Electrochemical Approaches to Interfacial Phenomena in Sodium and Lithium-Ion Batteries

Sophia E. Lee

doi:10.17918/00000442

Raising demand for high-density, long-duration energy storage is driving the development of new beyond-lithium-ion battery chemistries, including sodium-ion batteries which offer an affordable and earth-abundant alternative. The performance and lifetime of many non-aqueous battery chemistries depend on the efficient formation of the solid electrolyte interphase (SEI). During the first charge cycles, the electrolyte is reduced at the anode surface and insoluble degradation products form a passivating layer, allowing ion transport while preventing additional electrolyte degradation. Key differences between the properties of sodium and lithium, including increased solubility of degradation products, affect the efficiency of SEI formation. As a result, sodium-ion batteries are subject to lower columbic efficiency, faster capacity fade, and higher resistance growth than lithium-ion. Soluble degradation products can also diffuse to the cathode, a form of cross-talk with additional implications on battery lifetime. 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. Here we use electrochemistry to study the characteristics of the SEI and its implications on battery performance and design. In the first part, we examine the impact of system design on experimental results. First, we demonstrate how cross-talk of soluble products can impact the study of working electrode behavior in sodium-ion systems. We then investigate candidate reference electrodes to minimize the effects of cross-talk and voltage drift in the system. Additionally, we provide a benchmark of the compatibility between non-aqueous solvents and materials of construction for designing custom spectroscopic and electrochemical reactors. In the second part, we develop a new electrochemical approach to soluble product analysis. 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. Here, we fabricate novel interdigitated electrode arrays (IDAs) with a high aspect ratio of W_[gen] to W_[col] (10-40:1) using photolithographic techniques. Using geometry to bias diffusion we achieve low feedback while maintaining relatively high collection efficiency (25-40%). Through this work, we demonstrate electrochemical monitoring of soluble products in several SEI formation and product detection schemes. Utilizing this technique, it is possible to observe the proportion of electroactive soluble degradation products formed as a function of potential and evaluate electrolyte formulations. Finally, we turn our attention to parallels between the SEI and solid-state electrolytes to understand the role of interfacial potential on the nature of ionic conduction and electronic insolation.

Electrochemical Approaches to Interfacial Phenomena in Sodium and Lithium-Ion Batteries

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Electrochemical Approaches to Interfacial Phenomena in Sodium and Lithium-Ion Batteries

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