Cylindrical Li-ion Cells Research Articles

Optimal Fast-charging Strategy for Cylindrical Li-ion Cells

This paper presents an innovative approach to optimize the fast-charging strategy for cylindrical Li-ion NMC 3Ah cells, with a focus on enhancing both charging efficiency and thermal safety. Leveraging the power of Model Predictive Control (MPC), we introduce a cost function that approximates the thermal safety boundary of Li-ion batteries, revealing a relationship between temperature gradient and state of charge. Our proposed approach formulates the fast and safe charging problem as an optimal output regulator problem, incorporating thermal safety margin constraints. To solve the optimization problem, we develop an MPC algorithm. Our charge control structure incorporates an equivalent circuit model coupled with a thermal equation for battery state of charge and temperature estimation. Through numerical validation with real experimental data obtained from testing an NMC 3Ah cylindrical cell, we demonstrate that our approach respects the battery’s electrical and thermal constraints throughout the charging process.

Toxicity Assessment of Gas, Solid and Liquid Emissions from Li-Ion Cells of Different Chemistry Subjected to Thermal Abuse

Lithium-ion batteries (LIBs) are employed in a range of devices due to their high energy and power density. However, the increased power density of LIBs raises concerns regarding their safety when subjected to external abuse. The thermal behavior is influenced by a number of factors, i.e., the state of charge (SoC), the cell chemistry and the abuse conditions. In this study, three distinct cylindrical Li-ion cells, i.e., lithium nickel cobalt aluminum oxide (NCA), lithium titanate oxide (LTO), and lithium iron phosphate (LFP), were subjected to thermal abuse (heating rate of 5 °C/min) in an air flow reactor, with 100% SoC. Venting and thermal runaway (TR) were recorded in terms of temperature and pressure, while the emitted products (gas, solid, and liquid) were subjected to analysis by FT-IR and ICP-OES. The concentrations of the toxic gases (HF, CO) are significantly in excess of the Immediate Danger to Life or Health Limit (IDLH). Furthermore, it is observed that the solid particles are the result of electrode degradation (metallic nature), whereas the liquid aerosol is derived from the electrolyte solvent. It is therefore evident that in the event of a LIB fire, in order to enhance the safety of the emergency responders, it is necessary to use appropriate personal protective equipment (PPE) in order to minimize exposure to toxic substances, i.e., particles and aerosol.

Analytical study on highway thermal runaway propagation and inter-electrode dynamics in lithium-ion battery applications: Insights into battery safety

It is essential to ensure thermal safety in lithium-ion (Li-ion) batteries to facilitate their increased use in electric cars, thereby enhancing the safety of individual batteries. When high-power Li-ion batteries are subjected to abusive conditions, they frequently experience thermal runaway (TR) due to a sequence of exothermic reactions resulting from electrode contacts. These events generate a significant amount of heat and have the potential to cause thermal runaway propagation (TRP) throughout the battery module. This paper presents an in-depth TRP analysis approach, combining thermo-kinetic and electrode interaction assessments at the cell level. The method is implemented on the TR properties of a cylindrical Li-ion cell equipped with nickel-rich and silicon-carbon electrodes. The developed TRP approach, incorporating various heat dissipation techniques, is applied to a prototype high-energy Li-ion chamber to evaluate TRP susceptibility under different conditions, including ambient temperature, cell spacing, initiating cell location, external heating power, and heat dissipation rates. Additionally, this research provides insights into the various TRP pathways, focusing on aspects such as the propagation rate, the exothermic processes’ thermal, required thermal energy input, and external heat release. The comprehensive model, considering both heat transfer and chemical interactions between electrodes under TR, can address accurate TR- propagation examinations for batteries. Moreover, it can be expanded to larger battery systems, ultimately improving overall battery safety.

Optimal Fast-Charging Strategy for Cylindrical Li-Ion Cells at Different Temperatures

Ensuring efficiency and safety is critical when developing charging strategies for lithium-ion batteries. This paper introduces a novel method to optimize fast charging for cylindrical Li-ion NMC 3Ah cells, enhancing both their charging efficiency and thermal safety. Using Model Predictive Control (MPC), this study presents a cost function that estimates the thermal safety boundary of Li-ion batteries, emphasizing the relationship between the temperature gradient and the state of charge (SoC) at different temperatures. The charging control framework combines an equivalent circuit model (ECM) with minimal electro-thermal equations to estimate battery state and temperature. Optimization results indicate that at ambient temperatures, the optimal charging allows the cell’s temperature to self-regulate within a safe operating range, requiring only one additional minute to reach 80% SoC compared to a typical fast-charging protocol (high current profile). Validation through numerical simulations and real experimental data from an NMC 3Ah cylindrical cell demonstrates that the simple approach adheres to the battery’s electrical and thermal limitations during the charging process.

Thermal Optimization Strategies for Li-Ion Batteries: Predictive Temperature Algorithm

Abstract Performance, safety, and longevity of batteries are all strongly impacted by thermal management, which is an essential component of battery design and operation. This work examines how accurate temperature control can result in significant improvements in performance and reliability with a focus on battery thermal heating. Predicting the temperature achieved by the battery during operation not only avoids conditions that lead to thermal runaway but also guarantees that the battery is used optimally within an optimal temperature range. Within the optimal temperature range, several advantages are observed. First, battery efficiency improves significantly as electrochemical processes occur more efficiently. Furthermore, by lowering the possibility of short circuits and improving overall battery safety, thermal stability aids in the prevention of undesirable phenomena like dendrite growth. By lessening the deterioration brought on by thermal degradation processes, thermal optimization also affects battery longevity. Based on experimental tests, a finite element method (FEM) model is developed. A model for thermal runaway propagation is established by combining thermal runaway and conduction models with an Arrhenius law-based combustion model. The study employed a cylindrical Li-ion cell to conduct tests, taking into account three parameters: discharge rate (CRate), ambient temperature (Tamb), and initial battery temperature (T0). An algorithm based on the three variables was developed using the simulation results. The algorithm enables the accurate prediction of rising battery temperature during use, facilitating the setting of an optimal maximum discharge rate considering initial and ambient temperatures, thereby ensuring optimal performance within the desired temperature range.

Improving heat transfer in indirect liquid-based battery thermal management systems through turbulator-equipped conical flow paths

To guarantee the safe operation of Li-Ion Cells (LICs) in various applications, such as electric vehicles, an efficient Battery Thermal Management System (BTMS) is necessary. The design of BTMSs, as documented in the literature, typically involves the use of cooling channels with uniform and smooth configurations. However, these configurations encounter the challenge of temperature non-uniformity in the battery module, which stems from elevated coolant temperatures along the flow direction. In this study, a 3D numerical analysis is conducted to develop a novel indirect liquid-based cooling system for the BTMS of a module with cylindrical LICs. The study explores the effects of conical cooling channels, both with and without twisted turbulators, across various coolant mass flows during a 2C discharge process of LICs. Within the studied ranges, diverging cooling channels improve the overall thermal and hydraulic performance of the BTMS by an average of 40%, increasing the heat transfer coefficient by a maximum of 32.6%, and decreasing pressure drop by up to 35.1%. The application of turbulators proves effective in reducing the maximum temperature difference within the LICs, demonstrating a decrease of approximately 34.2% at the minimum mass flow rate and 74.4% at the maximum mass flow rate. This emphasizes the more substantial impact of turbulator utilization at higher coolant mass flow rates. An increased coolant flow rate leads to a significant enhancement of the heat transfer coefficient, ranging from 70.5% to 79.0% for the BTMS incorporating turbulators. Moreover, the study highlights a more pronounced impact of increasing mass flow rates in the BTMS designed with converging cooling channels compared to those designed with diverging channels. Finally, the results of this analysis demonstrate that the proposed system is capable of efficiently regulating the temperature of the entire battery module, ensuring that it consistently maintains a normal and safe operational range suitable for vehicular applications (below 40 °C).

Heterogeneity of Lithium Distribution in the Graphite Anode of 21700-Type Cylindrical Li-Ion Cells during Degradation

Structural and spatial aspects of cell degradation are studied using a combination of diffraction-and imaging-based tools applying laboratory X-rays, neutron scattering and synchrotron radiation with electrochemical and thermal characterization. Experimental characterization is carried out on cylindrical cells of 21700-type, where four regimes of cell degradation are identified, which are supplemented by an increased cell resistance and surface temperature during cell operation. The amount of intercalated lithium in the fully charged anodes in the fresh and aged states is determined by ex situ X-ray diffraction radiography and in situ X-ray diffraction computed tomography. The qualitatively similar character of the results revealed a loss of active lithium along with the development of a complex heterogeneous distribution over the electrode stripe.

The effect of liquid channels and cold plates on the performance of hybrid thermal management strategies for cylindrical Li-ion cells

In this paper, phase change material (PCM) based hybrid thermal management strategies are proposed for cylindrical Lithium-ion battery modules. Experimental and numerical studies are conducted to analyze the proposed strategies at a high discharge rate of 7 C. Three different types of thermal management configurations are developed. In all the configurations primary coolant is the PCM and secondary coolants are liquid and air. In one of the configuration vertical liquid coolant channels are placed between the PCM and airflow at the top of the cells. Whereas, in the two configurations different designs of cold plates are utilized in place of liquid channels. The experimental studies are conducted to obtain the temperature and heat flux profiles of the battery module. Moreover, a numerical model is developed and validated using the experimental data obtained. Using the numerical model, the thermal performance of the battery module was obtained for the proposed strategies. Based on the results, the maximum temperature of the battery module was limited to 29.1 °C and a high temperature uniformity was achieved by the end of the discharge cycle with the temperature difference limited to 1.2 °C. Additionally, the proposed hybrid thermal management strategy was capable of maintaining the required optimal thermal operating conditions without changing the phase of the PCM. Furthermore, it eliminates the requirement of a pump and reservoir since there is no flow of liquid within the battery module. This reduces the energy required for the operation of the thermal management system, thereby increasing the available energy for propulsion. Additionally, the developed strategy also increases the safety of the battery modules as it is capable of maintaining the required thermal environment in high discharge rates up to 7 C.

Experimental Assessment of Different Air-Based Battery Thermal Management System for Lithium-Ion Battery Pack

<div>Lithium-ion (LI) batteries are widely used to power electric vehicles (EVs), owing to their high charge density, to minimize the environmental pollution caused by fossil fuel-based engines. It experiences an enormous amount of heat generation during charging and discharging cycles, which results in higher operating temperatures and thermal nonuniformity. This affects performance, useful battery life, and operating costs. This can be mitigated by an effective battery thermal management system (BTMS) to dissipate the heat there by safeguarding the battery from adverse thermal effects and ensuring high performance, safety, and longevity of the battery. This article presents a method to estimate the cooling requirement of a given battery pack using calorimetry and discusses the effect of airflow path, flow rate, and inlet air temperature on the thermal behavior of a 4S4P battery pack consisting of 18650 type lithium iron phosphate (LFP) cylindrical Li-ion cells each of 2.55 Ah capacity. In battery pack operation without a cooling system, it is found that the interior cells experience a 3°C higher cell temperature relative to the rest of the battery pack. The cooling performance of the different air BTMS, viz., natural convection cooling, single suction fan cooling, two-fan air cooling, centralized inlet air cooling, and wet jute-based single suction fan air cooling, is discussed. It is observed that the peak temperature rise of the cells in the battery pack was found to be the least in centralized inlet air cooling, unlike other cooling configurations. The wet jute-based single suction fan air cooling is superior by achieving the highest COP of 6, with an 8.9°C rise in battery pack peak temperature for a permissible peak discharge rate. It is observed that the temperature rise of the battery pack is profoundly influenced by the temperature of the inlet air compared to the flow rate.</div>

State-of-Charge Implications of Thermal Runaway in Li-ion Cells and Modules

The thermal safety of lithium-ion (Li-ion) batteries for electric vehicles continues to remain a major concern. A comprehensive understanding of the thermal runaway (TR) mechanisms in Li-ion cells and modules due to intrinsic factors such as state-of-charge (SOC) and cell-to-cell arrangement under abuse scenarios such as external heating is critical toward the development of advanced battery thermal management systems. This study presents a hierarchical TR modeling framework to examine the TR behavior of Li-ion cells at various SOCs and probe its implications on the thermal runaway propagation (TRP) in a battery module. We perform accelerating rate calorimetry (ARC) experiments with 3.25 Ah cylindrical Li-ion cells at different SOCs and demonstrate the strong SOC dependence of TR characteristics such as the onset temperature, maximum cell temperature, and self-heating rate. The thermo-kinetic parameters extracted from the ARC experiments are used to develop a TR model that captures the SOC-induced TR response in Li-ion cells. The mechanistic information from the cell-level model is used to examine the pathways for TRP in a battery module consisting of cells with uniform and imbalanced SOCs, thereby demonstrating the underlying role of SOC variability on the resulting TRP under abuse conditions.

Tracking the Lifecycle of a 21700 Cell: A 4D Tomography and Digital Disassembly Study

Extending the lifetime of commercial Li-ion cells is amongst the most important challenge to facilitate the continued electrification of transport as demonstrated by the substantial volume of literature dedicated to identifying degradation mechanisms in batteries. Here, we conduct a long-term study on a cylindrical Li-ion cell, tracking the evolution of the structure of the cell using X-ray computed tomography. By evaluating the internal geometry of the cell over several hundreds of cycles we show a causal relationship between changes in the electrode structure and the capacity fade associated with cell ageing. The rapid aging which occurs as cells reach their end-of-life condition is mirrored in a significant acceleration in internal architecture changes. This work also shows the importance of consistent and accurate manufacturing processes with small defects in the jelly-roll being shown to act as nucleation sites for the structural degradation and by extension capacity fade.

Tracking Electrolyte Motion in Cylindrical Li-ion Cells Using Moment of Inertia Measurements

High energy density cylindrical Li-ion cells are densely packed with active materials, inactive materials and electrolyte. Injected electrolyte generally fills all pore spaces in the electrodes and separators of manufactured cells with possibly some excess. When such cells are charged, the overall volume of the electrode materials increases and therefore some electrolyte is pushed under hydraulic pressure to the spaces outside the electrode winding at the ends of the cylindrical can and also possibly into the hollow core of the cylindrical electrode winding. During discharge this electrolyte reenters the pore spaces of the electrodes as electrode particles contract. Therefore, the moment of inertia of the cell about an axis perpendicular to the axis of the cylindrical can change as the cell is charged and discharged. We have built a torsional oscillator that can measure the resonant frequency, and hence the moment of inertia, of a cylindrical Li-ion cell as it is charged and discharged. Because the moment of inertia of the cell depends on the electrolyte distribution, we can “watch” the electrolyte move within the cell. The design and operation of the instrument is described here as well as experiments that demonstrate the electrolyte motion that occurs in cylindrical cells. Consequences of this electrolyte motion are discussed.

Analysis of nanofluid flow and heat transfer behavior of Li-ion battery modules

The operating battery temperature significantly affects electric vehicle performance, reliability, and safety. Therefore, batteries need to keep within the operating temperature design. The 3D Eulerian model is applied to determine battery thermal behavior with five different flow directions of coolant throughout the battery pack jacket. The computational domain consists of sixty cylindrical Li-ion cells inserted into the cooling module socket with constant power input conditions. The predicted results are consistent with the experimental results, with an average error of 1.28%. Coolant-improved flow direction and thermophysical properties significantly affect the decreasing maximum operating temperature and temperature gradient across a cell. The highest temperatures of the battery module are 30.06 °C, 30.00 °C, 29.91 °C, 29.89 °C, and 29.49 °C for models II, IV, III, I, and V, respectively. In addition, for the maximum temperature gradient across a cell, models I, II, and III yield the highest value [0.42 °C], followed by models IV [0.40 °C] and model V [0.15 °C], respectively. The proposed battery nanofluid cooling pack can therefore optimize the thermal management system of the EV pack.

Investigation of internal gas pressure and internal temperature of cylindrical Li-ion cells to study thermodynamical and mechanical properties of hard case battery cells

This study is the first to measure the relationship between gas pressure, internal gas temperature and state of charge of cylindrical Li-ion battery cells. For this purpose, a new, simple and affordable setup for in-situ investigation of internal gas pressure and internal temperature of commercial cylindrical Li-ion cells is designed and tested. Setups to investigate the internal gas pressure have so far only been performed on pouch and prismatic cells or require complex and expensive measurement equipment. In this work, the mechanical effects of an increasing gas pressure in cylindrical Li-ion cells are addressed for the first time. It is shown that the measurement of internal gas pressure and internal temperature over multiple charging and discharging cycles is possible. Measurements at different current rates show the dependency of the gas pressure to the total amount of charge transferred and the temperature of the cell. The pressure, which depends on the degree of lithiation of the electrodes, relaxes after the Li-ion cells have been exposed to a high current. Additionally, phases of the phase transition are visible via differential capacity analysis. The differential pressure analysis shows an equivalent trend to the differential capacity. Thus demonstrates the predominant phase transformations, namely the restructuring of lithiated graphite, the lithiation of silicon, and the phase transformation of nickel–cobalt–manganese (NCM) oxide. These effects contribute significantly to the pressure change, as they cause electrode expansion, which results in a reduction of the gas volume of the Li-ion cell, leading to an increase in pressure.

Heat transfer experiments and correlations for vent gases emerging from a Li-ion battery and impinging on a flat surface

During thermal runaway of Li-ion battery cells, high-temperature gas jets may impinge onto nearby surfaces and may increase the risk of a propagating failure. In this paper, heat transfer rates of jets emerging from a cylindrical Li-ion cell and impinging on a flat surface were measured experimentally and empirical correlations were developed based on the resulting data. Experiments used compressed air as the working fluid, which issued from the isolated safety vent and impinged on a target plate. Infrared thermography was used to obtain the spatially-resolved, convective heat transfer distribution on the target plate. While heat transfer distributions showed local maxima at the site of jet impingement, complex geometric features within the vent resulted in variation in the convection rate when comparing the multiple impinging jets emerging from a single Li-ion cell. Heat transfer correlations were developed in the form of Nusselt number as a function of Reynolds number and may be used in thermal runaway models which seek to include the effects of venting and combustion as an alternative to resolving the impingement heat transfer rates with expensive 3-D computational fluid dynamics simulations.

Effect of the heating rate on the degassing and combustion of cylindrical Li-Ion cells

The thermal runaway (TR) of 18650 Li-ion cells under constant heating rate is investigated in a cone calorimeter. The effect of the heating rate was observed on the outgassing and cell casing temperatures, as well as on the thermal power of the flames produced and resulting smoke characteristics. Time-resolved analysis of outgassing composition based on four main gases was carried out using mass spectrometry and the size distribution of the particulate emission was measured with aerosol spectrometers. The results revealed that the heating rate had an impact on the course of TR events, with high heating rates resulting in more violent TR. The initial temperature of the first venting also increased with higher heating rates. Mass spectrometry measurements revealed that the first venting mainly released vaporized electrolyte (dimethyl carbonate, DMC) with a small quantity of CO2. During the combustion associated with the major venting event, the gases released were mainly CO2 and H2O, with small quantities of H2 and DMC, and sporadic detection of C3H6 and C6H6. The high-resolution size distributions obtained for the particulate emission showed a bimodal distribution, characteristic of very rich diffusion flames, but no significant effect of the heating rate.

(Invited, Digital Presentation) Multiscale Measurements and Characterization of Thermal Transport in Materials and across Interfaces in Li-Ion Cells

Li-ion cells are used universally for energy conversion and storage. Key application areas include electric vehicles, grid energy storage, air/space vehicles and consumer electronics. Overheating of Li-ion cells is a well-known problem, with serious concerns related to thermal runaway and fire. Additionally, Li-ion cells also under-perform at low temperatures, which is a particularly important problem for automotive and space applications. Advances in battery materials are needed to mitigate these concerns and improve the performance and safety of Li-ion cells. This talk will summarize research advances in understanding and optimizing thermal transport in Li-ion cell and battery pack materials. Measurement of thermal conductivity of cylindrical Li-ion cells, indicating the highly anisotropic nature of thermal conduction will be described. This will be followed by discussion of measurement of thermal properties of individual cell materials, such as the separator, as well as thermal contact resistance at the cathode-separator interface. This contact resistance will be shown to play a rate-limiting role in determining overall thermal transport properties of the cell, and is, in particular, responsible for anisotropic thermal conduction. Experiments to mitigate this thermal resistance through molecular bridging at the interface will be described. Good agreement with molecular dynamics simulations will be demonstrated. Challenges associated with practical implementation of such techniques will be discussed. Finally, numerical simulations to understand the nature of thermal transport in battery packs will be described, with the goal of minimizing thermal runaway propagation. These measurements and modeling techniques contribute towards a fundamental understanding of multiscale thermal transport in Li-ion cell materials, and may help optimize and improve thermal performance of practical energy conversion and storage devices.

Thickness change and jelly roll deformation and its impact on the aging and lifetime of commercial 18650 cylindrical Li-ion cells with silicon containing anodes and nickel-rich cathodes

Two types of commercial cylindrical cells in the format 18650 both with NCA cathode and anodes containing different silicon materials were electrochemically long-term cycled. Two voltage windows and their influence on the aging of the cells were investigated. Multiple CT images after a pre-determined number of completed cycles reveal volume expansion and jelly roll deformation during the aging of the cells. With an optical micrometer, the reversible and irreversible thickness change was studied in operando. Furthermore the electrode expansion was investigated with an electrochemical dilatometer. Depending on the type of silicon active material used, differences in the aging behavior of the cells were observed. These findings were linked to the rest of the non-destructive, destructive and post-mortem analyses. With the help of differential voltage analysis, pulse tests and different evaluations using the generated CT data, the inhomogeneous thickness change of the cells especially at critical spots could thus be revealed and reasons for capacity degradation or resistance increase can be directly seen. Due to post-mortem analysis a much better understanding of the underlying aging mechanisms was generated. With SEM/EDX analysis, ICP-OES and TGA, the formation of passivation layers on the electrodes was observed. By comparing the results with new cells, individual causes of failure were revealed. It was found that the main aging mechanism is the loss of lithium from the cathode together with the volume change of the anode. Cells with pure Si instead of SiOx have exhibited a faster degradation because of the additionally higher thickness change during charging and discharging. Delamination of the electrode coating because of particle swelling, binder failure and jelly roll deformation is seen after unrolling the aged cells. Limiting the voltage range leads to longer lifetime, less resistance increase and overall less aging of silicon in all tested cells.

Development of a hybrid cooling concept for cylindrical li-ion cells

In this paper, the issue of battery thermal management of cylindrical lithium-ion cells for electric and hybrid electric vehicles is studied. Hybrid concepts combining air and liquid cooling are examined in detail using numerical simulation. The first concept includes vortex generators and jet inlets to increase turbulence within the battery pack in order to improve mixing and air flow between adjacent battery cells as well as liquid jackets to each cell in order to improve temperature uniformity at the cell level. The second concept uses evaporative cooling by means of fiber channels soaked by a reservoir below the battery pack through capillary action. The results of the second concept indicated a significant improvement in the cooling at the pack level. The maximum temperature increase was reduced by ∼50% and the average temperature increase was reduced by ∼59%. The second concept removes the complexity of the multiple air-cooling passive techniques and eliminates the safety issue due to the leakage of liquid within the battery pack. It is beneficial in terms of thermal performance at both the cell level and pack level and the commercialization of the battery pack as it is much simpler and easier to implement.

Temperature-Dependent Time Constants of Li-ion Batteries

We investigate the effect of temperature on the time constants of Li-ion batteries (LIBs). Using the distribution of relaxation times (DRT), the time constants of three cylindrical Li-ion cells were determined. EIS (Electrochemical Impedance Spectroscopy) was conducted on the cells, and the measured impedance spectra were analyzed using DRT. The DRT analysis is usually formulated as a Ridge Regression optimization problem. While the regression tuning parameter has a significant impact on the results, the studies on selecting this parameter are very limited. This letter proposes novel cost functions to select the optimal regressions parameters. The cost functions include (i) Discrepancy, (ii) Cross-Discrepancy, and (iii) the Sum of Squared Errors. The first two criteria exploit the Kramers-Kronig relations, and they quantify the discrepancy of the reconstructed impedance spectra using only its real, imaginary, or both components. The last criterion quantifies the errors in real and imaginary components of the data from the reconstructed EIS. The method was applied to the impedance spectra of Li-ion cells at low and high temperatures and different state of charges (SOCs). We identified the time constants of the cells using the proposed criteria for different test conditions.