Generation of Novel Solid-State Electrolyte: Co-Crystals of Organic Solvent and Lithium (or Sodium) Salts

Abstract

The nature of the electrolyte plays an essential role in ionic transport of any lithium or sodium-ion battery. However, there are safety issues associated with these batteries, limiting their use in high power applications because of flammable liquid electrolytes, which can lead to fire and explosion. Solid-state electrolytes are inherently safer than liquid electrolytes, which are composed of ions dissolved in high-dielectric liquid organic solvents such as propylene carbonate or ethylene carbonate, and which are used in current lithium-ion batteries (LIBs). The safety and performance issues of lithium-ion batteries are respectively related to the flammability of the liquid organic solvent electrolytes that have a low flash point, and to the continuous growth of lithium metal dendrites on the anode, at which the lithium cations are reduced and deposited on the anode of LIBs during charging. The dendrite formation and growth on the anode of Lithium metal batteries (LMBs) is also observed on the anode of sodium metal batteries (NMBs). These issues, which impair the overall quality of both battery systems, can be addressed by developing novel electrode and electrolyte materials. This thesis has focused on the design, synthesis, characterization, and investigation of new lithium (or sodium) solid state electrolytes, co-crystalline electrolytes, as well as their application in lithium or sodium metal batteries. The first such material we have prepared, DMF: LiCl, has high conductivity for a pure soft solid lithium electrolyte. Additionally, in chapter 4, new sodium co-crystalline electrolytes of dimethylformamide (DMF) organic solvent and NaClO4 salt with different solvent salt ratios 2:1 ((DMF)2NaClO4) and 3:1 (DMF)3NaClO4 have been prepared. The synthesis, and physical and electrochemical properties of (DMF)3NaClO4 and (DMF)2NaClO4 are presented. The 3:1 co-crystal can be transformed into 2:1. The crystal structures of the materials reveal parallel channels of Na+ and ClO4- ions. The pressed pellet of (DMF)3NaClO4 has better ionic conductivity (10-4 S cm-1) than the pressed pellet of (DMF)2NaClO4 at RT, and over a broad temperature range (-77 ⁰C and 50 ⁰C), with a low activation barrier of 25 kJ mol-1. The SEM of (DMF)3NaClO4 reveals thin liquid interfacial contacts between crystalline grains, which promote ion mobility. The two materials have different melts that do not decompose, and upon cooling, they re-solidify as their solid structures, permitting melt casting of the electrolytes into thin films and the fabrication of cells in the liquid state, ensuring penetration of the electrolyte between the active electrode particles. Besides the high ionic conduction of the co-crystal of DMF and NaClO4, in chapter 6, we investigated the synthesis, and the physical and electrochemical properties of the co-crystal of adiponitrile (ADN) and NaClO4 salt. The crystal structure calculation of the material reveals 3:1 solvent salt ratio as (ADN)3NaClO4, a high ionic conductivity sodium solid electrolyte. The material possesses high thermal stability (up to 150 °C) and the ability to be melt-cast (Tm = 81 °C). The pressed pellet of (ADN)3NaClO4 has a high ionic conductivity of 2.2 × 10-4 S cm−1 at RT with a low activation barrier for ion conduction of 22 kJ mol-1. The high conductivity is the result of low-affinity ion-conduction channels in bulk, based on the X-ray crystal structure, and thanks to low grain boundary resistance, as well as possibly a grain-boundary percolating network due to a fluid-like nano liquid layer between the grains, observable by scanning electron microscopy and differential scanning calorimetry. When the liquid nanolayer is rinsed away or removed by excessive drying, the bulk room temperature ionic conductivity is 4 × 10-5 S cm-1 with an activation energy of 37 kJ mol-1, and the sodium ion transference number is 0.71. Scanning electron microscopy and classical molecular dynamics simulations suggest that these cocrystals form a fluid layer of ADN at the surface, which facilitates the Na+ ion migration between the grains. Density functional theory calculations are consistent with the possibility of ion conduction via a solvent−anion coordinated transition state through vacancy defects in the three symmetry equivalent ion channels along separate directions, suggesting the possibility of ionic conductivity in three dimensions. The last sodium electrolyte in this thesis (ADN)3NaPF6), in chapter 9, is investigated by co-crystallizing the organic solvent ADN with NaPF6 salt. The calculated crystal structure shows a ratio of 3:1 AND/NaPF6 salt. The stability of the material to sodium metal and its thermal and electrochemical properties show its application to sodium metal battery. The sodium ion is solvated by six -C≡N of the ADN. (ADN)3NaPF6 presents 3D linear parallel ionic channels of Na+ and PF6- ions, with distances between two successive Na+ (and PF6-) of 8.393, 8.393, and 11.527 Å, where the shortest distance between two successive Na+ in the b-crystallographic direction is 8.39 Å. The presence of 3D channels and the large gap between two Na+ ions in the complex may facilitate the migration of Na+ in the matrix. The (ADN)3NaPF6 cocrystal melts around 98 °C. The conductivity of the pressed pellets is 2 x 10-4 S/cm at RT, with an activation energy of 38.2 kJ mol-1. The CV scans and plating tests indicate that (ADN)3NaPF6 is electrochemically stable to sodium metal up to ~ 4V, meaning the crystal can be used in a sodium metal battery. This thesis also focuses on lithium co-crystalline electrolytes by co-crystallizing Lewis base organic solvents with common lithium salts used in electrolyte materials. In chapter 5, a soft solid crystal composed of isoquinoline (IQ) and LiCl was prepared based on the concept of Pearson’s hard-soft acid-base (HSAB) theory, and in addition to the crystal structure, the thermal and ionic conduction are investigated. Single-crystal X-ray diffraction best described the (IQ)3 •(LiCl)2 as consisting of molecular Li4Cl4(IQ)6 units, where the LiCl cluster is an array of edge-fused Li2Cl2 rhombs. The pressed pellet conductivity showed that ionic mobility occurs mainly through the bulk via a hopping mechanism, with a calculated activation energy of Ea = 67 kJ mol-1. The high value of the activation energy was due to Li4Cl4 clusters that were well separated by intervening IQ ligands in the crystal structure, requiring long hops for ions to migrate through the lattice. Another lithium co-crystalline material is investigated, in chapter 7, by co-crystallizing ADN, a highly thermally and electrochemically stable organic solvent with LiPF6, a thermally unstable salt. The physical and electrochemical properties of the material are experimentally and theoretically investigated. The calculated crystal structure shows a 2:1 solvent salt ratio as co-crystalline (ADN)2LiPF6. The complex forms linear parallel lithium channels, through which the Li+ ions can migrate. The Li+ ion is solvated by four -C≡N of the ADN, with no contact ion pairs with PF6-. High conductivity (σ ~ 10-4) results from weaker interactions between “hard” Li+ ions with “soft” -C≡N. Plane-wave DFT calculations show that the mechanism of ion migration is through the formation of an intermediate between two adjacent Li+ sites in the lattice and not through a hopping mechanism as is observed in inorganic ceramic electrolytes. A liquid-like layer is found at the grain boundaries that merge the grains, so that pellets are easily formed and do not require high pressure/temperature treatments to achieve high conductivities, as in the case of ceramics. The solid (ADN)2LiPF6 has a wide electrochemical stability window of 0 to 5 V. Li0/(ADN)2LiPF6/LiFePO4 half cells exhibit cycling for > 50 cycles at C/20, C/10, C/5 rates with capacities of 140 mAh/g to 100 mAh/g and efficiencies > 95%. In line with co-crystalline (ADN)2LiPF6, in chapter 8, we co-crystallized ADN organic solvent with two LiPF6 homologous salts, LiAsF6 and LiSbF6, and explore the physical and electrochemical properties of the three material. The new co-crystalline structures have the same solvent salt ratio 2:1 as (ADN)2LiPF6. The three materials have linear parallel lithium channels in their b crystallographic direction. In each complex, the Li+ ion is solvated by four -C≡N of the ADN, with no contact ion pairs to anions. Temperature-dependent ionic conductivities (σ) and lithium-ion transference numbers (tLi+) reveal that increasing the molar mass of the salt promotes lithium mobility in the matrix, meaning that there is a more significant contribution from the Li+ ion to the conductivity as the anion becomes larger (greater mass). TGA data indicates that co-crystalline (ADN)2LiPF6 thermally stabilizes the LiPF6 salt, which decomposes at low temperatures. In the case of (ADN)2LiAsF6 and (ADN)2LiSbF6, the thermal stability of the complexes is similar to that of ADN. The three compounds melt approximately at the same temperature, around 180 ºC. The electrochemical stability window of the complexes decreases as the electronegativity of the center atom (P, As, and Sb) decreases.