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