Tetragonal MF2 (M=Ni, Co) micro/nanocrystals anodes for lithium/sodium-ion capacitors

Tetragonal MF2 (M = Ni, Co) micro/nanocrystals have been synthesized by one-pot solvothermal method and further served as anodes for the lithium/sodium (Li/Na)-ion energy storage. The overall electro-chemical performance of MF2 candidates have been optimized by the orthogonal experiment conditions of reaction solvents, temperatures and times. The optimal MF2 anodes and commercial activated carbon (AC) cathodes have been used to construct the MF2//AC Li/Na-ion capacitors (LICs/NICs), which have been optimized by tuning the active mass ratios of positive and negative electrodes and working voltage windows of the capacitors. The NiF2//AC LICs demonstrate an overall superior energy/power densities and cycling behavior with the AC/NiF2 mass ratio of 1:2 under the voltages of 0.01-4 and 0.01-4.3 V, in comparison with the CoF2//AC LICs (1:2; 0.01-4/0.01-4.3 V). The MF2 anodes exhibit the dominant surface and diffusion control kinetics for Li-ion and Na-ion storage respectively, showing a major conversion mechanism for the charge storage processes. The work addresses the new insight of tetragonal MF2 (M = Ni, Co) micro/nanocrystals anodes with the dominant conversion mechanisms for advanced LICs and NICs. (C) 2019 Elsevier Ltd. All rights reserved.