Week 1 Understanding Different Battery Chemistry

Week 1 - Undestanding Different Battery Chemistry

Q) Prepare a table which includes materials & chemical reactions occurring at the anode and cathode of LCO, LMO, NCA, NMC, LFP and LTO type of lithium ion cells.

ANS:

Lithium Ion cells are a secondary type meaning that they are rechargeable and hence reusable which makes them ideally suited for automotive application and certain consumer electronics. When 2 or more lithium ion cells are connected together in a series arrangement, it is referred to as a lithium ion battery. Lithium ion batteries like other batteries rely on the chemical energy stored in the bonds and atoms of elements and compounds and utilises this energy in a chemical reaction occurring inside the cell. The chemical energy from this reaction is converted to electrical energy for us to use. Since lithium ion cells are rechargeable, they can perform the reverse reaction using electricity as well.

Internal Construction

Lithium ion batteries will consist of the following components:

• Anode
• Cathode

The anode and cathode are the 2 electrodes that are connected by a wire in an external circuit between which electrons will be transferred during the chemical reaction. This transfer of electrons will results in a flow of charge, meaning a current is produced. The electrode through which conventional current enters a polarized electrical device is referred to as the anode. The electrode through which conventional current leaves a polarized electrical device is referred to as the cathode. The anode and cathode are not defined by their voltage polarities but instead by the direction of current flowing through them. Often, the cathode is made from a lithium oxide, sulphide or some other compound whereas the anode is made of a pure graphite or hard carbon.

• Separator

A separator is needed as an insulating barrier between the anode and cathode and prevents internal short circuiting. The separator is permeable to ions but to electron due to its micro-porosity.

• Electrolyte

For there to be transfer of electrons, there needs to be a chemical reaction involving ions because ions are charged particles that are formed when electrons have been lost or gained by them. This is where an electrolyte is needed. An electrolyte is a medium through which ions can flow between the electrodes but not electrons. Electrolytes can be in the solid, liquid or aqueous states, but in lithium ion cells the electrolyte is a non aqueous type. The ions of the electrolyte will move toward an electrode depending on their polarities as opposites attract. So, a negative ion will move to a positive electrode and vice versa. So, electrolytes permit ionic conduction but not electronic conduction as electronic conduction will occur through the external circuit. The electrolyte used is a salt of lithium dissolved in an organic solvent.

• Current collector

Current collectors are electronic conductors that are mixed with the electrodes and allows for simple electronic connection to reduce the resistance of an electrode. In lithium ion cells, the current collector is copper foil at the graphite electrode and aluminium foil at the lithium oxide electrode and they receive electrons.

Working

There are 2 modes of operation for cells: charging and discharging.

Charging

In the charging mode, a power source is connected to the lithium ion cell. The positive and negative terminals of the power source will be connected to the lithium oxide and graphite electrodes respectively. The positive terminal will attract and remove electrons from the lithium oxide electrode and will flow from the positive terminal through the external circuit to the negative terminal and get collected at the graphite electrode. This results in the formation of positively charged lithium ions which will be attracted towards the negative terminal and get trapped in the space between graphite's atomic layers. Once all the lithium ions have crossed over to the graphite electrode, the cell is fully charged. This creates an unstable state. In this operation, the lithium oxide electrode is the positive anode and the graphite electrode is the negative cathode. This is because the direction of current is opposite to the flow of electrons. Electrons flow from the lithium oxide electrode to the graphite electrode, therefore current flows in reverse. By definition, the anode is the electrode through which conventional current enters a polarized device and the cathode is the electrode through which conventional current leaves a polarized device.

Discharging

In this mode, the power source is replaced with a load instead e.g. a motor. The terminal polarities of the cell are now reversed so that the negative terminal of load is connected to the lithium oxide electrode and the positive terminal is connected to the graphite electrode. Due to this, the lithium ions will be attracted back to the lithium oxide electrode so it can regain its stability. Due to the load presence, the electrons will also leave the graphite electrode and return to the lithium oxide electrode. Once all the lithium ions and electrons have returned to the lithium oxide electrode, the cell is fully charged and the lithium is now in a stable state. In this operation, the graphite electrode is now the anode and the lithium oxide electrode is the cathode.

The diagram below shows the working of the charging and discharging modes.

The electricity generated from Lithium ion cells ranges between 3 and 4.2V which is almost thrice of what Nickel Cadmium or Nickel Metal Hydride batteries are capable of. There are several types of Lithium ion cells, each with different constructions and chemical reactions occurring within them to produce different characteristics for different applications. They are listed below:

• NMC - Lithium Nickel Manganese Cobalt Oxide
• NCA - Lithium Nickel Cobalt Aluminium Oxide
• LMO - Lithium Manganese Oxide
• LCO - Lithium Cobalt Oxide
• LTO - Lithium Titanium Oxide
• LIPO or LFP - Lithium Iron Phosphate

There is a variety of factors to consider when using any of these lithium ion cells for a particular application such as:

• Specific energy
• Specific power
• Safety
• Performance
• Life span
• Cost

The five different types of Lithium ion battery are compared on the basis of their materials, electrode reactions and their characteristics in the table below:

	Lithium Cobalt Oxide (LCO)	Lithium Manganese Oxide (LMO)	Lithium Nickel Manganese Cobalt Oxide (NMC)	Lithium Iron Phosphate (LFP)	Lithium Nickel Cobalt Aluminium Oxide (NCA)	Lithium Titanium Oxide (LTO)
Chemical Formula	`LiCoO_2`	`LiMn_2O_4`	`LiNiMnCoO_2`	`LiFePO_4`	`LiNiCoAlO_2`	`Li_2TiO_3`
Anode Material	Graphite carbon	Graphite carbon	Graphite Carbon	Graphite carbon	Graphite carbon	Lithium Titanium Oxide
Cathode Material	Lithium Cobalt Oxide	Lithium Manganese Oxide Treated at `350^(circ)C`	Lithium Nickel Manganese Cobalt Oxide	Lithium Iron Phosphate	Lithium Nickel Cobalt Aluminium Oxide	Lithium Manganese Oxide or NMC
Electrolyte	Lithium salt in an organic solvent	Lithium Halide dissolved in an organic solvent	Lithium salt in an organic solvent	Lithium salt in an organic solvent	Lithium salt in an organic solvent	Lithium salt in an organic solvent
Anode Reaction	Charging (L to R) Discharging (R to L) `6C + xe^(-) + xLi^(+) leftrightarrow Li_xC_6`	Charging (L to R) Discharging (R to L) `xLi^(+) + xe^(-) + 6C leftrightarrow Li_xC_6`	Charging (L to R) Discharging (R to L) `xLi^(+) + xe^(-) + nC leftrightarrow Li_xC_n`	Charging (L to R) Discharging (R to L) `xLi^(+) + xe^(-) + 6C leftrightarrow Li_xC_6`	Charging (L to R) Discharging (R to L) `xLi^(+) + xe^(-) + 6C leftrightarrow Li_xC_6`	Charging (L to R) Discharging (R to L) `Ti_5O_12 + 4Li^(+) + 4e^(-) leftrightarrow Li_4Ti_5O_12`
Cathode Reaction	Charging (L to R) Discharging (R to L) `LiCoO_2 leftrightarrow Li_(1-x)CoO_2 + xLi^(+) +xe^(-)`	Charging (L to R) Discharging (R to L) `LiMnO_2 leftrightarrow Li_(1-x)Mn_2O_4 + xLi^(+) + xe^(-)`	Charging (L to R) Discharging (R to L) `LiMO_2 leftrightarrow Li_(1-x)MO_2 + xLi^(+) + xe^(-)`	Charging (L to R) Discharging (R to L) `LiFePO_4 leftrightarrow Li_(1-x)FePO_4 + xLi^(+) + xe^(-)`	Charging (L to R) Discharging (R to L) `LiNiCoAlO_2 leftrightarrow Li_(1-x)NiCoAlO_2 + xLi^(+) + xe^(-)`	Charging (L to R) Discharging (R to L) `4LiMn_2O_4 leftrightarrow 4Mn_2O_4 + 4Li^(+) + 4e^(-)`
Overall Reaction	`LiCoO_2 + 6C leftrightarrow CoO_2 + LiC_6`	`LiMn_2O_4 + 6C leftrightarrow LiC_6 + Mn_2O_4`	`Li_(1-x)MO_2 + Li_xC_n leftrightarrow LiMO_2 + nC`	`LiFePO_4 + 6C leftrightarrowLiC_6 + FePO_4`	`6C + LiNiCoAlO_2 leftrightarrow LiC_6 + NiCoAlO_2`	`Li_4Ti_5O_12 + 4Mn_2O_4 leftrightarrow 4LiMn_2O_4 + Ti_5O_12`
Voltage	Nominal Voltage = 3.60V Typical Operating Range = 3.0-4.2V per cell	Nominal Voltage = 3.70 - 3.80V Typical Operating Range = 3.0-4.2V per cell	Nominal Voltage = 3.60 - 3.70V Typical Operating Range = 3.0-4.2V per cell	Nominal Voltage = 3.20 - 3.30 V Typical Operating Range = 2.5 - 3.65V per cell	Nominal Voltage = 3.60V Typical Operating Range = 3.0-4.2V per cell	Nominal Voltage = 2.40V Typical Operating Range = 1.8 - 2.85V per cell
Cost	Relatively Expensive	Relatively Expensive	$420 per kWh	$580 per kWh	$350 per kWh	$1005 per kWh
Specific Energy	150 - 200 Wh/kg Special cells: 240Wh/kg	100 - 150 Wh/kg	150 - 220 Wh/kg	90 - 120 Wh/kg	200 - 260 Wh/kg	50 - 80 Wh/kg
Cycle Life	500 - 1000 cycles	300 - 700 cycles	1000 - 2000 cycles	>= 2000 cycles	500 cycles	3000 - 7000 cycles
Charge/Discharge Rate Thermal Runaway	Charge Rate = 0.7 - 1C Discharge Rate = 1C Thermal Runaway = `150^(circ) C`	Charge Rate = 0.7 - 1C Discharge Rate = 1C Thermal Runaway = `250^(circ) C`	Charge Rate = 0.7 - 1C Discharge Rate = 1 - 2C Thermal Runaway = `210^(circ)C`	Charge Rate = Discharge Rate = 1C Thermal Runaway = `270^(circ)C`	Charge Rate = 0.7C Discharge Rate = 1C Thermal Runaway = `150^(circ)C`	Charge Rate = 1 - 5C Discharge Rate = 10 - 30C Thermal Runaway = very safe
Application	Mobile Phones, Tablets, Laptops, Cameras	Power tools, medical devices, electric powertrains	E-Bikes, medical devices, EVs, industrial tools	Power tools, aviation products, automotive hybrid systems, PHEV conversions	Medical devices, industrial tools, EVs (Tesla)	UPS, Electric powertrain
Remarks	Very high specific energy Low specific power Not very safe Cobalt is expensive Short working life	High power but less capacity Safer than LCO Mixed with NMC to improve performance	High capacity High power	Very flat voltage discharge curve Low capacity Very safe Very long life	Similar characteristics to LCO But shorter life Higher specific energy	Long working life Fast charging Very safe Low specific energy Expensive

Characteristics

The diagrams below show the graphical representation of the performance characteristics of the different types of Lithium ion battery:

LCO

LMO

NMC

LFP

NCA

LTO