Introduction to lithium-ion batteries

Learn about the basics of lithium-ion batteries

Why batteries?

Batteries are essential for technologies such as: consumer electronics, electric vehicles and stationary energy storage for balancing renewable energy sources.

The size of these batteries varies significantly between applications from several Watt-Hours (Wh) to hundreds of MWh.

What is a Watt-Hour (Wh) and why is it useful?

A Wh is a unit of energy where 1 Wh = 3,600 J. It’s useful as it allows us to easily see how long an energy storage device can last for under different power loads.

For example a 1 Wh battery can provide 1 W for 1 hour, 2 W for 0.5 hour or 0.5 W for 2 hours.

Energy (Wh) = Power (W) x Time (h)

Different types of batteries

Various types of batteries exist with these being defined by the materials they use. Here, 2 key performance metrics are the energy density (Wh/L) and specific energy (Wh/kg).

Here, current lithium-ion batteries batteries typically use an anode made of graphite and a cathode made of a transition metal oxide, which results in a specific energy of ~250 Wh/kg and energy density of ~600 Wh/L, however this is constantly being improved upon.

How does this compare with petrol?

While petrol has a specific energy of ~12,800 Wh/kg and energy density of ~9,500 Wh/L this is not a fair comparison with a battery since petrol in a vehicle is often burnt in an internal combustion energy with an efficiency of ~25%. This would give an effective specific energy and energy density of ~3,200 Wh/kg and ~2,400 Wh/L, still higher than batteries but with a narrower gap.

Performance metrics

Targets from the Faraday Institution. Adapted from content from Professor David Greenwood

For mass market adoption, a practical battery not only requires a good energy density but also needs to consider other performance metrics such as cost, power density, safety, lifetime, operating temperature range, predictability and recyclability, amongst others.

Making a battery with a single one of these metrics is relatively straightforward, however the difficult challenge is the “AND” problem, of achieving high energy density and good lifetime with low cost (with other metrics).

The figure (left), indicates roughly where performance is right now and future targets.

The battery industry structure

The battery supply chain is complex and can be broadly split into the areas shown in figure (right). Vertically integration across all of these areas is challenging due to the cost of each one of these with some of the key considerations highlighted.

These supply chains are globally distributed. For instance, large amounts of lithium are located in Chile and Argentina with China then producing the vast majority of battery grade precursor material and cells for global automotive manufacturers.

To compound the complexity, key considerations such as the purity of the raw material can impact the eventual quality/cost of the battery. Furthermore, the exact composition of future batteries isn’t 100% clear due to constant innovations in battery materials, leading to uncertain future demand volumes for critical minerals and risk for investments in large capital projects such as new mines.

Adapted from material from Professor David Greenwood

How does a battery work?

There are various types of lithium ion-battery but inside a typical you’ll find multiple materials which include:

A copper current collector - This provides a pathway for electrons to get in and out the anode
A porous anode - This is the negative side of the battery which is usually made of graphite with a polymer binder and conductive additive
A porous polymer separator - This allows lithium-ions in the electrolyte to travel from the anode to the cathode, but prevents electrical contact between the two electrodes
A porous cathode - This is the positive side of the battery often made with a transition metal oxide with a polymer binder and conductive additive
An aluminium current collector - This provides a pathway for electrons to get in and out the cathode
An electrolyte which fills the anode, separator and cathode pore space which allows lithium-ions the move between the two electrodes

Charged battery - Discharging

When the battery is fully charged, the lithium-ions are intercalated (sit) in the anode. When the battery discharges, the lithium-ions move out from the anode and release an electron. Lithium-ions then intercalate into the cathode, but the electron has the move around an external circuit where useful electrical work can be extracted.

Discharged battery - Charging

When the battery is fully discharged, the lithium-ions are intercalated in the cathode. When the battery charges, the reverse reaction occurs and the lithium-ions move out from the cathode, with the lithium-ions going into the anode, with the electron again going round the external circuit, with the help of a power supply.

Rocking chair mechanism

This back and forth movement of the lithium-ions is often called the rocking chair mechanism, with the reversible rechargeability of batteries one of the main advantages of the technology.

Battery materials

Different types of batteries exist with these generally characterised by the materials they use. Here, the 3 main considerations in a battery are its anode, cathode and electrolyte. Here, it should be noted that there are other components which are in a battery such as the current collectors, binders, conductive additives and separator but much of the performance is often defined by the 3 main considerations.

Cathodes

A good cathode is one which has a high specific capacity (mAh/g) and high voltage (within the electrolyte stability window. Lithium cobalt oxide (LCO) was used in the first commercial lithium-ion battery, however suffers from relatively low lifetime and high cost. Many current batteries use a cathode of either lithium iron phosphate, lithium nickel manganese cobalt oxide (NMC) or lithium nickel cobalt aluminium oxide (NCA). Higher energy dense batteries often use a nickel rich cathode or strive to use alternative cathode materials such as sulfur or an oxygen based system, however these usually suffer from poor lifetime.

Anodes

A good anode is one which has a high specific capacity and low voltage. Here the majority of batteries use a graphite based anode due to its relatively low cost and good lifetime. Industry trends are currently aiming to blend in a small amount of silicon to increase the specific capacity, with future anodes aspiring to be lithium-metal or silicon based. Here, most lithiated anodes are thermodynamically unstable in common electrolytes and thus should not work well. However, a solid-electrolyte interphase (SEI) forms on the surface of these materials which both protects the anode, but also contributes to its degradation (capacity and power fade).

Electrolytes

The electrolyte is essential for enabling lithium-ions to move back and forth between the anode and cathode. Here, a good electrolyte has a wide stable voltage range, good thermal stability and ionic conductivity. Currently, the majority of batteries have carbonate based electrolytes however alternative systems exist with solid materials (oxides, sulfides and polymers) as well as others.

All

In addition to these considerations, all components should ideally be cheap, mass produced and non-flammable.

Adapted from content from Dr. Monica Marinescu

Cathodes

LMO = Lithium manganese oxide | LFP = Lithium iron phosphate | LCO = Lithium cobalt oxide | NMC = Lithium nickel manganese cobalt oxide | NCA = Lithium nickel cobalt aluminium oxide

The cathode has a significant influence on the specific energy of the resulting battery. Here, a number of different chemistries exist which have been commercialised, each with the pros and cons. Ultimately, the selection of the optimal chemistry will depend on the required combination of properties, but factors which are considered typically include: specific energy, specific power, safety, lifetime and cost, ideally with these optimised properties in the same device.

Note that the performance of these cathode are constantly being improved alongside new chemistries. Here, within the lithium-ion cathode space, lithium iron manganese phosphate (LMFP) is one notable example which has reportedly good specific energy alongside using low cost materials.

Voltage curves

Typically, lithium-ion batteries operate between a maximum voltage of 4.2 V and a minimum of 2.7 V, resulting in a nominal (average) voltage of ~3.6 V (though this depends on the chemistry). Charging above this can cause the electrolyte to decompose, and discharging below this can cause the copper current collector to dissolve, amongst other effect. Neither are good for battery lifetime and can cause safety issues.

The capacity of the cell (Ah) between these voltage limits is often a key performance metric for the cell. The capacity can be defined by the below equation and can give an estimate of the run time of a battery for a specific capacity and current.

Capacity (Ah) = Current (A) x Time (h)

For example, a 4.8 Ah battery can be discharged at:

2.4 A for 2 hours
4.8 A for 1 hour
9.6 A of 0.5 hours

To help communicate how fast a given current will charge/discharge a battery with a certain capacity, the C-rate is often used.

C-rate (1/h) = Current (A) / Capacity (Ah)

For example, for a 4.8 Ah battery the following currents translate to*:

2.4 A = 0.5 C = 2 hour charge/discharge
4.8 A = 1.0 C = 1 hour charge/discharge
9.6 A = 2.0 C = 0.5 hour charge/discharge

Here, C-rate is often quite useful when considering fast charging speeds, C-rates of ~4 C and beyond are currently being targeted which would give a theoretical charge time of 15 minutes.

*Note that the actual charge/discharge times will vary due, with charge times being longer and discharge times being shorter due to keeping to battery within its safe operating limits.

Form factors

Recycling lithium-ion batteries from electric vehicles. Harper et al. Nature. 2019

There are 3 main form factors (shapes) of batteries; cylindrical, prismatic and pouch, each with their pros and cons.

Cylindrical

Cylindrical cells have good mechanical stability due to their rigid cell cans and can be made rapidly as the electrodes can be rapidly rolled and inserted into the cans. A common form factor is the 18650 cell which has a diameter of 18 mm and length of 65 mm. Here, there is a trend to larger cells (e.g. 2170 and 4680) to increase the specific energy by reducing the mass contribution of the cell can which is an inactive component. A drawback of this cell format is that you can’t achieve a high packing density due to the cylindrical shape.

Prismatic

Prismatic cells also have good mechanical stability due to their rigid cases and can also be made rapidly, with the electrodes being made into a jelly roll (similar to a squished toilet roll of paper). These can often be found in higher capacity variants than cylindrical cells, meaning fewer large cells can be used for electric vehicles; reducing the number of connections, but posing challenges with managing their safety and lifetime. A key advantage here is the prismatic shape which enables a high packing density. The rigid case, however again reduces the specific energy of the cell.

Pouch

Pouch cells usually have a soft casing material and individual electrode layers which are welded together at the tabs. This form factor in theory gives the highest specific energy as the casing is minimal, however they lack mechanical rigidity, meaning that additional support structures may be needed, as well as compression on the electrodes for good lifetime.