High−Performance organic lithium−ion battery with plastic crystal electrolyte

Abstract

Organic quinone compounds are expected to become the main cathode materials for the next generation of energy storage devices due to their high capacity. However, they would soluble in conventional organic electrolytes, causing a reduction in electrochemical performance. Interestingly, the plastic crystal electrolyte (PCE) based on succinonitrile (SN) not only has high ionic conductivity, but also can alleviate the solubility problem of quinone compounds. In this study, the 5 mol% LiTFSI (lithium bis(trifluoromethane)sulfonimide)/SN PCE was firstly chosen to match with quinone compound calix[4]quinone (C4Q) to assemble lithium−ion batteries (LIBs). The SN−based electrolyte system greatly boosting batteries life and achieving good rate performance. With SN−based electrolyte, LIBs displayed an initial discharge specific capacity of 424 mAh g⁻¹ (95% of theoretical specific capacity) at 0.1 C, and the capacity retention rate was still as high as 60% after 1000 cycles with the coulombic efficiency always around 100%. At 2 C, the capacity maintained around 140 mAh g⁻¹. Our work has great application value in LIBs due to the low cost and the excellent electrochemical performance of C4Q in LiTFSI/SN PCE.

Introduction

In recent years, organic materials with redox activity have attracted much attention when they are used as cathodes for lithium−ion batteries (LIBs) due to their advantages of easy synthesis, diverse structure, low cost, environmentally friendly and adjustable electrochemical performance. So far, there are many kinds of organic cathode materials have been studied in LIBs. Among them, quinone compounds are considered as the most promising cathode materials because of the advantages of high theoretical specific capacity (C_theo), good electrochemical activity, large conjugated system and multiple carbonyl functional groups [[1], [2], [3], [4], [5], [6]].

Calix[4]quinone (C4Q) [7,8], a small molecule quinone compound with simple ring structure, is composed of four p−benzoquinones connected by methylene sites and has eight carbonyl groups, its C_theo can reach 446 mAh g⁻¹. According to the previous report, C4Q has been proved to be a prospective cathode material for LIBs. In 1 M LiPF₆ ethylene carbonate/dimethyl carbonate (EC/DMC) (v:v = 1:1) electrolyte, C4Q performed well in electrochemical tests. The initial discharge capacity was 431 mAh g⁻¹ at 0.2 C, up to 97% of the C_theo, verifying that the eight carbonyl sites of C4Q were participated in the electrochemical reaction. Inevitably, like most organic small molecule electrode materials, C4Q is easy to dissolve in traditional organic liquid electrolytes, resulting in rapidly capacity decay, poor cycle stability and low coulombic efficiency [[9], [10], [11], [12], [13], [14], [15]]. In addition, organic liquid electrolytes have the defects of volatile, toxic and flammable, etc., which make the battery has some safety risks during discharging−charging progress.

Electrolyte modification is a good choice to solve the above−mentioned problems. Huang et al. [7] proposed to replace the organic liquid electrolyte with a quasi−solid state electrolyte (poly(methacrylate)/poly(ethylene glycol) (PMA/PEG)−based gel polymer electrolyte (GPE) with LiClO₄/dimethyl sulfoxide (DMSO)) to reduce the dissolution rate of C4Q. The discharge capacity kept as 379 mAh g⁻¹ after 100 cycles at 0.2 C. Zhu et al. [16] synthesized another solid state electrolyte (a PMA/PEG−based composite polymer electrolyte (CPE) with 3 wt% SiO₂) to overcome the dissolution problems of pillar[5]quinone. The capacity was still retained 95% of the initial capacity at 0.2 C after 50 cycles. It is worth noting that the low conductivity, poor interface contacts between electrodes and solid−state electrolytes, and the large interface impedance during cycling would affect the ion shuttle rate. Therefore, these obstructions should also be considered when modifying electrolytes.

Furthermore, Huang et al. [17] found that compared with other solid−state electrolytes, the plastic crystal electrolyte (PCE) based on succinonitrile (SN) has good conductivity [18,19], which can effectively reduce the interface resistance between electrodes and electrolytes. In this system, the battery could still obtain excellent performance without adding liquid electrolyte to the PCE or running at high temperature. It is because that SN has the high dielectric constant (ε = 55), high conductivity (σ_SN = 3.1 × 10⁻⁷ S cm⁻¹ at room temperature), strong oxidation stability, excellent plasticity and good solubility to lithium salt. Additionally, the SN−based electrolyte is stable at low voltage and matches well with quinone electrode materials at low electrochemical window (~3.7 V) [20,21]. Thanks to these advantages, using SN−based electrolytes would make the rechargeable batteries have good electrochemical performance. Among our previous work [17], the conductivity of 5 mol% LiTFSI (lithium bis(trifluoromethane)sulfonimide)/SN was up to 4.58 mS cm⁻¹, which showed excellent electrochemical performance when matched with calix[6]quinone (C6Q) (405 mAh g⁻¹/500 cycles/0.1 C). This result well proves the above conclusion. However, C6Q needs to be synthesized in a strict experimental condition with oxygen−free. The entire reduction process is carried out under the protection of N₂ atmosphere, using oxygen−free water, and the yield is only about 9% [14,22,23]. Comparatively, the synthesis procedure of C4Q is simple, the experimental conditions are relatively loose and the yield of C4Q is more than 50% owing to the relatively matured synthesis technology. In short, compared with C6Q, C4Q is easier for mass production and practical application. Therefore, in this work, high−capacity organic cathode material C4Q and 5 mol% LiTFSI/SN PCE were selected as research objects to further explore their performance in the electrochemical cycle.