Thermal stability of charged LiNixMnyCozO2 (NMC, with x + y + z = 1, x:y:z = 4:3:3 (NMC433), 5:3:2 (NMC532), 6:2:2 (NMC622), and 8:1:1 (NMC811)) cathode materials is systematically studied using combined in situ time-resolved X-ray diffraction and mass spectroscopy (TR-XRD/MS) techniques upon heating up to 600 °C. The TR-XRD/MS results indicate that the content of Ni, Co, and Mn significantly affects both the structural changes and the oxygen release features during heating: the more Ni and less Co and Mn, the lower the onset temperature of the phase transition (i.e., thermal decomposition) and the larger amount of oxygen release. Interestingly, the NMC532 seems to be the optimized composition to maintain a reasonably good thermal stability, comparable to the low-nickel-content materials (e.g., NMC333 and NMC433), while having a high capacity close to the high-nickel-content materials (e.g., NMC811 and NMC622). The origin of the thermal decomposition of NMC cathode materials was elucidated by the changes in the oxidation states of each transition metal (TM) cations (i.e., Ni, Co, and Mn) and their site preferences during thermal decomposition. It is revealed that Mn ions mainly occupy the 3a octahedral sites of a layered structure (R3̅m) but Co ions prefer to migrate to the 8a tetrahedral sites of a spinel structure (Fd3̅m) during the thermal decomposition. Such element-dependent cation migration plays a very important role in the thermal stability of NMC cathode materials. The reasonably good thermal stability and high capacity characteristics of the NMC532 composition is originated from the well-balanced ratio of nickel content to manganese and cobalt contents. This systematic study provides insight into the rational design of NMC-based cathode materials with a desired balance between thermal stability and high energy density.

Abstract. We investigate the energy of arrangements of N points on the surface of a sphere in R 3 , interacting through a power law potential V = r α , −2 < α < 2, where r is Euclidean distance. For α = 0, we take V = log(1/r). An area-regular partitioning scheme of the sphere is devised for the purpose of obtaining bounds for the extremal (equilibrium) energy for such points. For α = 0, finer estimates are obtained for the dominant terms in the minimal energy by considering stereographical projections on the plane and analyzing certain logarithmic potentials. A general conjecture on the asymptotic form (as N → ∞) of the extremal energy, along with its supporting numerical evidence, is presented. Also we introduce explicit sets of points, called "generalized spiral points", that yield good estimates for the extremal energy. At least for N ≤ 12, 000 these points provide a reasonable solution to a problem of M. Shub and S. Smale arising in complexity theory.

Most P2-type layered oxides suffer from multiple voltage plateaus, due to Na+/vacancy-order superstructures caused by strong interplay between Na–Na electrostatic interactions and charge ordering in the transition metal layers. Here, Mg ions are successfully introduced into Na sites in addition to the conventional transition metal sites in P2-type Na0.7[Mn0.6Ni0.4]­O2 as new cathode materials for sodium-ion batteries. Mg ions in the Na layer serve as “pillars” to stabilize the layered structure, especially for high-voltage charging, meanwhile Mg ions in the transition metal layer can destroy charge ordering. More importantly, Mg ion occupation in both sodium and transition metal layers will be able to create “Na–O–Mg” and “Mg–O–Mg” configurations in layered structures, resulting in ionic O 2p character, which allocates these O 2p states on top of those interacting with transition metals in the O-valence band, thus promoting reversible oxygen redox. This innovative design contributes smooth voltage profiles and high structural stability. Na0.7Mg0.05[Mn0.6Ni0.2Mg0.15]­O2 exhibits superior electrochemical performance, especially good capacity retention at high current rate under a high cutoff voltage (4.2 V). A new P2 phase is formed after charge, rather than an O2 phase for the unsubstituted material. Besides, multiple intermediate phases are observed during high-rate charging. Na-ion transport kinetics are mainly affected by elemental-related redox couples and structural reorganization. These findings will open new opportunities for designing and optimizing layer-structured cathodes for sodium-ion batteries.

Using a combination of time-resolved X-ray diffraction (XRD), in situ transmission electron microscopy (TEM), and first principles calculations, we explore the structural origin of the overcharge induced thermal instability of two cathode materials, LiNi 0.8 Co 0.15 Al 0.05 O 2 and LiNi 1/3 -Co 1/3 Mn 1/3 O 2 , which exhibit significant difference in thermal stabilities. Detailed TEM analysis reveals, for the first time, a complex coreÀshell-surface structure of the particles in both materials that was not previously detected by XRD. Structural comparison indicates that the overcharged Li x Ni 0.8 Co 0.15 Al 0.05 O 2 (x < 0.15) particles consist of a rhombohedral core, a spinel shell, and a rock-salt structure at the surface, while the overcharged Li x Ni 1/3 Co 1/3 Mn 1/3 O 2 consists of a similar coreÀshell-surface structure but a very different CdI 2 -type surface structure. The thermal instability of Li x Ni 0.8 Co 0.15 Al 0.05 O 2 can be attributed to the release of oxygen because of the rapid growth of the rock-salt-type structure on the surface during heating. In contrast, the CdI 2 -type surface structure of the overcharged Li x Ni 1/3 Co 1/3 Mn 1/3 O 2 particles delays the oxygen-release reaction to a much higher temperature resulting in better stability. These results gave deep insight into the relationship between the local structural changes and the thermal stability of cathode materials, which is vital to the development of new cathode materials for the next generation of lithium-ion batteries.

For LiMO 2 (M ¼ Co, Ni, Mn) cathode materials, lattice parameters, a(b), contract during charge. Here we report such changes in opposite directions for lithium molybdenum trioxide (Li 2 MoO 3 ). A 'unit cell breathing' mechanism is proposed based on crystal and electronic structural changes of transition metal oxides during charge-discharge. Metal-metal bonding is used to explain such 'abnormal' behaviour and a generalized hypothesis is developed. The expansion of the metal-metal bond becomes the controlling factor for a(b) evolution during charge, in contrast to the shrinking metal-oxygen bond as controlling factor in 'normal' materials. The cation mixing caused by migration of molybdenum ions at higher oxidation state provides the benefits of reducing the c expansion range in the early stage of charging and suppressing the structure collapse at high voltage charge. These results may open a new strategy for designing layered cathode materials for high energy density lithium-ion batteries.