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

Researchers have been announcing advances in lithium-ion cathode materials and their manufacture, as Nick Flaherty reports Positive steps T here are three main types of lithium-ion batteries for e-mobility, all with different balances of energy capacity, charging rate and long-term stability. There are however also many different approaches to constructing cathodes, using different chemical compositions, crystal structures and methods of high- volume manufacturing. Principally, the cathode types are layered oxides with nickel, manganese and cobalt (NMC), and spinel and polyanion oxides such as lithium iron phosphate (LFP). The first layered oxide cathode used lithium cobalt dioxide (LiCoO 2 ), where Li+ and trivalent Co3+ ions are ordered on the alternate planes with a cubic close-packed array of oxide ion. This is referred to as the O3 structure. This has enabled a substantial increase in the operating voltage from less than 2.5V using earlier sulphide cathodes to around 4V, allowing a cell to be assembled without having to use a metallic lithium anode, and which has fast charge/discharge characteristics and good reversibility. The large charge and size differences between Li+ and Co3+ ions lead to good ordering of the ions, which is critical to support fast 2D lithium- ion diffusion and conductivity. That means LiCoO 2 remains one of the best cathodes to date, thanks to that 4 V operating voltage. The LiCoO 2 cathode solved two major challenges associated with the sulphide cathodes. As well as the higher voltage, it allows a lithium-free anode using materials such as graphite or silicon to be paired with the LiCoO 2 to give the modern-day lithium-ion cell pioneered by Sony in 1991. However, despite good electrochemical performance, the practical capacity of LiCoO 2 is limited to around 140mAh/g, says Professor Arumugam Manthiram, Chair in Engineering at the Walker Department of Mechanical Engineering at the University of Texas and developer of polyanion oxide technology. This capacity limit has led to a number of other layered LiCoO 2 materials being investigated over the years, driving the substitution of cobalt with manganese and nickel for NMC cathodes. Some of the oxides can be directly synthesised using direct high- temperature reactions, while others such as LiMnO 2 and LiFeO 2 do not crystallise in the O3 structure when synthesised by high-temperature reactions, but can be obtained by ion-exchanging the sodium analogues (NaMnO 2 ) with lithium salts. In NMC, the Mn3+ tends to become oxidised during synthesis to Mn4+ by reducing Ni3+ to Ni2+. The Mn4+ helps the incorporation of the nickel as a stable Ni2+ into NMC and serves as a structural stabiliser without participating in the charge/discharge process. In NMC, each transition-metal ion has its own advantages and disadvantages (see table) with chemical stability and structural stability, in which cobalt and manganese are diametrically opposed. Manganese does not suffer from any chemical instability involving oxygen release from the lattice, in contrast to cobalt. On the other hand, manganese suffers from structural instability, as it can readily migrate from the transition- metal plane to the lithium plane, leading to a voltage decay during cycling. However, it is more abundant and environmentally benign than cobalt, which can only be sourced from a small number of mines around the world, many of them in unstable economies. In contrast, cobalt enjoys good structural stability without such cation migration. Nickel lies between manganese and cobalt in all the five criteria in the table, and there is a trend to progressively increase the nickel content and decrease the cobalt content in NMC so that the capacity can be increased while lowering the cost. Materials for cathodes in lithium- ion batteries include this layered oxide sheet (Courtesy of Targray) 58 January/February 2023 | E-Mobility Engineering