Fast Li+ transport and superior interfacial chemistry within composite polymer electrolyte enables ultra-long cycling solid-state Li-metal batteries
Graphical abstract
A 3D ion-conducting metal-organic framework (MOF)-based network is rationally constructed in PEO-based composite polymer electrolyte, where the in-situ growth of densely packed MOF chains provides continuous pathways for fast Li+ transport, and the ionic liquid encapsulated in MOFs significantly improves the interfacial compatibility and Li+ migration kinetics, resulting in superior ionic conductivity and ultra-long cycling solid-state Li-metal batteries at room temperature.
Introduction
In the past ten years, rechargeable lithium metal batteries (LMBs), including Li-S and Li-O2 batteries, have become the most promising candidates for next-generation high-performance energy storage systems owing to the high specific capacity (3860 mAh g−1) and the lowest electrochemical potential (−3.04 V vs standard hydrogen electrode) of lithium metal [1], [2], [3], [4], [5], [6]. However, the conventional liquid electrolytes are not only highly flammable, but also spontaneously react with Li metal, which leads to the formation of lithium dendrites during cycling that can cause short circuits or even explosions [7,8]. Therefore, it is of great importance to develop safe and stable solid-state electrolytes (SSEs) to improve the safety of LMBs [9], [10], [11]. Generally, SSEs can be divided into two main categories: inorganic SSEs (including ceramic and sulfide-based electrolytes) and solid-state polymer electrolytes (SPEs) [9,12,13]. Although the inorganic SSEs have high ionic conductivity and thermal stability, their brittleness and poor interfacial compatibility make them difficult to be fabricated on large-scale and be deployed in practical applications [[13], [14]]. Comparing to inorganic SSEs, SPEs feature high flexibility, good processibility, light-weight, and excellent interfacial compatibility with the electrodes, holding the great promise for practical application in solid-state LMBs [[13], [15], [16], [17], [18], [19], [20]].
Unfortunately, suffering from low ionic conductivity, conventional SPEs cannot meet the requirements of operating solid-state LMBs at room temperature (RT) [9,18,19]. In addition, the inhomogeneous Li deposition would lead to the formation of Li dendrites, which has been considered as a long-term primary challenge for achieving long lifespan to the solid-state LMBs. Among various SPEs, poly(ethylene oxide) (PEO)-based SPEs have attracted intensive investigation owing to their high flexibility, excellent solvation capability for lithium salts, and good mechanical/chemical stability towards lithium metal [[16], [18], [21], [22], [23]]. However, PEO-based SPEs normally has low ionic conductivity (≤ 10−6 S cm−1) at RT because of high crystallinity of PEO, which restricts their practical application in solid-state LMBs [13,18,24].
Incorporation of inorganic fillers, such as SiO2 [17], CeO2 [18], and Li5La3Zr2O12 (LLZO) [16], into PEO matrix has been recognized as an effective approach not only reduces the crystallinity of PEO that enhances the ionic conductivity, but also improves its electrochemical stability and mechanical robustness. Compared with conventional inorganic fillers, metal-organic frameworks (MOFs) with tunable pores, versatile functionality, high surface area, and periodical crystalline structure offers great opportunities for manipulating ion transport, which have shown great potential to improve the properties of PEO-based composite polymer electrolytes (CPEs) [25,26]. To be specifically: (1) the pore structures (e.g., pore diameter, pore shape) of MOFs can be designed to restrict the transport of anions; (2) the tunable surface functionalities (e.g., Lewis acidity, polarity) promise a preferential anchoring of ether bond, which lowers the crystallinity of PEO; (3) the ultra-high surface area of MOFs enables a sufficient contact with lithium salts and PEO, promoting the ion transport; (4) the periodical crystalline structure of MOFs contributes to the homogeneous Li+ plating, which prevents the growth of Li dendrites.
However, by simply adding MOF nanoparticles to the PEO matrix to form MOF/PEO CPEs (Scheme 1a) [27,28], the ionic conductivity improvement is generally very limited (≤ 10−5 S cm−1 at RT), which is ascribed to the inherent agglomeration effect of nanoparticles as well as failure of forming continuous Li+ transport pathways. In addition, the randomly distributed MOFs in CPEs are not able to offer sufficient mechanical enhancement to suppress the growth of Li dendrites. To address these above issues, research efforts have been carried out to the spatial arrangement of the incorporated MOFs in CPEs by constructing 3D MOF networks, with the aim to build 3D continuous ion transport channel (Scheme 1b) [29,30]. For instance, Li et al. [29] recently developed a 3D interconnected MOF network-based SSE via electrospinning of MOFs, of which the 3D MOF network not only improves the ionic conductivity to 2.89 × 10−4 S cm−1, but also provides structural reinforcement to enhance the mechanical strength, enabling the stable operation of LMBs at 60 °C. Nevertheless, despite the significant reduce in Li+ migration barrier and avoidance in MOFs agglomeration, the ionic conductivity of the 3D MOF-based SSEs can only reach 4.87 × 10−5 S cm−1 at 30 °C, owing to the sluggish Li+ migration between different MOFs particles as well as between MOFs and polymer, which restricts their application at RT. Therefore, it is still in an urgent demand to construct a continuous Li+ transport channel between MOF particles and reduce the interfacial resistance between MOFs and PEO, which is of vital importance to realize the operation of PEO-based solid-state LMBs at RT.
Herein, we reported a rational design strategy to address the above challenges that significantly improves Li+ migration kinetics as well as reduces interfacial resistance in the PEO-based CPEs. As illustration in the Scheme 1c, to provide continuous linear pathways for ion transport, the MOF nanoparticles were first in-situ grown on the polyacrylonitrile (PAN) fibers to form closely packed MOF chains. Furthermore, by encapsulating ionic liquid (IL) in in-situ growth MOF chains, the rich pores of MOFs with confined IL transform MOFs from inert conductors to efficient ionic conductors, affording well-defined and continuous ion-conducting network for fast Li+ migration. Meanwhile, the anchoring of the anion by MOF metal sites and the confinement of the IL would contribute to uniform distribution of Li+ flux and high Li+ transference number (tLi+). Subsequently, infiltrated with PEO SPE, the resulting MOF@PAN/PEO/IL CPE exhibited a high ionic conductivity of 2.57 × 10−4 S cm−1 and high tLi+of 0.59 at 25 °C. Moreover, the rational designed MOF@PAN/PEO/IL CPE not only endows stable Li plating/stripping for over 600 cycles, but also enables the stable operation of various state-of-the-art cathodes, including LiFePO4 (LFP), LiNi0.8Co0.1Mn0.1O2 (NCM811), and 1,1′-iminodianthraquinone (IDAQ). Especially, the solid-state Li||LFP cell can stably cycle for over 2500 cycles with a high capacity retention of 86% at RT.
Section snippets
Preparation of MOF@PAN fibers
MOF@PAN fibers were prepared by electrospinning of PAN and MOF precursor solution first, and in-situ growth of MOFs later. Typically, 2.346 g PAN, 1.173 g 2-MIL and 19.092 g DMF were loaded in a glass reactor and stirred at room temperature to form a yellow transparent viscous solution. Transferred the solution to a syringe and the electrospinning was carried out. The size of electrospinning needle was 25 G, the voltage was 11 kV, and the flow rate was 0.8 mL/h. The obtained PAN fibers were
Rational design of CPEs with highly efficient ion-conducting network
PEO is one of the most promising polymers for SPEs due to its excellent compatibility with Li-metal and lithium salts [25,31,32]. However, the low ionic conductivity (<10−6 S cm−1, 25 °C) limits its practical application. As shown in Scheme 1a, previous studies have attempted to incorporate various MOFs to reduce the crystallinity of PEO (i.e., MOF/PEO CPE) [25,26,33,34]. However, in lack of continuous ion transport channels due to the existence of abundant interfaces between MOFs as well as
Conclusion
In summary, a PEO-based composite electrolyte (e.g., MOF@PAN/PEO/IL CPE) with highly efficient 3D ion-conducting network was rationally designed. The MOF nanoparticles in-situ grown on PAN fibers were densely packed to form MOF chains, which provide continuous pathways for Li+ transport. Moreover, the ionic liquid (IL) encapsulated in MOFs not only significantly reduces the energy barrier to enable continuous and fast Li+ migration, but also elegantly improves the interfacial compatibility for
CRediT authorship contribution statement
Xue-Liang Zhang: Conceptualization, Methodology, Formal analysis, Investigation, Writing – original draft. Fang-Ying Shen: Investigation, Validation, Software. Xin Long: Validation, Methodology, Software. Siyan Zheng: Methodology, Investigation. Zhiqin Ruan: Formal analysis, Validation. Yue-Peng Cai: Supervision, Writing – review & editing, Funding acquisition. Xu-Jia Hong: Supervision, Formal analysis, Writing – review & editing, Funding acquisition. Qifeng Zheng: Conceptualization,
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 supported by the National Natural Science Foundation of China (22001082), Natural Science Foundation of Guangdong Province (2019A1515010841, 2019A1515011460, 2019B1515120027, 2022B1515020005), Department of Science and Technology of Guangdong Province (2017B090917002, 2019A050510038, 2020B0101030005, 2021A0505030063), Guangzhou Science and Technology Association Young Talents Promotion Project (X20210201043), Basic and Applied Basic Research Projects of Guangzhou (202102020624).
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These authors contributed equally to this work.