The Role of Transition Metals on Chemo‐Mechanical Instabilities in Prussian Blue Analogues For K‐Ion Batteries: The Case Study on KNHCF Versus KMHCF

Abstract

Prussian blue analogues (PBAs) cathodes can host diverse monovalent and multivalent metal ions due to their tunable structure. However, their electrochemical performance suffers from poor cycle life associated with chemo‐mechanical instabilities. This study investigates the driving forces behind chemo‐mechanical instabilities in Ni‐ and Mn‐based PBAs cathodes for K‐ion batteries by combining electrochemical analysis, digital image correlation, and spectroscopy techniques. Capacity retention in Ni‐based PBA is 96% whereas it is 91.5% for Mn‐based PBA after 100 cycles at C/5 rate. During charge, the potassium nickel hexacyanoferrate (KNHCF) electrode experiences a positive strain generation whereas the potassium manganese hexacyanoferrate (KMHCF) electrode undergoes initially positive strain generation followed by a reduction in strains at a higher state of charge. Overall, both cathodes undergo similar reversible electrochemical strains in each charge–discharge cycle. There is ~0.80% irreversible strain generation in both cathodes after 5 cycles. XPS studies indicated richer organic layer compounds in the cathode‐electrolyte interface (CEI) layer formed on KMHCF cathodes compared to the KNHCF ones. Faster capacity fades in Mn‐based PBA, compared to Ni‐based ones, is attributed to the formation of richer organic compounds in CEI layers, rather than mechanical deformations. Understanding the driving forces behind instabilities provides a guideline to develop material‐based strategies for better electrochemical performance.

Impact of Alkali Metal-Ion Intercalation on Mechanical Deformations in Cobalt Oxide Cathodes

“…Furthermore, strain derivatives were calculated to correlate the electrochemical behavior with the mechanical deformations in the electrode. Potential-dependent strain derivatives have been linked with the nanoscale deformations in the electrode structure due to phase transformations during intercalation. − The peaks in d Q /d V align well with the peaks in strain derivatives in Supporting Information Figure 4, indicating Li-ion intercalation-induced deformations in the electrode.…”

Advancements in cathode materials for potassium-ion batteries: current landscape, obstacles, and prospects

“…Potassium-based Prussian analogues are hexacyanometallate compounds, 24–34 comprising cyanide-bridged coordination polymers formed by reacting transition metal cyanometallates with potassium ions. These compounds possess open three-dimensional (3D) crystalline frameworks and can be represented by the chemical composition K a L [ L ′(CN) 6 ] 1− b · c H 2 O (0 ≤ a ≤ 2, 0 ≤ b < 1, c ≥ 0), where L and L ′ represent various transition metals like Zn, Cu, Ni, Co, Fe, Cr, and Ti, or alkaline-earth metals such as Mg, coordinated to cyano groups.…”

“…), as well as Prussian analogues (e.g., K 2 MnFe(CN) 6 , K 2 FeFe(CN) 6 , K 2 NiFe(CN) 6 , etc.). [24][25][26][27][28][29][30][31][32][33][34] Conversely, the negative electrode can be constituted of carbonaceous materials such as graphite, which stands as a prevalent choice due to its proficient facilitation of reversible extraction/insertion of K-ions. As for the electrolyte, customary formulations comprise mixtures of organic carbonate solvents, such as diethyl carbonate or ethyl carbonate, in conjunction with non-coordinating anion salts, prominently exemplified by potassium hexafluorophosphate (KPF 6 ), potassium bis(fluorosulphonyl)imide (KN(SO 2 F) 2 (typically abbreviated as KFSI or KFSA)), potassium trifluoromethanesulphonate (KCF 3 SO 3 (KOTf)), potassium (3-methoxypropyl)((trifluoromethyl)-sulphonyl)imide (abbreviated as KMPSA) and potassium bis(trifluoromethanesulphonyl)imide (KN(CF 3 SO 2 ) 2 (KTFSI or KTFSA)).…”

“…Potassium-based Prussian analogues are hexacyanometallate compounds, [24][25][26][27][28][29][30][31][32][33][34] Both L and L 0 are typically redox-active, and the fraction of [L 0 (CN) 6 ] vacancies is denoted as b. By selecting different combinations of L and L 0 , along with the concentration of interstitial potassium ions, the electrochemistry of Prussian analogues can be tailored for various performance metrics.…”

“…Prussian analogues have garnered substantial attention due to their high specific capacity and sustainability. [24][25][26][27][28][29][30][31][32][33][34] Prussian white (K 2 Fe[Fe(CN) 6 ]) exhibits a greater potassium content than Prussian blue and Berlin green derivatives, resulting in a theoretical capacity twice that of Prussian blue. K 2 Fe[Fe(CN) 6 ] and K 2 Mn[Fe(CN) 6 ] consist of cost-effective elements and generally demonstrate high energy density (Fig.…”