Lithium cobalt phosphate (LiCoPO₄) with olivine structure has attracted much attention as a new positive electrode material for lithium ion batteries because of its high potential (4.8V vs. Li/Li⁺), theoretical capacity (167 mAh g^-1) and thermal stability due to the P-O covalent bonding. However, the practical use of LiCoPO₄ has been limited by the capacity fading during cycling due to the electrolyte oxidation and irreversible structure changes such as amorphization and anti-site defects of the material during charge and discharge¹⁾. Besides, it suffers from its poor rate performance related to the inherently low electron conductivity (<10^-9 S cm^-1)²⁾ and Li⁺ diffusivity (<10^-13 cm² s^-1)²⁾ due to its b-axis oriented diffusion path.

To overcome these limitations, we have synthesized Fe-substituted LiCoPO₄ (LiCo_0.8Fe_0.2PO₄) nanoparticles which are highly dispersed within a multi-walled carbon nanotube (MWCNT) matrix via an original in-situ material synthesis method called ultracentrifugation (UC) treatment^3,4). Here, we also report about the influence of Fe substitution on cycle and rate performances of LiCo_0.8Fe_0.2PO₄.

2. Experimental

 UC treatment was carried out on the prepared mixture of Co(CH₃COO)₂・4H₂O, Fe(CH₃COO)₂, CH₃COOLi, citric acid, MWCNT and H₃PO₄ aqueous solution. Obtained composites were electrochemically characterized using a 2032 coin half-cell with Li metal in 1M LiPF₆ / EC:PC:DMC (1:1:3, volume ratio). Physicochemical characterizations of the LiCo_0.8Fe_0.2PO₄/ MWCNT composite were conducted by HRTEM, XRD, XPS, XAFS measurements and Mössbauer spectroscopy.

3. Results and Discussion

The XRD pattern of LiCo_0.8Fe_0.2PO₄ / MWCNT composite indicates an olivine structure, without impurity phases. The combination of XRD, XPS, XAFS and Mössbauer analysis suggests that substituted Fe³⁺ into LiCoPO₄ crystals was not placed in the Co²⁺ site, but in the Li⁺ site. The result of HRTEM observation indicates that LiCo_0.8Fe_0.2PO₄ nanocrystals (ca. 100 nm) were highly dispersed within a MWCNT matrix. In addition, HRTEM images of LiCo_0.8Fe_0.2PO₄ / MWCNT composite showed that a substitution of Fe³⁺ into LiCoPO₄ crystals prevents degradations such as amorphization of LiCoPO₄and electrolyte decomposition during charge and discharge processes.

Then, LiCo_0.8Fe_0.2PO₄ / MWCNT composite showed a higher discharge capacity of 130 mAh g^-1 than LiCoPO₄ / MWCNT composite and enabled a 100C (36 seconds) rate discharge with 45 mAh g^-1. Furthermore, the obtained composite showed stable cycle performance with 81 % of capacity retention after 100 cycles at 0.2C. These enhancements were achieved by the combination of an entanglement between nanocrystalline LiCo_0.8Fe_0.2PO₄ and MWCNT matrix and Fe³⁺ substitution in the Li⁺ site of LiCoPO₄. The Fe³⁺substitution gives a “pillar effect” which effectively prevents an irreversible crystal structure change and electrolyte decomposition during charge and discharge processes, resulting in the stable cycle performance.

References

1) A. Boulineau et al., Chem. Mater., 27, 802 (2015).

2) S. Brutti et al., ACS Symposium Series, (2013).

3) K. Naoi et al., Acc. Chem. Res., 46, 1075 (2013).

4) K. Naoi et al., Energy Environ. Sci., 5, 9363 (2012).