In-Situ Characterization of Electrolyte Degradation Products with Lithographically Patterned Electrode Arrays
Authors: Lee Sophia E , Tang Maureen H. 
Published: February 2020

he successful operation of many beyond-lithium-ion battery chemistries depends on efficient formation of the solid electrolyte interphase (SEI). During the first charge cycles, electrolyte is reduced at the anode surface and insoluble degradation products form a passivating layer, allowing ion transport while preventing additional electrolyte degradation. The solubility of degradation products affects the efficiency of SEI formation. In particular, electrolyte degradation products are more soluble in sodium-ion systems compared to lithium. As a result, sodium-ion batteries are subject to lower columbic efficiency, faster capacity fade, and higher resistance growth than lithium-ion. Addressing this limitation requires improved understanding of degradation product solubility, including methods to measure the relative concentration of dissolved electrolyte products.
While differences in solubility have been studied through ex-situ spectroscopic techniques, in-situ detection of soluble degradation products can allow increased understanding of SEI dynamics and enable real time evaluation of the efficiency of SEI formation. 2 Interdigitated electrode arrays are frequently used for sensitive detection of electroactive species. Interdigitated electrode arrays (IDAs) make use of small diffusion lengths between electrodes to allow for a collector-generator arrangement, yielding similar behavior to rotating ring-disk electrodes without the noise of rotation, risk of SEI shearing or need for bulky equipment.3 IDAs are subject to increased feedback current, also known as redox cycling, due to diffusion of products from the collector back to the generator.4 This is useful in the detection of products in very low concentrations but is not desirable for accurate prediction of SEI formation efficiency.
Here, we fabricate customized designs of IDAs with a high aspect ratio of Wgen­ to Wcol (10-40:1) using photolithographic techniques. We are able to achieve a low feedback (1-1.1 X) while maintaining relatively high collection efficiency (25-40%). Through this work we demonstrate electrochemical monitoring of soluble products by chronoamperometric detection at the collector during SEI formation at the generator. Utilizing this technique, it is possible to observe the proportion of electroactive soluble degradation products formed as a function of potential and monitor changes in the system with time.
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Figure. SEI formation current at generator (blue) and amperometric detection at collector (orange) as a function of time

Figure 1