• Energy Research

Lithium-ion batteries are currently the pioneers of green transition in the transportation sector. The nickel-manganese-cobalt (NMC) technology, in particular, has the largest market share in electric vehicles (EVs), offering high specific energy, optimized power performance, and lifetime. The aging of different lithium-ion battery technologies has been a major research topic in the last decade, either to study the degradation behavior, identify the associated aging mechanisms, or to develop health prediction models. However, the lab-scale standard test protocols are mostly utilized for aging characterization, which was deemed not useful since batteries are supposed to age dynamically in real life, leading to aging heterogeneity. In this research, a commercial NMC variation (4-4-2) was aged with a pragmatic standard-drive profile to study aging behavior. The characterized measurable parameters were statistically investigated before performing an autopsy on the aged battery. Harvested samples of negative and positive electrodes were analyzed with Scanning Electron Microscopy (SEM) and the localized volumetric percentile of active materials was reported. Loss of lithium inventory was found to be the main aging mechanism linked to 20% faded capacity due to heavy electrolyte loss. Sparsely distributed fluorine from the lithium salt was found in both electrodes as a result of electrolyte decomposition.

The growing demand for enhanced batteries with higher energy density and safety is pushing lithium-ion battery technology towards solid-state batteries. Replacing the liquid with a solid electrolyte significantly improves safety by removing the possibility of leaking flammable organic solvents. Solid electrolytes also enable the use of lithium metal as anode material to obtain battery cells with higher energy density. This review summarizes the classification of all three state-of-the-art solid electrolyte types (inorganic, polymer and composite solid electrolytes) and their governing lithium ion transport mechanisms. Nevertheless, to make solid-state batteries applicable, improvements in ionic conductivity of the solid electrolyte, low electrode-electrolyte interfacial resistance and high compatibility of the solid electrolyte with the electrodes are required. This review paper discusses improvement strategies for solid electrolytes to achieve high ionic conductivity, good flexibility, and high electrode compatibility. Enhanced ionic conductivity can be obtained by suppressing the polymer phase's crystallization (e.g., copolymerization, inorganic fillers, adjusting polymer matrix) and optimizing the physicochemical parameters and the surface of the inorganic phase. Interfacial stability can be improved by using multilayered electrolytes or applying coatings and passivation layers on electrolyte or electrode particles.