Ni-rich Layered Cathode Materials Research Articles

Optimization of synthesis condition of water-resistant and thin titanium oxide layer-coated Ni-rich layered cathode materials and their cathode performance

In this study, in order to develop water-resistant LiNiaCobAl1−a−bO2 (a > 0.85, NCA) cathode materials which exhibit high-rate performance, the surface coating of NCA with titanium oxide (TiOx) was examined. The synthesis conditions for the TiOx-coated NCA cathode materials were investigated, by taking into account some essential factors in the surface coating of NCA by TiOx, with a view to improving the rate performance. We successfully prepared the TiOx-coated NCA cathode material, the rate performance of which is superior to that of the conventionally prepared NCA cathode materials, typically using a polyvinylidene difluoride (PVdF) binder and N-methyl-2-pyrrolidone (NMP) solvent. Their surface analysis suggested that the specific surface structure of TiOx layer coated on the NCA particle leads to both a water-resistant property and a high permeability of Li+ ions through it in the charging/discharging process.

Microstructural visualization of compositional changes induced by transition metal dissolution in Ni-rich layered cathode materials by high-resolution particle analysis

The dissolution of transition metals (TMs) from LiMO2 (M = Ni, Co, Mn) cathodes and their subsequent side reactions on the anode and in the electrolyte result in Li-ion battery capacity and power losses. Despite the importance of this process, the lack of adequate analysis methods for tracking the subtle compositional changes at specific locations with nano-meter spatial resolution has prevented the elucidation of its microstructural origin and mechanism. Herein, we studied the dissolution of TMs from a Ni-rich layered cathode and investigated their deposition on a graphite anode and reactions with the electrolyte, with focus on the microstructural aspects. Changes in TM and oxygen contents in Ni-rich LiNi0.87Co0.09Mn0.04O2 (NCM) cathode materials were two-dimensionally visualized on a micro-scale gathering by nano-scale analysis, which enabled high-resolution particle analysis, through transmission electron microscopy coupled with X-ray energy dispersive spectroscopy. Degraded (capacity retention < 80%) NCM particles featuring grain-boundary cracking caused by repeated volume expansion/contraction upon charge/discharge exhibited compositions similar to that of pristine particles, whereas sectionalized chemical composition mapping revealed that broken and pulverized NCM particles, i.e., those very heavily fractured and broken in such a way as to directly expose the particle surface to the electrolyte, exhibited decreased TM contents. Therefore, TM dissolution was concluded to occur at the cathode material–electrolyte interface and be one of the main reasons of electrode material degradation.

Facile Mn Surface Doping of Ni-Rich Layered Cathode Materials for Lithium Ion Batteries.

A facile Mn surface doping process is proposed to improve the thermal and structural stabilities of Ni-rich layered cathode materials (Ni ≥ 80%) for lithium-ion batteries in electric vehicles. Herein, we demonstrate that the surface structure of the Ni-rich layered cathode materials can be stabilized by the introduction of a thin Mn-rich surface layer. This layer effectively reduces the direct exposure of the highly reactive Ni on the surface of the cathode materials, thus enhancing thermal stability and mitigating side reactions associated with highly reactive Ni that causes the loss of reversible capacity. In practice, the Mn surface-doped Ni-rich layered cathode material exhibits a high specific capacity with an improved cycling stability even at a high temperature (60 °C). We believe that our simple approach offers more opportunities to upscale production without any extra caution.

Investigation on Relationship between the Amount of Ni4+ Ion and Thermal Stability in Ni-Rich Layered Cathode Materials

Recently, the lithium ion batteries (LIBs) have applied to electric vehicle (EV), energy storage system (ESS) as a part of smart grids, and various electronic devices. However, LIBs was faced with deficiency of cyclability and demand for improved energy and power density due to the development of advanced applications. The electrochemical performance of LIBs has been enhanced through the Ni-rich layered cathode materials, such as LiMO2 (M = Ni, Co, Mn, and Al, etc.) [1]. However, these cathode materials are exposed to the risk of combustion and performance degradation in a little thermally harsher environment, due to the inherent thermal instability [2]. To develop thermally enhanced Ni-rich cathode material, therefore, it is the significant challenge to explore how the primary factor determines the thermal stability of Ni-rich cathode materials for the safety and credibility of LIBs. The primary reason for the thermal instability in Ni-rich cathode materials was often noted as the reduction of Ni4+ ion, which occurs in the course of the exothermic reaction [3,4]. However, there was no deep discussion on how the reduction of Ni ions influences the thermal stability of cathode materials due to the ambiguity of interpretation and the uncertainty of equipment. In this study, the relationship between the amount of Ni4+ ion and thermal stability of LixNi0.5+yCo0.2Mn0.3-yO2 (x= 0.33, 1, y= 0, 0.1, 0.2) was investigated through synchrotron-based X-ray techniques; Time-resolved X-ray diffraction (TR-XRD), high-resolution powder diffraction (HRPD), and X-ray absorption spectroscopy (XAS) combined with differential scanning calorimeter (DSC). We observed that delithiated Ni-rich cathode material had a high degree of thermal expansion and a high possibility in formation of oxygen vacancy during heating process. They were identified through the comparison of lattice parameter and the k-weight (k = 1, 2, 3) dependent fitting using Ni K-edge EXAFS spectrum. Also, we specified the effect of amount of Ni4+ ion on the thermal expansion and formation of oxygen vacancy. Based on the results and well-known phase transition mechanism [5], we presented a complemented mechanism of 1st phase transition during heating process. Through the discussion, we will deliver how the primary factor determines the thermal stability of Ni-rich cathode materials. More details will be discussed in the meeting. Reference s : [1] Z. Liu, A. Yu and J. Y. Lee, Journal of Power Sources, 1999, 81, 416-419. [2]W.-S. Yoon, K. Y. Chung, M. Balasubramanian, J. Hanson, J. McBreen and X.-Q. Yang, Journal of power sources, 2006, 163, 219-222. [3] S.-M. Bak, K.-W. Nam, W. Chang, X. Yu, E. Hu, S. Hwang, E. A. Stach, K.-B. Kim, K. Y. Chung and X.-Q. Yang, Chemistry of Materials, 2013, 25, 337-351. [4] Y. Makimura, C. Okuda, T. Nonaka, Y. F. Nishimura, T. Sasaki and Y. Takeuchi, ECS Electrochemistry Letters, 2014, 3, A66-A68. [5] J. Reed and G. Ceder, Chemical reviews, 2004, 104, 4513-4534.

High-Performance and Industrially Feasible Ni-Rich Layered Cathode Materials by Integrating Coherent Interphase.

For developing the industrially feasible Ni-rich layered oxide cathode with extended cycle life, it is necessary to mitigate both the mechanical degradation due to intergranular cracking between primary particles and gas generation from the reaction between the electrolyte and residual Li in the cathode. To simultaneously resolve these two issues, we herein propose a simple but novel method to reinforce the primary particles in LiNi0.91Co0.06Mn0.03O2 by providing a Li-reactive material, whose spinel interphase is coherent with the surface of the cathode. The modified structure significantly outperforms analogous bare samples: they conserve more than 90% of the initial capacity after 50 cycles and also exhibit a greater rate capability. By tracking the same particle location during cycling, we confirmed that the current method significantly reduces crack generation because the provided coating material can penetrate inside the grain boundary of the secondary particle and help maintain the volume of the primary particle. Finally, first-principles calculations were implemented to determine the role of this spinel material, i.e., having intrinsically isotropic lattice parameters, superior mechanical properties, and only a small volume change during delithiation. We believe that the proposed method is straightforward and cost-effective; hence, it is directly applicable for the mass production of Ni-rich cathode material to enable its commercialization.

Reversibility of Ni-Rich Layered Cathode Material Revealed By Structural, Spectroscopic and Imaging Characterizations

Ni-rich layered cathode materials are important candidates for lithium-ion batteries used for high energy density applications, such as electric vehicles. However, its capacity fades during cycling, especially when a high cut off charge voltage is applied (>4.4 V). To understand the origin of this capacity fade, comprehensive diagnostic tools are applied to understand their structure, chemical and morphological properties during cycling. X-ray diffraction results show that while the LiNi0.6Mn0.2Co0.2O2 (NMC622) can mostly recover its original structure at fully discharged state, even after 0.95 Li is extracted (charged to 5.2 V), it does undergo slab sliding that leads to stacking fault (O1 type) which does not go away during cycling. In addition, transmission X-ray imaging (TXM) results indicate that cycling can induces Li content inhomogeneity within the particle. As a result, the strain builds up and is directly responsible for the cracking of primary and secondary particles. Hard X-ray absorption spectroscopy (XAS) results indicate that both Ni and Co contribute to the capacity by going through the Ni3+/Ni4+ and Co3+/Co4+ redox couples. A comparison is made between Ni-rich and the Li-rich layered materials, which is well known for the problem of oxygen release. It is found that for the Ni-rich system, the oxidization state of transition metals (bulk) are well maintained during cycling, whereas for the Li-rich system the average oxidization states keep decreasing (lower oxidation states at both fully charged and fully discharged states after cycling than the pristine sample), suggesting at least the bulk of Ni-rich material is robust against oxygen release. Soft XAS that differentiates the surface chemical information from the bulk indicates that inactive rock salt phase is formed during cycling. These results show that cycling induced strain, stacking fault and surface reconstruction are the main issues to be addressed for further improvement of this material. Acknowledgement This project was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies through Advanced Battery Material Research (BMR) program (Battery500 consortium) under Contract No. DESC0012704. This research used resources 8-ID and 28-ID-2 of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract no. DE-SC0012704. The authors would also like to acknowledge the use of beamline 6-2c of Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE-AC02-76SF00515.

Effective Strategies for Improving the Cycle Life of Ni-Rich LiNixMnyCozO2 Cathodes for Lithium-Ion Batteries

Lithium ion battery (LIB) is considered the most promising energy storage system for applications in electrical vehicles (EVs). However, the large-scale market penetration of EVs has been impeded by several challenges, particularly the limited energy density, high cost and safety concern of cathodes in LIB. Conventional layered oxide LiCoO2 (LCO) has been widely used in LIBs developed for consumer electronics, but the high cost of cobalt, low lithium utilization (~0.5 Li) has limited its application for EVs. To increase the driving range of EVs to over 300 miles at a single charge and promote the massive market penetration of EVs, low-cost electrode materials with high tap density, high-energy density, good rate capability, and long-term cycle life must be developed. Recently, Ni-rich layered structure cathode materials LiNixMnyCozO2 (NMC, x ≥ 0.6) have been receiving much attention as one of the most promising cathode materials for large scale applications owing to their high discharge capacity (200~220 mAh g-1) and energy density (~800 Wh kg-1) as compared to traditional LiCoO2 (~570 Wh kg-1). However, several technical challenges still exist with these Ni-rich NMCs due to their structural/interfacial instability and poor thermal stability. In this presentation, we will discuss several effective approaches, including lattice doping, grain boundary engineering, and use of novel electrolytes that enhance the structural/interfacial stability of Ni-rich NMC cathodes and significantly improved their long-term cycle life.

Precursor effects on structural ordering and electrochemical performances of Ni-rich layered LiNi0.8Co0.2O2 cathode materials for high-rate lithium ion batteries

Ni-rich layered oxides (LiNi1-xMxO2, M = Co, Mn, Al, etc., x < 0.5) are demonstrated as practical cathode materials for high-energy and high-power lithium ion batteries. However, various synthetic factors affect the degree of the structural ordering of layered LiNi1-xMxO2 oxides and their electrochemical lithium storage performances. Herein, we intentionally evaluate the transition metal precursor effects on manipulating the structural ordering and associated electrochemical performances of the Ni-rich LiNi0.8Co0.2O2 cathode material. The transition metal precursor in a special phase of carbonate hydroxide hydrate, i.e., Ni0.8Co0.2(CO3)1-a(OH)2a·bH2O together with a tridimensional network structure is demonstrated as the desirable precursor as compared with different contrasts. The resulting LiNi0.8Co0.2O2 cathode material shows significantly enhanced ordered layered structure and reveals remarkable high-rate capability along with excellent cycling stability. The LiNi0.8Co0.2O2 cathodes can deliver 195, 148 and 128 mAh g−1 at 0.1, 1 and 5 C rates (1 C = 200 mA g−1) and retain 95% of the initial discharge capacity after 100 electrochemical cycles at 1 C and 72% after 500 cycles at 5 C, respectively. This work sheds lights on understanding transition metal precursor effects on the synthetic control and maximizing electrochemical performances of Ni-rich layered cathode materials for high-rate lithium ion batteries.

An Ostwald ripening route towards Ni-rich layered cathode material with cobalt-rich surface for lithium ion battery

An Ostwald ripening-based route is proposed to prepare Ni-rich layered cathodes with Co-rich surface for lithium-ion batteries (LIBs). Commercially available Ni0.8Co0.1Mn0.1(OH)2 and spray pyrolysis derived porous Co3O4 are used as mixed precursors. During the lithiation reaction process under high-temperature, the porous Co3O4 microspheres scatter primary particles and spontaneously redeposit on the surface of Ni-rich spheres according to Ostwald ripening mechanism, forming the Ni-rich materials with Co-rich outer layers. When evaluated as cathode for LIBs, the resultant material shows ability to inhibit the cation disorder, relieves the phase transition from H2 to H3 and diminishes side reactions between the electrolyte and Ni-rich cathode material. As a result, the obtained material with Co-rich outer layers exhibits much more improved cycle and rate performance than the material without Co-rich outer layers. Particularly, NCM-Co-1 (molar ratio of Ni0.8Co0.1Mn0.1(OH)2/Co3O4 is 60:1) delivers a reversible capacity of 159.2 mA h g−1 with 90.5% capacity retention after 200 cycles at 1 C. This strategy provides a general and efficient way to produce gradient substances and to address the surface problems of Ni-rich cathode materials.

Suppressing the Structure Deterioration of Ni-Rich LiNi0.8Co0.1Mn0.1O2 through Atom-Scale Interfacial Integration of Self-Forming Hierarchical Spinel Layer with Ni Gradient Concentration.

Ni-rich layered cathodes have attracted great interest due to the high specific capacity, but they suffer from the layered structure deterioration and the resultant poor cyclability and inferior storage performance. Herein, we propose a novel facile strategy to in situ generate an integrated hierarchical spinel layer on the surface of layered LiNi0.8Co0.1Mn0.1O2 (SC-LNCMO) through a pH modulation induced gradient change of Mn ions valence in the precursor. The self-forming hierarchical spinel layer through this strategy is tightly integrated into the layered phase by atom-scale interfacial junctions, and a Ni gradient concentration from the outer to inner has also formed, which strengthens the interface bonding, reduces the surface layer-host phase mismatch, alleviates the Li+/Ni2+ mixing, and substantially enhances the structure stability of LiNi0.8Co0.1Mn0.1O2 during charge-discharge cycles. These contribute to the large improvement of the cycling stability, rate capability, and low-temperature performances. More importantly, the long-term storage stability of SC-LNCMO has also been significantly improved due to the effective suppression of the integrated spinel layer on the reduction of Ni3+ to Ni2+, cations migration and Li+/Ni2+ exchange, and Li2CO3 formation. This study not only offers a facile novel strategy to create tightly integrated spinel-layered high-performance cathode materials but also presents some new insights into the structure deterioration and the stabilization mechanism of Ni-rich layered cathode materials during charge/discharge cycles or long-term storage.

Capacity fading behavior of Ni-rich layered cathode materials in Li-ion full cells

We report the failure mechanism of a Ni-rich layered cathode material (LiNi0.8Co0.1Mn0.1O2) in Li-ion full cells. Laminated pouch-type full cells were cycled 300 times, after which the cells exhibited 16.3% capacity fading. After 1, 100, 200, and 300cycles, each full cell was disassembled and the cathode was investigated via a half-cell test, X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM). XRD analysis showed loss of the active lithium source in the cathode, and FE-SEM observation exhibited traces of mechanical failure in the Ni-rich secondary particles. Particle strength measurements on the cathode materials by applying compressive force demonstrated that the mechanical strength of particles significantly weakened after full-cell cycling. Thus, the capacity of the Ni-rich cathode may deteriorate during full-cell cycling because of decreases in compressive particle strength. This issue should be considered in materials preparation processes.

Development of Cathode Materials of Lithium-Ion Secondary Batteries for High Energy and Safety

As a consequence of shortage of fossil fuels, development of electrified vehicles (EVs) has further raised the performance requirements of rechargeable battery system. The research and development efforts in meeting these challenges have focused on identifying and designing new cathode materials with capacity higher than 200 mAh g −1 and improved safety characteristics. In order to improve the thermal and structural stability of the Ni-rich layered cathode materials, cathodes enriched in Mn at the outer layer were proposed to suppress the undesired Ni 4+ reduction because Mn 4+ is naturally stable (MnO2 is a stable compound) and does not participate in the redox reaction during the charge and discharge cycle. Recently, we developed a high-energy cathode material with not only full concentration gradient materials but also two-sloped full concentration gradient materials, in which the Ni concentration decreases and the Mn concentration increases toward the particle surface. These cathode materials are synthesized via a specially designed batch-type reactor. The cathode delivers high discharge capacity with excellent capacity retention. The cathode materials exhibits the best cycling stability, rate capability, and thermal stability compared to conventional cathode (CC) and Li[Ni0.8Co0.15Al0.05]O2 (NCA). While the chemical partitioning of the composition generated the observed high capacity and thermal stability, the unique microstructure of the FCG secondary particle provided excellent cycle stability and rate capability.

Organosilane Cathode Coatings for High-Voltage Lithium Ion Batteries

One of the more promising cathode materials for high-energy high-voltage lithium ion batteries (LIBs) are Ni-rich layered cathode materials such as LiNixMnyCozO2 (where x + y + z = 1). Although these cathode materials are attractive from a high capacity standpoint, they commonly suffer from poor structural stability at highly delithiated (charged) states leading to poor electrochemical performance.1-3 To help aid these issues, surface coatings are becoming a widely adopted method to improve the cycling performance of high-voltage cathode materials. More widely investigated coating materials include oxide coatings (Al2O3, TiO2, etc.)4 while comparatively fewer studies have investigated organic monomer or polymer coatings (PEDOT, polyimide, etc.).5 In this work, the surface of LiNi0.5Mn0.3Co0.2O2 cathode materials were coated with organosilane reagents to investigate their effect on cell electrochemical performance. The impact of the weight percentage of organosilane used and the functionality of the organosilane used will be presented. Characterization of the organosilane coating using XRD, XPS and FTIR confirm the presence of a Si-containing coating. Cycling performance for half cells (Li/ LiNi0.5Mn0.3Co0.2O2) using the organosilane coated cathodes will be shown. In conclusion, a new type of organic coating was utilized on Ni-rich cathode materials and this research explores a new type of coating for high voltage cathodes. 1) Son, I. H.; Park, J. H.; Kwon, S.; Mun, J. and Choi, J. W. Chem. Mat. 2015, 27, 7370-7379. 2) Bak, S.-M.; Hu, E.; Zhou, Y.; Yu, X.; Senanayake, S. D.; Cho, S.-J.; Kim, K.-B.; Chung, K. Y.; Yang, X.-Q. and Nam, K.-W. ACS Appl. Mater. Interfaces 2014, 6, 22594-22601. 3) Lin, F.; Markus, I. M.; Doeff, M. M.; Xin, H. L. Sci. Rep., 2014, 4, 5694. 4) Xu, S.; Jacobs, R. M.; Nguyen, H. M.; Hao, S.; Mahanthappa, M.; Wolverton, C.; Morgan, D. J. Mater. Chem. A, 2015, 3, 17248. 5) Zhang, J.; Lu, Q.; Fang, J.; Wang, J.; Yang, J. and NuLi, Y. ACS Appl. Mater. Interfaces 2014, 6, 17965-17973.

The Effects of Synthesis Conditions on the Performances of Ni-Rich LiNixMnyCozO2 Cathode Materials for Lithium-Ion Batteries

Rechargeable lithium-ion batteries (LIBs) have become the dominant power sources not only for the consumer electronic devices but also for the pure electric vehicles (EVs) and plug-in electric vehicles (PHEVs) because of their high energy density, light weight and long cycle life. The increasing need on minimizing global warming and environmental pollution have accelerated the development of EVs. With the state of the art LIBs used in automobile industry, the driving range of EVs is still far behind those of conventional vehicles using internal combustion engines. Therefore, there is an urgent need to further improve both energy density and power capability of LIBs, to improve the driving range and decrease the recharging time. To meet these requirements, a lot of effort has been made in the search of high energy density electrode materials, especially the cathode materials. Recently, Ni-rich layered cathode materials LiNixMnyCozO2 (x ≥ 0.6) have received great attention as a potential cathode alternative due to their high achievable discharge capacity (200~220 mAh g-1),1 representing a significant improvement in energy density (~780 Wh kg-1) as compared to LiCoO2 (~570 Wh kg-1) and spinel LiMn2O4 (~440 Wh kg-1). Ni-rich cathode materials also show much higher lithium ion diffusion coefficients, signifying a superior power density as compared to other LiNixMnyCozO2 cathodes with lower Ni contents.2 The other advantage of Ni-rich cathode materials is the reduction of production cost owing to the reduced Co content. However, there are still some technical challenges hindering the mass applications of Ni-rich cathode materials, including (i) Li/Ni cation mixing due to the difficulty in maintaining all the Ni in 3+ valence state, (ii) micro strain and micro crack formation due to the significant volume variation during charge/discharge processes, (iii) safety issue because of the aggressive thermal reactions between the delithiated Ni-rich materials and the electrolyte. Much effort has been devoted to improving the electrochemical performance of the Ni-rich cathode materials.3 Representative approaches include lattice doping, surface treatment/modification, tuning the material compositions, and a smart design of core-shell or concentration gradient structures. Other than these useful treatment and design, the synthetic conditions, such as co-precipitation pH value, particle size, lithium content, calcination temperature, calcination atmosphere (O2, air etc.), heating/cooling rates, also play an important role in manipulating the material structural properties and thus influencing the electrochemical performances of prepared Ni-rich cathode materials. Herein, we systematically investigated the effects of calcination temperature on the primary particle size, cation mixing and the electrochemical performances of Ni-rich cathode materials. It is found that higher calcination temperature leads to considerable increase of primary particle size, which is supposed to largely reduce the side reactions between cathode material and the electrolytes. However, the materials prepared at high temperatures exhibited much poorer long-term cycling stability both at room temperature and high temperature of 60oC, in addition to their inferior rate performance. Therefore, the fundamental reasons behind this phenomenon are investigated in detail, which will be presented and discussed during the meeting. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, the Advanced Battery Materials Research Programs of the U.S. Department of Energy (DOE). The microscopy and spectroscopy measurements were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. References 1) A. Manthiram, J.C. Knight, S-T, Myung, S-M Oh, Y-K Sun, Adv. Energy Mater., 1501010 (2015). 2) Y. Wei, J. Zheng, S. Cui, X. Song, Y. Su, W. Deng, Z. Wu, X. Wang, W. Wang, M. Rao, Y. Lin, C. Wang, K. Amine, F. Pang, J. Am. Chem. Soc., 137, 8364 (2015). 3) W. Liu, P. Oh, X. Liu, M-J Lee, W. Cho, S. Chae, Y. Kim, J. Cho, Angew. Chem. Int. Ed., 54, 4440 (2015).

Improved High Voltage Electrochemical Properties and Thermal Stability of LiNi0.6Co0.2Mn0.2 By Li3PO4 Surface Modification

LiCoO2 is the most commercialized in lithium ion batteries (LIBs) for portable electronics because of its high volumetric energy density compared to the other commercialized cathode materials. However, low specific capacity (145 mAh/g @ 4.3 V cut-off) hinders its application to emerging technologies, such as power tool and plug-in hybrid electric vehicles. As a result, layered Ni-rich LiNixCoyMn1 - x - yO2 materials have attracted much attention as promising alternative cathode materials because of their high capacity and relatively low cost. However, Ni-rich layered cathode materials suffer from structural instability by direct contact with electrolyte at high voltage (>4.3V), resulting in poor cyclability, rate capability and thermal instability. To solve these problems, a lot of researches have been carried out to coat the surface of layered cathode materials using oxide or phosphate, such as TiO2, Al2O3, and Li3PO4. Among these coating materials, Li3PO4 has attracted a great attention as coating material due to its high ionic conductivity and thermal stability due to strong P=O bonding. Recently, Zhao et al., reported on the Li3PO4 coating on LiFePO4 cathode material, but they used Li3PO4 nano particle as a coating material, resulting in non-uniform coating. Also, Xiong et el, tried to coat the Li3PO4 on LiNi0.8Co0.1Mn0.1O2 cathode material using the reaction between (NH4)2HPO4 and lithium residues, but the Li3PO4 coating layer is not confirmed. In this study, we report on the synthesis of Li3PO4-coated LiNi0.6Co0.2Mn0.2O2 using a citric acid assisted sol-gel method in which nm-thick Li3PO4 layer was uniformly coated on the LiNi0.6Co0.2Mn0.2O2. It exhibited improved cyclability at high voltage (4.7V) and rate capability than those of bare LiNi0.6Co0.2Mn0.2O2. In addition, Li3PO4-coated LiNi0.6Co0.2Mn0.2O2showed improved thermal stability at elevated temperature. More details on the synthetic procedure, electrochemical and physical properties will be presented at the meeting.

Mechanistic Insight in the Function of Phosphite Additives for Protection of LiNi0.5Co0.2Mn0.3O2 Cathode in High Voltage Li-Ion Cells.

Triethlylphosphite (TEP) and tris(2,2,2-trifluoroethyl) phosphite (TTFP) have been evaluated as electrolyte additives for high-voltage Li-ion battery cells using a Ni-rich layered cathode material LiNi0.5Co0.2Mn0.3O2 (NCM523) and the conventional carbonate electrolyte. The repeated charge/discharge cycling for cells containing 1 wt % of these additives was performed using an NCM523/graphite full cell operated at the voltage window from 3.0-4.6 V. During the initial charge process, these additives decompose on the cathode surface at a lower oxidation potential than the baseline electrolyte. Impedance spectroscopy and post-test analyses indicate the formation of protective coatings by both additives on the cathode surface that prevent oxidative breakdown of the electrolyte. However, only TTFP containing cells demonstrate the improved capacity retention and Coulombic efficiency. For TEP, the protective coating is also formed, but low Li(+) ion mobility through the interphase layer results in inferior performance. These observations are rationalized through the inhibition of electrocatalytic centers present on the cathode surface and the formation of organophosphate deposits isolating the cathode surface from the electrolyte. The difference between the two phosphites clearly originates in the different properties of the resulting phosphate coatings, which may be in Li(+) ion conductivity through such materials.

Organosilane Coatings for Ni-Rich High-Voltage Lithium Ion Batteries

One of the more promising cathode materials for high-energy high-voltage lithium ion batteries (LIBs) are Ni-rich layered cathode materials such as LiNixMnyCozO2 (where x + y + z = 1). Although these cathode materials are attractive from a high capacity standpoint, they commonly suffer from poor structural stability at highly delithiated (charged) states leading to poor electrochemical performance.1-3 While incorporation of additives into the electrolyte provides one way to improve cycling performance in LIBs, surface coatings are becoming a widely adopted method to improve the cycling performance of high-voltage cathode materials. More widely investigated coating materials include oxide coatings (Al2O3, TiO2, etc.)4 while comparatively fewer studies have investigated organic monomer or polymer coatings (PEDOT, polyimide, etc.).5 Additionally, the mechanism of the coating interaction with the material is intriguing due to its importance in creating a performance-enhancing coating. In this work, the surface of LiNi0.5Mn0.3Co0.2O2 cathode materials were coated with organosilane reagents to investigate their effect on cell electrochemical performance. The impact of the cathode pretreatment, the weight percentage of organosilane used and the functionality of the organosilane used will be presented. Characterization of the organosilane coating using SEM shows a slight morphological change in the cathode surface while EDX and FTIR confirm the presence of a Si-containing coating. Half cells (Li/ LiNi0.5Mn0.3Co0.2O2) using the organosilane coated cathodes operating in the voltage window of 3.0 - 4.4 V showed similar cycling behavior to that of non-coated material. In conclusion, a new type of organic coating was utilized on Ni-rich cathode materials and this research explores a new type of coating for high voltage cathodes. References 1) Son, I. H.; Park, J. H.; Kwon, S.; Mun, J. and Choi, J. W. Chem. Mat. 2015, 27, 7370-7379. 2) Bak, S.-M.; Hu, E.; Zhou, Y.; Yu, X.; Senanayake, S. D.; Cho, S.-J.; Kim, K.-B.; Chung, K. Y.; Yang, X.-Q. and Nam, K.-W. ACS Appl. Mater. Interfaces 2014, 6, 22594-22601. 3) Lin, F.; Markus, I. M.; Doeff, M. M.; Xin, H. L. Sci. Rep., 2014, 4, 5694. 4) Xu, S.; Jacobs, R. M.; Nguyen, H. M.; Hao, S.; Mahanthappa, M.; Wolverton, C.; Morgan, D. J. Mater. Chem. A, 2015, 3, 17248. 5) Zhang, J.; Lu, Q.; Fang, J.; Wang, J.; Yang, J. and NuLi, Y. ACS Appl. Mater. Interfaces 2014, 6, 17965-17973.

Improved Electrochemical Performances and Thermal Stability of LiNi0.6Co0.2Mn0.2 by Li3PO4 coating

LiCoO2 is the most commercialized in lithium ion batteries (LIBs) for portable electronics because of its high volumetric energy density compared to the other commercialized cathode materials. However, low specific capacity (145 mAh/g @ 4.3 V cut-off) hinders its application to emerging technologies, such as power tool and plug-in hybrid electric vehicles. As a result, layered Ni-rich LiNixCoyMn1 - x - yO2 materials have attracted much attention as promising alternative cathode materials because of their high capacity and relatively low cost. However, Ni-rich layered cathode materials suffer from structural instability by direct contact with electrolyte at high voltage (>4.3V), resulting in poor cyclability, rate capability and thermal instability. To solve these problems, a lot of researches have been carried out to coat the surface of layered cathode materials using oxide or phosphate, such as TiO2, Al2O3, and Li3PO4. Among these coating materials, Li3PO4 has attracted a great attention as coating material due to its high ionic conductivity and thermal stability due to strong P=O bonding. Recently, Zhao et al., reported on the Li3PO4 coating on LiFePO4 cathode material, but they used Li3PO4 nano particle as a coating material, resulting in non-uniform coating. Also, Xiong et el, tried to coat the Li3PO4 on LiNi0.8Co0.1Mn0.1O2 cathode material using the reaction between (NH4)2HPO4 and lithium residues, but the Li3PO4 coating layer is not confirmed. In this study, we report on the synthesis of Li3PO4-coated LiNi0.6Co0.2Mn0.2O2 using a citric acid assisted sol-gel method in which nm-thick Li3PO4 layer was uniformly coated on the LiNi0.6Co0.2Mn0.2O2. It exhibited improved cyclability at high voltage (4.7V) and rate capability than those of bare LiNi0.6Co0.2Mn0.2O2. In addition, Li3PO4-coated LiNi0.6Co0.2Mn0.2O2 showed improved thermal stability at elevated temperature. More details on the synthetic procedure, electrochemical and physical properties will be presented at the meeting.