Mapping the Entropic Coefficient of the NMC Pouch Cell at Various Temperatures and State-of-Charge Levels

A unique characteristic of lithium batteries, as compared to other cell chemistries, is the thermal behavior exhibited during cycling.  There are several sources of heat within a lithium cell but they ultimately can be categorized as being either irreversible (always exothermic) in nature or as being reversible.  The contribution of the entropic (reversible) heat source alternates between endothermic or exothermic in behavior depending on the state-of-charge (SOC).  Past researchers have attributed this to a phase change of the active material.¹The entropic coefficient itself is not a constant factor and its value varies with different SOC levels.  This makes heat generation within a cell a rather complicated study but one that must be taken.  The amount of heat generated within a cell affects its temperature and, in turn, affects its performance.

The role of the entropic coefficient in calculating the quantity of reversible heat, Q_rev, is described as follows:              

                    Q_rev = T_cell * (dV_oc/dT) * I                          (1)

where T_cell is the temperature of the cell, dV_oc/dT is the change in the open circuit voltage of the cell that results in a change of cell temperature, and I is the current in amps. dV_oc/dT is the entropic coefficient.  It is determined by measuring the open circuit voltage (OCV) of the battery at a set SOC value and noting its change as the temperature of the cell varies a given amount in an environmental chamber. 

The Varying Effects of Entropic Heat

The impact of entropic heat on the overall heat generation within a cell runs the gamut from being negligible to significant depending on the chemistry.  Kim et al.² found a strong influence of entropic heating at the 1C-rate for a LiMn₂O₄ spinel coin cell.  In contrast, Lu and Prakash³ found nearly no effect at the same 1C rate for the C/Li half-cell that utilized mesocarbon microbeads (MCMB).

In this study, the researchers focused on quantifying the entropic coefficient for the Nickel Manganese Cobalt (NMC) lithium-ion electrochemistry. Leading battery manufacturers are focusing more intently on integrating the NMC cell into powertrain applications.  By finding the right combination of these three metals for the cathode, they can offer a good balance between power, safety, and life.  As a result, this particular type of cell is commercially available in rather large capacities for both pouch and prismatic formats.

The objective of this research will be to fully map the varying entropic coefficient values of the NMC cell.  This value will be calculated from data taken at different SOC levels as well as different temperatures.  This map will be compared to one compiled for a lithium iron phosphate (LFP) pouch cell done previously by the authors.

Experimental Setup

The SOC levels range from full charge to full discharge in 5% increments.  The temperature levels will vary from  55 ⁰C to -20⁰C in 5⁰C increments.  A commercially available 12Ah lithium-ion Kokam pouch cell will be used.  Eventually the entropic map will be composed of 315 data points (21 SOC levels and 15 different temperature settings).  In order to accelerate the data collection process, three cells will be set at SOC levels that are 5% apart and all will be simultaneously exposed to the full range of temperature changes. Each new temperature setting will dictate that the cells have 8 hours of soak time to stabilize its OCV. The effects of self-discharge on OCV will also be quantified and removed from the measured data.  SOC levels and data acquisition is to be performed by a Maccor System Model 4200 10-channel cycler.

Entropic Coefficient Map for LFP

The data acquired by the authors in previous work has shown entropic behavior not uncovered by previous literature on the subject. Other studies that have used larger SOC and temperature increments found the entropic coefficient to be almost linear and independent of temperature.⁴   However, a study by the authors on a LFP cell found the entropic coefficient to be influenced by temperature at extreme SOC levels.⁵

References

[1] R. Tamamushi, Electrochemistry, second ed., Maruzen, Tokyo, 2001.

[2] J.-S. Kim, J. Prakash, and J.R. Selman, Electrochemical Solid State Letters, 4, A141 (2001)

[3] W. Lu and J. Prakash, Journal of the Electrochemical Society, 150, A262 (2003)

[4] K. Onda, H. Kameyama, T. Hanamoto, and K. Ito, Journal of the Electrochemical Society, 150 (2003) A285.

[5] S. Bazinski and X. Wang, The Influence of Cell Temperature on the Entropic Coefficient of a Lithium Iron Phosphate Pouch Cell across the Full State-of-Charge Spectrum, 221^st ECS Meeting, Seattle WA, May 2012.