Magnetic and magnetostrictive properties of RE-doped Cu-Co ferrite fabricated from spent lithium-ion batteries

Author’s Accepted Manuscript Magnetic and magnetostrictive properties of REdoped Cu-Co ferrite fabricated from spent lithiumion batteries Guoxi Xi, Lu Wang, Tingting Zhao www.elsevier.com/locate/jmmm

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S0304-8853(16)32091-1 http://dx.doi.org/10.1016/j.jmmm.2016.10.031 MAGMA61944

To appear in: Journal of Magnetism and Magnetic Materials Received date: 7 September 2016 Revised date: 29 September 2016 Accepted date: 6 October 2016 Cite this article as: Guoxi Xi, Lu Wang and Tingting Zhao, Magnetic and magnetostrictive properties of RE-doped Cu-Co ferrite fabricated from spent lithium-ion batteries, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.10.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Magnetic and magnetostrictive properties of RE-doped Cu-Co ferrite fabricated from spent lithium-ion batteries Guoxi Xi*, Lu Wang, Tingting Zhao Key Laboratory for Yellow River and Huai River Water Environment and Pollution Control, Ministry of Education, College of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang 453007, P. R. China [email protected] (G. Xi) [email protected] (L. Wang). * Corresponding author. Abstract Magnetostrictive Cu0.1Co0.9RExFe2-xO4 (RE = Ho, Gd or Sm) was fabricated by a sol-gel auto-combustion technique using spent lithium-ion batteries as raw materials. X-ray diffraction analysis confirmed the spinel structure of the RE-incorporated samples with limited RE solubility. Field-emission scanning electron microscopy and Fourier transform infrared spectroscopy revealed a layered structure composed of particles and the cation distribution. Magnetic hysteresis loops and magnetostriction strain curves showed that the saturation magnetization, magnetostriction coefficient and strain derivative were significantly modified due to the substitution of larger ionic radius RE3+ ions for Fe3+ ions, influencing the interaction between the tetrahedral and octahedral sites.

Keywords: Cu-Co ferrites; rare earth doping; magnetostrictive properties; spent lithium-ion batteries

1. Introduction It is well known that the cathodes of spent lithium-ion batteries (LIBs) contain valuable 1

nonferrous metals, such as cobalt and nickel, with concentration levels sometimes higher than those found in their natural ores [1-3]. On the basis of this, spent LIBs are good choices as raw materials for the preparation of magnetostrictive cobalt ferrite, as they not only prevent environmental pollution, but also alleviate resource shortages. Cubic spinel cobalt ferrite (CoFe2O4) has various advantages over its alloy-based counterparts as a magnetostrictive material for sensors and actuators due to its high magnetocrystalline anisotropy, high corrosion resistance, better mechanical properties and low cost [4-6]. However, for CoFe2O4, the value of the slope of the magnetostriction or the strain derivative (dλ/dH) at an applied magnetic field is still necessary to be enhanced [7, 8]. These properties can be modified by synthesis methods, doping the main lattice structure and by modifying the grain size and morphology. CoFe2O4 has two sub-lattices, namely, tetrahedral (A) and octahedral (B) sites. The magnetic properties of CoFe2O4 depend on the exchange interaction of the unpaired 3d electrons of the transition element and the cation distribution at the A- and B-sites [9]. Hence, doping the spinel lattice with rare earth (RE) elements with large ionic radii can adjust the structure, magnetic and magnetostrictive properties of CoFe2O4 [10, 11]. Another important factor for RE-doping of CoFe2O4 is related to the occupancy of the 4f electron shells (from 0 (La) to 14 (Lu)) and the magnetic moments (from 0 (La) to 10.6 μB (Dy)) of the RE ions. Due to their moderate elastic constants and large orbital components, RE elements display the largest known magnetostrictions [12]. The inclusion of RE elements in the spinel lattice produces RE3+-Fe3+ interactions (3d-4f coupling) that change the original superexchange interactions [13]. The high spin Co2+ ions at the B-sites exhibit a strong spin-orbital (L-S) coupling and introduce large magnetocrystalline

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anisotropy. The substitution of RE3+ ions for Fe3+ ions can manipulate the magnetic coupling [14]. Due to the larger ionic radii of RE3+ ions compared to Fe3+ ions, the tetrahedral and octahedral symmetries may become distorted, which affects the lattice parameter [15]. Recent studies have attempted to improve the magnetostrictive properties of CoFe2O4 by doping Al [6], Mn [7, 16], Cu [4] and Ni [17] ions into the host lattice. Xi and Xi [18] reported that the substitution of Fe3+ with Cu2+ promotes an increase in maximum magnetostriction (λmax) and maximum strain derivative (dλ/dHmax). However, magnetostrictive properties of CoFe2O4 modified by doping RE ions are rarely reported. On this basis, we intend to investigate the effect of RE-doping (RE = Ho, Gd or Sm) on the properties of Cu0.1Co0.9Fe2O4.

2. Experimental Samples of Cu0.1Co0.9Fe2O4 (code CF), Cu0.1Co0.9HoxFe2-xO4 (x = 0.05–0.25, code H1–H5), Cu0.1Co0.9GdxFe2-xO4 (x = 0.05–0.25, code G1–G5) and Cu0.1Co0.9SmxFe2-xO4 (x = 0.05–0.25, code S1–S5) were synthesized by following a sol-gel auto-combustion method using spent LIBs as raw materials. The spent LIBs were dismantled and the cathode materials were separated and dissolved with a 3 mol/L sulfuric acid solution. Precipitations were generated by adjusting the pH, filtered and re-dissolved using a nitric acid solution. Details of the treatment process have been reported by Xi et al. [19] and Yang et al. [20]. The appropriate amounts of analytical grade Co(NO3)2, Cu(NO3)2, Fe(NO3)3 and RE(NO3)3 (RE = Ho, Gd or Sm) were added to a solution containing mostly Co2+ to adjust the metal ion ratio to stoichiometric. Then, the addition of citric acid produced a metal ion to citric acid ratio of 1:1. The solution was continuously stirred at 60 ºC. Ammonia water was added to adjust the pH to 6.5. The temperature was adjusted to 80 ºC and the

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sol-gel process began. Finally, the obtained gel was dried at 110 ºC. The dry gel was subjected to auto-combustion. Then, the Cu0.1Co0.9RExFe2-xO4 (x = 0.00–0.25, 0.05 was used as a gradient) samples were fabricated. The experimental flow chart is depicted in Fig. 1.

Fig. 1 Flow chart of the preparation of Cu0.1Co0.9RExFe2-xO4 using spent LIBs

The as-obtained samples, with 10% polyvinyl alcohol (PVA) added, were compacted into cylindrical specimens (10 mm diameter and 20 mm length) by cold uniaxial pressing of the powders at a pressure of 15 MPa [21]. The cylinder samples were sintered at 1200 ºC for 6 h. The crystallographic properties of the prepared samples were analyzed using X-ray diffraction (XRD, D8-advance, BRUKER, Germany) with Cu-Kα radiation. The microstructural analysis was performed using field emission scanning electron microscopy (FE-SEM, SUPRA-40, Carl Zeiss, Germany). The absorption spectra were recorded in a wavenumber range of 400–4000 cm-1 using Fourier transform infrared spectroscopy (FTIR, NEXUS, Thermo Nicolet, USA). The magnetic

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properties were measured using a vibrating sample magnetometer (VSM, Versa Lab, Quantum Design, USA) at room temperature. The magnetostrictive properties were measured from 0 to 6000 Oe using a JDM-30 magnetostrictive coefficient measuring system.

3. Results and discussion XRD patterns of Cu0.1Co0.9RExFe2-xO4 that are before the sintering as synthesized by a sol-gel auto-combustion method are depicted in Fig. 2. It is found that the diffraction peaks of all samples are sharp, indicating the highly crystalline nature of the samples. No impurity peaks are observed with increasing Ho concentration. The pure phase Gd-doped Cu-Co ferrite is formed up to a concentration of 0.1. GdFeO3 is found to be formed as an impurity phase in samples G2 to G5. The impurity phase SmFeO3 is found in all samples of the Sm-doped Cu-Co ferrite. The appearance of the impurity phase RE orthoferrite may be due to the RE ionic radii, RE3+ > Fe3+ (0.64 Å [13]), which causes the limited solubility in the RE-doped Cu-Co ferrites. The different limiting amounts of Ho, Gd and Sm may also be a result of the RE ionic radii, Sm3+ (0.964 Å [15]) > Gd3+ (0.938 Å [22]) > Ho3+ (0.90 Å [13]). All diffraction peaks of the samples, except for impurity peaks, are consistent with the JCPDS 22-1086 standard [23]. The average crystallite size (DXRD) is calculated using the Scherrer equation [24]: DXRD = kλ/βcosθ

(1)

where λ is the wavelength equal to 1.5406 Å, k is the Scherrer constant equal to 0.9, θ is the Bragg's angle and β is the full width at half maxima (FWHM). The calculated average crystallite size is about 35 nm for CF. The average crystallite sizes of the RE-doped samples are in the range of 26.5–32.1 nm, which illustrates the constriction of RE-doped ferrites because of the significant energy required, at the expense of crystallization, to substitute the large RE3+ ions for the Fe3+ ions

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and form the RE-O band [10]. The lattice constant (a) of all samples is calculated using the standard relation [25]: a = λ(h2+k2+l2)1/2/2sinθ

(2)

The calculated lattice constants are 8.378–8.395 Å for the RE-doped samples, an enhancement compared to 8.37 Å for CF, providing evidence of the incorporation of RE3+ ions in the host spinel structure.

Fig. 2 XRD patterns of Cu0.1Co0.9RExFe2-xO4

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Fig. 3 FT-IR spectra of Cu0.1Co0.9Fe2O4 and Cu0.1Co0.9RE0.1Fe1.9O4

Fig. 4 EDS patterns of Cu0.1Co0.9RE0.1Fe1.9O4

The FTIR spectra of the Cu0.1Co0.9Fe2O4 and Cu0.1Co0.9RE0.1Fe1.9O4 samples shown in Fig. 3 are recorded in the range of 400–4000 cm-1. The dominant absorption band at around 573 cm-1 is characteristic of a ferrite and is assigned to the vibration of Fe3+-O2- at the tetrahedral A-sites. The

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broad peak at around 3450 cm-1 and the small peak at around 1650 cm-1 indicate the stretching modes and H-O-H bending vibration of the free or absorbed water molecules [10, 26]. Energy dispersive spectrometer (EDS) measurements, as depicted in Fig. 4, are carried out to provide the chemical composition and distribution of the RE-doped samples. The atomic ratios of metal elements Cu, Co, RE and Fe are approximately consistent with the theoretical stoichiometry of 1:9:1:19. All RE elements are uniformly distributed. The 2.14 keV peak is attributed to the conductive resin holding the sample [27]. EDS further illustrates the introduction of RE3+ ions into the Cu-Co ferrite.

Fig. 5 FESEM images of Cu0.1Co0.9Fe2O4 (a), Cu0.1Co0.9RE0.1Fe1.9O4 (b: Ho, c: Gd, d: Sm)

The effect of RE elements on the morphology is observed by FESEM. Figs. 5a–d demonstrate the micrographs of CF, H2, G2 and S2, respectively. It is clear that all of the samples have a flaky and porous morphology. During the combustion reaction, a large volume of gases liberated in a

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short time are responsible for the highly porous nature of the synthesized samples [5]. The intra-granular pores of the RE-substituted samples are reduced in comparison with the CF sample and gradually disappear from H2 to S2. This can be ascribed to the larger Sm3+ ionic radius and the formation of the SmFeO3 phase. The RE orthoferrite tends to accumulate at the grain boundaries. From the above results, it can be seen that different metal ions and solubility limits have a great influence on the morphology and structure of the ferrite.

Fig. 6 Magnetic hysteresis loops of sintered Cu0.1Co0.9RExFe2-xO4 measured at room temperature

Fig. 7 Magnetostriction strain curves of sintered Cu0.1Co0.9RExFe2-xO4

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The magnetic hysteresis loops of the sintered samples of Cu0.1Co0.9RExFe2-xO4 measured at room temperature are depicted in Fig. 6. The saturation magnetizations (Ms) extracted from M versus 1/H curve to 1/H = 0 are compared in Table 1. It is observed that Ms decreases with increasing RE content. The important factors may be related to the following points: (1) The reduction of Ms is correlated with the decrease of crystal size, exhibiting a large surface effect [14]. (2) The magnetic moment of Co2+ is aligned antiparallel to the RE3+ ions. Substitution of the magnetic Fe3+ ion by RE3+ ions at the B-sites, which may lead to the migration of Co2+ ions from the B-sites to the A-sites, reduces the overall magnetic moment in the spinel ferrite [28]. (3) The incorporation of RE3+ in the ferrite reduces the Fe3+-Fe3+ interactions that govern the magnetic response due to the weaker RE3+-RE3+ and RE3+-Fe3+ interactions [29, 30]. The variation of coercivity as a function of RE content in Cu0.1Co0.9RExFe2-xO4, as shown in Figs. 6a–c, follows the same behavior as that of Ms. The coercivity can be expressed as: Hc = 2K1/μ0Ms

(3)

where K1 is the magnetocrystalline anisotropy coefficient. From the relation of Hc and K1, it can be seen that K1 decreases with increasing RE concentration. The decrease of K1 is likely to be associated with the reduction of the A-O-B super-exchange interaction caused by RE-doping. Magnetostriction strain curves of the sintered samples measured at room temperature in the direction of the applied magnetic field are shown in Fig. 7. Maximum magnetostriction and strain derivative values are tabulated in Table 1. It can be seen that substitution of Fe by RE in Cu-Co ferrite has a strong influence on the magnetostriction characteristics. The maximum magnetostriction (λmax) is observed to decrease rapidly with increasing RE concentration. The decrements are 12.5% (Ho), 19.5% (Gd) and 14.8% (Sm), respectively. In cobalt ferrite-based

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systems, the large negative and positive magnetostrictive contributed by Co2+ and Fe3+ ions at octahedral sites and the magnetostrictive contributions by the cations at tetrahedral sites are less important and opposite to the octahedral site contributions [4, 16]. The substituted RE ions tend to occupy B-sites. This means that the RE3+ ions with large positive magnetostrictive contributions change the original magnetostrictive state. The maximum strain derivative (dλ/dHmax) has been observed to slightly decrease with x ≤ 0.15, but at higher concentrations of RE elements, dλ/dHmax sharply decreases. At lower concentrations, dλ/dHmax may be maintained at the expense of consuming magnetostriction. It is possible that higher concentrations of RE elements lead to serious distortions of the lattice and significant reductions in magnetocrystalline anisotropy. It can be seen from Fig. 7 that the RE-doped samples have lower strain sensitivity than that of the undoped sample, but the maximum strain sensitivity is obtained at lower magnetic fields than that for the latter. For sensor and actuator applications, ceramic oxide materials need not only good magnetostriction parameters, but also need to obtain parameters at low magnetic fields. The strain derivative can be expressed as [31]: dB/dσ = dλ/dH = 2μλsM/NK1

(4)

where dB/dσ is the sensitivity of the magnetization to stress, μ is the initial permeability, λs is the saturation magnetostriction, M is the magnetization and K1 is the magnetocrystalline anisotropy coefficient. The strain derivative is related to the stress sensitivity of the magnetization, which makes ferrites attractive for stress torque sensors [32]. Based on the above equation, λmax/K1 is an important factor for influencing the strain derivative of materials. But samples with significant porosity may also lead to low strain sensitivity due to the demagnetizing field exerted by the pores between the grains [33]. Thus, microstructure is also an important factor in affecting the

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magnetostriction parameters.

Table 1 Magnetization (Ms), maximum magnetostriction (λmax) and strain derivative (dλ/dHmax) λmax

dλ/dHmax

(emu/g)

(ppm)

(10-9A-1m)

CF

77.87

-186.9

-1.23

H1

75.86

-163.5

-1.28

H2

69.21

-127.3

-1.22

H3

64.30

-110.2

-1.20

H4

58.12

-100.1

-0.89

H5

57.95

-88

-0.68

G1

67.99

-150.5

-1.15

G2

62.37

-129.6

-1.22

G3

61.8

-109.9

-1.04

G4

59.27

-96.4

-0.82

G5

59.07

-81.8

-0.62

S1

67.2

-159.2

-1.12

S2

62.97

-126.5

-1.15

S3

57.9

-109.3

-1.08

S4

52.1

-96.5

-0.85

S5

51.33

-83.7

-0.64

Sample

Ms

4. Conclusions Nanocrystalline composite magnetostrictive Cu0.1Co0.9RExFe2-xO4 has been prepared by a sol-gel auto-combustion route using spent LIBs. The saturation magnetization and coercivity of the Cu-Co ferrite were reduced by doping with RE elements. The maximum magnetostriction was

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-163.5 ppm at x = 0.05 in the Cu0.1Co0.9HoxFe2-xO4 system. Strain derivative is an important factor of sensors and actuators design and applications. Higher strain derivative at low field is desirable. So adding low quantity of rare earth dopant can be a solution even if the magnetostriction decreases. The dλ/dHmax was maintained at x ≤ 0.15 at lower magnetic fields.

Acknowledgments The authors are thankful for financial support from the National Natural Science Foundation of China (grant No. 51174083).

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Highlights 1. Magnetostrictive Cu0.1Co0.9RExFe2-xO4 (RE = Ho, Gd or Sm, x = 0.0 - 0.25) nanocomposites were fabricated via sol-gel auto-combustion route using spent lithium-ion batteries as raw materials. 2. The RE elements doping had limited solubility. 3. The saturation magnetization (Ms) and maximum magnetostriction (λmax) were reduced and the lattice parameter (a) was increasing by increasing RE3+ substitution contents. 4. The relationship of maximum strain derivative (dλ/dHmax) after the incorporation of RE was Ho>Gd>Sm.

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