Molecular Dynamics Simulations on Li-Ion Conduction in Nasicon-Type Solid Electrolytes

Na super‐ionic conductor (NASICON)‐structured solid electrolyte Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) excels in mechanical strength, ionic conductivity, and commercialization prospect, yet its incompatibility against Li metal anode typically leads to interfacial failure, significantly reducing the overall electrochemical performance of fabricated solid‐state Li metal batteries (SSLMBs). Here, this issue is tackled by interposing an in situ cured poly(ethylene oxide) (PEO)‐LiTFSI@BN interfacial layer between LATP and Li metal. This layer exhibits an ionic conductivity of 7.76 × 10 −5 S cm −1 at room temperature, robust thermostability, as well as satisfactory stability against Li metal (critical current density: 1.5 mA cm −2 ). A Li|LATP|Li symmetric cell employing the interfacial layer demonstrates long cyclic stability over 1200 cycles with a low overpotential of ± 100 mV. Furthermore, this interfacial layer enables a LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523)|LATP|Li metal SSLMB to sustain 120 cycles of operation at a cutoff voltage of 4.2 V. This work provides a practical strategy for integrating the LATP solid electrolyte into SSLMBs.

Solid electrolyte is one of functional materials in which only single ion species can migrate in solids and is expected to be utilized for various applications such as rechargeable batteries and gas sensors etc. As the conducting ion species in solid electrolytes, the cations having higher valency than trivalent state have been considered to be extremely poor migrants because of the strong electrostatic interaction between the conducting cation and the surrounding anions comprising the framework of the structure, while monoor divalent ions are well known to migrate in solids. For the realization of such a trivalent cation conduction in solids, the cation whose valency is higher than tetravalent state is necessary to effectively reduce the above mentioned strong electrostatic interaction between mobile trivalent cations and the anions in addition to a large migrating pathway for ion conduction in the structure. In 1995, M2(WO4)3 tungstates (M = Al, Sc, Y, Er– Lu) [1–7] holding the Sc2(WO4)3 type structure have been clearly demonstrated to be trivalent ion conducting solid electrolytes. The M2(WO4)3 contains hexavalent W6+ cation and holds a quasi-layered structure which is one of the suitable structures for ion conduction. However, there remains two serious disadvantage in M2(WO4)3 solid electrolytes. One is that the W6+ cation might be reduced to lower valency state not only by reacting with metals by contact but also in a reducing atmosphere, resulting in an appearance of electronic conduction which makes the ionic conducting characteristics deteriorate greatly. The other is that the relative density of these tungstate series is too low to apply practically. Recently, R1/3Zr2(PO4)3 (R = rare earths) with NASICON type structure prepared by a sol-gel method were demonstrated to be new trivalent R3+ cationic conductors [8–10]. While the R1/3Zr2(PO4)3 has both a high chemical stability and a high relative density compared to the M2(WO4)3 solid electrolytes, the conductivity of the samples prepared by a sol-gel method was considerably lower than that for Sc2(WO4)3 because of their low crystallinity. In order to enhance the conductivities of the R1/3Zr2(PO4)3 solid electrolytes, it is considerably effective to improve the crystallinity and to create the well-ordered ion conducting pathway in the R1/3Zr2(PO4)3 grains. However, the crystallization (ca. 800 ◦C) and the decomposition temperature

Recently, demands on energy storage systems of high capacity and efficiency are continuously increased as the growth in commercialization of new renewable energy generation, such as wind and solar power, electric vehicle and portable electronic devices. Li-ion battery(LIB) has been introduced in diverse electronic products as one of the promising energy storage devices because of good energy density and long cycle life to answer these demands. For these reasons, study on LIB actively progressed to improve the performance of LIB. However, there are critical obstacles to make advance in energy density, such as increase in voltage and capacity, of conventionally used LIB. The commercialized LIB system use liquid electrolytes as for Li-ion migration between two electrodes. This liquid electrolyte deteriorate the stability of LIB, bring about overheating and explosion as temperature changes originated mainly from increase in operating voltage. In addition, leakage of organic based liquid electrolyte from the inside of LIB could have noxious effect on health. Therefore, alternatives on liquid electrolyte have been highly required to resolve these limitations. As increase in efforts to develop new electrolyte types, Inorganic solid electrolytes have received attentions to substitute it for liquid electrolytes. Inorganic solid electrolytes is one of the promising alternatives of liquid electrolyte because of their excellent thermal stability and good Li ion conductivity. Among inorganic electrolytes, Li10GeP2S12 (LGPS) indicates the great Li ion conductivity to be comparable with Li ion conductivity of liquid conductivity. Despite of this superior Li ion conductivity, the understanding of mechanism for Li ion diffusion in LGPS has not been clearly understood. This study explains correlation between Li ion diffusion and local environments of LGPS to understand mechanism of Li ion migration by first principles calculation. LGPS has enough Li ion concentrations and two types of migration paths. Li ion could migrate in frameworks of Ge-S and P-S(fig.1a). These frameworks secure c-axis and ab-plane paths for Li ion diffusions. There are also two types of methods for first principles calculation of Nudged Elastic Band(NEB) method of Li ion diffusion. One is defect migration (single ion migration) and the other is multi ion concentration migration(fig.1). Firstly, defect migration was analyzed by NEB. We setup three local environments for understanding the correlation in c-axis direction migration. The three situations are only one Li ion without any other Li ions in LGPS(fig.1b), one Li ion in the migration path around Li ions existent in the other non-migration paths(fig.1c) and only one vacancy with existent of Li ions in the migration path(fig.1b). Although the first and second cases of Li ion migration indicates similar tendency and energy barrier for Li ion migration, the third case shows that energy barrier in same migration path is more higher than the energy barrier of the other cases. This means that the factor influencing the Li ion diffusion is not the local Li ions in non-migration path but the near Li ions in the migration path. In addition to single ion diffusion, multi ion concentration migration is examined and this type has no vacancy and assumes that the all Li ions in the path diffuse all together. The energy barrier is lower than the energy barrier in all of single ion migration cases(fig.1f). This indicates that simultaneous migration of all Li ions in migration path is energetically beneficial. The ab-plane migration has two migration paths and the energy barriers are relatively higher than the energy barrier of c-axis migration path(fig.1e). This means that Li ion migration is mainly progressed in c-axis rather than ab-plane regardless of local environments in LGPS. Therefore, this understanding of mechanism for Li ion diffusion in LGPS would be effective guide line for developing the superior inorganic solid electrolyte based on LGPS. Figure 1

<p indent="0mm">Since the commercialization of lithium-ion batteries in the 1990s, lithium-ion batteries have been successfully applied in portable electronics, electric vehicles, and grid energy storage. Although current organic liquid electrolytes have high ionic conductivities, they are inherently flammable, volatile, and prone to leakage. Moreover, severe side reactions and dendrite growth on the surface of the lithium anode during the charge-discharge process can cause safety hazards, which greatly impede their applications in lithium metal batteries. Solid electrolytes, including inorganic solid electrolytes and polymer electrolytes, are regarded as effective alternatives to organic liquid electrolytes for the construction of lithium metal batteries with high energy density and safety. Among them, solid polymer electrolytes offer excellent flexibility, processability, and interfacial compatibility over inorganic solid electrolytes, and they are extraordinarily promising for lithium metal batteries with high energy density and safety. Ideal solid polymer electrolytes should have following features: (1) High ionic conductivity (&gt; <sc>10 ^–4 S cm ^–1) </sc> at room temperature; (2) high lithium ion transference number (~1) to reduce the concentration polarization and improve the rate performance of batteries; (3) intimate contact at the electrode/electrolyte interfaces; (4) wide electrochemical window <sc>(&gt;4.5 V</sc> vs. Li/Li ⁺) to match high-voltage cathodes and improve the energy density of batteries; (5) good mechanical stability to resist processing, buffer electrode volume change and inhibit dendrite growth; (6) good thermal stability to withstand environmental changes. Generally, the ionic conductivity of pure solid polymer electrolytes at room temperature is low <sc>(~10 ^–6 S cm ^–1). </sc> Researchers have tried to improve the ionic conductivities by adjusting the lithium salt concentration, such as developing “polymer-in-salt” solid electrolytes. However, increasing the concentration of lithium salt leads to the deterioration of the mechanical strength. Strategies such as developing novel lithium salts, modifying polymer matrix, and incorporating inorganic fillers into solid polymer electrolytes are proposed to promote ionic conductivities of solid polymer electrolytes. In particular, composite polymer electrolytes, fabricated by dispersing a certain amount of inorganic fillers into solid polymer electrolytes, have improved ionic conductivities without sacrificing their mechanical performances. Poor interfacial property between electrodes and electrolytes is also a critical issue for solid polymer electrolytes. On one hand, poor and uneven solid/solid contacts at the electrode/electrolyte interfaces lead to high resistance and sluggish ionic transport kinetics. Furthermore, the volume change of the positive and negative electrodes in the charge/discharge process deteriorates the interfacial contacts, blocks the ion and electron transport through the interfaces, and greatly reduces the electrochemical reaction kinetics. On the other hand, the electrochemical windows of solid polymer electrolytes are usually narrow <sc>(＜4.5 V).</sc> During cycling, redox reactions are prone to occur at the electrode/electrolyte interfaces, causing battery failure. Solid polymer electrolytes have also poor thermal and mechanical stabilities. Therefore, design and synthesis of polymer-based solid electrolytes with excellent comprehensive performances and construction of fast and stable ion transport channels at the electrolyte/electrode interfaces are of great significance for the successful development of solid-state lithium metal batteries. This paper presents a brief review of the research progress in solid polymer electrolytes from two aspects: Improving the ionic conductivities of solid polymer electrolytes and enhancing the interfacial performance at electrolyte/electrode interfaces. First, targeted optimization strategies on ionic conductivities of solid polymer electrolytes, including constructing continuously aligned ionic transport paths and shortening the ionic transport distance, are summarized. Second, interface optimization strategies, including constructing wetting interfaces and synthesizing asymmetric electrolytes, are presented to reduce the interface resistance and improve the interfacial contact. Finally, perspectives on the development of solid polymer electrolytes and high-performance solid-state lithium metal batteries are discussed, and key research directions and advanced test methods are proposed. This review may provide a comprehensive understanding and further guidance for not only the material design of solid polymer electrolytes, but also the structural design of lithium metal batteries with favorable electrochemical and interfacial performances.

Development of solid electrolytes with high Li ion conductivities are requested for safe and reliable all solid-state rechargeable Li ion (metal) batteries. A-site deficient perovskite-type La1/3NbO3 serves as a host structure for fast Li-ion conductors. However, the relationship between crystal structure and Li-ion conductivities in this compound is still unclear, because La1/3NbO3 shows complex modulation structure according to previous electron diffraction studies. Recently, we performed multiscale computational studies using Monte Carlo simulation and/or molecular dynamics (MD) calculations combined with highly accurate first-principles density functional theory (DFT) calculations. We successfully visualized the long-range La and vacancy ordering in real space at the first time. Interestingly, energetically comparable two types of La/vacancy ordering structures are indicated in the Monte Carlo simulation, and good accordance between experimental and computational electron diffraction patterns are confirmed by considering two ordering structures. In addition, MD calculations using meta-heuristically optimized force field parameters unveil the enhancement of Li-ion conductivity by modulated structure in comparison with random La/vacancy arrangement.

The low ionic and electronic conductivity between current solid electrolytes and high-capacity anodes limits the long-term cycling performance of all-solid-state lithium-ion batteries (ASSLIBs). Herein, this work reports the fabrication of an ultra-stable electrode-solid electrolyte composite for high-performance ASSLIBs enabled by the homogeneous coverage of ultrathin Mg(BH4 )2 layers on the surface of each MgH2 nanoparticle that are uniformly distributed on graphene. The initial discharge process of Mg(BH4 )2 layers results in uniform coverage of MgH2 nanoparticle with both LiBH4 as the solid electrolyte and Li2 B6 with even higher Li ion conductivity than LiBH4 . Consequently, the Li ion conductivity of graphene-supported MgH2 nanoparticles covered with ultrathin Mg(BH4 )2 layers is two orders of magnitude higher than that without Mg(BH4 )2 layers. Moreover, the thus-formed inactive Li2 B6 with strong adsorption capability toward LiBH4 , acts as a stabilizing framework, which, coupled with the structural support role of graphene, alleviates the volume change of MgH2 nanoparticles and facilitates the intimate contact between LiBH4 and individual MgH2 nanoparticles, leading to the formation of uniform stable interfaces with high ionic and electronic conductivity on each MgH2 nanoparticles. Hence, an ultrahigh specific capacity of 800 mAh g-1 is achieved for MgH2 at 2 A g-1 after 350 cycles.

Development of solid inorganic lithium (Li) ion conducting materials for the use as solid electrolytes is indispensable for the realization of next-generation all-solid-state Li batteries with high safety and reliability. Among various oxide-based solid electrolyte materials, a garnet-type oxide with the formula of Li7La3Zr2O12 (LLZO) has attracted much attention because of its high Li ion conductivity at room temperature, excellent thermal performance, and high stability against Li metal.1,2) However, the formation of a solid-solid interface between LLZO and the Li metal anode is challenging. Poor interfacial connection causes non-uniform Li plating and intergranular penetration of Li dendrite in polycrystalline LLZO when the cell is cycled particularly at high current densities, resulting in internal short-circuit failure.3–5) There is no doubt that the establishment of prevention technology for short-circuit failures is a top priority issue for the development of all-solid-state Li metal batteries.5) On the other hand, from the viewpoint of effective use of material resources, the possibility of reusing LLZO extracted from a solid-state battery after a short-circuit failure occurred is worth considering. In this work, we investigated the reusability of a Ta-doped Li6.55La3Zr1.55Ta0.45O12 (Ta-LLZO) solid electrolyte shorted by Li dendrite growth during electrochemical Li plating/stripping testing for a Li/Ta-LLZO/Li symmetric cell. Ta-LLZO was taken out of a tested cell after the degradation by Li dendrite growth occurred, and then annealed at 700 ºC in air. The annealing temperature was set to suppress possible excess Li loss from Ta-LLZO during post-annealing.6) In the first Li plating/stripping test, the cell was shorted at 0.85 mA cm-2 and the dark gray area with possible Li dendrite growth was confirmed on the surface of Ta-LLZO. This dark gray area turned white but slightly different from the original color of Ta-LLZO by post-annealing. The ionic conductivity of as-synthesized and post-annealed Ta-LLZO was measured and compared. Post-annealed Ta-LLZO retained high room temperature ionic conductivity of 0.82 mS cm-1, which is slightly lower than the conductivity of as-synthesized one (= 0.90 mS cm-1). We also prepared a symmetric cell with the post-annealed Ta-LLZO and Li metal electrodes and the second Li plating/stripping test was carried out. Symmetric cell with post-annealed Ta-LLZO showed stable voltage response. This indicates the possibility of reusing the degraded garnet-type solid electrolyte by Li dendrite growth for an another solid-state Li battery.

The development of solid-state batteries (SSBs) is crucial for advancing next-generation battery technology. Compared to conventional lithium-ion batteries using organic liquid electrolytes, SSBs offer higher energy density, improved safety, and wide operating temperature range. Nonetheless, constructing a practical SSB with lithium-metal anode (LMA) and optimal inorganic solid electrolyte (SE) presents significant challenges. In this presentation, we introduce our findings addressing the two primary obstacles impeding the remarkable advancement of SSBs.The electrochemical instability at the interface between LMA and SE is the first persistent issue to overcome. Solid materials constituting SSBs exhibit distinct physical and chemical properties. When these materials, in the form of electrodes or electrolytes, are assembled into an electrochemical cell configuration, they establish solid–solid interfaces within the energy storage system. Achieving electrochemically stable SSBs requires non-reactive interfaces between LMA and SE with conformal contact. Previous studies have predominantly focused on developing interface modification methods to enhance the long-term cycling stability of SSBs. However, such approaches often offer phenomenological solutions without a thorough understanding of the interface dynamics. Additionally, the role of charges within an electric field in the operation of lithium-ion batteries is often overlooked. The LMA-SE interface of SSBs experiences an in situ electric field during the charging process, where the SEs exhibit semiconducting properties distinct from liquid electrolytes. Consequently, identifying the governing mechanism of interface stabilization cannot rely solely on conventional electrochemical techniques or a materials science perspective alone.Interface modeling by tailoring energy band alignment is a fundamental approach to achieving the cycling stability of SSBs. In this study, we used Li1.3Al0.3Ti1.7(PO4)3 (LATP), a representative NASICON-type SE, alongside a Li-metal foil. Although LATP possesses excellent electrochemical stability with a high anodic limit (4.31 V) and small decomposition energy (- 65 meV per atom at 5 V), its direct contact with LMA leads to the formation of a reactive interface having mixed ionic-electronic conducting interphases (MCI). The presence of MCI expedites the reduction of LATP’s metallic constituents by enabling electron transfer across the interface. Given the heightened risk of interfacial instability in the electrochemical cell, controlling interfacial reactions is imperative for constructing SSBs. Herein, we deposited titanium (45 nm) onto the surface of LATP to stabilize the inherently reactive LMA-LATP interface. The pure transition metal interlayer transformed into an ion-conducting crystal structure through electrochemical interactions with Li-metal. This titanium compound self-induced interlayer (TSI) served as a stable Li-ion conductor and electron buffer between LMA and LATP. Lithium symmetric cells having TSI maintained constant overpotential over 1000 cycles and exhibited significantly reduced impedance.Another critical obstacle to realizing practical SSBs is the intrinsic structural instability of SEs. This physical instability shortens the lifespan of SSBs and severely diminishes their electrochemical performance at the typical ambient temperatures of electric vehicle power batteries (25-50 °C). Over the past decade, various solid-state Li-ion conductors have been developed such as Li7La3Zr2O12 (garnet-type oxide SE) and Li6PS5Cl (argyrodite-type sulfide SE). By associating lattice dynamics and ionic transport in these materials, previous studies could improve the ionic conductivity of SE through major substitution with large cations like tantalum or partial substitution with large anions. Recently, antiperovskite-type SEs with the anion-centered unit of Li3AB, where A represents hydroxyl and B represents halogen groups, have attracted attention. The representative composition Li2OHCl is renowned for its facile fabrication process through solid-state reaction, high ionic conductivity (10-3 S cm-1), and excellent compatibility with LMA. Still, significant limitations exist in its use for constructing practical SSBs, as is common with most other types of SEs.We found that substituting trace amounts of aliovalent cations to modify atomic interactions in SEs can significantly enhance the electrochemical reliability of SSBs. This strategy effectively resolves the intrinsically unstable structure of SEs and simultaneously enhances the LMA-SE interfaces. As a proof-of-concept, we substituted lithium in Li2OHCl with metal elements that do not experience crystal field stabilization energy. The resulting modified electrolytes led to prolonged lifespan and superior rate performance in lithium symmetric cells at room temperature. The cells with the modified SEs exhibited a remarkable critical current density exceeding 10.5 mA cm-2 and maintained a stable voltage profile for 3000 hours at 10.0 mA cm-2, whereas the pristine cells achieved a relatively low critical current density of 2.0 mA cm-2 and a short lifespan of 1000 hours at 1.0 mA cm-2. These findings underscore a strong correlation between the physical properties of inorganic materials and the electrochemical stability of SSBs. Finally, we anticipate that the revealing results will contribute to the advancement of high-power SSBs employing solid electrolytes and metallic electrodes.

Since solid electrolytes have a broad electrochemical stability window, are exceptionally electrochemically stable against Li metal, and function as a physical separator to prevent dendrite growth, they are at the forefront of alternate possibilities, further increasing the stability and energy density of Li-ion batteries. NASICON-type electrolytes are a promising candidate due to their negligible moisture sensitivity, which results in outstanding stability and a lower probability of Li2CO3 passivity under the ambient atmosphere. However, one of the most promising representatives, Li1+xYxZr2-x(PO4)3 (LYZP), has multiple stable phases with significant variation in their corresponding Li-ion conductivity. In this paper, we have successfully synthesized the highly ionically conductive rhombohedral phase of LYZP via spray-flame synthesis. Two different solvent mixtures (e.g., 2-ethyl hexanoic acid/ethanol, propanol/propanoic acid) were chosen to explore the effect of precursor composition and combustion enthalpy on the phase composition of the nanoparticle. The as-synthesized nanoparticles from spray-flame synthesis consisted of the crystalline tetragonal zirconia (t-ZrO2) phase, while lithium, yttrium, and phosphate were present on the nanoparticles' surface as amorphous phases. However, a short annealing step (1 h) was sufficient to obtain the NASICON phase. Moreover, we have shown the gradual phase conversion from orthorhombic β phase to rhombohedral α phase as the annealing temperature increased from 700 °C to 1300 °C (complete removal of β phase). In this context, Y3+ doping was also crucial, along with the appropriate solvent mixture and annealing temperature, for obtaining the much-desired rhombohedral α phase. Further, 0.2 at% Y3+ doping was added to the solvent mixture of 2-ethyl hexanoic acid/ethanol, and annealing at 1300 °C for 1 h resulted in a high ionic conductivity of 1.14∙10-5 S cm-1.

The safety issues concerning conventional Li+ ion batteries (LIBs) are attributed to the use of flammable organic liquid electrolyte (LE), which could result in fire and explosion. To address the safety issues and increase the durability, All-solid-state Li+ ion batteries (ASLBs) using inorganic solid electrolyte (SE) are considered as an ideal alternative for developing safe LIBs. Their wide range of operating temperature, from room temperature to over 90°C, is also an important advantage with regards to energy storage systems.SE can be classified into two types: sulfide-based SEs and oxide-based SEs. First, in the case of sulfide-based SEs, thio-LISICON, and Li-argyrodite are present and these have high ionic conductivity that are comparable to LE . Furthermore, a sulfide-based SE with a polymer electrolyte has been extensively investigated as well. However, one of the major disadvantages of a sulfide-based SE is that it is chemically unstable in air as it reacts with moisture in the air to produce H2S gas, which is a hazardous.On the contrary, oxide- based SEs including garnet, NASICON type may be attractive because they not only have high ionic conductivity but also, are chemically stable in air. Given these aspects, many researchers are devoted to the combination of oxide-based SEs and polymer electrolytes by adding oxide-based SE powders in a polymer matrix, as polymer electrolytes exhibit more suitable mechanical properties than those of ceramics, but show low ionic conductivity at room temperature.The combination of oxide-based SEs and polymers have paved new ways to create better electrolytes which incorporate both the high ionic conductivity of the oxide-based SE and the stable mechanical properties of polymers. In the case of composite electrolytes, many researchers anticipated that the higher the oxide-based SE (herein, ceramic) content in a composite electrolyte, the higher the Li+ ion conduction contribution from the ceramic. To investigate the effect of Li+ ion conduction from ceramic powders, analytical research studies according to the ceramic content have been conducted. Unexpectedly, even if the ratio of ceramic powder is increased, the ceramic’s contribution to Li+ ion conductivity could not be confirmed as there is no path for the Li+ ions to percolate through the ceramic due to high interfacial resistance in the ceramic powders. Furthermore, this ceramic powder acts as a resistor due to high interfacial resistances between the ceramic powder and polymer electrolyte. In this work, scalable and flexible composite electrolyte film (SFCEF) was fabricated based on a fiber-shaped ceramic and polymer support. Ceramic fibers of Li1.3Ti1.7Al0.3(PO4)3 (LATP) were prepared by sintering the precursor-coated sacrificial template and then infiltrated with a polyethylene oxide (PEO) polymer to obtain the FSCEF. The LATP fibers induced continuous Li+ ion channels, allowing the FSCEF to show an ionic conductivity exceeding 10−4 S cm−1 at 60 °C. The synergistic action of the ceramic frameworks and supportive PEO resulted in enhanced mechanical flexibility. Furthermore, the possibility of using a FSCEF in all-solid-state batteries was confirmed by conducting electrochemical performance tests on a Li/FSCEF/LCO (LiCoO2) cell. We expect that the herein reported findings will contribute to the synthesis of thin and flexible solid-state electrolyte films with manufacturing scalability for promising high-voltage all-solid-state batteries.

Free volume structure at interphase region of poly (ethylene oxide)-Al2O3 nanorods composites based solid polymer electrolyte and its direct correlation with Li ion conductivity

While the idea that fast charging batteries could be utilized in electric vehicles is enticing, current fast charging batteries face significant challenges that must be mitigated before this application is realized. Fast charging batteries currently suffer from capacity loss and degradation over time which not only affects battery life but poses a safety hazard. The current electrolyte, 1.2 M LiPF6 in 3:7 wt.% EC/EMC (Gen2), contains the salt LiPF6 that when exposed to moisture can form dangerous hydrofluoric acid and contains a carbonate-based solvent that when exposed to oxygen can ignite. In addition, batteries experience degradation over many cycles of fast charging which results in capacity fade and poor battery performance. However, previous studies have shown the potential for other electrolyte systems to mitigate these problems, specifically by using highly concentrated electrolytes1. These studies claim that using these electrolytes results in the formation of a solid electrolyte interphase (SEI) that passivates Li ions better than the Gen2 electrolyte. The SEI is a layer that forms on the anode within the battery and serves as a protective layer that allows for Li intercalation into graphite and can therefore protect against degradation. By using different electrolytes, a different SEI is formed, and different degradation mechanisms are observed. In this way, the SEI can be tuned to improve Li passivation and ionic conductivity by changing the electrolyte’s composition and concentration2. This research focuses on such alternative electrolyte systems and aims to characterize the different degradation mechanisms corresponding to each electrolyte. Favorable performance has been observed using 1.2 M LiFSI in 3:7 wt.% EC/EMC as an electrolyte. We are studying LiFSI in acetonitrile (AN) at higher concentrations than traditional Gen2 to quantify the SEI. We expect AN to form an anion derived SEI, a composition of which results in improved passivation of Li ions1. In this study, we characterize degradation of alternative electrolyte systems through x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Through these methods we can observe and quantify both crystalline, amorphous degradation products due to loss of Li inventory and loss of active material. XPS allows quantification of compositional changes in the SEI layer at the surface of the electrode. The SEM provides qualitative information on degradation such as visualizing lithium plating and dendrite formation. We implement quantitative XRD to measure the crystalline phases of the graphite anode, showing the degree of lithium intercalation and further quantifying degradation such as Li plating. Putting the results of these three characterization techniques together in tandem with electrochemical cycling data paints a picture of the degradation that happens on the anode of a fast-charging lithium-ion battery with respect to electrolyte composition.[1] Yamada, Y., Wang, J., Ko, S., Watanabe, E., & Yamada, A. (2019). Advances and issues in developing salt-concentrated battery electrolytes. Nature Energy. [2] Logan, E. R., & Dahn, J. R. (2020). Electrolyte Design for Fast-Charging Li-ion Batteries. Trends in Chemistry.

Interfacial analysis is critical to analyze the performance of Li-ion batteries, the prediction of reactions occurring at the interface electrode/electrolyte, the formation of the solid electrolyte interface (SEI), affecting the cycling life of the battery. These are issues that computational techniques can address prior to experiments and fabrication. We analyze interphases produced by liquid-solid and solid-solid electrolyte-anode interfaces using molecular dynamics simulations (MD);1-4 thus, experience from liquid electrolytes SEIs is extended to the analysis of alternative solid electrolytes and their SEIs with the anode. The solid electrolyte interphase formed due to a liquid-solid electrolyte-anode is modeled entirely as a LiF layer coating a LiSi nanoparticle (NP). We focus on the mechanical properties of the SEI layer during the expansion of the NP due to charging of a Li-ion battery.2 Several charging rates are tested presenting current dependent phenomena such as a surface reconstruction, amorphization and cracking of the SEI. Extrapolating C-rates, we identified an interval between cracking onset and critical cracking (SEI fully fragmented); this interval corresponds to a volume expansion from 180 to 320%, matching the experimental range of charging maximum of 2200 mAh/g and equivalent to ~200% volume expansion.5 The breaking of LiF bonds begins with those nearest to the SiLi-LiF interface, along a radial direction pointing to the outside of the NP, then forming larger and larger LiF rings until the SiLi alloy totally loses its protective shell. Due to cracking, the anode is exposed to the electrolyte allowing the formation of dendrites. Taking a sample of a cracked LiF, a MD simulation is performed to develop and test a protocol to show the formation of dendrites and what influences them, making them faster or slower to grow.1 In order to avoid, or at least reduce the effect, of the dendrites, we test solids materials as electrolyte. For the solid-solid interfaces, we analyze the ClN and Li (001) planes of the solid Li9N2Cl3 electrolyte interfacing the Li-metal (001) using ab initio MD to identify the electrochemical reactions that may occur at the formation of the interface of an initially discharged battery with or without out a charging power source.6 An atomic concentration analysis does not reveal any significant changes up to 20 ps in any of the two analyzed interfaces, meaning that there is no considerable migration of atoms from electrode to electrolyte and vice versa. Diffusion coefficient of all the atomic species involved decreases with a trend to reach a minimum value, indicating that the atoms reach a stable state, trying to avoid movement. Charge transfer on the Li sites at the interphases is observed, changing its charge from 0 to +1. On interface solid-ClN—Li-metal, N and Cl atoms form bonds with 7-8 Li around them, taking Li from the electrode to achieve a BCC structure with N and Cl as central atoms. On the solid-Li—Li-metal, one Li, originally from the electrolyte, diffuses to the electrode, however, this phenomenon has only occurred once during the whole simulation, indicating that it is possible but it is not recurrent; so at least during the equilibration, the loss of Li of the electrolyte to the electrode is not a considerable problem in these interfaces. However, other solid electrolytes react at the interface forming compounds. For example, Li5PS4Cl, Li7P2S8Br, and Li7P2S8I electrolytes, derivatives such as Li2S, Li3P and LiCl form in the Li/LPSC interface at 50 ps on each face of the solid electrolyte and from analyzing the faces of LPSC, the highest Li-metal consumption is on the (010) face. Author Contributions: D.E.G.-A analyzed the mechanical properties of the LiF solid electrolyte interface and studied the Li/Li9Na2Cl3 interface; L.A. S. performed the molecular dynamics simulation of dendrite formation; C.R.-V. did the interfacial study of Li/Li5PS4Cl2; F.F.-G. did the study of the interfacial Li/Li7P2S8I; M.G. did the study of the interfacial Li/Li7P2S8Br; and J.M.S. directed the research. Selis, L. A.; Seminario, J., RSC Advances 2018, 8 (10), 5255-5267.Galvez-Aranda, D. E.; Seminario, J. M., J. Electrochem. Soc. 2018, 165 (3), A717-A730.Galvez-Aranda, D. E.; Ponce, V.; Seminario, J. M., J. Mol. Model. 2017, 23 (4), 120.Ponce, V.; Galvez-Aranda, D. E.; Seminario, J. M., J. Phys. Chem. C 2017, 121 (23), 12959-12971.Yang, Y.; Wang, Z.; Zhou, R.; Guo, H.; Li, X., Materials Letters 2016, 184, 65-68.Roman-Vicharra, C.; Franco-Gallo, F.; Alaminsky, R.; Galvez-Aranda, D.; Balbuena, P.; Seminario, J., Crystals 2018, 8 (1), 33. Figure 1

Experimental Bayesian optimization for Li ion conductivity in NASICON-type LiZr2(PO4)3.

The transition to solid-state Li-ion batteries requires solid electrolytes with Li-ion conductivity exceeding 1 mS/cm. Solid composite electrolytes or SCEs consisting of an ionic conductor and a dielectric matrix offers an elegant strategy to enhance the ionic conductivity of electrolytes by engineering the interface conduction. At imec, we have developed a ternary nano-composite electrolyte composed of a non-volatile ionic liquid and organic Li-salt confined in mesoporous silica. The material is made by a one-step sol-gel process whereby the ionic liquid acts as template for the hydrolysis and polycondensation reaction leading to an aqueous gel. The process is similar to that for ionogels with that difference that no acid is used but water. The water and solvents are subsequently carefully removed to form a solid nano-composite electrolyte. The slow sol-gel reaction and drying allows the adsorption of an ordered molecular layer on the fully hydrolyzed silica surface. In this way, several nano-SCE with conductivities between 0.3 and 3 mS/cm have been synthesized, using TFSI-based ionic liquid electrolytes (ILE). We demonstrate that the Li-ion conductivity of nano-composites consisting of a mesoporous silica monolith with an ionic liquid electrolyte as filler can be several times higher than that of the pure ionic liquid electrolyte itself when the silica surface is appropriately hydroxylated. Interfacial ice layers induce strong adsorption and ordering of the ionic liquid molecules through H-bonding rendering it immobile and solid-like as for the interfacial ice layer itself. The dipole over the adsorbate results in solvation of the Li+ ions for enhanced conduction. We will show that the ice layer is electrochemically inactive, in contrast with water-in-salt electrolytes. Functional cells with LFP, LMO and NCA cathodes with Li, Li-alloy and LTO electrodes are demonstrated. As the ice-water layer was confirmed to be electrochemically inactive, it doesn’t cause degradation during cycling of the batteries. Furthermore, damage to the active electrode materials is avoided as our water-based sol-gel precursor does not contain corrosive acid compounds such as formic acid typically proposed as catalyst in literature. The cells are made by impregnation of the liquid sol-gel precursor solution inside the porous electrodes, very similar to how liquid cells are made. The sol-gel reaction and solidification is done in-situ inside the electrodes. By careful drying, the nano-SCE contracts the electrodes together and the electrolyte fills the spaces in the porous electrodes, providing an all around contact with the active material. As such, we have shown high capacity cells at C-rates up to 0.5C. C-rate and cycling performance of the solid-state cells with nano-SCE is shown. Figure caption: Functionality of nano-SCE as Li-ion electrolyte up to 4.3V: galvanic charge/discharge curves of Li/nano-SCE/ LiMn2O4 cell for 5 cycles at each C-rate of 1 C, 5 C, and 20 C. Figure 1