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

Polyanion-type phosphate materials, such as M3V2(PO4)(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 Mg3V4(PO4)(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 Li3PO4 in LIBs, while in SIBs/PIBs the material only experiences a solid solution with the reduction of V3+ to V2+. 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.

Automated summary

Polyanion-type phosphate materials, like M3V2(PO4)(3) (M = Li/Na/K), show promise as negative electrodes for monovalent-ion batteries. However, understanding the reaction mechanisms during monovalent-ion insertion remains a challenge.