Miniaturization of Reference Electrodes for Solid-State Lithium-Ion Batteries

Enabling simple three-electrode (3E) setups for solid-state battery cells is important allowing investigation of individual electrodes to shed more light on interface charge transfer and reactions occurring in solid-state battery cells. Two different 3E setups are compared, and their practical value is evaluated. A miniaturized reference electrode (mu-RE) is developed from lithium-plated gold wires with a tungsten core providing a stable potential. Cells with Li6PS5Cl as solid electrolyte, Li1-xNi0.85Co0.10Mn0.05O2 (NCM851005) as cathode active material and Li4Ti5O12/Li7Ti5O12 (LTO) or In/InLi as anode are investigated. The reference electrode provides a stable potential of 0 V vs Li+/Li, hence allowing the precise measurement of single electrode potentials. The setup leaves the usual cell geometry essentially unchanged and causes only minor additional work during cell assembly, allowing widespread application. Evidence is provided that 3E setups are needed to evaluate the rate capability of active materials correctly and that two-electrode (2E) setups can massively underestimate the rate capability of electrodes. The impedance of full cells is systematically analyzed based on separate anode and cathode impedances.