Session B20: Energy Storage: Electrolytes and Interfaces

Interfaces in Electrochemical Energy Storage

Electrochemical energy storage systems are inherently complex making detailed characterization of their operation challenging. We have taken a reductionist approach by using simple, but relevant, model systems than are amenable to detailed investigations. This talk will describe in situ X-ray scattering and spectroscopy experiments taking this approach and aimed at understanding the lithiation of Si single crystal anodes [1], the molecular ordering of organic solvent molecules at solid-electrolyte interfaces [2] and the plating and stripping of Li metal anodes. We used X-ray reflectivity to investigate, in real time, the electrochemical lithiation of single crystalline Si(100) electrodes, which allows us to gain nanoscale, mechanistic insight into the lithiation of Si and the formation of the solid electrolyte interphase (SEI) surface film on the Si anode. This approach was also used to determine the structure non-aqueous electrolytes at the solid-liquid sapphire (001) interface. We fianlly describe In situ X-ray diffraction and ex situ X-ray spectroscopy used to understand the nucleation and growth of Li on Cu current collectors.

[1] Cao et al., Nanolett , NanoLetts 16,7394 (2016).; Adv Mater Inter 4, 1700771 (2017).
[2] Steinruck et al., Ener Envron Sci., submitted.   

Computational Study of the Solid Electrolyte Material Li4PS4I 1

Several recent experimental studies have shown^2,3,4 that the incorporation of LiI into lithium thiophosphate electrolytes can improve their stability and ionic conductivity with possible positive implications for battery technology. In this presentation, we report the results of our first principles simulations of the newly sythesized and analyzed³ electrolyte Li₄PS₄I. The simulations help us understand the structural and ion mobility properties of this material and to study models of interfaces of the material with Li metal. Comparison with previous simulations studies of Li₃PS₄ electrolytes help identify the effects of LiI.
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²Rangasamy et al. JACS 137, 1384-1387 (2015).
³Sedlmaier et al. Chem. Mater. 29, 1830-1835 (2017).
⁴Phuc et al. J. Power Sources 365, 7-11 (2017).
  

Computational Understanding of Solid-Electrolyte Interphase Formation in Ca-ion Batteries

Multivalent ion batteries (MVIB) have garnered attention as alternatives to Li-ion batteries in applications where portability is not an issue, as they are energy dense, cost efficient, and utilize Earth-abundant elements. However, the development of MVIBs, especially Ca-ion, has been limited by the lack of suitable electrolytes that can reversibly plate metallic anodes. This is due to the fact that the passivating layer which forms between the electrolyte and anode (the solid-electrolyte interphase, SEI) does not allow for the migration of Ca²⁺ ions. In this work, we develop an understanding of the SEI in Ca-ion systems using a computational approach combining density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. We first identify the principle components of the SEI by studying the decomposition of the solvents and salts comprising various electrolytes on a Ca surface using AIMD. Following this, we identify electrolytes which can be used with a Ca metal anode by investigating the diffusion of Ca ions through the likely inorganic compounds produced using DFT. We anticipate the promising new electrolytes proposed in this work will help guide experimentalists in the development of rechargeable Ca cells.   

Solid Electrolytes for Stable Electrodeposition in Li Metal Anode-based Batteries

High energy density batteries require a functioning Li metal anode to realize their full capacity. However, Li deposition on Li metal anode leads to the growth of dendrites which are a grave safety concern. Solid electrolytes present a new opportunity for attacking this problem of unstable electrodeposition. It is known that Li/solid polymer electrolyte system can be stabilized if the polymer electrolyte has a sufficiently high shear modulus [1]. In this work, we show that the condition on shear modulus for stable electrodeposition depends on the relative density of the metal ion in the electrolyte and electrode, for both, isotropic and anisotropic mechanical properties at the interface. An anisotropic linear stability analysis is useful since Li metal has a high elastic anisotropy. We illustrate two different regimes of stability, the pressure-driven stability and density-driven stability through a two-parameter stability diagram [2]. Our results show that a soft solid electrolyte can suppress the growth of dendrites when the metal ion is more densely packed in the electrolyte than in the metallic form.

References
[1] C. Monroe and J. Newman, J. Electrochem. Soc. 152, A396 (2005).
[2] Z. Ahmad and V. Viswanathan, Phys. Rev. Lett. 119, 056003 (2017).   

Computational Study of the High Ion Conductivity Cubic Phases of Li2OHX Electrolytes1 for X=Cl and Br

Li₂OHCl, Li₂OHBr, and Li₂(OH)_0.9F_0.1Cl have recently been experimentally studied.^2,3,4 All three of these materials have a disordered cubic phase which has high ionic conductivity of possible interest for battery technology. First principles simulations of these materials also identify low temperature ordered phases⁵ which are seen experimentally in only some of the materials. In this presentation, we report the results of our analysis of the material specific contributions of the internal energies as well as the configurational and phonon entropies that effect order-disorder phase transitions in these materials.

² Hood et al. JACS 138, 1768-1771 (2016).
³ Schwering et al. CHEMPHYCHEM 4, 343-348 (2003).
⁴ Li et al. Angew. Chem. Int. Ed. 55 9965-9968 (2016).
⁵ Howard et al. Manuscript submitted to PR Materials (2017).   

Simulations by density functional + implicit classical solvation theories for electrode/solution interfaces under an applied voltage

In order to treat electrochemical reactions at electrode/solution interfaces, the influences of the applied bias voltage should be included in an appropriate computational model. We adopted density functional theory (DFT) with effective screening medium (ESM) method [1] and Laue-represented reference interaction site method (RISM); ESM-RISM method. The former is for applying a voltage and the latter is to represent the electrolyte solution. The ESM-RISM combined with DFT treats the electrode surface by quantum mechanical theory and implicit classical thermodynamics. This formulation automatically contains formation of electronic double layer (EDL) under the applied bias voltage. In the presentation, we will describe the detailed analysis of Pt/water interfaces. Water distribution compared with an experiment, the thickness of the EDL, the potential of zero charge (PZC) vs. standard hydrogen electrode (SHE) potential at Pt(111) surface, and so on, will be discussed. [1] M. Otani and O. Sugino, PRB 73, 115407 (2006). [2] S. Nishihara and M. Otani, PRB 96, 115429 (2017).   

High Ionic Conductiviy Composite Ceramic-Polymer Electrolyte

Solid-state electrolytes (SSE) such as the garnet LiLaZrO are of utmost interest. They offer high electrochemical stability and non-flammability. However, compared to liquid electrolytes, their ionic conductivity is low and their incorporation into battery fabrication is challenging. Integrating these ceramic materials into polymer matrixes to form composite electrolytes offers potential solutions to these problems. We have fabricated a cubic phase bismuth-doped LLZO ceramic (LLZBO) particles, and integrated them into poly (ethylene oxide) matrixes. The LLZBO average particle size was decreased by attrition milling to a range of 500 to 600 nm range. Several weight contents in the polymer were used to optimize the ionic conductivity. Size and the particle dispersion was characterized with SEM. An optimum weight content was found at 10%wt yielding an unprecedented high value of the ionic conductivity of 5.98x10^-3 S/ at 55C. Work by others in similar systems employing tetragonal phase LLZO in a PEO matrix yielded ionic conductivities of 4.42x10^-4 S/cm at the same temperature, but a weight content load in the polymer of at least 52.5% was needed. Experimental details of our composites will be presented and mechanisms will be described for the large ionic conductivities attained.   

Combining Experimental and Computational Techniques to Understand the Role of Native Oxide on the Solid Electrode Interphase Formation on Si Electrode in Li-Ion Batteries

With the current surge in energy demand for electrification of transportation, developing high energy storage devices is essential. Si anode based Li-ion batteries offer huge promise in this regard by providing high theoretical capacity of 3579 mAh/g, 10 times higher than graphite anode based Li-ion batteries (LIBs). One of the major contributor to capacity fade in Li-ion batteries is the uncontrolled growth of the solid-electrolyte interphase (SEI) layer. SEI is formed due to the decomposition of electrolytes and consumes Li ions. Despite a significant amount of work on SEI, our understanding of both its formation and growth is still limited. In this work, we studied the lithiation mechanism of the inevitable native oxide on the Si surface, identified the stable phases formed due to the lithiation and determined how these stable phases become part of the SEI by using Ab-initio molecular dynamics, X-Ray photoelectron spectroscopy (XPS) and X-Ray reflectivity (XRR). This fundamental understanding of SEI formation would enable to find better strategies for designing high-performance LIBs.
  

Discovering Physical Limits of Battery Materials with Physics-based Machine Learning

We compile data and physics-based machine learned models for solid Li-ion electrolyte performance to assess the state of materials discovery efforts in solid-state batteries. Candidate electrolyte materials must satisfy several requirements, chief among them fast ionic conductivity and robust electrochemical stability. In order to probe the interplay of these properties, we first build and validate a machine learning-based model for predicting ionic conductivity. We find this model offers a 3x improvement over trial-and-error searches, and successfully identifies several new materials that demonstrate exceptional ionic conductivity. Then, drawing on DFT-based electrochemical stability models, we examine the predicted performance of thousands of candidate materials and quantify the likelihood of breakthrough solid electrolyte discoveries. Among other insights, this analysis suggests that two electrolytes are likely to be necessary in solid-state Li-ion batteries with Li metal anodes. This work is an effort to extract as much information as possible from today’s limited existing data in order to provide a clear path forward for accelerating tomorrow’s efforts.   

Neutron scattering studies of ionic conducting amorphous xLi2SO4(x-1)LiPO3

We have probed the structure and dynamics of a series of lithium sulfate substituted phosphate glass materials (xLi₂SO₄(x-1)LiPO₃) using elastic, quasielastic, and inelastic neutron scattering. Glass-based Li ion conductors are attractive as electrolyte materials for all-solid state batteries because they are highly isotropic and chemically stable. Local disorder is expected to reduce barriers to ion migration—leading to improved ionic conductivity relative to their crystalline counterpart—while minimizing unwanted electron transport. We have conducted quasielastic neutron scattering measurements in order to determine the self-diffusion in these materials on a lengthscale comparable to the interatomic spacings, obtaining values ~5x10^-7 cm²/s at 450K. The addition of Li₂SO₄ is known to improve the conduction properties in the LiPO₃ glasses. We will discuss the structural and dynamic changes that occur within the Li₂SO₄ solubility range x≤0.6.   

X-ray Absorption Spectroscopy Study of the Lattice Site Occupancy of Bi Dopants in LiLaZrO Garnets

The garnet LiLaZrO is of great interest for solid-state electrolyte applications in energy storage devices. Dopants have been employed to stabilize the desired cubic phase with concomitant increments in ionic conductivity. We have obtained significant improvements the LiLaZrO ionic conductivity by Bi-doping. Taking advantage of the element specific nature of X-ray Absorption Spectroscopy (XAS), we have explored the site occupancy of bismuth additions to LLZO. XAS studies were carried out at beamline 20-BM (Advance Photon Source, Argonne National Laboratory). Analysis of the XAS data show that to within the accuracy of the measurement, bismuth additions occupy zirconium-type sites in the LLZO lattice. Additionally, bismuth perturbs the local environment of zirconium and lanthanum, and induces static disorder in the samples. Further details of the XAS experiment, analysis, and findings of the study will be discussed in the talk.   

Li-ion Transport in Amorphous Solid Electrolytes

With recent industry-wide investments into electric vehicles, much research & development has focused on designing safer, lighter, fast-charging batteries. In particular, researchers search for an electrolyte that is manufacturable, mechanically robust, dendrite-resistant, and ionically conductive. The success of LiPON thin films indicates promise in glassy and amorphous materials, which are known to have different transport properties than their crystalline counterparts. With computational study of transport mechanisms, we can develop design rules for new glassy electrolytes. Using molecular-dynamics simulations, we model Li-ion conduction in various families of crystalline and amorphous materials. We study the results and mechanisms of amorphous structures on ion diffusivity, conductivity, and correlation.