Li-ion battery performance and life cycle strongly depend on a passivation layer called solid-electrolyte interphase (SEI). Its structure and composition are studied in great details, while its formation process remains elusive due to difficulty of in situ measurements of battery electrodes. Here we provide a facile methodology for in situ atomic force microscopy (AFM) measurements of SEI formation on cross-sectioned composite battery electrodes allowing for direct observations of SEI formation on various types of carbonaceous negative electrode materials for Li-ion batteries. Using this approach, we observed SEI nucleation and growth on highly oriented pyrolytic graphite (HOPG), MesoCarbon MicroBeads (MCMB) graphite, and non-graphitizable amorphous carbon (hard carbon). Besides the details of the formation mechanism, the electrical and mechanical properties of the SEI layers were assessed. The comparative observations revealed that the electrode potentials for SEI formation differ depending on the nature of the electrode material, whereas the adhesion of SEI to the electrode surface clearly correlates with the surface roughness of the electrode. Finally, the same approach applied to a positive LiNi1/3Mn1/3Co1/3O2 electrode did not reveal any signature of cathodic SEI thus demonstrating fundamental differences in the stabilization mechanisms of the negative and positive electrodes in Li-ion batteries.

The effect of salt concentration in diglyme-based electrolytes on cycling performance of promising KVOPO 4 and K 1.69 Mn[Fe(CN) 6 ] 0.85 •0.4H 2 O positive electrodes (cathodes) and a hard carbon negative electrode (anode) for next-generation potassium-ion (K-ion) batteries is investigated. A decrease in free solvent molecule number with increasing electrolyte concentration is found, which results in a better aluminum current collector stability, formation of thinner solid electrolyte interface (SEI) passivation layers, and further inhibition of solvent degradation redox processes occurring at the electrode surface upon cycling. The KVOPO 4 and K 1.69 Mn[Fe(CN) 6 ] 0.85 •0.4H 2 O cathodes exhibit an enhanced specific discharge capacity (54 and 105 mA•h•g −1 , respectively) in K-ion cells at the highest electrolyte concentrations (2 and 2.5 M KPF 6 in diglyme, respectively) at a 0.1 C rate. However, the behavior of the hard carbon anode is noticeably affected by the salt concentration over the first few cycles, a phenomenon tentatively attributed to the SEI layer formation and the presence of irreversible intercalation sites for K + ions in the hard carbon framework. Finally, electrochemical tests on K-ion full cells consisting of the K 1.69 Mn[Fe(CN) 6 ] 0.85 •0.4H 2 O cathode, a hard carbon anode, and an ether-based electrolyte show capacity retention of 86% over 300 cycles at a 0.6 C rate.

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Hard carbon (HC) is considered as a negative electrode material for potassium-ion batteries, but it suffers from significant irreversible capacity loss at the first discharge cycle. Here, we investigated the possible reasons of this capacity loss with a combination of in situ AFM and various ex situ TEM techniques (high resolution TEM and high angle annular dark field scanning TEM imaging, and STEM-EELS and STEM-EDX spectroscopic mapping) targeting the electrode/electrolyte interphase formation process in the carbonate-based electrolyte with and without vinylene carbonate (VC) as an additive. The studied HC consists of curved graphitic layers arranged into short packets and round cages, the latter acting as traps for K+ ions causing low Coulombic efficiency between cycling. Our comparative study of solid electrolyte interphase (SEI) formation in the carbonate-based electrolyte with and without the VC additive revealed that in the pristine electrolyte, the SEI consists mostly of inorganic components, whereas adding VC introduces a polymeric organic component to the SEI, increasing its elasticity and stability against fracturing upon HC expansion/contraction during electrochemical cycling. Additionally, significant K+ loss occurs due to Na+ for K+ exchange in Na-carboxymethyl cellulose used as a binder. These findings reflect the cumulative impact of the internal HC structure, SEI properties, and binder nature into the electrochemical functional properties of the HC-based anodes for K-ion batteries.

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