Long Term Cycling Performance of High-Capacity Lithium-Ion Full Cells with Li1.2Ni0.15Mn0.55Co0.1O2/Li4Ti5O12
Authors: Li Yan , Bettge Martin , Zhu Ye , Li James C.M. , Abraham Daniel P 
Published: February 2020

High-capacity Li-ion battery systems bearing Li1.2Ni0.15Mn0.55Co0.1O2-based positive electrode and graphite-based negative electrode show severe capacity fade and large impedance rise during long term cycling.[1] Cell capacity fade mainly arises by lithium trapping in the solid electrolyte interphase (SEI) of the graphite-based negative electrode. The cells impedance rise and voltage fade are mainly from the Li1.2Ni0.15Mn0.55Co0.1O2-based positive electrode.[1-3]From a practical and scientific point of view, it is important to design and evaluate optimized Li-ion battery systems with minimal capacity fade and low impedance rise.
Li4Ti5O12 is an attractive anode material and shows a well-defined 1.55 V plateau vs. Li+/Li.[4] This relatively high voltage plateau prevents conventional SEI formation.[5] Therefore, replacing graphite with Li4Ti5O12 in our current Li-ion battery system is expected to achieve good capacity retention during cycling. Adding a small amount of LiDFOB to the Gen 2 electrolyte is an additional strategy to improve the cell performance. LiDFOB can be oxidized and forms a protective layer on the positive electrode surface; this layer slows down the impedance rise during cycling. [2]

Therefore, we evaluate cells with Li1.2Ni0.15Mn0.55Co0.1O2-based positive and Li4Ti5O12-based negative electrodes; the cells contain Celgard 2325 separator, and an EC: EMC (3:7, by wt.) + 1.2M LiPF6 + 2 wt% LiDFOB electrolyte. Cells are cycled between 0.75 to 3.15V at a ~C/3 rate, to 500 cycles. After 500 cycles, the capacity retention is better and impedance rise is lower than that of the graphite-bearing full cells.
Electrochemical performance evaluations are conducted on the harvested positive and negative electrodes. We identify that impedance rise still mainly arises from the Li1.2Ni0.15Mn0.55Co0.1O2-based positive electrode, after 500 cycles. The impedance rise on the Li4Ti5O12-based negative electrode is negligible after aging. The capacities of the harvested positive and negative electrodes, measured at slow rates, are also similar to that of the pristine electrodes. The progression of voltage fade and its implications will be also discussed.[6, 7]

References

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