Effects of Coated Separator Surface Morphology on Electrolyte Interfacial Wettability and Corresponding Li-Ion Battery Performance.

Ruijie Xu,Henghui Huang,Caihong Lei,Ziqin Tian,Jiayi Xie

doi:10.3390/polym12010117

Abstract

In order to study the effect of interfacial wettability of separator on electrochemical properties for lithium–ion batteries, two different kinds of polyvinylidene fluoride-hexafluoropropylene (PVDF–HFP) solution are prepared and used to coat onto a polypropylene (PP) microporous membrane. It is found that the cell performance of a coated separator using aqueous slurry (WPS) is better than that of the coated separator using acetone (APS) as the solvent. The separator with flat and pyknotic surface (PP and APS) has a strong polar action with the electrolyte, where the polar part is more than 80%. To the contrary, the WPS has a roughness surface and when the PVDF–HFP particles accumulate loose, it makes the apolar part plays a dominate role in surface free energy; the dispersive energy reaches to 40.17 mJ m−2. The WPS has the lowest immersion free energy, 31.9 mJ m−2 with the electrolyte, and this will accelerate electrolyte infiltration to the separator. The loose particle accumulation also increases the electrolyte weight uptake and interfacial wettability velocity, which plays a crucial role in improving the cell performance such as the ionic conductivity, discharge capacity and the C-rate capability.

Highlights

With the material science progressing, the properties of lithium–ion batteries (LIBs) have satisfied in many high demand areas, such as HEV or EV [1]
Surface and cross-section morphology of the coated separators were characterized by scanning electron microscopy (SEM; S3400N and SU8010, Hitachi, Tokyo, Japan)
In the electrochemical impedance spectroscopy (EIS) measurements, the excitation voltage applied to the cells was 5 mV and the frequency range was between 300 kHz and 0.05 Hz

Summary

Introduction

With the material science progressing, the properties of lithium–ion batteries (LIBs) have satisfied in many high demand areas, such as HEV or EV [1]. The diffusion resistances’ route from the separator and electrolyte are seldom paid attention This part of the resistance has a greater effect on the rapid conduction of lithium ions. The cell using the PVDF-coated separator showed good cycle life stability and excellent rate performance at room temperature [6] This improvement mainly comes from the surface polarity-induced electrolyte absorption increment, which is conducive to improve ionic conductivity and reduce the inner resistance [7,8]. The wetting gap between the separator and electrolyte will form very huge inner resistance and block Li+ ion transportation At this moment, the spread of the electrolyte on the separator caused by the interface interaction will greatly affect the performance of the battery, such as higher discharge capacity, rate capability and better low temperature performance. The target of this work is to clarify the relationship between membrane surface characterization and its effect on corresponding cell performance

Materials

Coated Membrane Preparation

Characterization of PVDF-HFP Coated Separators

Electrochemical Measurements of PVDF-HFP Coated Separators

Results and Discussion

Conclusions