Working electrode: SnSb, Sb and Cu2Sb Electrolytes and fiber glass Mössbauer spectroscopy of Na-ion batteries with SnSb, Sb and Cu2Sb Electrodes H-Y. Haha,b, C. E. Johnsona, J. A. Johnsona,b, L. Baggettoc, G. M. Veithc a. Center for Laser Applications, University of Tennessee Space Institute, Tullahoma, TN, 37388 b. Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee Space Institute, Tullahoma, TN, 37388 c. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. Motivation Discussion Results A good battery has a high storage capacity (charge per weight of Li or Na) and good cycle performance (i.e. returns to its original state after recharging).   1800 Volta pile 1836 Daniell cell 1859 lead-acid battery* 1900 zinc-carbon battery nickel-cadmium battery* 1970 lithium battery 1990 lithium-ion battery* 2??? Sodium-ion battery* *rechargeable  For a Li-ion battery the Li ions are provided by graphite with Li intercalated between layers.    However, Li is relatively rare and expensive, and also has safety problems in batteries. Recent developments use Na-ions since sodium is much more plentiful. Sb and Sn are potential electrodes since these atoms can store 3 and 3.75 atoms of Na respectively. At UTSI we have made Mössbauer measurements on the Sb as electrodes in Na-ion batteries at various stages of charge. These rechargeable batteries can be used in medical applications for pacemakers (both rechargeable and non-rechargeable) , robotic artificial limbs (for above joint amputations), see Figure 1 and cordless surgical tools. Figure 1 A robotic leg running on rechargeable batteries, required for above the knee amputations. Charging of Na-ion batteries results in desodiation or removal of sodium ions from the working electrode. Discharging the working electrode however causes sodiation to happen and sodium is inserted into the working electrode. Through the different stages of these two cycles of charging and discharging, the working electrode experiences a change in chemistry. It is this change in chemistry that is being measured here with Mössbauer spectroscopy. In Figure 2, it is shown that the isomer shift of the antimony spectra increases as the battery is discharged (sodiated) and decreases back close to its original starting point when fully charged (desodiated). Similarly in Figure 3 for the intermetallic compound of tin and antimony, the same reversibility of the charged/discharge cycles is shown. However, it is also important to notice the widths do not always return to its original narrowness, indicating that recharging cycles are not unlimited. Finally, it is noticeable some of the spectra contain more than two sites, which is an indication of Sb atoms of different valencies existing at one point or another in these working electrodes. Figure 2 Characterization of the reaction of pure Sb with Na using 121Sb Mössbauer spectroscopy. (a) Electrochemical profile of Sb during the first cycle and (b) corresponding 121Sb Mssbauer spectra measured at the positions highlighted by markers in (a). The top curve in (a) is the charging curve. Figure 3 Similarly in (a) the top black curve is the charging curve while the bottom black curve is the discharging curve.121Sb Mössbauer spectroscopy data for Sb powder, Cu2Sb powder, Cu2Sb thin film, electrode thin film charged at 1 V and discharged at 0 V, and Na3Sb reference powder. Figure 4 121Sb Mössbauer spectroscopy data for Sb powder, Cu2Sb powder, Cu2Sb thin film, electrode thin film charged at 1 V and discharged at 0 V, and Na3Sb reference powder. Charge Extract Na-ions Intermediate material, Na and working electrode mixture Insert Na-ions Discharge Working electrode: SnSb, Sb and Cu2Sb Electrolytes and fiber glass Sodium Conclusion We can see that by using the Mössbauer effect, we can track the changes that occur in the working electrode as the Na-ions are inserted or extracted. Most importantly, we note that some of the changes are reversible, which is important for a rechargeable battery.