Operando scanning electrochemical microscopy (SECM) is a powerful analytical technique used to investigate electrochemical activity at battery interfaces with high spatial resolution. This method enables real-time mapping of local electrochemical processes, such as ion transport, charge transfer reactions, and surface heterogeneity, under operating conditions. By providing insights into dynamic interfacial phenomena, operando SECM contributes to the understanding of battery degradation mechanisms, solid-electrolyte interphase (SEI) formation, and defect-related performance losses.
The core principle of SECM involves scanning a microelectrode probe across a sample surface while measuring faradaic currents resulting from redox reactions. In operando SECM, this is performed within a functioning battery cell, allowing researchers to correlate electrochemical activity with battery state variables like state of charge (SOC) or cycling history. The technique is particularly valuable for studying heterogeneous reactions, where localized variations in reactivity can significantly impact overall battery performance.
Probe design is critical for operando SECM measurements. Microelectrodes are typically fabricated from platinum, gold, or carbon fibers, with diameters ranging from 1 to 25 micrometers to achieve sufficient spatial resolution. Insulated probes with exposed disk-shaped tips are commonly used, although specialized designs such as nanopipettes or dual-electrode probes have been developed for specific applications. The small size of these probes enables localized measurements without significantly perturbing the electrochemical environment of the operating battery.
Several feedback modes are employed in operando SECM studies. In positive feedback mode, the probe current increases when approaching an electrochemically active surface due to regeneration of redox mediators by the sample. This mode is useful for mapping conductive domains and charge transfer sites. Negative feedback occurs when the probe approaches an insulating region, causing a decrease in current due to hindered diffusion. This mode helps identify inactive or passivated areas on electrode surfaces. Additional modes like redox competition or surface interrogation SECM provide complementary information about reaction kinetics and surface coverage of intermediates.
The spatial resolution of operando SECM depends on multiple factors, including probe size, working distance, and the nature of the electrochemical process being studied. Under optimal conditions, lateral resolution of 100-500 nm can be achieved, while the vertical resolution is typically in the nanometer range. This resolution is sufficient to map microstructural features and reaction heterogeneity in battery electrodes, though it is generally lower than that of scanning probe techniques like atomic force microscopy (AFM) or scanning tunneling microscopy (STM). However, SECM provides unique electrochemical information that these other techniques cannot access.
Applications of operando SECM in battery research are diverse. One key area is the study of SEI formation and evolution. By mapping electrochemical activity during cycling, researchers can identify locations where SEI growth is non-uniform or where breakdown occurs. This information helps correlate local electrochemical behavior with macroscopic performance metrics like capacity fade. Another application involves investigating lithium dendrite formation, where SECM can detect early-stage nucleation sites and propagation pathways that precede visible dendrite growth.
Heterogeneous reactions at composite electrodes are another focus area. Battery electrodes often consist of active material particles mixed with conductive additives and binders, creating complex reaction landscapes. Operando SECM can reveal how electrochemical activity varies between different components and interfaces, providing insights into limitations imposed by non-uniform current distribution. This capability is particularly valuable for optimizing electrode formulations and processing methods.
Defect characterization represents a third major application. Localized defects such as cracks, impurities, or coating irregularities can create hotspots of electrochemical activity or passivation. Operando SECM enables direct mapping of these defects and their evolution during cycling, offering a tool for quality control and failure analysis. The technique has been used to study how manufacturing defects propagate into performance limitations in commercial battery cells.
When comparing operando SECM to other microscopy techniques, several distinctions emerge. Unlike electron microscopy methods (SEM, TEM), SECM provides functional electrochemical information rather than purely structural data. While AFM can measure topography and mechanical properties with higher spatial resolution, it typically lacks the chemical specificity of SECM. Optical techniques like Raman microscopy offer molecular identification but often with lower spatial resolution and without direct electrochemical activity measurements. The strength of SECM lies in its ability to bridge these domains by providing spatially resolved electrochemical data under realistic operating conditions.
Challenges remain in implementing operando SECM for battery studies. Maintaining stable probe positioning during battery cycling requires careful cell design, as volume changes in electrodes can alter the probe-sample distance. The choice of redox mediator is also critical, as it must be compatible with battery electrolytes while providing measurable signals without interfering with normal cell operation. Recent advances in probe fabrication and positioning systems have addressed some of these challenges, expanding the range of battery systems that can be studied.
Future developments in operando SECM are likely to focus on improving spatial resolution through smaller probes and advanced positioning systems, as well as expanding the range of detectable electrochemical processes. Combining SECM with complementary techniques in multimodal approaches represents another promising direction. These advances will further enhance the technique's utility for understanding and optimizing battery interfaces.
The ability to map electrochemical activity under operating conditions makes operando SECM a unique tool in battery research. By revealing spatial variations in reactivity that are often averaged out in bulk measurements, this technique provides critical insights into the fundamental processes governing battery performance and degradation. As battery technologies continue to advance toward higher energy densities and longer lifetimes, operando SECM will play an increasingly important role in characterizing and optimizing these complex electrochemical systems.