A free-standing CeO₂/Co₃O₄ nanowires electrode featuring a controllable discharge/charge product evolution route with enhanced catalytic performance for Li-O₂ batteries

Highlights

• A controlled discharge/charge evolution route based on CeO₂/Co₃O₄ hybrid composite.

• Directly observed the oriented nucleation of Li₂O₂ on embedded CeO₂ phase as predicted.

• Characterized the morphology, surface composition and nucleation-growth evolutions of ORR/OER products during cycling.

• Ultra-long stable cycle durability of 500 cycles at 500 mA g⁻¹ was achieved.

Abstract

Although transition metal oxides are important potential catalytic cathode materials for Li-O₂ batteries (LOBs), their poor cycle durability at high current density, high overpotentials and side reaction are still the challenges to solve. Herein, CeO₂/Co₃O₄ nanowire arrays grown on Ni foam were fabricated as a free standing cathode of LOBs, featuring a controllable discharge/charge products evolution route. CeO₂ served as active sites for nucleation, initial growth and decomposition of Li₂O₂. The embedded CeO₂ nanocrystalline on Co₃O₄ substrate dominated the initial discharge/charge product evolution with multi-formation kinetics of crystal Li₂O₂ and Li_2-xO₂ at high current densities which leading to low overpotentials and efficient decomposition of discharge products. Owing to the stable structure, the CeO₂/Co₃O₄ nanowires were found to energetically favor the mass transport between the electrode/electrolyte interface during long cycle testing. As a consequence, excellent cyclability of 500 cycles at high current density (500 mA g⁻¹) under a fixed capacity of 500 mA h g⁻¹ with low overpotentials of 0.2 V and 1.0 V for discharge/charge process (after 500 cycles) were achieved. The present work provides a new strategy and intrinsic insight in designing high-performance metal oxides electrocatalysts with a fine-tuned structure for LOBs.

Introduction

Because of its high energy density, environmental friendliness and reversibility, LOBs are considered to be the most suitable energy storage system for booming electric vehicles (EV) and hybrid electric vehicles (HEVs) in the foreseeable future, which could efficiently alleviate the increasingly serious environmental pollution and energy crisis. Abraham [1] first proposed non-aqueous Li-O₂ system in 1996, which has gained much attention in academic fields and has been studied worldwide. The reversibility of LOBs under organic electrolyte system was proved by Bruce in 2006 [2].

A typical Li-O₂ battery consists of a metal lithium anode, positive porous carbon cathode, and conductive electrolyte, wherein the air positive electrode generally consists of a carbon matrix material and a catalyst. In the discharge process, Li ions from the lithium anode pass through the electrolyte, combines with oxygen from the external environment and electrons at the cathode to form Li₂O₂, which are stored in the cathode. While charging, the Li₂O₂ reversibly decomposes into oxygen and Li⁺. Actually, there are still substantial issues for LOBs to overcome at this stage, including low actual specific capacity, poor cycle performance, serious polarization phenomenon caused by sluggish kinetics, which seriously limit their practical implementations. Conventional cathodes consist of a catalyst supported on carbon black and a polymer binder. One of the critical problems is the instability of carbon in a highly oxidizing environment. Carbon materials spontaneously react with electrolyte and discharge product Li₂O₂ to form irreversible by-products. Moreover, due to the low OER catalytic activity of carbon, polarization tends to occur during the discharge/charge process, which eventually inevitably led to an increased overpotential. To eliminate the negative influences of the carbon and binder, free-standing cathodes fabricated by the in-situ growth of nano-sized oxides with various structures on the Ni foam were investigated. Such cathodes could not only avoid side reactions involved by conductive carbon and binder, but also provide a large specific surface area, good electrical conductivity and the ability to form smaller sized Li₂O₂ [[3], [4], [5], [6]]. Among candidate cathode catalytic materials for free-standing electrodes of LOBs, including carbon materials [[7], [8], [9]], precious metals [[10], [11], [12]], metal oxides [[13], [14], [15], [16]] and so on, transition metal oxides have been studied as promising materials for free-standing cathodes because they can in-situ grow on Ni foam by surface mechanisms, such as NiCo₂O₄, MnO₂ and NiO [[17], [18], [19]].

As a promising metal oxide candidate, Co₃O₄ has been intensively studied as the cathode catalyst for LOBs, because of its efficient bi-functional catalytic activity and remarkable stability for electrocatalytic performance [[20], [21], [22], [23]]. Based on this conception, free-standing Co₃O₄ electrodes were fabricated and delivered good electrochemical performance [4,24]. However, the transition metal oxides like Co₃O₄ exhibit low electrical conductivity and capacity, evidently limiting their electrochemical activity and cycle performance. Therefore, construction of Co₃O₄-based nanocomposites situ-grew on Ni foam as carbon-free cathode materials is commonly used to fulfill the Li-O₂ battery performance. It was reported that Ren Yanbiao et al. [25] fabricated Pd/Co₃O₄ on Ni foam. The presence of Pd nanoparticles effectively promotes the uniform growth of a fluffy and porous discharge product Li₂O₂ layer on the surface of electrode, resulting in a specific capacity of 1551 mA h g⁻¹ at 50 mA g⁻¹ and long cycle life over 72 cycles at 100 mA g⁻¹ with the capacity limited at 300 mA h g⁻¹. Han et al. [26] synthesized MnO₂/Co₃O₄ on Ni foam, achieving synergistic effects towards improved ORR and OER, which present a small discharge/charge voltage gap of 0.76 V and long cycle life of 170 cycles at 300 mA g⁻¹ with a limited capacity of 1000 mA h g⁻¹. However, the capacity and long-cycle performance of Co₃O₄-based oxides were still poor than other highly efficient catalysts for LOBs.

As a potential candidate, cerium oxide (CeO₂) has drawn a lot of attention recently because of its high oxygen ion conductivity and good mechanical resistance [[27], [28], [29]]. Furthermore, CeO₂ is generally regarded as an oxygen storage medium, owing to its easy shift between reduced and oxidized states caused by the variation in oxygen concentration [30]. Thus, CeO₂ acts as an “oxygen buffer” in the electrocatalytic reactions, facilitating both ORR and OER for LOBs [[31], [32], [33]]. One of the distinct features of CeO₂ is its non-stoichiometry, some Ce⁴⁺ ions have a tendency to reduce to Ce³⁺ ions and oxygen vacancies, then transfer to its surface, which causes active sites for adsorption of superoxide radicals and electrochemical reactions [34]. In our previous work [35], CeO₂ rods containing oxygen vacancies were synthesized and used as a cathode for LOBs which showed extended electrochemical stability of 200 cycles, and reduced overpotential of the ORR to 0.11 V.

In this work, we supposed that a controllable discharge/charge product evolution route can significantly enhance the electrocatalytic performance of cathode materials for LOBs. The CeO₂/Co₃O₄ hybrid nanowire arrays (NWAs) in-situ grew on a conductive Ni foam substrate were fabricated as a free standing cathode of LOBs. The evenly distributed CeO₂ nanocrystalline on the Co₃O₄ substrate served as active sites for nucleation, initial growth and decomposition of Li₂O₂. The rapid shift between reduced and oxidized states and well-matched interplanar spacing between (200) plane of CeO₂ (2.72 Å) and (100) plane of the Li₂O₂ (2.74 Å) allows Li₂O₂ to rapidly form on the surface of CeO₂ phase, reducing overpotentials during the electrochemical process. Benefiting from the synergistic effect of bifunctional catalytic Co₃O₄ in the following charge/discharge product evolution, the oxide hybrid shows excellent electrocatalytic performance, including large capacities, lower overpotentials, high stability and ultra-long cycle stability. For example, the free standing CeO₂/Co₃O₄ NWAs electrode delivered a remarkable cycle performance of more than 500 and 120 cycles at 500 mA g⁻¹ with a fixed capacity of 500 mA h g⁻¹ and 1000 mA h g⁻¹. Meanwhile, the specific capacity was only 1539 mA h g⁻¹ tested under a cut off voltage mode at 500 mA g⁻¹.