BATT Research Highlight: Designing High-Performance, High-Energy Cathode Materials
The Batteries for Advanced Transportation Technologies (BATT) Program is the premier fundamental research program in the U.S. for developing high-performance, rechargeable batteries for electric vehicles (EVs) and hybrid-electric vehicles (HEVs). BATT is supported by the U.S. Department of Energy Office of Vehicle Technologies. BATT investigators in top research universities and institutions work on the following task areas: Anodes, Cathodes, Liquid Electrolytes, Solid Electrolytes, Cell Analysis, Diagnostics, Modeling, lithium-air batteries and sodium-ion batteries. BATT funds research at institutions throughout the U.S. The BATT program publishes a quarterly report, from which the following research highlight is adapted.
Although the performance of electric vehicles continues to improve, the high cost of automotive batteries is still hindering their market penetration. To reduce costs and expand the market, researchers continue to explore avenues to increase battery energy density by developing cathode materials with higher voltages and/or higher capacities without compromising safety or cycle life.
The capacities of technically important electrode materials with layered structures such as NMCs (lithium nickel manganese cobalt oxides) can be improved by increasing the charging voltage limit. However, to prevent degradation during cycling to these high voltages requires: (1) reducing reactivity with the electrolyte, and/or (2) modifying the materials to improve their robustness at high states of charge. Lawrence Berkeley National Laboratory (Berkeley Lab) researcher Marca Doeff's team is pursuing these solutions by focusing on three goals: (1) exploring the feasibility of increasing utilization of NMC materials by substituting some of the cobalt (Co) with titanium (Ti), (2) using higher voltage cutoffs during cycling; and (3) working to better understand the origins of Ti-substitution effects in NMCs.
The Berkeley Lab team conducted a series of experiments in collaboration with researchers at the Stanford Synchrotron Radiation Lightsource, Brookhaven National Laboratory, and the University of California, Berkeley, to understand the surface and bulk characteristics of baseline and Ti-substituted NMCs. The materials were synthesized either by classic co-precipitation methods or by spray pyrolysis.
The NMC cathodes typically cycle well in lithium (Li) half-cells when an upper voltage limit of 4.3 V is used, but they lose capacity rapidly when cycled to higher voltages. Previous results indicated that Ti-substitution ameliorates the fading observed during high voltage cycling to some extent, so this work focused on understanding the phenomena responsible for the poor high-voltage performance, particularly at particle surfaces, and the role of Ti in ameliorating capacity fading.
Synchrotron x-ray absorption spectroscopy measurements showed that there is a gradient of oxidation states extending from the surface into the bulk, with the more reduced species at the surface. High resolution transmission electron microscopy revealed the formation of a rock salt phase on particle surfaces, which, along with a more diffuse reaction layer consisting of lithium fluoride and organic matter on NMC particle surfaces, is thought to be responsible for the rise in impedance and rapid capacity fading observed in the cells charged to high voltages. Surface reconstruction also occurs to some extent after long exposures to electrolytic solution even without electrochemical cycling, and increases with increasing cycle number. The degree to which it occurs depends on how materials were synthesized and cycling regimes, with more severe changes observed for materials cycled potentiostatically rather than galvanostatically. First principles calculations show that the tendency to form rock salt increases as more lithium is removed from the structure. At a lithium content of about 0.4 in baseline NMCs, rock salt is predicted to form. However, substitution with 2 percent Ti has a stabilizing influence, delaying the onset of transformation.
The team also conducted successful spray pyrolysis synthesis of NMCs, creating uniform morphologies of hollow spherical particles composed of nanometric primary particles. The electrochemical performance of the spray-pyrolyzed materials equals or surpasses that of materials made by co-precipitation. The reason for this improvement is, as yet, unknown, but it may involve the formation of conductive phases at particle-particle interfaces that promote the diffusion of lithium ions and electrons.
Future work will focus on the synthesis of Ti-substituted NMCs by spray pyrolysis, preparation of coated particles using atomic and molecular layer deposition, and an investigation of the efficacy of these strategies, using microscopy and synchrotron x-ray techniques.
Papers and Journal Articles
Design of High Performance, High Energy Cathode Materials. Marca M. Doeff. 2014 DOE Annual Peer Review Meeting Presentation.
"Influence of Synthesis Conditions on the Surface Passivation and Electrochemical Behavior of Layered Cathode Materials." Feng Lin, Dennis Nordlund, Taijun Pan, Isaac Markus, Tsu-Chien Weng, Huolin Xin, and Marca M. Doeff. J. Mater. Chem. A DOI: 10.1039/C4TA04497E (2014).
"Computational and Experimental Investigation of Ti Substitution in Li1(NixMnxCo1-x-yTiy)O2 for Lithium-Ion Batteries." Isaac Markus, Feng Lin, Kinson Kam, Mark Asta, and Marca Doeff. J. Phys. Chem. Lett. 5, 3649 DOI: 10.1021/jz5017526 (2014).
"Profiling the Nanoscale Gradient in Stoichiometric Layered Cathode Particles for Lithium-ion Batteries." Feng Lin, Isaac M. Markus, Dennis Nordlund, Tsu-Chien Weng, Huolin L. Xin, and Marca M. Doeff. Energy & Environ. Sci. 7, 3077 DOI: 10.1039/C4EE01400F (2014).
"Chemical and Structural Stability of Lithium-Ion Battery Electrode Materials under Electron Beam." Feng Lin,Isaac M. Markus, Marca M. Doeff,and Huolin L. Xin. Scientific Reports, 4, 5694; DOI:10.1038/srep05694 (2014).
"Surface Reconstruction and Chemical Evolution of Stoichiometric Layered Cathode Materials for Lithium-Ion Batteries." Feng Lin, Isaac Markus, Dennis Nordlund, Tsu-Chien Weng, Mark Asta, Huolin L. Xin, and Marca M. Doeff. Nature Commun. 5:3529, DOI: 10.1038/ncomms4529 (2014).
2014 Advanced Light Source Annual User Meeting. (Oct 7-Oct 8, 2014), Berkeley, California. Invited oral presentation by Feng Lin. "Revealing the Materials Transformation at Complementary Length Scales for High Performance Lithium Ion Batteries."
248th ACS National Meeting & Exposition. (Aug 10-14, 2014), San Francisco, California, Invited oral presentation by Feng Lin. "Characterization at Complementary Length Scales: Fundamental Correlation between Structure and Electrochemistry of Electrode Materials in Lithium-ion Batteries." Feng Lin, Isaac Markus, Dennis Nordlund, Tsu-Chien Weng, Mark Asta, Huolin Xin and Marca Doeff.