Phase Engineering via Aluminum Doping Enhances the Electrochemical Stability of Lithium-Rich Cobalt-Free Layered Oxides for Lithium-Ion Batteries.

Lithium-rich, cobalt-free oxides are promising potential positive electrode materials for lithium-ion batteries because of their high energy density, lower cost, and reduced environmental and ethical concerns. However, their commercial breakthrough is hindered because of their subpar electrochemical stability. This work studies the effect of aluminum doping on Li1.26Ni0.15Mn0.61O2 as a lithium-rich, cobalt-free layered oxide. Al doping suppresses voltage fade and improves the capacity retention from 46% for Li1.26Ni0.15Mn0.61O2 to 67% for Li1.26Ni0.15Mn0.56Al0.05O2 after 250 cycles at 0.2 C. The undoped material has a monoclinic Li2MnO3-type structure with spinel on the particle edges. In contrast, Al-doped materials (Li1.26Ni0.15Mn0.61-xAlxO2) consist of a more stable rhombohedral phase at the particle edges, with a monoclinic phase core. For this core-shell structure, the formation of Mn3+ is suppressed along with the material's decomposition to a disordered spinel, and the amount of the rhombohedral phase content increases during galvanostatic cycling. Whereas previous studies generally provided qualitative insight into the degradation mechanisms during electrochemical cycling, this work provides quantitative information on the stabilizing effect of the rhombohedral shell in the doped sample. As such, this study provides fundamental insight into the mechanisms through which Al doping increases the electrochemical stability of lithium-rich cobalt-free layered oxides.

Explore the Effects of Microstructural Defects on Voltage Fade of Li- and Mn-Rich Cathodes.

Li- and Mn-rich (LMR) cathode materials have been considered as promising candidates for energy storage applications due to high energy density. However, these materials suffer from a serious problem of voltage fade. Oxygen loss and the layered-to-spinel phase transition are two major contributors of such voltage fade. In this paper, using a combination of X-ray diffraction (XRD), pair distribution function (PDF), X-ray absorption (XAS) techniques, and aberration-corrected scanning transmission electron microscopy (STEM), we studied the effects of micro structural defects, especially the grain boundaries, on the oxygen loss and layered-to-spinel phase transition through prelithiation of a model compound Li2Ru0.5Mn0.5O3. It is found that the nanosized micro structural defects, especially the large amount of grain boundaries created by the prelithiation can greatly accelerate the oxygen loss and voltage fade. Defects (such as nanosized grain boundaries) and oxygen release form a positive feedback loop, promote each other during cycling, and accelerate the two major voltage fade contributors: the transition metal reduction and layered-to-spinel phase transition. These results clearly demonstrate the important relationships among the oxygen loss, microstructural defects and voltage fade. The importance of maintaining good crystallinity and protecting the surface of LMR material are also suggested.

Constructing a Cr-Substituted Co-Free Li-Rich Ternary Cathode with a Spinel-Layered Biphase Interface.

Lithium-rich manganese-based layered oxides (LRMOs) have recently attracted enormous attention on account of their remarkably big capacity and high working voltage. However, some inevitable inherent drawbacks impede their wide-scale commercial application. Herein, a kind of Cr-containing Co-free LRMO with a topical spinel phase (Li1.2Mn0.54Ni0.13Cr0.13O2) has been put forward. It has been found that the high valence of Cr6+ can reduce the Li+ ion content and induce the formation of a local spinel phase by combining more Li+ ions, which is beneficial to eliminate the phase boundary between the spinel phase and the bulk phase of the LRMO material, thus dramatically avoiding phase separation during the cycling process. In addition, the introduction of Cr can also expand the layer spacing and construct a stronger Cr-O bond compared with Mn-O, which enables to combine the transition metal (TM) slab to prevent the migration of TM ions and the transformation of the bulk phase to the spinel phase. Simultaneously, the synergistic effect of the successfully constructed spinel-layered biphase interface and the strong Cr-O bond can effectively impede the escape of lattice oxygen during the initial activation process of Li2MnO3 and provide the fast diffusion path for Li+ ion transmission, thus further reinforcing the configurable stability. Besides, Cr-LRMO presents an ultrahigh first discharge specific capacity of 310 mAh g-1, an initial Coulombic efficiency of as high as 92.09%, a good cycling stability (a capacity retention of 94.70% after 100 cycles at 1C), and a small voltage decay (3.655 mV per cycle), as well as a good rate capacity (up to 165.88 mAh g-1 at 5C).

Restriction of voltage decay by limiting low-voltage reduction in Li-rich oxide materials.

Li-rich layered oxides are recognized as promising candidates for next-generation Li-ion batteries owing to the high capacity of >250 mAh g-1, but the severe voltage fade has prevented their commercialization. It is widely known that high-voltage charge processes result in layered-to-spinel structural evolution and voltage fade in Li-rich layered oxides. This work emphasizes that limiting the low-voltage reduction can maintain the structure and voltage stability of Li-rich layered oxides after the 4.6 V high-voltage charge processes. A strategy of limiting the low-voltage (<2.8 V) reduction by cycling at 4.6-2.8 V was performed in traditional Li1.2Ni0.13Mn0.54Co0.13O2 and high-Ni Li1.2Ni0.222Mn0.504Co0.074O2. After 300 cycles, traditional Li1.2Ni0.13Mn0.54Co0.13O2 and high-Ni Li1.2Ni0.222Mn0.504Co0.074O2 cycling at 4.6-2 V showed midpoint discharge voltages of 2.83 V and 2.97 V with high voltage fade rates of 2.25 mV/cycle and 2.24 mV/cycle, respectively. While the two materials cycling at 4.6-2.8 V can maintain discharge midpoint voltages of 3.34 V and 3.49 V, with low voltage decay rates of 0.692 mV/cycle and 0.632 mV/cycle, respectively. To better understand the voltage performance, their electric structures were calculated by density functional theory. Physical characterizations were also used to analyze their differences in structural evolution. The results suggested that limiting low-voltage reduction in Li-rich layered oxides is highly necessary for maintaining their structure and voltage stability.

Phase Compatible NiFe2O4 Coating Tunes Oxygen Redox in Li-Rich Layered Oxide.

Li-rich layered oxides have attracted intense attention for lithium-ion batteries, as provide substantial capacity from transition metal cation redox simultaneous with reversible oxygen-anion redox. However, unregulated irreversible oxygen-anion redox leads to critical issues such as voltage fade and oxygen release. Here, we report a feasible NiFe2O4 (NFO) surface-coating strategy to turn the nonbonding coordination of surface oxygen into metal-oxygen decoordination. In particular, the surface simplex M-O (M = Ni, Co, Mn from MO6 octahedra) and N-O (N = Ni, Fe from NO6 octahedra) bonds are reconstructed in the form of M-O-N bonds. By applying both in operando and ex situ technologies, we found this heterostructural interface traps surface lattice oxygen, as well as restrains cation migration in Li-rich layered oxide during electrochemical cycling. Therefore, surface lattice oxygen behavior is significantly sustained. More interestingly, we directly observe the surface oxygen redox decouple with cation migration. In addition, the NFO-coating blocks HF produced from electrolyte decomposition, resulting in reducing the dissolution of Mn. With this strategy, higher cycle stability (91.8% at 1 C after 200 cycles) and higher rate capability (109.4 mA g-1 at 1 C) were achieved in this work, compared with pristine Li-rich layered oxide. Our work offers potential for designing electrode materials utilizing oxygen redox chemistry.

Protective Spinel Coating for Li1.17Ni0.17Mn0.50Co0.17O2 Cathode for Li-Ion Batteries through Single-Source Precursor Approach.

The Li1.17Ni0.17Mn0.50Co0.17O2 Li-rich NMC positive electrode (cathode) for lithium-ion batteries has been coated with nanocrystals of the LiMn1.5Co0.5O4 high-voltage spinel cathode material. The coating was applied through a single-source precursor approach by a deposition of the molecular precursor LiMn1.5Co0.5(thd)5 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) dissolved in diethyl ether, followed by thermal decomposition at 400 °C inair resulting in a chemically homogeneous cubic spinel. The structure and chemical composition of the coatings, deposited on the model SiO2 spheres and Li-rich NMC crystallites, were analyzed using powder X-ray diffraction, electron diffraction, high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy-dispersive X-ray (EDX) mapping. The coated material containing 12 wt.% of spinel demonstrates a significantly improved first cycle Coulombic efficiency of 92% with a high first cycle discharge capacity of 290 mAhg-1. The coating also improves the capacity and voltage retention monitored over 25 galvanostatic charge-discharge cycles, although a complete suppression of the capacity and voltage fade is not achieved.

In Situ Construction of Spinel Coating on the Surface of a Lithium-Rich Manganese-Based Single Crystal for Inhibiting Voltage Fade.

Layered lithium-rich transition-metal oxides (LRMs) have been considered as the most promising next-generation cathode materials for lithium-ion batteries. However, capacity fading, poor rate performance, and large voltage decays during cycles hinder their commercial application. Herein, a spinel membrane (SM) was first in situ constructed on the surface of the octahedral single crystal Li1.22Mn0.55Ni0.115Co0.115O2 (O-LRM) to form the O-LRM@SM composite with superior structural stability. The synergetic effects between the single crystal and spinel membrane are the origins of the enhancement of performance. On the one hand, the single crystal avoids the generation of inactive Li2MnO3-like phase domains, which is the main reason for capacity fading. On the other hand, the spinel membrane not only prevents the side reactions between the electrolyte and cathode materials but also increases the diffusion kinetics of lithium ions and inhibits the phase transformation on the electrode surface. Based on the beneficial structure, the O-LRM@SM electrode delivers a high discharge specific capacity and energy density (245.6 mA h g-1 and 852.1 W h kg-1 at 0.5 C), low voltage decay (0.38 V for 200 cycle), excellent rate performance, and cycle stability.