Scanning Electrochemical Microscopy Reveals That Model Silicon Anodes Demonstrate Global Solid Electrolyte Interphase Passivation Degradation during Calendar Aging

Silicon is a promising next-generation anode to increase energy density over commercial graphite anodes, but calendar life remains problematic. In this work, scanning electrochemical microscopy was used to track the site-specific reactivity of a silicon thin film surface over time to determine if undesirable Faradaic reactions were occurring at the formed solid electrolyte interphase (SEI) during calendar aging in four case scenarios: formation between 1.5 V and 100 mV with subsequent rest starting at (1) 1.5 V and (2) 100 mV and formation between 0.75 V and 100 mV with subsequent rest starting at (3) 0.75 V and (4) 100 mV. In all cases, the electrical passivation of silicon decreased with increasing time and potential relative to Li/Li+ over a 3 day period. Along with the decrease in passivation, the homogeneity of passivation over a 500 μm2 area decreased with time. Despite some local "hot spots" of reactivity, the areal uniformity of passivation suggests global SEI failure (e.g., SEI dissolution) rather than localized (e.g., cracking) failure. The silicon delithiated to 1.5 V vs Li/Li+ was less passivated than the lithiated silicon (at the beginning of rest, the forward rate constants, kf, for ferrocene redox were 7.19 × 10-5 and 3.17 × 10-7 m/s, respectively) and was also found to be more reactive than the pristine silicon surface (kf of 5 × 10-5 m/s). This reactivity was likely the result of SEI oxidation. When the cell was only delithiated up to 0.75 V versus Li/Li+, the surface was still passivating (kf of 6.11 × 10-6 m/s), but still less so than the lithiated surface (kf of 3.03 × 10-9 m/s). This indicates that the potential of the anode should be kept at or below ∼0.75 V vs Li/Li+ to prevent decreasing SEI passivation. This information will help with tuning the voltage windows for prelithiation in Si half cells and the operating voltage of Si full cells to optimize calendar life. The results provided should encourage the research community to investigate chemical, rather than mechanical, modes of failure during calendar aging and to stop using the typical convention of 1.5 V as a cutoff potential for cycling Si in half cells.

Fluorine Chemistry in Rechargeable Batteries: Challenges, Progress, and Perspectives

The renewable energy industry demands rechargeable batteries that can be manufactured at low cost using abundant resources while offering high energy density, good safety, wide operating temperature windows, and long lifespans. Utilizing fluorine chemistry to redesign battery configurations/components is considered a critical strategy to fulfill these requirements due to the natural abundance, robust bond strength, and extraordinary electronegativity of fluorine and the high free energy of fluoride formation, which enables the fluorinated components with cost effectiveness, nonflammability, and intrinsic stability. In particular, fluorinated materials and electrode|electrolyte interphases have been demonstrated to significantly affect reaction reversibility/kinetics, safety, and temperature tolerance of rechargeable batteries. However, the underlining principles governing material design and the mechanistic insights of interphases at the atomic level have been largely overlooked. This review covers a wide range of topics from the exploration of fluorine-containing electrodes, fluorinated electrolyte constituents, and other fluorinated battery components for metal-ion shuttle batteries to constructing fluoride-ion batteries, dual-ion batteries, and other new chemistries. In doing so, this review aims to provide a comprehensive understanding of the structure-property interactions, the features of fluorinated interphases, and cutting-edge techniques for elucidating the role of fluorine chemistry in rechargeable batteries. Further, we present current challenges and promising strategies for employing fluorine chemistry, aiming to advance the electrochemical performance, wide temperature operation, and safety attributes of rechargeable batteries.

Operando Investigation of Solid Electrolyte Interphase Formation, Dynamic Evolution, and Degradation During Lithium Plating/Stripping

The solid electrolyte interphase (SEI) dictates the stability and cycling performance of highly reactive battery electrodes. Characterization of the thin, dynamic, and environmentally sensitive nature of the SEI presents a formidable challenge, which calls for the use of microscopic, time-resolved operando methods. Herein, we employ scanning electrochemical microscopy (SECM) to directly probe the heterogeneous surface electronic conductivity during SEI formation and degradation. Complementary operando electrochemical quartz crystal microbalance (EQCM) and ex situ X-ray photoelectron spectroscopy (XPS) provide comprehensive analysis of the dynamic size and compositional evolution of the complex interfacial microstructure. We have found that stable anode passivation occurs at potentials of 0.5 V vs Li/Li+, even in cases where anion decomposition and interphase formation occur above 1.0 V. We investigated the bidirectional relationship between the SEI and lithium plating-stripping, finding that plating-stripping ruptures the SEI. The current efficiency of this reaction is correlated to the anodic stability of the SEI, highlighting the interdependent relationship between the two. We anticipate this work will provide critical insights on the rational design of stable and effective SEI layers for safe, fast-charging, and long-lifetime lithium metal batteries.

Interfacial Chemistry Effects in the Electrochemical Performance of Silicon Electrodes under Lithium-Ion Battery Conditions

Understanding the solid electrolyte interphase (SEI) formation and (de)lithiation phenomena at silicon (Si) electrodes is key to improving the performance and lifetime of Si-based lithium-ion batteries. However, these processes remain somewhat elusive, and, in particular, the role of Si surface termination merits further consideration. Here, scanning electrochemical cell microscopy (SECCM) is used in a glovebox, followed by secondary ion mass spectrometry (SIMS) at identical locations to study the local electrochemical behavior and associated SEI formation, comparing Si (100) with a native oxide layer (SiOx /Si) and etched with hydrofluoric acid (HF-Si). HF-Si shows greater spatial electrochemical heterogeneity and inferior lithiation reversibility than SiOx /Si. This is attributed to a weakly passivating SEI and irreversible lithium trapping at the Si surface. Combinatorial screening of charge/discharge cycling by SECCM with co-located SIMS reveals SEI chemistry as a function of depth. While the SEI thickness is relatively independent of the cycle number, the chemistry - particularly in the intermediate layers - depends on the number of cycles, revealing the SEI to be dynamic during cycling. This work serves as a foundation for the use of correlative SECCM/SIMS as a powerful approach to gain fundamental insights on complex battery processes at the nano- and microscales.

Insights into the solvation chemistry in liquid electrolytes for lithium-based rechargeable batteries

Lithium-based rechargeable batteries have dominated the energy storage field and attracted considerable research interest due to their excellent electrochemical performance. As indispensable and ubiquitous components, electrolytes play a pivotal role in not only transporting lithium ions, but also expanding the electrochemical stable potential window, suppressing the side reactions, and manipulating the redox mechanism, all of which are closely associated with the behavior of solvation chemistry in electrolytes. Thus, comprehensively understanding the solvation chemistry in electrolytes is of significant importance. Here we critically reviewed the development of electrolytes in various lithium-based rechargeable batteries including lithium-metal batteries (LMBs), nonaqueous lithium-ion batteries (LIBs), lithium-sulfur batteries (LSBs), lithium-oxygen batteries (LOBs), and aqueous lithium-ion batteries (ALIBs), and emphasized the effects of interactions between cations, anions, and solvents on solvation chemistry, and functions of solvation chemistry in different types of electrolytes (strong solvating electrolytes, moderate solvating electrolytes, and weak solvating electrolytes) on the electrochemical performance and redox mechanism in the abovementioned rechargeable batteries. Specifically, the significant effects of solvation chemistry on the stability of electrode-electrolyte interphases, suppression of lithium dendrites in LMBs, inhibition of the co-intercalation of solvents in LIBs, improvement of anodic stability at high cut-off voltages in LMBs, LIBs and ALIBs, regulation of redox pathways in LSBs and LOBs, and inhibition of hydrogen/oxygen evolution reactions in LOBs are thoroughly summarized. Finally, the review concludes with a prospective outlook, where practical issues of electrolytes, advanced in situ/operando techniques to illustrate the mechanism of solvation chemistry, and advanced theoretical calculation and simulation techniques such as "material knowledge informed machine learning" and "artificial intelligence (AI) + big data" driven strategies for high-performance electrolytes have been proposed.

Scanning Electrochemical Microscopy for Chemical Imaging and Understanding Redox Activities of Battery Materials

Improving the charge storage capacity and lifetime and charging/discharging efficiency of battery systems is essential for large-scale applications such as long-term grid storage and long-range automobiles. While there have been substantial improvements over the past decades, further fundamental research would help provide insights into improving the cost effectiveness of such systems. For example, it is critical to understand the redox activities of cathode and anode electrode materials and stability and the formation mechanism and roles of the solid-electrolyte interface (SEI) that forms at the electrode surface upon an external potential bias. The SEI plays a critical role in preventing electrolyte decay while still allowing charges to flow through the system while serving as a charge transfer barrier. While surface analytical techniques such as X-ray photoelectron (XPS), X-ray diffraction (XRD), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and atomic force microscopy (AFM) provide invaluable information on anode chemical composition, crystalline structure, and morphology, they are often performed ex situ, which can induce changes to the SEI layer after it is removed from the electrolyte. While there have been efforts to combine these techniques using pseudo-in situ approaches via vacuum-compatible devices and inert atmosphere chambers connected to glove boxes, there is still a need for true in situ techniques to obtain results with improved accuracy and precision. Scanning electrochemical microscopy (SECM) is an in situ scanning probe technique that can be combined with optical spectroscopy techniques such as Raman and photoluminescence spectroscopy methods to gain insights into the electronic changes of a material as a function of applied bias. This Review will highlight the potential of SECM and recent reports on combining spectroscopic measurements with SECM to gain insights into the SEI layer formation and redox activities of other battery electrode materials. These insights provide invaluable information for improving the performance of charge storage devices.

Impact of the Crystalline Li15Si4 Phase on the Self-Discharge Mechanism of Silicon Negative Electrodes in Organic Electrolytes

Because of their high specific capacity and rather low operating potential, silicon-based negative electrode materials for lithium-ion batteries have been the subject of extensive research over the past 2 decades. Although the understanding of the (de)lithiation behavior of silicon has significantly increased, several major challenges have not been solved yet, hindering its broad commercial application. One major issue is the low initial Coulombic efficiency and the ever-present self-discharge of silicon electrodes. Self-discharge itself affects the long-term stability of electrochemical storage systems and, additionally, must be taken into consideration for inevitable prelithiation approaches. The impact of the crystalline Li₁₅Si₄ phase is of great interest as the phase transformation between crystalline (c) and amorphous (a) phases not only increases the specific surface area but also causes huge polarization. Moreover, there is the possibility for electrochemical over-lithiation toward the Li_15+aSi₄ phase because of the electron-deficient Li₁₅Si₄ phase, which can be highly reactive toward the electrolyte. This poses the question about the impact of the c-Li₁₅Si₄ phase on the self-discharge behavior in comparison to its amorphous counterpart. Here, silicon thin films used as model electrodes are lithiated to cut-off potentials of 10 mV and 50 mV versus Li|Li⁺ (U_10mV and U_50mV) in order to systematically investigate their self-discharge mechanism via open-circuit potential (U_OCP) measurements and to visualize the solid electrolyte interphase (SEI) growth by means of scanning electrochemical microscopy. We show that the c-Li₁₅Si₄ phase is formed for the U_10mV electrode, while it is not found for the U_50mV electrode. In turn, the U_50mV electrode displays an almost linear self-discharge behavior, whereas the U_10mV electrode reaches a U_OCP plateau at ca. 380 mV versus Li|Li⁺, which is due to the phase transition from c-Li₁₅Si₄ to the a-Li_xSi phase. At this plateau potential, the phase transformation at the Si|electrolyte interface results in an electronically more insulating and more uniform SEI (U_10mV electrode), while the U_50mV electrode displays a less uniform SEI layer. In summary, the self-discharge mechanism of silicon electrodes and, hence, the irreversible decomposition of the electrolyte and the corresponding SEI formation process heavily depend on the structural nature of the underlying lithium-silicon phase.

In Situ Observation of Interface Evolution on a Graphite Anode by Scanning Electrochemical Microscopy

A well-formed solid electrolyte interface (SEI) is critical for achieving long-term cycling stability in lithium-ion batteries (LIBs). However, the SEI remains the poorly understood component in LIBs especially under dynamic conditions. Here, scanning electrochemical microscopy (SECM) was applied to study the spontaneous reaction on a graphite electrode, SEI formation in the first cycle, SEI evolution during 10 cycles, and the stability of the as-formed SEI in the electrolyte. The conversion, dissolution, stabilization, and growth behaviors of the SEI were determined. Moreover, the SECM results were analyzed in combination with ex situ material characterization to understand the SEI on the graphite electrode comprehensively.

Scanning Gel Electrochemical Microscopy (SGECM): Lateral Physical Resolution by Current and Shear Force Feedback

Scanning gel electrochemical microscopy (SGECM) is a novel technique measuring local electrochemistry based on a gel probe. The gel probe, which is fabricated by electrodeposition of hydrogel on a microdisk electrode, immobilizes the electrolyte, and constitutes a two-electrode system upon contact with the sample. The contact area determines the lateral physical resolution of the measurement, and considering the soft nature of the gel it is essential to be well analyzed. In this work, the lateral physical resolution of SGECM is quantitatively studied from two aspects: (1) marking single sampling points by locally oxidizing Ag to AgCl and measuring their size; (2) line scan over reference samples with periodic topography and composition. The gel probe is approached to the sample by either current or shear force feedback, and the physical resolution of them is compared. For the optimal gel probe based on 25 μm diameter Pt disk electrode of R_g ≈ 2, the lateral physical resolution of SGECM at contact position is ca. 50 μm for current feedback and ca. 63 μm for shear force feedback. More importantly, the lateral physical resolution of SGECM can be flexibly tuned in the range of 14-78 μm by pulling or pressing the gel probe after touching the sample. In general, current feedback is more sensitive to gel-sample contact than shear force feedback. But the latter is more versatile, which is also applicable to nonconductive samples.

Review of Recent Development of In Situ/Operando Characterization Techniques for Lithium Battery Research

The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions, and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X-ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high-energy-density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.

In situ analytical techniques for battery interface analysis

Interface is a key to high performance and safe lithium-ion batteries or lithium batteries.

Solid Electrolyte Interphase (SEI) at TiO2 Electrodes in Li-Ion Batteries: Defining Apparent and Effective SEI Based on Evidence from X-ray Photoemission Spectroscopy and Scanning Electrochemical Microscopy

The high (de)lithiation potential of TiO₂ (ca. 1.7 V vs Li/Li⁺ in 1 M Li⁺) decreases the voltage and, thus, the energy density of a corresponding Li-ion battery. On the other hand, it offers several advantages such as the (de)lithiation potential far from lithium deposition or absence of a solid electrolyte interphase (SEI). The latter is currently under controversial debate as several studies reported the presence of a SEI when operating TiO₂ electrodes at potentials above 1.0 V vs Li/Li⁺. We investigate the formation of a SEI at anatase TiO₂ electrodes by means of X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). The investigations were performed in different potential ranges, namely, during storage (without external polarization), between 3.0-2.0 V and 3.0-1.0 V vs Li/Li⁺, respectively. No SEI is formed when a completely dried and residues-free TiO₂ electrode is cycled between 3.0 and 2.0 V vs Li/Li⁺. A SEI is detected by XPS in the case of samples stored for 6 weeks or cycled between 3.0 and 1.0 V vs Li/Li⁺. With use of SECM, it is verified that this SEI does not possess the electrically insulating character as expected for a "classic" SEI. Therefore, we propose the term apparent SEI for TiO₂ electrodes to differentiate it from the protecting and effective SEI formed at graphite electrodes.

Running out of lithium? A route to differentiate between capacity losses and active lithium losses in lithium-ion batteries

Active lithium loss (ALL) resulting in a capacity loss (Q_ALL), which is caused by lithium consuming parasitic reactions like SEI formation, is a major reason for capacity fading and, thus, for a reduction of the usable energy density of lithium-ion batteries (LIBs).