Synergetic Effects of LiFe0.3Mn0.7PO4 - Spinel Blends: From Proof of Concept to Prototype Li-Ion Cells
Authors: Klein Andreas , Axmann Peter , Wohlfahrt-Mehrens Margret 
Published: February 2020

Due to increasing demands for lithium-ion batteries, especially for the use in automotive applications, many investigations were focused on increasing their capacity, cycle life and rate capability.

Among the commercially used active cathode materials, the lithium metal phosphate olivines are promising compounds with respect to lifetime and safety [1-2]. The olivine LiFe0.3Mn0.7PO4 (LFMP) is an environmental friendly low cost cathode material offering high capacities and a good rate capability. Yamada et al. demonstrated the structural and electrochemical properties of such olivines for different Mn : Fe ratios [3-4]. Unfortunately LiFe0.3Mn0.7PO4
suffers from voltage drop on the manganese plateau (4.1 V vs. Li/Li+) at high rates or during power pulses which results in a dramatic decrease in energy density [5].

Our idea is to increase the power pulse capability on the manganese plateau by adding a cathode material with faster kinetics and a redox plateau in the same potential range compared with LFMP. The LMO spinel meets these requirements and is therefore a promising material for the use in blend electrodes as demonstrated in Figure 1a. The capacity of spinel is rather low but there are other advantages as high abundance and low cost which make the material interesting for this kind of application.

The Blend electrodes are prepared by mixing LFMP and LMO in different ratios. The experimental data has been compared with calculations. In this work the results concentrate on the 50:50 cap% blend. The main results are summarized in Figure 1b.

Comparable electrodes are prepared. Electrode characteristics resemble the expected behavior according to pure physical mixtures of active materials. Synergetic effects are observed for the electrochemical properties especially at high current rates. The voltage drop on the Mn-Plateau at high constant current discharge is reduced. Increased power densities can be obtained at high rates. Pulse power capability can be improved because the blend electrodes resemble the behavior of pure spinel electrodes. Another advantage of this blend is that the spinel related manganese dissolution can be reduced dramatically. The latter can be ascribed to the basic surface of the LFMP, which can act as scavenger. The increased potential stability and the improved power density at high rates can be related to a reduction in electrode polarization.

Nevertheless, this could not explain the increased power pulse behavior. In order to investigate this matter in detail, an in-situ XRD-electrochemistry cell was developed with the aim to resolve dynamic processes during and after power pulses. The in-situ measurements show that the spinel is able to compensate high current pulses and is getting recharged by the LFMP during relaxation.

Based on these results, 500 mAh stacked prototype pouch cells were developed. On the cathode side a blend of LFMP and LMO (50:50 cap%) was used and on anode side a blend of amorphous carbon and graphite was chosen. The developed full cell is able to sustain current pulses up to 15C while capacity is still 80% of the 1C capacity as shown in Figure 1c.

References:

[1] A.K. Padhi, K.S. Nanjundaswamy and J.B. Goodenough, J. Electrochem. Soc., 144, 4, 1188 (1997).

[2] G. Li, H. Azuma and M. Tohda, Solid State Lett., 5, A135-A137 (2002).

[3] A. Yamada, Y. Kudo and K.-Y. Liu, J. Electrochem. Soc., 148, 7, A747 (2001).

[4] A. Yamada, K. Yoshihiro and K.-Y. Liu, J. Electrochem. Soc., 148, 10, A1153 (2001).

[5] A. Klein, P. Axmann and M. Wohlfahrt-Mehrens, J. Power Sources, 309, 169 (2016).

Figure 1: a) Galvanostatic curves demonstrating the voltage drop on the manganese plateau of LFMP at high rates and the concept how the spinel can help to buffer high current loads. b) Net-Chart with battery relevant features of LFMP, spinel and the 50:50 cap% blend, demonstrating the synergetic effects of this blend system. c) Rate capability test of a 500 mAh full cell prototype.

Figure 1