Component Development Towards Ultra-Fast Chargeable Lithium Ion Battery Using High Voltage Materials

摘要

In modern society, portable consumer electronics and e-mobility show an increasing market demand, which catches rising attention to one of the key components, the battery, as well. After several years of continuous enhancement, lithium ion batteries (LIBs) are the technology of choice to store electrochemical energy in many applications, in which the cell chemistry is customized according to the demands of the device. For example, fast charging is considered as an undeniable need for the mainstream adoption of LIBs in the area of e-mobility. Today's state-of-the-art LIBs typically use graphite (or graphite/silicon composite materials) as anode material, which has an equilibrium potential at ≈100 mV vs. Li/Li+. Large anode polarization at either high rate or low operating temperature can push graphite potential below the threshold for lithium plating, causing detrimental consequences, such as drastic capacity loss and severe safety hazards. One approach to counter these issues is the use of an alternative anode material such as LTO (Li5Ti4O12). Operating at a working potential of ≈1.55 V vs. Li/Li+, its implementation warrants the inhibition of metallic lithium plating even at low operating temperatures. Despite the undeniable advantages of LTO, its elevated working potential has an adverse effect on the energy density of the resulting battery. For example, a LTO-NMC (Li(NiCoMn)O2) based commercial battery presents an energy density of only 90 Wh/kg. The utilization of cathode active materials bearing a high working potential is one possibility in order to increase the energy density of LTO based batteries. For this, the high voltage spinel LNMO (LiNi0.5Mn1.5O4) with its operating potential of ≈4.7 V vs. Li/Li+ is a prominent candidate. However, there are two main challenges that have to be solved. On the one hand, LNMO operates outside of the theoretical electrochemical stability window of electrolytes based on organic carbonates and on the other hand, LTO suffers from gas formation. Consequently, all electrolyte and electrode components have to be adjusted towards this new and demanding cell chemistry. In this context, we present our latest results on performance determining characteristics of the electrolyte, the composite electrode and its active material, as well as the development of suitable separators. A particular focus was set on the particle size and conductivity of the active materials, electrical conductivity of the composite electrodes, diffusion length of the electrolyte inside the electrode, low resistivity and thickness of the separator, ion conductivity as well as electrochemical and chemical compatibility of the electrolyte in order to achieve a long-lasting battery with ultra-fast charging capability. Consequently, the electrode thickness is determined as a limiting factor for high power electrodes, next to the intrinsic properties of the active material, which can be countered by either doping or coating approaches. Furthermore, the ionic conductivity of the electrolyte is directly correlated to the charge-rate performance of the battery. In addition, electrolyte additives are identified in order to decrease the gassing issue on LTO-active material at elevated temperatures, prolonging the cycle life in LTO|LNMO-based batteries.