Solid Electrolyte Interphase Resistance Research Articles

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.

Lithium-Ion Battery Real-Time Diagnosis with Direct Current Impedance Spectroscopy

The health and safety of lithium-ion batteries are closely related to internal parameters. The rapid development of electric vehicles has boosted the demand for online battery diagnosis. As the most potential automotive battery diagnostic technology, AC impedance spectroscopy needs to face the problems of complex test environment and high system cost. Here, we propose a DC impedance spectroscopy (DCIS) method to achieve low-cost and high-precision diagnosis of automotive power batteries. According to the resistance–capacitance structure time constant, this method can detect the battery electrolyte resistance, the solid electrolyte interphase resistance and the charge transfer resistance by controlling the pulse time of the DC resistance measurement. Unlike AC impedance spectroscopy, DCIS does not rely on frequency domain impedance to obtain battery parameters. It is a time-domain impedance spectroscopy method that measures internal resistance through a time function. Through theoretical analysis and experimental data, the effectiveness of the DCIS method in battery diagnosis is verified. According to the characteristics of DCIS, we further propose a fast diagnostic method for power batteries. The working condition test results show that this method can be used to diagnose online battery life and safety.

Toward Low‐Temperature Lithium Batteries: Advances and Prospects of Unconventional Electrolytes

Lithium batteries have been widely used in various fields such as portable electronic devices, electric vehicles, and grid storages devices. However, the low temperature‐tolerant performances (−70 to 0 °C) of lithium batteries are still mainly hampered by low ionic conductivity of bulk electrolyte and interfacial issues. In general, there are four threats in developing low‐temperature lithium batteries when using traditional carbonate‐based electrolytes: 1) low ionic conductivity of bulk electrolyte, 2) increased resistance of solid electrolyte interphase (SEI), 3) sluggish kinetics of charge transfer, 4) slow Li diffusion throughout bulk electrodes. Meanwhile, conventional electrolytes have been close to the upper limit of optimum low‐temperature performance owing to their intrinsic molecular structural properties. As a result, it is urgent to design unconventional electrolytes with lower melting point and higher ionic conductivity. Herein, the recent key advances in regard to unconventional electrolytes including fluorinated ester, ethyl acetate, gamma‐butyrolactone, liquefied gas, ether, plastic crystal, and aqueous electrolytes are overviewed. Solvation structure modification and SEI optimization of unconventional electrolytes for low‐temperature lithium batteries are focused. Finally, aiming at the deficiencies in current understanding, the inherent limitations and envision the future prospects of low‐temperature lithium batteries are explored.

A novel dual time scale life prediction method for lithium‐ion batteries considering effects of temperature and state of charge

Life prediction facilitates efficient management and timely maintenance of lithium-ion batteries. Challenges are still faced in eliminating the effects of battery temperature or state of charge (SOC) on the life indicator to form a life prediction method for complex onboard working conditions. To fulfill the research gap, this paper focuses on three novelties about the life indicator, effect elimination, and life prediction method. First, impedance spectra at different temperatures, SOC, and aging cycles are comprehensively studied by experiments. By fitting the spectra with an equivalent circuit model, changes of ohmic resistance, solid electrolyte interphase resistance, and charge transfer resistance (CTR) are analyzed in detail. CTR is determined as a novel life indicator, and an empirical model describing the changing trend of CTR with aging cycles is established. Second, a multi-factor coupled CTR model is applied to eliminate the strong effects of temperature and SOC during the prediction. Third, the tracking of the effects and the changing trend of the CTR with the aging cycles form a composite life prediction method with dual time scales. The results show that the battery life can be accurately predicted and the errors converge to within ±5% even though the indicator CTR is obtained at different temperatures and SOC. With this method, life prediction no longer depends on the indicator obtained in a specific state. It has great potential to broaden the implementation of life prediction for onboard conditions.

Effect of LiTFSI and LiFSI on Cycling Performance of Lithium Metal Batteries Using Thermoplastic Polyurethane/Halloysite Nanotubes Solid Electrolyte

All-solid-state lithium batteries (ASSLB) are promising candidates for next-generation energy storage devices. Nevertheless, the large-scale commercial application of high energy density ASSLB with the polymer electrolyte still faces challenges. In this study, a thin solid polymer composite electrolyte (SPCE) is prepared through a facile and cost-effective strategy with an infiltration of thermoplastic polyurethane (TPU), lithium salt (LiTFSI or LiFSI), and halloysite nanotubes (HNTs) in a porous framework of polyethylene separator (PE) (TPU–HNTs–LiTFSI–PE or TPU–HNTs–LiFSI–PE). The composition, electrochemical performance, and especially the effect of anions (TFSI− and FSI−) on cycling performance are investigated. The results reveal that the flexible TPU–HNTs–LiTFSI–PE and TPU–HNTs–LiFSI–PE with a thickness of 34 μm exhibit wide electrochemical windows of 4.9 and 5.1 V (vs. Li+/Li) at 60 ℃, respectively. Reduction in FSI− tends to form more LiF and sulfur compounds at the interface between TPU–HNTs–LiFSI–PE and Li metal anode, thus enhancing the interfacial stability. As a result, cell composed of TPU–HNTs–LiFSI–PE exhibits a smaller increase in interfacial resistance of solid electrolyte interphase (SEI) with a distinct decrease in charge-transfer resistance during cycling. Li|Li symmetric cell with TPU–HNTs–LiFSI–PE could keep its stable overpotential profile for nearly 1300 h with a low hysteresis of approximately 39 mV at a current density of 0.1 mA cm−2, while a sudden voltage rise with internal cell impedance-surge signals was observed within 600 h for cell composed of TPU–HNTs–LiTFSI–PE. The initial capacities of NCM|TPU–HNTs–LiTFSI–PE|Li and NCM|TPU–HNTs–LiFSI–PE|Li cells were 149 and 114 mAh g−1, with capacity retention rates of 83.52% and 89.99% after 300 cycles at 0.5 C, respectively. This study provides a valuable guideline for designing flexible SPCE, which shows great application prospect in the practice of ASSLB.

Investigation of capacity fade for 18650-type lithium-ion batteries cycled in different state of charge (SoC) ranges

18650-type cells comprising of LiNi0.5Co0.2Mn0.3O2 and LiNi0.9Co0.05Al0.05O2 as cathode blend and graphite as anode are cycled in various SoC ranges. Differing capacity fade is found, indicating that cycling in a medium SoC range results in lower capacity loss and behaves better than including high or low SoC in the cycling ranges. Cycling to low SoC tends to have nonlinear capacity fade (sudden capacity drop). Non-destructive methods, i.e. in situ neutron powder diffraction (NPD) and alternating current (AC) impedance, are employed to study the degradation mechanisms. Lithiated cathode loss and loss of lithium inventory (LLI) are calculated from crystal structure parameters refined from the in situ NPD. LLI is the dominating degradation factor for the differing capacity fade of cells. Lithiated cathode loss and solid electrolyte interphase (SEI) growth are deemed to be main fatigue reasons behind the LLI. By investigating the sensitivity of impedance parameters concerning the change in battery capacity, both ohmic resistance (R0) and SEI resistance (R1) present a linear relationship with the change of capacity for both, linear and nonlinear degradation, i.e. R0 and R1 follow the sudden capacity drop, which is ascribed to the formation of new SEI as evidenced by scanning electron microscopy (SEM) images.

Operando Infrared Spectroscopy Study on Electrode-Electrolyte Interface in Lithium Secondary Batteries Using Polyethylene Oxide-Based Electrolyte

(Electro-)chemical reaction at the electrode-electrolyte interface, including the formation of solid electrolyte interphase (SEI), is a crucial element to improve the performance of lithium secondary batteries. Numerous operando measurements, for instance, XPS1, Raman2, and infrared spectroscopy (IR)3 technique have been performed to study the reaction proceeding on the electrode. Operando attenuated total reflection (ATR) infrared spectroscopy technique especially has received much attention due to its high sensitivity to organic molecules, and have successfully revealed the oxidation mechanisms of carbonate solvents on the common electrode (LiNixMnyCo1-x-yO2)4. On the contrary, the electrode-electrolyte interface of the polymer electrolytes, which is considered as a next-generation electrolyte owing to its flame retardancy and structural stability5, is not well understood.In this study, operando ATR-IR was newly employed to understand (electro-)chemical reaction at the interface between electrode and polymer electrolyte, combined with various electrochemical measurements, including electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV). We selected highly ion-conductive polyethylene oxide (PEO)-based electrolyte as a target electrolyte. (Electro)chemically inert Cu nanofilm was used as a model electrode to achieve good S/N while avoiding the undesirable side reaction.Linear sweep voltammetry (LSV), performed from 2.8 VLi to -0.25 VLi, confirmed (1) no side reaction related to Cu electrode and (2) successful Li deposition at -0.25 VLi. Furthermore, LSV showed a small reduction current at ca. 1.0 VLi and ca. 0.5 VLi, which may correspond to the growth of the SEI layer on the Cu electrode. Meanwhile, we observed two semicircles in the EIS spectra, which can be assigned to the bulk resistance of the electrolyte (high-frequency semicircle) and charge transfer and SEI resistance (low-frequency semicircle) due to the following reasons; No visible change was observed for high-frequency semicircle during LSV while the low-frequency semicircle changed significantly during LSV. The change in the low-frequency semicircle suggests the formation of the SEI layer, which is in agreement with the observation of the small reduction current during LSV. Operando ATR-IR spectra, obtained simultaneously with LSV, showed the voltage-dependent peaks at ca. 1000 cm-1 and ca. 1200 cm-1, further indicating the SEI layer formation.We will discuss the formation mechanisms of the SEI during the LSV, based on the detailed analysis of operando ATR-IR spectra. It will also be demonstrated that the holistic information about the electrode-electrolyte interface can provide additional knobs to design the highly-stable and highly-ion conductive polymer electrolyte, leading the implementation of all-solid-state battery technologies. Dedryvère, R. et al. J. Power Sources 174, 462–468 (2007).Wu, H. L., Huff, L. A. & Gewirth, A. A. ACS Appl. Mater. Interfaces 7, 1709–1719 (2015).Hongyou, K. et al. J. Power Sources 243, 72–77 (2013).Zhang, Y. et al. Energy Environ. Sci. 13, 183–199 (2020).Long, L., Wang, S., Xiao, M. & Meng, Y. J. Mater. Chem. A 4, 10038–10039 (2016).

Towards Quantifying Capacity Losses Due to Solid Electrolyte Interphase Evolution in Silicon Thin Film Batteries

To better understand failure mechanisms in lithium-ion batteries (LIBs), it is imperative to quantify the capacity fading contributions from active material loss and solid electrolyte interphase (SEI) growth. However, these two contributions are indistinguishable in traditional electrochemical measurements. In this study, we used a model system in combination with (1) precision measurements of the overall Coulombic efficiency via electrochemical experiments and (2) X-ray reflectivity (XRR) measurements of the active material losses. The model system consisted of a 515 Å thick amorphous silicon (a-Si) thin film on silicon carbide in a half-cell geometry using a carbonate electrolyte with LiPF6 salt. Schematics of the cells used in operando XRR and precision electrochemistry measurements are shown in Fig. (a) and (b) respectively.Specifically, we used surface/interface sensitive operando XRR to monitor the a-Si/LixSi thickness during (de)lithiation in order to quantify the capacity fading due to active material loss. Combined with electrochemical analysis, we quantified the capacity loss originating in SEI growth during each cycle (Fig. (c)). We have found that SEI continues to grow with cycling, and the SEI thickness is dependent on the time spent at low potentials (Fig. (d)). The continued SEI growth results in increasing overpotentials due to increased SEI resistance, which leads to a lower extent of lithiation when the cut-off voltage is reached. Finally, we extracted a proportionality constant for SEI growth following a parabolic growth law. Our results are in fair agreement with the capacity losses observed in full cell systems.We envision that the proposed methodology can be used as a qualitative measure of the ion transport properties of the SEI. The model-based approach allows for quantitative determination of lithium ion loss mechanisms in LIBs by separately tracking lithium ions within the active materials and will aid in developing ways to reduce parasitic losses. Figure 1

Comprehensive electrochemical impedance spectroscopy study of Si-Based anodes using distribution of relaxation times analysis

Silicon (Si) and Silicon Monoxide (SiO) have been considered as anode materials for the next-generation Li-ion batteries (LiBs) based on their high energy density. A combination of electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analysis is used to investigate the electrochemical behaviors of Si and SiO anodes. Systematic DRT analysis on EIS data collected using different cell geometries (e.g., half and symmetrical cells) allows to deconvolute the origins of individual polarization losses and their evolutions under different operating conditions such as temperature and state-of-charge (SOC). Predominant polarization losses were found to be contact impedance, solid-electrolyte interphase (SEI) layer related impedance, and charge transfer impedance based on calculated activation energy and time constant that are comparable with literature. The impedance behavior of Si and SiO showed unique patterns during cycling, which could be explained by the changes in SEI due to the presence of cracks according to previous experimental studies. In addition, the fitted SEI and charge transfer resistance of Si show larger variation than those of SiO at a given SOC. Recommendations for SOC operating range in commercial applications are given for Si and SiO to achieve the best electrode performance.

Competition and synergy of carbon nanomaterials in lithium-ion battery

This article reviewed the applications of carbon nanomaterials such as carbon black (CB), carbon nanotubes (CNTs) and graphene (Gr) in lithium-ion batteries based on their structural characteristics, focusing on the key scientific issues, including the conductive network, stability of solid electrolyte interphase (SEI) and dispersibility. We discussed the differences in the regulations of the conductive additives, conductivity of active materials, current collector/electrode contact resistance and stability of SEI through different carbon nanomaterials, proving the competition between “individual” characteristics. We also discussed the synergy of carbon nanomaterial “composites” which were prepared by physical compounding or in-situ synthesis in improving dispersibility, inhibiting volume expansion, optimizing conductive networks, etc . In addition, we summarized the industrialization of carbon nanomaterials in lithium-ion batteries, and forecasted its development tendency is from competition to synergy, and opposition to unity.

Artificial Solid Electrolyte Interphases for Lithium Metal Electrodes by Wet Processing: The Role of Metal Salt Concentration and Solvent Choice.

In this study, the artificial solid electrolyte interphase (SEI) formed on lithium metal when treated in ZnCl2 solutions is thoroughly investigated. The artificial SEI on lithium metal electrodes substantially decreases the interfacial resistance by ca. 80% and improves cycling stability in comparison to untreated lithium. The presence of a native SEI negatively affects the morphology and interfacial resistance of the artificial SEI. Increasing the ZnCl2 concentration in tetrahydrofuran (THF) (precursor solution) results in higher homogeneity of the surface morphology. Independent of the ZnCl2 concentrations, the artificial SEI is composed of Cx, CO, LiCl, Li2CO3, ZnCl2, and LixZny alloys. ZnCl2 (1 M) produces the most homogenous surface and additional surface species with carbonyl side groups. Nonetheless, the ZnCl2 concentration only has a small effect on the interfacial resistance or cycling stability. Using ethyl methyl carbonate (EMC) as the solvent significantly reduces the interfacial resistance to 7 Ω cm2, in comparison to 25 Ω cm2 for THF. The composition of the artificial SEIs varies depending on the solvent. Either way, the SEI consists of Cx LixC, LiCl, Li2CO3, ZnCl2, and LiZn alloys. The THF-based SEI additionally features ether and carbonyl groups, LiZnO, and Zn metal. For the artificial SEI formed with both solvents, the atomic percentage of the LiZn alloy increases close to the Li surface.

A novel quantitative electrochemical aging model considering side reactions for lithium-ion batteries

A novel quantitative electrochemical aging model for lithium-ion batteries considering side reactions is proposed in this paper. The resistance of solid electrolyte interphase and the thickness of deposited layer caused by side reactions are utilized as degradation representatives to explicitly quantify the aging effects. The aging model is established through deriving the transfer function relationship between the aging representatives and input current history. Therefore, the gap between macroscopic (battery operating mode) and microscopic (aging mechanism) can be well bridged. The aging mechanisms for the lithium-ion batteries are well identified by comprehensive post-mortem analysis. The experimental results demonstrate that the irreversible side reactions occurring at the surface of anode particles are the primary cause for performance degradation in this study. To verify the proposed aging model, the comparisons are made between experimental and simulated results at both macroscopic cell-level (cell voltage response, capacity fade, and solid-electrolyte interphase resistance increase) and microscopic-level (deposited-layer growth). The capacity decay error is bounded to 3% up to 400 cycles. The results demonstrate that the presented transfer-function type aging model is capable of predicting battery degradation severity precisely.

Toward quantifying capacity losses due to solid electrolyte interphase evolution in silicon thin film batteries.

To understand the origins of failure and limited cycle life in lithium-ion batteries (LIBs), it is imperative to quantitatively link capacity-fading mechanisms to electrochemical and chemical processes. This is extremely challenging in real systems where capacity is lost during each cycle to both active material loss and solid electrolyte interphase (SEI) evolution, two indistinguishable contributions in traditional electrochemical measurements. Here, we have used a model system in combination with (1) precision measurements of the overall Coulombic efficiency via electrochemical experiments and (2) x-ray reflectivity measurements of the active material losses. The model system consisted of a 515 Å thick amorphous silicon (a-Si) thin film on silicon carbide in half-cell geometry using a carbonate electrolyte with LiPF6 salt. This approach allowed us to quantify the capacity lost during each cycle due to SEI evolution. Combined with electrochemical analysis, we identify SEI growth as the major contribution to capacity fading. Specifically, the continued SEI growth results in increasing overpotentials due to increased SEI resistance, and this leads to lower extent of lithiation when the cutoff voltage is reached during lithiation. Our results suggest that SEI grows more with increased time spent at low voltages where electrolyte decomposition is favored. Finally, we extracted a proportionality constant for SEI growth following a parabolic growth law. Our methodology allows for the quantitative determination of lithium-ion loss mechanisms in LIBs by separately tracking lithium ions within the active materials and the SEI and offers a powerful method of quantitatively understanding LIB loss mechanisms.

Interphase Formation of PEO20:LiTFSI-Li6PS5Cl Composite Electrolytes with Lithium Metal.

Composite polymer electrolytes (CPEs), consisting of solid electrolyte particles embedded within a solid polymer electrolyte matrix, are promising materials for all-solid-state batteries because of their mechanical properties and scalable production processes. In this study, CPEs consisting of PEO20:LiTFSI blended with 1, 10, and 40 wt % (CPE40) of the Li6PS5Cl electrolyte filler are prepared by a slurry-based process. The incorporation of Li6PS5Cl improves the lithium-ion conductivity from 0.84 mS cm-1 (PEO20:LiTFSI) to 3.6 mS cm-1 (CPE40) at 80 °C. Surface-sensitive X-ray photoelectron spectroscopy (XPS) reveals LiF, polysulfides, and Li3PO4 on the CPE surface, originating from decomposition reactions between PEO20:LiTFSI and Li6PS5Cl. The decomposition products influence the formation of the solid electrolyte interphase (SEI) at the lithium metal | CPE interface, resulting in a reduced SEI resistance of 3.3 Ω cm2 (CPE40) compared to 5.8 Ω cm2 (PEO20:LiTFSI) at 80 °C. The SEI growth follows a parabolic rate law and the growth rate declines from 1.2 Ω cm2 h-0.5 (PEO20:LiTFSI) to 0.57 Ω cm2 h-0.5 (CPE40) during thermal aging at 80 °C. By substituting CPEs for PEO20:LiTFSI in lithium plating and stripping experiments, the increase in SEI resistance was reduced by more than 75%. In order to get a deeper understanding of the SEI formation process, in situ XPS measurements were carried out where the lithium metal is successively deposited on the CPE sample and XPS is measured after each deposition step. On the basis of these measurements, a multistep decomposition mechanism is postulated, including the formation of LiF and Li2S as key components of the SEI.

Four-electrode symmetric setup for electrochemical impedance spectroscopy study of Lithium–Sulfur batteries

Electrochemical impedance spectroscopy is a powerful tool to probe into the complexity of the internal resistance, which is critical in affecting the actual energy density of lithium-sulfur battery. However, due to the limit of cell setup, both two and three-electrode cells are unable to reveal the resistances of individual cell components directly. We analyze the resistance of the cell components by using a novel four-electrode symmetric setup. The in situ impedance responses unmask the resistance of solid electrolyte interphase on the lithium foil, the electrolyte resistance, the diffusion resistance of porous in the cathode, the charge transfer resistance and its coupling ionic resistance in pores of the four-electrode cell. The solid electrolyte interphase consists of two layers which have differences in ion transport properties. The stripping/plating of lithium has a significant effect on the anode resistance. The charge transfer resistance and pores resistance are sensitive to the polysulfides species and cathode passivation. Finally, the loss of energy caused by the internal resistance indicates the cell-level energy density will reduce rapidly when the operating current is higher than 1 C.

Unraveling the role of binder concentration on the electrochemical behavior of mesocarbon microbead anode in lithium–ion batteries: understanding the formation of the solid electrolyte interphase

Binders play a significant role in the electrochemical performance of anodes in lithium–ion batteries. In this study, mesocarbon microbead (MCMB) electrodes are fabricated with various binder concentrations of 4.5 wt% (MCMB-4.5), 7.5 wt% (MCMB-7.5), and 10 wt% (MCMB-10) to evaluate the optimum content of binder for a MCMB anode. The cyclic voltammetry (CV) profiles indicate a slow kinetic process of lithiation/delithiation in the MCMB-10 anode. The results demonstrate that MCMB-7.5 has the highest lithium diffusion coefficient, which represents the improved lithium diffusion kinetics of MCMB-7.5. The step-by-step electrochemical impedance spectroscopy (EIS) studies indicate that a highly conductive solid electrolyte interphase (SEI) film is formed from 0.1 to 0.01 V on MCMB-7.5 anode that not only makes up the increase of SEI resistance (RSEI), relating to the expansion of MCMB volume, but also leads to a decrease of the RSEI. The MCMB-4.5, MCMB-7.5, and MCMB-10 show self-discharge rates of 0.99, 0.28, and 1.09% per week, respectively. Thus, the MCMB-7.5 has the lowest level of parasitic reactions between the active material and electrolyte. Furthermore, the 7.5% binder concentration provides the best capacity retention along 100 cycles at 0.5 C rate, which can suggest as the optimum concentration of CMC/SBR for MCMB anodes.

Understanding the Effect of Polydopamine Interlayer on the Long‐Term Cycling Performance of Silicon Anodes: A Multiphysics‐Based Model Study

AbstractTo understand the effect of a polydopamine interlayer between a copper current collector and a silicon composite electrode, a physics‐based model is used to analyze the cycle performance of silicon‐based lithium‐ion half‐cells with bare and polydopamine‐treated copper current collectors. We investigate the capacity‐fading mechanisms of the two cell configurations by analyzing the model parameters that change with cycling. The major capacity‐fading mechanisms in the silicon‐based anodes are the increase in film resistance (solid electrolyte interphase resistance and contact resistance) and the isolation of silicon active material. The polydopamine interlayer reduced the contribution of the film resistance and isolation of the silicon active material to the capacity fade by 22 % and 10 %, respectively. The insulating‐nature of the polydopamine interlayer resulted in an increase in the charge transfer resistance contributing to 15 % reduction in the capacity retention. The efficacy of the physics‐based model is validated with experimental data obtained from silicon‐based half‐cells with bare and polydopamine‐treated copper current collectors.

Is the solid electrolyte interphase in lithium-ion batteries really a solid electrolyte? Transport experiments on lithium bis(oxalato)borate-based model interphases

In order to improve the properties of the solid electrolyte interphase (SEI) on the graphite anode in lithium-ion batteries, different electrolyte additives are used, such as lithium bis(oxalate)borate (LiBOB), vinylene carbonate, and fluoroethylene carbonate. It is known that LiBOB increases the SEI resistance, but there is very little fundamental knowledge about the influence of LiBOB on the structure of the SEI as well as on ion and molecule transport mechanisms in the SEI. Here, we study SEIs grown at the interface between a planar glassy carbon electrode and battery electrolytes containing different amounts of LiBOB. The SEIs are characterized by a combination of FIB-SEM, AFM, electrochemical impedance spectroscopy and redox probe experiments. The transport of Li+ ions and of redox molecules becomes slower with increasing LiBOB concentration in the electrolyte, but like observed for a LiBOB-free electrolyte, the effective diffusion coefficients of Li+ ions and ferrocene molecules in the SEIs are virtually identical and show the same temporal evolution after voltammetric SEI formation. This gives strong indication that both Li+ ions and molecules are transported in the liquid electrolyte phase inside the pores of the SEI and thus challenges the common view of a solid-electrolyte-type Li+ transport mechanism in SEIs.

Monitoring SEI Resistance during Formation of Li-Ion Batteries By Impedance Spectroscopy

Electrochemical impedance spectroscopy is a powerful tool to study electrode/electrolyte interfaces while they evolve, as measurements can be performed quickly and in-situ, i.e., in the battery cell without need for disassembly. To interpret electrochemical impedance spectroscopy (EIS) data is however not trivial, as all the diverse processes like electronic resistances, electrolyte ionic resistance, transport limitations within the electrodes, surface film resistances, and the actual charge transfer resistances of both electrodes contribute simultaneously to the EIS spectra. Recently, we developed a three-electrode Swagelok® type T-cell with a gold wire reference electrode (GWRE), which allows for a reliable deconvolution of anode and cathode impedance [1]. Using this GWRE, we were able to isolate the evolution of electrical contact resistance, pore resistance and charge transfer resistance of a lithium nickel manganese spinel (LNMO) cathode during cycling by impedance measurements at non-blocking and blocking conditions [2]. At the latter, the intercalation reaction is thermodynamically hindered, which increases the charge transfer resistance significantly and leads to a shift in its characteristic frequency, facilitating a separation of other impedance contributions. In the present study, we apply the concept of blocking conditions to a graphite anode in LFP/graphite cells equipped with a GWRE. To monitor the SEI evolution during formation, we use a potential stepping procedure, which lowers the potential of the graphite anode gradually with every step down to 0.01 V vs. Li/Li+ and then back up. After each step, the graphite potential is moved back to blocking conditions (2 V vs. Li/Li+), where impedance spectra are acquired. As the charge transfer resistance in blocking conditions appears at very low frequencies, we can unambiguously ascribe an emerging additional resistance in the mid-frequency range to the formation of a growing solid-electrolyte interphase (SEI) resistance. The evolution of SEI resistance as a function of potential is now obtained by fitting the recorded spectra to a transmission-line based equivalent circuit model. A comparison of the SEI resistance evolution with the reduction currents obtained by cyclic voltammetry (CV) in different electrolytes (pure LP57 or LP57 + 1 wt% vinylene carbonate (VC), fluoroethylene carbonate (FEC) or difluoroethylene carbonate (DiFEC)) confirms that the electrolyte/additive reduction potential coincides with an increase in the SEI resistance (see Figure 1). Lastly, we investigate how the SEI resistance develops during continuous cycling of LFP/graphite cells under different conditions.