Investigating Local Degradation and Thermal Stability of Charged Nickel-Based Cathode Materials through Real-Time Electron Microscopy

Abstract

In this work, we take advantage of in situ transmission electron microscopy (TEM) to investigate thermally induced decomposition of the surface of Li(x)Ni(0.8)Co(0.15)Al(0.05)O2 (NCA) cathode materials that have been subjected to different states of charge (SOC). While uncharged NCA is stable up to 400 °C, significant changes occur in charged NCA with increasing temperature. These include the development of surface porosity and changes in the oxygen K-edge electron energy loss spectra, with pre-edge peaks shifting to higher energy losses. These changes are closely related to O2 gas released from the structure, as well as to phase changes of NCA from the layered structure to the disordered spinel structure, and finally to the rock-salt structure. Although the temperatures where these changes initiate depend strongly on the state of charge, there also exist significant variations among particles with the same state of charge. Notably, when NCA is charged to x = 0.33 (the charge state that is the practical upper limit voltage in most applications), the surfaces of some particles undergo morphological and oxygen K-edge changes even at temperatures below 100 °C, a temperature that electronic devices containing lithium ion batteries (LIB) can possibly see during normal operation. Those particles that experience these changes are likely to be extremely unstable and may trigger thermal runaway at much lower temperatures than would be usually expected. These results demonstrate that in situ heating experiments are a unique tool not only to study the general thermal behavior of cathode materials but also to explore particle-to-particle variations, which are sometimes of critical importance in understanding the performance of the overall system.

Structural studies of Li1.2Mn0.55Ni0.15Co0.1O2 electrode material

“…[32, 33] This behavior is also similar to the effects caused by heating. [34] Accordingly, when imaging these materials the dose should be limited by operating at lower beam currents (dose rate) and reducing total exposure time (dose). During sample preparation preferably select minimally invasive techniques, or when using an invasive technique like ion milling identify best practices to minimize damage or remove damage through additional processing (e.g., chemical etches).…”

Narrowing the Gap between Theoretical and Practical Capacities in Li‐Ion Layered Oxide Cathode Materials

“…Experiments have found a tendency for Li(Ni 1−y Al y )O 2 to segregate into coherent Ni-rich and Al-rich domains for nondilute Al concentrations (y ≥ 0.1). Transformations to rocksalt/spinel-like structures have been extensively observed using S/TEM [173][174][175] and X-ray absorption spectroscopy (XAS). [172] NCA nevertheless experiences capacity fade associated with surface reactions and mechanical degradation.…”

Unraveling the Degradation Process of LiNi0.8Co0.15Al0.05O2 Electrodes in Commercial Lithium Ion Batteries by Electronic Structure Investigations

“…8,9,17,18 An increase in the impedance is associated with these structural changes. 6 It is assumed that the impedance rise is caused by a hindrance of the lithium-ion diffusion through the altered surface structure.…”

“…[36][37][38] Nevertheless, it was found that oxygen loss takes place in LNCAO at lithium contents x lower than 0.4. 18,35 This can also lead to a NiO like structure.…”