Sodium-ion batteries are moving closer to the center of the global energy-storage conversation, and a new optical sensor may help solve one of their most persistent problems: seeing exactly what happens inside a working battery. Researchers from Jinan University and Wuhan University have developed an implantable multiresonant fiber-optic grating sensor capable of tracking sodium-ion movement directly at the electrode–electrolyte interface. The technology can monitor ion transport in real time, reveal how electrodes charge and discharge, and provide a more physically grounded estimate of a battery’s state of charge, or SoC.

The breakthrough comes at a moment when demand for affordable, large-scale energy storage is accelerating. Sodium-ion batteries are attractive because sodium is abundant, widely distributed, and generally less expensive than lithium. Unlike lithium resources, which are concentrated in a limited number of regions and are subject to intense competition from electric vehicles and consumer electronics, sodium can be sourced more readily. Yet sodium-ion technology still faces performance and monitoring challenges. To make these batteries reliable for grid storage and other commercial applications, scientists need to understand how sodium ions move through the complex boundary where the electrode meets the electrolyte.

That boundary is where charging and discharging begin, but it is also one of the most difficult regions to observe. Sodium ions must leave or enter the electrode structure, travel through the electrolyte, and cross the interface under changing electrical and chemical conditions. The process is not a single smooth step. It can involve ion adsorption at the surface, rearrangement near the interface, and subsequent diffusion into the electrode. Small differences in these stages can determine how quickly a battery charges, how much energy it can deliver, and how rapidly its performance declines over time.

Until now, many methods used to study ion migration near battery electrodes have depended on offline analysis or large laboratory instruments. Such techniques can provide valuable snapshots, but they often require the battery to be disassembled or examined under highly controlled conditions. That makes it difficult to capture what is happening during real operation. The new sensor is designed for operando monitoring, meaning it can remain inside or alongside a functioning battery and collect information while the battery is charging and discharging.

At the heart of the device is a multiresonant fiber-optic grating, or MFG, engineered to interact with its immediate chemical environment through an evanescent optical field. This is a light field that extends only a very short distance beyond the surface of the optical structure. Because the field is confined to the near-surface region, it can detect tiny changes in the refractive index caused by variations in the local concentration and movement of sodium ions. In effect, the fiber acts as a microscopic optical probe positioned at the electrochemical reaction zone.

The researchers report that the sensor generates a sub-micron evanescent field and achieves a refractive-index resolution of 10⁻⁶ RIU, or refractive index units, together with sub-micron spatial resolution. These capabilities allow the system to distinguish changes that would be invisible to conventional, larger-scale measurements. Rather than averaging conditions across the entire battery, the sensor focuses on the narrow interfacial region where ion transport is initiated. This localized view could help researchers separate surface reactions from slower bulk diffusion and identify the physical causes of performance losses.

The team also tracked changes in the amplitude of a cut-off mode in the multiresonant grating. Optical resonances occur when specific wavelengths of light interact strongly with the structure of the grating. When the surrounding refractive index changes, the strength and behavior of these resonances change as well. By monitoring the resonance amplitude and, importantly, its time derivative, the researchers were able to decode the kinetics of sodium-ion motion. Their measurements revealed an “intermediate stage” between the initial adsorption of ions at the electrode surface and their later diffusion into the electrode material.

This intermediate stage may be a crucial missing link in the understanding of fast charging. According to the study, electrodes with a shorter intermediate stage showed better fast-charging performance. The finding suggests that rapid charging is not determined only by how quickly ions diffuse through the electrode. The earliest moments of interfacial interaction may also impose a significant bottleneck. If ions remain temporarily trapped, reorganize slowly, or encounter a high energy barrier before diffusion begins, the battery may charge less efficiently even when the electrode’s bulk structure appears favorable.

Beyond studying ion kinetics, the optical sensor was used to build an ion-flux-based model for calculating state of charge. SoC is one of the most important quantities in battery management, but it is notoriously difficult to determine precisely. Conventional approaches often estimate SoC from voltage, current, and accumulated charge. These signals can be distorted by temperature changes, electrode aging, hysteresis, and the strongly nonlinear behavior of batteries near very high or very low charge levels. A battery may therefore appear to have a particular SoC while its actual electrochemical condition is different.

The researchers found a highly linear relationship between the time integral of the optical signal amplitude and the battery’s SoC. In practical terms, the cumulative optical response over time was closely linked to the amount of sodium-ion transport occurring within the battery. This connection offers a direct physical route to SoC estimation rather than relying only on electrical proxies. If the approach can be miniaturized, standardized, and validated across different electrode chemistries and operating conditions, it could provide battery-management systems with a new source of real-time information.

The technology could eventually become part of a “lab-on-fiber” platform, in which optical fibers perform advanced chemical and physical measurements inside compact devices. Fiber-optic sensors are attractive for this role because they are small, lightweight, resistant to electromagnetic interference, and capable of transmitting measurements over long distances. A fiber integrated into a battery pack could potentially monitor ion transport without adding bulky equipment or interrupting normal operation. The same platform might also help identify abnormal behavior before it develops into irreversible degradation or a safety event.

For commercial batteries, that predictive capability could be transformative. Early warnings based on local ion-transport behavior might reveal the beginning of electrode deterioration, electrolyte instability, or abnormal charging conditions before conventional electrical measurements detect a serious problem. The information could support more accurate charging strategies, extend battery life, and improve the reliability of storage systems connected to renewable-energy networks. It could also help manufacturers compare electrode materials using a direct measurement of interfacial kinetics rather than relying solely on long cycling tests.

The researchers emphasize that the sensor provides more than a new way to observe sodium-ion batteries. It establishes a link between optical signals, microscopic ion dynamics, and practical battery parameters such as fast-charging capability and state of charge. That connection is particularly important because the most useful battery technologies will need both high performance and intelligent monitoring. A battery that stores energy efficiently but cannot be accurately diagnosed may remain difficult to deploy at scale.

The work, published in Light: Science & Applications, was led by Professor Tuan Guo and Professor Wenjie Mai of Jinan University and Professor Yongjin Fang of Wuhan University. Supported by China’s National Key Research and Development Program and the National Natural Science Foundation of China, the study points toward a future in which batteries are no longer treated as sealed black boxes. With an optical fiber observing the chemistry from within, researchers and engineers may be able to watch energy storage unfold in real time—and use that knowledge to make sodium-ion batteries faster, safer, and far more predictable.

Subject of Research: Operando optical monitoring of sodium-ion transport, interfacial ion kinetics, fast-charging behavior, and state-of-charge estimation in sodium-ion batteries.

Article Title: Operando tracking of ion kinetics and state-of-charge via multiresonant fiber-optic grating sensors in sodium-ion batteries

Web References: https://doi.org/10.1038/s41377-026-02388-1

References: Light: Science & Applications, “Operando tracking of ion kinetics and state-of-charge via multiresonant fiber-optic grating sensors in sodium-ion batteries,” DOI: 10.1038/s41377-026-02388-1

Image Credits: Tuan Guo et al.

Keywords

Sodium-ion batteries, fiber-optic sensors, battery monitoring, state of charge, ion transport, electrochemical interfaces, fast charging, operando measurement, optical sensing, battery management systems

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