Structure-Controlled Prussian Blue Analog as a Cathode Material for Long-Life Seawater Batteries
Authors: Jeong Hyebin , Jo Changshin 
Published: February 2024

Nowadays, as the use of fossil fuels is suppressed for achieving carbon neutrality, rechargeable battery is considered as a key factor to lead sustainable society. As far, the most developed and already commercialized batteries are lithium-ion batteries (LIBs). However, as the demand for LIBs is increasing, there are also several issues in the process of obtaining high-performance batteries. First, as the price of raw materials in LIB rises rapidly, a cost issue arises. In addition, as the fire accidents have occurred consistently due to overcharging or use of organic electrolyte, safety concerns are being raised. Finally, the environmental issues are emerging in the process of mining raw material and disposal of waste batteries. Therefore, a cheap, safe, and environmentally friendly battery system is needed as an alternative to LIBs.
In this situation, a battery system called seawater battery (SWB) using seawater as a raw material is attracting attention. Since seawater is super-abundant resource, accounting 97% of the earth’s water, the material and manufacturing cost can be effectively reduced. Also, since it is operated in seawater, it is free from fire hazard and environmentally benign. In addition, as natural seawater can be used without additional treatment, energy can be reduced used for refining raw materials in LIBs.
Likely to other battery systems, SWB consists of anode, cathode and separator. In anode part, Na-ions originated from NaCl in seawater are plated and stripped during cycling process. For facile metal plating, the anode part is filled with organic electrolyte, which also broaden the operation voltage window of overall cell. The ceramic solid electrolyte is used as separator, which is called Na-ion selective membrane, NASICON. The cathode part contains seawater which can act as an electrolyte or cathode as well (catholyte).
The reaction mechanism mechanisms of cathode part can be sorted into three types―Na-intercalation, oxygen evolution/reduction reaction, and Cl-capturing. Herein, we have focused on the intercalation cathode to realize long-life and stable SWB, which is critical properties for large-scale energy storage systems. For intercalation cathode material, Prussian blue analog (PBA) was selected. PBA is an analog form of Prussian blue (PB). Since PB/PBAs have frame-like structures consist of metal nodes and cyanide ligands, the big alkali ions including Na-ions can be inserted and released through interstitial sites. By substituting the iron of PB to other transition metals, various form of PBAs can be synthesized. Among them, Ni-based PBA (Na2Ni[Fe(CN)6] when fully sodiated) was selected, because it is known as stable in seawater and undergoing small volume change during repeating ion (de)insertion, which is advantageous properties for stable SWB application.
However, during synthetic process of PBAs, when metal ions and ligands meet rapidly, vacancies are easily created inside the structure. In these empty sites, the water molecules tend to coordinate, becoming crystal water. Since crystal water negatively affects the electrochemical performance of PB/PBAs, it should be controlled. Therefore, we have controlled the structure of Ni-based PBAs by chelation and improved the electrochemical performance of SWBs.
By aid of chelating agent, well-structured Ni-based PBA was successfully synthesized. The defect ratio was effectively reduced from 40% to 6%, followed by less crystal water and more Na-ions inside the structure. When applied to SWB system, the cycle performance of chelated PBA was superior to untreated PBA, exhibiting capacity retention of 92.8% after 2000 cycles at 5 C. In conclusion, showing stable and reversible operation during 2000 cycles at the first time in SWB field, PBA-based SWB showed potential to be a promising next-generation battery.

Figure 1