Guest Ion‐Dependent Reaction Mechanisms of New Pseudocapacitive Mg₃V₄(PO₄)₆/Carbon Composite as Negative Electrode for Monovalent‐Ion Batteries

Abstract Polyanion‐type phosphate materials, such as M 3 V 2 (PO 4 ) 3 (M = Li/Na/K), are promising as insertion‐type negative electrodes for monovalent‐ion batteries including Li/Na/K‐ion batteries (lithium‐ion batteries (LIBs), sodium‐ion batteries (SIBs), and potassium‐ion batteries (PIBs)) with fast charging/discharging and distinct redox peaks. However, it remains a great challenge to understand the reaction mechanism of materials upon monovalent‐ion insertion. Here, triclinic Mg 3 V 4 (PO 4 ) 6 /carbon composite (MgVP/C) with high thermal stability is synthesized via ball‐milling and carbon‐thermal reduction method and applied as a pseudocapacitive negative electrode in LIBs, SIBs, and PIBs. In operando and ex situ studies demonstrate the guest ion‐dependent reaction mechanisms of MgVP/C upon monovalent‐ion storage due to different sizes. MgVP/C undergoes an indirect conversion reaction to form Mg 0 , V 0 , and Li 3 PO 4 in LIBs, while in SIBs/PIBs the material only experiences a solid solution with the reduction of V 3+ to V 2+ . Moreover, in LIBs, MgVP/C delivers initial lithiation/delithiation capacities of 961/607 mAh g −1 (30/19 Li + ions) for the first cycle, despite its low initial Coulombic efficiency, fast capacity decay for the first 200 cycles, and limited reversible insertion/deinsertion of 2 Na + /K + ions in SIBs/PIBs. This work reveals a new pseudocapacitive material and provides an advanced understanding of polyanion phosphate negative material for monovalent‐ion batteries with guest ion‐dependent energy storage mechanisms.