Determination of an Efficient Formation Strategy -- The Impact of Different C-Rates

摘要

Objective: Lithium-Ion Batteries (LIBs) are strongly relying on the electrolyte decomposition layer forming at the anode (solid electrolyte interphase, SEI) and at the cathode (cathode electrolyte interphase, CEI) [1-5]. These protective layers are formed within the first cycles (usually depicted as: formation). The formation of batteries is a costly and time intensive production step. However, very little is known about the interaction of the formation procedure and the quality of the SEI and CEI. Hence, the objective is to find the most desirable structure, composition and thickness of the decomposition layers and the most cost efficient way to build them. Methodology: To study the effects of different C-rates within the formation procedure on the cell performance lab cells and 5 Ah pouch cells were formed with different C-rates (0.05 to 2C) for 1 cycle holding a CV step at 4.2 V with C/50. Afterwards electrochemical characterization using 1C CCCV long term cycling to determine effects on the capacity fade and pulse tests to determine effects on the cell resistance were conducted. The cells were composed of a NMC111 cathode and a MCMB anode. To determine a dependency of the used electrolyte on the optimal formation procedure cells were filled either with EC:EMC 3:7 1M LiPF6 or the same electrolyte plus 2% of VC. Results: Increasing the C-rate during the formation leads to decreased formation times. With 0.2C the formation takes 10.2 h while at 2C it takes 1.8 h. It can be noticed that the CV step is increased from 0.2 to 1.8 h. From 0.05C to 0.2 C no change in cell performance can be observed. From 0.2C to 2C in both electrolytes slight tendencies to lower discharge capacities at 1C CCCV at 20℃ can be observed. Regarding the resistance of the cells for the non-VC containing electrolyte no significant difference can be found, however, the VC containing electrolyte shows a slight increase <10% for the 0.5 and 1C formation which are at the same values. For 2C the VC containing electrolyte shows a more drastic increase by >20% compared to 0.2C. To investigate the influence of the charge and the discharge current during formation on the 1C CCCV discharge capacities 2 different asymmetric formation cycles were tested. The first one with 0.2C charge and 2C discharge, the second one with 2C Charge and 0.2C discharge. Discussion: From 0.2C on a slight impact of the C-rate up to 2C on the battery performance could be determined. Significant cost reduction can be obtained, however, depending on the application and the real production costs the optimal formation C-rate might depend. In future the effect of temperature, different active materials and electrolyte additives will be investigated to finally receive a strategy for an efficient formation, which can be applied for all relevant battery materials.