Surface films phenomena on vanadium-pentoxide cathodes for Li and Li-ion batteries: In situ AFM imaging

Vanadium oxides are attractive candidates as insertion cathode materials for lithium and lithium-ion batteries, because of their several possible oxidation states, which mean high capacity. We discovered unique surface phenomena related to V₂O₅ cathodes in lithium batteries when LiPF₆ solutions are in use, which may have strong impact on the electrochemical performance of lithium-ion batteries. Thin-film vanadium-pentoxide electrodes were investigated in LiPF₆ and LiClO₄ solutions (PC, EC-DMC) in the relevant potential window (3.0-3.6 V vs. Li/Li⁺). Slow-scan cyclic voltammetry measurements demonstrated better kinetics of intercalation of the lithium ions into the layered V₂O₅ cathodes in the LiClO₄ solutions than in the LiPF₆ solutions. Long-term cycling measurements had shown a better stability in the intercalation kinetics in the LiClO₄ solutions, while during cycling in the LiPF₆ solutions, one could recognize slow down of the kinetics and capacity fading of the electrodes. Using in situ AFM imaging, it was revealed that the presence of the PF ₆^- anions induce unique surface chemistry which has detrimental effect on the electrochemical performance: nano-size particles are deposited in LiPF₆ solutions on the boundaries of the V ₂O₅ grains, thus slowing the insertion of lithium ions into the layered matrix, while in the presence of the ClO₄ ^- anions, there were only negligible changes in the morphology, leading to intercalation of lithium ions only, as was demonstrated by EQCM measurements. The role of the PF₆^- anion in the anodic passivation phenomena is demonstrated and discussed, along with presentation of imaging of surface films formation on the cathode side of Li-ion battery systems.