Recovery of metals from spent lithium-ion batteries with organic acids as leaching reagents and environmental assessment

Abstract

A leaching process for the recovery of cobalt and lithium from spent lithium-ion batteries (LIB) is developed in this work. Three different organic acids, namely citric acid, malic acid and aspartic acid, are used as leaching reagents in the presence of hydrogen peroxide. The cathode active materials before and after acid leaching are characterized by X-ray diffraction and scanning electron microscopy. Recovery of cobalt and lithium is optimized by varying the leachant and H₂O₂ concentrations, the solid-to-liquid ratio, and the reaction temperature and duration. Whereas leaching with citric and malic acids recovered in excess of 90% of cobalt and lithium, leaching with aspartic acid recovered significantly less of these metals. The leaching mechanism likely begins with the dissolution of the active material (LiCoO₂) in the presence of H₂O₂ followed by chelation of Co(II) and Li with citrate, malate or aspartate. An environmental analysis of the process indicates that it may be less energy and greenhouse gas intensive to recover Co from spent LIBs than to produce virgin cobalt oxide.

Introduction

Applications of lithium-ion batteries (LIBs) as electrochemical power sources in consumer electronics and electric vehicles (EV) are increasing. LIBs have been available on the market from Sony Corp. since the early 1990s. [1] Desirable characteristics such as modest size and weight, high cell voltage, low self-discharge rates and significantly higher energy density have made LIBs preferable to typical nickel–cadmium or nickel–metal hydride batteries for mobile phones, laptops, electronic devices, and EVs. Graphite–LiCoO₂ has become the leading LIB system, which at present powers most portable electronic devices. [2] Another important application of LIBs is storage of energy from renewable but intermittent energy sources such as wind and solar. [3] World LIB production reached 500 million units in 2000 and almost 4.6 billion in 2010. [4] Consequently the end-of-life of waste LIB material is becoming an environmental burden. In China, consumer battery waste amounted to 200–500 tons year⁻¹ from 2002 to 2006 with significant amounts of metals, organic chemicals and plastics in the following proportions: 5–20% cobalt, 5–10% nickel, 5–7% lithium, 15% organic chemicals and 7% plastics. This composition varies slightly with different manufacturers [5], [6]. Aside from environmental motivations, the price of cobalt (Fig. 1), which fluctuates with the economy but has exhibited a generally increasing trend since 2000, [7] is a strong economic driver to increase LIB recycling. Lithium prices are significantly lower, but have been on the rise since 2006. [8] Therefore, the recycling of spent LIBs has strong potential to provide economic and environmental benefits in addition to conserving raw materials [9], [10], [11], [12], [13]. A LIB comprises a cathode, an anode, an organic electrolyte and a separator [14]. The cathode typically consists of Al foil covered by a fine layer of powdered LiCoO₂, while the anode is made from Cu foil covered by a fine layer of powdered graphitic carbon. Each electrode also contains polyvinylidene fluoride (PVDF), which holds the active material particles together. The electrolyte consists of a Li salt (normally LiPF₆), which is dissolved in an organic solvent. In LIBs, the anodes and cathodes are made from materials that allow the migration of Li ions through an electrolyte solution. The typical chemical composition of LIBs with LiCoO₂ as the cathode active material is shown in Fig. 2 [6].

Recycling processes typically pretreat spent LIBs with physical techniques [10] such as mechanical, thermal, mechanochemical, and dissolution processes. Next, battery components can be recovered via chemical means such as acid leaching or bioleaching, solvent extraction, chemical precipitation and electrochemical processes. Currently, the only large-volume commercial battery recycling technology employs smelting and recovers cobalt and nickel after further processing of smelter output via leaching and solvent extraction [15]. One drawback to this technology is the energy consumption of the high-temperature smelting step and its associated air pollution control equipment. Moreover, it does not recover lithium and aluminum. Acid leaching, on the other hand, is an important technique for recovering valuable metals that avoids these drawbacks. It brings metals into solution, assisted at times by a reductant (e.g., H₂O₂) that converts the metal to a more soluble oxidation state. Once in solution, the metals are more easily separated by electrochemical, precipitation or solvent extraction techniques [6], [16], [17]. Different leaching agents, such as H₂SO₄ [18], HCl [19], and HNO₃ [5] have been investigated with lithium and cobalt recoveries exceeding 99%. Notably, strong acid leachants release toxic gases like Cl₂, SO₃ and NO_x and the waste acid solution is harmful to the environment. To avoid adverse environmental impacts of battery recycling, more benign processes are under development [20].

Similarly, we are developing a low-environmental-impact recycling process using citric (C₆H₈O₇·H₂O), dl-malic (C₄H₅O₆) and l-aspartic (C₄H₇NO₄) acids as leachants to recover metals from spent-battery active materials. The three organic acids were selected because of their characteristics including easy natural degradation and the absence of toxic gases in the reaction process, and because of previous reports of other metal leaching processes [21], [22], [23], [24]. The acidity sequence of the three acids is citric acid > malic acid > aspartic acid. They are often used as raw materials in manufacturing. Sonmez and Kumar [21] studied the use of citric acid as a reagent in aqueous media to recover Pb and PbO from scrap battery paste. Wang used malic acid to dissolve the kaolinite in soil [22]. Marafi and Stanislaus [23] conducted ultrasonic-assisted leaching of Mo, V, and Ni from spent hydroprocessing catalysts in citric acid. They determined that citric acid was a superior leachant to H₂SO₄ under the conditions they examined, recovering over 95% of the three metals. Szymczycha-Medeja [24] investigated the kinetics of leaching Mo, Ni, V, and Al from spent hydrodesulphurization catalysts in an oxalic acid and hydrogen peroxide solution. We conducted the first preliminary studies into citric acid and dl-malic acid leaching [25], [26] of Co and Li from spent-battery active material (LiCoO₂) and have received a patent for this technology [27].

We have now expanded our investigation of acid leaching of spent LIBs to include aspartic acid. The effectiveness of the three organic acids at recovering lithium and cobalt under varying process conditions are compared. Furthermore, we conducted a preliminary study of the acid leaching mechanism. Finally, we conducted an environmental assessment of the process. The two most recent published analyses of LIB [28], [29] life-cycle environmental impacts do not include the impacts of battery recycling. While Rydh and Sandén [30] consider the impact of the use of recycled materials on the life-cycle energy consumption of batteries, they do not present an estimate of the energy consumption of the recycling process itself. The data in this paper could be used to address this gap and is a unique examination of the environmental impacts of a battery recycling process.