12Rechargeable lithium–air batteries- characteristics and prospects

Introduction to Lithium–Air Batteries

  • High Specific Energy Density: Lithium-air batteries are attracting attention due to their higher theoretical specific energy density compared to conventional batteries, positioning them as potential candidates for electric vehicles (EVs).

  • Types: Two reversible lithium-air battery systems have been proposed:

    • Non-Aqueous Systems: Battery reaction: 2Li + O2 → Li2O2

    • Aqueous Systems: Reaction involves water: 4Li + 6H2O + O2 → 4(LiOH • H2O)

Characteristics of Non-Aqueous Lithium-Air Batteries

  • Energy Density Calculations:

    • Specifically, the non-aqueous lithium-air battery can achieve:

      • Discharge state: 3460 Wh/kg and 6940 Wh/L; OCV of 2.96 V

      • Charged state: 11680 Wh/kg (comparable to gasoline).

  • Comparison to Lithium-Ion Batteries:

    • Energy density per mass: ~10x higher and per volume: ~6x higher than lithium-ion batteries.

Characteristics of Aqueous Lithium-Air Batteries

  • Chemical Reactions: Include the decomposition of LiOH during discharge, which saturates at roughly 5% discharge depth.

  • Performance Metrics:

    • Specific energy: 1910 Wh/kg (charged) and 2004 Wh/kg (discharged).

    • Compared to conventional lithium-ion batteries (energy density ~100 Wh/kg).

Practical Considerations for EV Applications

  • Comparative Energy Density Needs: To compete with internal combustion engines, batteries need an approximate energy density of 700 Wh/kg.

    • This requires a calculated density exceeding 2000 Wh/kg.

  • Challenges:

    • No existing practical battery has demonstrated the needed high power density and cycling capabilities.

Development History and Current State

  • Rechargeable Lithium-Air Cells:

    • First reported in 1996 with a lithium metal anode and polymer electrolyte.

    • Resurgence of interest began in 2006 with variations in electrolyte formulations.

  • Electrolyte Requirements: Must be stable with lithium metal, have high oxidation potential, and maintain stability in varying conditions.

  • Current Research Focus: Involves non-aqueous electrolytes (e.g., LiPF6 in propylene carbonate).

Electrolytes in Non-Aqueous Systems

  • Impact on Performance:

    • Discharge and rate capacities are significantly affected by the electrolyte formulation (e.g., carbonate vs. ether).

    • Stability and solubility of oxygen are critical parameters.

  • Recent Findings: Evidence suggests that certain organic carbonates face decomposition during battery operation, warranting further research for better alternatives.

Air Cathodes and Reaction Dynamics

  • Mechanism: Discharge reactions include forming Li2O2, leading to kinetic limitations due to oxygen evolution.

  • Reaction Steps:

    • O2 + e- → O2 (hydrogen evolution phases)

    • Li+ + O2 → LiO2

  • Performance Measurement: Voltage gaps between charge and discharge potentially limit efficiency.

Potential of Aqueous Lithium-Air Systems

  • Advantages: Aqueous systems could mitigate issues related to lithium corrosion and improve energy conversion efficiency.

  • Design Considerations: Development of water-stable lithium electrodes and efficient air electrodes critical to enhancing performance.

  • Materials for Ideal Systems: NASICON-type solid electrolytes provide promising avenues for electrolyte stability in aqueous systems.

Challenges and Future Prospects

  • Current Limitations: There is a pressing need for batteries to efficiently handle deep cycling and maintain high energy density at practical charge/discharge rates.

  • Research Needs: Continued exploration of materials and electrolytes to provide long-term stability and efficiency in rechargeable lithium-air batteries.

  • Outlook: While progress has been made, researchers remain cautious about the viability and scalability of lithium-air technology for practical applications.