Based on the lightest metal, lithium, and oxygen from ambient air, non-aqueous lithium–air batteries exhibit a super high theoretical capacity of 3.86 × 103 mA h g−1, attracting increased research focus in recent years [1]. However, this technology is still in its early stage and faces challenges, including low practical capacity and power density, which severely limit its application as a power source for electric vehicles. The electrochemical reactions in the battery involve the formation and decomposition of lithium peroxide (Li2O2) as

$$ 2{\text{L}}{{\text{i}}^ +} + 2{{\text{e}}^-} + {{\text{O}}_ 2} \leftrightarrow {\text{L}}{{\text{i}}_ 2}{{\text{O}}_ 2}. $$

For a cathode made of given materials, the power density depends primarily on (1) the area of the triple-phase boundaries among lithium ions, electrons, and oxygen and (2) the transport rate of the three species, both of which are closely related to the distribution of electrolyte within cathode pores. In addition, the battery’s capacity also depends on how well the triple-phase boundaries can be maintained during discharge. For a porous cathode with fixed pore shapes and sizes, triple-phase boundaries inside pores depend on how the electrolyte is distributed. Hence, a critical question is what is the ideal distribution of electrolyte inside cathode pores that can maximize the triple-phase boundaries and facilitate the transport of species?

In conventional non-aqueous lithium–air batteries, the cathode is fully saturated by the electrolyte [2], as shown in Fig. 1a. Oxygen dissolves into the electrolyte at the oxygen/electrolyte interface and is then transported to the reaction sites. Due to the limited transport rate of oxygen in the electrolyte [2], the oxygen concentration decreases substantially from the air side to the separator side. The insufficient supply of oxygen near the separator side limits the chemical reaction, leading to a lower power density output. Perhaps more importantly, the reaction results in a buildup of solid discharge product Li2O2 [3]. Since the oxygen level is higher toward the air side and lower on the separator side, as the battery operates, the cathode pores on the air side become progressively clogged by the product, further reducing the oxygen transport and eventually stopping the discharge process. This is a key factor responsible for lithium–air battery’s low practical capacity. To further compound the issues, the charge voltage can be affected by the product morphology; larger-sized product leads to a higher charge plateau [4]. In short, distribution of discharge product causes the accumulation of large particles/thick films on the air side, increasing the charge resistance and therefore lowering round-trip efficiency and shortening cycle life.

Fig. 1
figure 1

(Color online) Schematic of a non-aqueous lithium–air battery with different distributions of electrolyte inside cathode pores: a fully-saturated, b partially-wetted, and c fully-wetted, partially-saturated

Full size image

A possible remedy to the aforementioned problems is to replace the fully-saturated cathode into a partially-wetted one [5, 6], as shown in Fig. 1b. In this scenario, only the cathode surfaces near the separator are surrounded by electrolyte. As a consequence, oxygen can reach the cathode’s inner region. As the solid Li2O2 is formed, it pushes electrolyte away from the separator with an increase in the capacity. With the optimum quantity of electrolyte, such a structure can help adapt the electrolyte distribution while Li2O2 is produced [5]. The experimental results indicated that a partially-wetted cathode is able to reach a discharge capacity 60 % higher than that of a conventional fully-saturated cathode [6]. Of course, the described scenario does not offer a complete solution because the cathode surfaces are only partially covered by the electrolyte while the dry areas are unused during discharge, due to the lack of lithium ions. Thus, the power density is still limited. In addition, the decomposition of solid Li2O2 will cause shrinkage of the electrolyte during charge, which may eventually break down the lithium ion pathway and exacerbate charge performance.

Hence, a more desirable distribution of electrolyte inside cathode pores of non-aqueous lithium–air batteries should be aimed at creating a uniform concentration distribution of both oxygen and lithium ions on the entire cathode surface. Ideally, the cathode surface should be fully-wetted, but the cathode as a whole should only be partially-saturated, as shown in Fig. 1c. In this scenario, all surfaces of the cathode are fully wetted by a thin electrolyte layer. In addition, the pathway for gaseous oxygen from the air side to the inner side should be maintained so that the cathode is partially saturated. Due to the open gaseous oxygen pathway, gaseous oxygen can rapidly penetrate the entire pore and can be quickly transported to the reaction sites through the thick-reduced electrolyte. Since lithium ions are also present on the surface, all areas can be used. Consequently, both the power density and the discharge capacity can be significantly enhanced. Moreover, the uniform distribution of oxygen and lithium ions will result in a uniform distribution of discharge product with small particles/thin films, instead of large particles/thick films accumulated at the air side. The described Li2O2 morphology with increased contact interfaces is expected to decrease the charge voltage [4], which can further improve the round-trip efficiency and cycle life.

In short, the formation of solid Li2O2 repels the electrolyte during discharge and can clog the gaseous oxygen transport pathway. The thickness of the electrolyte layer is, therefore, a key parameter that should be well-designed to maintain the pathway for gaseous oxygen in the electrochemical processes. From these considerations, a fully-wetted, partially-saturated cathode can improve oxygen transport and offer significantly improved performance. To achieve the ideal distribution of electrolyte within the cathode pores with a fixed pore structure, several approaches can be adopted: (1) applying the electrolyte with low surface tension to reduce the wetted area, for example, using an imidazolium ion-based ionic liquid [7], enabling efficient cathode usage and more open pathway, and (2) modifying the active material surface to preserve the electrolyte layer, for example, by using a polydopamine coating [8], providing sufficient lithium ion pathways. Additionally, due to the enlarged oxygen/electrolyte interface, a functional membrane [9] can be adopted to suppress the evaporation of the liquid electrolyte. In the future, efforts should be focused on realizing the described distribution of electrolyte to achieve a non-aqueous lithium–air battery with greatly enhanced performance.