Enhanced thermal and electrochemical properties of PVDF-HFP/PMMA polymer electrolyte by TiO₂ nanoparticles

Highlights

• A facile method to prepare PVDF-HFP/PMMA/TiO₂ (0–7 wt.%) composite polymer electrolyte (CPE) is presented.

• The CPE has reticular porous fabric with suitable pore size.

• The CPE exhibits much higher ionic conductivity and wider electrochemical window.

• LiCoO₂/Li cells with CPE exhibit much better cyclic stability and C-rate performance.

Abstract

(Poly(vinylidene fluoride-co-hexafluoropropylene)/poly(methyl methacrylate)) PVDF-HFP/PMMA based gel polymer electrolyte comprising 0–7 wt.% TiO₂ nanoparticles has been synthesized. After introducing TiO₂ nanoparticles, the thermal properties of the PVDF-HFP/PMMA/TiO₂ composite polymer electrolyte (CPE) are remarkably improved: when the working temperature is up to 90 °C, the weight loss of CPE with 7% TiO₂ is only 12.8% of that without TiO₂, suggesting much higher thermal stability; the shrinkage of the CPE at 130 °C can also obviously decrease from 23.4% down to 14.4% after introducing TiO₂ nanoparticles. In addition, the CPE exhibits much higher ionic conductivity and better electrochemical stability. Furthermore, the LiCoO₂/Li cells with CPE exhibit good cyclic stability and C-rate performance: the capacity maintains 92.1% of the initial capacity after 50 cycles; the capacity can reach as large as ~ 80 mAhg^− 1 even at 5 C. It is promising for PVDF-HFP/PMMA/TiO₂ CPE to meet the practical demands of high-performance and thermal safety for lithium ion batteries.

Introduction

Rechargeable Li-ion batteries (LIBs) are key components of both hybrid electric vehicles (HEVs) and full electric vehicles (EVs) which require high energy and power capability [1], [2], [3]. The use of polymer electrolytes is widely regarded as a promising approach for Li-ion batteries [4], [5], [6], [7], due to their lack of leakage, high flexibility within the cell geometry and high physical and chemical stability [8], [9]. Among of all the polymer electrolytes, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and poly(methyl methacrylate) (PMMA) polymer matrixes have attracted much attention [10], [11], [12], [13], [14] due to the advantages of excellent mechanical, chemical stability [15], [16] and considerable wettability [17]. Nevertheless, they also have shown some drawbacks such as low thermal stability, poor electrochemical stability, low cyclic and rate performances, which limit the wide applications [18], [19], [20].

To overcome the above-mentioned disadvantages [21], [22], [23], considerable research efforts have been done. Yang et al. improved the thermal property of polymer electrolyte by using core-shell structured SiO₂–PMMA microspheres [17], [18], while Kim used gamma ray irradiation to improve the thermal properties of the polymer electrolyte [19]. Such methods seem a little complicated. Introducing inorganic nanoparticles such as SiO₂ [17], [18], [20], [24], [25], [26], Al₂O₃ [27], [28], [29], SnO₂ [30], TiO₂ [31], [32], [33], [34], [35] and CaCO₃ [10] into polymer electrolyte is another effective way to improve the ionic conductivity and enhance electrochemical performance of LIBs. The comprehensive effects of the composite polymer electrolyte (CPE) with various contents of inorganic nanoparticles on the porous structure, the thermal and electrochemical properties are still not so clear.

In this study, we perform a facile method to prepare PVDF-HFP/PMMA based gel polymer electrolyte comprising 0–7 wt.% TiO₂ nanoparticles. The effects of TiO₂ nanoparticles on the porous structure, the thermal and electrochemical properties have been investigated. The results show that after introducing TiO₂ nanoparticles, the composite polymer electrolyte exhibits much better thermal stability, higher ionic conductivity, and wider electrochemical window. The LiCoO₂/Li cells with CPE exhibit excellent cyclic stability and C-rate performance.