E-Mobility Engineering 017 l ECE Doosan electric excavators dossier l In Conversation: Matt Faulks l Battery testing focus l Battery Show North America 2022 report l Ariel Hipercar digest l Cathode materials insight l Thermal management focus

appealing. They have a wider range of compositions than spinel oxides owing to their inability to stabilise highly oxidised M3+/4+ states by conventional synthetic processes for the spinel oxides. The spinel cathodes are largely limited to LiMn 2 O 4 , and the high operating voltage of about 4.7V and the higher power capability is appealing, but production is hampered by the lack of adequate electrolytes that can survive at this higher operating voltage. More recently, increasing the nickel content and lowering or eliminating the cobalt content in NMC cathodes is becoming much more prominent. However, layered oxides with a high nickel content have three critical challenges – cycle instability, thermal instability and air instability. The deposition of metals on the graphite anodes compromises the electrolyte and leads to the formation of a thick solid electrolyte interphase (SEI) layer, which grows with the cycles as more transition-metal ions dissolve and migrate to the anode. After a certain number of cycles, typically more than 500, the SEI layer’s thickness also increases with increasing nickel content owing to the increasing transition- metal dissolution and deposition on the graphite anode. High-nickel cathodes also suffer from cracks during the charging cycles, which further exaggerates the surface reactivity with the electrolyte and increases the metal dissolution and SEI formation, resulting in rapid capacity fade as cycling progresses. However, doping the cathode with aluminium and using surface stabilisation strategies to minimise the volume changes, crack formation and surface reactivity can help to overcome the challenges with high-nickel cathodes. These layered oxide cathodes with high-nickel content have become appealing, but intuitive bulk and surface stabilisation strategies are needed to overcome the associated cycle, thermal and air instabilities. Altered microstructure To improve the energy per unit volume of cathode materials, researchers in Russia have developed two versions of NMC cathodes with a single-crystal microstructure. “Cathode materials are an important bottleneck as far as EV batteries are concerned,” says Professor Artem Abakumov, principal investigator at Skoltech Energy. “The cathodes in batteries powering electric cars tend to use layered transition metal oxides, including nickel- rich ones. We have improved two commonly used materials of this kind, achieving a 10-25% increase in energy density. That translates into smaller cathodes, more compact batteries and therefore greater energy storage capacity for the same volume. As an added bonus, the material is slower to deteriorate.” Without changing the chemical composition, Skoltech researchers improved NMC811 (eight parts nickel, one part manganese, one cobalt) and NMC622 (six parts nickel, two manganese, two cobalt) cathodes by tweaking their microstructures. Conventional NMCs are powdered polycrystalline materials, where each secondary particle consists of randomly oriented grains. The crystal structure within any given grain is nearly flawless, but no two grains fit perfectly together and there are some empty spaces at grain boundaries. Creating a single crystal eliminates these gaps. “Our material is a single-crystal NMC with spherical particles, which combines the best of both worlds as far as maximising density goes,” says Aleksandra Savina, a researcher on the project at Skoltech. “Unlike polycrystals, the powder particles don’t have an internal structure, so there are no wasted spaces at grain boundaries. On top of that, you can pack more spherically shaped single crystals into the same limited volume than octahedron-shaped ones, so you get more density too.” Besides denser packing, the spherical shape of the crystals reduces the area of contact with the electrolyte, minimising unwanted interactions that over time cause cathode degradation owing to crack formation in the particles of conventional NMCs. That should prolong the operating life of the cathode and battery. To create the single crystal, the researchers tweaked the synthesis procedure. This starts with a precursor with homogeneously distributed nickel, manganese and cobalt, mixing in lithium hydroxide or some other source of lithium. This is annealed at high temperatures. “What we do is after adding the lithium-containing compound, we also mix in an inert salt that has a low melting point, and we melt that mixture and anneal at high temperature,” says Alina Pavlova from the project. “We then wash away the salt and anneal again to get rid of the products of any unwanted reactions with water. “The key though is that, depending on which inert salt is used and in what amount, the particle geometry will change. With conventional Colour-coded, high-resolution microscopy shows the crystal grain boundaries in a complex, manganese-rich cathode material (Courtesy of Argonne National Laboratory) January/February 2023 | E-Mobility Engineering 61 Deep insight | Cathode materials