Effect of Formation Cycling Schemes on Li-Ion Cell Capacity

Abstract

Li-ion batteries (LIBs) have the highest gravimetric and volumetric energy density than other rechargeable batteries and are a potential candidate which can replace internal combustion engines (ICEs) in the present and future automobiles to reduce the transport sector’s problems of oil supplies and CO2 emission climate change. The limitations associated with adopting this green technology to completely replace the traditional propulsion technology from automobiles are higher cost, and capacity fade of batteries with cycling and time. The capital cost of the battery includes the material cost and production cost, including initial testing and formation cycle cost. Therefore, reducing the cost of electrode materials and battery fabrication together with developing high energy density materials and novel electrode processing techniques are promising challenges for the academic research community as well as battery manufacturers. Initial formation cycles of cells are required for growth and stabilisation of the solid electrolyte interphase (SEI) film over the surface of the electrode which protects the sites from further electrolyte reduction and thereby controls the irreversible loss of lithium and electrolyte. So far, the formation process is done at very low current, i.e., C/30~C/20, for three to five cycles followed by resting to achieve a uniform growth of the SEI film over the reactive surface of the electrodes1. Therefore it typically requires 1-3 weeks of industrial space, environmental chambers and battery cycler2, and leads to high costs. The cost of the formation period is the second most expensive process in cell manufacture after electrode processing3. A recent study by An et al. 2 proposed a fast formation cycling method and shortened the formation time six fold with improved cycling performance of the cell. However, a comprehensive study of the effect of operational current and voltage on formation cycling of Li-ion cells needs to be conducted to optimise the formation strategy. This optimisation can minimise the formation time without compromising the quality of SEI layer and cell performance that will finally contribute to overall cost reduction of the battery pack. The objectives of the present study are the quantitative and qualitative analysis of SEI film growth for various formation cycling approaches of the Graphite-Nickel Manganese Cobalt (NMC) oxide cells. The SEI film resistance/thickness will be gauged by electrochemical impedance spectroscopy (EIS), scanning electron spectroscopy (SEM) and X-ray photoelectron spectroscopy (XPS). This testing will be followed by the charge-discharge cycling of the cells to elucidate the effect of formation schemes on the subsequent capacity degradation. Furthermore, this experimental framework will also be simulated and comprehensively analysed using an electrolyte reduction based SEI growth model coupled with the porous electrode model. The results obtained through this study will be valuable to the cell manufacturers in the selection of the formation approach for the reduced cell and battery pack cost.