Revealing the Nanoscopic Corrosive Degradation Mechanism of Nickel-Rich Layered Oxide Cathodes at Low State-of-Charge Levels: Corrosion Cracking and Pitting.

Ni-rich layered oxides have received significant attention as promising cathode materials for Li-ion batteries due to their high reversible capacity. However, intergranular and intragranular cracks form at high state-of-charge (SOC) levels exceeding 4.2 V (vs. Li/Li+), representing a prominent failure mechanism of Ni-rich layered oxides. The nanoscale crack formation at high SOC levels is attributed to a significant volume change resulting from a phase transition between the H2 and H3 phases. Herein, in contrast to the electrochemical crack formation at high SOC levels, another mechanism of chemical crack and pit formation on a nanoscale is directly evidenced in fully lithiated Ni-rich layered oxides (low SOC levels). This mechanism is associated with intergranular stress corrosion cracking, driven by chemical corrosion at elevated temperatures. The nanoscopic chemical corrosion behavior of Ni-rich layered oxides during aging at elevated temperatures is investigated using high-resolution transmission electron microscopy, revealing that microcracks can develop through two distinct mechanisms: electrochemical cycling and chemical corrosion. Notably, chemical corrosion cracks can occur even in a fully discharged state (low SOC levels), whereas electrochemical cracks are observed only at high SOC levels. This finding provides a comprehensive understanding of the complex failure mechanisms of Ni-rich layered oxides and provides an opportunity to improve their electrochemical performance.

Plane-Selective Coating of Li2SnO3 on Li[NixCo1-x]O2 for High Power Li ion Batteries.

Interphase engineering is becoming increasingly important in improving the electrochemical performance of cathode materials for rechargeable batteries, including Li ion, Li metal, and all-solid-state batteries, because irreversible surface reactions, such as electrolyte decomposition, and transition metal dissolution, constitute one of these batteries' failure modes. In this connection, various surface-engineered cathode materials have been investigated to improve interfacial properties. No synthesis methods, however, have considered a plane-selective surface modification of cathode materials. Herein, we introduce the basal-plane-selective coating of Li2SnO3 on layered Li[NixCo1-x]O2 (x = 0 and 0.5) using the concept of the thermal phase segregation of Sn-doped Li[NixCo1-x]O2 due to the solubility variation of Sn in Li[NixCo1-x]O2 with respect to temperature. The plane-selective surface modification enables the formation of Li2SnO3 nanolayers on only the Li[NixCo1-x]O2 basal plane without hindering the charge transfer of Li+ ions. As a result, the vertical heterostructure of Li[NixCo1-x]O2-Li2SnO3 core-shells show promising electrochemical performance.