(Invited) Interfacial Engineering of Solid-State Batteries: Coupled Electrochemical, Morphological, and Mechanical Evolution

Solid-state batteries utilizing lithium metal anodes have the potential to enable batteries with a specific energy of >500 Wh/kg and an energy density of >1,500 Wh/L for thousands of cycles. When optimized they will improve the energy efficiency, operating temperature range, sustainability, and safety at a lower cost compared to projections for advanced Li-ion batteries. This improved performance is critical for the widespread adoption of electric vehicles and may enable future applications such as electric aviation. Expectations for solid-state batteries are high, there are significant materials and processing challenges that need to be overcome. Some of these challenges are well known; others are more subtle and are just becoming known. The challenges and their solutions must be clearly identified to realize high-energy solid-state Li metal batteries. In the United States, the U.S. Department of Energy (DOE) funds the majority of energy storage research and development, including solid state battery research. Web of Science publication analysis shows that worldwide DOE is second largest funder. Work on solid state batteries is distributed across different arms of DOE and is coordinated at headquarters to ensure a broad national portfolio for advanced batteries, including lithium batteries and solid-state battery devices. We note that university and national laboratory researchers in the US with a long-standing interest in solid state batteries is a small, integrated community with recognized impact for publications, patents and startup companies. The opportunity for this group to engage for research with international colleagues is growing, fostered by periodic bilateral meetings supported by DOE and by the recent joint US German research program for lithium battery interface studies. These are valuable opportunities to speed solid-state battery development. The ORNL led virtual workshop, overviewed here, confirms the community’s shared vision of the exciting advances, opportunities, and challenges. Further, we are seeing that this workshop has spawned several new informal collaborations and draft proposals. This encourages us to propose future discussion. On May 11, 2020, Oak Ridge National Laboratory (ORNL) hosted a 6-hour, on-line national workshop to discuss recent advances and most the prominent obstacles to realizing solid-state Li metal batteries. The workshop included more than 30+ experts from national laboratories, universities, and companies, all of whom have worked on solid-state batteries for multiple years. The participants shared recent advances, many not yet in print, illustrating that the community has gained significant new insights for solid-state Li battery materials over the last 5 years. In this report, the major outcomes of the workshop are organized to identify the gaps in our scientific knowledge for four core materials science areas: (1) Li metal anodes, (2) the solid electrolyte in contact with Li metal, (3) active cathode materials and solid-state composite cathodes, and (4) solid electrolytes. Illustrative examples and discussions are reported with the more comprehensive issues in the report. Discussion of additional challenges related to processing of solid-state battery materials and to the designs and architectures for mechanically robust, long-lived batteries received less attention due to time constraints. These are good themes for deeper discussion at a follow-on workshop, where a complete list of critical research topics can be identified.

The development of solid-state batteries (SSBs) with a Li metal anode (LMA) requires the exploration of various solid electrolytes (SEs), including sulfides, halides, and oxides. Oxide-based SEs, like LLZTO, allow the use of a Li metal anode (LMA) and high-voltage cathodes[1]. However, the limited compatibility of oxide electrolytes with existing cathode chemistries arises from high-temperature processes like co-sintering, which can induce degradation reactions between the materials and negatively impact performance.Recent investigations into hybrid cell concepts, utilizing dual solid electrolytes, aim to combine the benefits of different SE types. In these systems, LLZTO acts as a separator layer in contact with the LMA, while another SE is used as the catholyte mixed with the CAM. Softer SEs, such as sulfides and argyrodites, can be used to achieve the conformal contact at the heterotonic interface (SE1|SE2) as shown in previous studies.[2][3]Achieving interfacial resistance below 50 ohm.cm2 at low stack pressures (<1-2 MPa) remains crucial for the practical viability of hybrid cells[4].Herein, we investigate the hetero-ionic interface of recently discovered oxyhalide-based Li catholytes - LiNbOCl4 (LNOC), LiTaOCl4 (LTOC), and LiAlCl4-2x O x (LAOC) in contact with LLZTO in a symmetric cell configuration (i.e., oxyhalide | LLZTO | oxyhalide). Our findings highlight the importance of achieving conformal contact at the nanoscale level under practically achievable stack pressures. The charge transfer process across the hetero-ionic interface is shown to depend significantly on the passivation layer (Li2CO3/LiOH/LiCl) present on the LLZTO surface. For instance, the activation energy at the hetero-ionic interface is lower for heat-treated LLZTO compared to acid-treated LLZTO. Furthermore, the pressure-dependent interfacial resistance measurements (1-60 MPa) support the current-constriction concept.[5] The interfacial resistance of LLZTO | LAOC is lower than that of LLZTO | LNOC or LTOC, likely due to differences in the mechanical properties of the oxyhalides.This study also incorporates an in-depth investigation of such interfaces, combining techniques like impedance spectroscopy, nanoindentation, FIB-SEM, and XPS. Additionally, phase-field based computational methodology is employed to analyze the densification differences between argyrodite and oxyhalides at different fabrication pressures, based on their experimental hardness and Young’s modulus values.Our findings will be presented in this talk that emphasize the significance of softer Li-ion oxyhalide catholytes in enhancing the long-term stability of hetero-ionic interfaces under minimal stack pressures, positioning garnet | oxychloride combinations as promising candidates for the bilayer cell concept in Li metal SSBs. References : Balaish, M., Gonzalez-Rosillo, J. C., Kim, K. J., Zhu, Y., Hood, Z. D., & Rupp, J. L. (2021). Processing thin but robust electrolytes for solid-state batteries. Nature Energy, 6(3), 227-239.Eckhardt, J. K., Kremer, S., Merola, L., & Janek, J. (2024). Heteroionic Interfaces in Hybrid Solid-State Batteries─ Current Constriction at the Interface between Different Solid Electrolytes. ACS Applied Materials & Interfaces, 16(14), 18222-18235.Hüttl, J., Seidl, C., Auer, H., Nikolowski, K., Görne, A. L., Arnold, M., & Michaelis, A. (2021). Ultra-low LPS/LLZO interfacial resistance–towards stable hybrid solid-state batteries with Li-metal anodes. Energy Storage Materials, 40, 259-267.Janek, J., & Zeier, W. G. (2023). Challenges in speeding up solid-state battery development. Nature Energy, 8(3), 230-240.Merola, L., Singh, V. K., Palmer, M., Eckhardt, J. K., Benz, S. L., Fuchs, T., & Janek, J. (2024). Evaluation of Oxide| Sulfide Heteroionic Interface Stability for Developing Solid-State Batteries with a Lithium–Metal Electrode: The Case of LLZO| Li6PS5Cl and LLZO| Li7P3S11. ACS Applied Materials & Interfaces, 16(40), 54847-54863.

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.

Lithium (Li) metal anodes have become an important component of the next generation of high energy density batteries. However, the Li metal anode still has problems such as Li dendrite growth and unstable solid electrolyte interface layer. Herein, we present a functional electrolyte additive (PANHF) successfully synthesized from acrylonitrile and hexafluorobutyl methacrylate via a polymerization reaction. With extensive analytical characterization, it is found that the PANHF can improve the reversibility and Coulombic efficiency of the Li deposition/dissolution reaction and prevent the growth of Li dendrites by forming a solid electrolyte interphase rich in organic matter on the outer layer and LiF on the inner layer. The results show that the cycling performance of the Li/Li cell was greatly improved in the electrolyte containing 0.5 wt % PANHF. Specifically, the cycling stability of more than 700 cycles was achieved at a current density of 1.0 mA cm-2. Moreover, the Li/NCM811 cell with 0.5 wt % PANHF has a higher capacity of 137.7 mA h g-1 at 1.0 C and a capacity retention of 83.41% after 200 cycles. This work highlights the importance of protecting the Li metal anode with functional bipolymer additives for next-generation Li metal batteries.

Solid-state batteries could not only present a safer alternative to conventional Li-ion batteries, but also the possibility for drastic increases in energy density. Replacing flammable liquid electrolytes with a non-flammable solid electrolyte could improve safety and using metallic Li instead of state-of-the-art graphite would double the energy density. While solid electrolytes could enable advanced cathode chemistries, currently it is believed that state-of-the-art cathodes (e.g. NMC, NCA, spinel, olivine) are the most viable to achieve >1000Wh L-1, high cycle life, and cost targets. However, given that state-of-the-art cathodes are fully lithiated, any pre-deposited Li will add extraneous volume, negating the energy density benefits of the Li metal. Furthermore, due to the difficulty and cost of handling free-standing Li foils, incorporating metallic Li anodes with representative thicknesses, low interfacial resistance, and using scalable processes remains a major manufacturing challenge. In lithium phosphorus oxynitride (LiPON) solid-state systems, “Li-free” configurations have been feasibly demonstrated, where the Li metal anode is formed in operando upon charging, using the Li contained within the cathode. However, with bulk-processed electrolytes like the garnet Li7La3Zr2O12 (LLZO), in operando formation has not been feasibly demonstrated. This work evaluates the potential for “Li-free” battery manufacturing using LLZO electrolytes. It is demonstrated that Li metal anodes with thicknesses >20μm (>5mAh cm-2) can be formed in operando without any evidence of Li filament propagation, as characterized using electrochemical and materials characterization methods. Furthermore, the mechanics of Li electrodeposition at the solid-solid current-collector/electrolyte interface are investigated. Finally, the preliminary cycling performance of in operando formed Li anodes is evaluated. The results not only provide insight into the mechanisms of electrodeposition at solid-solid interfaces, but also demonstrates the feasibility of “Li-free” manufacturing of Li metal solid-state batteries.

There is a growing interest in all-solid-state lithium batteries(ASLBs) for their safety and high energy density. However, the power density for ASLBs is still far away from satisfactory on the account of high interfacial resistance at electrode / solid electrolyte interface. For example, Garnet-type solid-state electrolytes (SSEs) are considered as a good choice for solid-state batteries, yet the interfacial issues with metallic Li limit their applications. In my group, we propose an ultra-simple and effective strategy to enhance the interfacial connection between garnet SSEs and Li metal just by drawing a graphite-based soft interface with a pencil[1]. Both experimental analysis and theoretical calculations confirm that the reaction between graphite-based interfacial layer and metallic lithium forms a lithiated connection interface with good lithium-ionic and electronic conductivity. Compared to the reported interfacial materials, the graphite material provides a soft interface with better ductility and compressibility. With the improvement by this soft interface, the impedance of symmetric Li cells significantly decreases from 1350 to 105Ωcm2 and the cell can cycle for over 1000hoursat a current density of 300 μA/cm2 at room temperature. Moreover, a solid-state battery with Li-metal anode, ternary cathode NCM523 and graphite-based interface modified garnet SSEs is fabricated and displays excellent rate capability and long cycling performance. In addition, LGPS-type materials show extremely high Li+ conductivity. But when in contact to Li metal, they can be reduced to produce Li2S, Li3P and Li-M alloy and form a mixed ionic and electronic conducting layer at the interface, which hinders the application of Li metal in such solid electrolyte system. We report on a simple approach for the modification of the interface between Li and Li10SnP2S12 sulfide solid electrolyte by utilizing ionic liquid[2]. Our study shows that the addition of 1.5M LiTFSI/Pyr13TFSI ionic liquid can not only promote intimate contact but can in-situ form a stable SEI layer at the interface between Li and solid electrolyte as well. A stable SEI layer instead of mixed conducting layer can prevent sulfide solid electrolyte from further decomposition and improve the cycle performance of batteries.

Lithium (Li) metal is an ideal anode material for rechargeable batteries due to its extremely high theoretical specific capacity (3860 mAh/g), low density (0.59 g/cm3) and the lowest negative electrochemical potential (-3.04 V vs. the standard hydrogen electrode, SHE). Li battery based on Li metal anode is expected to have an order of magnitude higher energy density than today's Li-ion battery. This will enhance energy and power capability of battery devices used in portable electronics, hybrid electric cars, aerospace, telecommunication, military, etc.). The Li metal battery will fulfill the high energy demand that cannot be met through the current Li-ion batteries (e.g. all electric transportation or many other applications which require light weight, high energy density, high power density batteries with small battery footprint and safest). But, to date, uncontrolled dendritic Li growth and limited Coulombic efficiency (CE) during Li deposition/stripping of a bulk Li anode, are the main reason behind low cycle life, which has limited their practical applications. Furthermore, the rate of dendrite growth gets amplified when the cell is cycled at high charge-discharge current density, high charge voltage, and high areal discharge. Increasing anode surface area would reduce the effective charge/discharge current. Additionally, thin films and high surface area Li anodes exhibit high ionic and electronic conductivity contributing to the high efficiency of the current collection required for high capacity battery. Although low CE can be partially compensated by an excess amount of Li (at the expense of lower useable Li energy), fire and other hazards associated with the dendrite growth have limited the development of rechargeable Li metal batteries. Most approaches to dendrite prevention focus on improving the stability and uniformity of the solid-electrolyte interface (SEI) on the bulk Li surface by adjusting electrolyte components and optimizing SEI formation additives. It is very difficult to achieve sufficient passivation on Li electrode due to the thermodynamic instability of Li with electrolytes [1,2]. As an alternative, various mechanical barriers have been proposed to block dendrite penetration [3]. These approaches rely on a strong mechanical barrier provided by an SEI film or a separator to suppress Li dendrite penetration but do not change the fundamental, self-amplifying behavior of the dendrite growth. Also, it is hard to find a mechanical barrier material with required shear modulus (required > 109 Pa) with desired conductivity. In other words, these methods did not prevent Li dendrites from growing during long-term cycling at high charge-discharge current density, high charge voltage, and high areal discharge, or hardly improved the CE and safety in Li deposition/stripping which is not suitable for practical rechargeable Li batteries [1]. We have developed a novel ionic-electronic-mechanical membrane (an artificial SEI), which allows long-term Li cycleability and improved safety by reducing dendrite formation. The new artificial SEI is comprised of three different materials to fulfill the desired SEI criteria such as: (a) facilitate uniform Li ion dispersion and deposition on entire Li surface, (b) desired SEI conductivity (c) mechanical separation of Li and electrolyte and flexibility to contain anode volume change during Li stripping and deposition. The combined effect of the invented SEI result into more than 300% increase in number of cycles and 3-4 times increase in usable Li energy density (i.e. high Coulombic efficiency (CE)) of a wet battery composed of protected Li metal anode, liquid electrolyte (e.g. lithium hexafluoro arsenate, LiAsF6) and Li-ion cathode (e.g. lithium cobalt oxide, LCO) for the advancement of a battery beyond the state-of-the-art Li-ion battery. The added advantage of the invented SEI is improved safety due to suppression of dendrite growth by mechanically strong/flexible SEI and controlled, uniform deposition/stripping of Li from Li anode surface. The protected Li anode may be incorporated into wet cell batteries or solid state batteries having transition metal oxide cathodes (more specifically the state-of-the-art Li-ion cathodes) or Sulfur cathode (Li-S battery) or Oxygen cathode (Li-O2 battery). Moreover, similar methodologies can be applied to other rechargeable metal anodes (Na, Al, Mg, etc.) used in batteries.

Lithium (Li) penetration through solid electrolytes (SEs) induces short circuits in Li solid‐state batteries (SSBs), which is a critical issue that hinders the development of high energy density SSBs. While cracking in ceramic SEs has been often shown to accompany Li penetration, the interplay between Li deposition and cracking remains elusive. Here, we constructed a mesoscale SSB inside a focused ion beam‐scanning electron microscope (FIB‐SEM) for in situ observation of Li deposition‐induced cracking in SEs at nanometer resolution. Our results revealed that Li propagated predominantly along transgranular cracks in a garnet Li6.4La3Zr1.4Ta0.6O12 (LLZTO). Cracks appeared to initiate from the interior of LLZTO beneath the electrode surface and then propagated by curving toward the LLZTO surface. The resulting bowl‐shaped cracks resemble those from hydraulic fracture caused by high fluid pressure on the surface of internal cracks, suggesting that the Li deposition‐induced pressure is the major driving force of crack initiation and propagation. The high pressure generated by Li deposition is further supported by in situ observation of the flow of filled Li between the crack flanks, causing crack widening and propagation. This work unveils the dynamic interplay between Li deposition and cracking in SEs and provides insight into the mitigation of Li dendrite penetration in SSBs.

Solid-state batteries (SSBs) that use solid electrolytes instead of flammable liquid electrolytes have the potential to generate higher specific capacity and offer better safety. Magnesium (Mg) based SSBs with Mg metal anodes are considered to be one of the most promising energy storage candidates, because it gives high theoretical volumetric capacities of 3830 mAh cm-3 . Here, we demonstrate an atomic layer deposition (ALD) process with a double nitrogen plasma process that successfully produces nitrogen-incorporated magnesium phosphorus oxynitride (MgPON) solid-state electrolyte (SSE) thin films at a low deposition temperature of 125 °C. The ALD MgPON SSEs exhibit an ionic conductivity of 0.36 and 1.2 μS cm-1 at 450 and 500 °C, respectively. The proposed ALD strategy shows the ability of conformal deposition nitrogen-doped SSEs on pattered substrates and is attractive for using nitride ion-conducing films as protective or wetting interlayers in solid-state Mg and Li batteries.

Solid-state batteries are promising energy storage alternatives that can achieve high energy densities be enabling Li metal anode and high voltage cathodes. When combined with long cycle life, improved safety, and low cost (<$100/kWh) the value proposition of solid-state lithium metal battery becomes more and more relevant. There are, however, significant materials and processing challenges that disrupt the materialization of working SSBs at present. Recent reviews clearly informed that the energy and power densities of the reported SSBs fall short in comparison with the state-of-the-art Li-ion batteries. The technical barriers to be addressed in the long-term include achieving areal capacities in the range of 3-10 mAh cm-2 with less than 10% excess Li anode, and more than 70% active that material loading in solid-state composite cathodes that are assembled against thin electrolytes which can withstand current densities higher than 1 mA cm-2, and yet enables higher coulombic efficiencies. Cell design calculations show that solid electrolytes with <100 µm thicknesses and Li metal anode with <50 µm thickness are necessary to achieve energy densities comparable or higher than the state-of-the-art lithium-ion batteries. The processability and integration of thick composite cathodes, thin solid electrolytes, and thin lithium metal anode into cell configurations in scalable fashions are of utmost significance to improve the value proposition of near market SSBs.

Towards advanced lithium metal solid-state batteries: Durable and safe multilayer pouch cell enabled by a nanocomposite solid electrolyte

Solid-state batteries (SSBs) promise improved safety and energy density by using solid electrolytes (SEs) and high-voltage cathodes. However, the SEs are unstable at high voltages, leading to oxidative chemical decomposition at the SE/CAM (cathode active material) interface, resulting in the formation of an insulating cathode-electrolyte interface (CEI) on CAM particles [1]. The layered oxide cathodes (LiNixMnyCozO2; NMC), when cycled at elevated voltages (>4.3V), undergo irreversible structural and chemical deformations, including the formation of kinetically less active phases (spinel and rock-salt), oxygen evolution, and mechanical degradation (intergranular or intraparticle cracking) [2]. The instability of the SE, CAM particles, and SE/CAM interface, collectively, results in inferior electrochemical performance, including low initial Coulombic efficiency (CE), increased cell polarization and impedance, reduced rate capability, and capacity fade during extended cycling.Therefore, it is crucial to stabilize the SE/CAM interface and preserve the CAM structure in SSB cathodes, particularly during high-voltage cycling. This is primarily achieved by developing a protective coating on CAM particles. Niobium (Nb)-based coatings are promising to protect the SE/CAM interface, mostly developed by wet chemical and solid-state processing routes that involve high-temperature annealing. On the other hand, atomic layer deposition (ALD) is a powerful technique for coatings, providing precise thickness and composition control [3]. Unfortunately, these coatings may have ‘hot spots’ for local current focusing (crystalline defects, i.e., grain boundaries) and oxygen release (pinholes at particle-particle contacts) [4].In the present study, we developed an Nb-based thin conformal amorphous coating on single crystal NMC CAM particles using ALD equipped with a rotary bed attachment. Unlike traditional stationary ALD reactors, ALD with a rotary bed attachment ensures a pinhole-free continuous coating on CAM powders. The low-temperature ALD process results in an amorphous coating (chemically and structurally isotropic), which is expected to effectively withstand the volumetric changes occurring in CAM particles during repetitive charge/discharge cycling. Comparing the performance of uncoated and coated samples tested at high voltages (≥4.5V) shows that the coated cathodes display significant improvements, including high initial Coulombic efficiency (91% vs. 83%), enhanced rate capability (10x higher accessible capacity at a 2C rate), and improved capacity retention during extended cycling (99.4% after 500 cycles). These enhancements are attributed to reduced cell polarization and interfacial impedance in coated samples. Post-mortem electron microscopy and XRD analysis confirm that the coating remains intact, inhibiting oxygen release from the CAM structure and the formation of inactive phases (spinel and rock-salt), thereby preventing CAM particle cracking. These findings pave the way for stable and high-performance SSBs with high-voltage cathodes.Keywords: Solid-state batteries, composite cathodes, Nb-based coating, atomic layer deposition (ALD), high voltage stability.Reference: Xiao et al. Understanding interface stability in solid-state batteries. Nat. Rev. Mater. 5 (2020) 105.Wang et al. Single crystal cathodes enabling high-performance all-solid-state lithium-ion batteries. Energy Storage Mater. 30 (2020) 98.Koshtyal et al. Applications and Advantages of Atomic Layer Deposition for Lithium-Ion Batteries Cathodes: Review. Batteries 8 (2022) 184.Cheng et al. Materials Design Principles of Amorphous Cathode Coatings for Lithium-Ion Battery Applications. J. Mater. Chem. A 10 (2022) 22245.

Li metal is an ideal anode material for next-generation energy storage devices due to its extremely high theoretical specific capacity and the lowest negative electrochemical potential [1,2]. However, uncontrolled Li dendrite growth and the side reaction between Li metal and electrolytes have prevented its practical application in rechargeable Li metal batteries. In our contributions, artificial solid electrolyte interphase (SEI) layer and 3D current collector have been developed to address these issues [3,4]. In this presentation, we will introduce an artificial Li3PO4 SEI layer [3] fabricated by in situ reaction of polyphosphoric acid (PPA) with Li metal and its native film (Li2CO3, LiOH, and Li2O). The partial dissolution of the Li native film and formation of the SEI layer lead to a porous structure of the Li surface, thereby promoting Li dendrite formation and growth. In the ingenious design, the native film of Li metal is replaced by the stable and uniform Li3PO4 SEI layer. The artificial Li3PO4 SEI layer with thickness of 50 nm exhibits a smooth surface and a high Young’s modulus (10-11 GPa). Furthermore, the artificial Li3PO4 SEI layer is demonstrated to be stable in the electrolyte and during cycling in a Li|LiFePO4 battery system. The Li-conducting Li3PO4SEI layer can achieve uniform Li deposition, and reduce the side reaction between the Li metal and the electrolyte. Thus, the artificial SEI strategy we described is a promising route to tackle the intrinsic problems of Li metal anodes and other metal based anodes. Furthermore, we will show that Li metal anode can be accommodated in the reserved pores of the 3D current collector [4] with a submicron skeleton and high surface area, thereby suppressing the Li dendrite growth and solving the associated problems of Li metal anodes. Because of the high electroactive area of the submicron 3D structure, the 3D current collector can improve the Li plating/stripping efficiency and reduce the resistance of the Li metal anode. The utilization of the 3D architecture to accommodate the Li metal anode will highlight the importance of rational design of current collectors and reveal a new avenue for developing Li metal anodes with a long lifespan. References Y.-X. Yin, S. Xin, Y.-G. Guo, L.-J. Wan, Angew. Chem. Int. Ed. 2013, 52, 13186.W. Xu, J. L. Wang, F. Ding, X. L. Chen, E. Nasybutin, Y. H. Zhang, J. G. Zhang, Energ. Environ. Sci. 2014, 7, 513.N.-W. Li, Y.-X. Yin, C.-P. Yang, Y.-G. Guo, Adv. Mater. 2015, DOI: 10.1002/adma.201504526.C.-P. Yang, Y.-X. Yin, S.-F. Zhang, N.-W. Li, Y.-G. Guo, Nat. Commun. 2015, 6, 8058

Nanofiber-reinforced polymer electrolytes toward room temperature solid-state lithium batteries

The use of lithium (Li) metal anodes has been reconsidered because of the necessity for a higher energy density in secondary batteries. However, Li metal anodes suffer from ‘dead’ Li formation and surface deactivation which consequently form a porous layer of redundant Li aggregates. In this work, a fibrous metal felt (FMF) as a three-dimensional conductive interlayer was introduced between the separator and the Li metal anode to improve the reversibility of the Li metal anode. The FMF can facilitate charge transfer in the porous layer, rendering it electrochemically more active. In addition, the FMF acted as a robust scaffold to accommodate Li deposits compactly in its interstitial sites. The FMF-integrated Li metal (FMF/Li) electrode operated with a small polarisation even at a current density of 10 mA cm−2, and it exhibited a seven times longer cycle-life than that of an FMF-free Li electrode in a symmetric cell configuration. A Li metal battery (LMB) using the FMF/Li electrode and a LiFePO4 electrode exhibited a two-fold increase in cycling stability compared with that of a bare Li metal electrode, demonstrating the practical effectiveness of this approach for high performance LMBs.