Electrochemical Signatures of Potassium Plating and Stripping

Abstract

Alkali metals are attractive anodes for high energy density batteries, but practical utilization of these materials in rechargeable cells has so far been hindered by safety concerns, limited Coulombic efficiencies and short cycle lives. These issues are all connected to the uneven electrodeposition and stripping of alkali metals; dendritic metal deposits can short circuit the cell and deposits with large surface areas consume more electrolyte and active material in side reactions.[1] To circumvent these problems, one of the approaches which have been attempted is electrolyte engineering. For instance, electrolytes with higher salt concentration, so called highly concentrated electrolytes, have been shown to suppress both dendrite growth and increase the Coulombic efficiency of metal anodes.[2,3] When different metal anode stabilization strategies are evaluated, it is common to perform galvanostatic cycling experiments in symmetric or asymmetric cells. This allows the cycling stability and Coulombic efficiency of the plating/stripping to be analyzed. Additionally, it is common to use features of the voltage profile as signatures on the mechanisms responsible for changes in electrochemical performance. For instance, a sharp decrease in (over)voltage during plating/stripping is a sign of a short circuit. A gradually increasing overvoltage during cycling on the other hand can be a result of continuous solid electrolyte interphase (SEI) buildup. However, several processes often occur in parallel when symmetric/asymmetric cells are cycled galvanostatically, each giving different contributions to the voltage profile which need to be deconvoluted to make sense of the data. In this contribution, we use three-electrode cells in combination with simple electrochemical experiments like cyclic voltammetry, chronoamperometry and chronopotentiometry to disentangle the contributions to different features in the galvanostatic voltage profiles of alkali metal anodes.In particular, we dissect the voltage profiles of potassium-copper (K-Cu) cells with highly concentrated electrolytes. Due to the higher reactivity of K compared to other alkali metals, certain features in the voltage profiles of these cells become exaggerated compared to lithium or sodium metal cells. We show that during galvanostatic deposition on a fresh Cu substrate, SEI formation and nucleation occur partly in parallel. If an SEI-formation step is performed prior to the galvanostatic deposition, a much sharper ‘nucleation peak’ in the voltage profile can be observed, indicative of the higher rate of nucleation that needs to occur in this case. Further, the potassium metal anode exhibits a steplike voltage profile from the second cycle and onwards. We confirm that this feature arises because bulk K is covered by a resistive native layer, whereas another type of, less resistive, interphase covers the freshly deposited K. This inhomogeneous interphase will inevitably promote preferential and inhomogeneous plating/stripping.