Solid-state Lithium-ion Conductors Research Articles

Enhanced ionic conductivity of Li1.3Al0.3Ti1.7(PO4)3 under the influence of graphene oxide

As a new type of clean and efficient energy storage device, lithium-ion batteries have been widely used in electronic products, electric vehicles, and other fields, and the accompanying safety issues are of increasing concern. Therefore, solid-state lithium-ion conductors have become a current research hotspot. This paper used graphene oxide material to design and prepare Li1.3Al0.3Ti1.7(PO4)3 solid electrolytes by the co-precipitation method and performed some relevant characterizations. The results showed that the addition of graphene oxide optimized the electrolyte structure improved the density of the powder, promoted grain growth, improved the transport path of lithium ions, and thus increased the total ionic conductivity. The optimal doping ratio was 0.5 wt%, and the total ionic conductivity reached the maximum of 2.42 × 10−4 S/cm. Compared to pure Li1.3Al0.3Ti1.7(PO4)3, the ionic conductivity increased by 154% after using the graphene oxide. It is believed that larger grains, fewer grain boundaries, and denser microstructure resulted in a less distorted migration pathway for lithium ions. Under the influence of graphene oxide, Li1.3Al0.3Ti1.7(PO4)3 exhibited better air stability than pure Li1.3Al0.3Ti1.7(PO4)3 electrolyte. The electrolyte prepared in this study showed great application potential in all-solid-state batteries.

Space Charges in Solid State Batteries: Friend, Foe or Fantasy?

Thermodynamics tells us that all materials have an internal energy that determines how they act and react with all matter in the universe. Furthermore, it forces all matter to strive towards equilibrium with its surroundings. For solid-state lithium-ion conductors, the chemical potential of lithium is an important parameter that determines stability and influences conductivity. However, it also implies something deeper. The chemical potential of an element in a solid is determined by the bonds that surround it as well as its concentration. Whereas the former is an intrinsic property of the material, the latter can be altered. When two solids with different chemical potentials are placed in contact, a driving force is created for atoms to migrate across the surface and establish thermodynamic equilibrium by altering their concentration. For semiconductors, in which either only a hole or electron can migrate, something interesting occurs. The migrating entity moves across but the counter-charge cannot, resulting in excess charge at the interface. The migrating partner, unable to return to its original material due to thermodynamics, is now unable to dissipate into the new material due to electrostatics. The result is a thin, charged layer across the interface. Because solid electrolytes conduct lithium cations but no anions or electrons, this ‘space-charge layer’ is also thought to occur in solid-state batteries between electrolyte and electrode. Whereas its size is thought to only be a few nanometres, its (negative) influence could be very large as it can act as barrier for lithium transport across the surface. Yet experimental evidence of its existence is rare and investigation of its behaviour even more so. The space charge between two materials is very thin and chemically almost identical to the bulk. But just like how water can carve the stone, we employ liquids to crack a solid-state problem.We find that when mixing an argyrodite solid electrolyte (Li6PS5Cl) with an ionic liquid (1-Ethyl-3-methylimidazolium bis(trifluoromethyls ulfonyl) imide or EMIN TFSI) , we observe a new chemical environment with NMR (see figure) that cannot be attributed to dissolved precursors. We see an increase in the mobility of lithium ions in the solid electrolyte itself with NMR and an increase in conductivity of the mixture with EIS. Furthermore, exchange of lithium has been confirmed between argyrodite and the mystery environment. What does this all mean? Are we observing the illusive space charge layer? Or are we just staring at an empty promise in the well-known shape of a mysterious peak in a spectrum? Figure 1

Nanophase separated, grafted alternate copolymer styrene-maleic anhydride as an efficient room temperature solid state lithium ion conductor

All solid-state lithium metal batteries (ASSLMBs) based on polymer solid electrolyte and lithium metal anode have attracted much attention due to their high energy density and intrinsic safety. However, the low ionic conductivity at room temperature and poor mechanical properties of the solid polymer electrolyte result in increased polarization and poor cycling stability of the Li metal batteries. In order to improve the ionic conductivity at room temperature while maintaining mechanical strength, we combine the conductivity of short chain polyethylene oxide (PEO) and strength of styrene-maleic anhydride copolymer (SMA) to obtain a grafted block copolymer with nanophase separation structure, which has room temperature ionic conductivity up to 1.14 × 10−4 S/cm and tensile strength up to 1.4 MPa. Li||Li symmetric cell can work stably for more than 1500 h under the condition of 0.1 mA/cm2. Li||LiFePO4 full cells can deliver a high capacity of 151.4 mAh/g at 25 °C and 0.2 C/0.2 C charge/discharge conditions, showing 85.6% capacity retention after 400 cycles. Importantly, the all solid state Li||LiFePO4 pouch cell shows excellent safety performance under different abuse conditions. These results demonstrate that the nanophase separated, grafted alternate copolymer electrolyte has huge potential for application in Li metal batteries.

Increase of the ionic conductivity of perovskite-type lithium-ion conductor by bacterial cellulose templating

Solid-state lithium-ion conductors suffer the disadvantages of high grain-boundary resistance. Perovskite-type LLTO was prepared via bacterial cellulose templating in this paper. BET analysis indicated that a broad range of pores formed in the LLTO templated onto bacterial cellulose. XRD examinations showed that the crystallinity of LLTO reduced upon being templated onto bacterial cellulose. SEM observations indicated that grains of LLTO pellets with bacterial cellulose became much larger and well-connected. The boundaries between grains of LLTO with bacterial cellulose became much less sharp, and the grains were almost inseparable. Under the influence of BC, the average grain-boundary thickness decreased. LLTO with 2% bacterial cellulose exhibited the highest total conductivity, 9.38 × 10−5 S/cm, which increased by 146% compared with pure LLTO. Both the reduced crystallinity and the decreased grain-boundary thickness contributed to the increase of the total conductivity by facilitating the migration of lithium ions across the grain boundaries. An electrochemical test cell of Li4Ti5O12//LLTO-2%BC//LiCoO2 was demonstrated.

Garnet-Type Lithium Metal Fluorides: A Potential Solid Electrolyte for Solid-State Batteries

In this work, we introduced a garnet-type lithium metal fluoride, Li3Na3M2F12 (M = Al, Sc, In), as solid-state lithium-ion conductors for the first time. The mechanically milled Li3Na3M2F12 compounds crystallized in a cubic garnet-like structure (Ia3̅d, No. 230). The ionic conductivities of Li3Na3Al2F12, Li3Na3Sc2F12, and Li3Na3In2F12 are 1.7 × 10–6, 8.2 × 10–6, and 2.4 × 10–6 S/cm at 300 °C and 1.2 × 10–10, 2.6 × 10–9, and 1.8 × 10–10 S/cm at 100 °C, respectively. Even though these fluoride garnets’ conductivity is less, it is still better than those of the oxide analogues Li3Ln3Te2O12 (Ln = Er, Gd, Tb, Nd). Moreover, we explored why garnet-type Li3Na3M2F12 has low ionic conductivity and presented strategies for further improving conductivities.

Enhancement of ionic conductivity and fracture toughness by infiltrating porous Li0.33La0.56TiO3 pellets

Although solid-state lithium electrolytes have the potential to reduce the safety issues associated with organic liquid electrolytes, disadvantages such as low total conductivity, large interface impedance, and delamination of the interface due to cyclic stress still need to be addressed. The solid-state lithium-ion conductor Li0.33La0.56TiO3 (LLTO) was prepared via a hydrothermal route by using CTAB as templates in this paper. Perovskite LLTO with micro-porous channels was obtained and the total conductivity is comparable to the non-porous LLTO. Porous LLTO pellets are infiltrated with the non-porous LLTO precursor solution, and the total conductivities of the infiltrated porous LLTO are all higher than those without infiltration. After infiltration, the porous LLTO calcined at 600 °C achieves the highest total conductivity, 7.88 × 10−5 S/cm. The fracture toughness of the infiltrated LLTO is higher than that of the non-porous LLTO. The results demonstrate a new way to prepare solid-state lithium-ion conductors with high ionic conductivity and great tolerance to cyclic stress.

Control of Ionic Conductivity by Lithium Distribution in Cubic Oxide Argyrodites Li6+xP1-xSixO5Cl.

Argyrodite is a key structure type for ion-transporting materials. Oxide argyrodites are largely unexplored despite sulfide argyrodites being a leading family of solid-state lithium-ion conductors, in which the control of lithium distribution over a wide range of available sites strongly influences the conductivity. We present a new cubic Li-rich (>6 Li+ per formula unit) oxide argyrodite Li7SiO5Cl that crystallizes with an ordered cubic (P213) structure at room temperature, undergoing a transition at 473 K to a Li+ site disordered F4̅3m structure, consistent with the symmetry adopted by superionic sulfide argyrodites. Four different Li+ sites are occupied in Li7SiO5Cl (T5, T5a, T3, and T4), the combination of which is previously unreported for Li-containing argyrodites. The disordered F4̅3m structure is stabilized to room temperature via substitution of Si4+ with P5+ in Li6+xP1-xSixO5Cl (0.3 < x < 0.85) solid solution. The resulting delocalization of Li+ sites leads to a maximum ionic conductivity of 1.82(1) × 10-6 S cm-1 at x = 0.75, which is 3 orders of magnitude higher than the conductivities reported previously for oxide argyrodites. The variation of ionic conductivity with composition in Li6+xP1-xSixO5Cl is directly connected to structural changes occurring within the Li+ sublattice. These materials present superior atmospheric stability over analogous sulfide argyrodites and are stable against Li metal. The ability to control the ionic conductivity through structure and composition emphasizes the advances that can be made with further research in the open field of oxide argyrodites.

First-principles calculation of the configurational energy density of states for a solid-state ion conductor with a variant of the Wang and Landau algorithm.

In this work, a variant of the Wang and Landau algorithm for calculation of the configurational energy density of states is proposed. The algorithm was developed for the purpose of using first-principles simulations, such as density functional theory, to calculate the partition function of disordered sublattices in crystal materials. The expensive calculations of first-principles methods make a parallel algorithm necessary for a practical computation of the configurational energy density of states within a supercell approximation of a solid-state material. The algorithm developed in this work is tested with the two-dimensional (2d) Ising model to bench mark the algorithm and to help provide insight for implementation to a materials science application. Tests with the 2d Ising model revealed that the algorithm has good performance compared to the original Wang and Landau algorithm and the 1/t algorithm, in particular the short iteration performance. A proof of convergence is presented within an adiabatic assumption, and the analysis is able to correctly predict the time dependence of the modification factor to the density of states. The algorithm was then applied to the lithium and lanthanum sublattice of the solid-state lithium ion conductor Li_{0.5}La_{0.5}TiO_{3}. This was done to help understand the disordered nature of the lithium and lanthanum. The results find, overall, that the algorithm performs very well for the 2d Ising model and that the results for Li_{0.5}La_{0.5}TiO_{3} are consistent with experiment while providing additional insight into the lithium and lanthanum ordering in the material. The primary result is that the lithium and lanthanum become more mixed between layers along the c axis for increasing temperature. In part, the simulation of the disordered Li_{0.5}La_{0.5}TiO_{3} system serves as a benchmark for what size systems are currently and in the near future practical to calculate with density functional theory methods.

Data-Driven Discovery of New Materials for Solid-State Batteries

All-solid-state Li-ion battery based on solid electrolytes is a promising next-generation battery technology with high energy density, intrinsic safety, long cycle life, and wide operational temperatures. However, the lack of solid electrolyte materials that satisfy multiple requirements, such as high ionic conductivity, good stability, and interfacial compatibility with the electrode, are impeding the development of this new battery technology. To resolve these materials challenges, we develop and leverage an array of data-driven computation techniques including machine learning to discover and design novel solid-state Li-ion conductors as solid electrolytes for all-solid-state batteries. The data-driven approach enables rapid searching over a large materials space of tens of thousands of materials with highly diverse structures and chemistries. Dozens of novel solid-state conductors are discovered through our data-driven materials search. Our data-driven analyses provide unique insights into the fundamental understanding about solid-state Li-ion conductors beyond traditional physical mechanistic studies. Our study demonstrates a new paradigm of using machine learning techniques for materials discovery that overcome the data-scarcity challenges.Zhang et al., “Unsupervised Discovery of Solid-State Lithium Ion Conductors”, Nature Communications, 10, 5260 (2019) DOI: 10.1038/s41467-019-13214-1He et al., “Crystal Structural Framework of Lithium Super-Ionic Conductors”, Advanced Energy Materials 9, 1902078 (2019) DOI: 10.1002/aenm.201902078Nolan et al., “Computation-Accelerated Design of Materials and Interfaces for All-Solid-State Lithium-Ion Batteries”, Joule, 2, 2016-2046 (2018) DOI: 10.1016/j.joule.2018.08.017

(Invited) Local Structure of Glassy Lithium Phosphorus Oxynitride (LION) Thin Films and Their Interphases with Lithium Metal Anode

Lithium phosphorus oxynitride (LiPON) is a well-known solid-state lithium ion conductor generally produced through the amorphization of lithium phosphate in the presence of nitrogen. Despite its modest ionic conductivity, it remains popular due to its cyclability against lithium metal anodes. The nature of a stable interface beweenn LiPON and Li metal remains elusive due to the difficulty in understanding and characterizing the buried, highly reactive and beam-sensitive interface. Here we successfully preserve and probe the Li/LiPON interface by cryogenic electron microscopy (cryo-EM), one such interface capable of long-term cycling. The chemical and structural evolution along this interface is observed. A reaction mechanism for Li/LiPON interface is proposed based on the observations.

A New Class of Superionic Solid-State Lithium-Ion Conductors: Lithium-Phosphido Silicates, Germanates, and Aluminates

Superionic conductors as a solid-state electrolyte materials are essential for use for all-solid-state lithium ion batteries. Whereas intensive research on oxide and sulfide based lithium ion conductors resulted in optimized materials which reach ionic conductivities up to 25 mScm-1 other compound classes are disregarded. A design principle for fabricating new superionic conductors follows the ideas of increasing the carrier densities, changing the diffusion pathways of the mobile species, creating vacancies or increasing structural defects, and including anions that are more polarizable. For example to increase the carrier density, an effective way is especially the aliovalent substitution of cations: for example, in Li3PS4 making a formal substitution from “P5+” to “Ge4+” arises in Li3.25Ge0.25P0.75S4 with a four times higher ionic conductivity.We propose here phosphide-based materials as ionic conductors that result in principle from the aliovalent substitution of [TtS4]4- tetrahedra, which are an important key building block in sulfide-based electrolytes, by [TtP4]8- leading to the analogous complex anions based on phosphorus (Tt = tetrel element = Group-14 element). The resulting higher charge of the [TtP4]8- tetrahedra allows for twice the number of lithium ions per formula unit for charge compensation if compared to [TtS4]4- and in addition, the more negatively charged P3- is more polarizable than S2-. In this context we discovered for example Li8SiP4,[1] β-Li8GeP4,[2] and Li14SiP6 [3] with conductivities up to 1.1 mScm-1 . In addition we recently succeded to synthesize the lithium richest representative, obtained by formal aliovalent substitution of “Tt 4+” by “Al3+” and formation of in Li9AlP4.[4] The insertion of Al3+ leads to formally nine-fold negatively charged [AlP4]9- tetrahedra, resulting in an even higher lithium density per formula unit and a change in spatial extent of the diffusion pathways. This first representative of a novel compound class of lithium phosphidoaluminates has as an undoped material a remarkable fast ionic conductivity of ~ 3 mS cm-1 and a low activation energy of ~ 29 kJ mol−1 as determined by impedance spectroscopy. Temperature-dependent 7Li NMR spectroscopy supports the fast lithium motion. In addition, Li9AlP4 combines a very high lithium content with a very low theoretical density of 1.703 g·cm-3.In this paper we give an overview on the recent development in this material class. All crystalline materials are readily obtained as single phase materials by ball mill synthesis from the elements followed by moderate thermal treatment of the mixtures. In all cases the atomic structures are characterized by X-ray diffraction methods. The lithium-ion mobility was investigated by temperature-dependent nuclear magnetic resonance and electrochemical impedance spectroscopy revealing low activation energies for lithium hopping. In some cases lithium diffusion pathways via both, tetrahedral and octahedral voids are analyzed by temperature-dependent powder neutron diffraction measurements in combination with maximum entropy method (MEM) and DFT calculations. The role of lithium ion occupation in the various tetrahedral and octahedral sites that occur in those compounds is highlighted.

Fast Ion Conduction of Sintered Glass-Ceramic Lithium Ion Conductors Investigated by Impedance Spectroscopy and Coaxial Reflection Technique

As the ionic conductivity of solid-state lithium ion conductors rises, knowledge of the detailed conductivity mechanisms is harder to obtain due to the limited frequency resolution of the traditional impedance spectrometers. Moreover, the data is easily affected by the local microstructure (i.e. pores, grain-boundaries) and the preparation conditions. The aim of this work is to demonstrate the feasibility of the coaxial reflection technique as a reliable tool to study fast ionic conductors (i.e. σ > 10−4 S cm−1). Especially the relative permittivity can be determined more accurately at room temperature. For the first time the electrical performance of LATP and LLZO manufactured via a scalable top-down glass-ceramic route is evaluated. The density turns out to be a key parameter influencing both relative permittivity and resulting conductivities. For a 100% dense LATP sample the coaxial reflection technique reveals a high grain-core conductivity of 6 × 10−3 S cm−1 similar to the conductivity of ideal single crystals.

Local Structure of Glassy Lithium Phosphorus Oxynitride Thin Films: A Combined Experimental and Ab Initio Approach.

Lithium phosphorus oxynitride (LiPON) is an amorphous solid-state lithium ion conductor displaying exemplary cyclability against lithium metal anodes. There is no definitive explanation for this stability due to the limited understanding of the structure of LiPON. Herein, we provide a structural model of RF-sputtered LiPON. Information about the short-range structure results from 1D and 2D solid-state NMR experiments. These results are compared with first principles chemical shielding calculations of Li-P-O/N crystals and ab initio molecular dynamics-generated amorphous LiPON models to unequivocally identify the glassy structure as primarily isolated phosphate monomers with N incorporated in both apical and as bridging sites in phosphate dimers. Structural results suggest LiPON's stability is a result of its glassy character. Free-standing LiPON films are produced that exhibit a high degree of flexibility, highlighting the unique mechanical properties of glassy materials.

(Invited) Lithium Batteries Enabled By Thin Film Composite Solid Electrolyte Separators

Over the past decade, solid-state lithium ion conductors with conductivities on par and even exceeding the conductivities of conventional liquid electrolytes have been developed. However, practical solid-state lithium metal batteries have not yet been realized due to challenges associated with fabricating and integrating solid electrolyte separators into electrochemical cells. To achieve the power metrics and effective energy densities of existing lithium ion cells, solid electrolytes must be fabricated with comparable area specific resistances and thicknesses on the order of 10 μm, without a significant increase in cost. Thin film composite separators, where a dense solid electrolyte film is coated onto microporous membranes (separators), are one possible solution. The solid thin film electrolyte enables reversible cycling of Li metal by isolating it from the catholyte and inhibiting growth of dendritic structures, while the porous scaffold mechanically supports the thin film and hosts the catholyte within its porosity for efficient ion transport.In this work, lithium phosphate oxynitride (Lipon) thin films with thicknesses on the order of 50 - 100 nm were deposited onto microporous polypropylene separators. These composite separators were robust and flexible; the Lipon film didn’t fracture as the separator was handled, enabling the integration into standard lithium cells, such as coin cells. Due to its thinness, the area specific resistance (ASR) of the solid electrolyte layer was 5 - 10 Ω-cm2, and a relatively low interfacial resistance of 2.5 Ω-cm2 between the Lipon and liquid electrolyte was observed. The Lipon layer was shown to be chemically and physically stable in several common liquid electrolyte formulations. Negligible evolution of interfacial resistance and reaction products at the Lipon/liquid electrolyte were observed after extended exposure to these electrolytes. In Li cycling tests, the Lipon layer promoted smooth, dense Li morphologies in electrodeposits with areal capacities exceeding 15 mAh cm-2. Without the dense Lipon layer, the morphology of the Li electrodeposits was rough – consistent with “mossy” lithium morphology typically observed in liquid electrolytes. In operando impedance spectroscopy showed that a stable resistance of the Lipon bulk was maintained during Li plating, suggesting that the Lipon layer remains intact. The solid electrolyte membrane was also shown to inhibit the polysulfide shuttle in Li-S cells, preventing the reaction of the polysulfide with Li metal. The Li-S cell using the composite separators showed high coulombic efficiency and stable cycling performance. In LiMn2O4-Li cells, the thin film composite separators protected the Li metal from reactions with Mn ions dissolved in liquid electrolyte. The development of these composite solid electrolyte separator offers a promising path to energy dense lithium metal batteries. This work was supported by the ARPA-E IONICS program, U.S. Department of Energy, award DE-AR0000775.

Investigating the Effects of Lithium Phosphorous Oxynitride Lamination on PEO/PPC Blended Hybrid Electrolytes

Solid-state electrolytes play an important role in the development of new generations of technology including lithium metal batteries. Lithium conducting solid materials offer a solution to the safety issues that are present in lithium-ion batteries (LIBs). Conventional LIBs typically use highly flammable organic solvents as a transport medium for lithium ions, potential risks that arise due to these solvents include fire, volatilization, and explosions. Solid polymer electrolytes (SPEs) are great as electrolyte material because fabrication of the electrolyte films is cheap, and processing is simple. However, SPEs can have stability issues when in contact with lithium metal for extended periods which is a problem for cells required to offer long lifetimes. Poly (ethylene oxide) (PEO) has been extensively studied since the conception of SPEs, due to its ability to solvate ions due to its polar ether groups. However, PEO, being a semi-crystalline polymer, exhibits thermal history effects due to recrystallization behavior. Amorphous SPEs are highly desirable because there are no recrystallization kinetics at play, and the properties are consistent regardless of the thermal history. Poly(propylene carbonate) (PPC), an amorphous, biodegradable polymer, can offer high ionic conductivities. Blending PEO and PPC offers a solution to resolve the thermal history effects of PEO by interfering with the crystalline structures in blended electrolyte systems with the salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Inorganic solid electrolytes (ISEs) can offer better chemical stability and excellent mechanical properties compared to SPEs; however, brittleness may decrease the structural stability over time. Thin films ISE’s also tend to cost much more than SPEs due to the fabrication methods required such as electron beam evaporation, and chemical vapor deposition. Lithium phosphorous oxynitride (LiPON), produced through RF magnetron sputtering, was developed in the 1990s at Oakridge National Laboratories and offers great electrochemical stability against lithium and relatively high ionic conductivities on the order of 10-6 S/cm at room temperature. A new bilayer lithium-ion conducting hybrid solid electrolyte (HSE) consisting of a blended polymer electrolyte (BPE) coated with a thin layer of the ISE LiPON, has been prepared. PEO and PPC are blended in a 1:1 mass ratio, then the salt LiTFSI is incorporated. BPE films are then produced through a facile solution casting method using acetonitrile as the solvent. Structure-property relationships are then systematically evaluated to understand the effects of molecular weight and mass loading of LiTFSI on the BPE systems. Influences of molecular weight and salt loading on crystallinity is observed though X-ray diffraction and optical microscopy of the BPE films. Ionic conductivity is evaluated through impedance spectroscopy between two stainless steel blocking electrodes, thermal history measurements were taken by heating and cooling the SPE while recording EIS at specified temperatures. The best BPE based on ionic conductivity, consists of 100k molecular weight PEO, 50k molecular weight PPC, and 25(w/w)% LiTFSI, (denoted as PEO100PPC50LiTFSI25) and exhibits an ionic conductivity of ~2x10-5S/cm. Temperature dependent EIS shows that the lowest molecular weight polymers offer lower activation energies as a result of the higher degree of free volume and molecular motion of the polymer chain. The optimized BPE system is then coated with various thicknesses of LiPON. The effects of LiPON coating are evaluated and show promise for the development of lithium stable solid electrolytes. Linear sweep voltammetry (LSV) is used to assess the anodic voltage stability against lithium metal. Incorporating very thin layers of LiPON down to 20nm on the BPE shows an increase in the voltage window of the electrolyte system from 5.2 to 5.5V (vs Li/Li+). However, the incorporation of LiPON decreases the ionic conductivity of the BPE. Thinner layers of LiPON show the ionic conductivity approaches that of the BPE, with thicker LiPON layers offering lower ionic conductivies. To assess the stability against lithium metal over time, impedance spectroscopy is used. Cells consisting of the solid electrolyte against lithium metal are prepared, and impedance measurements are taken over a 20-day period to assess the changes in charge transfer resistance. From these measurements, it has been shown that only a 30nm thin layer of LiPON is required to improve the stability of the resulting HSE. Therefore, coating BPEs with a thin layer of LiPON is shown to be an effective strategy to improve the long-term stability of solid-state lithium ion conductors.

Design of High Voltage All-Solid-State Batteries Based on Sulfide Electrolytes

Ceramic sulfide solid-electrolytes are amongst the most promising materials for enabling solid-state lithium ion batteries. The ionic conductivities can meet or exceed liquid-electrolytes for such ceramic sulfides, however, there is a theoretical concern about the narrow electrochemical stability window of approximately 1.7-2.1 V vs Lithium metal. In addition, ceramic sulfides are frequently plagued by interfacial reactions when combined with common electrode active materials. In this talk, methods for the stabilization of both the bulk electrochemical decompositions and the interfacial reactions will be discussed. Solid-state cells stable up to 5V, including layered Li-Co-O and spinel Li-Mn-Ni-O, will be presented and the stabilization mechanisms will be discussed. Ceramic sulfides are known substantially swell during electrochemical decay. Such swelling has been shown to provide a viable means by which to stabilize electrochemical decomposition in lithium ion batteries[1]. Evidence and theoretical understanding of stability window expansion as the result of mechanical constriction will be discussed for both Li10GeP2S12 and Li10SiP2S12 systems. Further discussion on the impact of interfaces will focus on our novel high-throughput computational schema[2], which has enabled the evaluation of nearly 70,000 material DFT calculation for interfacial chemical and electrochemical stability with Li10SiP2S12. These new computational methods found over 2,000 high-voltage coating materials that are electrochemically inert against Li10SiP2S12 at standard cathode voltages. [1] Fan Wu, William Fitzhugh, Luhan Ye, Jiaxin Ning, Xin Li. Advanced Sulfide Solid Electrolyte by Core-Shell Structural Design. Nature Communications, (9), 4037 (2018). [2] William Fitzhugh, Fan Wu, Wenye Deng, Pengfei Qi, Luhan Ye, Xin Li. A High-Throughput Search for Functionally Stable Interfaces in Sulfide Solid-State Lithium Ion Conductors. Advanced Energy Materials. In press.

Design of High Voltage All-Solid-State Batteries Based on Sulfide Electrolytes

Ceramic sulfide solid-electrolytes are amongst the most promising materials for enabling solid-state lithium ion batteries. With ionic conductivities that can meet or exceed liquid-electrolytes, such ceramic sulfides are limited in practice chiefly by a narrow electrochemical stability window of approximately 1.7-2.1 V vs Lithium metal. In addition, ceramic sulfides are frequently plagued by interfacial reactions when combined with common electrode active materials. In this talk, methods for the stabilization of both the bulk electrochemical decompositions and the interfacial reactions will be discussed. Solid-state cells stable up to 5V, including layered Li-Co-O and spinel Li-Mn-Ni-O, will be presented and the stabilization mechanisms will be discussed from a detailed theoretical perspective. Ceramic sulfides are known substantially swell during electrochemical decay. Such swelling has been shown to provide a viable means by which to stabilize electrochemical decomposition in lithium ion batteries[1]. Evidence and theoretical understanding of stability window expansion as the result of mechanical constriction will be discussed for both Li10GeP2S12 and Li10SiP2S12 systems. Further discussion on the impact of interfaces will focus on our novel high-throughput computational schema[2], which has enabled the evaluation of nearly 70,000 material DFT calculation for interfacial stability with Li10SiP2S12. These new computational methods found over 2,000 high-voltage coating materials that are electrochemically inert against Li10SiP2S12 at standard cathode voltages. The culmination of these approaches, mechanical bulk stabilization combined with interfacial coating layers, is the design of solid-state full cells with high voltage stability and good cyclability. The principal focus of this talk will be a discussion on the theoretical underpinnings of these methods and a comparison with experimental data. [1] Fan Wu*, William Fitzhugh*, Luhan Ye, Jiaxin Ning, Xin Li. Advanced Sulfide Solid Electrolyte by Core-Shell Structural Design. Nature Communications, (9), 4037 (2018). [2] William Fitzhugh*, Fan Wu*, Wenye Deng, Pengfei Qi, Luhan Ye, Xin Li. A High-Throughput Search for Functionally Stable Interfaces in Sulfide Solid-State Lithium Ion Conductors. Advanced Energy Materials. Under review.

Recent Progress in All-Solid-State Lithium−Sulfur Batteries Using High Li-Ion Conductive Solid Electrolytes

Rechargeable lithium−sulfur (Li−S) batteries are one of the most promising next-generation energy storage systems due to their extremely high energy densities and low cost compared with state-of-the-art lithium-ion batteries. However, the main obstacles of conventional Li−S batteries arise from the dissolution of lithium polysulfides in organic liquid electrolytes and corresponding safety issues. To address these issues, an effective approach is to replace conventional liquid electrolytes with solid-state electrolytes. In this review, recent progress in the development of solid electrolytes, including solid polymer electrolytes and inorganic glass/ceramic solid electrolytes, along with corresponding all-solid-state Li−S batteries (ASSLSBs) and related interfacial issues at the electrode/electrolyte interface, will be systematically summarized. In addition, the importance of various solid-state lithium ion conductors in ASSLSBs will be discussed followed by detailed presentations on the development of various forms of sulfur-based positive electrode materials (e.g., elemental sulfur, lithium sulfide, metal sulfides, lithium thiophosphates, and lithium polysulfidophosphates) along with key interfacial challenges at the electrode/solid electrolyte interface (cathode/SE and anode/SE). Finally, this review will provide a brief outlook on the future research of ASSLSBs.

Electrochemical extraction of hydrogen isotopes from Li/LiT mixtures

In this study, we propose a new methodology for lithium hydride electrolysis which can be extended for the extraction of hydrogen isotopes from molten lithium. This new process utilizes a ceramic lithium ion conductor that can be directly immersed in molten lithium to facilitate the electrochemical extraction of hydrogen isotopes in the molten state. The extraction of hydrogen isotopes from lithium is necessary for the safe and reliable operation of fusion reactors that utilize molten metals as a coolant and/or as tritium breeder blanket in the system. A number of technologies for hydrogen isotope extraction from molten lithium have been proposed that can facilitate the extraction of hydrogen isotopes. However, they require a series of complicated mechanical steps (expensive mechanical parts with limited lifetimes) in order to carry out the separation and prevent impurities build up in the extraction process. Our proposed electrochemical process can potentially simplify the isotope extraction process by eliminating many of the mechanical parts and being able to integrate this technology within the fusion reactor to perform the extraction in-line. To determine the feasibility of this process, a proof of concept was demonstrated experimentally utilizing a solid state lithium ion conductor to facilitate the electrochemical extraction of hydrogen and deuterium from lithium in the molten state. The extraction of hydrogen and deuterium from molten lithium was verified during this experiment and indicates this new process is a potential alternative for other processes used for the extraction of hydrogen isotopes. Process modeling was also performed to understand the energy requirements and electrode areas needed for decomposing different concentrations of tritium.

(Invited) Influence of Mechanical Stress on Lithium Chemical Potential in Lithium Ion Battery Electrodes

In solid-state ionic devices, such as all-solid-state lithium ion batteries (ASSLIBs) and solid oxide fuel cells (SOFCs), mechanical stress is introduced at the electrode/electrolyte hetrointerfaces due to differences in the crystal structures and the chemical/thermal expansion coefficients. A mechanical strain at the heterointerface might affect thermodynamic and kinetic properties of the electrodes and the electrolyte, resulting in the performance deterioration or improvement of the devices. Although such a so-called “mechano-electro-chemical coupling effect” has been empirically demonstrated by many researches, the origins of the effect are not fully understood so far. In this presentation, we will discuss the influence of mechanical stress on thermodynamic properties of solid-state ionic materials. As a model case, lithium chemical potential in electrode materials for ASSLIBs under mechanical stress was investigated. A dense thin film electrode of LiCoO2, Li2MnO4 or LiFePO4 was deposited on a plate of solid-state lithium ion conductors, and the variation of the lithium chemical potential in the electrode was evaluated by measuring electromotive force under compressively- and tensely-strained conditions. Electromotive force accompanied by mechanical strain could be clearly observed, demonstrating a “mechano-electro-chemical coupling effect”. The influence of mechanical stress was different depending on the kinds of electrode material. The observed change in the electromotive force could be thermodynamically related to the lithium partial molar volume of the electrodes.