Performance Of Lithium-ion Cells Research Articles

A Review on Temperature-Dependent Electrochemical Properties, Aging, and Performance of Lithium-Ion Cells

Temperature heavily affects the behavior of any energy storage chemistries. In particular, lithium-ion batteries (LIBs) play a significant role in almost all storage application fields, including Electric Vehicles (EVs). Therefore, a full comprehension of the influence of the temperature on the key cell components and their governing equations is mandatory for the effective integration of LIBs into the application. If the battery is exposed to extreme thermal environments or the desired temperature cannot be maintained, the rates of chemical reactions and/or the mobility of the active species may change drastically. The alteration of properties of LIBs with temperature may create at best a performance problem and at worst a safety problem. Despite the presence of many reports on LIBs in the literature, their industrial realization has still been difficult, as the technologies developed in different labs have not been standardized yet. Thus, the field requires a systematic analysis of the effect of temperature on the critical properties of LIBs. In this paper, we report a comprehensive review of the effect of temperature on the properties of LIBs such as performance, cycle life, and safety. In addition, we focus on the alterations in resistances, energy losses, physicochemical properties, and aging mechanism when the temperature of LIBs are not under control.

Investigation of the thermal performance in lithium-ion cells during polyformaldehyde nail penetration

Investigation of the thermal performance in lithium-ion cells during polyformaldehyde nail penetration

A Facile Approach to High Precision Detection of Cell-to-Cell Variation for Li-ion Batteries

Over the past decade, it has been repeatedly demonstrated that homogeneity in electrochemical performance of lithium-ion cells plays a major role in determining the life and safety of lithium-ion battery modules or packs. Generally, the homogeneity of a battery pack is evaluated by characterizing the cells individually in terms of capacity, mass, impedance. Particularly, high quality electrochemical data heavily relies on the availability of high precision current source to minimize the discrepancy induced by the channel-to-channel variation. Here, a facile and precise measurement method is reported for screening cell-to-cell variations, in which voltage is the only indicator parameter independent of high precision current source. In detail, by connecting the cells in series (CiS), the measurement error of electrochemical data caused by stability and discrepancy of current sources among different charge/discharge equipment can be effectively avoided. The findings of this work showed that the cell-to-cell variations can be simply and sensitively detected with CiS configuration. For example, the relative standard deviation, which is the evaluation criterion of battery homogeneity, was 2.14% based on CiS while it was 0.43% based on individual measurements. The simple and precise CiS measurement is promising for evaluation of cell quality or module integration quality. In addition, this work can also provide a solid foundation for the development of detection algorithms for battery management systems to rapidly monitor battery homogeneity.

On the Implication of Porosity Configuration on Lithium-Ion Cell Performance: A Numerical Study

AbstractThe present study numerically investigates the implication of different porosity configurations, viz., uniform, algebraic, trigonometric, logarithmic, and stepwise constant porosities at the negative electrode on performance characteristics of Lithium-ion cell. We assess the merit of nonuniform porosity over uniform one in terms of cell performance characteristics, viz., specific energy, capacity, electrolyte salt concentration, local volumetric current density, power dissipation density, and solid lithium concentration. Our results reveal that specific energy and capacity are found to be maximum when the porosity increases logarithmically in the direction from the negative electrode–current collector to negative electrode–separator interface. Also, it is found that the variation of power dissipation density and electrolyte salt concentration characteristics are dictated by the interplay of the porosity and the length of the negative electrode. Furthermore, the effect of charging rates (quick charge, fast charge, and ultrafast charge) on cell performance is carried out. It is seen that the increment in C-rates strongly influences the cell performance. It is found that the average capacity increases by 44% at the higher C-rate, i.e., 5C when the porosity increases logarithmically. On the contrary, sinusoidal variation in porosity yields in the worst cell performance. The findings of the present study bear utility toward designing an efficient battery system that can operate for a higher number of cycles with minimal power dissipation density and can fit into the ultrafast charging technique.

A Thermal Design and Experimental Investigation for the Fast Charging Process of a Lithium-Ion Battery Module With Liquid Cooling

Abstract The appropriate temperature distribution is indispensable to lithium-ion battery module, especially during the fast charging of the sudden braking process. Thermal properties of each battery cell are obtained from numerical heat generation model and experimental data, and the deviation of thermophysical performance is analyzed by K-means clustering and hierarchical clustering to select battery cells with similar performance. Thermal performance of lithium-ion cells under different charging rates is investigated in experiments and the effects of different mini-channel designs discussed using numerical simulation, maximum temperature, maximum pressure, and temperature standard deviation are compared by both numerical calculation and experimental validation. Two kinds of cooling plates are selected, considering the uniformity of temperature distribution and energy consumption, respectively. All of these cooling plate designs have the ability to constrain the maximum temperature and temperature standard deviation within 306 K and 1.2 K, respectively. Additionally, this thermal management system does not need too much energy consumption. In experimental validation, deviation of maximum temperature is measured to be within 2.2 K and difference of temperature standard deviation is also within tolerance.

(Invited) Tuning Electrochemical Performance of Lithium-Ion Cells By Introducing Laser-Generated Electrode Architectures

Laser technology in lithium-ion battery (LIB) manufacturing could enable rapid manufacturing, high reliability, and a reduction of production costs. Laser cutting and welding processes for batteries already reached a high industrial readiness level. A new approach which is assigned to laser micro-structuring of battery materials has a huge impact on battery performance and operational lifetime of lithium-ion cells and an up-scaling for industrial production is currently investigated. So far, different types of laser structuring were used on metallic current collectors, separators, and thin or thick film composite electrodes. For thin metallic current collector foils, at anode and cathode sides, self-organized structuring by laser-induced periodical surface structures were successfully applied for improving electrode film adhesion. Direct laser ablation was applied for the generation of capillary structures, which was found to be most powerful in order to transform electrode and separator surfaces to superwicking. Thick film composite electrodes in lithium-ion cells are complex multi-material systems with defined material components, grain sizes, porosities, and pore size distributions in the micrometer and sub-micron range. The development of laser generated electrode architectures became a promising approach to overcome limitations in lithium-ion diffusion kinetics, high inter-electrode ohmic resistances, and mechanical stresses due to high volume changes during battery operation. It was shown, that structure sizes down to 10-30 µm and high aspect ratios enable advanced electrode architectures with huge benefits regarding capacity retention and cell life-time for high power operations during charging/discharging. For laser structured electrodes, the overall impedance was reduced and the diffusion kinetics could be enhanced which was proven by galvanostatic intermittent titration technique, cyclic voltammetry, and electrochemical impedance spectroscopy. Furthermore, the new electrode architecture provides new lithium-ion diffusion pathways, which could be indirectly proven by post-mortem analysis of entire electrodes by laser-induced breakdown spectroscopy (LIBS). It is evident that the increased active surface area provided by laser generated electrode architectures act as an attractor for lithium-ions which finally is responsible for the boost in battery performances. By increasing the cathode film thickness to values significantly larger than 150 μm an increase of the specific capacity or energy density at cell level was achieved. While the energy per area increases with the cathode layer thickness, the power density will in general decrease due to an increasing diffusion overpotential. In addition, the mechanical integrity of the films will decrease with increasing layer thickness and the migration of binders to the electrode surface will occur through altered drying conditions. The mechanical degradation of the cells due to the volume change inherent in the battery process in the electrodes will become more important as the layer thickness increases. Laser structuring of the electrodes can counteract these processes and will also improve the high current capability. Finally, LIBs with high-energy and high-power density can be realized by introducing laser generated electrode architectures (3D battery concept), i.e., at the same time, improved electrolyte transfer in thick film electrodes and improved lithium ion transport kinetics are achieved. For developing next generation batteries including the 3D battery concept for large footprint areas, high energy materials such as nickel-enriched Lithium-Nickel-Manganese-Cobalt-Oxide (NMC) for cathodes and silicon-based anodes are introduced.

The Impact of Calendering on the Electronic Conductivity Heterogeneity of Lithium-Ion Electrode Films

Electrical conductivity is a key metric in the performance of lithium-ion batteries. However, accurately measuring the electronic conductivity nondestructively is difficult. We have developed a micro-four-line probe that has been used to make this measurement and to demonstrate the heterogeneity of electrical conductivity of lithium-ion electrodes on a mm scale [1]. Such heterogeneity or non-uniformity can cause the lithium ion battery to degrade unevenly overtime, which is undesirable. The design of the micro-four-line probe has been improved into a flexible-four-line probe. This flexible measurement device is able to conform to the surface of the electrode, which improves the contact and the resulting electronic conductivity measurement. The flexible probe has been used to take accurate measurements of electronic conductivity and the contact resistance between the film and the current collector. The microstructure of lithium-ion electrodes is another important characteristic that determines the performance of lithium-ion cells, including electronic and ionic conductivity of electrodes. Calendering, an important step in the manufacturing process, affects the microstructure as the film is compressed to a uniform thickness. Prior to calendering there are more substantial variations in the thickness of the electrode film, which constitutes surface roughness, as illustrated in the included thickness profile (figure). The effect of calendering on the electronic heterogeneity of electrode films will be reported. The flexible probe will be used to measure the variation in electronic conductivity across the electrodes. A thickness analysis of the electrode films before and after calendering will complement the electrical measurements. This information increases our understanding of electrode manufacturing processes to allow for future optimization of these processes.

Cell Heat Generation and Dissipation: From Experimentation to Application for Cell Design.

Lithium ion batteries (LIBs) are increasingly important in ensuring sustainable mobility and reliable energy supply, storing and managing energy from renewable sources [1]. Temperature is a critical factor in LIBs performance optimisation where large temperature deviations within the cell could lead to accelerated degradation and in extreme cases, thermal runaway. Thermal management has therefore become the focus of intensive research in an attempt to improve battery performance and lifespan [2-5]. Despite the growing research interest in this area, cell heat generation and heat dissipation pathways are not usually considered when designing a cell. This typically leads to cells with thermal bottlenecks prone to internal thermal gradients. With the goal of improving performance and lifetime, a two-dimensional electro-thermal model has been developed to simulate cell performance and internal states under complex thermal boundary conditions [6]. This model can be used to assess different cooling strategies and parameters such us tab position and dimensions can be optimised from the thermal performance perspective for a particular cell chemistry and geometry. In this study, a novel experimental procedure is employed to evaluate cell heat generation and dissipation for various operation conditions. The two-dimensional electro-thermal model was employed to assess the internal temperature distribution during the measurements and to verify the heat dissipation patterns observed during the experiments. As a result, a new metric, the Cell Cooling Coefficient (CCC) is proposed to evaluate the thermal pathways of a cell cooled via its tabs. International Energy Agency. Tracking Clean Energy Progress 2017. 1–82 (2017). doi:10.1787/energy_tech-2014-enBandhauer, T. M., Garimella, S. & Fuller, T. F. A Critical Review of Thermal Issues in Lithium-Ion Batteries. J. Electrochem. Soc. 158, R1–R25 (2011).Troxler, Y., Wu, B., Marinescu, M., Yufit, V., Patel, Y., Marquis, J.A., Brandon, N.P. & Offer, G. J. The effect of thermal gradients on the performance of lithium-ion batteries. J. Power Sources 247, 1018–1025 (2014).Hunt, I. A., Zhao, Y., Patel, Y. & Offer, G. J. Surface Cooling Causes Accelerated Degradation Compared to Tab Cooling for Lithium-Ion Pouch Cells. J. Electrochem. Soc. 163, A1846–A1852 (2016).Khan, M. R., Swierczynski, M. J. & Kær, S. K. Towards an Ultimate Battery Thermal Management System : A Review. (2017). Batteries 3, 9 (2017)Zhao, Y., Patel, Y., Zhang, T. & Offer, G. J. Modeling the Effects of Thermal Gradients Induced by Tab and Surface Cooling on Lithium Ion Cell Performance. J. Electrochem. Soc. 165, A3169–A3178 (2018).

Effects of Lithium-Ion Conducting Solid Particles in Hybrid Electrolytes on Lithium-Ion Cell Performance

Development of lithium-ion batteries for automotive applications such as electric vehicles (EVs) has been focused on enhancement of energy density, power, safety, and life. In particular, all-solid-state batteries have been extensively studied in order to respond to these demands. However, the solid electrolytes such as sulfide or oxide materials have important subjects for practical uses. Sulfide solid electrolytes have low chemical stability in air and low mechanical strength for thin separator. Oxide solid electrolytes have low ionic conductivity and large resistance at interfaces between active material and solid electrolyte. We proposed a thin hybrid electrolyte system instead of solid electrolytes [1]. The thin hybrid electrolyte consisting of lithium-ion conducting solid particles of a cubic garnet-type Li7La3Zr2O12 (LLZ) and a gel polymer electrolyte exhibits smaller activation energy for ionic conductivity than a gel polymer electrolyte without LLZ. The fabricated cells using the hybrid electrolyte show high-rate charge-discharge performance [1, 2]. We have also investigated electrochemical properties of other hybrid electrolytes consisting of NASICON-type Li-Al-Ti-P-O (LATP) particles wetted with liquid electrolytes of LiPF6-PC/DEC. Figure 1 shows fast-charge performance of 1 Ah TiNb2O7/LiNi0.5Co0.2Mn0.3O2 cells using the hybrid and the liquid electrolyte at 10 C rate. The charge performance of the cell using the hybrid electrolyte was significantly superior to that of the cell using the liquid electrolyte, which indicates the existence of LATP particles contributing to enhancement of migration of lithium ions in the hybrid electrolyte. Figure 2 shows high-rate discharge properties of the cells using the hybrid and the liquid electrolytes containing different LiPF6 concentration. The voltage decline of the cells using the hybrid electrolytes were more gradual than that of the cell using the liquid electrolytes with LiPF6 concentration of 0.5 M and 1 M. The difference between hybrid electrolytes and liquid electrolytes was clearly observed in the cell with LiPF6 concentration of 0.5 M. In the case of a high LiPF6 concentration of 2 M, the voltage decline of the cells using the hybrid and the liquid electrolytes showed almost no difference. Electrochemical kinetics on the discharge were more effectively enhanced by the existence of LATP particles wetted with the liquid electrolyte containing smaller amount of LiPF6. From these results, we considered that lithium-ionic conducting solid particles in hybrid electrolyte have effects to enhance lithium-ion conductivity by promoting lithium-ion migration on the surface of the solid particles and the amount of lithium-ions for charge transfer on the electrodes. However the detail of this mechanism for effects has not been cleared yet. In order to understand the phenomenon, it is determined that properties of lithium-ion conducting and electrochemical performances of hybrid electrolytes with different ratio of solid particles and liquid electrolyte and so on. In this presentation, we will discuss the role of lithium-ion conducting solid particles in hybrid electrolytes.

A Tale of Two Additives: Effects of Glutaric and Citraconic Anhydrides on Lithium-Ion Cell Performance

The use of electrolyte additives is an important method to improve lithium-ion cell lifetime and performance without significantly affecting costs. This work evaluates two organic anhydrides, glutaric anhydride (GA) and citraconic anhydride (CA), as additives in Li(Ni0.6Mn0.2Co0.2)O2 (NMC622)/graphite and Li(Ni0.5Mn0.3Co0.2)O2 (NMC532)/graphite pouch cells, using ultrahigh precision coulometry and high-temperature storage. The additives were tested singly and in binary blends. GA-based additive blends give high coulombic efficiencies (CEs) and good storage performance. However, GA leads to substantial impedance during formation. Most notably, GA is extremely effective at suppressing gas during cell formation and storage. Whereas CA-containing blends yield good CEs, they show rapid voltage drop during storage. Both additives may provide specific benefits for target applications. Long-term cycling data indicates that GA is a negative electrode SEI-forming additive that is useful for capacity retention and limiting cell impedance growth when used as a binary blend with vinylene carbonate or lithium difluorophosphate. These results are also intended to facilitate comparison between chemically related additives in order to better understand the underlying chemistry behind their function in lithium-ion cells.

A Joint DFT and Experimental Study of an Imidazolidinone Additive in Lithium-Ion Cells

Electrolyte additives are a practical route to improving the lifetime and performance of lithium-ion cells. It is not well understood what makes a good additive; thus, the discovery of new additives poses a significant challenge. Computational methods have the potential to streamline the search for new additives, but it is important to compare predicted additive behavior with experimentally measured results. A new electrolyte additive, 1,3-dimethyl-2-imidazolidinone (DMI), has been evaluated in LiNi1-x-yMnxCoyO2 (NMC)/graphite pouch cells as a single additive and with the co-additive vinylene carbonate (VC). This work compares the density functional theory (DFT)-predicted behavior of DMI with experimental results, including differential capacity analysis (dQ/dV), electrochemical impedance spectroscopy (EIS), high-temperature storage, gas chromatography-mass spectrometry (GC-MS) and long-term cycling tests. The DFT-calculated reduction potential of DMI is −0.63 V vs Li/Li+, consistent with the experimental observation that it reduces at a lower potential than ethylene carbonate (EC), ∼0.80 V vs Li/Li+. Although DMI turns out not to be a competitively useful additive, the good match between many aspects of the experimental results and theoretical predictions is a good indication that it is possible to understand aspects of the behavior of additives. This can guide future researchers.

A New Consideration for Validating Battery Performance at Low Ambient Temperatures

Existing validation methods for equivalent circuit models (ECMs) do not capture the effects of operating lithium-ion cells over legislative drive cycles at low ambient temperatures. Unrealistic validation of an ECM may often lead to reduced accuracy in electric vehicle range estimation. In this study, current and power are used to illustrate the different approaches for validating ECMs when operating at low ambient temperatures (−15 °C to 25 °C). It was found that employing a current-based approach leads to under-testing of the performance of lithium-ion cells for various legislative drive cycles (NEDC; FTP75; US06; WLTP-3) compared to the actual vehicle. In terms of energy demands, this can be as much as ~21% for more aggressive drive cycles but even ~15% for more conservative drive cycles. In terms of peak power demands, this can range from ~27% for more conservative drive cycles to ~35% for more aggressive drive cycles. The research findings reported in this paper suggest that it is better to use a power-based approach (with dynamic voltage) rather than a current-based approach (with fixed voltage) to characterise and model the performance of lithium-ion cells for automotive applications, especially at low ambient temperatures. This evidence should help rationalize the approaches in a model-based design process leading to potential improvements in real-world applications for lithium-ion cells.

Surface modification of natural vein graphite for the anode application in Li-ion rechargeable batteries

Natural vein graphite with high purity and crystallinity is seldom used as anode material in lithium-ion rechargeable batteries (LIB) due to impurities and inherent surface structure. This study focuses on improving the surface properties of purified natural vein graphite surface by employing mild chemical oxidation. Needle-platy graphite sample with initial average carbon percentage of 99.83% was improved to 99.98% after treatment with 5 vol.% HCl. Surface modification of purified graphite was done by chemical oxidation with (NH4)(2)S2O8 and HNO3. Fourier-transform infrared spectra of graphite after chemical indicating surface oxidation of graphite surface. X-ray diffraction and scanning electron microscopic studies show the improvement of graphite structure without modification of crystalline structure. Electrochemical performance of lithium-ion cell assembled with developed anode material shows noticeable improvement of the reversible capacity and coulombic efficiency in the first cycle and cycling behavior after surface modification.

The Effect of Methyl Acetate, Ethylene Sulfate, and Carbonate Blends on the Parasitic Heat Flow of NMC532/Graphite Lithium Ion Pouch Cells

The formulation of solvent systems can have a severe impact on the lifetime, rate performance, and temperature performance of lithium ion cells. Methyl acetate (MA) has been found to increase rate and temperature performance of carbonate solvents, but decreased cell lifetime. This work used ultra-high precision coulometry, in-situ gas analysis, and isothermal microcalorimetry to investigate a recently reported promising high rate additive blend of fluoroethylene carbonate (FEC) and 1,3,2-dioxathiolane-2,2-dioxide (DTD) in LiNi0.5Mn0.3Co0.2O2/graphite pouch cells. Solvent systems composed of blends of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were investigated, with additions of 0%, 20% and 40% wt MA. MA was found to decrease the coulombic efficiency and increase slippage, parasitic heat flow, and gas volume. The addition of just 1% wt of DTD to 2% FEC improved the performance of cells containing 20% and 40% MA to that of cells containing no MA and 20% MA, respectively, under 4.3 V. Results suggest the negative impact of MA originates from increased electrolyte oxidation at the positive electrode. Additionally, blends of MA in EC:EMC and EC:DMC (3:7 wt) were tested head-to-head, yielding a small improvement to parasitic heat flow and UHPC performance when cells used EC:EMC.

Efficient Global Sensitivity Analysis of 3D Multiphysics Model for Li-Ion Batteries

Parameter sensitivity analysis of mechanistic battery models has the power to quantify the individual and joint effects of parameters on the performance of lithium-ion cells. This information can be beneficial for industrial cell designs, cell testing, and battery management system (BMS) configurations. The numerical quantification of these parameter sensitivities, however, is challenging in terms of computational costs and is an active field of research. In this paper, based on a 3D multiphysics model, we conduct a global sensitivity analysis for the utilizable cell discharge capacity and the maximum cell temperature at the discharge rate of 1C. The least angle regression version of the polynomial chaos expansion (PCE) concept has been identified as an optimal trade-off between approximation power and computational complexity. As a result, the sensitivities of all parameters in the 3D multiphysics model were studied using a hierarchical design and a stepwise design. We conclude that the cell discharge capacity and the thermal behavior at 1C discharge are most sensitive to the electrode parameters and their pore structure. The results reveal different dependencies and lead to new insights for cell design and operation.

A study into different cell-level cooling strategies for cylindrical lithium-ion cells in automotive applications

Previous research has identified that the ageing rate and performance of lithium-ion cells is negatively influenced by unfavourable cell thermal conditions, specifically, high ambient temperatures and large in-cell temperature gradients. In this paper, the effectiveness of different cell cooling strategies on reducing the in-cell temperature gradient within cylindrical cells is analysed through the development of a 2-D transient bulk layer thermal model displaying anisotropic thermal conductivity. The model is validated against experimental temperature measurements in which the peak error of the simulation was found to be 2% and 5% for the experimental test drive cycle and constant 1C discharge respectively. Results indicate that radial cooling with air or singular tab cooling with liquid may be inadequate in limiting cell temperature gradients to below 5 ℃ for HEV type 32113 cells when subject to 4 loops of the US06 drive cycle.

A study into different cell-level cooling strategies for cylindrical lithium-ion cells in automotive applications

Previous research has identified that the ageing rate and performance of lithium-ion cells is negatively influenced by unfavourable cell thermal conditions, specifically, high ambient temperatures and large in-cell temperature gradients. In this paper, the thermal performance of tab cooling cylindrical cells, which is not well understood within the literature, is compared to more common radial cooling strategies. The analysis is conducted through the development of a 2D transient bulk layer thermal model displaying anisotropic thermal conductivity. The model is validated against experimental temperature measurements, where the peak error of the simulation was found to be 2% and 5% for the experimental test drive cycle and constant 1 C discharge respectively. Results indicate that radial cooling with air or singular tab cooling with liquid is inadequate in limiting in-cell temperature gradients to below 5°C for HEV type 32113 cells when subject to four loops of the US06 drive cycle.

Modeling the Effects of Thermal Gradients Induced by Tab and Surface Cooling on Lithium Ion Cell Performance

Lithium ion batteries are increasingly important in large scale applications where thermal management is critical for safety and lifetime. Yet, the effect of different thermal boundary conditions on the performance and lifetime is still not fully understood. In this work, a two-dimensional electro-thermal model is developed to simulate cell performance and internal states under complex thermal boundary conditions. Attention was paid to model, not only the electrode stack but also the non-core components (e.g. tab weld points) and thermal boundaries, but also the experiments required to parameterize the thermal model, and the reversible heat generation. The model is comprehensively validated and the performance of tab and surface cooling strategies was evaluated across a wide range of operating conditions. Surface cooling was shown to keep the cell at a lower average temperature, but with a large thermal gradient for high C rates. Tab cooling provided much smaller thermal gradients but higher average temperatures caused by lower heat removing ability. The thermal resistance between the current collectors and tabs was found to be the most significant heat transfer bottleneck and efforts to improve this could have significant positive impacts on the performance of li-ion batteries considering the other advantages of tab cooling.

Comparative study of imide-based Li salts as electrolyte additives for Li-ion batteries

Herein, we report the results of a detailed study on the use of different Li imide salts (LiTFSI, LiFSI, and LiFTFSI) as electrolyte additives for lithium-ion batteries. The introduction of lithium imide salts in the electrolyte is shown to considerably improve the first cycle coulombic efficiency and the long-term cycling stability of graphite/LiFePO4 cells. Using LiTFSI, a capacity fading of only ∼2% occurred over 600 cycles while the control cell with the state-of-the-art additive (VC) lost ∼20% of the initial capacity at 20 °C. The results of the XPS and impedance spectroscopy measurements of graphite electrodes show that, after the formation cycle, the SEI obtained in the presence of imide salts is thinner, contains more LiF and is less resistive than that obtained using VC. Despite the beneficial effect of the imide salts on the lithium-ion cell performance, a slightly reduced thermal stability of the SEI is observed.

Parametric Optimization Study of a Lithium-ion Cell

Lithium-ion cell technology is well known for its high power and energy density in the automotive application. This paper presents development of a 1D electrochemical model which can be used to predict 18650 lithium-ion cell performance under different operating conditions. COMSOL Multiphysics 5.2a software has been utilized to develop the electrochemical model to predict the cell behaviour under various discharge rates. This tool uses the finite element method (FEM) to solve the conservation equations of charge and species in solid and electrolyte phase. And Butler-Volmer equation for reaction rates of lithium insertion and extraction. In an event that the electrochemical parameters of the cell are not known, determination of these parameters by measurements or experiments is a difficult and challenging task. An attempt has been made in this paper to estimate unknown cell parameters by two methods, first by performing a parametric study on cell parameters such as particle radius, diffusion coefficient, porosity etc. within a known range from literature studies and analyse the sensitivity of these parameters on the model results. Secondly, to improve the accuracy of the simulation results, COMSOL optimization module is used and the simulation results are validated against the experimental data. Apart from the discharge profiles, the proposed model can also be used to study the time dependent distribution of lithium-ion concentration, electrolyte concentration, lithium diffusivity and ionic conductivity in the cell.