A transmission electron microscopy study of crack formation and propagation in electrochemically cycled graphite electrode in lithium-ion cells

Subsurface defects and local compositional changes that occurred in graphite anodes subjected to cyclic voltammetry tests (vs. Li/Li⁺, using an electrolyte consisting of 1 M LiClO₄ in a 1:1 volumetric mixture of ethylene carbonate and 1,2-dimethoxy ethane) were investigated using high-resolution transmission electron microscopy (HR-TEM). Cross-sections of anodes prepared by focused ion beam (FIB) milling indicated that graphite layers adjacent to solid electrolyte (SEI)/graphite interface exhibited partial delamination due to the formation of interlayer cracks. The SEI layer formed on the graphite surface consisted of Li₂CO₃ that was identified by {1 1 0} and {0 0 2} crystallographic planes. Lithium compounds, LiC₆, Li₂CO₃ and Li₂O, were observed on the surfaces of separated graphite layers. Deposition of these co-intercalation compounds near the crack tip caused partial closure of propagating graphite cracks during electrochemical cycling, and possibly reduced the crack growth rate. Graphite fibres that were observed to bridge crack faces likely provided an additional mechanism for the retardation of crack propagation.     

A new look at the solid electrolyte interphase on graphite anodes in Li-ion batteries

The solid electrolyte interphase (SEI) of graphite electrodes has been extensively studied using surface sensitive techniques such as photoelectron spectroscopy (PES) and soft X-ray spectroscopy. By combining measurements of reference compounds with graphite electrodes cycled in different electrolytes and under different conditions, knowledge of the solid electrolyte interphase (SEI) chemistry can be obtained. In this article, conclusive results concerning the chemical composition of the inorganic part of the SEI is described. The results show that Li₂O often reported to be present in the SEI could be an artifact from abusive Ar⁺ sputtering. The presence of Li₂CO₃ is a matter of debate; the compound is not observed in anodes extracted from hermetically sealed cells that are never exposed to air. The results show that cell-design and sample handling are crucial to the observed chemical composition of the SEI.     

Electrochemical study of the formation of a solid electrolyte interface on graphite in a LiBC2O4F2-based electrolyte

The formation of a solid electrolyte interface (SEI) on the surface of graphite in a LiBC₂O₄F₂-based electrolyte was studied by galvanostatic cycling and electrochemical impedance spectroscopy (EIS). The results show that a short irreversible plateau at 1.5–1.7 V versus Li⁺/Li was inevitably present in the first cycle of graphite, which is attributed to the reduction of –OCOCOO⁻ pieces as a result of the chemical equilibrium of oxalatoborate ring-opening. This is the inherent property of LiBC₂O₄F₂ and it is independent of the type of electrode. EIS analyses suggest that the reduced products of LiBC₂O₄F₂ at 1.5–1.7 V participate into the formation of a preliminary SEI. Based on the distribution of the initial irreversible capacity and the correlation of the SEI resistance and graphite potential, it was concluded that the SEI formed at potentials below 0.25 V during which the lithiation takes place is most responsible for the long-term operation of the graphite electrode in Li-ion batteries. In addition, the results show that the charge-transfer resistance reflects well the kinetics of the electrode reactions, and that its value is in inverse proportion to the differential capacity of the electrode.     

Investigating the solid electrolyte interphase using binder-free graphite electrodes

Binder-free (BF) electrodes simplify interpretation of solid electrolyte interphase (SEI) data obtained from studies of graphite surfaces. In this work, we prepared BF-graphite electrodes by electrophoretic deposition (EPD), and the SEI layers formed on the electrode in lithium cells containing LiPF₆- and LiF₂BC₂O₄-bearing electrolytes were examined by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results showed that the dominant SEI species were lithium alkyl carbonates (ROCO₂Li) and lithium alkoxides (ROLi); Li₂CO₃ was conspicuously absent. Trigonal borate oligomers are most likely present in the SEI of graphite samples cycled in LiF₂BC₂O₄ electrolyte, while lithium fluorophosphates are present on graphite samples cycled in LiPF₆ electrolyte. The SEI layer coverage was greater on graphite samples cycled in LiF₂BC₂O₄ electrolyte than in the LiPF₆ electrolyte. Our results demonstrate that BF-graphite electrodes prepared by EPD are suitable for the study of SEI layer formed in various electrolyte systems.     

Surface structural disordering in graphite upon lithium intercalation/deintercalation

We report on the origin of the surface structural disordering in graphite anodes induced by lithium intercalation and deintercalation processes. Average Raman spectra of graphitic anodes reveal that cycling at potentials that correspond to low lithium concentrations in Li_xC (0 ≤ x < 0.16) is responsible for most of the structural damage observed at the graphite surface. The extent of surface structural disorder in graphite is significantly reduced for the anodes that were cycled at potentials where stage-1 and stage-2 compounds (x > 0.33) are present. Electrochemical impedance spectra show larger interfacial impedance for the electrodes that were fully delithiated during cycling as compared to electrodes that were cycled at lower potentials (U < 0.15 V vs. Li/Li⁺). Steep Li⁺ surface-bulk concentration gradients at the surface of graphite during early stages of intercalation processes, and the inherent increase of the Li_xC d-spacing tend to induce local stresses at the edges of graphene layers, and lead to the breakage of C-C bonds. The exposed graphite edge sites react with the electrolyte to (re)form the SEI layer, which leads to gradual degradation of the graphite anode, and causes reversible capacity loss in a lithium-ion battery.     

Differential pulse effects of solid electrolyte interface formation for improving performance on high-power lithium ion battery

Solid electrolyte interface (SEI) formation is a key that utilizes to protect the structure of graphite anode and enhances the redox stability of lithium-ion batteries before entering the market. The effect of SEI formation applies a differential pulse (DP) and constant current (CC) charging on charge-discharge performance and cycling behavior into brand new commercial lithium ion batteries is investigated. The morphologies and electrochemical properties on the anode surface are also inspected by employing SEM and EDS. The electrochemical impedance spectra of the anode electrode in both charging protocols shows that the interfacial resistance on graphite anodes whose SEI layer formed by DP charging is smaller than that of CC charging. Moreover, the cycle life result shows that the DP charging SEI formation is more helpful in increasing the long-term stability and maintaining the capacity of batteries even under high power rate charge-discharge cycling. The DP charging method can provide a SEI layer with ameliorated properties to improve the performance of lithium ion batteries.     

Ethyl isocyanate—An electrolyte additive from the large family of isocyanates for PC-based electrolytes in lithium-ion batteries

To avoid solvent co-intercalation into graphite, the presence of a solid electrolyte interphase (SEI) is required. This film is formed via the reductive decomposition of electrolyte species, i.e. a film forming electrolyte additives. In this contribution we focus on an isocyanate compound, ethyl isocyanate (EtNCO) which performs well in a propylene carbonate electrolyte at both graphite anode and LiCoO₂ cathode. EtNCO is investigated by in situ Fourier transform infrared (FTIR) spectroscopy. We conclude that the formation of a radical anion via electrochemical reduction of the electrolyte additive is the initiating step of the SEI formation process. The electro-polymerization of isocyanate monomers in small additive amounts in the PC electrolyte is critically discussed.     

Reconsideration of SEI stability: reversible lithium intercalation into graphite electrodes in trans-2,3-butylene carbonate

Lithium ion batteries with graphitic carbon anodes and LiCoO₂ cathodes are cycled reversibly in electrolytes based on trans-2,3-butylene carbonate (t-BC), even in the absence of ethylene carbonate. While the poor interfacial film (the solid electrolyte interface (SEI)) on the lithium electrode can be readily explained in terms of previous models of its stability, this highly reversible behavior of graphite is hard to account for. To explain this profound difference in the SEI stability of the two electrodes, we have taken into account the influence that the nature of the electrode (lithium metal versus graphite) and the type of the reaction site (basal plane versus edge sites) exert on the solvent reduction pathways.     

Study of poly(acrylonitrile-methyl methacrylate) as binder for graphite anode and LiMn2O4 cathode of Li-ion batteries

We evaluated poly(acrylonitrile-methyl methacrylate) (AMMA, AN/MMA=94:6) as a binder for the graphite anode and the LiMn₂O₄ cathode of Li-ion batteries by studying the cycling performance of lithium half-cells. The results showed that, using AMMA binder, both graphite and LiMn₂O₄ could be cycled well in 1 m LiPF₆ 3:3:4 (weight) PC/EC/EMC electrolyte with less capacity fading. AMMA is chemically more stable than poly(vinylidene fluoride) (PVDF) against the lithiated graphite. More importantly, AMMA can help graphite to form a stable solid electrolyte interface (SEI) film. An impedance study showed that the SEI film formed with AMMA is more stable than the one formed with PVDF. Therefore, self-delithiation of the lithiated graphite can be reduced by use of AMMA instead of PVDF, which improves the storage performance of Li-ion batteries.     

Effect of the atmosphere on chemical composition and electrochemical properties of solid electrolyte interface on electrodeposited Li metal

Many efforts have been paid to realize the superior anodes for future Li batteries in either the dry Ar atmosphere or the dry air atmosphere. In this work, in order to clarify the effects of such atmospheres, the most reactive anodes of Li were freshly electrodeposited under the dry Ar or under the dry air condition. The Solid Electrolyte Interface (SEI) formed during the electrodeposition of Li anodes is revealed to have a different chemical composition and protective feature. The Li deposited under the dry air was revealed to have longer cycle life in the electrolyte than that deposited in Ar, even in the electrolyte containing ionic liquid. From the XPS results, the SEI formed in dry air is proved to be different from that formed in Ar gas atmospheres, that is, the SEI formed in dry air consists of Li₂CO₃ and Li nitride. In order to improve the performance of the anodes, the atmosphere for the initial preparation of the anode/electrolyte interface should be tuned.     

Effect of vinylene carbonate (VC) as electrolyte additive on electrochemical performance of Si film anode for lithium ion batteries

The effect of VC as electrolyte additive on the electrochemical performance of Si film anode was studied in this paper. The charge/discharge test, scanning electron microscopy (SEM), electrochemical impedance spectrum (EIS), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to investigate the cycle performance and SEI layer of Si film anode. It was found that the SEI layer formed in VC-containing electrolyte possessed better properties. It was impermeable to electrolyte and its impedance kept almost invariant upon cycling. The presence of VC in electrolyte brought out the VC-reduced products and decreased the LiF content in SEI layer. The major components of SEI layer were similar in VC-free and VC-containing electrolytes, which contained lithium salt (e.g. ROCO₂Li, Li₂CO₃, LiF), polycarbonate and silicon oxide. It was newly found that silicon oxide could be formed in SEI layer of Si film anode due to the reaction of lithiated silicon with permeated electrolyte in both VC-free and VC-containing electrolytes.     

Electrochemical behaviors of a Li3N modified Li metal electrode in secondary lithium batteries

A lithium conductive Li₃N film is successfully prepared on Li metal surface by the direct reaction between Li and N₂ gas at room temperature. X-ray diffraction (XRD), Auger electron spectroscopy (AES), cyclic voltammetry (CV), scanning electron microscopy (SEM), AC impedance, cathodic polarization and galvanostatic charge/discharge cycling tests are applied to characterize the film. The experimental results show that the Li₃N protective film is tight and dense with high stability in the electrolyte. Its thickness is more than 159.4 nm and much bigger than that of a native SEI film formed on the lithium surface as received. An exchange current as low as 3.244 × 10⁻⁷ A demonstrates the formation of a complete SEI film at the electrode|electrolyte interface with Li₃N modification. The SEI film is very effective in preventing the corrosion of the Li electrode in liquid electrolyte, leading to a decreased Li|electrolyte interface resistance and an average short distance of 3.16 × 10⁻³ cm for Li ion diffusion from electrolyte to Li surface. The Li cycling efficiency depends on N₂ exposing time and is obviously enhanced by the Li₃N (1 h) modification. After cycling, a dense and homogeneous Li layer deposits on the Li₃N (1 h) modified Li surface, instead of a loose and inhomogeneous layer on the Li surface as received.     

Enhanced performance of Li-ion cell with LiBF4-PC based electrolyte by addition of small amount of LiBOB

LiBF₄-PC based electrolytes have superior potentials in enhancing cycling performance of Li-ion cells at low temperature and at high current rate, respectively. However, their application is prevented because of the poor capability in enabling graphite to form a solid electrolyte interface (SEI). In this work, we found that addition of small amount of lithium bis(oxalate)borate (LiBOB) into the LiBF₄-based electrolytes could significantly promote SEI formation on the surface of graphite even in solutions with a high content of PC. This is attributed to the fact that LiBOB participates in the formation chemistry of the SEI. Results showed that 1–5 mol% LiBOB is sufficient to promote graphite cycling reversibly in 1.0 m LiBF₄ 1:1:3 PC/EC/EMC and 1.0 m LiBF₄ 1:1 PC/EC, respectively, while the excellent cycling performance of LiBF₄-PC based electrolytes at low temperatures was maintained.     

Studies on the enhancement of solid electrolyte interphase formation on graphitized anodes in LiX-carbonate based electrolytes using Lewis acid additives for lithium-ion batteries

The new electrolyte systems utilizing one type of Lewis acids, the boron based anion receptors (BBARs) with LiF, Li₂O, or Li₂O₂ in carbonate solutions have been developed and reported by us. These systems open up a new approach in developing non-aqueous electrolytes with higher operating voltage and less moisture sensitivity for lithium-ion batteries. However, the formation of a stable solid electrolyte interphase (SEI) layer on the graphitized anodes is a serious problem needs to be solved for these new electrolyte systems, especially when propylene carbonate (PC) is used as a co-solvent. Using lithium bis(oxalato)borate (LiBOB) as an additives, the SEI layer formation on mesophase carbon microbeads (MCMB) anode is significantly enhanced in these new electrolytes containing boron-based anion receptors, such as tris(pentafluorophenyl) borane, and lithium salt such as LiF, or lithium oxides such as Li₂O or Li₂O₂ in PC and dimethyl carbonate (DMC) solvents. The cells using these electrolytes and MCMB anodes cycled very well and the PC co-intercalation was suppressed. Fourier transform infrared spectroscopy (FTIR) studies show that one of the electrochemical decomposition products of LiBOB, lithium carbonate (Li₂CO₃), plays a quite important role in the stablizing SEI layer formation.     

Lithium bis(fluorosulfonyl)imide-PYR14TFSI ionic liquid electrolyte compatible with graphite

Lithium bis(fluorosulfonyl)imide (LiFSI) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) was successfully tested as an electrolyte for graphite composite anodes at elevated temperature of 55 °C. The graphite anode showed a good cyclability during the galvanostatic testing at C/10 rate and 55 °C with the capacity close to theoretical. The formation of SEI in different electrolytes was the subject of study using impedance spectroscopy on symmetrical cells containing two lithium electrodes. The 0.7 m LiFSI in PYR₁₄TFSI exhibits a good ionic conductivity (5.9 mS cm⁻¹ at 55 °C) along with high electrochemical stability and high thermal stability. These properties allow their potential application in large-scale lithium ion batteries with improved safety.     

Surface film formation on a carbonaceous electrode: Influence of the binder chemistry

We have investigated the possible effect of carboxymethylcellulose (CMC) in the SEI film formation at the surface of a graphite composite electrode of LiCoO₂/graphite cells. The electrode/electrolyte interface was analyzed by XPS at different potentials of the first electrochemical cycle, and after simple contact of the electrode with the electrolyte. We could evidence a specific reactivity of CMC towards the electrolyte (LiPF₆ in a mixture of carbonate solvents), resulting in the formation of new species that contribute to the surface film composition. This result shows that the chemical reactivity of CMC towards the electrolyte takes part in the formation of the surface film, and contributes to the good properties of CMC as binder.     

The role of the interface of tin electrodes in lithium cells: An impedance study

In this work we investigate by impedance spectroscopy the characteristics of a film formed on the surface of Sn electrodes when cycled in a lithium cell. We show that this film is formed by electrolyte decomposition catalyzed by the tin surface and that its characteristics depend upon cycling rate as reported in the literature. By the infrared analysis we study the chemical characteristic of mentioned film. By reporting the cycling response of a complete lithium-ion battery using a Sn anode coupled with a LiNi_0.5Mn_1.5O₄ cathode, we show that the interfacial film may indeed influence the response of the batteries using conventional, tin-based anodes.     

Effect of electrolytes on the structure and evolution of the solid electrolyte interphase (SEI) in Li-ion batteries: A molecular dynamics study

We have studied the formation and growth of solid-electrolyte interphase (SEI) for the case of ethylene carbonate (EC), dimethyl carbonate (DMC) and mixtures of these electrolytes using molecular dynamics simulations. We have considered SEI growth on both Li metal surfaces and using a simulation framework that allows us to vary the Li surface density on the anode surface. Using our simulations we have obtained the detailed structure and distribution of different constituents in the SEI as a function of the distance from the anode surfaces. We find that SEI films formed in the presence of EC are rich in Li₂CO₃ and Li₂O, while LiOCH₃ is the primary constituent of DMC films. We find that dilithium ethylene dicarbonate, LiEDC, is formed in the presence of EC at low Li surface densities, but it quickly decomposes to inorganic salts during subsequent growth in Li rich environments. The surface films formed in our simulations have a multilayer structure with regions rich in inorganic and organic salts located near the anode surface and the electrolyte interface, respectively, in agreement with depth profiling experiments. Our computed formation potentials 1.0 V vs. Li/Li⁺ is also in excellent accord with experimental measurements. We have also calculated the elastic stiffness of the SEI films; we find that they are significantly stiffer than Li metal, but are somewhat more compliant compared to the graphite anode.     

Tris(pentafluorophenyl) borane as an electrolyte additive for LiFePO4 battery

The effects of tris(pentafluorophenyl) borane (TPFPB) additive in electrolyte at the LiFePO₄ cathode on the high temperature capacity fading were investigated by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), cyclability, SEM and Fourier transform infrared (FTIR). According to the study results, tris(pentafluorophenyl) borane has the ability to improve the cycle performance of LiFePO₄ at high temperature. LiFePO₄ electrodes cycled in the electrolyte without the TPFPB additive show a significant increase in charge transfer resistance by EIS analysis. SEM and FTIR disclose evidence of surface morphology change and solid electrolyte interface (SEI) formation. FTIR investigation shows various functional groups are found on the cathode material surface after high temperature cycling tests. The results showed an obvious improvement of high temperature cycle performance for LiFePO₄ cathode material due to the TPFPB additive. The observed improved cycling performance and improved lithium ion transport are attributed to decreased LiF content in the SEI film.     

Optimization of the forming conditions of the solid-state interface in the Li-ion batteries

We optimized the forming conditions of Li-ion batteries from the standpoint of the stability of the solid-state interface (SEI) on the surface of graphite. The optimization processes were focused on the initial two cycles of the Li/graphite half-cell in terms of the current density and the temperature. We evaluated the stability of the SEI from two aspects: (1) self-delithiation at 60 °C of the lithium intercalation compound, which reflects storage stability of the SEI and (2) cycling performance of the Li/graphite cell, which reflects cycling stability of the SEI. It was observed that the low current density and the low temperature favored increasing effectiveness of the SEI formation, and resulted in formation of a stable SEI. However, the SEI formed under such conditions had relatively high impedance. That is, there was a contradictory relationship between the ionic conductivity and the stability of the SEI. Based on the graphite and electrolyte used in this work, we found that the optimum conditions for the formation of the SEI were at the current densities of 10–20 mA/g and at the temperatures of 20–35 °C.