Theoretical Prediction of Surface Stability and Morphology of LiNiO2 Cathode for Li Ion Batteries

Universal Neural Network Potential-Driven Molecular Dynamics Study of CO2/O2 Evolution at the Ethylene Carbonate/Charged-Electrode Interface

Long-term durability and safety are required to develop Li-ion batteries that can operate at high voltages. However, side reactions, including the release of O2 from the electrode and CO2 from the organic electrolyte, occur at the positive-electrode/electrolyte interface during charging at high voltages. In this study, universal neural network potential (UNNP)-driven molecular dynamics (MD) calculations are used to investigate the mechanism of the reaction between LixCoO2 (0 ≤ x ≤ 1) or LixNiO2 (0 ≤ x ≤ 1), as the positive-electrode material, and an ethylene-carbonate-based electrolyte, with a solid-liquid interface composed of ∼1700 atoms. Molecular CO2 and O2 evolve from the partially or fully Li-deintercalated LixNiO2, while no gas-evolution reactions are observed for LixCoO2. Hence, compared LixNiO2, the LiCoO2 electrode is more stable toward the decomposition of ethylene carbonate in the charged state. The decomposition reactions at the solid-liquid interface during charging are also analyzed using a NN force field. This study provides a robust approach involving MD simulations using UNNP to better understand the side reactions in electrochemical devices, which can guide manufacturers in selecting appropriate materials.

Origins of High Air Sensitivity and Treatment Strategies in O3-Type NaMn1/3 Fe1/3Ni1/3O2

Sodium-ion layered oxides are one of the most highly regarded sodium-ion cathode materials and are expected to be used in electric vehicles and large-scale grid-level energy storage systems. However, highly air-sensitive issues limit sodium-ion layered oxide cathode materials to maximize cost advantages. Industrial and scientific researchers have been developing cost-effective air sensitivity treatment strategies with little success because the impurity formation mechanism is still unclear. Using density functional theory calculations and ab initio molecular dynamics simulations, this work shows that the poor air stability of O3-type NaMn1/3Fe1/3Ni1/3O2 (NMFNO) may be as follows: (1) low percentage of nonreactive (003) surface; (2) strong surface adsorption capacity and high surface reactivity; and (3) instability of the surface sodium ions. Our physical images point out that the high reactivity of the NMFNO surface originates from the increase in electron loss and unpaired electrons (magnetic moments) of the surface oxygen active site as well as the enhanced metal coactivation effect due to the large radius of the sodium ion. We also found that the hydrolysis reaction requires a higher reactivity of the surface oxygen active site, while the carbon hybridization mode transformation in carbonate formation depends mainly on metal activation and does not even require the involvement of surface oxygen active sites. Based on the calculation results and our proposed physical images, we discuss the feasibility of these treatment strategies (including surface morphology modulation, cation/anion substitution, and surface configuration design) for air-sensitive issues.

A novel cathode interphase formation methodology by preferential adsorption of a borate-based electrolyte additive

The coupling of high-capacity cathodes and lithium metal anodes promises to be the next generation of high-energy-density batteries. However, the fast-structural degradations of the cathode and anode challenge their practical application. Herein, we synthesize an electrolyte additive, tris(2,2,3,3,3-pentafluoropropyl) borane (TPFPB), for ultra-stable lithium (Li) metal||Ni-rich layered oxide batteries. It can be preferentially adsorbed on the cathode surface to form a stable (B and F)-rich cathode electrolyte interface film, which greatly suppresses the electrolyte-cathode side reactions and improves the stability of the cathode. In addition, the electrophilicity of B atoms in TPFPB enhances the solubility of LiNO3 by 30 times in ester electrolyte to significantly improve the stability of the Li metal anode. Thus, the Li||Ni-rich layered oxide full batteries using TPFPB show high stability and an ultralong cycle life (up to 1500 cycles), which also present excellent performance even under high voltage (4.8 V), high areal mass loading (30 mg cm-2) and wide temperature range (-30∼60°C). The Li||LiNi0.9Co0.05Mn0.05O2 (NCM90) pouch cell using TPFPB with a capacity of 3.1 Ah reaches a high energy density of 420 Wh kg-1 at 0.1 C and presents outstanding cycling performance.

Ga-Doped Ultrahigh-Nickel Oxide Microspheres with Radially Aligned Primary Grains as a Cathode for Stable Cycling Li-Ion Batteries

Owing to the high energy density, ultrahigh-nickel (Ni > 0.9) layered oxides are used as promising cathode materials for next-generation Li-ion batteries. Unfortunately, the serious pulverization and rapid capacity fading during cycling limit the commercial viability of an ultrahigh-nickel oxide cathode. Herein, the introduction of Ga into LiNi0.96Co0.04O2 brings a radially aligned microstructural change of oxide microspheres during the lithiation of the Ni0.96Co0.04(OH)2 precursor. As expected, such radially aligned needle-like primary grains on microspheres have a positive influence to reduce the anisotropic volume change and suppress the formation of microcracks of Ga-induced Li(Ni0.96Co0.04)0.99Ga0.01O2 during cycling. Specifically, compared with irregular primary grains of LiNi0.96Co0.04O2, Ga-induced oxide presents a high initial discharge capacity of 227.9 mA h g-1 at 0.1C rate between 2.8 and 4.3 V. Especially, Ga-induced oxide delivers higher initial discharge capacities of 233.9 and 240.3 mA h g-1 with higher cutoff charge voltages of 4.4 and 4.5 V at 0.1C, respectively. Furthermore, a good capacity retention of 74.1% at 1 C rate is obtained after 300 cycles, which is almost 85% higher than that of the pristine sample, mainly due to the generation of microcracks of oxide microspheres during the long-term cycle. Therefore, the introduction of Ga into LiNi0.96Co0.04O2 is a feasible approach for improving the microstructure and cycling stability of the ultrahigh-Ni layered oxides.

A review on nickel-rich nickel-cobalt-manganese ternary cathode materials LiNi0.6Co0.2Mn0.2O2 for lithium-ion batteries: performance enhancement by modification

The new energy era has put forward higher requirements for lithium-ion batteries, and the cathode material plays a major role in the determination of electrochemical performance. Due to the advantages of low cost, environmental friendliness, and reversible capacity, high-nickel ternary materials are considered to be one of ideal candidates for power batteries now and in the future. At present, the main design idea of ternary materials is to fully consider the structural stability and safety performance of batteries while maintaining high energy density. Ternary materials currently face problems such as low lithium-ion diffusion rate and irreversible collapse of the structure, although the battery performance can be improved utilizing coating, ion doping, etc., the actual demand requires a more effective modification method based on the intrinsic properties of the material. Based on the summary of the current research status of the ternary material LiNi0.6Co0.2Mn0.2O2 (NCM622), a comparative study of the modification paths of the material was conducted from the level of molecular action mechanism. Finally, the major problems of ternary cathode materials and the future development direction are pointed out to stimulate more innovative insights and facilitate their practical applications.

Vanadium Ferrocyanides as a Highly Stable Cathode for Lithium-Ion Batteries

Owing to their high redox potential and availability of numerous diffusion channels in metal-organic frameworks, Prussian blue analogs (PBAs) are attractive for metal ion storage applications. Recently, vanadium ferrocyanides (VFCN) have received a great deal of attention for application in sodium-ion batteries, as they demonstrate a stable capacity with high redox potential of ~3.3 V vs. Na/Na⁺. Nevertheless, there have been no reports on the application of VFCN in lithium-ion batteries (LIBs). In this work, a facile synthesis of VFCN was performed using a simple solvothermal method under ambient air conditions through the redox reaction of VCl₃ with K₃[Fe(CN)₆]. VFCN exhibited a high redox potential of ~3.7 V vs. Li/Li⁺ and a reversible capacity of ~50 mAh g^-1. The differential capacity plots revealed changes in the electrochemical properties of VFCN after 50 cycles, in which the low spin of Fe ions was partially converted to high spin. Ex situ X-ray diffraction measurements confirmed the unchanged VFCN structure during cycling. This demonstrated the high structural stability of VFCN. The low cost of precursors, simplicity of the process, high stability, and reversibility of VFCN suggest that it can be a candidate for large-scale production of cathode materials for LIBs.

Stabilization of crystal and interfacial structure of Ni-rich cathode material by vanadium-doping

Stable structure and interface of nickel-rich metal oxides is crucial for practical application of next generation lithium-ion batteries with high energy density. Bulk doping is the promising strategy to improve the structural and interfacial stability of the materials. Herein, we report the impact of vanadium-doping on the structure and electrochemical performance of LiNi_0.88Co_0.09Al_0.03O₂ (NCA88). Vanadium doped in high oxidation state (+5) would lead to alteration of the crystal lattice and Li⁺/Ni²⁺ cation mixing. Those are the main factors determining the cycling and rate capability of the materials. With optimization of vanadium-doping, the preservation of the integrity of the secondary particles of the materials, the enhancement of the diffusion of Li⁺ ions, and alleviation of the side reactions of the electrolyte can be efficiently achieved. As a result, NCA88 doped with vanadium of 1.5 mol % can provide superior cycling stability with capacity retention of 84.3% after 250 cycles at 2C, and rate capability with capacity retention of 65.5% at 10C, as compared to the corresponding values of 58.6% and 55% for the pristine counterpart, respectively. The results might be helpful to the selection of dopants in the design of the nickel-rich materials.

Comparison of LiTaO3 and LiNbO3 Surface Layers Prepared by Post- and Precursor-Based Coating Methods for Ni-Rich Cathodes of All-Solid-State Batteries

Surface coating is essential for the cathode materials applied in all-solid-state batteries (ASSBs) based on sulfide electrolytes because of the instability of the cathode/sulfide interface. In contrast with those for general lithium ion batteries (LIBs) using a liquid electrolyte, the coating materials for ASSBs require different functional properties such as high ionic conductivity, low reactivity with sulfide electrolytes, and low electronic conductivity. In addition to LiNbO₃, which is the most popular coating material for ASSBs, LiTaO₃ is another highly promising coating material, and both materials mostly satisfy these requirements. In this work, LiTaO₃ and LiNbO₃ were used to coat the surface of LiNi_0.82Co_0.12Mn_0.06O₂ cathodes for ASSBs. Further, the effects of two different coating methods, postcoating and precursor-based (PB) coating, were characterized and compared. The postcoating method simply forms a coating layer, whereas the PB coating method offers an additional doping effect owing to the diffusion of coating ions into the cathode structure. Surface coating considerably increased the capacity of the ASSB cathodes under all experimental conditions. With the same coating amount and method, the effect of the LiTaO₃ coating was similar or superior to that of the LiNbO₃ coating. Compared with the postcoating method, however, the PB coating method resulted in a superior rate capability and cyclic performance, which was mostly attributed to the doping effect of Ta or Nb. An X-ray photoelectron spectroscopy analysis confirmed that both the LiTaO₃ and LiNbO₃ coatings suppressed side reactions. Among the coatings we examined, the LiTaO₃ coating prepared by the PB method most effectively enhanced the electrochemical performance of the cathodes for sulfide electrolyte-based ASSBs.

Research Progress of Single-Crystal Nickel-Rich Cathode Materials for Lithium Ion Batteries

Single-crystal nickel-rich cathode materials (SC-NRCMs) are the most promising candidates for next-generation power batteries which enable longer driving range and reliable safety. In this review, the outstanding advantages of SC-NRCMs are discussed systematically in aspects of structural and thermal stabilities. Particularly, the intergranular-crack-free morphology exhibits superior cycling performance and negligible parasitic reactions even under severe conditions. Besides, various synthetic methods are summarized and the relation between precursor, sintering process, and final single-crystal products are revealed, providing a full view of synthetic methods. Then, challenges of SC-NRCMs in fields of kinetics of lithium diffusion and the one particularly occurred at high voltage (intragranular cracks and aggravated parasitic reactions) are discussed. The corresponding mechanism and modifications are also referred. Through this review, it is aimed to highlight the magical morphology of SC-NRCMs for application perspective and provide a reference for following researchers.

In Situ Monitoring of Thermally Induced Effects in Nickel-Rich Layered Oxide Cathode Materials at the Atomic Level

The thermal stability of cathode active materials (CAMs) is of major importance for the safety of lithium-ion batteries (LIBs). A thorough understanding of how commercially viable layered oxide CAMs behave at the atomic length scale upon heating is indispensable for the further development of LIBs. Here, structural changes of Li(Ni_0.85Co_0.15Mn_0.05)O₂ (NCM851005) at elevated temperatures are studied by in situ aberration-corrected scanning transmission electron microscopy (AC-STEM). Heating NCM851005 inside the microscope under vacuum conditions enables us to observe phase transitions and other structural changes at high spatial resolutions. This has been primarily possible by establishing low-dose electron beam conditions in STEM. Specific focus is put on the evolution of inherent nanopore defects found in the primary grains, which are believed to play an important role in LIB degradation. The onset temperature of structural changes is found to be ∼175 °C, resulting in phase transformation from a layered to a rock-salt-like structure, especially at the internal interfaces, and increasing intragrain inhomogeneity. The reducing environment and heat application lead to the formation and subsequent densification of {003}- and {014}-type facets. In the light of these results, postsynthesis electrode drying processes applied under reducing environment and heat, for example, in the preparation of solid-state batteries, should be re-examined carefully.

Stabilization of a Highly Ni-Rich Layered Oxide Cathode through Flower-Petal Grain Arrays

Nickel adds to the capacity of layered oxide cathodes of lithium-ion batteries but comprises their stability. We report a petal-grained Li[Ni_0.89Co_0.10Sb_0.01]O₂ cathode that is, nevertheless, stable. The stability originates from the ordering of the nanosized grains in a dense, flower-petal-like array, where the elongated and nearly parallel grains radiate from the center to the surface. The ordering of the grains prevents microcrack generation from abrupt lattice changes of the stressful H2-H3 phase transition. The tight packing of the nanograins is conserved upon cycling, preventing destructive seepage of the electrolytic solution into the particles. The half-cell, cycling between 2.7-4.3 V versus Li/Li⁺ at a 0.5 C rate retains 95.0% of its initial capacity of 220 mAh g^-1 after 100 cycles. The full-cell, cycling with a graphite anode and between 3.0-4.2 V at a 1 C rate, retains 83.9% of its initial capacity after 1000 cycles.

Understanding the electrochemical properties of bulk phase and surface structures of Na3TMPO4CO3 (TM = Fe, Mn, Co, Ni) from first principles calculations

First principles calculations were performed to investigate the electrochemical performance (voltage, cycling stability, electrical conductivity, mechanical properties and safety) of the bulk phase and surface structures of Na₃T^MPO₄CO₃ (T^M = Fe, Mn, Co, Ni). Na₃FePO₄CO₃ and Na₃MnPO₄CO₃ are estimated to be promising candidates for the cathode materials of sodium ion batteries because of the moderate voltages, good stability and high safety during the cycling process of two sodium ions per formula unit. For the purpose of improving the rate performances, Na₃MnPO₄CO₃ was chosen as an example to explore its surface performance. The surface energies, equilibrium morphology, redox potentials and electronic conductivities of surfaces are explored in detail. The results suggest that (010), (001), (111) and (110) orientations are the dominating surfaces in the Wulff shape, while the surfaces (010) and (001) possess high second surface redox potentials, corresponding to the unsatisfactory specific capacity and ionic conductivity. Moreover, low surface band gaps are discovered in all orientations, which gives a good explanation for the enhanced electronic conductivity as a consequence of decreasing particle size. In addition, the (110), (101) and (12-1) surfaces display significantly lower surface band gaps and comparatively lower second redox potentials, thus enlarging the relative surface areas of surfaces (110), (101) and (12-1) could be an efficient methodology to further improve the specific capacity and electronic conductivity of the Na₃MnPO₄CO₃ material.

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.

Ab Initio Study on Surface Segregation and Anisotropy of Ni-Rich LiNi1-2yCoyMnyO2 (NCM) (y ≤ 0.1) Cathodes

Advances in ex situ and in situ (operando) characteristic techniques have unraveled unprecedented atomic details in the electrochemical reaction of Li-ion batteries. To bridge the gap between emerging evidences and practical material development, an elaborate understanding on the electrochemical properties of cathode materials on the atomic scale is urgently needed. In this work, we perform comprehensive first-principle calculations within the density functional theory + U framework on the surface stability, morphology, and elastic anisotropy of Ni-rich LiNi_1-2yCo_yMn_yO₂ (NCM) (y ≤ 0.1) cathode materials, which are strongly related to the emerging evidence in the degradation of Li-ion batteries. On the basis of the surface stability results, the equilibrium particle morphology is obtained, which is mainly determined by the oxygen chemical potential. Ni-rich NCM particles are terminated mostly by the (012) and (001) surfaces for oxygen-poor conditions, whereas the termination corresponds to the (104) and (001) surfaces for oxygen-rich conditions. Besides, Ni surface segregation predominantly occurs on the (100), (110), and (104) nonpolar surfaces, showing a tendency to form a rocksalt NiO domain on the surface because of severe Li-Ni exchange. The observed elastic anisotropy reveals that an uneven deformation is more likely to be formed in the particles synthesized under poor-oxygen conditions, leading to crack generation and propagation. Our findings provide a deep understanding of the surface properties and degradation of Ni-rich NCM particles, thereby proposing possible solution mechanisms to the factors affecting degradation, such as synthesis conditions, coating, or novel nanostructures.