In Situ Crosslinked Gel Polymer Electrolytes for Li-Ion Capacitors
Authors: Lindberg Simon , Arnaiz María , Canal Rodríguez María , Martinez-Ibañez María , Ajuria Jon 
Published: February 2024

Supercapacitors are the choice when high-power performance is essential, however, for many applications the energy density of supercapacitors is too low to be used. To increase the energy density, it is possible to combine an insertion type anode of a Li-ion battery with a capacitor type double-layer electrode, forming a so-called Li-ion capacitor (LIC) [1]. There are however several issues with LICs that needs to be resolved to enable a more widespread use in applications. The most critical issue is the pre-lithiation of the anode, which can be either electrochemical or mechanical, this adds an extra processing step and increases the cost of assembly. Another issue is the stability at elevated temperatures and flammability of the organic electrolyte most often used in Li-ion capacitors. Recent progress dealing with the issue of lithiation of a hard carbon anode has been made by researchers at CIC energiGUNE, suggesting the use of a cheap and commercial dilithium squarate as a sacrificial salt [2]. For battery applications polymer electrolytes (PEs) have been suggested as a possible route to improve stability and increase the safety of the device. A similar approach using PEs could be used in LICs, but there are only a few publications for high power devices such as traditional supercapacitors [3]. Important properties of electrolytes in general for LICs are wide electrochemical stability window, high conductivity and a low viscosity to ensure a complete infiltration of activated carbon materials [4]. For a solid electrolyte to fulfill these demands the PE content should be low, the conductivity of the liquid component should be high and the electrolyte needs to be able to be crosslinked in situ, so that a PE can infiltrate and be formed throughout the electrodes of the cell. In this work, we have investigated a mixture of two monomers that can be covalently crosslinked as the polymer component in a EC:DMC based liquid electrolyte. This polymer mixture enables the formation of gel polymer electrolytes (GPEs) in situ, at very low concentrations, through the addition of an initiator and a mild heat treatment that enables the breaking of double bonds. Similar results have been shown in slightly different systems to provide a high conductivity and wide electrochemical stability windows [3][5]. In this work we show that the GPE does not affect the use of a sacrificial salt and that it would increase the safety of the device by eliminating the risk of leakage while maintaining the high-power performance of the LIC previously only showed for liquid electrolytes.

References

[1] Shao, Y. et al. (2018) ‘Design and Mechanisms of Asymmetric Supercapacitors’, Chemical Reviews, 118(18), pp. 9233–9280.
[2] Arnaiz, M. et al. (2020) ‘A transversal low-cost pre-metallation strategy enabling ultrafast and stable metal ion capacitor technologies’, Energy & Environmental Science, 13(8), pp. 2441–2449.
[3] Su, Y.-H. et al. (2021) ‘Postinjection gelation of an electrolyte with high storage permittivity and low loss permittivity for electrochemical capacitors’, Journal of Power Sources, 481, p. 228869.
[4] Li, B. et al. (2018) ‘Electrode Materials, Electrolytes, and Challenges in Nonaqueous Lithium-Ion Capacitors’, Advanced Materials, 30(17), p. 1705670.
[5] Wang, Q. et al. (2022) ‘Molecular engineering of a gel polymer electrolyte via in-situ polymerization for high performance lithium metal batteries’, Chemical Engineering Journal, 428, p. 131331.

Figure 1