Interestingly, an additional lithium can be inserted into an empty octahedral site (at a position called 16c) at 3 V to form a lithiated spinel called Li2Mn2O4. This gives an operating voltage of 4 V with a practical capacity of less than 130 mAh/g. The 3D lithium-ion diffusion pathway allows fast lithium-ion conductivity to provide even faster charge/discharge characteristics for Li1-xMn2O4 with good reversibility compared to LiCoO2, says Prof Manthiram. LiMn2O4, or spinel, has a cubic close-packed array of octahedral oxide ions. NMC333 has an energy density of 154.8 mAh/g, while 532 reaches 187.0 mAh/g at 4.5 V, 622 reaches 175 mAh/g and 811, the most popular, reaches 203.4 mAh/g at 4.3 V. Nickel lies between manganese and cobalt in all the five criteria in the table, and there is a trend is to progressively increase the nickel content and decrease the cobalt content in NMC so that the capacity can be increased while lowering the cost.įor example, NMC powders from materials supplier Targray include NMC333, 532, 622 and 811. In contrast, cobalt enjoys good structural stability without such cation migration. However, it is more abundant and environmentally benign than cobalt, which can only be sourced from a small number of mines around the world with many in unstable economies. 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. Manganese does not suffer from any chemical instability involving oxygen release from the lattice, in contrast to cobalt. 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. 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, the Mn3+ tends to become oxidised during synthesis to Mn4+ by reducing Ni3+ to Ni2+. Some of these oxides can be directly synthesised using direct high-temperature reactions, while others such as LiMnO2 and LiFeO2 do not crystallise in the O3 structure when synthesised by high-temperature reactions, but can be obtained by ion-exchanging the sodium analogues (NaMO2) with lithium salts. This capacity limit has led to a number of other layered LiCoO2 oxides being investigated over the years, driving the substitution of cobalt with manganese and nickel for NMC cathodes. However, despite good electrochemical performance, the practical capacity of LiCoO2 is limited to around 140 mAh/g, says Professor Arumugam Manthiram, Chair in Engineering at the Walker Department of Mechanical Engineering at the University of Texas and developer of the polyanion oxide technology. As well as the higher voltage, it allows a lithium-free anode using materials such as graphite or silicon to be paired with LiCoO2 to give the modern-day lithium-ion cell that was pioneered by Sony in 1991. The LiCoO2 cathode solved two major challenges associated with the sulphide cathodes. That means LiCoO2 remains one of the best cathodes to date, thanks to its 4 V operating voltage. 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. This enabled a substantial increase in the operating voltage from less than 2.5 V from earlier sulphide cathodes to around 4 V, allowing a cell to be assembled without having to use a metallic lithium anode, with fast charge/discharge characteristics and good reversibility. The first layered oxide cathode used lithium cobalt dioxide (LiCoO2), where Li+ and trivalent Co3+ ions are ordered on the alternate planes with a cubic close-packed array of oxide ion. Principally, the cathode types are layered oxides with nickel, manganese and cobalt (NMC), and spinel and polyanion oxides such as lithium iron phosphate (LFP). There are however also many different approaches to constructing cathodes, using different chemical compositions, crystal structures and methods of high-volume manufacturing. There are three main types lithium-ion batteries for e-mobility, all with different balances of energy capacity, charging rate and long-term stability. Researchers have been announcing advances in lithium-ion cathode materials and their manufacture, as Nick Flaherty reports Materials for cathodes in lithium-ion batteries include this layered oxide sheet

Nmc cathode melting point