Systematic Investigations of Surface Oxygen Functional Groups on Spherical Hard Carbon for Sodium-Ion Batteries
Authors: Bauer Felix , Scheiba Frieder 
Published: August 2024

The surface activity of hard carbons and its effect on the formation of a stable solid electrolyte interface (SEI) in sodium ion batteries has gained significant interest in the scientific community over the last years. The surface of hard carbons thus plays a pivotal role within the cell, as it constitutes the interface between the bulk material and the electrolyte. Following pyrolysis of the carbon-rich precursor at high temperatures, various oxygen functional groups (OFGs) and defects, such as five- or seven-membered rings, are present on the surface of the resulting hard carbons. The influence of OFGs on the surface is however controversially discussed. On the one hand, irreversible reactions of sodium ions with OFGs lower the initial coulombic efficiency (ICE), which leads to a decreased energy density. On the other hand, OFGs may also serve as nucleation sites for the SEI leading to a more conductive interfacial phase for sodium ions. Although many approaches of additional oxygen functionalization on the surface of hard carbons have already been reported, they mostly employ harsh reaction conditions disregarding the morphological impact on the carbon surface.
In our studies, oxo-functionalized polycyclic aromatic hydrocarbons (PAHs) are adsorbed on the surface to serve as model OFGs on surface oxygen deficient hard carbons. The versatility of functionalized pyrenes in combination with nondestructive adsorption enables independent investigations on the oxygen functionalization of the hard carbon surface. In the first step, native surface oxygen groups are cleaved from the pristine hard carbon through a high-temperature post-treatment in hydrogen atmosphere. Afterwards, selective re-introduction of surface OFGs is achieved through adsorption of oxo-functionalized pyrene derivatives such as 1-hydroxypyrene or 1-pyrenecarboxylic acid to simulate hydroxy or carboxylic surface functionalities. Additionally, various spatially configurations of surface OFGs can be mimicked by elongating the distance between the oxygen functionality and the pyrene backbone through a short alkyl chain (e.g. 1-pyrenebutyric acid).
Our systematic approach of selectively re-introducing surface OFGs allows the discrimination between different surface OFGs and their influence on the SEI. The surface of our modified hard carbons is characterized by X-Ray photoelectron spectroscopy (XPS) and Raman spectroscopy to evaluate the oxygen content and determine the defect concentration. The electrochemical performance is assessed by rate performance tests and cyclic voltammetry in three-electrode half-cells with sodium metal as counter and reference electrode. Initial findings validate the correlation between surface oxygen functionalization determined by XPS and the electrochemical formation potential of the SEI during the first cycle. After high-temperature treatment in a hydrogen atmosphere, the surface oxygen content decreases from ~5% to 2%, leading to a significant lowering of the plateau capacity. Simultaneously, the ICE decreases from 79% to 55%. Cyclic voltammetry of deoxygenated hard carbon reveals that the SEI formation potential is shifted from ~1.0 V vs. Na towards lower potentials of ~0.3 V vs. Na indicating that different decomposition products are formed. Furthermore, rate performance is substantially deteriorated by the development of a modified SEI. Ex-situ XPS of cycled electrodes and electrochemical impedance spectroscopy are used to identify differences in the SEI composition and link them to the electrochemical performance of our spherical hard carbons.
Figure: 1-Pyrenebutyric acid adsorbed on the surface of hard carbon highlighting the different possible spatially orientations of the oxygen functional group.

Figure 1