Microscopic Structure of Contact Ion Pairs in Concentrated LiCl- and LiClO4-Tetrahydrofuran Solutions Studied by Low-Frequency Isotropic Raman Scattering and Neutron Diffraction with 6Li/7Li Isotopic Substitution Methods

Extending Ring-Chain Coupling Empirical Law to Lithium-Mediated Electrochemical Ammonia Synthesis

With its efficient nitrogen fixation kinetics, electrochemical lithium-mediated nitrogen reduction reaction (LMNRR) holds promise for replacing Haber-Bosch process and realizing sustainable and green ammonia production. However, the general interface problem in lithium electrochemistry seriously impedes the further enhancement of LMNRR performance. Inspired by the development history of lithium battery electrolytes, here, we extend the ring-chain solvents coupling law to LMNRR system to rationally optimize the interface during the reaction process, achieving nearly a two-fold Faradaic efficiency up to 54.78±1.60 %. Systematic theoretical simulations and experimental analysis jointly decipher that the anion-rich Li+ solvation structure derived from ring tetrahydrofuran coupling with chain ether successfully suppresses the excessive passivation of electrolyte decomposition at the reaction interface, thus promoting the mass transfer of active species and enhancing the nitrogen fixation kinetics. This work offers a progressive insight into the electrolyte design of LMNRR system.

The role of ion solvation in lithium mediated nitrogen reduction

Since its verification in 2019, there have been numerous high-profile papers reporting improved efficiency of lithium-mediated electrochemical nitrogen reduction to make ammonia. However, the literature lacks any coherent investigation systematically linking bulk electrolyte properties to electrochemical performance and Solid Electrolyte Interphase (SEI) properties. In this study, we discover that the salt concentration has a remarkable effect on electrolyte stability: at concentrations of 0.6 M LiClO₄ and above the electrode potential is stable for at least 12 hours at an applied current density of -2 mA cm^-2 at ambient temperature and pressure. Conversely, at the lower concentrations explored in prior studies, the potential required to maintain a given N₂ reduction current increased by 8 V within a period of 1 hour under the same conditions. The behaviour is linked more coordination of the salt anion and cation with increasing salt concentration in the electrolyte observed via Raman spectroscopy. Time of flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy reveal a more inorganic, and therefore more stable, SEI layer is formed with increasing salt concentration. A drop in faradaic efficiency for nitrogen reduction is seen at concentrations higher than 0.6 M LiClO₄, which is attributed to a combination of a decrease in nitrogen solubility and diffusivity as well as increased SEI conductivity as measured by electrochemical impedance spectroscopy.

Elucidating the mechanism behind the infrared spectral features and dynamics observed in the carbonyl stretch region of organic carbonates interacting with lithium ions

Ultrafast infrared spectroscopy has become a very important tool for studying the structure and ultrafast dynamics in solution. In particular, it has been recently applied to investigate the molecular interactions and motions of lithium salts in organic carbonates. However, there has been a discrepancy in the molecular interpretation of the spectral features and dynamics derived from these spectroscopies. Hence, the mechanism behind spectral features appearing in the carbonyl stretching region was further investigated using linear and nonlinear spectroscopic tools and the co-solvent dilution strategy. Lithium perchlorate in a binary mixture of dimethyl carbonate (DMC) and tetrahydrofuran was used as part of the dilution strategy to identify the changes of the spectral features with the number of carbonates in the first solvation shell since both solvents have similar interaction energetics with the lithium ion. Experiments showed that more than one carbonate is always participating in the lithium ion solvation structures, even at the low concentration of DMC. Moreover, temperature-dependent study revealed that the exchange of the solvent molecules coordinating the lithium ion is not thermally accessible at room temperature. Furthermore, time-resolved IR experiments confirmed the presence of vibrationally coupled carbonyl stretches among coordinated DMC molecules and demonstrated that this process is significantly altered by limiting the number of carbonate molecules in the lithium ion solvation shell. Overall, the presented experimental findings strongly support the vibrational energy transfer as the mechanism behind the off-diagonal features appearing on the 2DIR spectra of solutions of lithium salt in organic carbonates.

Solvation Structure of Li+ in Concentrated Acetonitrile and N,N-Dimethylformamide Solutions Studied by Neutron Diffraction with 6Li/7Li Isotopic Substitution Methods

Neutron diffraction measurements on ⁶Li/⁷Li isotopically substituted 10 and 33 mol % *LiTFSA (lithium bis(trifluoromethylsulfonyl)amide)-AN-d₃ (acetonitrile-d₃) and 10 and 33 mol % *LiTFSA-DMF-d₇(N,N-dimethylformamide-d₇) solutions have been carried out in order to obtain structural insights on the first solvation shell of Li⁺ in highly concentrated organic solutions. Structural parameters concerning the local structure around Li⁺ have been determined from the least squares fitting analysis of the first-order difference function derived from the difference between carefully normalized scattering cross sections observed for ⁶Li-enriched and natural abundance solutions. In 10 mol % LiTFSA-AN-d₃ solution, 3.25 ± 0.04 AN molecules are coordinated to Li⁺ with a intermolecular Li⁺···N(AN) distance of 2.051 ± 0.007 Å. It has been revealed that 1.67 ± 0.07 AN molecules and 2.00 ± 0.01 TFSA^- are involved in the first solvation shell of Li⁺ in the 33 mol % LiTFSA-AN solution. The nearest neighbor Li⁺···N_AN and Li⁺···O_TFSA^- distances are obtained to be r(Li⁺···N) = 2.09 ± 0.01 Å and r(Li⁺···O) = 1.88 ± 0.01 Å, respectively. The first solvation shell of Li⁺ in the 10 mol % LiTFSA-DMF-d₇ solutions contains 3.4 ± 0.1 DMF molecules with an intermolecular Li⁺···O_DMF distance of 1.95 ± 0.02 Å. In highly concentrated 33 mol % LiTFSA-DMF-d₇ solutions, there are 1.3 ± 0.2 DMF molecules and 3.2 ± 0.2 TFSA^- in the first solvation shell of Li⁺ with intermolecular distances of r(Li⁺···O_DMF) = 1.90 ± 0.02 Å and r(Li⁺···O_TFSA^-) = 2.01 ± 0.01 Å, respectively. The Li⁺···TFSA^- contact ion pair stably exists in highly concentrated 33 mol % LiTFSA-AN and -DMF solutions.

Solvation Structure of Li+ in Methanol and 2-Propanol Solutions Studied by ATR-IR and Neutron Diffraction with 6Li/7Li Isotopic Substitution Methods

Neutron diffraction measurements have been carried out on 10 mol % LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) solutions in methanol- d₄ and 2-propanol- d₈ to obtain information on the solvation structure of Li⁺. The detailed coordination structure of solvent molecules within the first solvation shell of Li⁺ was determined through the least-squares fitting analysis of the difference function between normalized scattering cross sections observed for ⁶Li/⁷Li isotopically substituted sample solutions. The nearest-neighbor Li⁺···O distance and coordination number determined for the 10 mol % LiTFSA-methanol- d₄ solution are r_LiO = 1.98 ± 0.02 Å and n_LiO = 3.8 ± 0.6, respectively. In the 2-propanol- d₈ solution, it has been revealed that 2-propanol- d₈ molecules within the first solvation shell of Li⁺ take at least two different coordination geometries with the intermolecular nearest-neighbor Li⁺···O distance of r_LiO = 1.93 ± 0.04 Å. The Li⁺···O coordination number, n_LiO = 3.3 ± 0.3, is determined. Ion-pair formation in the LiTFSA-methanol and LiTFSA-2-propanol solutions has been investigated by the attenuated total reflection infrared spectroscopic method. Mole fractions of free, Li⁺-bound, and aggregated TFSA^- are derived from the peak deconvolution analysis of vibrational bands observed for TFSA^-.