Researchers funded by Nanostructures for Electrical Energy Storage (NEES), a DOE Energy Frontier Research Center, have developed a nanowire-based hybrid battery/capacitor that can be recharged hundreds of thousands of times. The team, based at the University of California, Irvine, coated gold nanowire in a manganese dioxide shell and encased the assembly in an electrolyte made of a Plexiglas-like gel. The combination is reliable and resistant to failure.
In a paper published in the journal ACS Energy Letters, they reported reversible cycle stability for up to 200 ,000 cycles with 94–96% average Coulombic efficiency for symmetrical δ-MnO2 nanowire capacitors operating across a 1.2 V voltage window in a poly(methyl methacrylate) (PMMA) gel electrolyte.
The nanowires have a Au@δ-MnO2 core@shell architecture in which a central gold nanowire current collector is surrounded by an electrodeposited layer of δ-MnO2 that has a thickness of between 143 and 300 nm. Identical capacitors operating in the absence of PMMA exhibited significantly reduced cycle stabilities ranging from 2000 to 8000 cycles. In the liquid PC electrolyte, the δ-MnO2 shell fractures, delaminates, and separates from the gold nanowire current collector. These deleterious processes are not observed in the PMMA electrolyte.
For electrode materials that rely on ion insertion for Faradaic charge storage, a nanowire morphology can enable higher power in either batteries or capacitors than is possible using a film of the same material. However, the Achilles heel of such nanowires for energy storage is cycle stability. The diminutive lateral dimension of nanowires increases their susceptibility to dissolution and corrosion, and these processes rapidly result in a loss of electrical continuity through the nanowire and an irreversible loss of capacity.
… Here, we report that the cycle stability of MnO2 all-nanowire capacitors can be extended from 2000 to 8000 cycles to more than 100000 cycles, simply by replacing a liquid electrolyte with a poly(methyl methacrylate) (PMMA) gel electrolyte.—Thai et al.
Based on SEM analysis of cycled Au@MnO2 core@shell nanowires, the researchers suggested that one mechanism by which PMMA gel may extend cycle lifetime is simply the mechanical confinement of the MnO2 shell material on the gold nanowire current collector.
The high viscosity and elasticity of the PMMA gel appears to prevent the separation of MnO2 from the current collector while remaining transparent to fluxes of Li+ involved in insertion and deinsertion. The researchers also suggested that the PMMA gel electrolyte reduces the propensity for fracture of the MnO2 shell, increasing its fracture toughness.
Mya Le Thai, Girija Thesma Chandran, Rajen K. Dutta, Xiaowei Li and Reginald M. Penner (2016) “100k Cycles and Beyond: Extraordinary Cycle Stability for MnO2 Nanowires Imparted by a Gel Electrolyte” ACS Energy Letters Vol. 1: Pages 57-63 doi: 10.1021/acsenergylett.6b00029