This paper discusses a new Cathode 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 3 types of Cathodes viz LFP, NMC, NCA
- LiFe PO₄ ( LFP) Lithium Ferrous Phosphate: Chinese manufacturers use this cathode in BEVs since the advent of EVs. The major advantage of LFP is that no Cobalt is used. Cobalt is an expensive and more importantly a non-sustainable material. The LFP batteries have lower energy densities compared to formulations with Cobalt and Nickel. However, recently a Chinese legislation mandated an energy density of 250 Wh/kg for EVs. The legislation excludes 2-Wheelers and 3 -wheelers where LFP will continue to be used.
- NMC: Lithium Nickel Manganese Cobalt Oxide: NMC is the most commonly used cathode in EV batteries. A higher nickel content gives higher energy densities and is preferred. 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%. This 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 and Aluminium 10 % each: Tesla’s 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.
Fig 1:Data taken from Ref: Un -Hyuck Kim et.al. ACS Energy Lett.4,576,2019
Table 1. Comparison of Cathodes used in Modern EVs. Most common Anode
used is Graphite ( but meso carbon microbeads in the present study)
Relative values are taken from the graph of Fig1.
|Lithiated Oxides of||NCA 89||NMC 89||NMCA 89|
|Initial capacity @ 0.1C 81% 93% 93%||81%||93%||93%|
|Cycle Life @1000cycles 40% 57% 89%||40%||57%||89%|
|Cycle Life @500 Cycles 45ᴼC 30% 33% 80%||30%||33%||80%|
|Resistance to microcracking 21% 29% 86%||21%||29%||86%|
|Heat Stability (phase change) DSC peak 87% 68% 90 %||87%||68%||90%|
|Ease of charge transfer: Inv Rct 46 % 54% 75%||46%||54%||75%|
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 is made on 6 parameters
- 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
- Cycle Life when discharged at 0.1C at 30ᴼC : NCA and NMC show only 68.2 % and 60.2% capacity respectively 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.
- 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 Aging test. Under these conditions, NMCA gives 80%, much higher than the very low level of 30% in the case of NMC and NCA.
- 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 cathode of LFP i.e discharged and charged condition
- 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
- 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 6 parameters, NMCA Cathode containing the 4 oxides is a SUPERIOR PRODUCT beyond compare as at present. It is understood that LG Chem, Korea presently using NMC 721 with 70% Nickel for EV batteries will start making NMCA from 2022. This information may be relevant when choosing an EV car. It is also of interest to those who would like to manufacture advanced Cathode Material. Possible method of manufacture is given elsewhere in this paper Other than the choice of the Cathode materials, the following stratagems could yield an excellent Cathode material
- 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.
- 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.
- 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.
- Search for solvents like ionic liquids or other solvents which are not flammable. Unfortunately, the ionic liquids have very low voltage of 2.4 V PC and therefore unacceptable for EVs
- 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
It can be seen that most manufacturers use NMC cathodes with different percentage of Nickel-NMC 532, NMC 622, NMC 721. Tesla uses NCA with high Nickel, probably 811.Indian cars are not shown due to paucity of available data.
Classification: Lithium-ion batteries are classified as “Dangerous Cargo”. This puts them at a higher risk level – more than the “hazardous” classification. For example, a Lead-acid battery is classified as Hazardous to the environment since it affects plant, animal and human lives.
Many instances of fire of Lithium-ion batteries are reported. Most of the fires and explosions are caused by the inflammable organic solvents used in the electrolytes. Another trigger is the runaway temperature observed mainly during charging. However, the main trigger could not be pin-pointed in many cases of reported fire.
Possible Reasons for fire with Lithium-ion Batteries:
Lithium-ion batteries can catch fire when not in use and also when being charged.
When not-in-use e.g in transit or storage:
The electrolyte used in the battery contains organic solvents which are inflammable. The organic solvents are used to dissolve the Lithium salts like Lithium hexafluorophosphate ( LPF₆) and form an integral part of the electrolyte. Replacing it with water will solve the problem, but water is not acceptable because when water is used as a solvent the cell voltage drops to 2 VPC compared to 3.7 v with organic solvents .
Some of the organic solvents have very low flashpoints. For example, Dimethyl Carbonate (DMC) commonly used has a flashpoint of 17 ⁰C. Therefore when a leak of electrolyte happens due to a puncture, the solvent will catch fire even at room temperature. This fire is a trigger and causes a conflagration.
Obnoxious gasses are evolved. This can happen in all chemistries including LFP ( LiFePO₄)
When the Lithium-ion Battery is on Charge :
When the battery is on charge, the Battery Management System (BMS), an essential adjunct protects the battery from overcharge, high currents in the normal course. There is a designed voltage window within which the battery has to function to avoid runaway conditions. This varies mainly with the chemistry of the cathode used and to some extent the stability of the solvents against higher charging voltage.
But BMS may malfunction during high rate fast charging. All or most Lithium may move to Anode. The cathode consisting of oxides of Transition metals like Cobalt, Nickel release oxygen. Again this is a trigger for the solvents to catch fire.
With higher currents, there is also some heating associated with it.
Heating is proportional to the square of the current (C²R).This shows the importance of controlling the charging current especially at high SOCs above 80%.
Possible Remedies :
Researchers on a global scale are working on developing solvents with substituted halogen (Fluoride for example) and other substituents. Cost and availability considerations prevent the widespread use of such chemicals.
Many other approaches are under investigation.
Slower charging or charging at lower currents will mitigate the problem to a large extent since the temperature does not increase unduly. Slow rate charging is a partial remedy. The question arises whether a longer duration of
charging with a lower current is acceptable. A Lithium battery works best under a partial state of charge since there is a lesser chance of decomposition of electrolytes under the designed voltage window of operation. A range of operation from say 5% of capacity for discharge and 95 % of the charge for maximum charge will be helpful to prevent runaway conditions. This is a loss of 10 % on the range of the vehicle per charge . Tables 2and 3 show designed usable capacities of 94% used by manufacturers of EV cars.
Manufacture of NMCA Cathode Material:
Several well-established procedures exist for the manufacture of MNC, 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 ions. 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.
- Moving through the electrolyte: This does not pose a problem for fast charge since the electrolytes have sufficient ionic conduction properties.
- The separator is very thin (15-20 micrometre 10¯⁶ m), allows lithium ions to pass through and does not hinder fast charging. When the temperature goes beyond a threshold, the separator self-destructs and stops current flow. This is a method of protection to avoid runaway temperatures
- The role of Anode is very complex in fast charging. Graphite containing 5 % Silicon has a good intercalation capacity. It can help in housing the incoming lithium ions as fast as they arrive. However, at very high charging currents the flux of lithium ions is high and all cannot be accommodated. The extra Lithium ions deposit as lithium metal outside of the anode and can cause shorting due to the dendrite formation. This again sets the limits for fast charging
- Lithium Titanate Oxide LTO for fast charging : LTO is used as an anode in place of the conventional Graphite Anode. Its main advantages are fast charging possibility, high stability and a very long Life cycle and higher safety. It can be coupled as Anode with many cathodes. It has a major disadvantage with a low voltage of 2.4 V per cell as compared to 3.7 V of NMC. Therefore LTO is not acceptable for high– end cars requiring long-range on a single charge. The reason for the stability and long life of LTO is the absence of SEI (Solid Electrolyte Interface) layer, unlike with Graphite Anodes. Graphite reacts with the electrolytes and forms an ion-permeable layer SEI during the initial charge of the cell. This increases in thickness on cycling and eventually becomes a barrier for ions to flow through it. The joint venture company of Leclanché and Exide – Nexcharge has the know-how to manufacture cells with LTO anodes and may introduce LTO anode for LIBs in their plant in Gujarat.
- The BMS has also a major role in fast charging. The many resistors used in the MOSFETs do not get heated up significantly at low charging currents. At high currents in fast charging, the heating is high and is proportional to the square of the current [Heat generated α C²R]. Therefore cooling becomes necessary to avoid temperature runaway conditions which will result in failure of battery or even cause a fire. Tables 2 and Table 3 show the methods employed by different carmakers to cool the battery passively or actively. Active cooling refers to forced air cooling or the circulation of a liquid coolant. Liquid (e,g. water + 10 % glycol) circulation around the cells is an effective method. Water has high specific heat and can take away the heat efficiently without undue temperature increase of coolant. The battery temperature should preferably not exceed 45 ⁰C to realise the designed life.
- When the design of BMS is proper and effective, most of the problems are avoided. This is the essential second line of defence operating at cell and battery level
- An increase of Temperature helps in faster movement of ions. But for safety, the temperature has to be kept well below the maximum temperature of 45ᴼC
By C.S.Ramanathan, Battery Consultant – firstname.lastname@example.org