Sodium ion batteries (SIBs), with their abundant resources, low cost, and high safety, hold broad application prospects in large-scale energy storage. Prussian blue and its analogues have been extensively investigated due to their high theoretical specific capacity and rapid ion diffusion rate. Nevertheless, the incomplete reaction of low-spin iron in Prussian blue often results in a lower actual specific capacity, thereby limiting its application.

Recently, a research team led by Prof. WANG Junhu from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. ZHENG Qiong from DICP, and Dr. Moulay Tahar Sougrati from the University of Montpellier, revealed low-spin iron activation mechanism of Prussian blue cathode materials through operando ⁵⁷Fe Mössbauer spectroscopy, which provides a new strategy for enhancing specific capacity within in SIBs.

This study was published in Chemical Engineering Journal on May 15^th.

Schematic diagram of HS/LS Fe redox and activation mechanism of LS Fe in the electrochemical reaction process (Image by WANG Zinan)

The researchers developed a rhombohedral phase Prussian blue by high-temperature calcination under nitrogen atmosphere. It exhibited a significantly longer high-voltage discharge plateau as cathode for SIBs. The specific capacities contributed by low-spin iron in the first and fifth cycles were 65 and 62 mAh g^-1, respectively, significantly higher than the 38 and 22 mAh g^-1 for the raw material.

Additionally, this research delves into the low-spin iron activation mechanism in this material using operando ⁵⁷Fe Mössbauer spectroscopy, ex situ soft X-ray absorption spectroscopy, and DFT calculations.

On one hand, in a half-charged state, electrons transfer from high-spin iron to low-spin iron, leading to a spin transition phenomenon. Meanwhile, the charge-accumulation state of low-spin iron is more susceptible to oxidation reactions. On the other hand, the calcined materials possess a narrower bandgap, facilitating electron transitions. These factors activate low-spin iron, significantly enhancing the specific capacity and specific energy of Prussian blue cathode materials.

“This work provides novel insights for developing Prussian blue cathodes with high specific capacity and energy,” said Prof. WANG.

This work was supported by the National Natural Science Foundation of China, the International Partnership Program of Chinese Academy of Sciences, the President’s International Fellowship Initiative (PIFI) of Chinese Academy of Sciences.

Keywords: Sodium-ion battery, Prussian blue, low-spin iron, spin transition, operando Mössbauer spectroscopy

Reference

Charge Transfer Induced Highly Active Low-spin Iron of Prussian blue Cathode through Calcination Strategy for High Performance Sodium-ion Batteries

https://doi.org/10.1016/j.cej.2024.151090