Review and Recent Advances in Mass Transfer in Positive Electrodes of Aprotic Li-O₂ Batteries

The widely used Li-ion batteries are insufficient for the rapid needs of high energy storage devices. Li-O₂ batteries are regarded as a promising candidate to meet the needs in the future. This technology has attracted tremendous attention and led to remarkable scientific advances. We consider aprotic Li-O₂ batteries in this review because they have drawn the most attention of scientific research. However, various challenges should be resolved, such as low rate capability, poor cyclability, and instability, before realizing this technology in practice. The mass transfer of O₂ and Li⁺ is a critical factor that affects the electrochemical performance of Li-O₂ batteries. In this review, we discuss the effects of aprotic electrolytes, porous structure, and operating conditions on the mass transfer, particularly the O₂ mass transfer. Fundamental advances in developing materials (e.g., organic electrolytes) and practical optimizations of the electrode structure and operating conditions are key to enhancing the mass transfer of O₂ and Li⁺. It is worthwhile to continue the research effort due to the benefits of Li-O₂ batteries. The research directions should be clarified to achieve breakthroughs in the future.