Electrolyte Effects on the Stabilization of Prussian Blue Analogue Electrodes in Aqueous Sodium-Ion Batteries

Abstract

Aqueous sodium-ion batteries based on Prussian Blue Analogues (PBA) are considered as promising and scalable candidates for stationary energy storage systems, where longevity and cycling stability are assigned utmost importance to maintain economic viability. Although degradation due to active material dissolution is a common issue of battery electrodes, it is hardly observable directly due to a lack of in operando techniques, making it challenging to optimize the performance of electrodes. By operating Na2Ni­[Fe­(CN)6] and Na2Co­[Fe­(CN)6] model electrodes in a flow-cell setup connected to an inductively coupled plasma mass spectrometer, in this work, the dynamics of constituent transition-metal dissolution during the charge–discharge cycles was monitored in real time. At neutral pHs, the extraction of nickel and cobalt was found to drive the degradation process during charge–discharge cycles. It was also found that the nature of anions present in the electrolytes has a significant impact on the degradation rate, determining the order ClO4 – > NO3 – > Cl– > SO4 2– with decreasing stability from the perchlorate to sulfate electrolytes. It is proposed that the dissolution process is initiated by detrimental specific adsorption of anions during the electrode oxidation, therefore scaling with their respective chemisorption affinity. This study involves an entire comparison of the effectiveness of common stabilization strategies for PBAs under very fast (dis)­charging conditions at 300C, emphasizing the superiority of highly concentrated NaClO4 with almost no capacity loss after 10 000 cycles for Na2Ni­[Fe­(CN)6].

High voltage and superior cyclability of indium hexacyanoferrate cathodes for aqueous Na-ion batteries enabled by superconcentrated NaClO4 electrolytes

“…2(D)), this effect likely goes hand in hand with the highly concentrated electrolytes suppressing the active material dissolution in aqueous systems. 32 As apparent from Fig. 3(B), the larger operational potential range gives access to a much higher specific capacity of 58 mA h g À1 compared to 37 mA h g À1 for the lower anodic cut-off potential of 1.15 V vs. SSC (1.71 V vs. RHE).…”

“…The investigation of exact mechanisms is beyond the scope of this study, however we could formulate a hypothesis that it is caused by either the adsorption of OH À or the emergence of H 3 O + during the OER process, as both species have detrimental effects on the stability of PBA materials. 32 Another possible degradation pathway could be the electrolysis of crystal coordination water within the InHCF lattice, which is indispensable for its structural integrity. 14 Electrodeposited thin film PBA electrodes serve as an ideal model system to investigate the inherent electrochemical properties of this battery material class without non-active electrode components, such as conductive additives and the binder material, as well as effects stemming from the porous nature of the usually employed slurry-casted electrodes.…”

“…The anion-dependent stability trend of ClO 4 − > NO 3 − > SO 4 2− is in accordance with previous findings for the PBA derivates, and gives further indication on an anion-chemisorption-induced degradation mechanism of such materials. 32 Nevertheless, the cycling stability of NaIn[Fe(CN) 6 ] in the 0.25 M Na + -electrolytes is yet poorer than for other PBA materials. 15–18,24,33…”

“…Simultaneously, the solution's uptake capability for dissolved electrode constituents is strongly decreased due to an entire incorporation of H 2 O molecules in the salt ions’ solvation shells, which results in the stabilization of the active material. 16,32…”

“…These results give further evidence, that highly concentrated electrolytes can effectively suppress the active material dissolution in aqueous systems. 32…”