Surface Polarization Effects on Collapse Transition of Polyelectrolyte Brushes

Polyelectrolyte brushes(PEBs)are commonly used to modify surface that have attracted great research interest.The dielectric permittivity of the grafted surface is typically significantly different from that of solution,which results in surface polarization(SP)effect with a jump of electric field.It is thus important to study how SP alters the PEB’s structure and properties.In this work,the SP effects on PEB structure was studied using a statistical thermodynamic theory.The free energy functional to describe SP effect was constructed by using the image-charge method.Meanwhile,the electrostatic potential was solved from a modified Poisson-Boltzmann equation taking the ion solvation effect into consideration.In the absence of SP,the thickness of PEB exhibited a continuous collapse transition when decreasing the solvent quality.In the presence of SP,the collapse became a jump-like transition.Free energy analysis showed that the long-range Coulombic interaction dominated the transition because of the enhanced counterion condensation in the presence of SP.The theory provides an effective tool to study SP effect on PEBs,and the results explain the underlying physics in PEB collapse transition.

Critical Factors Dictating Reversibility of the Zinc Metal Anode

With high energy density and improved safety,rechargeable battery chemistries with a zinc(Zn)metal anode offer promising and sustainable alternatives to those based on lithium metal or lithium-ion intercalation/alloying anode materials;however,the poor electrochemical reversibility of Zn plating/stripping,induced by parasitic reactions with both aqueous and non-aqueous electrolytes,presently limits the practical appeal of these systems.Although recent efforts in rechargeable Zn metal batteries(RZMBs)have achieved certain advancements in Zn metal reversibility,as quantified by the Coulombic efficiency(CE),a standard protocol for CE has not been established,and results across chemistries and systems are often conflicting.More importantly,there is still an insufficient understanding regarding the critical factors dictating Zn reversibility.In this work,a rigorous,established protocol for determining CE of lithium metal anodes is transplanted to the Zn chemistry and is used for systematically examining how a series of factors including current collector chemistry,current density,temperature,and the upper voltage limit during stripping affect the measured reversibility of different Zn electrolytes.With support from density functional theory calculations,this standardized Zn CE protocol is then leveraged to identify an important correlation between electrolyte solvation strength toward Zn2+and the measured Zn CE in the corresponding electrolyte,providing new guidance for future development and evaluation of Zn electrolytes.

Vibrational Spectroscopic Study on Ion Solvation and Association of Lithium Perchlorate in 4-Methoxymethyl-ethylene Carbonate

Solvation interaction and ion association in solutions of lithium perchlorate/4-methoxymethyl-ethylene carbonate （MEC） have been studied by using Infrared and Raman spectra as a function of concentration of lithium perchlorate. The splitting of ring deformation band and ring ether asymmetric stretching band, and the change of carbonyl stretching band suggest that there should be a strong interaction between Li^＋ and the solvent molecules, and the site of solvation should be the oxygen atom of carbonyl group. The apparent solvation number of Li^＋ was calculated by using band fitting technique. The solvation number was decreased from 3.3 to 1.1 with increasing the concentration of LiClO4/MEC solutions. On the other hand, the band fitting for the ClO4^- band revealed the presence of contact ion pair, and free ClO4^- anion in the concentrated solutions.