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Electrochemical Energy Reviews

, Volume 1, Issue 4, pp 461–482 | Cite as

The Recycling of Spent Lithium-Ion Batteries: a Review of Current Processes and Technologies

  • Li Li
  • Xiaoxiao Zhang
  • Matthew Li
  • Renjie Chen
  • Feng Wu
  • Khalil Amine
  • Jun Lu
Review article
  • 1.5k Downloads

Abstract

The application of lithium-ion batteries (LIBs) in consumer electronics and electric vehicles has been growing rapidly in recent years. This increased demand has greatly stimulated lithium-ion battery production, which subsequently has led to greatly increased quantities of spent LIBs. Because of this, considerable efforts are underway to minimize environmental pollution and reuse battery components. This article will review the current status of the main recycling processes for spent LIBs, including laboratory- and industrial-scale recycling processes. In addition, a brief review of the design and reaction mechanisms of LIBs will be provided, and typical physical, chemical, and bioleaching recycling processes will be discussed. The significance of recycling will also be emphasized in terms of economic benefits and environmental protection. Furthermore, due to the unprecedented development of electric vehicles, large quantities of retired power batteries are predicated to appear in the near future. And because of this, secondary uses of these retired power batteries will be discussed from an economic, technical, and environmental perspective. Finally, potential problems and challenges of current recycling processes and prospects of key recycling technologies will be addressed.

Graphical Abstract

Distribution of typical LIBs recycling companies around the world.

Keywords

Spent lithium-ion batteries Recycling Leaching Secondary use 

1 Introduction

Since the 1980s, the development of portable electronic devices (i.e., cellular phones, notebook computers, and video cameras) has led to a growing demand for rechargeable batteries with higher capacities or reduced size and weight for a given capacity [1, 2]. At the time however, conventional rechargeable batteries that were under development (e.g., lead–acid, nickel–cadmium, and nickel–metal hydride batteries) were facing limitations in their achievable energy densities [3, 4]. Because of this, there was an urgent need for the development of new electrode materials with higher energy densities. This urgent need was finally resolved when in 1979, Goodenough et al. [5] tested LiCoO2 (LCO) as a cathode material. And after further development, the lithium-ion battery (LIB) was commercialized by SONY in 1991 and by a joint venture of Asahi Kasei and Toshiba in 1992 [6].

Currently, LIBs, because of their high power and energy density, high voltage, long storage life, low self-discharge rate, and wide operating temperature range, have been extensively used as the electrochemical power source in electronic devices, electric vehicles (EVs), and energy storage systems (ESSs) [7, 8, 9, 10, 11, 12]. And as the most popular battery technology with the highest growth during the last decade, the production of commercial LIBs in 2015 was over 100 GWh, and it is forecasted that the growth of LIB technologies in global markets will be close to $32 billion by 2020 [13], in which more than 80% of the LIB market in 2020 is predicted to be occupied by ESSs and EVs. Accordingly, the quantity and weight of spent LIBs in 2020 are predicted to surpass 25 billion units and 500 thousand tons [14], containing significant amounts of valuable metals and toxic chemicals.

Among the precious metals in LIBs, Co is considered strategically important because it has many industrial and military uses [15, 16]. The use of Co in LIBs accounts for about 25% of the global Co demand and in 2010, the world resources for cobalt were estimated at about 15 million tons [15]. As seen in Fig. 1, Co is about twice as expensive as Ni and ten times more expensive than Mn [17] with the average price of Co being $55,496 per ton as reported by the London Metal Exchange in 2017. In addition, the average price of battery-grade Li2CO3 has tripled over the last decade in China, reaching almost $25,000 per ton in 2017 [18].
Fig. 1

Average prices of Co, Ni, Mn, Cu, and Al from 2010 to Jan. 2018.

Data obtained from [17]

Although LIBs are technically considered “green”, the solvent inside LIBs is flammable and toxic gases such as HF and PF5 can be released if LIBs are burned or exposed to air or water [15, 19]. In addition, Co and Ni that are used in LIBs are also classified as carcinogenic, mutagenic, and toxic to human reproduction [15]. Therefore, the obvious reason for recovering spent LIB metals is the reduction of virgin material production and the prevention of environmental pollution. LIB recycling is also important to conserve resources and necessary to recover high cost metals from an economic perspective [16, 20, 21]. Therefore, the recycling of spent LIBs is highly desirable because it prevents environmental pollution and provides alternative sources of high cost materials such as Li, Co, Cu, etc.

The purpose of this article is to review the progresses achieved so far for spent LIB recycling technologies and in this article, the design and reaction mechanisms of LIBs will first be summarized. Following this, main recycling processes will be systemically introduced, including physical, chemical, and bioleaching methods. Typical industrial-scale treatment methods of spent LIBs applied around the world will also be summarized. As a large number of retired power batteries will be appearing in the near future because of the unprecedented development of EVs, secondary uses of these retired power batteries will also be discussed in this article from an economic, technical, and environmental perspective. Finally, potential problems and challenges of current technologies and processes will be discussed and future prospects will be presented for the recycling of spent LIBs.

2 Structure and Reaction Mechanism of Lithium-Ion Battery

In its most conventional design, an LIB contains a cathode, an anode, and an electrolyte consisting of dissociated salts (e.g., LiPF6) in a mixed organic solvent imbedded in a separator [22]. The design parameters and chemical composition of a typical LIB are provided in Fig. 2 [23]. In general, the cathode of an LIB usually consists of an oxide formed by a lithium metal oxide (e.g., LiCoO2) that can produce higher potentials, and the anode usually consists of graphitic carbon, which can hold Li in its layers [3, 12]. A separator felt is also used to separate the electrodes, and a nonaqueous electrolyte is used to transport Li ions between electrodes [24, 25, 26]. One important characteristic of LIBs is that both electrodes can reversibly insert and remove Li ions from their respective structures, allowing the charge and discharge of the batteries in which Li ions can be removed from the layered oxide compound and intercalated into the graphite layers during the charging process, and reversed during the discharge process. Figure 3 shows a typical LIB design and the structures of commonly used cathodes [8]. The reaction mechanism in LIBs can be illustrated by using the following process involving the reversible extraction and insertion of Li ions between two electrodes with the concomitant removal and addition of electrons:
Fig. 2

Typical design parameters and chemical structure of a C/LiCoO2 18,650 cylindrical cell (PC = propylene carbonate, DEC = diethyl carbonate; PP = polypropylene).

Data obtained from [23, 88]

Fig. 3

Illustration of the reaction mechanism and typical nanotechnologies applied in various cathodes. Reproduced with permission from [8].

Copyright 2016 The Royal Society of Chemistry

$$ y{\text{C}} + {\text{LiCoO}}_{2} \leftrightarrow {\text{Li}}_{x} {\text{C}}_{y} + {\text{Li}}_{(1 - x)} {\text{CoO}}_{2}, x \sim 0.5,y = 6,{\text{ voltage}} \sim 3.7{\text{ V}} $$

3 Recycling Processes

The aim of recycling is to separate the constituents of a waste product into different fractions and reintroduce these fractions back into production. The goals of recycling are to produce useable products, minimize waste, and treat hazardous substances in an energy efficient and economical way [27, 28, 29, 30]. In the case of spent LIB recycling, the recovery of Co and Li is the primary objective because these metals are more precious and important [31]. In general, based on reaction features, the recycling processes of spent LIBs can be differentiated into three categories: physical, chemical, and biological. In practice however, different processes are usually combined to obtain satisfactory recycling efficiencies. And in addition to the actual recycling process, pretreatments are usually necessary for the sake of safety and convenience [32, 33, 34] and a discharge pretreatment step is commonly used to remove excess capacity [35, 36, 37]. For example, Li et al. [33] investigated the influence of NaCl concentrations and discharge times on LIB discharge efficiencies (Fig. 4a) [33] and their results showed that the highest discharge efficiency of ~ 72% can be obtained with a 10 wt% NaCl solution and a discharge time of 358 min.
Fig. 4

a Effects of NaCl concentration and time on discharge efficiencies. Reproduced with permission from [33], Copyright 2016 Elsevier. b Released gas concentration changes during different phases of a discontinuous crushing process. Reprinted with permission from [32], Copyright 2016 The Author(s). c The mass percentage BOMs (bill of materials) for LiCoO2 cathode batteries and the distribution of metallic components in each size fraction of the battery pack (materials with less than 5% content are combined in each fraction and labeled as ‘‘Others”). Reprinted with permission from [34].

Copyright 2015 Elsevier

3.1 Physical Processes

Physical recycling processes are usually applied as pretreatments to separate cathode materials from other components such as current collectors and binders so as to reduce impurities and facilitate subsequent recycling processes [38, 39]. And based on the different physical characteristics of LIBs (including density, solubility, magnetic property, etc.), mechanical separation and organic solvent dissolution are common physical methods applied in the recycling process.

3.1.1 Mechanical Separation

The mechanical separation of spent LIBs aims to remove outer plastics and shells to concentrate inner metallic components and commonly used methods include crushing [32, 34, 40, 41], sieving [34, 41, 42], flotation [43, 44, 45, 46], gravity separation [32, 40, 42, 47, 48], and magnetic separation [36].

In terms of crushing, potential hazards such as explosion and combustion can occur [32]. Therefore, precautions such as inert atmospheres and low temperature environments are applied for safety reasons [34, 38]. Gases are also released in the crushing process and Diekman et al. [32] found that dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and CO2 were the main active gas components released during crushing (Fig. 4b) in which both the electrolyte component and the state of battery health determined the concentration of gas released. In addition, crushing not only changes particle size and distribution of components, it can also change the surface properties of electrode materials, negatively influencing subsequent flotation separation processes [46].

Sieving is another preliminary physical method to separate different components of LIBs. For example, plastics, separators, Al foils, and Cu foils mainly exist in coarse fractions (> 1 mm), whereas active electrode materials exist in fine fractions (< 1 mm) [34]. And by applying crushing and sieving in a combined mechanical pretreatment process, various components can be separated into a controllable range of particle sizes, which according to Shin et al. [49], is beneficial to the acid leaching step. In a systematic investigation of sieving, Gaustad et al. [34] investigated five levels of size fractions (< 0.5 mm, 0.5~1 mm, 1~2.5 mm, 2.5~6 mm, and > 6 mm) obtained from sieving and studied the effects of pre-sorting on the segregation efficiency of spent LIBs. And in the results of the study (Fig. 4c), in the case of LCO, Co was found to be the only dominant element in the fine size fraction (< 1 mm), with a high portion of 85%, demonstrating the beneficial role of pre-sorting and sieving on overall separation efficiency.

Flotation separation is based on the wettability difference between cathode and anode materials, in which layered cathode materials are usually hydrophilic and graphite anode materials are usually hydrophobic. One drawback to this separation process however, is that electrode materials after crushing are often coated with organic layers from the decomposition of electrolytes (Fig. 5a, b) [46]. Therefore, modifications are required to regain wettability differences. Here, Fenton’s reagent has been shown to be effective in the assistance of flotation separation, in which under an optimal condition of a Fe2+/H2O2 ratio of 1:120 and a solid-to-liquid (S/L) ratio of 75:1, most organic layers can be removed (Fig. 5c, d) [43].
Fig. 5

a Structural schematic of the fine particles in crushed products of spent LIBs; b C 1s XPS spectra of the fine crushed products. Reproduced with permission from [46], Copyright 2014 Elsevier. TEM images of LCO particles before c and after d Fenton modified flotation. Reproduced with permission from [43].

Copyright 2016 Elsevier

Finally, components with different densities can be separated based on gravity separation, whereas magnetic impurities such as iron can be eliminated with the magnetic separation process. In laboratory-scale studies however, spent LIB cells are usually dismantled manually into different parts with knives and saws (Fig. 6), and experiments are conducted with protective measures such as safety glasses, masks, and gloves [39].
Fig. 6

Images of the different components dismantled manually from spent 18,650 type LIBs: a spent LIBs, b plastic and metallic shells, c cathode, anode, and separator, d Al foils after immersion in NMP, and e active materials after filtration and drying. Reproduced with permission from [39].

Copyright 2012 Elsevier

3.1.2 Dissolution Process

Because cathode and anode materials are attached to current collector plates (Cu and Al foil) by using solubilized polyvinylidene difluoride (PVDF) binders, organic solvents are required to dissolve PVDF binders to separate electrode materials from Al foils [50, 51, 52]. Based on this dissolution process, Contestabile et al. [50] experimented with various separation approaches and found that the best method of separation was a treatment with NMP at roughly 100 °C for 1 h. This is because NMP demonstrates good solubility for PVDF (around 200 g kg−1 of solvent) and possesses a high boiling point (about 200 °C). In addition, this treatment allows for the recovery of both Cu and Al in their metallic form through simple filtration from the NMP solution. Overall, submerging electrodes into N-methylpyrrolidone (NMP) is the most convenient method to separate active materials from Cu and Al, and in addition, NMP is easily vaporized for recovery.

To further improve the recovery efficiency of dissolution processes, ultrasonic-assisted technology was introduced by Li et al. [53] who proposed a new process of recovering Co from spent LIBs, in which ultrasonic washing was tested as an alternative for the first time to improve Co recovery efficiency and reduce energy consumption and pollution. Here, separation by ultrasonic washing should be conducted at temperatures below 55 °C, which conserves energy and produces better recovery results. Ultrasonic-assisted technology also has many applications in chemical separation processes as well. And although dissolution processes are effective at separating electrode materials from current collectors, high cost and high toxicity of organic solvents are issues that need to be addressed.

3.2 Chemical Processes

Chemical separation processes usually entail thermal treatment, mechanochemical processes, and leaching, followed by purification (e.g., chemical precipitation, solvent extraction, and electrodeposition) to obtain Co and Li compounds.

3.2.1 Thermal Treatment

Thermal treatments can be divided into two categories: (1) a thermal treatment process, which can be applied to remove organic components, binders, and electrolytes from spent LIBs and separate materials from current collectors [54, 55, 56, 57, 58]; and (2) pyrolysis, which can be used to produce compounds or electrode materials (will be discussed in the following re-synthesis section) [59, 60, 61, 62]. And depending on the nature of the components, products with different purities and compositions can be obtained by controlling the atmosphere and temperature of the thermal treatment. For example, Lee and Rhee [55] developed a two-step thermal treatment process in which spent LIB materials were thermally treated at 100~150 °C for 1 h to separate the materials from the current collector, followed by calcination at 500~900 °C for 0.5~2 h to burn off carbon and binders. And recently, in another example, an investigation was conducted into a vacuum pyrolysis method which demonstrated that the method can peel cathode powders (composed of LiCoO2 and CoO) completely from Al foils at a temperature of 600 °C followed by vacuum evaporation for 30 min at a residual gas pressure of 1.0 kPa.

Furthermore, an in situ reduction pyrolysis process was recently developed by Xu et al. [60] that showed great potential in industrial applications, in which LCO and graphite were in situ calcined at 1000 °C under N2 atmosphere for 30 min, and Co, Li2CO3, and graphite were formed as products, which were subsequently separated by using a wet magnetic separation method (Fig. 7a). Here, the researchers also conducted thermodynamic analysis of the process and confirmed the feasibility of the overall reaction (Fig. 7b). In addition, this in situ pyrolysis method was found to be also applicable to the recycling of LMO and NCM cathodes [61, 62].
Fig. 7

a Flowchart of the oxygen-free roasting/wet magnetic separation process for the recovery of Co, Li and C from spent LIBs; b Relationship between ΔG and temperature for different possible reactions among LCO, C, CoO, and O2. Reproduced with permission from [60].

Copyright 2015 Elsevier

Thermal treatment processes possess advantages such as a simple operation and lower costs but possess obvious disadvantages such as air pollution and high-energy consumption. In practice, this method is usually used as a pretreatment step in combination with other processes to recycle spent LIBs.

3.2.2 Mechanochemical Reaction

Mechanochemical (MC) reactions refer to chemical reactions induced by mechanical energy, which is usually provided by high-energy milling [63, 64]. These reactions can reduce particle sizes and break crystal structures of spent LIB materials and are usually applied as pretreatment processes to facilitate subsequent leaching processes through the formation of more soluble compounds [65, 66, 67, 68, 69]. The advantage of the MC method becomes especially evident when we are dealing with LFP cathodes, which are difficult to dissolve in acid due to its stable structure. For example, Yang et al. [70] recently investigated a MC reaction and used it as a pretreatment process to recycle spent LFP batteries (Fig. 8) in which EDTA-2Na was chosen as the grinding aid. In this study, the effects of various parameters on the MC reaction as well as the subsequent acid leaching process were systematically investigated, including activation times, mass ratios of cathode powder to grinding aid, acid concentrations, S/L ratios, and leaching times, and the researchers found that leaching efficiencies were improved significantly if the MC reaction was applied as a pretreatment process, in which Fe recovery efficiencies improved from 40% to 97.67% and Li recovery efficiencies improved from 60% to 94.29% (Fig. 8a). The XRD and Fourier transform infrared (FTIR) analyses (Fig. 8b, c) in this study also revealed that the crystal structure of LFP cathodes was destroyed during the MC reaction.
Fig. 8

a Leaching efficiencies of Fe and Li from different samples (MC ground sample, grinding time = 5 h, mass ratio of LFP to EDTA-2Na = 3:1; leaching parameters: H3PO4 = 0.5 M, S/L ratio = 40 g L−1, leaching time = 1 h); b FTIR patterns of LFP for the MC reaction at different grinding times; c XRD intensities of the lattice faces at different grinding times; d FWHMs (full width at half maximum) of the lattice faces at different grinding times. Reproduced with permission from [70].

Copyright 2017 American Chemical Society

3.2.3 Leaching

Leaching is generally used to transfer metals from active materials to solutions for further refining. In addition, leaching processes can be selective toward specific elements and acid leaching is a standard procedure in many recovery processes. As for the recycling of spent LIBs, acid leaching is used to transfer active materials such as Co and Li elements into solutions for further refining and alkaline leaching can be applied to dissolve Al foils in cases in which electrode materials have not been separated from current collectors in advance [35, 36]. Leaching can also be used to dissolve specific elements such as Ni and Co due to the formation of corresponding stable metal ammonia complexes [71, 72]. Like thermal treatment and mechanochemical reactions, leaching is also considered a pretreatment of the chemical process, and leaching efficiencies have important influences on the subsequent purification stage and the overall recovery of metals.

Currently, acid leaching with HCl [53, 73, 74, 75, 76, 77, 78], H2SO4 [35, 49, 54, 79, 80, 81, 82, 83, 84, 85, 86, 87], HNO3 [55, 88, 89], and H3PO4 [90, 91, 92] has been extensively investigated and high leaching efficiencies of both Co and Li have been achieved. Leaching conditions and efficiencies with different reagents and reductants are summarized in Table 1. In general, leaching with reductants such as H2O2, Na2S2O3, or glucose can improve leaching efficiencies by converting metal oxidation states to more soluble states (i.e., Co2+ is much more water soluble than Co3+ [79, 92, 93, 94, 95, 96, 97, 98]), and leaching with inorganic strong acids such as H2SO4, HCl, and HNO3 can cause significant secondary pollution, such as toxic gas emission. To eliminate this secondary pollution, several organic acids have been investigated as replacements without sacrificing leaching efficiencies [39, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114] and in particular, Li et al. [39] found that ascorbic acid is capable of acting as both a leaching agent and a reducing agent to improve Co recovery efficiencies [39]. In the investigation conducted by Li et al. [39], the optimum leaching conditions were determined to be an ascorbic acid concentration of 1.25 M, a leaching temperature of 70 °C, a leaching time of 20 min, and a S/L ratio of 25 g L−1. And under these experimental conditions, as much as 94.8% Co and 98.5% Li can be recovered at a relatively low temperature in a short time. This experimental result demonstrates that, compared with conventional methods, the overall recovery process by using leaching is simpler and more environmentally friendly with lower energy consumption.
Table 1

Typical leaching conditions of cathode materials from spent LIBs

Reagents

Spent electrodes

Conditions

(T, t, and S/L)

Efficiency (%)

4 M HCl [77]

LCO

80 °C, 1 h, 10 g L−1

Li = 99, Co = 99

1 M H2SO4 [79]

Mixed cathodes

95 °C, 4 h, 50 g L−1

Li = 93, Co = 66,

Ni = 96, Mn = 50

1 M HNO3 + 1.7 vol% H2O2 [88]

LCO

75 °C, 1 h, 20 g L−1

Li = 95, Co = 95

2 M H2SO4 + 5 vol% H2O2 [57]

LCO

80 °C, 1 h, 50 g L−1

Li = 99, Co = 99

2 M H2SO4 + 50 g/L glucose [97]

LCO

80 °C, 2 h, 35 g L−1

Li = 92, Co = 88

3 M H2SO4 + 0.25 M Na2S2O3 [98]

LCO

90 °C, 3 h, 66.7 g L−1

Li = 100, Co = 100

1.34 M H2SO4 + 0.45 g/g Na2S2O5 [95]

Mixed batteries

20 °C, 45 min, 10.9%

Mn = 94, Cd = 81,

Zn = 99, Co = 96, Ni = 68

1 M H2SO4 + 0.075 M NaHSO3 [94]

Mixed cathodes

95 °C, 4 h, 20 g L−1

Li = 97, Co = 92,

Ni = 96, Mn = 88

2%H3PO4 + 2 vol% H2O2 [91]

LCO

90 °C, 0.5 h, 8 g L−1

Li/Co > 95

1.5 M H3PO4 + 0.02 M glucose [92]

LCO

80 °C, 2 h, 20 g L−1

Li = 100, Co = 98

1.25 M citric acid + 1 vol% H2O2 [108]

LCO

90 °C, 0.5 h, 20 g L−1

Li = 100, Co > 90

1.5 M malic acid + 2 vol% H2O2 [109]

LCO

90 °C, 40 min, 20 g L−1

Li = 100, Co > 90

2 M formic acid + 6 vol% H2O2 [106]

NCM

60 °C, 2 h, 50 g L−1

Li = 100 Co/Ni/Mn = 85

2 M tartaric acid + 4 vol% H2O2 [104]

Mixed cathodes

70 °C, 0.5 h, 17 g L−1

Li = 99 Co/Ni/Mn = 99

1.5 M succinic acid + 4 vol% H2O2 [105]

LCO

70 °C, 40 min, 15 g L−1

Li > 96, Co = 100

3 M trichloroacetic acid + 4 vol% H2O2 [115]

NCM

60 °C, 0.5 h, 50 g L−1

Li = 100, Co = 92,

Ni = 93, Mn = 90

0.1 M citric acid + 0.02 M ascorbic acid [112]

LCO

80 °C, 6 h, 10 g L−1

Li = 100, Co = 80

2 M citric acid + 0.6 g/g H2O2

1.5 M citric acid + 0.4 g/g tea waste

1.5 M citric acid + 0.4 g/g Phytolacca Americana [101]

LCO

70 °C, 80 min, 50 g L−1

90 °C, 2 h, 30 g L−1

80 °C, 2 h, 40 g L−1

Li = 99, Co = 98

Li = 98, Co = 96

Li = 96, Co = 83

1.25 M ascorbic acid [39]

LCO

70 °C, 20 min, 25 g L−1

Li = 99, Co = 95

1 M oxalic acid [100]

LCO

95 °C, 2.5 h, 15 g L−1

Li = 98, Co = 97

Four leaching mechanism models, including the shrinking core [72, 85, 100, 104, 106], the empirical [79, 90, 94], Avrami [115], and the revised cubic rate law [87], have been investigated for kinetics studies. And in most cases, the shrinking core model has been found to be suitable for leaching mechanism analyses in which researchers speculate that particles undergo a loosening-breaking-shrinking change during the acid leaching process (Fig. 9) [99]. In addition, according to fitting results, leaching reactions can be controlled by liquid film mass transfers, surface chemical reactions, and residue layer diffusions.
Fig. 9

Diagrammatic sketch of the reaction mechanism in the leaching process. Reproduced with permission from [99].

Copyright 2017 Elsevier

3.2.4 Chemical Precipitation

The chemical precipitation of precious metals from leaching solutions yield insoluble compounds and in general, precipitants include sodium hydroxide, oxalic acid, ammonium oxalate, and sodium carbonate, all of which can react with cobalt and lithium ions to form insoluble precipitates of cobalt hydroxide [50, 116], cobalt oxalate [35, 81, 101, 107, 110, 112, 117, 118], lithium phosphate [70, 91, 101, 119], and lithium carbonate [35, 81, 93, 106]. Apart from the recycling of precious metals, chemical precipitation can also be used to remove trace amounts of impurities, such as iron, copper or aluminum [93, 118, 120, 121]. The chemical precipitation separation process is dependent on the different solubilities of compounds at certain pH values and temperatures, which need to be controlled carefully during precipitation.

For example, Sun and Qiu [57] developed a novel hydrometallurgical process based on vacuum pyrolysis, oxalate leaching, and precipitation in which a mixture of active materials in a 1.0 M oxalate solution at 80 °C with a S/L ratio of 50 g L−1 was leached for 120 min followed by the precipitation of Co from LiCoO2 to CoO directly as CoC2O4·2H2O. In this study, a reaction efficiency of more than 98% LiCoO2 was achieved. The combination of leaching and precipitation is a simple and adequate method to recover valuable metals from spent LIBs. For example, Wang et al. [74] conducted experiments to separate and recover metals such as Co, Mn, Ni, and Li from cathode active materials of spent LIBs. In this study, Mn was first recovered as MnO2 and Mn(OH)2 by using a KMnO4 reagent, followed by the selective precipitation of Co(OH)2 with the addition of a 1 M NaOH solution to reach a pH value of 11. Li in the remaining aqueous solution, readily recovered as Li2CO3, was subsequently precipitated by the addition of a saturated Na2CO3 solution and the purity of Li, Mn, Co, and Ni in the recovered powders was 96.97, 98.23, 96.94, and 97.43 wt% respectively.

Chemical precipitation has been widely applied because of its low equipment requirements, simple operations, and low costs. And in terms of recycling, key factors to consider are the selection of appropriate precipitants and the determination of optimal conditions to avoid precipitate dissolution.

3.2.5 Solvent Extraction

Solvent extraction is a process in which a two-phase system normally consisting of an organic phase and an aqueous phase is introduced. Here, separation can be achieved by the uneven distribution of the two phases in which after leaching, solvent extractants with high selectivity such as bis-(2,4,4-tri-methyl-pentyl) phosphinic acid (Cyanex 272) [122, 123, 124, 125, 126, 127, 128], trioctylamine (TOA) [129, 130], 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC-88A) [129, 131, 132], and di-(2-ethylhexyl) phosphoric acid (D2EHPA) [122, 126, 133, 134], can be used to separate specific transition metals from the leaching solution, allowing Li to remain in the solution.

In one study, Kang et al. [124] proposed a process combining acid leaching, precipitation, and extraction that resulted in an overall 92% Co recovery. In this proposed process, after leaching with 2 M H2SO4 and 6 vol% H2O2, Co is selectively extracted from the purified aqueous phase through equilibrating with 50% saponified 0.4 M Cyanex 272 at an equilibrium pH ~ 6. And following the stripping of the loaded organic phase with 2 M H2SO4, a solution of 96 g L−1 Co is left behind, from which pure pigment-grade cobalt sulfate can be recovered by using evaporation/recrystallization.

Solvent extraction can produce metals with high purity and high efficiency, and is usually conducted at room temperature within a short period of time. However, this process also possesses disadvantages such as complex operations and high costs of solvents. Therefore, future research efforts should focus on the development of cheap solvents and the cyclic utilization of solvents.

Aside from the main chemical processes discussed above, other processes such as electrodeposition and recrystallization methods have also been extensively investigated [84, 124, 135, 136, 137, 138] and in practice, different chemical processes are usually combined together to attain higher recovery efficiencies [31].

3.3 Bioleaching

Bioleaching is an emerging interdisciplinary process that includes chemistry, biology, and metallurgy. Recently, the bioleaching of spent secondary batteries has attracted great attention from researchers [139, 140, 141, 142, 143, 144]. In general, bioleaching is carried out by using chemolithotrophic and acidophilic bacteria, which utilizes elemental sulfur and ferrous ions as energy sources to produce metabolites such as sulfuric acid and ferric ions in a leaching medium [139, 143, 144] which can subsequently help dissolve metals from spent LIBs. In addition, fungi can also produce organic acids [140, 142].

For example, Xin et al. [144] investigated the bioleaching mechanisms of Co and Li for spent LIBs by a mixed a culture of sulfur- and iron-oxidizing bacteria. Here, the researchers found that acid dissolution was the main mechanism for Li bioleaching independent of energy types and that apart from acid dissolution, Fe2+-catalyzed reductions also take part in the bioleaching process. At present, bioleaching processes are still under investigation due to its vulnerability to pollution and long periods of bacterial cultures. However, this process might just be the most environmentally friendly method in the recycling of spent LIBs in the long run.

3.4 Re-synthesis of Cathode Materials

Solutions obtained after acid leaching can also be used to re-synthesize new cathode materials through thermal treatments [37, 56, 145, 146, 147, 148, 149, 150], co-precipitation [36, 120, 151, 152, 153, 154], sol–gel methods [155, 156, 157, 158], and so on. Such re-synthesis processes can realize the closed-loop recycling of spent LIBs and avoid the separation of multiple metal ions in solution.

In terms of the re-synthesis of cathode materials, the sol–gel method can fully reuse metal ions in leaching solutions and avoid the separation process. Here, the sol–gel method is more applicable if organic acids are used in the leaching step because organic acids can act as both a leaching and a chelation agent. As an example, a flowchart of a complete recycling process by using lactic acid in the leaching process is shown in Fig. 10 [155]. In addition, re-synthesized electrodes via the sol–gel method usually exhibit similar morphologies and electrochemical performances as compared with freshly synthesized electrodes (Fig. 11a, b) [156].
Fig. 10

Flowchart of the recycling process consisting of organic acid leaching and re-synthesis. Reproduced with permission from [155].

Copyright 2017 American Chemical Society

Fig. 11

a Cycling performances of re-synthesized and fresh-synthesized samples at 1C (2.75~4.25 V); b An SEM image of re-synthesized samples prepared by the sol–gel method. Reproduced with permission from [156], Copyright 2015 The Royal Society of Chemistry. c An SEM image of re-synthesized LiNi1/3Mn1/3Co1/3O2 with NaOH co-precipitation at different magnifications; d Cycling performances of NCM at 0.5C. Reproduced with permission from [154], Copyright 2015 Elsevier. Heat regeneration of different cathodes: e XRD patterns of LCO after regeneration at different temperatures; f Cycling performances of LCO after regeneration at different temperatures (30 mA g−1, 3~4.3 V). Reproduced with permission from [37], Copyright 2015 The Royal Society of Chemistry. g XRD patterns of as-purchased, end-of-life, and chemically re-lithiated LFP cathodes; h Voltage profiles of as-purchased and electrochemically re-lithiated end-of-life LFP cathodes. Reproduced with permission from [159].

Copyright 2014 Elsevier

Co-precipitation is another re-synthesis method which is simple and has potential in industrial applications. In one example, Wang et al. [151] successfully re-synthesized Ni1/3Mn1/3Co1/3(OH)2 precursors from a sulfuric acid leaching solution using co-precipitation (Fig. 11c, d) in which pH levels were controlled at 11, and the complete hydroxide co-precipitation of Ni, Co, and Mn was achieved. In this study, the final LiNi1/3Mn1/3Co1/3O2 cathode was found to possess a spherical shape with uniform size distribution, and exhibited an initial discharge capacity of 150 mAh g−1 and a capacity retention of about 88% after 100 cycles at 0.1C.

In cases in which elemental compositions are simple, such as in the cases of LCO and LFP, the thermal treatment is an effective method to regenerate spent cathode materials. As an example, Song et al. [37] developed a thermal treatment method to regenerate spent LCO cathodes and their results demonstrated that regenerated cathodes after calcination at 900 °C with Li2CO3 supplementation can exhibit electrochemical performances that are comparable to commercial LCO cathodes (Fig. 11e, f).

Re-lithiation is another effective re-synthesis process that can be used to regenerate cathodes in situations in which cathode scraps show little structure damage. For example, Gaustad et al. [159] conducted re-lithiation on an end-of-life LFP cathode using chemical and electrochemical methods and the resulting regenerated LFP delivered a discharge capacity of 155 mAh g−1 (Fig. 11g, h). In another example, Kim et al. [160] investigated a hydrothermal re-lithiation method using a concentrated LiOH solution at 200 °C without any scraping procedures to regenerate LCO cathodes and their results showed that although the obtained regenerated LCO possessed some electrochemically inactive impurities, it was capable of achieving an initial discharge capacity of 144.0 mAh g−1 and a capacity retention of 92.2% after 40 cycles.

Our group has also developed several different methods to re-synthesize LCO cathode materials. In our electrodeposition method [161], LCO films were regenerated in a single synthetic step by using a nitric acid leaching solution on nickel plates at a constant current. Here, the LCO phase with preferred (104) orientation was electrodeposited on a nickel substrate at a current density of 1 mA cm−2 for 20 h, and subsequent cell tests revealed that this obtained cathode material possessed acceptable characteristics in terms of charge and discharge capacities and cycling performances in which initial charge and discharge capacities were 130.8 and 127.2 mAh g−1 respectively and capacity retention was more than 96% after 30 cycles. In another method developed by our group, Li1.2Co0.13Ni0.13Mn0.54O2 cathode materials were successfully re-synthesized from an organic acid leaching solution of spent LIBs by using oxalic acid co-precipitation followed by hydrothermal and calcination processes [162]. In this study, the re-synthesized sample was found to be capable of delivering a reversible initial discharge capacity of 225.1 mAh g−1 and a capacity retention of 87% after 50 cycles. The results in this study also demonstrated that the organic acid leaching solution from spent LIBs can be used as a source of Co and Li elements, and that the performance of the re-synthesized Li1.2Co0.13Ni0.13Mn0.54O2 material is comparable to cathode materials newly synthesized from chemicals.

4 Typical Industrial Recycling of Spent Lithium-Ion Batteries Around the World

The main industrial methods of recycling LIBs used globally include pyro- and hydrometallurgical technologies, which can be used separately or in combination. In general, a complete industrial recycling process usually involves sorting, classification, mechanical pretreatment, pyro- or hydrometallurgical treatment, waste disposal [163, 164, 165, 166, 167], and recycling companies usually cooperate with recycling organizations such as Bebat and Call2recycle to ensure sufficient recycling amounts. The distribution and recycling technology features of LIB recycling companies around the world are shown in Fig. 12 and Table 2.
Fig. 12

Distribution of typical LIBs recycling companies around the world

Table 2

Summary of industrial LIB recycling processes

Company

Battery type

Methods

Features

Umicore

Li-, Ni-based

Pyrometallurgy and hydrometallurgy

UHT technology

Retriev

All

Pyrometallurgy (Ni-based)

Hydrometallurgy (Li-based)

Mechanical process (Pb-acid)

Liquid N2 protection

Batrec

Li-, Hg-based

Pyrometallurgy

CO2 protection

Accurec

All (except Pb and Hg)

Pyrometallurgy

Vacuum thermal recycling

SNAM

Li-, Ni-based

Pyrometallurgy

Pyrolysis–distillation–refining

Inmetco

Li-, Ni-based

Pyrometallurgy

Direct-reduced iron process

Recupyl

Li-, Zn-based

Hydrometallurgy

Inert gas protection

Sony & Sumitomo

All

Pyrometallurgy

High temperature calcination

Xstrata

Li-, Ni-based

Pyrometallurgy and hydrometallurgy

GEM

Li-, Ni-based

Hydrometallurgy

Brunp

Li-, Ni-based

Hydrometallurgy

In one example, the Umicore company applies a combined pyro- and hydrometallurgical process to recycle spent LIBs [168] in which spent LIBs are first fed into a smelter without any pretreatment. In the smelter, combustion occurs and advanced plasma technologies are applied to deal with generated gases from the smelting process to prevent the formation of dioxin, furan, and other environmental pollutants. After smelting, high-purity inert slags are produced that can be used as additives in construction industries, including the fabrication of concrete, ceramics, glass, and steel, and the main purification products, including Co and Ni, can be sent to refineries with hydrometallurgical processes to produce raw materials for LIBs such as lithium cobalt oxide and nickel hydroxide.

Retriev Technologies (formerly Toxco Inc.) is another LIB recycling company that uses advanced processing technologies and equipment certified by the U.S. Environmental Protection Agency to recycle batteries of all types and sizes [169]. In their process, spent batteries are initially discharged for safety reasons before processing. The battery cell packing is subsequently disassembled after discharge, and the materials are separated for appropriate recycling or scrapping. Here, Retriev Technologies uses a patented cryogenic process in which spent LIBs are chilled to – 200 °C in liquid nitrogen to maintain lithium at a relatively inert state. Hammer milling is then applied to crush remaining large pieces, which is subsequently placed in a caustic bath to dissolve the lithium salts and neutralize the acidic components. Finally, lithium carbonate is produced as a raw material through precipitation by using salts and dewatering in filter presses. The resulting sludge is compacted as cobalt cakes to produce LCO as new battery electrode materials.

Batrec Industrie AG has also developed a specialized process that guarantees the safe disposal of Li batteries, which can completely separate hazardous substances from metals in spent batteries [170]. In their process, spent LIBs are presorted and sent to a crushing unit in batches. Here, the crushing atmosphere is controlled and the released lithium is neutralized to allow battery processing to continue without atmospheric pollution. Individual components of the batteries, such as chrome-nickel steel, cobalt, nonferrous metal, manganese oxide, and plastic, are subsequently completely separated in a multistage separating plant for use as raw materials of new products for further processing.

Accurec is another recycling company that is dedicated to the recycling of used batteries, including all the types of industrial and consumer rechargeable batteries [171]. This company uses a newly developed and innovative process that allows for the recovery of high proportions of metal from portable LIBs at minimized cost. In their process, spent LIBs are first sorted and classified before they are safely disassembled. Auto-thermal vacuum pyrolysis and multi-step mechanical treatment is subsequently applied to separate Cu and Al fractions from the active mass of the electrodes. Finally, the vacuum evaporation of lithium is applied and lithium oxides and metal concentrates for existing industrial process routes are obtained.

Sony also developed an LIB recycling process in 1996 which includes two main steps: battery incineration, in which untreated batteries are incinerated at 1000 °C, followed by cobalt extraction performed at their Sumitomo plants. In this process, flammable components such as plastic casings and organic solvents are first burned off. The batteries are subsequently damaged and filtered, with the residues, containing Fe, Cu, and Al pieces, being disposed of with magnetic separation, leaving behind Co, which is finally recovered from the residual powder and returned to the battery production cycle.

GEM is a company that possesses advanced hydrometallurgical recycling technologies and has successfully produced hi-tech cobalt/nickel products from spent batteries through acid leaching, extraction activation, and reprocessing. GEM is now a pioneer enterprise in the recycling and reusing of spent batteries and is China’s largest manufacturer of ultra-fine cobalt and nickel powders [166].

5 Second Use of Retired Power Batteries

The increasing awareness of the importance of environmental protection associated with greenhouse gas (GHG) effects has inspired the development of green transportation. Because of this, the production of EVs and hybrid electric vehicles (HEV) by using LIBs as the power source has increased markedly in the last few years. As a result, the amount of retired power battery packs is expected to increase exponentially by 2030 [172]. And for the most part, automotive manufacturers recommend that batteries be retired if energy or power densities drop to 70%~80% of original values, which means that substantial amounts of electric energy will remain in these retired batteries. And because the cost of a power battery can account for a major portion of a total vehicle’s cost (approximately 50%), additional value can be extracted from these retired batteries through extending their total battery service lifetime. Here, stationary energy storage is an optimal application scenario and this post-application of retired power batteries is defined as battery second use (B2U) (Fig. 13) [173, 174, 175].
Fig. 13

Flowchart of a power LIB lifecycle with various choices after its first electric vehicle life

Thus far, the feasibility of B2U has been explored in terms of economic, technical, and environmental perspectives in which economic feasibility analyses are based on cost–benefit analysis [173, 175, 176, 177, 178]. The cost of B2U can be divided into two categories, with one being the repurposing cost, and the other being the cost of energy storage applications [176]. Here, the cost of new batteries in the future and the type of specific stationary application have great influences on the cost–benefit analysis results. As for the technical feasibility analysis, the most challenging aspect is the assessment of battery performance in its first vehicle life and second use life [174, 178]. This is because the second use life will involve a new energy storage system and systematic knowledge of the stationary application is required to match the two systems (Fig. 14) [179]. Moreover, the use of overlapping components from two systems is recommended to reduce costs [175]. Apart from the technical and economic analyses of B2U, a few reports have also investigated the environmental feasibility, and most of these studies have confirmed the positive effects of B2U in terms of energy use and GHG emission reduction [180, 181, 182, 183]. In summary, researchers are cautiously optimistic about B2U applications, but agree that thorough and deep analysis is necessary.
Fig. 14

Requirements of LIBs in EVs and stationary applications

6 Challenges and Future Prospect

The recycling of spent LIBs has been extensively studied globally and great progress has been made in the last few decades. However, problems and difficulties remain during the recycling process that need to be solved:
  1. (1)

    Dealing with potential dangers such as short circuiting and explosions in the pretreatment process, especially with large power battery packs. Methods need to be explored to safely dismantle large battery packs with automatic dismantling being urgently needed.

     
  2. (2)

    Controlling the type and amount of impurities in the final product. This is because impurities can negatively affect the performance of final products. And although solvent extraction or electrodeposition processes can achieve high-purity products, the processes are complex and costly.

     
  3. (3)

    Avoiding the production of toxic gases and waste liquids during the recycling process, especially toxic gases generated during thermal treatment.

     
  4. (4)

    Recycling of electrolytes and anodes. Currently, most recycling processes focus on cathode materials and only a few studies have dealt with the recycling of electrolytes [184, 185, 186, 187, 188, 189, 190] and anodes [191, 192, 193, 194, 195, 196, 197, 198]. This is because electrolytes are difficult to recycle because of its high volatility. However, they should still be properly recycled and disposed from an environmental protection viewpoint.

     
  5. (5)

    Finding more economical recycling processes. Economic benefits are the direct incentive for manufacturers to recycle spent LIBs with most of the benefits obtained from the precious metals contained in the final products. Therefore, the cost of the overall recycling process must be reduced.

     
  6. (6)

    Popularizing the life-cycle assessment [20, 199, 200, 201, 202] and assessing the environmental benefits. Although these are difficult to calculate, there is a need to evaluate the greenness of specific recycling processes and to popularize the concept of recycling LIBs.

     

The overall recycling of spent LIBs involves systematic engineering consisting of source control, processing control, and treatment. With growing environmental awareness, much focus will be put on the development of green and simple recycling processes. And in the long term, the fundamental path forward for the recycling of spent LIBs lies in the selection of newly developed materials as cathodes, anodes, and electrolytes, as well as battery designs [20]. Therefore, efforts must be focused on the use of abundant and nontoxic materials so that whatever developments are made will not create new environmental problems, with simpler battery designs facilitating better treatment of spent LIBs.

7 Conclusion

Currently, most recycling processes are focused on hydrometallurgical methods (acid leaching, chemical precipitation, solvent extraction, electrodeposition, etc.) which, despite being able to obtain high-purity products, are complex and difficult to scale up. In comparison, industrial-scale recycling processes of spent LIBs are mainly based on pyrometallurgical methods, which are simple to operate, but require higher energy consumption and produce the secondary pollution. It is probable that future recycling processes will implement both pyrometallurgical and hydrometallurgical methods to produce high value-added products that can be used in various industries, including the production of raw materials for LIBs. Here, future recycling processes should be both green and simple.

In addition, more attention should be paid to the secondary use of retired power batteries, which will become an inevitable issue in the near future. Systematic analysis is also needed in terms of economic, technical, and environmental perspectives. Ultimately, breakthroughs in battery material and design are necessary for both the future development of LIBs and the recycling of spent LIBs.

Notes

Acknowledgements

This work was supported by the Chinese National 973 Program (2015CB251106), the Joint Funds of the National Natural Science Foundation of China (U1564206), and the Major Achievements Transformation Project for Central University in Beijing. J. Lu and K. Amine gratefully acknowledge the support from the U.S. Department of Energy (DOE), the Office of Energy Efficiency and Renewable Energy, and the Vehicle Technologies Office. The Argonne National Laboratory is operated for the DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357. This work was especially made possible thanks to the US-China Electric Vehicle and Battery Technology program between Beijing Institute of Technology and Argonne National Laboratory.

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Copyright information

© Shanghai University and Periodicals Agency of Shanghai University 2018

Authors and Affiliations

  1. 1.Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science and EngineeringBeijing Institute of TechnologyBeijingChina
  2. 2.Chemical Sciences and Engineering DivisionArgonne National LaboratoryLemontUSA
  3. 3.Institute for Research and Medical ConsultationsImam Abdulrahman Bin Faisal UniversityDammamSaudi Arabia
  4. 4.Material Science and EngineeringStanford UniversityStanfordUSA

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