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Abstract:
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Lithium ion batteries , due to their relatively high energy density , are now widely used as the power source for portable electronics . Commercial lithium ion cells currently employ layered LiCoO₂ as a cathode but only 50 % of its theoretical capacity can be utilized . The factors that cause the limitation are not fully established in the literature . With this perspective , prompt gamma -ray activation analysis (PGAA ) has been employed to determine the hydrogen content in various oxide cathodes that have undergone chemical extraction of lithium (delithiation ) . The PGAA data is complemented by data obtained from atomic absorption spectroscopy (AAS ) , redox titration , thermogravimetric analysis (TGA ) , and mass spectroscopy to better understand the capacity limitations and failure mechanisms of lithium ion battery cathodes . As part of this work , the PGAA facility has been redesigned and reconstructed . The neutron and gamma -ray backgrounds have been reduced by more than an order of magnitude . Detection limits for elements have also been improved . Special attention was given to the experimental setup including potential sources of error and system calibration for the detection of hydrogen . Spectral interference with hydrogen arising from cobalt was identified and corrected for . Limits of detection as a function of cobalt mass present in a given sample are also discussed . The data indicates that while delithiated layered Li[subscript 1 -x]CoO₂ , Li[subscript 1 -x]Ni[subscript 1 /3]Mn[subscript 1 /3]Co[subscript 1 /3]O₂ , and Li[subscript 1 -x]Ni[subscript 0 .5]Mn[subscript 0 .5]O₂ take significant amounts of hydrogen into the lattice during deep extraction , orthorhombic Li[subscript 1 -x]MnO₂ , spinel Li[subscript 1 -x]Mn₂O₄ , and olivine Li[subscript 1 -x]FePO₄ do not . Layered LiCoO₂ , LiNi[subscript 0 .5]Mn[subscript 0 .5]O₂ , and LiNi[subscript 1 /3]Mn[subscript 1 /3]Co[subscript 1 /3]O₂ have been further analyzed to assess their relative chemical instabilities while undergoing stepped chemical delithiation . Each system takes increasing amounts of protons at lower lithium contents . The differences are attributed to the relative chemical instabilities of the various cathodes that could be related to the position of the transition metal band and the top of the O² - :2p band . Chemically delithiated layered Li[Li[subscript 0 .17]Mn[subscript 0 .33]Co[subscript0 .5 -y]Ni[subscript y]]O₂ cathodes have also been characterized . The first charge and discharge capacities decrease with increasing nickel content . The decrease in the capacity with increasing nickel content is due to a decrease in the lithium content present in the transition metal layer and a consequent decrease in the amount of oxygen irreversibly lost during the first charge . |