Debye Temperature Evaluation for Secondary Battery Cathode of α-Sn x Fe 1- x OOH Nanoparticles Derived from the 57 Fe- and 119 Sn-Mössbauer Spectra.

Debye temperatures of α -Sn x Fe 1- x OOH nanoparticles ( x = 0, 0.05, 0.10, 0.15 and 0.20, abbreviated as Sn100 x NPs) prepared by hydrothermal reaction were estimated with 57 Fe- and 119 Sn-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 Sn100 x NPs as a cathode were evaluated. 57 Fe-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 119 Sn-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 SnO 6 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 57 Fe-Mössbauer spectra measured at the low temperature were 210 K, 228 K, and 250 K for Sn10, Sn15, and Sn20, while that of α -Fe 2 O 3 was 324 K. Similarly, the Debye temperature of 199, 251, and 269 K for Sn10, Sn15, and Sn20 were estimated from the temperature-dependent 119 Sn-Mössbauer spectra, which were significantly smaller than that of BaSnO 3 (=658 K) and SnO 2 (=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 Sn100 x 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 Sn100 x NPs.