Stabilizing Lattice Oxygen in Slightly Li-Enriched Nickel Oxide Cathodes Toward High-Energy Batteries
Authors: Wang Han , Zhou Tong , Wang Yong , Zhang Wei , Li Linsen 
Published: November 2022

LiNiO2 (LNO, 100% Ni) is an old material first identified in the early 1990s (as a higher-capacity and lower-cost alternative to LiCoO2) but has yet to fulfill its potential. Despite intense research efforts for more than two decades, LNO still exhibits rapid capacity loss during cycling and poor thermal stability (1-3). The traditional LNO is generally prepared by solid-state reactions and recognized as Li-deficient Li1-yNi1+yO2 (4). Ex situ characterizations indicated that the performance degradation originates from the detrimental phase transition (layered to rock-salt structure) during electrochemical cycling (5-7), which is closely related to the lattice-oxygen release during charge (8, 9). To improve the performance of LNO, the structure is commonly modified by lattice doping or surface coating, which have led to improved cycle stability but at the cost of capacity loss (10). Meanwhile, these modification approaches failed to address the lattice oxygen instability, as the O2 release was still detected for the doped or surface-coated layered cathodes (11, 12).
Here, we demonstrate an Li-enrichment strategy to produce a trigonal-structured layered Li-enriched LNO (Li1.04Ni0.96O2, LR-LNO) with a slight excess of Li to occupy the Ni sites, which is a possible phase according to the Li-Ni-O phase diagram but has never been experimentally synthesized (Figure 1). LR-LNO (Figure 1) enables a combination of a high specific energy density of 904 Wh kg-1, outstanding cyclability (~80% capacity retention after 400 cycles in full cells versus 35 cycles for LNO), and significantly enhanced thermal stability (>70 °C increase in thermal-runaway temperature over LNO).
We further designed a double-tilt electrochemical liquid cell inside a transmission electron microscope (TEM) to track the local structural changes at the surface of individual particles during galvanostatic cycling (Figure 2), revealing the performance-enhancing mechanism behind the slight change in the material composition. Excess Li ions in the Ni layer promoted intralayer migration of Ni ions during delithiation in LR-LNO, generating vacancy clusters to trap the electrochemically oxidized molecular O2 in the near-surface lattice. Consequently, the oxygen redox reaction became highly reversible, and the detrimental layered-to-rock-salt phase transition are effectively inhibited, thus improving the structural reversibility of LR-LNO during cycling and the thermal stability.Our results provide a composition fine-tuning strategy to produce highly-reversible cathodes for high energy-density, low-cost and safe batteries. Beyond batteries, the double-tilt operando TEM technique will facilitate studies into complex phase transitions in a wide range of materials.

Figure 1 Pristine structure and outstanding performance of LR-LNO

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Figure 1