Hanlei Zhang

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.