Cross-Linked Gel Electrolytes with Self-Healing Functionalities for Smart Lithium Batteries

Abstract

Next-generation Li-ion batteries must guarantee improved durability, quality, reliability, and safety to satisfy the stringent technical requirements of crucial sectors such as e-mobility. One breakthrough strategy to overcome the degradation phenomena affecting the battery performance is the development of advanced materials integrating smart functionalities, such as self-healing units. Herein, we propose a gel electrolyte based on a uniform and highly cross-linked network, hosting a high amount of liquid electrolyte, with multiple advantages: (i) autonomous, fast self-healing, and a promising PF₅-scavenging role; (ii) solid-like mechanical stability despite the large fraction of entrapped liquid; and (iii) good Li⁺ transport. It is shown that such a gel electrolyte has very good conductivity (>1.0 mS cm^-1 at 40 °C) with low activation energy (0.25 eV) for the ion transport. The transport properties are easily restored in the case of physical damages, thanks to the outstanding capability of the polymer to intrinsically repair severe cracks or fractures. The good elastic modulus of the cross-linked network, combined with the high fraction of anions immobilized within the polymer backbone, guarantees stable Li electrodeposition, disfavoring the formation of mossy dendrites with the Li metal anode. We demonstrate the electrolyte performance in a full-cell configuration with a LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathode, obtaining good cycling performance and stability.

Keywords: autonomous self-healing; cross-linking; dynamic hydrogen bonding; gel electrolyte; lithium batteries; smart functionalities.

Figure 1

Synthesis of cross-linked gels PEGDA-UPy X (X = 50 and 67 wt %).

Figure 2

(Right) ¹³C MAS-NMR spectrum of the pristine sample and peak assignment. The peaks marked with stars at ∼70 and ∼160 ppm refer to the −CH₂ and −C=O groups of LP30 liquid electrolyte, respectively; (left) peak assignment of UPyPEG₃₅₀₀₀UPy (a), PEGDA (b), and the “ziplike” network originating from in situ polymerization (c).

Figure 3

SEM images in top-view (upper) and cross-section (down) modes of PEGDA (a and d), PEGDA-UPy 50 (b and e), and PEGDA-UPy 67 (c and f).

Figure 4

Frequency sweep test (a and b), master curve (c), and temperature sweep test (d) of the two samples PEGDA-UPy 50 and PEGDA-UPy 67.

Figure 5

(a) DSC analysis of liquid electrolytes LP30 (black line), PEGDA-UPy 50 electrolyte (red line), and PEGDA-UPy 67 (blue line). Scan from −80 to 50 °C (dotted line), scan from 50 to −80 °C (dashed line), and scan from −80 to 200 °C (solid line). (b) Conductivity vs temperature, T, for PEGDA-UPy 50 and PEGDA-UPy 67 in the range of −10 to 70 °C.

Figure 6

(a) ⁷Li MAS-NMR spectra of the pristine (top) and cycled (bottom) samples and (b) T₁ relaxation times vs temperature. In the cycled sample, populations A and B are highlighted in blue and red, respectively. The cycled sample refers to the gel recovered after galvanostatic cycling of the Li|gel|NMC 811 full cell (see the pertinent section).

Figure 7

Ionic conductivity and optical microscopy images of PEGDA-UPy 67 measured at 25 and 40 °C before and after applying a deep fracture with a blade (100% of the whole film thickness).

Figure 8

Galvanostatic stripping and plating experiments on symmetric Li|PEGDA-UPy 67|Li (a) and Li|LP30|Li (b) cells; (c) SEM images showing the cross-sectional view of the gel after the galvanostatic electrodeposition experiments.

Figure 9

Cycling performance of the Li|PEGDA-UPy 67|NMC811 (black and red circles) full cell at room temperature between 3.0 and 4.3 V. (a) Rate performance at different C rates; (b) voltage profiles at C/10, C/5, and C/2 for the GPE-based cell; (c) galvanostatic cycling at C/2 after two equilibration cycles at C/5.

Figure 10

³¹P MAS-NMR spectra of the pristine sample (left panel) and of the cycled one (right panel) at two different temperatures. The stars indicate spinning sidebands.