A Comparison of Electrode Surface Films Formed with Different Oxide Cathodes for Lithium-Ion Batteries

Abstract

High-nickel, Li-rich, and high-voltage spinel electrodes are lithium-ion battery cathodes commonly considered as candidates for near-term commercialization. Each of these cathode materials can offer higher energy densities than conventional lithium cobalt oxide cathode materials, at least during initial cycles. High-Ni cathodes of the form LiNiaMnbCo1-a-bO2 have the same layered structure as current commercial cathodes with alternating transition metal and lithium layers between oxide slabs. With increasing Ni content, High-Ni cathode materials generally can provide higher discharge capacities, reaching upwards of 240 mAh g-1. Li-rich electrode materials (Li1+xMO) have a similar layered structure as High-Ni materials but include excess Li+ in the transition metal layer that can be accessed for much higher discharge capacities, even exceeding 300 mAh g-1. High-voltage spinel electrodes of the form LiMnaNi2-aO4 operate at high voltages providing high energy densities, though at much lower discharge capacities, ca. 140 mAh g-1. While all three of these electrode classes offer exciting possibilities for future lithium-ion battery development, they all suffer from the inherent instability of their surface reactivities. High-Ni cathodes suffer from high surface reactivity and a slow growth of the cubic rocksalt phase throughout cycling that impedes lithium intercalation. The activation of Li-rich electrodes involves a complex surface passivation involving the evolution of molecular oxygen and the formation of a spinel-like phase. High-voltage spinel electrodes operate outside the electrolyte stability region, so also suffer from rapid oxidative breakdown of electrolyte components and impedance growth. Aside from the cathode interphase, the solid electrolyte interphase formed at the anode can be affected by the surface chemistry of these various cathodes; for instance, transition-metal ions may dissolve from the cathode, migrate to the anode side where they are reduced, and act as a catalyst for further electrolyte reduction. This presentation describes our recent characterization of cathode and anode surface electrolyte interphases from cells with High-Ni, high-voltage spinel, and Li-rich electrodes. Half cells vs. lithium anodes, as well as full cells with up to 1,500 cycles are presented. For these studies, we employ time of flight – secondary ion mass spectrometry (TOF-SIMS), a powerful characterization technique pioneered by our group for characterization of battery materials. TOF-SIMS provides sub-nanometer depth resolution of specific chemical fragments over micron-sized image areas that can also be used in a depth profiling method, through layer-by-layer ablation of surface species to form a 3D image (Figure 1). TOF-SIMS is used in conjunction with standard characterization techniques, such as X-ray photoelectron spectroscopy (XPS), galvanostatic cycling data, and electrochemical impedance spectroscopy. Figure 1: A 3D TOF-SIMS image of cycled high-Ni cathode particles with transition metals indicated in green, as well as decomposed organics (C-, CH-, CH2 - ) indicated in red. Figure 1