Synthesis and Characterization of LiFeBO3/C Composite By Spray Pyrolysis with Heat Treatment
Authors: Taniguchi Izumi , Suto Kenichi 
Published: February 2020

Introduction

Since the commercialization of lithium-ion batteries in the 1990s, they have widely utilized for energy storage in mobile electric devices owing to their large gravimetric and volumetric energy densities. Recently, attention has turned to the use of them as a power source for electric vehicles (EVs) and hybrid electric vehiches (HEVs). Large-scale batteries require safety, low-cost, long cycle-life, and high energy and power densities. Furthermore, it is well known that the cathode materials have a significant impact on battery capacity, cycle life, safety and cost. Among proposed cathode materials so far, lithium transition metal borates(LiMBO3,M=Fe, Mn and Co) are recently kinds of attractive cathode materials for lithium-ion batteries owing to relatively high theoretical capacity ( 220 mAh g-1) and little volume changes [1-2].
Thus far, we have investigated the synthesis of LiMPO4/C (M=Fe, Mn and Co) composites by a combination of aerosol and powder technologies followed by heat treatment, and then reported that the composite electrodes showed a good electrochemical performance [3-5]. In this work, the synthesis of LiFeBO3/C composites by the spray pyrolysis (SP) followed by heat treatment and their electrochemical properties are studied.

Experimental

The experimental setup was described in our previous paper [6]. The precursor solution was prepared by dissolving LiNO3, FeNO3·9H2O and B2O3in distilled water in stoichiometric ratio. A pinch of ascorbic acid was also added to the precursor solution.
The precursor solution was atomized at 1.7 MHz by an ultrasonic nebulizer. The atomized droplets were transported to the reactor by a 3% H2+Ar gas with a flow rate of 1 dm3 min-1, then heated at 600 °C by the electric furnace, and converted into solid particles through the evaporation of the solvent, the precipitation of the solute, drying and thermal decomposition within the laminar flow aerosol reactor. The resulting powders were collected at the reactor exit by an electrostatic precipitator which operated at 180 °C, while the gases were dried and cleaned by passing them through a bubbler and a cold trap. The collected powders were then annealed at various temperatures ranging from 500 to 750 oC in a 3% H2+Ar atmosphere.
The crystalline phase of the samples was studied by X-ray diffraction (XRD, Rigaku, Ultima IV with D/teX Ultra) analysis using Cu-Kα radiation. The surface morphology of the samples was examined by scanning electron microscopy (SEM, Keyence YE-8800) at 8 kV. The carbon content of the samples was determined using an element analyzer (Elementar, Vario Micro Cube).
The electrochemical performance of LiFeBO3/C composite was investigated using coin-type cells (CR2032). The cell comprised a lithium metal negative electrode and a LiFeBO3/C composite positive electrode, which were separated by a microporous polypropylene film. A 1 mol dm-3 LiPF6 solution in a solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) in 1:1 volume ratio (Tomiyama Pure Chemical Co., Ltd.) was used as the electrolyte. The cathode consisted of 80 wt.% LiFeBO3/C, 10 wt.% polyvinylidene fluoride (PVdF) as a binder and 10 wt.% acetylene black. The cells were cycled in a constant current-constant voltage mode at a 0.05 C rate (where 1 C = 220 mA g−1) to 4.5 V, held at 4.5 V until C/100, and then discharged to 1.5 V at a 0.05C rate. The current density and specific capacity were calculated from the weight of LiFeBO3 in the cathode.

Results and Discussion

Figure 1 shows the XRD patterns of the samples prepared by SP at 600°Cand then annealed at temperatures from 600 to 750 oC. The ICSD card patterns of LiFeBO3 are also shown in the figure. The diffraction peaks of the samples annealed at 700 oC or less are identified as those of a monoclinic structure with the space group C2/c, although several impurity peaks corresponding to Fe0 are observed in the diffraction peaks. It is clearly seen from the figure that LiFeBO3 with a smaller amount of Fe0 can be synthesized by SP at 600°C with heat treatment at 700°C. The chemical composition analysis was done by the element analyzer to confirm that the sample is a mixture of LiFeBO3 and carbon (LiFeBO3/C composite).The carbon content was 10 wt. % in the sample.

Figure 2 shows the SEM image of LiFeBO3/C composite prepared by SP at 600°C followed by heat treatment at 700°C. It can be seen from the SEM image that the spherical particles with a few micrometers in size could be prepared by the present method.

Figure 3a shows the charge and discharge profiles of the LiFeBO3/C composite prepared by SP at 600°C followed by heat treatment at 700°C. In the discharge profiles, the LiFeBO3 /C composite cathode shows a broad and flat voltage plateaus at approximately 2.5 V vs. Li/Li+, which may indicate a well-defined redox activity of the Fe3+/Fe2+ redox couple. Moreover the cell delivers discharge capacities of 178 mAh g-1 ( 80 % of its theoretical capacity) at 1st cycle, 146 mAh g-1 at 2nd cycle and 144 mAh g-1 at 10th cycle, respectively. The cycle performance of the LiFeBO3/C composite cathode was shown in Figure 3b. A capacity fading occurred in the initial stage of cycling, while the cathode showed a stable cycle performance from 2 to 30 cycles. This fact may be due to a small volume change of LiFeBO3in a Li insertion/deinsertion process.
From the above mentioned results, we could conclude that the LiFeBO3/C composite electrode with a good electrochemical performance was successfully synthesized by spray pyrolysis at 600°C followed by heat treatment at 700°C.

References

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