Application of Asymmetric Interdigitated Electrode Arrays to in-Situ Analysis of Soluble Electrolyte Degradation Products
Authors: Lee Sophia , Tang Maureen H. 
Published: December 2020

The efficient formation of the solid electrolyte interphase (SEI) plays a central role in the performance and lifetime of beyond-lithium-ion battery chemistries. The SEI forms as a result of insoluble products generated during electrolyte undergoes reduction at the anode surface during initial charge cycles. A preferential SEI prevents additional electrolyte reduction while allowing facile ion transport, while an improperly formed SEI can severely limit battery lifetime through continuous electrolyte reduction. Electrolyte degradation products are more soluble in sodium-ion systems compared to lithium, impacting the stability of the SEI. As a result, sodium-ion batteries are subject to lower columbic efficiency, faster capacity fade, and higher resistance growth than lithium-ion[1]. Better understanding of the formation and dissolution of soluble degradation products, including methods to measure the relative concentration of dissolved electrolyte products, is necessary to overcome these limitations.
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(IDAs) are frequently used for sensitive detection of electroactive species. These electrode arrays make use of small diffusion lengths between electrodes to enable communication over short time scales. When used in a collector-generator arrangement, IDAs yield similar electroanalytical capabilities to rotating ring-disk electrodes without the noise of rotation, risk of SEI shearing, or need for bulky equipment [3]. The small diffusion lengths of these devices also result in overlapping diffusion layers, causing a redox cycling effect[4]. While this is useful in the detection of products in very low concentrations but is not desirable for accurate prediction of SEI formation efficiency. Using photolithographic techniques we have fabricated a novel, highly-asymmetric interdigitated electrode array that utilizes geometry to bias diffusion and limit feedback while maintaining collection efficiencies greater than 25%.
Through this work, we demonstrate electrochemical analysis of soluble products by chronoamperometric and cyclic voltammetry detection at the collector during SEI formation at the generator. Using this technique, we can generate characteristic signatures of electrolytes with preferential and detrimental SEI formation characteristics and observe the proportion of electroactive soluble degradation products formed as a function of potential at the working electrode.
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Figure 1. (a) ) Voltage curves for charge at 1C and discharge at C/10 on generator and (b) product detection via cyclic voltammetry at the collector.

Figure 1