Session E47: Energy Storage: Electrolytes and Ionic Conductors

Nuclear Magnetic Resonance Investigation of Lithium Ion Transport in a Highly Conducting Solid Polymer Electrolyte: The Enabling Effect of High Salt Concentration

Solid polymer electrolytes based on poly(ethylene oxide) (PEO) for rechargeable lithium ion batteries significantly lack sufficiently high ionic conductivity for practical use at ambient temperature. Recent reports of high ionic conductivity and an expanded electrochemical window of aqueous electrolytes enabled by very high salt concentration (~ 20m) as well as earlier work on the “salt-in-polymer” concept have inspired this investigation of a solid PEO-salt-water system, where the dissolved salt (LiTFSI) concentration approaches its solubility limit and in combination with water association succeeds in inhibiting crystallization of the PEO matrix leading to higher conductivity. Room temperature ionic conductivity in these solid-like polymer electrolytes is about 2 mS-cm^-1, which is sufficiently high for battery application. Characterization of the ion transport process by NMR pulsed field gradient diffusion measurements (⁷Li and ¹⁹F for the cation and anion, respectively) and electrochemical impedance spectroscopy demonstrate an acceptable degree of salt ion dissociation and a lithium transport number, i.e. the Li ionic fraction of the current, exceeding 0.6, which is unusually high for polymer electrolytes. Additional electrochemical testing results will also be presented.   

Computational and experimental investigation of Na4P2S6 as a promising solid electrolyte material for sodium metal batteries

Recent experiments¹ have shown that Na₄P₂S₆, prepared from a hydrated precursor Na₄P₂S₆●6H₂O by heating under vacuum to 175 deg. C, has good ionic conductivity and stability with respect to metallic Na anodes, suggesting its promise as a solid state electrolyte for Na ion batteries. Examining the experimental results, we have extended previous computational work² on this material to further understand its structural, conductivity, and interface properties. Of particular interest is the observation^1,3 that Na₄P₂S₆ crystallizes to form monoclinic crystals having the space group C2/m which is distinct from the hexagonal or trigonal structures^4-6 analyzed for the related material Li₄P₂S₆. Computer modeling results suggest that the C2/m structure of Na₄P₂S₆ is stabilized by lattice vibrations.
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¹Hood et al. Manuscript in preparation. ²Rush et al. SSI 286, 45-50 (2016). ³Kuhn et al. Z. Anorg. Allg. Chem. 640, 689-692 (2014). ⁴Mercier et al. Solid State Chem. 82, 151-162 (1982). ⁵Hood et al. SSI 284, 61-70 (2015). ⁶Neuberger et al. Dalton Trans. 47, 11691-11695 (2018).   

First-principles prediction of potentials and space-charge layers in all-solid-state batteries

As all-solid-state batteries (SSBs) develop as an alternative to traditional cells, a thorough theoretical understanding of driving forces behind battery operation is needed. We present a fully first-principles-informed model of potential profiles in SSBs and apply the model to the Li/LiPON/Li_xCoO₂ system. These profiles, especially the interface dipoles, yield valuable information about lithium distribution and transport and the nature of interfacial electrical double layers. The results suggest design rules to minimize interfacial lithium transport resistance and optimize device performance.   

Structural Characterization of a Solid Electrolyte for 3D Microbatteries

Three-dimensional (3D) microbatteries have gained attention as viable compact power sources with applications in wearable and implantable microelectronics, and IoT functions. A key innovation from our lab is electrophoretic deposition (EPD) of all thin-film layers, enabling conformal deposition of electrodes and solid electrolytes on complex-shape, high-aspect-ratio perforated silicon and polymer substrates. This leads to a large capacity gain per footprint area of battery, compared with wafer surface deposition.

Using EPD, we prepared a novel composite polymer-in-ceramic electrolyte composed of 87% lithium aluminum germanium phosphate (Li_1.5Al_0.5Ge_1.5(PO₄)₃, LAGP), 13% polyethyleneimine (LAGP−PEI), saturated by LiI salt to produce high concentrations of mobile Li⁺ and I^- ions.

Solid state ^6,7Li, ²⁷Al, and ³¹P magic angle spinning nuclear magnetic resonance (MAS NMR) measurements showed structural changes the LAGP-PEI-LiI composite compared with base LAGP material, including the conversion of about 1/3 of the crystalline compound into an amorphous phase containing both two-fold and three-fold coordinated P, and a change in the ratio of five-fold to six-fold coordinated Al. ⁷Li NMR diffusometry revealed faster Li ion diffusion in the LAGP-PEI-LiI composite than in base LAGP.   

Sol-gel synthesis of a Fe-doped cubic phase Li7La3Zr2O12 Solid State Electrolyte

Cubic Li₇La₃Zr₂O₁₂ (LLZO) exhibits high ionic conductivity, chemical stability and thermal stability and a high transference number. These characteristics make it a promising material for integration in solid state batteries. Doping LLZO with certain elements like bismuth, gallium, aluminum and tantalum increases the ionic conductivity and reduces the stabilization temperature of the cubic phase. In this work a sol-gel method is used to synthesize Fe-doped LLZO (Li_7-3xFe_xLa₃Zr₂O₁₂), wherein a low calcination temperature of 700°C in air is used. XRD is used to characterize the cubic phase and presence of other impurity phases. SEM is used to determine the microstructure of the powder and the prepared pellets with different Fe stoichiometric content. Electrochemical impedance spectroscopy is used to determine the bulk conductivity of the Fe-doped LLZO pellets. The low synthesis and calcination temperature combined with the high ionic conductivity (and other favorable characteristics of the LLZO garnet) make this synthesis method and material highly attractive for lithium-ion solid state batteries. Future work will involve fabrication of polymer composite electrolytes with this material, looking for positive synergy of polymer and garnet characteristics.   

Theoretical modelling of lithium environment in composite solid electrolyte batteries investigated through x-ray absorption near edge spectroscopy

Liquid electrolytes in conventional batteries are volatile, flammable, and cause many of the fires and explosions of lithium ion batteries. Moving to solid state electrolytes not only removes these instabilities but also suppress dendritic growth of lithium that can short a battery. Ion transport is a main limitation in these solid-electrolytes. Composite electrolytes made of nanostructured combinations of ceramics and polymers show promise to increase ion transport at the ceramic-polymer interface^[1]. To investigate the mechanisms that preferentially transport lithium ions at this interface, x-ray absoprtion near edge spectroscopy (XANES) of lithium K-edge was performed experimentally and elucidated through theoretical simulation. The solid-electrolyte system investigated is lithium perchlorate (ClO₄^-) dissolved in poly(ethylene oxide) (PEO) in nanopores of anodized aluminum oxide. Simulation results explicate a complex relationship between the XANES edge shift and lithium environment, going beyond the preliminary expectation that coordination number of lithium with oxygen causes the XANES shift.
[1] X. Zhang et al., Nano Lett., 18   

Effects of solvent-salt charge-transfer complexes on oxidative stability of Li-ion battery electrolytes

Electrochemical stability windows of multi-component electrolytes, both solid polymer and organic liquid, largely determine the operating regime limits of Li-ion batteries. In order to increase energy densities and lifetimes of batteries, new electrolyte materials need to be discovered and optimized which requires better understanding of their degradation and oxidation.
Using computational tools, we present new insights into the oxidation mechanism that governs stability of multi-component polymer and liquid electrolytes. We find that explicitly including solvent molecules in the computation of the anion stability has a strong impact, and we show that this effect stems from electrostatic interactions between the molecules. Particularly, we find that across all chemistries studied, only one molecule in the system is oxidized. Building on this, we construct a model where two oxidation scenarios lead to different stability behaviors for the anion-solvent pairs, depending on their relative strength and geometry. This understanding of the microscopic details of oxidation allows one to formulate design rules and provides a good framework for screening electrolyte materials.   

The Role of Ion Correlation on Transport Properties of Concentrated Ionic Liquids for Battery Electrolytes

Recent pioneering work on Na batteries has drawn attention on ionic liquid electrolytes, yet most published works ignore the effects of ion-ion correlation. Using classical all-atom molecular dynamics and fully-correlated theory we highlight the drastic differences in fundamental understanding when using the systematically overlooked correlated approach. As a case study, we look at the promising Na/bis(fluorosulfonyl)imide (NaFSI) in room temperature ionic liquid N-propyl-N-methylpyrrolidinium/FSI. A previously undetected negative transference number emerges for sodium molar fractions lower than 0.2, effectively hinting to high degrees of ion-ion correlation that should not be dismissed. The spatial correlation is explored further by employing a variant of the single-linkage clustering algorithm. The approach we use enables us to explain the concentration-driven trends in ionic conductivity and transference numbers, and is generally able to correctly capture the transport physics of highly-correlated systems such as concentrated electrolytes.   

First-Principles Replica Monte Carlo Sampling of Dopant Disorder in Solid Electrolyte

Oxides such as ZrO₂ and BaZrO₃ are promising candidates for the electrolyte material in medium to high-temperature solid oxide fuel cells. In these materials, aliovalent cation doping is used to introduce ionic defects which act as ionic charge carriers. Optimization of doping conditions is imperative for obtaining sufficient and stable ionic conductivity, but it is yet unclear how dopants are distributed in the material under various processing conditions and how the details of the distribution affect the total conductivity. Since the experimentally found optimal doping concentration can run up to 20%, the degrees of freedom in substitutional dopant placement is enormous (for example, choosing 10 substitutional sites out of 50 corresponds to ₅₀C₁₀~10 billion degrees of freedom). In this work, we harness the power of modern-day supercomputers and perform thermodynamic sampling of dopant placement in BaZrO₃ by directly combining first-principles calculations and the replica exchange Monte Carlo method. We show that dopant-carrier association decreases with increasing dopant concentration, in contrast to popular belief that association effects are responsible for conductivity decrease at high dopant concentrations.   

Polyethylene oxide-Li7La3Zr2O12 composite solid-state electrolyte for advanced Li-ion batteries (LIBs)

Recently, conformal solid-state polymer-based electrolytes have been considered as promising alternatives for liquid electrolytes in LIBs because of their fire safety and ease of fabrication in thin film form with desirable mechanical, thermal and high electrochemical stability. Several researchers have investigated polyethylene oxide (PEO) based films complexed with Li-salts, such as LiClO₄, as polymer electrolytes. However, these electrolytes exhibit low ionic conductivity (~10^-6 - 10^-7 S cm^-1) at room temperature due to high degree of crystallinity. This issue has been addressed by adding plasticizers such as ethylene carbonate (EC) or using inorganic fillers. We have studied PEO based polymer electrolyte with Li₇La₃Zr₂O₁₂ (LLZO) as an inorganic filler, which is also a Li-ion conductor. We show that the addition of LLZO (20 wt%) and EC (20 wt%) increases the ionic conductivity of PEO-LLZO composite electrolyte by an order of magnitude, lowers the activation energy from 0.6 eV to 0.5 eV, and maintains a high voltage stability (5 V). The results of electrochemical impedance spectroscopy, linear sweep voltammetry, and chronoamperometry measurements will be presented.   

Investigate the mobility of Lithium-sulfur batteries via ab initio molecular dynamic with external force

Even though tremendous achievement has been made experimentally in the performance of Li-S battery, theoretical studies in this area are lagging behind due to the complexity of the Li-S systems. For this purpose, we have developed a new molecular dynamic method with external force applied on the Li atom to investigate the mobility. Our proposed computational framework not only opens a new avenue for understanding the key role played by solution and liquid electrolytes in Li-S battery, but also can be generally applied to other processes with liquids involved.
  

Design principles for multicomponent solid electrolytes for lithium metal anodes

The development of safe high energy density rechargeable lithium (Li) ion batteries is crucial for meeting the goal of clean sustainable energy. A major limiting factor in these batteries is the electrolyte – organic liquids are typically flammable and have limited electrochemical stability while most solid electrolytes have lower ionic conductivity and limited stability. Here, we focus on design principles for ionic transport in multicomponent solid electrolytes, either artificially created or naturally generated at the electrode-electrolyte interface. General tradeoffs prevent single component solid electrolytes from achieving the desired properties simultaneously [1] but may be broken through a multicomponent system by a careful use of the phase boundaries through the generation of space charge regions [2], confinement, ion adsorption etc. Together with the descriptors for ionic conduction, our design principles will be supplemented by theoretical spectroscopy studies based on x-ray absorption and emission to assist in experimental validation of samples with the required composition.

[1] Z. Ahmad, T. Xie, C. Maheshwari, J. C. Grossman, and V. Viswanathan, ACS Cent. Sci. 4, 996 (2018).
[2] J. Maier, Prog. Solid State Chem. 23, 171 (1995).   

Effect of lattice flexibility and ion-ion interactions on transport in superionic conductors

Superionic conductors (SICs) play an important role in the development of solid-state batteries and thermoelectrics. The atomic mechanisms for rapid ion transport in such materials, however, remain poorly understood. Here, we use ab-initio and classical molecular dynamics (MD) simulations to study the diffusion of Ag⁺ ions in AgCrSe₂, a common SIC. Our ab-initio simulations reveal the dominant diffusion pathway taken by ions as they hop across the a and b lattice sites. To study macroscopic diffusion across length and time scales inaccessible by the ab-initio simulations, we develop a molecular-mechanics force field to describe the interactions of the mobile ions and the lattice framework. Classical MD simulations based on this force field demonstrate the role of lattice flexibility in modulating the energy barrier and vibrational frequency of Ag⁺ ions, and the role of ion-ion interactions in facilitating the concerted hopping of neighboring mobile ions on the lattice. Such physical insights into the transport mechanisms of ion transport in SICs could help optimize their performance.