Electrospun CuO nanowires from an aqueous polymeric solution gave the highest specific capacitance so far achieved in this material when tested as a supercapacitor electrode.
Followed
by decades of successful efforts in developing cathode
materials for high specific capacity lithium-ion batteries, currently
the attention is on developing a high-voltage battery (>5 V vs
Li/Li+) with an aim to increase the energy density for
their many
fold advantages over conventional <4 V batteries. Among the various
cathode materials, phosphate polyanion materials (LiMPO4, where M is a single metal or a combination of metals) showed promising
candidacy given their high electrochemical potential (4.8–5
V vs Li/Li+), long cycle stability, low cost, and achieved
specific capacity (∼165 mAh·g–1) near
to its theoretical limit (170 mAh·g–1). In
this review, factors affecting the electrochemical potential of the
cathode materials are reviewed and discussed. Techniques to improve
the electrical and ionic conductivities of phosphate polyanion cathodes,
namely, surface coating, particle size reduction, doping, and morphology
engineering, are also discussed. A processing–property correlation
in phosphate polyanion materials is also undertaken to understand
relative merits and drawbacks of diverse processing techniques to
deliver a material with targeted functionality. Strategies required
for high-voltage phosphate polyanion cathode materials are envisioned,
which are expected to deliver lithium-ion battery cathodes with higher
working potential and gravimetric specific capacity.
A one-dimensional morphology comprising nanograins of two metal oxides, one with higher electrical conductivity (CuO) and the other with higher charge storability (CoO), is developed by electrospinning technique. The CuO-CoO nanocomposite nanowires thus formed show high specific capacitance, high rate capability, and high cycling stability compared to their single-component nanowire counterparts when used as a supercapacitor electrode. Practical symmetric (SSCs) and asymmetric (ASCs) supercapacitors are fabricated using commercial activated carbon, CuO, CoO, and CuO-CoO composite nanowires, and their properties are compared. A high energy density of ∼44 Wh kg at a power density of 14 kW kg is achieved in CuO-CoO ASCs employing aqueous alkaline electrolytes, enabling them to store high energy at a faster rate. The current methodology of hybrid nanowires of various functional materials could be applied to extend the performance limit of diverse electrical and electrochemical devices.
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