Investigation of Carbon-Coating Effect on the Electrochemical Properties of LiCoPO4 By Single Particle Measurement

Active materials for rechargeable lithium-ion batteries are generally evaluated as porous composite electrodes including binder and conductive carbon. However, this method sometimes makes it difficult to understand the intrinsic electrochemical properties of active material because the electrochemical response of composite electrode is strongly influenced by the electrode structure and composition. In order to overcome this problem, we have focused on single particle measurement, in which a single particle of active material can be evaluated ^[1]. Lithium metal phosphates have been investigated as promising cathode active materials with high thermal and structural stability. Though their electronic and Li⁺ ion conductivities are very low, those drawbacks can be improved by carbon coating and formation of fine particles, respectively. It has been demonstrated that the electrochemical performance of LiFePO₄ can be increased up to a practical use level by such particle design. However, its operating potential is low (around 3.5 V vs. Li /Li⁺) compared to conventional LiCoO₂. Thus, LiCoPO₄ have been particularly focused in recent years, due to its high operating potential (around 4.8 V vs. Li /Li⁺). In this study, the effect of carbon coating on the electrochemical properties of LiCoPO₄ was investigated by single particle measurement.

LiCoPO₄ was synthesized by hydrothermal method using Li₃PO₄ as Li and P sources and CoSO₄·7H₂O as a Co source, then coated with carbon by using sucrose as a carbon source. The morphology of LiCoPO_4­ particles was characterized with a scanning electron microscope (SEM). The amount of carbon on LiCoPO₄ was estimated by thermogravimetric analysis (TGA). The electrochemical properties of pristine and carbon-coated LiCoPO₄ particles were evaluated by single particle measurement (Fig. 1), using a grass coated Au wire (Φ10 μm diameter) as a micro current collector. The single particle measurement was performed in a potential range of 2.5 ~ 5.1 V vs. Li /Li⁺ at room temperature under Ar atmosphere. A mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (3:7 in volume) containing 1 mol dm^-3 LiPF₆ was used as an electrolyte solution.

Fig. 2 shows SEM images of pristine and carbon-coated LiCoPO₄. The pristine LiCoPO₄ particles were cubic and more than 5 μm in size, and those shape and size were maintained after the carbon coating. From TGA result, the amount of carbon on LiCoPO₄ was estimated to be 0.5 wt%. Fig. 3 shows the charge – discharge curves of pristine LiCoPO₄ particle at initial 3 cycles, in which the charge was carried out at 0.2 nA until the electrode potential reached to 5.1 V vs. Li /Li⁺ and then the potential was hold at 2 hours, followed by 0.2 nA discharge. It was hardly operated due to high ohmic resistance. Fig. 4 shows the charge – discharge curves of carbon-coated LiCoPO₄ particle with 20 μm diameter at initial 3 cycles, in which the charge was carried out at 3 nA until the electrode potential reached to 5.1 V vs. Li /Li⁺ and then the potential was hold until the charge current dropped to 0.3 nA, followed by 3 nA discharge. The carbon-coated LiCoPO₄ showed better electrochemical performance with the plateaus corresponding to Li⁺ extraction and insertion were clearly observed at 4.8 and 4.7 V vs. Li /Li⁺, respectively, although the irreversible capacity and capacity fading were observed due to the decomposition of electrolyte solution at high voltage ^[2]. Electrochemical properties of LiCoPO₄ was greatly improved by the small amount of carbon even though the particle size was as large as 20 μm, suggesting that LiCoPO₄ is a promising cathode material to realize high energy density lithium-ion batteries.

References

[1] H. Munakata et al. , Journal of Power Sources 217 (2012) 444 – 448.

[2] E. Markevich et al. , Electrochemistry Communications 15 (2012) 22–25.