Amphiphilic-triblock-copolymer-derived protective layer for stable-cycling lithium metal anodes

https://doi.org/10.1016/j.jiec.2022.01.003Get rights and content

Abstract

Lithium metal batteries have shown great potential for the development of efficient energy storage devices. However, the uncontrollable growth of lithium metal dendrites results in poor cycling efficiency and severe safety concerns. In this study, amphiphilic triblock copolymers (P123) were used for the direct polymerization of silica precursors, resulting in a well-ordered mesoporous silica structure SBA-15 with a large surface area and excellent absorbent properties for the protective layer. The protective layer consisting of SBA-15 stores Li ions in a large number of pores as an electrolyte reservoir, which exhibits sufficient ionic conductivity. Furthermore, it ensures a spatially uniform Li-ion flux on the anode surface and plays an important role in the physical blocking of dendrite growth. As proof of concept, the SAB-15 protective layer anode demonstrates a high average Coulomb efficiency, rate capability, and cycle stability at 1 mA h cm−2 over 1200 cycles. Moreover, SAB-15 can be cycled stably for more than 750 cycles in symmetric cells. The results provide insights for implementing stable lithium metal anodes in next-generation batteries.

Introduction

Owing to carbon dioxide emission and the depletion of fossil fuel reserves in many parts of the world, there is an increasing need for eco-friendly energy devices. Several studies have investigated the development of high-performance energy storage devices. For example, lithium-ion batteries (LIBs) using graphite anodes have been rapidly commercialized because of their stable charge/discharge cycles [1], [2], [3]. However, LIBs have limited energy density; thus, it is necessary to study next-generation anode materials that can replace them. Lithium metal batteries (LMBs) have attracted considerable research attention as potential candidates owing to their low electrochemical potential (-3.040 V vs. standard hydrogen electrode) and high capacity (3860 mA h g−1), which is approximately 10 times higher than the capacity (372 mA h g−1) of LIBs [3], [4].

Nevertheless, LMBs are difficult to commercialize owing to their poor cycling performance and safety issues. Unlike graphite anodes, which store Li ions via intercalation and de-intercalation, lithium metal anodes are deposited on the electrode surface without a storage host [5]. During charge/discharge cycling, non-uniform deposition of lithium metal causes an uneven electric field and lithium nucleation, resulting in a dendritic morphology. Dendritic lithium causes depletion of electrolytes due to the expansion of electrode volume and formation of an unstable solid electrolyte interphase (SEI) layer, resulting in a low Coulombic efficiency (CE) and short circuit of the cell [5], [6], [7], [8], [9], [10], [11], [12].

Various studies have been conducted to suppress dendrite growth and improve the cycling efficiency. Zheng et al. [8] proposed the introduction of a coating layer of hollow carbon spheres to inhibit dendrite growth. A thin amorphous carbon layer does not increase the resistance when charges are transferred, but it has a high mechanical stiffness that can inhibit dendrites. Some protective layers have been shown to have high adhesion energy that can effectively suppress dendrites. These studies exhibited that an adhesion energy of 0.5 J m−2 can suppress dendrite growth as demonstrated by phase field simulation [9]. A LiAl-rich interfacial layer has a high adhesion energy of ∼1.52 J m−2 and can remarkably improve cycling stability compared to a bare Li metal anode. However, the artificial protective layer (APL) still does not solve the fundamental problem of non-uniform lithium metal deposition. Moreover, the structure provides a relatively low surface area and insufficient electrolyte reservoir [13], [14]. Considering that Li ions cannot be transferred immediately after reduction near the anode during discharging, a continuous supply from the bulk electrolyte to the interface is essential. This is because the concentration gradient and limited diffusion lead to the growth of dendritic metals. Therefore, the key to reducing the possibility of lithium metal dendrite formation is to accelerate the diffusion rate of Li ions from bulk electrolytes and eradicate the concentration gradient near the electrode [15], [16]. For this purpose, a new strategy of using a soft-rigid APL composed of PVDF-HFP and LiF, and simultaneously possessing high mechanical strength and ionic conductivity, was proposed [17]. The APL also possesses abundant hollow pores with a bimodal distribution of pore size, which provide fast Li-ion diffusion. Additionally, their sizes being much smaller than micrometer-sized Li dendrites, prevent the possibility of dendrite growth and penetration in the pores.

In this study, porous SBA-15 has been used as the APL, which acts as an electrolyte reservoir as well as a physical barrier. Hexagonal mesoporous SBA-15 can be used as an adsorbent because of its high specific surface area and well-ordered porous structure [18], [19]. These mesoporous compounds were prepared from a micellar solution using a Pluronic P123 block copolymer, and the silica precursor condensed at the external surface of the micelles. The SAB-15 protective layer anode (SPLA) adsorbs the electrolyte and delivers Li ions uniformly during discharge through highly aligned porous structures. Moreover, the SPLA made of silica particles with a modulus of elasticity as high as ∼73 GPa has a sufficiently high mechanical modulus to inhibit dendrite growth [20], [21]. Theoretically, a modulus of elasticity higher than 6 GPa can suppress the growth of dendrites [21]. The synthesized SBA-15 was coated on a copper current collector using a doctor blade and investigated using galvanostatic methods at different areal current densities in half, symmetric, and full cells.

Section snippets

Preparation of SBA-15

Microporous SBA-15 was prepared using the procedure outlined in a previous study [22]. Typically, 150 mL of 1.6 M HCl solution was added to a round-beaker containing 4 g of Pluronic P123, which is a poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 triblock copolymer. The mixture was stirred at 37 °C until complete dissolution was achieved. Next, 8.5 g of tetraethyl orthosilicate (TEOS) was added dropwise into the beaker under vigorous stirring. The mixture was then placed

Synthesis of SBA-15

The synthesis process for SBA-15 silica is shown in Fig. 1. A Pluronic P123 amphiphilic triblock copolymer, comprising two hydrophilic polyethylene oxide chains and one hydrophobic polypropylene oxide chain, was micellized at the critical micelle concentration [22], [23], [24], [25]. The hydrolysis and condensation of TEOS occurred, and some oligomers interacted with the ethylene oxide groups on the surface of the micelles. When the van der Waals interaction between the hydrophobic cores in the

Conclusion

The protective layer composed of SBA-15 was demonstrated to significantly improve the electrochemical performance of LMBs. SBA-15, which has a porous and large specific area, acts as an electrolyte reservoir and facilitates the transport of Li ions. The SPLA, which has a large number of functional –OH groups on the surface, undergoes an electrolyte uptake of ∼160%, which is caused by its high ionic conductivity. Furthermore, the results of the half-cell test revealed that the SPLA yields a

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 study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2021R1A4A2001403).

References (43)

  • Y. Nishi

    J. Power Sources

    (2001)
  • W. Xu et al.

    Energy Environ. Sci.

    (2014)
  • T. Yang et al.

    Energy Storage Mater.

    (2020)
  • G. Zheng et al.

    Energy Storage Mater.

    (2020)
  • B. Xu et al.

    Nano Energy

    (2020)
  • Y. Liu et al.

    Nat. Commun.

    (2016)
  • S. Park et al.

    Chem. Eng. J.

    (2022)
  • Y. Feng et al.

    Energy Storage Mater.

    (2020)
  • D.H. Jeon

    Energy Storage Mater.

    (2019)
  • K. Xu

    Chem. Rev.

    (2004)
  • D. Aurbach et al.

    J. Electrochem. Soc.

    (1996)
  • X.-B. Cheng et al.

    Chem. Rev.

    (2017)
  • G. Zheng et al.

    Nat. Nanotechnol.

    (2014)
  • R. Pathak et al.

    Nat. Commun.

    (2020)
  • P. Shi et al.

    Adv. Mater.

    (2019)
  • J. Xiao science 336 2019 10.1126/science.aay8672 426...
  • P. Bai et al.

    Energy Environ. Sci.

    (2016)
  • R. Xu et al.

    Adv. Funct. Mater.

    (2018)
  • T. Kang et al.

    J. Mater. Chem.

    (2004)
  • T.M. Albayati et al.

    Heliyon

    (2019)
  • J.M. Rimsza et al.

    J. Am. Ceram. Soc.

    (2014)
  • Cited by (0)

    View full text