With the increasing demand for renewable energy grid integration and the miniaturization of portable electronic devices, the development of energy storage systems that combine high energy density, high power density, and long cycle life has become a major focus of global scientific research. Due to the decreasing reserves and rising costs of lithium resources, sodium-ion energy storage technology offers core advantages such as abundant sodium reserves and low cost, positioning it as an ideal alternative for large-scale, low-cost energy storage systems. Among various sodium-ion storage technologies, sodium-ion capacitors (SICs) achieve an excellent balance between energy density and power density by integrating the Faradaic reaction mechanism of battery-type anodes with the electric double-layer capacitance behavior of capacitor-type cathodes, demonstrating significant commercial application potential. Among these, K2Ti6O13 (KTO) is widely recognized as a highly promising anode material for sodium-ion capacitors due to its tunable 3D tunnel framework, low cost, and environmental compatibility.

The commercialization of sodium-ion capacitors is currently severely constrained by the insufficient kinetic performance and poor mechanical stability of the anode. On one hand, the sluggish ion diffusion and electron transport rates of battery-type anodes cannot match the fast response of capacitor-type cathodes. On the other hand, the large ionic radius of sodium ions easily leads to structural collapse, pulverization, or detachment of electrode materials during repeated charge-discharge cycles. Although KTO possesses a robust tunnel structure, its inherent electronic conductivity is poor, and its sodium storage mechanism—particularly the interaction between the pre-existing potassium ions in the structure and the intercalated sodium ions—remains controversial and lacks sufficient investigation. Moreover, while nanostructuring strategies can improve performance, the severe agglomeration that nanoparticles are prone to can significantly reduce the accessible active surface area and block ion transport channels, presenting a major challenge to be overcome in current research.

To address the above issues, a team from the University of Jinan, Xiamen University, and other institutions conducted systematic research using ZEPTOOLS' in-situ TEM measurement system. The team constructed a three-dimensional porous KTO@CNFs self-supporting hybrid film featuring an orthogonal charge transport network with axial electron conduction and radial ion permeation, inspired by the "seed germination" model. Furthermore, they deeply elucidated the zero-strain solid-solution sodium storage mechanism induced by irreversible sodium-potassium ion exchange.

Title: Multi-Level Design and Irreversible Ion Exchange Involved Sodium-Storage Mechanism of Zero-Strain K2Ti6O13 Toward Sodium-Ion Capacitors
Journal: Advanced Energy Materials
URL：https://doi.org/10.1002/aenm.70969

This study first addresses the core challenges commonly found in titanium-based anode materials, such as particle agglomeration and low electronic conductivity, by proposing an innovative multi-level optimization strategy. Inspired by the seed germination model, the researchers grew highly dispersed potassium titanate nanobelts in situ on the surface and inside of hierarchical porous carbon nanofibers using electrospinning combined with a hydrothermal method, successfully constructing a self-supporting KTO@CNFs hybrid film. This unique structure forms an intertwined orthogonal charge transport network, in which the carbon nanofibers provide axial fast electron conduction pathways, while the nanobelts with open tunnel structures are responsible for radial ion permeation. This multi-level design not only significantly increases the exposure of active sites but also shortens ion diffusion paths, fundamentally enhancing the kinetic performance of the material during high-rate charge-discharge processes.

To elucidate the structural evolution mechanism during sodium ion intercalation and verify the mechanical stability of the material, the research team employed high-resolution real-time characterization methods. In this experimental segment, the researchers utilized ZEPTOOLS' in-situ sample holder to perform in-situ sodiation and desodiation tests on individual nanobelts under a transmission electron microscope. Using this precision instrument, the researchers were able to directly observe the dynamic dimensional changes of the material during charge-discharge cycling. The experimental data conclusively demonstrated that the potassium titanate nanobelts exhibit excellent zero-strain characteristics, with a volume change rate of only approximately 0.15% during cycling. The visual evidence provided by ZEPTOOLS' in-situ characterization technology offers core physical mechanism support for achieving an ultra-long cycle life exceeding ten thousand cycles with this material.

This comprehensive optimization, spanning from material design to electrode construction, directly translates into outstanding electrochemical energy storage performance. Experimental results show that the KTO@CNFs film anode exhibits excellent rate capability, maintaining a significant reversible capacity even when the current density is increased by 100 times. When assembled into a full-cell sodium-ion capacitor with a self-activated carbon nanofiber cathode, the device achieves an energy density of 96.1 Wh/kg at a high power density of 10.2 kW/kg. Furthermore, after 10,000 cycles at a current density of 4 A/g, the device retains a capacitance of 92.4%, fully demonstrating its potential for commercial applications as a substitute for expensive lithium-based technologies in high-efficiency energy storage scenarios.

Through in-depth physicochemical characterization and density functional theory calculations, the research team further revealed the intrinsic reaction mechanism of the material. The study found that during the first charge-discharge cycle, an irreversible sodium-potassium ion exchange reaction occurs within the material, i.e., some sodium ions replace potassium ions in the crystal lattice, thereby generating an in-situ formed and more structurally stable KNTO solid solution phase. This newly formed electrochemically active phase benefits from the spring effect of Ti-O-Ti and K-O-Ti bonds, enabling the framework to maintain elastic recovery during ion migration. Theoretical calculation results indicate that this ion exchange mechanism effectively reduces the migration energy barrier for sodium ions and significantly accelerates ion diffusion kinetics, providing important theoretical guidance and a practical paradigm for designing next-generation high-performance sodium ion storage platforms.

As a Chinese high-end precision instrument company, ZEPTOOLS is a leading provider of in-situ electron microscopy characterization solutions. Its PicoFemto series of in-situ transmission electron microscopy characterization solutions have successively provided technical support for major research findings of users both domestically and internationally. The following image shows the ZEPTOOLS in-situ TEM sample holder product used in this research achievement: