Revealing the Thermal Safety of Prussian Blue Cathode for Safer Nonaqueous Batteries

Abstract

Prussian blue analogs (PBAs) are promising cathode materials for many next‐generation metal‐ion batteries due to their exceptional electrochemical performance. Their oxygen‐free structure avoids a common battery thermal runaway pathway which requires O2 liberation. Herein, the thermal runaway mechanisms of PBAs are studied from the level of material and full cell in nonaqueous sodium‐ and potassium‐ion batteries (SIBs and KIBs). Their hidden safety issue and a novel runaway mechanism that requires no oxygen evolution are identified. The cyanide groups are released (≈51.4 wt%) as toxic cyanides above 200 °C, which also exothermically react with the electrolyte and cause the runaway. The cyanide gas generation mechanism is proposed as cathode hydrolytic disproportionation by Raman spectroscopy, X‐ray photoelectron spectroscopy, in situ environmental transmission electron microscopy, and operando synchrotron X‐ray diffraction studies. In addition, full‐cell level calorimetric studies reveal mitigated heat generation but lower initiation temperature of runaway from such SIBs and KIBs than conventional LiCoO2–graphite system. These results change how PBA materials are evaluated from a safety standpoint, suggesting that they cannot be regarded as safe cathodes. They also indicate the correlations between thermal safety and their crystal defects or trapped water content. The proposed thermal runaway mechanism provides insights to assist in the building of safer next‐generation batteries.

Ordered assembly of potassium cobalt hexacyanoferrate hollow multivoid nanocuboid arrays for high-performance aqueous K-ion batteries towards all-climate energy storage

“…Among the available cathode candidates, Prussian blue analogues (PBAs) with the chemical formula of A x M 1 [M 2 (-CN) 6 ] y $mH 2 O (where A ¼ alkali metal; M ¼ transition metal; 0 # x # 2) have attracted intensive interest. [12][13][14][15][16][17] Bridged by cyanide groups (-C^N-) À , the open frameworks of PBAs not only provide large interstitial sites to host alkali ions, but also 3D channels for fast ion transport. 18,19 Both make them promising electrodes for energy storage.…”

Opportunities in Na/K [hexacyanoferrate] frameworks for sustainable non-aqueous Na+/K+ batteries

“…The AMHCF structures are intact below 200 °C, and minor thermal runaway can be observed due to lattice water decomposition; however, toxic HCN is released above 250 °C, and (CN) 2 and CO 2 are released above 350 °C due to the breakdown of the AMHCF structure. Studies conducted by Zheng Li and his team 189 using an AMHCF as a cathode and graphite as an anode coupled with organic electrolyte suggest that in K-MHCF, exothermic heat is generated at 127 °C due to the SEI degradation. Above 162 °C, the K intercalated, plated anode starts to react with the electrolyte, and evolution of cyanides was observed above 190 °C.…”

Progress on Fe‐Based Polyanionic Oxide Cathodes Materials toward Grid‐Scale Energy Storage for Sodium‐Ion Batteries

“…Besides, despite the Prussian blue material being non‐toxic in a natural environment, [ 24,25 ] it shows the potential to release poisonous cyanide anion when exposed to high temperature or strong acid. [ 26–30 ]…”