OCV Hysteresis in Li-Ion Batteries including Two-Phase Transition Materials

Michael A Roscher,Oliver Bohlen,Jens Vetter

doi:10.4061/2011/984320

Michael A Roscher, Oliver Bohlen + Show 1 more

Open Access

https://doi.org/10.4061/2011/984320

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Abstract

The relation between batteries' state of charge (SOC) and open-circuit voltage (OCV) is a specific feature of electrochemical energy storage devices. Especially NiMH batteries are well known to exhibit OCV hysteresis, and also several kinds of lithium-ion batteries show OCV hysteresis, which can be critical for reliable state estimation issues. Electrode potential hysteresis is known to result from thermodynamical entropic effects, mechanical stress, and microscopic distortions within the active electrode materials which perform a two-phase transition during lithium insertion/extraction. Hence, some Li-ion cells including two-phase transition active materials show pronounced hysteresis referring to their open-circuit voltage. This work points out how macroscopic effects, that is, diffusion limitations, superimpose the latte- mentioned microscopic mechanisms and lead to a shrinkage of OCV hysteresis, if cells are loaded with high current rates. To validate the mentioned interaction, Li-ion cells' state of charge is adjusted to 50% with various current rates, beginning from the fully charged and the discharged state, respectively. As a pronounced difference remains between the OCV after charge and discharge adjustment, obviously the hysteresis vanishes as the target SOC is adjusted with very high current rate.

Highlights

In the recent years, lithium-ion batteries (Li-ion) became the favorable choice for most portable energy-consuming applications
The open-circuit voltage (OCV) hysteresis becomes a major influencing factor for reliable OCV reconstruction, which is a critical task for the model-based state estimation as a part of a battery management system (BMS)
The voltage gap between both curves depends on the state of charge (SOC), where a maximum gap of approximately 40 mV can be found at SOC = 25%

Summary

Introduction

Lithium-ion batteries (Li-ion) became the favorable choice for most portable energy-consuming applications. The electrode material composition and morphology (e.g., grain sizes an distribution) strongly affects the electric characteristics of battery cells. The OCV hysteresis is a typical phenomenon for batteries and is well documented for nickel-metal-hydride (NiMH) battery systems [1, 2]. Even in Li-ion batteries, OCV hysteresis effects can be observed [2,3,4,5], which have a minor impact on battery cells’ OCV for cobalt, nickel, or manganese-based cathode systems, due to the high gradient in the specific state of charge (SOC) to OCV relation. Modern Li-ion cells comprise active materials showing only a slight gradient in the SOCOCV curves. Firstly the special OCV characteristics of two-phase transition materials are described with respect to hysteresis phenomena.

Two-Phase Transition Lithium Insertion

Li4x Ti5 O12

Experimental

Results and Discussion

Conclusion