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Numerical Analysis of Distribution and Evolution of Reaction Current Density in Discharge Process of Lithium-Ion Power Battery

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Abstract

The reaction current density is an important process parameter of lithium-ion battery, significantly influencing its electrochemical performance. In this study, aimed at the discharge process of lithium-ion power battery, an electrochemical-thermal model was established to analyze the distribution of the reaction current density at various parts of the cathode and its evolution with the time of discharge, and to probe into the causes of distribution and evolution. The investigation revealed that the electrochemical-thermal coupled model showed more accurate compared to the single electrochemical model, which was more obvious in high rate discharge. The results demonstrated that the conductivity of solid and liquid-phases was an important factor affecting the distribution of the reaction current density. Moreover, the uniformity of the distribution of the current density was related to the rate of utilization of the active materials in the electrodes. By optimizing the porosity and thickness of the electrode, not only the distribution of the current density was improved, but also the rate of utilization of the active materials in the electrodes and the energy density of batteries were significantly enhanced.

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Dissolution of active material is one of the primary reasons for capacity fade in lithium-ion batteries, particularly at elevated temperatures. The effects of the volume fraction changes due to dissolution in both the active and inert material phases in composite Li-ion electrodes are investigated by a thermal-electrochemical coupled model. The study reveals that the changes in effective transport properties result in a reduction in the electrochemical reaction rate and an increase in the cell resistance, reducing capacity. The simulation results are also used to map the nature of the effects of dissolution of the active particles on the capacity decrease during cycling with different conditions, including temperature and voltage range. (c) 2010 The Electrochemical Society. [DOI: 10.1149/1.3516619] All rights reserved.
Article
An isothermal model for the electrochemical behavior of a commercial graphite/LiFePO4 cell at 25 and 45 degrees C is developed. Although it does not embed any special feature of the porous electrodes and of the two-phase process of the LiFePO4 electrode, proper account of the experimental charge/discharge (from C/10 to 1C) and path-dependence effects of the commercial cell is achieved. The LiFePO4 electrode is treated based on a resistive-reactant concept with multiple particles whereas a single-particle approach is used to model the graphite electrode. In order to refine the model parameters for each electrode, half cells are made either from the recovered LiFePO4 or graphite electrodes vs. a Li counterelectrode. A detailed experiment/simulation analysis of half-cell and complete-cell data unfolds the impact of uniaxial pressure on the galvanostatic charge/discharge limitation and path dependence of the LiFePO4 electrode in the coin cell and the commercial cell.
Article
An electro-thermal cycle life model is developed by incorporating the dominant capacity fading mechanism to account for the capacity fading effect on the lithium ion battery performance. This model is comprehensively validated in three different aspects: (1) Electrochemical performance with different discharge C_rates (C/10, 1C, 3C) and different ambient temperature (0 °C, 25 °C, 45 °C, 60 °C); (2) battery surface temperature and center temperature during 6 C_rate constant current charge–constant voltage charge–constant current discharge (CC–CV–CC); (3) the amount of capacity fade indicated by the discharge potential curves of different cycle conditions. The feasibility of using the dominant factor only to represent the overall capacity fading in model is verified, and the model is used to predict OCV–SOC curves of cycled battery. To recover the capacity fade of cycled battery, either charging the negative electrode of fully discharged battery or charging the positive electrode of fully charged battery, could be an effective method, and possible capacity recovering design is presented. The decreasing trend of thermodynamic parameter Ea with increasing C_rate in the capacity fading function is deduced with increasing battery temperature when increasing discharge C_rate.
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Local current density is an important parameter in battery modeling, which affects the performance of lithium-ion batteries. In this study, we take LiFePO4 cathode material as an example. A simplified mathematical model has been developed to study the internal mechanism of the electrode. According to the results of the model, the local current density distribution has a regular change at different time in the discharge process. The parameter “critical thickness” as an optimized variable has been presented for battery design. By qualitative analysis to estimate the critical thickness under different condition, we can optimize the design parameter of a battery according to the practical demand.
Article
Olivine-structured LiFePO4 has been the focus of research in developing low cost, high performance cathode materials for lithium ion batteries. Various processes have been developed to synthesize LiFePO4 or C/LiFePO4 (carbon coating on LiFePO4), and some of them are being used to mass produce C/LiFePO4 at the commercial or pilot scale. Due to the low intrinsic electronic and ionic conductivities of LiFePO4, the decrease of particle size and the nano-layer of carbon coating on LiFePO4 particle surfaces are necessary to achieve a high electrochemical performance. Significant progress has been made in understanding and controlling phase purity, particle size and carbon coating of the C/LiFePO4 composite material in the past. However, there are still many challenges in achieving a high quality product with high consistency. In this review, we summarize some of the recent progress and advances based on selected reports from peer-reviewed journal publications. Several typical synthesis methods and the effect of carbon coating quality on the properties of C/LiFePO4 composite are reviewed. An insight into the future research and further development of C/LiFePO4 composite is also discussed.
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Research in the area of advanced batteries for electric-vehicle applications has increased steadily since the 1990 zero-emission-vehicle mandate of the California Air Resources Board. Due to their design flexibility and potentially high energy and power densities, lithium/polymer batteries are an emerging technology for electric-vehicle applications. Thermal modeling of lithium/polymer batteries is particularly important because the transport properties of the system depend exponentially on temperature. Two models have been presented for assessment of the thermal behavior of lithium/polymer batteries. The one-cell model predicts the cell potential, the concentration profiles, and the heat-generation rate during discharge. The cell-stack model predicts temperature profiles and heat transfer limitations of the battery. Due to the variation of ionic conductivity and salt diffusion coefficient with temperature, the performance of the lithium/polymer battery is greatly affected by temperature. Because of this variation, it is important to optimize the cell operating temperature and design a thermal management system for the battery. Since the thermal conductivity of the polymer electrolyte is very low, heat is not easily conducted in the direction perpendicular to cell layers. Temperature profiles in the cells are not as significant as expected because heat-generation rates in warmer areas of the cell stack are lower than heat-generation rates in cooler areas of the stack. This nonuniform heat-generation rate flattens the temperature profile. Temperature profiles as calculated by this model are not as steep as those calculated by previous models that assume a uniform heat-generation rate.
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The existing lithium ion battery model in Multiphysics (MP) software (COMSOL Inc., Palo Alto, CA) is extended to include the thermal effects. The thermal behavior of a lithium ion battery is studied during the galvanostatic discharge process with and without a pulse.
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A general form of the thermal energy equation for a battery system is derived based on first principles using the volume-averaging technique. A thermal-electrochemical coupled modeling approach is presented to simultaneously predict battery electrochemical and thermal behaviors. This approach couples the thermal energy equation with the previous multiphase micro-macroscopic electrochemical model via the heat generation and temperature-dependent physicochemical properties. The thermal-electrochemical model is multidimensional and capable of predicting the average cell temperature as well as the temperature distribution inside a cell. Numerical simulations are performed on a Ni-MH battery to demonstrate the significance of thermal-electrochemical coupling and to investigate the effects of thermal environment on battery electrochemical and thermal behaviors under various charging conditions.
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A general energy balance has been developed for insertion battery systems by using enthalpy potentials. This leads to a new calculation method for the heat-generation rate. The same result is also derived from an alternative model based on local heat generation in an electrochemical cell. A new concept, the effective open-circuit potential of an insertion battery was proposed to characterize the open-circuit state of the battery during galvanostatic discharge. Simulation results are presented for heat generation in a lithium cell under galvanostatic discharge. The analysis of these results focuses on effects of the shape of the open-circuit potential and ohmic losses in the electrolyte in the porous cathode. It is shown that a single reaction may look like two reactions due to the presence of two plateaus in the open-circuit potential.
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The galvanostatic charge and discharge of a lithium anode/solid polymer separator/insertion cathode cell is modeled using concentrated solution theory. The model is general enough to include a wide range of polymeric separator materials, lithium salts, and composite insertion cathodes. Insertion of lithium into the active cathode material is simulated using superposition, thus greatly simplifying the numerical calculations. Variable physical properties are permitted in the model. The results of a simulation of the charge/discharge behavior of the Li/PEO[sub 8]-LiCF[sub 3]SO[sub 3]/TiS[sub 2] system are presented. Criteria are established to assess the importance of diffusion in the solid matrix and transport in the electrolyte. Consideration is also given to various procedures for optimization of the utilization of active cathode material.
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The addition of double-layer capacitance into a mathematical model of a dual lithium ion insertion cell is investigated. Double-layer capacitance is introduced in both the negative electrode and the positive electrode. For the purposes of this paper, a LiâCâ{vert{underscore}bar}propylene carbonate + 1 M LiClOâ{vert{underscore}bar}Li{sub y}MnâOâ cell is used for the collection of simulation results. Simulation results on the effect of short-time pulses passed through the cell are presented. Differences in the transient potential response in the negative and positive electrodes at short times are made between an electrode with a resistive film and one without and between situations with small and large rates of change of the open-circuit potential with state of charge. A simpler resistive-capacitive model is developed which clarifies many features of the modified behavior due to the capacitance but without simultaneously dealing with the discharge of the active material and concentration gradients.
Article
The galvanostatic charge and discharge of a dual lithium ion insertion (rocking-chair) cell are modeled. Transport in the electrolyte is described with concentrated solution theory. Insertion of lithium into and out of the active electrode material is simulated using superposition, greatly simplifying the numerical calculations. Simulations results are presented for the LiâCâ{vert_bar}propylene carbonate + 1M LiClOâ{vert_bar}Li{sub y}MnâOâ cell, and these results are compared with experimental data from the literature. Criteria are established to assess the importance of diffusion in the solid matrix and of transport in the electrolyte solution. Various procedures to optimize the utilization of active material are considered. Simulation results for the dual lithium ion insertion cell are compared with those for a cell with a solid lithium negative electrode.
Article
Based on the resistive-reactant concept, a simple mathematical model for lithium intercalation/deintercalation in a lithium iron phosphate electrode is developed. Demonstrative experiments are provided to shed light on the resistive-reactant feature of this electrode. Without embedding any special feature of the two-phase process, the model consists of regular concentration-dependent lithium diffusion inside four groups of active-material particles that have different connectivities to the conductive matrix of the electrode. Model-experiment comparisons reveal the effectiveness of the resistive-reactant concept for a quantitative description of the charge/discharge as well as the path dependence observed in lithium iron phosphate electrodes.
Article
This paper develops a mathematical model for lithium intercalation and phase change in an iron phosphate-based lithium-ion cell in order to understand the cause for the low power capability of the material. The juxtaposition of the two phases is assumed to be in the form of a shrinking core, where a shell of one phase covers a core of the second phase. Diffusion of lithium through the shell and the movement of the phase interface are described and incorporated into a porous electrode model consisting of two different particle sizes. Open-circuit measurements are used to estimate the composition ranges of the single-phase region. Model-experimental comparisons under constant current show that ohmic drops in the matrix phase, contact resistances between the current collector and the porous matrix, and transport limitations in the iron phosphate particle limit the power capability of the cells. Various design options, consisting of decreasing the ohmic drops, using smaller particles, and substituting the liquid electrolyte by a gel are explored, and their relative importance discussed. The model developed in this paper can be used as a means of optimizing the cell design to suit a particular application. © 2004 The Electrochemical Society. All rights reserved.
Article
The paper presents a multidimensional modeling framework for simulating coupled thermal and electrochemical phenomena in lithium-ion batteries.Understanding the distribution of current density, potential, and temperature is critical for designing lithium-ion batteries for improved safety and durability, as well as for conducting effective design optimization studies. We have developed a model that retains the details of diffusion, migration, and charge-transfer kinetics in the various phases in a lithium-ion battery, while introducing computational techniques that allow us to efficiently calculate the transient current and temperature distributions in a three-dimensional stack. Several modeling example results are presented showing the influence of cooling system design and thermal effects on current distribution within a cell and between cells in a stack.
Article
A 1D electrochemical, lumped thermal model is used to explore pulse power limitations and thermal behavior of a 6 Ah, 72 cell, 276 V nominal Li-ion hybrid-electric vehicle (HEV) battery pack. Depleted/saturated active material Li surface concentrations in the negative/positive electrodes consistently cause end of high-rate (∼25 C) pulse discharge at the 2.7 V cell−1 minimum limit, indicating solid-state diffusion is the limiting mechanism. The 3.9 V cell−1 maximum limit, meant to protect the negative electrode from lithium deposition side reaction during charge, is overly conservative for high-rate (∼15 C) pulse charges initiated from states-of-charge (SOCs) less than 100%. Two-second maximum pulse charge rate from the 50% SOC initial condition can be increased by as much as 50% without risk of lithium deposition. Controlled to minimum/maximum voltage limits, the pack meets partnership for next generation vehicles (PNGV) power assist mode pulse power goals (at operating temperatures >16 °C), but falls short of the available energy goal.In a vehicle simulation, the pack generates heat at a 320 W rate on a US06 driving cycle at 25 °C, with more heat generated at lower temperatures. Less aggressive FUDS and HWFET cycles generate 6–12 times less heat. Contact resistance ohmic heating dominates all other mechanisms, followed by electrolyte phase ohmic heating. Reaction and electronic phase ohmic heats are negligible. A convective heat transfer coefficient of h = 10.1 W m−2 K−1 maintains cell temperature at or below the 52 °C PNGV operating limit under aggressive US06 driving.