Investigation of Plasticized Polyacrylonitrile-based Composite Electrolytes for High-performance Solid-state Lithium-ion Batteries

Abstract

Lithium-ion batteries (LIBs) are an indispensable energy storage device for portable electronics, electric vehicles, and grid electricity storage. The fabrication of solid-state lithium-ion batteries (SSLIBs) by replacing liquid electrolytes with solid-state electrolytes (SSEs), regarded as the next generation of energy storage, is intended to fundamentally fix the safety risks and the issue of lower energy density for conventional lithium-ion batteries. Due to the high ionic conductivity at room temperature (RT) and the tolerance to high-voltage cathodes, the poly-(acrylonitrile) (PAN)-based SSEs have attracted considerable interest. However, it is susceptible to severe passivation and poor compatibility against the Li-metal anode, which limits the enhancement of SSLIBs' energy density.

To address these challenges, this work introduces a novel PAN-based composite SSE with single-layer architecture via the traditional solution-casting method, incorporating Li_6.4La₃Zr1.₄Ta_0.6O₁₂ (LLZTO) nanoparticles as ceramic fillers and plasticizers of ethylene carbonate (EC) and fluoroethylene carbonate (FEC). Key findings include:

(1) Synergetic effect of LLZTO NPs and plasticizer of EC on interfacial compatibility between SSEs and Li-metal anode
This section of the work describes an effective method for producing single-layer flexible PAN-based composite solid-state electrolyte membranes without liquid electrolyte, and it is optimized by adding an EC additive, leading to an improved high ionic conductivity of 0.28 mS/cm at RT. Assembled with the optimized PAN-LLZTO-EC SSEs, the symmetric Li/Li cell presents different overpotentials of nearly 49 mV and stable cycling of nearly 1000 h at 0.2 mA/cm², 150 mV at 0.5 mA/cm² for 300 h, and 276 mV at 1.0 mA/cm² for 50 h, and possesses high critical current density (CCD) of 1.4 mA/cm² (RT), which indicates that in comparison to non-optimized SSEs, the optimized SSEs exhibit high interfacial stability against Li metal. The comprehensive characterizations and density functional theory (DFT) calculations imply that with the impurity of LiOH and Li₂CO₃ on the surface of LLZTO, EC could transform into another substance with ring opening, which could couple with the nitrile functional group in PAN, resulting in the drop of difference between HOMO and LUMO energy of PAN polymer matrix, and consequently inducing the improvement of ionic transportation and interfacial stability. The coin cell of Li/LFP assembled with plasticized SSEs present high discharge capacity of 149.5 and 149.6 mAh/g at 0.5 C and 2 C (RT), respectively, and maintain 96.0% and 92.7% of discharge capacity after 200 cycles.

(2) Enhancement of interfacial compatibility between SSEs and Li-metal anode with fluorine-doped EC
In this work, cyclic FEC is added to the PAN-based solid-state electrolyte to induce a more robust and stable SEI layer and prevent unfavorable side reactions between PAN and the lithium metal anode. The in-depth characterizations of physicochemical and electrochemical performance suggest the synergetic effect of LLZTO and FEC reduces the formation of nitrile-type passivation layers on the lithium metal surface during cycling. Additionally, the fluorinated EC facilitates the formation of a LiF passivation layer on the lithium metal surface, which effectively inhibits the side reaction induced by nitrile-based functional group of PAN and Li-metal anode. As the consequence, the optimized combination of LLZTO and FEC exhibits superior performance in symmetric cells compared to the EC-contained SSEs, which presents the high CCD of 2.4 mA/cm² and cycling stability of symmetric Li/Li cell at 0.2 mA/cm² for 500 h, 0.5 mA/cm² for 400 h, 1.0 mA/cm² for 150 h with the overpotential of 51, 89, 110 mV, respectively. Assembled coin cell of Li/LFP with fluorinated PAN-based SSEs provide stable discharge capacity of 150.3 mAh/g at 0.5 C and 140.2 mAh/g at 2 C, with the capacity retention of 94.5% after 200 cycles and 86.0% after 800 cycles, respectively. Moreover, pouch cells of the NCM811|| PAN-based SSEs || Gr and LFP|| PAN-based SSEs || Li exhibit the enduring cycling stability at room temperature.

In conclusion, the molecular interactions induced by the organic additives EC and FEC significantly improve the performance of PAN-based SSEs. This includes enhancing ionic conductivity at room temperature and interfacial compatibility with Li-metal anodes. The results from this work suggest a promising approach for producing high-performance, single-layer composite SSEs in solid-state batteries using a facile casting-solution method. This approach holds potential for scalable, cost-effective manufacturing of PAN-based SSEs, positioning them as viable candidates for large-scale production and commercialization in future lithium-ion solid-state battery applications.

Date of Award	11 Oct 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chi Yuen CHUNG (Supervisor)