Layered Lithium Transition Metal Oxides Research Articles

Electrochemical Characteristics of Layered Transition Metal Oxide Cathode Materials for Lithium Ion Batteries: Surface, Bulk Behavior, and Thermal Properties.

Layered lithium transition metal oxides, in particular, NMCs (LiNixCoyMnzO2) represent a family of prominent lithium ion battery cathode materials with the potential to increase energy densities and lifetime, reduce costs, and improve safety for electric vehicles and grid storage. Our work has focused on various strategies to improve performance and to understand the limitations to these strategies, which include altering compositions, utilizing cation substitutions, and charging to higher than usual potentials in cells. Understanding the effects of these strategies on surface and bulk behavior and correlating structure-performance relationships advance our understanding of NMC materials. This also provides information relevant to the efficacy of various approaches toward ensuring reliable operation of these materials in batteries intended for demanding traction and grid storage applications. In this Account, we start by comparing NMCs to the isostructural LiCoO2 cathode, which is widely used in consumer batteries. Effects of changing the metal content (Ni, Mn, Co) upon structure and performance of NMCs are briefly discussed. Our early work on the effects of partial substitution of Al, Fe, and Ti for Co on the electrochemical and bulk structural properties is then covered. The original aim of this work was to reduce the Co content (and thus the raw materials cost) and to determine the effect of the substitutions on the electrochemical and bulk structural properties. More recently, we have turned to the application of synchrotron and advanced microscopy techniques to understand both bulk and surface characteristics of the NMCs. Via nanoscale-to-macroscale spectroscopy and atomically resolved imaging techniques, we were able to determine that the surfaces of NMC undergo heterogeneous reconstruction from a layered structure to rock salt under a variety of conditions. Interestingly, formation of rock salt also occurs under abuse conditions. The surface structural and chemical changes affect the charge distribution, the charge compensation mechanisms, and ultimately, the battery performance. Surface reconstruction, cathode/electrolyte interface layer formation, and oxygen loss are intimately related, making it difficult to disentangle the effects of each of these phenomena. They are driven by the different redox activities of Ni and O on the surface and in the bulk; there is a greater tendency for charge compensation to occur on oxygen anions at particle surfaces rather than on Ni, whereas the Ni in the bulk is more redox active than on the surface. Finally, our latest research efforts are directed toward understanding the thermal properties of NMCs, which is highly relevant to their safety in operating cells.

(Battery Division Technology Award Address) Progress in High-Capacity Gradient Layered Li[NixCoyMnz]O2 Cathodes for Lithium-ion Batteries

Rechargeable lithium-ion batteries have great potential as a new large-scale power source for electric vehicles (EVs) due to their high energy density, high voltage, and long cycle life. However, commercialization of these batteries for the automobile industry requires further improvements in energy density and safety. Meeting this challenge has centered on finding new high-capacity electrode materials, especially cathode materials. A layered lithium transition metal oxide, Li[NixCoyMnz]O2 (x+y+z = 1) have been extensively utilized and studied as cathode materials for lithium-ion batteries due to their high capacity [1]. It is well known that an increase of the Ni content in the Li[NixCoyMnz]O2 results in an increase of capacity and total residual lithium content, but a decrease in capacity retention and safety [2]. To resolve the aforementioned drawbacks and to overcome the inherent chemical instability of Ni-rich layered cathodes and to improve capacity retention, we have hybridized Ni-rich and Ni-deficient Li[NixCoyMnz]O2 cathodes at the particle level by establishing concentration profiles within a single particle such that the Ni-rich center maximizes the capacity while the Ni-deficient particle surface ensures high thermal and structural stability in the form of a core-shell (with gradient) [3], full concentration gradient (FCG) [4], and two-slope full concentration gradient (TSFCG) lithium nickel-cobalt-manganese oxide [5]. In addition to increasing the structural stability, Al was incorporated into the FCG and TSFCG cathodes [6]. All the hybridized Li[NixCoyMnz]O2 cathodes demonstrated better electrochemical and thermal properties than their corresponding convention Li[NixCoyMnz]O2 cathodes without concentration (Figure 1). Reference s J. Katana Ngala, et al, J. Mater. Chem. 14, 214-220, 2004. H.-J. Noh, et al, J. Power Sources, 233, 121-130, 2013. Y.-K. Sun, et al, Nat. Mater. 8, 320-324, 2009. Y.-K. Sun, et al, Nat. Mater. 11, 942-947, 2012. B.-B. Lim, et al, Adv. Fun. Mater. 25, 4673-4680, 2015. U.-H. Kim, et al, Adv. Energy Mater. 6, 1601417, 2016. Figure 1. SEM and TEM images of hybridized Li[NixCoyMnz]O2 cathodes and their cell performances. Figure 1

Elucidating Effect of Transition Metal on Anionic Oxygen Activity in High-Energy Li-Rich Layered Oxides

In commercial Li-ion batteries, well-ordered close-packed oxides, particularly, layered lithium transition metal oxides, LiTMO2 (TM = Ni, Mn, Co, Al), are widely used. Despite the high theoretical capacity of these layered oxides (> 270 mAh/g), they are typically operated to deliver a capacity of less than 200 mAh/g to attain good cycling and safety attributes.1-6 Nowadays, strategies to push the capacity limit of such materials have led to the development of Li-rich layered oxides7, 8, which can consistently deliver a reversible capacity approaching 300 mAh/g. This exceptionally high capacity is far beyond the theoretical capacity from Ni and Co redox, for example, Ni redox (Ni2+/4+) can only account for a theoretical capacity of 127 mAh/g in a Co-free compound, Li1.2Ni0.2Mn0.6O2. This has been clearly directed to the participation of oxygen redox in the electrochemical reaction.9-14 In our work, we aimed to probe the electrochemical activity of anionic oxygen in Li-rich layered oxides from material perspective via tackling the effect of transition metal species. We have successfully synthesized a series of Li-rich layered oxides, Li2-x-yNixTmyO2 (TM is transition metal). Compounds with designed transition metals possess a similar crystal structure and enable a similar amount of Li removal and uptake during charge-discharge processes, but with a significantly different charge profile, characterized by the voltage plateau around 4.55 V. We performed a systematic study to capture the oxygen activity ranging from O2- in the lattice to O0 in gaseous phase in the as-produced compounds by combining a suite of advanced characterization techniques with in-situ differential electrochemical spectrometry (DEMS). We have observed completely different oxygen behaviors in such compounds with varied transition metals. We will present our experimental evidence on a reversible participation of electrons from oxygen in Li-, Mn-rich layered oxide. We hope these findings will provide additional insights into the complex mechanism of oxygen redox and the development of advanced high-capacity Li-ion cathodes. Reference 1. J. B. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587-603. 2. J. B. Goodenough and K.-S. Park, J. Am. Chem. Soc., 2013, 135, 1167-1176. 3. M. S. Whittingham, Chemical Reviews, 2004, 104, 4271-4301. 4. B. L. Ellis, K. T. Lee and L. F. Nazar, Chemistry of Materials, 2010, 22, 691-714. 5. M. S. Whittingham, Chemical Reviews, 2014, 114, 11414-11443. 6. J. Xu, F. Lin, M. M. Doeff and W. Tong, Journal of Materials Chemistry A, 2017, 5, 874-901. 7. S.-H. Kang, Y. K. Sun and K. Amine, Electrochemical and Solid-State Letters, 2003, 6, A183-A186. 8. Z. Lu, D. D. MacNeil and J. R. Dahn, Electrochemical and Solid-State Letters, 2001, 4, A191-A194. 9. C. Delmas, Nat Chem, 2016, 8, 641-643. 10. A. Grimaud, W. T. Hong, Y. Shao-Horn and J. M. Tarascon, Nat Mater, 2016, 15, 121-126. 11. K. Luo, M. R. Roberts, R. Hao, N. Guerrini, D. M. Pickup, Y.-S. Liu, K. Edström, J. Guo, A. V. Chadwick, L. C. Duda and P. G. Bruce, Nat Chem, 2016, 8, 684-691. 12. D.-H. Seo, J. Lee, A. Urban, R. Malik, S. Kang and G. Ceder, Nat Chem, 2016, 8, 692-697. 13. Z. Chenglong, W. Qidi, L. Yaxiang, H. Yong-Sheng, L. Baohua and C. Liquan, Journal of Physics D: Applied Physics, 2017, 50, 183001. 14. B. Qiu, M. Zhang, Y. Xia, Z. Liu and Y. S. Meng, Chemistry of Materials, 2017, 29, 908-915.

Electrochemical properties of high-energy-density Li[Ni0.5Co0.2Mn0.3]O2 cathode at full-cell level

Layered lithium transition metal oxide LiNi0.5Co0.2Mn0.3O2 cathode materials were successfully obtained through a simple carbonate co-precipitation method. Monodispersed Li[Ni0.5Co0.2Mn0.3]O2 spherical agglomeration consisted of nanoscale secondary particles enable material to be higher energy and powder density. Pouch-typed cells were assembled to investigate electrochemical performance of Li[Ni0.5Co0.2Mn0.3]O2 at level of full-cell. The results show that the assembled pouch-typed full-cells present higher capacity, excellent rate capacity, and cycle life, which is due to its monodispersed uniform morphology with nanoscale secondary particles.

In situ stress measurements during electrochemical cycling of lithium-rich cathodes

Layered lithium transition metal oxides (Li1+xM1-xO2, M = Ni, Mn, Co) are attractive cathode materials for lithium-ion batteries due to their high reversible capacity. However, they suffer from structural changes that lead to substantial voltage fade. In this study, we use stress as a novel way to track irreversible changes in Li1.2Mn0.55Ni0.125Co0.125O2 (LR-NMC) cathodes. A unique and unpredicted stress signature is observed during the first delithiation. Initially, a tensile stress is observed, consistent with volume contraction from lithium removal, however, the stress reverses and becomes compressive with continued charging beyond 4 V vs Li/Li+, indicating volume expansion; this phenomenon is present in the first cycle only. This irreversible stress during delithiation is likely to be at least partially due to oxygen loss and the resulting cation rearrangement. Raman spectroscopy provides evidence of the layered-to-spinel phase transition after cycling in the LR-NMC films, as well as recovery of the original spectra upon re-annealing in an oxygen environment.

Surface element segregation and electrical conductivity of lithium layered transition-metal oxide cathode materials

Surface element segregation and electric conductivity are critical in determining lithium storage ability of given cathode materials, which are poorly understood and not correlated with the structure and overall performance. Here, layered lithium transition-metal oxides, one of the state-of-the-art cathode materials for lithium ion batteries are chosen to study. A serial of LiNixCo1-2xMnxO2 samples were prepared via a solid state reaction and subsequently characterized by XRD in conjunction with structural refinement, XPS depth profiling, and AC impedance spectroscopy. Slightly different expansion rates are observed for lattice parameters (a and c/3) with varying of Ni content, which is attributed to the increase of average metal-ion radius and an increase of eg electron that enhances the columbic repulsion between transition metal and oxygen atoms. XPS depth profiling results show that surface composition is significantly deviated from bulk, in which Ni and Mn atoms tend to enrich in the surface region, while Co element is relatively deficient. Further, surface element segregation is alleviated by the increase of Ni/Mn content. Moreover, increasing the Ni/Mn content also raises the activation energy of bulk conduction.

Atomic Layer Deposition of Stable LiAlF4 Lithium Ion Conductive Interfacial Layer for Stable Cathode Cycling.

Modern lithium ion batteries are often desired to operate at a wide electrochemical window to maximize energy densities. While pushing the limit of cutoff potentials allows batteries to provide greater energy densities with enhanced specific capacities and higher voltage outputs, it raises key challenges with thermodynamic and kinetic stability in the battery. This is especially true for layered lithium transition-metal oxides, where capacities can improve but stabilities are compromised as wider electrochemical windows are applied. To overcome the above-mentioned challenges, we used atomic layer deposition to develop a LiAlF4 solid thin film with robust stability and satisfactory ion conductivity, which is superior to commonly used LiF and AlF3. With a predicted stable electrochemical window of approximately 2.0 ± 0.9 to 5.7 ± 0.7 V vs Li+/Li for LiAlF4, excellent stability was achieved for high Ni content LiNi0.8Mn0.1Co0.1O2 electrodes with LiAlF4 interfacial layer at a wide electrochemical window of 2.75-4.50 V vs Li+/Li.

Achieving high capacity and rate capability in layered lithium transition metal oxide cathodes for lithium-ion batteries

In this study, we target to find a new composition for a layered mixed metal oxide, which has a high structural stability and a good electrochemical performance. Our strategy is to alter the transition metal composition focusing on the relative amounts of redox active Ni and Co to the inactive Mn, based on highly-stabilized LiNi1/3Co1/3Mn1/3O2. X-ray absorption near-edge structure and X-ray diffraction analyses show that the degree of cation disorder decreases on increasing the ratio of Ni and Co to Mn, by the presence of Ni3+, suggesting that slightly higher Ni and Co contents lead to improved structural stability. Electrochemical studies demonstrate that LiNi0.4Co0.4Mn0.2O2 cathodes exhibit considerable improvements in both the reversible capacity and the rate capabilities at a voltage range of 2.5–4.6 V. In situ XRD measurements reveal that LiNi0.4Co0.4Mn0.2O2 maintains a single-phase and undergoes lesser structural variations compared to controlled compositions during a delithiation process up to 4.6 V, while achieving a high reversible capacity over 200 mAh g−1. As a result, LiNi0.4Co0.4Mn0.2O2 experiences fewer structural degradations during electrochemical cycling, which explains the excellent long-term cycling performance.

Improving cycle life of layered lithium transition metal oxide (Li M O 2 ) based positive electrodes for Li ion batteries by smart selection of the electrochemical charge conditions

Abstract Increasing the specific energy of a lithium ion battery and maintaining its cycle life is a predominant goal and major challenge for electrochemical energy storage applications. Focusing on the positive electrode as the specific energy bottleneck, cycle life characteristics of promising layered oxide type active materials (Li M O 2 ) has been thoroughly investigated. Comparing the variety of Li M O 2 compositions, it could be shown that the “Ni-rich” (Ni ≥ 60% for M in Li M O 2 ) electrodes expectably revealed best performance compromises between specific energy and cycle life at 20 °C, but only LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) could also maintain sufficient cycle performance at elevated temperatures. Focusing on NMC622, it could be demonstrated that the applied electrochemical conditions (charge capacity, delithiation amount) in the formation cycles significantly influence the subsequent cycling performance. Moreover, the insignificant transition metal dissolution, demonstrated by means of total X-ray fluorescence (TXRF) technique, and unchanged lithiation degree in the discharged state, determined by the measurement of the Li + content by means of the inductively coupled plasma optical emission spectroscopy (ICP-OES) technique, pointed to a delithiation (charge) hindrance capacity fade mechanism. Considering these insights, thoughtful modifications of the electrochemical charge conditions could significantly prolong the cycle life.

Atomic Resolution Structural and Chemical Imaging Revealing the Sequential Migration of Ni, Co, and Mn upon the Battery Cycling of Layered Cathode.

Layered lithium transition metal oxides (LTMO) are promising candidate cathode materials for next-generation high-energy density lithium ion battery. The challenge for using this category of cathode is the capacity and voltage fading, which is believed to be associated with the layered structure disordering, a process that is initiated from the surface or solid-electrolyte interface and facilitated by transition metal (TM) reduction and oxygen vacancy formation. However, the atomic level dynamic mechanism of such a layered structure disordering is still not fully clear. In this work, utilizing atomic resolution electron energy loss spectroscopy (EELS), we map, for the first time at atomic scale, the spatial evolution of Ni, Co and Mn in a cycled LiNi1/3Mn1/3Co1/3O2 layered cathode. In combination with atomic level structural imaging, we discovered the direct correlation of TM ions migration behavior with lattice disordering, featuring the residing of TM ions in the tetrahedral site and a sequential migration of Ni, Co, and Mn upon the increased lattice disordering of the layered structure. This work highlights that Ni ions, though acting as the dominant redox species in many LTMO, are labile to migrate to cause lattice disordering upon battery cycling, while the Mn ions are more stable as compared with Ni and Co and can act as pillar to stabilize layered structure. Direct visualization of the behavior of TM ions during the battery cycling provides insight for designing of cathode with high structural stability and correspondingly a superior performance.

Understanding Capacity Fading of LiNi0.8Co0.15Al0.05O2 Via Long-Term Operando X-Ray Diffraction Studies

Layered lithium transition metal oxides are promising high-capacity cathode materials for Li-ion batteries. However, capacity fading has restricted the practical capacity to less than 70% of the theoretical capacity. Higher initial capacity can be achieved by charging to a higher voltage, yet this initial gain in capacity is quickly offset by the rapid fade in capacity. Understanding the capacity fading mechanism will help develop strategies to mitigate this problem. We used operando X-ray powder diffraction to follow the reactions of the layered LiNi0.8Co0.15Al0.05O2 over extended charge-discharge cycles at a slow current rate. The operando result shows that the capacity loss can be explained by the heterogeneity developed in the electrode after extended charge-discharge cycles.

Dynamic behaviour of interphases and its implication on high-energy-density cathode materials in lithium-ion batteries

Undesired electrode–electrolyte interactions prevent the use of many high-energy-density cathode materials in practical lithium-ion batteries. Efforts to address their limited service life have predominantly focused on the active electrode materials and electrolytes. Here an advanced three-dimensional chemical and imaging analysis on a model material, the nickel-rich layered lithium transition-metal oxide, reveals the dynamic behaviour of cathode interphases driven by conductive carbon additives (carbon black) in a common nonaqueous electrolyte. Region-of-interest sensitive secondary-ion mass spectrometry shows that a cathode-electrolyte interphase, initially formed on carbon black with no electrochemical bias applied, readily passivates the cathode particles through mutual exchange of surface species. By tuning the interphase thickness, we demonstrate its robustness in suppressing the deterioration of the electrode/electrolyte interface during high-voltage cell operation. Our results provide insights on the formation and evolution of cathode interphases, facilitating development of in situ surface protection on high-energy-density cathode materials in lithium-based batteries.

Shape-controlled synthesis of hierarchically layered lithium transition-metal oxide cathode materials by shear exfoliation in continuous stirred-tank reactors

A scalable and continuous synthesis technique for hierarchically structured layered cathode materials with exposed {010} planes is proposed.

Understanding the Degradation Mechanism of Lithium Nickel Oxide Cathode for Li-Ion Batteries

Nowadays, Li-ion battery technology has become one of the best performing energy storage devices and holds the best promise to power the upcoming electrical vehicles (EV).1 Continuous search of cathode materials with even better performance and lower cost ($/Wh) led to numerous layered lithium transition metal oxides, such as LiNixMnyCozO2 and LiNixCoyAlzO2 (0 < x, y, z <1). Recently, there is an emerging interest on developing Ni-rich cathode materials because a higher level of Ni incorporation can lead to a higher practical capacity.2 This is resulted from the electronic configuration of Ni3+ (3d7), allowing the removal of electrons only from the eg band, therefore, the loss of oxygen occurs at a higher charge state.3 On the other hand, a higher Ni content may result in a severe capacity fade during cycling, likely due to the structural transformation in the bulk and at the surface.4, 5 A combination of different transition metals in these LiNixMnyCozO2 or LiNixCoyAlzO2 further increases the complexity of the problem because of the phase separations and side reactions with electrolyte resulting from the difference in chemical and electrochemical reactivity of the transition metals. We have reported the synthesis of high-performance LiNiO2 6, here, we will use it as a model system to reveal the intrinsic performance degradation mechanism originating from the high Ni content for those complex R-3m layered oxides. The phase transition, charge compensation and local chemical environment of Ni in the bulk and at the surface of LiNiO2 were investigated by a variety of bulk and surface characterization techniques for different terms of cycling. We revealed the phase transition from original hexagonal (H1) phase to another two hexagonal (H2+H3) phases and observed a gradual loss of H3 phase features over the extended charges. The reduction in Ni redox activity occurred in both charge and discharge states, and it appeared both in the bulk and at the surface over the extended cycles. The destruction in crystal structure, and Ni redox activity simultaneously contributed to the cycling performance decay of LiNiO2. Reference 1. M. S. Whittingham, Chemical Reviews, 2004, 104, 4271-4301. 2. M. H. Choi, C. S. Yoon, S. T. Myung, B. B. Lim, S. Komaba and Y. K. Sun, Journal of the Electrochemical Society, 2015, 162, A2313-A2318. 3. M. Balasubramanian, X. Sun, X. Q. Yang and J. McBreen, Journal of Power Sources, 2001, 92, 1-8. 4. P. F. Yan, J. M. Zheng, D. P. Lv, Y. Wei, J. X. Zheng, Z. G. Wang, S. Kuppan, J. G. Yu, L. L. Luo, D. Edwards, M. Olszta, K. Amine, J. Liu, J. Xiao, F. Pan, G. Y. Chen, J. G. Zhang and C. M. Wang, Chemistry of Materials, 2015, 27, 5393-5401. 5. J. M. Zheng, W. H. Kan and A. Manthiram, Acs Applied Materials & Interfaces, 2015, 7, 6926-6934. 6. J. Xu, F. Lin, D. Nordlund, E. J. Crumlin, F. Wang, J. Bai, M. M. Doeff and W. Tong, Chemical Communications, 2016, 52, 4239-4242.

Countering the Segregation of Transition-Metal Ions in LiMn1/3 Co1/3 Ni1/3 O2 Cathode for Ultralong Life and High-Energy Li-Ion Batteries.

High-voltage layered lithium transition-metal oxides are very promising cathodes for high-energy Li-ion batteries. However, these materials often suffer from a fast degradation of cycling stability due to structural evolutions. It seriously impedes the large-scale application of layered lithium transition-metal oxides. In this work, an ultralong life LiMn1/3 Co1/3 Ni1/3 O2 microspherical cathode is prepared by constructing an Mn-rich surface. Its capacity retention ratio at 700 mA g(-1) is as large as 92.9% after 600 cycles. The energy dispersive X-ray maps of electrodes after numerous cycles demonstrate that the ultralong life of the as-prepared cathode is attributed to the mitigation of TM-ions segregation. Additionally, it is discovered that layered lithium transition-metal oxide cathodes with an Mn-rich surface can mitigate the segregation of TM ions and the corrosion of active materials. This study provides a new strategy to counter the segregation of TM ions in layered lithium transition-metal oxides and will help to the design and development of high-energy cathodes with ultralong life.

Chemical and Structural Evolution of Layered Lithium-Transition Metal Oxide Cathode Material upon Cycling

With the rapid development of electronic devices and hybrid-electric/electric vehicles (H/EVs), lithium-ion batteries have become the most promising electrical storage system due to its high volumetric and gravimetric energy density. However, the high energy density of lithium-ion batteries is difficult to maintain after extended cycles due to capacity fading, which can be attributed to multiple possible reasons including SEI formation,[1] electrolyte decomposition,[2] and structural changes in electrode materials.[3] These issues generate important concerns to the stability and lifetime of the battery. Layered lithium-transition metal oxides represent a major type of cathode materials that are widely used in the commercial market. However, these materials are suggested to suffer structural transformation during electrochemical cycling. Previous studies suggest the possible surface structural transitions from the layered structure to spinel[4], and/or rock-salt structures,[5] or possible further decomposition of the transformed phase.[6]A clear explanation of these surface phenomenon is still under debate. An in-depth investigation of the structural reconstruction is necessary to elucidate the degradation mechanisms of the layered cathode materials. In the present study, we investigated the chemical evolution and structural transformation of a prominent layered cathode material for lithium-ion batteries, LiNi1/3Mn1/3Co1/3O2 (NMC). The redox reaction of NMC cathode during charge-discharge process was analyzed in detail using high-resolution electron-energy loss spectroscopy (EELS) and 7Li magic-angle spinning (MAS) NMR. Our results suggest that the charge compensation of the NMC cathode during the delithiation-lithiation process is mainly achieved by the oxidation and reduction of Ni2+↔Ni4+, whereas Mn4+ and Co3+ remain mostly unchanged. This is consistent with the NMR findings, from which a trend to lower chemical shift upon lithium extraction is well-correlated with the oxidation change from paramagnetic Ni2+ to diamagnetic Ni4+. Furthermore, the electronic structure of the NMC cathode during initial charging process is found to be inhomogeneous from the particle surface to the bulk using spatially resolved STEM-EELS technique (Figure 1a-1d). It is revealed that the particle surface is at lower oxidation state compared with the bulk region, indicating that the surface evolution of NMC cathode material occurs during the initial cycle. This surface evolution is further analyzed at different numbers of cycles using atomic-resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) imaging combined with EELS and nano-beam electron diffraction (NBED). Figure 1e shows a HAADF-STEM image of the NMC particle after 50 cycles. It can be clearly seen from the image that the bulk of the cycled NMC particle shows identical layered structure symmetry, the NBED (Figure 1h) obtained from the bulk can be indexed to the R-3m space group. In contrast, the Li layers are occupied by transition metal ions at the particle surface, and this surface reconstruction layer thickens with extended cycles. When probing to the outer-most surface, the corresponding NBED can be indexed to an Fm-3m rock-salt structure, as shown in Figure 1f. The valence change of the transition metal cations is analyzed using STEM-EELS. The results indicate that the transition metal ions are reduced to divalent states at the surface. In addition, the TM:O ratio increases almost twice at the surface than the bulk. The results indicating that the surface layer is a MO-type rock-salt phase with small amount of residual Li ions and/or vacancies. In addition, a transition zone is observed (Figure 1e) between the bulk layered region and the surface rock-salt layer, where the Li sites are partially occupied by the transition metal ions (some are marked by the red arrows), and the diffraction spots corresponding to the alternating arrangement of Li-containing layers and transition metal layers (some are marked with arrows) become weaker (Figure 1g). 7Li NMR studies of the local Li-environments, following ten cycles and ending at the top of charge, indicate that the order of the TM layers remains unchanged in the bulk of the material. This highlights the importance of complementary studies that are sensitive to different length scales. Further data, including EELS and NMR of the NMC cathode at different cycling state and electrolyte effect will be discussed in the presentation. [1] K. Edström, T. Gustafsson, J. Thomas, Electrochim. Acta 2004, 50, 397. [2] L. Terborg, S. Weber, F. Blaske, et al. J. Power Sources 2013, 242, 832. [3] B. Xu, C.R. Fell, M. Chi, et al. Energy Environ. Sci. 2011, 4, 2223. [4] A. Boulineau, L. Simonin, J.F. Colin, et al. Chem. Mater. 2012, 24, 3558. [5] F. Lin, I. M. Markus, D. Nordlund, et al. Nat. Commun. 2014, 5, 3529. [6] J. Zheng, M. Gu, J. Xiao, et al. Nano Lett. 2013, 13, 3824. Figure 1

Insight into the Vibrational and Thermodynamic Properties of Layered Lithium Transition-Metal Oxides LiMO2(M = Co, Ni, Mn): A First-Principles Study

Evaluation of the finite-temperature thermodynamic properties of the electrode materials generally helps to accurately describe the performance of Li-ion battery (LIBs). To know the characteristics of the layered lithium transition-metal oxides LiMO2 (M = Co, Ni, Mn) comprehensively, herein, the vibrational and related thermodynamic quantities of these electrode materials are investigated by using density functional perturbation theory (DFPT). Local density approximation (LDA) and generalized gradient approximation with the Hubbard model correction (GGA+U) yield similar results, either for the phonon dispersion or for the thermodynamic functions. Among the three layered lithium transition-metal oxides, the vibrational and thermodynamic properties of LiNiO2 is more close to that of LiMnO2, while relatively far away from that of LiCoO2, due to the same crystal structure of LiNiO2 and LiMnO2, which is different from that of LiCoO2. In addition, the corrections of average intercalation voltage as a function o...

Defect Physics and Chemistry in Layered Mixed Transition Metal Oxide Cathode Materials: (Ni,Co,Mn) vs (Ni,Co,Al)

Although layered lithium transition-metal oxides with different compositions of (Ni,Co,Mn) [NCM] or (Ni,Co,Al) [NCA] have been used in commercial lithium-ion battery cathodes, their defect physics and chemistry is still not well understood, despite having important ramifications for cycling properties, particularly capacity fade. Herein we report a hybrid density functional study of the crystal and electronic structures of and intrinsic point defects in the compositions LiNi1/3Co1/3Mn1/3O2 (NCM1/3) and LiNi1/3Co1/3Al1/3O2 (NCA1/3), which also serve as model compounds for NCM and NCA. We find that the transition metals can exist in different charge and spin states at different lattice sites. In NCM1/3, nickel/lithium antisite pairs, i.e., NiLi–LiNi, are estimated to be about 3% in samples prepared at 1000 °C. In NCA1/3, in addition to nickel antisites NiLi, aluminum antisites AlLi can also occur with a very high concentration. The AlLi defect has a high energy except when it is located between two Al atoms...

Effect of Sodium Content on the Reversible Lithium Intercalation into Sodium-Deficient Cobalt–Nickel–Manganese Oxides NaxCo1/3Ni1/3Mn1/3O2 (0.38 ≤ x ≤ 0.75) with a P3 Type of Structure

Layered lithium transition metal oxides with optimized nickel–manganese content are nowadays of primary interest as electrode materials for lithium ion batteries, since they are able to deliver a high capacity at a low cost. Herein we report a new class of less expensive cathode materials, which comprise sodium-deficient cobalt–nickel–manganese oxides NaxCo1/3Ni1/3Mn1/3O2 characterized with a layered structure and broad concentration range of sodium solubility. NaxCo1/3Ni1/3Mn1/3O2 oxides are obtained by thermal decomposition of mixed acetate–oxalate precursors, followed by thermal annealing between 700 and 800 °C. In the concentration range of 0.33 < x ≤ 0.75, NaxCo1/3Ni1/3Mn1/3O2 oxides assume a layered structure with a three-layer stacking (i.e., P3 type of structure). Based on electron paramagnetic resonance spectroscopy operating in the X-band (9.4 GHz), it is found that the charge compensation of Na deficiency is achieved by preferential oxidation of Ni2+ to Ni3+ and Ni4+, while Co and Mn ions retain their oxidation state of 3+ and 4+ within the whole concentration range. The electrochemical performance of NaxCo1/3Ni1/3Mn1/3O2 in model lithium cells is simply controlled by the amount of sodium content in the pristine compositions: a higher reversible capacity is achieved for sodium-rich oxides (i.e., 0.75 ≥ x ≥ 0.67), while sodium-poor oxides (i.e., 0.38 ≤ x ≤ 0.50) display a lower reversible capacity and improved cycling stability. The mechanism of the lithium intercalation into NaxCo1/3Ni1/3Mn1/3O2 is discussed on the basis of ex situ XRD, HRTEM, and X-ray photoelectron spectroscopy analyses.

Spatially resolved surface valence gradient and structural transformation of lithium transition metal oxides in lithium-ion batteries

Layered lithium transition metal oxides are one of the most important types of cathode materials in lithium-ion batteries (LIBs) that possess high capacity and relatively low cost. Nevertheless, these layered cathode materials suffer structural changes during electrochemical cycling that could adversely affect the battery performance. Clear explanations of the cathode degradation process and its initiation, however, are still under debate and not yet fully understood. We herein systematically investigate the chemical evolution and structural transformation of the LiNixMnyCo1-x-yO2 (NMC) cathode material in order to understand the battery performance deterioration driven by the cathode degradation upon cycling. Using high-resolution electron energy loss spectroscopy (HR-EELS) we clarify the role of transition metals in the charge compensation mechanism, particularly the controversial Ni2+ (active) and Co3+ (stable) ions, at different states-of-charge (SOC) under 4.6 V operation voltage. The cathode evolution is studied in detail from the first-charge to long-term cycling using complementary diagnostic tools. With the bulk sensitive 7Li nuclear magnetic resonance (NMR) measurements, we show that the local ordering of transition metal and Li layers (R3[combining macron]m structure) is well retained in the bulk material upon cycling. In complement to the bulk measurements, we locally probe the valence state distribution of cations and the surface structure of NMC particles using EELS and scanning transmission electron microscopy (STEM). The results reveal that the surface evolution of NMC is initiated in the first-charging step with a surface reduction layer formed at the particle surface. The NMC surface undergoes phase transformation from the layered structure to a poor electronic and ionic conducting transition-metal oxide rock-salt phase (R3[combining macron]m → Fm3[combining macron]m), accompanied by irreversible lithium and oxygen loss. In addition to the electrochemical cycling effect, electrolyte exposure also shows non-negligible influence on cathode surface degradation. These chemical and structural changes of the NMC cathode could contribute to the first-cycle coulombic inefficiency, restrict the charge transfer characteristics and ultimately impact the cell capacity.