High-Precision Measurement of Side Reaction Current on Lithium Insertion Electrodes in Lithium-Ion Cells

Abstract

To achieve long-life lithium-ion batteries for automobile and stationary applications, capacity fading caused by imbalance in state of charges between positive and negative electrodes is a critical issue because such capacity fading cannot be avoided by applying electrode materials without any deterioration during cycling. In our previous paper, imbalance in state of charges was due to a difference in side reaction currents (rates of side reaction) of positive and negative electrodes by taking mass balance into account in a lithium-ion cell, and lifetime of lithium-ion cells can be extended by adjusting side reaction currents on both electrodes to be the same. A side reaction current on a lithium insertion electrode was measured by using a zero-volt lithium-ion cell with symmetric parallel-plate electrode configuration (SPEC), in which fully-charged and fully-discharged electrodes were combined in a cell. In this case, side reaction current measured in a symmetrical cell corresponds to “intrinsic” rate of side reaction, because there is no influence of a counter electrode. Side reaction currents on lithium insertion electrodes of Li[Li0.1Al0.1Mn1.8]O4 (LAMO), Li[Li1/3Ti5/3]O4 (LTO) and Li[Ni1/2Mn3/2]O4 (LiNiMO) by using symmetrical cells were ascending in the order of LAMO < LTO < LiNiMO at any temperature. In a lithium-ion cell, relation of side reaction currents on positive and negative electrodes can be estimated by comparing state of charges between positive and negative electrodes after cycle tests. According to the results on long-term cycle tests of LTO/LAMO and LTO/LiNiMO cells, the side reaction currents of LTO and LiNiMO in a LTO/LiNiMO cell show a different order from those in a symmetrical cell, LTO > LiNiMO, whereas side reaction currents of LTO and LAMO show the same order, LAMO < LTO. These results suggest that “real” rate of side reaction in a lithium-ion cell with different electrodes is deviated from “intrinsic” rate of side reaction in a symmetrical cell with the same electrodes. In order to measure a “real” side reaction currents on lithium insertion electrodes in lithium-ion cells, we fabricated battery cycler in high accuracy and stability of external current during charge and discharge and verified the battery cycler by using a LTO/LTO symmetrical cell. In a symmetrical cell side reaction currents on both electrodes are thought to be the same, consequently charge-end and discharge-end capacities in a cumulative capacity plot, in which charge and discharge capacities are cumulated from the initial cycle with charge capacity in plus value and discharge capacity in minus value, should change in the same degree. In a cumulative capacity plot, side reaction currents on each electrode can be calculated from the slope of charge-end and discharge-end capacities. As can be seen in the figure, slopes of time profiles of charge-end and discharge-end capacities are virtually the same; 0.24 μA and 0.20 μA. Difference in side reaction current of the electrodes is only 0.04 μA, which corresponds to an error in charge-discharge current supplied by the home-made battery cycler fabricated in this study. Since the error caused by the battery cycler is enough low compared with side reaction current on lithium insertion electrode of LTO, the battery cycler can allow us to measure side reaction currents in lithium-ion cells with high accuracy. In this paper we will report a method to measure side reaction currents on lithium insertion electrodes in lithium-ion cells and discuss our strategy toward extending cycle-life of lithium-ion batteries based on mass balance in the cell. Figure 1