Composite Titanate–Graphite Negative Electrode for Improved State-of-Charge Estimation of Lithium-Ion Batteries

Abstract

We have constructed a negative-electrode material system that provides an abrupt voltage change near the end of discharge, providing a clear marker for the voltage vs state of charge (SOC) relationship. Distinctive voltage features during the operation of a battery enable an accurate state estimation of the cell, and thus efficient energy and power management as well as health assessment. The approach we have developed should be useful for lithium-ion batteries with cell voltages remaining nearly constant over a large SOC range such as cells constructed with graphite negative electrodes and iron phosphate positive electrodes. Specifically, we describe the incorporation of a lithium titanate (LTO) material into a substantially graphitic negative electrode so as to provide a distinctive voltage change at a preselected SOC. A simple analytic procedure based on juxtaposing open-circuit potential curves is shown to represent well the combined system open-circuit potential, thereby confirming the efficacy of the approach. Fabricated full cells comprising lithium iron phosphate positive and carbon–LTO negative electrodes are used to demonstrate the effectiveness of the concept in a battery.