Khim Karki

TUNING THE ACTIVITY OF OXYGEN IN LINI0.8CO0.15AL0.05O2 BATTERY ELECTRODES

Layered transition metal oxides such as LiNi0.8Co 0.15Al0.05O2 (NCA) are highly desirable battery electrodes. However, these materials suffer from thermal runaway caused by deleterious oxygen loss and surface phase transitions when in highly overcharged and overheated conditions, prompting serious safety concerns. Using in situ environmental transmission electron microscopy techniques, we demonstrate that surface oxygen loss and structural changes in the highly overcharged NCA particles are suppressed by exposing them to an oxygen-rich environment. The onset temperature for the loss of oxygen from the electrode particle is delayed to 350 °C at oxygen gas overpressure of 400 mTorr. Similar heating of the particles in a reducing hydrogen gas demonstrated a quick onset of oxygen loss at 150 °C and rapid surface degradation of the particles. The results reported here illustrate the fundamental mechanism governing the failure processes of electrode particles and highlight possible strategies to circumvent such issues.

In-Situ TEM Study of Phase Evolution in Individual Battery Materials

There has been significant interest in understanding the mechanism of structural phase evolution occurring in individual battery materials at different state of charge (SOC) levels and at various environmental constraints [1, 2]. For example, a commercially-important LiNi0.8Co0.15Al0.05O2 (NCA) cathode material when over-discharged (> 4.2 V) and over-heated can lead to the loss of stoichiometric oxygen from the surface [1, 2]. The loss of oxygen is detrimental as it can react with inflammable liquid electrolyte and cause thermal runaway. Furthermore, the loss of oxygen is accompanied by the migration/re-ordering of transition metal (TM) ions, which leads to complex phase transformation: (R3̅m) → disordered spinel (Fd3̅m) → disordered rock salt (Fm3̅m). The spinel/rock-salt phase that forms on the surface increases the impedance and degrades the electrochemical activity of the electrode. The local probing of the structural and chemical changes that occur within the individual battery material is thus important. Conventional X-ray techniques are insensitive to localized phase transformation, as they provide only average information from ensemble of particles. In-situ environmental transmission electron microscopy (ETEM) provides a unique platform where individual nanoparticles can be investigated for any morphological, structural or chemical changes, under external stimuli, in real-time [3]. Furthermore, the aberration-corrected ETEM with a differential pumping apparatus allows high spatial resolution of < 0.1 nm even in a high-pressure gas environment (e.g., O2, H2) in the system.

The Intermediate State of the Layered → Spinel Phase Transformation in LiNi0.80Co0.15Al0.05O2 Cathode

Layered LiNi0.80Co0.15Al0.05O2 (NCA) is a promising cathode material for lithium ion batteries (LIBs), which has a high rate capability, a long lifetime and theoretically a high specific capacity. The aluminum addition prevents the NCA layered structure from collapsing into an inactive rock-salt phase, but it also accelerates the spinel phase transformation. The spinel phase formed in the surface region increases the impedance of NCA, reduces the electrochemical activity and diminishes the overall capacity.

ETEM Study of Oxygen Activity in LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) Cathode Materials at Various States of Charge

Lithium ion batteries (LIBs) have been predominantly used in the consumer electronics and other power devices. The drive to use LIBs in the large-scale applications such as electric vehicles (EVs) and smart grids has spurred significant research activity, particularly concerning the cathode electrodes. The current cathode materials, LiCoO2 (~140 mAh/g) or LiFePO4 (~160 mAh/g), have relatively low energy densities, and can hardly match with the capacities of the next generation of anode materials (e.g. Si, ~4200 mAh/g). To improve upon this limitation, layered materials such as LiNi0.8Co0.15Al0.05O2 (NCA) & LiNixMn1-x-yCoyO2 (NCM) – which have discharge capacity ~200 mAh/g – have been actively pursed as potential replacements. However, these cathode materials suffer from rapid capacity fade and poor thermal instability, thus raising serious safety concerns. For example, these materials in a highly delithiated state (overcharged) can readily release oxygen at high temperature, and lead to complex phase transitions: layered (R-3m)  disordered spinel (Fd-3m)  rock-salt (Fm-3m). The released O2 can react with the flammable electrolyte, leading to thermal-runaway and catastrophic battery failure. Therefore, it is critical to understand the role that the oxygen release plays in the migration of transition metal (TM) cations (Ni, Co, & Mn) during the various phase transition processes. Environmental transmission electron microscopy (ETEM) provides a unique platform where individual nanoparticles can be investigated for any morphological, structural or chemical changes, under external stimuli, in real-time. Furthermore, the aberration-corrected ETEM with a differential pumping apparatus allows high spatial resolution of < 0.1 nm even in a high-pressure gas environment (e.g., O2, H2) in the system. Here, we use in-situ ETEM to understand the role that oxygen plays in the rearrangement of the TM ions both at the surface & in the bulk of the NCA materials at elevated temperatures.