A Self-Healing Plastic Ceramic Electrolyte for Lithium Metal Batteries

By Nidhi DhullReviewed by Laura ThomsonNov 28 2024

A recent article in Nature Communications introduced a plastic ceramic electrolyte (PCE) synthesized by hybridizing a dynamically crosslinked aprotic polymer with ionically conductive ceramics. In situ synchrotron X-ray techniques and cryogenic transmission electron microscopy (cryo-TEM) revealed the PCE's self-healing capability.

Background

Solid-state electrolytes (SSE) offer better energy density, dendrite resistance, and safety than conventional flammable, volatile, and leaky liquid electrolytes.

Among various types of SSE, oxide-based ceramic electrolytes (OCEs) have the advantages of better elastic modulus and electrochemical stability than sulfide-based SSE. However, the wide application of OCEs in solid-state Li⁰ batteries (SSLMB) is prevented by several entangled chemical, mechanical, and electrochemical challenges.

Consequently, current OCEs typically exhibit low current density, small operating areal capacity, high stack pressure requirement, and poor durability.

Successfully applying the SSLMB technology requires simultaneously addressing all the basic challenges of dendrite growth, conductivity, interphase, stack pressure, and fabrication. Therefore, this study proposed fabricating a PCE by embedding commercial Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP, 70 wt.%) powder in a self-healing solid polymer electrolyte (SH-SPE, 30 wt.%) with an aprotic dynamic bonding network.

Methods

A solvent-free, one-pot ultraviolet (UV)-polymerization method was used to synthesize the SH-SPE.

Ethyl acrylate and lithium-methacrylate- bis(trifluoromethane)sulfonimide (LiMTSFI) were employed as the monomers, succinonitrile as a solid crystal plasticizer, and 4-fluoro-1,3-dioxolan-2-one as the solid electrolyte interface (SEI) forming additive.

The PCE was synthesized by hand-milling the SH-SPE and LATP and roll-pressing them repeatedly to obtain a homogenous solid membrane. Simultaneously, a polyacrylate-based PA-SPE was synthesized via thermal polymerization.

The LFP and high-Ni, zero-Co, zero-strain cathode were prepared via slurry-casting. Alternatively, the high-loading LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathode was obtained from a laboratory. Li⁰ foil was employed as the anode, and stainless steel (SS) as the working electrode. Thus, solid-state batteries were assembled using a 2032-type coin cell configuration.

Electrochemical impedance spectroscopy profiles of Li⁰|SSE|Li⁰ or SS|SSE|SS cells were obtained under a frequency range from 1 MHz to 1 Hz and polarization voltage of 5 mV. Their electrochemical stability window (ESW) was also determined using cyclic voltammetry (CV).

The Li⁰ deposits for scanning electron microscopy (SEM) were obtained after discharging a Li⁰|PCE|Cu cell or Li⁰|H-SSE|Cu. A tube battery with Li⁰ as the reference electrode, PCE as the electrolyte, and SS as the working electrode was assembled for X-ray fluorescence (XRF) microscopy and X-ray absorption spectroscopy (XAS). For cryo-TEM experiments, a single-tilt liquid nitrogen holder was used to transfer the samples under frost-free conditions at −196 °C.

Results and Discussion

XRF images revealed that the PCE's self-healing rate remained almost constant across different cycling conditions, signifying that the healing process was primarily diffusion-limited. During cycling, no electrode-electrolyte crack formation or delamination was evident at the PCE-electrode interface. Alternatively, the electrode surface became gradually wet and covered by the PCE due to the latter’s excellent flexibility and self-infiltration ability.

XAS data further revealed the chemistry at the PCE-Li⁰ interface. The electrochemical reduction of LiMTFSI was considered responsible for forming a stable SEI. This was further supported by ex-situ X-ray photoelectron spectroscopy results demonstrating S–O_x and Li₂S_x in the SEI's S 2p profiles.

The SEI was enriched with LiF, Li₂O, Li₃N, and Li₂CO₃, suggesting the contribution of the SH-SPE in forming a stable SEI.

XAS results proved the effectiveness of this SEI in inhibiting LATP degradation, exhibiting no variations in the pre-edge energy and peak shape.

These results confirmed the self-infiltrating ability of the PCE's dynamically crosslinked main chain. This was probably the first study to reveal the self-healing mechanism of a hybrid SSE due to the polymer's dynamic crosslinking and self-infiltration.

Cryo-TEM experiments revealed dendrite-free Li0 deposition and uniform SEI morphology, which were ascribed to PCE's SEI-forming and self-healing ability. This corroborated the PCE’s excellent electrochemical properties and full-cell durability.

Notably, the Li⁰|PCE|Li⁰ cell exhibited a constant and low charge transfer resistance, while the Li⁰|LATP|Li⁰ cell demonstrated high charge transfer resistance due to the uncontrolled side reactions at the interface.

Considering the long-term durability, the Li⁰|LATP|Li⁰ cell failed within 100 hours at a small current density. While the Li⁰|SH-SPE|Li⁰ cell exhibited improved durability, it short-circuited at 1000 hours. On the other hand, the Li⁰|PCE|Li⁰ cell cycled stably for >4000 hours with steady overpotential.

Conclusion

Overall, the researchers successfully developed a cold-milled PCE, avoiding conventional OCE fabrication's high-temperature, high-pressure, hot-press sintering. The PCE's self-healing capability was attributed to the aprotic and dynamically crosslinked polymer network.

The results of this study may facilitate solving the electrochemical or mechanical failures of inorganic electrolytes via combining polymer electrolytes with functionality like self-healing, SEI-forming, and stimuli-responsiveness. However, despite good cycling durability, the PCE requires further improvement for low initial coulombic efficiency and discharge capacity. These issues were potentially attributed to the side reactions at the cathode-electrolyte interface and high room-temperature resistance, contributing to voltage hysteresis.

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