Untersuchungen zur Erhöhung der Strombelastbarkeit manganbasierter Kathodenmaterialien für Lithium-Ionen Batterien

Abstract

Lithium-ion battery technology is one of the key enabler for electromobility. High cyclic stability, high capacity and rate capability as well as low costs are demanded. In this thesis two approaches are being attempt concentrating on manganese based cathode materials due to their environmental benign, abundance and low costs. The first part describes the optimization of Li2MnO3 on material level. This material exhibits high initial capacities but suffers from manganese dissolution and capacity fading. The second part describes the optimization of LiFe0.3Mn0.7PO4 (LFMP) on electrode level by blending with other materials. LFMP shows high capacities and high energy densities. However, this material suffers from a dramatic potential drop on the manganese plateau (4 V) at high C-rates. In order to improve the electrochemical properties of Li2MnO3, different methods and different carbon coatings were applied and structural as well as electrochemical properties have been investigated. Pyrrolization of different carbon sources on the particle surface results in the partial decomposition of the material leading to electrochemical active LiMnO2 which converts from a layered to a spinel-like structure during cycling. In terms of carbon coating the most promising results are obtained by ball milling Li2MnO3 with carbon black. Besides carbon coating also a surface modification with nickel nitrate has been applied. A phase pure material with a homogenous distribution of nickel over the sample has been synthesized. A material composition of 0.8 Li2MnO3 · 0.2 Li(Mn0.5Ni0.5)O2 has been determined. Structural changes related to the treatment before and after cycling as well as electrochemical performance are investigated. A reduced transition from layered to spinel-like structure as well as reduced manganese dissolution can be observed leading to enhanced rate capability and cycling stability. For LFMP the concepts for improving the electrochemical properties focuses on the optimization on electrode level. Composite cathodes are prepared by blending the olivine LFMP and the spinel LiMn1.9Al0.1O4 (LMO) as well as the layered oxide LiMn1/3Ni1/3Co1/3O2 (NMC) in order to combine complementary material properties. Experimental data are compared with theoretical calculations. While the LFMP/NMC blends show an expected behavior according to the calculations, remarkable positive synergetic effects are observed for the LFMP/LMO blends related to the electrochemical performance at high rates (3C). Electrode polarization could be reduced, falling even below theoretically calculated expectations. Pulse power behavior could be improved resembling characteristics of pure spinel electrodes. The origin of this behavior has been investigated with a new developed in situ XRD / electrochemistry cell and could be related to particle-to-particle interactions between LFMP and LMO. In addition, the spinel-related manganese dissolution can be drastically reduced by blending spinel with LFMP due to the basic surface of LFMP, which can act as scavenger for Mn2+ and H+.