High-energy Cathode Materials Research Articles

Graphene-Incorporated Nano-Structured Iron Oxyfluoride with Excellent Performance As Cathode Materials for Li-Ion Batteries

Lithium-ion batteries (LIBs) are the major power sources for electronic and transportation applications due to their high specific energy, good cyclability and environmental friendliness1, 2 Intercalation compounds were commercially employed as the positive (cathode) electrode material. However, they have a limited capacity due to the mono-valence change of host materials and the accompanied 1-e- transfer process, which accommodates 1-Li ion intercalation. These materials can’t meet the growing demand on higher specific energy and energy/power density. To achieve high specific energy cathode materials, efforts are being made to find alternative cathode materials for LIBs: (1) materials with transition metal ions capable of multi valence changes, and (2) materials with high potentials (vs. Li/Li+). Compared with intercalation cathode materials, conversion cathode materials can store multiple electrons instead of 1 electron of the intercalation materials. However, the conversion materials have not been used as practical LIB cathode materials due to the low electric conductivity, and structural stability. Iron oxyfluoride (FeOF) was proposed3, 4 as a promising candidate because its high theoretical specific capacity of 885 mAh/g, leading to an exceptionally high theoretical specific energy of 2938 Wh/kg for 3- electron reactions. Graphene was employed to solve the low conductivity and structural stability. As the results from the improved FeOF structure, the nanostructured FeOF with the incorporated graphene sheets shows the superior performance to its blank. The FeOF/G initial specific capacity reached 621 mAh/g (Fig. 1) with an initial Coulombic efficiency of 94.7%, while the blank FeOF initial specific capacity can reach 586 mAh/g but has a drastically low Coulombic efficiency of 38.7%. SEM and TEM (Fig. 2) were employed to observe the morphology. The structure evolution during charge/discharge process were also characterized by in situ XAS and high resolution XRD. The results showed that the graphene nano-sheets serve as substrates to stabilize the structure of FeOF and form a framework to stabilize the discharged species through bonding them to their original sites without migration, in which the graphene sheets serve as a “cage” to trap the discharged species. Thus, the FeOF/Graphene composite can keep the (de)lithiation reaction reversible during discharge and charge process and could be a promising cathode material for Li-ion batteries. Fig. 1 Charge/discharge curves of FeOF (BLK) and FeOF/graphene (GRP). Fig. 2 SEM image (a) and TEM image (b) of FeOF/Graphene hybrid material. J. B. Goodenough and K.-S. Park, Journal of the American Chemical Society, 2013, 135, 1167-1176.J.-M. Tarascon and M. Armand, Nature, 2001, 414, 359-367.A. Kitajou, H. Komatsu, R. Nagano and S. Okada, Journal of Power Sources, 2013, 243, 494-498.K. M. Wiaderek, O. J. Borkiewicz, E. Castillo-Martínez, R. Robert, N. Pereira, G. G. Amatucci, C. P. Grey, P. J. Chupas and K. W. Chapman, Journal of the American Chemical Society, 2013, 135, 4070-4078. Figure 1

Gas Formation and Crossover Effect in High Voltage Li-Ion Full Cells with Ni-Rich NMC Cathodes

High voltage high energy cathode materials for Li-ion battery are highly needed. High nickel content NMC (e.g. NMC811) is believed to be a promising candidate. When raised the upper cutoff voltage of NMC811 cathode to 4.5 V vs. Li+/Li, the cathode materials could deliver a capacity above 215 mAh g-1, which is very attractive to the electric vehicle application. However, raising the upper cutoff voltage will also greatly accelerate the capacity fade of the full cell. Of the various causes proposed by the battery community, a large volume of gases generated at high voltage is believed to be one of the major issues. In this study, we constructed full pouch cells composed of graphite anode, NMC811 cathode and standard GEN2 electrolyte which is 1.2M LiPF6 in EC/EMC (3/7 by weight). We also constructed anode and cathode symmetric pouch cells to investigate the gas evolution and crosstalk at specific sides. Using Archimedes’ Principle, we measured the generated gas volume along cycles within the pouch cells with different cutoff voltages. We found out that the full cells using 4.2 V cutoff voltage barely generated gases along the course of 100 cycles while the ones using 4.4 V cutoff voltage generated a large amount of gases in the first 20 cycles, but the gases would be gradually consumed in the subsequent cycles. By using GC-MS, we determined the main gas species generated in the full cell to be carbon dioxide and alkanes. Moreover, the relative amount of carbon dioxide decreased along cycles while the amount of alkanes would increase. In cathode symmetric cells, CO2 remained the major component after 100 cycles and its signal was much stronger than in full cells. Interestingly, some fluorinated alkanes not seen in the full cells were also identified in cathode symmetric cells, suggesting potential involvement and loss of LiPF6 from electrolyte due to the parasitic side reactions on the cathode. In anode symmetric cells, hydrocarbons attributed to electrolyte decomposition were the main products. Unlike CO2 and fluorinated alkanes formed on the cathode, these species were relatively stable and remained in the full cell after extensive cycling. It was also shown that gas crossover to the anode leads to the depletion of gaseous products during cycling, which could stabilize the cell cycle performance. Although some inactive surface reconstruction layer was formed on the cathode at high voltage operation, impedance buildup and active lithium inventory loss contributed to the significant capacity loss in the full cell. Our study revealed the important role of crossover effect in stabilizing Ni-rich layered oxide cathode, and provided a better understanding on capacity fading to take more effective measures for improving the full cell cycle life.

Improvements to Disordered Rock-Salt Li-Excess Cathode Materials

Cathode materials with higher energy density than layered oxide materials are required for future demands of vehicle electrification. Disordered rock-salt Li-excess structures, such as Li3NbO4, have been demonstrated to achieve capacities of greater than 300 mAh/g reversible capacities at elevated temperatures. The high capacity is believed to be due to reversible redox chemistry of the oxide anions. This new class of high energy cathode materials provides an opportunity for a step change increase in cell level energy density. However, improvements are still required in material conductivity and stability. In this presentation, we demonstrate material improvements which enable high specific capacity at room temperature and extended cycle life.

Layered Iron Vanadate As a High-Energy Cathode Material for Nonaqueous Calcium-Ion Batteries

Calcium-ion batteries (CIBs) are one of the promising candidates for post-lithium-ion batteries. However, only a few types of host materials are known to intercalate calcium ions reversibly in a nonaqueous electrolyte. Here, we demonstrate the electrochemical performances and evidence of calcium-ion intercalation into iron vanadate, FeV3O9∙xH2O, as a cathode material for CIBs at ambient temperature. FeV3O9∙xH2O was prepared via a facile water-bath method. It shows reversible calcium intercalation with a first discharge capacity of 334.4 mAh g−1 at 20 mA g−1 rate with an average voltage of ~2.26 V (vs. Ca/Ca2+) and excellent capacity retention in an electrolyte of 0.5 M Ca(ClO4)2 in acetonitrile. The high performance of the material with a layered structure and a wide range of vanadium oxidation states provides insights into the search for new oxide-based high-energy cathode materials for CIBs.

Analysis of Minerals as Electrode Materials for Ca-based Rechargeable Batteries

Rechargeable lithium-ion batteries dominate the consumer electronics and electric vehicle markets. However, concerns on Li availability have prompted the development of alternative high energy density electrochemical energy storage systems. Rechargeable batteries based on a Ca metal anode can exhibit advantages in terms of energy density, safety and cost. The development of rechargeable Ca metal batteries requires the identification of suitable high specific energy cathode materials. This work focuses on Ca-bearing minerals because they represent stable and abundant compounds. Suitable minerals should contain a transition metal able of being reversibly reduced and oxidized, which points to several major classes of silicates and carbonates: olivine (CaFeSiO4; kirschsteinite), pyroxene (CaFe/MnSi2O6; hedenbergite and johannsenite, respectively), garnet (Ca3Fe/Cr2Si3O12; andradite and uvarovite, respectively), amphibole (Ca2Fe5Si8O22(OH)2; ferroactinolite) and double carbonates (CaMn(CO3)2; kutnahorite and CaFe(CO3)2; ankerite). This work discusses their electrode characteristics based on crystal chemistry analysis and density functional theory (DFT) calculations. The results indicate that upon Ca deintercalation, compounds such as pyroxene, garnet and double carbonate minerals could display high theoretical energy densities (ranging from 780 to 1500 Wh/kg) with moderate structural modifications. As a downside, DFT calculations indicate a hampered Ca mobility in their crystal structures. The overall analysis then disregards olivine, garnet, pyroxene, amphibole and double carbonates as structural types for future Ca-cathode materials design.

Revealing the Electrochemical Mechanism of Cationic/Anionic Redox on Li-Rich Layered Oxides via Controlling the Distribution of Primary Particle Size.

Lithium-manganese-rich layered oxides have sparked intense interest on high-energy-density lithium-ion batteries because of their unique anionic redox. While making efforts to adjust the primary particle size for the improved electrochemical kinetics, current research is insufficient to explain how the anionic/cationic interplay governs the electrochemical behavior except for ion diffusion. Here, fully ordered Li1.2Mn0.54Ni0.13Co0.13O2 spheres of shortened primary particle size were synthesized via the coprecipitation method for use as cathodes. A high discharge capacity of 303.2 mA h g-1 was achieved for the first cycle. Optimized nanostructures reduce the lithium ion diffusion length and increase electronic conductivity unsurprisingly, contributing to excellent electrochemical activity and rate capability. Furthermore, the decreased primary particle size accelerated the redox reactivity of cations and the reversibility of anionic redox on path dependence. This work clarifies the electrochemical mechanism of cationic/anionic redox of these Li-rich layered oxides and provides a new vision for a unique design of high-energy cathode materials with better application.

Preparation and electrochemical performances of Ni-rich LiNi0.91Co0.06Mn0.03O2 cathode for high-energy LIBs

Abstract One important challenge of lithium ion batteries is to improve the energy density while maintaining long-term cyclability. The energy density is strongly dependent on the Ni content in LiNi1-x-yCoxMnyO2. Herein, Ni-rich LiNi0.91Co0.06Mn0.03O2 has been synthesized as a high energy cathode material by co-precipitation method and the electrochemical performance of the LiNi0.91Co0.06Mn0.03O2 has been investigated. The granule morphology LiNi0.91Co0.06Mn0.03O2 with high crystallinity is obtained and which delivers a discharge capacity of 208.3 mA h g−1 with cyclability of 61.9%, after 100 cycles and rate performance of 85.6%, at 2 C. These findings indicate that LiNi0.91Co0.06Mn0.03O2 is one of the promising candidate cathode for high-energy lithium ion batteries.

Precise Surface Engineering of Cathode Materials for Improved Stability of Lithium-Ion Batteries.

As lithium-ion batteries continue to climb to even higher energy density, they meanwhile cause serious concerns on their stability and reliability during operation. To make sure the electrode materials, particularly cathode materials, are stable upon extended cycles, surface modification becomes indispensable to minimize the undesirable side reaction at the electrolyte-cathode interface, which is known as a critical factor to jeopardizing the electrode performance. This Review is targeted at a precise surface control of cathode materials with focus on the synthetic strategies suitable for a maximized surface protection ensured by a uniform and conformal surface coating. Detailed discussions are taken on the formation mechanism of the designated surface species achieved by either wet-chemistry routes or instrumental ones, with attention to the optimized electrochemical performance as a result of the surface control, accordingly drawing a clear image to describe the synthesis-structure-performance relationship to facilitate further understanding of functional electrode materials. Finally, perspectives regarding the most promising and/or most urgent developments for the surface control of high-energy cathode materials are provided.

Synthesis of $$\hbox {V}_{2} \hbox {O}_{5}$$ V 2 O 5 nanoparticles: cathode materials for lithium-ion batteries

Vanadium oxide-based nanomaterials have been showing great promise as cathode materials for lithium-ion batteries (LIBs). Among these, nanostructured $$\hbox {V}_{2}\hbox {O}_{5}$$ shows a high discharge capacity due to its layer structure and thermodynamically stable form. This work reports the synthesis of $$\hbox {V}_{2}\hbox {O}_{5 }$$ nanoparticles via a simple low temperature hydrothermal method using ammonium vanadate and quinol. The reduced size of $$\hbox {V}_{2}\hbox {O}_{5 }$$ has resulted in the blue shift of the absorption spectrum. The material has been examined as a cathode material to study lithium intercalation/deintercalation. It shows an initial discharge capacity of $$310 \hbox { mAh } \hbox {g}^{-1}$$ at a current density of $$0.1 \hbox { mA } \hbox {g}^{-1}$$ at 1.5–4 V and retains a specific discharge capacity of $$184 \hbox { mAh } \hbox {g}^{-1}$$ even after 58 cycles. The present study manifests how the nanostructured size $$\hbox {V}_{2}\hbox {O}_{5}$$ could be applied as a high-energy cathode material for LIBs.

Phase Control on Surface for the Stabilization of High Energy Cathode Materials of Lithium Ion Batteries.

The development of high energy electrode materials for lithium ion batteries is challenged by their inherent instabilities, which become more aggravated as the energy densities continue to climb, accordingly causing increasing concerns on battery safety and reliability. Here, taking the high voltage cathode of LiNi0.5Mn1.5O4 as an example, we demonstrate a protocol to stabilize this cathode through a systematic phase modulating on its particle surface. We are able to transfer the spinel surface into a 30 nm shell composed of two functional phases including a rock-salt one and a layered one. The former is electrochemically inert for surface stabilization while the latter is designated to provide necessary electrochemical activity. The precise synthesis control enables us to tune the ratio of these two phases, and achieve an optimized balance between improved stability against structural degradation without sacrificing its capacity. This study highlights the critical importance of well-tailored surface phase property for the cathode stabilization of high energy lithium ion batteries.

High-Energy and Stable Li[Ni0.90Co0.05Mn0.05]O2 Cathode for Lithium-Ion Batteries through Microstructure Modification By Boron Doping

Lithium-ion batteries (LIBs) are emerging as one of the most promising power sources for portable devices, as well as electric vehicles (EVs) and large energy storage systems (ESS), owing to their high energy density and excellent cycle life. However, the application of LIBs in EVs to reduce CO2 emissions in internal combustion engine vehicles (ICEVs) involves some critical technical challenges, such as the driving range, safety, and cost concerns due to the properties of LIBs.1-3 For this reason, research interests and industrial efforts have focused on developing low-cost, high-energy, and safe cathode materials, which are the main component determining LIB performance and cost. Transition metal layered oxides belonging to the LiNixCoyAlzO2 (NCA) or LiNixCoyMnzO2 (NCM) family, which can increase their intrinsic capacity (above 200 mAh g-1) by increasing Ni content, are emerging as promising new cathode materials.4 However, the intrinsic drawbacks of Ni-rich cathodes, such as low Li intercalation stability over extended cycles, should be improved. In this study, we present the first successful boron doping strategy for a Ni-rich cathode material with very high Ni content. For the boron doping, we used a simple method where only a small amount of B2O3 was added during the calcination process. In addition, the structural and morphological stability of the boron-doped Ni-rich NCM cathode material was investigated before and after testing to evaluate the improved electrochemical performance; specifically, the significantly enhanced cycle performance. Thus, we conclude that the boron doping strategy is an effective and simple way to mitigate the poor stability problem of Ni-rich cathode materials for high-energy and stable LIBs. References D.-W. Jun, C. S. Yoon, U.-H. Kim, Y.-K. Sun, Chem. Mater. 2017, 29, 5048.J. H. Lee, C. S. Yoon, J.-Y. Hwang, S.-J. Kim, F. Maglia, P. Lamp, S.-T. Myung, Y.-K. Sun, Energy Environ. Sci. 2016, 9, 2152.S.-T. Myung, F. Maglia, K.-J. Park, C. S. Yoon, P. Lamp, S.-J. Kim, Y.-K. Sun, ACS Energy Lett. 2017, 2, 196.K.-S. Lee, S.-T. Myung, K. Amine, H. Yashiro, Y.-K. Sun, J. Electrochem. Soc. 2007, 154, A971.

High capacity sodium-rich layered oxide cathode for sodium-ion batteries**Project suppoted by the National Natural Science Foundation of China (Grant Nos. 11774017, 51761135129, and 51472010) and Beijing Municipal High Level Innovative Team Building Program, China (Grant No. IDHT20170502).

Sodium-ion batteries have attracted significant recent attention currently considering the limited available lithium resource. However, the energy density of sodium-ion batteries is still insufficient compared to lithium-ion batteries, mainly because of the unavailability of high-energy cathode materials. In this work, a novel sodium-rich layered oxide material (Na2MnO3) is reported with a dynamical stability similar to that of the Li2MnO3 structure and a high capacity of , based on first-principles calculations. Sodium ion de-intercalation and anionic reaction processes are systematically investigated, in association with sodium ions migration phenomenon and structure stability during cycling of NaxMnO3 (). In addition, the charge compensation during the initial charging process is mainly contributed by oxygen, where the small differences of the energy barriers of the paths , , , , and indicate the reversible sodium ion occupancy in transitional metal and sodium layers. Moreover, the slow decrease of the elastic constants is a clear indication of the high cycle stability. These results provide a framework to exploit the potential of sodium-rich layered oxide, which may facilitate the development of high-performance electrode materials for sodium-ion batteries.

H2V3O8 as a High Energy Cathode Material for Nonaqueous Magnesium-Ion Batteries

Magnesium-ion batteries (MIBs) suffer from a low energy density of cathode materials in a conventional nonaqueous electrolyte, contrary to the expectation due to the divalent Mg ion. Here, we report H2V3O8, or V3O7·H2O, as a high-energy cathode material for MIBs. It exhibits reversible magnesiation–demagnesiation behavior with an initial discharge capacity of 231 mAh g–1 at 60 °C, and an average discharge voltage of ∼1.9 V vs Mg/Mg2+ in an electrolyte of 0.5 M Mg(ClO4)2 in acetonitrile, resulting in a high energy density of 440 Wh kg–1. The structural water remains stable during cycling. The crystal structure for Mg0.97H2V3O8 is determined for the first time. Bond valence sum difference mapping shows facile conduction pathways for Mg ions in the structure. The high performance of this material with its distinct crystal structure employing water–metal bonding and hydrogen bonding provides insights to search for new oxide-based stable and high-energy materials for MIBs.

(Invited) Recent Advances in the DFT-Based Search of Cathode Materials for Ca Batteries

In the last years, concerns on Li availability have prompted the search for suitable alternatives to rechargeable Li-ion batteries. Batteries based on multivalent charge carriers (Mn+) can exhibit advantages in terms of energy density since for a certain amount of M carriers the capacity is doubled (n=2) or tripled (n=3) when compared to single valent carriers like Li+ or Na+. Calcium is an attractive candidate due to its low cost, natural abundance and low reduction potential. At this very early stage of the Ca-batteries technology, the quest for a high specific energy cathode material is an active field. The general goal of our work is the identification of materials able to function as positive electrode in Ca batteries. DFT is a unique tool to quickly screen a variety of structures and compositions to identify potential cathode materials for this novel technology. The general strategy is firstly to calculate average voltage and volume changes associated to the Ca insertion reaction. Further, DFT theory could be applied to gather additional information regarding electronic and ionic conductivities, crystal structure of intercalated phases and precise voltage-composition-curves at various temperatures. In this communication, we will present a survey of recent results for oxides, silicates and sulphides as cathode for Ca batteries. Special attention is given to the calculation and analysis of Ca migration pathways using the NEB method. In good agreement with parallel experimental investigations [1-3], we found that the low ionic mobility is the main handicap for the insertion/deinsertion of Ca ions in most structures. Yet, for some materials, Ca intercalation is predicted at low C rates. Acknowledgments: Authors are grateful for financial support from European Union H2020-FETOPEN funded project CARBAT-766617. References; [1]. M.E. Arroyo-de Dompablo, C. Krich, J. Nava-Avendaño, M.R. Palacin, F. Barde. Phys. Chem. Chem. Phys. 18 (2016) 19966. [2]. M.E. Arroyo-de Dompablo, C. Krich, J. Nava-Avendaño, N. Biskup, M.R. Palacin, F. Barde. Chem. Mater. 28 (2016) 6886. [3]. D. Tchitchekova, A. Ponrouch, R. Verrelli, T. Broux, C. Frontera, A. Sorrentino, F. Barde, N. Biskup, M.E. Arroyo-de Dompablo, M.R. Palacin. Chem. Mater. 30 (2018) 847.

Dual Coatings, Triple the Benefit: A Study in ALD Surface-Modifications to Nickel-Rich Architectures

NMC 811 has elicited great attention as a high energy cathode material for next generation lithium ion batteries. With 80% Ni content, NMC 811 has surpassed energy densities of previous generations of NMC cathodes. However, there exist inherent drawbacks to increasing nickel content, which limit the long-term cell performance. As compared to structurally similar NMCs, 811 suffers from poor cycle life and decreased stability due to the surface reactivity of now heightened nickel in the fully oxidized state. A number of surface-modification methods have been adopted to address these significant degradation issues including atomic layer deposition (ALD). ALD is a platform technology that has been widely demonstrated throughout the literature to impart significant processing and performance gains on lithium ion battery technologies, having notable impacts on surface stabilization of the positive electrode. In this work we will explore ALD as a means of controlling surface phenomena in cell architectures containing NMC 811, and present evidence for improved performance under high voltage conditions. In addition, we will demonstrate the synergistic effects of dually coated cells with significant enhancement resulting upon pairing ALD-enhanced 811 with surface-modified natural graphite. Alongside observations of high voltage stability, ALD-modified cells exhibit upwards of 30% increased cycle life and superior rate capabilities, increasing the attractiveness of this material for next generation LIBs.

Isolating Surface Reactivity of High Energy Cathode Materials through Model Thin Films

Increasing demands for improved energy density in lithium ion batteries for electric vehicles and grid storage has spurred the development of high energy cathode materials. Ni-rich LiNixMnyCozO2 (x > 0.5, x + y + z = 1) cathode systems are frontrunners among the next-generation of Li-ion battery cathode active materials. Despite the material’s high theoretical capacity (~200 mAh/g), commercial adoption has been hindered by a lack of understanding of complex interactions between the electrolyte and the binder, conductive additives, and active material present in composite electrodes. In this study, we isolated the surface reactivity of LiNi0.6Mn0.2Co0.2O2 (NMC622) by physical vapor deposition of thin film electrodes (thickness < 1.5 µm) for characterization and electrochemistry without influence from conductive additives and binders. Radio Frequency Magnetron Sputtering of homemade NMC622 targets was used to deposit thin films of NMC622 onto Pt-coated Al2O3 substrates which were annealed to achieve the desired layered structure. LiCoO2 thin films were deposited in a similar manner as a baseline for the process. The dependence of crystal structure and stoichiometry on deposition and annealing conditions of these films were determined by XRD, Raman, and ICP-AES. Cathodes were cycled against Li metal in a standard commercial electrolyte of 1.2 M LiPF6 in EC:EMC (3:7 wt%) and extracted in an inert atmosphere glove box for surface characterization after rinsing in DMC. Ex situ XPS and FT-IR analysis of pristine and cycled cells traced a substantial difference in surface species dependent on annealing environments and cycling conditions of thin films. This provided a model system to investigate the influence of thin film coatings such as Al2O3, LiPON, and ZrO2 on the active material alone. The approach developed here may be extended to other promising cathode systems for which the surface chemistry of the active material is not well understood. Figure 1

Engineering Surface Vacancy to Stabilize High-Voltage Battery Cathodes

In this issue of Chem, Cao and co-workers report a stabilization strategy for high-voltage lithium-ion battery cathodes down to the atomic scale. Using LiNi0.5Mn1.5O4 as a model, the authors demonstrated a protocol for precisely modulating the interstitial lattice sites on the material surface, showing the promising capability to improve electrochemical cyclability.

Rational design of a high-energy LiNi0.8Co0.15Al0.05O2 cathode for Li-ion batteries

The energy density of lithium ion batteries heavily relies on the loading of electrochemically active materials in their cathode. LiNi0.8Co0.15Al0.05O2 (NCA) as a promising high-energy cathode material has a practical specific capacity of about 200 mAh g−1 which, however, is often compromised by the additives in the electrode, such as the conductive carbon and binder. To harvest a higher energy density NCA electrode, cyclic voltammetry and galvanostatic intermittent titration techniques are employed herein to evaluate the apparent chemical diffusion coefficient of Li+ and maximize the loading of NCA. Our results show that the carbon content can be reduced to 0.2 wt% (with 97.8 wt% of NCA), making a high-energy cathode for Li-ion batteries. This protocol would also set a precedent for the energy density optimization of other cathodes.

Improvement of Nickel-Rich Lithium Transition-Metal Oxide By Surface Modifications

Nickel-rich lithium transition-metal oxides for Li-ion batteries have been interested under intense commercialization as high-energy cathode materials because of their high specific capacity and relatively low cost. However, it still demands the necessities of higher energy density and more improved thermal stability to compete with requirements of smart mobile applications and x-EVs (extended electric vehicles). Accordingly, further research to increase both Ni content over 60 mol% and the cut off voltage to more than 4.5 V are being carried out to increase the energy density of Li-ion cells. However, two critical problems (structural instability both at >60 °C cycling and fast exothermic oxygen evolution at >200 °C) affecting such Ni contents should be solved. Here, we present electrochemical performance of a nickel-rich lithium transition-metal oxide (Ni content from 60 mol% to 91 mol %. Furthermore, by employing various surface chemistry on the cathode materials such as dry powder mixing, direct precipitation, colloidal deposition, Improvement of structural instability have been studied. The degradation of LiMO2 (M = Co, Ni, and Mn) cathode materials was strongly related to their surface chemistry. Among the proposed degradation mechanisms, the irreversible formation of inactive di- or tri-valant Ni species, particularly those localized in surface regions, are the products of such reactions. In other words, when the cathode materials are charged above 4.3 V, the cathodes could be partially transformed into an electrochemically inactive NiO-like phase or spinel structure with volumetric changes due to the migration of the Ni cations from the original transition metal (TM) layers. Our surface treatment cathode materials suppressed formation of NiO-like phase or spinel structure over 5% compared with bare cathode materials. The experimental results suggest one of way for applications that require high energy, long maintain life and excellent safety.

Boron-Doped Ni-Rich Layered Cathodes with Enhanced Cycling Stability

Lithium-ion batteries (LIBs) are emerging as one of the most promising power sources for portable devices, as well as electric vehicles (EVs) and large energy storage systems (ESS), owing to their high energy density and excellent cycle life. However, the application of LIBs in EVs to reduce CO2 emissions in internal combustion engine vehicles (ICEVs) involves some critical technical challenges, such as the driving range, safety, and cost concerns due to the properties of LIBs.1-3 For this reason, research interests and industrial efforts have focused on developing low-cost, high-energy, and safe cathode materials, which are the main component determining LIB performance and cost. Transition metal layered oxides belonging to the LiNixCoyAlzO2 (NCA) or LiNixCoyMnzO2 (NCM) family, which can increase their intrinsic capacity (above 200 mAh g-1) by increasing Ni content, are emerging as promising new cathode materials.4 However, the intrinsic drawbacks of Ni-rich cathodes, such as low Li intercalation stability over extended cycles, should be improved. In this study, we present the first successful boron doping strategy for a Ni-rich cathode material with very high Ni content. For the boron doping, we used a simple method where only a small amount of B2O3 was added during the calcination process. In addition, the structural and morphological stability of the boron-doped Ni-rich NCM cathode material was investigated before and after testing to evaluate the improved electrochemical performance; specifically, the significantly enhanced cycle performance. Thus, we conclude that the boron doping strategy is an effective and simple way to mitigate the poor stability problem of Ni-rich cathode materials for high-energy and stable LIBs. References D.-W. Jun, C. S. Yoon, U.-H. Kim, Y.-K. Sun, Chem. Mater. 2017, 29, 5048.J. H. Lee, C. S. Yoon, J.-Y. Hwang, S.-J. Kim, F. Maglia, P. Lamp, S.-T. Myung, Y.-K. Sun, Energy Environ. Sci. 2016, 9, 2152.S.-T. Myung, F. Maglia, K.-J. Park, C. S. Yoon, P. Lamp, S.-J. Kim, Y.-K. Sun, ACS Energy Lett. 2017, 2, 196.4. K.-S. Lee, S.-T. Myung, K. Amine, H. Yashiro, Y.-K. Sun, J. Electrochem. Soc. 2007, 154, A971.