Quantifying the Distribution of Electrolyte Decomposition Products in Silicon-Graphite Electrodes by Neutron Depth Profiling

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//LiFePO 4 pseudofull cells, using a capacitively oversized positive electrode and an electrolyte mixture consisting of 1 M LiPF 6 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.

Integration of Graphite and Silicon Anodes for the Commercialization of High‐Energy Lithium‐Ion Batteries

“…The irremovable residual water generates HF from the hydrolytic cycle of LiPF 6 . Notably, small HF molecules are able to permeate through the SEI layer, and corrode the Si surface . Therefore, the degradation can be seriously accelerated by a coarse SEI layer.…”

“…A pure Si anode suffers from clogging of the electrode pores and electrical isolation by accumulation of the SEI, such that the Si could not be homogeneously utilized despite the addition of conductive additives such as carbon black or carbon fibers . In contrast, Si and graphite can be evenly utilized throughout the electrode during cycling as a consequence of the sufficient electronic connection and the favorable void formation resulting from the presence of large graphite particles …”

Graphit‐ und‐Silicium‐Anoden für Lithiumionen‐ Hochenergiebatterien

“…Über den hydrolytischen Zyklus von LiPF 6 führen die unentfernbaren Wasserspuren zur Bildung von HF . Das kleine HF‐Molekül kann durch die SEI‐Schicht permeieren und die Si‐Oberfläche korrodieren . Das Vorliegen einer rauen SEI‐Schicht kann diesen Zerfall stark beschleunigen.…”

“…Die Anreicherung der SEI führt bei einer Si‐Anode zum Verstopfen der Poren der Elektrode und zu elektrischer Isolation, wobei Si trotz des Zusatzes leitfähiger Mittel wie Carbon Black und Kohlefasern nicht homogen verwertet werden kann . Im Gegensatz dazu können Si und Graphit, aufgrund ihrer ausreichenden elektronischen Verbindung und weil große Graphitpartikel und damit vorteilhafte Hohlräume vorhanden sind, über die gesamte Elektrode hinweg gleichmäßig verwertet werden …”

“…Mit steigender Zyklenanzahl wächst das Gesamtporenvolumen der Elektrode durch die Änderung der Si‐Morphologie und die Quellung der Elektrode stetig . Das große Porenvolumen begünstigt das Eindringen des Elektrolyten in die Elektrode und ermöglicht den Transport von Lithiumionen durch die Elektrode.…”