The cathode is a lithium transition metal oxide, eg manganese or cobalt or a combination of transitional metals. The anode is a graphite-based material, which can intercalate or release lithium.
When discharge begins the lithiated carbon releases a Li+ ion and a free electron. Electrolyte, that can readily transports ions, contains a lithium salt that is dissolved in an organic solvent. The Li+ ion, which moves towards the electrolyte, replaces another Li + ion from the electrolyte, which moves towards the cathode. At the cathode/electrolyte interface, Li+ ions then become intercalated into the cathode and the associated electron is used by the external device.
- Lithium Cobalt Oxide
- Capacity ~274mAh/g (theoretical) ~140mAh/g (practical limit)
- Lithium Iron Phosphate
- Voltage range 2.0V to 3.6V
- Capacity ~170mAh/g (theoretical)
- Energy density at cell level ~125Wh/kg (2021)
- Maximum theoretical cell level energy density ~170Wh/kg
High cycle life and great for stationary storage systems.
The low energy density meant it wasn’t used for electric vehicles much until the BYD Blade design showed how to increase the system level density.
Enabled by the fact that LFP takes a lot higher temperature to provoke it into thermal runaway.
- lithium manganese iron phosphate
- 15 to 20% higher energy density than LFP
- Maximum theoretical cell level energy density ~230Wh/kg
- lithium ion manganese oxide (LiMn2O4)
- Capacity ~148mAh/g (theoretical)
- Lower cost and lower toxicity than LCO
- Energy density at cell level 150 to 220Wh/kg
- Lithium Nickel Manganese Oxide
- Lithium Nickel-Cobalt-Aluminum Oxide (LiNixCoyAlzO2)
- Capacity ~279mAh/g (theoretical) 180 to 200mAh/g (practical)
- NMC and NCM are the same thing.
- Lithium-Nickel-Manganese-Cobalt-Oxide (LiNiMnCoO2)
- Voltage range 2.7V to 4.2V with graphite anode.
- NMC333 = 33% nickel, 33% manganese and 33% cobalt
- NMC622 = 60% nickel, 20% manganese and 20% cobalt
- Capacity ~ 154 to 203mAh/g (practical)
- Trend is to reduce Cobalt based on cost and increased capacity
- Higher Nickel content => higher capacity, more heat and faster capacity fade
- Energy density at cell level ~280Wh/kg (2021)
- Maximum energy density at cell level with graphite anode ~350Wh/kg
- Capacity 372mAh/g (theoretical)
- Lithium Titanate or Lithium Titanium Oxide
- Lower energy density, typically ~80Wh/kg at cell level
- Wider operating temperatures
- Low operating voltage 1.9V to 2.9V
- High discharge rates
- Capacity 3580 mAh/g (theoretical) ~10x the theoretical capacity of graphite
- Volume expansion/contraction during insertion/extraction (∼400%)
As you charge a cell it expands, when you discharge a cell it contracts and as the cell ages over its lifetime we see a continuing cell expansion. Thus the cell expansion can be divided into:
- Reversible cell expansion
- Irreversible cell expansion
There are a lot of companies and startups looking at the addition of Niobium to battery chemistry to improve stability, increase capacity, coatings and faster charging. This is being added to anode and cathode materials, some in research, but first perhaps we should start with the Toshiba SCiB technology as this is in production.
The cathode layer in a lithium-ion battery is a composite of solid charge storing particles, a polymeric binder, and a conductive additive. Together, they are well dispersed in a solvent and spread like paint on a conductive substrate, an effective and pleasingly simple solution that works across various chemistries and cell designs.