Mitigating Voltage Fade in Cathode Materials By Improving Atomic Level Spatial Uniformity of Chemical Species

Lithium- and manganese-rich (LMR) layered-structure cathode materials deliver a much higher energy density than traditional cathode materials such as LiMn₂O₄ spinel and LiCoO₂. However, significant challenges, including voltage fade and limited cycle life in LMR cathodes still remain to be overcome prior to their large-scale market penetration.

Here, we report on the direct correlation between voltage fade and atomic level spatial distribution of chemical species among LMR cathode materials (Li[Li_0.2Ni_0.2M_0.6]O₂) prepared by several methods, including co-precipitation (CP), sol-gel (SG) and hydrothermal assisted (HA) methods. Quantitative chemical composition analysis was performed on these materials using large-area X-ray energy dispersive spectroscopy (XEDS) mapping. Meanwhile, we used aberration-corrected scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) to acquire detailed crystallographic data on Li[Li_0.2Ni_0.2Mn_0.6]O₂ cathode prepared by different methods. The atomic contrast in high-angle annular dark field (HAADF) STEM imaging identified the details of stacking and ordering in the particles while EELS chemical analysis revealed important chemistry information. We found that the materials prepared by the CP and SG methods exhibit a relatively high propensity for Ni segregation, leading to fast voltage fade and poor cycling stability. However, material prepared by the HA method exhibits very uniform Ni distribution and greatly reduced voltage fade. To the best of our knowledge, this is the first direct atomic-scale evidence to correlate atomic level cation uniformities with the electrochemical performance of cathode materials.   More importantly, the voltage fade and energy degradation of LMR cathode materials can be significantly mitigated by improving the uniformity of chemical species in atomic level that are strongly affected by the preparation methods and synthesis conditions.  These results also shine the light on the current debate regarding to the average/local structure of LMR cathode materials.  LMR cathodes prepared by co-precipitation and sol-gel methods exhibit significant Ni segregation and dominated by LiMO₂ R-3m phase, where the preferential segregation of Ni blocks lithium ion diffusion channels, weaken nickel-manganese interactions, lead to easy reduction of the manganese ions and fast voltage/capacity fade.  In contrast, LMR cathodes prepared by the hydrothermal assisted method form a solid solution dominated by Li₂MO₃C2/m monoclinic symmetry with largely suppressed Ni segregation and are more stable against excessive lithium ion removal, enhance Ni-Mn interaction and stabilize crystal structure, leads to greatly reduced voltage fade and excellent cycling stability of LMR cathodes. Therefore, LMR cathodes with uniform distribution of chemical species (minimal Ni segregation) are very promising for use in high-energy Li-ion batteries for large-scale practical applications. The fundamental correlation between the atomic level spatial distribution of the chemical species and the functional stability of the materials found in this work also provides new perspective on the design and development of other functional materials with significantly enhanced stability.

Acknowledgements

This work is supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U. S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 18769, under the Batteries for Advanced Transportation Technologies program. The microscopic study described in this paper is supported by the Laboratory Directed Research and Development Program as part of the Chemical Imaging Initiative at Pacific Northwest National Laboratory (PNNL). The work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830. The authors also would like to thank M. M. Thackeray for useful discussions.