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.     

Surface analysis of the solid electrolyte interface formed by additives on graphite electrodes in Li‐ion batteries using XPS,FE‐AES,and XHR‐SEM techniques

We investigate the formation and distribution of the solid electrolyte interface (SEI) layer on a graphite anode with two additives [vinylethylene carbonate (VEC) and vinylene carbonate (VC)] in a formation process using XPS, field emission AES, and extreme high‐resolution SEM (XHR‐SEM) techniques, and we studied what factors play an important role in determining the formation of the SEI layer. The VEC‐derived SEI behaviors (morphology, thickness, compound, and balance over electrode position) on a graphite anode largely depend on the elevated temperature. The VC‐derived SEI layer is mostly formed in the initial charging step, showing simple growth (formation) behavior. It is suggested that the properties of the additives are important for SEI bonding configurations at the nanoscale film surface, and to achieve the stable SEI layer, there appears to be an effective formation process for the additive properties. This research highlights the challenges of developing a stable SEI layer with additives in the formation process for electric vehicle batteries and would make a contribution to the understanding of how formation conditions affect an SEI layer with respect to additive properties. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Critical assessment of a new in situ spectroelectrochemical cell designed for the study of interfacial reactions between a porous graphite anode and alkyl carbonate solution

This article critically evaluates the characteristics of a new in situ spectroelectrochemical cell with an optimized path of the IR beam, designed in our laboratory for study of the solid electrolyte interphase (SEI) layer formed between a porous graphite anode and alkyl carbonate solution for lithium-ion batteries. The cell was designed in view of the optical principles underlying the way the in situ cell works, to give depth of penetration of the evanescent IR beam through the attenuated total internal reflectance crystal into the electrolyte at such a small value, ranging from 0.277 to 2.77 μm, that it was possible to minimize the "masking effect" of the ethylene carbonate/diethyl carbonate solvent. Moreover, the "local compositional change" which may arise significantly from the "thin layer electrolyte configuration" cell also could be fairly avoided, since only the electrolyte in the vicinity of the electrode composed of graphite particles is reduced to form the SEI layer to a thickness of at most 0.1 μm during the application of potentials. Thus, it was possible to measure the in situ FT-IR spectra in the cell, which represents the real chemical composition and structure of the SEI layer. Taking the application of the designed in situ cell as an example, this article reports the effect of salt type and electrolyte temperature on the chemical composition and structure of the SEI layer between graphite particles and alkyl carbonate solution with the help of various measured in situ FT-IR spectra. Electronic Publication     

Effects of current densities on the formation of LiCoO<Subscript>2</Subscript>/graphite lithium ion battery

The activation characteristics and the effects of current densities on the formation of a separate LiCoO₂ and graphite electrode were investigated and the behavior also was compared with that of the full LiCoO₂/graphite batteries using various electrochemical techniques. The results showed that the formation current densities obviously influenced the electrochemical impedance spectrum of Li/graphite, LiCoO₂/Li, and LiCoO₂/graphite cells. The electrolyte was reduced on the surface of graphite anode between 2.5 and 3.6 V to form a preliminary solid electrolyte interphase (SEI) film of anode during the formation of the LiCoO₂/graphite batteries. The electrolyte was oxidized from 3.95 V vs Li⁺/Li on the surface of LiCoO₂ to form a SEI film of cathode. A highly conducting SEI film could be formed gradually on the surface of graphite anode, whereas the SEI film of LiCoO₂ cathode had high resistance. The LiCoO₂ cathode could be activated completely at the first cycle, while the activation of the graphite anode needed several cycles. The columbic efficiency of the first cycle increased, but that of the second decreased with the increase in the formation current of LiCoO₂/graphite batteries. The formation current influenced the cycling performance of batteries, especially the high-temperature cycling performance. Therefore, the batteries should be activated with proper current densities to ensure an excellent formation of SEI film on the anode surface.     

Atomic force microscopy study on the stability of a surface film formed on a graphite negative electrode at elevated temperatures

The stability at elevated temperatures of a solid electrolyte interphase (SEI) formed on a graphite negative electrode in lithium ion batteries was investigated by storage tests and in situ atomic force microscopy (AFM) observation. When the fully discharged graphite electrode was stored at elevated temperatures, the irreversible capacity in the following cycle increased remarkably. On the other hand, when the electrode was stored at the fully charged state at elevated temperatures, it was severely self-discharged during storage. AFM observation of the SEI layer formed on a model electrode of highly oriented pyrolytic graphite revealed two important facts on the stability of the SEI at elevated temperatures: (i) dissolution and agglomeration of the SEI layer at the discharged state and (ii) serious SEI growth at the charged state. These phenomena well explain the results of the charge and discharge tests. It was also shown that the addition of vinylene carbonate greatly improves the stability of the SEI at elevated temperatures, and gives good charge and discharge performance after storage.     

Pre-constructed SEI on graphite-based interface enables long cycle stability for dual ion sodium batteries

Lithium batteries have been widely used in all over the world for its high energy density, long-term cycle stability. While the resources of lithium metal and transition metal are limited, which restrict their applications in the grid energy storage. Dual ion sodium batteries (DISBs) possess higher energy density, especially owning high power density for its higher operating voltage (> 4.5 V). Nevertheless, the poor oxidation tolerance of carbonate electrolyte and the co-intercalation of solvents accompanied with anions are main obstacles to make the DISBs commercialization. Herein, a physical barrier (artificial SEI film) is pre-constructed in the Na||graphite batteries to solve these thorny problems. With the CSMG (covered SEI on modified graphite), batteries deliver higher capacity 40 mAh/g even under the current density of 300 mA/g and the capacity retention maintains very well after 100 cycles at a high operating voltage. Moreover, the function mechanism was revealed by in-situ XRD, demonstrating that the pre-constructed SEI can effectively suppress the irreversible phase transition and exfoliation of graphite, resulting from the co-intercalation of anions. Additionally, the work voltage windows of carbonate electrolyte are significantly broadened by establishing electrode/electrolyte interphase. This method opens up an avenue for the practical application of DISBs on the grid energy storage and other fields.     

Highly Stable Lithium Metal Batteries Enabled by Regulating the Solvation of Lithium Ions in Nonaqueous Electrolytes

Safe and rechargeable lithium metal batteries have been difficult to achieve because of the formation of lithium dendrites. Herein an emerging electrolyte based on a simple solvation strategy is proposed for highly stable lithium metal anodes in both coin and pouch cells. Fluoroethylene carbonate (FEC) and lithium nitrate (LiNO₃) were concurrently introduced into an electrolyte, thus altering the solvation sheath of lithium ions, and forming a uniform solid electrolyte interphase (SEI), with an abundance of LiF and LiN_xO_y on a working lithium metal anode with dendrite‐free lithium deposition. Ultrahigh Coulombic efficiency (99.96 %) and long lifespans (1000 cycles) were achieved when the FEC/LiNO₃ electrolyte was applied in working batteries. The solvation chemistry of electrolyte was further explored by molecular dynamics simulations and first‐principles calculations. This work provides insight into understanding the critical role of the solvation of lithium ions in forming the SEI and delivering an effective route to optimize electrolytes for safe lithium metal batteries.     

Interfacial Evolution of Lithium Dendrites and Their Solid Electrolyte Interphase Shells of Quasi‐Solid‐State Lithium‐Metal Batteries

Unstable electrode/solid‐state electrolyte interfaces and internal lithium dendrite penetration hamper the applications of solid‐state lithium‐metal batteries (SSLMBs), and the underlying mechanisms are not well understood. Herein, in situ optical microscopy provides insights into the lithium plating/stripping processes in a gel polymer electrolyte and reveals its dynamic evolution. Spherical lithium deposits evolve into moss‐like and branch‐shaped lithium dendrites with increasing current densities. Remarkably, the on‐site‐formed solid electrolyte interphase (SEI) shell on the lithium dendrite is distinctly captured after lithium stripping. Inducing an on‐site‐formed SEI shell with an enhanced modulus to wrap the lithium precipitation densely and uniformly can regulate dendrite‐free behaviors. An in‐depth understanding of lithium dendrite evolution and its functional SEI shell will aid in the optimization of SSLMBs.     

A review on electrode and electrolyte for lithium ion batteries under low temperature

Under low temperature (LT) conditions (−80 °C∼0 °C), lithium-ion batteries (LIBs) may experience the formation of an extensive solid electrolyte interface (SEI), which can cause a series of detrimental effects such as Li⁺ deposition and irregular dendritic filament growth on the electrolyte surface. These issues ultimately lead to the degradation of the LT performance of LIBs. As a result, new electrode/electrolyte materials are necessary to address these challenges and enable the proper functioning of LIBs at LT. Given that most electrochemical reactions in lithium-ion batteries occur at the electrode/electrolyte interface, finding solutions to mitigate the negative impact caused by SEI is crucial to improve the LT performance of LIBs. In this article, we analyze and summarize the recent studies on electrode and electrolyte materials for low temperature lithium-ion batteries (LIBs). These materials include both metallic materials like tin, manganese, and cobalt, as well as non-metallic materials such as graphite and graphene. Modified materials, such as those with nano or alloying characteristics, generally exhibit better properties than raw materials. For instance, Sn nanowire-Si nanoparticles (SiNPs−In-SnNWs) and tin dioxide carbon nanotubes (SnO₂@CNT) have faster Li⁺ transport rates and higher reversible capacity at LT. However, it′s important to note that when operating under LT, the electrolyte may solidify, leading to difficulty in Li⁺ transmission. The compatibility between the electrolyte and electrode can affect the formation of the solid electrolyte interphase (SEI) and the stability of the electrode/electrolyte system. Therefore, a good electrode/electrolyte system is crucial for successful operation of LIBs at LT.     

In situ Observation of Evolving H2 and Solid Electrolyte Interphase Development at Potassium Insertion Materials within Highly Concentrated Aqueous Electrolytes

The solid-electrolyte interphase (SEI) is key to stable, high voltage lithium-ion batteries (LIBs) as a protective barrier that prevents electrolyte decomposition. The SEI is thought to play a similar role in highly concentrated water-in-salt electrolytes (WISEs) for emerging aqueous batteries, but its properties remain unknown. In this work, we utilized advanced scanning electrochemical microscopy (SECM) and operando electrochemical mass spectrometry (OEMS) techniques to gain deeper insight into the SEI that occurs within highly concentrated WISEs. As a model, we focus on a 55 mol/kg K(FSA)_0.6(OTf)_0.4 electrolyte and a 3,4,9,10-perylenetetracarboxylic diimide negative electrode. For the first time, our work showed distinctly passivating structures with slow apparent electron transfer rates alike to the SEI found in LIBs. In situ analyses indicated stable passivating structures when PTCDI was stepped to low potentials (≈−1.3 V vs. Ag/AgCl). However, the observed SEI was discontinuous at the surface and H₂ evolution occurred as the electrode reached more extreme potentials. OEMS measurements further confirmed a shift in the evolution of detectable H₂ from −0.9 V to <−1.4 V vs. Ag/AgCl when changing from dilute to concentrated electrolytes. In all, our work shows a combined approach of traditional battery measurements with in situ analyses for improving characterization of other unknown SEI structures.     

磷酸三甲酯和碳酸亚乙烯酯对锂离子电池的复合作用

应用循环伏安、交流阻抗、扫描电子显微镜和锂离子电池性能检测装置研究了阻燃添加剂磷酸三甲酯(TMP)和成膜添加剂碳酸亚乙烯酯(VC)对锂离子电池的复合作用.结果表明,复合使用TMP和VC不仅能提高电池的安全性而且能改善电池的循环性能,原因可能是在电池首次充放电过程中VC优先还原,还原产物在负极表面聚合形成良好的SEI膜,有效地制约了因TMP在石墨负极表面的分解而造成负极石墨的脱落,同时提高了SEI膜的稳定性.     

A Flexible Solid Electrolyte Interphase Layer for Long‐Life Lithium Metal Anodes

Lithium (Li) metal is a promising anode material for high‐energy density batteries. However, the unstable and static solid electrolyte interphase (SEI) can be destroyed by the dynamic Li plating/stripping behavior on the Li anode surface, leading to side reactions and Li dendrites growth. Herein, we design a smart Li polyacrylic acid (LiPAA) SEI layer high elasticity to address the dynamic Li plating/stripping processes by self‐adapting interface regulation, which is demonstrated by in situ AFM. With the high binding ability and excellent stability of the LiPAA polymer, the smart SEI can significantly reduce the side reactions and improve battery safety markedly. Stable cycling of 700 h is achieved in the LiPAA‐Li/LiPAA‐Li symmetrical cell. The innovative strategy of self‐adapting SEI design is broadly applicable, providing opportunities for use in Li metal anodes     

Solvation Rule for Solid‐Electrolyte Interphase Enabler in Lithium‐Metal Batteries

Despite the exceptionally high energy density of lithium metal anodes, the practical application of lithium‐metal batteries (LMBs) is still impeded by the instability of the interphase between the lithium metal and the electrolyte. To formulate a functional electrolyte system that can stabilize the lithium‐metal anode, the solvation behavior of the solvent molecules must be understood because the electrochemical properties of a solvent can be heavily influenced by its solvation status. We unambiguously demonstrated the solvation rule for the solid‐electrolyte interphase (SEI) enabler in an electrolyte system. In this study, fluoroethylene carbonate was used as the SEI enabler due to its ability to form a robust SEI on the lithium metal surface, allowing relatively stable LMB cycling. The results revealed that the solvation number of fluoroethylene carbonate must be ≥1 to ensure the formation of a stable SEI in which the sacrificial reduction of the SEI enabler subsequently leads to the stable cycling of LMBs.     

锂离子电池中固体电解质界面膜(SEI)研究进展

本文综述了锂离子电池中固体电解质界面膜(SEI膜)的研究进展.在总结SEI膜的形成机理及模型的基础上,讨论了对SEI膜可能的影响因素及其改性方法,以及各种表征技术、特别是原位分析技术在SEI膜研究中的实际应用.指出在今后的研究中,正极表面与电解液间的界面膜,以及引入水溶性粘合剂体系后正负极表面与电解液间的相互作用将成为人们关注的热点。     

Electrochemical SPM investigation of the solid electrolyte interphase film formed on HOPG electrodes

Solid electrolyte interphase (SEI) film formation on graphite electrodes was studied on highly oriented pyrolytic graphite (HOPG) in nonaqueous electrolyte by in situ electrochemical atomic force microscopy (AFM). For potentials negative to 0.7 V versus Li|Li⁺ a SEI film is formed on the HOPG electrode surface. After the first cycle the film is rough and covers the surface of the HOPG electrode only partially. After the second cycle the HOPG surface is fully covered by a compact film. The thickness of the SEI film was measured by increasing the pressure of the AFM tip and thus scraping a part of the electrode surface. In this way a thickness of about 25 nm was found for the SEI film formed after two scan cycles between 3 and 0.01 V versus Li|Li⁺.     

Ultrathin CuF2-Rich Solid-Electrolyte Interphase Induced by Cation-Tailored Double Electrical Layer toward Durable Sodium Storage

Solid-electrolyte interphase (SEI) seriously affects battery's cycling life, especially for high-capacity anode due to excessive electrolyte decomposition from particle fracture. Herein, we report an ultrathin SEI (3–4 nm) induced by Cu⁺-tailored double electrical layer (EDL) to suppress electrolyte consumption and enhance cycling stability of CuS anode in sodium-ion batteries. Unique EDL with SO₃CF₃-Cu complex absorbing on CuS in NaSO₃CF₃/diglyme electrolyte is demonstrated by in situ surface-enhanced Raman, Cyro-TEM and theoretical calculation, in which SO₃CF₃-Cu could be reduced to CuF₂-rich SEI. Dispersed CuF₂ and F-containing compound can provide good interfacial contact for formation of ultrathin and stable SEI film to minimize electrolyte consumption and reduce activation energy of Na⁺ transport. As a result, the modified CuS delivers high capacity of 402.8 mAh g⁻¹ after 7000 cycles without capacity decay. The insights of SEI construction pave a way for high-stability electrode.     

Understanding High‐Rate K+‐Solvent Co‐Intercalation in Natural Graphite for Potassium‐Ion Batteries

Graphite shows great potential as an anode material for rechargeable metal‐ion batteries because of its high abundance and low cost. However, the electrochemical performance of graphite anode materials for rechargeable potassium‐ion batteries needs to be further improved. Reported herein is a natural graphite with superior rate performance and cycling stability obtained through a unique K⁺‐solvent co‐intercalation mechanism in a 1 m KCF₃SO₃ diethylene glycol dimethyl ether electrolyte. The co‐intercalation mechanism was demonstrated by ex situ Fourier transform infrared spectroscopy and in situ X‐ray diffraction. Moreover, the structure of the [K‐solvent]⁺ complexes intercalated with the graphite and the conditions for reversible K⁺‐solvent co‐intercalation into graphite are proposed based on the experimental results and first‐principles calculations. This work provides important insights into the design of natural graphite for high‐performance rechargeable potassium‐ion batteries.     

Li Plating Regulation on Fast-Charging Graphite Anodes by a Triglyme-LiNO3 Synergistic Electrolyte Additive

Graphite anodes are prone to dangerous Li plating during fast charging, but the difficulty to identify the rate-limiting step has made a challenging to eliminate Li plating thoroughly. Thus, the inherent thinking on inhibiting Li plating needs to be compromised. Herein, an elastic solid electrolyte interphase (SEI) with uniform Li-ion flux is constructed on graphite anode by introducing a triglyme (G3)-LiNO₃ synergistic additive (GLN) to commercial carbonate electrolyte, for realizing a dendrite-free and highly-reversible Li plating under high rates. The cross-linked oligomeric ether and Li₃N particles derived from the GLN greatly improve the stability of the SEI before and after Li plating and facilitate the uniform Li deposition. When 51 % of lithiation capacity is contributed from Li plating, the graphite anode in the electrolyte with 5 vol.% GLN achieved an average 99.6 % Li plating reversibility over 100 cycles. In addition, the 1.2-Ah LiFePO₄ | graphite pouch cell with GLN-added electrolyte stably operated over 150 cycles at 3 C, firmly demonstrating the promise of GLN in commercial Li-ion batteries for fast-charging applications.     

Electrolyte Design for Improving Mechanical Stability of Solid Electrolyte Interphase in Lithium–Sulfur Batteries

Practical lithium–sulfur (Li−S) batteries are severely plagued by the instability of solid electrolyte interphase (SEI) formed in routine ether electrolytes. Herein, an electrolyte with 1,3,5-trioxane (TO) and 1,2-dimethoxyethane (DME) as co-solvents is proposed to construct a high-mechanical-stability SEI by enriching organic components in Li−S batteries. The high-mechanical-stability SEI works compatibly in Li−S batteries. TO with high polymerization capability can preferentially decompose and form organic-rich SEI, strengthening mechanical stability of SEI, which mitigates crack and regeneration of SEI and reduces the consumption rate of active Li, Li polysulfides, and electrolytes. Meanwhile, DME ensures high specific capacity of S cathodes. Accordingly, the lifespan of Li−S batteries increases from 75 cycles in routine ether electrolyte to 216 cycles in TO-based electrolyte. Furthermore, a 417 Wh kg⁻¹ Li−S pouch cell undergoes 20 cycles. This work provides an emerging electrolyte design for practical Li−S batteries.