MXene-mediated regulation of local electric field surrounding polyoxometalate nanoparticles for improved lithium storage

Due to their capability of reversibly accepting multi lithium ions, polyoxometalates (POMs) have been widely regarded as promising candidates for electrochemical lithium storage. Nevertheless, the insulating nature of POMs hinders fast migration kinetics of lithium within the bulk of these materials. Herein, we propose the introduction of a local electric field surrounding the POM nanoparticles consisting of Mn and V where the concomitant Coulomb forces can accelerate the migration of lithium ions. After rationally hybridizing POMs with MXene nanosheets, the imbalanced charge distribution emerging at their interface produces the local electric field, thereby leading to a 250-fold increase of lithium diffusion coefficient. In this regard, a capacitive contribution as high as 81.7% at 1.0 mV s−1 is observed. Moreover, the POM nanoparticles could densely assemble on the surface of MXene nanosheets, offering highly packed electrodes and thus high volumetric capacities. Due to the improved lithiumion transfer kinetics, the POMs/MXenes composites are paired with activated carbon to produce lithium-ion capacitors which could offer a high energy density of 195.5 W h kg−1 and a large power capability of 3800 W kg−1. The findings in this work could build a clear relationship between materials with different conductivities for designing electrode materials.