Gas evolution characterization and phase transformation during thermal treatment of cathode plates from spent LiFePO4 batteries

Journal Pre-proof Gas evolution characterization and phase transformation during thermal treatment of cathode plates from spent LiFePO4 batteries Yafei Jie (Methodology) (Investigation) (Data curation) (Visualization), Shenghai Yang (Supervision) (Conceptualization) (Funding acquisition), Fang Hu (Investigation) (Data curation), Yun Li (Software) (Writing - review and editing) (Validation), Longgang Ye (Resources) (Formal analysis), Duoqiang Zhao (Conceptualization) (Validation), Wei Jin (Software) (Resources), Cong Chang (Resources) (Supervision), Yanqing Lai (Supervision) (Validation), Yongming Chen (Supervision) (Writing - review and editing) (Project administration) (Validation)

PII:

S0040-6031(19)30820-2

DOI:

https://doi.org/10.1016/j.tca.2019.178483

Reference:

TCA 178483

To appear in:

Thermochimica Acta

Received Date:

10 September 2019

Revised Date:

9 December 2019

Accepted Date:

11 December 2019

Please cite this article as: Jie Y, Yang S, Hu F, Li Y, Ye L, Zhao D, Jin W, Chang C, Lai Y, Chen Y, Gas evolution characterization and phase transformation during thermal treatment of cathode plates from spent LiFePO4 batteries, Thermochimica Acta (2019), doi: https://doi.org/10.1016/j.tca.2019.178483

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Gas evolution characterization and phase transformation during thermal treatment of cathode plates from spent LiFePO4 batteries

Yafei Jie a, Shenghai Yang a, Fang Hu a,b, Yun Li a, Longgang Ye c, Duoqiang Zhao a, Wei Jin a

, Cong Chang a, Yanqing Lai a, Yongming Chena,* [email protected]

a

School of Metallurgy and Environment, Central South University, Changsha, 410083, China Hydrometallurgy and Corrosion, Department of Chemical and Metallurgical Engineering (CMET), School of Chemical Engineering, Aalto University, P.O. Box 16200, FI-00076 AALTO, Finland c College of Metallurgy and Material Engineering, Hunan University of Technology, Zhuzhou, 41200, China *

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b

Corresponding Authors.

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Graphical abstract

Highlights



An on-line TG-DSC-EI-MS system was innovatively employed.



Around 10 kinds of pyrolysis volatile gases species were clearly identified.

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The pyrolysis characteristics of organic compounds were in detail evaluated.

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The thermal reaction mechanism of spent LFPBs cathode plates were deduced. 1

Abstract In this paper, thermal reaction behaviors and gas evolution characteristics of cathode electrodes separated from spent LiFePO4 batteries were systematically characterized using thermogravimetric-differential scanning calorimetry analysis coupled with mass spectrometry equipped with electron ionization system (TG-DSC-EI-MS). TG-DSC-EI-MS analysis indicated tail gases were mainly released in a low-temperature range of 60-250 °C, which attributed to the hydrolysis and decomposition reactions of lithium salt electrolyte. Additionally, H2O, HF and CO2 were detected at the range of 380-600 °C, which responded to

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thermal decomposition of the binder. H2O and CO2 were the main gaseous products at around 520 °C, which might be released from the oxidation combustion reactions of the binder. At the same time, phase transformation of the cathode active material during thermal treatment was

further investigated by SEM-EDS, FT-IR, XRD and Mössbauer spectrum analysis techniques.

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The results indicated LiFePO4 was oxidized to Li3Fe2(PO4)3 and Fe2O3 during the heating process.

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Keywords: Spent LFPBs; Cathode electrodes; Thermal treatment; Gas evolution; Phase

1. Introduction

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transformation

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Electric vehicles (EVs) and hybrid electric vehicles (HEVs) have exceptional advantages in relieving the pressure of energy shortage and environmental pollution[1, 2]. LiFePO4 battery

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is chosen as one of the main lithium-ion batteries owing to its inherent advantages of low cost, high thermal safety, nontoxicity and high reversibility[3-5]. In 2015, a fast growth (10.84 GWh[6]) in the sales volume of LiFePO4 batteries appeared due to the rapid development of EVs and HEVs. Moreover, it was estimated that the LiFePO4 battery market would continue expanding and the growth rate would remain at around 20% in the next five years. Lithium iron phosphate batteries (LFPBs) have been put to use in commerce for around 6 years, a huge 2

amount of spent LFPBs will be recycled after terminal lifespans. It was predicted that the amount of spent batteries will reach 1.132 million tons by 2025, in which LFPBs will account for 87%[6]. If disposed improperly, the metals contained, toxic organic compounds (consisted of alkyl carbonates like ethylene carbonate (EC) and methyl ethyl carbonate (EMC)), and electrolyte lithium salts (LiPF6) in the spent batteries will bring an inescapable risk to the environment and public health[7, 8]. However, if these components can be recycled properly, the residual value of spent LFPBs will be significantly enhanced and great economic benefits

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will be brought to the recycling enterprise[4, 9]. As reported in various papers, mostly leading LFPBs recycling systems prefer to recover metallic components, especially Li due to its high market value[10]. In fact, recycling is not

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just a matter of recovering recyclable materials, it should be a total ecological system. The

definition of recycling loop is difficult because there are many factors to consider, including

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complicated end-of-life products and environmental management in recycling process. At

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present, pyrometallurgical, hydrometallurgical LFPBs recycling methods and direct regeneration have been reported [11]. During the recycled process, spent LFPBs usually are completely discharged firstly, and then dismantled to separate cathode plates, anode plates,

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plastic separators and metals. Among them, cathode plates have a highest recycling value. It is commonly comprised of cathode active materials, aluminum plates, electric conductor,

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polyvinylidene fluoride (PVDF) binder and additives[12, 13]. In order to separate the cathode

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active materials, different kinds of separation processes, such as thermal treatment[14, 15], alkali solution dissolution leaching[16-20] and organic solvent dissolution[21, 22], have been adopted in practice for the cathode plates treatment. Thermal treatment, an easily operated process, has also been widely used in LFPBs recycling. It can burn PVDF and separate Al foils simultaneously. Several thermal treatment processes have been developed for pretreatment of LFPBs, but it involved in different optimum operation temperature. The Belgian company

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Umicore started Val’Eas recycling process with thermal treatment. During the process, the battery cells were gradually heated until reached the maximum temperature 300 °C[23]. Accurec utilized thermal treatment process to recycle spent lithium-ion batteries (LIBs) by vacuum chamber, and the treatment temperature was 250 °C[24]. It was also reported that PVDF began to decompose at 350 °C under a certain oxygen atmosphere. Furthermore, some investigators suggested that 600 °C was the best temperature for PVDF decomposition[15, 24]. Besides, although it was believed that some harmful gaseous combustion products would be

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exhausted during the heat treatment process, the thermal decomposition characteristics of organic compounds, PVDF binder and LiPF6 have not been thoroughly and distinctly explained in the existing papers. Therefore, the confirmation of the gaseous composition of combustion

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products will be helpful for efficient purification process development and environmental protection.

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In this study, the gas evolution of cathode plates from spent LiFePO4 batteries were

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investigated by on-line TG-DSC-EI-MS analysis system during the heat treatment process. Meanwhile, the in phase composition changes and microstructural identification of the cathode active powder from spent LFPBs cathode plates were further investigated by SEM-EDS, FTIR,

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XRD, and Mössbauer spectrum. It is expected to systematically establish a theoretical analysis of decomposition profiles of cathode plates during thermal treatment, and to provide theoretical

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recycling.

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guidance for process optimization and environmental management during spent LIBs

2. Experimental

2.1. Materials and procedure The flowsheet of the experimental procedure are illustrated in Fig. 1. Firstly, spent LFPBs were discharged by immersing into a 10 wt% NaCl solution for 24 h to avoid potential danger of short-circuit or self-ignition. Secondly, the discharged batteries were further manually 4

dismantled to different components to separate cathode plates, anode plates, polymer separator and aluminum shell. The cathode plates were cut into small pieces (3.0×3.0 mm) and used as raw material in the following TG-MS tests. Then, some cathode plates were put in the tray of the thermal analyser and heated up to 850 °C at a heating rate of 10 °C·min-1. Meanwhile, mixed gases (20% O2 + 80% Ar) with a flow rate of 100 ml/min were injected into the sample chamber. After TG-MS test, the cathode active material were collected and characterized by SEM-EDS, FT-IR, XRD and Mössbauer spectrum to investigate the phase transformation of

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cathode active material during the thermal treatment process. The main components and physicochemical properties of relative compounds existed in the spent LFPBs cathode plates

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are shown in Table 1.

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2.2. Experimental setup

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STA449F3+QMS403C analyzer was employed in the TG-MS test (Netzsch, Germany). The second part of Fig. 1 depicts the schematic diagram of TG-DSC-EI-MS system. It mainly

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contained a horizontal thermogravimetry-differential thermal analyzer (TG-DSC) and a cylindrical quadrupole mass spectrometry (MS). The TG-DSC system was coupled with a mass

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spectrometer by a heated line with quartz capillary tubes. The MS system was tested in a vacuum atmosphere and it detected the characteristic fragment ion intensity of the volatiles by

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their respective mass to charge ratios (m/z). FT-IR spectra were taken down on a Nicolet FT-IR spectrophotometer (Nicolet 6700, Thermo Fisher Scientific, USA) in a range of 4000-400 cm-1. The phase composition of cathode active material was detected by X-ray diffraction (XRD, Rigaku D/max-2500). The valence of iron was analyzed by Mössbauer spectrum at room temperature and recorded in the transmission mode composed of 57Co(Pd) source and multichannel analyzer. The morphology and elemental compositions of cathode active material were 5

characterized by scanning electron microscopy (SEM, TESCAN MIRA3 LMU) and energydispersive X-ray spectrometer (EDS, Oxford X-Max20). 3. Results and discussion 3.1. Thermogravimetric analysis The thermogravimetric analysis results of spent cathode plates are shown in Fig. 2. The figure shows that three weight change zones were recorded in the tested temperature range.

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The first weight loss of 3.1%, was detected in the range of 43-175 °C, and the corresponding DTG curve presented two evident weight loss peaks at 100.6 °C and 159.5 °C, respectively.

The weight loss might be caused either by volatilization or combination of the volatilization

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and decomposition of the electrolyte solvent[27]. It was a complex process in the second 380485.4 °C zone. As the temperature increased, the DTG curve showed a small weight loss peak

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at 382.3 °C, and correspondingly, an exothermic peak appeared in the DSC curve at 390.6 °C. This might be caused by the thermal decomposition of the binder, which was supported by the

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decomposition temperature of the PVDF[14]. A weight increase of 1.2% was observed in the range of 405-485.4 °C, which corresponded to an exothermic peak at 475.1 °C in the DSC

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curve and a significant weight gain peak in the DTG curve at 470.6 °C. The obvious weight increase was resulted from the oxidization of LiFePO4 cathode active material. It was proved

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by the color change of cathode active material during the annealing process in air. In addition, it was worth noting that the weight increase of 1.2% was the result of thermal decomposition of

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PVDF and oxidization of LiFePO4 cathode active material. The third weight loss of 3.7% started from 485.4 °C to 545 °C, where the DTG curve appeared a clear weight loss peak at 525.6 °C, and the DSC curve presented a corresponding exothermic peak at 530.4 °C. It was ascribed to the intense oxidative decomposition of the binder. Besides, an obvious endothermic peak was observed at around 656.7 °C, which might be resulted from the smelting of Al foil[28]. 6

3.2. Gas evolution characterization TG-MS test of cathode plates obtained from spent LFPBs was conducted in the high and low channels of m/z=64-128 and m/z=0-64, and measured by electron ionization (EI) method. A normalized ion current intensity was performed based on the equivalent characteristic spectrum analysis (ECSA) method proposed by Xia et al[29]. The normalized ion current intensities of the generated gases at high channel and low channel were plotted against m/z and temperature, which is shown in Fig.3a, Fig.3b and 3c (in low channel), respectively.

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It can be seen from Fig. 3a that the macromolecular gases in high channel with m/z of 85, 88, and 104 corresponded to C3H4O3 and C4H8O3/OPF3, respectively. In Fig. 3b and 3c, the

gases in low channel with m/z of 2, 18, 30, 38 and 44 were identified as H2, H2O, C2H6, C3H2,

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and CO2, respectively. The gases with m/z of 12, 19, 31, 41, 43, 69 and 85 were identified as fragments 12C+, 19F+, 31CH3O+, 41C3H5+, 43C3H7+/43C2H3O+, 69PF2+ and 85OPF2+ respectively. To

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explore the possible reaction mechanism of organic compounds in cathode active materials of

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spent LPBs, the evolution characteristics of alkane gases, fluorine-containing gases, H2O and CO2 released above were further described and discussed in detail.

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3.2.1. Alkane gases evolution

Fig. 4 plotted the ion current intensity curves of alkane gases against temperature and

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DTG curve of cathode plates. It can be seen that the escape patterns of alkane gases and their fragments are obvious, and mainly took place at around 88.8 °C, 163.4 °C and 520.0 °C. At

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around 88.8 °C, the ion current intensity curve of m/z=104 firstly reached the maximum intensity. Correspondingly, a sharp peak of mass loss was detected at around 100.6 °C. Combined with the physicochemical properties of EMC (the low boiling point is 107 °C) which added in the cathode plates, it was believed that the ion current intensity peak at around 88.8 °C was mainly due to the volatilization of EMC (C4H8O3). When the temperature increased to 163.4 °C, the ion current intensity curves of other alkane gases also reached their 7

first maximum ion current intensity peaks. At the same time, the DTG curve of cathode plates reached the maximum peak of mass loss at around 159.5 °C. The correlation between the weight loss peak in DTG curve and ion current intensity curves indicated that the volatilization and decomposition of the electrolyte solvent might be triggered by heating operation. The ion current intensity peak of 44C3H4O3+ at around 163.4 °C further confirmed that the occurrence of volatilization reaction. The co-existence of 44CO2+ and 31CH3O+ fragments implied that the decomposition of organic solutions EC (C3H4O3)/EMC (C4H8O3). This was supported by the

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studies of Mogi R[30] and R. Notario[31]. As demonstrated in the studies, the presence of methanol in the decomposition products attributed to the cleavage of the C-C bonds of EC

upon reduction. Therefore, 44CO2+ and CH3O+ fragments might come from the decomposition

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of EC (C3H4O3) and EMC (C4H8O3). In the third temperature range of 163-520.0 °C, it was observed that the ion current intensity curve of 31CH3O+ appeared a downward peak, which

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was different from other alkane gases. It might be caused by its participation in the oxidative

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decomposition of the binder.

Therefore, based on the analysis of the evolution law of alkane gases, the thermal reaction mechanism of organic components can be deduced. The main volatile organic compounds

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(VOC) C3H4O3 and C4H8O3 were volatilized and decomposed at around 60-190 °C. The chemical reaction equations can be presented as: volatilization

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C3 H4 O3 /C4 H8 O3 (l) →

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nC3 H4 O3 /C4 H8 O3 (l) →

C3 H4 O3 /C4 H8 O3 (g)

decomposition

nCH3 OH(g) + nCO2 (g)

(2)

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(1)

3.2.2. Fluorine-containing gases evolution Fig. 5 depicts the ion current intensity curves of 104OPF3+/104C4H8O3+, 85OPF2+, 69PF2+, 19 +

F , as well as 44CO2+ and 18H2O+ against temperature and DTG curves of the cathode plates

and PVDF.

It is evident that the ion current intensity curves of 19F+ and 18H2O+ started at around 70 °C, and gradually enhanced and reached their maximum intensity at around 111.9 °C,

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149.3 °C and 163.4 °C, respectively. This was possibly caused by the reaction of LiPF6 and water, which could generate HF and OPF3[32], owning to the physicochemical properties of LiPF6 added in the cathode plates. Therefore, based on the above analysis, the characteristic

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peak of m/z=104 was believed to be derived from the volatilization of EMC (C4H8O3) and the release of OPF3. However, as the temperature increased from 88.8 °C to 163.4 °C, the ion

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current intensity of m/z=104 did not increase as that of 19F+. It was believed that the OPF3 generated continued to react with water. Therefore, the characteristic peak of m/z=104

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appeared was resulted from the co-escape of C4H8O3 and OPF3, and the volatilization of C4H8O3 dominated. It was worth noting that the ion current intensity curves of fragments PF2+ and 85OPF2+ followed a similar trend as OPF3, indicating that the fragments 69PF2+ and

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OPF2+ came from generated OPF3 gas. Furthermore, the DTG curve of PVDF appeared a

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small weight loss peak at 386.4 °C, which corresponded to the weight loss peak of cathode plates at 382.3 °C. Based on the peaks of 18H2O+, 19F+ and 44CO2+ at around 386 °C, it could be

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inferred that the above gases were mainly generated by the thermal decomposition of PVDF. The maximum peak of 44CO2 appeared at 520 °C. Comparing with the DTG curves of PVDF and cathode plates, it indicated that the escaping gases were produced by the oxidation combustion reaction of PVDF. Therefore, based on the analysis of the evolution law of fluorine-containing gases, the reaction mechanism of lithium salt electrolyte, PVDF binder below can be deduced. The 9

hydrolysis reactions between lithium salt electrolyte and water occurred at 70-163.4 °C. The chemical reaction equations were: LiPF6 (s) + H2 O(g) →

hydrolysis

OPF3 (g) + 3H2 O(g) →

LiF(s) + OPF3 (g) + 2HF(g)

hydrolysis

(3)

3HF(g) + H3 PO4 (l)

(4)

The decomposition and oxidation combustion reactions about PVDF occurred at 380600 °C. The chemical reaction equation was: combustion

nCO2 (g) + nH2 O(g) + nHF(g)

(5)

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(CH2 CF2 )n (s) + nO2 (g) →

3.3. Cathode active material characterization

In order to further investigate the change of cathode active material during the thermal

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SEM-EDS, FT-IR, XRD and Mössbauer spectra.

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treatment, the cathode active material separated from aluminium foil were characterized by

Fig. 6a shows the SEM images of raw cathode active material. The lithium iron phosphate

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cathode active material particles were distributed in oval or circular shape with a size range of 300 nm-800 nm. Furthermore, it was found that cathode active powder agglomerated seriously

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because of the existence of binder. The EDS image in Fig. 6b reveals that the raw cathode material was primarily comprised of O, Fe, P, C, and F. The mole fraction of C and F were as

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high as 13.14% and 3.78%, respectively. This suggested that the massive fluorocarbon materials (e.g. binder and conductive additive) possibly consisted in cathode active material.

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Correspondingly, the elemental mapping proved that Fe, P, O, C and F were unevenly distributed in the raw cathode material.

Fig 7a presents the SEM images of the roasted cathode active material. Compared with raw cathode active material, the agglomeration significantly decreased in lithium iron phosphate cathode active material particles. At the same time, the surface of cathode active

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material became rough due to the form of small irregular particles. The elemental composition of cathode active powder above, as shown in Fig 7b, was mainly O, P and Fe, which could be inferred that the small irregular particles might be Fe2O3 and Li3Fe2(PO4)3. The elemental composition of Fig 7b was quite different from that in Fig 6b, and the mole fractions of C and F in roasted cathode material were 0.31% and 0.47%, far below than those in raw cathode active material. It can be proved from the corresponding mapping that C and F nearly disappeared, indicating that the fluorine-containing organic compounded together.

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FT-IR spectra of raw cathode active material and roasted cathode material are described in Fig 8. These bands basically matched the internal stretching, internal bending and external oscillations modes of PO43-, respectively[33]. Wavenumber region at 400-700 cm-1 was

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bending vibration bands of O-P-O, and wavenumber region at 940-1250 cm-1 was asymmetric stretching vibration peaks of -PO4 tetrahedron, indicating that raw cathode active material and

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roasted cathode material were phosphates[34, 35]. Identical conclusions were obtained in

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the studies on FT-IR features of lithium iron phosphates, which was as electrode materials for rechargeable lithium batteries[36]. In addition, there were also two absorption peaks distributed in wavenumber range 1500-2000 cm-1 in the middle infrared region.

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Wavenumber around 1639 was characteristic absorption peak of O-H stretching vibration in the H2O molecules. Wavenumber around 1772 cm-1 was associated with the characteristic

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absorption peak of C=C stretching vibration, which was inferred from H2C=CF-R in the

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polymer[37, 38]. However, it could be discovered that absorption peak around 1772 cm-1 disappeared after thermal treatment, indicating that the binder was removed after the heating process due to the oxidation combustion reaction of PVDF. This was consistent with the observation from the analysis of the evolution law of fluorine-containing gases. Fig. 9 shows XRD patterns of the raw cathode active material and the roasted cathode active material separated from aluminum foil. The phase in Fig. 9a was identified as LiFePO4.

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Compared with the XRD pattern of raw cathode active material, there were distinct differences in roasted cathode active material (shown in Fig. 9b). Based on the characteristic diffraction peaks of Li3Fe2(PO4)3 at 20.65o, 24.31o and 27.37o, indicating that the main phase composition of roasted cathode material was Li3Fe2(PO4)3. Meanwhile, a few diffraction peaks of Fe2O3 were observed, confirming the formation of Fe2O3 during the the heating process. Therefore, it could be inferred that the LiFePO4 cathode active material was oxidized to form Li3Fe2(PO4)3

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and Fe2O3 during thermal treatment.

As shown in Fig. 10, the Mössbauer spectra of cathode active material after thermal

treatment were used to further determine the valence of iron. It is known that different spectra

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are depicted by different subspectra according to different positions of iron atoms. The spectra of roasted cathode materials were fitted with a single doublet (Line a) and a single sextet (Line

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b). The single doublet was dominant with IS (isomer shift) =0.44 mm/s, QS (quadrupole splitting) =0.39 mm/s and Г (Lorentzians of width) =0.32 mm/s. Additionally, the single sextet

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had an IS value of 0.39 mm/s, a QS value of 0.19 mm/s and a Г value of 0.4 mm/s. These Mössbauer parameters for the spectra demonstrated that the single sextet (Line b) corresponded

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to iron in hematite, and the single doublet (Line a) corresponded to iron in Li3Fe2(PO4)3. This is supported by the previous Mössbauer studies[39, 40] regarding Li3Fe2(PO4)3 and Fe2O3. It also

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agreed with the XRD analysis results in Fig. 9.

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4. Conclusion

The thermal reaction behaviors of spent LFPBs cathode plates and the characterization of

escaping gases and phase transition of cathode materials were investigated by TG-DTG-DSC, TG-MS, SEM-EDS, FT-IR, XRD and Mössbauer spectrum analysis. The TG curve showed two weight loss platforms and a weight gain platform in the thermal treatment process, and the corresponding weight change were -3.1%, +1.2% and -3.7%, respectively. The DSC curve

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appeared two endothermic peaks at 159.5 °C and 656.7 °C as well as two exothermic peaks at 475.1 °C and 530.4 °C. Around 10 gases were detected in TG-MS systems, including alkane gases, fluorine-containing gases, H2, H2O and CO2, mainly at 60-250 °C and 430-540 °C. The alkane gases included C3H4O3, C4H8O3, CH3OH, C2H6, etc, and the fluorine-containing gases contained OPF3 and HF, etc. The volatilization and decomposition of organic solutions mainly took place in a low-temperature range from 60.0 °C to 250.0 °C, and C4H8O3, C3H4O3, CH3OH, C2H6, H2, CO2, H2O and other alkane gases, etc were released. Similarly, the

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hydrolysis and decomposition of lithium salt electrolytes took place in the same temperature range and it released OPF3, HF. Moreover, the thermal decomposition of binder mainly

occurred at around 386 °C, and the evolved gas components mostly contained H2O, HF and

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CO2. The oxidation combustion reaction of binder mainly took place at 380-600 °C, producing H2O and CO2. The SEM-EDS and FT-IR analysis indicated that the organic compounds were

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removed after the heating process. The agglomeration significantly decreased in lithium iron

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phosphate cathode active material particles, and the mole fraction of C decreased from 13.14% to 0.31%, while that of F decreased from 3.78% to 0.47%. Besides, LiFePO4 was oxidized to

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Li3Fe2(PO4)3 and Fe2O3 during the heating process.

Author Contributions

Methodology, Investigation, Data curation, Writing, Visualization.

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Yafei Jie:

Shenghai Yang: Supervision, Conceptualization, Funding acquisition. Fang Hu: Investigation, Data curation. Yun Li: Software, Reviewing and Editing, Validation. Longgang Ye: Resources, Formal analysis.

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Duoqiang Zhao: Conceptualization, Validation. Wei Jin: Software, Resources, Cong Chang: Resources, Supervision. Yangqing Lai: Supervision, Validation.

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Yongming Chen: Supervision, Reviewing and Editing, Project administration, Validation.

Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be

Acknowledgements

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considered as potential competing interests:

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This work was supported financially by Anhui Province Research and Development Innovation Project for Automotive Power Battery Efficient Recycling System. The authors also

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gratefully acknowledge the real-time TG-MS analysis provided from Institute of Engineering Thermophysics (Chinese Academy of Sciences, Beijing 100190, China). Hongde Xia and Kai

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Wei of the CAS Institute of Engineering Thermophysics are thanked for the TG-DSC-EI-MS experiments and helpful discussion.

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References

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Fig. 1 Illustrations of the flowsheet of experimental procedures

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Fig. 2 TG-DTG-DSC analysis of cathode plates.

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PF2+; 85OPF2+; 88C3H4O3+; 104C4H8O3+/104OPF3+

H2+; 12C+; 19F+; 31CH3O+; 38C3H2+; 44CO2+

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(a)

H2O+; 30C2H6+; 41C3H5+; 43C3H7+/43C2H3O+

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Fig. 3 3D mass spectrum graph of cathode plates against temperature and mass-to-charge ratio (m/z), (a) measured at high channel, (b) and (c) measured at low channel.

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Fig. 4 Ion current intensity curves of alkane gases plotted against temperature.

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Fig. 5 Ion current intensity curves of fluorine-containing gases against temperature.

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Fig. 6 SEM-EDS images of raw cathode active material, elemental mapping of Fe, P, O, C, F for raw

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cathode active material.

Fig. 7 SEM-EDS images of roasted cathode material, elemental mapping of Fe, P, O, C, F for roasted cathode active material.

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Fig. 8 FTIR spectra of cathode active material: (a) raw cathode active material, (b) roasted cathode active

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Fig. 9 XRD patterns of cathode active material: (a) raw cathode active material, (b) roasted cathode active

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material.

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Fig. 10 Room temperature Mössbauer spectrum of roasted cathode material.

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Table 1 Physicochemical properties of main components in spent LFPBs cathode plates[25, 26] Compounds

Component

Chemical formula C3H4O3

Formula weight 88.02

Melting /boiling point 36.4 °C/248 °C

C4H8O3

104.05

-14 °C/107 °C

LiPF6

151.98

200 °C/-

-CH2-CF2C

12.01

156-165 °C/3550 °C/-

Binder Conductive additive

Ethylene carbonate (EC) Methyl ethyl carbonate (EMC) Lithium hexafluorophosphate PVDF Acetylene black

Al foil

Aluminum

Al

26.98

660 °C/2327 °C

Cathode material

Lithium iron phosphate

LiFePO4

157.76

-

Electrolyte

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Lithium salts

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