New ORNL protocol reduces Li-ion battery formation time 6x or more without affecting battery performance
A new process developed by Oak Ridge National Laboratory could alleviate a bottleneck in battery manufacturing and deliver higher capacity batteries for electric vehicles and consumer devices. (Earlier post.) The ORNL method, published as an open-access paper in the Journal of Power Sources, also conserves lithium, which improves battery capacity. The process is applicable to all lithium-ion batteries and can be tuned for other chemistries as well, said principal investigator David Wood.
The formation process—where batteries undergo repeated cycling to stabilize and activate them for use—typically takes several days or more, and it is necessary for providing a stable solid electrolyte interphase on the anode (at low potentials vs. Li/Li+) for preventing irreversible consumption of electrolyte and lithium ions. An analogous layer known as the cathode electrolyte interphase layer forms at the cathode at high potentials vs. Li/Li+.
However, the time required for this process either results in lower battery production rates or a prohibitively large size of charging-discharging equipment and space (i.e. excessive capital cost).
The largest contributor to processing cost during LIB production is the electrolyte interphase formation step. The anode solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) form when the electrolyte is accessible to electrons at the electrode and, simultaneously the electrolyte experiences an unstable voltage range. During a charging cycle, the electrolyte decomposes and precipitates at low potentials at the anode via reduction reactions and at high potential on cathode via oxidation reactions.
Irreversible capacity loss indicating electrolyte interphase formation is the highest after the first charge/discharge cycle (ca. 10% in the case of graphite anode), significantly lower after the second cycle, and even lower after the third cycle and so on (less than 0.05%). The irreversible capacity loss varies depending on negative-to-positive capacity ratio, surface area of particles, operation conditions, etc.
Most electrolyte interphase forms during the first charge/discharge cycle because the pristine anode and cathode do not have previously formed passivation layers that electronically insulate the electrode from the electrolyte. If after the first cycle, the anode graphite was not significantly exfoliated, further cycling results in significantly lower electrolyte interphase formation because the preformed interphase layer (from the first charging cycle) impedes solvent molecule diffusion towards the electrode surface and electron transfer between the electrode and electrolyte.
Besides material cost … the electrolyte wetting and SEI formation steps are the most expensive processes ($2.2/kWh for electrode processing and $7.5/kWh for wetting/formation cycling) because of the slow wetting and slow charge/discharge rates (e.g. 3–5 cycles at C-rate of C/20 and 3–5 cycles at higher C-rate at a higher temperature). This process may take up to 1.5–3 weeks, depending on the cell manufacturer and cell chemistry, requiring a tremendous number of charge/discharge cycles for mass production of LIBs, large floor space, and intense energy for the cyclers and environmental chambers. These processes are a major production bottleneck; therefore, it is important to reduce wetting and formation time for cost and production rate benefits.—An et al.
The ORNL team developed a fast and effective electrolyte interphase formation protocol and, in the study, compared it with an Oak Ridge National Laboratory baseline protocol.
For the study, the team used graphite anodes, NMC 532 cathodes, and 1.2 M LiPF6 in ethylene carbonate: diethyl carbonate as the electrolyte.
The proposed formation protocol shortened formation time by 6 times or more without compromising cell performance. Results from electrochemical impedance spectroscopy showed the new protocol reduced surface film (electrolyte interphase) resistances, and 1,300 aging cycles showed an improvement in capacity retention.
Seong Jin An, Jianlin Li, Zhijia Du, Claus Daniel, David L. Wood III (2017) “Fast formation cycling for lithium ion batteries,” Journal of Power Sources, Volume 342, Pages 846-852 doi: 10.1016/j.jpowsour.2017.01.011