Elsevier

Solid State Ionics

Volume 380, July 2022, 115923
Solid State Ionics

Star-shaped polyethylene glycol methyl ether methacrylate-co-polyhedral oligomeric silsesquioxane modified poly(ethylene oxide)-based solid polymer electrolyte for lithium secondary battery

https://doi.org/10.1016/j.ssi.2022.115923Get rights and content

Highlights

  • The star-shaped POSS-(PPEGMEM)8 was synthesized.

  • The ionic conductivity of the blends electrolyte is significantly improved.

  • The blends electrolyte has a wide electrochemical window and outstanding tLi+.

  • LiFePO4‖Li showed 90% capacity retention after 100 cycles at 60 °C at a rate of 0.1C.

Abstract

Poly(ethylene oxide) (PEO)-based solid polymer electrolyte (SPE), as a key component connecting the internal channels, can improve the safety performance for lithium secondary battery, but it is still challenged by limited ionic conductance due to high polymer crystallinity at room temperature. Here, we develop a SPE by blending PEO with star-shaped polymer of polyethylene glycol methyl ether methacrylate-co-polyhedral oligomeric silsesquioxane (POSS-(PPEGMEM)8), which is synthesized by atom transfer radical polymerization (ATRP) under octa (2-bromoisobutyryloxyethyl sulfide) octasilsesquioxane (POSS-Br8) as initiator and poly(ethylene glycol) methyl ether methacrylate (PEGMEM) as polymerization monomer. The unique star-shaped structure of POSS-(PPEGMEM)8 is helpful to increasing the segmental motion by reducing the crystallinity of PEO polymer and supporting more ion transport channels to obtain higher ion conductivity. The mixed electrolyte containing 40% POSS-(PPEGMEM)8 exhibits the highest ionic conductivity (1.4 × 10−4 S cm−1, 50 °C) with a Li+ transference number of 0.51 and a wide electrochemical window (5.0 V vs. Li/Li+) and a high mechanical strength. The LiFePO4||Li battery, assembled with PEO/POSS-(PPEGMEM)8–40%, carries out 100 cycles at a current density of 0.1C, provides a stable reversible capacity of 129.4 mAh g−1 and a high capacity retention rate due to the high ratio of free volume of polymer electrolyte formed by the star-shaped POSS-(PPEGMEM)8, which indicates that PEO/POSS-(PPEGMEM)8 has good application prospects in high-performance all-solid-state lithium secondary battery.

Introduction

The rechargeable lithium-ion batteries (LIBs) are widely utilized in portable electronics, electric vehicles and large-scale grid applications since its high operating voltage, high energy density and little memory effect [[1], [2], [3], [4]]. The electrolyte system in the battery is an important component to connect anode and cathode and provide ion channels [5]. However, safety issues caused by volatility and flammability of liquid electrolyte have always been a stumbling block for the application of LIBs [[6], [7], [8], [9]]. The solid polymer electrolytes (SPEs), which mix lithium salts into polymer matrix, could reduce the security risk and increase the power density [9]. Poly(ethylene oxide) (PEO) was extensively investigated in solid electrolyte compared with polyvinylidene fluoride (PVDF) [10], polymethyl methacrylate (PMMA) [11] and polyacrylonitrile (PAN) [12] due to the low glass transition temperature (Tg) and excellent “dissolving” alkaline salt properties [13]. However, the high crystallinity of the PEO polymer chain prevents the amorphous regions surrounded by weak conductivity crystal region from forming an interconnected conductive network, resulting in extremely low ionic conductivity at ambient temperature [14,15].

Polymer blending can suppress crystallization of PEO to improve ion conductivity of SPEs [16]. Patla et al. observed that a bit PVDF in PEO could enhance the ion-polymer interaction to form more dissociated ions to increase the ionic conductivity to 10−6 S cm−1 at room temperature. The PVDF acts as a plasticizer to reduce the proportion of crystalline phase in the matrix and enhance the ionic conduction. However, the content of PVDF has a greater impact on the electrolyte performance and the PVDF–rich electrolyte exhibited low ionic conductivity owing to the complex microstructure [17]. Inorganic particle doping is another a prominent and feasible technique to increase the ionic conductivity of SPEs [18,19]. PMMA-b-PS-based electrolyte modified by organic rectorite (OREC) was studied in our previous work [20]. The OREC in the electrolyte increased the ionic conductivity by 18 times at ordinary temperature. Ding et al. prepared an electrolyte with 5% MnO2 in the PEO. The inorganic particles in the matrix change the environment of the polymer chain, leading to a higher ionic conductivity (1.5 times) than pristine PEO solid electrolyte at 60 °C [21]. Nevertheless, the inorganic nanoparticles with high surface energy could cause the aggregation of excessive particles in the polymer matrix, which hinders the segmental motion of PEO, resulting in the degradation of electrolyte performance [22].

Polyhedral oligomeric silsesquioxane (POSS) is a special organic-inorganic hybrid nanoparticle with a cage structure composed of Si-O-Si and an organic substituent covalently bonded to silicon [[23], [24], [25], [26], [27]]. The organic groups of poly(ethylene glycol) (PEG) with a low molecular weight at each silicon of POSS core can promote the dispersion of POSS in polymer to enhance the compatibility with PEO matrix, which could reduce the crystallinity to increase the ionic conductivity. Ma and co-workers synthesized a star-shaped POSS-(PEGw)8 (w = 1 K g mol−1) with linear PEG on eight silicon vertices [28]. The ionic conductivity of the composite electrolyte containing 60% POSS-(PEG1K)8 in PEO was two orders of magnitude higher than that of pure PEO at room temperature. The further researches have shown that linear polymer chains with different degrees of polymerization on the POSS polymer arms can affect the crystallinity of the electrolyte by changing the spatial freedom of the POSS polymer to affect the electrical properties [29].

In this work, the large free volume star-shaped polyethylene glycol methyl ether methacrylate-co-polyhedral oligomeric silsesquioxane (POSS-(PPEGMEM)8) with comb PEG chains on the eight silicons of the POSS core was synthesized by atom transfer radical polymerization (ATRP) to blend with PEO to reduce the crystallinity of PEO and improve Li+ conductivity of PEO electrolyte. The star-shaped polymer with comb-shaped PEG segments on the arms has larger free volume, better fluidity and compatibility with electrodes than linear polymers with similar structures, which is helpful for improving ionic conductivity and stability of electrolyte [30]. It's found that when the POSS-(PPEGMEM)8 reaches 40%, the ionic conductivity of blends electrolyte at 50 °C is 8 times higher than that of pure PEO. Simultaneously, the all-solid-state lithium second battery assembled with PEO/POSS-(PPEGMEM)8–40% has good cycle stability at 60 °C. Our work indicates that the blends electrolyte has a bright future in promoting the commercial application of high performance all-solid-state batteries.

Section snippets

Materials

PEO (Mw = 1000,000), acetonitrile (99.5%), Poly(ethylene glycol) methyl ether methacrylate (PEGMEM, average Mw = 475) and Lithium perchlorate (LiClO4, 98%) were purchased from Aladdin Industrial Co., Ltd. Cuprous bromide (CuBr) and N,N,N′,N″,N″-Pentamethyldiethylenetriamine (PMDETA) were received from Aladdin Industrial Co., Ltd. 1,4-dioxane was supplied by Guangdong Guanghua Sci-Tech Co., Ltd. PEO and LiClO4 were dried under vacuum at 60 °C for 24 h before use. All other chemicals were used as

Structural characterization of POSS-(PPEGMEM)8

The large volume and high spatial freedom of POSS-(PPEGMEM)8 was synthesized by ATRP of POSS-Br8 with eight identical active sites and polymerized monomer PEGMEM (Fig. 1(a)). Structural information for POSS-(PPEGMEM)8 is determined by FT-IR spectroscopy and 1H NMR analysis. As shown in Fig. 1(b), the absorption peaks at 2956 cm−1 and 1446 cm−1 are assigned to the Csingle bondH groups and the broad peaks at 1731 cm−1 is attributed to the carbonyl stretching vibration of Cdouble bondO groups on PEGMEM, and the

Conclusion

In summary, a high-performance poly(ethylene oxide) (PEO) based polymer electrolyte can be prepared by blending PEO with a star-shaped polyethylene glycol methyl ether methacrylate-co-polyhedral oligomeric silsesquioxane (POSS-(PPEGMEM)8), which is synthesized by grafting polyethylene glycol methyl ether methacrylate (PEGMEM) on the polyhedral cage silsesquioxane groups via atom transfer radical polymerization. The PEO/POSS-(PPEGMEM)8 with 40% POSS-(PPEGMEM)8 possesses high ionic conductivity

CRediT authorship contribution statement

Jingyu Ma: Conceptualization, Methodology, Validation, Investigation, Formal analysis, Writing – original draft. Xiaoyan Ma: Investigation, Formal analysis, Validation, Writing – review & editing. Qi Zhang: Investigation, Formal analysis, Validation, Writing – review & editing. Xinghua Guan: Investigation, Formal analysis, Validation, Writing – review & editing. Fang Chen: Conceptualization, Methodology, Formal analysis, Writing – review & editing, Supervision, Project administration. Peiran

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 NSAF (Grant No. U2130118) and the Natural Science Foundation Joint program of the Shaanxi province (2020GY-297).

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