In situ observation of solid electrolyte interphase evolution in a lithium metal battery

Extended Battery Compatibility Consideration from an Electrolyte Perspective

The performance of electrochemical batteries is intricately tied to the physicochemical environments established by their employed electrolytes. Traditional battery designs utilizing a single electrolyte often impose identical anodic and cathodic redox conditions, limiting the ability to optimize redox environments for both anode and cathode materials. Consequently, advancements in electrolyte technologies are pivotal for addressing these challenges and fostering the development of next-generation high-performance electrochemical batteries. This review categorizes perspectives on electrolyte technology into three key areas: additives engineering, comprehensive component analysis encompassing solvents and solutes, and the effects of concentration. By summarizing significant studies, the efficacy of electrolyte engineering is highlighted, and the review advocates for further exploration of optimized component combinations. This review primarily focuses on liquid electrolyte technologies, briefly touching upon solid-state electrolytes due to the former greater vulnerability to electrode and electrolyte interfacial effects. The ultimate goal is to generate increased awareness within the battery community regarding the holistic improvement of battery components through optimized combinations.

Monitoring the Behavior of Na Ions and Solid Electrolyte Interphase Formation at an Aluminum/Ionic Liquid Electrode/Electrolyte Interface via Operando Electrochemical X-ray Photoelectron Spectroscopy

In electrochemical energy storage devices, the interface between the electrode and the electrolyte plays a crucial role. A solid electrolyte interphase (SEI) is formed on the electrode surface due to spontaneous decomposition of the electrolyte, which in turn controls the dynamics of ion migration during charge and discharge cycles. However, the dynamic nature of the SEI means that its chemical structure evolves over time and as a function of the applied bias; thus, a true operando study is extremely valuable. X-ray photoelectron spectroscopy (XPS) is a widely used technique to understand the surface electronic and chemical properties, but the use of ultrahigh vacuum in standard instruments is a major hurdle for their utilization in measuring wet electrochemical processes. Herein, we introduce a 3-electrode electrochemical cell to probe the behavior of Na ions and the formation of SEI at the interface of an ionic liquid (IL) electrolyte and an aluminum electrode under operando conditions. A system containing 0.5 molar NaTFSI dissolved in the IL [BMIM][TFSI] was investigated using an Al working electrode and Pt counter and reference electrodes. By optimizing the scan rate of both XPS and cyclic voltammetry (CV) techniques, we captured the formation and evolution of SEI chemistry using real-time spectra acquisition techniques. A CV scan rate of 2 mVs-1 was coupled with XPS snapshot spectra collected at 10 s per core level. The technique demonstrated here provides a platform for the chemical analysis of materials beyond batteries.

Review of the Real-Time Monitoring Technologies for Lithium Dendrites in Lithium-Ion Batteries

Lithium-ion batteries (LIBs) have the advantage of high energy density, which has attracted the wide attention of researchers. Nevertheless, the growth of lithium dendrites on the anode surface causes short life and poor safety, which limits their application. Therefore, it is necessary to deeply understand the growth mechanism of lithium dendrites. Here, the growth mechanism of lithium dendrites is briefly summarized, and the real-time monitoring technologies of lithium dendrite growth in recent years are reviewed. The real-time monitoring technologies summarized here include in situ X-ray, in situ Raman, in situ resonance, in situ microscopy, in situ neutrons, and sensors, and their representative studies are summarized. This paper is expected to provide some guidance for the research of lithium dendrites, so as to promote the development of LIBs.

Self-Assembly Monolayer Inspired Stable Artificial Solid Electrolyte Interphase Design for Next-Generation Lithium Metal Batteries

Lithium metal is widely regarded as the "ultimate" anode for energy-dense Li batteries, but its high reactivity and delicate interface make it prone to dendrite formation, limiting its practical use. Inspired by self-assembled monolayers on metal surfaces, we propose a facile yet effective strategy to stabilize Li metal anodes by creating an artificial solid electrolyte interphase (SEI). Our method involves dip-coating Li metal in MPDMS to create an SEI layer that is rich in inorganic components, allowing uniform Li plating/stripping under a low overpotential over 500 cycles in carbonate electrolytes. In comparison, pristine Li metal shows a rapid increase in overpotential after merely 300 cycles, leading to failure soon after. Molecular dynamics simulations demonstrate that this uniform artificial SEI suppresses Li dendrite formation. We further demonstrated its enhanced stability pairing with LiFePO₄ and LiNi_1-x-yCo_xMn_yO₂ cathodes, highlighting the proposed strategy as a promising solution for practical Li metal batteries.

Fluorinating the Solid Electrolyte Interphase by Rational Molecular Design for Practical Lithium-Metal Batteries

The lifespan of practical lithium (Li)-metal batteries is severely hindered by the instability of Li-metal anodes. Fluorinated solid electrolyte interphase (SEI) emerges as a promising strategy to improve the stability of Li-metal anodes. The rational design of fluorinated molecules is pivotal to construct fluorinated SEI. Herein, design principles of fluorinated molecules are proposed. Fluoroalkyl (-CF₂ CF₂ -) is selected as an enriched F reservoir and the defluorination of the C-F bond is driven by leaving groups on β-sites. An activated fluoroalkyl molecule (AFA), 2,2,3,3-tetrafluorobutane-1,4-diol dinitrate is unprecedentedly proposed to render fast and complete defluorination and generate uniform fluorinated SEI on Li-metal anodes. In Li-sulfur (Li-S) batteries under practical conditions, the fluorinated SEI constructed by AFA undergoes 183 cycles, which is three times the SEI formed by LiNO₃ . Furthermore, a Li-S pouch cell of 360 Wh kg^-1 delivers 25 cycles with AFA. This work demonstrates rational molecular design principles of fluorinated molecules to construct fluorinated SEI for practical Li-metal batteries.

Expanding the active charge carriers of polymer electrolytes in lithium-based batteries using an anion-hosting cathode

Ionic-conductive polymers are appealing electrolyte materials for solid-state lithium-based batteries. However, these polymers are detrimentally affected by the electrochemically-inactive anion migration that limits the ionic conductivity and accelerates cell failure. To circumvent this issue, we propose the use of polyvinyl ferrocene (PVF) as positive electrode active material. The PVF acts as an anion-acceptor during redox processes, thus simultaneously setting anions and lithium ions as effective charge carriers. We report the testing of various Li||PVF lab-scale cells using polyethylene oxide (PEO) matrix and Li-containing salts with different anions. Interestingly, the cells using the PEO-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solid electrolyte deliver an initial capacity of 108 mAh g⁻¹ at 100 μA cm⁻² and 60 °C, and a discharge capacity retention of 70% (i.e., 70 mAh g⁻¹) after 2800 cycles at 300 μA cm⁻² and 60 °C. The Li|PEO-LiTFSI|PVF cells tested at 50 μA cm⁻² and 30 °C can also deliver an initial discharge capacity of around 98 mAh g⁻¹ with an electrolyte ionic conductivity in the order of 10⁻⁵S cm⁻¹.

The energy content of secondary batteries is often limited by the charge carriers available in the system. Here, the authors employed an anion acceptor cathode for simultaneous use of electrolyte anions and cations as effective charge carriers in solid polymer electrolytes for lithium-based batteries.

Nanoscale interface engineering of inorganic Solid-State electrolytes for High-Performance alkali metal batteries

All-solid-state metal batteries (ASSMBs) have been regarded as the ideal candidate for the next-generation high-energy storage system due to their ultrahigh specific capacity and the lowest redox potential. However, the uncontrollable chemical reactivity during cycling which directly determines the growth behaviour of metal dendrites, the low coulombic efficiency and the safety concerns severely limit their real-world applications.. Crystallographic optimization based on solid-state electrolytes (SSEs) provides an atomic-scale and fundamental solution for the inhibition of dendrite growth in metal anodes, which has attracted widespread attentions. From this perspective, we summarize the recent advance of the crystallographic optimization for various classes of solid-state electrolytes. We highlight the recent experimental findings of crystallographic optimization for a new generation of all-solid-state batteries, including lithium-ion batteries, sodium-ion batteries, magnesium-ion batteries, with the aim of providing a deeper understanding of the crystallographic reactions in ASSMBs. The challenges and prospects for the future design and engineering of crystallographic optimization of SSEs are discussed, providing ideas for further research into crystallographic optimization to improve the performance of rechargeable batteries.

In Situ and In Operando Techniques to Study Li-Ion and Solid-State Batteries: Micro to Atomic Level

This work summarizes the most commonly used in situ techniques for the study of Li-ion batteries from the micro to the atomic level. In situ analysis has attracted a great deal of interest owing to its ability to provide a wide range of information about the cycling behavior of batteries from the beginning until the end of cycling. The in situ techniques that are covered are: X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Scanning Transmission Electron Microscopy (STEM). An optimized setup is required to be able to use any of these in situ techniques in battery applications. Depending on the type of data required, the available setup, and the type of battery, more than one of these techniques might be needed. This study organizes these techniques from the micro to the atomic level, and shows the types of data that can be obtained using these techniques, their advantages and their challenges, and possible strategies for overcoming these challenges.

Thermal evolution of NASICON type solid-state electrolytes with lithium at high temperature via in situ scanning electron microscopy

We present the thermal evolution of two NASICON-type ceramics namely LATP (Li_1+xAl_xTi_2-x(PO₄)₃) and LAGP (Li_1+xAl_xGe_2-x(PO₄)₃) by monitoring the electrode-electrolyte interfaces (i.e., Li/LATP and Li/LAGP) at temperatures up to 330 °C via in situ scanning electron microscopy, post-mortem energy-dispersive spectroscopy, and X-ray diffraction. Upon melting of Li and contacting electrolytes, LAGP decomposes completely to form Li based alloys, while LATP is partially decomposed without alloying.

Interfacial processes in electrochemical energy systems

Electrochemical energy systems such as batteries, water electrolyzers, and fuel cells are considered as promising and sustainable energy storage and conversion devices due to their high energy densities and zero or negative carbon dioxide emission. However, their widespread applications are hindered by many technical challenges, such as the low efficiency and poor long-term cyclability, which are mostly affected by the changes at the reactant/electrode/electrolyte interfaces. These interfacial processes involve ion/electron transfer, molecular/ion adsorption/desorption, and complex interface restructuring, which lead to irreversible modifications to the electrodes and the electrolyte. The understanding of these interfacial processes is thus crucial to provide strategies for solving those problems. In this review, we will discuss different interfacial processes at three representative interfaces, namely, solid-gas, solid-liquid, and solid-solid, in various electrochemical energy systems, and how they could influence the performance of electrochemical systems.

Inorganic Fillers in Composite Gel Polymer Electrolytes for High-Performance Lithium and Non-Lithium Polymer Batteries

Among the various types of polymer electrolytes, gel polymer electrolytes have been considered as promising electrolytes for high-performance lithium and non-lithium batteries. The introduction of inorganic fillers into the polymer-salt system of gel polymer electrolytes has emerged as an effective strategy to achieve high ionic conductivity and excellent interfacial contact with the electrode. In this review, the detailed roles of inorganic fillers in composite gel polymer electrolytes are presented based on their physical and electrochemical properties in lithium and non-lithium polymer batteries. First, we summarize the historical developments of gel polymer electrolytes. Then, a list of detailed fillers applied in gel polymer electrolytes is presented. Possible mechanisms of conductivity enhancement by the addition of inorganic fillers are discussed for each inorganic filler. Subsequently, inorganic filler/polymer composite electrolytes studied for use in various battery systems, including Li-, Na-, Mg-, and Zn-ion batteries, are discussed. Finally, the future perspectives and requirements of the current composite gel polymer electrolyte technologies are highlighted.

Carbon and germanium nanocages as anode electrodes in sodium-ion and potassium-ion batteries

Here, the potential of C₃₆, C₄₈, and Ge₄₈ nanocages as anodes of Na-ion battery (IB) and K-IB are investigated by DFT/M06-2X and DFT/B3LYP in gas and solvent. The E_Formation and E_Gap of C₃₆, C₄₈, and Ge₄₈ nanocages are investigated by theoretical methods. The vertical and adiabatic EA and IP of C₃₆, C₄₈, and Ge₄₈ nanocages are examined in gas and solvent. The E_ad of Na⁺ and K⁺ on inner and outer positions of C₃₆, C₄₈, and Ge₄₈ nanocages are investigated. The V_cell and C_Theory of C₃₆, C₄₈, and Ge₄₈ as anodes of batteries are investigated. The results of this paper proposed the nano materials (C₄₈ and Ge₄₈ nanocage) as anodes of Na-IB and K-IB with higher C_Theory and V_cell than graphene nanosheet.

On high-temperature evolution of passivation layer in Li-10 wt % Mg alloy via in situ SEM-EBSD

Li-10 wt % Mg alloy (Li-10 Mg) is used as an anode material for a solid-state battery with excellent electrochemical performance and no evidence of dendrite formation during cycling. Thermal treatment of Li metal during manufacturing improves the interfacial contact between a Li metal electrode and solid electrolyte to achieve an all solid-state battery with increased performance. To understand the properties of the alloy passivation layer, this paper presents the first direct observation of its evolution at elevated temperatures (up to 325°C) by in situ scanning electron microscopy. We found that the morphology of the surface passivation layer was unchanged above the alloy melting point, while the bulk of the material below the surface was melted at the expected melting point, as confirmed by in situ electron backscatter diffraction. In situ heat treatment of Li-based materials could be a key method to improve battery performance.

Direct observation of lithium metal dendrites with ceramic solid electrolyte

Dendrite formation, which could cause a battery short circuit, occurs in batteries that contain lithium metal anodes. In order to suppress dendrite growth, the use of electrolytes with a high shear modulus is suggested as an ionic conductive separator in batteries. One promising candidate for this application is Li₇La₃Zr₂O₁₂ (LLZO) because it has excellent mechanical properties and chemical stability. In this work, in situ scanning electron microscopy (SEM) technique was employed to monitor the interface behavior between lithium metal and LLZO electrolyte during cycling with pressure. Using the obtained SEM images, videos were created that show the inhomogeneous dissolution and deposition of lithium, which induce dendrite growth. The energy dispersive spectroscopy analyses of dendrites indicate the presence of Li, C, and O elements. Moreover, the cross-section mapping comparison of the LLZO shows the inhomogeneous distribution of La, Zr, and C after cycling that was caused by lithium loss near the Li electrode and possible side reactions. This work demonstrates the morphological and chemical evolution that occurs during cycling in a symmetrical Li–Li cell that contains LLZO. Although the superior mechanical properties of LLZO make it an excellent electrolyte candidate for batteries, the further improvement of the electrochemical stabilization of the garnet–lithium metal interface is suggested.

Temperature-Dependent Chemical and Physical Microstructure of Li Metal Anodes Revealed through Synchrotron-Based Imaging Techniques

The Li metal anode has been long sought-after for application in Li metal batteries due to its high specific capacity (3860 mAh g^-1 ) and low electrochemical potential (-3.04 V vs the standard hydrogen electrode). Nevertheless, the behavior of Li metal in different environments has been scarcely reported. Herein, the temperature-dependent behavior of Li metal anodes in carbonate electrolyte from the micro- to macroscales are explored with advanced synchrotron-based characterization techniques such as X-ray computed tomography and energy-dependent X-ray fluorescence mapping. The importance of testing methodology is exemplified, and the electrochemical behavior and failure modes of Li anodes cycled at different temperatures are discussed. Moreover, the origin of cycling performance at different temperatures is identified through analysis of Coulombic efficiencies, surface morphology, and the chemical composition of the solid electrolyte interphase in quasi-3D space with energy-dependent X-ray fluorescence mappings coupled with micro-X-ray absorption near edge structure. This work provides new characterization methods for Li metal anodes and serves as an important basis toward the understanding of their electrochemical behavior in carbonate electrolytes at different temperatures.

Behavior of Solid Electrolyte in Li-Polymer Battery with NMC Cathode via in-Situ Scanning Electron Microscopy

We present the first results of in situ scanning electron microscopy (SEM) of an all-solid Li battery with a nickel-manganese-cobalt-oxide (NMC-622) cathode at 50 °C and an operating voltage of 2.7-4.3 V. Experiments were conducted under a constant current at several C rates (nC rate: cycling in 1/n h): C/12, C/6, and C/3. The microstructure evolution during cycling was monitored by continuous secondary electron imaging. We found that the chemical degradation of the solid polymer electrolyte (SPE) was the main mechanism for battery failure. This degradation was observed in the form of a gradual thinning of the SPE as a function of cycling time, resulting in gas generation from the cell. We also present various dynamic electrochemical and mechanical phenomena, as observed by SEM images, and compare the performance of this battery with that of an all-solid Li battery with a LiFePO₄ cathode.