Operando SoC tracking in sodium-ion batteries now offers a direct way to monitor ion motion and charge state inside a working cell. In this 2026 study, researchers developed a multiresonant fiber-optic grating sensor that sits close to the electrode surface and measures local ion transport in real time. As a result, the platform captures fast electrochemical changes without interrupting battery operation. It also delivers state-of-charge accuracy above 98%, which makes it highly relevant for advanced battery diagnostics and control.
Operando SoC Tracking in Sodium-Ion Batteries: Why It Matters
Sodium-ion batteries continue to gain attention for grid storage and large-scale energy systems. They offer attractive material availability and strong practical potential. However, battery performance still depends on how quickly sodium ions move at the electrode-electrolyte interface. Therefore, researchers need tools that can observe this motion during actual cycling.
This study addresses that need with an implanted optical fiber sensor. Unlike bulky lab methods, the sensor works inside an operating Sodium-ion Battery. Moreover, it tracks both ion kinetics and state of charge with high sensitivity. The design uses a single multiresonant fiber-optic grating, or MFG, to detect tiny refractive index changes caused by local sodium-ion concentration shifts.
How the Operando SoC Tracking Sensor Works
The MFG sensor couples light from the fiber core into many narrow cladding resonances. These resonances react strongly to the surrounding electrolyte. In particular, the cut-off mode responds to local ion concentration changes near the electrode surface. Because of this behavior, the sensor reaches a refractive index resolution of about 10-6 RIU.
The system also provides a temperature resolution of 0.1b0C. In addition, it offers sub-second temporal resolution and sub-micron spatial sensitivity. The evanescent field extends about 1.5 b5m beyond the fiber surface. Thus, it probes the active diffusion layer where ion transport and electron transfer occur most strongly.
The sensor probe measures 10 mm in length and 125 b5m in diameter. Researchers placed it near the working electrode inside a sodium-ion battery. They used SnO2/BaTiO3/C nanofiber electrodes for the main validation tests. They also compared the results with hard carbon electrodes.
Operando SoC Tracking in Sodium-Ion Batteries Reveals Ion Kinetics
The optical signal tracked the galvanostatic charge-discharge response with strong consistency. At current densities of 100, 200, and 400 mA g-1, the maximum optical intensity increased as current density increased. Furthermore, the relationship between maximum optical intensity and current density showed excellent linearity, with an R2 of 99.6%.
The researchers also examined the derivative of optical intensity. This derivative highlighted the rate of sodium-ion migration at the interface. As current density rose, the negative derivative peak shifted to lower voltage during discharge. Meanwhile, the positive peak moved to higher voltage during charging. These shifts reflected faster interfacial ion dynamics.
Most importantly, the sensor identified three distinct sodium storage stages. First, sodium ions moved rapidly toward the interface in an adsorption stage. Next, the system entered an intermediate stage. Finally, it reached a diffusion-dominated stage. The intermediate stage had not been clearly demonstrated before in this way.
Stage Contributions at Different Current Densities
At 100 mA g-1, the capacity contributions were 4% for adsorption, 18% for the intermediate stage, and 78% for diffusion. At 200 mA g-1, the values changed to 10%, 32%, and 58%. At 400 mA g-1, they became 13%, 43%, and 44%.
These numbers show a clear trend. As current density increases, the intermediate stage takes a larger share of the total process. Therefore, this stage becomes a useful marker for fast-charging behavior. A shorter intermediate stage points to stronger kinetic performance under the same operating conditions.
Operando SoC Tracking in Sodium-Ion Batteries Achieves High Accuracy
The team also integrated the optical intensity over time to quantify state of charge. This method links the measured concentration change to the total quantity of transported sodium ions. As a result, the integrated optical signal serves as a direct operando SoC indicator.
For SoC values above 10%, the relationship between integrated optical intensity and SoC remained highly linear across all tested current densities. The reported linearity exceeded R2 99.8%. In addition, all tested accuracies remained above 98.4% over repeated charge-discharge cycles.
At SoC values of 10% or less, the relationship became quadratic instead of linear. The study linked this effect to the early infiltration of sodium-rich electrolyte into the porous electrode structure. Even so, after a square-root transformation, the data again showed strong linearity, with R2 above 99.5% and accuracy above 98.4%.
Why Operando SoC Tracking in Sodium-Ion Batteries Stands Out
This sensing approach combines compact size, fast response, and strong compatibility with real battery environments. It avoids interference from electromagnetic noise. It also supports real-time optical and electrochemical monitoring in one setup. Because the core mode remains stable, the system can also compensate for temperature effects when needed.
In practical terms, this makes the method promising for battery management systems in vehicles, home energy storage, and power stations. It can help engineers monitor ion transport, improve SoC estimation, and better understand charging behavior inside working sodium-ion batteries.
Key Takeaways on Operando SoC Tracking in Sodium-Ion Batteries
This 2026 research shows that a multiresonant fiber-optic grating sensor can track ion kinetics and state of charge directly inside a sodium-ion battery. It reveals an intermediate ion-transport stage between adsorption and diffusion. It also achieves more than 98% SoC accuracy in real time. Therefore, operando SoC tracking in sodium-ion batteries stands out as a practical and precise tool for next-generation battery diagnostics.
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