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Cathode Catalysts for Non-Aqueous Lithium-Air Batteries

Mengting Xu , Yanqing Wang , Ya Mao , Jingjuan Li , Zhidong Jiang , Xianxia Yuan

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (10) : 1679 -1692.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (10) : 1679-1692 DOI: 10.7536/PC200942
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Cathode Catalysts for Non-Aqueous Lithium-Air Batteries
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Abstract

With expanding demand for high-performance batteries, Li-air batteries have been in spotlight due to the ultra-high theoretical energy density. Although some breakthroughs have been accomplished recently, their commercial level applications are still constrained by intense problems and challenges. Among them, sluggish reaction kinetics is assented as one of the serious issues responsible for unsatisfactory battery performance. Thus, an immense measure of exploration on cathode catalysts has been performed lately to improve the battery performance and facilitate the development. In this work, primary advancement and accomplishments on cathode catalysts for non-aqueous Li-air batteries have been summarized and comparatively discussed, and the future directions and developments have likewise been proposed.

Contents

1 Introduction

2 Working principle of non-aqueous lithium-air batteries

3 Cathode catalysts for non-aqueous lithium-air batteries

3.1 Noble metal and the alloys

3.2 Transition metal oxides

3.3 Transition metal sulfides

3.4 Soluble redox mediators

4 Conclusion and outlook

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Keywords

non-aqueous lithium-air batteries / cathode catalysts / noble metals and the alloys / transition metal compounds / soluble redox mediators

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Mengting Xu, Yanqing Wang, Ya Mao, Jingjuan Li, Zhidong Jiang, Xianxia Yuan. Cathode Catalysts for Non-Aqueous Lithium-Air Batteries. Progress in Chemistry, 2021, 33(10): 1679-1692 DOI:10.7536/PC200942

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1 Introduction

Energy and environmental problems have become an important factor in the sustainable development of human beings, and it is urgent to research and develop new energy technologies that are clean, efficient and can replace fossil resources. In recent years, the rapid development and wide application of electric vehicles (EVs) have put forward higher and higher requirements for the energy density and power density of their power sources (including secondary batteries, fuel cells, supercapacitors and other electrochemical systems)[1]. At present, lithium-ion batteries are widely used as the main power source of electric vehicles, but due to the limitations of its electrochemical principle and many other factors, the performance of lithium-ion power batteries has basically reached the limit, and the improvement space is very small, which greatly limits the development and application of electric vehicles. Therefore, the search and development of secondary batteries with higher energy density than lithium ion batteries has become a hot research topic[2,3].
As a new type of energy storage and conversion device, lithium-air batteries with metal lithium and oxygen/air as negative and positive active materials respectively have attracted much attention because of their theoretical energy density comparable to gasoline (Figure 1), clean environmental protection and low cost, and their theoretical energy density is more than ten times that of lithium-ion batteries[4]. Therefore, lithium-air battery is considered to be one of the most promising energy storage and conversion systems, which is expected to replace lithium-ion batteries and become a potential power source to solve the problem of electric vehicle endurance[5].
According to the different electrolytes, the current lithium-air batteries can be divided into four categories (Figure 2): non-aqueous lithium-air batteries, aqueous lithium-air batteries, solid-state lithium air batteries and composite lithium-air battery[6][7][8][9][10]. Among them, water-based lithium-air batteries have low cost and do not have the problem of blockage of discharge products, but the corrosion of lithium metal will cause poor safety of batteries. Solid-state lithium-air batteries have high stability and long cycle life, but high internal resistance and poor rate capability. The composite lithium-air battery combines the advantages of both aqueous and non-aqueous lithium-air batteries, but the battery system is complex and there is little research at present. Non-aqueous lithium-air battery has the advantages of high energy density and good reversibility, which is the earliest, most in-depth and relatively mature lithium-air battery system. Therefore, non-aqueous lithium-air batteries are reviewed in this paper.
Although the performance of non-aqueous lithium-air batteries has made great progress in recent years, they still face the problems of low energy conversion efficiency, poor rate capability and short cycle life, which seriously restrict their large-scale application. Fundamentally, this is mainly determined by the kinetic rate and reversibility of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the positive electrode of lithium-air batteries. The slow kinetic rate results in poor rate capability of the battery; If the discharge products can not be effectively and reversibly decomposed during charging, the cycle stability of the battery will be affected, and the accumulation of undecomposed discharge products on the positive electrode will lead to the blockage of the porous electrode, which will lead to the obstruction of oxygen transport, the covering of catalyst active sites and the acceleration of battery performance degradation[11]. Therefore, it is very important to develop bifunctional catalysts which can catalyze both ORR and OER reactions efficiently and promote the reversible formation and decomposition of discharge products during battery operation to improve the overall performance of lithium-air batteries.
In this paper, the working principle of nonaqueous lithium-air battery was briefly introduced, and the main research progress of oxygen electrode catalysts for nonaqueous lithium-air battery in recent years was reviewed, and its future development direction was reviewed and prospected.

2 Working Principle of Non-aqueous Lithium-air Battery

Fig. 3 is a schematic diagram of the working principle of the non-aqueous lithium-air battery, and the specific electrode reactions can be represented by Formulae 1 to 3[4][12,13].
负极反应: Li↔Li++e-
正极反应: O2+2Li++2e-↔Li2O2↓
电池反应: 2Li+O2↔Li2O2↓
More specifically, the positive electrode reaction (Formula 2) during discharge includes several steps as shown in Formulas 4 to 6, in which the O2 is first reduced to the O 2 -,And then forms a LiO2 on the surface of the electrode with the Li+ transmitted through the external circuit, and the unstable LiO2 is subsequently converted into a more stable Li2O2, so the Li2O2 is the final discharge product. During the charging process, the Li2O2 is directly decomposed into the Li+ and the O2 without generating the intermediate LiO2[14].
O2+e-→ O 2 -
O 2 -+Li+→LiO2
2LiO2→Li2O2+O2
However, due to differences in battery environment (such as catalyst, electrolyte, cell/electrode structure, etc.), the working process of lithium-air batteries may proceed according to Formula 7, in which lithium oxide (Li2O) is the final discharge product. The poor reversibility of the Li2O during the charge and discharge process greatly affects the cycle stability of the battery.
4Li+O2↔2Li2O↓
Therefore, the catalyst of lithium-air battery should not only have efficient ORR/OER catalysis, promote the reversible formation and decomposition of Li2O2, but also effectively inhibit the formation of Li2O during discharge.

3 Anode catalyst of non-aqueous lithium air battery

Carbon materials have the advantages of large specific surface area, high conductivity and low price, which are the earliest and widely used catalyst materials in the research of lithium-air batteries, but their catalytic effect is limited and their stability is poor during the working process of batteries.In recent years, it is often used as a support material for catalysts, and the research on catalysts mainly focuses on precious metals and their alloys, transition metal oxides, transition metal sulfides and soluble redox intermediates.

3.1 Noble metal and its alloy catalyst

Noble metals and their alloys are ideal catalyst materials because of their excellent performance, but because of their high price and insufficient reserves, the research on these catalysts is relatively small. Among them, precious metals mainly include Pt, Au and Ru.Domestic and foreign researchers mainly focus on the structural design of precious metals, the alloying of precious metals with other metals, and the formation of composites of precious metals and functional carbon materials, in order to reduce the amount of noble metals and improve their utilization. Song et al. Synthesized anisotropic Pt particles, and the specific discharge capacity of the lithium-air battery with the catalyst reached 12 985 mAh·g-1 at a current density of 200 mA·g-1, which was more than twice the capacity (6272 mAh·g-1) of the battery with commercial Pt/C as the catalyst, and the overpotential of the battery was 0.8 V lower than that with commercial Pt/C[15]. Wu et al. Supported 10 wt% Pt, 40 wt% Pt and 20 wt% Pt/10 wt% Ru on XC-72 carbon materials, and compared the effects of these three catalysts on the morphology and structure of Li2O2[16]. They found that compared with the other two catalysts, when the mass fraction of Pt reached 40 wt%, it was easier to form small particles of ring-shaped Li2O2, which would cover the electrode and cause the decline of discharge capacity. When Ru was introduced into the Pt-Ru/C catalyst, the Pt-Ru/C catalyst was beneficial to the formation and decomposition of large ring-shaped Li2O2 with poor crystallinity, thus reducing the charge overpotential and prolonging the cycle life of the battery. Hayashi et al. Prepared highly ordered mesoporous Pt-Ru/C catalyst materials by hard template method, and compared the electrochemical performance of Pt-Ru/C with different pore sizes (10, 40 and 100 nm) as cathode catalysts for lithium-air batteries[17]. They found that the cycle performance of Pt-Ru loaded batteries was significantly improved compared with pure carbon carriers, and the Pt-Ru/C catalyst with a pore size of 10 nm showed the best discharge specific capacity, cycle performance and rate performance. They further explored the effect of pore size on the growth behavior of discharge products and found that materials with smaller pore size could maintain porous structure and high conductivity after cycling, which also explained why Pt-Ru/C catalysts with pore size of 10 nm showed the best overall performance. Kang et al. Used DFT calculations to study the effects of Pt3M bimetallic alloys (M = 3D, 4D, and 5d transition metals) on the kinetic rates of ORR and OER in lithium-air batteries, and found that the fifth subgroup elements (V, Nb, and Ta, in the 3D, 4D, and 5d periods, respectively) were the most effective alloy components to obtain high catalytic activity, which originated from the electron-rich surface[18].
Wu et al. Synthesized hollow graphene nanocages modified by ultrafine Pt particles (~ 1 nm), and the charging voltage plateau of the lithium-air battery using the graphene nanocages as the catalyst dropped to 3. 2 V at a current density of 100 mA·g-1, and remained below 3. 5 V when the current density increased to 500 mA·g-1[19]. As shown in Figure 4, they believe that this is mainly due to the fact that the substrate of hollow graphene nanocages can not only provide multiple nanoscale three-phase regions as active sites for ORR, but also provide meso-scale pore structure for rapid oxygen diffusion[19]. Sevim et al. Supported bimetallic Pt alloy nanoparticles on reduced graphene oxide and prepared rGO-Co48Pt52, rGO-Ni47Pt53 and rGO-Cu49Pt51 catalysts (the mass fraction of metal is about 16 wt%)[20]. Among them, the lithium-air battery with rGO-Co48Pt52 as the catalyst showed the lowest OER overpotential, and its discharge specific capacity reached 9898 mAh·g-1 at the current density of 0.15 mA·cm-2, and could be stably cycled 80 times at the current density and the limit specific capacity of 0.75 mAh·cm-2. Zhao et al. Successfully prepared RuO2@CNFs composite materials by growing RuO2 clusters on carbon nanofibers. The study found that RuO2 had excellent catalytic activity for the OER process of lithium-air batteries, and its unique cluster structure effectively alleviated the deactivation of batteries caused by the coverage of catalyst active sites by discharge products[21]. Therefore, the lithium-air battery using the RuO2@CNFs as the cathode catalyst has high specific capacity, excellent rate capability and cycle stability, and low overpotential. Under the condition that the specific capacity is limited to 1000 mAh·g-1 and the current density is 200 mA·g-1, the battery can be stably cycled for 300 times.
In recent years, some noble metal catalyst materials with new structures for lithium-air batteries have been gradually developed. Wang et al. Designed a sandwich structure of graphene/Au nanoparticles/Au nanosheets (G/Au-NP/Au-NS), which can control the growth of Li2O2 in space and structure, so that the growth of Li2O2 is limited to the surface of Au nanoparticles between graphene and Au nanosheets.The battery performance degradation caused by the accumulation of the Li2O2 is effectively inhibited, and the contact of the Li2O2 with the graphene and the electrolyte is greatly reduced, so that the occurrence of side reactions such as decomposition of the carbon material and the electrolyte is slowed down[22]. Under the condition of limited specific capacity of 500 mAh·g-1, the lithium-air battery with the catalyst can be stably cycled for 300 times at the current density of 400 mA·g-1.

3.2 Transition metal oxide catalyst

Transition metal oxides have been widely studied due to their low cost, simple preparation and excellent catalytic properties. As a large class of oxygen electrode catalysts for lithium-air batteries, transition metal oxides include single metal oxides and mixed metal oxides, in which the transition metal elements generally have multiple valences and different crystal structures. Uch as :MnO2, MnOx, Co3O4, NiCo2O4,CuCo2O4, MnCo2O4, LaNiO3, LaMnO3, etc.All of these transition metal oxides exhibit significant ORR/OER catalytic activity.
Manganese oxides are the earliest and most intensively studied transition metal oxide catalysts. The results show that the electrochemical properties of manganese oxides with different compositions, crystal forms and morphologies are significantly different, and the properties can be further improved by changing the crystal form of the material, increasing the specific surface area of the material or compounding with carbon materials. Electrolytic manganese dioxide (EMD) was the earliest manganese oxide used in lithium-air batteries. Bruce et al. Then compared and studied the effects of crystal form and morphology of MnO2 on its performance as a catalyst for lithium-air batteries, and found that α-MnO2 showed the highest specific discharge capacity (3000 mAh·g-1) at a current density of 70 mA·g-1[23][24]. Lu et al. Synthesized α-MnO2 nanowires, and the specific discharge capacity of lithium-air batteries using them as catalysts reached 12 000 mAh·g-1 at a current density of 100 mA·g-1[25]. Park et al compared the effects of nanowire α-MnO2 and nanopowder α-MnO2 as catalysts on the performance of lithium-air batteries, and found that at the current density of 10 mA·cm-2, the specific discharge capacity of the battery using the former was 511 mAh·g-1, while that of the battery using the latter was only 273 mAh·g-1[26]. They believe that this is mainly because the nanowire-like MnO2 can expose more active sites and shorten the distance of electrolyte diffusion into the carbon substrate. Wang et al. Uniformly grew petal-shaped nano-flake MnO2 on the surface of carbon nanotubes (CNT) to form a coaxial core-shell structure, which prevented the direct contact between CNT and Li2O2, thus reducing the oxidative decomposition of CNT and the formation of Li2CO3 during charging[27]. When the current density is 200 mA·g-1 and the limiting specific capacity is 500 mAh·g-1, the lithium-air battery using the composite material as a catalyst can be cycled for more than 200 times. Tu et al. Synthesized a cailiao carbon submicron tube array material. When the current density is 800 mA·g-1 and the limiting specific capacity is 1000 mAh·g-1, the lithium-air battery using it as the catalyst can be cycled more than 300 times[28]. Alam et al studied the formation and decomposition of Li2O2 on the surface of α-MnO2(100) during the discharge/charge process of lithium-air battery by using density functional theory, and calculated the overpotential of the battery under two possible Li2O2 formation/decomposition mechanisms[29]. They found that during the discharge process, the Li2O2 preferentially formed on the α-MnO2, but the binding force between the Li2O2 and the oxide was weak, and it could be completely decomposed at the charging potential of 3.37 V. They also explained the reasons for the excellent performance of α-MnO2 in lithium-air batteries from the perspective of calculation, and pointed out that exposing more (100) crystal planes would help reduce the loss of battery capacity.
In addition to MnO2, other manganese oxides are also used in lithium-air batteries as cathode catalysts. For example, Zhang et al. synthesized MnOx microspheres with a core-multi-shell structure as shown in fig. 5, whose spheres were assembled from ultrafine manganese oxide nanocrystals containing mixed valences (MnⅡ, MnⅢ, and MnⅣ)[30][30]. Under the conditions of 100 mA·g-1 current density and limited specific capacity of 1000 mAh·g-1, the lithium-air battery with the catalyst can be cycled 320 times; The battery can withstand more than 120 cycles even under the current density of the 200 mA·g-1 and the limited specific capacity of the 2000 mAh·g-1. They believe that the excellent electrochemical performance of the material is mainly attributed to the intrinsic high catalytic activity of MnOx and the increase of catalytic active sites by mixed-valence Mn ions. Wei et al. Prepared a stainless steel self-supporting MnO2-x nanosheet electrode (MnO2-x/SS) by electrodeposition-solvothermal method[31]. At the current density of 200 mA·g-1, the specific discharge capacity of the lithium-air battery using the composite as the positive electrode is 7300 mAh·g-1, which is 39% higher than that of the lithium-air battery using the MnO2/SS electrode. They believe that this is mainly due to the Mn (III) and oxygen vacancies produced in the MnO2-x nanosheets, which significantly increase their conductivity and catalytic activity, improve the kinetic rate of ORR reaction, and make the discharge products uniformly distributed on the electrode surface. Lang et al. Prepared a new type of Mn3O4/MnO nanosphere as a bifunctional catalyst for lithium-air batteries by vacuum DC arc method. The material has higher conductivity and catalytic activity than the traditional MnO2 catalyst. Electrochemical impedance spectroscopy shows that the charge transfer resistance is smaller when the Mn3O4/MnO catalyst is used. The discharge voltage platform of lithium-air batteries with Mn3O4/MnO as catalyst can reach 2.7 V[32].
Spinel oxides are another type of transition metal oxides that have been widely studied in lithium-air batteries. Among them, the most studied is Co3O4, in which the Co element exists in both Co2+ and Co3+ valences, which improves its redox capacity. Kim et al. Found that the morphology of Co3O4 has a significant impact on its performance as a catalyst for lithium-air batteries[33]. When the current density is 0.4 mA·cm-2, the specific capacity of the lithium-air battery using the fluff Co3O4 as the catalyst is the highest (2900 mAh·g-1), while the specific capacity of the battery using the flower-like and cube-like Co3O4 is 1800 and 2600 mAh·g-1, respectively. They believe that this is mainly because the fluffy Co3O4 particles are composed of many nanorods with a size of about 100 nm, which can expose a large number of active sites and provide enough space for the storage of discharge products. Ming et al. Prepared a dense hollow porous Co3O4 material. Under the conditions of current density of 200 mA·g-1 and limited specific capacity of 2000 mAh·g-1, the charging voltage of the lithium-air battery with the catalyst is 4.0 V, the discharging voltage is as high as 2.74 V, and the cycle time is more than 100 times[34]. These studies have shown that the specific surface area of Co3O4 can be increased by changing its morphology, thus exposing more active sites and improving its catalytic performance. However, these measures do not change the fact that the intrinsic conductivity of the Co3O4 is poor. In order to solve this problem, researchers have compounded Co3O4 with carbon nanotubes, graphene and other carbon materials in order to improve the conductivity and thus improve the overall performance of the battery. Li et al. Synthesized mesoporous rod-like Co3O4- carbonized polyaniline nanotubes (cPANI) by a simple hydrothermal method[35]. On the one hand, its unique structure provides abundant space for the deposition of Li2O2, and provides a large number of oxygen and electrolyte transport channels. On the other hand, the synergistic effect of Co3O4 and cPANI makes the Co3O4-cPANI have high conductivity and excellent ORR/OER catalytic activity. Compared with pure Co3O4 and pure cPANI,Co3O4-cPANI, the performance of the battery is significantly improved. When the current density is 100 mA·g-1, the discharge specific capacity of the lithium-air battery with the Co3O4-cPANI as the catalyst is as high as 32 478.3 mAh·g-1, and the battery can be stably cycled for 430 times under the conditions that the discharge specific capacity is 500 mAh·g-1 and the current density is 500 mA·g-1. Liu et al. Prepared N-doped graphene supported Co3O4 nanoparticles (CNG) catalyst materials, whose unique structure provides high catalytic activity, open active sites and abundant pore structure for oxygen electrode to accommodate discharge products[36]. When the current density is 0.05 mA·cm-2, the discharge specific capacity of the battery using CNG as the positive electrode catalyst reaches 8843 mAh·g-1, and the battery can be stably cycled for 135 times under the conditions that the discharge specific capacity is 1000 mAh·g-1 and the current density is 0.1 mA·cm-2. Cui et al. prepared a nickel foam supported Co3O4 self-supporting electrode, which showed higher discharge voltage (2.95 V), lower charge voltage (3.44 V) and higher specific capacity (4000 mAh·g-1) compared with the electrode using Co3O4 particles as catalyst[37]. Park et al. Used hydrothermal method to directly grow binary Mn/Co oxide with spherical flower-like layered structure on the surface of nickel foam skeleton, which eliminated the influence of conductive agent and adhesive on the electrochemical performance of lithium-air batteries[38]. When the current density is 200 mA·g-1, the discharge specific capacity of the lithium-air battery with the catalyst reaches 9690 mAh·g-1, and when the discharge specific capacity is 1000 mAh·g-1 and the current density is 600 mA·g-1, the battery can be stably cycled for 66 times.
In addition to monometallic spinel oxides, bimetallic spinel oxides have been widely used in lithium-air batteries in recent years. Compared with monometallic oxides, bimetallic spinel oxides have higher redox capacity due to the existence of multiple mixed valences (A3+/A2+ and B3+/B2+) in the crystal structure and the synergistic effect of A and B cations. NiCo2O4 is a typical example of bimetallic spinel oxides. Li et al. Prepared ordered mesoporous NiCo2O4 materials using KIT-6 as a hard template, and compared the effects of its content as a cathode catalyst on the performance of lithium-air batteries[39]. At the current density of 0.1 mA·cm-2, when the content of NiCo2O4 is 20%, 45% and 70%, the specific capacity is 4357, 4120 and 1881 mAh·g-1, respectively. They believe that the reduction of specific capacity is mainly due to the small specific surface area of NiCo2O4. However, when the content of NiCo2O4 increased from 20% to 70%, the charge voltage of the cell decreased from 4.0 V to 3.75 V, while the discharge voltage remained stable at 2.75 ~ 2.80 V, which was attributed to the excellent bifunctional catalytic properties of NiCo2O4. Liu et al. Reported a new method to synthesize ultrafine bimetallic oxide NiCo2O4 by electrochemical pre-lithiation process[40]. As shown in Figure 6, this method reduced the size of the NiCo2O4 particles from 20-30 nm to ~ 2 nm, and the morphology and particle size remained highly stable over long cycles[40]. It was found that the prepared ultrafine NiCo2O4 nanoparticles had the best catalytic performance when the pre-lithiation voltage was 0. 5 V. The initial discharge specific capacity of the lithium-air battery using the ultrafine NiCo2O4 nanoparticles as the catalyst reached 29 280 mAh·g-1 at a current density of 0.1 mA·cm-2, and the specific capacity was still greater than 1000 mAh·g-1 after 100 cycles. Xue et al. studied mesoporous nitrogen-doped carbon nanofibers modified by NiCo2O4 nanoparticles. The lithium-air battery with the catalyst has a discharge specific capacity of 5304 mAh·g-1 at a current density of 200 mA·g-1, and can be stably cycled for nearly 100 cycles at a discharge specific capacity of 1000 mAh·g-1[41].
They further improved the cycle performance of this kind of material by in-situ growth of mesoporous NiCo2O4 nanowire arrays on carbon cloth. The lithium-air battery using it as catalyst has little change in cut-off voltage after 200 cycles at a current density of 200 mA·g-1 and a limited specific capacity of 1000 mAh·g-1[42]. Similarly, Wang et al. Synthesized mesoporous CuCo2O4 materials and found that at 100 mA·g-1 current density, lithium-air batteries using them as catalysts showed lower overpotential than those using bulk CuCo2O4. They believed that this was mainly due to the high conductivity, easy mass transfer and high OER catalytic activity of mesoporous CuCo2O4[43]. The hierarchical flower-like CuCo2O4 self-assembled by porous nanosheets developed by Niu et al. Shows excellent cycle stability. The lithium-air battery using the catalyst shows little change in discharge cut-off voltage after 120 cycles at a current density of 100 mA·g-1 and a limited specific capacity of 1000 mAh·g-1[44]. Li et al. Prepared NiMn2O4-PH with positive spinel structure and NiMn2O4-FT with mixed spinel structure with different morphologies by co-precipitation and ball milling[45]. When used as a positive electrode catalyst, the lithium-air battery using the NiMn2O4-FT exhibits higher discharge capacity and energy conversion efficiency, better rate capability, and cycle stability than the battery using the NiMn2O4-PH. They believe that the main reason for the difference in performance between the two is their different lattice structures, and the mixed spinel structure is conducive to the generation of redox couples due to its unique atomic substitution, thus improving the overall performance of the battery. Wu et al. prepared single-walled and double-walled gadolinium nanotubes as shown in Figure 7 by electrospinning method, and found that double-walled materials have higher specific surface area, in which the macropore volume is almost twice that of single-walled materials, thus providing more catalytic active sites and sufficient storage space for discharge products[46][46]. The lithium air battery using the double-wall MnCo2O4 can release the capacity of 8100 mAh·g-1 at the current density of 100 mA·g-1, and achieves more than 278 cycles at the current density of 400 mA·g-1 and the limited specific capacity of 1000 mAh·g-1. Using carbon cloth as a carrier, Li et al. Directly prepared MnCo2O4 catalysts with three-dimensional porous network structure by electrodeposition and calcination, which were free of adhesives and interconnected by nanosheets[47]. When the current density is 340 mA·g-1 and the limiting specific capacity is 500 mAh·g-1, the MnCo2O4@ carbon cloth electrode can be cycled 108 times; When the current density is 200 mA·g-1, the discharge specific capacity of the battery is 7238 mAh·g-1. They believe that the good catalytic activity of carbon cloth electrode is mainly due to the high valence of Mn3+ and Mn4+, as well as the three-dimensional interconnected mesoporous structure, which is conducive to the diffusion of oxygen and the penetration of electrolyte. Karkera et al. Reported a nanoporous MnCo2O4 material composited with graphene, and the specific discharge capacity of a lithium-air battery using the material as a catalyst reached 10 092 mAh·g-1 at a current density of 100 mA·g-1[48]; Under the conditions of 800 mA·g-1 current density and 1000 mAh·g-1 limiting specific capacity, the cycle life is more than 250 times. Kim et al. Prepared ZnCo2O4 nanofibers with an average diameter of 162 nm, and the lithium-air battery with the nanofibers as the catalyst could be cycled 116 times under the conditions of 500 mA·g-1 current density and 1000 mAh·g-1 limited specific capacity[49]. In order to further improve its catalytic performance, they developed porous ZnCo2O4 nanofibers with doubled specific surface area by selective etching, which can reduce overpotential, increase energy conversion efficiency and extend cycle performance to 226 times when used in lithium-air batteries. Song et al. Grown three-dimensional network structure materials with crossed nanowires on carbon cloth (CC) by a simple hydrothermal method[50]. Among them, the battery using NiCo2O4/CC as catalyst showed the best cycle performance (200 cycles) and the lowest overpotential (0. 94 V) because of its higher specific surface area and more mesoporous structure. In addition, the NiFe2O4 developed by Jadhav et al. And Zhang et al. Also showed some catalytic performance in lithium-air batteries[51][52].
Perovskite oxides, which are widely used in solid oxide fuel cells (SOFCs), have been gradually used as oxygen electrode catalysts in lithium-air batteries in recent years. The general molecular formula of the perovskite oxide is that the ABO3,A site is a rare earth element, the B site is an alkaline earth metal element, and the A and B sites can also be doped, and the electrocatalytic performance of the perovskite oxide is influenced by the selection and stoichiometric ratio of transition metals. Francia et al.'S comparative study of La0.65X0.35-MnO3(X=Pb, Ba, Sr) materials found that La0.65Pb0.35MnO3 as a catalyst for lithium-air batteries has the best overall performance[53]. Fu et al. Prepared high-purity perovskite oxide materials g-La0.8Sr0.2MnO3 and s-La0.8Sr0.2MnO3 by sol-gel method and solid-state reaction method respectively, and found that g-La0.8Sr0.2MnO3 had better performance when used as cathode catalyst for lithium-air batteries. They believed that nanostructure was the main factor to enhance the electrochemical performance of catalyst materials[54]. Cheng et al. Found that compared with La0.6Sr0.4CoO3 and La0.6Sr0.4Co0.2Fe0.8O3,La0.6Sr0.4-Co0.2Mn0.8O3, the catalyst for lithium-air batteries has better comprehensive performance, and the batteries show higher capacity and longer cycle life[55]. Xu et al. Synthesized LaNiO3 nanoparticles by reverse homogeneous precipitation method, and the discharge voltage of the lithium-air battery with the nanoparticles as the catalyst remained above 2.51 V after 155 cycles at a current density of 100 mA·g-1 and a limited specific capacity of 500 mAh·g-1[56]. Similarly, Zhang et al. prepared a nitrogen-doped LaNiO3 material, in which the introduction of N added additional oxygen vacancies in the perovskite lattice to promote the formation/decomposition of Li2O2 (fig. 8)[57][57]. Lin et al. Synthesized three-dimensional ordered macroporous LaMnO3 materials, which increased the specific surface area and specific pore volume compared with the LaMnO3 prepared by traditional methods[58]. Compared with the battery using bulk LaMnO3, the lithium-air battery using the composite cathode catalyst has higher discharge specific capacity, lower overpotential and higher coulombic efficiency.
Perovskite oxides are also often compounded with carbon materials or other metal oxides to further enhance catalytic performance. Kim et al. prepared a composite material LaCo0.8-Fe0.2O3/rGO of perovskite material and reduced graphene oxide, and the specific discharge capacity of the lithium-air battery using the composite material as a catalyst reached 7088 mAh·g-1 at a current density of 200 mA·g-1, which was significantly higher than that of the battery using only LaCo0.8Fe0.2O3(5497 mAh·g-1)[59]. Chen et al. Synthesized a composite La0.8Sr0.2MnO3/NrGO of nitrogen-doped reduced graphene oxide and perovskite materials, and the lithium-air battery with the composite as the catalyst still had a discharge voltage higher than 2. 31 V after 360 cycles at a current density of 50 mA·g-1 and a limited specific capacity of 600 mAh·g-1[60]. They believe that its excellent catalytic performance is mainly attributed to its large specific surface area (152.24 m2g-1) and mesoporous structure, which can promote the transport of O2. Luo et al. Grown nano-NiCo2O4(NCO) uniformly on the surface of La0.8Sr0.2MnO3(LSM) nanorods to form a core-shell material NCO @ LSM, and found that the LSM core mainly catalyzed the ORR process, while the nano-NCO shell mainly catalyzed the OER process.The lithium-air battery using the material as the catalyst showed much better performance than the battery using only LSM, and its discharge voltage remained unchanged after 80 cycles under the conditions of 200 mA·g-1 current density and limited discharge capacity of 1000 mAh·g-1[61]. Gong et al. Prepared RuO2 modified La0.6Sr0.4Co0.2Fe0.8O3 nanofibers (RuO2@LSCF-NFs) by electrospinning, and the discharge specific capacity and energy conversion efficiency of lithium-air batteries using them as catalysts were much higher than those of batteries using only LSCF-NFs[62].

3.3 Transition metal sulfide catalyst

Sulfur and oxygen are the same main group elements, so transition metal sulfides and oxides have chemical similarities, but the larger electronegativity and atomic radius of sulfur make sulfides have better conductivity and larger unit cell volume.Therefore, in recent years, it has been gradually applied to lithium-air batteries as cathode catalyst materials in order to further improve the overall performance of batteries. Ma et al. applied transition metal sulfides to lithium-air batteries for the first time. They prepared NiS materials with flower-like and rod-like morphologies respectively (Fig. 9 a ~ d). Through comparative study, it was found that flower-like NiS with higher specific surface area showed better catalytic activity for the formation and decomposition of Li2O2 than rod-like NiS catalysts[63][63]. Since then, the use of sulfides as catalysts for lithium-air batteries has attracted wide attention, and more and more research work has been reported. Among them, MoS2 is the most studied transition metal sulfide material. Asadi et al. Prepared defect-free, single-phase layered MoS2 nanosheets by liquid phase exfoliation method, and the lithium-air battery with the nanosheets as catalyst and 1 mol/L LiTFSI/[EMIM][BF4] solution as electrolyte showed excellent electrochemical performance[64]. They believe that MoS2/IL(IL (ionic liquid) is a highly efficient composite catalyst, the boundary Mo atom is the active center, and the adsorption of EMIM+ on the boundary Mo atom promotes the catalytic performance of MoS2. Sadighi et al. Prepared two-dimensional trigonal MoS2 nanosheets by in-situ liquid phase exfoliation method, and the lithium-air battery assembled with CNTs could be stably cycled for more than 100 times under the conditions of 200 mA·g-1 current density and 500 mAh·g-1 limiting specific capacity[65]. Similarly, Hu et al. Synthesized a three-dimensional porous network material composed of ultra-thin MoS2 nanosheets and CNTs. The specific discharge capacity of the lithium-air battery with the catalyst was 6904 mAh·g-1 at a current density of 200 mA·g-1, and could be cycled 132 times at a specific discharge capacity of 1000 mAh·g-1[66]. Sun et al. Synthesized carbon fibers (CTs) supported flower-like MoS2 microspheres (MoS2@CTs) by hydrothermal method, and obtained Def-MoS2@CTs with different concentrations of sulfur vacancies after annealing treatment. Comparative study found that the Def-MoS2@CTs with higher concentrations of sulfur vacancies showed better comprehensive performance, mainly manifested in higher specific discharge capacity and better cycle stability of the corresponding batteries[67]. They believe that high sulfur vacancy concentration makes the catalyst surface have stronger O2 adsorption capacity, thus affecting the morphology and nucleation mechanism of Li2O2 and improving the performance of batteries.
Sennu et al. Prepared two-dimensional Co3S4 materials with mesopores on the surface by sacrificial template method, and the capacity of lithium-air batteries using them as catalysts reached nearly 6000 mAh·g-1 at the current density of 0.1 mA·cm-2[68]. They believe that the unique structure of the catalyst is conducive to the formation and accumulation of Li2O2 during discharge and the decomposition of Li2O2 during charging. Lin et al. Designed and synthesized a sisal-like Co9S8 material with an open structure. Combined with the DFT calculation results, it was found that the material had good oxygen affinity and could induce oxygen to react on its surface to form Li2O2, forming an excellent Li2O2/ electrode contact interface, which was conducive to the complete decomposition of Li2O2 during charging[69]. Lyu et al. Supported CoS2 on the surface of reduced graphene oxide, and the lithium-air battery with the catalyst had a discharge voltage plateau of 2.8 V and a charge voltage plateau of 4.2 V at a current density of 0.1 A·g-1[70]. Hou et al. Successfully synthesized flower-like SnS2 microspheres and applied them to lithium-air batteries as cathode catalysts, and found that the specific discharge capacity, rate capability and cycle performance of the batteries were significantly improved[71]. They believe that the improvement of the overall performance of the battery is mainly attributed to the unique graphene-like layered structure of flower-like SnS2 microspheres, which provides sufficient reaction sites for oxygen electrode reaction, while the open pore structure is conducive to the transmission of oxygen and the mass transfer process between electrolyte and ions, and provides abundant storage space for discharge products. In addition, the performance differences of the flaky and flower-like SnS2 materials (Fig. 9e-h) used as lithium-air battery catalysts were further compared and studied, and it was found that the catalytic effect of the flaky SnS2 was better[72][72]. They believe that this is mainly due to the different active crystal planes exposed by the two kinds of SnS2. The plate-like SnS2 is mainly { 100 }, while the flower-like SnS2 is mainly { 001 }. The more the relative content of { 100 }, the better the catalytic performance. Li et al. compared the performances of α-MnS and γ-MnS as cathode catalyst materials for lithium-air batteries, and the results showed that both of them had catalytic activity for ORR/OER, but the performance of α-MnS was better, and the batteries with α-MnS as catalyst had higher energy conversion efficiency, larger discharge specific capacity and better cycle stability[73]. Similar to spinel oxide catalysts, the application of spinel sulfide as a catalyst for lithium-air batteries has gradually attracted attention. Xu et al. synthesized two kinds of NiCo2S4 materials with sea urchin-like and core-shell structures (Fig. 9i ~ J). Through comparative study, it was found that the catalytic performance of sea urchin-like NiCo2S4 was better than that of core-shell materials when they were used as catalysts for lithium-air batteries, which was specifically manifested in lower charge-discharge overpotential, higher specific capacity and longer cycle life of the corresponding batteries[74][74]. They believe that this is mainly because the urchin-like structure has a larger specific surface area, increases the electrochemical active sites, and provides enough space to accommodate the discharge product Li2O2. Sadighi et al. Used electrodeposition method to grow 3D network MnCo2S4 nanosheets on carbon paper, because the material can adapt to the volume effect caused by the generation and decomposition of Li2O2 during battery charging and discharging, which greatly improves the cycle stability of batteries[75]. Long et al. Prepared three-dimensional CuCo2S4 nanosheet arrays supported on nickel foam, and the lithium-air battery with these arrays as catalysts could be stably cycled 164 times at a current density of 200 mA·g-1 and a limited specific capacity of 500 mAh·g-1[76]. They believe that the three-dimensional nanosheet structure directly grown on the foam nickel skeleton can not only reduce the internal resistance of the battery without using adhesives, but also promote gas diffusion, provide abundant catalytic active sites and sufficient space to store discharge products. Hou et al. Successfully prepared flower-like ZnCo2S4 on nickel foam by hydrothermal method. The ultrathin sheet structure exposed enough active sites, and the nickel foam skeleton provided abundant pore structure, which was conducive to the diffusion of oxygen and the penetration of electrolyte, thus significantly improving the kinetic performance of ORR and OER and reducing the reaction overpotential (0. 78 V)[77]. The self-supporting positive electrode structure without conductive agent and adhesive improves the stability of the battery and can be stably cycled for 500 times. Similarly, Hu et al. Directly grew mesoporous CoNi2S4 nanorod arrays CNS-RAs/CT on carbon cloth through a two-step hydrothermal synthesis method, and found that the direct contact between CNS-RAs and highly conductive CT could effectively promote the electron transfer process. The discharge specific capacity of the lithium-air battery using CNS-RA as the catalyst reached 5438 mAh·g-1, and it could be stably cycled 588 times at a current density of 500 mA·g-1 and a limiting specific capacity of 1000 mAh·g-1[78]. In addition, the material has shown feasibility as a power source for next-generation flexible and wearable electronic devices.

3.4 Oluble redox intermediate catalyst

Usually, the discharge product of lithium-air battery, Li2O2, is not dissolved in the electrolyte, but accumulated on the surface of the electrode in the form of solid particles, which will inevitably cover the active sites on the surface of the catalyst and prevent it from contacting with reactant oxygen and Li+, thus making it difficult to play a catalytic role.Moreover, with the extension of time in the charging process, the Li2O2 in direct contact with its surface decomposes, resulting in the gap between the catalyst and the undecomposed Li2O2, which can not directly contact, so it can not play a catalytic role. All these will lead to the reduction of the comprehensive performance of lithium-air batteries. In order to solve this problem, some researchers have proposed to use soluble redox intermediates (RM) as catalysts, which are dissolved in the electrolyte and can reach various pores in the electrode with the electrolyte, thus ensuring the direct contact between the catalyst and the reaction species to the maximum extent, which is conducive to the improvement of battery performance[79,80]. The specific working principle is shown in Figure 10. The redox intermediate RM, as the carrier of electron hole transfer, is electrochemically oxidized on the surface of the positive electrode in advance during charging, and then acts as an oxidant to chemically oxidize lithium peroxide to decompose it. At the same time, it is reduced to the initial state, so that the charging voltage of the battery is significantly reduced[81,82]. During the discharge process, RM is first electrochemically reduced on the surface of the positive electrode, then reacts with oxygen and lithium ions to form lithium peroxide, and at the same time, it is oxidized to its initial state, thus increasing the specific discharge capacity of the battery.
In 2013, Bruce et al. First reported that the charge voltage of lithium-air batteries containing redox intermediate tetrathiafulvalene (TTF) remained almost unchanged after 100 cycles at current densities of 0.078, 0.196, 0.313 and 1 mA·cm-2, respectively. Fourier transform infrared spectroscopy (FTIR) and in-situ surface-enhanced Raman spectroscopy (SERS) were used to prove that the Li2O2 was completely decomposed after 100 cycles[79]. Since then, the application of soluble redox intermediate catalysts in lithium-air batteries has attracted wide attention, and a large number of studies have been reported. Among them, organic RM is one of the most studied. Bergner et al. developed TEMPO (2,2,6,6-tetramethylpiperidine-N-oxide) as a RM catalyst, and its application reduced the charge voltage of lithium-air batteries by about 500 mV[83]. At the same time, compared with the battery using TTF as RM, the battery using TEMPO has a longer cycle life and a lower charge voltage during the cycle. Feng et al. Used MPT (N-methylphenothiazine) as RM to reduce the charge overpotential of lithium-air batteries by 0.67 V, while reducing side reactions in the battery[84]. Kundu et al. Found that TDPA (Tris [4- (diethylamino) phenyl] amine) has two redox States of TDPA/TDPA+ and TDPA+/TDPA2+, which can reduce the charging voltage to below 3.7 V and increase the energy conversion efficiency to more than 80% when applied to lithium-air batteries[85]. In addition, Nasybulin et al. Found that the use of PEDOT (poly (3,4-ethylenedioxythiophene)) as RM can reduce the charging voltage of lithium-air batteries by 0. 7 ~ 0.8 V, but a more stable electrolyte system matching it needs to be developed[86].
The soluble inorganic lithium salts LiI and LiBr have attracted the attention of lithium air battery researchers because of their X-/ X 3 - and X 3 - /X2. The charging voltage of the Li-air battery assembled by Lim et al. Using LiI as the catalyst is much lower than that of the battery using Pt as the catalyst. The battery can be cycled more than 900 times at a current density of 2 A·g-1 and a limited capacity of 1000 mAh·g-1, and can be cycled 300 times at a limited specific capacity of 3000 mAh·g-1 (fig. 11)[87][87]. Kwak et al. Found that adding LiBr can reduce the charge overpotential of lithium-air batteries to 3. 5 V. The Li2O2 without LiBr is in the shape of a film, while the discharge product with LiBr is in the shape of a ring[88]. Similarly, Liang et al. Found that LiBr can not only improve the efficiency of O2 evolution during charging, but also reduce the formation of by-products[89]. When LiBr is not added, the Li2-xO2 of active intermediates will be produced after the electrochemical oxidation of the Li2O2, and they can react with the electrolyte to produce a variety of by-products, while the addition of LiBr can bypass the formation of active intermediates and reduce the formation of by-products, which is more obvious with the increase of charge current.
In addition to the RM catalysts with catalytic activity for OER process, some RM catalysts with significant catalytic effect for ORR process have been reported successively. Owen et al. Dissolved the EtV2+ (ethyl ionine) in the electrolyte with BMPTFSI as the solvent and applied it to the lithium-air battery, and found that the oxygen reduction activity was significantly improved[90]. During the discharge process, the EtV2+ is first reduced to EtV+ on the surface of the electrode, and then chemically reacts with oxygen to reduce it to superoxide ions, while itself is oxidized. Bruce et al. Reported that the quinone additive DBBQ (2,5-di-tert-butyl-1,4-benzoquinone) was used as RM. During the discharge process, DBBQ was preferentially reduced on the electrode surface, and then reacted with oxygen to form a complex LiDBBQO2, which was further reduced in the solution to form a Li2O2, which promoted the liquid phase growth of lithium peroxide, and the specific capacity of the battery was increased to 80-100 times[91]. Tesio et al. Proposed that a carbon radical compound TTM (Tris (2,4,6-trichlorophenyl) methyl radical) can be used in ether electrolytes to double the capacity of batteries[92].
Bifunctional RM catalysts with catalytic activity for both OER and ORR processes have become the target of researchers. In 2014, Goodenough et al. Reported and confirmed that FePc (iron phthalocyanine) can be used as a bifunctional RM catalyst, which can not only act as an electron transport medium between the electrode and lithium peroxide to reduce the charge overpotential of the battery, but also combine with superoxide ions to promote the growth of lithium peroxide, and can be stably cycled 136 times at a limited capacity of 1000 mAh·g-1[93]. Iron porphyrins have also been shown to improve the reduction efficiency of oxygen and the oxidation efficiency of lithium peroxide[94]. Zhou et al. Introduced copper ions as a bifunctional redox intermediate into lithium-air batteries, which greatly improved the specific discharge capacity by Cu+/Cu redox pairs during discharge[95]. During the charging process, the Cu+/Cu2+ redox couple effectively lowers the charging potential to 3.5 V. Bruce et al. Used DBBQ and TEMPO as liquid phase catalysts in the discharge and charge processes to effectively improve the cycle performance of the battery[96].
Although the use of RM catalysts can effectively reduce the charge voltage of lithium-air batteries and improve their discharge voltage and specific discharge capacity, they will inevitably shuttle through the diaphragm to the lithium cathode and react with lithium metal during the long-term operation of the battery.Resulting in the loss of RM and the reduction of cell energy efficiency, which makes it an important task to mitigate or avoid the shuttle effect of RM catalysts[97,98]. Zhang et al. Used InI3 instead of LiI as RM, and the working principle is shown in fig. 12[99][99]. When LiI is used, the I 3 - on the positive electrode side will shuttle to the lithium negative electrode and react with metal lithium to form LiI, and part of LiI will dissolve to form I-, which will return to the positive electrode and continue to be oxidized to I 3 -, which reduces the utilization rate of I 3 -. When the InI3 is used, the In3+ from the shuttle first deposits on the surface of the lithium anode, forming a thin layer of indium. The indium layer can effectively resist the attack of the shuttling I 3 - on the lithium metal, thereby protecting the lithium metal electrode.
In addition, the indium layer can also reduce the growth of lithium dendrite and improve the cycle performance of the battery. Similarly, Lee et al. Developed CsI as a highly efficient RM catalyst for lithium-air batteries, in which I- can promote the decomposition of Li2O2 and reduce the overpotential of batteries, and Cs+ can effectively inhibit the growth of lithium dendrites[100]. Sharon et al. Found that the addition of LiNO3 to ether electrolytes not only improves the stability of the O 2 -,It can also reduce N O 3 - by lithium to form soluble N O 2 -, which is oxidized to NO2,NO2 at 3. 6 ~ 3.8 V and is easy to oxidize Li2O2 and decompose[101]. At the same time, they also found that LiNO3 can promote the "top-down" growth mechanism of Li2O2“, promote the accumulation of discharge products in the liquid phase, and facilitate the transfer of charges on particles. In addition, the introduction of LiNO3 has also been shown to effectively mitigate the shuttle effect of redox intermediates by forming a Li2O layer, thus protecting lithium metal[102]. Another widely used measure to mitigate redox intermediate shuttling is to coat the lithium metal anode with a protective layer. For example, Lee et al. Used TEMPO as RM catalyst and Al2O3/PVdF-HFP composite as protective layer to coat on the surface of lithium metal anode, combined with electrochemical data and SEM, XPS analysis.It was found that the protective layer inhibited the reaction between TEMPO and lithium metal on the surface of the negative electrode, and effectively reduced the charge voltage and internal resistance of the battery during long-term cycling, thus improving the energy conversion efficiency of the battery[103]. Kwak et al. Used LiBr as RM and introduced graphene-polydopamine composite layer as lithium metal protective layer, which reduced the charge voltage of lithium-air battery to 3. 4 V and significantly improved the cycle stability of the battery, maintaining an energy conversion efficiency of more than 80% after 150 cycles[104].

4 Conclusion and prospect

Lithium-air battery is one of the most promising energy storage and conversion devices due to its ultra-high energy density. However, it is still in the early stage of development, and many challenges need to be overcome before practical application, including low energy conversion efficiency, low actual discharge specific capacity, poor cycle performance and poor rate capability. One of the important measures to solve these problems is to develop oxygen electrode catalysts with high activity and long life. Based on the brief introduction of the working principle of non-aqueous lithium-air battery, the main research progress of oxygen electrode catalysts for lithium-air battery in recent years was reviewed from the aspects of noble metals and their alloys, transition metal oxides, transition metal sulfides and soluble redox intermediates. In addition, the main problems and future research directions of these catalysts are as follows: (1) The limited resources and high cost of precious metal and its alloy catalysts are still the key problems restricting their practical application.Ternary or more alloy catalysts with low loading will be the main development direction of this kind of catalysts in the future because of the synergistic effect of each metal in them. (2) The transition metal mixed-row spinel oxides, double perovskite oxides and their composites with array structure will be the research focus of transition metal oxide catalysts in the future; (3) The research on transition metal sulfides in lithium-air batteries is relatively rare, but their higher conductivity and larger lattice structure compared with transition metal oxides make them potential oxygen electrode catalyst materials for lithium-air batteries, which deserve further study; (4) Soluble redox mediator catalysts have both ORR and OER catalytic properties, but they will shuttle to the negative electrode and react with lithium metal during use, which will adversely affect the performance of batteries. Therefore, the RM catalysts based on lithium protection will be an important research field of lithium-air battery catalysts in the future.

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Funding

National Natural Science Foundation of China(21776176)

National Natural Science Foundation of China(21476138)

Aerospace Science and Technology Innovation Fund(SAST2017-134)

Shanghai Sailing Program(17YF1412400)

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