Lithium-ion battery (LIB) technology is the key enabler of energy storage and electric propulsion. Today´s LIB technology was primarily developed for applications in consumer electronics and is now transferred into automotive applications. These applications demand high specific capacity in order to provide a long driving range as well as high power densities for acceleration, recharging and recuperation. So far, state-of-the-art Lithium-ion batteries do not fully meet the requirements of automotive applications with respect to energy density, power density, extended life time, cost and safety. The commercialized LiFePO₄ exhibits good rate capability. Energy density is below that of commercially used layered oxides due to its redox plateau at 3.5 V. A lot of interest is therefore drawn to the isostructural LiMnPO₄ due to its higher redox plateau of 4.1 V vs. Li/Li⁺, which is able to provide higher energy densities [1,2]. Unfortunately, pure LiMnPO₄ suffers from low rate capability. On material level, Yamada et al. demonstrated that the partial substitution of manganese in LiMnPO₄ with iron, Li(Mn_xFe_1-x)PO₄ (LFMP) leads to an improved electrochemical performance and increased energy density [3,4]. On electrode level, a promising strategy to further adjust the electrochemical behavior is to custom-tailor properties by blending complementary types of cathode materials [5-7]. In order to enhance the limited kinetics of manganese based olivine, blending with LiMn₂O₄-Spinel is expected to result in a promising cathode material for two reasons: a) spinel show fast kinetics, as can be observed from a less pronounced voltage decay at higher C-rates and b) provide a redox plateau in a range comparable to LFMP.

Blend electrodes are prepared by mixing the olivine LiFe_0.3Mn_0.7PO₄ (LFMP) and LiMn_1.9Al_0.1O₄ spinel (LMO) in order to obtain a composite electrode, combining the high capacity of LFMP and the rate capability of the spinel. Electrode characteristics are compared with theoretical calculations based on experimental data obtained from electrodes of the single components LFMP and LMO. While tap density of the powders and electrode parameters show a linear dependency on the blend ratio, remarkable synergetic effects can be observed regarding the electrochemical performance at high rates (3C). Potential curves of blend electrodes at rates of 3C reveal a less pronounced voltage decay for the Mn^2+/3+plateau than expected from theoretical calculations as depicted in Figure 1a.  This buffer effect is also observed for high current pulses (5C) where blend electrodes resemble the behavior of pure spinel electrodes. In terms of power density at high states of charge (SoC), the performance of the investigated blends exceeds even that of pure spinel. In addition, the spinel-related manganese dissolution can be drastically reduced by blending spinel with LFMP. Structural changes in blend electrodes during pulse power discharge of the manganese redox plateau are investigated with in-situ XRD measurements. Shifts in XRD pattern during relaxation (I = 0) gives us indication of particle-to-particle interactions between LFMP and LMO. Figure 1b illustrates a comparison between the battery relevant features of LFMP, spinel and a blend with LFMP:LMO = 50:50 cap%.

This study shows the expected and synergetic effects of LFMP/spinel blends and compares the results with theoretical calculations [8]. The complementary properties of the single component materials LFMP and spinel result in a cathode material which can be promising for 48V starter batteries in automotive applications, where high power densities at high states of charge are required in order to start the internal combustion engine.

References:

[1] A.K. Padhi, K.S. Nanjundaswamy, J. B. Goodenough, J.Electrochem. Soc. 144, 4 (1997) 1188-1194.

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

[3] A. Yamada, Y. Kudo, K.-Y. Liu, J. Electrochem. Soc. 148, 7 (2001) A747–A754.

[4] A. Yamada, K. Yoshihiro, K.-Y. Liu, J. Electrochem. Soc. 148, 10 (2001) A1153–A1158.

[5] K.G. Gallagher, S.-H. Kang, S.U. Park, S.Y. Han, J.Power Sources 196 (2011) 9702–9707.

[6] J.F. Whitacre, K. Zaghib, W.C. West, B.V. Ratnakumar, J. Power Sources, 177 (2008) 528–536.

[7] S.B. Chikkannanavar, D.M. Bernardi, L. Liu, J. Power Sources 248 (2014) 91–100.

[8] A. Klein, P. Axmann, M. Wohlfahrt-Mehrens, J. Power Sources (2016) submitted.

Figure 1: a) Comparison of calculated and experimentally obtained galvanostatic curves of Blend(50_LFMP/50_LMO cap%) and b) radar chart comparing relevant material features of LMFP, spinel and Blend(50_LFMP/50_LMOcap%).