Cathode Crystal Structures

Cathode materials consist of highly organized crystal structures which determine where lithium ions can sit, how they can move through the material, and how well the structure holds itself together as lithium repeatedly enters and leaves. Most conventional lithium-ion cathodes store energy through intercalation, whereby lithium ions are reversibly inserted into available sites within a solid crystal structure without completely destroying or rebuilding that structure.

For that to work, the cathode needs a crystal structure that is both stable enough to remain intact as lithium is removed and reinserted, and open enough to provide sites and pathways for lithium ions.

Many cathode materials achieve this using frameworks containing transition metals and oxygen.

These both serve different purposes. Transition metals such as nickel, manganese, cobalt, and iron can exist in multiple oxidation states. This allows them to participate in the reversible redox reactions that store and release electrical energy. Oxygen, meanwhile, helps form and stabilize the crystal framework around these transition metals.

Cathode materials are grouped into three main crystal structures: layered, spinel, and olivine. These groupings are determined by the arrangement of transition metals, oxygen, and lithium within the material, which in turn affects various battery characteristics, including:

• Diffusion dimensionality: the number of directions available for lithium-ion transport through the material

• Structural bonding and stability: how the atoms are connected within the material and how well the framework remains intact as lithium is removed and reinserted

Spinel Structure - LMO, LNMO

In a spinel structure, transition-metal ions are surrounded by six oxygen atoms, forming transition metal-oxygen octahedra. These octahedra connect with neighbouring octahedra to create a rigid interconnected 3D framework. This arrangement of oxygen atoms also creates tetrahedral sites, where the centre of the site is surrounded by four oxygen atoms. Lithium ions occupy these tetrahedral sites. 

Due to the interconnected pathways in all three dimensions, lithium transport through the crystal structure occurs through 3D diffusion. Lithium ions can move between tetrahedral sites by passing through neighbouring vacant octahedral sites. An added advantage of 3D diffusion is that if movement along one direction is blocked, alternative pathways through the material may still be available.

The interconnected transition metal-oxygen framework also provides good structural stability, helping the structure remain intact as lithium moves in and out. Overall, these interconnected lithium-transport pathways support rapid lithium insertion and extraction, which is one reason spinel materials such as LMO and LNMO are attractive for applications requiring high power.

The trade-off is that while the spinel structure's interconnected 3D pathways enable fast lithium transport and high power, spinel cathodes generally offer lower energy density than layered cathodes. Their open structure provides excellent pathways for lithium movement but generally allows less lithium to be reversibly stored per unit mass. Depending on the material and operating conditions, other degradation mechanisms can also occur, such as manganese dissolution in LMO.

Layered Structure - NMC, LCO, NCA 

Layered cathode materials take a different approach. Instead of creating an interconnected 3D lithium network like spinel cathodes, their transition metal-oxygen octahedra connect to form sheets. These sheets alternate with layers containing lithium ions. 

Lithium ions can move relatively freely within their own layers. However, due to the transition metal framework,  trying to move perpendicular to the layers in the z-plane would require lithium ions to break through the dense transition metal-oxygen sheets which is not an energetically favorable pathway.  As such,  lithium ion movement is mostly restricted to the x-y plane, giving layered cathodes 2D diffusion. 

Layered cathode structures can store a lot of lithium while operating at relatively high voltages. This combination of high capacity and high voltage gives materials such as NMC and NCA their characteristically high energy density.

On the flip side, layered cathodes can become less stable as more lithium is removed during charging. Lithium helps maintain the spacing and stability between the transition metal-oxygen layers, so removing large amounts can cause the layers to shift or change shape. At very high states of charge, this can lead to more significant structural changes and further degradation.

Olivine Structure - LFP, LFMP

Olivine materials take yet another approach. LFP is the best-known example. Its framework contains two structural units: transition metal octahedra and PO₄ tetrahedra. In the transition metal octahedra, the transition metal atom is surrounded by six oxygen atoms, while in the PO₄ tetrahedra, a phosphorus atom is surrounded by four oxygen atoms. These octahedra and tetrahedra connect together to create a rigid 3D framework. Lithium ions occupy their own octahedral sites within this framework, separate from the sites occupied by the transition metal.

The available lithium sites are arranged into continuous channels primarily along one direction through the crystal. Lithium ions therefore move from one lithium site to the next through a 1D diffusion pathway. As such, even though the olivine framework itself is three-dimensional, lithium ion diffusion is one-dimensional. 

The defining characteristic of the olivine structure is its stability. The PO₄ tetrahedra have strong phosphorus-oxygen bonds, which help hold the framework together and keep oxygen tightly bound within the structure. Together with the interconnected octahedra and tetrahedra, this creates a rigid framework that remains relatively stable as lithium moves in and out. This contributes to the excellent structural and thermal stability associated with LFP.

The trade-off is that its 1D lithium pathways provide fewer alternative routes for lithium movement. If a pathway is disrupted by a defect, for example, lithium has fewer ways to move around that obstruction than it would in a 2D or 3D diffusion network.

In Essence

Spinel materials are known for high power, layered materials for high energy, and olivine materials for high stability. These differences are closely linked to their crystal structures: spinel materials provide interconnected 3D pathways for fast lithium transport; layered materials provide 2D lithium-ion pathways and high lithium storage; olivine materials have more restricted 1D pathways but a strongly bonded and stable framework.

Cathode crystal structure is only one of many factors that determine battery performance. However, understanding these structural differences provides a foundation for understanding the trade-offs between energy density, power capability, stability, and lithium-ion transport that ultimately influence how different cathode materials perform.