Three dopants, one cathode: what Co, Mn and Al each do to LiNiO₂

An electric vehicle that goes farther on one charge needs a cathode that stores more energy per gram. Lithium nickel oxide (LiNiO₂, or LNO) is one of the best candidates. Its theoretical capacity is 275 mAh/g, and nickel is cheaper than cobalt. The problem is that LNO loses capacity quickly. In our cells, undoped LNO kept only about 42% of its initial discharge capacity after 100 cycles.

What goes wrong?

LNO is a layered material. Sheets of nickel and oxygen are stacked like the floors of a building, and the lithium ions sit between them like pillars. Charging the battery removes the lithium.

For most of the charge, the floors move apart a little, because the oxygen layers repel each other. Near the end of the charge, at about 4.3 V, most of the pillars are gone and the floors suddenly move closer together. This step is called the H2–H3 phase transition. In our measurements, the unit cell of LNO lost about 9.8% of its volume during this transition.

A particle that shrinks and expands this much in every cycle accumulates strain, and with time it cracks.

What did we do?

A common way to stabilize LNO is doping: replacing a small fraction of the nickel with another element. Cobalt (Co), manganese (Mn) and aluminum (Al) are the most used, and each one is thought to help in a different way. We wanted to see those differences directly.

We compared four cathodes: undoped LNO, and LNO with 5% of Co, Mn or Al. Our collaborators at UT Austin synthesized all of them by the same method. We tested them with the same electrolyte, the same loading and the same cycling protocol, between 2.8 and 4.4 V. We chose these conditions to compare the dopants fairly, not to reach the highest possible capacity.

Then we used three X-ray techniques at the Stanford Synchrotron Radiation Lightsource (SSRL). Each one looks at a different length scale.

• X-ray diffraction (XRD), during cycling (operando) and after it (ex situ), follows the distance between the layers.

• X-ray absorption spectroscopy (XAS) follows the oxidation state of nickel and the Ni–O bonds around it.

• Transmission X-ray microscopy (TXM) maps the oxidation state inside single particles, with a resolution of about 30 nm.

What did we find?

Cobalt helps at the beginning. It is electrochemically active, it improves the electronic conductivity, and it delays the mixing of lithium and nickel between the layers. But the protection fades. After 100 cycles the Co-doped cathode kept about 55% of its capacity, and XRD still showed a mixture of phases in the discharged state.

Manganese is in the middle. The collapse at high voltage is more gradual, with a volume change of about 7.7%. The cathode kept about 58% of its capacity.

Aluminum gave the best result, which is interesting because Al does not store any charge. With 5% Al, the layer spacing changed smoothly during the whole charge, and the abrupt H2–H3 step almost disappeared. After 100 cycles this cathode kept 72.5% of its capacity. In the XAS data, about 91% of the nickel signal returned to the discharged state, against roughly half for undoped LNO.

One thing doping did not solve: the TXM images showed microcracks in the particles of all four materials. We imaged only one to five particles per sample, so we cannot say which composition cracks less.

What does it mean?

Each dopant does a different job. Co improves the conductivity, Mn moderates the changes in the oxygen framework, and Al keeps the structure rigid. Al gives up a little capacity at the start and pays it back in cycle life.

For anyone designing a high-nickel cathode, the next question is which combination of these three roles the application needs.

Full paper: E. Espinosa-Villatoro, O. Marques, Z. Cui, P. Hyde, Z. Liang, M. B. Preefer, K. Stone, A. Manthiram, J. Nelson Weker, "Disentangling the role of Al, Co, and Mn dopants in LiNiO₂ cathodes via synchrotron-based probes", Journal of Materials Chemistry A (2026) 14 (42): 28483–28498. DOI: 10.1039/d6ta02574a