Forum Article www.acsami.org
Ammonium Additives to Dissolve Lithium Sulfide through Hydrogen Binding for High-Energy Lithium−Sulfur Batteries Huilin Pan, Kee Sung Han, Vijayakumar Murugesan, Jie Xiao, Ruiguo Cao, Junzheng Chen, Jiguang Zhang, Karl T. Mueller, Yuyan Shao,* and Jun Liu* Joint Center for Energy Storage Research, Pacific Northwest National Laboratory, Richland, Washington 99354, United States S Supporting Information *
ABSTRACT: In rechargeable Li−S batteries, the uncontrollable passivation of electrodes by highly insulating Li2S limits sulfur utilization, increases polarization, and decreases cycling stability. Dissolving Li2S in organic electrolyte is a facile solution to maintain the active reaction interface between electrolyte and sulfur cathode, and thus address the above issues. Herein, ammonium salts are demonstrated as effective additives to promote the dissolution of Li2S to 1.25 M in DMSO solvent at room temperature. NMR measurements show that the strong hydrogen binding effect of N−H groups plays a critical role in dissolving Li2S by forming complex ligands with S2− anions coupled with the solvent’s solvating surrounding. Ammonium additives in electrolyte can also significantly improve the oxidation kinetics of Li2S, and therefore enable the direct use of Li2S as cathode material in Li−S battery system in the future. This provides a new approach to manage the solubility of lithium sulfides through cation coordination with sulfide anion. KEYWORDS: lithium sulfur batteries, lithium sulfide, solubility, ammonium additive, NMR
■
utilization and polarization in Li−S batteries,25 but rarely investigated in literature. The passivation of electrode (carbon host) surface by repeated and uncontrolled deposition of insulating Li2S during cycling will continuously reduce the active interface and cause cell failure.26 This is quite similar to the electrode passivation process by insulating Li2O2/Li2O in lithium−air batteries.7,27,28 Besides, the large initial activation potential (up to 4 V) for Li2S is also the main obstacle to directly use Li2S as cathode for future Li metal free sulfur battery.29 Enhancing the solubility of Li2S in organic electrolyte is a direct and feasible approach to sustain the active reaction interface between electrolyte and sulfur cathode and increase sulfur utilization.30 However, crystalline Li2S (Scheme 1) is merely soluble in commonly used organic electrolytes.30−32 In the past, the dissolution chemistry of polysulfides has been tuned through manipulating the coordination between Li+ cation and solvent molecular and/or special anion groups.33 For example, triflate anion (CF3SO3−) ,which has strong ionic association strength, has been reported by a few groups
INTRODUCTION High-energy battery technologies are highly desired for modern society.1 Li−S battery is one of the promising candidates that has attracted intensive interest in recent years. The conversion reaction chemistry between Li and S is fundamentally different from the intercalation reaction in Li-ion batteries.2−5 This difference leads to superior energy storage capability of Li−S batteries.6,7 Meanwhile, it also makes the Li−S system more complicated because of the involvement of intermediate different chain length of polysulfide species.3,8 On one hand, highly soluble long-chain polysulfide species have been viewed as the main obstacle in Li−S batteries, leading to gradual capacity loss and low efficiency.9,10 A number of concepts have been proposed to address this issue.6,11−16 Nevertheless, the real application of Li−S batteries is still largely unreachable currently.17−19 Meanwhile, long-chain polysulfides have also been proposed as additives or catholyte to improve the kinetics and stability of sulfur cathode and lithium anode.20−22 Therefore, the soluble polysulfides seem a double-edged sword for Li−S batteries; understanding and controllable management of polysulfide solubility are needed to achieve high performance of Li−S batteries. On the other hand, shortchain polysulfides, e.g., Li2S2/Li2S, are insoluble and highly insulating, which accounts for the poor kinetics of the secondplateau reaction (corresponds to 3/4 of the theoretical capacity).23,24 Especially, the chemistry of end discharge product of insoluble Li2S is closely related to the sulfur © XXXX American Chemical Society
Special Issue: New Materials and Approaches for Beyond Li-ion Batteries Received: April 22, 2016 Accepted: June 20, 2016
A
DOI: 10.1021/acsami.6b04158 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Forum Article
ACS Applied Materials & Interfaces
and 7Li nuclear magnetic resonance (NMR) spectra were obtained using single pulse excitation on a 600 MHz NMR spectrometer (Agilent, USA) equipped with 5 mm liquid NMR probe at 25 °C for the solutions of Li2S dissolved in (DMSO-d6)n(NH4NO3). The observed 1H, 17O and 7Li NMR shifts were calibrated using tetramethylsilane (TMS), H2O and 1 M LiCl in H2O as an external standard, respectively. Li/dissolved Li2S cells were assembled using 0.2 M Li2S and 1.5 M LiTFSI in (DMSO)50(NH4NO3) as catholyte, carbon paper (FuelCellStore) as current collector, celgard 3501 as separator, Li metal as anode. Cyclic volytammogram (CV) was performed on CHI600D (CH Instrument) within a voltage range of 1.7−3 V at a scanning rate of 0.1 mV s−1. Charge/discharge was performed on LANHE tester during a voltage range of 1.7−2.9 V. Quantum chemistry calculations were carried out using the Amsterdam Density Functional (ADF-2014) program. All DFT calculations were performed under hybrid-Generalized Gradient Approximation (GGA) based B3LYP function with recent Grimme dispersion correction (DFT-D3). The TZP (triple Z, polarization functions, all electrons) basis set with Slater type functional implemented in the ADF program is used for both geometry optimization and binding energy calculations. The DFT calculations were carried out using both explicit (8 DMSO molecules) and implicit conductor-like screening model-based model with DMSO as solvent system.
Scheme 1. Interaction between Crystal Structure Ionic Li2S and Ammonium Ion with N−H Groups in Organic Solvent
including ourselves to efficiently modify the dissolution of polysulfide species, but failed to dissolve Li2S.34−37 Herein, we propose a completely different and new approach to tune Li2S solution chemistry through the coordination between sulfide anion S2− and ammonium additives and solvent molecules. This finding opens a new pathway to the efficient application of Li2S for high energy Li−S batteries and gives insight into modulate the polysulfide chemistry in whole Li−S system. Scheme 1 shows the molecular structure of NH4+ ion, which is composed of four ammonium groups (N−H) with nitrogen and the smallest atom hydrogen (37 pm). In N−H groups, highly electronegative nitrogen atom bonded with small hydrogen atom pulls electrons away from the hydrogen atom, resulting in tightly concentrated positive charge on hydrogen atom and an unusual dipole−dipole interaction. This is the reason for the extreme polar strength of N−H bonds in ammonium-based additives, which facilitates the dissolution of ionic Li2S crystal through the strong interaction between NH 4+ and S 2− (N―H····S2−) via hydrogen binding (Scheme 1).
■
■
RESULTS AND DISCUSSION Figure 1a shows the solubility of Li2S in DMSO with different contents of NH4NO3 additive. Despite the high polar nature of DMSO solvent, pure DMSO barely dissolves ionic compound Li2S (
