A study explores the engineering of cathode electrolyte interphase (CEI) layers in Prussian blue analogues (PBAs) for lithium-ion batteries (LIBs) to enhance their electrochemical performance. The research focuses on iron hexacyanoferrate (FeHCFe) and demonstrates the formation of a polymeric CEI layer through the ring-opening reaction of ethylene carbonate (EC) triggered by hydroxide radicals from structural water. This approach significantly mitigates sluggish electrochemical kinetics in organic electrolytes and improves the stability and capacity of FeHCFe. The CEI layer, formed by EC ring-opening, activates Fe low-spin states and enhances structural integrity, leading to a specific capacity of 125 mAh g⁻¹ and stable performance over 500 cycles. The study highlights the importance of water content in PBAs, which influences structural stability and ion insertion efficiency. The CEI layer also prevents water and hydronium ions from penetrating the structure, preserving active sites and enhancing Fe low-spin activation. The research demonstrates that electrolyte engineering, particularly the formation of a CEI layer, can stabilize PBAs in aqueous Li-ion systems, making them viable cathode materials for commercial use. The study also compares the performance of FeHCFe in different electrolytes, showing that hybrid electrolytes (combining aqueous and organic components) offer the best performance due to the activation of both Fe high-spin and low-spin states. The results indicate that advanced electrolyte engineering and targeted CEI formation can significantly improve the performance and longevity of PBAs in LIBs.A study explores the engineering of cathode electrolyte interphase (CEI) layers in Prussian blue analogues (PBAs) for lithium-ion batteries (LIBs) to enhance their electrochemical performance. The research focuses on iron hexacyanoferrate (FeHCFe) and demonstrates the formation of a polymeric CEI layer through the ring-opening reaction of ethylene carbonate (EC) triggered by hydroxide radicals from structural water. This approach significantly mitigates sluggish electrochemical kinetics in organic electrolytes and improves the stability and capacity of FeHCFe. The CEI layer, formed by EC ring-opening, activates Fe low-spin states and enhances structural integrity, leading to a specific capacity of 125 mAh g⁻¹ and stable performance over 500 cycles. The study highlights the importance of water content in PBAs, which influences structural stability and ion insertion efficiency. The CEI layer also prevents water and hydronium ions from penetrating the structure, preserving active sites and enhancing Fe low-spin activation. The research demonstrates that electrolyte engineering, particularly the formation of a CEI layer, can stabilize PBAs in aqueous Li-ion systems, making them viable cathode materials for commercial use. The study also compares the performance of FeHCFe in different electrolytes, showing that hybrid electrolytes (combining aqueous and organic components) offer the best performance due to the activation of both Fe high-spin and low-spin states. The results indicate that advanced electrolyte engineering and targeted CEI formation can significantly improve the performance and longevity of PBAs in LIBs.