Electrochemical properties of SnO 2 NRs

2.3.3 Electrochemical properties of SnO2 NRs

Half-cells composed of a Li foil, as the negative electrode (anode), and SnO2 NRs, as the positive electrode (cathode), were assembled into test cells for the following electrochemical studies. To understand the electrochemical reactions during the cell cycling, CV measurements were performed and presented in Figure 2.8a. In the first cathodic sweep, a broad peak at 0.59 V is attributed to the reduction of SnO2 to form Sn, as described in eqn (1-1), and the formation of the SEI layer.^28,29 In the following

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cycles, the peak disappears while two peaks, at 0.88 V and 1.15 V, are observed.

Another peak at 0.07 V is found in the first cycle. It shifts slightly in the subsequent scans to 0.2 V with reduced peak current. The observations are attributed to the occurrence of irreversible processes initially and the formation of various LixSn alloys, as suggested by eqn (2-2), during the cathodic sweeps.^30,31 In the anodic sweeps, two peaks are found. The one at ca. 0.61 V is assigned to the dealloying process of LixSn, the reverse reaction in eqn (2-2). The other peak is at ca.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, indicating the reduction of the oxides to Sn metal.²⁸ Figure 2.8b depicts the specific capacity and the columbic efficiency of the discharge–charge process of the half-cell with a cycling rate 100 mA g⁻¹ (0.13 C). The first discharge and charge steps deliver specific capacities 1583 and 1044 mA h g⁻¹, respectively. The large initial capacity loss 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.^28–

31 These materials are attributed to the major components in the inactive amorphous byproduct matrix formed among the NRs and will be discussed more below. Obviously, the capacity dropped swiftly for the first twenty five cycles. In the later cycles, the specific capacity and the columbic efficiency stay relatively stable. At the end of the one hundredth cycle, a respectable specific capacity 435 mA h g⁻¹ and a columbic efficiency over 98% are observed. In contrast, the cycling performances of half-cells constructed from commercial SnO2 powders (particle sizes 1–10 μm and 100 nm) at 100 mA g⁻¹ are poor, as shown in Figure 2.9a. Figure 2.9b displays the discharge capacities of the device fabricated from SnO2 NRs 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) capacities are found to be 357, 290 and 215 mA h g⁻¹, respectively. In Figure

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2.8c, a capacity 997 mA h g⁻¹ is observed after the battery is cycled at 100 mA g⁻¹ for five times. Then, after it is cycled at 500 mA g⁻¹, 1000 mA g⁻¹, and finally 3000 mA g⁻¹ for five times each, the half-cell shows a capacity 510 mA h g⁻¹ at 100 mA g⁻¹, very close to value found in the twenty fifth measurement shown in Figure 2.8b, 518 mA h g⁻¹. These observations demonstrate that even after the fast discharge–charge cycles at 3000 mA g⁻¹, the electrode did not degrade severely so that the half-cell still exhibited excellent cycling properties.

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Figure 2.8 (a) CV of a SnO2 NR electrode scanned at 0.5 mV s⁻¹. (b) Specific capacity and columbic efficiency of a SnO2 NR electrode cycled at 100 mA g⁻¹. (c) Discharge capacity of a SnO2 NR electrode as a function of discharge rate (100–3000 mA g⁻¹). All experiments were cycled between 0.005 V and 2.0 V vs. Li/Li⁺.

(a)

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Figure 2.9 (a) Discharge capacities of electrodes fabricated from commercial SnO2

powders (sizes: 1 - 10 m and 100 nm) at a cycling rate 100 mA g^-1. (b) Electrochemical performance of a SnO2 NR electrode cycled between 0.005 V and 2.0 V vs. Li/Li⁺ after 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 ().

Clearly, the half-cells constructed from the SnO2 NRs demonstrate much better performance than the ones from the commercial SnO2 powders do. To understand the alteration of the electrode material after repeated lithiation and de-lithiation processes, a SEM image of the electrode after one hundred discharge–charge cycles is shown in Figure 2.10a. Clearly, many NRs still maintain their original 1-D morphology when they are compared to the image of the original electrode shown in Figure 2.10b. In addition, the EDX and the XRD data of the electrode material after one hundred cycles are displayed in Figure 2.11. The EDX spectra in Figure 2.11a suggest that both Sn and

(b)

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O atoms are the major components of the electrode. The XRD pattern in Figure 2.11b indicates the presence of Sn and Cu metals, which is the foil for the electrode contact. This suggests that the SnO2 NRs has been completely converted into Sn metal NRs during the discharge–charge cycles. Based on the results, we assign the O signal found in the EDX to Li2O, 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.^20,28–33 Due to its light mass, Li cannot be observed by EDX. These inactive components appear to be amorphous because no related XRD signals can be found in Figure 2.11b. We assume that the amorphous byproduct matrix played an important role for maintaining the cell performance over extended discharge–

charge cycles.³³ The soft and low density matrix appears to intersperse uniformly among the NRs. The separations could effectively minimize the aggregation of the as-formed Sn NRs. Also, due to the even distribution of the voids among the inactive matrix, the mechanical stress caused by the volume changes in the lithiation and de-lithiation process could be alleviated, as shown in Scheme 2.1. In contrast, the electrode fabricated from commercial SnO2 show severe aggregations after fifty discharge–

charge cycles, as demonstrated in the SEM image shown in Figure 2.12. Considering a relatively wide voltage window applied in this study, we summarize the enhanced capacity of the SnO2 NRs based cells at long cycles and variable rates to the following reasons. First of all, the amorphous byproduct matrix in the voids among the NRs might effectively buffer the drastic volume changes during the lithiation and de-lithiation process. Also, due to the presence of the matrix, the NR structure was maintained after SnO2 was reduced to Sn. The NR structure may provide effective electrolyte/electrode contact surfaces which shorten the transport lengths for both electrons and Li⁺ ions. In addition, the diffusion time of ions could be reduced in the nanocomposite so that the rates of phase transitions are increased.

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Figure 2.10 SEM images of (a) an electrode after one hundred cycles of lithiation and de-lithiation (without being washed) and (b) the original SnO2 NR electrode. The electrode was fabricated from a mixture of SnO2NRs, carbon black, and binder.

(a)

(b)

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Figure 2.11 (a) EDX of a SnO2 NR electrode after 100 cycles of lithiation and de-lithiation. The upper result was obtained from the whole-scan of the area shown in Figure 2.10a. The Cl content was low. The Pt signal was from the sputtered Pt metal, used to enhance the sample conductivity. The lower result was from the centre–point of one NR. (b) XRD patterns of sample A before and after 100 cycles of lithiation and de-lithiation. Related XRD patterns and the corresponding JCPDS file numbers are shown also.

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Figure 2.12 SEM image of an electrode fabricated from a mixture of commercial SnO2

powder, carbon black, and binder after 50 cycles of lithiation and de-lithiation.

To understand the effect of the SnO2 morphology on the electrochemical performances further, EIS studies were carried out.^34,35 As shown in Figure 2.13a, the EIS spectra of the half-cells constructed from the NRs and the commercial powder of SnO2 exhibit typical Nyquist plots. Each one consists of a high frequency semicircle (100 k-10 Hz) and a low frequency inclined line (10-0.1 Hz). The high frequency semicircle represents the charge transfer resistance of the electrochemical reactions across the interface between the electrolyte and the electrode surface, and the contact resistance among the components on the electrode. The semicircle from the SnO2 NRs-based cell shows a smaller diameter, implying its better electrochemical performance.

To quantify the experimental EIS results, the spectra were fitted with the equivalent electrical circuit shown in Figure 2.13b.³⁵ It consists of a serial connection of Re, R(sf+ct) + W//CPE, and Rf//C. Here, Re is the electrolyte resistance, R(sf+ct) is the surface film and charge transfer resistance, Rf is the polarization resistance, CPE (constant phase element) is the indicator for the roughness, porosity, and inhomogeneity of the electrode surface, W is the Warburg impedance, and C is the intercalation capacitance. The fitted results are listed in Table 2.3. R(sf+ct) of the cell value 81 Ω while the cell constructed from the commercial powder is high, 424 Ω. The observation

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implies that in the cell fabricated from the SnO2 NRs, the charge transfer and Li⁺ ion fabricated from the SnO2 NRs exhibit a low diffusion pathways are efficient. Thus, the enhanced electrode performance can be attributed to the presence of the LiO2/Sn NRs nanocomposite structure which not only enhances the diffusion and the charge transfer but also buffers the large volume changes during the discharge–charge cycles.

Figure 2.13 (a) Nyquist plots from coin cells composed of as-fabricated NR and commercial powder of SnO2. (b) 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.

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Table 2.3 Fitted impedance parameters obtained from EIS using the circuit in Figure 2.13b

Electrode materials Re () R(sf+ct) () CPE (F) Rf () C (F)

NR 3 81 488 22 8

Commercial

powder 5 424 2493 111 4