Formation Impact Study of Lithium-ion Battery Capacity,
Cycle Life, and
Jeff Q. Xu
Inclusive Dates: 01/01/12 – Current
Background — A "stable" surface film is formed on graphite electrodes upon the first charging of lithium-ion battery cells. This is called formation, which is a critical step during the battery manufacturing process. The formation of this solid electrolyte interface (SEI) layer passivates the graphite surface against further solvent decomposition. The SEI acts to decelerate the cell aging process and improve performance. To date, much of the SEI formation process is still not fully understood and the process is not well monitored during the formation step. Therefore, a direct and effective approach is needed for monitoring the formation process to develop effective and efficient formation protocols.
Approach — The objective of this project is to develop and evaluate the static and dynamic formation protocol impacts to lithium-ion cell performance such as capacity and cycle life. The approach is to leverage the thermal characteristics of the cell produced when the battery cell is under the first charging or formation cycle. By comparing various thermal or heat profiles at different formation rates, one or more clear voltage transition points can be identified as reference points to guide the changes of formation rates. An illustration of this approach is demonstrated in Figure 1.
Accomplishments — Use of a heat profile measurement/analysis tool during the cell formation stage can offer an insight for developing a suitable dynamic formation protocol for any type of graphite anode chemistry based lithium-ion cell (potentially can be used for silicon or carbon silicon alloy based anodes). The cells formed using the dynamic formation protocol have proved that thermal or heat measurement is effective during the study. The project was able to identify a specific voltage window that is most important to form an SEI layer effectively by using dynamic charge currents to allow reactions to take place with reduced battery cell degradation. A flexible formation current could be adopted to expedite the process. This tool can be applied to investigate novel electrolyte composition and additive functions, electrode material selection and formulation to the improvement of battery capacity, cycle life, and safety as well. Figure 2a and 2b are examples of the experimental setup.