Research Article: High Ion‐Conducting Solid‐State Composite Electrolytes with Carbon Quantum Dot Nanofillers

Date Published: March 01, 2018

Publisher: John Wiley and Sons Inc.

Author(s): Cheng Ma, Kuan Dai, Hongshuai Hou, Xiaobo Ji, Libao Chen, Douglas G. Ivey, Weifeng Wei.

http://doi.org/10.1002/advs.201700996

Abstract

Solid‐state polymer electrolytes (SPEs) with high ionic conductivity are desirable for next generation lithium‐ and sodium‐ion batteries with enhanced safety and energy density. Nanoscale fillers such as alumina, silica, and titania nanoparticles are known to improve the ionic conduction of SPEs and the conductivity enhancement is more favorable for nanofillers with a smaller size. However, aggregation of nanoscale fillers in SPEs limits particle size reduction and, in turn, hinders ionic conductivity improvement. Here, a novel poly(ethylene oxide) (PEO)‐based nanocomposite polymer electrolyte (NPE) is exploited with carbon quantum dots (CQDs) that are enriched with oxygen‐containing functional groups. Well‐dispersed, 2.0–3.0 nm diameter CQDs offer numerous Lewis acid sites that effectively increase the dissociation degree of lithium and sodium salts, adsorption of anions, and the amorphicity of the PEO matrix. Thus, the PEO/CQDs‐Li electrolyte exhibits an exceptionally high ionic conductivity of 1.39 × 10−4 S cm−1 and a high lithium transference number of 0.48. In addition, the PEO/CQDs‐Na electrolyte has ionic conductivity and sodium ion transference number values of 7.17 × 10−5 S cm−1 and 0.42, respectively. It is further showed that all solid‐state lithium/sodium rechargeable batteries assembled with PEO/CQDs NPEs display excellent rate performance and cycling stability.

Partial Text

Solid polymer electrolytes (SPEs) for all solid‐state lithium/sodium‐ion rechargeable battery applications have been extensively investigated to satisfy several special requirements for new generation energy storage devices.1, 2 Compared with a traditional liquid electrolyte system that has safety risks caused by leakage of flammable and volatile organic solvents, SPEs possess some distinct advantages.3 In addition to improved safety, SPEs have excellent thermal and electrochemical stability which can extend their operating conditions to higher temperatures and higher working voltages. In addition, their flexibility allows for assembly of batteries in various package styles and their mechanical strength enables SPEs to mitigate volumetric expansion of active electrode materials and block the growth of Li dendrites.4, 5 Hence, innovative and optimized SPEs are becoming increasingly attractive, but they face several challenges.

Well‐dispersed CQDs with oxygen‐containing functional groups were fabricated through a simple and low energy consumption method. Highly dispersed CQDs and strong Lewis acid–base interactions effectively increased the dissociation degree of LiClO4 or NaClO4, adsorption of ClO4− anions as well as the amorphous phase content of PEO. As such, nanocomposite PEO‐based polymer electrolytes of PEO/CQDs‐Li and PEO/CQDs‐Na exhibit high ionic conductivities of 1.39 × 10−4 and 7.17 × 10−5 S cm−1, respectively, and a high Li ion transference number of 0.48 (PEO/CQDs‐Li) and Na ion transference number of 0.42 (PEO/CQDs‐Na). Moreover, PEO/CQDs NPEs with excellent electrochemical characteristics enable the fabrication of all solid‐state Li and Na batteries with significantly enhanced cycling stability and rate performance. Both the simple fabrication process and the outstanding electrochemical performance of PEO/CQDs NPEs make them promising candidates for application in next generation solid‐state rechargeable batteries.

Preparation of CQDs: The CQDs were prepared by a facile aldol condensation process, as reported in our previous study.13 Specifically, 8 g of NaOH was dissolved into 30 mL of acetone (C3H6O, analytical reagent (AR) grade) under constant stirring for 1 h, followed by natural aging at ambient temperature in air. After 96 h, the resulting solid mixture was neutralized with dilute HCl solution, separated centrifugally, and washed with deionized water. Then the final product was dried in a vacuum at 100 °C for 12 h producing a brown CQD powder.

The authors declare no conflict of interest.

 

Source:

http://doi.org/10.1002/advs.201700996

 

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