Low C-rates Research Articles

Effect of Temperature on Intermediate Phases of Na3V2(PO4)3 during Cycling by Operando X-ray Diffraction.

Na3V2(PO4)3 (NVP) has gained a lot of attention due to its remarkable properties, such as its robust crystal structure, cycle life, rate capabilities, and so on. Nevertheless, NVP undergoes a substantial decrease in its rate capability at low temperatures, which limits its practical applications. In this study, the performance of NVP at low, room, and high temperatures during cycling is thoroughly investigated using synchrotron operando X-ray diffraction. The (de)insertion of two sodium ions from Na3V2(PO4)3 to Na1V2(PO4)3 appeared to occur via two intermediate phases (Na2V2(PO4)3 and Na1.64V2(PO4)3). The Na1.64V2(PO4)3 phase which is observed for the first-time during operando XRD measurements of NVP, exhibited limited stability at high temperatures. The increase in the quantity of these intermediate phases from high to low temperatures, especially at high C-rates, could be anticipated to be one of the contributing factors of poor rate capabilities of NVP at low temperatures. This study encourages the exploration of suitable strategies to enhance the performance of NVP at low temperatures and high C-rates.

A Comparison of Standard SEI Growth Models in the Context of Battery Formation

Growth of the Solid Electrolyte Interphase (SEI) layer on negative electrode particles during the formation cycle is one of the most complex and least understood steps of lithium-ion battery manufacturing. This initial SEI formation significantly impacts battery performance, lifetime, and degradation. Zero-dimensional models, which reduce the complexity of SEI’s morphology, material, and structure, are commonly used to study long-term SEI growth rates and capacity fade. These models are derived based on limiting mechanisms. We aim to compare the most common SEI growth models, focusing on the first few cycles at low C-rates representing formation protocols. Using consistent parameters across models, we seek to understand if they can capture the dynamics of SEI formation. We conducted qualitative comparisons with experimental measurements of Coulombic efficiency in 2032-type coin cells at low C-rate. Our analysis shows that the models predict SEI growth in the first cycle to be higher than in subsequent cycles. However, the difference between cycles in these models is insufficient to explain the experimental results, which indicate a capacity fade during the first cycle that is two orders of magnitude higher than in later cycles. This suggests new models are needed to accurately describe the physics of the formation cycle.

Methods for faster estimation of the entropy profile of a lithium-ion battery: A comparison of accelerated potentiometry and the estimation of entropy through thermal signatures

The operating temperature of a lithium-ion battery (LIB) has strong effects on its electrochemical performance, safety, and lifetime. Reliable predictions of cell temperature and heat generation rate are required for the design and thermal management of battery systems. The entropy variation of a LIB has a significant influence on the heat generation and the cell temperature variation, especially at a lower C-rate. Since the entropy is a function of State-of-Charge (SoC), and may change during ageing, the entropy profile may as well be used as an indicator for battery State-of-Health diagnostics. The present experimental methods, such as potentiometric or calorimetric methods, for determining the entropy variation throughout a battery's full SoC window are time-consuming and require specialized equipment with good thermal insulation and instrumentation. Hence, faster and more convenient methods are needed. In this paper, we compare an accelerated potentiometric method with a new entropy characterization method where the entropy profile is extracted from the thermal signature of a LIB during charge and discharge at low C-rates. The accelerated potentiometric method aims to obtain the entropy from thermal cycling while the cell is still not completely relaxed to its stable OCV. The thermal signature method monitors the variations of a cell's outer surface temperature during a low constant current charge or discharge, from which the continuous variation of entropy with the SoC can be extracted. The obtained entropy profile was validated through a modeling approach that combines a P2D-electrochemical model for battery operation and a 3D-finite-element model for the thermal behavior during battery operation. Good agreement between the model predictions and experiments was achieved on the temporal variation of the cell's outer surface temperature.

Application of Deep Learning to Optimize Gradient Porosity Profile for Improved Energy Density of Lithium-Ion Batteries

Lithium-ion batteries with high active material loading can yield a high energy density at low C-rates. However, the sluggish ion transport caused by longer and more tortuous pathways hinders high energy delivery when extracting high power. This study presents the implementation of neural networks to optimize the gradient active material distribution profile throughout the thickness of electrodes to enhance energy density. The profiles were randomly generated, while maintaining a constant average active material in each electrode. An electrochemical–thermal model was used to investigate the impact of different profiles. A neural network model was then developed to establish the connection between the profiles and the resulting energy density for various electrode thicknesses and C-rates, utilizing a limited amount of simulation data. The neural network model could replicate the performance of the electrochemical–thermal model, but with significantly reduced computational time. This enabled the possibility of efficiently exploring a vast number of candidate profiles to identify the most optimal one for each of the positive and negative electrodes. The results showed that the gradient profiles were mostly influenced by the average active material, rather than the thickness of the electrode. Finally, at high currents, the optimal gradient profiles increased the energy density by over four times compared to uniform electrodes.

Assessment of Hysteresis Models for LiFePO4

The selection of the cathode material is an important task in the design phase of batteries for mobile applications. One material of increasing interest is LiFePO4 (LFP) due to its improved energy density, good thermal stability and durability. In contrast to other commonly used cathode materials, LFP features a clear phase separation between the lithium rich and the lithium poor phase, leading to a pronounced voltage plateau over wide ranges of state of charge (SOC). The level of this plateau also changes for charge and discharge operation, even at very low C-rates. This voltage hysteresis in combination with the wide plateau gives some ambiguity to the correlation between voltage and SOC, which is a challenge for battery management systems. Thus its correct description is crucial if a cell model is meant to serve as virtual twin supporting control function development and calibration. Beyond that, a highly dynamic battery development process requires models that run significantly faster than real-time and parametrization processes which allow for accurate validation and easy adaptation to modified electrodes. A simulation approach complying with these requirements is the electrochemical pseudo-two dimensional (P2D) model, though adaptions to the core model are needed to appropriately capture hysteresis-affected voltage plateau characteristics for phase-separating materials such as LFP.The goal of this study is to extend an existing P2D framework with two approaches capable of describing hysteresis effects and analyzing these extensions regarding result plausibility, computational cost and effort of parametrization. The first model of interest is based on the one-state hysteresis model proposed by Plett. This approach allows for fast simulation and accurate reproduction of the charge and discharge voltages at low C-rates but lacks deeper physical consideration of the phase-separating origin of the hysteresis and thus might lose accuracy to predict system responses on dynamic pulses. In this approach, the voltage hysteresis is not a simulation result, it is due to the input of two separate open circuit voltages for charge and discharge. The second model of interest applies variable solid diffusion (VSSD) featuring a reduced Fickian diffusion coefficient in the plateau region. Lower diffusion at a given C-rate leads to steeper concentration gradients between lithium rich and lithium poor phase. With this approach, effective phase boundaries within the representative particles can be obtained. In the limiting case, when the Fickian diffusion coefficient becomes zero or even negative at some lithium concentration, phase separation persists also when no current is applied. In this setting, the voltage hysteresis at low C-rates is an emergent phenomenon, it is not an input. The physical basis of the approach also allows to consider mechanical stresses and their impact on the voltage gap between charge and discharge. Concentration gradients cause a mismatch of volume demands and thus lead to local stresses, which in turn alter the chemical potential.Results emphasizing advantages and disadvantages of the two hysteresis models are presented. Charge and discharge simulations are performed for several C-rates and the voltage hysteresis is extrapolated to infinitely small currents. Furthermore, the effect of resting phases with zero current as in Galvanostatic Intermittent Titration Technique measurements is discussed. As hysteresis models are often tested only on charge/discharge cycles with constant currents over large SOC regions, the responses of the simulation models are also assessed in pulse tests and dynamic profiles giving a realistic picture of automotive applications. Here special focus is put on energetical aspects of the one-state model and on intra-particle concentration profiles for the VSSD model.

Effects of Non-Uniform Temperature Distribution on the Degradation of Liquid-Cooled Parallel-Connected Lithium-Ion Cells

Our previous work on an air-cooled stack of five pouch-format lithium-ion (Li-ion) cells showed that non-uniform temperature can cause accelerated degradation, especially of the middle cell. In this work, a stack of five similar cells was cycled at a higher C-rate and water-cooled to create a larger temperature gradient for comparison with the air-cooled stack. It was hypothesized that the larger temperature gradient in the water-cooled stack would exacerbate the degradation of the middle cell. However, the results showed that the middle cell degraded slightly slower than the side cells in the water-cooled stack. This trend is opposite to that in the air-cooled stack. This difference could be attributed to the combined effects of a smaller temperature rise and larger temperature gradient in the water-cooled stack than in the air-cooled stack. Post-mortem analysis of cycled cells and a fresh cell showed that the degradation mainly came from the anode. Increased lithium plating and decreased porosity in the side cells are possible mechanisms for the faster degradation compared with the middle cell. It was also found that all the cells in the water-cooled stack experienced a phenomenon of capacity drop and recovery after a low C-rate reference performance test and extended rest. This phenomenon can be attributed to lithium diffusion between the anode active area and the anode overhang area.

Materials recovery from NMC batteries with water as the sole solvent

We report an environmentally benign recycling approach for large-capacity nickel manganese cobalt (NMC) batteries through the electrochemical concentration of lithium on the anode and subsequent recovery with only water. Cycling of the NMC pouch cells indicated the potential for maximum lithium recovery at a 5C charging rate. The anodes extracted from discharged and disassembled cells were submerged in deionized water, resulting in lithium dissolution and graphite recovery from the copper foils. A maximum of 13 mg of lithium salts per 100 mg of the anode, copper current collector, and separator was obtained from NMC pouch cell cycled at a 4C charging rate. The lithium salts extracted from batteries cycled at low C-rates were richer in lithium carbonate, while the salts from batteries cycled at high C-rates were richer in lithium oxides and peroxides, as determined by X-Ray photoelectron spectroscopy. The present method can be successfully used to recover all the pouch cell components: lithium, graphite, copper, and aluminum current collectors, separator, and the cathode active material.

Modeling of Li-Ion Battery Electrodes Accounting for Microstructure Properties: The Newman’s Model Revisited

The most established lithium-ion battery (LIB) porous-based model is the Newman's pseudo-two-dimensional (P2D) model used as a good trade-off between numerical computational cost and physics precision. However, this model does not resolve the local fluctuations of physical quantities on the microstructure and can give inaccurate results especially at high C-rate. In this work, we revisit the P2D model by applying the method of volume averaging to mass and charge transport equations of LIB microstructure models. The outcome is a non-classical homogenized model where the effective properties do not only depend on geometry, but are shown to be functions of the local current density. The model reduces to the classical P2D at low C-rate, but extends the validity range of the P2D at high C-rate for complex microstructures. Moreover, the new model allows to reconstruct the local fluctuations from the averaged variables, whereas this information is not available from the classical P2D model.

Mechanistic insight into the impact of dry-state prelithiated SiOx thin-film anode toward extremely fast-charging and long-term stability

Silicon suboxides (SiOx) have achieved partial success in commercialization as high-capacity anode materials to replace graphite because of optimal electrochemical properties over silicon. Unfortunately, the further development has been sluggish, jeopardizing the urgent need of green energy. The daunting challenges to tackle include low initial coulombic efficiency (ICE) and low charging rate due to poor electron and ionic conductivity. In the context of thin film microbattery, we propose to precisely control the chemical states of SiOx films through varying sputtering power followed by dry-state pre-lithiation through Li-metal thermal evaporation, superior to the current pre-lithiation approaches for SiOx thin-film anodes. We addressed the leading roles of the surface chemical states of the as-deposited SiOx in the formation of the high ionic conductive Li4SiO4 phase as the pre-solid electrolyte interphase during the proposed dry-state prelithiation process. Ultimately, the prelithiated SiOx successfully mitigated the most confronted issues as low ICE and C-rate limitation, achieving an unprecedented rate performance of 72.8 % at 20 C, and ultra-long-cycle retention of 54.2 % over 5000 cycles at 10 C. These results strongly prove that appropriate pre-lithiation provides tremendous advantages to SiOx thin film anode not only in prolonging electrode stability but also promoting a significant fast-cycling.

Measuring Heat Dissipation and Entropic Potential in Battery Cathodes Made with Conjugated and Conventional Polymer Binders Using Operando Calorimetry.

This study explores the influence of electronic and ionic conductivities on the behavior of conjugated polymer binders through the measurement of entropic potential and heat generation in an operating lithium-ion battery. Specifically, the traditional poly(vinylidene fluoride) (PVDF) binder in LiNi0.8Co0.15Al0.05O2 (NCA) cathode electrodes was replaced with semiconducting polymer binders based on poly(3,4-propylenedioxythiophene). Two conjugated polymers were explored: one is a homopolymer with all aliphatic side chains, and the other is a copolymer with both aliphatic and ethylene oxide side chains. We have shown previously that both polymers have high electronic conductivity in the potential range of NCA redox, but the copolymer has a higher ionic conductivity and a slightly lower electronic conductivity. Entropic potential measurements during battery cycling revealed consistent trends during delithiation for all of the binders, indicating that the binders did not modify the expected NCA solid solution deintercalation process. The entropic signature of polymer doping to form the conductive state could be clearly observed at potentials below NCA oxidation, however. Operando isothermal calorimetric measurements showed that the conductive binders resulted in less Joule heating compared to PVDF and that the net electrical energy was entirely dissipated as heat. In a comparison of the two conjugated polymer binders, the heat dissipation was lower for the homopolymer binder at lower C-rates, suggesting that electronic conductivity rather than ionic conductivity was the most important for reducing Joule heating at lower rates, but that ionic conductivity became more important at higher rates.

A particle packing electrode model for microstructure optimization of lithium-ion batteries

The microstructure of porous electrodes has a significant impact on the performance of lithium-ion batteries (LIBs). The critical challenge in theoretical research of porous electrodes is to construct an electrode model that properly describes its microstructure while ensuring solving efficiency. In this work, a particle packing electrode (PPE) model is used to analyze the charging dynamics of LIBs, which is constructed based on the randomness distribution of the active materials. After accurately predicting the experimental data, the PPE model illustrates two key factors governing the C-rate performance: The C-rate performance depends on the Li+ transfer in active material at low C-rate and depends on Li+ transfer in the electrolyte at high C-rate. By optimizing the uniformity of the particle size and the orderliness of the pore structure, LIBs achieve improved performance. This model offers novel possibilities for designing and optimizing electrode microstructures of LIBs.

Silicon-tin thin-film anodes for low and high power-density lithium-ion batteries

Silicon (Si) is a promising replacement material for graphite, the commonly utilized anode material in lithium-ion batteries (LIBs). Si has a theoretical specific capacity up to 11 times higher than that of graphite. However, pure Si has two main issues: high electronic resistivity and significant volume change during charge/discharge cycling. To overcome these issues, Si was alloyed with high-conductivity materials. In this contribution, physical vapor deposition was utilized to produce binderless silicon-tin anodes for LIBs. Tin is a lithium-active alloying material that has high electrical conductivity and high specific capacity (i.e., up to three times that of graphite). Electronic resistivity, residual film stress, X-ray photoelectron spectroscopy, Raman spectroscopy, and cyclic voltammetry measurements were performed to thoroughly analyze the deposited Si-Sn films and help understand their charge/discharge cycling performance at low and high C-rates. At a low C-rate of 0.5 C, the alloyed films did not show any improvement compared to pure Si. However, the alloyed films showed better performance compared to pure Si when tested at a high C-rate of 10 C. Si-Sn films with (40 – 50 wt.% Sn) were able to retain a specific capacity of around 680 mAh g−1 after 1000 cycles.

A modified high C-rate battery equivalent circuit model based on current dependence and concentration modification

The current battery equivalent circuit models are mainly suitable for room temperature or low C-rate conditions. However, the polarization phenomenon is serious at high C-rate, which results in the obvious voltage drop and significantly reduces the model accuracy at high C-rate. In this paper, the characteristics analysis of different RC models is firstly studied. It is found that the first-order RC circuit model is more suitable for high C-rate conditions due to the coupling of model parameter extraction. Based on the first-order RC model, a basic equivalent circuit model adapted to high C-rate condition is then determined. Secondly, the influences of sampling frequency and the identified time domain on instantaneous impedance and polarization resistance are researched. Thirdly, the change rules of different polarizations at high C-rate are analyzed. Furthermore, the relationship between the change rule of the model parameters and that of different overpotentials are researched. Subsequently, a modified high C-rate model is innovatively established, which considers the current dependence and concentration/temperature modification of instantaneous impedance and polarization resistance. Finally, the accuracy of the modified high C-rate model is verified at 5 °C, 25 °C, and 55 °C, respectively. Results show that the modified high C-rate model can improve the simulation accuracy of the voltage at high C-rate (≤6C), and the mean absolute error (MAE) and Root Mean Squared Error (RMSE) are within 68 mV. Compared with the existing high C-rate model, the accuracy of the modified high C-rate model proposed in this paper is improved and the complexity is also significantly reduced.

“Zero” Porosity High Loading NMC622 Positive Electrodes for Li-Ion Batteries

LiNi0.6Mn0.2Co0.2O2 (NMC622) is a widely used positive electrode material for lithium-ion batteries, including electric vehicles. In this work, we investigated the effects of porosity, ranging from “zero” to the typical 35%, on the electrochemical behavior of high-loading NMC622 electrodes. Although it is well known that the energy density of the electrode increases with increasing areal capacity and decreasing porosity, NMC-positive electrodes with exceedingly low porosity (e.g., near zero) and high loading (e.g., 4 mAh cm−2) have not been investigated. Here, we report an intriguing observation that the “zero porosity” NMC electrode can have higher capacity at low C-rates, and the volumetric energy density significantly increases to 1739 Wh l−1 compared to 805 Wh l−1 of conventional electrodes of 35% porosity. We performed cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to help understand this observation. This work provides new insights into the effects of porosity on the electrochemical behavior of high-loading positive electrodes.

Insights into the Development and Performances of Ionogel-Based Electrolytes for Solid-State Lithium-Based Batteries

Solid-state electrolytes (SSEs) aim to overcome safety issues, decrease package volume, and increase the energy density of next-generation energy storage devices, such as Li-ion and post-lithium batteries. Among different types of SSEs, the ionogel (or ion gel) systems, in which polymer electrolytes are doped with room-temperature ionic liquids (RTILs), are amongst the most attractive choices to reach ionic conductivity values comparable to those of liquid electrolytes, ensuring better safety and electrode/electrolyte interface stability [1]. In our previous studies, several ionogel samples were prepared by in-situ UV-curing [2]. Here, we present the results of newly optimized methacrylate-based solid-state electrolyte systems conceived for ambient temperature cycling with high-energy cathodes. We characterized the electrolyte formulations using various physico-chemical and electrochemical methods, including gel content, FTIR, rheology, DMTA, TGA, SEM, voltammetry, impedance spectroscopy, and galvanostatic cycling. The focus is particularly on the influence of using two different RTILs as reaction media on the properties of the resulting materials and their electrochemical behaviours. The achieved results indicate that viscosity affects the polymerization kinetics of the ionogels, which in turn affects their thermal stability and galvanostatic cycling behavior. The obtained ready-to-use and self-standing crosslinked ionogel electrolyte membranes (~100 µm thickness) showed relatively high ionic conductivity (0.8 to 2 mS/cm at 20 °C) and anodic stability (up to 4.5-5 V vs. Li). Lithium lab-scale cells were assembled with ionogel-polymerized-on-NMC-cathodes for galvanostatic cycling measurements, and they demonstrated initial discharge capacities of around 180 mAh/g at low C-rate in the voltage range of 3.0-4.3 V at room temperature. Furthermore, motivated by the demand of increasing the overall performance of the SSEs, different types of composite solid electrolytes are now under our investigation and further development, including the utilization of active and inert inorganic fillers. For example, we introduced LLZO ceramic microparticles into the aforementioned ionogel systems for fabricating composite SSEs via in-situ photopolymerization. So far, the best achieved preliminary cycling results show enhanced and more stable cycling behaviour at higher C-rates up to 0.2C at room temperature, suggesting the promising prospects of our composite SSEs and their potential contribution to the fabrication of solid-state, high-energy lithium-based batteries.Acknowledgements: This work has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 860403.

Fast Estimation of Entropy Profiles for Lithium-Ion Batteries

The operating temperature of a lithium-ion battery (LIB) has strong effects on its electrochemical performance, safety, and lifetime. Reliable predictions of cell temperature and heat generation are required for the design and thermal management of battery systems. During the normal operation of LIBs, heat sources can be classified into irreversible heat from resistive losses such as ohmic resistance, charge-transfer overpotentials, and mass-transfer limitations; and reversible heat from entropy change . The irreversible heat generation is related to the cell's internal resistance and the applied current. The reversible heat depends on the entropy variation of electrode materials, the state of charge (SoC), and the temperature. An accurate measurement of entropy-variation will not only help to improve the battery operation under optimal thermal conditions but can also be used to diagnose the battery aging since it can reveal crystal structure changes in the electrodes [1, 2].Techniques for measuring the entropy of a battery can mainly be classified into potentiometric [3] and calorimetric methods. The potentiometric method measures the open circuit voltage (OCV) changes while the cell temperature is varied at different SoCs. This method requires a long relaxation time to obtain the OCV equilibrium state following each SoC point and temperature step. The calorimetric method [4-6] is based on measuring the heat generation during charge and discharge, and various models are applied to differentiate between and quantify the reversible and irreversible heat sources. The entropy variation with SOC is obtained via the reversible heat rate.Recent improved methods include e.g., Schmidt et al. [4] who proposed a reversible heat extraction method based on electrothermal impedance spectroscopy (ETIS) and Damay et al. [5] proposing a fast calorimetric method to obtain a continuous entropy profile. The cell surface temperature is monitored while the battery is charged and discharged continuously. A thermal balance is used to estimate the heat generation from the monitored cell temperature changes during charge and discharge. The benefit of this method can be low cost, fast and high resolution. Bedürftig et al. [6] proposed a calorimetric method named ‘Double Pulse Method’ to measure reversible heat in lithium-ion battery cells by exciting the cell with charge and discharge current pulses.Here we present a new entropy characterization method via charge and discharge cycles of a LIB at low constant C-rates using simple insulating condition. Through monitoring the variations of cell surface temperature and voltage with the state of charge, the continuous variation of entropy with the state of charge can be extracted. These results were compared to classical entropy measurements obtained through the potentiometric method as shown in Figure 1. In addition, the extracted entropy variation data is input into a LIB modelling approach, combining a P2D-electrochemical sub-model for battery operation and a 3D-finite-element sub-model for thermal behavior while battery charge and discharge. The model predictions on the cell outer surface temperature variation while charge and discharge under different C-rates are compared with the data obtained in the battery characterization tests as shown in Figure 2. Good agreements between the model predictions and tests are achieved.

Cracking and associated volumetric expansion of NMC811 secondary particles

Secondary particles comprising a large number of nickel-rich single crystal primary particles are extensively used as storage particles in cathodes of lithium-ion batteries. It is well-established that crack formation in secondary particles is an important degradation mode that contributes to decline in battery performance. Recent X-ray tomographic observations suggest that, at very low C-rates, concentration gradients of lithium within an NMC811 secondary particle are negligible yet cracking still occurs. Additionally, during delithiation the primary particles shrink yet a volumetric expansion of the secondary particle occurs. These observations are explained by a numerical model of distributed cracking due to the extreme anisotropy of lithiation strain of primary particles. The incompatible deformation from grain to grain induces large self-stresses even in the absence of spatial gradients in the lithium concentration. The stress state is sufficient to drive a dynamic catastrophic fracture event, and the associated kinetic energy acquired by the primary particles moves them apart (akin to an explosive event) with the carbon and binder domain surrounding each secondary particle restricting the outward motion of the primary particles. It is predicted that a volume expansion of the secondary particles on the order of 20 % accompanies cracking, in agreement with recently reported observations.

Electrolyte/Electrode Interface Reaction in Polysulfides Insoluble Lithium-Sulfur Battery Using Li-Glyme Solvate Ionic Liquid

Lithium-Sulfur (Li-S) battery is attracting attention as a new generation secondary battery. However, one of the major problems is dissolution of discharge products of lithium polysulfides (Li2Sn: n = 1, 2, 4, 6 and 8) that cause the self-discharge. Solvate ionic liquids (SIL), which consist of a solvate ion and its counter ion, are a subclass of ionic liquids. Li-Gn SIL is yielded by equimolar mixing lithium bis-(trifluoromethanesulfonyl)amide (LiTFSA) and glyme, an oligoether such as triglyme and tetraglyme (Gn : CH3O-(CH2CH2O) n -CH3, n = 3, 4). Watanabe and Dokko et al. [1] reported that the Li-glyme SIL had low solubility of the polysulfides, and proposed polysulfides insoluble Li-S batteries using these electrolyte solutions. Recently, a sulfolane-based super-concentrated electrolyte solution (SCES) also exhibits low solubility of the polysulfides and has been applied to Li-S battery [2]. Both of Li-Gn SIL and the SCES are polysulfide insoluble electrolytes, however, the Li-S batteries with the SCES has better battery performance than that with Li-Gn SIL, implying that the different discharge reaction paths. In this contribution, in situ electrochemical impedance spectroscopy (EIS), which can acquire charge-discharge curves and impedance spectra simultaneously, is applied to the Li-S battery using the Li-Gn SIL to obtain further insight into the discharge behavior.A positive electrode slurry was obtained by mixing a sulfur S8, a Ketjen black (KB, EC600JD, Lion Corporation) and a carboxymethyl cellulose (CMC2200, Daicel) at a weight ratio of 60 : 30 : 10. The slurry was spread on an Al foil to obtain a KB-S composite electrode. The typical loading of the sulfur was 1.0 mg cm-1. A negative electrode and a separator were used a lithium metal foil (Honjo Metal) and a glass separator (GA-55, Advantec), respectively. The Li-Gn SIL electrolyte solution was prepared by mixing LiTFSA, G4 and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) as the molar ratio of 1 : 1 : 4. The Li-S batteries were assembled using the CR 2032 coin-type cell in an Ar-filled glove box. In situ EIS measurements were conducted at different C-rate under constant-current discharging/charging condition using an electrochemical measurement system (SP-150, BioLogic). The current values were 336 and 84 μA. The DC current was superimposed with a small AC current adjusted so that the response voltage amplitude did not exceed 10 mV. The measurements were carried out in the frequency range from 500 kHz to 10 mHz.The voltage drop is observed at the DOD = 20-35 %, in the discharge profile measured at the high C-rate (336 μA), whereas that measured at the low C-rate (84 μA) has no voltage drop. The compensation for the changes over time [3,4] was performed on the obtained impedance spectra to determine instantaneous impedance spectra at a specific DOD (depth of discharge). Two capacitive semicircles and a straight line are observed for all of the investigated systems. The semicircle in the high frequency range (< 10 Hz) is asymmetrical, as if two semicircles in the high frequency range from 500 kHz to 1 kHz and in the middle frequency range from 1 kHz to 10 Hz are overlapped. For the high C-rate, the diameter of the semicircle in the middle frequency range increases with increasing DOD and remarkably increases at DOD = 20-35 %. By contrast, in the low C-rate, the diameter does not change during discharging. This semicircle is assigned to the charge transfer resistance of the negative electrode. According to the previous studies [5, 6], when the charge transfer reaction of lithium is faster than the diffusion rate of G4, the solvation structure changes due to the lack of G4 coordinating to Li+. This solvation structure change causes suppression of Li+ elution to increase the charge transfer resistance of the negative electrode. References 1) K. Dokko et al., J. Electrochem. Soc., 160, A1304-A1310 (2013).2) A. Nakanishi et al., J. Phys. Chem. C, 123, 14229-14238 (2019).3) Z. B. Stoynov et al., J. Electroanal. Chem., 183, 133-144 (1985).4) M. Itagaki et al., J. Power Sources, 135, 255-261 (2004).5) N. Serizawa et al., J. Electrochem. Soc., 160(9), A1529–A1533, (2013).6) A. Miki et al., J. Mater. Chem. A, 9, 14700–14709, (2021).

Mathematical Toolbox for Joint Structural and Electrochemical Analysis of Lithium-Ion Battery Cells

Increasing interest in the automotive field for electrification has led to an increased demand for lithium-ion batteries (LIB). LIB remain the most popular and widely applied energy storage technology. The microstructure of the positive electrode of LIBs governs its electrochemical performance. For this reason, it is quite imperative to determine an optimal electrode microstructure design to achieve the best possible energy properties. Mechanistic simulation models can be a powerful tool in analyzing the effect of microstructural parameters on the battery cell performance. Not only do they negate costs for necessary materials, but they also allow analysis and optimization while avoiding time-consuming experiments.Doyle-Fuller-Newman (DFN) models, being pseudo-2-dimensional (P2D), are computationally less expensive than their higher dimension counterparts; however, they often apply the Bruggeman relation to compute effective parameters such as the electrode conductivity of the electrode. Despite its simple implementation (i. e. a correction factor to reflect effective parameters), the Bruggeman relation delivers inaccurate results. [1, 2]In this work [1], we coupled a DFN model with the structure surrogate model from [2], excluding the Bruggeman relation. The model predicts the effective transport parameters and thus the battery performance more accurately. With this coupled model, we investigated the effect of the cathode mass loading and density on the achievable volumetric discharge energy density. [1] Furthermore, we developed a numerical optimization tool, which can predict the optimal structural parameters for maximum energy density. Our findings suggested different optimal designs for different current densities; whereas higher mass loading was preferred at lower C-rates, it was less favorable at higher C-rates. The concentration profiles of lithium-ions across the cathode thickness at the end of discharge suggest transport limitations to be the main reason behind this. As a further step, we carried out an uncertainty analysis to determine the robustness of the optimal designs to structural fluctuations with respect to their energy density.Our findings provide a deeper understanding into the influence of structural parameters on the discharge performance of lithium-ion battery cells, using only simulative methods. Additionally, our methodology of numerical optimization and subsequent uncertainty analysis allows for a quick and simple method for determining optimal structural parameters and their robustness against fluctuations. This approach can assist to produce battery cell electrodes with an optimized design in industry as well as help simulative and experimental researchers to further investigate optimal electrode design.[1] Karaki et al., Energy Technology 2022 [2] Laue et al., E. Acta 2019 Figure 1

Crosslinked Hyperbranched Polyglycerol-Based Polymer Electrolytes for Lithium Metal Batteries

Tailored partially methylated and methacrylated hyperbranched polyglycerols (hbPG-MAx/OMey) combined with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as conducting salt were investigated after crosslinking with respect to their application as solid polymer electrolytes (SPE) in lithium metal batteries. For sample preparation and coating, a straightforward solvent-free photopolymerization method was applied. With the aim of finding the right balance between mechanical and electrochemical properties, electrolytes with different crosslinking densities were studied. High crosslink density increases mechanical integrity but reduces local chain motion and thus ionic conductivity at the same time. Differential scanning calorimetry (DSC), chronoamperometric and impedance measurements show that the hyperbranched polyether structure interacts strongly with lithium cations. Finally, the SPE with the lowest crosslinking density was selected and investigated in cycling tests due to the parameters of highest absolute values in conductivity (2.1 × 10−6 S cm−1 at 30 °C; 2.0 × 10−5 S cm−1 at 60 °C), lowest Tg (from DSC: −39 °C), electrochemical stability window (4.3 V vs. Li/Li+) and mechanical strength (1.6 ± 0.4 MPa at 25 °C). At low C-rates and elevated temperatures (60 °C), cells were cycled with high Coulombic efficiency. At high C-rates, a distinct decrease in specific capacity was observed due to insufficient ionic conductivity.