Spectroscopic studies of cathode materials for lithium-ion batteries.

摘要

Structural changes that occur during electrochemical cycling of lithium-ion battery cathode materials have been investigated using in situ spectroscopic techniques.; A new method was developed for the preparation of carbon and binder free cathodes utilizing powder materials of interest for commercial batteries. The extraordinary quality of the cyclic voltammetric curves recorded for this type of electrodes during the in situ measurements allows direct correlations to be made between the state of charge of the material and its structural and electronic characteristics.; LiCoO₂, LiMn₂O₄ and LiCo_0.15Ni _0.85O₂ electrodes were evaluated using cycling voltammetry and the mean diffusion coefficient for Li-ions in the lattice (D_Li) was calculated for LiMn₂O₄.; LiMn₂O₄ electrodes prepared by this technique have been studied in situ using Mn K-edge XAS. Data analysis for the species formed at different potentials indicated a contraction of the lattice associated with the increase in the oxidation state of manganese.; In situ Raman spectra of particles of LiMn₂O ₄, and LiCoO₂ embedded in Au and also of KS-44 graphite and carbon microfibers MCF28 embedded in thermally annealed Ni have been recorded as a function of the applied potential.; Fe K-edge XAFS of pyrite electrodes in a Li/PEO(LiClO₄)/FeS ₂ cell and S K-edge XANES measurements of a FeS₂ electrode in a non-aqueous electrolyte have been acquired as a function of the state of charge. The studies have clearly evidenced the formation of metallic Fe and Li₂S as intermediates after 4 e− discharge and the formation of Li₂FeS₂ after 2 e− recharge. While Fe K-edge studies have indicated that there is no change in the Fe environment and oxidation state upon 4 e− recharge, the results obtained from S K-edge studies are inconclusive for this stage.; Finally, in situ Co K-edge XAFS data were obtained for the first time during the electrochemical cycling of electrodeposited Co(OH) ₂ films in alkaline solutions. The results support previous claims based only on electrochemical data and ex situ XRD and IR measurements that the oxidation of Co(OH)₂ leads to the formation of an inert CoOOH phase impervious to further reduction.