Epitaxial Nickel Ferrocyanide Stabilizes Jahn–Teller Distortions of Manganese Ferrocyanide for Sodium‐Ion Batteries

Abstract

Manganese‐based Prussian Blue, Na2−δMn[Fe(CN)6] (MnPB), is a good candidate for sodium‐ion battery cathode materials due to its high capacity. However, it suffers from severe capacity decay during battery cycling due to the destabilizing Jahn–Teller distortions it undergoes as Mn2+ is oxidized to Mn3+. Herein, the structure is stabilized by a thin epitaxial surface layer of nickel‐based Prussian Blue (Na2−δNi[Fe(CN)6]). The one‐pot synthesis relies on a chelating agent with an unequal affinity for Mn2+ and Ni2+ ions, which prevents Ni2+ from reacting until the Mn2+ is consumed. This is a new and simpler synthesis of core–shell materials, which usually needs several steps. The material has an electrochemical capacity of 93 mA h g−1, of which it retains 96 % after 500 charge–discharge cycles (vs. 37 % for MnPB). Its rate capability is also remarkable: at 4 A g−1 (ca. 55 C) it can reversibly store 70 mA h g−1, which is also reflected in its diffusion coefficient of ca. 10−8 cm2 s−1. The epitaxial outer layer appears to exert an anisotropic strain on the inner layer, preventing the Jahn–Teller distortions it normally undergoes during de‐sodiation.

Ball Milling Solid‐State Synthesis of Highly Crystalline Prussian Blue Analogue Na2−xMnFe(CN)6 Cathodes for All‐Climate Sodium‐Ion Batteries

“…The all‐climate performance of MnHCF‐S‐170 is better than those of MnHCF‐L and MnHCF‐S, which may mainly benefit from its intact crystal structure and suppress interstitial water content (Supporting Information Figures S16 and S17). The results indicate that neither low nor high temperature has any further influence on the crystal structure of the MnHCF‐S‐170 except for the kinetics factors [52–54] . The reasons for the better all‐climate performance of the MnHCF‐S‐170 compare to those of other two samples may be as following: Compared to other two samples, the highly crystallized MnHCF‐S‐170 with unusual monoclinic phase possesses higher sodium‐ion contents, which means that the MnHCF‐S‐170 can provide sufficient Na ions during the charge and discharge process, and further achieve high specific capacity.…”

Advanced Characterization Techniques Paving the Way for Commercialization of Low‐Cost Prussian Blue Analog Cathodes

“…Researchers have made extensive efforts to curb these difficulties associated with unsatisfactory performance in SIBs. Considering the above-mentioned issues, on the one hand, the optimized design of synergetic strategies is essential in terms of crystallization control, [17][18][19][20][21] dehydration, [22,23] morphology engineering, [24][25][26][27][28] surface modification, [29][30][31][32][33][34][35][36][37] transition metal (TM) doping or elemental substitution, [38][39][40] high entropy, [41,42] etc. On the other hand, more importantly, various advanced characterization techniques, such as in situ and operando techniques, are urgently needed to understand the details of the reaction mechanisms of electrodes due to their unique advantages relative to conventional ex situ characterization techniques.…”

Structural complexity in Prussian blue analogues

“…61 Coating the particles could certainly be a promising solution since it also prevents dissolution of Mn ions. 68,69 Vacancy network correlation could also play a role since they appear to improve ionic conductivity. 28,53…”