Morphology and modulus evolution of graphite anode in lithium ion battery: An in situ AFM investigation

We investigated the interfacial electrochemical processes on graphite anode of lithium ion battery by using highly oriented pyrolytic graphite(HOPG)as a model system.In situ electrochemical atomic force microscopy experiments were performed in 1M lithium bis(trifluoromethanesulfonyl)imide/ethylene carbonate/diethyl carbonate to reveal the formation process of solid electrolyte interphase(SEI)on HOPG basal plane during potential variation.At 1.45 V,the initial deposition of SEI began at the defects of HOPG surface.After that,direct solvent decomposition took place at about 1.3 V,and the whole surface was covered with SEI.The thickness of SEI was 10.4±0.2 nm after one cycle,and increased to 13.8±0.2 nm in the second cycle,which is due to the insufficient electron blocking ability of the surface film.The Young’s modulus of SEI was measured by a peak force quantitative nanomechanical mapping(QNM).The Young’s modulus of SEI is inhomogeneous.The statistic value is 45±22 MPa,which is in agreement with the organic property of SEI on basal plane of HOPG.     

Inhibition of lithium dendrites and dead lithium by an ionic liquid additive toward safe and stable lithium metal anodes

The uncontrolled growth of lithium dendrites and accumulation of “dead lithium” upon cycling are among the main obstacles that hinder the widespread application of lithium metal anodes. Herein, an ionic liquid (IL) consisting of 1-methyl-1-propylpiperidinium cation (Pp₁₃⁺) and bis(fluorosulfonyl)imide anion (FSI^?), was chosen as the additive in propylene carbonate (PC)-based liquid electrolytes to circumvent the shortcoming of lithium metal anodes. The optimal 1% Pp₁₃FSI acts as the role of electrostatic shielding, lithiophobic effect and participating in the formation of solid electrolyte interface (SEI) layer with enhanced properties. The in-situ optical microscopy records that the addition of IL can effectively inhibit the growth of lithium dendrites and the corrosion of lithium anode. This study delivers an effective modification to optimize electrolytes for stable lithium metal batteries.     

Anion receptor for enhanced thermal stability of the graphite anode interface in a Li-ion battery

The thermal stability of the solid electrolyte interphase (SEI) formed on a graphite anode has been enhanced by adding an anion receptor, tris(pentafluorophenyl)borane (TPFPB), to the electrolyte. The investigated electrolyte was LiBF₄ in a 2:1 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). Two concentrations of TPFPB have been investigated, 0.2 and 0.8 M. Galvanostatic cycling and differential scanning calorimetry (DSC) were used to study the effect of TPFPB on the electrochemical performance and thermal stability of graphite anodes. The best performance is obtained for a graphite anode cycled in an electrolyte with 0.2 M TPFPB: cyclability is improved, and the onset temperature for the first thermally activated reaction is increased by more than 60 °C up to 140–160 °C. X-ray photoelectron spectroscopy (XPS) has been used to examine the composition of the SEI formed in the different electrolytes; the improved performance for the graphite cycled with 0.2 M TPFPB is attributed to a reduced amount of LiF in the SEI.     

In situ AFM studies of SEI formation at a Sn electrode

Early stages of the solid electrolyte interphase (SEI) formation at a tin foil electrode in an ethylene carbonate (EC) based electrolyte were investigated by in situ AFM and cyclic voltammetry (CV) at potentials >0.7 V, i.e., above the potential of Sn–Li alloying. We detected and observed initial steps of the surface film formation at ～2.8 V vs. Li/Li⁺ followed by gradual film morphology changes at potentials 0.7 < U < 2.5 V. The SEI layer undergoes continuous reformation during the following CV cycles between 0.7 and 2.5 V. The surface film on Sn does not effectively prevent the electrolyte reduction and a large fraction of the reaction products dissolve in the electrolyte. The unstable SEI layer on Sn in EC-based electrolytes may compromise the use of tin-based anodes in Li-ion battery systems unless the interfacial chemistry of the electrode and/or electrolyte is modified.     

Decreasing Irreversible Capacity of Graphite Electrodes in Lithium-Ion Batteries by Direct Contact of Graphite with Metallic Lithium

The feasibility of reducing the irreversible capacity of negative graphite electrodes in lithium-ion batteries by a direct contact of such electrodes with lithium in the electrolyte is studied. It is shown that the dynamics of the formation of the passive film on graphite and the degree of the decrease in the irreversible capacity depend on the ratio between weights of graphite and lithium in contact. This method of reducing the irreversible capacity does not diminish the reversible capacity of graphite during the cycling. The irreversible capacity of the initial graphite cycled in 1 M LiPF₆ in a mixture of propylene carbonate and diethyl carbonate at a current density of 20 mA g^–1 is 550–1150 mA h g^–1. The reversible capacity of electrodes cycled in the same conditions reaches 290 mA h g^–1.     

In situ analysis of interfacial reactions between negative MCMB,lithium electrodes,and gel polymer electrolyte

The interfacial properties of mesocarbon-microbeads (MCMB) and lithium electrodes during charge process in poly (vinylidenefluoride-co-hexafluoropropylene)-based gel electrolyte were investigated by in situ Raman microscopy, in situ Fourier transform-infrared (FTIR) spectroscopic methods, and charge–discharge, electrochemical impedance spectroscopy techniques. For MCMB electrode, the series phase transitions from initial formation of the dilute stage 1 graphite intercalation compound (GIC) to a stage 4 GIC, then through a stage 3 to stage 2, and finally to stage 1 GIC was proved by in situ Raman spectroscopic measurement. The formation of solid electrolyte interface (SEI) films formed on MCMB and metal lithium electrode was studied by in situ reflectance FTIR spectroscopic method. At MCMB electrode surface, the solvent (mostly ethylene carbonate) decomposed during charging process and ROCO₂Li may be the product. ROCO₂Li, ROLi, and Li₂CO₃ were the main composites of SEI film formed on lithium electrode, not on electrodeposited lithium electrode or lithium foil electrode.     

Reforming the Uniformity of Solid Electrolyte Interphase by Nanoscale Structure Regulation for Stable Lithium Metal Batteries

The stability of high-energy-density lithium metal batteries depends on the uniformity of solid electrolyte interphase (SEI) on lithium metal anodes. Rationally improving SEI uniformity is hindered by poorly understanding the effect of structure and components of SEI on its uniformity. Herein, a bilayer structure of SEI formed by isosorbide dinitrate (ISDN) additives in localized high-concentration electrolytes was demonstrated to improve SEI uniformity. In the bilayer SEI, LiN_xO_y generated by ISDN occupies top layer and LiF dominates bottom layer next to anode. The uniformity of lithium deposition is remarkably improved with the bilayer SEI, mitigating the consumption rate of active lithium and electrolytes. The cycle life of lithium metal batteries with bilayer SEI is three times as that with common anion-derived SEI under practical conditions. A prototype lithium metal pouch cell of 430 Wh kg⁻¹ undergoes 173 cycles. This work demonstrates the effect of a reasonable structure of SEI on reforming SEI uniformity.     

Characterization of SEI layer formed on high performance Si–Cu anode in ionic liquid battery electrolyte

Formation of the SEI layer on Si–Cu film electrode in the ionic liquid electrolyte of 1 M lithium bis(trifluoromethylsulfonyl)imide/1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide (LiTFSI/MPP-TFSI) was investigated using ex-situ ATR FTIR and X-ray photoelectron spectroscopy. The SEI layer is found to be composed of organic and inorganic compounds that are the decomposition products of MPP cation and TFSI anion, and effectively passivate the electrode surface during initial cycling. Formation of a stable SEI layer leads to an excellent capacity retention 98% of the maximum discharge capacity, delivering discharge capacities of > 1620 mAhg^? 1 over 200 cycles. The data contribute to a basic understanding of SEI formation and composition responsible for the cycling performance of Si-based alloy anodes in ionic liquid electrolyte-based rechargeable lithium batteries.     

Initial solid electrolyte interphase formation process of graphite anode in LiPF6 electrolyte: an in situ ECSTM investigation

Understanding the structure and formation dynamics of the solid electrolyte interphase (SEI) on the electrode/electrolyte interface is of great importance for lithium ion batteries, as the properties of the SEI remarkably affect the performances of lithium ion batteries such as power capabilities, cycling life, and safety issues. Herein, we report an in situ electrochemical scanning tunnelling microscopy (ECSTM) study of the surface morphology changes of a highly oriented pyrolytic graphite (HOPG) anode during initial lithium uptake in 1 M LiPF(6) dissolved in the solvents of ethylene carbonate plus dimethyl carbonate. The exfoliation of the graphite originating from the step edge occurs when the potential is more negative than 1.5 V vs. Li(+)/Li. Within the range from 0.8 to 0.7 V vs. Li(+)/Li, the growth of clusters on the step edge, the decoration of the terrace with small island-like clusters, and the exfoliation of graphite layers take place on the surface simultaneously. The surface morphology change in the initial lithium uptake process can be recovered when the potential is switched back to 2.0 V. Control experiments indicate that the surface morphology change can be attributed to the electrochemical reduction of solvent molecules. The findings may lead to a better understanding of SEI formation on graphite anodes, optimized electrolyte systems for it, as well as the use of in situ ECSTM for interface studies in lithium ion batteries.     

Suppression of lithium deposition at sub-zero temperatures on graphite by surface modification

Lithium deposition on graphite anodes is considered as a main reason for failures and safety for lithium ion batteries (LIB). Different amounts of carbon coating on the surface of natural graphite are used in this work to suppress the amount of lithium deposited at − 10 °C. Pulse polarization experiments reveal relative polarization of graphite anodes at various temperatures and show that lithium deposition is accelerated at lowered temperatures. Electrochemical experiments, along with photographs, scanning electron microscopy (SEM) images and ex-situ X-ray diffraction (XRD) data suggest that carbon coating not only suppresses the lithium deposition but also enhances the formation of LiC₆ at − 10 °C. The homogeneous potential profile on the graphite surface attained by the carbon coating explains such an improved low temperature performance, as it allows efficient Solid Electrolyte Interface (SEI) film formation, which is a prerequisite for safety LIB.     

Significant Strain Dissipation via Stiff-Tough Solid Electrolyte Interphase Design for Highly Stable Alloying Anodes

The pulverization of alloying anodes significantly restricts their use in lithium-ion batteries (LIBs). This study presents a dual-phase solid electrolyte interphase (SEI) design that incorporates finely dispersed Al nanoparticles within the LiPON matrix. This distinctive dual-phase structure imparts high stiffness and toughness to the integrated SEI film. In comparison to single-phase LiPON film, the optimized Al/LiPON dual-phase SEI film demonstrates a remarkable increase in fracture toughness by 317.8 %, while maintaining stiffness, achieved through the substantial dissipation of strain energy. Application of the dual-phase SEI film on an Al anode leads to a 450 % enhancement in cycling stability for lithium storage in dual-ion batteries. A similar enhancement in cycling stability for silicon anodes, which face severe volume expansion issues, is also observed, demonstrating the broad applicability of the dual-phase SEI design. Specifically, homogeneous Li−Al alloying has been observed in conventional LIBs, even when paired with a high mass loading LiNi_0.5Co_0.3Mn_0.2O₂ cathode (7 mg cm⁻²). The dual-phase SEI film design can also accelerate the diffusion kinetics of Li-ions through interface electronic structure regulation. This dual-phase design can integrate stiffness and toughness into a single SEI film, providing a pathway to enhance both the structural stability and rate capability of alloying anodes.     

Effect of sulfolane on the morphology and chemical composition of the solid electrolyte interphase layer in lithium bis(oxalato)borate‐based electrolyte

To discuss the source of sulfolane (SL) in decreasing the interface resistance of Li/mesophase carbon microbeads cell with lithium bis(oxalate)borate (LiBOB)‐based electrolyte, the morphology and the composition of the solid electrolyte interphase (SEI) layer on the surface of carbonaceous anode material have been investigated. Compared with the cell with 0.7 mol l^?1 LiBOB‐ethylene carbonate/ethyl methyl carbonate (EMC) (1 : 1, v/v) electrolyte, the cell with 0.7 mol l^?1 LiBOB‐SL/EMC (1 : 1, v/v) electrolyte shows better film‐forming characteristics in SEM (SEI) spectra. According to the results obtained from Fourier transform infrared spectroscopy, XPS, and density functional theory calculations, SL is reduced to Li₂SO₃ and LiO₂S(CH₂)₈SO₂Li through electrochemical processes, which happens prior to the reduction of either ethylene carbonate or EMC. It is believed that the root of impedance reduction benefits from the rich existence of sulfurous compounds in SEI layer, which are better conductors of Li⁺ ions than analogical carbonates. Copyright © 2013 John Wiley & Sons, Ltd.     

Studies of the interfacial properties of an electroplated Sn thin film electrode/electrolyte using in situ MFTIRS and EQCM

Sn thin film electrodes were prepared by electroplating in an acidic sulfate bath containing SnSO4. During charge/discharge processes, the interfacial properties between a Sn thin film electrode and an electrolyte of 1 mol.L(-1) LiPF6 in a mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 vol %) were investigated by using cyclic voltammetry (CV), electrochemical quartz crystal microbalance (EQCM), and in situ microscope Fourier transform infrared reflection spectroscopy (in situ MFTIRS). The processes of alloying/dealloying of lithium with Sn and the decomposition of the electrolyte on the Sn electrode were characterized quantitatively by surface mass change and at the molecule level. EQCM studies demonstrated that the mass accumulated per mole of electrons (mpe) was varied in different electrochemical processes. In the process of electrolyte decomposition, the measured mpe is smaller than the theoretical value, whereas it is higher than the theoretical value in the process of alloying/dealloying. The reduction products, ROCO2Li, of the electrolyte involved in charge/discharge processes were determined by in situ MFTIRS. The solvation/desolvation of lithium ion with solvent molecules, which is induced by the alloying/dealloying of lithium with Sn, was evidenced by shifts of relevant IR bands of C=O, C-O, and C-H. The current studies clearly revealed the details of interfacial reactions involved in lithium ion batteries employing a Sn thin film as the anode.     

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.     

Effect of the Structure of Nonuniform Conducting Polymer Films on Their Electrochemical Impedance Response

Electrochemical impedance spectroscopy is used to characterize thin p-doped polypyrrole (PPy) films in propylene carbonate (PC) solutions and poly(trifluorophenyl)thiophene (PTFPT), in solutions based on sulfolane (SF). It appears that the latter film is much less swelled compared to the former one. One consequence of this difference is that the PTFPT film shows a much higher bulk resistance compared to that for the PPy film. Another important consequence is that the swelling of the PTFPT film is essentially physically non-homogeneous. Two parallel, uncoupled paths, with different chemical diffusion coefficients, model the experimental results adequately. In order to quantify the impedance spectra for both polymer films, we use a model proposed by Rubinstein et al. explaining the difference in the diffusion coefficients of Ru(bpy)^3+/2+ ₃ within a thin Nafion film. The model can also predict the impedance spectra for composite powdery electrodes containing different particle sizes, such as composite cathodes and graphite anodes used in lithium batteries.     

Effects of solid polymer electrolyte coating on the composition and morphology of the solid electrolyte interphase on Sn anodes

In order to discuss the effect of polymer coating layer on the Sn anode, the composition and morphology of the solid electrolyte interphase (SEI) film on the surface of Sn and Sn@PEO anode materials have been investigated. Compared with the bare cycled Sn electrode, the SEI on the surface of cycled Sn@PEO electrode is thinner, smoother, and more stable. Therefore, the Sn@PEO nanoparticles can basically keep the original appearance during cycling. Based on the results obtained from X-ray photoelectron spectroscopy (XPS), the SEI formed on the Sn@PEO electrode is characterized by inorganic components (Li₂CO₃)-rich outer layer and organic components-rich inner which could make the SEI more stable and inhibit the electrolyte immerging into the active materials. In particular, the elastic ion-conductive polyethylene oxide (PEO) coating could increase the toughness of SEI and allow the SEI to endure the stress variation in repetitive lithium insertion and extraction process. As a result, the Sn@PEO electrodes show significantly better capacity retention than bare Sn electrodes. The findings can serve as the theoretical foundation for the design of lithium-ion battery electrode with high energy density and long cycle life.     

Irreversible processes during the lithium intercalation into graphite: the passive film formation

Comparative studies of three type of carbonaceous materials—the modified oxidized graphite, thermoexpanded graphite, and carbon paper—prior to and after galvanostatic cycling in 1 M LiClO₄ solution in propylene carbonate-dimethoxyethane mixture are carried out using standard porosimetry. It was shown that the mean (effective) thickness of the passive film [solid electrolyte interface (SEI)] at the electrodes of the modified oxidized graphite and thermoexpanded graphite equals a few nanometers. The comparison of porosimetric and electrochemical data shows that the passive film comprises both lithium carbonate and alkylcarbonates. Additionally, this comparison allows corroborating the concept on the formation of polymer (or oligomer) component of the passive film at least at the thermoexpanded graphite electrodes.     

Dimethyl carbonate (DMC) electrolytes – the effect of solvent purity on Li–ion intercalation into graphite anodes

Graphite electrodes cycled in single solvent electrolytes based on dimethyl carbonate (DMC) exhibit surprising and unfamiliar behavior. The electrochemical performance of graphite anodes cycled vs. Li metal in DMC electrolytes, containing 1 M LiPF₆ is strongly dependent on the solvent purity. The behavior of the graphite anodes in electrolytes containing “pure” DMC is dependent mainly on the identity of the contamination present in the native solvent. It was found that methanol is causing deterioration in the electrochemical performance of the cycled graphite electrode, while carbon dioxide and mainly carbonochloridic acid methyl ester are enhancing the performance. It was established that it is essential to investigate and understand the processes and materials used during the production of battery grade solvents, in order to clarify the roll of traces of chemical compounds responsible for the variation and modification in the electrochemical behavior of the cycled electrodes.     

Thermal reactions of lithiated graphite anode in LiPF6-based electrolyte

The thermal reactions of a lithiated graphite anode with and without 1.3 M lithium hexafluorophosphate (LiPF₆) in a solvent mixture of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) were investigated by means of differential scanning calorimetry (DSC). The products of the thermal decomposition occurring on the lithiated graphite anode were characterized by Fourier transform infrared (FT-IR) analysis. The lithiated graphite anode showed two broad exothermic peaks at 270 and 325 °C, respectively, in the absence of electrolyte. It was demonstrated that the first peak could be assigned to the thermal reactions of PF₅ with various linear alkyl carbonates in the solid electrolyte interphase (SEI) and that the second peak was closely related to the thermal decomposition of the polyvinylidene fluoride (PVdF) binder. In the presence of electrolyte, the lithiated graphite anode showed the onset of an additional exothermic peak at 90 °C associated with the thermal decomposition reactions of the SEI layer with the organic solvents.     

Surface film formation on a graphite electrode in Li‐ion batteries: AFM and XPS study

The formation of a passivation film (solid electrolyte interphase, SEI) at the surface of the negative electrode of full LiCoO₂/graphite lithium‐ion cells using LiPF₆ (1M ) in carbonate solvents as electrolyte was investigated by means of x‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The analyses were carried out at different potentials of the first and the fifth cycles, showing the potential‐dependent character of the surface‐film species formation. These species were mainly identified as Li₂CO₃ up to 3.8 V and LiF up to 4.2 V. This study shows the formation of the SEI during charging and its partial dissolution during discharge. Copyright © 2005 John Wiley & Sons, Ltd.