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

form a lithiated spinel known as Li 2 [16c] Mn 2 [16d]O 4 . Spinel is of particular interest for solid-state batteries, as materials such as LiNi 0.5 Mn 1.5 O 4 (LNMO) have a high energy density, are low in cost and are environmentally friendly as they use no cobalt. However, sulphide solid electrolytes are not compatible with an LNMO cathode, so work is being done on sulphurising LNMO for an essentially new LNMOS cathode. This can improve the compatibility of the layers in a solid-state battery and provide three times higher initial discharge capacity along with improved cycle stability. Other versions of spinel use titanium or vanadium, but one titanium material, LiTi 2 O 4 , operates at around 1.5V so it is not suitable as a cathode. Nor is the vanadium material LiV 2 O 4 a practical choice, since it suffers from structural changes and a lower voltage, of around 3V, Prof Manthiram says. One important advantage with moving from LiCoO 2 to LiMn 2 O 4 is the significant reduction in cost, as manganese is as much as 100 times cheaper than cobalt. However, one critical issue with LiMn 2 O 4 is the dissolution of manganese from the lattice into the electrolyte in presence of trace amounts (ppm levels) of hydrogen ions if the electrolyte is too acidic. That can degrade the cathode but also promotes the migration of manganese to the anode and can severely poison a graphite anode, and as a result reduce the cycle life of lithium-ion cells. There have been efforts to prepare both LiCo 2 O 4 and LiNi 2 O 4 spinel oxides by chemically extracting 50% lithium from LiCoO 2 and LiNiO 2 to obtain Li 0.5 CoO 2 and Li 0.5 NiO 2 , followed by heating at temperatures of 200-400 ºC to transform the layered phase to the spinel phase. However, such attempts result in either incomplete transformation at low temperatures or loss of oxygen. That creates a mixture of spinel-like phases and reduced Co 3 O 4 or NiO phases owing to the instability of Co3+/4+ and Ni3+/4+. These spinel-like phases also exhibit poor electrochemical performance owing to a lack of well- formed crystalline structures. Another approach has been to partially substitute manganese with other ions such as chromium, cobalt and nickel. One example is LiMn 1.5 Ni 0.5 O 4 spinel, where the manganese exists as Mn4+ and the nickel exists as Ni2+, as in NMC cathodes. LiMn 1.5 Ni 0.5 O 4 operates at about 4.7V with a reversible capacity of about 135mAh/g. However, LiMn 1.5 Ni 0.5 O 4 spinel suffers from capacity fade owing to the lack of suitable stable electrolyte at 4.7 V. Polyanion oxides The third class of cathode is the polyanion oxides using molybdenum and tungsten. Both Fe 2 (MoO 4 ) 3 and Fe 2 (WO 4 ) 3 cathodes exhibit a flat discharge voltage of 3V, which is higher than that seen with simple oxides such as Fe 2 O 3 or Fe 3 O 4 (less than 2.5V). This ability to increase the operating voltage by going from simple oxides For example, NMC powders from materials supplier Targray include NMC333, 532, 622 and 811. 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, achieves 203.4 mAh/g at 4.3 V. LiMn 2 O 4 , or spinel, has a cubic close- packed array of octahedral oxide ions. The 3D lithium-ion diffusion pathway allows fast lithium-ion conductivity to provide even faster charge/discharge characteristics for Li 1-x Mn 2 O 4 with good reversibility compared with LiCoO 2 , says Prof Manthiram. This gives an operating voltage of 4V with a practical capacity of less than 130mAh/g. Interestingly, additional lithium can be inserted into an empty octahedral site (at a position called 16c) at 3V to Different forms of lithium oxide cathodes offer varying ways of increasing cell voltage (Courtesy of Arumugam Manthiram) Spinel oxides Chemical stability Mn > Ni > Co Structural stability Co > Ni > Mn Electrical conductivity Co > Ni > Mn Abundance Mn > Ni > Co Environmental benignity Mn > Ni > Co January/February 2023 | E-Mobility Engineering 59 Deep insight | Cathode materials