Cell Discharge Testing to Calibrate a Positive-Electrode Open-Circuit-Potential Model for Lithium-Ion Cells

Abstract

Previous work by our research team has proposed methods to estimate the open-circuit-potential (OCP) vs electrode stoichiometry relationships required by physics-based models of lithium-ion battery cells. It has also shown how to estimate the boundaries of the stoichiometric operating windows of both electrodes as the cell is cycled between 0% and 100% state of charge. In this earlier work, we sought to calibrate the OCP relationships by fitting them to a physics-based multi-species multi-reaction model; however, we have found that even this fitting is insufficient for producing unbiased estimates of the OCP relationships for some common positive-electrode materials such as LCO, NCA, and NMC. The primary contribution of this paper is to propose a new method that can calibrate the positive-electrode OCP relationship and its stoichiometric operating boundaries. It requires only commonly available constant-current discharge data from the cell, and when combined with methods from the earlier papers promises to mitigate the need for cell teardown to find the OCP relationships of a physical cell in many cases. The method is first validated in simulation and is then applied to a physical graphite//NMC cell. Results agree closely with others in the literature that were found using a teardown approach.