No abstract

No abstract

No abstract

The main purpose of this work is to study the electronic structure of Co and Ni in deintercalated and reintercalated LiCo 0.5 Ni 0.5 O 2 . The technique used in the study was Co 2p and Ni 2p X-ray absorption spectroscopy (XAS). The results show that the Co ions are relatively unaffected by the Li deintercalation. On the other hand, there are clear changes in the electronic state of the Ni ions. It is worth noting that the changes observed in the Ni ions are reversible upon Li reintercalation.LiCo 0.5 Ni 0.5 O 2 is emerging as a useful cathode material for the production of lithium-ion batteries, an electrochemical system where the electrodes play a fundamental role. 1 This material is cheaper than the commonly used LiCoO 2 , it can be more easily prepared than the alternative LiNiO 2 , and it seems to be relatively stable upon Li reintercalation. Moreover, the LiCo 0.5 Ni 0.5 O 2 -based electrodes present a large potential variation (E > 3 eV with respect to the Li potential) as well as a relatively large and reversible charge capacity. 2 For all these reasons, the LiCo 0.5 Ni 0.5 O 2 compound is attracting considerable attention.The electronic structure of the transition metal ions in LiCo 0.5 Ni 0.5 O 2 is controversial. It is usually said that both the Co and Ni ions are in a trivalent state, 3 although recent spectroscopic results shows that the Ni ions are in a divalent state 4 (the charge missing at the Ni site is compensated by a less negative O ion). The changes in the electronic structure upon Li deintercalation-reintercalation have received little attention. This information is important for understanding the oxidation-reduction processes during these reactions, as well as the performance of the lithium-ion batteries that use LiCo 0.5 Ni 0.5 O 2 as a cathode. This provides the motivation to study the electronic structure of LiCo 0.5 Ni 0.5 O 2 in more detail.XAS is a useful tool in the study of electronic structure. This technique provides site and symmetry selected information on the unoccupied electronic states. 5,6 The transition metal 2p absorption edges are particularly useful in the case of transition metal oxides. The shapes of the spectra are directly related to the ground state of the transition metal ion. These spectra can be analyzed using atomic multiplet plus crystal field calculations. 7-10 The potential of this method was demonstrated in studies of the related LaCoO 3 11 and NiO nanoparticles. 12 ExperimentalThe LiCo 0.5 Ni 0.5 O 2 powder was prepared in atmosphere using the carbonate method (CoCO 3 , NiCO 3 и2Ni(OH) 2 и4H 2 O, and LiCO 3 , Fluka). Three consecutive thermal treatments were applied; 450ЊC for 36 h, 750ЊC for 36 h, and 800ЊC for 48 h. All thermal treatments were done in a Pt crucible and the sample was thoroughly milled between each step.The delithiation of the sample was carried out by chemical method, using HCl (1 N) at low temperature (T ϭ 0ЊC) to avoid intercalation of protons. 13 The samples were dried in a vacuum at 80ЊC for 24 h and stored in a dry box filled with Ar...

No abstract

The phonon properties of the lithium transition metal phosphates LiMPO 4 (M = Ni, Co, Fe) with the olivine structure were studied using a computational simulation. The calculation employs a normal coordinate analysis based on the Wilson's FG matrix method. The method applied to LiNiPO 4 allowed for the fitting of a set of stretching force constants and angle bond strengths that resulted in Raman wavenumbers comparing with experiments within a 5% average deviation. This set was assumed fixed for the isostructural Co-containing and Fe-containing compounds, as a first approximation. The calculated LiCoPO 4 Raman wavenumbers also compared well with experimental values (average deviation 9%). Using this procedure, the 36 Raman wavenumbers of LiFePO 4 were predicted and assigned to vibrational motions and symmetry species of the crystal group. LiFePO 4 unpolarized Raman data agree very well with the predicted values.

The mechanical deformation of thin films and membrane carbon electrodes during galvanostatic charge-discharge cycles has been measured by a simple optical technique. We have considered the deformations of coke and of graphite membrane electrodes to be due to the variations in the volume of grains and to the surface forces resulting from the electrochemical double layer. For such composite electrodes the minimum stress can be associated with the potential of zero charge of the electrode. The results show that the stress is due mainly to variations of volume in nonporous electrodes, to surface forces, and to the presence of passivating layers in very porous electrodes. The stress observed in the membrane electrodes after Li intercalation consists of an irreversible compressive tension and results in a permanent deformation of the electrodes. Infroduction

No abstract