high capacity graphite anodes for li-ion battery applications using tin microencapsulation basker...
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High Capacity Graphite Anodes for Li-Ion battery applications
using Tin microencapsulation
Basker Veeraraghavan, Anand Durairajan, Bala Haran
Ralph White and Branko Popov
University of South Carolina, Columbia, SC 29208
and
Ronald Guidotti
Sandia National Laboratories
Albuquerque, NM 87185-0614
Introduction
Graphite has good cycle life but low theoretical capacity (372 mAh/g)
Tin has high theoretical capacity (991 mAh/g)
Tin based anodes have poor cycling characteristics due to density changes of Tin
Reducing the Sn particle size may mitigate the problem
Objectives
To obtain an anode material with high specific capacity, better rate capability and good cycle life
To use electroless deposition for preparing Sn-C composites and to optimize the deposition conditions
To optimize the Sn loading on graphite based on discharge characteristics
To study the effect of Sn loading on the electrochemical performance of the composite
Preparation of Sn/Graphite compositesElectroless deposition of Sn using
hypophosphite bathpH-10 (using NaOH) and T-50C
Cell Preparation for testing1/2” T-cells used for electrochemical testingElectrodes prepared by cold rolling using
PTFE binder (10wt%)Whatman fiber used as separator and Li-foil
used as counter and reference electrode1M LiPF6 in EC/DMC (1:1 v/v) used as
electrolyte
Experimental
Experimental (Cont’d.)
Electrochemical characterizationCharge-discharge and cycling behavior
Cycling was performed between 2V and 5 mV at C/15 rate (0.1mA/cm2)
Electrochemical Impedance Spectroscopy (EIS)100kHz to 1mHz with 5mV sinusoidal signal
Cyclic VoltammetryCVs were performed in the potential range 1.6V to
0.01V at 0.05 mV/s
Physical characterizationSEM, EDAX and XRD
XRD analysis of 15% sn-coated SFG10 samples as a function of heat treatment temperature
40 41 42 43 44 45 46 47 48 49 50
2
0
250
500
750
1000In
tens
ity
Bare100oC200oC300oC400oC
XRD patterns of SFG10 with 15% Sn heat treated at different temperatures.
The XRD analysis of Bare SFG10 is shown for comparison.
Charge discharge studies of 15% sn-coated SFG10 samples as a function of heat treatment temperature
-100 100 300 500 700
Specific Capacity (mAh/g)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Pote
ntia
l (V
vs
Li/
Li+
)
100oC (332 mAh/g)200oC (400 mAh/g)300oC (343 mAh/g)400oC (337 mAh/g)
Specific Capacities of SFG10 coated with 15% Sn as a function of temperature of drying
Comparison of charge-discharge curves of bare and 15 wt% sn-coated graphite.
0 1000 2000 3000 4000 5000
Specific capacity (mAh/g)
0 .0
1.0
2.0
3.0
4.0P
ote
ntia
l ( V
vs
Li/L
i+)
Bare15% Sn
Charge-Discharge curves of bare and sn-coated SFG10 samples
100 300 500 700
Specific Capacity (mAh/g)
-0.2
0.3
0.8
1.3
1.8
Pote
ntia
l (V
vs
Li/
Li+
)
Bare5% Sn
10% 20%
15%
Specific Capacity as a function of Sn loading at C/15 rate
Percentage increase in reversible capacity as a function of composition of sn
0 5 10 15 20 25
Composition of Sn (%)
0
10
20
30
40
50
60
70
Perc
enta
ge in
crea
se in
reve
rsib
le c
apac
ity (%
)
Percentage increase in reversible capacity as function of Sn compositionfor two different rates
Utilization of sn in the coated samples as a function of the composition of tin
Sample Reversible
Capacity (mAh/g)
Capacity due to Sn (mAh/g)
Utilization of Sn1 (%)
Specific Surface
area (m2/g)
Volumetric Surface
area (m2/cm3)
Volumetric Capacity
(mAh/cm3)
Bare
5% Sn
10% Sn
15% Sn
20% Sn
284.6
327.8
374.6
433.2
381.7
-
57.4
118.5
191.3
154.0
-
55.0
53.8
54.7
31.1
9.84
8.52
8.30
7.61
7.12
21.65
20.49
21.66
21.42
21.50
626.1
788.4
977.7
1219.5
1152.7
1Utilization of tin = (Capacity due to tin/weight of tin in the composite)/Theoretical capacity of tin (991 mAh/g)*100
Impedance plots for the bare and sn-coated SFG10 samples at fully discharged state
0 1 2 3 4
Real Z (g)
0.0
0.5
1.0
1.5
2.0
Imag
inar
y Z
(g
Impedance plot of SFG10 samplesImpedance plot of SFG10 samples
Bare
5% Sn
10% Sn15% Sn
20% Sn
Cyclic Voltammograms of bare and 15% sn coated SFG10 samples for the reversible cycle
0.0 0.4 0.8 1.2 1.6 2.0
Potential (V vs Li/Li+)
-300
-100
100
300
Spec
ific
Cur
rent
(mA
/g)
15% SnBare
Cycle life studies of bare and 15% sn coated SFG10 samples at C/15 rate
5 10 15 20 25 30 35
Cycle number
0
100
200
300
400
500
Spec
ific
Cap
acit
y (m
Ah/
g)
Bare
15% Sn
Rate Capability studies of bare and 15% sn coated SFG10 samples
0.0 0.5 1.0 1.5 2.0
Discharge current density in mA/cm2
0
100
200
300
400
500Sp
ecifi
c C
apac
ity (m
Ah/
g)
C/15
C/6
C/3
C
2C
Bare15% Sn
Conclusions Tin encapsulation on SFG10 graphite results in high
performance anodes for use in Li-ion batteriesReversible capacities are improved upto 15% Sn, relative to
bare graphite Cycle life of the bare graphite is improved on Sn-encapsulation
The optimum heat treatment temperature was found to be 200 CCrystallinity increases with temperature
Sn-C based anodes show better conductivity and lower polarization resistance compared to virgin carbon
Addition of Polypyrrole reduces irreversible capacity and further studies need to be done to optimize the amount of polypyrrole
Acknowledgements
This work was funded by the Dept. of Energy division of Chemical Science, Office of Basic Energy Sciences and, in part, by Sandia National Laboratories
(Sandia National Laboratories is a multi-program laboratory operated by Sandia corp., a Lockheed Martin Company, for the U.S. Dept. of Energy under Contract DE-AC04-94AL85000.)