PNb 9 O 25 , a Wadsley-Roth compound whose structure is obtained by appropriate crystallographic shear of the ReO 3 structure, is a high-power electrode material that can reach 85 % of the equilibrium capacity in 30 minutes and 67% in 6 minutes. Here we show that multielectron redox, as observed through X-ray absorption spectroscopy and X-ray photoelectron spectroscopy, and an insulator-to-metal transition upon lithium insertion, as suggested by a number of complementary techniques, contribute to the impressive performance. Chemically tuning the tetrahedral site between phosphorus and vanadium leads to significant changes in the electrochemistry and kinetics of lithium insertion in the structure, pointing to larger implications for the use of crystallographic shear phases as fast-charging electrode materials.
Mixed
electron- and ion-conducting polymers serve as excellent
candidates for polymer binders in lithium-ion batteries (LIBs) because
of an extension of functionality beyond simple mechanical adhesion.
Such dual conduction was observed in our recent report on dihexyl-substituted
poly(3,4-propylenedioxythiophene) (PProDOT-Hx2), which
showed excellent performance as a cathode binder for LiNi0.8Co0.15Al0.05O2 (NCA). However, ionic
conductivity was found to be significantly lower than that of its
electronic counterpart. To enhance mixed conduction, here we report
a family of synthetically tunable, electrochemically stable, random
copolymers based on PProDOT-Hx2, in which the hexyl (Hex)
side chains are replaced to varying extents with oligoether (OE) side
chains, generating a series of (Hex:OE) PProDOTs. When OE content
was varied from 5 to 35%, the resulting copolymers were insoluble
in the battery electrolyte and were stable after 100 electrochemical
doping/dedoping cycles. Electron paramagnetic resonance and electrochemical
kinetics studies were performed to illustrate the reversible and fast
electrochemical doping process of (Hex:OE) PProDOTs. Electronic and
ionic conductivity measurements as a function of electrochemical potential
show a decrease in electronic conductivity and a concurrent increase
in ionic conductivity with increasing incorporation of OE side chains.
X-ray scattering studies on electrochemically doped polymers indicate
a decline in crystalline ordering with the increase in OE content
of the (Hex:OE) PProDOTs, suggesting that decreasing crystallinity
is responsible for both the increased ionic and reduced electronic
conductivity. Compounding these structural changes, swelling studies
show a linear mass increase with OE content upon electrolyte exposure,
indicating that solvent-induced swelling and electrolyte uptake play
a significant role in the ability of these polymers to conduct ions.
Finally, rigorous cell testing was performed by employing electrochemical
impedance spectroscopy, galvanostatic charge–discharge, rate
capability tests, and differential capacity vs voltage analysis, using
NCA cathodes to understand the role of these polymers as mixed electron-
and Li+-ion-conducting polymer binders in LIBs in comparison
to the commonly used polyvinylidene fluoride. It is observed that
(75:25) PProDOT containing 25% of OE side chains achieves the highest
rate capability and fastest charging and discharging under symmetric
testing conditions. The synthetic flexibility to fine-tune electronic
and ionic conductivity makes (Hex:OE) PProDOTs a promising new class
of mixed conducting polymers for electrochemical energy-storage application.
The effects of shear planes in perovskitic materials have been studied in order to identify their role in the electrochemical behavior and structural evolution of Li+ intercalation hosts.
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