High-temperature (HT) LiCoO₂ recycled from spent lithium ion batteries as catalyst for oxygen evolution reaction

Highlights

• Thermal synthesis of HT LiCoO₂ from Co(OH)₂ recycled from spent Li-ion batteries.

• HT LiCoO₂ is used as an electrocatalyst for the oxygen evolution reaction.

• Cyclic voltammetry and chronoamperometry tests of HT LiCoO₂ for OER.

• The activation free energy of the reaction of OER.

Abstract

Development of novel materials based on inexpensive metals is crucial for renewable energy storage technologies. Furthermore, the recycling of compounds, such as lithium cobaltate (LiCoO₂) from Li-ion battery cathodes, contributes to the recovery of important elements, such as Co and Li. The applicability of high-temperature (HT) LiCoO₂ as a multifunctional material has been tested. In this work, HT LiCoO₂ is used as an electrocatalyst for the oxygen evolution reaction (OER). Cyclic voltammetry and chronoamperometry tests show that the evolution of oxygen starts at 0.35 V accompanied by the formation of Co⁴⁺ ions. The activation free energy of the reaction calculated using Tafel plot is 28.0 kJ mol⁻¹ and electrochemical impedance spectroscopy elucidates an equivalent circuit with a transfer charge resistance of 1.55 Ω, Warburg impedance of 150.3 Ω, and constant phase elements of 3.50 and 1.35 m F inside the pores and at the double layer, respectively.

Introduction

The oxygen evolution reaction (OER) plays an important role in devices for energy conversion and storage, such as fuel cells, metal-air batteries, and water electrolysers. However, the large- scale use of such devices is limited by the sluggish kinetics of the OER. Research on efficient electrocatalysts is extremely important [[1], [2], [3], [4]]. Pt, IrO₂, and RuO₂ have conventionally been used as are catalysts for OER. However, owing to the high cost and scarcity of their components, their extensive use is not feasible. Hence, the material cost, electrocatalytic activity, and long-term stability of catalysts for OER must be taken into account [1,3,4]. In alkaline media, the OER process is kinetically favorable and takes place as a multi-step reaction with single-electron transfer at each step. As shown in Eq. (1), O₂ detachment requires the transfer of four electrons in a multiple electron transfer mechanism. One electron is involved per step, which causes an accumulation of energy leading to the sluggish kinetics of the OER and a large overpotential that has to be overcome [5]. The main reaction for O₂ production requires the transfer of four electrons according to the reaction

$$\text{4O}{\text{H}}^{-}\to {\text{O}}_2\text{+ 2}{\text{H}}_2\text{O + 4}{\text{e}}^{-}$$

An alkaline medium is generally preferred over an acidic one for the electrolysis of water. In an alkaline medium, the materials required for construction are less expensive and less susceptible to corrosion. Relatively inexpensive elements, such as Fe, Mn, and Pb, present lower electrochemical performance for OER compared to more expensive elements, such as Pt, Ir, Ru, and Co. Cobalt is a non-precious metal and forms promising electrocatalysts such as Co₃O₄ [3,4,[6], [7], [8], [9]]. The redox couple Co³⁺/Co⁴⁺ has been extensively studied for OER application in Co₃O₄ compounds with different synthesis methods and morphologies [3,[10], [11], [12], [13]]. Cobalt oxide (Co₃O₄) has Co²⁺ and Co³⁺ on the tetrahedral and octahedral sites, respectively. Many studies have shown that the Co²⁺ would be electro-oxidized into Co³⁺ during the electrochemical process, and the Co³⁺ are further oxidized into Co⁴⁺ before the onset of the OER. The Co⁴⁺ ions thus formed are the actual active OER species [7,14]. From this point of view, many Co compounds, such as sulfides and doped oxides, have been investigated for their potential application in electrocatalysis using the redox couple Co³⁺/Co⁴⁺ [7].

Lithium cobaltate (LiCoO₂) is one of the most important Co oxides because it has long been used as cathode material in Li-ion batteries owing to its high specific capacity, long cycle life, and high potential [[15], [16], [17], [18]]. With a view to reducing the cobalt dependence, many other oxides, such as LiNiO₂ and LiNi_1-xCo_xO₂, have been studied. Since 2010, LiMn_1/3Ni_1/3Co_1/3O₂ has been increasingly used as the cathode in commercial Li-ion batteries [[19], [20], [21]]. Recycling spent Li-ion batteries is an alternative for reducing the environmental impact of metal extraction processes, and it has been proven to be technically feasible [[22], [23], [24], [25], [26], [27], [28]]. LiCoO₂ can exist in two forms: low-temperature (LT) LiCoO₂ and high-temperature (HT) LiCoO₂. LT LiCoO₂ has a spinel-type structure with the space group Fd3 m. It is obtained at temperatures below 400 °C. HT LiCoO₂ has a hexagonal and lamellar structure with the space group R-3 m. It is obtained at temperatures both below and above 400 °C [15,[29], [30], [31]].

HT LiCoO₂ is widely used as cathode material. The HT LiCoO₂ used in this study was previously synthesized by recycling a Li-ion battery, and its capacitance was suitable for its applicability as a promising cathode material. However, its application as an electrocatalyst for OER can be studied by considering different synthesis methods and precursor materials. In a previous study by this group, synthesized HT LiCoO₂ was characterized using thermogravimetric analysis (TGA/DTG), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and its electrochemical performance as a cathode material was tested [22]. In this work, cobalt is recycled from a spent Li-ion battery as Co(OH)₂, which serves as a precursor material for synthesizing HT LiCoO₂ using Li₂CO₃ as a reagent. OER activities of Ni and Pt in alkaline media are widely known, and in this work, they were tested for comparison [5,32]. The HT LiCoO₂ was used as a catalyst for OER. Electrochemical tests were accompanied by cyclic voltammetry (CV), chronoamperometry, generation of Tafel slopes for different temperatures, and electrochemical impedance spectroscopy (EIS).