Research articles
Magnetic and magnetostrictive properties of non-stoichiometric cobalt ferrite synthesized from spent Li-ion batteries

https://doi.org/10.1016/j.jmmm.2020.167185Get rights and content

Highlights

  • Co1+xFe2−2x/3O4 were synthesized by a convenient method using spent LIBs.

  • This process can reduce environmental pollution and alleviate resource shortage.

  • Co1+xFe2−2x/3O4 (x = 0.15) is a candidate for magnetostrictive sensing applications.

Abstract

A series of Co1+xFe2−2x/3O4 (x = 0, 0.05, 0.1, 0.15, 0.2) samples, in which Fe3+ was progressively replaced by Co2+, were prepared through the sol-gel auto-combustion method using spent Li-ion batteries as raw materials. The structure, morphology, magnetic and magnetostrictive properties of the samples were studied. The purpose of the present work is to make cobalt ferrite more suitable for its application in non-contact sensors and actuators. X-ray diffraction (XRD) patterns show that all the samples have a pure single-phase cubic structure. The non-stoichiometric cobalt ferrite has different crystallite size and lattice constant compared with pure CoFe2O4. Field emission scanning electron microscopy (FE-SEM) was carried out to investigate the amount and size of pores in the sintered samples. The cationic oxidation state and cation distribution of the spinel structure was investigated by means of X-ray photoelectron spectroscopy (XPS). The variations in structure and morphology were found to lead to significant changes in the magnetic and magnetostrictive properties. With the increase of cobalt content, the saturation magnetization and the maximum magnetostrictive coefficient of the samples initially increase and then decrease. Given the different behaviors of the magnetostriction coefficient and strain sensitivity of the samples, their magnetostrictive performance was quantified through a “figure of merit” defined as the product between the two above mentioned parameters.

Introduction

Li-ion batteries, which are an essential energy source, have been attracting increasing attention because of their high energy density, high capacity, low self-discharge rate, and the absence of memory effect [1]. However, the life of Li-ion batteries ends when their capacity drops by 70% of their nominal value. The abundance of spent Li-ion batteries represents a serious environmental issue, especially when land availability is limited. Moreover, this characteristic leads to a great waste of natural resources, especially of high-value and recyclable elements such as Co [2], [3], [4]. Reprocessing techniques for spent Li-ion batteries have been the focus of research all over the world. The recycling of high-value components concerns mostly cobalt, which represents 5–15% of the total mass of the Li-ion batteries [2].

In recent years, some works have considered the use of spent batteries as raw materials for the preparation of ferrites, among which cobalt ferrite is the most versatile and hard ferrimagnetic material [5], [6]. It is also a good candidate for application in the biomedical field and for magnetic devices due to its unique properties, such as high density magnetic recording, high magneto-crystalline anisotropy, large coercive field (Hc), moderate saturation magnetization (Ms), and excellent magnetostrictive coefficient [7]. Recent studies present a number of preparation methods for cobalt ferrite, such as standard solid-state ceramic processing [8], auto-combustion [9], coprecipitation [10], microwave synthesis [11], citrate precursor method [12], Pechini sol-gel [13], and sol-gel auto-combustion [14]. Sol-gel auto-combustion, developed from both the sol-gel method and self-propagating high-temperature synthesis, is among the most promising methods to prepare cobalt ferrite because it is easy to operate and occurs without calcination.

Cobalt ferrite has a spinel structure composed of a unit cell with 64 tetrahedral sites (A-sites), surrounded by four oxygen atoms, and 32 octahedral sites (B-sites), surrounded by six oxygen atoms. However, Co2+ and Fe3+ do not occupy the spinel structure sites completely, a characteristic that is helpful for substituting processes [15].The cation distribution between the A and B sites of the spinel lattice affects the magnetostrictive properties of CoFe2O4 [16], [17], which also depend on the factors such as the synthesis methods [18], heat treatment conditions [19], sintering atmosphere and time [20], and microstructure and density of the sintered samples [21], [22]. Furthermore, the magnetostrictive properties are associated with the A-O-B super exchange between metal ions caused by the overlap between the 2p orbital of the O2− ion of cobalt ferrite with the 3d orbital of transition metal ions. Cobalt ferrite presents the highest magnetostrictive properties in oxide-based materials due to the high spin orbit coupling and high magnetocrystalline anisotropy of Co2+ [7], [23], [24].

The present investigation aims at making full use of the role of cobalt ions in spent Li-ion batteries. The variation of structure, magnetic properties, and magnetostrictive properties of a non-stoichiometric ferrite were explored by progressively increasing the degree of substitution of Fe3+ with Co2+.

Section snippets

Materials and reagents

The spent Li-ion batteries with LiCoO2 as cathode material were kindly supplied from the solid-waste collection station of Henan Normal University (Xinxiang, China). Hydrogen peroxide (SCRC, 30%) was employed as a reductive reagent. Citric acid (SCRC, 99.5%) was used as complexing agent in the process. The auxiliary reagents Co(NO3)2·6H2O (SCRC, 98.5%), Fe(NO3)3·9H2O (SCRC, 98.5%) were used to regulate the proportion of metal ions in the cobalt ferrite. And all the reagents were used

XRD analysis

The structural characteristics of the Co1+xFe2−2x/3O4 (x = 0, 0.05, 0.1, 0.2) samples were analyzed by XRD in the 2θ range of 20–80° with a scanning rate of 0.02°/s. The recorded patterns are presented in Fig. 2. The XRD patterns show that all the main diffraction peaks were well matched with the JCPDS card NO. 22-1086. In all the samples, the characteristic diffraction peaks indexed as the (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3) planes of the cubic spinel ferrites [26]

Conclusions

Co1+xFe2−2x/3O4 (x = 0, 0.05, 0.1, 0.15, 0.2) samples were successfully synthesized by sol-gel auto-combustion using spent Li-ion batteries as raw materials. Although the crystal lattice of the non-stoichiometric ferrite samples were distorted, XRD patterns still showed the formation of a cubic spinel structure for all samples. FE-SEM images revealed the variation of the quantity and size of pores with the content of Co2+. XPS analysis confirmed the oxidation state of the metal element and

Declaration of interests

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.

CRediT authorship contribution statement

Changwei Dun: Conceptualization, Methodology, Software, Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Guoxi Xi: Conceptualization, Methodology, Supervision. Ye Zhang: Investigation, Validation, Resources. Xiaoying Heng: Investigation, Validation, Visualization, Data curation. Yumin Liu: Writing - review & editing, Project administration, Data curation, Supervision. Xinyan Xing: Writing - review & editing, Project administration. Rui Liang: Writing -

Acknowledgements

The authors are grateful to the financial support from the National Natural Science Foundation of China (no. 51174083) and the Specialized Fund for graduate research and innovation projects (no. YL201728).

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