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A new study involving researchers from the Chemistry Department has shed light on how the internal structure of high-nickel battery materials influences their ability to withstand repeated high-voltage charging. The researchers found that shaping these materials into tiny nanorods helped their crystal structure recover more effectively after charging, offering new insights into the development of higher-energy, longer-lasting batteries.

As demand grows for batteries that can store more energy, researchers are exploring ways to improve the materials used inside lithium-ion batteries. This is particularly important for electric vehicles, where increasing the amount of energy a battery can store could help extend driving range without increasing its size or weight. High-nickel materials are promising candidates because they can deliver high energy densities, yet maintaining their performance over repeated charging cycles remains a challenge.

One of the main difficulties occurs when these materials are charged to higher voltages. As a battery charges, the crystal structure of its positive electrode material undergoes a series of changes. At high voltages, it undergoes a particularly significant transformation known as the H2 to H3 phase transition, which causes the layers within the crystal structure to contract substantially. While limiting the charging voltage can reduce this structural stress, it also prevents the battery from making full use of the material’s potential energy.

In their paper, ‘, published in the journal Nature Communications, researchers led by the Chemistry Department’s Dr Xiao Hua, with Dr Hekang Zhu as the first author, set out to investigate whether changing the structure of the electrode material could help overcome this challenge. Funded by the Faraday Institution, the team produced high-nickel materials in the form of tiny nanorods and compared them with conventional microcrystalline materials of the same composition.

The results showed a clear difference between the two structures. At a voltage range of 2.7-4.7V, the nanorod materials retained around 93-95 per cent of their capacity after 100 charging cycles, compared with 33 per cent for one conventional material and 75 per cent for the other.

To understand why the nanorods performed better, the team examined the materials before and after cycling. Microscopy showed that the improvement was not simply due to preventing particles from cracking. Instead, detailed analysis of the crystal structure provided a different explanation.

Both materials underwent the H2 to H3 phase transition at high voltage, but the nanorods were better able to recover their original crystal structure when the battery was discharged. In the conventional materials, some of these structural changes became irreversible, contributing to capacity loss and limiting the voltage at which they could be operated.

The findings therefore show that improved structural reversibility within the bulk of the material is a key factor behind the enhanced cycling performance of the nanorods. The team also demonstrated that the nanorod approach could be applied to high-nickel sodium-ion materials in the future.

The research highlights how controlling the structure of battery materials at a very small scale can influence how they respond to high-voltage charging. By improving understanding of the structural changes that contribute to battery degradation, the study provides new insights into the development of higher-energy and more durable battery materials for future energy technologies, including those used in electric vehicles.

“We are very excited by these results because they show that structural changes in high-nickel cathodes at high voltages do not necessarily have to lead to rapid degradation”, said Dr Xiao Hua. “By controlling the material structure and morphology, we can make these transformations much more reversible, allowing the cathode to recover and maintain its performance over prolonged cycling. Moving forward, we hope to build on this understanding to develop high-nickel cathode materials that combine higher energy density with the long-term stability needed for practical lithium-ion batteries.”