Structural Defects and the Impact of Battery Ageing - How Sensitive Are Entropy Profile Measurements, and What Do They Show?

We report on entropy profile measurements at lithium ion full and half cells.¹ Those profiles are derived from voltage responses on temperature variations for various states of charge. For LiCoO₂ cathodes, measured in half cells, we demonstrate that the entropy profiles are sensitive to particle sizes as varied through the synthesis method, including calcination (Fig. 1). The origin of those variations is further elucidated by in-situ XRD studies and thermodynamic calculations. We discuss, in how far those variations reflect an impact of the particle sizes on the stability of ordered and disordered phases, or rather are related to defects in the crystal structure. The density of such defects may as much depend on the synthesis route as the particle sizes do.
Furthermore, we demonstrate that entropy profile measurements are suitable to track the impact of cycle and thermal ageing² on full cells. We show how full cell entropy profiles of new and aged cells can be calculated based on the half cell profiles of the fresh electrode materials. As confirmed by in-situ XRD measurements of new and aged cathodes (here: NMC and NCA), the changes observed in the entropy profiles can be explained without considering changes in the crystal structure of anode or cathode.

¹ P. J. Osswald, M. del Rosario, J. Garche, A. Jossen, H. E. Hoster, Electrochimica Acta, DOI: 10.1016/j.electacta.2015.01.191
² K. Maher, R. Yazami, J. Power Sources 261 (2014) 389