Preparation of PVDF/PEO-PPO-PEO blend microporous membranes for lithium ion batteries via thermally induced phase separation process

Abstract

Microporous poly(vinylidene fluoride)/polyethylene oxide-co-polypropylene oxide-co-polyethylene oxide (PVDF/PEO-PPO-PEO, or PVDF/F127) blend membranes were prepared via thermally induced phase separation (TIPS) process using sulfolane as the diluent. Then they were soaked in a liquid electrolyte to form polymer electrolytes. The effects of F127 weight fraction on the morphology, crystallinity and porosity of the blend membranes were studied. It was found that both electrolyte uptake of blend membranes and ionic conductivity of corresponding polymer electrolytes increased with the increase of F127 weight fraction. The maximum ionic conductivity was found to reach 2.94 ± 0.02 × 10⁻³ S/cm at 20 °C. Electrochemical stability window was stable up to 4.7 V (vs. Li⁺/Li). The testing results indicated that the PVDF/F127 blend membranes prepared via TIPS process can be used as the polymer microporous matrices of polymer electrolytes for lithium ion batteries.

Introduction

PVDF has been widely used as the polymer microporous matrix (or membrane) of polymer electrolyte in lithium ion batteries due to its appealing properties. PVDF-based polymer electrolytes with ionic conductivity of 10⁻³ S/cm at 25 °C have been obtained [1]. Currently, polymer matrix is usually prepared by non-solvent-induced phase separation (NIPS) process, such as phase inversion process [2], immersion precipitation technique [3], liquid extraction/activation [4], evaporation phase inversion [5] and solvent–solvent extraction process [6]. Although the membranes with different porous structures, such as “sponge-like”, “cellular-like”, “spherulite-like” or “finger-like” structure, can be obtained via NIPS process, there are many factors influencing porous structure. Therefore, this process is difficult in controlling porous structure. It is well known that for porous polymer electrolytes prepared by immersing microporous membranes into liquid electrolyte, their performances intensely depend on the porous structure. Therefore, NIPS process is not a good candidate to prepare microporous membranes used as polymer electrolytes.

Thermally induced phase separation (TIPS) process is another method to prepare microporous membranes [7]. In this process, a polymer is dissolved in a diluent at high temperature, and then the solution is cooled to induce phase separation. The diluent is extracted to yield a microporous structure. Compared with NIPS process, the main advantage of this method is the ease of controlling porous structure because the factors of influencing porous structure are fewer [8]. Ji et al. [8] have shown that TIPS process is an effective method to prepare PVDF matrix used as polymer electrolyte, and ionic conductivity of corresponding polymer electrolyte at room temperature reached 10⁻³ S/cm. However, in order to further increase ionic conductivity, some measures should be taken. One simple way is to increase electrolyte uptake, because the higher the electrolyte uptake is, the higher the ionic conductivity would be. An applicable method is to add material with higher affinity toward the solvent of liquid electrolyte into polymer matrix. Wu et al. [9] have suggested that polyethylene oxide-co-polypropylene oxide-co-polyethylene oxide (PEO-PPO-PEO, or F127) is a suitable candidate because it not only has a good affinity with liquid electrolyte, but also is compatible with PVDF (PPO segment is miscible with PVDF). Moreover, F127 can also tune the porous structure of membranes and thus improve the conducting performance of polymer electrolytes [10].

PVDF/F127 blend membranes have been prepared via immersion precipitation process, and the ionic conductivity of corresponding polymer electrolytes reached 1.0 × 10⁻³ S/cm at room temperature [9], [10]. In this work, TIPS process was employed to prepare PVDF/F127 blend membranes using sulfolane as the diluent. Then the effects of F127 weight fraction on the morphology, crystallinity and porosity of blend membranes were studied. Finally, electrochemical properties of corresponding polymer electrolytes were investigated.