Batteries used for electric vehicles are required to meet the ever-increasing demand for longer ranges and faster charging. The emphasis is on improving the energy density to realise a longer range between two charge events. Preventing runaway temperatures and ensuring the safety of the vehicle and prevention of fire is another major concern, especially during charging and use.
Types of batteries used by some global EV manufacturers
It can be seen that most manufacturers use NMC cathodes with different percentages of Nickel – NMC 532, NMC 622, NMC 721. Tesla uses NCA with high Nickel, probably 811. Indian cars are not shown due to the paucity of available data.
This article discusses a new cathode (NMCA) of higher stability and longer life invented by the researchers from premier Scientific Institutions in Germany and South Korea [Un.Hyuck Kim et al. “Quaternary Layered Ni-Rich NCMA Cathode for Lithium-ion Batteries” in ACS Energy Letters: Vol 4, p 576-562, 2019]
Characteristics of Cathodes
Modern EVs use the following three types of Cathodes viz LFP, NMC, NCA:
LiFePO₄ ( LFP) Lithium Ferrous Phosphate
Chinese manufacturers have used this cathode in BEVs (battery electric vehicles) since the advent of EVs. The major advantage of LFP is that no Cobalt is used. Cobalt is an expensive and, more importantly, an unsustainable material. LFP batteries have lower energy densities compared to formulations with Cobalt and Nickel. However, recently Chinese legislation mandated an energy density of 250 Wh/kg for EVs. The legislation excludes 2Ws and 3Ws where LFP will continue to be used.
NMC: Lithium Nickel Manganese Cobalt Oxide
NMC is the most commonly used cathode in EV batteries. A maximum of 60% Nickel (say NMC 622 – Nickel 60%, Manganese 20% and Cobalt 20%) is considered a safe choice. Some manufacturers increase Nickel to 70%, which is the maximum used in NMC cathodes. But there is a sustained research effort to increase Nickel content to above 80% and thus reduce the use of cobalt. Nickel and Cobalt oxides are reactive and yield higher energy densities. But the same reactivity results in uncontrolled oxidation at higher potentials during the end of charge. This is one cause of fire seen in many instances. LFP does not react similarly since it has no oxidative property. Therefore, LFP is safer during charge. But there is a possibility of migration of Iron from cathode to anode during overcharge. The BMS software programmes will come into play and prevent such high voltage or currents in the normal course.
NCA 811 with Nickel 80%, Cobalt 10% and Aluminium 10%
Tesla EVs use NCA 811. The high Nickel gives higher energy densities required for a longer range. Aluminium does not take part in the electrochemical reaction and does not contribute to capacity. The role of Aluminium is to stabilise the cathode. When most of the Lithium moves out on full charge, the Nickel and Cobalt oxides become unstable. Aluminium oxide present in the cathode is not affected and contributes to the stability of the cathode. Since Aluminium does not contribute to capacity, too much of it is not acceptable. It is a ‘diluent’of Cathode. Therefore, in later designs, the quantity of Aluminium is reduced to 1.5%.
The new quaternary Cathode NMCA is discussed below.
Comparison of Cathodes used in Modern EVs. The most common anode used is Graphite (but meso carbon microbeads in this study). Relative values are taken from the graph of Fig1 below.
Nickel content approx 89% (molecular Ratio) in all Cathodes + 1 mole Lithium
NCA 89 – Nickel 88.5%+ Cobalt 10% + Aluminium 1.5%
NMC 89 – Nickel 90 % + Cobalt 5% + Manganese 5%
NMCA 89 – Nickel 89% + Cobalt 5% + Manganese 5% + Aluminium 1%
Mol. Wt- Li=7; Nickel=58.7 Cobalt=58.9; Manganese= 54.9; Aluminium=27
The comparison between NCA, NMC and NMCA cathodes is made on six parameters:
1. Ah capacity at a Discharge current of 0.1C (10 A for 100 Ah battery)
The Ah capacity shown above is low for NCA and almost the same for the others. Therefore this is not a distinguishing feature.
2. Cycle Life when discharged at 0.1C at 30ᴼC
NCA and NMC show only 68.2 % and 60.2% capacity after 1000 cycles, whereas NMCA yields 84.5%. This is a remarkable difference and should be one reason for choosing NMCA over the other two.
3. Cycle life when discharged at 45 ᴼC
45 ᴼC is the maximum temperature at which a LIB cell can operate without deterioration. Cycling at this temperature is, therefore, an accelerated ageing test. Under these conditions, NMCA gives 80%, much higher than the very low level of 30% in the case of NMC and NCA.
4. Resistance to Microcracking
Lithium ions move out of the cathode structure during charge and return during discharge (called Intercalation). The cathodes containing Cobalt and Nickel have a layered structure. The lithium ions occupy the interspace (gap) between the layers. The cathode crystals expand on discharge and contract during charge. Many cycles of charge and discharge result in microcracks. The electrolyte enters the cracks causing failure. The NMCA shows the least tendency for microcracks. When the expansion is small, as in LFP, cycle life is high. There is little difference in the structure of lithiated and delithiated cathodes of LFP, i.e. discharged and charged condition.
5. Heat Stability (DSC peak temperature)
Differential Scanning Calorimetry – DSC is a procedure that traces changes in the battery, particularly in cathode structure during heating. In this test, both NCA and NMCA show good results. This appears to be one reason for TESLA cars to choose NCA cathode.
6. Ease of Charge Transfer (Inverse Rct, Inverse Charge Transfer Resistance)
Only NMCA has a high score of 76% in this test, much higher than the other NCA and NMC. This means that the new cathode NMCA is suited for fast charging compared to the other two.
Thus in all six parameters, NMCA Cathode containing the four oxides is a SUPERIOR PRODUCT beyond compare as at present.
Prospects of adoption of NMCA for EV cars
There is no doubt that NMCA cathodes will be the material of choice for LIBs in cars. The sheer superiority of the product will ensure adoption. Advantages are longer cycle life without deterioration in the range on a single charge, faster charge capability compared to either NMC or NCA.
Among the manufacturers of batteries for EV cars, it is understood that LG Chem, Korea, presently using NMC 721 with 70% Nickel for EV batteries, will start making NMCA in 2022. LG Chem is currently known to supply batteries to Renault, Hyundai, Chevrolet, Jaguar. These EV makers are therefore likely to change over to NMCA in the near future.
Manufacture of NMCA cathode material
Several well-established procedures exist for the manufacture of NMC, NCA, LFP. The manufacture of NMCA will follow a similar process. Only one of the procedures called SPRAY PYROLYSIS is discussed here.
Spray pyrolysis is chosen for its simplicity and universality. Most of the types of Cathode material can be made using this process. Water-soluble salts of all components, including Lithium salt, are mixed in molecular proportion (as per formula ) and made to pass through a fine nozzle. Ultrasonic Vibrations or Carrier gas like compressed air (Venturi Effect) will carry the mixed solution and force it through the nozzle. Droplets of the solution will form at the nozzle. The size of the droplet will depend upon the size of the nozzle. The droplets impinge on a pre-heated surface kept at 800-1000 ᴼC. The solution evaporates instantly to form a powder. A polycrystalline material is generally obtained. Further treatment of polycrystals (Annealing) is necessary to get a Single crystal or monocrystal.
Properties of Cathode for fast charging
– The Lithium–ions should be able to move faster when a charging current is applied. The movement is called diffusion and depends upon the concentration gradient of lithium ions within the cells. The Li-ions have to move from inside the crystal, reach the solid-liquid interface and then pass through the electrolyte, through the separator, again the electrolyte, a layer called SEI (SOLID ELECTROLYTE INTERFACE) and then reach the anode. The lithium ions then enter the Graphite anode (INTERCALATION). Graphite anode has a layered structure, and the Lithium-ion stay between the layers. This is a simplistic view for easy understanding. The diffusion of Lithium ions takes finite time and limits the fast charge capability.
– A single crystal cathode in place of a polycrystalline solid cathode helps in the smooth working/movement of Li-ons. The single-crystal structure is still preferred even though the path length within the crystal is longer. The single crystals are more stable and take longer for deterioration. Hence the preference.
– The path the Lithium–ions travel within the cathode crystal is longer or shorter depending on the size of the cathode particle. Therefore particle sizes of nanometres (10¯⁹ m) are used. This size can be obtained from micron-sized particles (10¯⁶ m) obtained in manufacture. A high-speed milling process makes the size reduction of 1000 times possible.
Other than the choice of the Cathode materials, the following stratagems could yield an excellent Cathode material.
1. Using single-crystal cathodes in place of polycrystalline. The method involves heating the precursor Polycrystalline material at 800-1000 ᴼC for a few hours. This is called Annealing.
2. Doping or adding a material belonging to the transition metals or an inert material like Aluminium. NMCA is itself an example – called Alloying. This increases the voltage and voltage window of operation. Higher energy density is also realised.
3. Then there are many possible areas like improving the safety of operation of LIBs. LFP, for example, can withstand higher temperatures compared to the oxides of Cobalt, Nickel. But it still contains inflammable organic solvents and can catch fire when punctured. This extends to handling the spent batteries during recycling too.
4. Search for solvents like ionic liquids or other solvents which are not flammable. Unfortunately, the ionic liquids have a very low voltage of 2.4V PC and, therefore, are unacceptable for EVs.
5. Continuous research is in progress all over the globe on all aspects of LIBs –Anode, Cathode, Electrolytes etc. One such breakthrough is the development of the NMCA cathode. Many manufacturers are likely to choose NMCA due to its superiority in the next few years.
About the author – C.S.Ramanathan is a Battery Consultant and holds M.Sc; D.I.I.Sc. Degrees from the Indian Institute of Science. Earlier, he was Head of R&D at AMCO Batteries Ltd and has been a consultant to many battery manufacturers in India and abroad.
Team EVreporter would like to thank Paladugu Chandrasekhar (CEO, Futurelite Batteries) for reviewing this article.
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