Investigating the Distribution of Electrolyte Decomposition Products (SEI) in Silicon Electrodes By Neutron Depth Profiling

Abstract

The electrochemical performance of silicon-based anodes in lithium-ion batteries is largely determined by the ongoing decomposition of electrolyte compounds at the silicon particle surface and the subsequent formation of a solid-electrolyte-interphase (SEI).1 Yet, the significant extent of electrolyte decomposition products which accumulate in the anodes as a result of the severe morphological changes of the silicon particles upon cycling requires to rethink the concept of a conformal SEI layer as it was proposed for state-of-the-art graphite anodes.2,3 Based on previous studies,4,5 our current characterization is concerned with the relation of the morphological changes and the resulting electrochemical performance of practical silicon-graphite electrodes. Hence, we apply ex-situ neutron depth profiling (NDP) to monitor the distribution of the lithium-containing electrolyte decomposition products across the thickness of silicon-graphite electrodes as a function of the cycle number. In contrast to conventional analytical techniques, NDP is a non-destructive method with a very high sensitivity to lithium, which allows to determine lithium concentration gradients across the thickness of the entire electrode.6 For our investigation, we prepared silicon-graphite (SiG) electrodes with a practical areal capacity of ~1.8 mAh cm-2 (initial thickness: ~19 µm), consisting of 35 wt% nanometer-sized silicon (~200 nm diameter) and 45 wt% graphite (~20 µm diameter). Vapor grown carbon fibers and lithiated poly(acrylic acid) binder accounted for the remaining 20 wt%.4 Utilizing SiG//LiFePO4 coin-cells with a capacitively oversized positive electrode (~3.5 mAh cm-2) and a relatively stable reference potential of ~3.45 V vs. Li/Li+, the silicon-graphite electrodes were aged by galvanostatic charge-discharge cycling at C/2 (~0.9 mA cm-2) up to 140 cycles. In the next step, we fully delithiated the active materials in the electrodes by applying a very slow C-rate of C/50 (~0.04 mA cm-2) and a high delithiation cutoff potential of ~2.0 V vs. Li/Li+. As a result, any residual lithium in these electrodes either originates from lithium-containing electrolyte decomposition products or the LiPAA binder. Based on the NDP spectra of the silicon-graphite electrodes after a different number of cycles, we demonstrate that the irreversible capacity obtained from charge-discharge cycling is directly proportional to the total amount of lithium-containing electrolyte decomposition products that accumulated in the electrodes during the same period. Further, we show that this accumulation is also reflected by the increase of the entire mass loading (coating + SEI) and the significant swelling of the electrodes (+140% in the delithiated state), thus following the two major degradation phenomena of silicon-graphite electrodes, which we discussed in our previous publication.4 In addition, we support our analysis by complementing the NDP spectra with high-resolution cross-sectional scanning electron microscopy (SEM) images of the silicon-graphite electrodes after different numbers of cycles. Finally, we analyze the energy loss of the 3H particles emitted by the nuclear reaction of 6Li with thermal neutrons as a function of the electrode depth, determining the evolution of the lithium concentration gradients across the thickness of the electrodes and discussing the implications on the utilization of the active materials near the separator/anode and the anode/current collector interface. Our analysis allows to shape a refined explanation of the accumulation of electrolyte decomposition products in silicon-graphite electrodes and its implication both on the changes in electrode morphology and the resulting electrochemical performance.