Synthesis of nanocubic lithium cobalt ferrite toward high-performance lithium-ion battery

The synthesis of Li1.1Co0.3Fe2.1O4 ferrite nanoparticles has been successfully fabricated by the citrate auto combustion technique. Numerous characterization techniques as X-ray Diffraction (XRD), High- Resolution Transmission Electron Microscopy (HRTEM), Field emission scanning electron microscope (FESEM), and Raman Spectroscopy are achieved. The homogeneous formation of the cubic phase is ratified through HRTEM. Five Raman-active modes A1g, 3F2g, Eg. are detected for the examined samples. In addition, X-ray photoelectron spectroscopy (XPS) is carried out to identify the various ions existing in samples and their oxidation states. The investigated ferrite nanoparticles manifest large capacity (until 1150 mAh g−1), stellar coulombic efficiency, and superb cycle stability (443 mAh g−1 after 50 cycles). Finally, the cheap and non-toxic Li1.1Co0.3Fe2.1O4 has been employed as an anode for lithium-ion batteries (LIBs), demonstrating superior electrochemical in terms of specific capacity, cycle performance, and rate capability.


Introduction
The widely used electrode material for the lithium-ion battery (LIB) is graphite (anode) and LiCoO 2 , LiNiMnCoO 2 and LiFePO 4 (cathode). Their theoretical lithium storage capacities are 372, 140 (3-4.2 V), 150 (3.5-5 V) and 170 mA h g −1 (2.5-4.2 V), respectively [1]. The small theoretical capacity is the main disadvantage and the demerit of the graphite-based anode [2]. Consequently, it is highly necessary to seek alternative materials that possess higher theoretical capacities. The non-toxic, low-cost high energy density (HED), with better cycling stability, are crucial demands for new electrode materials [3,4].
Lithium ferrite is an important transition metal (TM) spinel nanoparticle with various advantages [5,6] making it a good candidate for utilization as an electrode. It has been widely testified for various technological applications such as the components in microwave equipment and cathode materials in LIB [7,8]. Among the numerous electrode materials, lithium cobalt oxide LiCoO 2 and Iron-based compounds including. LiFeO 2 have high theoretical capacities and are non-toxic, and environmentally friendly [9,10].
The magnetic property of some ferrite materials can be adjusted through electrochemical studies [11,12]. The manufacture of modulable instruments controlled by Li-ions is highly affected by the existence of magnetization and ion storage ability. During the battery cycle, the TM electrodes in LIB will experience, successions of redox reactions. Accordingly, the state of the 3d electrons, as well as the magnetization, will be varied. Therefore, the management of magnetic strength by intercalation/deintercalation of Li-ions is expectable [13,14].
Most of the research is focused on the change of the structure during the battery cycle. However, the magnetism evolution through 3d transition-metal (TM) oxides didn't catch the researchers' attention. In spite of the importance of structural classification of the electrodes, the magnetic measurement appears to be an accurate, sensitive, and appropriate description technique that achieves data on the detailed atomic interaction during the intercalation/ deintercalation of Li-ions. Numerous [18]. Compared to other battery systems, the LIB battery offers the highest development potential for electric and hybrid electric vehicles. This is mainly because lithium has a low electronegativity (0.98 on Pauling scale) and the lowest atomic weight (6.94 g mol −1 ) of all metals. Li 1.1 Co 0.3 Fe 2.1 O 4 (LiCoFeO) sample has a clear advantage in terms of cost, higher electrochemical potential, the HED and it is relatively less toxic [19].
Widespread commercial use of nanoparticles in energy devices will require more sophisticated electrode formulations and better recognition of the nature of charge storage. Consequently, this work is motivated by a need for a deeper understanding of nanoparticles for use as active material in LIBs.
In the current work, the LiCoFeO nano ferrite was prepared via the citrate auto combustion technique. The structure, morphology, oxidation states of numerous ions and the appropriateness of the investigated samples to be used as LIB electrodes were testified in detail, especially specific capacity, cycle performance, high reversible lithium storage capacity and rate capability.

Preparation of LiCoFeO
LiCoFeO was synthesized using the citrate auto combustion approach through the blending of non-stoichiometric amounts of lithium, cobalt, and iron nitrates with the estimated amount of citric acid, as stated earlier [20], with certain modifications as indicated in Fig. 1. All the chemical reagents were obtained from Sigma-Aldrich Company and used without additional treatment.

Electrode fabrication and electrochemical measurements
The electrochemical behavior of LiCoFeO ferrite nanoparticles as an anode material for LIB uses was testified utilizing the two-electrode coin cell (CR 2032). The working electrode was prepared in a glove box (I-Lab, inert, USA) under Ar atmosphere through the dissolving of 70:20:10 mass ratios of active electrode material LiCoFeO ferrite, acetylene black as the conductive material, and polyvinylidene fluoride in N-methyl-2-pyrrolidone as a binder. Thereafter, this mixture was smoothly deposited onto the copper foil (current collector) then left to dry at 95 °C overnight before pressing to 16 mm diameter. As reference and counter electrode, Li metal foil was utilized. A glassy membrane was employed as a separator. 1 M LiPF 6 in a 1:1 blend of dimethyl carbonate and ethylene carbonate was utilized as an electrolyte. The electrodes completed half cells were immersed in the electrolyte overnight to ensure their saturation. Cyclic voltammetry (CV) was measured from 0.01 to 3.0 V (vs. Li/Li + ) at . At an open circuit voltage, electrochemical impedance spectroscopy (EIS) was performed at frequency ranging from 100 kHz to 100 MHz. A NEWARE BTS-4000 battery testing equipment was used to perform the galvanostatic charge-discharge (GCD) tests at numerous current densities.

Characterization tools
The X-ray diffractometer (Xpert PRO MPD) was used to analyze the XRD patterns of ferrite nanoparticles using CuKα radiation. High-Resolution Transmission Electron Microscopy (HRTEM) was operated to investigate the morphology and size distribution of the as-prepared ferrites. The morphology of the nanoparticles was investigated using Field Emission Scanning Electron Microscope (FESEM, model Quanta 250). The energy-dispersive X-ray analysis (EDAX) and the elemental mapping were carried out through Scanning Electron Microscope (Quanta 250 FEG) linked with EDAX system. To investigate the oxidation states of various elements in the as-prepared ferrites, X-ray photoelectron spectroscopy (XPS) analysis was performed via K-ALPHA (Thermo Fisher Scientific, USA). Figure 2a demonstrates the XRD chart of the LiCoFeO sample. This chart is recorded at room temperature, and all the diffraction lines are indexed for the cubic spinel phase with space group (Fd-3 m). The crystallite size (D XRD ) of the sample is detected according to the Williamson-Hall equations as mentioned in the previous work [20]. The obtained data assure that the sample is prepared in nano scale with crystallite size 30 nm. The XRD data is compared with JCPDS card 04-022-8066 [20]. Figure 2b shows the Raman spectra for LiCoFeO nanoparticles. In general, the cubic-inverse spinel structure belonging to the symmetry O 7 h with a space group (Fd3m). It has 16 cations in octahedral sites, 8 cations in tetrahedral sites, and 32 oxygen anions [21]. White et al. [22,23] predict the following vibrational normal modes according to the group theory as given as:

Raman spectra
where the (R) and (IR) identify Raman-active and infraredactive vibrational species, respectively, and the rest of the modes are silent modes.
There are five Raman-active modes [24] that should be observed for the cubic spinel phase as shown in Equation: Here A, E, and F signify one, two, and three-dimensional representations, and g signifies the symmetry of the center of inversion. The three Raman-active F 2g modes are labeled F 2g (1), F 2g (2), and F 2g (3), where F 2g (1) is the lowest frequency F 2g mode and F 2g (3) is the highest frequency mode of this vibrational species.
As shown from the figure, three various spectral intervals can be noticed in the region 100-800 cm −1 for LiCoFeO as follows (i) 600-800 cm −1 is the region of A 1g modes; (ii) 410-550 cm −1 is the region of F 2g (3) modes, and (iii) 260-380 cm −1 is the region of F 2g (2) modes. Eg and F 2g (1) may be observed at a lower frequency. The highest-frequency A 1g mode is assigned to the symmetric stretching of Fe 3+ -O at the tetrahedral (A) site within the spinel lattice [25]. All the other low-frequency modes are attributed to vibrations inside the octahedral sites (BO 6 ) [26].
Besides the highest-frequency, Raman-active F 2g (3) mode is alternatively attributed to the antisymmetric bending of the tetrahedral unit [27] or the asymmetric bending motion of the oxygens bonded to the tetrahedral cations [28]. A vibrational frequency side is dependent on both the bond strength (degree of covalence and bond length affect the bond strength) and the reduced mass of the vibrational species.
The F 2g (2) mode is due to the opposite motion of cation and oxygen along one direction of the lattice [29].The E g mode is assigned to the symmetric bending motion of the oxygen anions with respect to ferric cations at the tetrahedral (A) site in agreement with other researchers [30]. Usually, the morphology and electrochemical performance of the spinel ferrites are susceptible to the fabrication process. The shape of the LiCoFeO nanoparticle is roughly spherical and highly agglomerated due to the magnetic nature of the cobalt. The SAED approves a highly crystallinity of the sample [31]. The mean size can be detected from Fig. 3c. It is equal to 70 nm, which is proportionate with a large surface area of the sample.

Field emission scanning electron microscope (FESEM)
The morphology of the sample is studied using a Field Emission Scanning Electron Microscope (FESEM) as demonstrated in (Fig. 4a-b). The nano size is ratified from the images. The grains have a rocky-like shape with porous nature. The presence of pores in the investigated sample as a result of escaping of gases during the preparation technique [32]. The morphological features recommend the use of LiCoFeO in water treatment, gas sensors and LIBs.

Energy-dispersive X-ray spectroscopy (EDAX) and mapping analyses
Figure 5a EDAX analysis for the LiCoFeO sample. EDAX analysis confirms the presence of elements Fe, Co, and O. While the Li element cannot be detected due to its low atomic number [33].The dispersive X-Ray spectroscope for the present objective does not have a sensing nature to reveal the energy released by a very small lithium-ion, which has a K shell for transition for auger electrons. The EDAX data detects that the sample is prepared without any impurity. The inset table gives a quantitative estimation of elements attained directly from the spectrum through its weight and atomic percentages. Figure 5b illustrates the elemental mapping of the LiCoFeO 4 sample. The elements cobalt, iron, and oxygen are present in the homogenous distribution

X-ray photoelectron spectroscopy (XPS)
The binding energy of numerous atoms is related to the chemical state of the atom and the molecular environment. The XPS technique denoted the elemental composition, the oxidation state of numerous cations, and the state in the synthesized sample. Figure 6 describes the taken XPS spectra for the LiCoFeO nanoparticles. The main peak, Fe2p 3/2 , with  Fig. 6b. According to the integrated intensity of the fitted doublets, the distribution of Fe 3+ ions for both samples is obtained to be about 70% in octahedral sites and about 30% in tetrahedral sites. The Li 1 s contribution from the active LiCoFeO is present as a weak component at ∼66 eV (Fig. 6c) the weak Li signal relative to the Fe and Co signals is partly due to the difference in the photoionization cross sections for Li 1 s compared to Fe and Co.
In Fig. 6d, the Co 2p 3/2 XPS spectra are well fitted into three peaks situated on 780, 783, and 785 eV. The binding energies at 779.8 and 781.9 eV are assigned to Co 3+ in B sites and Co 2+ in A site, respectively. The signal at 785.9 eV can be ascribed to the satellite peak of Co 2p 3/2 main line [34].
The O 1 s XPS signals shown in Fig. 6e are divided into three peaks at 528.5, 531, and 532 eV. The contribution of the crystal lattice oxygen is signified by the main peak at 528.5 eV. However, the exact assignment of the higher binding energy peaks is rather complex and controversial as numerous issues like contaminants, impurities, surface defects, or chemisorbed oxygen species. Finally, the existence of LiCoFeO in the sample can be seen from the XPS spectrum, which further designates that the sample has been successfully synthesized.

Electrochemical performance
The performance of Li 1.1 Co 0.3 Fe 2.1 O 4 powder as an electrode material for LIBs is characterized electrochemically. Figure 7 shows the CV curves for the first five cycles between 0.01 and 3.0 V (vs. Li/Li + ) at a scan rate of 0.25 mV s −1 . During the initial discharge process, the reduction peak at 0.52 V (vs. Li/Li + ) can be linked to a reduction reaction of Fe 3+ and Co 2+ with Li and the formation of Li 2 O [35,36]. The oxidation processes of metallic Fe and Co are responsible for the broad anodic peak among all [37].
Further, a broad peak is observed in the anodic polarization process at around 1.05 V (vs. Li/Li + ), which matched the oxidation of Fe 0 to Fe 2+ and then to Fe 3+ [37]. The CV curves for the Li 1.1 Co 0.3 Fe 2.1 O 4 electrode are highly stable after the second cycle, showing improved steadiness through the lithiation and de lithiation operations.
The corresponding charge-discharge patterns of the LiCoFeO electrode in the voltage window of 3-0.01 V (vs. Li/Li + ) (from 1st cycle to 100th cycle is shown in Fig. 8a at a current density of 100 mA g −1 . A lengthy plateau emerges at around 0.85 V (vs. Li/Li + ) during the initial discharge cycle, suggesting a reduction of Fe 3+ and Co 2+ to Fe 0 and Co 0 as well as the formation of Li 2 O, respectively [38,39]. The electrode has an initial charge/discharge specific capacity of 770/1300 mAh g −1 , equating to a 59.2% initial coulombic efficiency. The coulombic efficiency reaches 98.77% after 100 cycles, suggesting that the LiCoFeO is more stable due to the creation of solid electrolyte interphase (SEI) layers on the electrode surface and hence can successfully handle  [40,41] as presented in Fig. 8b. The LiCoFeO electrode initial capacity loss was 40.8%, which may be due to the irreversible surface reactions, such as (i) Lithium ions capture inside the lattice, (ii) Li + ions intercalation/deintercalation into/out of an acetylene carbon black conductor iii) solid electrolyte interphase (lithium alkyl carbonates) formed on the electrode [42].
Another significant characteristic of high-performance LIBs is their rate capabilities. The rate capability performance of the LiCoFeO electrode is investigated at varied current densities of 100, 200, 500, 800, 1000, and 2000 mA g −1 , as shown in Fig. 8c. After 42 cycles, the LiCoFeO electrode reversible discharge specific capacity is 1150, 390, 265, 170, 140 and 110 mAh g −1 , at current densities of 100, 200, 500, 800, 1000 and 2000 mA g −1 , sequentially. When 50 cycles, the LiCoFeO electrode exhibits a high rate performance with a specific capacity of 443 mAh g −1 after the current density progressively recovered to 100 mA g −1 . The improvement in rate capability might be attributed to the presence of the highly conducting Li ions and the small size of LiCoFeO ferrite nanoparticles which facilitates the Li-ions mobility [35]. Moreover, the cubic 3D structure of LiCoFeO is favorable for the preferable moistening of the electrode material via the electrolyte, thus facilitating the lithiation and delithiation across the electrolyte/ electrolyte interface [36].
The performance of the electrode materials is examined through EIS by elucidation of the physicochemical processes. Figure 8d shows the Nyquist plot, revealing that the electrodes have two different components. The equivalent circuit includes constant phase elements (CPE) of double-layer capacitance, charge/transfer resistance (R ct ), electrolyte resistance (R 1 ), and Warburg impedance (W o ). The charge transport impedance at the electrode/ electrolyte contact is responsible for the semicircular in the high-frequency region. The straight line (Warburg tail) is ascribed to Li + prevalence in the active electrode material in the low-frequency region. Before and after 100 cycles, the cells are exposed to an EIS examination, as revealed in Fig. 8d. The charge-transfer resistance (R ct ) after 100 cycles looks constant (changed from 114.6 to 115.2 Ω) because the contact between the electrolyte and the electrode is continually stimulated through the cycles in the high-frequency area [43].

Conclusion
The citrate auto combustion technique has been used to fabricate novel LiCoFeO nanoparticles with a size of ≅ 70 nm. XPS measurements support the existence of Co and Fe ions as (Co 3+ , Fe 3+ ), and (Co 2+ , Fe 2+ ) in octahedral and tetrahedral, respectively. The five Ramanactive modes are detected for LiCoFeO with the cubic spinel structure. The electrochemical measurements show that the investigated nanoparticles as electrodes exhibit an excellent high capacity in the initial discharge, a high reversible capacity, and good cycle stability. Additionally, at 58 cycles, the LiCoFeO electrode exhibits a high-rate performance with a specific capacity of 443 mA h g −1 . The EIS parameters of the LiCoFeO electrode after one and 100 cycles are almost the same.

Declarations
Conflict of interest The authors declare that they have no conflict of interest.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.