Effect of composition on the structure of lithium- and manganese-rich transition metal oxides

The choice of chemical composition of lithium- and manganese-rich transition metal oxides used as cathode materials in lithium-ion batteries can significantly impact their long-term viability as storage solutions for clean energy automotive applications. Their structure has been widely debated: conflicting conclusions drawn from individual studies often considering different compositions have made it challenging to reach a consensus and inform future research. Here, complementary electron microscopy techniques over a wide range of length scales reveal the effect of lithium-to-transition metal-ratio on the surface and bulk structure of these materials. We found that decreasing the lithium-to-transition metal-ratio resulted in a significant change in terms of order and atomic-level local composition in the bulk of these cathode materials. However, throughout the composition range studied, the materials consisted solely of a monoclinic phase, with lower lithium content materials showing more chemical ordering defects. In contrast, the spinel-structured surface present on specific crystallographic facets exhibited no noticeable structural change when varying the ratio of lithium to transition metal. The structural observations from this study warrant a reexamination of commonly assumed models linking poor electrochemical performance with bulk and surface structure

Single-Crystal Based Diagnostics for Li-Ion Battery Cathode Development

Lithium transition-metal oxides (Li-TM oxides) are presently the most promising high energy density cathode materials for rechargeable lithium batteries. When operated at high voltages (&gt;4.3 V), however, these oxides suffer from structural instability, extensive side reactions with the electrolyte, poor cyclability and severe thermal runaway reactions. [1, 2] In order to develop successful strategies addressing these issues, there is a clear need in fundamental knowledge of the relationships between the material’s specific physical properties/reaction mechanisms and its reactivity. This is difficult to obtain on conventional aggregated secondary particles because isolating specific physical properties of interest from less controlled ones, such as particle porosity, grain boundaries, primary particle size and size distribution, is often challenging if not impossible. To this end, we developed a unique diagnostic approach combining carefully prepared cathode model samples and the start-of-the-art analysis techniques with high spatial resolution and chemical specificity. In this presentation, we will discuss the synthesis of high-voltage Ni/Mn spinel (LMNO) and high-capacity Li-and Mn-rich layered oxide (LMR-NMC) crystals with various physical characteristics. [3] We demonstrate the effective use of complementary microscopy and spectroscopy techniques at multi-length scale, such as aberration corrected (scanning) transmission electron microscopy, electron energy loss spectroscopy, X-ray energy dispersive spectroscopy, surface sensitive soft X-ray absorption spectroscopy and X-ray photoelectron spectroscopy, full-field transmission X-ray microscopy and X-ray absorption near edge structure imaging, to determine elemental, chemical and atomic-level structural make-up of the entire bulk as well as the surface properties of the crystals. [4, 5] Combined with electrochemical analysis, insights on the phase transformation mechanism during Li extraction/reinsertion and the key factors influencing the reactions occurring at the cathode/electrolyte interface were obtained. [6] Future directions and research in addressing some of the pressing challenges facing Li-TM oxide cathode development will also be discussed. References: 1. K. Edström, T. Gustafsson, and J. O. Thomas, The Cathode-Electrolyte Interface in the Li-ion Battery, Electrochimica Acta, 50, 397 (2004) 2. F. Lin, I. M. Markus, D. Nordlund, T.-C. Weng, M. D. Asta, H.L. Xin, and M. M. Doeff, Surface Reconstruction and Chemical Evolution of Stoichiometric Layered Cathode Materials for Lithium-ion Batteries, Nat. Commun., 5, 3529 (2014). 3. B. Hai, A. K. Shukla, H. Duncan and G. Chen, The Effect of Particle Surface Facets on the Kinetic Properties of LiMn1.5Ni0.5O4 Cathode Materials, J. Mater. Chem. A, 1, 759 (2013). 4. A. K. Shukla, Q. M. Ramasse, C. Ophus, H. Duncan, F. Hage, and G. Chen, Unravelling Structural Ambiguities in Lithium- and Manganese-Rich Transition Metal Oxides, Nat. Commun., 6, 8711 (2015). 5. P. Yan, J. Zheng, J. Zheng, Z. Wang, G. Teng, S. Kuppan, J. Xiao, G. Chen, F. Pan, J.-G. Zhang, and C.-M. Wang, “Ni and Co Segregations on Selective Surface Facets and Their Composition Dependence in Layered Lithium Transition-Metal Oxide Cathodes,” Advanced Energy Materials, 6, 1502455 (2016). 6. S. Kuppan, H. Duncan, and G. Chen, Controlling Side Reactions and Self-Discharge in High-Voltage Spinel Cathodes: The Critical Role of Surface Crystallographic Facets, Phys. Chem. Chem. Phys., 17, 26471 (2015). Acknowledgment This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. </jats:p

Bulk Structure and Surface Properties of Lithium- and Manganese-Rich Layered Oxides and Their Impact on Electrochemical Performance

Lithium- and manganese-rich layered oxides, Li1+x M1-x O2 (M=Mn, Ni, Co), are promising cathode materials for lithium-ion batteries due to their high capacities, typically more than 250 mAh/g. Despite the intense research and development in the past decade, their commercial adoption remains hindered by severe shortcomings including a large first-cycle irreversible capacity loss, [1] voltage and capacity fade, [2] DC resistance rise at low state of charge (SOC) and transition metal dissolution. [3] Studies have attributed these issues to the structural changes occurring during first-cycle activation and prolonged cycling, yet the crystal structure of the pristine oxides is still a matter of debate.  Fundamental studies on the conventionally synthesized, aggregated secondary particles often lead to ambiguous results due to the structural complexity in this class of materials.  In this presentation, we report the synthesis of well-formed, discrete layered Li1+x M1-x O2 crystals with various morphologies and surface properties using a molten salt method. Complementary microscopy and spectroscopy techniques at multi-length scale, including aberration corrected (scanning) transmission electron microscopy, electron energy loss spectroscopy and X-ray energy dispersive spectroscopy, were used to reveal the structural make-up of the bulk as well as the surface of the primary particles.  Surface sensitive soft X-ray absorption spectroscopy was further used to reveal the roles of key surface properties, particularly surface composition, surface area and surface crystalline orientation, in the material challenges facing Li1+x M1-x O2 cathodes.  Future directions and research to enable this class of materials will also be discussed. References: 1. Z. Lua and J. R. Dahn, Understanding the Anomalous Capacity of Li/Li[Ni x Li(1/3-2x/3)Mn(2/3-x/3)]O2 Cells Using In Situ X-Ray Diffraction and Electrochemical Studies, J. Electrochem. Soc., 149, A815 (2002). 2. J. R. Croy, S.-H. Kang, M. Balasubramanian, and M. M. Thackeray, Li2MnO3-Based Composite Cathodes for Lithium Batteries: A Novel Synthesis Approach and New Structures, Electrochem. Communications, 13, 1063 (2011). 3. S.-H. Kang and M. M. Thackeray, Enhancing The Rate Capability of High Capacity xLi2MnO3 .(1-x)LiMO2 (M = Mn, Ni, Co) Electrodes by Li–Ni–PO4 Treatment, Electrochem. Communications, 11, 748 (2009).  Acknowledgment This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. </jats:p

Structural Ambiguity in Li- and Mn-Rich Transition Metal Oxides: Trigonal, Monoclinic, or Both?

Li- and Mn-rich transition metal oxides have been extensively studied recently for their potential application in Li-ion batteries. These materials have attracted a lot of interest due to the high capacity offered by them. However, the structure of these materials in their pristine state is not clearly understood. Several reports have assigned their structure to be trigonal (R-3m), monoclinic (c2/m), or a combination of both (composite). The present study discusses the structure of Li1.2(Ni0.13Mn0.54Co0.13) O2 prepared with two different morphologies: plates and needles, using the results obtained from aberration corrected scanning transmission electron microscopy (STEM) imaging, electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (XEDS). 3-dimensional information was obtained by imaging the material at different zone axes and it was found that the primary particles are made up of a single phase, consisting of domains that correspond to variants of monoclinic structure, save for rare localized defects and a thin surface layer on certain crystallographic facets. These findings were further confirmed on two commercially available Li-and Mn-rich transition metal oxides, namely TODA HE5050 and Envia HCMR XLE2 . It will be shown how diffraction-based techniques can lead to ambiguous interpretation of structure in this class of materials. The study not only solves the ambiguity in the structure determination of this class of complex oxides but also highlights the importance of atomic resolution imaging performed on multiple zone axes when it is used for similar investigations. Figure 1 <jats:p /