The rhombohedral sodium manganese hexacyanoferrates (MnHCF) is regarded as a scalable, low-cost, and high-energy cathode material for Na-ion batteries due to the absence of crystal water.  

However, the unexpected Jahn-teller effect and significant phase transformation causes Mn dissolution and anisotropic volume change, leading to capacity loss and structural instability. Thus, it is important to build a robust and full coverage coating on the surface of MnHCF particles to address cycling instability. 

A research group led by Prof. ZHAO Junmei from the Institute of Process Engineering (IPE) of the Chinese Academy of Sciences reported a simple room-temperature route to construct a magical Co_xB skin on the surface of MnHCF, demonstrating thousands-cycle level cycling lifespan and high energy density close to LiFePO₄ for Li-ion batteries.  

The study was published in Angewandte Chemie on Jan. 31 (DOI：10.1002/anie.202217761). 

"Room-temperature synthesized Co_xB seems to be tailor-made as a coating substance of MnHCF cathode," said Prof. ZHAO. "Meanwhile, Co_xB is a metallic borate glass, and due to high corrosion and wear resistance, it shows good mechanical flexibility, which is expected to prevent the fracture or fragmentation of cathode particles."  

Moreover, Co_xB can work as a double conductor of mixed electrons and ions. These unique advantages make the optimal Co_xB coated MnHCF cathode (MnHCF-5%Co_xB) deliver an initial capacity of ~133 mA h g^-1 at 10 C, higher than that of bare MnHCF (~110 mA h g^-1).  

More importantly, the coated samples can remain a capacity retention of over 80%, far superior to that of the pristine one (41%). In the full-cell configurations, the MnHCF-5%Co_xB//HC delivers a high energy density of 310 Wh kg^-1 based on the total mass of cathode/anode active substances, which is comparable to the commercial LFP cathode (~330 Wh kg^-1).  

"MnHCF-5%Co_xB//HC shows an impressive capacity retention of ~71% at 5 C after cycling 1000 cycles, which is a significant breakthrough among the nonaqueous MnHCF full cells," said ZHAO. 

Figure. Schematic illustration of the proposed protective mechanism of Co_xB to suppress the Mn dissolution and generation of microcracks of MnHCF cathode (Image by ZHAO Junmei).