Combinatorial Studies of Fe-Zn and Fe-Si-Zn Alloy Negative Electrodes for Li-ion Batteries

Abstract

In this work Fe-Zn and Fe-Si-Zn alloy systems were studied for use as negative electrodes in Li-ion batteries. The aim was to understand the electrochemistry of these alloy systems and observe if the addition of Fe and Zn to Si thin-film electrodes leads to improvements in performance by reducing volume expansion and suppressing formation of Li15Si4. Thin-film libraries were fabricated using combinatorial sputtering. X-Ray diffraction, Mössbauer effect spectroscopy, and electron microprobe were used to characterize structure of the thin-films. Electrochemical cycling measurements were used to determine if alloys based on these systems are practical for use in Li-ion batteries. Electrochemical performance of Fe-Zn electrodes was strongly dependent on composition. The cycle life and coulombic efficiency improved as Fe concentration in electrodes increased to x = 0.12. A simple macroscopic atom model predicted that compositions with up to 50 atomic percent Fe would be active toward Li. However, the capacity decreased as the iron content increased and the FexZn1-x alloys became completely inactive when the Fe content was above 12 atom %. Ex-situ X-ray diffraction and Mössbauer measurements were used to explain the structural changes that occur during cycling. A large amorphous region exists in Si-rich compositions of the Fe-Si-Zn system. The electrochemistry of numerous compositions of Fe-Si-Zn materials was determined near room temperature. Voltage curves and capacity of the films was strongly dependent on composition. ICE and hysteresis improved with increasing Si concentration in the electrodes. Both Zn and Fe additions to Si were found to suppress the formation of Li15Si4 during cycling.