Study On Preparation And Magnetostrictive Properties Of High Quality Cobalt Ferrite From Spent Li-ion Batteries

In the field of battery technology,lithium ion batteries?LIBs?have many advantages,such as high energy density,high voltage,wide operating temperature range,good cycle performance,no memory effect,light weight,low self-discharge rate and environmental friendliness,compared with the traditional nickel-cadmium batteries and nickel-hydride batteries.In recent years,with the rapid development of electronic products and electric vehicles,the market demand for lithium ion batteries has increased significantly.The wide use of lithium ion batteries will lead to the rapid increase of spent lithium ion batteries.If they are not handled properly,they will definitely pollute the soil,air,water,etc.,causing immeasurable harm to the environment and human health.The spent lithium ion batteries contain a large number of metal elements.Some metal content is higher than that in the natural ore,and relatively concentrated.Therefore,it is urgent and necessary to recycle the spent lithium ion batteries.At present,many researches are focused on the separation and purification of metal elements in the cathode materials of spent lithium ion batteries through various physical,chemical and biological means.These processes are relatively complex.In this paper,we prepare a magnetostrictive material cobalt ferrite using the leaching liquid of the cathode materials of spent lithium ion batteries directly,avoiding the complex separation and recovery process.The product cobalt ferrite can be applied in magnetostrictive sensors and actuators,which has higher added value.This process broadens the recycling channels of spent lithium ion batteries and lays a foundation for the industrial recycling of spent lithium ion batteries.It is of great significance in environmental protection and resource utilization,and has good application prospects.Compared with rare earth magnetostrictive materials,cobalt ferrite with spinel structure has the advantages of good corrosion resistance,easy processing and good mechanical properties.However,the maximum magnetostrictive coefficient and maximum strain derivative are not high enough.Therefore,it is necessary to improve its magnetostrictive property,synthesize high-quality cobalt ferrite,i.e.,make it more sensitive to the external magnetic field,require less external magnetic field strength,so as to facilitate its application.Substitution is a common method to improve the magnetostrictive property of cobalt ferrite.The rare earth metal ion Er³⁺has some unique properties,for example,a large ion radius and unpaired 4f electrons.The 4f-3d coupling with the transition metal ion can cause large magnetocrystalline anisotropy.It can be magnetic at low temperature?88 K?and non magnetic at room temperature.Li⁺is the inherent metal element of the cathode material of the spent lithium ion batteries,which can enter the spinel lattice,and affect the structure and performance of cobalt ferrite.The radius of Bi³⁺is larger than that of Er³⁺,and there is a difference in whether it can enter the spinel lattice.Moreover,Bi₂O₃ is often used as a sintering additive,which can produce liquid phase in high temperature,thus affecting the porosity.The radius of In³⁺is large and tend to occupy the tetrahedral site of spinel structure.In addition,the distribution of metal ions in tetrahedral and octahedral sites of the spinel lattice is also affected by the synthesis methods,which can also modify the magnetostrictive property of cobalt ferrite.Thus,the main research contents and conclusions of this paper are as follows:Taking the nitric acid solution of the cathode material of the spent lithium ion batteries as the raw material,a small amount of auxiliary reagents were added and sol-gel method was used to prepare Er³⁺substituted cobalt ferrite CoFe_2-xEr_xO₄?x=0,0.02,0.04,0.06,0.08,0.1?and Bi³⁺substituted cobalt ferrite CoFe_2-xBi_xO₄?x=0,0.05,0.1,0.15,0.2?.The sol-gel auto-combustion method was used to prepare Li⁺substituted cobalt ferrite Co_1-xLi_xFe₂O₄?x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.7?and In³⁺substituted cobalt ferrite CoFe_2-xIn_xO₄?x=0,0.1,0.2,0.3,0.4,0.5?.Pure cobalt ferrite was synthesized by sol-gel method,coprecipitation method and microwave-assisted hydrothermal method.The optimal calcining temperature of the dry gel for the preparation of cobalt ferrite by sol-gel method was determined by thermogravimetric-differential scanning calorimetry analysis?TG-DSC?,and the crystal style and structure of the products were analyzed by X-ray diffraction?XRD?.Field emission scanning electron microscopy?FESEM?was used to observe the morphology of the cobalt ferrite nanoparticles and cross-section of the sintered cylindrical rods.High resolution transmission electron microscopy?HRTEM?was used to determine the lattice interplanar distances of the samples.Fourier transform infrared spectroscopy?FT-IR?was used to analyze the functional groups contained in the samples.The metal elements and their valence states were determined by X-ray photoelectron spectroscopy?XPS?.The magnetic parameters of the samples,such as saturation magnetization?M_s?,coercivity?H_c?and residual magnetization?M_r?,were analyzed by using the vibration sample magnetometer?VSM?,and M-T curves were measured under the variable temperature to determine the Curie temperature?T_c?of the samples.Magnetostriction coefficient of the sintered cobalt ferrite cylindrical rods along the direction parallel to the magnetic field is analyzed by magnetostriction measuring instrument.Er³⁺and Bi³⁺enter the octahedral?B?site in spinel structure of cobalt ferrite and substituted Fe³⁺.Li⁺enter the the octahedral?B?site in spinel structure of cobalt ferrite and substituted Co²⁺,In³⁺enter the tetrahedral?A?site and substituted Fe³⁺.Then study the influence of Er³⁺,Bi³⁺,Li⁺and In³⁺substitution on the structure,morphology,magnetic and magnetostritive properties of cobalt ferrtie and determine the best substituted quantity for different ions in order to obtain the best magnetostrictive property of cobalt ferrite.Compare the structure,grain size,magnetic properties and magnetostrictive properties of cobalt ferrite synthesized by three different methods and discern the best route for their synthesis,taking their different application prospects into account.The results shows that:?1?In the case of Er³⁺substitution,the lattice constant of the samples first increases and then decreases,the grain size decreases,the number of pores formed in the high-temperature sintered cylindrical rods increases,the shape changes,the saturation magnetization decreases,and the coercivity shows a nonlinear change.The magnetostrictive property of CoFe_1.9Er_0.1O₄ is the best.When Li⁺was substituted,the grain size range of the samples was between 30 and 40 nm,the lattice constant,Curie temperature and saturation magnetization gradually decreased,the coercivity first dropped sharply,then changed irregularly,and the magnetostriction of Co_0.8Li_0.2Fe₂O₄ was the best.?2?When Bi³⁺is used for substitution,x?0.1,Bi³⁺can completely enter the lattice without impurity phase formation.x>0.1,the generated Bi₂O₃ gathered in the grain boundary,changing the structure and properties of cobalt ferrite,making the maximum magnetostriction coefficient and maximum strain derivative of CoFe_1.8Bi_0.2O₄ cylindrical rod sintered under 900 ^oC reach 200 ppm and 1.53×10^-9 A^-1 m.They are comparable to the value of the cylindrical rods sintered under high temperature 1200 ^oC,indicating that Bi³⁺substitution can reduce sintering temperature and save energy.In case of the In³⁺substitution,x<0.2,no impurity phase is generated;In₂O₃ aggregates in the grain boundary after x?0.2.The lattice constant increases,grain size,Curie temperature and maximum magnetostriction coefficient gradually decrease,and the maximum strain derivative changes irregularly,the magnetic field required to achieve the maximum strain derivative gradually decreases.?3?The distribution of metal ions in tetrahedral and octahedral sites in the spinel lattice of cobalt ferrite can be affected by different synthesized methods directly,thus affecting the magnetic and magnetostrictive properties of cobalt ferrite.The maximum magnetostrictive coefficient of cobalt ferrite synthesized by sol-gel method?S-CFO?is 181 ppm and S-CFO can be used in actuator applications.By contrast,the maximum strain derivative of cobalt ferrite synthesized by microwave-assisted hydrothermal method?M-CFO?is 1.40×10^-9 A^-1m and M-CFO can be a suitable candidate for magnetostrictive sensor applications.