Evaluation of electrochemical performances of ZnFe2O4/[gamma]-Fe2O3 nanoparticles prepared by laser pyrolysis

A ZnFe 2 O 4 /γ-Fe 2 O 3 nanocomposite (theoretical specific capacity: 1002 mAh/g) was successfully synthesized by laser pyrolysis, a very attractive nanosynthesis technique characterized by high versatility and flexibility. The obtained nanopowder was thoroughly characterized by XRD, XPS, Mössbauer spectroscopy and HRTEM, which confirmed the presence of the two phases. A bimodal size distribution with small particles (tens of nanometers) and large ones (above 500 nm) was revealed by SEM and TEM. The ZnFe 2 O 4 /Fe 2 O 3 nanocomposite was tested as negative electrode material for lithium-ion batteries, showing significantly improved lithium storage properties with a high reversible capacity and rate capability compared to pure ZnFe 2 O 4 electrode. A capacity exceeding 1200 mAh/g is sustained after 100 cycles at 100 mA/g, with a gradual increase of the capacity during cycling. At 500 mA/g current rate, a reversible and stable capacity of 360 mAh/g is observed after 300 cycles. Electrochemical measurements with several electrolytes and electrode formulations were also conducted in order to explore the origin of the extra capacity and its increase with cycling.


Introduction
Lithium-ion batteries have been widely used in portable electronic devices for years. With the depletion of fossil fuels and the growing environmental concerns, lithium-ion batteries also received considerable attention to contribute to the development of electric vehicles and the storage of renewable energies. Most commercial lithium-ion batteries use graphite as anode material, which has a good cycling stability but suffers from a low theoretical capacity (372 mAh/g), limiting the energy density for nomad applications.
New high performance negative electrode materials are thus needed for next generation lithium-ion batteries to address the increasing demand of energy. Due to their high theoretical capacities, conversion materials have been investigated as potential substitutes to graphite. In particular, a great attention has been devoted to 3d transition metal oxides (AxOy with A=Fe, Co, Ni, Mn …) 1 .
Transition metal oxides such as cobalt or iron oxides have been studied because of their high theoretical capacities, which can be as high as 1000 mAh/g. Good performance was obtained with nanostructured oxides: for example, α-Fe2O3 nanoflakes exhibit a stable capacity of 680 mAh/g for 80 cycles at C/10 (100 mA/g) current rate 2 , porous Co3O4 nanotubes show a reversible capacity of 677 mAh/g for 60 cycles at 1C rate 3 and porous hollow Fe3O4 beads are able to maintain a reversible capacity of 500 mAh/g for 100 cycles at C/10 4 . However, these materials are not the ideal choice to replace graphite due to their high working voltage vs. Li + /Li (between 2.1 and 1.9 V vs. Li + /Li for cobalt and iron oxides) and the toxicity and cost of cobalt. On the contrary, mixed-transition metal oxides with an AB2O4 spinel crystal structure (A, B=Fe, Zn, Mn …) provide not only similar theoretical capacities but also the feasibility to tune the working voltage, depending on the chosen transition metals. For instance, ZnFe2O4 has a working voltage of 1.5 V vs. Li + /Li which is lower than Fe2O3 and Fe3O4 working voltages 5 ; also the addition of zinc vs lithium allows the formation of ZnLi alloy during the storage mechanism, leading to an increase of capacity.
Moreover, as an abundant, cheap, non-toxic and environmental-friendly material with a high theoretical capacity (1000 mAh/g), zinc ferrite is an interesting substitute for graphite as negative electrode material in lithium-ion batteries.
Several works have reported very promising results for zinc ferrite negative electrodes made with a variety of synthesis processes and with different particles morphologies. ZnFe2O4 hollow spheres obtained by hydrothermal synthesis are able to cycle for 50 cycles keeping a reversible capacity of 900 mAh/g 6 . Electrospinning technique allows the synthesis of zinc ferrite nanofibers which are able to reach a capacity higher than 700 mAh/g after 30 cycles 7 . To enhance ZnFe2O4 performance, some studies also focused their efforts on carbon addition to improve the material conductivity and maintain the structure of the material. This can lead to very high capacity materials: ZnFe2O4 nanoparticles with graphene nanosheets with a reversible capacity around 950 mAh/g after 50 cycles at C/10 current rate 8 or ZnFe2O4 microspheres with a carbon coating showing a capacity of 1187 mAh/g after hundred cycles at C/20 9 . The latter showed a capacity higher than the theoretical one and increasing during the first cycles. Several explanations are proposed for these two phenomena: (i) a morphological re-configuration of the electrode during the charge/discharge process that activates additional oxides particles which were not initially reachable by the lithium, 9,10 (ii) a contribution of carbon to the capacity 9,11 or (iii) a reversible reaction of a polymeric layer on the nanoparticles 11 .
Various synthetic processes are available for the synthesis of zinc iron oxides nanopowders with different morphologies including hydrothermal synthesis, 6,12 co-precipitation, 10 urea combustion process 13 and electrospinning 7 .
In this paper, CO2 laser pyrolysis was used for the first time to synthesize zinc iron oxide nanoparticles. This process is an advantageous one-step versatile and flexible nanosynthesis technique 14 . The control of different operational parameters such as laser power, choice of gases and tuning of their flow rates, as well as the nature and concentration of the precursors, allows the production of nanopowders with various morphologies and crystallinities. It is also a very interesting technique as the precursors can be solid, liquid or gaseous, giving a large range of different possibilities for the tailored synthesis of any compound. It is a scalable route to produce high purity nanomaterials, with a good reproducibility which can be adapted for industrial production. One of the major drawbacks of laser pyrolysis, however, is the need to find a precursor with an absorption band at 10.6 µm, or the use of a sensitizer (mainly C2H4 or NH3) sometimes leading to carbon or nitrogen pollution.
Herein, the original synthesis of ZnFe2O4/γ-Fe2O3 nanopowders by laser pyrolysis is proposed, and their electrochemical performance as negative electrode materials for Li-ion batteries is evaluated. The effect of different electrolytes and electrode formulations was also studied in order to better understand the cycling mechanism and the cycle life of this promising material.

Experimental section
-ZnFe2O4/γ-Fe2O3 synthesis by laser pyrolysis During the laser pyrolysis synthesis, an aerosol of the solution containing the precursors produced by a nebulizer is flown into a reactor with a carrier gas. In the reactor, a 10.6 µm CO2 laser beam decomposes the precursors producing nanopowders which are then collected on a filter ( fig.1). Further details about this process were given by D'Amato et al. 14 .

Figure 1. Laser pyrolysis experimental setup
A solution of Zn(NO3)2.6H2O and Fe(NO3)3.9H2O precursors (1 mol of zinc nitrate for 2 mol of Fe nitrate) dissolved in deionized water was used for the synthesis of ZnFe2O4/γ-Fe2O3 nanoparticles. The amount of precursors was limited to 50% wt. of the total solution in order to avoid clogging of the spraying nozzle upon long lasting experiments. Ethylene was used as a sensitizer gas to absorb the CO2 laser and allow the decomposition of the precursors which do not absorb this laser wavelength. Air was used as the carrier gas, it was chosen for its combustion reaction with C2H4 that enhances oxidization of Zn and Fe (avoiding suboxide phases formation), increases the reaction temperature (enhancing crystallization), and limits carbon pollution deriving from C2H4 decomposition.
Carrier air flow also serves as spraying gas in the nozzle: as a consequence its flow has to be high enough to generate the aerosol. This latter condition fixed the air flow instead of stoichiometry consideration regarding combustion reaction with C2H4. Experimental conditions are summarized in the table 1.  -Electrochemical measurements Electrodes were prepared using ZnFe2O4/γ-Fe2O3 (with their specific amount deduced from Mössbauer spectrometry) as the active material (70% wt.), CMC (carboxymethyl cellulose) as the binder (12% wt.), and different types of carbon additives to increase conductivity (18% wt.  Galvanostatic cycling tests at different current rates (from 20 to 1000 mA/g, respectively around C/50 and 1C) and C-rate from 0.01V to 3.0V were performed with the different electrodes and electrolytes to compare their influence on the battery performance (MTI Neware). Cyclic voltammetry curves were studied to understand the storage mechanism and were performed with a scan rate of 0.1 mV/s between 0.01 and 3.0V (Solartron). Besides, HRTEM identifies the two phases in both small and large particles, highlighting that there is no preferential composition corresponding to one population size. In summary, these results clearly indicate that the obtained powder is composed of a homogeneous and intimate mixture of separate particles of the two phases.    The presence of both ZnFe2O4 and Fe2O3 was not expected as the solution containing the precursors was prepared in a stoichiometric ratio. However, the pyrolytic decomposition process of ferrite powders is known to lead to many intermediate products such as ZnO, Fe2O3 and stoichiometric ZnFe2O4 18,21 . Moreover, what can be highlighted in these results is the specific morphology with the two size populations which is not commonly observed for laser pyrolysis grown nanoparticles. This appears to be a specific trend when using Fe nitrate precursor while chloride or acetylacetonate precursors lead to only one size population with small nanoparticles 22 . The two populations could possibly result from two different but simultaneous growth processes. When the aerosol droplets enter the reaction zone, the solvent is vaporized and the dissolved salts form dry spherical objects. Upon heating, a solid phase reaction can take place within these particles leading to large nanostructured spherical particles. In the specific case of laser pyrolysis, the heating rate is very fast upon interacting with the laser beam and the energy density encountered by the dried particles is very high, especially with a focused beam. In that case, part of the solid salt could be vaporized before reacting, leading to a gas phase reaction (nucleation and growth) which would form small nanoparticles agglomerated in chain-like poorly dense structures 14 . This latter vaporization phenomenon could take place at the surface of the dried particles simultaneously to the solid state reaction occurring in the core of the particles (fig.5).  .6a). A coulombic efficiency of 66% was obtained after the first cycle, which is low but rather common for transition metal oxides electrodes 7,24,25 . The first lithiation capacity of 1706 mAh/g is higher than the theoretical capacity and the additional capacity could be explained by some parasitic reactions of the electrolyte at low potential that create a solid electrolyte interphase (SEI) as well as a polymeric gel-type layer around the nanoparticles, which irreversibly trap a part of the lithium 26 . These two phenomena lead to an irreversible capacity of 576 mAh/g at the first cycle, with a charge capacity of 1130 mAh/g that is still higher than the theoretical capacity. A slow decrease of the reversible capacity is then observed for the 16 following cycles, probably due to additional lithium trapping into the unstable solid electrolyte interface. Nevertheless, after about 20 cycles, the reversible capacity progressively increases to about 1210 mAh/g after 100 cycles which is still 200 mAh/g higher than the theoretical capacity. Such behavior is not uncommon for transition metal oxides and has been reported for Fe2O3 27   To test the cyclability of the material at a higher current rate, electrodes similar to the previous one were cycled at C/90 current rate for the first cycle (which helps to form a more stable SEI at the beginning and have a fully lithiated material) and then 200 cycles at C/2 (500 mA/g) current rate have been performed ( fig.7). A first discharge capacity of 1520 mAh/g was obtained at C/90 current rate which is quite close to the capacity measured at C/10. At C/2, the reversible capacity decreases steadily during the first hundred cycles before stabilizing around 360 mAh/g in the following 200 cycles. The average coulombic efficiency in the last 200 cycles is stable at 99.25%. Although the capacity is drastically reduced at C/2, the ZnFe2O4/Fe2O3 electrode is able to give a very stable capacity for hundreds cycles with a good efficiency.

Figure 7. Capacity retention at C/2 (after one cycle at C/90)
A study of the rate capability was also performed to demonstrate the stability of the material. The current density was increased in several steps after every five cycles from C/45 (20 mA/g) to 4C (4000 mA/g) for the fastest cycles and then decreased directly to C/45 ( fig.8). The specific capacity decreases with increasing current rate. The electrode shows a very good cyclability with a reversible capacity varying from 876 to 422 mAh/g. At the second C/45 current density, the material recovers a higher specific capacity than for the first cycles. To better identify the potential of each electrochemical process during the lithiation and delithiation processes, cyclic voltamogramms were realized using an electrode containing both CB and VGCF and EC PC 3DMC + 1M LiPF6 electrolyte ( fig.9). The first lithiation curve shows a reduction peak around 0.55 V which can probably be attributed to the insertion of lithium inside the crystalline structure leading to its amorphization and to the conversion reaction with Zn(II) and Fe(III) which are reduced to Zn(0) and Fe(0). The first delithiation curve then shows an anodic peak at 1.6 V which may correspond to the oxidization of Zn(0) and Fe(0) to Zn(II) and Fe(III) 33,34,35,36 . After the first cycle, the cathodic peak shifts from 0.55 to 0.94 V. This is in agreement with the discharge potential plateau located approximately at 1.0 V after the first cycle in fig.5b. This potential shift is characteristic of conversion-type reactions and is related to the replacement of the conversion potential plateau in first discharge by a mostly sloping curve corresponding to the cycling of the in situ formed nanomaterial. Another contribution to the drastic changes in voltage between discharge and charge in the first cycle has an origin in the amorphous character for the Li2O/M (M=Fe, Zn) nanocomposite (see the proposed mechanism below) at the end of discharge which implies differences in free energy, and, therefore, in equilibrium reaction potential. Concerning the lithium storage mechanism in ZnFe2O4, assumptions were made by different authors 5,9 : insertion of lithium in the material should occur first, leading to a LixZnFe2O4 phase with x ≤ 2; on advancing lithiation, the material undergoes the conversion reaction, with a complete loss of the crystalline structure, and the expected formation of Zn and Fe nanoparticles embedded in a Li2O matrix. In the final part of the discharge, Zn is expected to alloy with Li to form ZnLi. It is also supposed that, after the first cycle, ZnFe2O4 is not recovered but instead nanometric forms of ZnO and Fe2O3 are formed as the final delithiated phases. Subsequent cycling is then between these phases and the species obtained at the end of the first discharge, which explains the difference in electrochemical signature between the first and the following lithiations: (1 st cycle only) ZnFe2O4 + 8Li + + 8e -→ 4Li2O + Zn + 2Fe Zn + Li + + e -↔ ZnLi Zn + 2Fe + 4Li2O ↔ 8Li + + 8e -+ ZnO + Fe2O3 These reactions, however, do not explain the reversible capacity exceeding the theoretical one and gradually increasing for our material. Several assumptions can be made to explain this phenomenon: the specific nanosized morphology obtained by laser pyrolysis, for instance, may allow specific surface related storage even after the first conversion reaction, which produces a massive morphological re-configuration of the cycling material 10 ; the presence of carbon additives in the electrode can also contribute to the capacity (taking into account that there is 18% of carbon additives in the electrode, an additional reversible contribution up to 60 mAh/g can be considered 37 ). Besides, with the electrolyte degradation during cycling, the reversible formation of a polymeric gel-type layer around the nanoparticles can also explain the excess capacity delivered at low potential 38 , another assumption is that the solid electrolyte interphase can work as an extra-charge reservoir for lithium 39 . To understand if either the nature of the electrolyte or of the carbon additives have an impact on the increasing capacity, galvanostatic cycles at C/10 were realized with and without VGCF and using a different electrolyte: EC DMC + 1M LiPF6 (fig.10a). The results reveal that the change of solvent mixture in the electrolyte has no effect on the increase of reversible capacity in cycling but it seems that EC PC 3DMC + 1M LiPF6 produces a higher specific capacity than EC DMC + 1M LiPF6. Concerning the influence of VGCF at C/10, no increase of capacity is observed without VGCF during the first hundred cycles. However, at 1C current rate ( fig.10b), the cycling was made with a CB electrode and a slight but significant increase of capacity can be observed even without VGCF. Reversible reactions with the electrolyte may occurred during the cycling, leading to this increase phenomenon, sometimes enhanced by the VGCF addition and favored by the specific particles morphology obtained by laser pyrolysis.

Conclusion
To summarize, ZnFe2O4/Fe2O3 nanopowder was synthesized by laser pyrolysis. The obtained powder shows a particular morphology constituted by agglomerated particles with a bimodal size distribution, which is attributed to the use of iron nitrate precursor. Compared with performances of pure ZnFe2O4, this ZnFe2O4/Fe2O3 nanocomposite shows a good performance as negative electrode material in half cells vs. lithium metal: at C/10 rate, the material exhibits a first lithiation capacity above 1700 mAh/g and more than 1100 mAh/g still remains after the first delithiation. Surprisingly the reversible capacity increases with the number of cycles at C/10 current rate leading to more than 1200 mAh/g after 100 cycles, which is significantly higher than the theoretical capacity. This overcapacity can be explained by an additional reversible redox reaction caused by electrolyte degradation, which is perhaps favored by the particular morphology of the particles and by the VGCF additive in the electrode formulation. This phenomenon is not observed at higher current rates. The influence of the electrolyte and of the addition of VGCF on the battery performance was also studied. The use of EC PC 3DMC + 1M LiPF6 electrolyte allows the material to keep a higher specific capacity and does not affect the increase phenomenon. The VGCF helps obtaining a higher reversible capacity after several cycles and seems to have an effect on the overcapacity phenomenon depending on the chosen current rate. In spite of these observations, the ZnFe2O4/Fe2O3 composite prepared by laser pyrolysis demonstrates a promising performance as a possible next generation of lithium-ion battery negative electrode.