Stable, hierarchical MWCNT/δ&γ-MnO₂ composite cathode for high-performance solid-state Li–CO₂ batteries

Li–CO₂ batteries (LCBs) hold great promise for energy storage and CO₂ utilization, yet their practical use is hindered by limited reversibility, high polarization, and safety concerns associated with liquid electrolytes. To address these issues, we introduced an all-solid-state LCB incorporating a NASICON-type lithium aluminum titanium phosphate (LATP) solid electrolyte and a novel MWCNT/δ&γ-MnO₂ composite cathode, where the distinct roles of the MnO₂ phases were systematically investigated, with δ-MnO₂ enhancing CO₂ adsorption and γ-MnO₂ improving Li-ion diffusion. The developed cell exhibits remarkable electrochemical performance, achieving a high full-depth discharge capacity of 35,502 mAh g⁻¹, stable cycling for over 1000 h at 100 mA g⁻¹, and substantially reduced polarization. These results are attributed to the synergistic interactions of the cathode components, with the δ-MnO₂ facilitating the CO₂ adsorption, γ-MnO₂ promoting the Li-ion diffusion, and the conductive MWCNT network boosting the electronic conductivity. Additionally, the high density and robust microstructure of the LATP electrolyte ensures reliable long-term operation and contributes to the high capacity by enabling a wider stable electrochemical window compared to liquid electrolytes. Comprehensive post-cycle analyses confirmed the improved performance, effective Li₂CO₃ formation/decomposition, and catalyst structural integrity. These findings offer a viable strategy, rooted in a well-defined synergistic catalytic mechanism, to overcome critical challenges, enabling safer and more efficient LCBs.