Elucidating the Sodiation Mechanism in Hard Carbon by Operando Raman Spectroscopy

Abstract

Operando microbeam Raman spectroscopy is used to map the changes in hard carbon during sodiation and desodiation in unprecedented detail, elucidating several important and unresolved aspects of the sodiation mechanism. On sodiation a substantial, reversible decrease in G-peak energy is observed, which corresponds directly to the sloping part of the voltage profile and we argue can only be due to steady intercalation of sodium between the turbostratic layers of the hard carbon. Corresponding reversibility of the D-peak energy change is consistent with intercalation rather than representing a permanent increase in disorder. No change in energy of the graphitic phonons occurs over the low voltage plateau, indicating that intercalation saturates before sodium clusters form in micropores in this region. At the start of the initial sodiation there is no change in G-and D-peak energy as the Solid Electrolyte Interphase (SEI) forms. After SEI formation, the background slope of the spectra increases irreversibly, due to fluorescence. The importance of in situ/operando experiments over ex situ studies is demonstrated; washing the samples or air exposure causes the G-and D-peaks to revert back to their original states owing to SEI removal and sodium de-intercalation and confirming no permanent damage to the carbon structure.

Homogenous metallic deposition regulated by defect-rich skeletons for sodium metal batteries

“…For the main sloping region of sodiation (40.1 V vs. Na + /Na), both peaks are found to become broader, while the D-peak is gradually shifting to a higher wavenumber (blue shift), meaning that in this region, Na + ions are forming chemical bonding with atoms at in-plane defect sites such as carbon vacancies, substitutional impurities, interstitial impurities. 55,56 These findings can also support that the longest sloping region in the GCD curve of L700 results from the interaction of Na + ions with defect sites. The position of the G-peak remains almost unchanged during the main sloping region of sodiation, suggesting no significant electrochemical interaction of Na + ions with the graphitic bands since the G-peak position and width are highly sensitive to electron or ion doping.…”

“…The position of the G-peak remains almost unchanged during the main sloping region of sodiation, suggesting no significant electrochemical interaction of Na + ions with the graphitic bands since the G-peak position and width are highly sensitive to electron or ion doping. 55 When the potential is close to 0 V vs. Na + /Na, the decrease of G-peak View Article Online position can be measured concomitant with an increase in its width because Na + ions are interacting with graphitic bands in this region. The transfer of charge from the sodium atoms to p* antibonding orbitals of carbons affects the length of the C-C bonds.…”

Assessing the Reactivity of Hard Carbon Anodes: Linking Material Properties with Electrochemical Response Upon Sodium‐ and Lithium‐Ion Storage

“…It is well known that Li + ions intercalate into the graphite structure forming Li x C 6 , which generally results in a shift to the lower binding energy of the C=C bond [37] . Likewise, it has been recently reported that also the insertion of Na + ions into the turbostratically layered pseudo‐graphitic nano domain (TLPG‐ND) of HCs causes a shift of the peak position [21] . Therefore, selecting the pseudo‐graphitic‐like peak as a reference for the binding energy calibration would lead to unprecise peak attribution.…”

“…Despite the promising properties, several disadvantages affect the performance of HC as anode material, such as the poor initial coulombic efficiency (ICE), the low power performance, and the instability of the solid electrolyte interphase (SEI) [17,18] . For this reason, a renewed attention has been recently focused towards the fundamental understanding of the sodiation mechanism, aiming at elucidating a direct relation among structural properties of HC and its electrochemical behavior especially by employing operando characterization methods [10,19–22] . However, the controversial nature of the HC structure hinders a comprehensive understanding of the structure‐function correlation [20,22–29] …”

Characterizing Batteries by In Situ Electrochemical Atomic Force Microscopy: A Critical Review

“…[ 99 ] For graphite anodes in LIBs, analysis of the G‐peak has been used during operando and in situ charging studies to reveal that the intercalation occurs in distinct steps forming high stage graphite intercalation compounds (where ‘stage’ refers to the number of graphite layers between each of intercalant) eventually forming stage 1 LiC 6 at full charging. [ 100–102 ] For hard carbon‐based electrodes, by fitting the G‐peak position and width with doping, it has been shown that Na‐ion intercalation is incremental (and associated with the sloping part of the voltage profile), [ 103,104 ] similar to the incremental charging found in single to few layer graphene. [ 105 ] However, the lateral resolution limit of Raman scattering is far larger than for AFM, being limited by the diffraction limit of light, thus blind to the detailed atomic‐scale changes that can occur.…”