Layered Ni-rich LiNi_xMn_yCo_zO₂ (NMC, x ≥ 0.6, x+y+z=1) cathode have attracted significant interest as the cathode materials for rechargeable lithium batteries (LIBs) owing to their high capacity, excellent rate capability and low cost. However, some drawbacks such as capacity fade during cycling and low thermal-abuse tolerance at elevated temperature, prohibiting their use in practical batteries. In addition, when in highly charged states, the reduction of Ni ions during thermal heating releases oxygen from the crystal structure, which can lead thermal runaway and violent reactions with the flammable electrolyte.

To overcome performance degradation and improve battery safety, several strategies including lattice doping, surface treatment/modification, tuning the material compositions has been proposed and demonstrated. As one of promising approaches, concentration gradient LiNi_xMn_yCo_zO₂ (CG-NMC) cathode material, in which the Ni concentration decreases and the Mn concentration increases linearly toward the particle surface, has received considerable attention due to its high capacity and improved structural stability. [1-3] The concept of CG-NMC opens many new possibilities for developing high Ni content NMC cathode with high capacity and improved safety characteristic for practical LIB applications. It is well-accepted that the material design of Ni-rich core and Mn-rich surface leads to the high capacity and better thermal stability, compared with normal NMC cathode materials. However, more detailed working mechanisms of CG-NMC cathode have not been fully understood yet.

In this study, the structural and chemical changes of normal NMC and CG-NMC cathode materials with various testing conditions including cycling numbers, different cut-off voltages, thermal heating, will be investigated by using synchrotron based time-resolved X-ray diffraction (TR-XRD), hard and soft X-ray absorption spectroscopy (XAS) techniques. Due to its structural and chemical inhomogeneity of CG-NMC cathode materials, the combined use of various types of characterization techniques is essential. Combinations of TR-XRD, hard and soft XAS techniques allow us to distinguish the electronic and crystal structure differences between the bulk and surface of cathode materials in various conditions. The result will provide valuable information for the development and optimize Ni-rich NMC cathode materials with increased capacity and better thermal stability.

Acknowledgement

The work at Brookhaven National Lab. was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies (BMR and VTO Battery500 projects) under Contract Number DE-AC02-98CH10886. The work was collaborated with UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science Laboratory, is operated under Contract No. DE-AC02-06CH11357.

References

1. Y.-K. Sun, S.-T. Myung, B.-C. Park, J. Prakash, I. Belharouak, and K. Amine, High-Energy Cathode Material for Long-Life and Safe Lithium Batteries, Nat. Mater. 2009, 8, 320.

2. B.-B. Lim, S.-J. Yoon, K.-J. Park, C.-S. Yoon, S.-J. Kim, J. J. Lee, and Y.-K. Sun, Advanced Concentration Gradient Cathode Material with Two-Slope for High-Energy and Safe Lithium Batteries, Adv. Funct. Mater. 2015, 5, 4673

3. U.-H. Kim, E.-J. Lee, C. S. Yoon, S.-T. Myung, Y.-K. Sun, Compositionally Graded Cathode Material with Long-Term Cycle Stability for Electric Vehicles Applications, Adv. Energy Mater. 2016, 1601417.