Single Li ion conducting solid-state polymer electrolytes based on carbon quantum dots for Li-metal batteries
Graphical Abstract
A new type of CQD is synthesized by pyrolysis, and the corresponding SLIC-SPEs is also developed. The large size of CQD anions makes their migration in the polyethylene oxide (PEO) matrix difficult. The SPE possesses a high lithium ion transport number (0.9446) and room-temperature ionic conductivity of 2.02 × 10−4 S cm−1. This material shows outstanding power performance and cycling stability in a battery prototype.
Introduction
Li-metal batteries (LMBs) are important the next-generation energy storage devices due to their high energy densities [1]. However, traditional commercial liquid electrolytes cannot inhibit the formation of lithium dendrites, and the flammable organic solvents pose a potential safety hazard [2], [3]. These issues have hindered the development of LMBs in the automotive industry and in large-format batteries. In comparison, Solid polymer electrolytes (SPEs) have many comparative advantages, including greater safety, excellent flexibility and improved system energy density from their low weights [4], [5], [6], [7], [8]. SPEs shaped by melt processing show good interfacial contact with the Li anode, facilitated by the molten-state fluidity [9], [10], [11], [12]. Further, flexible and wearable electronics, such as smart watches, artificial skins, implantable medical devices and epidermal sensors were bright into social life, it is urgent to develop reliable power sources with high flexibility and stretchability [13], [14]. Due to good flexibility and durability, SPEs have shown great potential application in flexible high-density power sources [15], [16], [17].
Conventional SPEs comprise polymer hosts, such as polyethylene oxide (PEO) and lithium salts. In these dual-ion conductors, both the anions and Li+ move under an electric field. However, Li+ contributes only ~ 20% of the overall ionic current because of its low motility [18]. This results in a strong concentration gradient that has deleterious effects, such as favored dendritic growth and limited power delivery. Single lithium ion conducting SPEs (SLIC-SPEs) are able to resolve the formation of lithium dendrites and other safety problems. The common approaches to fabricate SLIC-SPEs are classified into three types: (1) covalent grafting of anions to the polymer backbone, (2) attaching anions to an inorganic backbone, and (3) adding anion acceptors to the polymer chains to limit the motion of anions [18], [19], [20], [21], [22], [23].
As early as the 1980s, Bannister et al. proposed using two polyanionic addition polymers containing alkyl sulfonic acid and perfluoroalkyl carboxylic acid side groups to inhibit anion mobility in SLIC-SPEs. While the fabricated SPE had a high Li+ transport number (tLi+), the very low dissociation of carboxylate anion and Li+ led to low ion conductivities even at elevated temperatures (10−6 S cm−1 at 60 °C) [20]. On the other hand, the high room-temperature crystallinity of PEO restricts Li+ transport, further affecting ion conduction [24], [25], [26]. Bouchet et al. designed a single-ion conductor based on triblock copolymers. Poly(styrene trifluoromethanesulfonylimide of lithium) was used as the anion, because its structure allowed negative-charge delocalization; the values of tLi+ and conductivity reached 0.85 and 1.3 × 10−5 S cm−1 at 60 °C, respectively [18]. Our group synthesized a single-ion polymer conductor with an alternating structure of maleic anhydride and lithium 4-styrenesulfonyl(phenylsulfonyl)imide [27]. SPEs demonstrated a high ionic conductivity and Li+ transport number (δLi+ = 3.08 × 10−4 S cm−1 and tLi+ = 0.93) [28]. In order to improve the rate capability, the lithium salt was modified with ethyl glycinate, and the δLi+ reached to 1.42 × 10−4 S cm−1 at 25 °C [29]. However, despite their good electrochemical properties, the synthesis of these SPEs involves costly raw materials and complex procedures, and their mechanical properties are also poor.
Adding nanofillers to a polymer matrix is a proven effective strategy to improve the electrochemical properties and thermal stabilities of materials without reducing their mechanical properties [30], [31], [32], [33], [34], [35], [36], [37], [38]. Lee et al. synthesized the core-shell structured SiO2(Li+) nanoparticles with uniform spherical shape and used as functional fillers in the composite gel polymer electrolytes. The SiO2(Li+) particles are intrinsic single ion conductors, since the sulfonate anions (–SO3−) are anchored to pendant group on the polymer around silica core. The generation of a free volume at the interface of the dispersed SiO2(Li+) nanoparticles may also contribute to the improved ionic conductivity. In addition, they also found that the tLi+ and δLi+ decreased with the increase of the particle size of SiO2. This may be related to the accumulation of bulky SiO2 (> 100 nm) insulating particles that reduces the lithium ions transport channel space as well as uneven distribution of Li+ [39], [40]. In hence, using a smaller nanostructured material instead of SiO2 may significantly enhance tLi+ and δLi+. For instance, Wei et al. reported a novel carbon nanostructure of carbon quantum dots (CQDs) in a PEO matrix, and used it to fabricate a composite solid electrolyte. Their results showed that the addition of CQDs helps reduce the crystallinity of the system and significantly improve the room temperature conductivity and ductility of SPEs [41]. However, they only added CQDs to the system as nano-filler, CQDs and Li+ conductor without closely linking, result in a low tLi+. Therefore, anchoring the anion on the surface of the CQDs is an effective method to limit the movement of the anion and increase the tLi+; while providing a large free volume at the interface may further improve the δLi+ due to the smaller particle size of CQDs (diameter < 10 nm) is more favorable for the uniform distribution of Li+ than SiO2 at the nanometer scale.
Based on these considerations, in this study, we design a novel lithium conductor based on CQDs and incorporate it with a PEO matrix to form SPEs. The CQDs were synthesized by single-step pyrolysis of poly(lithium 4-styrene sulfonate) (PLSS) and citric acid (CA), and named PLSSCQD. Unlike previous attempts to reduce the movement of anions by binding them, this method simply increases the volume of individual anions. This means that SLIC-SPEs can be prepared without complex synthesis and that different polymer matrix can be more freely selected for various application conditions. Meanwhile, the generation of a free volume at the interface of the dispersed CQDs may also contribute to the improved ionic conductivity. The addition of CQDs also reduce the crystallinity of SPEs while making the membranes more flexible, which both inhibit the growth of lithium dendrites and hinder puncturing of the electrolyte, which can cause battery short-circuit. PLSS and CA are commercially available and inexpensive, the CQDs prepared by this method are stable and efficient, and the battery cycling tests reveal superior cycle stability and power performance. The CQD SPEs also exhibit excellent environmental stability and the SPE membranes can be prepared in air. Considering the above advantages, these CQD-based SPEs are promising candidates for next-generation functional electrolytes in lithium batteries.
Section snippets
Preparation of PLSSCQD
The preparation of synthesis of the CQDs by pyrolysis has been widely reported in the previous studies. Herein, we design a type of CQDs on the surface passivated by PLSS. Fig. 1a depicts the synthesis and molecular configuration of PLSSCQD prepared by the pyrolysis of low-cost PLSS and CA at a temperature high enough for CA–CQD conversion but too low to allow the decomposition of PLSS (Fig. S1) [42]. Thus, the main chemical structure and groups of PLSS were retained [43]. The detail is
Characterization of PLSSCQD
In order to prove the successful preparation of CQDs, we observed the morphology of the samples and verified their photoluminescence properties. TEM images of PLSSCQD showed well-dispersed spherical particles with an average size of 2.90 nm (Fig. 1b) and a normal size distribution (Fig. 1c). Here, the anions are CQDs with diameters of 1.8–4.2 nm, exceeding those of traditional anions by ~10 × (the effective radii of typical diffusing species are 0.229–0.402 nm) [44], [45]. Steric hindrance from
Conclusions
In summary, SLIC-SPE membranes with different PLSSCQD contents were prepared from safe and low-cost materials without using complex chemical synthesis steps. The PLSSCQD could be produced in larger batches than those possible by conventional methods. The large anion size in the PLSSCQD/PEO membrane and the low crystallization enhanced δLi+ and tLi+. Among the prepared membranes, the one with PLSSCQD:PEO = 4:10 (w/w) had the highest room-temperature δLi+ (2.02 × 10−4 S cm−1 at room temperature)
CRediT authorship contribution statement
Zeyu Li: Conceptualization, Methodology, Investigation, Writing - original draft preparation. Feng Liu: Visualization, Investigation. Shao shan Chen: Investigation, Formal analysis. Fei Zhai: Validation, Formal analysis. Yu Li: Supervision, Visualization. Yiyu Feng: Supervision. Wei Feng: Supervision, Visualization, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the National Key R&D Program of China (No. 2016YFA0202302), the State Key Program of National Natural Science Foundation of China (No. 51633007), and the National Natural Science Foundation of China (No. 51773147 and 51973151).
Zeyu Li received his BS degree in Chemistry from Zhengzhou University (China) in 2015 and completed his MS degree under the supervision of Prof. Kun Dai at Zhengzhou University (China) in 2017. He is now pursuing Ph.D. under the tutelage of Prof. Wei Feng at the School of Materials Science and Engineering, Tianjin University. Currently, his research is focused on carbon dots and solid polymer electrolytes.
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Cited by (0)
Zeyu Li received his BS degree in Chemistry from Zhengzhou University (China) in 2015 and completed his MS degree under the supervision of Prof. Kun Dai at Zhengzhou University (China) in 2017. He is now pursuing Ph.D. under the tutelage of Prof. Wei Feng at the School of Materials Science and Engineering, Tianjin University. Currently, his research is focused on carbon dots and solid polymer electrolytes.
Feng Liu is currently a master of polymer materials and engineering in the Functional Organic Carbon Composites Laboratory of Tianjin University, under the supervision of Professor Wei Feng. His research interests include the synthesis, photoluminescence mechanism and applications of carbon dots.
Shaoshan Chen received his BS degree in Polymer Science and Engineering from Beijing University of Chemical Technology (China) in 2014. He joined in the School of Materials Science and Engineering, Tianjin University to pursue Ph.D. with Prof. Wei Feng and Yiyu Feng in 2018 on solid polymer electrolytes for lithium metal batteries.
Fei Zhai received his BS degree in Chemistry from Jilin University (China) in 2009 and completed his MS degree under the supervision of Prof. Haizhu Sun at Northeast Normal University (China) and State Key Laboratory of Supramolecular Structure and Materials (Jilin University, China). He is now pursuing Ph.D. under the tutelage of Prof. Wei Feng at the School of Materials Science and Engineering, Tianjin University. Currently, his research is focused on photo-responsive molecules and polymers, modified graphene, and nanocomposites applied for solar-thermal fuels and stimuli-response.
Yu Li is a lecturer at the School of Materials Science and Engineering, Tianjin University. He obtained his Ph.D. degree from the same department of Tianjin University in 2011 and he worked as a postdoctoral research fellow at Tianjin University from 2011 to 2014. His current research is focused on conductive polymer and nano energy storage materials.
Yiyu Feng is a professor in the School of Materials Science and Engineering at Tianjin University. He obtained his Ph.D. from Tianjin University in 2009 and held an academic position at Tianjin University. He has authored and co-authored over 100 academic articles and reviews. Currently, his research is focused on carbon-based materials or composites for solar-thermal fuels, interfacial heat dissipation and structural self-healing.
Wei Feng is a professor at the School of Materials Science and Engineering in Tianjin University. He obtained his Ph.D. degree from the Xi’an Jiaotong University (China) in 2000. Then, he worked at Osaka University and Tsinghua University as a JSPS fellow and postdoctoral researcher, respectively. In 2004, he became a full professor at Tianjin University. He has obtained the support of the National Science Fund for Distinguished Young Scholars in China. His research interests include photo-responsive organic molecules and their derivatives, thermal-conductive and high-strength carbon-based composites, and two-dimensional fluorinated carbon materials and polymers.