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 PF6- 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 V2O5 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. (C) 2004 Elsevier B.V. All rights reserved.