CHAPTER II. NATURAL POLYSACCHARIDE BINDER FOR HIGH
3.3. Results and discussion
3.3.4. Electrochemical performances of the HC@c-Si@a-Si
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3.4a, amorphous Si is conformally coated on the surface of HC and maintains the structural integrity of the HC@c-Si@a-Si. This ideal structure prevents particles from pulverization during cycling even at the high current densities. From those results, the combination of the high rate HC particles and the high capacity c-Si@a-Si layers leads to the outstanding electrochemical performances including a high specific capacity, a long-term cycling stability, and good rate capabilities.
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Figure 3.5. The HRTEM images of a) the carbon coated nano-sized crystalline Silicon (n-Si, ~200 nm), b) the magnified n-Si with carbon coating layer of about 5 nm)
a b
10 nm
50 nm
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Figure 3.6. Electrochemical performances of HC@c-Si@a-Si electrodes. (a) First discharge/charge curves of the HC@c-Si@a-Si and HC/n-Si at a rate of 0.05 C between 0.005 V and 2.0 V; Cycle performances of HC@c-Si@a-Si and HC/n-Si at a rate of (b) 0.2C and (c) 1C discharge/charge; (d) Rate capabilities of HC@c-Si@a-Si and HC/n-Si (the same discharge/charge rate was used from 1 to 20 C).
0 50 100 150 200
0 200 400 600 800 1000
Cycle Number Capacity (mAh g-1 )
60 70 80 90 100
Coulombic efficiency (%)
0 10 20 30 40 50
0 200 400 600 800 1000 1200
Cycle Number Capacity (mAh g-1 )
60 70 80 90 100
Coulombic efficiency (%)
0 10 20 30 40 50 60 70 80 90 0
200 400 600 800 1000 1200
Capacity (mAh g-1 )
Cycle Number 0 200 400 600 800 1000
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Voltage (V)
Capacity (mAh g-1)
a) b)
c) d)
HC@c-Si@a-Si HC/n-Si
●HC@c-Si@a-Si
●HC/n-Si
●HC@c-Si@a-Si
●HC/n-Si
0.2C
0.5C 1C
3C
5C
10C15C 20C
●HC@c-Si@a-Si
●HC/n-Si 0.2C
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Figure 3.7. The SEM image of pristine electrodes of a) the HC@c-Si@a-Si electrodes and b) the magnified HC@c-Si@a-Si electrodes; The SEM image of pristine electrodes of a) the HC/n-Si electrodes and b) the magnified HC/n-Si electrodes.
5 µm
a
2 µm
b
c d
5 µm 2 µm
n-Si@C HC
HC@c-Si@a-Si
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In addition to electrochemical properties of Si-based anode materials, volume expansion of the electrodes is one of the critical factors to use in practical LIB applications.50,51 Figure III-8 shows cross- sectional SEM images of both HC@c-Si@a-Si and HC/n-Si electrodes after 200 cycles at a rate of 1 C discharge/charge. As-expected, the HC@c-Si@a-Si electrodes show a significantly reduced volume expansion (53%), compared to that of the HC/n-Si electrodes (100%). It can be explained that the Si shell of the HC@c-Si@a-Si particles is effectively maintained without detachment from the HC core.
On the contrary, n-Si particles (~200 nm in diameter) are only composed of crystalline Si and they are already separated from the HC in the HC/n-Si electrodes before cycling. Therefore, the n-Si particles undergo more severe pulverization and they are easily detached from the HC than a-Si. As a result, the volume expansion of the HC@c-Si@a-Si electrodes is much smaller than the HC/n-Si electrodes.
To further investigate the superior rate capabilities of the HC@c-Si@a-Si electrodes, electrochemical impedance spectroscopy (EIS) was conducted. The Nyquist impedance plots are obtained after 1st and after 200th cycle for each sample (Figure 3.9). The semicircle appearing in the high-frequency region is attributed to the existence of contact resistance by the formation of SEI layer.52 The medium-frequency semicircle is assigned to the charge transfer resistance, and the straight line in the low-frequency region corresponds to the mass transfer of lithium ions.53,54 The SEI film resistance (RSEI) and charge-transfer resistance (Rct) of the HC@c-Si@a-Si electrodes are smaller than those of HC/n-Si electrodes both after 1st and 200th cycle at a rate of 1C. The results indicate that the HC@c- Si@a-Si formed and maintained more stable SEI layer than HC/c-Si. Therefore, the structure of the HC@c-Si@a-Si is more suitable for the Li+ diffusion.
To confirm the effectiveness of the HC@c-Si@a-Si for practical use in LIB applications, the cycling and rate performance of the HC@c-Si@a-Si was conducted using coin-type (2016 R-type) full cells. LiCoO2 was used as cathode materials in the full-cell test. The voltage profiles of the LiCoO2/HC@c-Si@a-Si and LiCoO2/HC/n-Si cells at a rate of 1 C charge/discharge in the range of 2.3–4.1 V are displayed in Figure 3.10a and b. The LiCoO2/HC@c-Si@a-Si cell shows very slow degradation of its capacity during cycling. On the other hand, capacity of the LiCoO2/HC/n-Si cell is rapidly reduced after few cycles. Figure 3.10c is the long term cycling performance of the full cells at a rate of 1C discharge/charge in the range of 2.3–4.1 V. The capacity retention for the LiCoO2/HC@c- Si@a-Si cell is 80% after 160 cycles. Meanwhile, the LiCoO2/HC/c-Si cell shows more severe degradation of its capacity and only 52% of capacity retention is obtained after 160 cycles at the same rate. Similar aspects obviously appear in the rate performance test under at various rates ranging from 0.5 C to 15 C discharge/charge as well (Figure 3.10d). The LiCoO2/HC@c-Si@a-Si cell shows much improved capacity retention under high current density and almost fully recovers to its initial capacity at a rate of 0.2 C after cycling at a rate of 15 C discharge/charge. These results are consistent with the results of above the half-cell test.
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Figure 3.8. Characterization of volume expansion. Cross-sectional SEM images of pristine electrodes ((a) HC@c-Si@a-Si electrode and (c) HC/n-Si) and electrodes after 200 cycles ((b) HC@c-Si@a-Si and (d) HC/n-Si).
50 µm
a b
c d
45 m
45 m
50 µm
69 m
90 m
50 µm
50 µm
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Figure 3.9. Nyquist plots of the electrochemical impedance spectra of HC@c-Si@a-Si and HC/n-Si. a) after 1st cycle, and b) after 200th cycles at a rate of 1C.
0 10 20 30 40 50 60 0
10 20 30 40 50 60
-Z" (ohm)
Z (ohm)
0 20 40 60 80 100 0
20 40 60 80 100
-Z" / ohm
Z / ohm
●HC@c-Si@a-Si
●HC/n-Si
●HC@c-Si@a-Si
●HC/n-Si
a) b)
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Figure 3.10. Electrochemical performances of the LiCoO2/HC@c-Si@a-Si and the LiCoO2/HC/c-Si full-cells. Variation in the charge/discharge profiles of a) the LiCoO2/HC@c-Si@a-Si and b) the LiCoO2/HC/n-Si full cell during cycling; c) Cycle performances at a rate of 0.2 C discharge/charge and d) rate capabilities of both electrodes. The same discharge/charge rate was used from 0.2-20 C between 2.3–4.1 V.
0 20 40 60 80 100 120 140 2.0
2.4 2.8 3.2 3.6 4.0 4.4
Voltage (V)
Capacity (mAh g-1)
0 20 40 60 80 100 120 140 2.0
2.4 2.8 3.2 3.6 4.0 4.4
Voltage (V)
Capacity (mAh g-1)
a) b)
0 40 80 120 160
0 20 40 60 80 100 120 140
Capacity (mAh g-1 )
Cycle Number
●HC@c-Si@a-Si
●HC/n-Si
0 10 20 30 40 50 60 70 0
20 40 60 80 100 120 140
Capacity (mAh g-1 )
Cycle Number
0.5C 1C 3C 5C 10C 15C 0.5C
HC@c-Si@a-Si HC/n-Si
c) d)
160, 120, 80, 50, 10, 1 cycle 160, 120, 80, 50, 10, 1 cycle
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