Implementation of Stable Surface Structures; A Promising Key to Solve Capacity Fading Issues for the High-Voltage LiNi0.5Mn1.5O4
Authors: Yoon Hyojung , Cho Hyung-Man , Qian Danna , Meng Shirley 
Published: February 2020

High-voltage spinel LiNi0.5Mn1.5O4 (LNMO) cathode materials allow three-dimensional lithium-ion diffusion in its lattice, which makes it unique among various high-voltage cathode materials. The structural changes and Mn ion dissolution of the LNMO cathode materials are some of the critical properties affecting its performance degradation, although it is considered to be a promising high-energy and high-power density cathode material for Li-ion batteries.1-3

In this study, our group extensively investigated the structural changes at the surface and bulk, where we adopted polyol process (Fig. 1) as a new method of synthesizing LNMO materials. New findings on the structural stability related to the transition metal (TM) migration are discovered for LNMO synthesized via conventional sol-gel method (SG-) versus polyol process (P-). Fig. 2a presents the relation between specific capacity and cycle number of SG- and P-LNMO. Initially, SG-LNMO has larger capacity, but the capacity of SG-LNMO degrades drastically (>50% loss of its initial capacity) even in 100cycles under high temperature 55 oC.
In-depth local structural variations at the surface and bulk around Mn during cycled-aging were studied via in situ extended X-ray absorption fine structure (EXAFS) spectra. The Fourier-transformed (FT) EXAFS spectra from both transmission electron yield (TEY) and fluorescence yield (FY) mode (k3-weighted in k-space, but not phase-corrected FT, causing shorter bond lengths in the plots than for the actual bonds) at Mn K-edge are studied. For the Mn K-edge results shown in FY mode, there were negligible alterations in both the Mn-O and Mn-TM bond length from the pristine to 20th cycled state, indicating that Mn in both materials is stable and maintains its original structure within the bulk site. On the other hand, the spectral changes during cycled-aging in TEY mode, the shift of the Mn-TM bond length and the emergence of the Mn-TMtet can be observed only on the SG-LNMO. It means the migration of Mn cations (mostly Mn2+) to the surface, which is well-known phase transition phenomenon in LNMO cathode materials. Surprisingly, P-LNMO does show structural and phase stability by looking at the spectra on both at the bulk and surface even after 20th cycled.
The local atomic-level crystal structures were further investigated via aberration-corrected scanning transmission electron microscopy (STEM) taken along the [110] crystallographic direction. Surface regions (~1 nm) of the cycle-aged SG-LNMO particles showed two types of local atomic-level transition metal (TM) ions migration, which is closely related to Mn ion dissolution. TM ions replace tetrahedral Li sites to form a Mn3O4-like structure, and fill the empty octahedral sites to form a rocksalt-like structure, whereas the cycle-aged P-LNMO maintained its original spinel structure. It points to a pathway toward the promising solution to prevent capacity degradation with increased structural stability against Mn dissolution.
We correlated such capacity fading issues to local structural changes and more detailed results of them will be presented at the meeting.

Acknowledgements

H. Yoon and H. Cho acknowledge S. Khalid for technical assistance at the National Synchrotron Light Source for the use of beamline X18B. The double aberration-corrected scanning TEM (TEAM0.5) was performed with an approval of the National Center for Electron Microscopy at the Lawrence Berkeley National Laboratory (Proposal No. 3162).

Reference

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Figure 1