Electroplated Tin-Based Alloy Anodes And Their Properties For Lithium-ion Battery

Tin and tin-based alloys have been suggested as promising alternative anode materials for their high gravitational and volumetric capacity. The main disadvantage of tin and tin-based alloys is large volume expansion, which caused exfoliation, and poor cycleability. Up to now, the main methods to resolve this problem include using nano-materials, alloying with other active or inactive element, and coating with active or inactive materials. Besides these, the study of the surface morphology and porosity of alloy electrodes, and interfacial properties of electrode/electrolyte is also important to improve electrochemical performance. In this paper, we invented the method to prepare large tin content Sn-Cu alloy (Cu₆Sn₅ alloy) in cyanide-free solution. We have extensively studied the electrochemical performance, especially the relation of cycleability and current collectors with different structure, and the compatibility of electrode and commercial electrolyte. We also investigated the SEI film and phase transformation of these electrodes using electrochemical impedance spectroscopy. The main experiments and results are given as follow.(1) The electrochemical performance of Sn-Cu alloy on smooth copper sheet, copper foam and rough copper foil. The results of XRD indicated that all of them are comprised of Cu₆Sn₅ and pure Sn. Charge/discharge results revealed that they all appeared two potential plateaus at about 0.4 V and 0.1 V, which are the characteristic potential plateaus of Cu₆Sn₅ alloy. Charge/discharge results also revealed that the cycleability of Sn-Cu alloy on rough copper foil was better than that of other two electrodes. From SEM results, we observed that there appeared serious cracks and exfoliation on the surface of Sn-Cu alloy on smooth copper sheet after 39 cycles, and there are few cracks and no exfoliation for Sn-Cu alloy on copper foam and rough copper foil after 50 cycles. The Nyquist spectra of different electrode potentials indicated that the arc appearing in high frequency region represents the SEI film at 1.2 V. When the electrode potential was polarized to about 0.4 V, an arc in the middle frequency region and an arc in the low frequency region appear, corresponding respectively to charge transferring and phase transformation. The phase transformation impedance arc disappeared at about 0.3 V, and appeared again at about 0.1 V. The Nyquist spectra acquired at different stages of discharge under galvanostatic condition revealed that when the electrode is discharged for 5 mAh/g, the arc of high frequency arc associating to the impedance of SEI film is obeserved. When the electrode discharged 55 mAh/g, there appeared middle frequency arc and low frequency arc which represent charge transferring impedance and phase transformation impedance respectively. From the Nyquist spectra of the Sn-Cu electrode after different cycles, we can observe that the charge transferring impedance decreased first and then increased with the increasing of cycles, which revealed a process of activity to failure.(2) Preparation and properties of Sn-Co alloy on smooth copper sheet and rough copper foil. The results of XRD indicated that the Sn-Co alloy on smooth copper sheet is amorphous, and the Sn-Co alloy on rough copper foil is comprised of intermetallic composites of SnCo and Co₃Sn₂. Charge/discharge results revealed that one potential plateau at about 0.22 V is observed in all cases in the first cycle, and is shifted to 0.4 V in the second cycle. There appears no obvious decreasing for gravitational specific capacity after 70 cycles for Sn-Co alloy on rough copper foil, which is better than that of Sn-Co alloy on smooth copper sheet. From SEM results, we observed that there appeared serious cracks and exfoliation on the surface of Sn-Co alloy on smooth copper sheet after 20 cycles, and there are few cracks and no exfoliation for Sn-Co alloy on rough copper foil even after 70 cycles. The Nyquist spectra at different polarized potential indicated that there appeared a high frequency arc which represents the impedance of transferring through SEI film of lithium-ion at the potential of 1.1 V. When the electrode potential was polarized to about 0.4 V, there appeared a middle frequency arc and a low frequency arc which represent charge transferring impedance and phase transformation impedance respectively. When the electrode potential was polarized to 0.125 V, the middle frequency arc disappeared, which is different from that of Sn-Cu alloy electrode. From the Nyquist spectra of the Sn-Co electrode recorded in different cycles, we observed that the charge transfer impedance is gradually increased with the increase of cycles.(3) The synthesis of porous copper foam and the electrochemical performance of Sn-Cu and Sn- Co alloy on this porous copper foam current collector were studied. Variety of porous copper foams with highly open porous walls have been successfully sculptured using the gas evolved in an electrochemical deposition process. The pore sizes and wall structures of the foams are tunable by adjusting the deposition conditions. The contact between porous copper foams and substrate was strengthened by annealing porous copper foams at high temperature. Charge/discharge results revealed that the cycleability of Sn-Cu alloy on porous copper foams after annealing is better than that of the Sn-Cu alloy deposited on copper porous foam without annealing. The first discharge and charge capacity of Sn-Cu alloy on porous copper foams after annealing is 735 mAh/g and 571 mAh/g respectively, and the charge capacity remained 342 mAh/g after 50 cycles. The Nyquist spectra of different polarized potential at different temperature indicated that there appeared a high frequency arc which represents the impedance of transferring through SEI film of lithium-ion at the potential of 1.2 V. When the electrode potential was polarized to about 0.4 V, there appeared a middle frequency arc and a low frequency arc which represent charge transfer impedance and phase transformation impedance respectively. Simulation result revealed the phase transformation resistance is smallest at the potential region of phase transformation. Charge/discharge results revealed that the first discharge and charge capacity of Sn-Co alloy on porous copper foams after annealing is 726 mAh/g and 563 mAh/g respectively, and the first coulomb efficiency is 77.6%. After 50 cycles, the capacity remained 70% of that of the second cycle. When the electrode potential was polarized to about 0.4 V, the Nyquist spectra are comprised of a high frequency arc, a middle frequency arc and a low frequency arc which represent SEI film impedance, charge transfer impedance and phase transformation impedance respectively. Similar to Sn-Cu alloy, the phase transformation resistance is smallest at the potential region of phase transformation. When Sn-Co alloy electrode suffer Short circuit, there appear inductive loop in the Nyquist spectra, which caused by heterogeneity. The diameter of inductive resistance increased with the decreasing of electrode potential in the initial lithiation, and decreased with the increasing of electrode potential in the initial delithiation.The results of this thesis throw insight into electrode/electrolyte interface and failure mechanism of tin and tin-based alloy, and are of significance in developing relevant fundamental theory. The extensively study on the invention of cyanide-free electroplating tin-based alloy, and the intrinsic relation of surface structure of current collector and properties of them is also great importance in improving electrochemical performance of them and application in next generation lithium-ion battery.