterBase plateFrameworkVialRefrigerant tankElectrodeSteel balls Spring DBDP (Dielectric Barrier Discharge Plasma) power supply.The weight ratio of ball to powder is 50:1.The ball mill was a vibratory type and the milling cylinder vibrated with a double amplitude of 7 mm and a frequency of 24 Hz.The milling was under the protection of pure argon gas.A.C. electricity (frequency 13 kHz) with the tension higher than 22 kV was supplied to the electrodes. (depends on the experiment requirement)The tension and frequency of DBD was 24 kV and 14.4 kHz, respectively.(synthesis nano powders of Al, Fe, W and WC) Journal of Alloys and Compounds 478 (2009) 624629

Schematic diagram of the p-milling process, electric motor assisted vibratery mecanical ball milling, plasma discharge done using high tension power supply 24kV & 14.4kHz b/w 2 electrodes, one is vial another is kept inside vial. These are the some process details.. Ball weight, vibration specifications, argon environment, plasma details.4Schematic illustration of P-milling

Schematic diagram for making Nano Si- embedded GN. Synergic effect of heat from plasma discharge and mechanical stress from steel balls and cutting effect of hard nano Si lead to thermal, mechanical forces peels graphite to gn and disperse the nano si in it.5Characterization- overviewCONFORMATION & OPTIMIZATIONXRDRaman SpectroscopySEMTEMBET Surface AreaCHEMICAL ACTIVITYDSC-TGAELECTROCHEMICAL CHARECTERIZATIONGalvanostatic CyclingElectrochemical ImpedanceCyclic VoltammetryLSV (for Resistance measurement)

Charecterization done for the material produced, optimized the milling time using XRD, Raman, SEM, BET and Galvanostatic Cycling. Chemical activity investigated using DSC and TGA. And EC charecterization done to prove the better performance of the optimized material.6XRDRAMAN

XRD and Raman for different duration p-milling shown, 0hr is manually mixed graphite and nano Si. As duration increases C(002) peak decreases which confirms graphites structure change same in Raman spectra D band intensity grows where G band intensity weaken. D for disordered carbon and G for sp2 graphite. 2 peaks for Si corresponding to nano and hard Si.7SEMTEM

SEM image of p-0h shows Si dispersed around graphites layered structure where p-20h image shows broken graphite layers and Si dispersion in it. Hresolution TEM of P-20h sample confirms Si dispersed in GN layers as it is clear spherical Si encapsulated by GN layers. And Cutting effect of Si also explained in TEM image.8Galvanostatic Cycling

Cycle life data of nano si compared with p-milled samples, Si even in nano structure could not give good even 40mAh till 50 cycles. 9TGA & DSC

Chemical activity of nano Si/GN is by studying thermal oxidation of the material. Fig shows decrease of oxidation temp as increase of milling time infers gr to gn structure change and in TG weight 40% wt loss happen 500-587 which is the formation temp. In the p-20 Dsc curve 2 peaks indicates two different structure present in the compound gn, gr. Confirms residual gr.10Conductivity is HighNyquist Plots: Comparing the impedance spectra after the 1st cycle of Nano Si and Nano Si/GN composite charge transfer resistance is four times reduced compare to Si.

Impedance of Si and P20 sample compared shows p-20 has 4 times lesser charge transfer resistance due to the gn base structure conductivity improved. And conductivity is checked by finding the bulk material resistance. Resisitance of Si 6.7e7 4.49e6 is very high compare to nano Si/GN 5ohm and 0.33 ohm m11

Volume change is Less during Li- alloying and de alloyingCross Sectional Images of Electrode: Shows volume change of the electrode made using before p-milling(0 hr) sample and 20h p- milling sample.

MaterialBefore CyclingAfter 10 CyclesChange in %P-0h9m19m211%P-20h10m12m120%Fig shows cross sectinal images of electrode before cycling and after 10 cycles to show volume change in Nano Si/Gn is pretty less compare to P-0h sample12Nano-Si Dominant ContributorInitial five cycles (charge/discharge) Cyclic voltammetry peaks confirms Li-Si alloying and Li-Si de alloying.First Cycle shows the SEI formation, irreversible reduction of Li from electrolyte at the surface of anode, which disappears from next cycles.

CycleColumbic Efficiency171.6%294%2098%CV confirms nano Si/GN reversible, and Si is contributing capacity by showing Si-Li alloying and de alloying peaks. First cycle 71.6%there is loss of capacity due to sei formation by reacting with electrolyte and irreversible consumption of Li occur which cause high irreversible capacity loss in it. First. 2th, 20th cycle 94, 98% ch disch curve shows good reversibility as curves overlap each other and 1st cycle differs due to sei formation.13Good Reversibility & Rate Capability Discharge capacity for 50 cycles plotted at different current rates shows stable reversible capacities at different current rates.

Galvanostatic cycling test done, by assembling cr2016 coin cells with Li reference(half cells) compared at different current densities. As current density decrease show high reversible capacity14Full Cell with LiMn2O4 CathodeCycle life test in full cell reproduces the same results good reversible capacity, 600mAh at the end of 30 cycles, Working Voltage greater than the graphite. Capacity is lower than half cell as cathode could not provide sufficient Li+.

Encouraged by good performance in halfcell, full cell assembled with comercially avialable LiMn2O4 cathode, tested gives good reversible capacity. Capacity is 400 mAh less compare to half cell due to in sufficient li from cathode.15CritiqueProcess optimized for good performance is 20hr p-milling is long duration, other process parameters could be discussed, which may reduce the hours of milling time.In DSC/TG, Raman analysis it is confirmed that residual graphite presence, it could tried for 100% conversion, which may yield better reversible capacityFrom CV it is explained that Si mainly contributes for the high capacity, contribution of graphite and graphene in capacity is could be added.Cycle life test done in half cell for 50 cycles and full cell only for 30 cycles are very less to show the material as promising anode.Influence of other parameters and reducing duration not discussedContribution of graphite and graphite for capacity is not Presence of graphite is usefull or not? Info could be more useful to understandCycle life data could be given for more numbers as to show material is promising anode.16THANK YOUReferences1. Synergism of mechanical milling and dielectric barrier discharge plasma on the fabrication of nano-powders of pure metals and tungsten carbide, Journal of Alloys and Compounds 478 (2009) 624629

2. Enhancing the performance of SnC nanocomposite as lithium ion anode by discharge plasma assisted milling, J. Mater. Chem., 2012, 22, 80228028