Thick Solid Electrolyte Interphase Research Articles

Optimizing lithium-silver alloy phases for enhanced energy density and electrochemical performance

Lithium (Li) metal batteries though with high energy density are still facing issues like Li dendrite growth, dead Li formation, and thick solid electrolyte interphase (SEI) formation, hindering their long-term stability. Recently, Li-Ag alloys have been reported to potentially address these challenges possibly due to their superior conductivity, lithiophilicity, and mechanical stability. In the pursuit of high-energy-density batteries, Li-Ag alloys typically employ a high Li content phase (γ1). In this study, we applied density functional theory (DFT) calculations to compare the thermodynamic stability, Li adsorption, and Li diffusion of Ag-rich Li-Ag alloy within the γ1 phase (AR-γ1), Ag-poor Li-Ag alloy within the γ1 phase (AP-γ1), and pure Li. AR-γ1 showed better thermodynamic stability and improved Li adsorption and diffusion properties compared to AP-γ1 and pure Li. Electrochemical tests further confirmed the advantages of AR-γ1 in terms of electrode kinetics and cell stability compared to AP-γ1 and pure Li. Our study offers guidance for the selection of the most suitable Li-Ag alloys that can be utilized in high-energy-density lithium batteries.

The Influence of Cathode Degradation Products on the Anode Interface in Lithium-Ion Batteries.

Degradation of cathode materials in lithium-ion batteries results in the presence of transition metal ions in the electrolyte, and these ions are known to play a major role in capacity fade and cell failure. Yet, while it is known that transition metal ions migrate from the metal oxide cathode and deposit on the graphite anode, their specific influence on anode reactions and structures, such as the solid electrolyte interphase (SEI), is still quite poorly understood due to the complexity in studying this interface in operational cells. In this work we combine operando electrochemical atomic force microscopy (EC-AFM), electrochemical quartz crystal microbalance (EQCM), and electrochemical impedance spectroscopy (EIS) measurements to probe the influence of a range of transition metal ions on the morphological, mechanical, chemical, and electrical properties of the SEI. By adding representative concentrations of Ni2+, Mn2+, and Co2+ ions into a commercially relevant battery electrolyte, the impacts of each on the formation and stability of the anode interface layer is revealed; all are shown to pose a threat to battery performance and stability. Mn2+, in particular, is shown to induce a thick, soft, and unstable SEI layer, which is known to cause severe degradation of batteries, while Co2+ and Ni2+ significantly impact interfacial conductivity. When transition metal ions are mixed, SEI degradation is amplified, suggesting a synergistic effect on the cell stability. Hence, by uncovering the roles these cathode degradation products play in operational batteries, we have provided a foundation upon which strategies to mitigate or eliminate these degradation products can be developed.

Solid–electrolyte interphase formation during Li metal deposition in LiN(SO2F)2-based solvate ionic liquids

AbstractSolvate ionic liquids (SILs) are promising electrolytes for Li metal batteries. In this study, Li plating-stripping reactions in SILs were investigated using electrochemical quartz crystal microbalance measurements and X-ray photoelectron spectroscopy (XPS). SILs were prepared by mixing Li salt and glyme (triglyme or tetraglyme) in a 1:1 molar ratio. During the Li plating-stripping reaction, a reversible mass change was observed in LiN(SO2F)2 (LiFSA) based SILs. In contrast, a considerably higher mass change than the theoretical value calculated from the Faraday current was observed in the LiN(SO2CF3)2 (LiTFSA) based SILs, owing to the accumulation of decomposition products of electrolytes on the electrode surface. XPS depth profiling for the deposited Li suggested the formation of thin solid-electrolyte interphase (SEI) in the LiFSA-based SILs while considerably thicker SEI was formed in the LiTFSA-based SILs. The SEI formed in LiFSA-based SILs effectively suppressed the decomposition of electrolytes. Thus, LiFSA-based SILs are favorable for achieving highly reversible charge–discharge of Li electrodes in Li metal batteries.

Electrochemical Investigations of Sulfur-Decorated Organic Materials as Cathodes for Alkali Batteries.

Alkali metal-sulfur batteries (particularly, lithium/sodium- sulfur (Li/Na-S)) have attracted much attention because of their high energy density, the natural abundance of sulfur, and environmental friendliness. However, Li/Na-S batteries still face big challenges, such as limited cycle life, poor conductivity, large volume changes, and the "shuttle effect" caused by the high solubility of Li/Na-polysulfides. Herein, novel organosulfur-containing materials, i.e., bis(4-hydroxy-2,2,6,6-tetramethylpiperidin-1-yl)disulfide (BiTEMPS-OH) and 2,4-thiophene/arene copolymer (TAC) are proposed as cathode materials for Li and Na batteries. BiTEMPS-OH shows an initial discharge/charge capacity of 353/192mAhg-1 and a capacity of 62mAhg-1 after 200 cycles at 100mAg-1 in ether-based Li-ion electrolyte. Meanwhile, TAC has an initial discharge/charge capacity of 270/248mAhg-1 and better cycling performance (106mAhg-1 after 200 cycles) than BiTEMPS-OH in the same electrolyte. However, the rate capability of TAC is limited by the slow diffusion of Li-ions. Both materials show inferior electrochemical performances in Na battery cells compared to the Li analogs. X-ray powder diffraction reveals that BiTEMPS-OH loses its crystalline structure permanently upon cycling in Li battery cells. X-ray photoelectron spectroscopy demonstrates the cleavage and partially reversible formation of S-S bonds in BiTEMPS-OH and the formation/decomposition of thick solid electrolyte interphase on the electrode surface of TAC.

Revealing the aging process of solid electrolyte interphase on SiOx anode

As one of the most promising alternatives to graphite negative electrodes, silicon oxide (SiOx) has been hindered by its fast capacity fading. Solid electrolyte interphase (SEI) aging on silicon SiOx has been recognized as the most critical yet least understood facet. Herein, leveraging 3D focused ion beam-scanning electron microscopy (FIB-SEM) tomographic imaging, we reveal an exceptionally characteristic SEI microstructure with an incompact inner region and a dense outer region, which overturns the prevailing belief that SEIs are homogeneous structure and reveals the SEI evolution process. Through combining nanoprobe and electron energy loss spectroscopy (EELS), it is also discovered that the electronic conductivity of thick SEI relies on the percolation network within composed of conductive agents (e.g., carbon black particles), which are embedded into the SEI upon its growth. Therefore, the free growth of SEI will gradually attenuate this electron percolation network, thereby causing capacity decay of SiOx. Based on these findings, a proof-of-concept strategy is adopted to mechanically restrict the SEI growth via applying a confining layer on top of the electrode. Through shedding light on the fundamental understanding of SEI aging for SiOx anodes, this work could potentially inspire viable improving strategies in the future.

Maximizing Electrostatic Polarity of Non‐Sacrificial Electrolyte Additives Enables Stable Zinc‐Metal Anodes for Aqueous Batteries

AbstractAlthough additives are widely used in aqueous electrolytes to inhibit the formation of dendrites and hydrogen evolution reactions on Zn anodes, there is a lack of rational design principles and systematic mechanistic studies on how to select a suitable additive to regulate reversible Zn plating/stripping chemistry. Here, using saccharides as the representatives, we reveal that the electrostatic polarity of non‐sacrificial additives is a critical descriptor for their ability to stabilize Zn anodes. Non‐sacrificial additives are found to continuously modulate the solvation structure of Zn ions and form a molecular adsorption layer (MAL) for uniform Zn deposition, avoiding the thick solid electrolyte interphase layer due to the decomposition of sacrificial additives. A high electrostatic polarity renders sucrose the best hydrated Zn2+ desolvation ability and facilitates the MAL formation, resulting in the best cycling stability with a long‐term reversible plating/stripping cycle life of thousands of hours. This study provides theoretical guidance for the screening of optimal additives for high‐performance ZIBs.

Maximizing Electrostatic Polarity of Non-Sacrificial Electrolyte Additives Enables Stable Zinc-Metal Anodes for Aqueous Batteries.

Although additives are widely used in aqueous electrolytes to inhibit the formation of dendrites and hydrogen evolution reactions on Zn anodes, there is a lack of rational design principles and systematic mechanistic studies on how to select a suitable additive to regulate reversible Zn plating/stripping chemistry. Here, using saccharides as the representatives, we reveal that the electrostatic polarity of non-sacrificial additives is a critical descriptor for their ability to stabilize Zn anodes. Non-sacrificial additives are found to continuously modulate the solvation structure of Zn ions and form a molecular adsorption layer (MAL) for uniform Zn deposition, avoiding the thick solid electrolyte interphase layer due to the decomposition of sacrificial additives. A high electrostatic polarity renders sucrose the best hydrated Zn2+ desolvation ability and facilitates the MAL formation, resulting in the best cycling stability with a long-term reversible plating/stripping cycle life of thousands of hours. This study provides theoretical guidance for the screening of optimal additives for high-performance ZIBs.

Effect of Carbonization Behaviour of Cotton Biomass in Electrodes for Sodium‐Ion Batteries

AbstractFree‐standing hard carbon electrodes are produced from cotton biomass using a low‐cost, one‐step pathway. The free‐standing feature of the electrode eliminates the use of binders and toxic solvents. The electrochemical performance of the electrodes is tested to study the correlation between Na storage and the structural properties of the hard carbon material. A remarkable specific capacity of 272 mAh g−1 at a current density of 50 mA g−1 is obtained with a high initial Coulombic efficiency of 75 % for the cotton fabric (CF) sample pyrolyzed at 1000 °C for 5 min (CF5 min). The excellent performance of the free‐standing electrode is attributed to a large interlayer spacing between the graphene layers, and a high number of oxygen‐containing functional groups on the surface. X‐ray photoelectron spectroscopy (XPS) surface characterisation shows that a thin and uniformly distributed SEI (solid electrolyte interphase) layer, mainly composed of NaF and Na2O, is formed on the CF5 min surface, whereas a thick SEI layer with a long Na+ diffusion pathway is formed on the sample pyrolyzed at 1000 °C for 10 h (CF10 h), which leads to slower reaction kinetics and poor electrochemical performance. This work proposes a scalable and economically feasible strategy to produce sodium ion anode materials with a focus on environmental sustainability and value addition to waste streams.

Effect of Salt Concentration on the Interfacial Solvation Structure and Early Stage of Solid–Electrolyte Interphase Formation in Ca(BH4)2/THF for Ca Batteries

The Ca2+ solvation structure at the electrolyte/electrode interface is of central importance to understand electroreduction stability and solid-electrolyte interphase (SEI) formation for the novel multivalent Ca battery systems. Using an exemplar electrolyte, the concentration-dependent solvation structure of Ca(BH4)2-tetrahydrofuran on a gold model electrode has been investigated with various electrolyte concentrations via electrochemical quartz crystal microbalance with dissipation (EQCM-D) and X-ray photoelectron spectroscopy (XPS). For the first time, in situ EQCM-D results prove that the prevalent species adsorbed at the interface is CaBH4+ across all concentrations. As the salt concentration increases, the number of BH4- anions associated with Ca2+ increases, and much larger solvated complexes such as CaBH4+·4THF or Ca(BH4)3-·4THF form at the interface at high concentrations prior to Ca plating. Different interfacial chemistries lead to the formation of SEIs with different components demonstrated by XPS. High electrolyte concentrations reduce the solvent decomposition and promote the formation of thick, uniform, and inorganic-rich (i.e., CaO) SEI layers, which contribute to improved Ca plating efficiency and current density in electrochemical measurements.

Characterization of the Solid Electrolyte Interphase at the Li Metal–Ionic Liquid Interface

AbstractThe solid electrolyte interphase (SEI) forms on electrode surfaces from decomposition of the electrolyte. However, there is almost no atomistic detail of SEI formation on Li metal anode, a major obstacle in understanding the highly complex battery electrochemistry sufficiently to design high performance batteries. Herein, a realistic atomistic model (39 000 atoms) for the SEI formation at the interface between the Li metal anode and ionic liquid electrolyte using reactive molecular dynamics simulations is provided. A ≈10 nm thick SEI composed of a dense ordered inorganic layer near the Li‐metal anode and a porous organic layer near the electrolyte is found. These results provide new insights into a deeper understanding of the complex SEI that should be useful in developing a new generation of highly efficient batteries.

Interwoven carbon nanotube-poly(acrylic acid) network scaffolds for stable Si microparticle battery anode

A highly stable silicon microparticle (μSi) anode with high energy density is reported using a hybrid binder made of functionalized carbon nanotubes (FCNTs) and poly(acrylic acid) (PAA). The FCNTs with high electrical conductivity can be stably dispersed in water after partial oxidation of the carbon nanotubes (CNTs), forming a macroporous scaffold in an aqueous dispersion. By mixing the FCNT scaffold with PAA and μSi particles, FCNT/PAA uniformly encapsulates μSi and can be easily coated on Cu foil via a practical bar-coating method without the use of harsh solvents, stabilizers, or additional processes. The μSi-FCNT-PAA electrode exhibits stable performance, with an initial reversible capacity of 3560 mAh g−1 at 1C and a specific capacity of 1764 mAh g−1 at 200 cycles in a half-cell test. Full-cell test with NCM 712 cathode also shows stable capacity retention at 1C, exhibiting an initial reversible capacity of 170 mAh g−1 and a specific capacity of 81 mAh g−1 at 200 cycles. The FCNT network with enhanced mechanical properties due to the addition of PAA is effective in preserving the Si anode structure during cycling tests by enduring its volume changes and preventing thick solid-electrolyte interphase (SEI) layer formation.

Enhanced Structural Stability and Capacity Retention of Ni-Rich LiNi0.91Co0.06Mn0.03O2 Cathode Material By Dual Doping and Coating for Libs

Nickel-rich layered cathode material has been received a lot of attention due to their superior cycle-ability, high energy density and improved safety. However, there are some issues which are needed to be resolved to ensure the long term application. The hostile attack of electrolyte with the charged cathode electrode results in the growth of thick solid electrolyte interphase (SEI) at the expense of Li+ consumption and increase in interfacial resistance. In addition, de/intercalation of Li+ leads to the continuous contraction and expansion of lattice structure that leads to the crack formation in the active material particles during extended cycling.In this work, a facile technique has been adopted to resolve the mentioned problems. In this study, Li3BO3 has been coated on the NCM surface to protect the active material against the continuous attack of electrolyte and boron is doped to enhance the structural stability of LiNi0.91Co0.06Mn0.03O2 NCM cathode material. The samples were characterized by XPS, XRD, SEM and TEM and the electrochemical properties were measured by assembling the coin cells. The XRD results confirm the shifting of (003) diffraction peak towards low angle which validates the boron doping in the crystal structure. The presence of B1s is confirmed on the surface of modified NCM by XPS analysis. The FETEM images shows a uniform and distinct coating layer of 12 nm on the surface of 0.05 wt% coated NCM. The 0.05 wt% NCM samples offers the highest discharge capacity of 215.3 mAh g-1 at 0.1C and capacity retention of 81.6% at 0.5C (1C = 202 mA g-1) after 100 cycles. The reason behind enhanced cycling is the high bond dissociation energy of B−O (ΔHf298 = 402 kJ mol−1) than Ni–O (ΔHf298 = 391.6 kJ mol−1), Co–O (ΔHf298 = 368 kJ mol−1) and even Mn–O (ΔHf298 = 402 kJ mol−1). Even at 2C cycling, LBO-0.05 NCM sample demonstrated a capacity retention of 79.4% after 100 cycles while pristine shows only 67.3%. The modified samples displayed superior rate performance as compare to pristine NCM, especially at high current density. The cyclic voltammetry and EIS results are in-line with the cycling data. The presence of B3+ doping in host structure and Li3BO3 coating on the NCM surface results in the superior electrochemical performance of modified NCM.

Ion-Solvent Interplay in Concentrated Electrolytes Enables Subzero Temperature Li-Ion Battery Operations.

Despite the essential role of ethylene carbonate (EC) in solid electrolyte interphase (SEI) formation, the high Li+ desolvation barrier and melting point (36 °C) of EC impede lithium-ion battery operation at low temperatures and induce sluggish Li+ reaction kinetics. Here, we demonstrate an EC-free high salt concentration electrolyte (HSCE) composed of lithium bis(fluorosulfonyl)imide salt and tetrahydrofuran solvent with enhanced subzero temperature operation originating from unusually rapid low-temperature Li+ transport. Experimental and theoretical characterizations reveal the dominance of intra-aggregate ion transport in the HSCE that enables efficient low-temperature transport by increasing the exchange rate of solvating counterions relative to that of solvent molecules. This electrolyte also produces a <5 nm thick anion-derived LiF-rich SEI layer with excellent graphite electrode compatibility and electrochemical performance at subzero temperature in half-cells. Full cells based on LiNi0.6Co0.2Mn0.2O2||graphite with tailored HSCE electrolytes outperform state-of-the-art cells comprising conventional EC electrolytes during charge-discharge operation at an extreme temperature of -40 °C. These results demonstrate the opportunities for creating intrinsically robust low-temperature Li+ technology.

Mechanistic Insight into Li–Host Interactions in Carbon Hosts for Reversible Li Metal Storage

In recent years, the industrial requirements for rechargeable batteries in terms of energy density have continuously increased owing to the widespread adoption of portable electronic devices and electric vehicles. Li metal has been identified as a promising anode material to meet the ever-increasing demand for rechargeable batteries with high energy densities because of its low redox potential (−3.04 V vs. standard hydrogen electrode)and high specific capacity (3860 mAh g−1) [1]. Despite these advantages, the commercial use of Li metal anodes has been hindered by unresolved critical issues, such as the uncontrollable growth of Li dendrites and the severe volume changes during cycling. In practice, the dendritic growth of Li deposits causes the formation of dead Li species and triggers internal short-circuiting of batteries during cycling, leading to a significant loss of reversibility and raising safety concerns. Moreover, large volume changes in the Li metal anode during Li plating–stripping, which originate from the “hostless” character of the Li metal, impose severe stresses on the solid–electrolyte interphase (SEI) layers, and thus reducing the mechanical stability of SEIs. Once the SEI layers are cracked or fractured, bare Li metal is exposed to the electrolyte, leading to the continuous formation of thick SEI layers on the anode surface. This is mainly responsible for the continuous, irreversible loss of active Li, i.e., low Coulombic efficiency and short cycle lifetimes.3D “host” architectures are beneficial for securing the dimensional stability of Li metal anodes by utilizing internal pores (i.e., free-spaces) as reservoirs for Li storage. Moreover, they provide large active surface areas and hence reduce the effective current densities, suppressing uneven Li plating (dendritic growth) during high-rate operations. Metal-organic framework (MOF)-derived carbon offers many advantages over conventional carbon materials in terms of specific surface area, pore volume, and structural tunability. Furthermore, the critical role of atomically dispersed heteroatoms has been verified in reducing the energy barriers for the nucleation and growth of Li [2].Herein, we report MOF-derived porous carbon hosts with strong Li–host interactions achieved by galvanically displaced lithiophilic seeds. Owing to the distinctive chemical and structural nature of zeolitic imidazolate frameworks (ZIF-8), atomically dispersed Zn can be spontaneously replaced by Ag with a strong lithiophilicity via a galvanic displacement (GD) reaction. Based on a comparative study of carbon hosts with different sizes, morphologies, and distributions of Ag, in particular, we assess the efficacy of lithiophilic Ag in promoting reversible Li storage inside porous carbon hosts. The strong Li–host interactions enabled by lithiophilic Ag effectively reduce the overpotentials for nucleation and growth of Li, and more importantly, the role of lithiophilic Ag in governing the reversibility of Li plating–stripping is discussed in terms of the spatial distribution characteristics of Ag. The atomically dispersed Ag triggers the outward growth of Li from the internal pores of the carbon host, showing a stable cycling performance (>745 cycles). Contrarily, the surface-anchored Ag nanoparticles induce uneven Li plating on the outer surface of the carbon host, resulting in a rapid performance drop (~305 cycles) during cycling. This work provides new insights into the development of advanced 3D host materials for reversible Li metal anodes by utilizing strong Li–host interactions.

Structural Restoration of Degraded LiFePO 4 Cathode with Enhanced Kinetics Using Residual Lithium in Spent Graphite Anodes

Structural Restoration of Degraded LiFePO ₄ Cathode with Enhanced Kinetics Using Residual Lithium in Spent Graphite Anodes

Tailoring of a reinforcing and artificial self-assembled alkyl sulfonic acid layer electrolyte interphase on silicon as an anode for high-energy-density lithium-ion batteries

Making anodes from silicon results in various longstanding challenges, including drastic volume expansion, formation of a thick and high-impedance solid electrolyte interphase (SEI), rapid capacity decay, and poor rate performance. The use of these anodes in high-energy-density lithium-ion batteries is thus limited. Herein, an artificial SEI with a self-assembled alkyl sulfonic acid (SAASA) reinforcement structure is deposited onto the surface of Si through an organosilane approach (to obtain a Si-SAASA electrode). A coupling agent, 3-mercaptopropyl trimethoxysilane (MPTMS), was tailored on the surface of Si to produce strong siloxane (Si–O–Si) bonds. The thiol group (-SH) in the MPTMS molecule consequently oxidized into sulfonic acid (-SO3H), resulting in sulfonated artificial SEI reinforcement on the Si surface. With sulfonated MPTMS, the Si anode was improved through the formation of -SO3Li, which enhanced Li+-ion diffusion and ionic mobility. The Si-SAASA electrode has the highest discharge capacity of 1507.1 mAh g−1 at 0.5 C after 400 cycles with a retention capacity of 74.3%, high rate capability, and a high lithium-ion diffusion coefficient (2.88 × 10−12 cm2 s−1). This study demonstrated that the SAASA reinforcement, acting as an artificial SEI, gives the electrode mechanical integrity, meaning that during cycling, expansion of the Si's volume can be accommodated and the fragmentation of Si particles can be prevented through the strong siloxane bonds. Additionally, the SAASA serves as a protection against parasitic reactions between the electrolyte and active interface, thus preventing the repeated growth of a thick and high-impedance SEI layer. The developed approach will be exceedingly effective and minimize the cost of developing high-performance Si anodes for next-generation lithium-ion batteries.

Electrochemical Pretreatment of Solid‐Electrolyte Interphase Formation for Enhanced Li4Ti5O12 Anode Performance in a Molten Li−Ca Binary Salt Hydrate Electrolyte

AbstractWater‐in‐salt electrolytes have been widely explored because of their expanded electrochemical stability (&gt;3.0 V). However, the instability of solid‐electrolyte interphase (SEI) in aqueous electrolytes leads to their reductive decomposition on the negative electrodes of low‐potential anode materials. Here, we demonstrate significant improvement in the cycle performance of a Li4Ti5O12 electrode using a Li−Ca binary salt hydrate (LCH) electrolyte in combination with an optimized electrochemical pretreatment process. Compared with a hydrate‐melt electrolyte, the LCH electrolyte provided less water‐soluble Ca‐based SEI components, and careful pretreatment process enabled the formation of a thicker SEI layer on the Li4Ti5O12 electrode. Protected with the hardly soluble, thick SEI layer, the Li4Ti5O12 electrode effectively mitigated unfavorable side reactions and achieved 95.5 % capacity retention over 50 cycles. These results offer insight into a promising route for stable SEI layer formation for the practical use of low‐potential anode materials in aqueous rechargeable lithium‐ion batteries.

Comparative X-ray Photoelectron Spectroscopy Study of the SEI and CEI in Three Different Lithium Ion Cell Formats

The solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) of three lithium ion cell formats, i.e., coin, lab-scale pouch and multi-layer pouch, are compared. Half the cells are additionally dried prior to electrolyte filling and cycling. The highest water content per cell, determined by Karl Fischer titration, is found for lab-scale pouch cells due to their disadvantageous ratio of cell housing area to electrode area. The water content influences the performance during electrochemical formation as well as the impedance. This is linked to increasing lithium fluoride concentration, as determined by X-ray photoelectron spectroscopy. For dried cells, this is not the case because there is less conducting salt hydrolysis. The CEI thickness decreases for dried pouch cells, while the organic SEI thickness increases in all cell formats for dried cells. It is concluded that the initial thickness of the porous organic SEI depends on the insulation of the dense inorganic SEI close to the electrode surface. Organic species are more likely to contribute to negative electrode passivation when the extent of conducting salt hydrolysis is low. For coin cells, the presence of atmospheric gases during formation results in thicker SEI and CEI, no matter whether cells are additionally dried.

Electrochemical Synthesis of Germanium-Polypyrrole Composite Nanomaterials in Ionic Liquids for the Fabrication of Lithium-Ion Batteries

Herein, we report the coating of nanostructured germanium using a polypyrrole (PPy) polymer coat as a composite anode material for the fabrication of lithium-ion batteries. The Ge/PPy composites were synthesized following the direct electrochemical deposition method in an ionic liquid (IL). The results revealed that the coating of PPy on Ge helped realize stable battery cycling and reversible capacities, which were not observed in uncoated Ge. The PPy layers could effectively inhibit side reactions between the electrode and electrolyte. The composition of the solid electrolyte interphase (SEI) formed after lithiation/delithiation cycles were analyzed using the X-ray photoelectron spectroscopy (XPS). Compact SEI layers consisted of decomposed TFSI− anion products such as LiF, Li2S, Li2NS2O4, and Li2CO3 at the Ge-PPy/IL interphase. In contrast, thick SEI layers consisted of not only decomposed TFSI− anion and [Py1,4]+ cation products but also chemically or physically adsorbed IL compounds at the Ge/IL interphase. In addition, the PPy coating could effectively inhibit Ge oxidation, resulting in improved battery capacity.

Multiscale Observations of Inhomogeneous Bilayer SEI Film on a Conversion‐Alloying SnO2 Anode

AbstractSnO2, storing Li through conversion and alloying reactions, has been regarded as one of the most typical anode materials and has been widely studied for both mechanism exploring and performance tuning. However, the structure of the solid electrolyte interphase (SEI) formed on the SnO2 electrode and its evolution process are rarely focused and still poorly understood. Herein, time of flight secondary ion mass spectrometry is used to observe the bilayer hybrid structure of SEI formed on a SnO2 film. Multiscale observations reveal the SEI accumulation after alloying reactions and distinct dissolving of the organic layer at potentials above de‐conversion reactions, which results in the inorganic layer being directly exposed to the electrolyte and thus becoming thick and inhomogeneous. The broken and thick SEI causes rapid capacity decay and low Coulombic efficiencies (CEs) of 97.5%. Accordingly, it is demonstrated that, as the SnO2 is precoated with LiF or Li2CO3, a robust and thin SEI layer is induced to form and is stabilized in the continuous cycles, resulting in enhanced cycling stability and promoted CEs to 99.5%. This work adds new insights to the SEI evolution mechanisms on SnO2‐based anodes and suggests an effective strategy to create high performance metal oxide anode for Li‐ion batteries.