Synthesis and Performance of Disordered Positive Electrode Materials for Lithium Ion Batteries

Disordered rocksalt materials of the form Li_1+xM_y(1-x)N_(1-y)(1-x)O₂have been largely overlooked as a potential candidate for high performance positive electrode materials in lithium ion batteries due to the inability of lithium to percolate through the disordered structure.  Recent works [1, 2] have shown these materials are able to form percolation networks if sufficient lithium and cation mixing is present in the structure. As these materials become more lithium rich, the probability of finding a network of favourable Li channels increases. However, for many transition metal choices for M and N the available redox capacity decreases. This must be considered in order to design compositions for optimal performance.

This work explores these recent theories and looks to improve the design and understanding of disordered materials. Many elemental combination series are synthesized between x = 0.00 to 0.33 in Li_1+xM_y(1-x)N_(1-y)(1-x)O₂ using solid state synthesis methods with varying temperature and excess Li content to explore the role of synthesis in disordering. Choices for M and N include V, Ti, Mn, Fe, Mo, and Cr in order to investigate capacity relationships due to available redox and Li channels.  Materials are characterized by X-Ray Diffraction (XRD), in-situ and ex-situ XRD, scanning electron microscopy, Mossbauer spectroscopy, induced coupled plasma optical emission spectroscopy and electrochemical testing. The structural and electrochemical data is compared with theoretical models based on available redox capacity and percolation theory.

Figure 1 shows that synthesis temperature plays an important role in the disordering process. Figure 2 demonstrates a series of Li_1+xTi_y(1-x)Fe_(1-y)(1-x)O₂ (0.00 ≤ x ≤ 0.25) heated at 800 °C. Electrochemical results were found to agree with current and proposed theories of disordered materials.

Discussion will include the importance of synthesis in disordered materials, as well as the performance of select series and how they compare to theoretical models.

References:

1.  A. Urban, J. Lee, G. Ceder, Adv. Energy Mater., 4 1400478 (2014)

2.  J. Lee, A. Urban, X. Li, D. Su, G. Hautier, G. Ceder, Science, 343 519-522 (2014)