Synthesis of LiFePO₄/C composite electrode with enhanced electrochemical performance

HU Guo-rong(胡国荣), GAO Xu-guang(高旭光), PENG Zhong-dong(彭忠东),

CHEN Zhao-yong(陈召勇), TAN Xian-yan(谭显艳), YU Xiao-yuan(禹筱元)

(School of Metallurgy Science and Engineering, Central South University, Changsha 410083, China)

Abstract: LiFePO₄/C composite was synthesized by high temperature solid-state reaction using iron(Ⅱ) oxalate, ammonium di-hydrogen phosphate and lithium carbonate with a kind of carbohydrate dissolved in the dispersant(ethanol) as carbon sources added to the synthetic precursor. The samples were characterized by X-ray diffraction(XRD), scanning electron microscopy observations(SEM), charge/discharge test, cyclic voltammetry(CV) and carbon analysis. The results show that the synthesis of LiFePO₄/C has ordered olivine structure. The carbon has two advantages: optimizing particle size of LiFePO₄ and increasing the electronic conductivity and high Li⁺ diffusivity. The cathode material can demonstrate a charge/discharge flat voltage of 3.4V(vs Li⁺/Li). Especially the active material with 15% and 20% carbohydrate added according to the final product of lithium iron phosphate shows very good electrochemical performance delivering about initial 150.2mA·h·g^-1 and 162.0mA·h·g^-1 specific capacity respectively at 0.1C rate and the carbon contents in the final production are only 5.17% and 5.29%, respectively.

Key words: Li-ion battery; cathode materials; LiFePO₄; carbon-coated CLC number: TM911.1

Document code: A

1 INTRODUCTION

The ever-growing demand for portable batteries with high energy density is exerting pressure for the development of advanced lithium-ion batteries. Commercial Li-ion batteries rely on the application of one of the well-known lithium insertion hosts, i.e. LiCoO₂, LiMn₂O₄ and LiNiO₂. However the high cost of cobalt resource, low specific capacity of LiMn₂O₄ and LiNiO₂ is known to be difficult to synthesize and its multi-phase reaction during electrochemical cycling leads to structural degradation. For large-scale applications such as electric and hybrid vehicle systems, the vital issue is the availability of advanced materials. The high temperature performance is also critical because these batteries may be operated at elevated temperature.

LiMPO₄ as cathode materials (M=Fe, Mn, FeyMn_1-y, Co, Ni, etc.) has attracted much attention as a candidate for the battery with high energy density since Padhi et al^[1] established the baseline characteristics of LiFePO₄ cathodes for lithium-ion batteries in 1996. LiFePO₄ has a high lithium intercalation voltage (3.4V, vs Li /Li⁺), high theoretical capacity (170mA·h·g^-1), excellence in a high temperature application^[2], low cost^[3], ease of synthesis, and stability when used with common organic electrolyte systems^[4-7]. Unfortunately, it should be noted that there is an intrinsic negative aspects of olivine-type materials for lithium battery cathodes—very poor conductivity^[1] and Li⁺ can only be partially extracted/inserted at large charge/discharge rates. About two years ago, Yamada et al^[8] and MacNeil et al^[9] considered that LiFePO₄ reacted very weakly with electrolyte at elevated temperatures compared with other common cathode materials and therefore it may be suitable for large cell applications. In recent optimization of LiFePO₄ is focusing on the increase of its electronic conductivity and the synthesis routes^[10-12]. In recent years, improvements have been made to improve the conductivity of LiFePO₄ through optimization of synthesis techniques to minimize the particle size, doping to improve the intrinsic conductivity^{[13, 14]}, addition of metal powder^[15]or carbon particles^{[16, 17]} during synthesis or incorporating organic or polymeric additives^[18] to form conductive carbon coatings on the particles during firing. In this paper, a study of pure and modified LiFePO₄ materials was described. The goal was to prepare active materials with excellent electrochemical performance.

2 EXPERIMENTAL

2.1 Synthesis of cathode materials

The cathode materials were prepared by solid-state reaction of FeC₂O₄·2H₂O(Aldrich), NH₄H₂PO₄ (Aldrich), Li₂CO₃ (Aldrich) and carbohydrate (Aldrich).The stoichiometric mixtures of the raw materials were mixed by ball-milling (500r/min) for 12h with agate balls in alcohol, followed by drying. Then these mixtures were ground with a mortar and pellets were calcined at 350℃ for 8h in flowing Ar. Final firing for crystallization of the olivine phase was made at 750℃ for 24h in Ar flowing ambience. The samples of L0, L1, L2, L3 and L4 correspond to LiFePO₄ with 0%(uncoated), 10%, 15%, 20% and 25% carbohydrate mixed respectively according to the product of LiFePO₄.

2.2 Characteristic of cathode materials

2.2.1 Phase identification and particle size analysis

The phase analysis and particle size of all samples were determined by using X-ray diffraction(XRD, D/max-r A type CuKα1140kV, 300mA, 10°-70°, 8°/min, Japan).

2.2.2 Microstructures of cathode material

Scanning electron microscope(SEM) was used to examine the microstructures of the cathode material(SEM, KYKY 2800, Japan).

2.2.3 Electrochemical measurements

The electrochemical cycling performances of the LiFePO₄ powders were evaluated at room temperature (20℃) with laboratory-scale Li/LiFePO₄ button cells including a lithium metal foil as counter electrode, a composite of 80% LiFePO₄, 10% acetylene black (AB) and 10%(mass fraction) polytetrofluornethelene(PTFE) binder as a cathode. A micro-porous polypropylene film (Celgard 2400) was used as a separator and 1mol/L LiPF₆ solution with the 1∶1 volume ratio of ethylene carbonate-dimethyl carbonate (EC-DMC) was used as the electrolyte. All cells were assembled inside a glove box filled with ultra-pure argon. Charge/discharge characteristics of the cells were recorded in the potential range of 2.5V and 4.1V using a LAND computer-controlled galvanostat. Currents and specific capacities were calculated based on the mass of LiFePO₄ but not LiFePO₄/C in the electrode.

The cyclic voltammgram of the powder microelectrode test cell was performed between 2.2V and 4.2V vs lithium using a Potentiostat/Galvanostat system, Model 273A, EG&G instrument. The scan rate was chosen to be 0.5mV/s to 20mV/s. A piece of Pt wire (100μm in diameter) was embedded in glass tubing. Part of the Pt wire was etched by aqua regia, so that a micro-cavity was packed with powders of the cathode material. By using the powder microelectrode as the working electrode, lithium metal foils were chosen as the counter electrode and 1mol/L LiPF₆/EC+DMC (1∶1 in volume) as the electrolyte solution, and the test cells were assembled in argon on dry box. As the surface area of the counter electrode was 200 times larger than that of the working microelectrode, the counter electrode could also be used as the reference electrode without serious error.

2.2.4 Carbon concentration

The carbon concentration of the composite was analyzed using CS-444 carbon/sulfur determinator (LECO Co. USA).

3 RESULTS AND DISCUSSION

The phases of all the samples L0, L1, L2, L3 and L4 are shown in Fig.1. From the powder X-ray diffraction (XRD) patters, it can be seen that the synthetic LiFePO₄ phase belongs to ordered olivine-type structure. From Table 1, it can be seen that the diffraction intensity of 311 peak of LiFePO₄ compound declines with the more carbohydrate mixed in the LiFePO₄/C composites. This means that with the increasing amount of carbon coated the amorphous phases in the composite also increase. But their diffraction peaks are in the same positions indicating that carbon coating does not have effect on the inner crystal structure of LiFePO₄.

Fig.1  XRD patterns of samples L0, L1, L2, L3 and L4

Table 1 Relative intensity of 311 peak

   From the SEM photographs of Fig.2, it can also be known that the addition of carbohydrate has an evident effect on controlling the growing particles of cathode powder, and the particle size of LiFePO₄/C(L2, L3) is more well-proportioned than that of the pure LiFePO₄(L0). The refined LiFePO₄ particle coated by carbon can help to increase the diffusivity of lithium ion during the electrode process. A wide size distribution ranging from 0.5μm to 5μm for the LiFePO₄ particles can be recognized in Fig.2(a). And the distribution is not uniform, which will make the Li⁺ of the larger particle not be discharged completely during the electrode process and leads to the low specific capacity of sample L0.

Fig.2  SEM micrographs for samples

  The electrochemical insertion/deinsertion of lithium ions into LiFePO₄ can be described as the following formula:

Li_1-xFePO₄=Li_1-xFePO₄+xLi⁺+xe

This reaction occurs on a flat plateau at 3.4V vs Li, as a two-phase process of FePO₄/ LiFePO₄, and the complete extraction of lithium(x=1) corresponds to a theoretical specific capacity of 170mA·h·g^-1.

From Fig.4, it can be clearly known that the initial capacity decreases with the more than 20% carbohydrate added according to the product of LiFePO₄. It can be seen that the pure LiFePO₄ (L0) can only keep the specific capacity of 70.5mA·h·g^-1 after 20 cycles with the initial specific capacity of 120.4mA·h·g^-1 owing to the low electric conductivity^[1]. The specific capacity loss is more than 40% while the LiFePO₄/C composites show excellent electrochemical behavior. Sample L3 can retain 156.90mA·h·g^-1 after 20 cycles with the initial capacity of 162.0mA·h·g^-1 and the carbon mass fraction is only 5.29% (Table 2). The capacity loss is very small. The reason is that the carbohydrate as the carbon precursor can improve the active material conductivity and the Li⁺ diffusivity.

The influence of the scan rate on electrochemical performance of sample L0 and sample L3 was investigated by using power micro-electrode cycle voltammetry. The pair of peaks, consisting of an anodic and a cathodic peak, observed around 3.4V vs Li/Li⁺ corresponded to the two-phase charge/

Fig.3  Initial charge-discharge(a) and cycling behaviors(b) profiles of samples L0, L1, L2, L3 and L4

Table 2 Carbon content in samples L2 and L3

Fig.4  Effects of potential scan rate on cyclic voltammogram at 1, 3, 5, 8, 10, 15, and 20mV/s for sample L0 (a); 0.2, 0.5, 4, 8, 10, 15 and 20mV/s for sample L3(b); and relationship between anodic peak current of cyclic voltammogram and square root of scan rate v^0.5(c)

discharge reaction of the Fe²⁺/Fe³⁺ redox couple. This voltammogram indicates only one electrochemical reaction. Figs.4(a) and (b) show the effect of the potential scanning rate on the cycle voltammograms measured at 1.0, 3.0, 5.0, 8.0, 10.0, 15.0 and 20.0mV/s for sample L0 and 0.2, 0.5, 4.0, 8.0, 10.0, 15.0 and 20.0mv/s for sample L3 respectively. Although the wave shape of the anodic and cathodic peak is almost symmetrical, the potential separation between the two peaks increases as the scanning rate increases. This result means that the Fe²⁺/Fe³⁺ redox reaction is a quasi-reversible process. Fig.4(c) shows the relationship between the peak current and the square root of the scan rate(v^0.5). The peak current is proportional to v^0.5 with the scan rate is less than about 10mV/s. This means that the electrochemical process is limited by lithium diffusion. When the scan rate exceeds 10mV/s the relationship between I_p and v^0.5 is not proportional. This indicates that the electrochemical process is controlled by a similar blend process^[19].

While with the increase of the carbon content the volumetric energy density and tap density decrease unfortunately^[20]. Huang et al^[21] made LiFePO₄/C composite by mixing raw materials with carbon gel before heating. In this composite, LiFePO₄ has a particle size of 100-200nm and there is about 15% carbon. Consequently the volumetric energy density decreases greatly.

4 CONCLUSIONS

The adding carbohydrate to the synthetic precursor can help to improve the electrical conductivity of LiFePO₄ and prevent growth of the particle. The most important thing is that the production of LiFePO₄ will be covered by refined carbon uniformity after the precursor has been sintered. It contributes to the transfer of electron and Li⁺ during the electrode process. Meanwhile the existing carbon can prevent the production of Fe³⁺ phase to some extent. Sample L3 shows excellent capacity and cycle capability exhibiting about 95% theoretic capacity, 162.0mA·h·g^-1, at room temperature. This indicates that LiFePO₄/C composite will be an attractive candidate as cathode material for Li-ion battery.

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(Edited by LI Xiang-qun)

Foundation item: Project(04JJ0388) supported by the Natural Science Foundation of Hunan Province, China

Received date: 2004-11-26; Accepted date: 2005-03-03

Correspondence: HU Guo-rong, Professor, PhD; Tel: +86-731-8830474; E-mail: hgrhsj@263.net