Invited Presentation: Materials and Electrode Design for Long Life Lithium-Ion Batteries

Long life, efficient and safe high energy storage systems are the key components for new car concepts (hybrid and full electric cars) as well as for intermediate storage of renewable energy sources. However, present lithium ion technology does not fulfil completely the requirements in terms of energy density, wide operation temperature, long life behaviour and safety even under abuse conditions. In the last twenty years the improvement of energy density of portable lithium ion batteries has mainly been achieved by optimization of electrode microstructure and cell design and not by introducing new electrode materials.

The life time and safety of the cells depend strongly on both cell chemistry and proper cell design. Most life time and safety studies focus either on the stability of single components like positive or negative materials from half cell measurements or on performance or safety tests of complete batteries. However, for a better understanding of aging mechanisms and safety issues it is important to analyse the full range from components up to battery level and especially the contribution of negative and positive electrode and their interactions in the complete cell under various operation conditions.

We studied the aging behaviour of different complete cells (pouch or 18650) with various cell chemistries by monitoring the voltage profile of the individual electrodes with a reference electrode. The results clearly demonstrate that cycling life and safety are strongly influenced by a proper adjustment of the electrode microstructure and kinetics of the negative and positive electrode. Abuse tests, accelerating rate calorimetry (ARC) and differential scanning calorimetry (DSC) techniques were combined to study potential safety hazards. Gas chromatography (GC) provides powerful information about the chemical decomposition of electrolyte and of interactions within the battery. This combination of methods leads to a detailed description of life-time and safety-relevant processes at different temperatures and rates. The results clearly demonstrate that the safety behaviour of the cells changes with prolonged cycling and capacity fading.

Based on the analytical results the microstructure for both electrodes anode and cathode has been optimized in order to minimize aging effects. On the cathode side blend electrodes of different sorts of materials and particle sizes can be used to optimize reversible capacity, rate capability, long term behaviour. For example LFP/NCM blend electrodes combine high mass loading (higher energy density) with excellent cycling life and improved safety. It has been demonstrated that a pouch cell containing synthetic graphite as negative and a NMC/LFP (3:1) blend as positive gives > 96% of initial capacity after 5760 cycles charged and discharged with 1 C rate between 3 – 4.2 V.

The presentation will summarize various strategies to improve the cycling stability for high energy and high voltage lithium ion batteries with various materials combinations.