On the Electroanalytical Characterization of Li x CoO 2 , Li x NiO 2 and LiMn 2 O 4 (Spinel) Electrodes in Repeated Lith
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ABSTRACT This paper reports on electroanalytical studies of the intercalation-deintercalation of lithium into lithiated transition metal oxides which are used as cathodes for Li ion batteries. These include LixCoO 2 Li.NiO 2 and Li.Mn 20 4 spinel. The basic electroanalytical response of these systems in LiAsF6 IM/EC-DMC solutions was obtained from the simultaneous use of slow and fast scan cyclic voltammetry (SSCV), potentiostatic intermittent titration (PITT) (from which D vs. E was calculated), and impedance spectroscopy (EIS). Surface sensitive FTIR spectroscopy and XRD were also used for surface and 3D characterization, respectively. A large and important denominator was found in the electrochemical behavior of lithium intercalation-deintercalation into these transition metal oxides and graphite. The use of the electroanalytical response of these systems as a tool for the study of stabilization and failure mechanisms of these materials as cathodes in rechargeable Li batteries is demonstrated and discussed. INTRODUCTION The most popular, important and commonly used cathode materials for lithium ion batteries are lithiated cobalt, nickel and manganese oxides: LiCoO 2, LiNiO 2 and LiMn 204 spinel. These compounds deintercalate/intercalate lithium reversibly at potentials around 4 V vs. Li/Li÷, and they are thereby very attractive for Li ion batteries with lithiated carbon anodes (which intercalate lithium in the 0 - I V vs. Li/Li' range). A tremendous amount of work has been dedicated so far to the correlation between preparation modes and performance of these materials." From practical experience accumulated, it is clear that the reversibility of these cathodes is limited due to fading mechanisms which lead to deterioration of their capacity upon cycling.( 2) This work was aimed at the study of the basic electroanalytical behavior of these compounds as reflected from the simultaneous application of slow scan-rate cyclic voltammetry (SSCV), potentiostatic intermittent titration (PITT), and electrochemical impedance spectroscopy (EIS), and the use of the electroanalytical response of these electrodes upon cycling as a tool for a rigorous study of their possible capacity fading mechanisms. The above is demonstrated for one type of stoichiometric LiMn 20 4 spinel electrodes in which a capacity fading is observed upon prolonged cycling. EXPERIMENTAL CONSIDERATIONS LiCoO 2, LiNiO 2 and several types of LiMn 20 4 of very high quality, as well as standard solution components including ethylene and dimethyl carbonates (EC, DMC), and LiAsF6 (Li battery grade), were obtained from Merck. Standard, thin electrodes were 435 Mat. Res. Soc. Symp. Proc. Vol. 496 © 1998 Materials Research Society
prepared using aluminum foil (current collector), carbon black (15% by weight for conductivity) and PVDF (5%, binder), and the active mass (85%). These were studied in standard solutions EC-DMC 1:3/LiAsF 6 IM vs. Li and lithiated carbon anodes. SSCV, PITT and EIS were performed using computerized instrumentation from Arbin, EG&G and Sch
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