Cation Mixing Research Articles

Effect of Al doped in Ni-rich LiNi0.6Co0.3Ti0.1O2 cathode materials and their electrochemical performance for rechargeable lithium-ion batteries

The growth of rechargeable lithium-ion batteries (LIBs) for portable electronic devices has attracted the researchers’ attention to improvise the cathode material for better battery performance. With the aim of obtaining high energy density and low cation mixing in the battery system, Al doping was implemented in a Ni-rich cathode system. In this work, undoped and Al-doped into nickel site of LiNi0.6Co0.3Ti0.1O2 (LNCT) nanostructures were successfully synthesized by a combustion method, producing LiNi0.59Co0.3Ti0.1Al0.01O2, LiNi0.57Co0.3Ti0.1Al0.03O2 and LiNi0.55Co0.3Ti0.1Al0.05O2 cathode materials denoted as LNA1, LNA3 and LNA5 respectively. The characterization of the materials was done by using X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). Based on the XRD results, all materials showed pure and single-phase layered structure. Meanwhile, all materials owned polyhedral-like shape as picturized by FESEM results, and as expected, the Al-doped materials show a smaller crystallite size than the undoped material. As for the electrochemical performance, it was found that 1% Al doped in LNCT (LNA1) shows the highest initial discharge capacity of 214.7 mAhg−1 with capacity fading of 23.1% after 50th cycles as compared to other sample. The excellence electrochemical performance of LNA1 sample is due to lower cation mixing and smaller crystallite size which gave them advantage for ease Li+ ions diffusion during delithiated/lithiated process. These findings prove that LNA1 sample has the potential to be used as cathode materials for lithium ion batteries.

Clarifying the Roles of Cobalt and Nickel in the Structural Evolution of Layered Cathodes for Sodium-Ion Batteries.

Layered sodium manganese-based oxides are appealing cathode candidates due to their high capacity and cost-effectiveness, yet performance degradation related with unwanted structural evolution still remains a disturbing disadvantage. Herein, atomic resolution STEM (scanning transmission electron microscopy) images of Na-extracted Na2/3NixCo1/3-xMn2/3O2 (x = 0, 1/6, 1/3) are collected and analyzed, to decipher the effect of cobalt and nickel substitution on the structural integrity of layered manganese-based oxides. Cobalt substitution is demonstrated to alleviate the lattice stress and retain the layered structure after sodium removal, and only a local P2-to-O2 phase transition could be identified. By contrast, various types of defects and phase transformation, including rarely reported P2-to-O3, are discovered in the Ni-substituted oxides. The generation of spinel and rock-salt phases is the critical evidence of cation mixing that leads to unrecoverable capacity loss. The interplay of different transition metals is complex, and compositional optimization is encouraged to minimize the effect of the concomitant phase transition.

A Baseline Kinetic Study of Co-Free Layered Li1+x(Ni0.5Mn0.5)1−xO2 Positive Electrode Materials for Lithium-Ion Batteries

Variations of Li chemical diffusion coefficient () with voltage in a series of Co-free Li1+x(Ni0.5Mn0.5)1−xO2, 0 ≤ x ≤ 0.12, materials were systematically investigated using the recently developed “Atlung Method for Intercalant Diffusion”. The effects of primary and secondary particle sizes, excess Li content, heating temperature, and synthesis atmospheres on were measured. Li-ion kinetics can be enhanced by an order of magnitude by lowering the amount of Ni atoms in the Li layers (cation mixing) from 10% to 4%. Decreasing cation mixing can be accomplished by either increasing the excess Li content or heating temperature. When cycled to 4.6 V, higher specific capacities were obtained, but with a penalty to due to transition metal migration to the Li layers. The primary particles control the Li diffusion length in these materials, regardless of the secondary particle size indicating that grain boundary diffusion must be very rapid. The general trends observed in this work are of great value for the development of higher Mn-containing, Co-free materials. It should be possible to increase energy/power density by making large secondary particles, composed of small primary particles to minimize the solid-state diffusion length while maximizing grain boundary diffusion.

Alkyl-Aryl Cation Mixing in Chiral 2D Perovskites.

We report 2D hybrid perovskites comprising a blend of chiral arylammonium and achiral alkylammonium spacer cations (1:1 mole ratio). These new perovskites feature an unprecedented combination of chirality and alkyl-aryl functionality alongside noncovalent intermolecular interactions (e.g., CH···π interactions), determined by their crystal structures. The mixed-cation perovskites exhibit a circular dichroism that is markedly different from the purely chiral cation analogues, offering new avenues to tune the chiroptical properties of known chiral perovskites, instead of solely relying on otherwise complex chemical syntheses of new useable chiral cations. Further, the ability to dilute the density of chiral cations by mixing with achiral cations may offer a potential way to tailor the spin-based properties in 2D hybrid perovskites, such as Rashba-Dresselhaus spin splitting and chirality-induced spin selectivity and magnetization effects.

(Invited) Operando X-Ray Absorption Fine Structure Studies of Polymer Electrolyte Membrane Electrolysis and Alkaline Water Electrolysis

With increasing global environmental problems, hydrogen, produced by renewable energies, has been considered a sustainable and clean fuel for next-generation energy sources[. Among the many proposed methods to produce hydrogen, the most renewable method is water electrolysis. However, the four electrons/protons needed for the oxygen evolution reaction (OER) in water electrolysis result in sluggish kinetics and low efficiency, representing the biggest obstacle for widespread applications. Polymer electrolysis membrane (PEM) electrolysis is expected to be commercialized due to its high current density, compact system, and high gas purity. Nevertheless, catalyst corrosion in acidic environments and high maintenance costs limit their general application. In contrast, alkaline water electrolysis (AWE) has been considered an alternative method for hydrogen production because earth-abundant transition-metal oxides (TM = Fe, Co, and Ni) can be used as catalysts, and certain states of TM cations accelerate the OER. Clarification of the electronic and local structure of catalysts during water electrolysis is important for understanding the activity and durability of catalysts. We have developed an operando X-ray Absorption Fine Structure (XAS) method that can be used to measure at high current densities under actual water electrolysis conditions. In this presentation, we will present examples of catalyst development using the established operando XAS method.Previous studies have focused on Ni-based oxides, including Ni(OH)2 and doped NiOOH, due to their low material cost, easily mediated structure, and high OER activity originating from various dopants, making them promising catalysts to replace noble metal oxides. Compared to Co-containing perovskite-type oxides, relatively few studies correlating the electronic states of Ni-based oxides with their OER activities have been reported. Recent studies on nickel oxides have elaborated the doping effect of lithium on catalyst activity, such as in LiNiO2 where lithium doping changed the local electronic structure of nickel, improving OER activity. LiNiO2 has a nominal oxidation state of Ni3+ and exhibits excellent OER activity considering the eg state descriptor. Moreover, because the energy levels of the Ni 3d orbitals of LiNiO2 are close to those of the O 2p orbitals, they can strongly hybridize. Therefore, LiNiO2 offers a suitable model to study the relationship between electronic structure and OER activity. However, for practical catalysts, the durability is also important and the deterioration mechanism of the LiNiO2 catalyst has not yet been clarified. LiNiO2, having an ordered rock salt structure, is a well-known cathode material for lithium-ion batteries, and exhibits extremely high lithium-ion mobility. Thus, catalyst degradation may be caused by de-intercalation of lithium ions during anodic polarization. In the Li-ion battery field, many studies have shown that the electrochemical properties of LiNiO2 cathodes are extremely dependent on the degree of cation mixing, primarily due to the presence of Ni2+ at Li+ and Ni3+ sites because of the similarity in their ionic radii. Cation mixing is a disadvantage for cathode materials because the presence of Ni2+ at Li+ sites hinders Li+ diffusion. Inspired by this, hindrance of the lithium diffusion path by cation mixing of Li and Ni is expected to be an effective solution when LiNiO2 is applied as an OER catalyst to improve stability. Herein, the cation mixing effect on OER activity and stability was studied using LixNi2-xO2. A combination of transmission electron microscopy (TEM), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) investigations were conducted to probe the electronic structure of the materials. The valence changes of Ni during OER were further examined using operando XAS using a home-made flow-type cell. The degradation process was also investigated in detail using operando XAS.

High-Performance LiNi0.8Mn0.1Co0.1O2 Cathodes By Atomic-Layer-Deposited Sulfide Coatings

Due to its higher capacity compared to the commercialized LiNixMnyCo1-x-yO2 (NMCs, 0< x, y <1) with lower Ni content (x≤0.6), LiNi0.8Mn0.1Co0.1O2 (NMC811) is now considered as the next promising cathode for lithium-ion batteries (LIBs). However, it still suffers from more serious performance degradation and safety hazard than the commercial ones. These existing issues are reflected as cation mixing, oxygen evolution, irreversible phase-transition, transition metal ion dissolution, and microcracking. Aimed at addressing these problems, surface coating has proven to be an important and effective approach. In recent years, atomic layer deposition (ALD) has emerged as an accurate tool to apply uniform and conformal coatings over NMCs at the atomic level, which have shown remarkable effects on the improved performance of batteries.1-4 Herein, for the first time we deposited ultrathin ALD Li2S coatings5 conformally over NMC811 composite electrodes. It was found that the ALD Li2S coatings enable to enhance performance of NMC811 cathodes with much better cyclability and rate capability. To clarify the effects of the ALD Li2S coatings, we utilized a suite of characterization tools including X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, and transmission X-ray microscopy. The results revealed that the ALD Li2S coatings can perfectly maintain the NMC structure during cycling, thus significantly suppress the crack formation. Furthermore, the Li2S layer can react with and eliminate H2O in electrolyte and O2 released from the NMC structure. In addition, the measurements of electrochemical impedance spectroscopy were applied, which confirms that the Li2S coatings serve as physical protection layers to prevent the interfacial reactions between the NMC cathode and liquid electrolyte. It can be foreseen that sulfides would be a new class of coating materials for addressing issues of NMC cathodes.1. Meng, X.; Yang, X. Q.; Sun, X. L., Advanced Materials 2012, 24 (27), 3589-3615.2. Liu, Y.; Wang, X.; Cai, J.; Han, X.; Geng, D.; Li, J.; Meng, X., Journal of Materials Science & Technology 2020, 54, 77-86.3. Wang, X.; Cai, J.; Liu, Y.; Han, X.; Ren, Y.; Li, J.; Liu, Y.; Meng, X., Nanotechnology 2020, 32 (11), 115401.4. Gao, H.; Cai, J.; Xu, G.-L.; Li, L.; Ren, Y.; Meng, X.; Amine, K.; Chen, Z., Chem. Mater. 2019, 31 (8), 2723-2730.5. Meng, X.; Comstock, D. J.; Fister, T. T.; Elam, J. W., ACS Nano 2014, 8 (10), 10963-10972.

NMC811 Carbon Core Shell Materials for High-Performance 18650 Ni-Rich Li-Ion Batteries

Rechargeable Li-ion batteries (LIB) have been extensively studied and become one of the essential energy storage devices for many applications, especially for electric vehicles (EVs). To date, Ni-rich materials such as lithium nickel manganese cobalt oxide (LiNixMnyCo1-x-yO2; x ≥ 0.6) are believed to be the next-generation cathodes due to their low-cost, high capacity, and high operating voltage.2-4 However, the practical application of these materials has not yet been fully developed because of many intrinsic drawbacks. Herein, a surface coating approach using a green scalable solvent-free mechanofusion process is introduced to prepare an infusing thin layer carbon nanosphere shell to improve the intrinsic drawbacks of NMC811 including poor electrical conductivity, Li+/Ni2+ cation mixing, parasitic reactions with electrolytes, and micro-crack formation. The in operando XRD measurement confirms the fast and uniform phase transition with low Li diffusion/kinetics hindrance. The structural integrity of NMC811cs is not affected by the abrupt shrinkage in c-axis as it shows better cycling stability compared to the pristine NMC811 with less variation in c-direction. The formation of CO2 and H2 at high SOC over 4.0 V can also be minimized as observed by online DEMS. Finally, the practical application of the NMC811cs is demonstrated by the 18650 battery prototype which shows higher specific capacity and energy as well as excellent cycling stability compared to the pristine NMC811. This work provides both fundamental understanding and economical strategy towards highly stable and practical high-energy Li-ion batteries.

Is Aluminum Useful in Ni-Rich Li-Ni-Mn-O Positive Electrode Materials for Lithium-Ion Batteries?

The low cost and high specific capacity of Ni-rich cathode active materials have made them promising materials for affordable, high performance Li-ion batteries. However, increasing the Ni-content in the cathode results in lower cycling performance and lower thermal stability.1 Meanwhile, lithium nickel manganese oxides (Li-Ni-Mn-O) are also inexpensive materials that benefit from high thermal stability and high energy density due to their high operation potential.2 Although manganese improves the performance of Ni-based materials, it also enables more cation mixing and reduces the rate capability of the cathodeAl-doping of cathode materials, although it lowers the initial capacity of the cell, has shown to reduce changes in the lattice of the material during delithiation resulting in improved cycling stability. The presence of Al in the lattice also decreases the charge-transfer resistance by facilitating Li-diffusion.3 Although the presence of Al has proved its virtue in commercial cathode materials such as LiNixCoyAl1-x-yO2 (NCA) and LiNixMnyCozAl1-x-y-zO2 (NMCA) it has yet to be used commercially in Co-free systems. In this work, Li(NixMn1-x)O2 (NM) systems of 3 different particle sizes (3.5 μm, 13 μm and 16 μm) as well as their Al-doped counterparts, Li(NixMn1-x-yAly)O2 (NMA), were studied. Figure 1 shows that the array of layered materials used in this study were optimally synthesized to be stoichiometric, single-phase materials with minimal cation mixing. Half cells and full cells were then constructed to evaluate the effect of Al on cycling stability, Li diffusion measurements were made to study rate capability and accelerated rate calorimetry was used to compare thermal stability. This head-to-head comparison of Li-Ni-Mn-systems with and without Al dopant will determine the effectiveness of Al in Ni-rich, Co-free cathode materials.References Li, H. et al. An Unavoidable Challenge for Ni-Rich Positive Electrode Materials for Lithium-Ion Batteries. (2019) doi: 10.1021/acs.chemmater.9b02372 Dou, S. Review and prospect of layered lithium nickel manganese oxide as cathode materials for Li-ion batteries. 911–926 (2013) doi.org/10.1007/s10008-012-1977-z Wang, G. X., Zhong, S., Bradhurst, D. H., Dou, S. X. & Liu, H. K. LiAlδNi1-δO2 solid solutions as cathodic materials for rechargeable lithium batteries. Solid State Ionics 116, 271–277 (1999). doi.org/10.1016/S0167-2738(98)00351-8 Figure 1

Investigation of a Novel Electrolyte Additive in Improving NCM811/Li Cell Performance at Elevated Temperature

Lithium-ion batteries are the most promising and reliable candidate to be adopted by the electric vehicle industry. In order to be widely used in the electric vehicle industry, there is a growing demand for material that delivers high capacity and high energy density. LiNi0.8Co0.1M0.1O2 (NCM 811) with a layered structure has attracted a significant interest as a cathode material due to its ability to supply a high capacity. Although, the increased Ni content in the NCM 811 results in delivering a higher capacity, it has reduced cycling stability at elevated temperatures and voltages. Active material degradation, phase changes and cation mixing are some of the underlying causes for cycling instabilities in Ni rich NCM811. One method for improving the performance of NCM 811 at high temperature and elevated temperatures involves the incorporation of electrolyte additives that are sacrificially oxidized on the surface of the electrode to generate a passivation film in stabilizing the surface. In this study, the role of a novel additive in improving the long term and high temperature performance of NCM811/Li cells has been investigated. Combination of X-ray photoelectron spectroscopy and ATR_IR has been used to characterize the differences of surface films generated with the additive.

Surface Controlled Nanomaterials Coating NCM for the Improved Stability of Cathodes

Globally, the demand for energy storage technology that satisfies high capacity and high stability is increasing due to the rapid growth of emerging markets such as ESS and electric vehicles and the expansion of IoT, wearable electronics, and smart devices related to the 4th industrial revolution. Among various energy storage devices, lithium-ion batteries (LIBs) are regarded as the best power supplies for portable electronic devices, electric vehicles (EVs) and hybrid electric vehicles (HEVs) based on their excellent energy density. Among the cathode materials of lithium-ion batteries, Ni-rich NCM is expected to be commercialized and expanded the most due to its excellent performance. However, in the case of Ni-rich NCM cathode materials, stability remains a problem. To overcome this limitation, doping of an electrochemically inert element with high polarity and strong binding force with oxygen may be considered. Doping increases electron and ionic conductivity and restricts the movement of Ni2+ ions, thereby reducing cation mixing and maintaining structural stability. In addition, it is possible to form a stable oxygen-tight structure to slow down the capacity reduction of lithium excess oxide. By supplementing the disadvantages of NCM cathode material through doping, it is possible to solve the stability issue of Ni-rich lithium-ion battery and improve performance.References Du, Kai, et al. Nano Energy 83 (2021): 105775.Seong, Won Mo, et al. Angewandte Chemie International Edition 59.42 (2020): 18662-18669.

Synthesis and Characterization of N-Rich Cathodes Metal Oxides Core-Shell Materials for High-Performance Li-Ion Batteries

One of the state-of-the-art cathode materials for lithium-ion batteries is nickel (Ni)-rich lithium nickel manganese cobalt oxide since it has high specific energy and cost-effectiveness. However, the challenges of the Ni-rich cathodes includes cation mixing, micro-crack, metal dissolution into the electrolyte, and parasitic reaction that cause battery failure. To address such issues, this work studies the synergistic effects of the protection layer of the cathode with the core–shell structure using a solvent-free mechano-fusion process and the pillar by doping high-valent metal ions into the structure of the N-rich cathode by annealing at high temperature, which can enhance the performance and life cycle of 18650 cylindrical battery cells.

Design Principles of Large Cation Incorporation in Halide Perovskites.

Perovskites have stood out as excellent photoactive materials with high efficiencies and stabilities, achieved via cation mixing techniques. Overcoming challenges to the stabilization of Perovskite solar cells calls for the development of design principles of large cation incorporation in halide perovskite to accelerate the discovery of optimal stable compositions. Large fluorinated organic cations incorporation is an attractive method for enhancing the intrinsic stability of halide perovskites due to their high dipole moment and moisture-resistant nature. However, a fluorinated cation has a larger ionic size than its non-fluorinated counterpart, falling within the upper boundary of the mixed-cation incorporation. Here, we report on the intrinsic stability of mixed Methylammonium (MA) lead halides at different concentrations of large cation incorporation, namely, ehtylammonium (EA; [CH3CH2NH3]+) and 2-fluoroethylammonium (FEA; [CH2FCH2NH3]+). Density functional theory (DFT) calculations of the enthalpy of the mixing and analysis of the perovskite structural features enable us to narrow down the compositional search domain for EA and FEA cations around concentrations that preserve the perovskite structure while pointing towards the maximal stability. This work paves the way to developing design principles of a large cation mixture guided by data analysis of DFT data. Finally, we present the automated search of the minimum enthalpy of mixing by implementing Bayesian optimization over the compositional search domain. We introduce and validate an automated workflow designed to accelerate the compositional search, enabling researchers to cut down the computational expense and bias to search for optimal compositions.

Fuzzy logic: about the origins of fast ion dynamics in crystalline solids.

Nuclear magnetic resonance offers a wide range of tools to analyse ionic jump processes in crystalline and amorphous solids. Both high-resolution and time-domain , , , NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. It is well accepted that ions are subjected to extremely slow exchange processes in compounds with strong site preferences. The loss of this site preference may lead to rapid cation diffusion, as is also well known for glassy materials. Further examples that benefit from this effect include, e.g. cation-mixed, high-entropy fluorides , Li-bearing garnets () and thiophosphates such as . In non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of defects will affect both the activation energy and the corresponding attempt frequencies. Whereas in () cation mixing influences F anion dynamics, in () the potential landscape can be manipulated by anion site disorder. On the other hand, in the mixed conductor cation-cation repulsions immediately lead to a boost in diffusivity at the early stages of chemical lithiation. Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined (or low-dimensional) dimensions, as is the case in layer-structured or . Diffusion on fractal systems complements this type of diffusion.This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.

Improving high-temperature performance of lithium-rich cathode by roll-to-roll atomic layer deposition of titania nanocoating for lithium-ion batteries

Lithium-rich layered oxides are attractive cathode materials for next-generation lithium-ion batteries thanks to their ultra-high specific capacities commonly surpassing 200 mAh g − 1. However, poor cycling stability and sluggish reaction kinetics inhibit their widespread applications. To alleviate this critical drawback, in this work, we have developed a unique roll-to-roll atomic layer deposition (R2R ALD) apparatus to continuously coat TiO2 nanolayers on the Li-rich cathode sheets. To confirm the efficacy of the design, the TiO2-coated Li-rich cathodes were electrochemically cycled within an aggressive voltage range (2.0–4.8 V) at 25 and 60 °C for 200 cycles. The rate capability, cyclic stability, and electrode polarization were significantly improved for high-temperature operation via coating a finely tuned TiO2 nanolayer on the layered cathodes. The TiO2 nanolayer effectively alleviates the metal migration and cation mixing within the Li-rich layered structure during high-temperature cycling. The R2R ALD technique engineered in this study enables continuous coating of TiO2 nanolayers on the Li-rich electrodes with ultra-high production rates (> 1.2 m min−1).

Synergistic Effect of Microstructure Engineering and Local Crystal Structure Tuning to Improve the Cycling Stability of Ni-Rich Cathodes

Ultrahigh Ni-rich layered oxides have been regarded as one of the most promising cathode candidates. However, cycling instability induced by interfacial reactions and irreversible H2-H3 lattice distortion is yet to be demonstrated by an effective strategy that could construct a stable grain interface and microstructure. Here, Ni-rich cathode LiNi0.92Co0.05Mn0.03O2 is modified by B and Ti to realize the synchronous regulation of a microstructure and the oxygen framework robustness. Compared with the large equiaxed crystalline grains for the pristine cathode, highly elongated grains with a strong radially oriented crystallographic texture in which the (003) facet is maximized are produced for Ti and B-modified LiNi0.92Co0.05Mn0.03O2. With the suppressed H2-H3 phase transition and cation mixing provided by radially oriented grains and turned local crystal oxygen framework robustness during cycling, the co-modified cathode exhibits enhanced Li+ diffusion kinetics and a capacity retention of 78.3% after 100 cycles, which outperformed the 38.5% for the pristine cathode. The improved cycling performance suggests the significance of the turned microstructure and local crystal structure in suppressing internal strain and crystal structure degradation. The synchronous realization of microstructure engineering and local crystal structure turning by optimal element combination would provide a heuristic solution for the construction of high perform Ni-rich cathodes.

Dual-Element-Modified Single-Crystal LiNi0.6Co0.2Mn0.2O2 as a Highly Stable Cathode for Lithium-Ion Batteries.

Single-crystalline LiNi0.6Co0.2Mn0.2O2 cathodes have received great attention due to their high discharge capacity and better electrochemical performance. However, the single-crystal materials are suffering from severe lattice distortion and electrode/electrolyte interface side reactions when cycling at high voltage. Herein, a unique single-crystal LiNi0.6Co0.2Mn0.2O2 with Al and Zr doping in the bulk and a self-formed coating layer of Li2ZrO3 in the surface has been constructed by a facile strategy. The optimized cathode material exhibits excellent structural stability and cycling performance at room/elevated temperatures after long-term cycling. Specifically, even after 100 cycles (1C, 3.0-4.4 V) at 50 °C, the capacity retention for the Al and Zr co-doped sample reaches 92.1%, which is much higher than those of the single Al-doped (85.4%), single Zr-doped (87.1%), and bare samples (76.3%). The characterization results and first-principles calculations reveal that the excellent electrochemical properties are attributed to the stable structure and interface, in which the Al and Zr co-doping hinders cation mixing and suppresses detrimental phase transformations to reduce internal stress and mitigate microcracks, and the coating layer of Li2ZrO3 can protect the surface and suppress interfacial parasitic reactions. Overall, this work provides important insights into how to simultaneously build a stable bulk structure and interface for the single-crystal NCM cathode via a facile preparation process.

Correlating Cation Mixing with Li Kinetics: Electrochemical and Li Diffusion Measurements on Li-Deficient LiNiO2 and Li-Excess LiNi0.5Mn0.5O2

Cation mixing in Li-based layered positive electrode materials has been reported to negatively affect the electrochemical performance and transport properties of intercalated Li. However, no previous reports have systematically correlated the impact of cation mixing (Ni atoms in the Li layer) on the electrochemical properties and Li transport. Herein, a series of Li-deficient LNO (Li1−xNi1+xO2) materials with different amounts of Ni in the Li layers ranging from ca. 1.5%–6.0% was intentionally prepared by varying the Li/Ni ratio during synthesis. An order of magnitude decrease in the Li chemical diffusion coefficient was found between samples with 1.5% and 6% Ni in the Li layer. A similar dependence of the diffusion constant on the amount of Ni in the Li layer was also observed in the Li-excess materials for x = 0, 0.04, 0.08, 0.12, suggesting that, in general, larger amounts of Ni in the Li layer will lead to worse kinetics. This work quantitatively demonstrates that the amount of Ni in the Li layer needs to be carefully considered for the development of high-energy Ni-containing layered positive electrode materials as it directly affects overall electrochemical performance, phase transitions, and Li diffusion, leading to worse kinetics and seriously hindering rate capability.

Three-dimensional Li-ion transportation in Li2MnO3-integrated LiNi0.8Co0.1Mn0.1O2

Ni-rich layered cathodes (LiNixCoyMnzO2) have recently drawn much attention due to their high specific capacities. However, the poor rate capability of LiNixCoyMnzO2, which is mainly originated from the two-dimensional diffusion of Li ions in the Li slab and Li+/Ni2+ cation mixing that hinder the Li+ diffusion, has limited their practical application where high power density is needed. Here we integrated Li2MnO3 nanodomains into the layered structure of a typical Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) material, which minimized the Li+/Ni2+ cationic disordering, and more importantly, established grain boundaries within the NCM811 matrix, thus providing a three-dimensional diffusion channel for Li ions. Accordingly, an average Li-ion diffusion coefficient (DLi+) of the Li2MnO3-integrated LiNi0.8Co0.1Mn0.1O2 (NCM811-I) during charge/discharge was calculated to be approximately 6*10−10 cm2 S−1, two times of that in the bare NCM811 (3*10−10 cm2 S−1). The capacity delivered by the NCM811-I (154.5 mAh g−1) was higher than that of NCM811 (141.3 mAh g−1) at 2 C, and the capacity retention of NCM811-I increased by 13.6% after 100 cycles at 0.1 C and 13.4% after 500 cycles at 1 C compared to NCM811. This work provides a valuable routine to improve the rate capability of Ni-rich cathode materials, which may be applied to other oxide cathodes with sluggish Li-ion transportation.

Enhancing the structural stability and capacity retention of Ni-rich LiNi0.7Co0.3O2 cathode materials via Ti doping for rechargeable Li-ion batteries: Experimental and computational approaches

The Ni-rich cathodes are considered as the next generation candidate cathode material of lithium-ion batteries due to their high-energy–density and environmentally friendly. In this study, the Ni-rich, LiNi0.7Co0.3O2 cathodes material was doped by titanium (Ti) via a self-propagating combustion method. The role of Ti species and its effects on the structural and electrochemical performance of the cathode materials were investigated through experimental and first principles studies. From the XRD results, all materials possessed a single phase with a hexagonal layered structure of the R-3m space group. The Rietveld refinement revealed that the lattice of the Ti-doped LiNi0.7Co0.3O2 sample was found to be expanded in the c-axis and has a lower cation mixing as compared to the pristine samples, which can ease the movement of Li-ions during the delithiation/lithiation process. After 70th cycle, the discharge capacity of the Ti-doped LiNi0.7Co0.3O2 sample possess an excellent capacity retention of 91.9% with a specific discharge capacity of 132.3 mAhg−1 as compared to the pristine sample with only has 86.2 mAhg−1. This can be explained by first-principles study where it was found that the Li-O distances for LNCT become expand after the delithiation process which ease the Li-ions diffusion during cycling. As a whole, the addition of Ti species into the Ni-rich layered cathode materials was found to stabilise the crystal structure of LiNi0.7Co0.3O2 and subsequently improved the lithium-ion kinetics of the layered cathode materials. Apart from lithium nickel manganese cobalt oxide (NMC) and lithium nickel cobalt aluminium oxide (NCA), this study demonstrated that LiNi0.6Co0.3Ti0.1O2 can also serve as potential cathode material for rechargeable Li-ion batteries.

An effective modification strategy enhancing the structure stability and electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode material for lithium-ion batteries

LiNixCoyMn1−x-yO2 (NCM) (x ≥ 0.5) have received more and more attention due to high energy density, which have been widely utilized in the fields of mobile electronic equipment and new energy vehicles. Nevertheless, these materials suffer severe capacity attenuation and inferior rate performance, hindering their commercialization. In this work, we design an effective strategy: co-modification of Nb doping and LiBO2 coating (NB-NCM) to stabilize the internal structure, reduce cation mixing, alleviate external side reactions and improve electrochemical performance. Material characterizations and electrochemical measurements were operated to comprehensively value the effect of co-modification on the structure, topography and electrochemical properties of NCM materials. As a result, Nb is doped into the bulk of the material and the surface of particle is uniformly coated with LiBO2. In addition, for the NB-NCM electrode, the discharge capacity retained 90.90% at 0.5 C rate after 50 cycles, which proves that co-modification is an effective method to stabilize structure and enhance electrochemical performance.