Design of Nickel-Based Cation-Disordered Rock-Salt Oxides: The Effect of Transition Metal (M = V, Ti, Zr) Substitution in LiNi0.5M0.5O2 Binary Systems

Over-Stoichiometric Metastabilization of Cation-Disordered Rock Salts

Cation-disordered rock salts (DRXs) are well known for their potential to realize the goal of achieving scalable Ni- and Co-free high-energy-density Li-ion batteries. Unlike in most cathode materials, the disordered cation distribution may lead to more factors that control the electrochemistry of DRXs. An important variable that is not emphasized by research community is regarding whether a DRX exists in a more thermodynamically stable form or a more metastable form. Moreover, within the scope of metastable DRXs, over-stoichiometric DRXs, which allow relaxation of the site balance constraint of a rock salt structure, are particularly underexplored. In this work, these findings are reported in locating a generally applicable approach to "metastabilize" thermodynamically stable Mn-based DRXs to metastable ones by introducing Li over-stoichiometry. The over-stoichiometric metastabilization greatly stimulates more redox activities, enables better reversibility of Li deintercalation/intercalation, and changes the energy storage mechanism. The metastabilized DRXs can be transformed back to the thermodynamically stable form, which also reverts the electrochemical properties, further contrasting the two categories of DRXs. This work enriches the structural and compositional space of DRX families and adds new pathways for rationally tuning the properties of DRX cathodes.

Regulating the Anion Redox and Suppressing the Structural Distortion of Cation-Disordered Rock-Salt Cathode Materials to Improve Cycling Durability through Chlorine Substitution

Owing to the capacity boost from anion redox activities, cation-disordered rock-salt oxides are considered as potential candidates for the next-generation of high energy density Li-ion cathode materials. Unfortunately, the anion redox process that affords ultra-high specific capacity often triggers irreversible O₂ release, which brings about structural degradation and rapid capacity decay. In this study, we present a partial chlorine (Cl) substitution strategy to synthesize a new cation-disordered rock-salt compound of Li_1.225Ti_0.45Mn_0.325O_1.9Cl_0.1 and investigate the impact of Cl substitution on the oxygen redox process and the structural stability of cation-disordered rock-salt cathodes. We find that partial replacement of O^2- by Cl^- expands the cell volume and promotes anion redox reaction reversibility, thus increasing the Li⁺ ion diffusion rate and suppressing irreversible lattice oxygen loss. As a result, the Li_1.225Ti_0.45Mn_0.325O_1.9Cl_0.1 cathode exhibits significantly improved cycling durability at high current densities, compared with the pristine Li_1.225Ti_0.45Mn_0.325O₂ cathode. This work demonstrates the promising feasibility of the Cl substitution process for advanced cation-disordered rock-salt cathode materials.

ZnO under Pressure: From Nanoparticles to Single Crystals

In the present review, new approaches for the stabilization of metastable phases of zinc oxide and the growth of ZnO single crystals under high pressures and high temperatures are considered. The problems of the stabilization of the cubic modification of ZnO as well as solid solutions on its basis are discussed. A thermodynamic approach to the description of zinc oxide melting at high pressures is described which opens up new possibilities for the growth of both undoped and doped (for example, with elements of group V) single crystals of zinc oxide. The possibilities of using high pressure to vary phase and elemental composition in order to create ZnO-based materials are demonstrated.

Identifying the effect of fluorination on cation and anion redox activity in Mn based cation-disordered cathode

Li-rich disordered rock-salt cathode (DRX) materials with advantage of low cost, long cycle life, nature abundant resource and high power and energy density attracted a great deal of scholarly attention. However, the poor cycle stability and the unclear realization of cation and anion redox activity in low-cost element system have severely hindered the construction of high-performance DRX. Herein, a promising class of Ti-Mn based cathode materials Li_1.25Mn_0.25Nb_0.25Ti_0.25O₂ and Li_1.25Mn_0.25Ti_0.5O_1.75F_0.25 were designed and successfully synthesized to construct high energy density DRX and investigate the effect of fluorination on cation and anion redox activity. The results show that both fluoridized and unfluoridized DRX possess a similar structure (Fm-3 m), but distinctly different charge/discharge profiles. The fluoridized cathode shows high initial charge/discharge capacity of 317.3/283.9 mAh g^-1, specific energy density of 1370.4/735.5 Wh kg^-1 and stable capacity retention with a discharge capacity of 202.6 mAh g^-1 after 20 cycles at 20 mA g^-1. Combining relevant spectroscopic results and HRTEM images, we revealed that the excellent cyclability of Li_1.25Mn_0.25Ti_0.5O_1.75F_0.25 is rooted in the weakened adverse effects of moderated oxygen redox and the reduced Jahn-Teller distortion effect resulting from Mn³⁺, endowing the fluoridized DRX with better structural stability and larger Mn²⁺/Mn⁴⁺ reservoir. The strategy of constructing low cost oxyfluoride and the understanding of the mechanism of fluorination induced cation and anion redox activity would provide reference for the development of high-performance DRX materials.

First-Principles Study of the Surfaces and Equilibrium Shape of Discharge Products in Li-Air Batteries

Li-air batteries are a promising alternative to Li-ion batteries as they theoretically provide the highest possible specific energy density. Mainly, Li₂O₂ (lithium peroxide) and to a lesser extent, Li₂O (lithium oxide) are assumed to be the discharge products of these batteries formed with the soluble LiO₂ (lithium superoxide) considered to be an intermediate product. Bulk Li₂O₂ is an electronic insulator, and the precipitation of this compound on the cathode is thought to be the main limiting factor in achieving high capacities in lithium-oxygen cells. For the most promising electrolytes including solvents with high donor numbers, microscopy observations frequently reveal crystallite morphologies of Li₂O₂ compounds, rather than uniform layers covering the electrode surface. The precise morphologies of Li₂O and Li₂O₂ particles, and their effect and their extent of contact with the electrode, which may all affect the capacity and rechargeability, however, remain largely undetermined. Here, we address the stability of various Li₂O and Li₂O₂ surfaces and consequently, their crystallite morphologies using density functional theory calculations and ab initio thermodynamics. In contrast to previous studies, we also consider high-index surface terminations, which exhibit surprisingly low surface energies. We carefully analyze the reasons for the stability of these high-index surfaces, which also prominently influence the equilibrium shape of the particles, at least for Li₂O₂, and discuss the consequences for the observed morphology of the reaction products.

Cation-Disordered Lithium-Excess Li-Fe-Ti Oxide Cathode Materials for Enhanced Li-Ion Storage

Cation-disordered Li-excess lithium-transition metal (Li-TM) oxides designed based on the percolation theory are regarded as a promising new type of high-performance cathode material for Li-ion batteries. Herein, cation-disordered rocksalt-type Li-Fe-Ti oxides of Li_0.89Fe_0.44Ti_0.45O₂, Li_1.18Fe_0.34Ti_0.45O₂, and Li_1.24Fe_0.38Ti_0.38O₂ with different Li-to-transition metal ratios (Li/TM = 1, 1.49, or 1.63) are investigated to understand the effect of a Li excess on the electrochemical Li-ion storage properties. The Li excess leads to local structural fluctuations of the as-prepared Li-Fe-Ti oxides, contributing to the formation of 0-TM diffusion channels for rapid Li-ion migration. The as-prepared Li-excess Li-Fe-Ti oxide cathodes (Li/TM = 1.49 and Li/TM = 1.63) deliver a higher reversible capacity of over 220 mAh g^-1 and a better rate capability compared to the Li/TM = 1 electrode, which possesses a maximum discharge capacity of only about 165 mAh g^-1. The redox reactions of Fe²⁺/Fe³⁺ and O^2-/O₂^2- achieve the main capacity of the Li-excess Li-Fe-Ti oxide cathodes during cycling, as supported by ⁵⁷Fe Mössbauer spectroscopy, O 1s X-ray photoelectron spectroscopy, and O K-edge soft X-ray absorption spectroscopy.

Design and Tuning of the Electrochemical Properties of Vanadium-Based Cation-Disordered Rock-Salt Oxide Positive Electrode Material for Lithium-Ion Batteries

Disordered rock-salt compounds are becoming increasingly important due to their potential as high-capacity positive electrode materials for lithium-ion batteries. Thereby, a significant number of studies have focused on increasing the accessible Li capacity, but studies to manipulate the electrochemical potential are limited. This work explores the effect of transition-metal substitution on the electrochemistry of ternary disordered rock-salt-type compounds with LiM²⁺_0.5V_0.5⁴⁺O₂ stoichiometry (M = Mn, Fe, Co) directly synthesized through mechanochemistry. Rietveld refinements of synchrotron X-ray diffraction patterns confirm the disordered rock-salt structures. First-principles density functional theory study is used to predict the impact of the cation substitution on the expected average voltage and the electronic structures of these materials are used to analyze the underlying redox processes. For LiM²⁺_0.5V⁴⁺_0.5O₂ (M = Mn, Fe, Co), discharge voltages increase in the order of Mn < Fe < Co with 2.28, 2.41, and 2.51 V, exhibiting discharge capacities of 219, 207, and 234 mAh g^-1, respectively. In comparison, for the disordered rock-salt Li₂VO_3, an average discharge voltage of ∼2.2 V with V^5+/4+ redox couple has been reported. However, detrimental electrode-electrolyte interactions manifested as transition-metal dissolution has been found to result in severe capacity fading. Thereto, the use of a concentrated 5.5 M LiFSI increased the cycling stability significantly, effectively reducing transition-metal dissolution. The underlying reasons for the capacity fading of disordered rock salts are yet unclear. We stress the importance of cathode-electrolyte interactions, thus opening new directions for the improvement of cation-disordered materials.

Synthesis and Redox Mechanism of Cation-Disordered, Rock-Salt Cathode-Material Li-Ni-Ti-Nb-O Compounds for a Li-Ion Battery

Cation-disordered oxide materials working as cathodes for Li-ion batteries have been at a standstill because of their structurally limited specific capacities (below 175 mAh g^-1 in most cases). In this work, we have introduced 4d⁰ Nb⁵⁺ into host material LiNi_0.5Ti_0.5O₂ to synthesize Ni-based cation-disordered Fm3̅m Li-Ni-Ti-Nb-O compounds of Li_1+x/100Ni_1/2-x/100Ti_1/2-x/100Nb_x/100O₂ (x = 0, 5, 10, 15, 20) through a sol-gel method, showing particle sizes of less than 200 nm. Taking Li_1.2Ni_0.3Ti_0.3Nb_0.2O₂ with the best performance (an average voltage of ∼2.7 V and high discharge capacity of 221.5 mAh g^-1) among oxides as a model, we study the relationship between the structure, morphology, redox mechanism, and electrochemical performance of cation-disordered oxides through a combination of X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption near-edge spectroscopy tests and in situ XRD with electrochemistry. The obtained results indicate that the improved capacity is mainly ascribed to Nb⁵⁺, which optimizes the Ni²⁺/Ni⁴⁺ practical capacity and effectively stabilizes the O^2-/O^- redox reaction. The results emphasize that Li-Ni-Ti-Nb-O compounds are promising members in the family of cation-disordered transition-metal oxide materials.

There and Back Again-The Journey of LiNiO2 as a Cathode Active Material

This Review provides a comprehensive overview of LiNiO₂ (LNO), almost 30 years after its introduction as a cathode active material. We aim to highlight the physicochemical peculiarities that make LNO a complex material in every aspect. We specifically stress the effect of the Li off-stoichiometry (Li_1-z Ni_1+z O₂ ) on every property of LNO, especially the electrochemical ones. The key instability issues that plague the compound and the strategies that have been implemented so far to overcome them are discussed in detail. Finally, the open questions that remain to be addressed by the scientific community are summarized, and the research directions that seem the most promising to enable LNO to be fully exploited are elucidated.

Enhanced Cycling Stability of Cation Disordered Rock-Salt Li1.2Ti0.4Mn0.4O2 Material by Surface Modification With Al2O3

Cation disordered rock-salt lithium-excess oxides are promising candidate cathode materials for next-generation electric vehicles due to their extra high capacities. However, one major issue for these materials is the distinct decline of discharge capacities during charge/discharge cycles. In this study, Al₂O₃ layers were coated on cation disordered Li_1.2Ti_0.4Mn_0.4O₂ (LTMO) using atomic layer deposition (ALD) method to optimize its electrochemical performance. The discharge capacity after 15 cycles increased from 228.1 to 266.7 mAh g^-1 for LTMO after coated with Al₂O₃ for 24 ALD cycles, and the corresponding capacity retention enhanced from 79.7 to 90.9%. The improved cycling stability of the coated sample was ascribed to the alleviation of oxygen release and the inhibition on the undesirable side reactions. Our work has provided a new possible solution to address some of the capacity fading issues related to the cation disordered rock-salt cathode materials.

A New Type of Li-Rich Rock-Salt Oxide Li2 Ni1/3 Ru2/3 O3 with Reversible Anionic Redox Chemistry

Li-rich oxide cathodes are of prime importance for the development of high-energy lithium-ion batteries (LIBs). Li-rich layered oxides, however, always undergo irreversible structural evolution, leading to inevitable capacity and voltage decay during cycling. Meanwhile, Li-rich cation-disordered rock-salt oxides usually exhibit sluggish kinetics and inferior cycling stability, despite their firm structure and stable voltage output. Herein, a new Li-rich rock-salt oxide Li₂ Ni_1/3 Ru_2/3 O₃ with Fd-3m space group, where partial cation-ordering arrangement exists in cationic sites, is reported. Results demonstrate that a cathode fabricated from Li₂ Ni_1/3 Ru_2/3 O₃ delivers a large capacity, outstanding rate capability as well as good cycling performance with negligible voltage decay, in contrast to the common cations disordered oxides with space group Fm-3m. First principle calculations also indicate that rock-salt oxide with space group Fd-3m possesses oxygen activity potential at the state of delithiation, and good kinetics with more 0-TM (TM = transition metals) percolation networks. In situ Raman results confirm the reversible anionic redox chemistry, confirming O^2- /O^- evolution during cycles in Li-rich rock-salt cathode for the first time. These findings open up the opportunity to design high-performance oxide cathodes and promote the development of high-energy LIBs.