How pressure improves battery materials in ball milling

Ball milling

Ball milling has been widely used to make next-generation battery materials, such as high-capacity electrode materials. However, until now, scientists did not fully understand how ball milling affects the synthesis of these materials.

The researchers discovered that the collisions between the balls and the materials create an important pressure effect in causing the changes. They also found that some of these changes are reversible by applying heat, indicating that pressure is a crucial variable in the process.

Professor Peter Slater, a Professor of Materials Chemistry and Co-Director of the Birmingham Centre for Energy Storage at the University of Birmingham, said: "We stumbled upon this discovery by accident. We were using lithium molybdate as a model system to study oxygen redox in batteries, and we noticed that it transformed into a high-pressure spinel polymorph, a specific crystal structure that had only been made under very high pressure before.

"We realized that local heating alone could not account for this transformation. We repeated the experiment with three other battery materials to confirm our hypothesis and got similar results. This showed us that local heating was not the only factor involved in these changes."

High-pressure spinel polymorphs

The researchers also demonstrated that ball milling can produce high-pressure spinel polymorphs of other compounds, such as Li2MoO4, which generally require very high temperature and pressure conditions to form. These polymorphs have better properties for battery applications than their conventional counterparts.

Dr Elizabeth Driscoll, a co-author of the study, said: "This discovery opens up new opportunities for battery manufacturers to develop cheaper and more energy efficient processes, as well as to explore new materials. For example, we also found that ball milling can create disordered rocksalt phases, which could be the key to making better-performing batteries.

"This improved understanding of the effect of ball milling on battery materials is fascinating for researchers in this field, but also for the future of battery development. We showed that we can achieve transformations normally requiring expensive and specialized equipment with just five minutes of ball milling.

Expanding our knowledge of battery technology remains crucial in transitioning to a more electric-dependent future aimed at reducing pollution and achieving net-zero emissions. Dr. Elizabeth Driscoll added that this newfound understanding opens doors to a realm of possibilities and discoveries, potentially contributing to a greener and more sustainable future for all.

The study was published in the journal RSC Energy Environmental Science.

Study abstract:

Synthesis of Li-ion battery materials via ball milling has been a huge area of growth, leading to new high-capacity electrode materials, such as a number of promising disordered rocksalt (DRS) phases. In prior work, it was generally assumed that the synthesis was facilitated simply by local heating effects during the milling process. In this work, we show that ball milling Li2MoO4 leads to a phase transformation to the high-pressure spinel polymorph and we report electrochemical data for this phase. This observation of the formation of a high pressure polymorph shows that local heating effects alone cannot explain the phase transformation observed (phenakite to spinel) and so indicates the importance of other effects. In particular, we propose that when the milling balls collide with the material, the resulting shockwaves exert a localised pressure effect, in addition to local heating. To provide further support for this, we additionally report ball milling results for a number of case studies (Li2MnO3,Li2SnO3, Nb2O5) which reinforces the conclusion that local heating alone cannot explain the phase transformations observed. The work presented thus provides greater fundamental understanding of milling as a synthetic pathway and suggests potential strategies to prepare such samples without milling (e.g., doping to create internal chemical pressure). In addition, we suggest that further research is needed into the effect of the use of milling as a route to smaller particles, since we believe that such milling may also be affecting the surface structure of the particles through the influence of the shockwaves generated.