No abstract

The effect of vinylene carbonate (VC) as electrolyte additive on the formation mechanisms of passivation films covering both electrodes in lithium-ion batteries was investigated by X-ray photoelectron spectroscopy (XPS). LiConormalO2 /graphite coin cells using a LiPnormalF6 /ethylene carbonate:diethyl carbonate:dimethyl carbonate liquid electrolyte with or without VC were charged at 20 and 60°C . The identification of VC-derived products formed at the surface of the electrodes was carried out by a dual experimental/theoretical approach. From a classical XPS core peak analysis completed by a detailed interpretation and simulation of valence spectra supported by ab initio calculations, and through direct synthesis of the VC polymer, we could evidence the formation of the radical poly(VC) at the electrode/electrolyte interfaces. We showed that the radical polymerization is the main reaction mechanism of VC contributing to the formation of the passivation layers at the surface of both electrodes.

No abstract

The impact of aluminum oxide coatings on LiCoO 2 materials for commercial lithium ion batteries has been investigated by X-ray photoelectron spectroscopy ͑XPS͒. A low binding energy component in the Al 2p core peak spectra was observed and attributed to the formation of a LiAl x Co ͑1−x͒ O 2 solid solution interphase for both Al 2 O 3 -and AlPO 4 -coated LiCoO 2 . The surface chemistry of pristine and Al 2 O 3 -coated LiCoO 2 cathodes and graphite anodes have been investigated after cycling up to 4.2 V or 4.4 V cutoff voltage and after various levels of capacity fade. The Al 2 O 3 coating enhances the capacity retention at both 4.2 V and 4.4 V cutoff voltages. XPS analyses provided evidence of the inhibition of cobalt dissolution from the LiCoO 2 positive electrode by the aluminum coatings. Moreover, the Al 2 O 3 coating lowers the kinetics of degradation of electrolyte species, especially the LiPF 6 salt.

Li y (Ni 0.425 Mn 0.425 Co 0.15 ) 0.88 O 2 materials were synthesized by a slow rate electrochemical deintercalation from Li 1.12 (Ni 0.425 Mn 0.425 Co 0.15 ) 0.88 O 2 during the first charge and the first discharge in order to study the structural modifications occurring during the first cycle and especially during the irreversible "plateau" observed in charge at 4.5 V vs Li + /Li. Chemical Li titrations showed that the lithium ions are actually deintercalated from the material during the entire first charge process, excluding the possibility that electrolyte decomposition causes the "plateau". Redox titrations revealed that the average transition metal oxidation state is almost constant during the "plateau", despite further lithium ion deintercalation. 1 H MAS NMR data showed that no Li + /H + exchange was associated to the "plateau" itself. Rietveld refinement of the XRD pattern for a material reintercalated after being deintercalated at the end of the "plateau", as well as redox titrations, revealed an M/O ratio larger than that of the pristine material, which is consistent with the oxygen loss proposed by Dahn and coauthors for the LiNi x Li (1/3-2x/3) Mn (2/3-x/3) O 2 materials to explain the irreversible overcapacity phenomenon observed upon overcharge. X-ray and electron diffraction showed that the transition metal ordering initially present within the slabs is lost during the "plateau" due to a cation redistribution. To explain this behavior a cation migration to the vacancies formed by the lithium deintercalation from the transition metal sites (3a) is assumed, leading to a material densification.

No abstract

No abstract

New LiNi 1Ϫy Mg y O 2 (0 Յ y Յ 0.20) layered oxides were synthesized by a coprecipitation method followed by a high-temperature thermal treatment. Rietveld refinements of their X-ray diffraction patterns showed that they exhibit a quasi-two-dimensional structure, isostructural to LiNiO 2 , for small substitution amounts (y Յ 0.10). For larger amounts (y ϭ 0.15, 0.20), the Li/(Ni ϩ Mg) ratio is significantly lower than unity. In all cases, the extra ions located in the inter-slab space for lithium deficiency compensation are preferentially Mg 2ϩ ions. A magnetic study confirmed the cationic distributions which result from the size difference between Ni 3ϩ and Mg 2ϩ ions. An electrochemical study showed reversible behavior for all materials. A high capacity (Ն150 Ah kg Ϫ1 ) was found for LiNi 1Ϫy Mg y O 2 phases (y Յ 0.02), which decreased when y increased. The presence of Mg 2ϩ cations in the inter-slab space, which cannot be oxidized and have a size close to Li ϩ , prevents the local collapses of the structure which occurs for the Li 1Ϫz Ni 1ϩz O 2 system; therefore good cycling stability is observed.