Isaac M. Markus

Surface reconstruction and chemical evolution of stoichiometric layered cathode materials for lithium-ion batteries

The present study sheds light on the long-standing challenges associated with high-voltage operation of LiNi(x)Mn(x)Co(1-2x)O2 cathode materials for lithium-ion batteries. Using correlated ensemble-averaged high-throughput X-ray absorption spectroscopy and spatially resolved electron microscopy and spectroscopy, here we report structural reconstruction (formation of a surface reduced layer, to transition) and chemical evolution (formation of a surface reaction layer) at the surface of LiNi(x)Mn(x)Co(1-2x)O2 particles. These are primarily responsible for the prevailing capacity fading and impedance buildup under high-voltage cycling conditions, as well as the first-cycle coulombic inefficiency. It was found that the surface reconstruction exhibits a strong anisotropic characteristic, which predominantly occurs along lithium diffusion channels. Furthermore, the surface reaction layer is composed of lithium fluoride embedded in a complex organic matrix. This work sets a refined example for the study of surface reconstruction and chemical evolution in battery materials using combined diagnostic tools at complementary length scales.

Profiling the nanoscale gradient in stoichiometric layered cathode particles for lithium-ion batteries

Chemical and structural evolution in battery materials influences properties relevant to ionic and electronic transport and ultimately impacts the battery performance. Although chemical and structural gradients have been observed in several cathode materials, the origin(s) of these phenomena are poorly understood. Via high-throughput core-level spectroscopies {i.e., X-ray absorption spectroscopy (XAS), depth-profiled X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS)}, as well as scanning transmission electron microscopy (STEM), the present study seeks to achieve mechanistic understanding for these phenomena in a stoichiometric Rm layered cathode material (e.g., LiNixMnxCo1−2xO2, NMC). We observed that the surfaces of particles in the composite electrode are complicated by the presence of a surface reaction layer resulting from electrolyte decomposition. In large particle ensembles, the global nickel oxidation state switches between Ni2+ and Ni2+x (x = 1–2) during charging/discharging processes, and hole states are also created at the O2p level due to the TM3d–O2p hybridization states. In primary particles, the surface is less oxidized than the bulk counterpart of the same particle whenever the particle has been cycled. This is partially attributed to the reconstruction from an Rm structure to an Fmm structure at the surfaces of NMC particles. This work provides a unique insight into correlating crystal structures with charge compensation mechanisms and performance fading in stoichiometric layered cathode materials.

Chemical and Structural Stability of Lithium-Ion Battery Electrode Materials under Electron Beam

The investigation of chemical and structural dynamics in battery materials is essential to elucidation of structure-property relationships for rational design of advanced battery materials. Spatially resolved techniques, such as scanning/transmission electron microscopy (S/TEM), are widely applied to address this challenge. However, battery materials are susceptible to electron beam damage, complicating the data interpretation. In this study, we demonstrate that, under electron beam irradiation, the surface and bulk of battery materials undergo chemical and structural evolution equivalent to that observed during charge-discharge cycling. In a lithiated NiO nanosheet, a Li2CO3-containing surface reaction layer (SRL) was gradually decomposed during electron energy loss spectroscopy (EELS) acquisition. For cycled LiNi0.4Mn0.4Co0.18Ti0.02O2 particles, repeated electron beam irradiation induced a phase transition from an layered structure to an rock-salt structure, which is attributed to the stoichiometric lithium and oxygen removal from 3a and 6c sites, respectively. Nevertheless, it is still feasible to preserve pristine chemical environments by minimizing electron beam damage, for example, using fast electron imaging and spectroscopy. Finally, the present study provides examples of electron beam damage on lithium-ion battery materials and suggests that special attention is necessary to prevent misinterpretation of experimental results.

Computational and Experimental Investigation of Ti Substitution in Li1(NixMnxCo1–2x–yTiy)O2 for Lithium Ion Batteries

Aliovalent substitutions in layered transition-metal cathode materials has been demonstrated to improve the energy densities of lithium ion batteries, with the mechanisms underlying such effects incompletely understood. Performance enhancement associated with Ti substitution of Co in the cathode material Li1(NixMnxCo1-2x)O2 were investigated using density functional theory calculations, including Hubbard-U corrections. An examination of the structural and electronic modifications revealed that Ti substitution reduces the structural distortions occurring during delithiation due to the larger cation radius of Ti(4+) relative to Co(3+) and the presence of an electron polaron on Mn cations induced by aliovalent Ti substitution. The structural differences were found to correlate with a decrease in the lithium intercalation voltage at lower lithium concentrations, which is consistent with quasi-equilibrium voltages obtained by integrating data from stepped potential experiments. Further, Ti is found to suppress the formation of a secondary rock salt phase at high voltage. Our results provide insights into how selective substitutions can enhance the performance of cathodes, maximizing the energy density and lifetime of current Li ion batteries.

Influence of synthesis conditions on the surface passivation and electrochemical behavior of layered cathode materials

Understanding the relationship between materials synthesis and electrochemical behaviors should provide valuable knowledge to further the advancement of lithium-ion batteries. In this work, layered cathode materials {e.g., LiNi0.4Mn0.4Co0.18Ti0.02O2 (NMCs)} were prepared under three different annealing conditions, i.e., 900 °C for 6 hours, 8 hours, and 12 hours, respectively. The resulting materials exhibit equivalent crystal structures and morphologies yet likely different surface chemical environments. These materials show distinctively different resistances against the surface passivation/reconstruction (reduction of the transition metals in the layered structure to form rock-salt and/or spinel phases) during electrochemical cycling (2.0–4.7 V vs. Li+/Li). In general, the materials annealed for longer durations exhibited lower tendencies to form the surface passivation layer. Furthermore, the surface passivation became less severe when the electrode materials were cycled under mild conditions, such as slow constant current charging–discharging as opposed to cyclic voltammetry. The present study correlates the synthetic conditions with the surface instability and the electrochemical performance in cathode materials, and provides new insights into improving synthetic protocols for battery materials.

High temperature investigation of electrochemical lithium insertion into Li4Ti5O12

Li4Ti5O12 was synthesized via a solid state reaction and lithiated at 400 °C in a custom built galvanostatic cell consisting of a molten LiCl-KCl electrolyte and Li-Al alloy wires as counter and reference electrodes. The material exhibits decreased rate capability at 400 °C compared to the room temperature behavior. Electrochemical lithiation at C/20 exhibits a discharge profile with both a sloping curve and flat plateau, which is indicative of a solid solution behavior before reaching a two phase region. This electrochemical behavior is shown to be correlated with reversible formation of the cubic Li2TiO3 phase.