Towards Understanding Heat Generation Characteristics of Li-Ion Batteries by Calorimetry, Impedance, and Potentiometry Studies

Abstract

The ability of Li-ion batteries to deliver high power and energy is accompanied by heat generation and thermal runaway resulting in detrimental effects. While capacity fade and the consequential reduced cycle life are prime issues in Li-ion batteries, the impending need for safety in handling them and need for safe failure in off-nominal conditions are critically important. In this contribution, we report heat generation in Li-ion batteries containing cathodes such as NCA (LiNi 0.8 Co 0.15 Al 0.05 O 2 ), NMCs (LiNi 0.2 Mn 0.2 Co 0.6 O 2 ; LiNi 0.6 Mn 0.2 Co 0.2 O 2 ) against graphite anode. The total heat generation is determined using an Accelerating Rate Calorimeter and the reversible heat generation is determined using entropic coefficient measurement. The internal resistance contributing to the irreversible heat generation is estimated by AC impedance studies. Further, variation in the components of internal resistance across different States of Charge determined by fitting an equivalent circuit model is presented. The cell skin temperature recorded at low C rates in 18650-type full cells is correlated with the entropic coefficient in corresponding half-cells to understand thermodynamics during phase transition in cathodes and structural transformation in graphite. The findings from the experiments are helpful in the electrochemical and thermal modeling of Li-ion batteries which in turn would aid better design of thermal management systems.

Heat Generation in NMC622 Coin Cells during Electrochemical Cycling: Separation of Reversible and Irreversible Heat Effects

“…Although thermodynamic properties like the entropy change, which is responsible for the reversible heat generation during the intercalation or deintercalation processes in Li-ion cells, can be determined by temperature-dependent measurements of the open circuit voltage [11], the application of calorimetric methods can give additional insights for the thermal characterization [8,9]. Furthermore, since the calorimetry measures the integral heat generated of the cell, the combination of calorimetry, impedance, and potentiometric or galvanostatic methods [8,12,13] can provide further information about the details of the heat generation processes. With this combination, the individual contributions to total overpotential, which is composed of the ohmic loss; the charge transfer resistance; and the mass transport limitation due to diffusion can determined and the corresponding fractions of the irreversible heat generation rate can be estimated [14].…”

“…where x is the molar fraction, and with these definitions, the total volumetric heat generation rate can be written in terms of the overpotential η under current load [13]:…”

Uncovering hydrogen anion triggered thermal runaway mechanism of a high-energy LiNi0.5Co0.2Mn0.3O2/graphite pouch cell

“…Irreversible heat refers to the ohmic heat from the polarization or overpotential of the cell, while reversible heat is determined by the measurement of cell entropic coe cient depending on the intrinsic nature of the electrode materials (relating with the atom arrangement in the crystal lattice). 48 Deciphering the heat generation law of a single cell is essential to design and optimize the battery thermal management system, which ensures batteries in a pack or module running in an ideal temperature range. Here, two boundary scenarios of adiabatic and isothermal environment, representing the worst and best case for the heat management, respectively, are considered for heat determination of the pouch cell with dual-salt electrolyte.…”

Universal Li-Ion Cell Electrothermal Model

“…The entropic coefficient can be empirically calculated through analyzing change in open circuit voltage with temperature, as a function of SoC [40], [43], [44], [49], [51], [52]. As this would require very accurate true OCV readings across the measurement range, the values representative for NMC/graphite were taken from a literature example that performed extensive testing [53]. For the convection part of the model, several fluid properties were required for air, which were taken from [45].…”