Influence of the lithium salt electrolyte on the electrochemical performance of copper/LiFePO4 composites

In this work, we studied the influence of the electrolyte salt, LiPF6 or LiClO4, on the electrochemical properties of copper/LiFePO4 composites. We found a different stability voltage window for the two electrolytes that was remarkably wide for LiPF6. Also, copper addition is commonly accepted to increase electrode conductivity, which is beneficial for electrochemical purposes. However, copper is always oxidised to a variable extent depending on the particular electrolyte during the charge phase. Oxidation of the electrolyte solvent (especially with LiClO4) was also observed during the first charge. In the first cycle, copper was more or less efficiently removed from the electrode surface. In subsequent cycles, however, these phenomena failed to occur (LiPF6) or were weaker (LiClO4). In all these configurations, iron is partially dissolved, to an extent dependent on the amount of copper present in the composite and differing with the particular electrolyte used. Electrochemical impedance spectroscopy allowed us to identify the process taking place close to 3.6 V in LiPF6 configuration: Fe(II) from the composites are oxidised and irreversibly complexed by the joint action of HF formed from LiPF6 and water traces and Cu(I) formed upon charging. Our hypothesis accurately explains the results observed in terms of charge/discharge profiles, capacities provided and capacities evolution upon cycling. Also, our test results testify to the importance of using low contents of copper in the composites and the good properties of LiPF6 as electrolyte solvent.