Imaging solid-electrolyte interphase dynamics using operando reflection interference microscopy

High-Throughput Combinatorial Analysis of the Spatiotemporal Dynamics of Nanoscale Lithium Metal Plating

The development of Li metal batteries requires a detailed understanding of complex nucleation and growth processes during electrodeposition. In situ techniques offer a framework to study these phenomena by visualizing structural dynamics that can inform the design of uniform plating morphologies. Herein, we combine scanning electrochemical cell microscopy (SECCM) with in situ interference reflection microscopy (IRM) for a comprehensive investigation of Li nucleation and growth on lithiophilic thin-film gold electrodes. This multimicroscopy approach enables nanoscale spatiotemporal monitoring of Li plating and stripping, along with high-throughput capabilities for screening experimental conditions. We reveal the accumulation of inactive Li nanoparticles in specific electrode regions, yet these regions remain functional in subsequent plating cycles, suggesting that growth does not preferentially occur from particle tips. Optical-electrochemical correlations enabled nanoscale mapping of Coulombic Efficiency (CE), showing that regions prone to inactive Li accumulation require more cycles to achieve higher CE. We demonstrate that electrochemical nucleation time (tnuc) is a lagging indicator of nucleation and introduce an optical method to determine tnuc at earlier stages with nanoscale resolution. Plating at higher current densities yielded smaller Li nanoparticles and increased areal density, and was not affected by heterogeneous topographical features, being potentially beneficial to achieve a more uniform plating at longer time scales. These results enhance the understanding of Li plating on lithiophilic surfaces and offer promising strategies for uniform nucleation and growth. Our multimicroscopy approach has broad applicability to study nanoscale metal plating and stripping phenomena, with relevance in the battery and electroplating fields.

Dual-Mode Imaging of Dynamic Interaction between Bubbles and Single Nanoplates during the Electrocatalytic Hydrogen Evolution Process

Gas bubble formation at electrochemical interfaces can significantly affect the efficiency and durability of electrocatalysts. However, obtaining comprehensive details on bubble evolution dynamics, particularly their dynamic interaction with high-performance structured electrocatalysts, poses a considerable challenge. Herein, dual-mode interference/total internal reflection fluorescence microscopy is introduced, which allows for the simultaneous capture of the evolution pathway of bubbles and the 3D motion of nanoplate electrocatalysts, providing high-resolution and accurate spatiotemporal information. During the hydrogen evolution reaction, the dynamics of hydrogen bubble generation and their interactions with single nanoplate electrocatalysts at the electrochemical interface are observed. The results unveiled that, under constant potential, bubbles initially manifest as fast-moving nanobubbles, transforming into stationary microbubbles subsequently. The morphology of stationary nanoplates regulates the trajectories of these moving nanobubbles while the pinned microbubbles induce the motion of the electrocatalysts. The dual-mode microscopy can be employed to scrutinize numerous multiphase electrochemical interactions with high spatiotemporal resolution, which can facilitate the rational design of high-performance electrocatalysts.

Radially distributed charging time constants at an electrode-solution interface

An electrochemically homogeneous electrode-solution interface should be understood as spatially invariant in both terms of intrinsic reactivity for the electrode side and electrical resistance mainly for the solution side. The latter remains presumably assumed in almost all cases. However, by using optical microscopy to spatially resolve the classic redox electrochemistry occurring at the whole surface of a gold macroelectrode, we discover that the electron transfer occurs always significantly sooner (by milliseconds), rather than faster in essence, at the radial coordinates closer to the electrode periphery than the very center. So is the charging process when there is no electron transfer. Based on optical measurements of the interfacial impedance, this spatially unsynchronized electron transfer is attributed to a radially non-uniform distribution of solution resistance. We accordingly manage to eliminate the heterogeneity by engineering the solution resistance distribution. The revealed spatially-dependent charging time 'constant' (to be questioned) would help paint our overall fundamental picture of electrode kinetics.

In Situ Constructing of Rigid-Soft Coupling Solid-Electrolyte Interphase on Silicon Electrode toward High-Performance Lithium Ion Batteries

The application of Si anodes is hindered by some critical issues such as large volume changes of bare Si and fragile solid-electrolyte interface (SEI), resulting in low coulombic efficiency and rapid capacity decay. Herein, a multifunctional SEI film with high content of LiF is in situ constructed via the surface grafting of carbon-fluorine functionalized groups on silicon nanoparticles (SiNPs) during cycling. Mechanical study demonstrates that the incorporation of LiF with high modulus and unbroken carbon-fluorine groups with highly elastic guarantee the rigid-soft coupling SEI film on Si electrode. Furthermore, it is demonstrated that the rigid-soft coupling SEI film can effectively accommodate the volume expansion of Si nanoparticles during lithiation process, with the electrode expanding rate of only 114.16% after 100 cycles (263.87% for bare Si without surface modification). Afterward, with the aid of well-designed rigid-soft coupling SEI, the initial Coulomb efficiency of 89.8% is achieved, showing a reversible capacity of 1477 mAh g-1 after 200 cycles at 1.2 A g-1 . This work provides a simple and efficient solution that can potentially facilitate the practical application of Si anodes.

In-Situ-Generated Electron-Blocking LiH Enabling an Unprecedented Critical Current Density of Over 15 mA cm-2 for Solid-State Hydride Electrolytes

LiBH4 is a promising solid-state electrolyte (SE) due to its thermodynamic stability to Li. However, poor Li-ion conductivities at room temperature, low oxidative stabilities, and severe dendrite growth hamper its application. In this work, a partial dehydrogenation strategy is adopted to in situ generate an electronic blocking layer dispersed of LiH, addressing the above three issues simultaneously. The electrically insulated LiH reduces the electronic conductivity by two orders of magnitude, leading to a 32.0-times higher critical electrical bias for dendrite growth on the particle surfaces than that of the counterpart. Additionally, this layer not only promotes the Li-ion conductance by stimulating coordinated rotations of BH4 - and B12 H12 2- , contributing to a Li-ion conductivity of 1.38 × 10-3 S cm-1 at 25 °C, but also greatly enhances oxidation stability by localizing the electron density on BH4 - , extending its voltage window to 6.0 V. Consequently, this electrolyte exhibits an unprecedented critical current density (CCD) of 15.12 mA cm-2 at 25 °C, long-term Li plating and stripping stability for 2700 h, and a wide temperature window for dendrite inhibition from -30 to 150 °C. Its Li-LiCoO2 cell displays high reversibility within 3.0-5.0 V. It is believed that this work provides a clear direction for solid-state hydride electrolytes.

In Situ Mapping of Activity Distribution of V(II)/V(III) and Onset Potential Distribution of Hydrogen Evolution Side Reaction in Vanadium Flow Batteries

Vanadium flow batteries (VFBs) face a challenge with the low reaction rates of the V(II)/V(III) redox couple, which limits the performance of VFBs. Additionally, the negative electrode in VFBs is often accompanied by the persistent hydrogen evolution reaction (HER), which is difficult to eliminate. Therefore, understanding the spatial distribution of activity on the negative electrode and the HER side reaction on the electrode surface is of critical importance. This study proposes a weak measurement imaging method to characterize the spatial distribution of surface activity and HER onset potential on the negative electrode in VFBs). This method enables the visualization and in situ detection of key parameters such as the absolute values of |ipa |, |ipc |, |∆E|, |ipc /ipa |, and the HER onset potential. By comparing three different types of graphite felts with varying activity levels, it validates the feasibility of this method. Furthermore, electrochemical stability tests are conducted to study the electrodes repeatability, uniformity, and durability. This method holds promise in guiding the design of electrodes with enhanced activity, good reversibility, minimized HER side reactions, and uniform distribution.

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.

Three-dimensional operando optical imaging of particle and electrolyte heterogeneities inside Li-ion batteries

Understanding (de)lithiation heterogeneities in battery materials is key to ensure optimal electrochemical performance. However, this remains challenging due to the three-dimensional morphology of electrode particles, the involvement of both solid- and liquid-phase reactants and a range of relevant timescales (seconds to hours). Here we overcome this problem and demonstrate the use of confocal microscopy for the simultaneous three-dimensional operando measurement of lithium-ion dynamics in individual agglomerate particles, and the electrolyte in batteries. We examine two technologically important cathode materials: LixCoO2 and LixNi0.8Mn0.1Co0.1O2. The surface-to-core transport velocity of Li-phase fronts and volume changes are captured as a function of cycling rate. Additionally, we visualize heterogeneities in the bulk and at agglomerate surfaces during cycling, and image microscopic liquid electrolyte concentration gradients. We discover that surface-limited reactions and intra-agglomerate competing rates control (de)lithiation and structural heterogeneities in agglomerate-based electrodes. Importantly, the conditions under which optical imaging can be performed inside the complex environments of battery electrodes are outlined.

Dynamic Molecular Investigation of the Solid-Electrolyte Interphase of an Anode-Free Lithium Metal Battery Using In Situ Liquid SIMS and Cryo-TEM

We use in situ liquid secondary ion mass spectroscopy, cryogenic transmission electron microscopy, and density functional theory calculation to delineate the molecular process in the formation of the solid-electrolyte interphase (SEI) layer under the dynamic operating conditions. We discover that the onset potential for SEI layer formation and the thickness of the SEI show dependence on the solvation shell structure. On a Cu film anode, the SEI is noticed to start to form at around 2.0 V (nominal cell voltage) with a final thickness of about 40-50 nm in the 1.0 M LiPF6/EC-DMC electrolyte, while for the case of 1.0 M LiFSI/DME, the SEI starts to form at around 1.5 V with a final thickness of about 20 nm. Our observations clearly indicate the inner and outer SEI layer formation and dissipation upon charging and discharging, implying a continued evolution of electrolyte structure with extended cycling.

Progressive and instantaneous nature of lithium nucleation discovered by dynamic and operando imaging

The understanding of lithium (Li) nucleation and growth is important to design better electrodes for high-performance batteries. However, the study of Li nucleation process is still limited because of the lack of imaging tools that can provide information of the entire dynamic process. We developed and used an operando reflection interference microscope (RIM) that enables real-time imaging and tracking the Li nucleation dynamics at a single nanoparticle level. This dynamic and operando imaging platform provides us with critical capabilities to continuously monitor and study the Li nucleation process. We find that the formation of initial Li nuclei is not at the exact same time point, and Li nucleation process shows the properties of both progressive and instantaneous nucleation. In addition, the RIM allows us to track the individual Li nucleus's growth and extract spatially resolved overpotential map. The nonuniform overpotential map indicates that the localized electrochemical environments substantially influence the Li nucleation.