Chapter 3 Template-Free Hydrothermal Syntheses of SnO 2 Structures for Anodes
3.3.3 Electrochemical properties of SnO 2
3.3.3 Electrochemical properties of SnO2
For the purpose of demonstrating the effect of morphological modification on the electrochemical performance of SnO2, two different types of SnO2 materials (HSs and NSs) were synthesized for use as anode materials. When SnO2 is used as the active component in the LIBs, the electrochemical reactions are comprised of irreversible and reversible steps, (3-1) and (3-2), respectively:8-16
SnO2 + 4 Li+ + 4 e- 2 Li2O + Sn (3-1)
Sn + x Li+ + x e- ⇄ LixSn (0 x 4.4) (3-2) In the studies, half-cells composed of a Li foil, as the negative electrode (anode), and SnO2 materials, as the positive electrode (cathode), were assembled into test cells for the following electrochemical studies. To identify the electrochemical reactions during cycling, cyclic voltammetry (CV) measurements were performed on the SnO2
HSs and the results are presented in Figure 3.6a. In the first cathodic sweep, the HSs show two apparent reduction peaks around 0.65 V and 0.11 V, respectively. The peak at 0.65 V which can be derived from Li2O formation and the reduction of SnO2 to form Sn when the SnO2 HSs react with Li+ as described in eqn (3-1), and the formation of a solid electrolyte interface (SEI) layer.27 The peak disappear in the following cycles, indicating the irreversibility processes. The reversible peaks appear in the cathodic sweep at 0.11 V, 0.86 and 1.12V which can be attributed to the formation of LixSn alloys as described in eqn (3-2). In the anodic sweeps, two peaks are found. The one at ca.
0.60 V is assigned to the dealloying process of LixSn, the reverse reaction in eqn (3-2).
The other peak is at 1.34 V, which is associated with partial oxidation of Sn to form tin oxides. The signal corresponds to a small peak at ca. 0.55 V in the cathodic scans,
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indicating the reduction of the oxides to Sn metal.28 In Figure 3.6b, the alloy/dealloy behavior of SnO2 NSs for the first four cycles was also studied that found the same peaks in the CV profiles for the SnO2 NSs over the same potential range and a similar mechanism was observed. Thus, it was confirmed that the different morphologies of materials make no difference on the redox reaction.
It is well-known that the electrochemical performance of LIBs is highly dependent on cycling rate. In this study, the cyclabilities of SnO2 HSs and NSs at different cycling rate were compared with a voltage window of 0.005−2.0 V as shown in Figure 3.6c-3.6f and in Figure 3.7. Figure 3.6c and 3.6d depicts the specific capacity and the columbic efficiency of the discharge–charge process of the SnO2 HSs and NSs half-cell with a cycling rate 100 mA g-1 (0.13 C). In Figure 3.6c, the first discharge and charge of SnO2 HSs deliver specific capacities 1877 and 1126 mA h g-1, respectively. In Figure 3.6d, the first discharge and charge of SnO2 NSs deliver specific capacities 1751 and 1138 mA h g-1, respectively. Both specific capacities are higher than the theoretical value and lose large initial capacity that can be attributed to the reduction of SnO2 to form Sn, the formation of the SEI layer on the electrode surface during the first discharge step, and the storage of Li+ ions in the EC/DMC-based electrolytes.27-30 Additionally, the SnO2 HSs are more apparent due to its larger surface area. In the later cycles, the specific capacity and the columbic efficiency stay relatively stable.
Specifically, after one hundred cycles, a higher reversible capacity of 522 mA h g−1 is delivered by SnO2 HSs as shown in Figure 3.6c and compared to 490 mA h g−1 for SnO2
NSs as shown in Figure 3.6d. In contrast, the cycling performances of half-cells constructed from commercial SnO2 powders are poor as shown in previous report.17
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Figure 3.6 Electrochemical studies of (a, c, e) HSs and (b, d, f) NSs of SnO2. (a, b) CVs of SnO2 electrodes scanned at 0.5 mV s-1. (c, d) Specific capacity and columbic efficiency of SnO2 electrodes cycled at 100 mA g-1. (e, f) Discharge capacities of SnO2
electrodes at various discharge rates 100-3000 mA g-1. All experiments were cycled at 0.005-2.0 V vs. Li/Li+.
Figure 3.7 displays the discharge capacities of the device fabricated from SnO2
HSs and NSs at high current rates 500, 1000, and 3000 mA g-1 (0.63, 1.26 and 3.78 C).
After one hundred cycles, the discharge (Li alloying) of SnO2 HSs capacities are found to be 436, 325 and 235 mA h g-1, respectively. And the discharge (Li alloying) of SnO2
NSs capacities are found to be 415, 315 and 226 mA h g-1, respectively. The rate
(a)
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capability of the SnO2 with two different morphologies is evaluated at various cycling rates in Figure 3.6e and 3.6f. The specific capacity slightly decreases as the cycling rates increases, the HSs and NSs still shows a discharge capacity of 461 and 435 mA h g-1 at a current density of 3000 mA g-1. These observations demonstrate that even after the fast discharge-charge cycles at high cycling rate, the electrode did not degrade severely so that the half-cell still exhibited excellent cycling properties. SnO2 HSs show the better rate performance compared with NSs at all cycling rates.
Figure 3.7 Electrochemical studies of (a) HSs and (b) NSs of SnO2. Electrochemical performances of SnO2 electrodes cycled at 0.005-2.0 V vs. Li/Li+. The first ten cycles were cycled at 100 mA g-1. : 100 mA g-1, : 500 mA g-1, : 1000 mA g-1, and :
3000 mA g-1.
In this studies, to understand the alteration of the SnO2 HSs and NSs electrode material after repeated lithiation and de-lithiation processes, SEM studies and XRD of the electrode after one hundred discharge–charge cycles is shown in Figure 3.8-3.11.
Based on the results and the previous wok, the inactive amorphous byproduct matrix were present in the materials that formed from the reduction of SnO2 by Li, the irreversible decomposition of the electrolyte, and the SEI layer on the surface of the active material formed during the cell cyclings. In addition, the significant improvement of the electrochemical performance is also attributed to the unique structure of SnO2
HSs NSs
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HSs and NSs with a large contact area with short diffusion lengths, improved charge transfer and the electrolyte efficient Li+ ion transport into the active materials. And the pores and void space between unique structures could effectively accommodate volume variations of the Sn phase during cycling, while the unique structure assemblies would probably better maintain the integrity of the electrode. Clearly, many HSs and NSs still maintain their original morphology when they are compared to the image of the original electrode shown in Figure 3.8 and 3.10. This observation lead a good cycling performance and high rate capability.
Figure 3.8 SEM images of (a) the original SnO2 HS electrode and (b, c) the electrode after one hundred cycles of lithiation and de-lithiation (without being washed). (d) EDX of the SnO2 HS electrode after 100 cycles of lithiation and de-lithiation. The Pt signal was from the sputtered Pt metal, used to enhance the sample conductivity. The electrode was fabricated from a mixture of SnO2, carbon black, and binder.
(a) (b)
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Figure 3.9 XRD patterns of the SnO2 HS electrode before and after 100 cycles of lithiation and de-lithiation. Related XRD patterns and the corresponding JCPDS file numbers are shown also.
Figure 3.10 SEM images of (a) the original SnO2 NS electrode and (b, c) the electrode after one hundred cycles of lithiation and de-lithiation (without being washed). (d) EDX of the SnO2 NS electrode after 100 cycles of lithiation and de-lithiation. The Pt signal was from the sputtered Pt metal, used to enhance the sample conductivity. The electrode was fabricated from a mixture of SnO2, carbon black, and binder.
20 30 40 50 60
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Figure 3.11 XRD patterns of the SnO2 NS electrode before and after 100 cycles of lithiation and de-lithiation. Related XRD patterns and the corresponding JCPDS file numbers are shown also.
Electrochemical impedance spectroscopy (EIS) was conducted to determine the Li+ transfer behavior in SnO2 HSs and NSs.31 In Figure 3.12, the EIS spectra of the half-cells constructed from the HSs and NSs exhibit typical Nyquist plots which were fitted with the equivalent electrical circuit shown in Figure3.13. It can be observed that the NSs have a larger resistance in the high-frequency region. According to the impedance values and the shape of the curves, the HSs exhibit the lowest value of R(sf+ct) of 16 and the NSs are 35 , respectively. This may be related to the disparity between the HSs and NSs in their surface area and electronic conductivity. The value is smaller relative to commercial SnO2 powders.
Summary, in this studies and the previous work, we test electrochemical performances in three types of SnO2 including hollow, 2D and 1D nanomaterials such as HSs, NSs and NRs. The relationships between different structural features such as surface area and electronic conductivity from different crystallographic structures and observed electrochemical properties are discussed. Based on results, changes in the morphology of SnO2 are not closely related with its electrochemical performance just
After
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only a slight difference between them. We anticipate development tin-based nanocomposites containing suitably chosen matrix elements to achieve higher performance and reduce irreversibility processes.32-34
Figure 3.12 Nyquist plots from coin cells composed of as-fabricated HSs and NSs of SnO2.
Figure 3.13 Equivalent circuit for experimental data fitting. Re: electrolyte resistance;
R(sf+ct): surface film and charge transfer resistance; Rf: polarization resistance; CPE:
constant phase elements; W: Warburg impedance; C: intercalation capacitance.
3.4 Conclusions
In this studies, we have successfully developed a simple and template-free method to prepare SnO2 nanomaterials by using hydrothermal. The morphologies of SnO2 HSs and NSs could be easily controlled by adjusting Sn+4/+2 precursors. Electrochemical
0 25 50 75 100 125 150
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performances revealed that the SnO2 HSs exhibited better cyclability and rate capability, when compared with SnO2 NSs. The reversible capacity of 522 mA h g−1 was delivered after one hundred cycles at a cycling rate of 100 mA g−1. These enhanced performances were caused by the unique structure, which could exhibited excellent cycling properties.
The use of SnO2 nanomaterials gives an important indication for the further improvement of the electrochemical properties of SnO2 systems.
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