Structure Distortion Induced Monoclinic Nickel Hexacyanoferrate as High-Performance Cathode for Na-Ion Batteries

Prussian blue analogs with an open framework are ideal cathodes for Na-ion batteries. A superior high-rate and highly stable monoclinic nickel hexacyanoferrate (NiHCF-3) is synthesized via a facile one-step crystallization-controlled co-precipitation method. It gives a high specific capacity of 85.7 mAh g(-1), nearly to its theoretical value. It also exhibits an excellent rate capability with a high capacity retention ratio of 78% at 50 C and a stable cycling performance over 1200 cycles. Through the ex situ X-ray diffraction and pair distribution function measurements, it is found that the monoclinic structure with distorted framework is greatly related to the high Na content. The electronic structure studies by density functional theory (DFT) calculation demonstrate that NiHCF-3 deformation promotes the framework conductivity and improves the electrochemical activity of Fe, which results in an ultrahigh-rate performance of monoclinic phase. Furthermore, the high-quality monoclinic (NiHCF-3) exhibits excellent compatibility with both hard carbon and NaTi2(PO4)(3) anodes in full cells, which shows great prospects for the application in the large-scale energy storage systems.