High Performance of Li-CO2 Batteries Using the Oxide Solid-State Electrolyte
Authors: Yoon Baeksang , Na Dan , Yu Dohyeon , Kim Hwan , Seo Inseok 
Published: December 2024

Global warming and climate change are urgent international issues, mainly stemming from the overreliance on fossil fuels. To combat this, researchers are actively striving for carbon neutrality [1]. Energy storage devices are crucial in advancing future renewable energy systems. Commercial Li-ion batteries, utilizing a LiCoO2 cathode and a carbon anode, offer a specific energy density limited to 387 Wh kg-1. Li-air batteries have emerged as promising energy storage solutions, boasting impressive energy densities of 1910 Wh kg-1 and 3460 Wh kg-1 with liquid and solid electrolytes, respectively [2-3].

In 2011, Takechi et al. reported the first functional Li-O2/CO2 battery prototype [4], revealing that employing CO2 as a working gas enhances discharge capacity compared to Li-O2 batteries. Consequently, several studies have focused on the electrochemical performance of Li-CO2 batteries. Despite their impressive discharge capacity, these batteries grapple with challenges such as limited cycle life, heightened volatility of electrolytes, and sluggish reaction kinetics. Therefore, understanding the electrochemical reactions in Li-CO2 batteries and their degradation mechanisms during cycling is essential. In addition, selecting the optimal electrode-electrolyte configurations in Li-CO2 batteries is equally vital to address any safety concerns that have arisen to date.

Using a simple solution-based method, this study synthesized a solid electrolyte, Li1.4Al0.4Ge0.1Ti1.5(PO4)3 (LAGTP). Stoichiometric amounts of LiCl (98.2%, SAMCHUN), Al(NO3)3∙9H2O (98%, SAMCHUN), GeO2 (99.9%, ALDRICH), C16H36O4Ti (97%, SAMCHUN), and NH4H2(PO4)3 (98%, SAMCHUN) were thoroughly mixed in deionized (DI) water. The solution was stirred magnetically and then ball-milled (Pulverisette 5, Fritsch) for 2 h. Subsequently, the solution was dried at 80 °C for 12 h to remove the solvent. It was then ground into a fine powder and calcined at 800 °C for 12 h. This heat treatment helped complete the chemical reaction and release volatile impurities to form pure LAGTP powder. Following heat treatment, the powder was mixed with 2 wt.% polyvinyl alcohol (PVA) binder and compressed into circular pellets with diameters of 15 mm. Finally, the pellets were sintered at 900 °C for 4 h in air to reduce porosity and increase relative density. LAGTP exhibited outstanding ionic conductivity (1.05 × 10-3 S cm-1) and a low activation energy (0.237 eV).

The LAGTP pellet was utilized as a solid-state electrolyte in a Li-CO2 battery, demonstrating charge-discharge characteristics through a reversible electrochemical reaction (4Li+ + 3CO2 ↔ 2Li2CO3 + C). Multi-walled carbon nanotubes, drop-cast on carbon cloth, served as the cathode material. The Li-CO2 mesh-type coin cell assembly comprised a Li anode, an MWCNT drop-cast carbon cloth cathode, and a LAGTP solid electrolyte. Initially, the cathode was prepared by mixing MWCNTs (>98%, 12 nm diameter, seven layers), polypyrrole, and polyvinylidene fluoride (PVDF) in a 7:2:1 weight ratio in an N-methyl-2-pyrrolidone (NMP) solvent. For electrochemical characterization, a Swagelok-type Li-CO2 test kit was assembled in a glove box filled with Ar, maintaining H2O and O2 levels below 1 ppm. Finally, the current density and capacity were normalized to the weight of the MWCNTs on the carbon-cloth cathode. The battery underwent 60 cycles with a cut-off capacity of 1000 mAh g-1 at various current densities, and a comprehensive charge and discharge test was performed at 100 mA g-1. Throughout the charge/discharge process, the particle size of inactive lithium increased on the cathode surface, obstructing active sites for conversion. Post-cycling analyses were conducted to elucidate the cathode degradation mechanism. The integration of LAGTP substantially improved battery cycle life and safety, positioning it as a viable option for next-generation, high-performance Li-CO2 batteries.

References

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