Influence of Li Content on the Topological Inhibition of Oxygen Loss in Li-Rich Cathode Materials.

Lithium-rich layer oxide cathodes are promising energy storage materials due to their high energy densities. However, the oxygen loss during cycling limits their practical applications. Here, the essential role of Li content on the topological inhibition of oxygen loss in lithium-rich cathode materials and the relationship between the migration network of oxygen ions and the transition metal (TM) component are revealed. Utilizing first-principles calculations in combination with percolation theory and Monte Carlo simulations, it is found that TM ions can effectively encage the oxidized oxygen species when the TM concentration in TM layer exceeds 5/6, which hinders the formation of a percolating oxygen migration network. This study demonstrates the significance of rational compositional design in lithium-rich cathodes for effectively suppressing irreversible oxygen release and enhancing cathode cycling performance.

Origin of the Seriously Limited Anionic Redox Reaction of Li-Rich Cathodes in Sulfide All-Solid-State Batteries.

Li-rich layered oxide (LLO) cathode materials with mixed cationic and anionic redox reactions display much higher specific capacity than other traditional layered oxide materials. However, the practical specific capacity of LLO during the first cycle in sulfide all-solid-state lithium-ion batteries (ASSLBs) is extremely low. Herein, the capacity contribution of each redox reaction in LLO during the first charging process is qualitatively and quantitatively analyzed by comprehensive electrochemical and structural measurements. The results demonstrate that the cationic redox of the LiTMO2 (TM = Ni, Co, Mn) phase is almost complete, while the anionic redox of the Li2MnO3 phase is seriously limited due to the sluggish transport kinetics and severe LLO/Li6PS5Cl interface reaction at high voltage. Therefore, the poor intrinsic conductivity and interface stability during the anionic redox jointly restrict the capacity release or delithiation/lithiation degree of LLO during the first cycle in sulfide ASSLBs. This study reveals the origin of the seriously limited anionic redox reaction in LLO, providing valuable guidance for the bulk and interface design of high-energy-density ASSLBs.

Accumulated Lattice Strain as an Intrinsic Trigger for the First-Cycle Voltage Decay in Li-Rich 3d Layered Oxides.

Li- and Mn-rich layered oxides (LMLOs) are promising cathode materials for Li-ion batteries (LIBs) owing to their high discharge capacity of above 250 mA h g-1. A high voltage plateau related to the oxidation of lattice oxygen appears upon the first charge, but it cannot be recovered during discharge, resulting in the so-called voltage decay. Disappearance of the honeycomb superstructure of the layered structure at a slow C-rate (e.g., 0.1 C) has been proposed to cause the first-cycle voltage decay. By comparing the structural evolution of Li[Li0.2Ni0.2Mn0.6]O2 (LLNMO) at various current densities, the operando synchrotron-based X-ray diffraction results show that the lattice strain in bulk LLNMO is continuously increased over cycling, resulting in the first-cycle voltage loss upon Li-ion insertion. Unlike the LLNMO, the accumulated average lattice strain of LiNi0.8Co0.1Mn0.1O2 (NCM811) and LiNi0.6Co0.2Mn0.2O2 (NCM622) from the open-circuit voltage to 4.8 V could be released on discharge. These findings help to gain a deep understanding of the voltage decay in LMLOs.

A Mechanistic Insight into the Oxygen Redox of Li-Rich Layered Cathodes and their Related Electronic/Atomic Behaviors Upon Cycling.

Li-rich cathodes are extensively investigated as their energy density is superior to Li stoichiometric cathode materials. In addition to the transition metal redox, this intriguing electrochemical performance originates from the redox reaction of the anionic sublattice. This new redox process, the so-called anionic redox or, more directly, oxygen redox in the case of oxides, almost doubles the energy density of Li-rich cathodes compared to conventional cathodes. Numerous theoretical and experimental investigations have thoroughly established the current understanding of the oxygen redox of Li-rich cathodes. However, different reports are occasionally contradictory, indicating that current knowledge remains incomplete. Moreover, several practical issues still hinder the real-world application of Li-rich cathodes. As these issues are related to phenomena resulting from the electronic to atomic evolution induced by unstable oxygen redox, a fundamental multiscale understanding is essential for solving the problem. In this review, the current mechanistic understanding of oxygen redox, the origin of the practical problems, and how current studies tackle the issues are summarized.

Revealing the Effect of High Ni Content in Li-Rich Cathode Materials: Mitigating Voltage Decay or Increasing Intrinsic Reactivity.

Li-rich layered oxides are considered as one of the most promising cathode materials for secondary lithium batteries due to their high specific capacities, but the issue of continuous voltage decay during cycling hinders their market entry. Increasing the Ni content in Li-rich materials is assumed to be an effective way to address this issue and attracts recent research interests. However, a high Ni content may induce increased intrinsic reactivity of materials, resulting in severe side reactions with the electrolyte. Thus, a comprehensive study to differentiate the two effects of the Ni content on the cell performance with Li-rich cathode is carried out in this work. Herein, it is demonstrated that a properly dosed amount of Ni can effectively suppress the voltage decay in Li-rich cathodes, while over-loading of Ni, on the contrary, can cause structural instability, Ni dissolution, and nonuniform Li deposition during cycling as well as severe oxygen loss. This work offers a deep understanding on the impacts of Ni content in Li-rich materials, which can be a good guidance for the future design of such cathodes for high energy density lithium batteries.

Stabilizing Cobalt-free Li-rich Layered Oxide Cathodes through Oxygen Lattice Regulation by Two-phase Ru Doping.

The application of Li-rich layered oxides is hindered by their dramatic capacity and voltage decay on cycling. This work comprehensively studies the mechanistic behaviour of cobalt-free Li1.2 Ni0.2 Mn0.6 O2 and demonstrates the positive impact of two-phase Ru doping. A mechanistic transition from the monoclinic to the hexagonal behaviour is found for the structural evolution of Li1.2 Ni0.2 Mn0.6 O2, and the improvement mechanism of Ru doping is understood using the combination of in operando and post-mortem synchrotron analyses. The two-phase Ru doping improves the structural reversibility in the first cycle and restrains structural degradation during cycling by stabilizing oxygen (O2- ) redox and reducing Mn reduction, thus enabling high structural stability, an extraordinarily stable voltage (decay rate <0.45 mV per cycle), and a high capacity-retention rate during long-term cycling. The understanding of the structure-function relationship of Li1.2 Ni0.2 Mn0.6 O2 sheds light on the selective doping strategy and rational materials design for better-performance Li-rich layered oxides.

Quantifying the Anomalous Local and Nanostructure Evolutions Induced by Lattice Oxygen Redox in Lithium-Rich Cathodes.

Due to their accessible lattice oxygen redox (l-OR) at high voltages, Li-rich layered transition metal (TM) oxides have shown promising potential as candidate cathodes for high-energy-density Li-ion batteries. However, this l-OR process is also associated with unusual electrochemical issues such as voltage hysteresis and long-term voltage decay. The structure response mechanism to the l-OR behavior also remains unclear, hindering rational structure optimizations that would enable practical Li-rich cathodes. Here, this study reveals a strong coupling between l-OR and structure dynamic evolutions, as well as their effects on the electrochemical properties. Using the technique of neutron total scattering with pair distribution function analysis and small-angle neutron scattering, this study quantifies the local TM migration and formation of nanopores that accompany the l-OR. These experiments demonstrate the causal relationships among l-OR, the local/nanostructure evolutions, and the unusual electrochemistry. The TM migration triggered by the l-OR can change local oxygen coordination environments, which results in voltage hysteresis. Coupled with formed oxygen vacancies, it will accelerate the formation of nanopores, inducing a phase transition, and leading to irreversible capacity and long-cycling voltage fade.

Sulfuration of Li-Rich Mn-Based Cathode Materials for Multianionic Redox and Stabilized Coordination Environment.

Lithium-rich transition metal oxides (LLOs) can deliver high specific capacity over 250 mAh g-1 , stemming from additional contribution of oxygen redox. However, the formation of O(2- n )- (0 < n < 2) species and even oxygen gas during the deep oxidation stage leads to progressive structural transformation that cause voltage decay/hysteresis, sluggish kinetics, and poor thermostability, preventing real-world application of LLOs. Therefore, the substantive key relies on enhancing the anionic redox stability in LLOs. Here, a sulfuration procedure of LLOs (S-LLOs) is proposed, in which sulfur anions are incorporated into oxygen sites in the lattice structure and form polyanions on the surface. Proved by structural characterizations and density functional theory (DFT) calculations, sulfur anions in the interior lattice can reversibly participate in the redox process and enhance the integral coordination stability by mitigating undesired oxygen redox. Moreover, S polyanions at the surface form a protecting layer for interfacial stability. The electrochemical measurements indicate that S-LLO demonstrates a high discharge capacity of 307.8 mAh g-1 , an outstanding capacity retention rate of 91.5% after 200 cycles, along with excellent voltage maintenance, rate capability, and thermostability. The sulfuration process of LLOs with multianionic redox mechanism highlights a promising strategy to design novel high-energy-density cathode materials with superior cycling performance.

Li-Rich Li2 [Ni0.8 Co0.1 Mn0.1 ]O2 for Anode-Free Lithium Metal Batteries.

Anode-free lithium metal batteries can maximize the energy density at the cell level. However, without the Li compensation from the anode side, it faces much more challenging to achieve a long cycling life with a competitive energy density than Li metal-based batteries. Here, we prolong the lifespan of an anode-free Li metal battery by introducing Li-rich Li2 [Ni0.8 Co0.1 Mn0.1 ]O2 into the cathode as a Li-ions extender. The Li2 [Ni0.8 Co0.1 Mn0.1 ]O2 can release a large amount of Li-ions during the first charging process to supplement the Li loss in the anode, then convert into NCM811, thus extending the lifespan of the battery without the introduction of inactive elements. By the benefit of Li-rich cathode and high reversibility of Li metal on Cu foil, the anode-free pouch cells enable to achieve 447 Wh kg-1 energy density and 84 % capacity retention after 100 cycles in the condition of limited electrolyte addition (E/C ratio of 2 g Ah-1 ).

Local Electric-Field-Driven Fast Li Diffusion Kinetics at the Piezoelectric LiTaO3 Modified Li-Rich Cathode-Electrolyte Interphase.

As one of the most promising cathodes for next-generation lithium ion batteries (LIBs), Li-rich materials have been extensively investigated for their high energy densities. However, the practical application of Li-rich cathodes is extremely retarded by the sluggish electrode-electrolyte interface kinetics and structure instability. In this context, piezoelectric LiTaO3 is employed to functionalize the surface of Li1.2Ni0.17Mn0.56Co0.07O2 (LNMCO), aiming to boost the interfacial Li+ transport process in LIBs. The results demonstrate that the 2 wt% LiTaO3-LNMCO electrode exhibits a stable capacity of 209.2 mAh g-1 at 0.1 C after 200 cycles and 172.4 mAh g-1 at 3 C. Further investigation reveals that such superior electrochemical performances of the LiTaO3 modified electrode results from the additional driving force from the piezoelectric LiTaO3 layer in promoting Li+ diffusion at the interface, as well as the stabilized bulk structure of LNMCO. The supplemented LiTaO3 layer on the LNMCO surface herein, sheds new light on the development of better Li-rich cathodes toward high energy density applications.

Revealing Grain-Boundary-Induced Degradation Mechanisms in Li-Rich Cathode Materials.

Despite their high energy densities, Li- and Mn-rich, layered-layered, xLi2MnO3·(1 - x)LiTMO2 (TM = Ni, Mn, Co) (LMR-NMC) cathodes require further development in order to overcome issues related to bulk and surface instabilities such as Mn dissolution, impedance rise, and voltage fade. One promising strategy to modify LMR-NMC properties has been the incorporation of spinel-type, local domains to create "layered-layered-spinel" cathodes. However, precise control of local structure and composition, as well as subsequent characterization of such materials, is challenging and elucidating structure-property relationships is not trivial. Therefore, detailed studies of atomic structures within these materials are still critical to their development. Herein, aberration corrected-scanning transmission electron microscopy (AC-STEM) is utilized to study atomic structures, prior to and subsequent to electrochemical cycling, of LMR-NMC materials having integrated spinel-type components. The results demonstrate that strained grain boundaries with various atomic configurations, including spinel-type structures, can exist. These high energy boundaries appear to induce cracking and promote dissolution of Mn by increasing the contact surface area to electrolyte as well as migration of Ni during cycling, thereby accelerating performance degradation. These results present insights into the important role that local structures can play in the macroscopic degradation of the cathode structures and reiterate the complexity of how synthesis and composition affect structure-electrochemical property relationships of advanced cathode designs.

Low-Dose Aberration-Free Imaging of Li-Rich Cathode Materials at Various States of Charge Using Electron Ptychography.

Imaging the complete atomic structure of materials, including light elements, with minimal beam-induced damage of the sample is a long-standing challenge in electron microscopy. Annular bright-field scanning transmission electron microscopy is often used to image elements with low atomic numbers, but due to its low efficiency and high sensitivity to precise imaging parameters it comes at the price of potentially significant beam damage. In this paper, we show that electron ptychography is a powerful technique to retrieve reconstructed phase images that provide the full structure of beam-sensitive materials containing light and heavy elements. Due to its much higher efficiency, we can reduce the beam currents used down to the subpicoampere range. Electron ptychography also allows residual lens aberrations to be corrected at the postprocessing stage, which avoids the need for fine-tuning of the probe that would result in further beam damage and provides aberration-free reconstructed phase images. We have used electron ptychography to obtain structural information from aberration-free reconstructed phase images in the technologically relevant lithium-rich transition metal oxides at different states of charge. We can unambiguously determine the position of the lithium and oxygen atomic columns while amorphization of the surface, formation of beam-induced surface reconstruction layers, or migration of transition metals to the alkali layers are drastically reduced.

Improved Cycling Stability and Fast Charge-Discharge Performance of Cobalt-Free Lithium-Rich Oxides by Magnesium-Doping.

Layered Li-rich, Co-free, and Mn-based cathode material, Li1.17Ni0.25-xMn0.58MgxO2 (0 ≤ x ≤ 0.05), was successfully synthesized by a coprecipitation method. All prepared samples have typical Li-rich layered structure, and Mg has been doped in the Li1.17Ni0.25Mn0.58O2 material successfully and homogeneously. The morphology and the grain size of all material are not changed by Mg doping. All materials have a estimated size of about 200 nm with a narrow particle size distribution. The electrochemical property results show that Li1.17Ni0.25-xMn0.58MgxO2 (x = 0.01 and 0.02) electrodes exhibit higher rate capability than that of the pristine one. Li1.17Ni0.25-xMn0.58MgxO2 (x = 0.02) indicates the largest reversible capacity of 148.3 mAh g-1 and best cycling stability (capacity retention of 95.1%) after 100 cycles at 2C charge-discharge rate. Li1.17Ni0.25-xMn0.58MgxO2 (x = 0.02) also shows the largest discharge capacity of 149.2 mAh g-1 discharged at 1C rate at elevated temperature (55 °C) after 50 cycles. The improved electrochemical performances may be attributed to the decreased polarization, reduced charge transfer resistance, enhanced the reversibility of Li+ ion insertion/extraction, and increased lithium ion diffusion coefficient. This promising result gives a new understanding for designing the structure and improving the electrochemical performance of Li-rich cathode materials for the next-generation lithium-ion battery with high rate cycling performance.

Considering Critical Factors of Li-rich Cathode and Si Anode Materials for Practical Li-ion Cell Applications.

In order to keep pace with increasing energy demands for advanced electronic devices and to achieve commercialization of electric vehicles and energy-storage systems, improvements in high-energy battery technologies are required. Among the various types of batteries, lithium ion batteries (LIBs) are among the most well-developed and commercialized of energy-storage systems. LIBs with Si anodes and Li-rich cathodes are one of the most promising alternative electrode materials for next-generation, high-energy batteries. Si and Li-rich materials exhibit high reversible capacities of <2000 mAh g(-1) and >240 mAh g(-1) , respectively. However, both materials have intrinsic drawbacks and practical limitations that prevent them from being utilized directly as active materials in high-energy LIBs. Examples for Li-rich materials include phase distortion during cycling and side reactions caused by the electrolyte at the surface, and for Si, large volume changes during cycling and low conductivity are observed. Recent progress and important approaches adopted for overcoming and alleviating these drawbacks are described in this article. A perspective on these matters is suggested and the requirements for each material are delineated, in addition to introducing a full-cell prototype utilizing a Li-rich cathode and Si anode.