3D porous network gel polymer electrolyte with high transference number for dendrite-free LiO₂ batteries

Highlights

• The PHPG electrolyte shows a 3D porous structure with a high ionic conductivity at room temperature.

• PHPG effectively reduce the crystallinity of the polymer and thus improve the Li ion transference number.

• PHPG based Li-O₂ batteries showing good stability and security due to the remarkable suppression of Li dendrite growth.

Abstract

Solid-state lithium battery is considered as the next generation energy storage device with high security and outstanding energy density. However the low ionic conductivity of the electrolyte seriously limits its practical application. Here, we reported a high-performance flexible gel electrolyte based on graphene oxide (GO) doped poly (vinylidene fluoride-hexafluoro pentaene) cross-linked with poly (ethylene oxide) (PHPG) through weak bond interaction. The polymer electrolyte shows a 3D porous structure with an ionic conductivity of 3.4 × 10⁻⁴ S cm⁻¹ at room temperature. Moreover, Li ion transference number is about 0.58, which could greatly accelerate the fast transmission of cations and anions. As a result, the PHPG-based Li symmetrical battery (Li/PHPG/Li) can run stably >750 h at 0.5 mA cm⁻², showing good stability and security due to the remarkable suppression of Li dendrite growth. LiO₂ batteries assembled with PHPG membrane as electrolyte can operate over 300 cycles at high current density (1 mA cm⁻²), indicating excellent cycling performance. The excellent performance of PHPG makes PHPG a very promising electrolyte for next-generation solid-state LiO₂ batteries.

Introduction

The development of novel energy storage and conversion system is the prerequisite for the widespread application of renewable energy. Li batteries are regarded as promising energy storage devices for renewable energy at present. They provide a significant leap forward in energy density for electric vehicles and smart grid applications [[1], [2], [3], [4]]. Lithium ion batteries (LIB), which have been widely used since it was developed in 1991, are limited by low theoretical energy density due to the inherent characteristics of the electrode [[5], [6], [7], [8]]. As an alternative, the theoretical energy density of LiO₂ batteries is as high as 3505 Wh kg⁻¹, which is comparable to gasoline and much higher than the currently used LIB [[9], [10], [11], [12]]. Oxygen in the air is used as the feedstock in LiO₂ batteries, thus both weight and volume of LiO₂ battery are simultaneously reduced, thereby effectively enhancing the energy density of batteries [[13], [14], [15]]. The charge and discharge process of LiO₂ batteries is based on the formation and decomposition process of Li₂O₂. The charge and discharge reaction in the batteries is 2Li + O₂ ⇄ Li₂O₂ [16,17]. The commercialization of LiO₂ batteries needs to solve the technical issues, including low rate capacity, unsatisfactory energy efficiency, poor cycleability as well as the serious security issues [[18], [19], [20], [21]]. As a result, a large number of efforts, such as cathode catalyst design, anode and electrolyte modification, are devoted to improve safe and efficient cycle performance of LiO₂ batteries [[22], [23], [24]]. Among these strategies, electrolyte modification is considered to be one of the most promising approaches in developing high performance LiO₂ batteries with excellent security [25,26].

Because the LiO₂ battery adopts a semi-open system, the commonly used organic liquid electrolytes (LEs) will cause serious safety problems due to its leakage and flammability as well as oxygen penetrability. Another serious problem of LiO₂ battery with LEs is the inevitable formation of lithium dendrites due to the “tip effect” [[27], [28], [29]]. The effect of lithium dendrite on the batteries is destructive because dendrite growth can penetrate the membrane and directly cause short circuit of the battery and a series of safety accidents. The proper strategies to inhibit lithium dendrite are the prerequisite of the practical application of LiO₂ batteries [30]. At present, an effective method is to use gel polymer electrolytes (GPEs) with high safety to replace LEs [[31], [32], [33], [34], [35]]. Unlike LEs, GPEs do not require the installation of a temperature-rise and short-circuit-resistant safety device in the practical batteries. Besides, most of GPEs are non-combustible, non-corrosive and non-volatile. In addition, they overcome the phenomenon of lithium dendrite and alleviate the problem of volume expansion during charging and discharging, thus gaining extremely high safety [[36], [37], [38], [39]].

However, GPEs has its unique disadvantages, such as poor mechanical properties. In addition, ion transport kinetics in solid electrolytes is lower at room temperature due to the high crystallinity of polymers, resulting in lower ionic conductivity [34,[40], [41], [42]]. A lot of researches have been done in order to improve the ionic conductivity of GPEs at room temperature [[43], [44], [45], [46], [47]]. Among them, cross-linking of multiple types of polymers is considered as a promising approach. The cross-link between polymers can disrupt the fixed molecular chain structure and effectively reduce the crystallinity of the polymer and thus improve the ionic conductivity.

Hence, in this paper, we report a flexible gel electrolyte with a 3D porous structure. By combining PVDF-HFP with PEO, the polymer was crosslinked to effectively inhibit the crystallization of PEO at room temperature, thus improving the ionic conductivity. At the same time, GO was inserted into the cross-linked polymer and jointly constructed a high-speed transport pathway for Li⁺. GO can also improve the porosity of the electrolyte membrane and shorten the pore diameter while ensuring high ionic conductivity, so as to obtain high liquid absorption rate and liquid retention rate [48]. Based on the deliberately design, the ionic conductivity of the as-prepared PHPG is as high as 3.4 × 10⁻⁴ S cm ⁻¹ at 25 °C and the achieved Li⁺ transference number is close to 0.58. In terms of electrochemical properties, the Li|PHPG|Li symmetric cell can run stably for over 400 h at 0.5 mA cm⁻². The LiO₂ battery based on the PHPG can operate for >300 times at high current density of 1 mA cm⁻² with low polarization, showing excellent cycling performance. Besides, the large-scale LiO₂ battery with PHPG can work continuously under extreme conditions. This study provides a valuable approach for the advance electrolytes in LiO₂ batteries.