Capacity Fade and Impedance Evolution in Tin Phosphide Anodes for Sodium-Ion Batteries

Abstract

Sodium-ion batteries (SIBs) offer a potentially more cost effective and environmentally friendly alternative to Lithium-ion batteries for room temperature high density energy storage. To maintain the benefits of SIBs, earth abundance for all the batteries components is paramount. One promising earth abundant anode material for SIBs is tin phosphide (Sn4P3). Tin phosphides are able to retain a high specific capacity for an extended cycle life. Understanding the changes in electrochemistry within a battery as a result of cycling is crucial in the complete understanding of capacity fade mechanisms and subsequent improvement and implementation as an anode material. In this work, the main technique employed to gain this understanding is electrochemical impedance spectroscopy with distribution of relaxation times analysis (EIS-DRT). This mathematical approach to understanding traditional EIS datasets helps distinguish impedance contributions from the following processes: diffusion, counter electrodes, solid electrolyte Interfaces (SEI), and contact resistances.Tin phosphide active material was synthesized by mechanical milling, with material structure confirmed by X-ray diffraction (XRD). Half cells were assembled with sodium counter electrodes and cycled with intermittent EIS assessment. Capacity retention was tracked during cycling to assess electrode operation. EIS-DRT analysis was performed every 5 cycles and after the end of cycling across the anode’s voltage range (0.01-1.5 V vs Na/Na+). After cycling, cyclic voltammetry (CV) and XRD were employed to further understand changes in electrochemical activity and crystal structure associated with the end of cycle life.The evolution of impedance signatures across the initial five cycles exhibits significant shifting within the SEI and counter electrode peak at 1000 Hz. Impedance trends across the voltage range switched, in which lower voltages at higher levels of sodiation exhibit larger impedance contributions to the system. These trends are confirmed by a base understanding of SEI formation and sodiation trends. Relative impedance contributions from the diffusion process increase from pristine to cycled electrodes, with diffusion peaks at becoming the dominant impedance contribution as cycling progresses. Significant changes in DRT trends plateaued after the initial SEI formation over the first five cycles.Abrupt and early failure during cycling of a subset of the cells was marked by a plateau in the working electrode voltage, an increase in contact resistance within EIS-DRT plots and decreased electrochemical activity within CV. Visual post mortem checks correlated well with these findings, with an apparent delamination of the electorate coating from the current collector. Understanding the physical and electrochemical phenomenon within a battery though nondestructive in situ characterization is crucial to simplify work flows in battery testing.