Impact of Cation Ordering and Doping on Thermal- and Electrochemical- Stability of High Voltage Spinel Cathodes

Recently, LiNi_0.5Mn_1.5O₄, denoted as LNMO, has attracted a lot of research attention as a promising high-energy density cathode material based on its higher operating voltage at ~4.7V vs. Li⁺/Li compared to the parent material, LiMn₂O₄.[1] On the other hand, the poor cycle and calendar life of LNMO, especially at elevated temperatures, still remain one of the major challenges in its widespread usage. Extensive research has addressed some key factors determining its capacity and rate performance, such as cation ordering, route of synthesis, stoichiometry, heat treatment, particle morphology, particle size, transition metal substitution, and the Li-insertion/de-insertion mechanism of this material.[2-7]

 Unlike the widely studied electrochemical performance and reaction mechanism, the thermal stability of LNMO, which could greatly impact the safety of LIBs, has received little attention. This lack of interest probably could be attributed to the assumption that the excellent thermal stability of the delithiated LNMO can be naturally inherited from its parent material LiMn₂O₄, for which only a subtle structural rearrangement takes place without the oxygen release up to 500 °C in the fully delithiated state.[8] Therefore, Li_xMn₂O₄ has been regarded as a thermally safer cathode material than layered materials, such as Li_xCoO₂, Li_xNi_0.8Co_0.15Al_0.05O₂ and Li_xNi_1/3Co_1/3Mn_1/3O₂. All of these structurally layered materials undergo a series of phase transitions with accompanied oxygen release below 300°C in their charged states. However, for LNMO, what was overlooked at is that when a quarter of the Mn is replaced by Ni, the thermodynamics of the material inevitably changes yielding a very different thermal stability than its parent LiMn₂O₄. Unfortunately, little research has been published on the thermal stability of LNMO materials and their doped derivatives; the research focus has been on their reactivity with the electrolyte using calorimetric measurements thus far.[9-11] There are very few studies correlating thermal stability neither with structural differences (ordered or disordered) nor with oxygen-releasing structure changes during heating for LNMO.

 To further understand the thermal stability of both ordered (o-) and disordered (d-) LNMO in the delithiated state, we applied a combination of in situ synchrotron time-resolved x-ray diffraction (TR-XRD) coupled with mass spectroscopy (MS) and in situ x-ray absorption spectroscopy (XAS) during heating. This combination allowed us to simultaneously monitor the phase transformations (by TR-XRD) and the accompanying gas evolution (e.g., oxygen by MS) as well as the local- and electronic-structural changes with an elemental selective capability (by XAS) during thermal decomposition. Through this systematic investigation, the mechanism of thermal decomposition and the oxygen release behavior of (electrochemically) delithiated d- and o-LNMO during heating have been explored in terms of changes in crystal structures, chemical compositions and valance of the transition metals. We also investigated the impact of doping (e.g., Zn and Fe) on the thermal- and electrochemical cycling- stability of LNMO materials using the above in situ x-ray tools. Some of preliminary results regarding doped LNMO materials will also be presented in the meeting.

Acknowledgement

This work was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies under Contract No. DE-AC02-98CH10886. Use of the National Synchrotron Light Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

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