Use of Electrochemical Impedance Spectroscopy to Reveal Ageing Mechanisms in Silicon Anodes for Lithium-Ion Batteries

Abstract

The key for the exploitation of renewable energy and thus the replacement of fossil fuels and traditional energy sources is the achievement of effective energy storage strategies. In this regard, rechargeable Lithium-ion batteries (LIBs) play a significant role due to the high gravimetric and volumetric energy, high power density, long cycle life and low self-discharge property. However, the new applications of LIBs need higher energy densities than the present LIB technology can offer. Replacing the commercially used carbon anodes with Silicon anodes can increase the capacity more than ten times. Unfortunately, implementing Silicon anodes is still challenging due to the poor cycling stability caused by the severe volume changes of Silicon upon lithiation and delithiation during charge and discharge respectively. To improve the cycling stability a fundamental understanding of the complex processes and the degradation mechanisms is necessary. The processes in a Silicon anode during cycling have been studied by using electrochemical impedance spectroscopy (EIS) as it is a powerful, noninvasive technique to resolve the various processes in an electrode. For the investigation, a Silicon electrode using 1 M LiPF6 in EC/DEC electrolyte was cycled between the voltage potential limits of 50 mV and 1 V at a constant current. EIS measurements have been conducted at different state of charge during 200 discharge/charge cycles. The experimentally obtained impedance data were analyzed by a semi-mathematical model based on a porous electrode model. The model simulates the impedance response of a porous Silicon electrode, composed of spherical intercalation particles. The radii of the particles are dynamically changing because of expansion and contraction due to lithiation and delithiation, respectively. The impedance of a single particle is calculated considering diffusion of the Lithium cations (Li+ ions) in the particles, the solid electrolyte interphase (SEI) film and interfacial process. The particle size distribution of the Silicon powder and porous electrode theory is used to simulate the total impedance of the Silicon anode. All complex spectra obtained from experiments and simulation showed the same elements reported in literature for Silicon anodes before[1,2]. The mathematical model was used to identify important parameters to analyze the experimental data on a fundamental basis. Evaluating the experimental data revealed the large impact of the porosity reduction on the impedance during cycling. It was shown that after an expected increase in active surface area with lithiation due to the expansion of the Silicon particles, the active surface area suddenly drops. This drop in active surface area was found to correlate to a critical porosity of 0.1. It was concluded that the decrease in porosity leads to a diffusion limitation of the ions in the electrode bulk and therefore to a severe decrease in accessible surface area. Increased surface area and reduced porosity after the same lithiation time with progressive cycling indicates that the expansion of the Silicon particle is not fully reversed with delithiation. This indicates an ageing mechanism of trapped Lithium in the electrode[3]. Discontinuous diffusion behavior were correlated to cracking of the Si particle which leads to new ions diffusion pathways. Furthermore, the results indicate a formation of new SEI with lithiation and a partially redissolution with delithiation, which was also reported by Philippe et al.[4]. The attached figure shows the ratio of the experimentally obtained active surface area Aexp and the modelled active surface area Amod (based on expansion due to lithiation) and the diffusion coefficient of Li+ ions in the electrode. The results are shown for cycle 12 during the lithiation from 1 V to 50 mV (a) and for the cycle 1 to 200 after a lithiation to 50 mV (b).