Deconvoluting Li-Ion Battery Electrode Crosstalk with Electrochemical Generator-Collector Measurements

Abstract

Accelerated capacity fade in high-voltage Li-ion batteries is attributed to crosstalk between the positive and negative electrodes. During low potential operation of the negative electrode and high potential operation of the positive electrode, the nonaqueous electrolyte is reduced and oxidized, respectively, forming soluble, insoluble, and gaseous byproducts. Additionally, during electrolyte oxidation transition metals dissolve from active material of the cathode. After cycling, soluble species from the positive electrode are detectable at the negative electrode and are suspected to play an important role in disrupting the formation, growth, and performance of the solid-electrolyte interphase (SEI), leading to unmitigated capacity fade. In full-cell studies the effects of electrolyte decomposition and metal dissolution on battery lifetime and coulombic efficiency are well-documented, however the precise nature of crosstalk species and the mechanism of capacity fade remain to be understood. Here we present two applications of our generator-collector approach to understanding battery crosstalk. First, we employ rotating-ring disk electrode (RRDE) voltammetry to study the effects of metal dissolution from LNMO (LiNi0.5Mn1.5O4) on SEI performance. By cycling an LNMO-coated glassy carbon disk to high operating potentials, we stimulate metal dissolution and collect these soluble species on the downstream glassy carbon ring electrode, which is cycling to low potentials to form and grow an SEI. The effects of metal contamination on SEI passivation are characterized via a redox mediator method and quantified using finite element simulations. We find that metal dissolution has a more profound impact on electronic than morphological SEI properties. Interpreting these results with physics-based models of SEI passivation imply that the inorganic layer is critical to passivation, while organic layers are inert at best. In the second application, we demonstrate an improved method for performing generator-collector measurements on composite electrodes via the development of a microfluidic electrochemical cell. Our design incorporates commercial high-voltage positive electrodes, a graphite negative electrode, and commercial Li-ion battery electrolyte. Forced convection from electrolyte flow isolates the upstream electrode from crosstalk species generated at the downstream electrode, and deconvolutes the role of electrolyte oxidation and reduction on capacity fade. We study the effects of flow rate and flow direction on charge/discharge behavior for each electrode, and correlate coulombic efficiencies to the extent of crosstalk. These electrochemical generator-collector studies provide a greater understanding of the mechanism behind transition metal travel from the positive to negative electrode in a full-cell, as well as the importance of electrolyte decomposition in this transport mechanism.