A Phenomenological Model of Bulk Force in a Li-Ion Battery Pack and Its Application to State of Charge Estimation

Abstract

A phenomenological model of the bulk force exerted by a lithium ion cell during various charge, discharge, and temperature operating conditions is developed. The measured and modeled force resembles the carbon expansion behavior associated with the phase changes during intercalation, as there are ranges of state of charge (SOC) with a gradual force increase and ranges of SOC with very small change in force. The model includes the influence of temperature on the observed force capturing the underlying thermal expansion phenomena. Moreover the model is capable of describing the changes in force during thermal transients, when internal battery heating due to high C-rates or rapid changes in the ambient temperature, which create a mismatch in the temperature of the cell and the holding fixture. It is finally shown that the bulk force model can be very useful for a more accurate and robust SOC estimation based on fusing information from voltage and force (or pressure) measurements. Lithium intercalation and de-intercalation result in volumetric changes in both electrodes of a li-ion battery cell. At the anode, carbon particles can swell by as much as 12% during lithium intercalation, and the resulting stress can be large.6 Commercial battery packs involve numerous cells assembled to occupy a fixed space as shown in Fig. 1 and held in mild compression to resist changes in volume associated with lithium intercalation and deintercalation. A small compression prevents de-lamination and associated deterioration of electronic conductivity of the electrodes. A large compression, however, can decrease the separator thickness and lead to degradation and power reduction due to the separator pore closing. 18The effect of expansion and the system's mechanical response on the cell performance and life 18,5,2,24 are under intense investigation with studies ranging from the micro-scale, 4,6,24 the particle level, 4 and multiple electrode layers. 22,8 While progress toward predicting the multi-scale phenomena is accelerating, 9,28 the full prediction of a cell expansion and its implications to cell performance depends heavily on the boundary conditions associated with the cell construction and electrode tabbing and crimping. Moreover, the wide range of conditions with respect to C-rates and temperatures that automotive battery cells must operate make the physics-based modeling approach very challenging. 17 Finally, measuring and quantifying the internal stress or strain to tune or validate the multi-scale models requires complex instrumentation. 23,30 In contrast to the micro-scale, the macro-scale stress and strain responses are directly observable and measured with high accuracy, 16,19,26 thus could be used to develop phenomenological models inspired by the underlying physics.To this end, a phenomenological model is developed in this paper in an attempt to mimic the evolution of bulk force/stress and to quantify the contributions of state of charge dependent intercalation and thermal expansion. A rudimentary version of this model ...

The Effects of Defects on Localized Plating in Lithium-Ion Batteries

“…14 Separator deformation can be caused during the manufacturing process of the separator or of the battery cell, as well as during operation as a result of internal mechanical stress accumulation during charging [15][16][17][18] and aging. 19,3,20,21 The resulting mechanical deformation of the separator is understood to cause transport restrictions through a pore-closure mechanism, 22,14,23 which can occur in a spatially localized manner due to non-uniform mechanical stresses arising from the cell's architecture 24 as well as spatial variations in local electrode structure and mechanical properties.…”

In Situ Stress Measurement Techniques on Li-ion Battery Electrodes: A Review

“…MuKai et al [60] validated that lithium manganese oxide spinels had average coefficients of volumetric thermal expansion by the maximum value of 2.39 × 10 −5 K −1 with X-ray diffraction measurements performed in the temperature range from 100 to 400 K. Because of thermal expansion of cathode, anode, and separator in a cell, Oh and Epureanu [61] found a 5Ah NMC cathode Li-ion cell swelling linearly in the through plane from 25 to 45 • C. Stress generation of a NMC cathode battery stack caused by temperature has been measured by Mohan et al [62], who found the bulky force at the battery stack surface increase with temperature rising from −5 to 45 • C at the same SOC level.…”

“…Although the availability of published data from in situ electrode stress/strain measurements is limited, factors that influence electrode stress generation also include the effects due to thermal expansion/contraction [60][61][62] and gas generation originated from electrochemical decomposition of electrolyte solvents by the SEI layer formation, high temperature operation, and overcharge [63][64][65]. The winding process of electrodes is the important production technique for Li-ion battery fabrication [66], which can also cause electrode stress generation from external forces and body forces.…”

Effects of Cycling Ranges on Stress and Capacity Fade in Lithium-Ion Pouch Cells

“…11 as it most accurately represents our experimental stress relaxations shown in Figure 5a. [11] σ decrease = σ relax = −ct m [12] Combining film growth in Eq. 1-2, 8-9 and stress relaxation in Eq.…”