Effect of Impurities on Electrochemical Performance of Low-Purity Natural Graphite As Anode Active Material for Lithium Ion Batteries

Natural graphite (NG) has been attracted as a promising anode material for lithium ion batteries due to its appropriate charge/discharge profile, high reversible capacity and low cost. However, high irreversible capacity and low capacity retention at first cycle have influenced on its practical use. Although low cost is the main advantage of natural graphite, the material cost of natural graphite should be reduced further in order to be used for electric vehicles (EVs) and energy saving systems (ESS). Generally, pristine natural graphite contains various impurities such as Al, Fe, and Si. For commercial use, pristine natural graphite should be refined since the impurities would have a negative effect on both electrolyte and electrode of lithium ion batteries. The purity grade of natural graphite can be classified based on the purification process. As the requirement for high purity increases, more purification steps are needed, resulting in high manufacturing cost. Therefore, the main issue for the application of natural graphite as an anode active material is to use low-purity natural graphite with purification process as less as possible. In this regard, effect of Fe as impurity on the electrochemical performance of the low-purity natural graphite as anode active material for lithium ion batteries was investigated in this study. Natural graphite powders with 5 wt% Fe (05Fe) and 10 wt% (10Fe) were synthesized by combustion method from the raw materials of Fe(III)(NO3)3 9H2O (Alfa Aesar) and high-purity spherical natural graphite (POSCO CHEMTECH) by calcination at 500 °C in air atmosphere. The morphology of the natural graphite powders was observed by scanning electron microscopy (SEM, JSM-5900, JEOL, Japan). The particle size of each powder was measured by a dynamic light scattering method (ELS 6000 zeta potential and particle size analyzer, Otsuka Electronics, Japan). Powder X-ray diffraction (XRD, MAX-2500, RIGAKU, Japan) analysis was conducted using Cu Kα radiation with a wavelength λ = 1.5406 Å. The crystallite sizes (La and Lc) were calculated on the basis of the d002XRD lines by application of the Scherrer’s equation. The crystallinity of the natural graphite powders was investigated by Raman spectroscopy (LabRAM, Horiba Jobin-Yvon, Japan). The concentrations of impurities in the natural graphite were determined by an inductively coupled plasma atomic emission spectrophotometer (ICP-AES). A working electrode paste was fabricated from a mixture of natural graphite with a binder consisting of carboxymethylcellulose (CMC)/styrene-butadiene rubber (SBR) and carbon black (Super-p) as a conductive agent dissolved in D.I. water. The weight ratio of graphite to binder (CMC:SBR:Super-p=2:2:1) was 95:2:2:1. The prepared paste was coated onto 10㎛ Cu foil by using a doctor blade and then dried under vacuum at 120 °C for 12 h. Electrochemical performance was evaluated using CR2032 coin-type cells with a 20 μm thick Cellgard 2300 porous membrane separator and 1 M LiPF6-EC/DMC (1:1 in volume ratio) electrolyte. Lithium metal foil was used as a counter electrode. All of the samples studied in this work were treated by sphericalization and the sphericalized natural graphite maintained a spherical shape after the calcination process at 500 °C for 4 h. All the samples have both hexagonal and rhombohedral phases which are the typical structure of natural graphite. Fe2O3 peaks (JCPDS card #33-0664) were indexed at 2θ = 33.1°, 35.6°, 62.4°, and 63.9°, respectively. The most of Fe2O3particles were located on a surface of natural graphite, based on the EDX mapping and back-scattered electron (BSE) images. The irreversible capacity during the charge-discharge reactions increased with increasing the Fe content. However, the cycle retention of the 05Fe, 10Fe, and NG are comparable. Therefore, it may be possible to use unrefined natural graphite as an anode active material for lithium-ion rechargeable batteries.