Solid Electrolyte Interphase Resistance Research Articles

Li-Ion Battery Material Impedance Analysis II: Graphite and Solid Electrolyte Interphase Kinetics

Li-ion battery graphite electrodes form a solid-electrolyte-interphase (SEI) which is vital in protecting the stability and efficiency of the cell. The SEI properties have been studied extensively in the context of formation and additives, however studying its kinetic features after formation have been neglected. In this study we show the dynamic resistive behavior of the SEI after formation. Via electrochemical impedance spectroscopy measurements on Cu-foil after SEI formation we show how the SEI shows a potential-dependent resistance which can be explained by a change in charge carriers (Li+) in the SEI. Additional measurements on graphite exhibit a similar behavior and allow us to separate the charge transfer kinetics from the SEI resistance, showing that the SEI resistance is the dominating resistance in the graphite kinetics. Measurements on pre-formed electrodes also show how the SEI resistance changes when in contact with electrolyte of different LiPF6 salt concentrations, with the resistance decreasing for increasing salt concentrations. Ultimately, we show that the SEI resistance affects Li-plating by acting as an offset to the plating reaction but does not affect the nucleation overpotential itself.

Non-invasive diagnosis of solid electrolyte interphase and charge transfers resistances for degraded lithium-ion batteries

Modeling the electrical behavior of cells is often complex because several parameters have to be determined simultaneously, which may lead to inconsistencies and inaccuracies. In the present paper, we modeled the “surface resistance” by using a method based on rules of dependence, already presented in the literature, which makes it possible to separate the contributions of the SEI (Solid Electrolyte Interphase) layer and the charge transfers as a function of the temperature and the current. A physical model is able to provide a consistent estimate of the surface resistance values at untested operating points. We transposed this model to NCA (LiNixCoyAl1−x−yO2) and NMC-622 (LiNi0.6Mn0.2Co0.2O2)/graphite+Si cells and we characterized them for different states of health (SOH). For a SOH of 87%, we found that the charge transfers resistance was multiplied at least by 5.8, while the SEI resistance was multiplied at least by 1.5. As the cell ages, the proposed model should be easier to recalibrate than lookup tables. For that, we propose two methods, one that provides more detailed and accurate results and one that is more practical from an engineering point of view.

Investigating the Change in the Microporous Structure of the Graphite Electrode during Formation

Till today, the graphite-based electrode is the most widely used negative electrode in commercially available lithium-ion batteries (LIBs).[1] While extended research has been conducted in the investigation of the graphite electrode’s properties, especially the formation of the solid-electrolyte interphase (SEI) within the first cycles of LIBs remains a field of ongoing research.[2] The SEI forms during the first cycle(s) on the anode surface at potentials below the stability window of the electrolyte components, due to the reductive decomposition of the respective solvents, additives, and conductive salts.[2,3] The initially formed SEI has an influence both on the long-term cycling stability of the cell, depending on how good the passivation, i.e., electronic insulation, of the graphite surface is,[4] and on the rate performance of the anode, depending on the thickness of the SEI, its chemical composition, and its lithium-ion conductivity, referred to as SEI resistance.[5] Additionally, with a thickness of 10-20 nm when employing additives such as vinylene carbonate (VC), the SEI takes up parts of the void volume of the pristine electrode and may change the pores and the electrolyte conduction pathways within the electrode (see Figure 1).[6] In this study, we investigated the change of the microporous structure of an artificial graphite (MAGE-5, Hitachi, Japan) with an LP572 electrolyte (1 M LiPF6 in EC:EMC 3:7 w:w with 2 wt% VC, Gotion, USA) during formation at 45 °C. The electrode’s microstructure is often connected to the macroscopic properties using the McMullin number N M in the form of:[7] N M = τ / ε = R Ion ·A · κ / d, (1)correlating the porosity ε, electrodes thickness d, ionic pore resistance R Ion, area of the electrode A, conductivity of the electrolyte κ, and the tortuosity τ (see Figure 1).Here, we analyzed graphite electrodes with varying initial porosities from 20 to 50 %, employing electrochemical impedance spectroscopy (EIS) in a MAGE//Li half-cell setup with a micro-reference electrode and a free-standing graphite electrode.[8,9] Measuring EIS at temperatures of -5 °C allowed the reliable determination of the ionic resistance before and after formation. Due to the formed SEI covering the graphite particles and partially filling the void of the porous electrode, we discovered an increase of the ionic pore resistance throughout the whole range of initial porosities.Next, we determined a porosity dependent increase in thickness of the electrodes from electrodes cycled at the same conditions in coin cells that were subsequently harvested from the cells and analyzed for thickness changes. Additionally, mercury intrusion porosimetry (MIP) was used to determine the porosity of formed graphite electrodes, revealing a decrease in porosity after formation for all samples. The combination of these findings - using Equation 1 as described above with a constant electrode area A and assuming a constant electrolyte conductivity κ - allowed us to evaluate the change in the tortuosity of the graphite electrodes, giving insights into the overall changes in the graphite anode properties due to SEI formation.

Understanding the Electrolyte Chemistry Induced Enhanced Stability of Si Anodes in Li-Ion Batteries based on Physico-Chemical Changes, Impedance, and Stress Evolution during SEI Formation

The volume expansion/contraction of Si-based anodes during electrochemical lithiation/delithiation cycles causes a loss in mechanical integrity and accrued instability of the solid electrolyte interphase (SEI) layer, culminating into capacity fade. Electrolyte additives like fluoroethylene carbonate (FEC) improve SEI stability, but the associated causes still under debate. This work reveals some of the roles of FEC via post-mortem observations/analyses, operando stress measurements and a comprehensive study of the impedance associated with the formation/evolution of SEI during lithiation/delithiation. Usage of 10 vol.% FEC as electrolyte additive leads to significant improvements in cyclic stability, Coulombic efficiency and facilitates smoother/compact/crack-free surface/SEI, in contrast to the cracked/pitted/uneven surface upon non-usage of FEC. Operando stress measurements during SEI formation reveal compressive stress development, followed by loss in mechanical integrity, upon non-usage of electrolyte additive, in contrast to insignificant stress development associated with SEI formation upon usage of FEC. The EIS model proposed here facilitates good fit with the impedance data at all states-of-charges, with the SEI resistance and capacitance exhibiting expected variations with cycling and the SEI resistance progressively decreasing with cycle number in the presence of FEC. By contrast, in the absence of FEC, severe fluctuations observed with the SEI resistance and capacitance indicate instability.

Conveniently Revealing Lithium-Ion Battery Reaction Kinetics Using Nonlinear Frequency Response Analysis

Nonlinear Frequency Response Analysis (NFRA) can be considered a nonlinear modification of galvanostatic EIS. Due to an increased AC excitation, higher harmonics are additionally present in the voltage response of the cell [1]. Here, a single particle model (SPM) [2] in the frequency domain is used for the simulation of the NFR of lithium-ion batteries (LiB). The time-dependent processes (double-layer charging, solid diffusion) are linear in currently available SPMs. Hence, they can be described algebraically as impedances in the frequency domain. From the nonlinear, time-independent equations (Butler-Volmer, Nernst), for the case of sinusoidal excitations, a system of algebraic equations for the fundamental and higher harmonics up to a freely selectable order N is derived. By combining these systems with the impedances, we obtain a still nonlinear SPM in the frequency domain, which can be solved in a purely algebraic way for the higher harmonics. The proposed model enables researchers to conveniently analyze the NFR of LiBs. This is especially beneficial for investigations of the reaction kinetics since charge transfer coefficients can be obtained directly by parameterizing the model with NFR measurements.The abilities of the model are shown with two exemplary cell setups. At first, a symmetric cell built from metallic lithium is analyzed. The measured impedance is used to parameterize a simple equivalent circuit model (ECM), including transport through the solid electrolyte interphase (SEI). The resulting exchange current densities are used within the frequency domain SPM, which represents a nonlinear version of the ECM. Subsequently, the charge transfer coefficients of the plating/stripping reaction can be obtained conveniently by comparing the NFR measurements with the SPM response. Since the sum of obtained charge transfer coefficients is smaller than one, they are interpreted in the sense of Marcus-Hush-Chidsey kinetics [3], which is possible within a certain range of overpotential. This leads to a distinct value for the solvent reorganization energy. The calculated value is in coincidence with literature values, which are relying on high scan rate CVs with ultramicroelectrodes [4].Secondly, a full pouch cell (NMC622/Graphite) was analyzed in a similar manner. Even though the SPM cannot capture microstructural effects of the porous electrode, a sufficient fit for the EIS measurement was found. After a subsequent parameterization of the charge transfer coefficients, the NFR measurement and simulation again agree well. The resulting values for intercalation reorganization energies at the graphite anode and NMC cathode are even lower than for the plating/stripping reaction. The reorganization energies could not be validated due to the lack of literature values. However, available values for LFP [5] and a value based on unpublished data for graphite [6], are also lower compared to the plating/stripping reaction.The proposed method is very promising since it can directly capture the complete nonlinear kinetics of an electrode. Due to the frequency dependence of LiBs, the separation of kinetic and diffusive effects remains as convenient as it is for EIS.If reorganization energies are considered as given, two persistent challenges in EIS analysis could be tackled: The separation of SEI resistance and charge transfer resistance since the latter is considered as nonlinear. In addition, the ionic transport resistance and charge transfer resistance could be separated for the same reason.In the future, microstructural effects should be integrated into the model. Vidaković-Koch, T.; Miličić, T.; Živković, L. A.; Chan, H. S.; Krewer, U.; Petkovska, M. Nonlinear Frequency Response Analysis: A Recent Review and Perspectives. Opin. Electrochem. 2021, 30, 100851. https://doi.org/10.1016/J.COELEC.2021.100851.Ning, G.; Popov, B. N. Cycle Life Modeling of Lithium-Ion Batteries. Electrochem. Soc. 2004, 151 (10), A1584. https://doi.org/10.1149/1.1787631/XML.Henstridge, M. C.; Laborda, E.; Rees, N. V.; Compton, R. G. Marcus–Hush–Chidsey Theory of Electron Transfer Applied to Voltammetry: A Review. Acta 2012, 84, 12–20. https://doi.org/10.1016/J.ELECTACTA.2011.10.026.Boyle, D. T.; Kong, X.; Pei, A.; Rudnicki, P. E.; Shi, F.; Huang, W.; Bao, Z.; Qin, J.; Cui, Y. Transient Voltammetry with Ultramicroelectrodes Reveals the Electron Transfer Kinetics of Lithium Metal Anodes. ACS Energy Lett. 2020, 5 (3), 701–709. https://doi.org/10.1021/acsenergylett.0c00031.Bai, P.; Bazant, M. Z. Charge Transfer Kinetics at the Solid-Solid Interface in Porous Electrodes. Commun. 2014, 5 (1), 1–7. https://doi.org/10.1038/ncomms4585.Gao, T.; Han, Y.; Fraggedakis, D.; Das, S.; Zhou, T.; Yeh, C. N.; Xu, S.; Chueh, W. C.; Li, J.; Bazant, M. Z. Interplay of Lithium Intercalation and Plating on a Single Graphite Particle. Joule 2021, 5 (2), 393–414. https://doi.org/10.1016/J.JOULE.2020.12.020. Figure 1

Concentrated Electrolyte for Stable Lithium Metal Anode

Secondary lithium-ion batteries have been widely used as energy storage systems for portable electronics and electric vehicles. Instead of applying graphite as the anode, lithium (Li) metal has an ultrahigh theoretical capacity (3860 mAh/g) that could significantly increase battery’s energy density. Li dendrite formation is considered the main issue when using Li-metal anodes. Therefore, anode-free batteries are suggested as a solution to this obstacle. However, the major problem with anode-free batteries is their inferior cycling performance than reported lithium metal batteries, resulting from the formation of a fragile and fractured Solid-Electrolyte Interphase (SEI).To establish a robust SEI, novel lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) and LiNO3-based electrolytes with four different ratios were prepared in the co-solvent system (1,3-dioxolane and 1,2-dimethoxyethane). Compared to the other three electrolytes, concentrated electrolyte (4M LiTFSI + 2 wt.% LiNO3 or 4+2) in Li||LiFePO4 cells indicated the highest initial discharge capacity of 154 mAh/g with a capacity retention of 78% after 50 cycles. Scanning electron microscopy was conducted to investigate the SEI morphology after the initial charge/discharge process. The deposited Li on the Li metal was highly dense, and no spiky or needle-like Li clusters were detected. Additionally, electrochemical impedance spectroscopy of the cells was measured after 1, 3, 5, and 10 cycles, respectively. The cells with 4+2 showed the smallest SEI resistance after different cycles, corresponding to a stable and less-resistive SEI formation.

Formation and Aging of the Solid Electrolyte Interphase on Lithium in Bis(fluorosulfonyl)amide-Type Ionic Liquids

The formation and aging of the solid electrolyte interphase (SEI) formed on a Li and Cu electrode in two bis(fluorosulfonyl)amide (FSA–)-type ionic liquids (ILs), 1-methyl-1-butylpyrrolidinium bis(fluorosulfonyl)amide and 1-methyl-1-methoxyethylpyrrolidinium bis(fluorosulfonyl)amide, containing a high concentration of LiFSA were investigated using electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy. The intensity of peaks corresponding to the decomposition of the electrolyte in XPS spectra changed with time. The thicknesses of the SEI formed on a Cu electrode in both ILs were estimated to be 20–100 nm by TEM observation. The resistance of SEI increased with the lapse of time although the change in the thickness of the SEI was small, suggesting the aging of the SEI in the ILs was considered to depend on the composition of the SEI rather than the thickness of the SEI. The Li deposits shrank during dissolution. The Li anode became sparse and porous after deposition and dissolution cycles, probably due to the inhomogeneous reaction through the aged SEI. The SEI formation on the surface of the porous deposits was considered to lead to an increase in the polarization and the isolation of the deposits, i.e., the formation of “dead lithium”.

Fluorinated ether-based co-solvent electrolytes for lithium-metal batteries: High ionic conductivity and suppressed dissolution of fragmented anions

Advanced electrolyte solutions for lithium (Li) metal batteries have been developed to improve their comparability with Li electrodes and enhance high oxidative stability. While the solvation structures of Li+ are critical for determining the quality of the solid electrolyte interphase (SEI) layer and cyclability, there is limited understanding of these correlations for co-solvent systems and long-term cycles. Herein, we investigate the role of fluorinated ether-based co-solvents consisting of 2,2,3,3-tetrafluoro-1,4-dimethoxylbutane (FDMB) and 1,2-diethoxyethane (DEE) with 1 M lithium bis(trifluoromethanesulfonyl)imide (LiFSI). FDMB exhibits high oxidative stability and low flammability, and the addition of DEE improves ionic conductivity by sharing Li+ coordination with FDMB. SEI formation is derived from FSI−. Notably, increasing the DEE volume ratio promotes lithium fluoride (LiF) deposition and dissolution of fragmented FSI− into the electrolyte solution. FDMB inhibits such dissolution and enhances the formation of lithium nitride (Li3N) and lithium oxide (Li2O) in the SEI. Thus, the DEE/FDMB co-solvents complemented the weaknesses of the two solvents and offer the low SEI resistance. It explains optimal cycling performance for Li|Li and Li|LiFePO4 cells with 1:6 DEE/FDMB with 1 M LiFSI compared to that of the single DEE and FDMB.

Atomic Layer Deposition of Ultrathin Al2O3 Layer on Carbon Nanotube Anodes for Potassium Ion Batteries with High Initial Coulombic Efficiency and Electrochemical Performance

AbstractPotassium ion batteries (PIBs) have emerged as one of the most attractive alternatives to lithium‐ion batteries (LIBs) to satisfy the burgeoning energy demand. The continuous growth of solid electrolyte interphase (SEI) and consumption of potassium resources cause poor initial Coulombic efficiency (ICE), unstable cycling performance, and even safety issues. Herein, the fabrication of an artificial SEI of an ultrathin Al2O3 layer by atomic layer deposition (ALD) on a multi‐walled carbon nanotubes (MWCNTs) anode for high‐performance PIBs is reported. The SEI resistance and the charge transfer resistance in the MWCNTs electrode are significantly reduced according to electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration (GITT) measurements. The MWCNT electrode with a 2‐nm Al2O3 layer exhibits a much higher ICE of up to 85.9% than that of the uncoated one (20.8 %). It also maintains a remarkably high reversible capacity of 284 mA h g−1 at 100 mA g−1 after 100 cycles. In particular, the anode delivers superior rate capability of 558, 303, 251, 203, and 154 mAh g−1 at a current density of 50, 100, 200, 400, and 800 mA g−1, respectively. This work offers an effective approach for fabricating PIBs with high ICE and electrochemical performance.

A benign strategy toward mesoporous carbon coated Sb nanoparticles: A high-performance Li-ion/Na-ion batteries anode

Antimony (Sb)-based anodes can offer excellent gravimetric capacity (∼660 mAhg−1) in lithium-ion/sodium-ion batteries (LIBs/SIBs) fabricated using carbonate-based electrolytes complexed with lithium/sodium salt. However, high first-cycle irreversible loss (35–40%) and gradual capacity fade (25–30%/cycle) originate from solid electrolyte interphase (SEI), and severe volumetric stress (∼300%) associated with alloyed phase(s) impede real-life applications. Herein, we devise a benign strategy to develop mesoporous carbon coating onto antimony nanoparticles (Sb@C) based core-shell architecture for LIBs/SIBs anode. In particular, ∼30–50 nm thick mesoporous carbon spheres (∼1 ± 0.5 μm) were obtained from resorcinol-formaldehyde (RF)-based polycondensation reaction by sol-gel chemistry engulfing Sb nanoparticles by suitably controlling Cetyltrimethylammonium bromide (CTAB)-induced steric stabilization and pH modulation during synthesis. The core-shell Sb@C helps faster Li+/Na+-ion migration preventing the structural collapse of Sb during electrochemical cycling and thereby improving the capacity fade. Electrochemical results demonstrate Sb@C can deliver a specific capacity of ∼536 mAhg−1 and ∼ 291 mAhg−1 at 0.1C current rate in LIBs and SIBs, respectively, up to 200 cycles. Electrochemical impedance spectroscopy (EIS) indicates lower charge transfer (Rct) and SEI resistance (RSEI) of Sb@C cycled electrode than the bare Sb-NPs was the probable reason for improved Li/Na-ion storage in Sb@C anode. A detailed galvanostatic intermittent titration technique (GITT) and internal resistance measurements during 1st and 2nd cycles shed light on distinguishably different Li-ion/Na-ion storage behavior. The bulk Li+/Na+-ion diffusion coefficients found diminishes at reaction voltages (0.9 V/0.6 V for lithiation and 0.6 V/0.4 V for sodiation) corresponding with alloyed phase(s) concurrent with a drop in internal resistance at the quasi-open-circuit voltage (QOCV) during 1st and 2nd discharge cycle. On the contrary, de-alloying phenomena from the fully lithiated/sodiated phase(s) display an entirely opposite trend. The Li+ diffusion coefficient reaches minima at ∼1.1 V with a sudden jump in the internal resistance at QOCV during 1st and 2nd charge cycle. However, Na+ diffusion coefficient gradually drops along with a steep increase in the internal resistance, indicating partial Na-ion trapping and irreversible capacity loss.

Estimation of SoH and internal resistances of Lithium ion battery based on LSTM network

State of Health (SoH) and internal resistances, including the solid electrolyte interphase (SEI) resistance and charge transfer resistance, are important parameters that change in the long-term representation of the aging state of Lithium-ion batteries. Using long short-term memory (LSTM) network, a neural network with the ability to remember long-term data features, this paper presents a method for estimating SoH and internal resistances of Lithium-ion batteries using LSTM network with deep learning mechanism. Based on experimental data including voltage, current, temperature with 03 charge/discharge scenarios and measuring impedance, input/output data structure is set up to reflect aging features used for estimating SoH and internal resistances by LSTM. The first LSTM network is designed to estimate SoH, then the data including current, voltage, temperature and estimated SoH will be used to estimate the SEI resistance and charge transfer resistance by the second LSTM network. With this structure, the estimation of internal resistances in practice will become simpler as it does not require measuring capacity and impedance spectroscopy. Comparing the estimation errors using LSTM and FNN with 03 performance metrics including mean absolute percentage error (MAPE), mean percentage error (MPE) and root mean square error (RMSE) shows that the estimation results of SoH and internal resistances of the cell by LSTM have higher accuracy than the estimation by Feedforward Neural Network (FNN).

Influence of Electrolytic Cell System on Graphite Electrode Interface Properties

Cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS) tests are used to investigate the performance of different cell systems. For two electrode button type cell, as the lack of reference electrode and its large contact resistance, it does not have the conditions on study the electrode/electrolyte interface performance effectively. Although the three-electrode button type cell shows some improvement on potential detection of the system which can partially eliminate the adverse effects caused by electrode polarization and reduce the contact resistance, there are some limitation on the investigation of the resistance of solid electrolyte interphase (SEI) film and charge transfer process. However, it can show the variation of kinetic parameters well in the three-electrode glass type cell, which is the premise on studying the reaction mechanism, failure mechanism and electrode interface characteristics of electrode materials.

Understanding SEI evolution during the cycling test of anode-free lithium-metal batteries with LiDFOB salt.

Anode-free lithium-metal batteries (AFLMBs) have the potential to double the energy density of Li-ion batteries, but face the challenges of mossy dendritic lithium plating and an unstable solid electrolyte interphase (SEI). Previous studies have shown that the AFLMBs with an electrolyte containing lithium difluoro(oxalato)borate (LiDFOB) salt outperform those with lithium hexafluorophosphate (LiPF6), but the mechanism behind this improvement is not fully understood. In this study, X-ray photoelectron spectroscopy (XPS) depth profile analysis and electrochemical impedance spectroscopy (EIS) were conducted to investigate the SEI on plated Li from the two conducting salts and their evolution in Cu‖NMC full cells during cycling. XPS results revealed that an inorganic-rich SEI layer is formed in the cell with LiDFOB-based electrolyte, with a low carbon/oxygen ratio of 0.56 compared to 1.42 in the LiPF6-based cell. With the inorganic-rich SEI, a dense electroplated Li with a shining surface on the Cu substrate can be retained after ten cycles. The inorganic-rich SEI enhances the reversibility of Li plating and stripping, with a high average CE of ∼98% and a stable charge/discharge voltage profile. The changes in SEI resistance and cathode electrolyte interphase resistance are more prominent compared to the changes in solution and charge transfer resistances, which further validate the role of the passivation films on Li deposits and NMC cathode surfaces in stabilizing AFLMB cycling performance.

Effective Approach by Computational Chemical Prediction and Experimental Verification to Elucidate SEI Formation Mechanism in LiPF6-, LiFSI-, and LiBF4-Containing Electrolyte Solutions

Solvation structures of ethylene carbonate-based electrolyte solutions containing 1 mol/L LiPF6, LiN(SO2F)2 (LiFSI), or LiBF4 are experimentally determined, and their electron affinity and lowest unoccupied molecular orbital (LUMO) distribution are calculated by density functional theory (DFT). The stability of the electrolyte solutions against reduction can be evaluated using electron affinity of the solvates, and the formation mechanism of the solid electrolyte interphase (SEI) can be predicted by their LUMO distributions. The calculation results suggest that 1 mol/L LiPF6 and LiFSI electrolyte solutions should form SEI consisting of carbonate and carboxylate which derives from solvent decomposition and LiF originating from anion decomposition. These predictions are verified experimentally by scanning electron microscopy–energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy measurements. On the other hand, intriguingly, the reductive decomposition of 1 mol/L LiBF4 electrolyte solution does not simply proceed as theoretically predicted, and the subsequent reactions of SEI need to be considered: further decomposition of the organic SEI and formation of inorganic particles containing lithium, fluoride, oxygen, and boron. These results are consistent with an increase in the resistance of SEI and interfacial lithium ion transfer at the graphite negative electrode. Thus, a combination of DFT prediction and experimental analysis is an effective approach to reveal SEI formation mechanisms and can be utilized as a versatile approach to develop new solvents, electrolyte salts, and additives and to design electrolyte solutions with an appropriate concentration.

Binder Chemistry Dependent Electrolyte Reduction in Potassium‐Ion Batteries: A Successive, Two‐Step Reduction Way

AbstractControlling electrode/electrolyte interfacial chemistry is critically important for improved K+ storage, but the influences of binder chemistry on electrolyte decomposition and interfacial properties are still poorly understood. Herein, sodium carboxymethyl cellulose (CMC)‐based, and polyvinylidene fluoride (PVDF)‐based graphite electrodes are introduced as model systems to quantify the electrolyte decomposition, solid electrolyte interphase (SEI) formation, and the corresponding kinetic evolution transition. A noncatalytic electrolyte reduction path on the CMC‐based electrode and a catalytic reduction path on the PVDF‐based electrode are identified, in terms of the reduction overpotential and product selectivity. The electrolyte reduction and/or SEI formation are found to occur in a successive, two‐step manner, starting with the electrochemical reduction at a potential above 0.35 V where no potassiation has happened (step I), and followed by the thermodynamically accelerated electrolyte reduction at a potential below 0.35 V (step II). Kinetics analysis reveals the former is charge transfer‐controlled for both CMC and PVDF‐based electrodes, and the latter involves a kinetic transition to SEI resistance controlled for the PVDF system, while it is charge transfer‐controlled for the CMC system. All these examples, highlight that binder chemistry plays a dominant role in the electrolyte decomposition and electrode/electrolyte interfacial properties, and promote a better fundamental understanding of electrolyte reduction.

Mechanism of Improved Lithium-Sulfur Battery Performance by Oxidation Treatment to Microporous Carbon as Sulfur Matrix

1. IntroductionLithium-sulfur (Li-S) batteries are rechargeable devices assembled with a sulfur cathode and a lithium metal anode. Li-S batteries have twice the volumetric energy density and 5 times the gravimetric energy density of lithium-ion batteries (LIB). Hence, Li-S batteries are expected to be applied to stationary power sources and EV vehicles [1]. However, Li-S batteries have the following issues:・Sulfur and the final discharge product (Li2S) are insulators.・In the discharge process, sulfur expands up to 1.8 times, so the structure of batteries is unstable.・Intermediate products (Li2Sx (x = 4 – 8)) dissolve in an electrolyte; Li2Sx (x = 4 – 8) diffused to an anode to provide an insulating layer at the anode surface.・In the charge process, Li2Sx (x = 4 – 8) causes redox shuttling.As a result, Li-S batteries cannot charge and discharge stably. In order to deal with this problem, Nazar et al. reported the method that sulfur is confined in porous carbon [2]. This approach provides a cathode realizing good electronic conductivity, restriction of sulfur expansion, and suppression of Li2Sx (x = 4 – 8) dissolution. Although these improved characteristics allow Li-S batteries to operate, The discharge capacity of Li-S batteries is still not high enough and this needs to be addressed.In our previous study, we reported that oxidation treatment to microporous carbon (MC) with HNO3 improves Li-S batteries' discharge capacity [3]. Moreover, we clarified that the discharge capacity of Li-S batteries has an approximate proportional relation with the amount of oxygen-containing functional groups on the MC surface [4].This work attempts to elucidate the mechanism of improved Li-S battery performance by oxidation treatment to MC. Our report would lead to the proposal of a novel strategy to improve the performance of Li-S batteries.2. Method2.1 Preparation of Oxidized MC-Sulfur composite (Ox MC-S)MC was added into 69 wt.% HNO3 and refluxed at 120ºC for 2 h. By vacuum filtration and washing with deionized water, Ox MC was obtained. Ox MC dried in vacuum at 80ºC overnight was mixed with sulfur at a weight ratio of Ox MC: S = 48: 52. The mixture was thermally annealed at 155ºC for 5 h (Ox MC-S). Untreated MC was also composited with sulfur by the same method (MC-S).2.2 Assembling of CellsEach MC-S cathode was prepared by mixing the MC-S, acetylene black, carboxymethyl cellulose, and styrene butadiene rubber at a respective weight ratio of 89: 5: 3: 3 and coating the resulting aqueous slurry on an Al foil current collector. The cells with the MC-S electrode and Li metal foil as an anode were assembled in a glove box filled with Ar. Lithium bis(trifluorosulfonyl)imide (LiTFSI): tetraglyme (G4): hydrofluoroether (HFE) = 10: 8: 40 (by mol) was used as the electrolyte.2.3 Electrochemical Impedance Spectroscopy (EIS)To elucidate the effect of oxidation treatment on the internal resistance of Li-S batteries, EIS was carried out at various potentials (Discharge 2.0 – 1.0 V and Charge 1.0 – 3.0 V). The obtained Nyquist plots were used for the evaluation of solid electrolyte interphase (SEI) resistance (Rsei), charge-transfer resistance (Rct), and Warburg impedance (Rw). Rw was investigated with the calculation of Warburg coefficient (σ).3. Major results and conclusionSince oxidation treatment to MC significantly increased the discharge capacity of Li-S batteries [3][4], it was expected that oxidation treatment would lower the internal resistance of Li-S batteries. EIS of MC-S and Ox MC-S at various potentials showed that oxidation treatment reduced Rsei by an average of 12.2 Ω. This indicates that the SEI thickness was reduced, or the SEI was composed of highly ion-conductive components by the oxidation treatment. Rct decreased only at lower potentials, and the Warburg coefficient decreased except at the end of charge and discharge potential. These results suggest that the oxidation treatment decreases overall resistance, but especially SEI resistance and Warburg impedance, which may improve the discharge capacity of Li-S batteries.We will also report the activation energy of Rsei and Rct and mechanism analysis of decreasing Rsei by oxidation to MC. This work was supported by “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING [JPMJAL1301])” from JST.[1] Y. Guo et al., Angew. Chem. Int. Ed., 52 (2013) 13186.[2] X. Ji et al., Nat. Mater., 8 (2009) 500.[3] S. Okabe et al., Electrochemistry, 85 (2017) 671.[4] L. Yoshida et al., ECS 238th PRiME Meeting s (2020).

Temperature Dependent Formation of the Graphite SEI with Vinylene Carbonate Electrolyte Additive

The charge/discharge efficiency and the cycle life of lithium-ion batteries (LiBs) depends on a large number of competing interfacial processes. Probably the best-known interface in LiBs is the solid-electrolyte-interphase (SEI), which is a passivating layer formed at the negative electrode by electrolyte decomposition during the first cycles.[1–3] Since the SEI stability is a deciding factor with regards to cycle life, understanding and improving the SEI formation process is crucial. In order to yield an effective SEI, commercial battery cells undergo an extensive so-called formation procedure after cell assembly, which generally consist of multiple voltage-holds and current-steps at various temperatures.[4,5] During the first cycles of a LiB, the SEI is generated by the reduction products of the commonly used carbonate-based electrolyte components, such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).[6,7] Additionally, electrolyte additives are commonly added to form an SEI layer with increased cycling stability and reduced irreversible capacity loss during the first cycle.[6,8–10] Among a variety of additives, vinylene carbonate (VC) is the most prominent. Since the reduction of VC initiates at higher reduction potentials compared to alkyl carbonate solvents (e.g., EC), it is reduced preferentially during the first charge and thus strongly affects the SEI composition.[6,11] In the present study we investigated the effect of the formation temperature in a range between 10-60 °C on the performance characteristics of full-cells with a graphite anode (MAGE) and an NCM831205 cathode as well as of MAGE/Li half-cells, both with LP572 (1 M LiPF6 in EC:EMC 3:7wt/wt + 2wt% VC) electrolyte. These include the first-cycle irreversible capacity loss, the negative electrode impedance build-up, the gas evolution during formation, the rate capability, and the cycle life.Employing electrochemical impedance spectroscopy using a µ-reference electrode allowed us to determine the graphite intercalation resistance (R Int), representing the sum of the SEI resistance (R SEI) and of the charge transfer resistance (R CT), after two 0.1C formation cycles at a given temperature. The impedance spectrum was recorded at 25 °C and 40% SOC referenced to 190 mAh/gNCM reversible capacity. As depicted in Figure 1, both R Int of the graphite electrode in the MAGE/NCM83125 full-cell and the first-cycle irreversible Li-loss determined a MAGE/Li half-cell increase with increasing formation temperature.Additionally, on-line electrochemical mass spectrometry (OEMS) was applied to identify and quantify the gaseous reduction products for LP572 in a MAGE/NCM831205 full-cell during one 0.1C formation cycle at different temperatures. As displayed in Figure 1, the relative increase of the total gas evolved, which is the sum of hydrogen, ethylene, carbon monoxide, and carbon dioxide, follows the relative increase of the intercalation resistance. Next to the overall gas amounts, changes in the quantity of the individual gases give insights into the highly temperature dependent reduction mechanisms of VC. Acknowledgement: The authors thank BMW AG for their financial support.

State-of-Health of Li-Ion Battery Estimation Based on the Efficiency of the Charge Transfer Extracted from Impedance Spectra

Several studies show that impedance spectroscopy is a suitable method for online battery diagnosis and State-of-Health (SoH) estimation. However, the most common method is to model the acquired impedance spectrum with equivalent circuits and focus on the most sensitive parameters, namely the charge-transfer resistance. This paper introduces first a detailed model of a battery cell, which is then simplified and adapted to the observable spectrum behavior. Based on the physical meaning of the model parameters, we propose a novel approach for SoH assessment combining parameters of the impedance spectrum by building the ratio of the solid electrolyte interphase (SEI) resistance to the total resistance of SEI and the charge transfer. This ratio characterizes the charge-transfer efficiency at the electrodes’ surfaces and should decrease systematically with SoH. Four different cells of the same type were cycled 400 times for the method validation, and impedance spectroscopy was performed at every 50th cycle. The results show a systematic correlation between the proposed ratio and the number of cycles on individual cell parameters, which build the basis of a novel online method of SoH assessment.

In-situ Analysis of the Solid-Electrolyte Interphase Formed in Li[N(CF3SO2)2] and Li[N(FSO2)2] Tetraglyme Solvate Ionic Liquids

Li metal has attracted attention as the negative electrode for secondary batteries due to its low density and negative redox potential. However, its negative operating potential also leads to the decomposition of electrolytes, resulting in the formation of some interfacial phase on the anode, i.e., solid-electrolyte interphase (SEI). The decomposition of the electrolyte causes the low coulombic efficiency of the Li anode. Therefore, the characterization of SEI is necessary in order to utilize the Li anode.Solvate ionic liquids (SILs) composed of 1:1 equimolar mixture of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) or bis(fluorosulfonyl)amide (LiFSA) and glyme have been investigated as the alternative electrolytes for lithium secondary batteries because of their high Li+ concentration [1, 2]. The deposition and dissolution of Li were reported to be possible in the SILs, although the cathodic decomposition of the electrolyte also proceeded at the potential more positive than that of Li(I)/Li. We have already reported the improvement of the morphology of Li deposits and the cyclability of the Li anode in LiTFSA-triglyme (G3) by the formation of SEI on a Cu electrode at 0 V vs. Li|Li(I) prior to the deposition of Li [3]. The cyclability of the Li anode in LiFSA-tetraglyme (G4) was found to be superior to that in LiTFSA-G4. However, the difference in the stability of Li deposits in these SILs has not been elucidated. In the present study, the formation of SEI on a Cu electrode and the interfacial reaction of the Li deposits were investigated in LiTFSA-G4 and LiFSA-G4 using electrochemical impedance spectroscopy (EIS) combined with electrochemical quartz crystal microbalance (EQCM).The solvate ionic liquids were prepared by mixing the equimolar quantities of G4 and LiTFSA or LiFSA. EIS and EQCM measurement were conducted on the same Cu-coated EQCM electrode (fundamental frequency: 9 MHz) using an impedance-type EQCM system (SEIKO EG&G, QCM922A) and a computerized electrochemical system (Hokuto Denko, HZ-7000). Li foil was used as a reference and counter electrode, respectively. SEI was formed at 0 V vs. Li|Li(I) on the Cu-coated EQCM electrode prior to the galvanostatic deposition of Li. EIS was performed at the open circuit potential (OCP) in the frequency range from 20 kHz to 1.0 Hz with an amplitude of 5 mV rms during SEI formation and after Li deposition.The resonance frequency (f) of a Cu-coated EQCM electrode gradually decreased with keeping the potential of the electrode at 0 V. A semicircle corresponding to the parallel circuit of the resistance and capacitance of SEI was observed in the Nyquist plots of the Cu electrode after keeping the potential at 0 V. The resistance of SEI, R SEI, increased with the lapse of time over 24 hours, indicating the formation and growth of SEI on the Cu electrode at 0 V in LiTFSA-G4 and LiFSA-G4. The changes in f and R SEI in LiFSA-G4 were smaller than those in LiTFSA-G4, suggesting the SEI formed in LiFSA-G4 was lighter and more conductive.The larger change in f of the Cu-EQCM electrode than that calculated from the passed electric charge implied that SEI was also formed on Li deposits during the galvanostatic cathodic reduction at 0.01 mA cm–2, as observed during cyclic voltammetry in LiTFSA-G3 [4]. Δf in LiFSA-G4 was smaller while the changes in the resonance resistance mainly reflecting the surface roughness of the electrode were close in the SILs, suggesting the reactivity against Li was low in LiFSA-G4. The electrode potential of the Cu-EQCM electrode changed over from that of Li(I)/Li after few tens of hours, suggesting the SEI formed in SILs did not suppress completely the reaction between Li and the electrolyte at the OCP. The transition time of the potential in LiFSA-G4 was longer than that in LiTFSA-G4, indicating Li deposits were more stable in LiFSA-G4.References H. Moon, et al., J. Phys. Chem. C, 118, 20246 (2014).S. Terada, et al., Aust. J. Chem., 72, 70 (2019).N. Serizawa, et al., J. Electrochem. Soc., 167, 110560 (2020).N. Serizawa, et al., J. Electrochem. Soc., 160, A1529 (2013). AcknowledgementThis study was supported in part by the JSPS KAKENHI (Grant No. 19K15682) and Advanced Low Carbon Technology Research and Development Program (ALCA, JPMJAL1301).

Deposition and Dissolution of Lithium in 1-Methyl-1-methoxyethylpyrrolidinium Bis(fluorosulfonyl)amide Ionic Liquid Electrolyte with Different Compositions

The solid electrolyte interphase (SEI) between Li and ionic liquid electrolytes was investigated in 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide (MPPFSA) and 1-methyl-1-methoxyethylpyrrolidinium bis(fluorosulfonyl)amide (MOEMPFSA) with different LiFSA concentrations. The physicochemical properties and the coordination states of Li+ with FSA– with increasing the LiFSA concentration were similar between xLiFSA-MPPFSA and MOEMPFSA. On the other hand, the SEI resistance obtained by electrochemical impedance spectroscopy using a Li∣Li symmetrical cell increased with the lapse of time in xLiFSA-MPPFSA more significantly than in xLiFSA-MOEMPFSA, suggesting that the SEI formed in xLiFSA-MOEMPFSA was more conductive than that in xLiFSA-MPPFSA. The SEI formed on Li was considered to be composed mainly of the ions in the electrolytes and their decomposed products by X-ray photoelectron spectroscopy. The cyclability of deposition and dissolution of Li was improved with increasing the LiFSA concentration in both xLiFSA-MPPFSA and MOEMPFSA. An increase in the conductivity of the SEI with increasing the LiFSA concentration may lead to the promotion of homogeneous growth of Li rather than dendritic growth of Li.