Lithium-ion batteries using Li[Li1/3Ti5/3]O-4 and Li[Li0.1Al0.1Mn1.8]O-4 (LTO/LAMO) are very attractive for long-life and high-power applications. Capacity fading of the batteries mainly results from the deviation in the state of charges (SOCs) of LTO and LAMO electrodes caused by the difference in the side reaction currents of the electrodes. In this study, "real" and "intrinsic" side reaction currents are measured in LTO/LAMO cells and symmetric cells of LTO/LTO and LAMO/LAMO, respectively. An observed significant difference in the side reaction currents is affected by the coupled electrode. The real side reaction currents for both electrodes are a few times larger than the intrinsic side reaction currents at any temperature, which can be explained by taking into account the influence of oxidation (or reduction) products generated at the positive (or negative) electrode on the negative (or positive) electrode. A mechanism of the side reaction inside the cell is proposed to explain the above phenomenon. The mechanism indicates the importance for understanding the "secondary" effects of side reactions, which are influenced by decomposition products on the other electrode, in addition to "primary" effects of side reactions such as the formation of solid electrolyte interphase. (C) 2019 The Electrochemical Society.