Exploring Electrochemistry and Interface Characteristics of Lithium-Ion Cells with Li1.2Ni0.15Mn0.55Co0.1O2 Positive and Li4Ti5O12 Negative Electrodes

Lithium-ion cells containing Li1.2Ni0.15Mn0.55Co0.1O2-based positive and Li4Ti5O12-based negative electrodes were electrochemically cycled in the 0.75-2.55 V and 0.75-3.15 V voltage windows. The maximum voltage of the Li1.2Ni0.55Mn0.55Co0.1O2 in the 0.75-2.55 V range is similar to 4.1 V vs. Li/Li+, well below the oxide's activation plateau; in contrast, in the 0.75-3.15 V range the oxide potential at top of charge is similar to 4.7 V vs. Li/Li+, beyond the activation plateau. Our results show that the discharge energy density decreases after 500 cycles from 154.2 to 147.8 mWh/g (similar to 4.1% reduction), and from 484.9 to 435.3 mWh/g (similar to 10.2% reduction), in the 0.75-2.55 V and the 0.75-3.15 V ranges, respectively. Almost all of this energy density decrease, in both cycling ranges, can be attributed to voltage fade in the Li1.2Ni0.15Mn0.55Co0.1O2 because cell capacity loss during cycling is small. An investigation of harvested electrolytes from cycled cells showed a noticeable, yet very small (ppb levels), increase in Mn, Ni and Co contents. In contrast, the content of these elements show a significant increase at the negative, indicating a preference for deposition at this electrode over accumulations in the electrolyte. In addition, XPS data showed significant amounts of organic and inorganic species at the negative electrode, which increased On cycling; this finding is consistent with previously reported data for Li4Ti5O12 electrodes. Surface films, containing products of electrolyte degradation, were also observed on the positive electrode. Although transition metal element dissolution occurs at this electrode, the loss of Mn, Ni and Co is not sufficient to significantly alter the oxide's composition and/or its capacity. (C) The Author(s) 2015. Published by ECS. All rights reserved.