Communication Uniform-dispersed ZnS quantum dots loading on graphene as a promising anode for potassium-ion batteries Yaqin Qi a,b , Yong Yang a,b , Qian Hou a,b , Kun Zhang a,b , Hui Zhao a,b , Haijun Su a,b , Lijiao Zhou a,b , Xingrui Liu b , Chao Shen a,b, *, Keyu Xie a,b a Research & Development Institute of Northwestern Polytechnical University in Shenzhen, Northwestern Polytechnical University, Shenzhen 518057, China b State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU), Xi’an 710072, China Diff tag not allowed inside this tag A R T I C L E I N F O Article history: Received 2 July 2020 Received in revised form 1 August 2020 Accepted 18 August 2020 Available online xxx Keywords: ZnS quantum dots Morphology-controllable Volume effect Anode Potassium-ion batteries A B S T R A C T The potassium-ion batteries (PIBs) have become the promising energy storage devices due to their relatively moderate cost and plenteous potassium resources. Whereas, the main drawback of PIBs is unsatisfactory electrochemical performance induced by the larger ionic radius of potassium ion. Herein, we report a well-designed, uniform-dispersed, and morphology-controllable zinc sulfide (ZnS) quantum dots loading on graphene as an anode in the PIBs. The directed uniform dispersion of the in-situ growing ZnS quantum dots (2.8 nm in size) on graphene can mitigate the volume effect during the insertion- extraction process and shorten the migration path of potassium ions. As a result, the battery exhibits superior cycling stability (350.4 mAh/g over 200 cycles at 0.1 A/g) and rate performance (98.8 mAh/g at 2.0 A/g). We believe the design of active material with quantum dot-minimized size provides a novel route into PIBs and contributes to eliminating the major electrode failure issues of the system. © 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved. Recently, PIBs have been widely researched as an extremely ascendant energy technology resulted from the moderate price and abundant potassium resources [1]. Whereas, the sluggish electrochemical reaction dynamics and electrode pulverization caused by the bigger radius of potassium-ion (r(K + ) =1.38 Å) have severely hindered its development [1–4]. Up to now, only a few materials including carbon-based [5–8], alloying/conversion-type [9–13] and organic composite [14] anodes have been reported for PIBs. Thus, the accelerated research of anode materials with small volume change and excellent cycle stability is urgent. Metal sulfides, with the features of non-toxic, low cost, high theoretical capacity, are a kind of materials commonly used as the anode in lithium/sodium-ion batteries [15–18]. However, their stupendous volume variations and intrinsic insulation have resulted in structural disruptions and poor electrochemical performance. Especially, when using metal sulfides as anode for PIBs, the electrode deterioration caused by the tardiness of K + migration will be amplified [19,20]. Minimizing the size of metal sulfides could be a possible way towards these problems. Considering that the bitty diameter (< 10 nm) of quantum dots is beneficial to the electron/ion migration, it will be helpful to obtain excellent potassium ion storage performance [21,22]. As a proof-of-concept, the uniform-dispersed ZnS quantum dots loading on reduced graphene oxide (ZnS QDs-rGO) composite was successfully prepared in this work [23–27]. ZnS quantum dots with the average diameter as small as 2.8 nm were in situ formed on rGO nanosheets. The design of minimized-dots loading on the conductive matrix can combine the advantages of quantum dots and graphene showing shorter potassium ions diffusion channels, smaller volume expansion, and higher electrode conductivity. On this basis, the electrochemical performance and reaction mecha- nism of ZnS QDs-rGO anode were studied. Eventually, the anode delivers an outstanding capacity of 122 mAh/g at 1.0 A/g over 500 cycles. The ZnS QDs-rGO composite was prepared through a solvo- thermal method (Fig. 1a). In the ethylene glycol solvent, the surface of graphene oxide shows a negative charge state due to its rich oxygen-containing groups [28,29]. During solvothermal processes, the graphene oxide adsorbs the positively charged zinc ions at first. Then, the zinc ions in situ react with sulfur ions to form anchored ZnS quantum dots. Meanwhile, the graphene oxide was reduced in the process [29,30]. Although these oxygen-containing groups * Corresponding author at: Research & Development Institute of Northwestern Polytechnical University in Shenzhen, Northwestern Polytechnical University, Shenzhen 518057, China. E-mail address: shenchao@nwpu.edu.cn (C. Shen). https://doi.org/10.1016/j.cclet.2020.08.030 1001-8417/ © 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved. Chinese Chemical Letters xxx (2020) xxx–xxx G Model CCLET-5807; No. of Pages 4 Please cite this article in press as: Y. Qi, et al., Uniform-dispersed ZnS quantum dots loading on graphene as a promising anode for potassium- ion batteries, Chin. Chem. Lett. (2020), https://doi.org/10.1016/j.cclet.2020.08.030 Contents lists available at ScienceDirect Chinese Chemical Letters journal homepa ge: www.elsevier.com/locate/cclet