Abstract

Currently, lithium-ion batteries are widely used as energy storage systems for mobile applications. However, a better understanding of their nature is still required to improve battery management systems (BMS). Overvoltages and open-circuit voltage (OCV) hysteresis provide valuable information regarding battery performance, but estimations of these parameters are generally inaccurate, leading to errors in BMS. Studies on hysteresis are commonly avoided because the hysteresis depends on the state of charge and degradation level and requires time-consuming measurements. We have investigated hysteresis and overvoltages in Li(NiMnCo)O2/graphite and LiFePO4/graphite commercial cells. Here we report a direct relationship between an increase in OCV hysteresis and an increase in charge overvoltage when the cells are degraded by cycling. We find that the hysteresis is related to diffusion and increases with the formation of pure phases, being primarily related to the graphite electrode. These findings indicate that the graphite electrode is a determining factor for cell efficiency.

Highlights

  • Lithium-ion batteries are widely used as energy storage systems for mobile applications

  • Degradation and internal losses worsens the performance of battery-monitoring systems (BMSs) with cycling[1], where battery management systems (BMS) rely on the open-circuit voltage (OCV), operational voltage (Vcell) and delivered charge

  • This dependence can be clearly observed in the discharge overvoltage of an aged cell if we compare the results for an state of charge (SoC) of 64% at C/50 (Fig. 2b) and C/25 (Fig. 2d), in which a large variation is observed compared with neighbouring SoCs

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Summary

Introduction

Lithium-ion batteries are widely used as energy storage systems for mobile applications. We report a direct relationship between an increase in OCV hysteresis and an increase in charge overvoltage when the cells are degraded by cycling. Degradation and internal losses worsens the performance of battery-monitoring systems (BMSs) with cycling[1], where BMSs rely on the open-circuit voltage (OCV), operational voltage (Vcell) and delivered charge. Losses in batteries are attributed to the overvoltage and OCV hysteresis observed during charging and discharging. Despite the available research related to hysteresis, controversy remains regarding the causes and effects of hysteresis When structural defects, such as distortion, plastic deformation or cracks, occur, changes in the electrical, thermal and mechanical properties and in the ionic diffusion and conduction of the electrodes are expected

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