Abstract
Silicon-based anodes for lithium-ion batteries exhibit severe volumetric changes of the active material particles during (de-)lithiation, resulting in continuously occurring side reactions at the silicon/electrolyte interface over extended charge/discharge cycling. The thus formed and accumulating electrolyte decomposition products lead to a growth of the solid-electrolyte-interphase (SEI) on the silicon particles. This results not only in an ongoing loss of electrolyte but also in a significant swelling and impedance increase of silicon-based anodes which significantly compromises their cycle-life. In the present study, neutron depth profiling (NDP) is used post mortem as a non-destructive, highly lithium-sensitive technique to (i) quantify the amount of lithium-containing electrolyte decomposition products in silicon-graphite (SiG) electrodes (35 wt% silicon, areal capacity ∼1.7 mAh cm−2), (ii) monitor their distribution across the SiG electrode thickness, and (iii) determine the active material utilization across the electrode over 140 cycles. Hence, SiG negative electrodes are aged and characterized by means of galvanostatic cycling in SiG//LiFePO4 pseudo-full cells, using a capacitively oversized positive electrode and an electrolyte mixture consisting of 1 M LiPF6 in EC:EMC with 5 wt% FEC. High-resolution cross-sectional SEM images and post-mortem characterization of the SiG electrodes with respect to changes in electrode mass thickness complement the analysis.
Highlights
To cite this article: Morten Wetjen et al 2018 J
The cycling stability of the SiG electrodes is characterized by a distinct capacity decay to ∼0.9 mAh cm−2 within the first 60 cycles (∼65% capacity retention, referenced to the 3rd cycle at 0.5 h−1), perhaps more evident by the steep increase in the total irreversible capacity within the first 60 cycles, which can be used as a measure for the accumulation of electrolyte decomposition products in the porous electrode
In our earlier work,[10] it was demonstrated that the increase in the electrode impedance and the loss of interparticle electrical contact is caused by the drastic expansion of the surface area of the silicon particles over the initial charge/discharge cycles, accompanied by an initially high formation rate of electrolyte decomposition products and high SEI growth as well as by a significant swelling of the SiG electrode
Summary
To cite this article: Morten Wetjen et al 2018 J. SiG electrodes were aged and characterized by means of galvanostatic cycling in SiG//LiFePO4 pseudo-full cells, using a capacitively oversized positive electrode and an electrolyte mixture consisting of 1 M LiPF6 in EC:EMC with 5 wt% fluoroethylene carbonate (FEC).[7] Over the course of charge/discharge cycling, side reactions occurring at the silicon/electrolyte interface result in the continuous preferential consumption of FEC which is accompanied by the accumulation of lithium-containing electrolyte decomposition products,[6,8] consisting of LiF, Li2CO3, Li2O, and lithium alkoxides.[18,31,32] After different numbers of cycles, fully delithiated SiG electrodes were harvested from the cells and characterized by ex-situ NDP In this case, residual lithium in the electrodes mainly originates from the lithium poly(acrylate) (LiPAA) binder and from the lithium-containing electrolyte decomposition products, independent of their chemical state.[7] The NDP analysis method was recently implemented and validated using pristine and aged SiG electrodes by Trunk et al.[33] at the newly constructed neutron depth profiling instrument (N4DP) at the Prompt Gamma-ray Activation Analysis (PGAA) facility of the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, Germany. Each electrode was evaluated at fifteen positions along the entire cross-section to obtain an average thickness and its standard deviation
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