Lithium-Ion Batteries

They work using a Lithium Metal Oxide cathode, which is oxidized during charging and that forms the anode, which is comprised of Lithium Ions held in place by a Graphite Lattice and Electrons.

Exceptions: Lithium Titanate batteries replace the Graphite anode with Titanium Oxide. Solid State Li-Ion batteries change the anode to either crystalline Silicon or solid Lithium and may change the cathode to Lithium Sulphide.

The electrolyte is a solution made of Lithium salts and organic solvents.

The electrolyte is also protected from degradation by interaction with electrons by the formation of a Solid Electrolyte Interface (SEI).

Resources

• A reflection on lithium-ion battery cathode chemistry | Nature Communications (Open Access): Differences between Li-Ion battery chemistries
• Electrochemical Potential
• Comparing Performances of Battery Technologies
• Comparison of Lithium Ion Batteries
• Details about Lithium Ion Electrolyte Materials

Cathode

• \(\text{LiMO}\) (MO = Metal Oxide)
  • The metals are 3d transition metals on the right end due their highest positive electronegativity values, which increases their electrode potential. So basically Cobalt, Nickel, Manganese, Iron.
  • Recap: Electronegativity goes lower down a group, and higher across a period in the periodic table of chemistry.

Reaction During Charging:
\(\text{LiMO} \rightarrow \text{MO} + \text{Li}^+ + \text{e}^-\) (\(-\text{E}^\ominus_\text{cathode} = -\text{1.0 V}\))

Examples of Cathode Materials

Battery cell descriptions like NMCXYZ means NMC with proportions of Ni:Mn:Co in the ratio X:Y:Z.

Anode

• \(\text{LiY}\) (Y=Lattice Material)

Reaction During Charging:
\(\text{C}_\text{6} + \text{Li}^+ + \text{e}^- \rightarrow \text{LiC}_\text{6}\) (\(-\text{E}^\ominus_\text{anode} = \text{3.05 V}\))

Examples of Anode Materials

• \(\text{LiC}_\text{6}\) (Most common, Has a maximum theoretical specific charge of about 372 \(\text{mAh/g}\))
• \(\text{Li}_\text{4}\text{Ti}_\text{5}\text{O}_\text{12}\) / \(\text{Li}_\text{2}\text{TiO}_\text{3}\) (Durable, used in LTO)
• \(\text{Li}_\text{15}\text{Si}_\text{4}\) (Used in Solid State Batteries, In development, Maximum theoretical specific charge of about 3600 \(\text{mAh/g}\). Each crystalline Silicon atom can hold about 3.75 Lithium ions compared to Graphite, which can only hold 1 Lithium ion for every 6 Carbon atoms at best.)
• \(\text{Li (s)}\) (Used in Solid State Batteries, in development, Maximum theoretical specific charge of about 3863 \(\text{mAh/g}\))

Electrolyte

The electrolyte is the medium through which the ions move between electrodes.

Its characteristics, such as temperature range, passivation, conductivity, etc. also sets the maximum allowed voltage (or decomposition voltage), beyond which the electrolyte breaks down.

Other roles of the electrolyte include:

• Coulombic efficiency (charge efficiency)
• Dendrite formation of Li–metal
• Degradation of electrolytes by the intermediate and final reaction products in \(\text{Li–S}\) and \(\text{Li–Air}\) batteries

Formula: A mixture of \(\text{LiX}\) (\(\text X\) = Anion) salts + Organic Solvent

Examples of Lithium Salts

• \(\ce{LiPF6}\) (Lithium Hexafluorophosphate) (main conductive salt, decomposes at high temperatures, reacts with water traces to form toxic \(\ce{HF}\))
• \(\ce{LiBr}\) (common additive salt)
• More (details in separate note): \(\ce{LiAsF6}\), \(\ce{LiClO4}\), \(\ce{LiBF4}\), \(\ce{LiAlCl4}\), Li-TFS, Li-TFSI, Li-TFSM, Li-DMSI, Li-HPSI, Li-FSI, Li-BOB, Li-FAP, Li-BETI, Li-DFOB, Li-BFMB, Li-Bison, Li-DCTA, Li-TDI, Li-PDI

Example of organic solvents

Cyclic Esters

• Ethylene Carbonate (EC)
• Propylene Carbonate (PC)
• γ-Butyrolactone (GBL)

Linear Esters

• Dimethyl Carbonate (DMC)
• Ethyl Methyl Carbonate (EMC)
• Diethyl Carbonate (DEC)
• Tetrahydrofuran (THF)
• 2-Methyl tetrahydrofuran (2-MTHF)

Cyclic Ethers

• 1-3 Dioxolane (DOL)

Linear Ethers

• 1,2-Dimethoxyethane (DME / monoglyme / G1)
• Tetraethylene glycol dimethyl ether (TEGDME / tetraglyme / G4)

Common mixtures of solvents

• EC-PC-DMC (20%:20%:60% vol.)

Charging and Formation of Solid Electrolyte Interface

Basically, upon initial charging, the positive plate attracts the electrons and they move through the wires and towards the negative plate and into the anode while the Lithium ions move through the electrolyte into the anode.

The electrolyte only expects Lithium ions to pass through. If the electrons move through the electrolyte, they will react and destroy the electrolyte. But through an accidental discovery, the Lithium Ions passing through the electrolyte seemed to carry the electrolyte around it, and together with Graphite, the initial batch of Lithium ions reacted to form a protective layer called the Solid Electrolyte Interface (SEI). The SEI keeps the electrons from passing through the electrolyte, while the other Lithium Ions can pass in through to the anode.

The Lithium Ions are stored in a stable concentration by the Graphite/Silicon lattice. When the potential is removed, the electrons will be stuck, wanting to move back into the cathode, but are not able to move through the electrolyte due to the SEI. During this process, a chunk of Lithium Ions are lost, however this happens at the factory and end-users will not notice a degradation in performance unless they exert it in other ways.

Reactions (the opposite of the spontaneous reaction):
\(\text{LiMO} \rightarrow \text{MO} + \text{Li}^+ + \text{e}^- (-\text{E}^\ominus_\text{cathode} = -\text{1.15 V} = -\text{E}^\ominus_\text{reduction}\)) (The oxidizing agent gets forcibly oxidized)
\(\text{C}_\text{6} + \text{Li}^+ + \text{e}^- \rightarrow \text{LiC}_\text{6}\) (\(-\text{E}^\ominus_\text{anode} = \text{3.05 V} = \text{E}^\ominus_\text{oxidation}\)) (The reducing agent gets forcibly reduced)
\(\text{E}^\ominus_\text{charger} = -\text{E}^\ominus_\text{cell} = -(\text{E}^\ominus_\text{cathode} - \text{E}^\ominus_\text{anode}) = -\text{4.2 V}\)

Discharging

When a load is connected, the electron current flows through it to the cathode, and when an electron reaches the cathode, it also makes a Lithium Ion from the other side flow in, returning the cathode to it's original discharged state, as Lithium Metal Oxide.

Reactions:
\(\text{MO} + \text{Li}^+ + \text{e}^- \rightarrow \text{LiMO}\) (\(\text{E}^\ominus_\text{cathode} = \text{1.15 V} = \text{E}^\ominus_\text{reduction}\)) (The oxidizing agent is spontaneously reduced)
\(\text{LiC}_\text{6} \rightarrow \text{C}_\text{6} + \text{Li}^+ + \text{e}^-\) (\(\text{E}^\ominus_\text{anode} = -\text{3.05 V} = -\text{E}^\ominus_\text{oxidation}\)) (The reducing agent is spontaneously oxidized)
\(\text{E}^\ominus_\text{cell} = \text{E}^\ominus_\text{cathode} -\text{E}^\ominus_\text{anode} = \text{4.2 V}\)

Considerations

• Overcharging is bad for the battery, and besides that, it may even explode.
• A Constant Current-Constant Voltage (CC-CV) charger is to be used along with a Battery Management System (BMS) to carefully handle charging and discharging cycles of the battery.
• A BMS is sometimes included in Li-Ion battery packs, but they definitely should be included in any battery that combines Li-Ion cells. They are usually excluded in favour of using custom BMSs.
• Overdischarging supersaturates the Metal Oxide and forms Lithium Oxide, which is irreversible.

Variations

• Lithium Ion (Li-Ion) is the standard type of Li-Ion batteries, as the name implies. It has a theoretical maximum specific energy of 370 \(\text{Wh/kg}\), when using a Lithium Cobaltate cathode and a Graphite anode.
• Lithium Polymer (Li-Po) has a relatively higher specific energy, but is more inflammable and can explode if it is overcharged. It uses a semisolid polymer gel electrolyte, but the electrode chemistry is the same as that of Li-Ion cells.
• Solid State Batteries (SSBs) has a much higher specific energy and is being researched on. They either use solid Lithium or Silicon anode to achieve these high specific energies. They may also use solid electrolytes, and are also called All-Solid-State Batteries (ASSBs). A good SSB can show a specific charge of about 2890 \(\text{mAh/g}\) and it has a maximum theoretical specific energy of about 1440 \(\text{Wh/kg}\) when using a Sulphide cathode. Practical specific energies come to around 500-900 \(\text{Wh/kg}\).
• Lithium Sulphur (Li-S) batteries use a Lithium Sulphide (\(\text{Li}_\text{2}\text{S}\)) cathode for its high specific energy. The cathode is coated with Carbon to help with electrical conductivity. During charging, the movement of Lithium ions cause the cathode to turn into Sulphur (\(\text{S}_8\)). Sulphur is very cheap, and with its high energy density is very appealing to use for battery cells. For the anode side, Solid Lithium is used. One of the shortfalls of these batteries is that while Lithium Sulphide and Sulphur are non-reactive with the electrolytes, some intermediary polysulphides aren't, and they end up destroying the electrolytes. For this reason, sometimes a protective coating with something like Teflon is used at the anode side to protect the electrolytes. Electrolytes commonly used in the Li-Ion batteries, based on organic carbonates (such as Polycarbonate, Ethylene Carbonate and Diethyl Carbonate) are not compatible with (Li-S) batteries. They have a maximum theoretical specific charge of about 1675 \(\text{mAh/g}\) and a specific energy of about 500 \(\text{Wh/kg}\).

Actually Li-S isn't exclusive from SSBs. An SSB would typically make use of a Sulphur/Oxide anode, Solid Electrolyte and Lithium/Silicon Anode.

Physical Design

The thickness and proportions of the battery internals are designed after careful studies and optimizations based on the industrial engineering data.

Form Factors

• Wrapped Cylindrical Cell (18560, etc.)
• Pouch (Mainly for Li-Po, flexible in design and has no standard footprints)
• Prismatic (Stacked Li-Ion Cells, used in EVs)