Lithium nickel manganese cobalt oxide cathodes come in several commercial types and are identified by the ratio of transition metals as its component. For example, NMC 111 has equal parts of nickel, manganese, and cobalt, while NMC 532 has 5 parts nickel, 3 parts manganese and 2 parts cobalt. As of 2019, NMC 532 and NMC 622 were the preferred low-cobalt option for electric vehicles. At that time, NMC 811, with an even lower cobalt ratio in its composition saw increasing usage to mitigate dependency on cobalt. This initiative to reduce cobalt usage is based on the fact that cobalt is highly toxic and expensive, making it an undesirable component to continue using in battery materials.
Elements beside lithium in cathode materials generally act as intercalation compounds. In other words, the nickel, manganese, and cobalt oxides in NMC provide the framework through which lithium ions diffuse during the length of battery usage. The cathode material is selected based on desired battery performance, most commonly pertaining to energy and power density. Other factors, such as cost, will also be considered when the material is intended for commercial use.
Currently, NMC electrodes are designed with high specific energy in mind. The secret of NMC's high specific energy lies in the combination between nickel and manganese. Nickel is known for its high specific energy, but poor stability. Meanwhile, manganese has low specific energy but offers the ability to form spinel structures that allow low internal resistance. Along with cobalt, these three transition metals work with a synergistic ternary effect that contributes to the high performance capabilities of NMC. The mix of these three elements often vary from one manufacturer to another and is usually a company secret. Generally, most companies have switched from NMC 111 to NMC 442 or NMC 622 in order to increase the discharge capacity of their batteries. And now, NMC 811, with an even higher discharge capacity and lower cobalt content, is slated for introduction to the market. Batteries containing NMC with high discharge capacity are in high demand, especially by EV producers such as Nissan, Cheverolet, or BMW.
The history of NMC started with the partial substitution of cobalt with nickel and manganese in one of the first and most famous cathode materials, LiCoO2, to improve electrochemical performance and reduce material cost. NMC's multiple performance specifications can be carefully adjusted through a careful control of Ni, Mn, and Co compositions. Ni-rich NMC tends to demonstrate high discharge capacity, Mn-rich compositions maintain better cycle life and thermal safety, while Co-rich compositions provide excellent rate capability. However, leaning too much on one of the constituents can lead to adverse effects. For example, Ni-rich cathode suffer from structural degradation due to nickel mixing with lithium sites, and Mn-rich cathodes has a reduced capacity because of an ample supply of inactive Mn4+ species during reactions. Therefore, there is a large design space for composition optimization to reach a balanced behavior.
Over the past decade, the performance of Li-ion batteries have improved greatly while costs have reduced significantly due to increased demand. However, emerging battery applications demand increasingly higher energy density, lower cost, longer life cycles, and higher safety. As stated beforehand, NMC 811 is increasing in significance due to desirable performance specifications and low cobalt content. There is an increasing number of studies into alternative synthesis methods that may give rise to better performance of nickel rich NMC 811. These studies also aim to reduce technical issues such as cracking, low thermal stability, cation mixing, and parasitic reactions.
|Date||:||07 December 2020|