Editorial

, Volume: 12( 2)

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Nanocatalysts in Lithium Oxygen Batteries

*Correspondence:

Chawla N , Carnegie Mellon University, USA, E-Mail: nehac@andrew.cmu.edu

Abstract

Introduction

Lithium ion batteries are currently used in several areas of technology, including transport, portable electronics, medical devices, power tools, and storage of electricity produced by renewable sources like solar, wind, etc. With the increasing demand and usage of high-performance batteries in hybrid and electric cars, and more demanding electronic gadgets, the physical limits of the materials used in batteries are being tested. While these batteries deliver high energy density, they have limited cycle life and power density. Lithium oxygen (Li-O₂) batteries have attracted interest as energy storage devices due to their high energy and power density. The first non-aqueous Li-O₂ battery was developed using a polyacrylonitrile (PAN) polymer based electrolyte by Abraham in 1996 [1]. These batteries when used in electric vehicles will significantly increase the driving range thus revolutionizing the automobile industry. Li-O₂ batteries have high theoretical specific energy density of 11,140 Wh/kg and they can achieve four times higher energy density than the current lithium ion batteries [1,2].

A typical Li-O₂ battery consists of a lithium anode, porous cathode which is open to oxygen and Li⁺ ion conducting electrolyte separating the electrodes[3]. The electrochemical reaction taking place in Li-O₂ battery is as follows [4]:

2Li⁺ O₂ ↔ Li₂O₂

Where Li₂O₂ is deposited on the surface of cathode during discharge (oxygen reduction reaction), and the reverse reaction (oxygen evolution reaction) takes place during charging to decompose Li₂O₂ on the surface of cathode. A few reports demonstrate the Li₂O formation during discharge which is difficult to reverse during charging [5,6]. Aqueous electrolytes involve formation of OH− and then LiOH at the cathode on discharge according to equation [4]:

2Li⁺ + O₂ + H₂O + 2e⁻ ↔ 2LiOH

Although extensive research has been done to improve the performance of Li-O₂ batteries, they are still in their developmental stages and many technical challenges must be addressed before they can be commercialized. The most common problems hindering the development of Li-O₂ batteries are low rate capability, poor recyclability and low round-trip efficiency [3,7,8] Catalysts have shown to improve the battery capacity and the recyclability of these batteries when used in cathodes. Metal nanoparticles consist of a large surface area and are hence used in various functions and applications.

Intrinsic properties of these are determined by shape, composition, size and crystallinity [9]. Nanotechnology can enhance and provide a new lease of life for these materials. In this paper, I will discuss a few nanoparticles that are used as nanocatalysts for Li-O₂ batteries.

Several nanoparticles such as gold, rhodium, nickel, platinum, silver, palladium, ruthenium, cobalt, and other inorganic compound nanoparticles have been used as nanocatalysts for Li-O₂ batteries [8]. Debart et al. used several catalysts for lithium oxygen batteries with non-aqueous electrolyte. They used Pt, La0.8Sr0.2MnO₃, Fe₂O₃, NiO, CuO, CoFe₂O₄. CO₃O₄ as electrocatalysts and exhibited high discharge capacity and good cycling performance. They also studied manganese oxides as catalysts for lithium oxygen batteries. Mn₃O₄, bulk Mn₂O₃, bulk α, β, λ, γ-MnO₂, α-MnO₂ nanowires and β-MnO₂ nanowires were used as catalysts [9]. The air cathode with α-MnO₂ as the catalyst showed the highest capacity of about 3000 mA.h/g (carbon).

For my research, I investigated the battery performance of Li-O₂ batteries by coating and filling the carbon nanotubes [3]. It was observed that the Pd-filled CNTs showed a 6-fold improvement in the first discharge cycle over pristine CNTs and 35% increase when compared to Pd-coated CNTs. High discharge capacity of 11,152 mA.h/g was obtained when using Pd-filled CNTs. Encapsulation of the catalyst in CNTs assisted in improving the stability of the electrolyte by reduction in Li₂CO₃ formation.

Zhang et al. used palladium coated α-MnO₂ as a catalyst in the Li-O₂ batteries with initial discharge of 1220 mA.h/g and capacity retention rate of 47.3% after 13 cycles [10,11]. The work of Zhao et al. gives a mechanistic insight into catalytic effect between metal oxide and noble metal [12]. They designed a hybrid nanostructure using 2D Birnessite MnO₂ (δ-MnO₂) and palladium nanocrystals. The Pd/δ-MnO₂catlysed Li-O₂ batteries exhibit high capacity, low polarization and stable cycling of 133 cycles at a fixed capacity 1000 mA.h/g at a current density of 200 mA/g. The DFT calculations indicated that the presence of Pd on one side of δ-MnO₂ sheets promoted the formation of Li₂O₂ and induced the formation of stable, electronically conductive LiO₂ at the opposite side of the δ-MnO₂ sheets. MnO₂ coated on hexagonal nanoplatelets were synthesized as bifunctional catalysts by Yun Jung Li et al. for Li-O₂ batteries [13]. The hetero-structured catalyst (HSC) hybrid enhanced the electrochemical performance of the battery. They obtained 60 cycles at a fixed capacity of 1000 mA.h/g and current density of 100 mA/g.

Lu et al. developed the cathode by atomic layer deposition (ALD) of palladium nanoparticles on carbon surface with an alumina coating for passivation of carbon defect sites [14]. Pd nanoparticles on ALD surface were used as electrocatalysts to promote Li₂O₂ formation by improving electronic transport. Super P Li conductive carbon was used to support the catalyst material. 1M LiCF₃SO₃ in tetra-ethylene glycol dimethyl ether (TEGDME) electrolyte on glass fiber separator and lithium metal anode was used for the charge discharge cycles. After discharging to 1000 mA.h/g and charging, the charging potential was reduced to 3.2 V which is comparable to the theoretical of 3 V. The low potential could be maintained up to 10 cycles at 500 mA.h/g. The full discharge capacity obtained was 2750 mA.h/g. Degradation of lithium anode and poisoning of Pd catalyst by contaminants or passivation could be the reasons for the battery failure. Zahoor et al. synthesized α-MnO₂ nanorods and modified them with Pd nanoparticles to form Pd deposited α-MnO₂ nanostructures and used them as electrocatalyst in the cathode for Li-O₂ battery [15]. Catalyst by contaminants or passivation could be the reasons for the battery failure. Zahoor et al. synthesized α-MnO₂ nanorods and modified them with Pd nanoparticles to form Pd deposited α-MnO₂ nanostructures and used them as electrocatalyst in the cathode for Li-O₂ battery [15]. Ketjen black cathode with the Pd deposited α-MnO₂ catalyst and LiTFSi salt in TEGDME as electrolyte and lithium metal anode. A discharge and charge capacity of 8526 mA.h/g at a constant current density of 0.1 mA/cm2 was obtained with an overpotential of 1.6 V (discharge plateau at 2.7 V and charge plateau at 4.3 V).

G. Zhao et al. fabricated TiO₂ nanotube arrays on Ti foam and used them as substrate for Li-O₂ batteries [16]. RuO₂/TiO₂ cathodes exhibit good catalytic activity towards OER and exhibit a cycle performance of 130 cycles at a current of 1.77 A/g under 1000 mA.h/g.

A uniformly coated RuO₂ shell on the surface of core CNT was used as catalyst to prevent direct contact between CNT and Li₂O₂ thus reducing formation of Li₂CO₃ [17]. RuO₂-CNT catalyst increased the specific capacity to about 4350 mA.h/g at a current of 385 mA/g and exhibited a discharge and charge overpotentials of 0.21 and 0.51 V. 20 cycles were obtained at a fixed capacity of 500 mA.h/g at the current rate of 100 mA/g.

Bui et al. fabricated carbon nanofiber with platinum nanoparticles on the surface by coaxial electrospinning and observed improvement in specific capacity, rate capability, energy efficiency, cycle stability, and O₂ efficiency [18]. They obtained 163 cycles at the capacity limit of 1000 mA.h/g and current density of 500 mA/g.

Zhang et al. developed a novel method to synthesize NiO/CoO catalysts by pre-lithiation (PL) of pristine ternary metal oxide spinel material NiCo₂O₄ (NCO) nanowires at appropriate conditions [19]. They fabricated the PL-NCO nanoparticles on the carbon fabric. An extremely high initial capacity of ∼29 280 mA.h/g was obtained with the PL-NCO NWs/CF electrodes pre-lithiated at 0.50 V. the batteries cycled for 100 cycles at a specific capacity of >1000 mA.h/g.

Brush-like CO₄N nanorods were uniformly anchored on conductive electrospun carbon nanofiber (CNF) paper via hydrothermal growth of Co(OH)F nanorods followed by nitridation step by Il-Doo Kim et Al [20]. The resulting cathode exhibited cycling stability of over 177 cycles in Li-O₂ cells. This cycling stability was attributed to metallic CO₄N nanorods providing sufficient accessible reaction sites and facile electron transport pathway through the well-networked CNF. Thin oxide layer (<10 nm) formation on the surface of CO₄N nanorods assists in reversibility of Li₂O₂, thus reducing the overpotential gap to ∼1.23 V at 700 mA.h/g.

Sun et al. reported inverse spinel Co[Co,Fe]O₄/nitrogen-doped graphene (NG) composite as a catalyst for rechargeable Li-O₂ batteries [21]. Co[Co,Fe]O₄/NG catalyst in Li-O₂ battery exhibited an initial discharge capacity of 8236 mA.h/g and 110 cycles at a fixed capacity of 1000 mA.h/g at a current density of 100 mA/g.

We discussed a few nanocatalysts that have shown improvement in the Li-O₂ battery performance. This technology is still in developmental stages. Most of the research in the field of nano-enhanced Li-O₂ batteries in the coming years will focus on further improving the electrochemical properties of the batteries, making them safer and viable for large scale production.

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