Atomic Insight into the Layered/Spinel Phase Transformation in Charged LiNi_0.80Co_0.15Al_0.05O₂ Cathode Particles

Layered LiNi_0.80Co_0.15Al_0.05O₂ (NCA) holds great promise as a potential cathode material for high energy density lithium ion batteries. However, its high capacity is heavily dependent on the stability of its layered structure, which suffers from a severe structure degradation resulting from a not fully understood layered → spinel phase transformation. Using high-resolution transmission electron microscopy and electron diffraction, we probe the atomic structure evolution induced by the layered → spinel phase transformation in the NCA cathode. We show that the phase transformation results in the development of a particle structure with the formation of complete spinel, spinel domains, and intermediate spinel from the surface to the subsurface region. The lattice planes of the complete and intermediate spinel phases are highly interwoven in the subsurface region. The layered → spinel transformation occurs via the migration of transition metal (TM) atoms from the TM layer into the lithium layer. Incomplete migration leads to the formation of the intermediate spinel phase, which is featured by tetrahedral occupancy of TM cations in the lithium layer. The crystallographic structure of the intermediate spinel is discussed and verified by the simulation of electron diffraction patterns

Tuning the Activity of Oxygen in LiNi_0.8Co_0.15Al_0.05O₂ Battery Electrodes

Layered transition metal oxides such as LiNi_0.8Co _0.15Al_0.05O₂ (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

What Happens to LiMnPO₄ upon Chemical Delithiation?

Olivine MnPO₄ is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO₄, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO₄. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO₄, which causes amorphization of olivine MnPO₄. The properties of crystalline MnPO₄ obtained from carbon-coated LiMnPO₄ and of the amorphous product resulting from delithiation of pure LiMnPO₄ were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP₂O₇ and MnH₂P₂O₇ from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO₄ is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal

Electrochemical Performance of Nanosized Disordered LiVOPO₄

ε-LiVOPO₄ is a promising multielectron cathode material for Li-ion batteries that can accommodate two electrons per vanadium, leading to higher energy densities. However, poor electronic conductivity and low lithium ion diffusivity currently result in low rate capability and poor cycle life. To enhance the electrochemical performance of ε-LiVOPO₄, in this work, we optimized its solid-state synthesis route using in situ synchrotron X-ray diffraction and applied a combination of high-energy ball-milling with electronically and ionically conductive coatings aiming to improve bulk and surface Li diffusion. We show that high-energy ball-milling, while reducing the particle size also introduces structural disorder, as evidenced by ⁷Li and ³¹P NMR and X-ray absorption spectroscopy. We also show that a combination of electronically and ionically conductive coatings helps to utilize close to theoretical capacity for ε-LiVOPO₄ at C/50 (1 C = 153 mA h g^–1) and to enhance rate performance and capacity retention. The optimized ε-LiVOPO₄/Li₃VO₄/acetylene black composite yields the high cycling capacity of 250 mA h g^–1 at C/5 for over 70 cycles