Construction of phosphorus-doping with spontaneously developed selenium vacancies: Inducing superior ion-diffusion kinetics in hollow Cu2Se@C nanospheres for efficient sodium storage

Achieving high-efficiency sodium storage in metal selenides is still severely constrained in consideration of their inferior electronic conductivity and inadequate Na+ insertion pathways and active sites. Heteroatom doping accompanied by spontaneously developed lattice defects can effectively tune electronic structure of metal selenides, which give a strong effect to motivate fast charge transfer and Na+ accessibility. Herein, we finely designed and successfully constructed a fascinating phosphorus-doped Cu2Se@C hollow nanosphere with abundant vacancy defects (Cu2PxSe1−x@C) through a combination strategy of selenization of Cu2O nanosphere template, self-polymerization of dopamine, and subsequent phosphorization treatment. Such exquisite composite possesses enriched active sites, superior conductivity, and sufficient Na+ insertion channel, which enable much faster Na+ diffusion rates and more remarkable pseudocapacitive features. Satisfyingly, the Cu2PxSe1−x@C composites manifest the supernormal sodium-storage capabilities, that is, a reversible capacity of 403.7 mA h g−1 at 1.0 A g−1 over 100 cycles, and an ultrastable cyclic lifespan over 1000 cycles at 20.0 A g−1 with a high capacity-retention of about 249.7 mA h g−1. The phase transformation of the Cu2PxSe1−x@C involving the intercalation of Na+ into Cu2Se and the following conversion of NaCuSe to Cu and Na2Se were further demonstrated through a series of ex-situ characterization methods. DFT results demonstrate that the coexistence of phosphorus-doping and vacancy defects within Cu2Se results in the reduction of Na+ adsorption energy from −1.47 to −1.56 eV improving the conductivity of Cu2Se to further accelerate fast Na+ mobility.