Chemical States of Overcharged LiCoO₂ Particle Surfaces and Interiors Observed Using Electron Energy-Loss Spectroscopy

Abstract

Deterioration mechanisms of LiCoO 2 electrode materials for lithium ion batteries remain unclear. Using electron energy-loss spectroscopy and transmission electron microscopy, this study investigated chemical states of LiCoO 2 particles on first overcharging. We present a scheme for quantification of the Li/ Co atomic ratio. Using quantitative Li mapping and comprehensive probing of Li−K, Co−M 2,3 , Co−L 3 , and O−K edges, we observed that overcharging causes the progression of Co 3+ /Co 2+ reduction with oxygen extraction from the particle surface to the interior. A gradual change in the chemical composition at and around the particle surfaces after charging of 60% revealed the presence of Co 3 O 4 -like and CoO-like phases at surface regions. We also observed nanocracks with deficient Li ions. These results are key factors affecting degradation on overcharging.

Surface Engineering Strategies of Layered LiCoO2 Cathode Material to Realize High‐Energy and High‐Voltage Li‐Ion Cells

“…Based upon maximum obtainable electrode density of the electrode and voltage profiles, LiCoO 2 showed the highest value of 2.80 W h cc −1 , while NCA and OLO showed 2.57 and 2.10 W h cc −1 , respectively at 0.1 C. The MP‐LCO exhibited improved cyclic stability at 60 ºC with a capacity retention of 86.5% after 100 cycles, excellent rate capability with a discharge capacity of 145 mA h g −1 at 10 C in lithium half‐cells, which is ≈25 times higher than bare‐LCO as shown in Figure b,f. Such an inferior cycling and rate performance of the bare‐LCO cathode can be assigned to the structural degradation (formation of micro‐cracks initiated from surface to the core of particle as shown in the Figure g) at elevated temperatures together with adverse cut‐off voltages, significant side reactions on the surface (see Figure h), and Co dissolution into the electrolyte because of HF attack, are evident from our post‐mortem analyses using focused‐ion beam (FIB), time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS), and also confirmed from the literature …”

Unveiling Nickel Chemistry in Stabilizing High‐Voltage Cobalt‐Rich Cathodes for Lithium‐Ion Batteries

“…Elevating the upper cutoff voltage in charging is the most straightforward method to further increase the energy density of LCO, but it unfortunately leads to poor cyclability if charged to >4.40 V versus Li/Li + (x ≥ 0.6 in the form of Li 1−x CoO 2 ). [8][9][10][11][12][13] Extensive researches in the past decades seek to address this critical issue. It is known that oxygen redox (O 2− ↔O 1− ) starts to contribute capacity at these higher voltages, since the O 2p orbitals hybridizes with the Co 3d orbitals in the Co 3+/4+ :t 2g & O 2p resonant band at lower electronic energies.…”

LiCoO2Degradation Behavior in the High-Voltage Phase Transition Region and Improved Reversibility with Surface Coating

“…This is because the LiCoO 2 electrodes rapidly degrade at higher charging voltages. Until now, many different mechanisms have been proposed for the degradation of LiCoO 2 , such as a transition to the monoclinic phase, 4,24,25 dissolution of Co,6,26 formation of the spinel phase, 27-29 attack by HF, 9,11,15,16 and reduction of Co. 20,29,30 Additionally, at charge voltages exceeding 4.5 V, LiCoO 2 exhibits transitions from the O3 phase to the H1-3 and O1 phases.…”