Lithium-ion batteries have aided the revolution in portable electronic devices, and they are now being intensively pursued for electric vehicles and grid storage. Cost, safety, cycle life, energy, power, and environmental impact are the major factors for adopting a specific battery chemistry for a specific application, and often tradeoffs among the factors are needed in the selection. Although the layered LiMO2
(M = Ni and Co) oxides with mixed lithium-ion and electronic conductivity were first demonstrated more than 30 years ago as a lithium-insertion host, they still remain to be the dominant candidate for lithium-ion technology due to several appealing features displayed by them. With an aim to increase the charge-storage capacity and energy density, layered oxides with a high Ni content (> 50%) are now at the forefront in the battery industry around the world. However, compositions with high Ni content are hampered by high surface reactivity with the organic liquid electrolyte and the consequent impedance rise and fast capacity fade during cycling. Furthermore, they also suffer from high surface reactivity with ambient air, forming lithium hydroxide and lithium carbonate on the particle surface and thereby hampering the electrode fabrication process. A firm fundamental understanding of the origin of the fast capacity fade with advanced characterization methodologies and design and development of nickel-rich cathode compositions with robustness in contact with the liquid electrolyte and ambient air are critically needed to make the technology practically move forward. Accordingly, this presentation will focus on the characterization of nickel-rich layered oxides with advanced surface techniques and development of less reactive compositions with robust surface structures.
To develop a fundamental understanding of the degradation mechanisms, the nickel-rich layered oxide cathodes are investigated before and after long-term cycling in full cells with X-ray photoelectron spectroscopy (XPS), time-of-flight – secondary ion mass spectroscopy (TOF-SIMS), and high-resolution transmission electron microscopy. In-depth surface characterization as a function of sputtering time and cross-sectional analyses reveal that surface active mass dissolution caused by the acidic species in the electrolyte during cell operation and migration of dissolved metal ions from the cathode to the graphite anode cause impedance rise at the graphite anode, lithium dendrite growth, and capacity fade. Utilizing the in-depth understanding gained, stabilized full cells are designed and the degradation is being mitigated. Promise of cathode compositions with Ni contents as high as 94% and capacities as high as ~ 220 mAh/g with high power capability will be presented. The progress paves the way to design and develop high-power lithium-ion cells with high volumetric and gravimetric energy densities for portable, vehicle, and grid storage applications.