Exploiting the Condensation Reactions of Acetophenone to Engineer Carbon‐Encapsulated Nb₂O₅ Nanocrystals for High‐Performance Li and Na Energy Storage Systems

Efficient synthetic methods to produce high‐performance electrode‐active materials are crucial for developing energy storage devices for large‐scale applications, such as hybrid supercapacitors (HSCs). Here, an effective approach to obtain controllable carbon‐encapsulated T‐Nb 2 O 5 nanocrystals (NCs) is presented, based on the solvothermal treatment of NbCl 5 in acetophenone. Two separate condensation reactions of acetophenone generate an intimate and homogeneous mixture of Nb 2 O 5 particles and 1,3,5‐triphenylbenzene (TPB), which acts as a unique carbon precursor. The electrochemical performance of the resulting composites as anode electrode materials can be tuned by varying the Nb 2 O 5 /TPB ratio. Remarkable performances are achieved for Li‐ion and Na‐ion energy storage systems at high charge–discharge rates (specific capacities of ≈90 mAh g −1 at 100 C rate for lithium and ≈125 mAh g −1 at 20 C for sodium). High energy and power densities are also achieved with Li‐ and Na‐ion HSC devices constructed by using the Nb 2 O 5 /C composites as anode and activated carbon (YPF‐50) as cathode, demonstrating the excellent electrochemical properties of the materials synthesized with this approach.