The progressive advancements in communication and transportation has changed human daily life to a great extent. While important advancements in battery technology has come since its first demonstration, the high energy demands needed to electrify the automotive industry have not yet been met with the current technology. One possible cathode material are the Li-rich layered oxide compounds xLi₂MnO₃.(1-x)LiMO₂ (M= Ni, Mn, Co) (0.5=<x=<1.0) that exhibit capacities over 280 mAh g^-1. In this class of compounds, lithium ions reside in both lithium layer and transition metal layer of close packed oxygen framework, typical from O3 type layered oxides like LiCoO₂.^{1, 2} Large irreversible capacities are often observed in these materials due to irreversible oxygen loss or side reactions stemming for the electrolyte. It has been also observed using ex-situ NMR that lithium reinsertion back into the transition metal layer is little to none. However, these studies have not revealed the dynamic process of lithium migration for the Li-rich material under operandoelectrochemical cycling conditions.

Neutron scattering has several distinct advantages for battery studies: 1) The sensitivity of neutron to light elements such as lithium and oxygen are significant in order to determine their position in the crystal structure; 2) Compares to the X-ray, the neutron shows larger scattering contrast between neighboring elements in the periodic table specifically the scattering lengths; and 3) The deep penetration capability of neutron allows simultaneous observation of the cathode and anode.³ However, challenges exist in broadening the application of operando neutron diffraction for Li-ion batteries research. First, limited by the generation reactions of neutrons, the neutron flux is usually several orders of magntitude lower than X-rays. In another words, longer acquisition times as well as larger amounts of samples are required for neutron diffraction experiments. In addition, the existence of hydrogen, which has a large incoherent neutron-scattering cross-section, is detrimental to the signal-to-noise ratio of neutron diffraction pattern. Separators (polyethylene based porous membrane) and poly carbonate based electrolyte solutions contain a considerate amount of hydrogen. These two major reasons pose significant challenges to operando neutron diffraction for lithium ion battery research although it is such a powerful technique for light elements like lithium.

In order to gain more in-depth insights about the lithium (de-)intercalation mechanisms in Li-rich layered oxides, track particularly the lithium ions in transition metal layer, operando neutron diffraction experiments were designed to quantitatively observe lithium migration in this type of oxides during the electrochemical process.  In this study, we use amorphous silicon as an anode for the neutron diffraction battery design in order to avoid any overlap of signal that may be associated with the anode material. We perform operando neutron diffraction to probe lithium and oxygen for a high Li-rich (HLR), Li[Li_x/3Ni_(3/8-3x/8)Co_(1/4-x/4)Mn_(3/8+7x/24)O₂ (x = 0.6) material, and low Li-rich (LLR), Li[Li_x/3Ni_(1/3-x/3)Co_(1/3-x/3)Mn_(1/3+x/3)O₂ (x = 0.24) material with varying degrees of the high voltage plateau. In conjunction with the operando neutron diffraction, density functional theory (DFT) calculations were used to explore the incorporation of dilute oxygen vacancy, its affect on the lattice mechanics and oxygen positions. We also observe site-dependent lithium migration taking place during different stage of charging/discharge processes. Furthermore, this work demonstrates the potential of investigating dynamic changes of light elements in large format (10-100 times larger format than the typical operando cells for synchrotron X-ray diffraction) prismatic and cylindrical batteries under realistic cycling condition via operando neutron diffraction method. 

Acknowledgements

UCSD’s efforts are supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231, Subcontract No. 7073923, under the Advanced Battery Materials Research (BMR) Program. The neutron experiments benefited from the SNS user facility, sponsored by the office of Basic Energy Sciences (BES), the Office of Science of the DOE. H.L. acknowledges the financial support from the China Scholarship Council under Award No. 2011631005. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575.

 

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