Size and charge effects on the structural stability of LiMO2 (M = transition metal) compounds

Manipulating disorder within cathodes of alkali-ion batteries

The fact that ordered materials are rarely perfectly crystalline is widely acknowledged among materials scientists, but its impact is often overlooked or underestimated when studying how structure relates to properties. Various investigations demonstrate that intrinsic and extrinsic defects, and disorder generated by physicochemical reactions, are responsible for unexpectedly detrimental or beneficial functionalities. The task remains to modulate the disorder to produce desired properties in materials. As disorder is often correlated with local interactions, it is controllable. In this Review, we explore the structural disorder in cathode materials as a novel approach for improving their electrochemical performance. We revisit cathode materials for alkali-ion batteries and outline the origins and beneficial consequences of disorder. Focusing on layered, cubic rocksalt and other metal oxides, we discuss how disorder improves electrochemical properties of cathode materials and which interactions generate the disorder. We also present the potential pitfalls of disorder that must be considered. We conclude with perspectives for enhancing the electrochemical performance of cathode materials by using disorder.

A New Class of High-Capacity Fe-Based Cation-Disordered Oxide for Li-Ion Batteries: Li-Fe-Ti-Mo Oxide

Low-cost Fe can be used for forming cation-disordered rocksalt Li-excess (DRX) materials instead of high-cost d⁰ -species and then the Fe-based DRX can be promising electrode materials because they can theoretically achieve high capacity, resulting from additional oxygen redox reaction and stable cation-disordered structure. However, Fe-based DRX materials suffer from large voltage hysteresis, low electrochemical activity, and poor cyclability, so it is highly challenging to utilize them as practical electrode materials for a cell. Here, novel high-capacity Li-Fe-Ti-Mo electrode materials (LFTMO) with high average discharge voltage and reasonable stability are reported. The effect of Ti/Mo on electrochemical reactions in Fe-based DRX materials (LFTMO) is studied by controlling its composition ratio and using techniques for analyzing the local environment to find the key factors that improve its activity. It is found out that the introduction of appropriate quantity of redox-active Mo^4+/5+ to Fe-based DRX materials can help stabilize the oxygen redox reaction via changing a local structure and can suppress a Fe redox reaction, which can cause poor performance. The understandings will help develop high capacity and long cyclability Fe-based DRX electrode materials.

Tuning Bulk Redox and Altering Interfacial Reactivity in Highly Fluorinated Cation-Disordered Rocksalt Cathodes

Lithium-excess, cation-disordered rocksalt (DRX) materials have been subject to intense scrutiny and development in recent years as potential cathode materials for Li-ion batteries. Despite their compositional flexibility and high initial capacity, they suffer from poorly understood parasitic degradation reactions at the cathode-electrolyte interface. These interfacial degradation reactions deteriorate both the DRX material and electrolyte, ultimately leading to capacity fade and voltage hysteresis during cycling. In this work, differential electrochemical mass spectrometry (DEMS) and titration mass spectrometry are combined to quantify the extent of bulk redox and surface degradation reactions for a set of Mn^2+/4+-based DRX oxyfluorides during initial cycling with a high-voltage charging cutoff (4.8 V vs Li/Li⁺). Increasing the fluorine content from 7.5 to 33.75% is shown to diminish oxygen redox and suppresses high-voltage O₂ evolution from the DRX surface. Additionally, electrolyte degradation processes resulting in the formation of both gaseous species and electrolyte-soluble protic species are observed. Subsequently, DEMS is paired with a fluoride-scavenging additive to demonstrate that increasing fluorine content leads to increased dissolution of fluorine from the DRX material into the electrolyte. Finally, a suite of ex situ spectroscopy techniques (X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectroscopy, and solid-state nuclear magnetic resonance spectroscopy) are employed to study the change in DRX composition during charging, revealing the dissolution of manganese and fluorine from the DRX material at high voltages. This work provides insight into the degradation processes occurring at the DRX-electrolyte interface and points toward potential routes of interfacial stabilization.

A Comparison of Order-Disorder in Several Families of Cubic Oxides

Order-disorder on both cation and oxygen sites is a hallmark of fluorite-derived structures, including pyrochlores. Ordering can occur on long- and short-range scales and can result in persistent metastable states. In various cubic oxide systems, different types of disorder are seen. The purpose of this paper is to review and compare the types and energetics of order-disorder phenomena in several families of cubic oxides having pyrochlore, weberite, defect fluorite, perovskite, rocksalt, and spinel structures. The goal is to better understand how structure, composition, and thermodynamic parameters (enthalpy and entropy) determine the feasibility of different competing ordering processes and structures in these diverse systems.

Insights into Layered Oxide Cathodes for Rechargeable Batteries

Layered intercalation compounds are the dominant cathode materials for rechargeable Li-ion batteries. In this article we summarize in a pedagogical way our work in understanding how the structure’s topology, electronic structure, and chemistry interact to determine its electrochemical performance. We discuss how alkali–alkali interactions within the Li layer influence the voltage profile, the role of the transition metal electronic structure in dictating O3-structural stability, and the mechanism for alkali diffusion. We then briefly delve into emerging, next-generation Li-ion cathodes that move beyond layered intercalation hosts by discussing disordered rocksalt Li-excess structures, a class of materials which may be essential in circumventing impending resource limitations in our era of clean energy technology.

Promises and Challenges of Next-Generation "Beyond Li-ion" Batteries for Electric Vehicles and Grid Decarbonization

The tremendous improvement in performance and cost of lithium-ion batteries (LIBs) have made them the technology of choice for electrical energy storage. While established battery chemistries and cell architectures for Li-ion batteries achieve good power and energy density, LIBs are unlikely to meet all the performance, cost, and scaling targets required for energy storage, in particular, in large-scale applications such as electrified transportation and grids. The demand to further reduce cost and/or increase energy density, as well as the growing concern related to natural resource needs for Li-ion have accelerated the investigation of so-called "beyond Li-ion" technologies. In this review, we will discuss the recent achievements, challenges, and opportunities of four important "beyond Li-ion" technologies: Na-ion batteries, K-ion batteries, all-solid-state batteries, and multivalent batteries. The fundamental science behind the challenges, and potential solutions toward the goals of a low-cost and/or high-energy-density future, are discussed in detail for each technology. While it is unlikely that any given new technology will fully replace Li-ion in the near future, "beyond Li-ion" technologies should be thought of as opportunities for energy storage to grow into mid/large-scale applications.

Ni/Li Disordering in Layered Transition Metal Oxide: Electrochemical Impact, Origin, and Control

Lithium ion batteries (LIBs) not only power most of today's hybrid electric vehicles (HEV) and electric vehicles (EV) but also are considered as a promising system for grid-level storage. Large-scale applications for LIBs require substantial improvement in energy density, cost, and lifetime. Layered lithium transition metal (TM) oxides, in particular, Li(Ni_xMn_yCo_z)O₂ (NMC, x + y + z = 1) are the most promising candidates as cathode materials with the potential to increase energy densities and lifetime, reduce costs, and improve safety. In order to further boost Li storage capacity, a great deal of attention has been directed toward developing Ni-rich layered TM oxides. However, structural disorder as a result of Ni/Li exchange in octahedral sites becomes a critical issue when Ni content increases to high values, as it leads to a detrimental effect on Li diffusivity, cycling stability, first-cycle efficiency, and overall electrode performance. Increasing effort has been dedicated to improving the electrochemical performance of layered TM oxides via reduction of cationic mixing. Therefore, it is important to summarize this research field and provide in-depth insight into the impact of Ni/Li disordering on electrochemical characteristics in layered TM oxides and its origin to accelerate the future development of layered TM oxides with high performance. In this Account, we start by introducing the Ni/Li disordering in LiNiO₂, the experimental characterization of Ni/Li disordering, and analyzing the impact of Ni/Li disordering on electrochemical characteristics of layered TM oxides. The antisite Ni in the Li layer can limit the rate performance by impeding the Li ion transport. It will also degrade the cycling stability by inducing anisotropic stress in the bulk structure. Nevertheless, the antisite Ni ions do not always bring drawbacks to the electrochemical performance; some studies including our works found that it can improve the thermal stability and the cycling structure stability of Ni-rich NMC materials. We next discuss the driving forces and the kinetic advantages accounting for the Ni/Li exchange and conclude that the steric effect of cation size and the magnetic interactions between TM cations are the two main driving forces to promote the Ni/Li exchange during synthesis and the electrochemical cycling, and the low energy barrier of Ni²⁺ migration from the 3a site in the TM layer to the 3b site in the Li layer further provides a kinetic advantage. Based on this understanding, we then review the progress made to control the Ni/Li disordering through three main ways: (i) suppressing the driving force from the steric effect by ion exchange; (ii) tuning the magnetic interaction by cationic substitution; (iii) kinetically controlling Ni migration. Finally, our brief outlook on the future development of layered TM oxides with controlled Ni/Li disordering is provided. It is believed that this Account will provide significant understanding and inspirations toward developing high-performance layered TM oxide cathodes.

Critical Role of Cations in Lithium Sites on Extended Electrochemical Reversibility of Co-Rich Layered Oxide

Only a very limited amount of the high theoretical energy density of LiCoO₂ as a cathode material has been realized, due to its irreversible deterioration when more than 0.6 mol of lithium ions are extracted. In this study, new insights into the origin of such low electrochemical reversibility, namely the structural collapse caused by electrostatic repulsion between oxygen ions during the charge process are suggested. By incorporating the partial cation migration of LiNiO₂ , which produces a screen effect of cations in the 3b-Li site, the phase distortion of LiCoO₂ is successfully delayed which in turn expands its electrochemical reversibility. This study elucidates the relationship between the structural reversibility and electrochemical behavior of layered cathode materials and enables new design of Co-rich layered materials for cathodes with high energy density.

A study of the structure–activity relationship of the electrochemical performance and Li/Ni mixing of lithium-rich materials by neutron diffraction

4 mol% Mg-doped and 1 mol% Al-doped lithium-rich 0.3Li₂MnO₃·0.7LiNi_0.5Mn_0.5O₂ materials exhibit enhanced electrochemical performance due to reduced Li/Ni mixing.

Effect of electrostatic and size on dopant occupancy in lithium niobate single crystal

We proposed a simple and an effective method to predict the site occupancy and threshold concentration of metal ions in lithium niobate (LiNbO3, LN) single crystal. The ionic energy parameter E(i), defined by the ionic electronegativity and ionic radius, was proposed to describe the electrostatic and size effects of cations on the structural stability of LN. The dopant location can be easily identified by comparing the E(i) deviation of dopant from those of host cations Li(+) and Nb(5+), and the dopant prefers to occupy the lattice site with the smaller deviation of E(i). Our calculated occupancies agree well with those experimental results, which demonstrate the predictive power of our present method. We in this work predicted the preferred occupancies of 60 metal ions in LN single crystal. Further, the threshold concentrations of some frequently used dopants were calculated on the basis of the assumption that all doped LN crystals can endure the same variation of E(i).