Energy storage has in recent times become a critical area of research due to the widespread adoption of portable electronic devices, and the need to derive more energy from sources other than fossil fuels. Alternative, renewable energy sources such as wind or solar are only able to produce power intermittently and thus the development of large-scale energy storage systems and the associated infrastructure to deliver a constant supply of energy will be key to a successful transition away from fossil fuels.[1, 2]

Lithium-ion batteries (LIBs) appear ubiquitously in a variety of electronic devices where it is favoured for its high energy density and dependable cycling characteristics. Although recent work has also shown the viability of other battery systems, e.g. Na-ion, K-ion, Li-air, the number of publications regarding lithium-ion battery systems continues to outnumber any other battery chemistry,[3] which highlights that the LIB system is still an area with room for further innovation despite it being a well-established technology. In commercial LIBs, graphite has remained the negative electrode material of choice since first generation LIBs, where graphite was coupled with a LiCoO₂ positive electrode. Second generation systems involving LiFePO₄ (LFP) positive electrodes were later developed, and more recently third generation systems which utilise Li(Ni_xMn_yCo_z)O₂ or Li(Ni_xCo_yAl_z)O₂ positive electrodes are beginning to be deployed in commercially produced LIBs.[4, 5] These systems have been optimised by their commercial vendors for electrochemical performance and production, however there is significant room for further investigation in regards to how these materials evolve and perform in devices, especially under different conditions or extreme conditions. Ex situ data and recent in situwork undertaken by our group has provided some insight,[6-9] however many open questions about battery performance and its relation to electrode materials remain unanswered. For example: phase evolution as a function of current rate, temperature, and transition metal composition (i.e., Ni:Co:Mn ratio), overcharging/undercharging and the structural consequences, or the influence of nano-sizing to phase evolution and its relationship to applied current. It is therefore important to characterise these systems thoroughly in order to find methods by which to improve their performance, e.g. optimal transition metal ratios and voltage cut-offs, and provide a foundation for the next-generation of LIBs which will be required to meet the growing demand for portable electronic devices.

To answer the questions proposed above, we use in operando neutron diffraction to study fully-functioning, unmodified devices. Neutron diffraction is ideal for performing operando experiments as the neutron beam can easily penetrate the battery casing and components, and the high intensity allows for short collection times which provides data with excellent time resolution. By correlating the electrochemical data with the diffraction data we are able to establish relationships between the electrochemical state of the device and the structural evolution taking place. Here we present findings from recent studies of commercial LIBs with different cathode materials. We have investigated devices using LiFePO₄ , where we were able to establish a novel relationship between the structural kinetics of the electrode materials and the cycling history of the battery,[10] as well as devices using Li(Ni_xMn_yCo_z)O₂with various Ni:Mn:Co ratios in order to compare their performance and structural stability at high voltage.[7]

[1] H. Pan, Y.-S. Hu, L. Chen, Energy & Environmental Science, 6 (2013) 2338.

[2] D. Lindley, Nature, 463 (2010) 18.

[3] J.C. Pramudita, D. Sehrawat, D. Goonetilleke, N. Sharma, Advanced Energy Materials, (2017) 1-20.

[4] M.S. Whittingham, Chemical Reviews, 104 (2004) 4271-4302.

[5] J.W. Fergus, Journal of Power Sources, 195 (2010) 939-954.

[6] J. Li, R. Petibon, S. Glazier, N. Sharma, W.K. Pang, V.K. Peterson, J.R. Dahn, Electrochimica Acta, 180 (2015) 234-240.

[7] R. Petibon, J. Li, N. Sharma, W.K. Pang, V.K. Peterson, J.R. Dahn, Electrochimica Acta, 174 (2015) 417-423.

[8] N. Sharma, W.K. Pang, Z. Guo, V.K. Peterson, ChemSusChem, 8 (2015) 2826-2853.

[9] N. Sharma, V.K. Peterson, Journal of Solid State Electrochemistry, 16 (2012) 1849-1856.

[10] D. Goonetilleke, J.C. Pramudita, M. Hagan, O.K. Al Bahri, W.K. Pang, V.K. Peterson, J. Groot, H. Berg, N. Sharma, Journal of Power Sources, 343 (2017) 446-457.