Debye Temperature Evaluation for Secondary Battery Cathode of α-Sn x Fe1- xOOH Nanoparticles Derived from the 57Fe- and 119Sn-Mössbauer Spectra

Abstract

Debye temperatures of α-Sn_xFe_1-xOOH nanoparticles (x = 0, 0.05, 0.10, 0.15 and 0.20, abbreviated as Sn100x NPs) prepared by hydrothermal reaction were estimated with ⁵⁷Fe- and ¹¹⁹Sn-Mössbauer spectra measured by varying the temperature from 20 to 300 K. Electrical properties were studied by solid-state impedance spectroscopy (SS-IS). Together, the charge-discharge capacity of Li- and Na-ion batteries containing Sn100x NPs as a cathode were evaluated. ⁵⁷Fe-Mössbauer spectra of Sn10, Sn15, and Sn20 measured at 300 K showed only one doublet due to the superparamagnetic doublet, while the doublet decomposed into a sextet due to goethite at the temperature below 50 K for Sn 10, 200 K for Sn15, and 100 K for Sn20. These results suggest that Sn10, Sn15 and Sn20 had smaller particles than Sn0. On the other hand, 20 K ¹¹⁹Sn-Mössbauer spectra of Sn15 were composed of a paramagnetic doublet with an isomer shift (δ) of 0.24 mm s^-1 and quadrupole splitting (∆) of 3.52 mm s^-1. These values were larger than those of Sn10 (δ: 0.08 mm s^-1, ∆: 0.00 mm s^-1) and Sn20 (δ: 0.10 mm s^-1, ∆: 0.00 mm s^-1), suggesting that the Sn^IV-O chemical bond is shorter and the distortion of octahedral SnO₆ is larger in Sn15 than in Sn10 and Sn20 due to the increase in the covalency and polarization of the Sn^IV-O chemical bond. Debye temperatures determined from ⁵⁷Fe-Mössbauer spectra measured at the low temperature were 210 K, 228 K, and 250 K for Sn10, Sn15, and Sn20, while that of α-Fe₂O₃ was 324 K. Similarly, the Debye temperature of 199, 251, and 269 K for Sn10, Sn15, and Sn20 were estimated from the temperature-dependent ¹¹⁹Sn-Mössbauer spectra, which were significantly smaller than that of BaSnO₃ (=658 K) and SnO₂ (=382 K). These results suggest that Fe and Sn are a weakly bound lattice in goethite NPs with low crystallinity. Modification of NPs and addition of Sn has a positive effect, resulting in an increase in DC conductivity of almost 5 orders of magnitude, from a σ_DC value of 9.37 × 10^-7 (Ω cm)^-1 for pure goethite Sn (Sn0) up to DC plateau for samples containing 0.15 and 0.20 Sn (Sn15 and Sn20) with a DC value of ~4 × 10^-7 (Ω cm)^-1 @423 K. This non-linear conductivity pattern and levelling at a higher Sn content suggests that structural modifications have a notable impact on electron transport, which is primarily governed by the thermally activated via three-dimensional hopping of small polarons (SPH). Measurements of SIB performance, including the Sn100x cathode under a current density of 50 mA g^-1, showed initial capacities of 81 and 85 mAh g^-1 for Sn0 and Sn15, which were larger than the others. The large initial capacities were measured at a current density of 5 mA g^-1 found at 170 and 182 mAh g^-1 for Sn15 and Sn20, respectively. It is concluded that tin-goethite NPs are an excellent material for a secondary battery cathode and that Sn15 is the best cathode among the studied Sn100x NPs.

Keywords: Mössbauer spectroscopy; hydrothermal reaction; secondary battery cathode; tin-goethite nanoparticles.

Figure 1

⁵⁷Fe-Mössbauer spectra of α-Sn_xFe_1-xOOH NPs with ‘x’ of 0 (a), 0.10 (b), 0.15 (c), and 0.20 (d) measured at 300, 250, 200, 150, 100, 50, and 20 K. The subspectra with the different colors in the Figure are matched with the corresponding Mössbauer parameters shown in Table 1, Table 2, Table 3 and Table 4 respectively.

Figure 1

⁵⁷Fe-Mössbauer spectra of α-Sn_xFe_1-xOOH NPs with ‘x’ of 0 (a), 0.10 (b), 0.15 (c), and 0.20 (d) measured at 300, 250, 200, 150, 100, 50, and 20 K. The subspectra with the different colors in the Figure are matched with the corresponding Mössbauer parameters shown in Table 1, Table 2, Table 3 and Table 4 respectively.

Figure 2

¹¹⁹Sn-Mössbauer spectra of α-Sn_xFe_1-xOOH NPs with ‘x’ of 0.10 (a), 0.15 (b), and 0.20 (c) measured at 300, 250, 200, 150, 100, 50, and 20 K.

Figure 3

The temperature dependence of the normalized absorption area for (a) iron oxide, (b) tin compounds, and (c) α-Sn_xFe_1-xOOH NPs derived from the ⁵⁷Fe- and (d) ¹¹⁹Sn-Mössbauer spectra of α-Sn_xFe_1-xOOH NPs with ‘x’ of 0.10, 0.15, and 0.20 measured at 300, 250, 200, 150, 100, 50, and 20 K.

Figure 4

Conductivity spectra for (a) Sn0, (b) Sn15 from α-Sn_xFe_1−xOOH NPs series, and (c) complex impedance plane (Nyquist plot) and spectra for Sn0 @various temperatures with corresponding equivalent circuit used for fitting the data. Empty symbols represent the experimental data, while the lines depict the fit obtained through EEC modelling.

Figure 5

(a) Arrhenius plots of DC conductivity and (b) conductivity isotherms measured @383 K for individual samples from α-Sn_xFe_1−xOOH NPs series; (c) compositional dependence of DC conductivity at various temperatures for individual α-Sn_xFe_1−xOOH NPs. Lines in (a) are obtained through linear regression, while the lines connecting data points in (c) are drawn as guides for the eye.

Figure 6

Charge–discharge curves of Li-ion battery (a) Sn0, (b) Sn10, and (c) Sn15 and Na-ion battery (d) Sn0, (e) Sn5, (f) Sn10, (g) Sn15, and (h) Sn20 as cathode recorded under current density of 50 mA g⁻¹.

Figure 7

Cyclability of Li-ion battery (LIB) and Na-ion battery (SIB) containing α-Sn_xFe_1−xOOH NPs cathode recorded under current density of 5 mA g⁻¹ and 50 mA g⁻¹, (a) LIB at 5 mA g⁻¹, (b) LIB at 50 mA g⁻¹, (c) SIB at 5 mA g⁻¹, and (d) SIB at 50 mA g⁻¹.

Figure 8

Fe-K edge XANES and EXAFS spectra of α-Sn_xFe_1−xOOH NPs cathode with ‘x’ 0.10 and 0.15 in a Na-ion battery before and after charge–discharge processes of 30 times, (a) XANES and (b) EXAFS.

Scheme 1

Preparation procedure of α-Sn_xFe_1−xOOH NPs.

Scheme 2

Structure of Na-ion battery.