Characterization methods and modelling for Li-ion batteries: entropy, impedance, pressure, diffusivity & temperature swings induced aging

Lithium-ion batteries are commonly used for energy storage systems, and temperature is one key impact factor on the cell performance and lifetime. In literature, the focus has been on the ambient temperature of the test condition, and there is little discussion about how the temperature swing during cycling affects the battery lifetime. In this study, lithium-ion cells are cycled with the same current but different temperature swings for more than two years. The results show that the cells cycled with a high temperature swing aged faster. Moreover, pure thermal cycling does not introduce significant aging to the cell.During the study, a series of characterization methods were also developed, including a physics-based circuit model, a convenient method to measure the entropic coefficient, an on-line impedance measurement technique, an effective reference performance test procedure, and methods to measure the cellpressure and thickness change during cycling. A selection of results are that the physics-based circuit model could predict key quantities, such as overpotential, concentrations etc., with less than 0.05% deviation compared with a state-of-art model. Furthermore, the on-line impedance measurement technique managed to extract the battery pack impedance between 0.01 Hz and 5 Hz based on CAN signals. In addition, the cell pressure and thickness change during cycling of a commercial cell were found to be up to 60 kPa and 150 μm

In situ key aging parameter determination of a vehicle battery using only CAN signals in commercial vehicles

In this article, an on-line impedance measurement technique for a battery pack is demonstrated and proofed, using only the already existing sensors and accessible bus data in an electrified vehicle. A sufficient amount of AC harmonics in the DC-link current, for the identification purposes, is created in normal driving conditions. Fourier analysis is used to process the data and extract the impedance information. It is found that the proposed on-line method can accurately measure the battery pack impedance at a low frequency range (5 Hz to 10 mHz) with 40 Hz sampling frequency in the bus data. A key impedance value in the electrochemical impedance spectroscopy can be captured clearly in different conditions, which can be used to track the battery state of health. A recorded current waveform during an on-road test is reproduced by a state-of-art battery tester in a lab and the obtained results are compared with impedance values measured by a classic potentiostat. The results from the on-road test have an excellent agreement with lab measurements

Bridging physics-based and equivalent circuit models for lithium-ion batteries

In this article, a novel implementation of a widely used pseudo-two-dimensional (P2D) model for lithium-ion battery simulation is presented with a transmission line circuit structure. This implementation represents an interplay between physical and equivalent circuit models. The discharge processes of an LiNi0.33Mn0.33Co0.33O2-graphite lithium-ion battery under different currents are simulated, and it is seen the results from the circuit model agree well with the results obtained from a physical simulation carried out in COMSOL Multiphysics, including both terminal voltage and concentration distributions. Finally we demonstrated how the circuit model can contribute to the understanding of the cell electrochemistry, exemplified by an analysis of the overpotential contributions by various processes

An Integrated Flow–Electric–Thermal Model for a Cylindrical Li-Ion Battery Module with a Direct Liquid Cooling Strategy

An integrated model is constructed for a Li-ion battery module composed of cylindrical cells by coupling individual first-order equivalent circuit models (ECMs) with a 3D heat transfer model, also considering the fluid flow dynamics of the applied cooling liquid, and bench-marked against experimental data. This model simulates a representative unit of the battery module with direct liquid cooling in a parallel configuration. Instead of assigning specific values to the featured parameters involved in the ECMs, they are here defined as 4D arrays. This makes it possible to simultaneously consider the effect of the state of charge, current rate, and temperature on the battery dynamics, making the model more adaptive, versatile, and connectable to the battery cell electrochemistry. According to the simulation results, the model employing state-dependent battery properties fits better with the experimental cooling results. Additionally, the temperature uniformity of the module with a parallel cooling configuration is improved compared to a serial configuration. However, the increase of the absolute core temperature cannot be directly controlled by the surface cooling due to the slow heat transport rate across the battery material. The simulations also provide directions for the modification of module design, to the potential benefit of battery pack developers

A multicell structural battery composite laminate

Multifunctional materials facilitate lightweight and slender structural solutions for numerous applications. In transportation, construction materials that can act as a battery, and store electrical energy, will contribute to realization of highly energy efficient vehicles and aircraft. Herein, a multicell structural battery composite laminate, with three state-of-the-art structural battery composite cells connected in series is demonstrated. The experimental results show that the capacity of the structural battery composite cells is only moderately affected by tensile loading up to 0.36% strain. The multicell structural battery laminate is made embedding the three connected structural battery composite cells between carbon fiber/glass fiber composite face sheets. Electrochemical performance of the multicell structural battery is demonstrated experimentally. High charge transfer resistance for the pack as well as the individual cells is reported. Mechanical performance of the structural battery laminate is estimated by classical laminate theory. Computed engineering in-plane moduli for the multicell structural battery laminate are on par with conventional glass fiber composite multiaxial laminates

Implementing intermittent current interruption into Li-ion cell modelling for improved battery diagnostics

A novel electroanalytical method, the intermittent current interruption (ICI) technique, has recently been promoted as a versatile tool for battery analysis and diagnostics. The technique enables frequent and continuous measurement of battery resistance, which then undergoes statistical analysis. Here, this method is implemented for commercial Li-ion cylindrical cells, and combined with a physics-based finite element model (FEM) of the battery to better interpret the measured resistances. Ageing phenomena such as solid electrolyte interphase (SEI) formation and metallic Li plating on the surface of the negative graphite particles are considered in the model. After validation, a long-term cycling simulation is conducted to mimic the ageing scenario of commercial cylindrical 21700 cells. The large number of internal resistance measurements obtained are subsequently visualized by creating a ‘resistance map’ as a function of both capacity and cycle numbers, providing a straight-forward image of their continuous evolution. By correlating the observed ageing scenarios with specific physical processes, the origins of ageing are investigated. The result shows that a decrease of the electrolyte volume fraction contributes significantly to the increase of internal resistance and affect the electrolyte diffusivity properties. Additionally, effects of porosity and particle radius of the different electrodes are investigated, providing valuable suggestions for battery design

Influence of contouring the lithium metal/solid electrolyte interface on the critical current for dendrites

Contouring or structuring of the lithium/ceramic electrolyte interface and therefore increasing its surface area has been considered as a possible strategy to increase the charging current in solid-state batteries without lithium dendrite formation and short-circuit. By coupling together lithium deposition kinetics and the me chanics of lithium creep within calculations of the current distribution at the interface, and leveraging a model for lithium dendrite growth, we show that efforts to avoid dendrites on charging by increasing the interfacial surface area come with significant limitations associated with the topography of rough surfaces. These limitations are sufficiently severe such that it is very unlikely contouring could increase charging currents while avoiding dendrites and short-circuit to the levels required. For example, we show a sinusoidal surface topography can only raise the charging current before dendrites occur by approx. 50% over a flat interface

Onboard Impedance Diagnostics Method of Li-ion Traction Batteries using Pseudo-Random Binary Sequence

Environmental and economic reasons have lead automotive companies towards the direction of EVs and HEVs. Stricter emission legislations along with the consumer needs for more cost-efficient and environmental friendly vehicles have increased immensely the amount of hybrid and electric vehicles available in the market. It is essential though for Li-ion batteries, the main propulsion force of EVs and HEVs, to be able to read the battery characteristics in a high accuracy manner, predict life expectancy and behaviour and act accordingly. The following thesis constitutes a concept study of a battery diagnostics method. The method is based on the notion of a pseudo-random binary signal used as the current input and from its voltage response, the impedance is used for the estimation of parameters such as the state of charge and more. The feasibility of the PRBS method at a battery cell has been examined through various tests, both in an experimental manner at the lab but also in a simulation manner. The method is compared for validation against the electrochemical impedance spectroscopy method which is being used as a reference. For both the experimental and the simulation examinations, the PRBS method has been validated and proven to work. No matter the change in the parameters of the system, the method behaves in a similar manner as in the reference EIS method. The level of detail in the research and the performed experiments is what makes the significance of the results of high importance. The method in all ways has been proven to work in the concept study and based on the findings, if implemented on an EV’s or HEV’s electric drive line and the same functionality is observed, be used as a diagnostics method of the battery of the vehicle

On-board impedance diagnostics method of Li-ion traction batteries using pseudo-random binary sequences. Method evaluation and feasibility study of concept.

This thesis deals with the on-board impedance measurements of Li-ion batteries on hybrid electric vehicles/electric vehicles by using pseudo-random binary sequences (PRBSs). The impedance of the battery can be related to its state of charge but the accurate impedance measurements are difficult to perform in the vehicles. By using an excitation signal like PRBS, it is possible to extract the impedance information of the battery packs. Both experiments and simulations are performed with different set-ups to verify the PRBS method. A non-parametric method is used to process the data and extract the impedance measurement. Experiments in the laboratory at different SOC levels and temperatures are made to validate the PRBS method. In the simulations, the noise sensitivity is analyzed. It is shown that the PRBS method can produce a valid electrochemical impedance spectrum in a limited frequency range, similar to the result from a high accuracy laboratory impedance analyzer. The method is stable at different SOC levels and temperatures. However, the battery impedance at high frequency is difficult to obtain with the PRBS method in the experiments. A simulation of the excitation signal in the vehicle is performed where the electric motor is used as the load. It shows that it is possible to some extent to use the drive line in a hybrid electric vehicle/electric vehicle to perform an on-board battery impedance measurement

A Time- and Cost-Effective Method for Entropic Coefficient Determination of a Large Commercial Battery Cell

The entropic coefficient of a lithium-ion battery cell is used to calculate the reversible heat of a battery during operation, which is a nonnegligible part of the battery thermal modeling. The contribution of this article is to propose a novel method to establish the entropic coefficient profile of a 26-Ah commercial pouch cell and compare the results with those obtained from the traditional potentiometric and calorimetric methods, and all are found to be in a good agreement. The originality of this article is to use a method, which consists of supplying a square pulse current waveform at a certain frequency, and thus, the resulting heat variation could be successfully linked to the input current using Fourier analysis. The current magnitudes used were 1 and 1.5 C, which are representative of the normal operation current in an electrified vehicle application. The method proposed is found to be cost efficient with a short experiment time and simple experiment setup. In fact, it can be used to characterize cells that are already mounted in a pack without access to a climate chamber or calorimeter