Lithium nickel manganese cobalt oxides

Lithium nickel manganese cobalt oxides (abbreviated NMC, Li-NMC, LNMC, or NCM) are mixed metal oxides of lithium, nickel, manganese and cobalt with the general formula LiNi_xMn_yCo_1-x-yO₂. These materials are commonly used in lithium-ion batteries for mobile devices and electric vehicles, acting as the positively charged cathode.

A general schematic of a lithium-ion battery. Lithium ions intercalate into the cathode or anode during charging and discharging.

There is a particular interest in optimizing NMC for electric vehicle applications because of the material's high energy density and operating voltage. Reducing the cobalt content in NMC is also a current target, owing to ethical issues with cobalt mining and the metal's high cost. Furthermore, an increased nickel content provides more capacity within the stable operation window.

Structure

Example of a layered structure. Lithium ions can move in and out between the layers.

NMC materials have layered structures similar to the individual metal oxide compound lithium cobalt oxide (LiCoO₂). Lithium ions intercalate between the layers upon discharging, remaining between the lattice planes until the battery gets charged, at which point the lithium de-intercalates and moves to the anode.

Points in a solid solution phase diagram between the end members LiCoO₂, LiMnO₂, and LiNiO₂ represent stoichiometric NMC cathodes. Three numbers immediately following the NMC abbreviation indicate the relative stoichiometry of the three defining metals. For example, an NMC molar composition of 33% nickel, 33% manganese, and 33% cobalt would abbreviate to NMC111 (also NMC333 or NCM333) and have a chemical formula of LiNi _0.33Mn_0.33Co _0.33O₂. A composition of 50% nickel, 30% manganese, and 20% cobalt would be called NMC532 (or NCM523) and have the formula LiNi_0.5Mn_0.3Co_0.2O₂. Other common compositions are NMC622 and NMC811. The general lithium content typically remains around 1:1 with the total transition metal content, with commercial NMC samples usually containing less than 5% excess lithium.

For NMC111, the ideal oxidation states for charge distribution are Mn⁴⁺, Co³⁺, and Ni²⁺. Cobalt and nickel oxidize partially to Co⁴⁺ and Ni⁴⁺ during charging, while Mn⁴⁺ remains inactive and maintains structural stability. Modifying the transition metal stoichiometry changes the material's properties, providing a way to adjust cathode performance. Most notably, increasing the nickel content in NMC increases its initial discharge capacity, but lowers its thermal stability and capacity retention. Increasing cobalt content comes at the cost of replacing either higher-energy nickel or chemically stable manganese while also being expensive. Oxygen can generate from the metal oxide at 300 °C when fully discharged, degrading the lattice. Higher nickel content decreases the oxygen generation temperature while also increasing the heat generation during battery operation. Cation mixing, a process in which Li⁺ substitutes Ni²⁺ ions in the lattice, increases as nickel concentration increases as well. The similar size of Ni²⁺ (0.69 Å) and Li⁺ (0.76 Å) facilitates cation mixing. Displacing nickel from the layered structure can alter the material's bonding characteristics, forming undesirable phases and lowering its capacity.

Synthesis

The crystallinity, particle size distribution, morphology, and composition all affect the performance of NMC materials, and these parameters can be tuned by using different synthesis methods. The first report of nickel manganese cobalt oxide used a coprecipitation method, which is still commonly used today. This method involves dissolving the desired amount of metal precursors together and then drying them to remove the solvent. This material is then blended with a lithium source and heated to temperatures up to 900 °C under oxygen in a process called calcination. Hydroxides, oxalic acid, and carbonates are the most common coprecipitation agents.

Sol-gel methods are another common NMC synthesis method. In this method, transition metal precursors are dissolved in a nitrate or acetate solution, then combined with a lithium nitrate or lithium acetate and citric acid solution. This mixture is stirred and heated to about 80 °C under basic conditions until a viscous gel forms. The gel is dried at around 120 °C and calcined twice, once at 450 °C and again at 800-900 °C, to obtain NMC material.

Hydrothermal treatment can be paired with either the coprecipitation or sol-gel routes. It involves heating the coprecipitate or gel precursors in an autoclave. The treated precursors are then filtered off and calcined normally. Hydrothermal treatments before calcination improves the crystallinity of NMC, which increases the material's performance in cells. However, this comes at the cost of longer material processing times.

History

NMC cathode materials are historically related to John B. Goodenough's 1980s work on lithium cobalt oxide (LiCoO₂), and can be represented as an intergrowth between a layered NaFeO₂-type oxide and a closely related lithium rich Li₂MnO₃ oxide whose amount is related to the initial lithium excess. The invention(s) of Li-rich NCM cathode material(s) was reported ca. 2000–2001 independently by four research teams:

1. At Argonne National Laboratory in the USA a group led by Michael M. Thackeray reported these lithium-rich cathodes with the intergrowth structure.
2. At Pacific Lithium in New Zealand a team led by Brett Amundsen reported a series of Li(Li_xCr_yMn_z)O₂ layered electrochemically active compounds.
3. At Dalhousie University in Canada a team led by Jeff Dahn reported a series of layered cathode materials based on a solid solution formulation of Li(Li_xM_yMn_z)O₂, where metal M is not chromium.
4. A group at Osaka City University led by Tsutomu Ohzuku, who also developed lithium nickel cobalt aluminium oxides.

Properties

The cell voltage of lithium-ion batteries with NMC cathodes is 3.6–3.7 V.

Arumugam Manthiram has reported that the relative positioning of the metals' 3d bands to the oxygen 2p band leads to each metal's role within NMC cathode materials. The manganese 3d band is above the oxygen 2p band, resulting in manganese's high chemical stability. The cobalt and nickel 3d bands overlap the oxygen 2p band, allowing them to charge to their 4+ oxidation states without the oxygen ions losing electron density.

Usage

Audi e-tron Sportback, a car that uses NMC-based batteries as a power source.

Many electric cars use NMC cathode batteries. NMC batteries were installed in the BMW ActiveE in 2011, and in the BMW i8 starting from 2013. Other electric cars with NMC batteries include, as of 2020: Audi e-tron GE, BAIC EU5 R550, BMW i3, BYD Yuan EV535, Chevrolet Bolt, Hyundai Kona Electric, Jaguar I-Pace, Jiangling Motors JMC E200L, NIO ES6, Nissan Leaf S Plus, Renault ZOE, Roewe Ei5, VW e-Golf and VW ID.3. Only a few electric car manufacturers do not use NMC cathodes in their traction batteries. Tesla is a significant exception, as they use nickel cobalt aluminium oxide and lithium iron phosphate batteries for their vehicles. In 2015, Elon Musk reported that the home storage Tesla Powerwall is based on NMC in order to increase the number of charge/discharge cycles over the life of the units.

Mobile electronics such as mobile phones/smartphones, laptops, and pedelecs can also use NMC-based batteries. These applications almost exclusively used lithium cobalt oxide batteries previously. Another application of NMC batteries is battery storage power stations. Two such storage systems were installed in Korea in 2016 with a combined capacity of 15 MWh. In 2017, a 35 MW NMC battery with a capacity of 11 MWh was installed and commissioned in Newman in the Australian state of Western Australia.

See also

• Lithium-ion battery
• Lithium cobalt oxide
• Lithium iron phosphate
• Karim Zaghib