Transition-metal
chalcogenides have emerged as a promising class
of materials for energy storage applications due to their earth abundance,
high theoretical capacity, and high electrical conductivity. Herein,
we introduce a facile and one-pot electrodeposition method to prepare
high-performance nickel selenide Ni
x
Se
y
(0.5 ≤ x/y ≤ 1.5) nanostructures (specific capacity = 180.3
mA h g–1 at 1 A g–1). The as-synthesized
nickel selenide (NS) nanostructure is however converted to other polymorphs
of nickel selenide including orthorhombic NiSe2, trigonal
Ni3Se2, hexagonal NiSe, and orthorhombic Ni6Se5 over cycling. Interestingly, NiSe2 and Ni3Se2 polymorphs that display a more
metallic character and superior energy storage performance are the
predominant phases after a few hundred cycles. We fabricated a hybrid
device using activated carbon (AC) as a supercapacitor-type negative
electrode and NS as a high-rate battery-type positive electrode (AC||NS).
This hybrid device provides a high specific energy of 71 W h kg–1, an excellent specific power of up to 31 400
W kg–1, and exceptional cycling stability (80% retention
of the initial capacity after 20 000 cycles). The higher energy
storage performance of the device is a result of the development of
high-performance NiSe2 and Ni3Se2 polymorphs. Moreover, the reduction of the critical dimension of
the NS particles to the nanoscale partially induces an extrinsic pseudocapacitive
behavior that improves the rate capability and durability of the device.
We also explored the origin of the superior energy storage performance
of the NS polymorphs using density functional theory calculations
in terms of the computed density of states around the Fermi level,
electrical conductivity, and quantum capacitance that follows the
trend NiSe2 > Ni3Se2 > NiSe
> Ni6Se5. The present study thus provides
an appealing
approach for tailoring the phase composition of NS as an alternative
to the commonly used templated synthesis methods.