Interfacial Reaction Mechanisms on Graphite Anodes for K-Ion Batteries

Abstract

Potassium-ion (K-ion) batteries (KIBs) potentially offer numerous advantages over conventional lithium-ion batteries as a result of the high natural abundance of potassium and its lower positive charge density compared with lithium. This introduces the possibility of using K-ion in fast charging applications, in which cost effectiveness is also a major factor. Unlike in sodium-ion batteries, graphite can be used as an anode in K-ion cells, for which an extensive supply chain, electrode manufacturing infrastructure, and knowledge already exist. However, the performance of graphite anodes in K-ion cells does not meet expectations, with rapid capacity fading and poor first cycle irreversible capacities often reported. Here, we investigate the formation and composition of the solid electrolyte interphase (SEI) as well as K+ insertion in graphite anodes in KIBs. Through the use of energy-tuned synchrotron-based X-ray photoelectron spectroscopy, we make a detailed analysis at three probing depths up to ∼50 nm of graphite anodes cycled to various potentials on the first discharge–charge cycle. Extensive SEI formation from a KPF6/DEC/EC electrolyte system is found to occur at low potentials during the insertion of potassium ions into graphite. During the subsequent removal of potassium ions from the structure, the thick SEI is partially stripped from the electrode, demonstrating that the SEI layer is unstable and contributes to a significant proportion of the capacity upon both discharge and charge. With this in mind, further work is required to develop an electrolyte system with stable SEI layer formation on graphite in order to advance the KIB technology.

Microstructure‐Dependent K+ Storage in Porous Hard Carbon

“…A tremendous augment of R SEI from 423 to 5595 Ω is identified for HC‐C, resulting from the repeated formation/destruction of SEI layer induced by the continuous collapsed architecture. [ 27 ] Notably, the continuous solvent/salt decomposition and the repeated regeneration of SEI layer usually cause the partial loss of electronic connection between active materials and current collector, giving rise to the increase of R ct upon long cycling, in addition to R SEI . This is evident from Figure 3h, in which, the R ct of HC‐C increases sharply from 175 (1st cycle) to 3623 Ω (100th cycle), ascribing to the interfacial changes or side reactions.…”

Strategies for Mitigating Dissolution of Solid Electrolyte Interphases in Sodium‐Ion Batteries

“…To shed light on the SEI chemistry in reference electrolytes based on EC:DEC and PC,s ynchrotron-based soft X-ray photoelectron spectroscopy (SOXPES) with photon energy of 1090 eV was used ( Figures S9-S11). [36] TheC Vs in Figures S1 and S12 show al arge reduction current due to solvent and water reduction. Thea mount of water in the electrolyte can affect the contents of inorganic and organic species in the SEI dependent on what solvent system was used.…”

Strategies for Mitigating Dissolution of Solid Electrolyte Interphases in Sodium‐Ion Batteries

“…To shed light on the SEI chemistry in reference electrolytes based on EC:DEC and PC, synchrotron‐based soft X‐ray photoelectron spectroscopy (SOXPES) with photon energy of 1090 eV was used (Figures S9–S11) [36] . The CVs in Figures S1 and S12 show a large reduction current due to solvent and water reduction.…”