Microscopic Protective Covering for Better Batteries

batteries
2D model of Ni-rich core-shell particles: Image a) Crystallographic orientation of the individual grains in a core-shell particle Image b) The mesh used for the calculation Images c) to f) Calculated tensile and compressive stress distributions in the charged state of the battery for normal (c,e) and core-shell particles with reduced intensity maxima at the edge (d,f)Copyright: Robert Mücke / Forschungszentrum Jülich (zum Vergrößern auf das Bild klicken; Bilder dürfen mit Quellenangabe für die Berichterstattung verwendet werden / click to enlarge, images may be used for reporting puposes if source is acknowledged)

Lithium-ion batteries are far superior to other rechargeable battery systems due to their long lifetime and high energy density. However, for many applications, such as electric cars, they still fall short of the mark. One of the reasons for this is the cathode material of these batteries. Scientists from Jülich and South Korea are conducting research on a material that could make the batteries more powerful in future.

The battery is the heart of every electric vehicle. At present, electric vehicles almost exclusively use lithium-ion batteries. Such batteries can tolerate numerous charging cycles, and their energy density – or discharge capacity – has more than doubled since their introduction in the early 1990s. Nevertheless, even state-of-the-art lithium batteries are insufficient for electric cars to satisfy the needs of a broader consumer base. This is due to the high cost of batteries and, more importantly, insufficient drive range per charge that they offer.

Both the cost of lithium-ion batteries and overall performance are largely determined by the cathode material. To become less dependent on cobalt (expensive) and increase storage capacity nickel-rich LiNixCoyMnzO2 (NCM) materials are preferred. However, this is at the expense of the lifetime of the cathodes: with increasing Ni content, capacity losses and overheating problems occur.

The loss of capacity in Ni-rich NCM is due to large anisotropic expansions and contractions of the cathode material’s crystal lattice, which are in turn caused by the phase transition that occurs in Ni-rich NCM cathodes during charge/discharge. The internal stresses produce cracks in the material, through which the liquid electrolyte of the batteries can penetrate and destabilize the material.

Robert Mücke and Payam Kaghazchi from the modelling team at Forschungszentrum Jülich’s Institute of Energy and Climate Research – Materials Synthesis and Processing (IEK-1), together with South Korean Hanyang University’s Division of Materials Science & Engineering, have investigated the performance of Ni-rich NCM particles with a multi-compositional core–shell structure.

They modelled the distribution of stress in Ni-rich multi-compositional core–shell NCM particles, which were synthesized by South Korean researchers. Their simulations showed that the unique composition and spatial arrangement of these cathode materials can mitigate the internal stresses caused by the phase transition. Cycle performance and thermochemical stability were significantly improved by the optimized microstructure.

Original publication: Microstructure‐Controlled Ni‐Rich Cathode Material by Microscale Compositional Partition for Next‐Generation Electric Vehicles, by Un‐Hyuck Kim, Hoon‐Hee Ryu, Jae‐Hyung Kim, Robert Mücke, Payam Kaghazchi, Chong S. Yoon, Yang‐Kook Sun, Advanced Energy Materials, Februar 2019, DOI: 10.1002/aenm.201803902