Investigation on Formation Process of Lithium-Ion Batteries

Abstract

To increase the energy density and the power density of lithium ion battery cell, it is important to design the capacity of each electrode accurately. Ideal lithium-ion battery should be formed SEI film through the first charge/discharge, after then should maintains 100% coulombic efficiency1. In reality, such a perfect formation cannot be achieved within one cycle, and reduction in cell capacity occurs with repeating charge/discharge cycle. This makes hard to measure the irreversible capacity accurately, so it is important to finish formation process in one cycle for cell design and process design. In this work, we confirmed that the 0.1C charge as the formation protocol does not complete the formation with electrochemical impedance spectroscopy using graphite half-cell. In addition, we investigated how the cell capacity and the coulombic efficiency changes as the cell with each protocol is cycled by varying the current applied in the CC process or time taken for the CV process during CC-CV charging protocol by graphite half-cell and Li [Ni0.6Co0.2Mn0.2]O2 / graphite cells. We investigated whether we could accelerate the formation of lithium-ion batteries with variation of charging protocol. According to the experimental results, the film resistance of the graphite electrode decreased as the cell formation completed. When the film resistance of graphite decreases, equilibrium potential increases so the utilization of graphite active material would be increased in the subsequent cycles. This is followed by the abnormal behavior of capacity and coulombic efficiency. In charging/discharging graphite half-cell, we confirmed that higher portion time taken of charging with CV in first charge leads to earlier completion of formation. Change in film resistance, capacity, and coulombic efficiency will be discussed.