3-Electrode Setup for the Operando Detection of Side Reactions in Li-Ion Batteries: The Quantification of Released Lattice Oxygen and Transition Metal Ions from NCA

Detecting parasitic side reactions is paramount for developing stable cathode active materials (CAMs) for Li-ion batteries. This study presents a method for the quantification of released lattice oxygen and transition metal ions (TMII+ ions). It is based on a 3-electrode cell design employing a Vulcan carbon-based sense electrode (SE) that is held at a controlled voltage against a partially delithiated lithium iron phosphate (LFP) counter electrode (CE). At this SE, reductive currents can be measured while polarizing a CAM working electrode (WE), here a LiNi0.80Co0.15Al0.05O2 (NCA), against the same LFP CE. In voltammetric scans, we show how the SE potential can be selected to specifically detect a given side reaction during CAM charge/discharge, allowing, e.g., to discriminate between lattice oxygen and dissolved TMs. Furthermore, it is shown via online electrochemical mass spectrometry (OEMS) that O-2 reduction in the here-used LP47 electrolyte consumes similar to 2.3 electrons/O-2. Using this value, the lattice oxygen release deduced from the 3-electrode setup upon charging of the NCA WE is in good agreement with OEMS measurements up to NCA potentials >4.65 V-Li. At higher potentials, the contributions from the reduction of TMII+ ions can be quantified by comparing the integrated SE current with the O-2 evolution from OEMS.