The Impact of Interlayer Spacing on Alkali Metal Insertion in Layered Electrode Materials

The Marbella Lab's research focuses on understanding the electrochemical behavior of lithium, sodium, and potassium ion batteries, specifically the insertion mechanisms of monovalent alkali metals (Li and K) into layered electrode materials. Despite their potential cost benefits, the primary challenge with K batteries is the shorter cycle life and lower energy density, likely attributable to the larger atomic size of potassium. With tunable synthetic properties, the Re6Se8X2 two-dimensional van der Waals material offers an avenue to investigate these mechanisms. Comparative studies were conducted using K and Li half cells with Re6Se8Cl2 and LiNi0.33Mn0.33Co0.33O2 (NMC111) cathodes, respectively. Key methods included cyclic voltammetry, galvanostatic cycling, X-ray photoelectron spectroscopy (XPS) to study the solid electrolyte interphase, and X-ray diffraction (XRD) to evaluate crystalline structures and interlayer spacing changes. Results from the cyclic voltammetry showed apparent differences between K and Li in Re6Se8Cl2: while Li had two discernible intercalation events, K exhibited just one. This suggests that potassium's larger atomic size complicates its intercalation process, producing broader peaks and eliminating a second intercalation event.

Further, the discharge cycle analysis showed Li undergoing two stable intercalation plateaus compared to K's single event. Screening effects amplified by K's ionic size result in enhanced ion-ion repulsion, impairing its transportation in the cathode. The slope data established less screening in the K cell compared to the Li cell, hinting that repulsions significantly hindered K's reversibility. An intriguing observation was the difference in interlayer spacing in Re6Se8(S-Ph)2 and Re6Se8Cl2. The former's larger spacing allowed for dual K ion intercalation without subsequent deintercalation. K's insertions are size-dependent. Our study underscores the size of potassium ions as a primary factor affecting their behavior in Re6Se8Cl2. Enhancing our understanding of these mechanisms is paramount to advancing the potential use of K batteries in the industry.