ABSTRACT
Understanding the kinetics of interfacial ion speciation could inform battery designs. However, this knowledge gap persists, largely due to the challenge of experimentally interrogating the evolution of ions near electrode interfaces in a sea of bulk signals. We report here the very first kinetically resolved correlation between interfacial ion speciation and lithium-ion storage in a model system, by applying global target analysis to in situ attenuated total reflectance (ATR) Fourier-Transform infrared (FTIR) spectroelectrochemical data. Our results suggest that it may be more kinetically viable for lithium to be extracted from contact ion pairs (CIPs) to contribute to faster electrode charging compared to fully solvated lithium. As the search for fast-charging lithium-ion batteries and supercapacitors wages on, this discovery suggests that manipulating the ion pairing within the electrolyte could be one effective strategy for promoting faster-charging kinetics.
ABSTRACT
In this study, microscopic Raman spectroscopy and Ab initio quantum chemical calculation were used to determine the structural details of ion pairs and their transformation in concentrated K[B(OH)4] droplets. The Raman experiment shows that the vsym-B(OH)4- undergoes a downward shift with the decrease of WSR. The contact ion pairs (CIPs) change to solvent shared ion pairs when the molar water-to-solute ratio (WSR) is bigger than 6; CIPs are the dominant species when 1.33 < WSR < 6, where K+ bonds to [B(OH)4-] in bidentate form (CIP-II); the CIPs quickly dehydrate and associate to triple ion pairs (TIPs) when WSR < 5. Raman experiment and ab initio quantum chemical calculation show that TIPs are mainly present in "anionic" type such as {[B(OH)4-]K+[B(OH)4-](H2O)n}, where K+ bonds to two [B(OH)4-] in bidentate or/and tridentate form (TIP-a-II or/and TIP-a-III). When WSR <1.33, most TIPs convert to complex clusters such as chain-like structure. The remaining TIPs associate to six-membered ring structure [B3O3(OH)4-] and the relative content increases from 0 to 20% when the WSR ranges from 1.33 to 0.55.
ABSTRACT
In this study, ion pairs in a single sodium tetrahydroxyborate [Na [B(OH)4] droplet were analyzed using an "in-situ strategy" in which a sample-droplet of nanogram mass was deposited on a hydrophobic substrate and droplet was forced to enter into a supersaturated state by decreasing the relative humidity (RH) of the environment. The structure of the solvated [B(OH)4-] ionic moiety with various molar water-to-solute ratios (WSR) was analyzed using Raman spectroscopy. To confirm the structural changes in the droplet, electronic structure calculations were carried out using density functional theory (DFT). The frequencies calculated for the totally symmetric BO stretching vibration (vsym(BO)) of the [B(OH)4-] moiety were compared with those of the fundamental bands observed in the Raman spectra recorded of the droplets. The following results have been obtained: (i) when WSR is reduced from 9 to 0.1, the frequency of the band that corresponds to vsym(BO) shifts from 745 to 746â¯cm-1, and its full-width at half-maximum value increases from 19.7 to 20.5â¯cm-1; (ii) when WSR ≥7, the solvent-shared ion pair (SIP) is predominantly present in the solution, whereas in the case of WSRâ¯<â¯7, SIP transforms into a contact ion pair (CIP) formed by Na+ and [B(OH)4-] in bidentate coordination; (iii) when WSRâ¯=â¯3, most of the CIPs transform into a cationic type of triple ion pair (TIP) composed of two Na+ and one [B(OH)4-] in bidentate coordination; (iv) when WSR is further reduced, most TIP continually associate to form a more complex structure and with a small amount of six-membered ring complex also formed. These results will help us understand the ion association mechanism during dehydration process of Na[B(OH)4] droplets.