Loss Of Cyclable Lithium Research Articles

Modeling the Effect of the Loss of Cyclable Lithium on the Performance Degradation of a Lithium-Ion Battery

This paper reports a modeling methodology to predict the effect of the loss of cyclable lithium of a lithium-ion battery (LIB) cell comprised of a LiNi0.6Co0.2Mn0.2O2 cathode, natural graphite anode, and an organic electrolyte on the discharge behavior. A one-dimensional model based on a finite element method is presented to calculate the discharge behaviors of an LIB cell during galvanostatic discharge for various levels of the loss of cyclable lithium. Modeling results for the variation of the cell voltage of the LIB cell are compared with experimental measurements during galvanostatic discharge at various discharge rates for three different levels of the loss of cyclable lithium to validate the model. The calculation results obtained from the model are in good agreement with the experimental measurements. On the basis of the validated modeling approach, the effects of the loss of cyclable lithium on the discharge capacity and available discharge power of the LIB cell are estimated. The modeling results exhibit strong dependencies of the discharge behavior of an LIB cell on the discharge C-rate and the loss of cyclable lithium.

Quantification of aging mechanisms and inhomogeneity in cycled lithium-ion cells by differential voltage analysis

Understanding the aging behavior in lithium-ion cells for automotive applications and its underlying physical-chemical degradation processes that lead to a decrease in energy and power density is crucial for the prediction of battery lifetime. Furthermore, the process of parameterization for physical-chemical motivated models with implemented aging mechanisms is a challenging task. In this work, we present a technique to quantify the loss of cyclable lithium and the decrease of electrode capacities. In addition, the inhomogeneous distribution of lithium inside the cell is estimated by introducing a fit factor to describe the vanishing of peaks in the first derivative of the open circuit voltage curve in respect to state of charge. Differential voltage analysis is performed using an algorithm inspired by pattern search. The algorithm uses the idea of superpositioning multiple open circuit voltage curves with individual distribution of losses. A dataset of ten automotive pouch cells with NMC-622 cathode and graphite anode that were cycled with varying load profiles is used for testing and validation of the method. The method reveals significant differences in the aging behavior and state of homogeneity of cells that cannot be obtained by solely analyzing capacity and resistance evolution.

Nanoscale X-ray imaging of ageing in automotive lithium ion battery cells

Due to an increasing number of electric vehicles, battery pack endurance over the cell lifetime is an important issue of public interest, because it determines to a large degree the value of the vehicle. The exact state of health of a battery is still a major challenge for analysis, especially since the individual ageing factors are not well understood and depend on the cell chemistry and design. In this analysis, X-ray computer tomographic (CT) images of new and aged battery electrodes are compared to determine structural changes over lifetime. The X-ray CT images with sub-micrometer resolution show the internal pore structure of the electrodes and allow the identification of the morphology of the materials. Organic residues and depositions responsible for reduced porosity in aged anodes are quantified and visualized. Furthermore, particle cracking and current collector corrosion is detected on the cathode side. All these are ageing effects, which might influence the cell capacity and impedance. A quantitative analysis of the lithium distribution completes the interpretation of the ageing study confirming that the degradation can be attributed to the loss of cyclable lithium which is bound in the particle surface layer of the anode.

Review of Capacity Fade Models for Lithium-Ion Batteries

Degradation in lithium-ion batteries occur mainly due to the loss of cyclable lithium and loss of active material in the electrodes.1 The lifetime of a battery is limited by the reduction in capacity as the battery is cycled, which can occur through mechanisms such as formation and growth of the solid interface layer (SEI), electrolyte decomposition, dissolution of transition metals at the cathode, lithium deposition at the anode, and porosity changes.2 , 3 While there are many physics-based degradation models that exist in the literature, the ability to accurately predict capacity and power fade over many cycles is limited. This is because not all capacity fade mechanisms are well understood or incorporated in a single model, and a wide range of parameters and different operating conditions make it difficult to arrive at a consistent model.2 , 4 In this study, we look at SEI models in the literature such as Ramadass’ kinetically limited model,5 Ploehn’s diffusion limited model,6 and Safari’s mixed model,7 and apply them to the single particle model (SPM). Mechanisms proposed by Lin8 and Delacourt4 that model side reactions at the cathode will also be explored. We look at the effects of charging/discharging rates and depth of discharge on the degradation during cycling, and the effects of temperature and state of charge on degradation during storage. We will also analyze how predicted degradation changes for different parameter ranges. A rubric of qualitative trends that have been observed experimentally will be used to determine the consistency of existing models. References P. Arora, R. E. White, and M. Doyle, J. Electrochem. Soc., 145, 3647–3667 (1998).V. Ramadesigan, K. Chen, N. A. Burns, V. Boovaragavan, R. D. Braatz, and V. R. Subramanian, J. Electrochem. Soc., 158, A1048–A1054 (2011).X. G. Yang, Y. Leng, G. Zhang, S. Ge, and C. Y. Wang, J. Power Sources, 360, 28–40 (2017).C. Delacourt and M. Safari, J. Electrochem. Soc., 159, A1283–A1291 (2012).P. Ramadass, B. Haran, R. White, and B. N. Popov, J. Power Sources, 123, 230–240 (2003).H. J. Ploehn, P. Ramadass, and R. E. White, J. Electrochem. Soc., 151, A456–A462 (2004).M. Safari, M. Morcrette, A. Teyssot, and C. Delacourt, J. Electrochem. Soc., 156, A145–A153 (2009).X. Lin, J. Park, L. Liu, Y. Lee, A. M. Sastry, and W. Lu, J. Electrochem. Soc., 160, A1701–A1710 (2013).

Low-temperature aging mechanisms of commercial graphite/LiFePO4 cells cycled with a simulated electric vehicle load profile—A post-mortem study

Abstract Reduced cycle life is one of the issues hindering the adoption of large lithium-ion battery systems in cold-climate countries. Thus, the aging mechanisms of commercial graphite/LiFePO4 (lithium iron phosphate) cells at low temperatures (room temperature, 0 °C and−18 °C) are investigated here through an extended post-mortem analysis. The cylindrical 2.3 Ah cells were cycled with a simulated battery electric vehicle load profile, and the aged cells were then disassembled inside an argon-filled glove box. A non-cycled cell was also dismantled as a reference. Half-cell testing was utilized to evaluate the degradation of the electrochemical performance of the electrodes, whereas X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, inductively coupled plasma optical emission spectroscopy and Raman spectroscopy were used to characterize the changes in the materials properties. The full-cell performance loss was mostly seen as capacity fade whereas significant changes in the cell impedance were not observed. Depending on the cycling temperature, loss of cyclable lithium due to solid electrolyte interphase growth and/or lithium plating on the graphite electrode were observed, and they are attributed as the main mechanisms responsible for the capacity loss. Furthermore, increased disordering of the graphite electrode was observed for the cell cycled at −18 °C. The graphite disordering was hypothesized to result from diffusion-induced stress and the mechanical stress caused by severe lithium plating. In contrast, the LiFePO4 electrodes showed only minimal signs of degradation regardless of the cycling temperature.

Assessing the potential of a hybrid battery system to reduce battery aging in an electric vehicle by studying the cycle life of a graphite∣NCA high energy and a LTO∣metal oxide high power battery cell considering realistic test profiles

The utilization of a hybrid battery system (combination of different battery packs via dc/dc converter) in an electric vehicle application is discussed. It is investigated whether battery aging in an electric vehicle can be reduced by using a hybrid battery system. Cycle aging measurements of lithium-ion battery cells were performed at 23 °C against the background of the latter application. Recommendations for hybrid battery system electric vehicle operation are given. For Panasonic NCR 18650 BD cylindrical high energy cells (graphite anode, Li(NiCoAl) O2 cathode), three cycle aging campaigns were conducted systematically, evaluating the impact of charging as well as discharging loads with different time scales and microcycling per driving distance. A significant impact of recuperation pulse duration on aging per driving distance could be observed, whereas varied discharge load characteristics did not vary the aging characteristics. On the basis of differential open circuit voltage analysis, possible degradation mechanisms are discussed. The main driver of capacity loss and resistance increase in cycle aging campaigns with real world driving cycles appears to be the loss of cyclable lithium. Within the operating conditions investigated here, anode aging is intensified with increasing recuperation pulse duration. Another cycle aging campaign with symmetric current rate of 10C for a prismatic high power battery cell (Li4Ti5O12 anode, metal oxide cathode) yielded excellent cycle performance of this cell.

Insights into Cyclable Lithium Loss as a Key Factor in Accelerated Capacity Fade of Lithiated Silicon-Sulfur Full Cells.

To evade the hurdles of dendrite growth and low Coulombic efficiency resulting from lithium metal anodes, integrating lithiated silicon anodes with sulfur cathodes to configure a lithiated silicon-sulfur (Si-S) full cell is a promising strategy to develop high-energy and high-safety rechargeable lithium batteries. Nevertheless, Si-S full cells always suffer accelerated capacity decay, even when excellent electrochemical performance of Li-S and Li-Si half cells is achieved. Herein, we report a comprehensive investigation of the capacity fade mechanism of Si-S full cells. It is revealed that cyclable lithium loss plays a key role in the accelerated capacity fade of Si-S full cells. In addition, cyclable lithium loss in Si-S full cells is mainly divided into irreversible lithium loss by forming inactive lithium compounds due to polysulfide shuttling and other side reactions, and restricted lithium because the Si/C anode cannot be fully delithiated when the Si-S full cell reaches the discharge cutoff voltage. From the 1st cycle to the 100th cycle, the irreversible lithium loss is determined to have increased from 21.7 to 54.5%, whereas the restricted lithium decreased from 24.6 to 16.7%, respectively. The high specific surface area of a Si/C anode leads to a remarkable irreversible lithium loss due to serious polysulfide shuttling in Si-S full cells. This work will help advance the practical application of Li-S batteries.

Aging in 18650-type Li-ion cells examined with neutron diffraction, electrochemical analysis and physico-chemical modeling

Aging in NMC/C cells (NMC-Liy(Ni0.33Mn0.33Co0.33)O2), cycled 1000 times at a 1C rate, has been characterized by in situ neutron diffraction and electrochemical analysis. These experimental results have been validated by a physico-chemical aging model, which attributes capacity fade to growth of a continuous SEI film on the anode. Neutron diffraction of the cells indicate a cyclable lithium loss corresponding to a capacity fade of about 23% in both electrodes of the cycled cell. The cycled cell suffers an anode stoichiometry shift from x = 0.84 to x = 0.65 in LixC6 (0 ⩽ x ⩽ 1) in its fully charged state and a cathode stoichiometry shift from y = 0.89 to y = 0.81 in Liy(Ni0.33Mn0.33Co0.33)O2 (0 < y ⩽ 1.05) in its fully discharged state. Anode (x = 0) as well as cathode stoichiometries (y = 0.54) remain practically unchanged in the cell's fully discharged and charged states, respectively. These stoichiometry shifts match well with those derived from the model, and both neutron diffraction and model are in good agreement to the electrically determined capacity fade of 21%. In fact, cyclable lithium losses slightly exceed this value. Thus, capacity fade in these cells is mainly due to loss of cyclable lithium into the continuous growth of a SEI film on the anode surface.

Fast-charging to a partial state of charge in lithium-ion batteries: A comparative ageing study

At electric vehicle fast-charging stations, it is generally recommended to avoid charging beyond ∼80% State-of-Charge (SOC) since topping-off to full capacity disproportionately increases the charging time. This necessitates studying its long-term impact compared to slower rate charging to full capacity typical of home or residential charging. Here we present the long-term ageing effects on commercial 18650 NMC-LMO/graphite cell cycled between 2.6–4.2V at three different charging protocols: 1.5 C-rate fast-partial charging (to 82.5% SOC), 0.5 C-rate slow standard charging without or with a constant-voltage step (to 93% or 100% SOC). Quantitative discharge-curve and postmortem analyses are used to evaluate ageing. The results show that ageing rate increases in the order: fast-partial charging < standard charging < standard charging with constant-voltage period, indicating that higher SOC-range near full capacity is more detrimental to battery life than fast-charging. The capacity fade is totally dominated by cyclable-lithium loss. The ∼8% NMC-LMO active material loss has negligible impact on the cell capacity fade due to the electrodes excess material in the fresh cell and its moderate loss rate with ageing compared to the cyclable-lithium. Similar ageing modes in terms of capacity fade and impedance rise are found irrespective of the charging protocol.

Calendar Aging and Gas Generation in Commercial Graphite/NMC-LMO Lithium-Ion Pouch Cell

The calendar life of a commercial graphite/NMC-LMO lithium-ion battery is assessed under various storage temperature (35°C and 60°C) and state-of-charge (0% and 100%) conditions. Virtually no degradation is observed for cells stored at 0% SOC as expected, whereas those aged for 9 months in a fully charged state at 35°C and 60°C lose ∼10% and ∼43% of their capacity, respectively. Differential-voltage analysis of periodic cell cycling data and post-mortem examination of the aged electrodes are used to identify degradation modes. Three main sources of high-temperature capacity loss are identified: i) electrode dry-out due to gas formation (∼30%), ii) loss of cyclable lithium (∼10%) and iii) cathode active material loss (∼3%). Cells stored at a lower temperature of 35°C experience little gas generation and degrade primarily through the loss of Li inventory. The analysis shows that similar amounts of cyclable Li are consumed by side reactions during calendar aging at both storage temperatures of 35°C and 60°C. Non-destructive compression (1.0–5.0 psi) of the aged pouch cells during discharge is shown to improve their capacity by ∼15%. This effect is attributed to the redistribution of gas bubbles inside the pouch cell by the applied pressure.

Calendar Aging of NCA Lithium-Ion Batteries Investigated by Differential Voltage Analysis and Coulomb Tracking

Calendar aging of lithium-ion cells is investigated by storing commercial 18650 cells with NCA cathode and graphite anode at different states of charge and temperatures. The resulting capacity fades are analyzed by differential voltage analysis (DVA) and coulometry. DVA reveals that the capacity fade results mainly from a shift in the electrode balancing due to a reduced inventory of cyclable lithium. Moreover, DVA confirms that the capacity fade strongly correlates with the anode potential. The observed loss of cyclable lithium is further analyzed by coulomb tracking, which stands for creating a continuous ampere-hour balance from all individual measurements performed with an examined cell and tracking the slippage of charging and discharging endpoints. It reveals the extent of anodic and cathodic side reactions during the storage periods and their effect on the inventory of cyclable lithium. Anodic side reactions, which are related to electrolyte reduction and passivation layer growth, can be identified as the main driver of capacity fade. Coulomb tracking also discloses that increasing cathodic side reactions can reduce the irreversible capacity fade, particularly for storage at very high SoC, which is likely to be misinterpreted as decelerated aging reactions. Evaluating also the reversible capacity fade prevents such a misconception.

Calendar and Cycle Aging of Commercial NCA Lithium-Ion Cells

In extensive experimental aging studies, focusing on the application of lithium-ion cells in electric vehicles, we have investigated the calendar and cycle aging of commercial 18650 lithium-ion cells with graphite anode and NCA cathode for almost three years. Calendar aging was examined by storing the cells at various states-of-charge (SoCs) and temperatures. Cycle aging was investigated for different charging and discharging loads at various temperatures, SoCs and cycle depths. For example, the impact of regenerative braking was examined by applying dynamic highway driving load profiles with different maximum recharging currents during braking periods [1]. To identify the dominant aging mechanisms for the different load conditions, the following non-invasive, non-destructive techniques were used:• Constant-current-constant-voltage capacity measurements• Differential voltage analysis (DVA) [2]• Coulometry [3,4]• DC pulse resistance measurements• Electrochemical impedance spectroscopy (EIS) Results for calendar aging In the storage tests, the capacity measurements exhibit SoC regions in which the capacity fade is similar. Hence, reducing the SoC does not reduce calendar aging in these regions. DVA discloses a strong correlation between the plateaus in the capacity fade and the plateaus in the anode potential. Together with the results from coulometry, anode side reactions are identified as the main driver of capacity fade from calendar aging. They cause an irreversible loss of cyclable lithium, which results in a shift in the electrode balancing. This shift also affects the pulse resistances at low SoC. Moreover, coulometry enables to analyze the reversible self-discharge for storage above 80% SoC. Results for cycle aging Larger cycle depths increase the capacity fade from cycle aging. EIS and pulse measurements demonstrate that the resistance increase also aggravates with larger cycle depths. Especially, deep discharging to 0% SoC massively increases the cell impedance. The results from EIS demonstrate that mainly the cathode resistances increase. Whereas calendar aging increases with temperature, the additional degradation from cycle aging decreases with higher temperature. For charging and discharging at different temperatures (0°C and 25°C), Figure 1 shows a strong degradation for the cells discharged with a dynamic highway driving profile at 0°C. The spectra from DVA reveal that the storage capabilities of the anode have remained largely intact. This demonstrates that discharging at low temperatures does not degrade the anode. A certain cathode degradation can be identified by EIS, but the predominant part of the capacity fade origins from a loss of cyclable lithium. From the aging mechanisms identified for calendar and cycle aging, strategies for optimal battery operation are derived.

(Invited) The Use of Modeling to Reveal Root-Cause and Quantify Degradation Effects in Electrochemical Systems

Electrochemical systems are characterized by coupled, non-linear behavior of several phenomena. Typically, thermodynamics, kinetics, and mass-transfer all come in to play in describing the current voltage response of a system. This strong coupling can make interpretation of experimental results confounding and cause our intuition to fail us. Through, mathematical modeling, the nature of the response is more easily revealed. This concept is explored through two real-world applications of electrochemical systems: start-stop phenomena in fuel cells and power fade in lithium-ion batteries. Corrosion of carbon support during start-up and shut-down of proton-exchange-membrane fuel cells is well-known and widely reported in the literature. Today, this is an accepted degradation mechanism. However, the journey from severe performance loss observed in a fuel-cell vehicle to our present understanding was difficult. In fact, measurements of cell potential and current did not help elucidate the mechanism at all. It was only through mathematical modeling that the corrosion process was clarified. No new physics was needed, just the careful assembly of existing knowledge of kinetics and transport for a start-up transient. Capacity fade in lithium-ion batteries is also a well-known issue. Generally, the formation and slow growth of the solid electrolyte interphase (SEI) results in irreversible consumption of lithium. This loss of cyclable lithium translates into a reduced capacity of the battery with age. In addition to capacity fade, power fade is critical for many applications. A prime example is the battery for a charge-sustaining hybrid-electric vehicle. Here, the energy storage system is used for recovering energy and power assist, not sustained propulsion of the vehicle. Therefore, power capability is at least if not more important than capacity. The power capability is characterized by “cell resistance.” As the SEI thickness grows, the ohmic resistance of the cell increases. However, the magnitude of the increase cannot explain the power fade. Typically, for a pulse of current of ten or so seconds is used in practice to measure “cell resistance.” For transients of this length, this quantity is not simply the ohmic resistance of the cell--concentration polarizations are significant. This behavior is explored with well-established mathematical models of lithium-ion cells. Again, a mathematical model that uses existing physics is able to shed light on the nature of the power fade.

Calendar Aging of Lithium-Ion Batteries

In this study, the calendar aging of lithium-ion batteries is investigated at different temperatures for 16 states of charge (SoCs) from 0 to 100%. Three types of 18650 lithium-ion cells, containing different cathode materials, have been examined. Our study demonstrates that calendar aging does not increase steadily with the SoC. Instead, plateau regions, covering SoC intervals of more than 20%–30% of the cell capacity, are observed wherein the capacity fade is similar. Differential voltage analyses confirm that the capacity fade is mainly caused by a shift in the electrode balancing. Furthermore, our study reveals the high impact of the graphite electrode on calendar aging. Lower anode potentials, which aggravate electrolyte reduction and thus promote solid electrolyte interphase growth, have been identified as the main driver of capacity fade during storage. In the high SoC regime where the graphite anode is lithiated more than 50%, the low anode potential accelerates the loss of cyclable lithium, which in turn distorts the electrode balancing. Aging mechanisms induced by high cell potential, such as electrolyte oxidation or transition-metal dissolution, seem to play only a minor role. To maximize battery life, high storage SoCs corresponding to low anode potential should be avoided.

Uneven Aging in Graphite Electrodes of Commercial Depht Cycled Li-Ion Battery Cells

In this paper we discuss some newly reported results1-2 on uneven aging within the graphite porous electrode of a cycled commercial cylindrical LiFePO4/graphite cell. Graphite is commonly used as the negative electrode material in Li-ion battery cells intended for both consumer electronic products and electrified vehicles, even though it is known that aging of this type of cells can cause capacity fade from lithium consuming side reactions on the graphite surface due to reductive decomposition of the organic electrolyte at low potentials.3 The side reactions rates decrease during the first formation cycles as the reaction products form a passive surface film, a so called solid electrolyte interphase (SEI), which in an ideal case would prevent further electrolyte reduction. However, gradual capacity fade related to loss of cyclable lithium during battery cycling is frequently reported in literature, showing that continuous film growth occurs during cycling. On the scale of porous electrodes, the SEI is often considered a rather evenly formed film on all graphite particles and of nanometer scale thickness. Recently, however, we reported on anomalous film growth on graphite electrode samples obtained from the inner parts of a commercial cylindrical LiFePO4/graphite cell that had been subjected to 3.75 C-rate constant-current-, wide state-of-charge (SOC)-cycling during approximately 2.5 months.1 In order to evaluate the distribution of the apparently thick film across depth of the electrode, surface and cross-section analyses of morphological changes and elemental compositions were performed by means of scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) on manually ripped as well as on a polished, focused ion beam (FIB) milled cross sections. In addition, ion depth profiling using time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to analyze the chemical modifications on the electrode induced by cycling compared to a calendar-aged electrode. Finally, to identify the modified specific electrode properties, such as local resistances, porosity, or available surface area and the type of limitation on the electrode performance caused by the film growth, the electrochemical data was evaluated using physics-based impedance modeling. The results show that the film of several micrometer thickness, is concentrated on top of the porous electrode and within the electrode regions close to the electrode / separator interface. The film is enriched in P-O and carbonate species but is otherwise similar in composition to the thin SEI formed on a calendar-aged electrode. FIB-SEM shows that the film is clogging the pores in the graphite electrode. A local porosity decrease, impeding the effective electrolyte transport in the electrode was also supported by performance evaluation using physics-based EIS modeling. The local variation of electrode properties implies that current distribution in the porous electrode under these cycling conditions causes inefficient material utilization and sustained uneven electrode degradation. The study of realistic systems, regarding cell design (larger cylindrical cells and porous electrodes) and cycling conditions, is important when targeting aging phenomena in specific applications. Detailed physical and electrochemical studies of aging phenomena in these cells can highlight the relevance of different degradation mechanisms observed in laboratory model systems.

Aging Analysis of LiNi1/3Mn1/3Co1/3O2-Graphite Cells Via X-Ray Diffraction

Although lithium ion batteries (LiBs) are being used successfully in portable electronics, automobiles, and stationary power storages, safety and degradation issues are still impeding their long term application. The main LiB aging mechanisms that have been reported include structural changes of the cathode, loss of active material, loss of cycleable lithium due to SEI growth, and impedance rise of the cell [1]. A study by Dubarry et al. and Burns et al. on LiNi1/3Mn1/3Co1/3O2 (NMC) based cells showed that during early cycles, the loss of lithium inventory is the main cause of capacity fade and that it follows a linear relationship. A rapid capacity roll-over failure, however, was attributed to deterioration of the interfacial kinetics [2,3]. Further studies on the graphite/NMC system showed that also transition metal dissolution has to be considered when cycling to high charging potential limits [4] or at elevated temperatures [5]. In both cases, the capacity loss which can be contributed to the loss of cyclable lithium was not quantified. In this work, we focus on the performance degradation of graphite/NMC cells charged and discharged for 300 cycles at different temperatures (25 °C and 60 °C) and with different charging voltage limits (4.2 V, 4.6 V). Cycling data clearly suggest that two degradation mechanisms can be distinguished: (i) a linear capacity fade at a 4.2 V limit (at both 25 and 60 °C) and, (ii) a rapid capacity roll-over failure at the more positive 4.6 V limit. To analyze the major contribution to cell failure, different diagnostic techniques comprising structural investigations via powder X-ray diffraction (XRD) and neutron based elemental analysis via prompt gamma activation analysis (PGAA) to determine transition metal deposition on graphite are applied. As NMC shows no severe structural changes like phase transition after cycling (ex-situ XRD), the material exhibits good crystal structure stability at the different tested operating conditions, which is consistent with Zheng et al. [4]. Slight differences observed in peak shifts are directly associated with changes in the lattice constants a and c. Figure 1 shows an in-situ XRD analysis of the Li/NMC system. XRD pattern collection during delithiation and lithiation of NMC demonstrates a clear correlation between the lattice ratio c/a and the corresponding lithium content of the cathode. We will show that this correlation allows for the quantification of the loss of cyclable lithium via post-mortem ex-situ XRD measurements. In addition, Fig. 1 also shows that NMC exhibits an irreversible capacity loss (ICL) in the initial cycle which goes along with structural irreversibility. As this is important for the in-situ XRD calibration, this issue will be discussed in more detail. In conclusion, loss of lithium inventory is most detrimental for NMC/graphite cells cycled at different temperatures, whereas transition metal dissolution and active material inactivation is most severe when cycling with a charge limit of 4.6 V. Figure 1: In-situ XRD calibration: Potential profile of Li/NMC (top) and the corresponding ratio of the lattice parameters a and c (bottom) determined by in-situ XRD during OCV steps (indicated by spikes) as a linear function of lithium content.

Simulations with an Accurately Calibrated Electrochemical Model of Li-Ion Battery to Determine Additional Aging Mechanisms

The growing number of electrified vehicles remains particularly constrained by the performance of the energy storage system, which is a complex and expensive component in the vehicle. Li-ion batteries seem to be the best candidates due to their suitable performances for plug in or full electric vehicle application. However, the aging phenomena in those batteries will have a direct impact on the autonomy, power and maintenance costs of the vehicle and are not yet clearly understood. For automotive industry, this kind of information is critical in determining the performance and durability of the battery in traction applications. For that reason, there is an urgent need of validated predictive models for the long term behavior of those electrochemical components, ensuring an optimal battery sizing on one hand, and a proper energy management onboard on the other hand. The aging of a Li-ion battery is mainly due to a modification of its electrochemical properties which will lead to a loss of capacity (decrease of the autonomy) and an increase of its internal impedance (loss of power). A simplified electrochemical model has already been developed at IFP Energies Nouvelles[1] taking into account the loss of cyclable lithium due to the growth of the Solid electrolyte Interphase (SEI) and the modification of the porosity of the graphite electrode as aging mechanisms. This model allows us to estimate the evolution of the performance (power and autonomy) of a Graphite/LiFePO4-based battery as if it was used on a typical hybrid or electric vehicle. In order to ensure the reliability of this model, a sensitivity analysis based on Zhang et al. work[2] has been performed to find out the impact of the variation of each parameters on the model output (Voltage, Temperature) and determine the optimal testing conditions for this model calibration. Moreover, the discrepancies observed during the validation of our properly calibrated model with experimental data highlighted an additional capacity loss due to another aging mechanism than SEI growth. According to our previous research and to the literature, we decided to add a mechanical stress based mechanism involving cracking into the electrode that leads to a loss of active materials or enhances the loss of lithium inventory. Therefore, specific experiments inducing stress during intercalation of lithium ions in the negative electrode have been carried out to gather data to identify parameters of the new mechanism added to our model. Afterwards, the behavior of this new aging model will be validated on dynamic PHEV duty profiles to ensure its good reliability.

Electrochemical Model-Based State of Charge and Capacity Estimation for a Composite Electrode Lithium-Ion Battery

Increased demand for hybrid and electric vehicles has motivated research to improve onboard state of charge (SOC) and state of health estimation (SOH). In particular, batteries with composite electrodes have become popular for automotive applications due to their ability to balance energy density, power density, and cost by adjusting the amount of each material within the electrode. SOH algorithms that do not use electrochemical-based models may have more difficulty maintaining an accurate battery model as the cell ages under varying degradation modes, such as lithium consumption at the solid-electrolyte interface or active material dissolution. Furthermore, efforts to validate electrochemical model-based state estimation algorithms with experimental aging data are limited, particularly for composite electrode cells. In this paper, we first present a reduced-order electrochemical model for a composite LiMn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -LiNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/3</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/3</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> electrode battery that predicts the surface and bulk lithium concentration of each material in the composite electrode, as well as the current split between each material. The model is then used in dual-nonlinear observers to estimate the cell SOC and loss of cyclable lithium over time. Three different observer types are compared: 1) the extended Kalman filter; 2) fixed interval Kalman smoother; and 3) particle filter. Finally, an experimental aging campaign is used to compare the estimated capacities for five different cells with the measured capacities over time.

Examination of Graphite Particle Cracking as a Failure Mode in Lithium-Ion Batteries: A Model-Experimental Study

Capacity fade in lithium-ion batteries remains an area of active research, with failure of the graphite anode thought to be an important contributor. While the formation of the solid electrolyte interphase and the subsequent loss of cyclable lithium have been well studied, mechanical degradation remains an area where ambiguity remains. While there appears to be little experimental evidence that suggest that macroscopic particle cracking occurs, mathematical models have suggested that this phenomenon is likely. The goal of this paper is to clarify this ambiguity by combining experimental cycling, mathematical stress modeling, and post-mortem microscopy. We experimentally determine an average diffusion coefficient of lithium in graphite using a thin-layer electrode and use this information in a diffusion-induced stress model. Our results suggest that cracking is not likely during lithiation due to the proximity of equilibrium potential to the cutoff potential. On delithiation, at 25°C, even at 30 C rate cracking is unlikely while at −10°C, a rate of 10 C can lead to particle cracking. By extrapolating the results of the thin-layer electrode to a thick porous electrode, we found that graphite cracking is unlikely to occur during typical vehicle operations.

Lithium plating in a commercial lithium-ion battery – A low-temperature aging study

The formation of metallic lithium on the negative graphite electrode in a lithium-ion (Li-ion) battery, also known as lithium plating, leads to severe performance degradation and may also affect the cell safety.This study is focused on the nondestructive characterization of the aging behavior during long-term cycling at plating conditions, i.e. low temperature and high charge rate. A commercial graphite/LiFePO4 Li-ion battery is investigated in order to elucidate the aging effects of lithium plating for real-world purposes. It is shown that lithium plating can be observed as a loss of cyclable lithium which affects the capacity balance of the electrodes. In this way, lithium plating counteracts its own occurrence during prolonged cycling. The capacity losses due to lithium plating are therefore decreasing at higher cycle numbers and the capacity retention curve exhibits an inflection point. It is further shown that the observed capacity fade is partly reversible. Electrochemical impedance spectroscopy (EIS) reveals a significant increase of the ohmic cell resistance due to electrolyte consumption during surface film formation on the plated lithium. Additional cell opening provides important quantitative information regarding the thickness of the lithium layer and the corresponding mass of the plated lithium.