High-Rate Structure-Gradient Ni-Rich Cathode Material for Lithium-Ion Batteries

Microstructures of layered Ni-rich cathodes for lithium-ion batteries

Millions of electric vehicles (EVs) on the road are powered by lithium-ion batteries (LIBs) based on nickel-rich layered oxide (NRLO) cathodes, and they suffer from a limited driving range and safety concerns. Increasing the Ni content is a key way to boost the energy densities of LIBs and alleviate the EV range anxiety, which are, however, compromised by the rapid performance fading. One unique challenge lies in the worsening of the microstructural stability with a rising Ni-content in the cathode. In this review, we focus on the latest advances in the understanding of NLRO microstructures, particularly the microstructural degradation mechanisms, state-of-the-art stabilization strategies, and advanced characterization methods. We first elaborate on the fundamental mechanisms underlying the microstructural failures of NRLOs, including anisotropic lattice evolution, microcracking, and surface degradation, as a result of which other degradation processes, such as electrolyte decomposition and transition metal dissolution, can be severely aggravated. Afterwards, we discuss representative stabilization strategies, including the surface treatment and construction of radial concentration gradients in polycrystalline secondary particles, the fabrication of rod-shaped primary particles, and the development of single-crystal NRLO cathodes. We then introduce emerging microstructural characterization techniques, especially for identification of the particle orientation, dynamic changes, and elemental distributions in NRLO microstructures. Finally, we provide perspectives on the remaining challenges and opportunities for the development of stable NRLO cathodes for the zero-carbon future.

Exploring the Role of an Electrolyte Additive in Suppressing Surface Reconstruction of a Ni-Rich NMC Cathode at Ultrahigh Voltage via Enhanced In Situ and Operando Characterization Methods

Vinylene carbonate (VC) is a widely used electrolyte additive in lithium-ion batteries for enhanced solid electrolyte interphase formation on the anode side. However, the cathode electrolyte interphase (CEI) formation with VC has received a lot less attention. This study presents a comprehensive investigation employing advanced in situ/operando-based Raman and X-ray absorption spectroscopy (XAS) to explore the effect of electrolyte composition on the CEI formation and suppression of surface reconstruction of LixNiyMnzCo1-y-zO2 (NMC) cathodes. A novel chemical pathway via VC polymerization is proposed based on experimental results. In situ Raman spectra revealed a new peak at 995 cm-1, indicating the presence of C-O semi-carbonates resulting from the radical polymerization of VC. Operando Raman analysis unveiled the formation of NiO at 490 cm-1 in the baseline system under ultrahigh voltage (up to 5.2 V). However, this peak was conspicuously absent in the VC electrolyte, signifying the effectiveness of VC in suppressing surface reconstruction. Further investigation was carried out utilizing in situ XAS compared X-ray absorption near edge structure spectra from cells of 3 and 20 cycles in both electrolytes at different operating voltages. The observed shift at the Ni K-edge confirmed a more substantial reduction of Ni in the baseline electrolyte compared to that in the VC electrolyte, thus indicating less CEI protection in the former. A sophisticated extended X-ray absorption fine structure analysis quantitatively confirmed the effective suppression of rock-salt formation with the VC electrolyte during the charging process, consistent with the operando Raman results. The in situ XAS results thus provided additional support for the key findings of this study, establishing the crucial role of VC polymerization in enhancing CEI stability and mitigating surface reconstruction on NMC cathodes. This work clarifies the relationship between the enhanced CEI layer and NMC degradation and inspires rational electrolyte design for long-cycling NMC cathodes.

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.

Recent achievements toward the development of Ni-based layered oxide cathodes for fast-charging Li-ion batteries

The driving mileage of electric vehicles (EVs) has been substantially improved in recent years with the adoption of Ni-based layered oxide materials as the battery cathode. The average charging period of EVs is still time-consuming, compared with the short refueling time of an internal combustion engine vehicle. With the guidance from the United States Department of Energy, the charging time of refilling 60% of the battery capacity should be less than 6 min for EVs, indicating that the corresponding charging rate for the cathode materials is to be greater than 6C. However, the sluggish kinetic conditions and insufficient thermal stability of the Ni-based layered oxide materials hinder further application in fast-charging operations. Most of the recent review articles regarding Ni-based layered oxide materials as cathodes for lithium-ion batteries (LIBs) only touch degradation mechanisms under slow charging conditions. Of note, the fading mechanisms of the cathode materials for fast-charging, of which the importance abruptly increases due to the development of electric vehicles, may be significantly different from those of slow charging conditions. There are a few review articles regarding fast-charging; however, their perspectives are limited mostly to battery thermal management simulations, lacking experimental validations such as microscale structure degradations of Ni-based layered oxide cathode materials. In this review, a general and fundamental definition of fast-charging is discussed at first, and then we summarize the rate capability required in EVs and the electrochemical and kinetic properties of Ni-based layered oxide cathode materials. Next, the degradation mechanisms of LIBs leveraging Ni-based cathodes under fast-charging operation are systematically discussed from the electrode scale to the particle scale and finally the atom scale (lattice oxygen-level investigation). Then, various strategies to achieve higher rate capability, such as optimizing the synthesis process of cathode particles, fabricating single-crystalline particles, employing electrolyte additives, doping foreign ions, coating protective layers, and engineering the cathode architecture, are detailed. All these strategies need to be considered to enhance the electrochemical performance of Ni-based oxide cathode materials under fast-charging conditions.

Cathode Electrolyte Interphase-Forming Additive for Improving Cycling Performance and Thermal Stability of Ni-Rich LiNixCoyMn1-x-yO2 Cathode Materials

High-capacity Ni-rich LiNi_xCo_yMn_1-x-yO₂ (NCM) has been investigated as a promising cathode active material for improving the energy density of lithium-ion batteries (LIBs); however, its practical application is limited by its structural instability and low thermal stability. In this study, we synthesized tetrakis(methacryloyloxyethyl)pyrophosphate (TMAEPPi) as a cathode electrolyte interphase (CEI) additive to enhance the cycling characteristics and thermal stability of the LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) material. TMAEPPi was oxidized to form a uniform Li⁺-ion-conductive CEI on the cathode surface during initial cycles. A lithium-ion cell (graphite/NCM811) employing a liquid electrolyte containing 0.5 wt % TMAEPPi exhibited superior capacity retention (82.2% after 300 cycles at a 1.0 C rate) and enhanced high-rate performance compared with the cell using a baseline liquid electrolyte. The TMAEPPi-derived CEI layer on NCM811 suppressed electrolyte decomposition and reduced the microcracking of the NCM811 particles. Our results reveal that TMAEPPi is a promising additive for forming stable CEIs and thereby improving the cycling performance and thermal stability of LIBs employing high-capacity NCM cathode materials.

Controlled Synthesis of Single-Crystalline Ni-Rich Cathodes for High-Performance Lithium-Ion Batteries

Single-crystalline LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) has been considered as one of the most promising cathode materials. It addresses the pulverization issue present in its polycrystalline counterpart by eliminating intergranular cracks. However, synthesis of high-performance single-crystalline NCM is still a challenge owing to the lower structure stability of NCM811 at high calcination temperatures (≥900 °C), which is often required to grow single crystals. Herein, we report a synthesis process for microsized single-crystalline NCM811 particles with exposed (010) facets on their lateral sides [named as SC-NCM(010)], which includes the preparation of a well-dispersed microblock-like Ni_0.8Co_0.1Mn_0.1(OH)₂ precursor through coprecipitation assisted with addition of PVP and Na₂SiO₃ and subsequent lithiation of the precursor at 800 °C. The SC-NCM(010) cathode exhibits an excellent capacity retention rate (91.6% after 200 cycles at 1 C, 4.3 V) and a high rate capability (152.2 mAh/g at 20 C, 4.4 V), much superior to those of polycrystalline NCM811 cathodes. However, despite the removal of interparticle boundaries, when cycled between 2.8 and 4.5 V, the SC-NCM(010) cathode still suffers from structural changes including lattice gliding and intragranular cracking. These structural changes are correlated with the interior structural inhomogeneity, which is evidenced by the coexistence of H2 and H3 phases in the fully deintercalated state.

Scalable Advanced Li(Ni0.8Co0.1Mn0.1)O2 Cathode Materials from a Slug Flow Continuous Process

Li[Ni_0.8Co_0.1Mn_0.1]O₂ (LNCMO811) is the most studied cathode material for next-generation lithium-ion batteries with high energy density. However, available synthesis methods are time-consuming and complex, restricting their mass production. A scalable manufacturing process for producing NCM811 hydroxide precursors is vital for commercialization of the material. In this work, a three-phase slug flow reactor, which has been demonstrated for its ease of scale-up, better synthetic control, and excellent uniform mixing, was developed to control the initial stage of the coprecipitation of NCM811 hydroxide. Furthermore, an equilibrium model was established to predict the yield and composition of the final product. The homogeneous slurry from the slug flow system was obtained and then transferred into a ripening vessel for the necessary ripening process. Finally, the lithium-nickel-cobalt-manganese oxide was obtained through the calcination of the slug flow-derived precursor with lithium hydroxide, having a tap density of 1.3 g cm^-3 with a well-layered structure. As-synthesized LNCMO811 shows a high specific capacity of 169.5 mAh g^-1 at a current rate of 0.1C and a long cycling stability of 1000 cycling with good capacity retention. This demonstration provides a pathway toward scaling up the cathode synthesis process for large-scale battery applications.

F-Doped Ni-Rich Layered Cathode Material with Improved Rate Performance for Lithium-Ion Batteries

Ni-rich layered cathode materials for lithium-ion batteries have received widespread attention due to their large capacity and low cost; however, the structural stability of the material needs to be improved. Herein, F-doped and undoped cathode materials prepared with an advanced co-precipitation method were used to measure the effect of F doping on the material. Compared to the undoped sample, the F-doped cathode materials exhibited an improved rate performance, because the porous structure of F-doped cathode materials is favorable for the infiltration of the electrolyte and the material, and the F-doped cathode material has a larger (003) crystal plane and a smaller Li+ migration barrier energy. This simple F-doping treatment strategy provides a promising way to improve the performance of Ni-rich layered cathode materials for lithium-ion batteries.

3D Correlative Imaging of Lithium Ion Concentration in a Vertically Oriented Electrode Microstructure with a Density Gradient

The performance of Li⁺ ion batteries (LIBs) is hindered by steep Li⁺ ion concentration gradients in the electrodes. Although thick electrodes (≥300 µm) have the potential for reducing the proportion of inactive components inside LIBs and increasing battery energy density, the Li⁺ ion concentration gradient problem is exacerbated. Most understanding of Li⁺ ion diffusion in the electrodes is based on computational modeling because of the low atomic number (Z) of Li. There are few experimental methods to visualize Li⁺ ion concentration distribution of the electrode within a battery of typical configurations, for example, coin cells with stainless steel casing. Here, for the first time, an interrupted in situ correlative imaging technique is developed, combining novel, full-field X-ray Compton scattering imaging with X-ray computed tomography that allows 3D pixel-by-pixel mapping of both Li⁺ stoichiometry and electrode microstructure of a LiNi_0.8 Mn_0.1 Co_0.1 O₂ cathode to correlate the chemical and physical properties of the electrode inside a working coin cell battery. An electrode microstructure containing vertically oriented pore arrays and a density gradient is fabricated. It is shown how the designed electrode microstructure improves Li⁺ ion diffusivity, homogenizes Li⁺ ion concentration through the ultra-thick electrode (1 mm), and improves utilization of electrode active materials.

Electrolyte Additives for Improving the High-Temperature Storage Performance of Li-Ion Battery NCM523∥Graphite with Overcharge Protection

The overcharge safety performance of lithium-ion batteries has been the major bottleneck in the widespread deployment of this promising technology. Pushing the limitations further may jeopardize cell safety when it is performed at high-temperature storage. On the basis of the lacking systematic research on overcharge protection electrolyte additives with high-temperature storage capacity, we explore the promotion effect of overcharge additives on electrolyte decomposition at 60 °C. Specifically, the addition of tris(trimethylsily) phosphite (TMSP) and lithium difluoro(oxalato)borate (LiDFOB) in the electrolyte can not only form the robust cathode electrolyte interface/solid electrolyte interphase (CEI/SEI) but also improve the thermal stability of the electrolyte. Therefore, we promote the electrolyte system to realize the 18,650 LIB storage at 60 °C for 50 days by optimizing the formula in the electrolyte containing biphenyl (BP) and cyclohexylbenzene (CHB) overcharge protection additives, and the capacity retention rate can reach more than 90% with overcharge safety. Further, the optimized electrolyte system has also been implemented to commercial 18,650 LIBs and demonstrates the widening of the route to the widespread application of the electrolyte under extreme conditions.

A Universal Method for Enhancing the Structural Stability of Ni-Rich Cathodes Via the Synergistic Effect of Dual-Element Cosubstitution

Ni-rich layered cathodes suffer detrimental structural changes due to irreversible phase transformation (IPT). Precisely surface structural reconstruction through foreign element doping is a potential method to alleviate IPT propagation. The structure of surface reconstructed layer is greatly determined by the foreign element content and species. Herein, small doses of Ti and Al were co-substituted in LiNi_0.92Co_0.08O₂ to synergistically regulate the surface reductive Ni distribution, consequently constructing thin rock salt phase at the particle surface. This homogeneous rock salt phase combined with the strong Ti-O and Al-O bonds generated a reversible H2-H3 phase transition and further eliminated IPT propagation. Moreover, the suppressed IPT propagation converted the two-phase (H2 and H3) coexistence to a quasi-single-phase transition. This eliminated the strong internal strains caused by a significant lattice mismatch. The Ti and Al co-substituted LiNi_0.92Co_0.08O₂ exhibited outstanding capacity retention and excellent structural stability. Similar improvements were observed with W or Zr and Al cosubstitution in Ni-rich layered cathodes. This study proposes a universal method for comprehensive improvement of structural stability based on the synergistic effect of dual-element cosubstitution in Ni-rich layered oxide cathodes, which is being explored for production of high-cycle-stability lithium-ion batteries.

Roles of Fast-Ion Conductor LiTaO3 Modifying Ni-rich Cathode Material for Li-Ion Batteries

Limited cycling stability hampers the commercial application of Ni-rich materials, which are regarded as one of the most promising cathode materials for Li-ion batteries. Ni-rich LiNi_0.9 Co_0.06 Mn_0.04 O₂ layered cathode was modified with different amounts of LiTaO₃ , and the influences of fast-ion conductor material on cathode materials were explored. Detailed analysis of the materials revealed the formation of a uniformly epitaxial LiTaO₃ coating layer and a little Ta⁵⁺ doping into the lattice structure of Ni-rich materials. The coating-layer thickness increased with the amount of LiTaO₃ added, protecting the electrode from erosion by electrolyte and suppressing undesired parasitic reactions on the cathode-electrolyte interface. Meanwhile, the doped Ta⁵⁺ increased the interplanar spacing of materials, accelerating Li⁺ transfer. Using the positive synergistic effects of LiTaO₃ -coating and Ta⁵⁺ -doping, improved capacity retentions of the modified materials, especially for 0.25 and 0.5 wt%-coated Ni-rich materials, were obtained after long-term cycling, showing the potential applications of LiTaO₃ modification. Further, the relations between one excessively thick coating layer and transfer of Li⁺ /electron between the cathode and electrolyte was established, proving that very thick coating layers, even layers containing Li ions, have adverse effects on electrochemical performances. This finding may help to understand the roles of the coating layer better.

Kinetic Control of the Li0.9Mn1.6Ni0.4O4 Spinel Structure with Enhanced Electrochemical Performance

The development of more sustainable societies has become an urgent goal worldwide. Electrical batteries are currently seen as one of the most important energy storage technologies for the development of decarbonized societies. However, many lithium-ion battery manufacturers currently utilize cobalt, a toxic and hazardous mineral, in their batteries. Lithium-deficient manganese nickel oxide spinels are considered promising candidates owing to their high potential and environmental friendliness. Their electrochemical performance highly depends on their average and local structures, such as phase purities, lattice parameters, and cation sites. Thus, a synthesis protocol should be designed to control these structural parameters to improve their electrochemical performance. In this study, we controlled the average and local structures of Li_0.9Mn_1.6Ni_0.4O₄ spinels obtained by co-precipitation by optimizing their cooling rates. High-resolution techniques, including transmission electron microscopy, synchrotron X-ray diffraction, and Auger-composition analysis combined with density functional theory calculations, X-ray absorption spectroscopy, and electrochemical analysis, were used to understand the average and local structural variations and their effects on the electrochemical properties. As a result, the control of oxygen diffusion at different cooling rates can promote the rearrangement of the structure, resulting in a cation-disordered spinel with minimal variations in lattice parameters and composition. Excellent electrochemical properties were noted in the cation-disordered spinel with high crystallinity and a slightly oxygen-rich surface produced via optimized cooling rates.

New Insights into the Mechanism of Enhanced Performance of Li[Ni0.8Co0.1Mn0.1]O2 with a Polyacrylic Acid-Modified Binder

A binder is an important component in lithium-ion batteries and plays a significant role in maintaining the properties of active substances. Most studies in the field of binders have only focussed on physical properties such as bonding performance. Here, a polyacrylic acid-modified binder was designed and adapted to Li[Ni_0.8Co_0.1Mn_0.1]O₂, which enhanced the electrochemical stability of Li[Ni_0.8Co_0.1Mn_0.1]O₂ from 30.2 to 66.6% (300 cycles at 1 C). We for the first time discovered that this was caused by a chemical reaction between polyacrylic acid and the residual lithium on the surface during the cycling, which formed a lithium propionic acid coating layer and maintained the stability of the layered structure.

High-Temperature Storage Deterioration Mechanism of Cylindrical 21700-Type Batteries Using Ni-Rich Cathodes under Different SOCs

The safety and energy density of lithium-ion batteries (LIBs) are important concerns. The use of high-capacity cathode materials, such as Ni-rich cathodes, can greatly improve the energy density of LIBs, but it also brings some safety hazards. Cylindrical 21700-type batteries using Ni-rich cathodes were employed here to investigate their high-temperature storage deterioration mechanism under different states of charge (SOCs). Electrolyte decomposition was identified as the main problem. It can be worsened by elevated storage temperatures and battery SOCs, with the latter having a more significant influence. Specifically, the decomposition of the LiPF₆ solute and the carbonate solvent will induce hydrofluoric acid (HF) formation and solid-electrolyte interphase (SEI) film regeneration, respectively. HF erosion will aggravate the dissolution of transition metal ions and structural degradation of cathode materials, while the destruction/regeneration of SEI films will consume active lithium and hinder Li⁺ diffusion at the anode side. Besides, the self-discharge behavior will also enlarge the graphite layer spacing, thus decreasing the graphitization degree of graphite anodes and causing anode failure. These findings will aid in the development of strategies for improving the safety of LIBs with high energy density.

Nickel-Rich Layered Cathode Materials for Lithium-Ion Batteries

Nickel-rich layered transition metal oxides are considered as promising cathode candidates to construct next-generation lithium-ion batteries to satisfy the demands of electrical vehicles, because of the high energy density, low cost, and environment friendliness. However, some problems related to rate capability, structure stability, and safety still hamper their commercial application. In this Review, beginning with the relationships between the physicochemical properties and electrochemical performance, the underlying mechanisms of the capacity/voltage fade and the unstable structure of Ni-rich cathodes are deeply analyzed. Furthermore, the recent research progress of Ni-rich oxide cathode materials through element doping, surface modification, and structure tuning are summarized. Finally, this review concludes by discussing new insights to expand the field of Ni-rich oxides and promote practical applications.

Research Progress on the Surface of High-Nickel Nickel-Cobalt-Manganese Ternary Cathode Materials: A Mini Review

To address increasingly prominent energy problems, lithium-ion batteries have been widely developed. The high-nickel type nickel–cobalt–manganese (NCM) ternary cathode material has attracted attention because of its high energy density, but it has problems such as cation mixing. To address these issues, it is necessary to start from the surface and interface of the cathode material, explore the mechanism underlying the material's structural change and the occurrence of side reactions, and propose corresponding optimization schemes. This article reviews the defects caused by cation mixing and energy bands in high-nickel NCM ternary cathode materials. This review discusses the reasons why the core-shell structure has become an optimized high-nickel ternary cathode material in recent years and the research progress of core-shell materials. The synthesis method of high-nickel NCM ternary cathode material is summarized. A good theoretical basis for future experimental exploration is provided.

Riveting Dislocation Motion: The Inspiring Role of Oxygen Vacancies in the Structural Stability of Ni-Rich Cathode Materials

In Ni-rich cathode materials, dislocation can be generated at the surface of primary grains because of the accumulation of stress fields. The migration of dislocation into grains, accelerating the annihilation of reverse dislocation as well as oxygen loss, is considered as the principal origin of crack nucleation, phase transformation, and consequent fast capacity decay. Thus, reducing the dislocation would be effective for improving cathode stability. Here, we report the inspiring role of oxygen vacancies in blocking and anchoring the dislocation. Specifically, a large number of oxygen vacancies can assemble to form dense dislocation layers at the surface of grains. Thanks to the dislocation interaction mechanism, preformed dense dislocation at the surface can effectively rivet the newly developed dislocation during cycling. Ex situ transmission electron microscopy analysis indicates that the intragranular cracks and phase transformation were hindered by the riveted effect, which in turn improved the structural and cycling stability of the Ni-rich cathode. Overall, this work provides novel crystallographic design and understanding of the enhanced mechanical strength of Ni-rich cathode materials.

Clean the Ni-Rich Cathode Material Surface With Boric Acid to Improve Its Storage Performance

The existence of residual lithium compounds (RLCs) on the surface of layered Ni-rich materials will deteriorate the electrochemical properties and cause safety problem. This work presents an effective surface washing method to remove the RLCs from LiNi_0.90Co_0.06Mn_0.04O₂ material surface, via ethyl alcohol solution that contains low concentration of boric acid. It is a low-cost process because the filter liquor can be recycled. The optimal parameters including washing time, boric acid concentration, and solid-liquid ratio were systematically studied. It has been determined by powder pH and Fourier transform infrared spectra results that the amount of RLCs was reduced effectively, and the storage performance was significantly enhanced for the washed samples. The 150th capacity retentions after storing had increased from 68.39% of pristine material to 85.46-94.84% of the washed materials. The performance enhancements should be ascribed to the surface washing process, which removed not only the RLCs, but also the loose primary particles effectively.

Structural and Electrochemical Properties of Low-Cobalt-Content LiNi0.6+xCo0.2-xMn0.2O2 (0.0 ≤ x ≤ 0.1) Cathodes for Lithium-Ion Batteries

The layered oxides LiNi_0.6+xCo_0.2-xMn_0.2O₂ are promising cathode materials for Li-ion batteries (LIBs) owing to their moderate energy densities and structure stabilities. In this study, we systematically investigate the effects of substitution of Co by Ni on the structures, morphologies, and electrochemical properties of LiNi_0.6+xCo_0.2-xMn_0.2O₂ (0.0 ≤ x ≤ 0.1). The physical characteristics of these materials are studied by particle size analysis, scanning electron microscopy, inductively coupled plasma-atomic emission spectroscopy, Rietveld refinement of X-ray diffraction data, and X-ray photoelectron spectroscopy. The electrochemical properties are investigated by charge-discharge cycling, galvanostatic intermittent titration, and electrochemical impedance spectroscopy. As the Co content decreases and the Ni content increases, the discharge capacity and voltage platform are slightly improved, while the initial efficiency, cycling performance, rate capability, and thermal stability gradually decrease. The decreased kinetic performance is attributed to the increased degree of cation mixing and resistance, which decreases the Li⁺ diffusivity. Moreover, the activation energy gradually increases with the decrease in the Co content, which decreases the low-temperature performance. Considering its cost, energy density, cycling lifetime, kinetic performance, and safety properties, LiNi_0.65Co_0.15Mn_0.2O₂ is a promising cathode candidate for use in LIBs.

Improving the Structure and Cycling Stability of Ni-Rich Layered Cathodes by Dual Modification of Yttrium Doping and Surface Coating

A crucial challenge for the commercialization of Ni-rich layered cathodes (LiNi_0.88Co_0.09Al_0.03O₂) is capacity decay during the cycling process, which originates from their interfacial instability and structural degradation. Herein, a one-step, dual-modified strategy is put forward to in situ synthesize the yttrium (Y)-doped and yttrium orthophosphate (YPO₄)-modified LiNi_0.88Co_0.09Al_0.03O₂ cathode material. It is confirmed that the YPO₄ coating layer as a good ion conductor can stabilize the solid-electrolyte interface, while the formative strong Y-O bond can bridle TM-O slabs to intensify the lattice structure in the state of deep delithium (>4.3 V). In particular, both the combined advantages effectively withstand the anisotropic strain generated upon the H2-H3 phase transition and further alleviate the crack generation in unit-cell dimensions, assuring a high-capacity delivery and fast Li⁺ diffusion kinetics. This dual-modified cathode shows advanced cycling stability (94.1% at 1C after 100 cycles in 2.7-4.3 V), even at a high cutoff voltage and high rate, and advanced rate capability (159.7 mAh g^-1 at 10C). Therefore, it provides a novel solution to achieve both high capacity and highly stable cyclability in Ni-rich cathode materials.

Hierarchical Fusiform Microrods Constructed by Parallelly Arranged Nanoplatelets of LiCoO2 Material with Ultrahigh Rate Performance

The past few decades have witnessed the unprecedented success of the commercialized LiCoO₂ layered cathode in consumer electronics, but it still faces the poor rate capability and cycling performance because of its hexagonal layered α-NaFeO₂ structure and the high energy of electrochemically active crystal planes. In a bid to address these problems, we report the delicate design and synthesis of hierarchical fusiform LiCoO₂ microrods constructed by directionally assembled nanoplatelets along the [001] direction via a self-template route (PAHF-LCO). Remarkably, it is the first time that almost all the exposed surfaces of layered cathodes are dominated by the consistent {010} facets, which enable the express channels of Li⁺ diffusion to penetrate throughout the entire fusiform microrods. The as-obtained PAHF-LCO cathode material delivers specific capacities of 113 and 106 mA h g^-1 at 10 and 20 C after 200 cycles, respectively. Even under the high rate of 50 C, the discharge capacity initializes around 105 mA h g^-1 and ends around 80 mA h g^-1 after 200 cycles. The improvement mechanisms to the high-rate performance through crystal habit tuning have also been unraveled. The enhanced electrochemical performance can be attributed to the hierarchical fusiform structure as well as the coordinated crystal orientation of {010} facets.

Ambient Air Stable Ni-Rich Layered Oxides Enabled by Hydrophobic Self-Assembled Monolayer

Ni-rich layered oxides, such as LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811), are considered as promising cathode materials for lithium-ion batteries due to their high energy density. However, Ni-rich layered oxides are prone to react with water and carbon dioxide in ambient air forming residual lithium compounds, resulting in deterioration of electrochemical performance and bringing a challenge to the cathode electrode preparation. In this work, we have, for the first time, demonstrated that the chemical stability of the NCM811 material in ambient air can be significantly enhanced by passivating the surface with a hydrophobic self-assembled monolayer (SAM) of octadecyl phosphate (OPA). As a result, the degradation reaction between the NCM811 material and ambient air and thus the electrochemical performance deterioration were significantly suppressed during ambient air exposure. Specifically, the 5C-rate capacity retention deterioration of the NCM811 sample during 14-day ambient air exposure has been decreased from 12 to 2% by OPA passivation. Furthermore, the 200-cycle capacity retention deterioration of the NCM811 sample after 7-day ambient air exposure has been improved from 23 to 0.7% by OPA passivation. These results are very important for the practical application of Ni-rich oxide since no need for controlling of humidity is required on the cathode manufacture; thus, the cost can be reduced. The concept of molecular self-assembly on the NCM811 material also open vast possibilities to design reagents for surface passivation of Ni-rich layered oxides.

Structure and electrochemical performance modulation of a LiNi0.8Co0.1Mn0.1O2 cathode material by anion and cation co-doping for lithium ion batteries

Ni-rich layered transition metal oxides show great energy density but suffer poor thermal stability and inferior cycling performance, which limit their practical application. In this work, a minor content of Co and B were co-doped into the crystal of a Ni-rich cathode (LiNi_0.8Co_0.1Mn_0.1O₂) using cobalt acetate and boric acid as dopants. The results analyzed by XRD, TEM, XPS and SEM reveal that the modified sample shows a reduced energy barrier for Li⁺ insertion/extraction and alleviated Li⁺/Ni²⁺ cation mixing. With the doping of B and Co, corresponding enhanced cycle stability was achieved with a high capacity retention of 86.1% at 1.0C after 300 cycles in the range of 2.7 and 4.3 V at 25 °C, which obviously outperformed the pristine cathode (52.9%). When cycled after 300 cycles at 5C, the material exhibits significantly enhanced cycle stability with a capacity retention of 81.9%. This strategy for the enhancement of the electrochemical performance may provide some guiding significance for the practical application of high nickel content cathodes.

Ni-rich layered transition metal oxides show great energy density but suffer poor thermal stability and inferior cycling performance, which limit their practical application.