Phase Evolution and Intermittent Disorder in Electrochemically Lithiated Graphite Determined Using in Operando Neutron Diffraction
Copyright © 2020 American Chemical Society. Since their commercialization in 1991, lithium-ion batteries (LIBs) have revolutionized our way of life, with LIB pioneers being awarded the 2019 Nobel Prize in Chemistry. Despite the widespread use of LIBs, many LIB applications are not realized due to performance limitations, determined largely by the ability of electrode materials to reversibly host lithium ions. Overcoming such limitations requires knowledge of the fundamental mechanism for reversible ion intercalation in electrode materials. In this work, the still-debated structure of the most common commercial electrode material, graphite, during electrochemical lithiation is revisited using in operando neutron powder diffraction of a commercial 18650 lithium-ion battery. We extract new structural information and present a comprehensive overview of the phase evolution for lithiated graphite. Charge-discharge asymmetry and structural disorder in the lithiation process are observed, particularly surrounding phase transitions, and the phase evolution is found to be kinetically influenced. Notably, we observe pronounced asymmetry over the composition range 0.5 > x > 0.2, in which the stage 2L phase forms on discharge (delithiation) but not charge (lithiation), likely as a result of the slow formation of the stage 2L phase and the closeness of the stage 2L and stage 2 phase potentials. We reconcile our measurements of this transition with a stage 2L stacking disorder model containing an intergrown stage 2 and 2L phase. We resolve debate surrounding the intercalation mechanism in the stage 3L and stage 4L phase region, observing stage-specific reflections that support a first-order phase transition over the 0.2 > x > 0.04 range, in agreement with minor changes in the slope of the stacking axis length, despite relatively unchanging 00l reflection broadening. Our data support the previously proposed /ABA/ACA/ stacking for the stage 3L phase and an /ABAB/BABA/ stacking sequence of the stage 4L phase alongside experimentally derived atomic parameters. Finally, at low lithium content 0 < x < 0.04, we find an apparently homogeneous modification of the structure during both charge and discharge. Understanding the phase evolution and mechanism of structural response of graphite to lithiation under battery working conditions through in operando measurements may provide the information needed for the development of alternative higher performance electrode materials.