Mesoscale Chemomechanical Interplay of the LiNi0.8Co0.15Al0.05O2 Cathode in Solid-State Polymer Batteries

Abstract

Presently, the challenges faced by batteries are twofold. Firstly, they are expected to possess high energy and power density for powering zero emission electric vehicles. Secondly, they are also expected to be inexpensive and safe for deployment in electric vehicles and consumer applications. A potential candidate ideally suited for this case are solid-state polymer lithium-ion batteries coupled with layered nickel-rich oxides such as LiNi0.8Co0.15Al0.05O2 (NCA) or LiNi1−x−yCoxMnyO2 (NCM). Replacing the non-aqueous, liquid electrolytic solution that is comprised of flammable components, with a solid-state electrolyte such as an inexpensive polymer is highly advantageous. However, layered nickel-rich oxides such as NCA are not only known for their high reversible capacity, but also their inherent structural and thermal instability at charged state, which leads to several degradation mechanisms including intergranular cracking. While this phenomenon has been studied extensively for lithium-ion batteries consisting of a non-aqueous, liquid electrolyte, it has not been studied for solid polymer batteries. Secondary NCA particles harvested from cycled NCA-poly(ethylene oxide) batteries were subjected to a combination of two- and three-dimensional full-field transmission X-ray microscopy (2D/3D-FF-TXM) and high resolution broad ion beam- and focuses ion beam milling-scanning electron microscopy (BIB-/FIB-SEM). Doing so, the complex chemomechanical interplay within the hierarchically structured lithium-ion battery was investigated to reveal morphological defects and chemical heterogeneity in the secondary particles. Intergranular cracking was found to be extremely apparent after only a few cycles. It was found, that the absence of liquid electrolyte severely affects transport in fractured particles and may also hastens and accelerates fracture due to lower uniformity in current density. Intergranular cracks significantly increase diffusion path lengths of both charge carriers, electrons and lithium ions. While in liquid lithium-ion batteries, electrolyte filling the cracks should maintain lithium ion transport pathways but not electron transport pathways, in solid-state batteries, intergranular cracking compromises not only the electronic pathways, but also the ionic pathways. Eventually, intergranular cracking in polymer batteries seems to lead to isolated and inactivated primary grains within the core of secondary particles. Figure 1