Intergranular Shielding for Ultrafine-Grained Mo-Doped Ni-Rich Li[Ni0.96 Co0.04 ]O2 Cathode for Li-Ion Batteries with High Energy Density and Long Life.

Deploying Ni-enriched (Ni≥95 %) layered cathodes for high energy-density lithium-ion batteries (LIBs) requires resolving a series of technical challenges. Among them, the structural weaknesses of the cathode, vigorous reactivity of the labile Ni4+ ion species, gas evolution and associated cell swelling, and thermal instability issues are critical obstacles that must be solved. Herein, we propose an intuitive strategy that can effectively ameliorate the degradation of an extremely high-Ni-layered cathode, the construction of ultrafine-scale microstructure and subsequent intergranular shielding of grains. The formation of ultrafine grains in the Ni-enriched Li[Ni0.96 Co0.04 ]O2 (NC96) cathode, achieved by impeding particle coarsening during cathode calcination, noticeably improved the mechanical durability and electrochemical performance of the cathode. However, the buildup of the strain-resistant microstructure in Mo-doped NC96 concurrently increased the cathode-electrolyte contact area at the secondary particle surface, which adversely accelerated parasitic reactions with the electrolyte. The intergranular protection of the refined microstructure resolved the remaining chemical instability of the Mo-doped NC96 cathode by forming an F-induced coating layer, effectively alleviating structural degradation and gas generation, thereby extending the battery's lifespan. The proposed strategies synergistically improved the structural and chemical durability of the NC96 cathode, satisfying the energy density, life cycle performance, and safety requirements for next-generation LIBs.

Prelithiation of Lithium Peroxide for Silicon Anode: Achieving a High Activation Rate.

The use of lithium peroxide (Li2O2) as a cost-effective low-weight prelithiation cathode additive was successfully demonstrated. Through a series of studies on the chemical stability of Li2O2 and the activation process of Li2O2 on the cathode, we revealed that Li2O2 is more compatible with conventional electrolyte and cathode laminate slurry than lithium oxide. Due to the significantly smaller size of commercial Li2O2, it can be used directly as a cathode additive. Moreover, the activation of Li2O2 on the cathode leads to the impedance growth of the cathode possibly resulting from the release of dioxygen and evacuation of Li2O2 inside the cathode. With the introduction of a new Li2O2 spread-coating technique on the cathode, the capacity loss was suppressed. Si||NMC full cells using Li2O2 spread-coated cathode demonstrated a highly promising activation rate of Li2O2 and significantly enhanced specific capacity and cycling stability compared to the uncoated full cells.

Probing Depth-Dependent Transition-Metal Redox of Lithium Nickel, Manganese, and Cobalt Oxides in Li-Ion Batteries.

Layered lithium nickel, manganese, and cobalt oxides (NMC) are among the most promising commercial positive electrodes in the past decades. Understanding the detailed surface and bulk redox processes of Ni-rich NMC can provide useful insights into material design options to boost reversible capacity and cycle life. Both hard X-ray absorption (XAS) of metal K-edges and soft XAS of metal L-edges collected from charged LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811) showed that the charge capacity up to removing ∼0.7 Li/f.u. was accompanied with Ni oxidation in bulk and near the surface (up to 100 nm). Of significance to note is that nickel oxidation is primarily responsible for the charge capacity of NMC622 and 811 up to similar lithium removal (∼0.7 Li/f.u.) albeit charged to different potentials, beyond which was followed by Ni reduction near the surface (up to 100 nm) due to oxygen release and electrolyte parasitic reactions. This observation points toward several new strategies to enhance reversible redox capacities of Ni-rich and/or Co-free electrodes for high-energy Li-ion batteries.

The role of an interface in stabilizing reaction intermediates for hydrogen evolution in aprotic electrolytes.

By combining idealized experiments with realistic quantum mechanical simulations of an interface, we investigate electro-reduction reactions of HF, water and methanesulfonic acid (MSA) on the single crystal (111) facets of Au, Pt, Ir and Cu in organic aprotic electrolytes, 1 M LiPF6 in EC/EMC 3:7W (LP57), the aprotic electrolyte commonly used in Li-ion batteries, 1 M LiClO4 in EC/EMC 3:7W and 0.2 M TBAPF6 in 3 : 7 EC/EMC. In our previous work, we have established that LiF formation, accompanied by H2 evolution, is caused by a reduction of HF impurities and requires the presence of Li at the interface, which catalyzes the HF dissociation. In the present paper, we find that the measured potential of the electrochemical response for these reduction reactions correlates with the work function of the electrode surfaces and that the work function determines the potential for Li+ adsorption. The reaction path is investigated further by electrochemical simulations suggesting that the overpotential of the reaction is related to stabilizing the active structure of the interface having adsorbed Li+. Li+ is needed to facilitate the dissociation of HF which is the source of protons. Further experiments on other proton sources, water and methanesulfonic acid, show that if the hydrogen evolution involves negatively charged intermediates, F- or HO-, a cation at the interface can stabilize them and facilitate the reaction kinetics. When the proton source is already significantly dissociated (in the case of a strong acid), there is no negatively charged intermediate and thus the hydrogen evolution can proceed at much lower overpotentials. This reveals a situation where the overpotential for electrocatalysis is related to stabilizing the active structure of the interface, facilitating the reaction rather than providing the reaction energy.

Enhanced Cycling Performance of Ni-Rich Positive Electrodes (NMC) in Li-Ion Batteries by Reducing Electrolyte Free-Solvent Activity.

The interfacial (electro)chemical reactions between electrode and electrolyte dictate the cycling stability of Li-ion batteries. Previous experimental and computational results have shown that replacing Mn and Co with Ni in layered LiNixMnyCo1-x-yO2 (NMC) positive electrodes promotes the dehydrogenation of carbonate-based electrolytes on the oxide surface, which generates protic species to decompose LiPF6 in the electrolyte. In this study, we utilized this understanding to stabilize LiNi0.8Mn0.1Co0.1O2 (NMC811) by decreasing free-solvent activity in the electrolyte through controlling salt concentration and salt dissociativity. Infrared spectroscopy revealed that highly concentrated electrolytes with low free-solvent activity had no dehydrogenation of ethylene carbonate, which could be attributed to slow kinetics of dissociative adsorption of Li+-coordinated solvents on oxide surfaces. The increased stability of the concentrated electrolyte against solvent dehydrogenation gave rise to high capacity retention of NMC811 with capacities greater than 150 mA h g-1 (77% retention) after 500 cycles without oxide-coating and Ni-concentration gradients or electrolyte additives.

High-Throughput Screening of Solid-State Li-Ion Conductors Using Lattice-Dynamics Descriptors.

Low lithium-ion migration barriers have recently been associated with low average vibrational frequencies or phonon band centers, further helping identify descriptors for superionic conduction. To further explore this correlation, here we present the computational screening of ∼14,000 Li-containing compounds in the Materials Project database using a descriptor based on lattice dynamics reported recently to identify new promising Li-ion conductors. An efficient computational approach was optimized to compute the average vibrational frequency or phonon band center of ∼1,200 compounds obtained after pre-screening based on structural stability, band gap, and their composition. Combining a low computed Li phonon band center with large computed electrochemical stability window and structural stability, 18 compounds were predicted to be promising Li-ion conductors, one of which, Li3ErCl6, has been synthesized and exhibits a reasonably high room-temperature conductivity of 0.05-0.3 mS/cm, which shows the promise of Li-ion conductor discovery based on lattice dynamics.

Revealing Electronic Signature of Lattice Oxygen Redox in Lithium Ruthenates and Implications for High-Energy Li-ion Battery Material Designs.

Anion redox in lithium transition metal oxides such as Li2RuO3 and Li2MnO3, has catalyzed intensive research efforts to find transition metal oxides with anion redox that may boost the energy density of lithium-ion batteries. The physical origin of observed anion redox remains debated, and more direct experimental evidence is needed. In this work, we have shown electronic signatures of oxygen-oxygen coupling, direct evidence central to lattice oxygen redox (O2-/(O2)n-), in charged Li2-xRuO3 after Ru oxidation (Ru4+/Ru5+) upon first-electron removal with lithium de-intercalation. Experimental Ru L3-edge high-energy-resolution fluorescence detected X-ray absorption spectra (HERFD-XAS), supported by ab-initio simulations, revealed that the increased intensity in the high-energy shoulder upon lithium de-intercalation resulted from increased O-O coupling, inducing (O-O) σ*-like states with π overlap with Ru d-manifolds, in agreement with O K-edge XAS spectra. Experimental and simulated O K-edge X-ray emission spectra (XES) further supported this observation with the broadening of the oxygen non-bonding feature upon charging, also originated from (O-O) σ* states. This lattice oxygen redox of Li2-xRuO3 was accompanied by a small amount of O2 evolution in the first charge from differential electrochemistry mass spectrometry (DEMS) but diminished in the subsequent cycles, in agreement with the more reduced states of Ru in later cycles from Ru L3-edge HERFD-XAS. These observations indicated that Ru redox contributed more to discharge capacities after the first cycle. This study has pinpointed the key spectral fingerprints related to lattice oxygen redox from a molecular level and constructed a transferrable framework to rationally interpret the spectroscopic features by combining advanced experiments and theoretical calculations to design materials for Li-ion batteries and electrocatalysis applications.

Mechanistic Study of Electrolyte Additives to Stabilize High-Voltage Cathode-Electrolyte Interface in Lithium-Ion Batteries.

Current developments of electrolyte additives to stabilize electrode-electrolyte interface in lithium-ion batteries highly rely on a trial-and-error search, which involves repetitive testing and intensive amount of resources. The lack of understandings on the fundamental protection mechanisms of the additives significantly increases the difficulty for the transformational development of new additives. In this study, we investigated two types of individual protection routes to build a robust cathode-electrolyte interphase at high potentials: (i) a direct reduction in the catalytic decomposition of the electrolyte solvent; and (ii) formation of a "corrosion inhibitor film" that prevents severely attack and passivation from protons that generated from the solvent oxidation, even the decomposition of solvent cannot be mitigated. Effect of two exemplary electrolyte additives, lithium difluoro(oxalato)borate (LiDFOB) and 3-hexylthiophene (3HT), on LiNi0.6Mn0.2Co0.2O2 (NMC 622) cathode were investigated to validate our hypothesis. It is demonstrated that understandings of both electrolyte additives and solvent are essential and careful balance between the cathode protection mechanism of additives and their side effects is critical to obtain optimum results. More importantly, this study opens up new directions of rational design of functional electrolyte additives for the next-generation high-energy-density lithium-ion chemistries.

Systematic control of α-Fe2O3 crystal growth direction for improved electrochemical performance of lithium-ion battery anodes.

α-Fe2O3 nanomaterials with an elongated nanorod morphology exhibiting superior electrochemical performance were obtained through hydrothermal synthesis assisted by diamine derivatives as shape-controlling agents (SCAs) for application as anodes in lithium-ion batteries (LIBs). The physicochemical characteristics were investigated via XRD and FESEM, revealing well-crystallized α-Fe2O3 with adjustable nanorod lengths between 240 and 400 nm and aspect ratios in the range from 2.6 to 5.7. The electrochemical performance was evaluated by cyclic voltammetry and charge-discharge measurements. A SCA test series, including ethylenediamine, 1,2-diaminopropane, 2,3-diaminobutane, and N-methylethylenediamine, was implemented in terms of the impact on the nanorod aspect ratio. Varied substituents on the vicinal diamine structure were examined towards an optimized reaction center in terms of electron density and steric hindrance. Possible interaction mechanisms of the diamine derivatives with ferric species and the correlation between the aspect ratio and electrochemical performance are discussed. Intermediate-sized α-Fe2O3 nanorods with length/aspect ratios of ≈240 nm/≈2.6 and ≈280 nm/≈3.0 were found to have excellent electrochemical characteristics with reversible discharge capacities of 1086 and 1072 mAh g-1 at 0.1 C after 50 cycles.

Chemical versus Electrochemical Electrolyte Oxidation on NMC111, NMC622, NMC811, LNMO, and Conductive Carbon.

We compare the stability of alkyl carbonate electrolyte on NMC111, -622, and -811, LNMO, and conductive carbon electrodes. We prove that CO2 and CO evolution onset potentials depend on the electrode material and increase in the order NMC811 < NMC111 ≈ NMC622 < conductive carbon ≈ LNMO, which we rationalize by two fundamentally different oxidation mechanisms, the chemical and the electrochemical electrolyte oxidation. Additionally, in contrast to the widespread understanding that transition metals in cathode active materials catalyze the electrolyte oxidation, we will prove that such a catalytic effect on the electrochemical electrolyte oxidation does not exist.

Chemical Reactivity Descriptor for the Oxide-Electrolyte Interface in Li-Ion Batteries.

Understanding electrochemical and chemical reactions at the electrode-electrolyte interface is of fundamental importance for the safety and cycle life of Li-ion batteries. Positive electrode materials such as layered transition metal oxides exhibit different degrees of chemical reactivity with commonly used carbonate-based electrolytes. Here we employed density functional theory methods to compare the energetics of four different chemical reactions between ethylene carbonate (EC) and layered (LixMO2) and rocksalt (MO) oxide surfaces. EC dissociation on layered oxides was found energetically more favorable than nucleophilic attack, electrophilic attack, and EC dissociation with oxygen extraction from the oxide surface. In addition, EC dissociation became energetically more favorable on the oxide surfaces with transition metal ions from left to right on the periodic table or by increasing transition metal valence in the oxides, where higher degree of EC dissociation was found as the Fermi level was lowered into the oxide O 2p band.

Kinetic Study of Parasitic Reactions in Lithium-Ion Batteries: A Case Study on LiNi(0.6)Mn(0.2)Co(0.2)O2.

The side reactions between the electrode materials and the nonaqueous electrolytes have been the major contributor to the degradation of electrochemical performance of lithium-ion batteries. A home-built high-precision leakage current measuring system was deployed to investigate the reaction kinetics between the delithiated LiNi(0.6)Mn(0.2)Co(0.2)O2 and a conventional nonaqueous electrolyte. It was found that the rate of parasitic reaction had strong dependence on the upper cutoff potential of the cathode material. The kinetic data also indicated a change of reaction mode at about 4.5 V vs Li(+)/Li.

Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction.

This Review is focused on ion-transport mechanisms and fundamental properties of solid-state electrolytes to be used in electrochemical energy-storage systems. Properties of the migrating species significantly affecting diffusion, including the valency and ionic radius, are discussed. The natures of the ligand and metal composing the skeleton of the host framework are analyzed and shown to have large impacts on the performance of solid-state electrolytes. A comprehensive identification of the candidate migrating species and structures is carried out. Not only the bulk properties of the conductors are explored, but the concept of tuning the conductivity through interfacial effects-specifically controlling grain boundaries and strain at the interfaces-is introduced. High-frequency dielectric constants and frequencies of low-energy optical phonons are shown as examples of properties that correlate with activation energy across many classes of ionic conductors. Experimental studies and theoretical results are discussed in parallel to give a pathway for further improvement of solid-state electrolytes. Through this discussion, the present Review aims to provide insight into the physical parameters affecting the diffusion process, to allow for more efficient and target-oriented research on improving solid-state ion conductors.

Electrode-electrolyte interface in Li-ion batteries: current understanding and new insights.

Understanding reactions at the electrode/electrolyte interface (EEI) is essential to developing strategies to enhance cycle life and safety of lithium batteries. Despite research in the past four decades, there is still limited understanding by what means different components are formed at the EEI and how they influence EEI layer properties. We review findings used to establish the well-known mosaic structure model for the EEI (often referred to as solid electrolyte interphase or SEI) on negative electrodes including lithium, graphite, tin, and silicon. Much less understanding exists for EEI layers for positive electrodes. High-capacity Li-rich layered oxides yLi2-xMnO3·(1-y)Li1-xMO2, which can generate highly reactive species toward the electrolyte via oxygen anion redox, highlight the critical need to understand reactions with the electrolyte and EEI layers for advanced positive electrodes. Recent advances in in situ characterization of well-defined electrode surfaces can provide mechanistic insights and strategies to tailor EEI layer composition and properties.

Simulation study of wave propagation instabilities for the combustion synthesis of transition metals aluminides.

Interest in the mode of propagation of self-sustaining reactions has been motivated by the influence of the mode on the microstructure and composition of the final product. However, comprehensive studies relating the onset of the various propagation modes to the chemical and phase transformations taking place in the sample are still lacking. In the present work propagation instabilities in self-propagating high-temperature synthesis (SHS) of transition metal aluminides are studied using a computer simulation approach. The results are presented for the SHS of NiAl, CoAl, TiAl, and NbAl(3). Particular emphasis is made with respect to the influence of process variables and system parameters on the onset of propagation instabilities, in relation to the physicochemical processes taking place during the propagation of the combustion front.

Incorporation of trivalent cations in synthetic garnets A3B5O12 (A = Y, Lu-La, B = Al, Fe, Ga).

Static-lattice atomistic calculations have been used to study the solution energy for the incorporation of 13 foreign cations at 3 different lattice positions of 12 synthetic garnets. Trends have been obtained as a function of the ionic radius of the dopant cation, and the predictions about site preference have been compared with both literature and experimental data. The preferred substitution site is mainly determined by the ionic size and has been correctly predicted in all cases. Moreover, the energy difference between the preferred substitution site and the next favored site is relatively small in several cases, and hence the foreign ions can be inserted at two different positions by using the correct stoichiometry. A remarkably different behavior has been encountered for Al garnets, due to the smaller size of the unit cell. In particular, some cations, such as Fe3+ and Ga3+, can be inserted at the dodecahedral position usually occupied by the rare-earth ion. Despite the limitations of the static-lattice approach, the results of the present simulations help in the understanding of the defect chemistry of garnets, which is strongly responsible for the physicochemical properties (such as luminescence and ferrimagnetism) that make garnets interesting for technological applications. Such results lead to the possibility of tuning the optical and luminescence properties of garnets by the formation of different types of solid solutions.