Properties Of Solid Electrolyte Interphase Research Articles

Approaching in-Situ Investigation of SEI Growth Dynamics By Isotope-Assisted TOF-SIMS

Among the many challenges for Li-ion batteries (LIBs) to achieve long cycle life, formation of high quality solid electrolyte interphase (SEI) is of critical importance. SEI forms on LIB anode during the initial cycles, and its properties directly impact battery life and power performance. In-depth understanding of SEI growth mechanism can provide critical parameter input for modeling and simulation, set foundation for the SEI properties and performance correlation, and lead to the ultimate effective artificial SEI or coating design to achieve satisfactory cell cycle efficiency. However, due to the nm thickness and air/moisture sensitivity of SEI, designing and performing in-situ diagnostics are technically challenging. To date, most studies on SEI properties are performed on electrodes harvested after SEI formation, and the time-dependent growth mechanism of SEI during its formation remains to be elucidated. In this talk, we will present an approximation of in-situ diagnostic method, by utilizing isotope-assisted time-of-flight secondary ion mass spectrometry (TOF-SIMS). This method is capable of obtaining time-dependent information during SEI growth, even after the growth process has completed. The model battery design includes a Cu electrode, 6Li labeled electrolyte (1M 6LiBF4 in EC:DEC 1:2) and a 7Li metal counter electrode. As SEI grows on the Cu electrode, the 6Li:7Li isotope ratio in the electrolyte decreases due to 6LiBF4 consumption. The time scale of SEI growth thus can be represented by the 6Li:7Li ratio in the SEI, and each 6Li:7Li ratio corresponds to a snapshot during SEI growth. The isotope ratio are measured with TOF-SIMS after harvesting the SEI. With this method, we have found that the SEI portion formed first (higher 6Li:7Li ratio) presents at the electrolyte/SEI interface, and the SEI formed last (lower 6Li:7Li ratio) exists at the SEI/electrode interface. These results also indicate that new SEI forms at the SEI/electrode interface. Further chemistry analyses with TOF-SIMS and XPS show that the portion formed first are mostly organic compounds while the portion formed last are mostly inorganic compounds. This method could also be applied in other fields where growth dynamics are of interest, such as investigating Li dendrite growth. Figure 1

A Non-Linear Multi-Phase Field Model for Li Plating and Dendrite Growth at Li Metal Anode Incorporating Solid Electrolyte Interphase (SEI) Layer

Despite the highest specific capacity and lowest voltage of all secondary Li battery anode candidates, the application of Li metal anode has been a challenge due to dendritic Li electrodeposition in liquid electrolytes, leading to battery degradation and even safety issues. We previously generated a dendritic pattern map using a non-linear phase field model for a Li metal/liquid electrolyte binary system. However, the solid electrolyte interphase (SEI), the key to understand the mechanism of Li dendrite formation, is not explicitly included in the model. In this study, we hereby further advance our phase field model by incorporating the effect of SEI layer (or any artificial electrode protective coating layers (PCLs)) as an additional phase. A new set of phase-fields are re-defined to distinguish the spatial distribution of each individual phase in the electrode/SEI/electrolyte microstructure. Butler-Volmer type non-linear electrochemical kinetics and the SEI phase rigid body translation are tested to validate the new model. The impacts of different SEI properties (e.g. conductivity, Li diffusion coefficient, electron tunneling range, etc.) on Li plating and dendrite formation will be investigated. The possibility of incorporating the SEI mechanical effects on Li plating will also be discussed.

Interfacial Chemistry at the Surface of Li-Metal Anodes

Efficient energy storage is essential not only for the success of renewable energies, but also for many of daily society needs including communications, electronics, and medical and biomedical applications, among others. Electrochemical cells are complex systems integrated by several materials and their corresponding interfaces whose physical-chemical behavior is dominated by highly coupled mass and electron transport and chemical and electrochemical reactions. During lithium ion battery operation for example, during charge, lithium ions move in the electrolyte towards the anode surrounded by coordination spheres that may include solvent, salts, and other species such as additives. Once the solvated ion (inserted in a usually multicomponent network) reaches the anode surface, many additional events will take place because of the sequential and sometimes almost simultaneous reduction of the cation and that of the various components of the coordination shell. To characterize such complex environment, we analyze the Li ion coordination shell as a function of the distance to the electrode surface and electrode potential. We use density functional theory (DFT) and ab initio molecular dynamics (AIMD), to identify structural properties, as well as reactions and reaction mechanisms. Among the various “additional” species we consider the presence of long-chain polysulfides, that migrate in the electrolyte towards the Li metal anode in the Li-S chemistry. In this talk we will discuss physical, chemical, and electrochemical phenomena occurring at solid/electrolyte interfaces of Li-ion batteries and how the methods of first-principles computational analysis may be used to understand them and suggest strategies for practical solutions. In particular we will focus on the solid electrolyte interphase (SEI) occurring due to electrolyte reduction on or near Li anode surfaces and the role of the nature of the solvents, salts, and additives on the resultant SEI properties as well as on dendrite formation and battery performance.

Mechanical insights into the stability of heterogeneous solid electrolyte interphase on an electrode particle

Solid electrolyte interphase (SEI) is known to be heterogeneous which comprises inorganic and organic decomposition products. To assess the effects of heterogeneity on the stability of the SEI, we establish a mathematical model in simulating the stress of heterogeneous SEI. Comparing with the analytical solution of stress in the homogeneous film, the heterogeneity of SEI is identified to be essential in the stress calculation of the inorganic layer, which is proven decisive for the stability of SEI. Further, the peak tensile stress within the inorganic layer is found in the bilayer SEI. It generates at the interface between the active material and the inorganic layer when the battery is fully charged. In addition, the impacts of the interface parameter, modulus, Poisson’s ratio, thickness of SEI and lithiation properties of active material on SEI stress are systematically investigated. Based on the simulation results, this work provides insights into the stress analysis of the heterogeneous SEI, as well as suggestions in pursuing a well-designed SEI, i.e., a functional gradient heterogeneous SEI in which the inner layer close to the active material provides sufficient mechanical performance while the outer layer near the electrolyte performs good chemical and electrochemical behaviors, to enhance the battery performance.

Comparative surface analysis study of the solid electrolyte interphase formation on graphite anodes in lithium‐ion batteries depending on the electrolyte composition

The composition of the solid electrolyte interphase (SEI) on graphite anodes is characterized within a comparative surface analytical study varying systematically the electrolyte composition and the cycling conditions. In particular, the conducting salts lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)imide as well as vinylene carbonate and 1‐fluoroethylene carbonate as different electrolyte additives are compared regarding the SEI formation under different cycling conditions. A comprehensive study using X‐ray photoelectron spectroscopy revealed pronounced differences of the SEI compositions at different aging stages. Both additives significantly influence the SEI composition and are able to prevent from parasitic side reactions as well as from decomposition of the conducting salt lithium hexafluorophosphate. This study suggests a promising approach to improve the SEI properties to enhance long‐term stability of lithium‐ion batteries by changing the electrolyte composition. Copyright © 2016 John Wiley & Sons, Ltd.

Connecting the irreversible capacity loss in Li-ion batteries with the electronic insulating properties of solid electrolyte interphase (SEI) components

The formation and continuous growth of a solid electrolyte interphase (SEI) layer are responsible for the irreversible capacity loss of batteries in the initial and subsequent cycles, respectively. In this article, the electron tunneling barriers from Li metal through three insulating SEI components, namely Li2CO3, LiF and Li3PO4, are computed by density function theory (DFT) approaches. Based on electron tunneling theory, it is estimated that sufficient to block electron tunneling. It is also found that the band gap decreases under tension while the work function remains the same, and thus the tunneling barrier decreases under tension and increases under compression. A new parameter, η, characterizing the average distances between anions, is proposed to unify the variation of band gap with strain under different loading conditions into a single linear function of η. An analytical model based on the tunneling results is developed to connect the irreversible capacity loss, due to the Li ions consumed in forming these SEI component layers on the surface of negative electrodes. The agreement between the model predictions and experimental results suggests that only the initial irreversible capacity loss is due to the self-limiting electron tunneling property of the SEI.

Comparison of Spatiotemporal Changes of SEI Properties on Different Anodes By SECM

Li ion batteries are widely used in our society. Mainly during the first charging process the solid electrolyte interphase (SEI) between the electrode and electrolyte is formed by the decomposition of electrolyte components at the lithiated graphite. This layer is critical for the performance and safety of the Li ion batteries.[1] The aim of this study is to characterize the electron transport across the SEI, which determines the ongoing reduction of electrolyte components. Thus, understanding the electron transport across the SEI helps to improve Li ion batteries. Characterization of the SEI is challenging, because of the variety of chemically similar components and enclosed electrolyte species. Furthermore, ex situ analysis of the SEI requires separation and isolation of the SEI, which may change the content and the structure of the SEI.[2] Recently we used the feedback mode of scanning electrochemical microscopy (SECM) to investigate in situ the electron transport at the lithiated graphite (Figure).[3] 2,5-di-tert-butyl-1,4-dimethoxy benzene was identified as an useful SECM mediator providing sufficient stability and sensitivity to study the passivation properties of the SEI. Our results by SECM show gradual and significant short-term spatiotemporal changes of the SEI properties and demonstrate the dynamic and spontaneous behavior of SEI formation, damage and reformation under open circuit conditions above lithiated graphite anodes. The results emphasize that spatiotemporal changes of the passivating SEI properties are highly localized and occur preferentially in between the gaps of graphite particles. A HOPG model electrode shows significant differences to a graphite composite electrode, indicating the impact of stressed graphite particles on the SEI stability. In addition, imaging experiments show the formation, detachment and reformation of gas bubbles. The outcome of significant short-term spatiotemporal changes of the SEI properties clarifies that electrolyte reduction still occurs after SEI formation at localized spots. All measurements were conducted under open circuit conditions and thus the charging state remained constant. Consequently, the SEI is unstable even under constant charging state. The detection of gas bubbles is in line with the electrolyte reduction events nearby, since electrolyte reduction is a requirement for gas formation. Further research using this methodology focuses on Li and Si electrodes. H.B. thanks the Lower Saxony Ministry of Science and Art for funding within the Graduate Program GEENI - Graduate school of electromobility and energy storage in Lower Saxony. M.S. and M. W. gratefully acknowledge the provision of anode substrates by Infineon Technologies Austria AG.

Chemical, Electrochemical and Physical Properties of the Solid Electrolyte Interphase on HOPG Surface: Mechanisms of Formation and Transition Metal Deposition

Both the reaction pathways that lead to the formation of the solid electrolyte interphase (SEI) on the negative electrode of a lithium ion battery and the chemical, electrochemical and physical properties of the SEI are not well understood. Consequently, a full control of the SEI composition and properties through the selection of appropriate formation conditions is far from being achieved. In this contribution we present the influence of the formation potential on the chemical and electrochemical properties of SEIs formed on model carbon electrodes. Our results show that, on HOPG, SEI is formed via a two-step process, where the step occurs at respectively ~700 and ~400 mV vs. Li/Li+. The SEIs that are formed at potentials corresponding to the first or the second potential step exhibit drastically different characteristics in terms of chemical composition, thickness and transport properties. Our results are consistent with electrochemical investigations that are performed on commercial composite carbon electrodes. The SEI formed at higher potentials appears to be disadvantageous for Li+ ion intercalation. A further reduction of the SEI at lower potential is required to improve the intercalation. The still debated mechanism of transition metal deposition on the anode of Li-ion batteries was also investigated on model HOPG electrode surfaces exposed to contaminated electrolyte after completion of the SEI formation. Our results indicate, that the deposition of transition metals is strongly affected by the nature of the SEI, in particular by its thickness. By introducing transition metal cations in our model system we demonstrate, that the respective deposition takes place at the SEI compact layer surface via an electron tunneling mechanism. Figure 1

Cyclic Carbonate Additives Effect on the Solid Electrolyte Interphase of Li Ion Batteries

We report on the effect of vinylene carbonate (VC) and vinyl ethylene carbonate (VEC) electrolyte additives on the formation, stability and Li+ transport properties of solid electrolyte interphase (SEI). The SEI is thought to consist of an inner inorganic part with an outer organic layer, likely formed via oligomerization of the solvent, which may change in the presence of additives. Our analysis of a Au model anode system in a 1 M LiClO4/propylene carbonate electrolyte solution with and without additive molecules shows that additives influence the lithiation current, as well as the mass and rigidity of the SEI film. Addition of the additive molecules increases the lithiation current. VC-containing solutions exhibit the highest lithiation currents. Interestingly, when comparing the additive containing electrolytes, increasing the concentration of additives causes an over-potential for the lithiation, which results in lower lithiation currents. These preventive effects are more pronounced in VEC containing electrolytes. Our results suggest that addition of additive molecules, influences the performance of Li ion batteries in part by modifying the SEI rigidity. Addition of VC decreases film rigidity, while providing a stable protective layer, which improves stability of the SEI film.

Comparison of Spatiotemporal Changes of Solid Electrolyte Interphase (SEI) Properties on Different Anodes By Scanning Electrochemical Microscopy (SECM)

In our society Li ion batteries are widely used. Mainly during the first charging process the solid electrolyte interphase (SEI) between the electrode and electrolyte is formed by the decomposition of electrolyte components at the lithiated graphite. This layer is critical for the performance and safety of the Li ion batteries.[1] The aim of this study is to characterize the electron transport across the SEI, which determines the ongoing reduction of electrolyte components. Thus, understanding the electron transport across the SEI helps to improve Lithium ion batteries. Characterization of the SEI is a challenge, because of the variety of chemically similar components and enclosed electrolyte species. Furthermore, ex situ analysis of the SEI requires separation and isolation of the SEI, which may change the content and the structure of the SEI.[2] Recently we used the feedback mode of scanning electrochemical microscopy (SECM) to investigate in situthe electron transport at the lithiated graphite (Figure).[3] 2,5-di-tert-butyl-1,4-dimethoxy benzene was identified as an useful SECM mediator providing sufficient stability and sensitivity to study passivation properties of the SEI. Our results by SECM show gradual and significant short-term spatiotemporal changes of the SEI properties and demonstrate the dynamic and spontaneous behavior of SEI formation, damage and reformation under open circuit conditions above lithiated graphite anodes. The results emphasize that spatiotemporalchanges of the passivating SEI properties are highly localized and occur preferentially in between the gaps of graphite particles. A HOPG model electrode shows significant differences to a graphite composite electrode, indicating the impact of stressed graphite particles on the SEI stability. In addition, imaging experiments show the formation, detachment and reformation of gas bubbles. The outcome of significant short-term spatiotemporalchanges of the SEI properties clarifies that electrolyte reduction still occurs after SEI formation at localized spots. All measurements were conducted under open circuit conditions and thus the charging state remained constant. Further research using this methodology focuses on Li and Si electrodes. H.B. thanks the Lower Saxony Ministry of Science and Art for funding within the Graduate Program GEENI - Graduate school of electromobility and energy Storage in Lower Saxony. M.S. and M. W. gratefully acknowledge the provision of anode substrates by Infineon Technologies Austria AG.

Why Bis(fluorosulfonyl)imide Is a “Magic Anion” for Electrochemistry

Ionic liquids consisting of bis(fluorosulfonyl)imide (FSI–) anion show promise as electrolytes for Li-ion-based electric storage devices, as they exhibit relatively low viscosity, high chemical stability, and form robust solid–electrolyte interphase (SEI) protecting liquid electrolyte from further breakdown on the electrode. These ionic liquids have been reported to inhibit dendrite formation on lithium metal and lithiated graphite electrodes, which also relates to the unusual SEI properties. In this study, we examine the chemistry aspects that may account for this behavior. Radiolysis was used to induce redox reactions of FSI– anions in model systems, and matrix isolation electron paramagnetic resonance was used to identify radical (ion) intermediates generated in these reactions. Our results suggest that qualitative differences between such ionic liquid electrolytes versus common carbonate electrolytes reflect ease of mineralization of the reduced anion without the concurrent generation of organic radic...

Design of Solid Electrolyte Interphase to Improve Li-Ion Battery Cycle Efficiency

Among the many challenges to enable large scale commercialization of Li-ion batteries in plug-in hybrid electric vehicles (PHEVs) and full electric vehicles (EVs), formation of high quality solid electrolyte interphase (SEI) is one critical issue to achieve long term battery durability and safety. As a nano-meter scale thin film which natively forms on the negative electrode due to electrolyte reduction, SEI is a complex system composed of various lithium salts and organic reduction products. Most published works have focused on identifying the SEI composition, but the understanding of correlation between SEI properties and battery performance is still lacking.Previously, we have identified that SEI impedance is closely related with its composition. Briefly, the inorganic components (e.g. Li2O and Li2CO3, etc) constitute a low impedance and less porous part of SEI, while the organic reduction products tend to be more porous and resistive.1 In this talk, we will advance the findings and evaluate characteristic Li salts using isotope exchange method. Typical SEI components, e.g. Li2CO3, LiF, etc. are found to behave differently in passing Li. With these understandings, we will also demonstrate that the SEI composition can be modified by surface functionalization and coating on graphite electrode. By designing the SEI chemistry, the electrode/electrolyte interface is more stable and the cycling efficiency can be improved. Figure. Isotope profiles of three Li salt thin films after soaked in 1M 6LiClO4 (DMC) solution for 30s. The natural abundance of 6Li:7Li is ~0.08. The different isotope ratios in the films indicate that SEI components may behave differently in passing Li.Reference 1. Lu, P., Li, C., Schneider, E. W., Harris, S. J., J. Phys. Chem. C, 2014, 118, 896-903

Spatiotemporal changes of the solid electrolyte interphase in lithium-ion batteries detected by scanning electrochemical microscopy.

The solid electrolyte interphase (SEI) in lithium-ion batteries separates the highly reductive lithiated graphite from reducible electrolyte components. It is critical for the performance, durability, and safe operation of batteries. In situ imaging of the SEI is demonstrated using the feedback mode of scanning electrochemical microscopy (SECM) with 2,5-di-tert-butyl-1,4-dimethoxy benzene as mediator. The formation of the SEI is indicated by a decrease of the mediator regeneration rate. Prolonged imaging of the same region revealed fluctuation of the passivating properties on time scales between 2 min and 20 h with an inhomogeneous distribution over the sample. The implications of the approach for in situ assessment of local SEI properties on graphite electrodes are discussed with respect to studying the influence of mechanical stress on SEI reliability and the mode of action of electrolyte additives aiming at improving SEI properties.

Electrochemical Sodiation and Solid Electrolyte Interphase for Phosphorus Electrodes

Phosphorus (P), an element of the fifth group in the periodic table, has three main allotropes: white, red, and black. P as electrode materials for Na cells has a considerable advantage in terms of materials abundance and relatively small volume change by sodiation as Na binary compounds and alloys.[1] However, the volumetric change of P upon sodiation/de-sodiation cycles at low voltage (0.3 - 0.7 V vs. Na/Na+) caused electrolyte degradation which remains the main factor limiting the cycling life of electrodes.[1] The surface reactions of electrolytes with P in sodium ion cells have been examined. Coin cells (red or black P // Na) were cycled in electrolytes containing NaPF6 salt and ethylene carbonate/dimethyl carbonate solvent without and with the electrolyte additive (FEC or VC). Capacity retention is significantly better for the cells cycled with the electrolyte additive than for the cells cycled without the electrolyte additive, and the cells with the additive show high discharge capacity above 1450 mAh g-1 with the high coulombic efficiency than 98% after 25th cycles (Figure 1-A). To further examine the understanding of the formation of a solid electrolyte interphase (SEI) and of its evolution during cycling, powerful surface characterization techniques are combined for studying the electrode/electrolyte interphase of black P/AB electrodes, such as SEM, TEM, time-of-flight secondary ion mass spectrometry (TOF-SIMS), hard/soft X-ray photoelectron spectroscopy (HAXPES/SXPES). In particular, the effect of the electrolyte additives on the interphase chemistry was carefully explored by HAXPES and TOF-SIMS. The HAXPES analysis shows that organic species is enriched from the reduction of electrolyte solvents (EC, DEC) and electrolyte additives (FEC, VC) confirming a formation of SEI passive film. The thicker SEI layer was formed after cycling in additive-free electrolyte as compared to electrolyte with FEC or VC additive (Figure 1-B). TOF-SIMS studies reveal the SEI of the FEC-based electrolyte consists of only inorganic species, however the SEI of the VC-based electrolyte consists of organics and inorganics species. In addition, considerable modification of the surface morphology for the both electrodes was observed by SEM. Electrochemical impedance spectroscopy spectra and charge/discharge profiles of the P electrodes are analyzed to examine the kinetics evolution of electrochemical processes at different stages for sodiation. Further investigations on black or red P electrodes to improve its overall electrochemical performance (capacity, cyclability, coulombic efficiency, and voltage profile) are being carried out, including the electrode structures, effect of binders, and electrolyte/additive dependency. To summarize analytical and electrochemical results, we will discuss the potential of P as the advanced negative electrode in Na-ion batteries, concerning charge-transfer mechanisms and SEI properties.

Reduction mechanisms of additives on Si anodes of Li-ion batteries.

Solid-electrolyte interphase (SEI) layers are films deposited on the surface of Li-ion battery electrodes during battery charge and discharge processes. They are due to electrochemical instability of the electrolyte which causes electron transfer from (to) the anode (cathode) surfaces. The films could have a protective passivating role and therefore understanding the detailed reduction (oxidation) processes is essential. Here density functional theory and ab initio molecular dynamics simulations are used to investigate the reduction mechanisms of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) on lithiated silicon surfaces. These species are frequently used as "additives" to improve the SEI properties. It is found that on lithiated Si anodes (with low to intermediate degrees of lithiation) VC may be reduced via a 2e(-) mechanism yielding an opened VC(2-) anion. At higher degrees of lithiation, such a species receives two extra electrons from the surface resulting in an adsorbed CO(2-)(ads) anion and a radical anion ˙OC2H2O(2-). Additionally, in agreement with experimental observations, it is shown that CO2 can be generated from reaction of VC with the CO3(2-)anion, a product of the reduction of the main solvent, ethylene carbonate (EC). On the other hand, FEC reduction on LixSiy surfaces is found to be independent of the degree of lithiation, and occurs through three mechanisms. One of them leads to an adsorbed VC(2-) anion upon release from the FEC molecule and adsorption on the surface of F(-) and one H atom. Thus in some cases, the reduction of FEC may lead to the exact same reduction products as that of VC, which explains similarities in SEI layers formed in the presence of these additives. However, FEC may be reduced via two other multi-electron transfer mechanisms that result in formation of either CO2(2-), F(-), and ˙CH2CHO(-) or CO(2-), F(-), and ˙OCH2CHO(-). These alternative reduction products may oligomerize and form SEI layers with different components than those formed in the presence of VC. In all cases, FEC reduction also leads to formation of LiF moieties on the anode surface, in agreement with reported experimental data. The crucial role of the surface in each of these mechanisms is thoroughly explained.

A new approach to surface properties of solid electrolyte interphase on a graphite negative electrode

A new method for observing the outermost surface morphology and the thickness of solid electrolyte interphase (SEI) are presented. Low acceleration voltage in extreme high resolution scanning electron microscopy (XHR-SEM) provides detailed information on the SEI surface morphology due to secondary electron contrast mechanism. Also, XHR-SEM under low acceleration voltage with low energy induced ion etching in x-ray photoelectron spectroscopy (XPS) studies provides valuable information on the SEI layer thickness. More detailed properties of the SEI layer were investigated by transmission electron microscopy (TEM).

Properties of solid electrolyte interphase formed by prop-1-ene-1,3-sultone on graphite anode of Li-ion batteries

The physical and chemical properties of the solid electrolyte interphase (SEI) formed by prop-1-ene-1,3-sultone (PES) on graphite anode in propylene carbonate (PC) based electrolyte for lithium ion battery were investigated by charge–discharge test, scanning electron spectroscopy with energy dispersive X-ray spectroscopy (SEM–EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). It is found that the charge–discharge performance of the cell LiCoO2/natural graphite (NG) using PC-based electrolyte containing 3wt% PES is superior to that containing 6wt% propane sultone (PS), an SEI formation additive that has the similar molecular structure to PES but is reduced not as easily as PES. The results from SEM–EDS, FTIR and XPS show that the structure of graphite has been protected and some S-containing species are proven to be components of the SEI, suggesting that the preferable reduction of PES plays an important role in the formation of a protective SEI on NG.

Modeling the Effects of Additives and Oxides on Solid Electrolyte Interphase Properties on Silicon Anode Surfaces

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Effect of VC additive on MFA-based electrolyte in Li-ion batteries

Rapid capacity fading of a graphite electrode was obtained with LiPF6/MFA electrolyte due to poor properties of solid electrolyte interphase (SEI). Vinylene carbonate (VC) was therefore applied to the MFA-based electrolyte as an SEI modification additive. The effects of VC concentration on the electrochemical properties of the graphite/Li half-cell were investigated at 25 and 60 °C. At 25 °C, the graphite electrode in MFA + 1% VC-based electrolyte showed good capacity retention of 300 mA h g−1 after 50 cycles, which was comparable with that in EC–DMC-based electrolyte; at 60 °C, the electrode in MFA + 3% VC-based electrolyte showed capacity retention of 240 mA h g−1 after 50 cycles, which was much better than that in EC–DMC-based electrolyte. Overabundant VC additives showed negative effects because of the formation of too-thick and/or too-dense SEI. DSC measurements were carried out to study the thermal behavior of MFA-based electrolyte coexisting with lithiated graphite. VC-containing MFA-based electrolyte still showed much better thermal stability than EC–DMC-based electrolyte. TG–MS analysis was performed on delithiated graphite cycled in MFA-based electrolyte containing different VC concentrations to analyze VC's effect on SEI formation. In summary, VC-added MFA-based electrolyte was a promising electrolyte for the development of safer Li-ion batteries, especially at elevated temperature.

Lithium difluoro(oxalato)borate: A promising salt for lithium metal based secondary batteries?

This work is a comparative study on lithium cycling on copper in solutions based on ethylene carbonate (EC) and diethyl carbonate (DEC) (3:7, by wt.) and two lithium salts, lithium hexafluorophosphate (LiPF6) and lithium difluoro(oxalato)borate (LiDFOB). Coulombic efficiencies of the long term lithium deposition/dissolution experiments and dissolution-rate (D-rate) tests on copper demonstrated clearly the superior behavior of the LiDFOB-based electrolyte. To clarify the impact of the formed solid electrolyte interphase (SEI) on the Coulombic efficiencies achieved in the electrolytes, voltage drop values of the D-rate tests were compared with measured values of conductivities and AC impedance measurements of the electrolytes. The formed SEI has a larger influence on voltage drop values of a cell than the conductivity of the electrolyte. The correlation between surface chemistry, morphology, and Coulombic efficiencies of lithium deposition on copper was investigated by scanning electron microscopy (SEM) and Raman spectroscopy. All methods demonstrated the strong influence of the investigated lithium salts on the lithium deposition/dissolution performance on copper substrates. The LiDFOB-based electrolyte showed superior SEI properties than the LiPF6-based electrolyte.