Recycling of spent lithium-ion batteries in view of lithium recovery: A critical review

Journal of Cleaner Production 228 (2019) 801e813

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Review

Recycling of spent lithium-ion batteries in view of lithium recovery: A critical review Chunwei Liu a, Jiao Lin a, b, Hongbin Cao a, Yi Zhang a, Zhi Sun a, * a

Beijing Engineering Research Center of Process Pollution Control and National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Division of Environment Technology and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China b School of Materials Science & Engineering, Beijing Institute of Technology, Beijing, 100081, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 February 2019 Received in revised form 23 April 2019 Accepted 23 April 2019 Available online 24 April 2019

Due to the rapid expanding of plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs) and electric vehicles (EVs), the projectfed demand for lithium-ion batteries (LIBs) is huge and might result in supply risks for natural lithium-containing reserves. After the service life, spent LIBs continuously accumulate in the market, and they are excellent secondary resources for lithium recovery. To alleviate resource shortage and to decrease potential environmental pollution caused by improper solid waste disposal, recycling of spent LIBs is motivated world widely in recent years. Previous studies have usually focused on the recovery of cobalt and nickel, which create high economic benefit. Recovery of lithium, however, has not been highlighted. In this article, state-of-the-art on spent LIBs recycling is discussed with emphasis on lithium recovery. In addition to understanding underlying mechanisms and physiochemistry features of various recycling methods, the possibility for industrial realization of each method is also evaluated. The complex processing steps limit the industrial implementation of hydrometallurgy-dominant methods, which usually reclaim lithium in the last step, resulting in a poor recovery efficiency of lithium. The pyrometallurgy-dominant approach is readily to scale up but lithium is lost in the slag phase. Therefore, the mild recycling (cleaner production) methods are recommended for future study since they take advantages of traditional pyrometallurgy and hydrometallurgy, and could decrease treatment temperature as well as acid/alkaline usage. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Recycling Spent lithium-ion batteries Review Lithium recovery

Contents 1. 2.

3. 4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 802 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 802 2.1. Criteria for selection of relevant literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 802 2.2. Extraction of useful information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 803 2.3. Categorization of recycling methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 803 Construction of lithium-ion batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 803 State-of-the-art for spent LIBs valorization in view of lithium recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 803 4.1. Hydrometallurgy-dominant process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 805 4.2. Pyrometallurgy-dominant process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 806 4.3. Mild recycling methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 807 4.4. Comparison of different methods with respect to recycling lithium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 809 Current industrial recycling process in view of lithium recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 809 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810 Declarations of interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811

* Corresponding author. National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology Institute of Process Engineering, Chinese Academy of Sciences, No. 1 Beierjie, Zhongguancun, Beijing, China. E-mail address: [email protected] (Z. Sun). https://doi.org/10.1016/j.jclepro.2019.04.304 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

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Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811

1. Introduction Since the 1990s, lithium batteries have widely been used in portable electronic instruments and more importantly, the lithiumion batteries (LIBs) are used to power the next generation of electric vehicles (EVs) with the aim to be environmentally friendly (Dunn et al., 2012). It is estimated that the penetration of plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs) and EVs into the vehicle market will be at 20% in 2020, resulting in as much as a threefold increase of production for the cathode material (Grey and Tarascon, 2016; Resea, 2009; Swain, 2016). Therefore, the consumption of lithium is predicted to continuously increase (Sun et al., 2017). Meanwhile, end-of-life LIBs will accumulate, and recycling of spent LIBs is a major technological challenge in the near future. Improper disposal of LIBs triggers threats to the environment and human health because they contain a high percentage of heavy metals and toxic electrolyte (Dorella and Mansur, 2007). The deposited lithium on the anode generated during each subsequent discharge-recharge cycle can react with water intensively, resulting in a potential threat. Lithium, viewed as a rare element (Sun et al., 2016), is an essential element for various LIBs due to its low density (0.534 g/ cm3) and high electrochemical potential (
3.04 V versus standard hydrogen electrode) (Shriver and Atkins, 2009). As shown in Fig. 1, since 2016, batteries, ceramics/glass, and lubricating greases are the major end-use markets for lithium, among which batteries represent the largest market value of lithium. It is projected that the requirement of lithium carbonate will increase from 265,000 tons in 2015 to 498,000 tons in 2025 (Choubey et al., 2017). Hence, lithium is forecast to witness a supply shortage in the commodity market after 2023 (Choubey et al., 2016; Sonoc et al., 2015). The huge gap between market supplies against demand leads to a continuous increase in the price of lithium carbonate. To avoid the supply risk and to decrease the production cost, it is indispensable to push metallurgical recovery of lithium from all possible resources. The concentration of lithium in spent LIBs (5e7 wt%) is far higher than in the natural resource (Kitajou et al., 2005; Swain,

2017). Thereby, the spent LIBs can be considered as a huge reservoir of lithium. In recent years, recycling of spent LIBs has been investigated more frequently with an emphasis on the extraction of cobalt, which is a high value-added metal (Bertuol et al., 2016; Dorella and Mansur, 2007; Golmohammadzadeh et al., 2017; Kim et al., 2004; Li et al., 2016; Li et al., 2010; Nayaka et al., 2016b; Paulino et al., 2008; Santos et al., 2012; Wang et al., 2017a; Zheng et al., 2016). In a typical hydrometallurgical approach, cobalt is recuperated in priority by leaching and precipitation, but the lithium usually left in the leaching solution, associated with various impurities introduced in the previous steps. Current pyrometallurgical methods aim to recycle cobalt and nickel but lithium is lost in the slag. By reviewing the previous studies on recycling of spent LIBs, it is found that nearly no articles focus on selective recycling of lithium from spent LIBs. For the sustainable management of natural lithium resources, achieving cleaner production as well as closing the loop for a circular economy, it is of great significance to emphasize and develop revolutionary recycling process to reclaim lithium. The incentive of work is to understand the process fundamentals and physiochemistry features of lithium-containing compounds in various recycling approaches. An overview of previous studies on recycling LIBs, including labscale investigations and industrial practice, is provided in light of lithium recovery. This article also offers insight toward the development of techno-economically feasible, environment-friendly and sustainable solutions to recycle spent LIBs, especially for lithium. 2. Method With the aim to identify the current state of academic insight with regards to recycling of lithium from spent LIBs, the literature reviewed in this article were selected through three steps: (i) searching the relevant literature by setting criteria in databases for identifying the pieces of target literature; (ii) extracting the useful information of the selected literature by listing a set of questions; (iii) categorizing the literature by the differences of major separation steps used in the recycling methods. These steps are described below. 2.1. Criteria for selection of relevant literature

Fig. 1. Estimated global market shares for lithium in its major applications. Data from the U.S. Geological Survey.

We searched for literature in the Google Scholar, Scopus, and Science Direct databases in November and December of 2018, using the following criteria and boundaries: (“spent lithium ion batteries” OR “spent LIBs” OR “used lithium ion batteries” OR “used LIBs” OR “end-of-life lithium ion batteries” OR “end-of-life LIBs”) AND (recycling OR reuse OR recovery OR regeneration) since the year of 1980. Filters for document type have been set to “articles”, “review” and “patents”. As a next step, articles have been considered for review that have the record of at least 80 times citations published between 1980 and 1990 or have the record of at least 50 times citations published between 1990 and 2000. Subsequently, the articles obtained from the original search were screened manually by reading the abstract. We excluded studies that (i) did not focus on reasons and mechanisms for the extraction of valuable metals; (ii) lithium recovery were not considered, and (iii) solely reuse and regeneration were investigated (recovery and recycling were not studied). To include as more relevant literature as possible, the

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literature was further expanded by reading the references of the articles encountered when reviewing other studies. In addition, to deliver an overview of the industrial recycling processes, the company websites were scanned.

pioneered and conceptual studies to guide the future efforts to drive the development.

2.2. Extraction of useful information

A LIB is composed of a cathode, an anode, an organic electrolyte and a separator (Lain, 2001). Taking a cylindrical LIB as an example, Fig. 2 (a) shows the schematic drawing representing its shape and components (Tarascon and Armand, 2001). A typical housing of the battery is iron or aluminum can body (Wakihara, 2001). An anode is usually a copper foil coated with a mixture of graphite, a conductor, binder, and electrolyte. The binder is usually made of polyvinylidene fluoride (PVDF) and the electrolyte is a solution of lithium-containing salt (such as LiPF6, LiClO4) dissolved in an organic solvent (such as ethylene carbonate, dimethyl carbonate). Similarly, the cathode is an aluminum foil coated with cathode materials, a conductor, a PVDF binder and fluoride salt (Zhang et al., 2014a). In order to prevent a short circuit between two electrodes, a separator is placed between the anode and cathode as a barrier. Goodenough et al. (Amos et al., 2016; Goodenough and Kim, 2010; Goodenough and Park, 2013; Mizushima et al., 1980; Thackeray et al., 1983; Yuan et al., 2011) have developed cathode materials such as LixMO2 (where M is Co, Ni or Mn) and LiFePO4, whose families of compounds are still used almost exclusively in today's LIBs. Generally, the LIBs are named after the cathode material, i.e. LCO (lithium cobalt oxide), NCM (lithium nickel manganese cobalt oxide), NCA (lithium nickel cobalt aluminum oxide), LFP (lithium iron phosphate), to name but a few. Fig. 2 (b) shows the schematic representation of a lithium-ion battery during discharging. Liþ migrates from negative to positive electrodes, while the current in the outside circuit is in an adverse direction. The success of lithium-ion technology depends largely on its excellent properties such as the cost, safety, cycle life, energy, and power. The major lithium-containing components of most frequently used LIBs are listed in Table 1. Each system has its own characteristics, determining its application areas. Cathode materials play a vital role in controlling batteries properties.

The useful information of the selected literature were extraction by listing a set of questions. What problem dose the article aim to solve? What methods are used? What apparatus are used (developed)? Are strong acidic and alkaline solution used? Is high temperature used? What are the major secondary pollution? Are the secondary pollution reduced, prevented or treated? Are the environmental impact studied? Are the proposed methods sustainable? Which metals are recovered? Are the recovered metals mixed or separated? Which step does the elemental separation take place? How the lithium is recovered? What is the reason/mechanism for the separation of these metals? What is the energy consumption in the recycling method? Does the study identify challenges for future research? 2.3. Categorization of recycling methods To categorize the research that have been performed to recycling spent LIBs, the recycling methods were summarized as three sub-categories. The categorization is based on the difference of key processing steps which control elemental separation, as well as the development trends. Hydrometallurgy-dominant methods. The separation of valuable metals are facilitated by leaching, precipitation and solvent extraction in the solution system, which is typically composed by acid/alkali/organics. Pyrometallurgy-dominant methods. High temperature is used to enhance the physicochemical transformation to separate valuable metals. This type of research usually carried out at temperature higher than 1400  C. Mild recycling methods. The advantages of hydrometallurgydominant and pyrometallurgy-dominant methods are integrated: the use of acid/alkali/organics are reduced or even avoided; the high temperature are lowered. This type of research may include

3. Construction of lithium-ion batteries

4. State-of-the-art for spent LIBs valorization in view of lithium recycling Valorization of spent LIBs mainly includes recovery, repair, and regeneration. Fig. 3 presents a summary of conventional paths for the recovery of spent LIBs. Prior to processing, the spent LIBs have to be discharged to prevent short-circuiting and self-ignition, consequently to avoid an explosion (Nie et al., 2015). To discharge

Fig. 2. (a) Schematic drawing showing the shape and components of cylindrical LIBs; (b) Schematic representation and operating principles of Li-ion batteries during discharging; Adapted from the reference of J. M. Tarascon (Grey and Tarascon, 2016; Tarascon and Armand, 2001).

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Table 1 Major components of common LIBs and their features. Component

Material

Weight percent of Li in the component

Properties

Ref.

Cathode

LiCoO2

7.09

LiMn2O4

3.84

Easy preparation, good capacity, and satisfactory stability; more expansive than others. Easy preparation, safe, ecological, cheap and long life; lower capacity than LiCoO2.

LiNiO2

7.11

LiNiCoMnO2 LiNiCoAlO2 LiFePO4

a

(Belov and Yang, 2008a,b; Doh et al., 2008; Fergus, 2010; Wakihara, 2001) (Fergus, 2010; Molenda et al., 2007; Thackeray, 1999; Thackeray et al., 1987; Wakihara, 2001) (Fergus, 2010; Rougier et al., 1996; Wakihara, 2001; Yamada et al., 2001) (Bak et al., 2014; Nitta et al., 2015; Yoon et al., 2012) (Amin and Maier, 2008; Biendicho and West, 2011; Wakihara, 2001) (Nishi, 2001; Scrosati and Garche, 2010; Xu, 2004)

Electrolyte a

~7.20

4.40

Lithium salt (LiPF6, Li[PF3(C2F5)3])

Cheaper, higher energy density (15% higher by volume, 20% higher by weight); less stable than LiCoO2. Cheap, stable, high capacity, stable (higher Ni content allows for higher Li extraction without structure deterioration); Cheap, stable, ecological; low capacity and low Li-ion diffusion. High ionic conductivity, good electrochemical stability

Varies depending on the specific composition of NCM and NCA.

Fig. 3. Conventional flow sheet of spent LIBs treatment processes.

the spent LIBs, salt-saturated solutions such as NaCl and Na2SO4 are commonly used (Nie et al., 2015). Once the electrolyte contacts the salt solution, highly corrosive HF will be generated through reaction 1. In addition to the release of HF, lithium incorporated in the electrolyte is lost in the solution, which is a waste of lithium.

LiPF6 þ H2 O/LiF þ POF3 þ 2HF[

(1)

Recycling of electrolyte is limited by the high recycling costs. Organic solvents extraction is an effective method to recover electrolytes from spent LIBs at the expense of introducing solvent impurities and subsequently hindering the purification operations (Bankole et al., 2013; Contestabile et al., 2001). Extraction with supercritical carbon dioxide has been confirmed as a promising method because it can decrease the solvent impurities and appears to be more moderate to heat sensitive LiPF6 (DeSimone, 2002; Grützke et al., 2014; Liu et al., 2014). By using supercritical carbon dioxide, it is possible to reuse the electrolytes since the electrolytes do not differ significantly in this approach (Bankole et al., 2013; Contestabile et al., 2001; Liu et al., 2017). To improve the recycling efficiency of valuable metals in

subsequent steps, it is suggested to apply a preliminary treatment before further processing. The pretreatment technology mainly includes mechanical separation (Pagnanelli et al., 2017; Shin et al., 2005; Vanitha and Balasubramanian, 2013), thermal process (Granata et al., 2012a; Hanisch et al., 2015; Paulino et al., 2008; Sun and Qiu, 2011), dissolution process (He et al., 2015; Li et al., 2009, 2010; Song et al., 2014) and mechanochemical methods (Guan et al., 2016; Tan and Li, 2015; Wang et al., 2016c, 2017). Since the cathode materials are covered/encapsulated by plastic or iron shell, it is required to dismantle the battery using mechanical separation. The mechanical separation is usually done either by crushing or shredding until acceptable size fractions are obtained (Georgi-Maschler et al., 2012). Valuable components of the cells are released after dry or wet crushing (Zhang et al., 2013). Separation of the fragments can be accomplished by using sorting and sieving steps, where magnetic separation and air separation methods can be employed. Due to the significant physical difference of Cu, Al and anode, they can be easily recuperated through pretreatment. The separation of anode materials can be achieved easily by struck owing to the low bonding force between copper foil and graphite caused by their different malleable property

C. Liu et al. / Journal of Cleaner Production 228 (2019) 801e813

(Shuguang et al., 2011; Zhang et al., 2014a, 2014b). A technical challenge in the mechanical separation process is to remove particulate coating from the current collector foils. A more intensive, second milling and screening step then can be applied to remove the coating. But the impurities such as the Al foils may contaminate the active battery materials after the further milling and screening (Chagnes and Pospiech, 2013). Alternatively, to separate cathode materials from Al foil and the organic binder, a thermal process is commonly used due to its simple and convenient operation. By heating the samples at 150e500  C for 1 h, organic binders can be eliminated via decomposition (Zeng et al., 2014). However, the decomposition releases hazardous gases such as HF, which needs a further cleaning system. Because of the disadvantage of the thermal process, an alternative method to peel off binders using organic reagents was investigated. N-Methylpyrrolidine, g-Butyrolactone, dimethylformamide, and dimethyl sulfoxide like toxic solvents are commonly used to dissolve the binders (Song et al., 2014). Recently, citrus fruit juice was explored as a green reagent for binder removal, although a relatively high temperature (90  C) is required (Pant and Dolker, 2017). The mechanochemical process takes advantage of high-energy ball milling to induce physical and chemical changes of active materials (Guan et al., 2016; Wang et al., 2016c). As a result, mechanochemical process changes or disrupts the inherent crystal structure of spent materials and enhance the leaching efficiency. More recently, vacuum pyrolysis is considered as an alternative pretreatment method. In the course of vacuum pyrolysis at elevated temperature, the organic materials including electrolyte, separator and binders are thermally destroyed and decompose to low molecular products (liquids or gases), which can be reused as fuel or chemical feedstock. Compared to the traditional thermal process, vacuum pyrolysis prevents toxic gases from releasing to the open environment at a lower temperature than under atmospheric pressure. Due to the oxygen-free atmosphere, Cu and Al avoid being oxidized and can be recycled in the metallic forms. More importantly, pyrolysis products or volatiles from lithium-containing organic compounds could be collected (Sun and Qiu, 2011). Recently, vacuum pyrolysis was combined with the carbothermic reduction to enhance lithium recycling efficiency, which will be will be addressed in the section of mild recycling methods. After pretreatment, the valorization approaches can be categorized as recycling (Routes 1a and 1b), repair (Route 2) and reuse (Route 3). In Route 2, spent cathode materials are repaired and regenerated using a solid phase sintering method (Song et al., 2013; Yang et al., 2017a). This method is usually cost-effective and it avoids the loss of valuable metals during the extraction process (Chen et al., 2016a; Song et al., 2017). Route 3 typically reuses Cu/Al foils and plastics directly after disassembly, and the rest materials go through Route 1. Since the present work focuses on the recycling (extraction) of lithium from spent LIBs, the following sections discuss Routes 1 in detail. According to the difference of key processing steps which control elemental separation, recycling of spent LIBs is categorized as hydrometallurgy-dominant (Route 1a) and pyrometallurgydominant (Route 1b) paths. 4.1. Hydrometallurgy-dominant process In a conventional hydrometallurgy-dominant process, the keys are leaching, precipitation and solvent extraction. Leaching of LIBs, like other metallurgical processes, is the dissolution of target active materials by leaching regents. The leaching regents mainly include inorganic acids, organic acids, and alkaline solutions. At an early stage, inorganic acids like HCl, HNO3, and H2SO4 were studied intensively.

805

Taking LiCoO2 as an example, the leaching mechanism by HCl (Wang et al., 2009; Zhang et al., 1998), HNO3 (Li et al., 2011; Myoung et al., 2002), and H2SO4 (Ferreira et al., 2009; Nan et al., 2005) can be represented as reactions (2), (3) and (4), respectively. Through these reactions, lithium and cobalt can be dissolved into the solution.

3HCl þ LiCO2 ¼ LiCl þ CoCl2 þ 1:5H2 O þ 0:25O2

(2)

3HNO3 þ LiCO2 ¼ LiNO3 þ CoðNO3 Þ2 þ 1:5H2 O þ 0:25O2

(3)

3H2 SO4 þ 2LiCO2 þ H2 O2 ¼ Li2 SO4 þ 2CoSO4 þ 5H2 O þ 1:5O2 (4) According to previous studies (Lv et al., 2018; Zeng et al., 2014), Co3þ is dominated in the cathode materials, but Co2þ is more readily dissolved than Co3þ in the aqueous phase at room temperature. In the absence of a reductant, the leaching efficiency of Co is in HCl is higher than in that of HNO3 and H2SO4, since HCl has a relative higher reducibility. The typical reductant includes hydrogen peroxide (H2O2) (Gratz et al., 2014; Guo et al., 2016; Li et al., 2015; Sa et al., 2015; Zou et al., 2013), sodium thiosulfate (Na2S2O3) (Zhang et al., 2015), and sodium bisulfite (NaHSO3) (Meshram et al., 2015; Pagnanelli et al., 2014). As illustrated by reaction (3), the application of H2O2 can facilitate the reduction of Co3þ in the case of H2SO4 leaching. Strong organic acids, such as citric acid (Li et al., 2013), formic acid (Gao et al., 2017), malic acid (Li et al., 2013), aspartic acid (Li et al., 2013), ascorbic acid (Nayaka et al., 2016a), oxalic acid (Zeng et al., 2015), and glycine (Nayaka et al., 2016b) are also effective to leach the cathode material. The leaching reaction, for instance using citric acid as leachant, could be described by reaction (5) (Chen et al., 2016b). Other organic acids present a similar reaction mechanism.

18H3 Cit þ 18LiNi1=3 Co1=3 Mn1=3 þ C6 H12 O6 ¼ 6Li3 Cit þ 2Ni3 ðCitÞ2 þ 2Co3 ðCitÞ2 þ 2Mn3 ðCitÞ2 þ 33H2 O þ 6CO2 (5) In addition, the alkaline system has been explored with respect to selectively leach Ni, Co, Cu and Li from the low-grade ore and different waste material (Meng and Han, 1996; Sun et al., 2015), whereas Mn is seldom dissolved out (Ku et al., 2016; Zheng et al., 2017). The formation of Ni and Co-containing ammine complexes under proper pH values can be described as reactions (6) and (7).

Ni2þ þ nNH3 ¼ NiðNH3 Þ2þ n

(6)

Co2þ þ nNH3 ¼ CoðNH3 Þ2þ n

(7)

Although the leaching efficiency in the ammoniacal system is usually questionable, the significance of ammonia leaching lies in its leaching selectivity, i.e., Ni, Co, Zn and Cu are more readily to be leached out due to their better complexation ability with ammonia compared with Fe, Mg, Mn and Ca. It should be noted that both Ni, Co, Mn and Li ions are leached in inorganic, organic and/or alkaline medium, indicating the separation of Li from the mixed solution is necessary for the following steps. In a hydrometallurgy-dominant process, the leaching kinetics mainly depends on leaching conditions, such as leachant and reductant concentrations, agitation speed, temperature, reaction time and solid-to-liquid ratio. In general, the leaching performance would be improved by increasing leachant and reductant

806

C. Liu et al. / Journal of Cleaner Production 228 (2019) 801e813

where A
Org þ 2ðHAÞ2Org indicates the extractant saponified by the following reaction

could change the whole process, while a physical process will not be so impacted (Danai, 2014). The low solid to liquid ratio for high leaching efficiency may reduce the process throughput (Al-Thyabat et al., 2013). In addition, a complex purification process for separating metals from the solutions is costly and causes a significant concern of extra wastewater emission. The extra cost for environmental improvement of secondary pollutions is very high. Therefore, from an economic point of view, hydrometallurgical processes are sometimes questionable because of the cost and handling of the reagents (especially if they are not recovered) (Danai, 2014). With respect to lithium recycling, by summarizing previous studies, it can be found that Li-ion is easily concentrated in the leaching solution after acid and/or alkaline leaching. Separation of Ni, Co, and Mn from the leaching solution through precipitation and/or solvent extraction has been explored and successfully developed, yet the recovery of Li was not paid much attention, i.e. recovery of Li is usually the last step of the process, as indicated by Fig. 4. In the course of separating Ni, Co and Mn, loss of Li is inevitable and accumulated. After extracting Ni, Co and Mn, the solutions generated from washing procedure in each stage of solvent extraction are merged into the raffinate, leading to a very low concentration of lithium (0.5e3 g L
1), associated with ammonium and sodium of approximately 5 g L
1 (Li et al., 2016). Hence, the overall recovery efficiency of Li is dependent on all the previous separation phases (Valio, 2017) and not satisfactory, particularly considered the market potential and value of lithium salts.

þ 2Naþ Aq þ ðHAÞ2Org ¼ 2NaAOrg þ 2HAq

4.2. Pyrometallurgy-dominant process

concentrations, agitation speed, temperature, as well as reaction time. The increase of solid-to-liquid ratio, however, decreases the leaching efficiency and kinetics significantly. Most of the previous studies focus on optimization of these factors, despite the battery types and leaching regents. In practical operation, environmental influence and processing costs should also be taken into consideration when optimizing these factors. Therefore, a comprehensive understanding of the leaching mechanism, kinetics, and processing costs will guide future efforts in optimizing hydrometallurgydominant process towards practical operation. Solvent extraction is used to obtain pure metal or metal compounds by taking advantage of the different relative solubilities of compounds in immiscible liquids. The extraction mechanism, taking di-(2-ethylhexyl) phosphoric acid (D2EHPA) and bis (2,4,4trimethylphenyl) phosphinic acid (Cyanex 272) as examples, can be described by reactions (8) and (9) (Granata et al., 2012b; Swain et al., 2008).
þ M2þ Aq þ AOrg þ 2ðHAÞ2Org ¼ MA2 ,3HAOrg þ HAq

(8)

or þ MOHþ Aq þ 2ðHAÞ2Org ¼ MðOHÞA,3HAOrg þ HAq

(9)

(10)

Extractants normally have the selectivity of different metal ions, but the selectivity is highly dependent on the equilibrium pH. For instance, D2EHPA can selectively extract Cu and Mn ions, whereas its selectivity on extracting Co ion is poor at pH 2.2e3.0 (Wang et al., 2016b). In addition, phase modifier, such as tributyl phosphate (TBP) or trioctylamine (TOA) (Darvishi et al., 2005; Virolainen et al., 2017), has been demonstrated as an effective addition to modify the pH towards a mild operation condition. From the practical operation point of view, a mild condition is preferred. Thereof, studies on the extractants and phase modifiers should be further carried out in the future. Precipitation has been widely used to extract metal or metal compounds from liquid systems. Due to the complexity of the leaching solution of LIBs, it is a challenge to precipitate a single metal ion. It has been reported that dimethylglyoxime reagent (DMG, C4H8N2O2) and ammonium oxalate are capable to effectively precipitate Ni2þ and Co2þ from the mixed solutions (Chen et al., 2015). The precipitation mechanisms are shown by following reactions.

Ni2þ þ 2C4 H8 N2 O2 ¼ NiðC4 H7 N2 O2 Þ2 þ 2Hþ

(11)

Co2þ þ ðNH4 Þ2 C2 O4 þ 2OH
¼ CoC2 O4 ,2H2 O þ 2NH3

(12)

2þ

2þ

The pyrometallurgy is usually used to extract target metals from ore and concentrates through physical and chemical transformations enhanced by high temperature. In a pyrometallurgydominant process, the recovery of valuables metals from spent batteries is achieved with the application of high temperatures and is usually associated with high amounts of atmospheric emissions (unless the emissions are controlled and treated using specific system), i.e. dioxins, fluoride compounds. Meanwhile, volatile metals can be recuperated during the process. Smelting and reduction are the key processes in a typical

1þ

After removing Ni and Co , Li can be separated by adding Na2CO3 or H3PO4, as given by reactions (16) and (17) (Chen et al., 2015; Pant and Dolker, 2017). Although the impurities associated with the Li1þ in the remaining solution may be disturbing, this method provide a feasible approach to recycle lithium from spent LIBs.

2Liþ þ CO2
3 ¼ Li2 CO3

(13)

3Liþ þ PO3
4 ¼ Li3 PO4

(14)

In hydrometallurgical techniques, a change in composition

Fig. 4. Recovery of active cathode materials from spent LIBs in a hydrometallurgydominant process.

C. Liu et al. / Journal of Cleaner Production 228 (2019) 801e813

pyrometallurgical-dominant path (Bernardes et al., 2004). Batteries are smelted with the addition of slag formers at a temperature higher than 1400  C (Georgi-Maschler et al., 2012). The typical reductant is graphite, which is a crystalline form of carbon and is the most stable form of carbon under standard conditions. The graphite may come from the anode material or external addition. After the reduction, Co is reduced to metallic form by graphite and carbon monoxide, while Li can be recovered as Li2CO3. With the application of temperature higher than the melting point of target metals, the corresponding metals form alloys while other impurities move to slag phase or form gases. Metals with low boiling points, for instance, Zn, Cd and Hg will evaporate at a high temperature. After leaving the molten phase, through control of the atmosphere, these vapors can either be recovered as metal after condensation or react with oxygen and generate dust (Müller et al., 2000). It has been demonstrated that it is possible to reclaim Zn, Ni, Cd, Pd, and other metals from spent Ni-Cd and/or Zn-Mn batteries (David, 1995; Espinosa et al., 2004). Slag formers are added before or during smelting to process a slag carrying the waste (lithium included) and off-gases. In the pyrometallurgy industry, CaO, SiO2, Al2O3, and MgO are the most frequently used oxides in slag systems. To minimize the capacity of slag for target metals (usually Co, Mn), a simple binary or ternary slag system is preferred. T. GeorgiMaschler et al. (2012) used FactSage to calculate the theoretical distribution coefficient for cobalt and manganese between metal and slag, in order to evaluate the thermodynamic property of various slag systems. The thermodynamic calculation indicates that 54.5 wt% CaO-45.5 wt% SiO2 and 50.0 wt% Al2O3-45.0 wt% CaO5.0 wt% MgO are appropriate to recycle cobalt and manganese from spent LIBs (Georgi-Maschler et al., 2012). Ren et al. (2017) applied a fayalite slag (FeO-SiO2-Al2O3) in a smelting reduction process to recycle spent Libs with Al shells. The optimized slag composition is FeO/SiO2 ¼ 0.58e1.03 (w/w), and 17.19e21.52 wt% Al2O3. After smelting at 1450  C for 30 min, an alloy of Fe-Co-Cu-Ni was obtained while the slag was composed of fayalite and hercynite. Yet the recovery of lithium was not clarified. In general, a conventional pyrometallurgical-dominant path to recycle spent LIBs is short and easy to scale up. It can also accept many variations of LIBs (Hendrickson et al., 2015). There is usually no safety risk from the leakage of electrolyte. However, a major drawback of the pyrometallurgical recycling process is that lithium cannot be effectively recovered (Georgi-Maschler et al., 2012). Lithium usually ends up in slag, associated with other ignoble metals (Sun et al., 2016). Although some researchers have investigated the recycling of lithium through the hydrometallurgy method using the slag, the energy consumption is too high (Yazami and Touzain, 1983). Usually, high temperature triggers the generation and emission of off-gas, e.g. carbon dioxide, carbon monoxide, sulfur dioxide, volatile organic compounds, and dust. Hence, the pyrometallurgical-dominant path requires a large amount of energy and a gas cleaning system, which are capital-intensive. 4.3. Mild recycling methods In addition to the traditional pyrometallurgical-dominant approaches, there are also other recently developed methods to recycle the cathode materials (so-called mild recycling method). The mild recycling methods aim to recycle all valuable metals (lithium included), decrease the energy/acid/alkaline consumption, while increase efficiencies in the uses of energy. They are usually pyrometallurgical-dominant and combines with acid/alkaline free hydrometallurgy. Vacuum carbothermal reduction and sulfation roasting are the main mild recycling methods. Fig. 5 presents the standard free energy change during carbothermal reduction of battery material

807

Fig. 5. Carbothermal reduction of battery material metal oxide metal oxides at atmospheric pressure, calculated by HSC 6.0.

metal oxides. The CO line cuts across the lines of NiO, CoO and MnO in the calculated temperature range (0e2000  C), indicating that C is a useful reducing agent to reduce the corresponding metal oxide to metal. Theoretically, NiO and CoO can be reduced at 400e500  C, while MnO is stable until 1300  C. However, Li2O cannot be reduced by C under 2000  C. Considering that reactions kinetics can be enhanced by high temperature, most of the mild recycling methods aim to decrease the temperature of traditional pyrometallurgicaldominant approaches to 500e1000  C. Roasting under reduced pressure (vacuum) was initially explored to prevent generating toxic volatile metals and gases
s et al., 2006). In reduced pressure, the gas-involved re(Konda actions are enhanced. Taking CoO as an example, the reduction of CoO by graphite is described by reaction (15).

CoO þ C ¼ Co þ CO

(15)

The Gibbs free energy of reaction (16) is




.

DG ¼ DGQ þ RTln pCO pQ



(16)

where DGQ the change of standard Gibbs free energy (J$mol
1), T temperature (K), pQ the standard pressure (101325 Pa), pCO partithe al pressure of CO (Pa). Under reduced pressure (pCO < pQ ), DG is always smaller than DGQ , indicating that the reduction reaction tends to be easier to take place. By assuming pCO ¼ pQ  10
m is the total pressure of the system, Eq. (17) can be calculated as

DG
DGQ ¼
19:146mT

(17)

Fig. 6 presents the influence of reduced pressure and temperature on reaction (15), as calculated according to Eqs. (16) and (17). By reducing pressure or increasing temperature, Gibbs free energy of the reduction reaction keeps decreasing, indicating the reaction is easier to take place. When the reactions reach equilibrium (DG ¼ 0), DGQ ¼ 19:146mT. Then the following equation is derived.

T¼

DGQ 19:146m

(18)

It is clear that with reducing gas pressure (increasing m), the equilibrium temperature is decreased, making vacuum carbothermal reduction a possible approach to recycle cathode materials as a mild recycling method. To recycle spent LiMnO2 batteries, the carbothermal reduction was carried out under reduced pressure at 650  C, which splits the

808

C. Liu et al. / Journal of Cleaner Production 228 (2019) 801e813

Fig. 6. Thermodynamic calculation on the influence of reduced pressure and temperature on the reduction of CoO by graphite.

casings and deactivated the batteries by reduction of LiMnO2 and MnO2 with residual lithium metal and graphite to form MnO and Li2CO3. The resultant lithium carbonate was selectively solubilized in water with the manganese remaining in the residue. High purity Li2CO3 was subsequently recovered by controlled evaporation
s et al., 2006). (Jandova et al., 2012; Konda Li et al. (Li et al., 2016; Xiao et al., 2017a,b) investigated recycling of spent LiCoO2, LiMn2O4 and LiCoxMnyNizO2 by using vacuum carbothermal reduction. Graphite, stripped from the anode, was used as a reductant. The process was “in situ” recycle which means to transfer the spent LIBs into resources (cobalt or lithium salt) by its own electrode materials. The furnace was pumped to achieve a vacuum environment, then the carbothermal reduction was performed at a temperature below 1000  C for 30 min, followed by wet magnetic separation. The gas production is quite low in this process. After the reaction, these gaseous products can be collected to use as clean energy to avoid disposal cost (Georgi-Maschler et al., ~ ez et al., 2016). Nickle and cobalt are recovered in the 2012; Ordon

metallic form while manganese is recovered as MnO. Li2CO3 is leached from roasted powders by water and finally, high valueadded Li2CO3 crystals are further gained by evaporating the filtered liquid. The reaction mechanism was explored by taking LiMn2O4 as an example, and the plausible pathway of the conversion reaction is illustrated in Fig. 7 (Xiao et al., 2017c). Cubic spinel LiMn2O4 starts collapsing and releasing Li elements, resulting in a distorted tetragonal spinel Mn3O4 over 400  C because the skeleton O tends to escape away as O2 at oxygen-free conditions (Hwang et al., 2001; Treuil et al., 1999). In this stage, some Li elements released from the O-tetrahedron of collapsed LiMn2O4 transferred into empty O-tetrahedrons of other cubic spinel LiMn2O4 to form distorted Li2Mn2O4. Furthermore, when the temperature rises from 600 to 700  C, the skeleton O was captured by C/CO and the distorted tetragonal spinel Mn3O4 and LiMnO2 were further collapsed. Finally, distorted Mn3O4 and LiMnO2 were totally collapsed and converted into the NaCl-structured MnO. The Li element was fully released and reacted with CO2 to form Li2CO3. After vacuum carbothermal reduction, Li is recycled as Li2CO3. But the solubility of Li2CO3 in water is relatively low (Ullmann, 2003), leading to a low concentration of lithium salt (<0.5 g L
1) and poor leaching efficiency. To overcome such a significant drawback, Hu et al. applied carbonated water leaching to treat the roasted products, then lithium in the roasted products could be leached as LiHCO3 (Hu et al., 2017) via reaction (19).

Li2 CO3 þ CO2 þ H2 O ¼ 2LiHCO3

(19)

By injecting CO2 gas into water, Li2CO3 in the roasted products transforms to more soluble LiHCO3. Consequently, the leaching efficiency can be improved. Wang et al. reported that lithium concentration was increased to 4.4 g L
1 by carbonated water leaching under a CO2 flow rate of 20 mL min
1, the liquid-solid ratio of 10:1, leaching time of 2 h at ambient temperature. It should be noted that prior to reduction roasting experiments, Hu et al. (2017) mixed the cathode powder with a certain amount of lignite by a

Fig. 7. Plausible pathways for the conversion of mixed powders from spent LiMn2O4 batteries at enclosed vacuum conditions.

C. Liu et al. / Journal of Cleaner Production 228 (2019) 801e813

planetary ball mill for 1 h, yet it is not clarified whether the mechanochemical treatment effect the reduction reaction and leaching. Previous studies have reported that by using mechanochemical activation, the crystal face which is more readily destroyed may transform to disordered states, and subsequently, the leaching efficiency is significantly enhanced (Guan et al., 2016; Wang et al., 2016c, 2017; Yang et al., 2017b). Therefore, a comprehensive understanding of the fundamentals of mechanochemical activation and carbothermal reduction on recycling of spent LIBs is necessary, especially for lithium recovery. Besides vacuum carbothermal reduction, sulfation roasting is also a promising method to recycle spent LIBs in mild operating conditions. Combination of sulfation roasting and water leaching has been successfully applied to recycle rare-earth elements (REEs) € from bauxite residue (Borra et al., 2016) and NbFeB magnets (Onal et al., 2015), as well as to recycle nickel from laterite and chromiferous overburden (Kar and Swamy, 2000). In general, sulfation roasting is mainly composed of three stages, namely sulfation, roasting, and leaching. Sulfation employs sulphuric acid or aggressive salt sulfate to transfer powdered samples into a sulfate mixture, i.e., most oxides are converted to their corresponding sulfates during the sulfation process. During subsequent roasting, some unstable sulfates decompose to oxides. Other sulfates, on the other hand, are relatively stable during roasting. In the final phase, the stable sulfate can dissolve during water leaching, leaving the other oxides in the residue. Most of the researches focus on the effects of temperature, roasting time, amount of acid/salt and external additions on leaching of the different elements. Wang et al. (Wang et al., 2014; Wang et al., 2016a) studied the elemental transfer during sulfation roasting of spent LiCoO2. The sulfation was done by mixing a certain amount of K2S2O7 with LiCoO2 powder, then the mixture was roasted for 0.5 h at the temperature less than 600  C. It was demonstrated that Co and Li gradually transfer to KLiSO4, K2Co2(SO4)3, accompanied with SO2, O2 gas releasing. However, Wang et al. aimed to convert all the metals elements into water-soluble salts, but the decomposition of sulfates was neglected. Obviously, the efficiency of sulfation roasting is determined by the thermodynamic stability of most of the considered metal sulfates, which is calculated (see the supplementary S1) and listed below in increasing order (in the temperature from 500 to 1000  C) (Saikkonen, 1984). Al2(SO4)3 < Fe2(SO4)3 < CuSO4 < NiSO4 < CoSO4 < MnSO4 < Li2SO4 Li2SO4 is the most stable one among these sulfates. Hence, with an appropriate selective roasting treatment. It is possible to transform the sulfates of almost all impurity metals (except for lithium) into their respective oxides while lithium remains in its sulfate form. However, the relative stability of metal sulfates cannot be predicted simply by thermodynamic data. It should also be noted that the thermal stability of any metal sulfate is also dependent on other factors (e.g., heating rate) that could shift its decomposition

809

range from the reported cases (Poston et al., 2003; Wendlandt, 1958; Wendlandt and George, 1961). Although selectivity of roasting with aggressive salt sulfate is poor, it shows the feasibility to recycle lithium and lower the energy consumption. To provide a better selectivity, it is suggested to employ sulphuric acid instead of sulfate salt to react with spent cathode materials, then the mixture is subjected to roasting and water leaching. Compared to the traditional pyrometallurgydominant process, sulfation roasting consumes lower energy, but the application of sulfur triggers aggressive corrosion of the furnace due to the release of SO2 and SO3. Hence, it is still a challenge to implement sulfation roasting technologies in industrial production. 4.4. Comparison of different methods with respect to recycling lithium Table 2 presents the comparison of different treatment methods with emphasis on the recycling of lithium. Overall, it is possible to recycling high-purity lithium using hydrometallurgy-dominant method at the expense of long processing flow. In a traditional pyrometallurgy-dominant process, recycling of lithium cannot be effectively achieved since it easily ends up in the slag phase. Mild recycling method integrates the pyrometallurgy and hydrometallurgy treatments, indicating promising research and application prospects. 5. Current industrial recycling process in view of lithium recycling Recycling of spent LIBs is relatively new in comparison with the recycling of other sorts of batteries, for instance, NiCd and Pb-acid batteries. At present, the recycling of spent LIBs is combined with existing large-scale processes (e.g. extractive cobalt or nickel metallurgy), which are normally not dedicated to battery recycling. This approach is common practice and very often an economical advantage (Georgi-Maschler et al., 2012). Table 3 summarizes the current industrial recycling process with emphasis on lithium recycling. Umicore technology deploys a typical pyrometallurgydominant path, where potentially hazardous pretreatment is avoided (Daniel and Sven, 2014; Meshram et al., 2014). The spent batteries are directly melted down using a unique ultra-high temperature (UHT) technology with the addition of slag formers. After that, an alloy of valuable materials (Co, Ni, Cu, Fe) is obtained (Sonoc et al., 2015). Plastics, solvent and graphite are burnt and leave as gases. The lithium, aluminum and manganese are not recovered and end up in the slag (Georgi-Maschler et al., 2012). The UHT technology is designed to safely treat large volumes of different types of complex metal-based waste streams. Batteries are combined with coke and slag formers (limestone, sand) and fed into the furnace. The feed should contain 30-50 wt% spent batteries in order to produce a product with an economically viable content of cobalt and nickel (Daniel and Sven, 2014). Air, pre-heated to 500  C, is fed

Table 2 Comparison of different methods with respect to recycling lithium. Different treatments

Advantages

Disadvantages

Hydrometallurgy-dominant process

Possible to recycle lithium; room temperature operation; purity is high;

Pyrometallurgy-dominant process

Easy to scale up; simple pretreatment; acid/alkaline-free; Relatively lower energy; satisfactory recycling efficiency;

Low concentration of lithium (0.5e3 g L
1) with high impurities in the solution after extraction of other metals; recycling efficiency is questionable; consumption of acid/alkaline solution; High energy consumption; emission of off-gas; lithium ends up in the slag (further treatment is needed); Emission of off-gas;

Mild recycling method

810

C. Liu et al. / Journal of Cleaner Production 228 (2019) 801e813

Table 3 Current major commercial processes for recycling spent LIBs (2018a; 2018b; 2018c; 2018d; 2018e; 2018f; Knights and Saloojee, 2015; Lv et al., 2018; Pistoia et al., 2001). Company/process

Capacity (tonnes/year)

Main products

Technology

Li recovery

TOXCO (Retriev) Accurec GmbH Inmetco Green Eco-Manufacture Hi-Tech Co Akkuser Ltd Bangpu Ni/Co High-Tech Co. Sumitomo-Sony Batrec AG SNAM ERAMET (Valdi) Nippon Recycle Center Co. DK Recycling und Roheisen GmbH Umicore Glencore plc. (Xstrata)

4500 6000 6000 20,000 4000 3600 150 200 300 20,000 5000 NR 7000 7000

Li2CO3, mixed metal oxides Co alloy, Li2CO3 Co alloy NiCo alloy, Ni, Co, Co3O4 Metal powder Cathode materials, Co3O4 Co alloy, Co metal Battery scraps NR Raw materials for special steel Raw materials for special steel NR Ni-Co alloy, NiCO3, NiSO4, CoCO3, CoSO4 Co alloy

Hydro-dominant Pyro-dominant Pyro-dominant Hydro-dominant Pyro-dominant Hydro-dominant Pyro-dominant Pyro-dominant Pyro-dominant Pyro-dominant Pyro-dominant Pyro-dominant Pyro-dominant Pyro-dominant

Yes Yes No No No No No No No No No NR No No

NR: not reported.

through the bottom of the furnace. The furnace is divided into three temperature zones: the pre-heating zone, the plastic pyrolyzing zone and the smelting zone, as shown in Fig. 8 (Knights and Saloojee, 2015). As shown in Table 3, most of the industrial process focus on recycling Co and Ni, except for TOXCO and Accurec GmbH process which also aim to recycle lithium. TOXCO process is one of the early hydrometallurgy-dominant processes that are devoted to the recycling of LIBs (McLaughlin and Adams, 1999). It includes submerging spent batteries in liquid nitrogen followed by crushing and placing the batteries in aqueous solution with high pH controlled by lithium hydroxide. Different lithium salts were precipitated, filtered, and finally, lithium carbonate was formed by pumping CO2 into the solution. The Accurec GmbH process uses the pyrometallurgical method to recover cobalt manganese alloy and lithium chloride. The process starts with the removal of electronics and plastic casings followed by vacuum thermal treatment and pyrolysis. The electrode material is pressed to form briquettes which are put in a smelting furnace for reduction. As a result, a metallic cobalt manganese alloy and a slag that contains the lithium are obtained. The slag is leached with acid and the lithium is precipitated as lithium chloride (Gabriel, 2016). Recently, to recover all battery components especially Li, and realize cost-neutral and sustainable, Accurec GmbH cooperates

Fig. 8. Temperature zones in the UHT technology used in Umicore.

with other researchers (Tr€ ager et al., 2015) launches a project which shows that a pyrometallurgical recovery of lithium from lithiumion traction batteries is possible. The underlying decomposition and evaporation reactions take place under vacuum and under a nitrogen atmosphere. They start already at about 1400  C and can be accelerated at a higher temperature, but the decomposition and evaporation of lithium compounds from automotive Li-ion batteries are not explored yet. 6. Conclusions and outlook Recycling of spent LIBs alleviates environmental pollution and reliefs the shortage of natural resources, contributing to the sustainability of the environment, economy, and humans. Most of the previous studies have focused on recycling cobalt and nickel, but few highlights the selective extraction of lithium. The underlying mechanism and physiochemistry features of various recycling methods have been summarized. Based on the difference of major separation steps, these recycling approaches are categories as pyrometallurgy-dominant, hydrometallurgy-dominant, and mild recycling methods. Both lab-scale investigations and industrial practice are reviewed with emphasis on lithium recovery. At present, seldom lithium extraction is industrially implemented from spent LIBs. Therefore, future efforts are suggested to focus on not only shortening the process steps but also improving the efficiency of metal extraction and separation, especially for lithium recovery. Specifically, the following conclusions could be reached: (1) High recycling costs limits lithium recovery from the electrolyte (LiPF6). Improper disposal of the spent electrolyte is a waste of lithium. Supercritical carbon dioxide method appears to be a promising method to recover lithium from the electrolyte, but more efforts in this field are needed. (2) Recent lab-scale studies have developed based on hydrometallurgy-dominant methods, which are normally complex to recover the valuable metals in a long process with high cost. Though literature investigation shows that recycling efficiency using hydrometallurgy-dominant methods is satisfied, whether the lab-scale technologies can be implemented at industrial scale is questionable. Moreover, Li-ion is easily concentrated in the leaching solution, associated with Ni, Co and Mn ions. Most of the previous studies highlight recovery of Ni, Co and Mn, but Li is usually the last step of the process. Consequently, the recovery efficiency of Li is not satisfactory. Therefore, future studies are suggested to consider process intensification while addressing selective extraction of Li.

C. Liu et al. / Journal of Cleaner Production 228 (2019) 801e813

(3) The pyrometallurgy-dominant approach is the most frequently used method in the industry since it is short and readily to scale up. But the energy consumption is high and lithium usually lost in slag. Treatment of off-gas and dust results in longer processing steps. Hence, points that should be considered include decrease the energy consumption and recycle lithium effectively. (4) As an alternative method to traditional recycling methods, mild recycling method to recycle the cathode materials has been explored. The mild recycling methods combine lowtemperature pyrometallurgy (below 1000  C) and mild condition hydrometallurgy (acid and alkaline free). With the aim to recycle lithium, decrease the energy consumption, and avoid the application of acid or alkaline, vacuum carbothermal reduction and sulfation roasting have shown promising potentials, although the mechanisms of physical and chemical changes in the recycling process need to be deeply explored. Also, the mechanochemistry-assisted carbothermal reduction should be further studied.

Declarations of interest None. Acknowledgment The authors acknowledge the financial support on this research from the National Key Research and Development Program of China (2017YFB0403300/2017YFB043305), National Natural Science Foundation of China under Grant No. 51425405 and 51874269, as well as 1000 Talents Program of China (Z.S.). Chunwei Liu gratefully acknowledges the financial support of China Postdoctoral Science Foundation (2018M631579) and Postdoctoral International Exchange Program (199100). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.04.304. References Danai, M., 2014. An Approach to Beneficiation of Spent Lithium-Ion Batteries for Recovery of Materials. Metallurgical and Materials Engineering. Colorado School of Mines, p. 179. Al-Thyabat, S., Nakamura, T., Shibata, E., Iizuka, A., 2013. Adaptation of minerals processing operations for lithium-ion (LiBs) and nickel metal hydride (NiMH) batteries recycling: critical review. Miner. Eng. 45, 4e17. Amin, R., Maier, J., 2008. Effect of annealing on transport properties of LiFePO4: towards a defect chemical model. Solid State Ionics 178 (35), 1831e1836. Amos, C.D., Roldan, M.A., Varela, M., Goodenough, J.B., Ferreira, P.J., 2016. Revealing the reconstructed surface of Li[Mn2]O4. Nano Lett. 16 (5), 2899e2906. Bak, S.M., Hu, E., Zhou, Y., Yu, X., Senanayake, S.D., Cho, S.J., Kim, K.B., Chung, K.Y., Yang, X.Q., Nam, K.W., 2014. Structural changes and thermal stability of charged LiNixMnyCozO2 cathode materials studied by combined in situ time-resolved XRD and mass spectroscopy. ACS Appl. Mater. Interfaces 6 (24), 22594e22601. Bankole, O.E., Gong, C., Lei, L., 2013. Battery recycling technologies: recycling waste lithium ion batteries with the impact on the environment in-view. J. Environ. Ecol. 4 (1), 14e18. Belov, D., Yang, M.-H., 2008a. Failure mechanism of Li-ion battery at overcharge conditions. J. Solid State Electrochem. 12 (7), 885e894. Belov, D., Yang, M.-H., 2008b. Investigation of the kinetic mechanism in overcharge process for Li-ion battery. Solid State Ionics 179 (27), 1816e1821.
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