Sodium-ion batteries (SIBs), with their abundant resources, low cost, and high safety, have become the preferred energy storage technology for large-scale energy storage and low-speed electric vehicles. Prussian blue and its analogues, with open three-dimensional structure, have been widely applied as cathode materials. However, the vacancies and water in their structure can compromise the material's stability. Especially at high current densities, the structure of the electrode material becomes particularly susceptible to damage, leading to a decline in the battery's rate performance.

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 sodium storage mechanism of Prussan blue analogues cathode materials through operando ⁵⁷Fe Mössbauer spectroscopy, which provides a new strategy for enhancing rate performance of SIBs.

This study was published in ACS Applied Materials & Interfaces on April 9^th.

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

The researchers developed Prussian white with nickel iron hexacyanoferrate (PW@NiFeHCF) composite materials and exhibited outstanding rate performance as cathode for SIBs. The optimized cathode material delivered specific capacities of 120 and 95 mAh g^-1 under the ultra-high current densities of 1.2 A g^-1 and 6 A g^-1, respectively, with capacity retention rates of 94.5% and 72.5% with respect to the first cycle.

Additionally, this research delves into the sodium storage mechanism in the composite material using operando ⁵⁷Fe Mössbauer spectroscopy, ex situ X-ray diffraction and kinetic tests. The reaction of high-spin iron is correlated with a diffusion and capacitance jointly controlled process, while the low-spin iron is correlated with merely capacitive process. The substitution of Ni to Fe increases the content of low-spin iron and facilitates its complete reaction, which leads to a capacitive sodium storage mechanism. Furthermore, the crystal structure changes into monoclinic during the second coprecipitation process of the composite material. The variation of crystal parameter a is less than 3% during sodiation/desodiation process for the monoclinic composite material, which is the main reason for intercalation pseudocapacitance and thereby enhancing the sodium diffusion kinetics.

“This work provides a new horizon for developing Prussian blue analoguse cathodes with high rate performance” 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: Prussian blue analogues, sodium-ion batteries, operando ⁵⁷Fe Mössbauer spectroscopy, sodium storage mechanism, rate performance

Reference

Capacitive-controlled Prussian white with nickel iron hexacyanoferrate composite cathode for rapid sodium diffusion

https://pubs.acs.org/doi/10.1021/acsami.4c00885