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

An Effective Catholyte for Sulfide-Based All-Solid-State Batteries Utilizing Gas Absorbents

All-solid-state batteries (ASSBs) possess the advantage of ensuring safety while simultaneously maximizing energy density, making them suitable for next-generation battery models. In particular, sulfide solid electrolytes (SSEs) are viewed as promising candidates for ASSB electrolytes due to their excellent ionic conductivity. However, a limitation exists in the form of interfacial side reactions occurring between the SSEs and cathode active materials (CAMs), as well as the generation of sulfide-based gases within the SSE. These issues lead to a reduction in the capacity of CAMs and an increase in internal resistance within the cell. To address these challenges, cathode composite materials incorporating zinc oxide (ZnO) are fabricated, effectively reducing various side reactions occurring in CAMs. Acting as a semiconductor, ZnO helps mitigate the rapid oxidation of the solid electrolyte facilitated by an electronic pathway, thereby minimizing side reactions, while maintaining electron pathways to the active material. Additionally, it absorbs sulfide-based gases, thus protecting the lithium ions within CAMs. In this study, the mass spectrometer is employed to observe gas generation phenomena within the ASSB cell. Furthermore, a clear elucidation of the side reactions occurring at the cathode and the causes of capacity reduction in ASSB are provided through density functional theory calculations.

Tuning the Solvation Structure in Water-Based Solution Enables Surface Reconstruction of Layered Oxide Cathodes toward Long Lifespan Sodium-Ion Batteries

Layered oxides of sodium-ion batteries suffer from severe side reactions on the electrode/electrolyte interface, leading to fast capacity degradation. Although surface reconstruction strategies are widely used to solve the above issues, the utilization of the low-cost wet chemical method is extremely challenging for moisture-sensitive Na-based oxide materials. Here, the solvation tuning strategy is proposed to overcome the deterioration of NaNi1/3Mn1/3Fe1/3O2 in water-based solution and conduct the surface reconstruction. When capturing the water molecules by the solvation structure of cations, here is Li+, the structural collapse and degradation of layered oxides in water-based solvents are greatly mitigated. Furthermore, Li(H2O)3EA+ promotes the profitable Li+/Na+ exchange to build a robust surface, which hampers the decomposition of electrolytes and the structural evolution upon cycling. Accordingly, the lifespan of Li-reinforced materials is prolonged to three times that of the pristine one. This work represents a step forward in understanding the surface reconstruction operated in a water-based solution for high-performance sodium layered oxide cathodes.

Origin of oxygen-redox and transition metals dissolution in Ni-rich LixNi0.8Co0.1Mn0.1O2 cathode

Recently, Ni-rich LiNi_xCo_yMn_1-x-yO₂ (x ≥ 0.8) draw significant research attention as cathode materials in lithium-ion batteries due to their superiority in energy density. However, the oxygen release and the transition metals (TMs) dissolution during the (dis)charging process lead to serious safety issues and capacity loss, which highly prevent its application. In this work, we systematically explored the stability of lattice oxygen and TM sites in LiNi_0.8Co_0.1Mn_0.1O₂(NCM811) cathode via investigating various vacancy formations during lithiation/delithiation, and properties such as the number of unpaired spins (NUS), net charges, and d band center were comprehensively studied. In the process of delithiation (x = 1 → 0.75 → 0), the vacancy formation energy of lattice oxygen [E_vac(O)] has been identified to follow the order of E_vac(O-Mn) > E_vac(O-Co) > E_vac(O-Ni), and E_vac(TMs) shows a consistent trend with the sequence of E_vac(Mn) > E_vac(Co) > E_vac(Ni), demonstrating the importance of Mn to stabilize the structural skeleton. Furthermore, the |NUS| and net charge are proved to be good descriptors for measuring E_vac(O/TMs), which show linear correlations with E_vac(O) and E_vac(TMs), respectively. Li vacancy plays a pivotal role on E_vac(O/TMs). E_vac(O/TMs) at x = 0.75 vary extremely between the NiCoMnO layer (NCM layer) and the NiO layer (Ni layer), which correlates well with |NUS| and net charge in the NCM layer but aggregates in a small region in the Ni layer due to the effect of Li vacancies. In general, this work provides an in-depth understanding of the instability of lattice oxygen and transition metal sites on the (104) surface of Ni-rich NCM811, which might give new insights into oxygen release and transition metal dissolution in this system.

Single-Crystalline Cathodes for Advanced Li-Ion Batteries: Progress and Challenges

Single-crystalline cathodes are the most promising candidates for high-energy-density lithium-ion batteries (LIBs). Compared to their polycrystalline counterparts, single-crystalline cathodes have advantages over liquid-electrolyte-based LIBs in terms of cycle life, structural stability, thermal stability, safety, and storage but also have a potential application in solid-state LIBs. In this review, the development history and recent progress of single-crystalline cathodes are reviewed, focusing on properties, synthesis, challenges, solutions, and characterization. Synthesis of single-crystalline cathodes usually involves preparing precursors and subsequent calcination, which are summarized in the details. In the following sections, the development issues of single-crystalline cathodes, including kinetic limitations, interfacial side reactions, safety issues, reversible planar gliding and micro-cracking, and particle size distribution and agglomeration, are systematically analyzed, followed by current solutions and characterization techniques. Finally, this review is concluded with proposed research thrusts for the future development of single-crystalline cathodes.

Single-Crystal LiNix Mny Co1- x - y O2 Cathodes for Extreme Fast Charging

Ni-rich layered LiNi_x Mn_y Co_{1- x - y} O₂ (NMCs, x ≥ 0.8) are poised to be the dominating cathode materials for lithium-ion batteries for the foreseeable future. Conventional polycrystalline NMCs, however, suffer from severe cracking along the grain boundaries of primary particles and capacity loss under high charge and/or discharge rates, hindering their implementation in fast-charging electric vehicular (EV) batteries. Single-crystal (SC) NMCs are attractive alternatives as they eliminate intergranular cracking and allow for grain-level surface optimization for fast Li transport. In the present study, the authors report synthetic approaches to produce SC LiNi_0.8 Co_0.1 Mn_0.1 O₂ (NMC811) samples with different morphologies: Oct-SC811 with predominating (012)-family surface and Poly-SC811 with predominating (104)-family surface. Poly-SC811, representing the first experimentally synthesized NMC811 single crystals with (104) surface, delivers superior performance even at the ultra-high rate of 6 C. Through detailed X-ray analysis and electron microscopy characterization, it is shown that the enhanced performance originates from better chemical and structural stabilities, faster Li⁺ diffusion kinetics, suppressed side reactions with electrolyte, and excellent cracking resistance. These insights provide important design guidelines in the future development of fast-charging NMC-type cathode materials.

Effects of doping high-valence transition metal (V, Nb and Zr) ions on the structure and electrochemical performance of LIB cathode material LiNi0.8Co0.1Mn0.1O2

Ni-rich layered oxides, like LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811), have been widely investigated as cathodes due to their high energy density. However, gradual structural transformation during cycling can lead to capacity degradation and potential decay of cathode materials. Herein, we doped high-valence transition metal (TM) ions (V⁵⁺, Nb⁵⁺, and Zr⁴⁺) at the Ni site of NCM811 by first principles simulations and explored the mechanism of doping TMs in NCMs for enhancing the electrochemical performance. Analysis of the calculations shows that doping V, Nb and Zr has an efficient influence on alleviating the Ni oxidation, reducing the loss of oxygen, and facilitating Li⁺ migration. Moreover, V doping can further suppress the lattice distortion due to the radius of V⁵⁺ being close to the radius of Mn⁴⁺. In particular, compared with the barrier of the pristine NCM in Li divacancy, the barrier of V-doped NCM reaches the lowest. In conclusion, V is the most favorable dopant for NCM811 to improve the electrochemical properties and achieve both high capacity and cycling stability.

Simultaneous Enhancement of Interfacial Stability and Kinetics of Single-Crystal LiNi0.6Mn0.2Co0.2O2 through Optimized Surface Coating and Doping

Balancing interfacial stability and Li⁺ transfer kinetics through surface engineering is a key challenge in developing high-performance battery materials. Although conformal coating enabled by atomic layer deposition (ALD) has shown great promise in controlling impedance increase upon cycling by minimizing side reactions at the electrode-electrolyte interface, the coating layer itself usually exhibits poor Li⁺ conductivity and impedes surface charge transfer. In this work, we have shown that by carefully controlling postannealing temperature of an ultrathin ZrO₂ film prepared by ALD, Zr⁴⁺ surface doping could be achieved for Ni-rich layered oxides to accelerate the charge transfer yet provide sufficient protection. Using single-crystal LiNi_0.6Mn_0.2Co_0.2O₂ as a model material, we have shown that surface Zr⁴⁺ doping combined with ZrO₂ coating can enhance both the cycle performance and rate capability during high-voltage operation. Surface doping via controllable postannealing of ALD surface coating layer reveals an attractive path toward developing stable and Li⁺-conductive interfaces for single-crystal battery materials.

Synthesis and Manipulation of Single-Crystalline Lithium Nickel Manganese Cobalt Oxide Cathodes: A Review of Growth Mechanism

Lithium nickel manganese cobalt oxide (NMC) cathodes are of great importance for the development of lithium ion batteries with high energy density. Currently, most commercially available NMC products are polycrystalline secondary particles, which are aggregated by anisotropic primary particles. Although the polycrystalline NMC particles have demonstrated large gravimetric capacity and good rate capabilities, the volumetric energy density, cycling stability as well as production adaptability are not satisfactory. Well-dispersed single-crystalline NMC is therefore proposed to be an alternative solution for further development of high-energy-density batteries. Various techniques have been explored to synthesize the single-crystalline NMC product, but the fundamental mechanisms behind these techniques are still fragmented and incoherent. In this manuscript, we start a journey from the fundamental crystal growth theory, compare the crystal growth of NMC among different techniques, and disclose the key factors governing the growth of single-crystalline NMC. We expect that the more generalized growth mechanism drawn from invaluable previous works could enhance the rational design and the synthesis of cathode materials with superior energy density.

Mitigating the Microcracks of High-Ni Oxides by In Situ Formation of Binder between Anisotropic Grains for Lithium-Ion Batteries

Increasing attention has been paid to layered high-Ni oxides with high capacity as a promising cathode for high-energy lithium-ion batteries. However, the undesirable microcracks in secondary particles usually occur due to the volume changes of anisotropic primary grains during cycles, which lead to the decay of electrochemical performance. Here, for the first time, a functional electrolyte with di-sec-butoxyaluminoxytriethoxysilane additive integrating the functions of silane and aluminate is proposed to in situ form the binder-like filler between anisotropic primary grains for mitigating the microcracks of high-Ni oxides. It is demonstrated that Li-containing aluminosilicate as a glue layer between the gaps of grains and as a coating layer on the surface of the grains is generated, and these features further enhance the interfacial bonding and surface stability of anisotropic primary grains. Consequently, the microcracks along with side reactions and phase transitions of high-Ni oxides are mitigated. As anticipated, the electrochemical performance and thermal stability of high-Ni oxides are improved, and there is also a capacity retention of 75.4% even after 300 cycles and large reversible capacity of ∼160 mA h g^-1 at 5 C. The functional electrolyte offers a simple, efficient, and scalable method to promote the electrochemical properties and applicability of high-Ni oxide cathodes in high-energy lithium-ion batteries.

Effect of Sodium Content on the Electrochemical Performance of Li-Substituted, Manganese-Based, Sodium-Ion Layered Oxide Cathodes

Li-substituted, manganese-based, layered oxides Na_xLi_0.18Mn_0.66Co_0.17Ni_0.17O_2+δ (x = 0.54, 0.66, 0.78, and 0.90) have been investigated as one kind of high-performance cathode materials for sodium ion batteries (SIBs). Phase compositions and local structures of the cathode materials with varying sodium content were elucidated by powder X-ray diffraction (PXRD) and atomic-scale high angle annular dark field scanning transmission electron microscope (HAADF-STEM), which demonstrates that a Li-O'3 phase was aroused in P2-type sodium layered oxide matrix forming a Na-P2/Li-O'3 hybrid structure. More importantly, the effect of sodium content on the preferential exposure of (102) and (104) facets and surface morphology of the cathode particles has been comprehensively studied, as well as their relationship with electrochemical performance. It reveals that, in addition to the preferential growth of (102) and (104) facets that has been proved to enhance the capacity and rate performance of the layered oxides, the smooth surface finish of the particles also plays a vital role in deciding the electrochemical performance. The layered sodium cathode material with a sodium content of 0.66 possesses sufficient exposure of (102) and (104) facets and smooth side surface, resulting in the superior capacities under various C rates (187 mAh/g at 0.2 C and 114 mAh/g at 5 C) comparing to the cathode materials with all other sodium contents. The mechanism has also been proposed in this study. These findings presented herein open up new strategies to design high performance sodium layered oxide cathode.

Highly Stabilized Ni-Rich Cathode Material with Mo Induced Epitaxially Grown Nanostructured Hybrid Surface for High-Performance Lithium-Ion Batteries

Capacity fading induced by unstable surface chemical properties and intrinsic structural degradation is a critical challenge for the commercial utilization of Ni-rich cathodes. Here, a highly stabilized Ni-rich cathode with enhanced rate capability and cycling life is constructed by coating the molybdenum compound on the surface of LiNi_0.815Co_0.15Al_0.035O₂ secondary particles. The infused Mo ions in the boundaries not only induce the Li₂MoO₄ layer in the outermost but also form an epitaxially grown outer surface region with a NiO-like phase and an enriched content of Mo⁶⁺ on the bulk phase. The Li₂MoO₄ layer is expected to reduce residential lithium species and promote the Li⁺ transfer kinetics. The transition NiO-like phase, as a pillaring layer, could maintain the integrity of the crystal structure. With the suppressed electrolyte-cathode interfacial side reactions, structure degradation, and intergranular cracking, the modified cathode with 1% Mo exhibits a superior discharge capacity of 140 mAh g^-1 at 10 C, a superior cycling performance with a capacity retention of 95.7% at 5 C after 250 cycles, and a high thermal stability.