About Microwave Solvothermal Synthesis of Solid Solutions LiFexMn1-xPO4/C and Their Electrochemical Properties

In 1997, Goodenough and coworkers (1) discovered phospho-olivines LiMPO₄ (M = Fe, Mn, Ni, Co) as promising positive electrode material for rechargeable lithium-ion batteries. Due to their flat voltage profile, high theoretical capacity (170 mAh*g^-1 for LiFePO₄), excellent thermal stability and environmental friendliness they attracted attention for both portable devices and automotive applications. Most research activities focused on the improvement of the electrochemical performance of LiFePO₄, since LiMnPO₄suffers from worse lithium-ion diffusion kinetics and Jahn-Teller distortion of its crystal lattice (2).

It is believed that the formation of solid solutions (3) between LiFePO₄ and LiMnPO₄ (LiFe_xMn_1-xPO₄) remarkably improves the electronic conductivity. This strategy seems auspicious, since the higher potential of LiMnPO₄ (4.1V vs. Li/Li⁺) compared to LiFePO₄ (3.5V vs. Li/Li⁺) leads to a superior energy density of the resulting lithium-ion battery.

Therefore, we report a microwave solvothermal synthesis of phospho-olivines LiMPO₄(M= Fe, Mn, Co, Ni) and mixtures thereof within 10 minutes of reaction time. It is common for increasing the electronic conductivity of phospho-olivines to use one of the following problem-solving approaches: Coating with a conducting phase (4), minimizing of the particles in size (5) and doping with positive divalent\trivalent metal ions and\or fluorine (6).

Morphology of the samples is analyzed by scanning electron microscopy (SEM) as shown in figure 1. Additionally all powders are analyzed by X-ray powder diffraction (XRD), energy dispersive X-ray analysis (EDX), cyclic voltammetry (CV) and galvanostatic cycling (CC) experiments.

The occurrence of Fe and Mn in the phospho-olivine and their influence on the cell performance will be discussed (cf. figure 2). Furthermore, Mg is incorporated into the lattice and its influence on electrochemical behavior will be shown as well (cf. figure 3).

Acknowledgement

Herbert Quandt Foundation is gratefully acknowledged for funding.

References

1. A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc., 144, 1188 (1997).

2. J. Hong, F. Wang, X. Wang, J. Graetz, J. Power Sources, 196, 3659 (2011).

3. C.-K. Back, R.-Z. Yin, Y.-S. Kim, J. Electrochem. Soc., 160, A1551 (2013).

4. H.C. Shin, W.I. Cho, H. Jang, Electrochim. Acta, 52, 1472 (2006).

5. I. Bilecka, A. Hintennach, M. D. Rossell, D. Vie, P. Novák, M. Niederberger, J. Mater. Chem., 21, 5881 (2011).

6. Y. Huang, Y. Xu, X. Yang, Electrochim. Acta, 113, 156 (2013).