Improved Cycle Life Research Articles

Rational Design of Protective Film for Long-Life and Stable Lithium Metal Anode Via Molecular Layer Deposition

It is urgent to develop efficient energy storage system for the increasing energy demands of human society. Batteries with high energy density is one of the most promising solutions. Metallic Li anode is believed to be the most promising electrode candidate for high-energy-density batteries due to its ultrahigh capacity and low electrochemical potential [1]. However, it has still not been successfully commercialized because of several great challenges, including poor safety and dendrite growth in plating/stripping process. Highly reactive nature of Li metal anode towards organic electrolyte leads to severe side reactions and prompt the formation of solid electrolyte interphase (SEI). Although SEI can passivate the Li surface and serve as a protective layer on the electrode, it cannot prevent the dendrite growth due to its heterogeneous and unstable properties [2]. In recent years, there are large amount of reported works on stabilizing SEI for Li metal anode via in-situ or ex-situ approach. In-situ approach is realized by optimization of electrolyte using electrolyte additives or changing concentration of Li salt, while ex-situ approach is achieved by building protective layer, such as metal oxides, lithium salts, and organic molecules on Li before electrochemical cycling [3]. However, up to now it still remains very challenging to achieve conformal, stable, and ultrathin layer as ideal SEI for long-term electrochemical cycling.Our group has been dedicated to build effective protective films for high performance Li metal anodes using Atomic layer deposition (ALD) or Molecular layer deposition (MLD). ALD is a widely used coating technique for the deposition of inorganic materials with great advantages including conformal coverage and accurate control over thickness in nanoscale. MLD can be employed to produce inorganic-organic hybrid or pure polymer coatings as an analogue technique of ALD. Therefore, MLD maintained the advantages of ALD and provides more advantages such as tunable composition and improved mechanical properties. Previously reported works from our group includes inorganic metal oxide and inorganic-organic metalcones synthesized by ALD/MLD for improved stability of Li metal anodes [4-5]. In this talk, we report a novel pure polymer film of MLD Polyurea (PU) as protective layer for Li-metal anodes for the first time [6]. The depth profile of PU layer on Li was illustrated by TOF-SIMS and the detailed surface chemistry was investigated by XPS for both the anodes before and after electrochemical cycling. Compared to bare Li, the polyurea coated Li shows highly improved cycle life and stability. Our results showed this electrically nonconductive PU film can effectively suppress the dendrite growth as a protective barrier and remain stable upon the repeated Li plating/stripping process. Meanwhile, the nitrogen-containing polar groups in PU can effectively regulate the Li-ion flux and lead to a uniform Li deposition. Owing to these advantages, the Li metal anode coated with PU layer enables greatly prolonged lifetime in the symmetric cells at different current densities and capacities. The full cells were tested using LiFePO4 (LFP) as the cathode and showed improved capacity retention and rate performance. Therefore, this work sheds new light on the design of protective layer for Li-metal anodes for high-energy-density next-generation batteries.[1] D. Lin, Y. Cui, et al, Nature Nanotechnology , 2017, 12, 194-206[2] X. Cheng, Q. Zhang, et al, Chemical Reviews, 2017, 117, 10403-10473[3] X. Cheng, Q. Zhang, et al, Adv. Sci., 2016, 3, 1500213[4] Y. Zhao, X. Sun et al, Small Methods, 2018, 2, 1700417[5] K. R. Adair, X. Sun et al, Angew. Chem. Int. Ed., 2019, 58, 15797.[6] Y. Sun, X. Sun, et al, Advanced Materials , 2019, 31, 1806541 Figure 1

Electrochemical Preparation of Si Based Anode Material in Molten CaCl2

Graphite, which is commercially used as anode material in lithium-ion battery (LIB), has a low theoretical capacity of 372 mAh/g, and therefore should be replaced by alternative material with high capacity for the toward vehicles and other applications. Not only that, the cost and environmental issues should be considered in the production of alternative materials. In this regard, silicon materials have received the extensive research interest due to their high specific capacity of 3579 mAh/g, a low lithiation reaction voltage plateau (vs. Li/Li+) and abundant sources. Unfortunately, Si electrode typically suffers from irreversible capacity fading during cycling from their volume expansion and low intrinsic electronic conductivity. Recently, development of nanostructural design with various morphologies, Si nanowires, Si nanoparticles, and porous structures has received attention as a promising way to solve these problems. Nanostructural design largely improve cycle life by providing a sufficient free volume to accommodate silicon electrode expansion. However, commercially utilization of Si electrode is still insurmountable owing to some critical issues such as high costs, poor scalability and hazardous precursors. Furthermore, nanostructure Si includes severe side reactions due to a large surface area, low tap density. Here, we simply use the SiO2+Graphite mixture as the starting material and synthesize Si@SiC@Graphite composite material through electro-deoxidation method applied with the FFC Cambridge process, a simple approach for the high-purity extraction of metals from solid oxides via molten salt electrolysis that is known to be energy-saving and environmentally friendly. The electro-deoxidation was carried out in a molten CaCl2 at 1123 K in cell voltage of 2.6 V and 2.9 V. As an anode material, the prepared Si@SiC@graphite composite resolves all of the aforementioned critical issues and shows 900 mAh/g after 300 cycles without capacity fading. Furthermore, we fabricated high tap-density Si@SiC@graphite powder (~ 0.76 g/ml) using a mechanical pressing method, resulting in a high capacity (> 3.0 mAh/cm2) at the large loading of cathode, verifying the successful design of stable electron transfer and high dense structure.

Significantly stable organic cathode for lithium-ion battery based on nanoconfined poly(anthraquinonyl sulfide)@MOF-derived microporous carbon

The organic materials are auspicious alternative to the inorganic counterparts as cathode materials for lithium-ion batteries (LIBs) owing to their unusual capacity and sustainability. However, the widespread application of organic cathode in most of the cases have been hampered by their intrinsically easy solubility in electrolyte and natural insulating property, resulting in the poor practical capacity and low rate capacity. Herein, we demonstrate the rational ‘ship in bottle’ strategy to overcome these problems by confining the organic materials into microporous host. Poly(anthraquinonyl sulfide) (PAQS) was successfully polymerized inside a MOF-derived 3D microporous carbon (3D-C) to obtain PAQS@3D-C via ‘ship in bottle’ strategy. The novel organic cathode for LIBs shows a higher reversible specific capacity of 219 mAh g−1 at 0.2 C and greatly improved cycle life with low capacity fading speed of ∼0.015% per cycle. Specifically, PAQS@3D-C shows a remarkable high-rate capacity of 112 mAh g−1 even at 20 C. The superior electrochemical performance is mainly ascribed to the novel structure via ‘ship in bottle’ strategy. This unveils efficient pathways for the development of new generation super-stable organic cathode materials with excellent battery performance.

A Comparison of the Performance of Different Morphologies of LiNi0.8Mn0.1Co0.1O2 Using Isothermal Microcalorimetry, Ultra-High Precision Coulometry, and Long-Term Cycling

Ni-rich positive electrode materials for Li-ion batteries have the dual benefit of achieving high energy density while reducing the amount of Co used in cells. However, limitations in cycle life are still an issue for the widespread adoption of these materials. The benefit of using single crystal materials has been demonstrated for LiNi0.5Mn0.3Co0.2O2 (NMC532), LiNi0.6Mn0.2Co0.2O2 (NMC622), and now LiNi0.8Mn0.1Co0.1O2 (NMC811). This work uses long-term cycling, ultra-high precision coulometry (UHPC), and isothermal microcalorimetry to investigate the effect of particle morphology on the lifetime of NMC811/graphite pouch cells. NMC811 with uncoated single crystal (SC) particles, coated polycrystalline (PC) particles, and a composite “bimodal” (BM) material are studied with electrolyte systems that have shown excellent cycle life in other NMC materials. Results from this work show that SC cells have improved cycle life in long-term cycling, as well as higher coulombic efficiency (CE) and lower charge endpoint capacity slippage as seen in the UHPC measurements. This correlates well with the isothermal microcalorimetry results, in which SC cells show the lowest parasitic heat flow over a range of upper-cutoff voltages. This study suggests excellent lifetimes can be achieved in single crystal NMC811/graphite cells with further electrolyte optimization.

Initiated Chemical Vapor Deposition of Ultrathin Polymer Coatings at Graphite Electrodes for Enhanced Performance in Li-Ion Batteries

The solid-electrolyte interphases (SEIs) that form in situ at Li-ion battery electrodes play a critical role in determining cell- and system-level characteristics, but may not contribute optimized performance. Power, cycle life, and safety are improved when the electrodes are deliberately modified with nanoscale, Li+-conducting coatings. Herein, we use initiated chemical vapor deposition (iCVD) to apply ultrathin (<40 nm) polymer coatings, based on the monomer, 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (V3D3), at the surfaces of powder-composite graphite electrodes. The conformal nature of the polymer deposition is verified by scanning electron microscopy (SEM), while X-ray photoelectron spectroscopy (XPS) confirms the expected chemical structure and nanoscale nature of the polymer. Poly(V3D3)-coated graphite electrodes are electrochemically evaluated in coin cells that include conventional liquid Li-ion electrolytes and LiCoO2-based cathodes. Galvanostatic tests on these cells demonstrate significant improvements in specific power, coulombic efficiency, cycle life, and tolerance to overcharge conditions vs equivalent cells containing unmodified graphite anodes, while impedance analysis shows reduced charge-transfer resistance and more consistent cell-to-cell behavior with polymer-coated graphite anodes. Finally, we use XPS with depth profiling to demonstrate that some conventional SEI products do form within/beneath the poly(V3D3) coating after extensive cycling, but in speciations such as LiF, that are favorable for electrochemical performance.

Communication—Pressure Evolution in Constrained Rechargeable Lithium-metal Pouch Cells

Understanding the role of pressure on improving cycle life for high energy rechargeable lithium metal batteries (LMBs) is crucial for their adoption in practical applications. In this work, a fixed-gap test fixture is developed to monitor stack pressure of a multi-layer, 300 Wh kg−1 LMB over 245 cycles. During early cycling, a linear fade in discharge capacity is mirrored by a steady increase in baseline pressure. In later cycles, cell failure is linked to a large increase in baseline pressure due to irreversible side reactions such as continued SEI formation and gas evolution, as corroborated by incremental capacity analysis.

Coupling of a conductive Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 metal-organic framework with silicon nanoparticles for use in high-capacity lithium-ion batteries.

A composite of Si nanoparticles (SiNPs) and a two-dimensional (2D) porous conductive Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 (Ni3(HITP)2) metal-organic framework (MOF), namely Si/Ni3(HITP)2, is suggested as a potential anode material for Li-ion batteries (LIBs). The Ni3(HITP)2 MOF with a carbon backbone and evenly dispersed Ni and N heteroatoms showed high potential for mitigating the volume expansion of Si and enhancing the electronic conductivity as well as Li storage ability of the Si/Ni3(HITP)2 anode. The Si/Ni3(HITP)2 electrode delivered a reversible capacity of 2657 mA h g-1 after 100 cycles of discharge-charge at a rate of 0.1C. Moreover, at a high rate of 1C, the Si/Ni3(HITP)2 electrode maintained a reversible capacity of 876 mA h g-1 even after 1000 cycles. The different rate capacities were 1655, 1129, and 721 mA h g-1 at 5C, 10C and 20C, respectively. The excellent electrochemical performance of the Si/Ni3(HITP)2 electrode in terms of improved cycle life and rate capability results from the open channels of the MOF network, which are beneficial for the movement of Li+ ions through the electrolyte to the electrode and the mitigation of stress by volume expansion of Si. We believe that the coupling of conductive Ni3(HITP)2 with Si is a potential way to make an anode for high-performance LIBs.

Designing Highly Conductive Functional Groups Improving Guest-Host Interactions in Li/S Batteries.

Li-sulfur batteries are of great interest due to their potential to surpass the energy densities of other battery types, but the low electronic conductivity of both sulfur and its discharge products requires the use of a conductive host material. The most common is the use of different porous carbons which normally are hydrophobic and hardly retain the polar discharge products of the Li/S reaction, such as Li2 S and lithium polysulfides (LiPs), at the working electrode. Functionalized hosts have been proposed as a strategy to improve LiPs interactions, including the use of heteroatom doping, organic frameworks, metals, metal oxides, sulfide particles, and conductive polymers. Despite demonstrating an improved cycle life, the functionalized structures often have an intrinsic limitation related to a low electronic conductivity resulting in slow kinetics and poor rate capability of Li/S cells. Herein, recent research trends aimed at designing sulfur electrodes with highly conductive functional groups on nanostructured hosts surface are reviewed. The main concepts, key developments, and parameters for building 3D hosts architectures that enable fast charge rates and long cycle life at high sulfur loadings are discussed.

Scalable synthesis of lotus-seed-pod-like Si/SiOx@CNF: Applications in freestanding electrode and flexible full lithium-ion batteries

Intensive attempts have been devoted to solving the inferior cycling stability of Si-based electrode induced by the large volume change of Si. However, the complex synthesis procedures make many strategies much low practical significances. Together with the inferior cycling stability, an easy and scalable fabrication strategy is still a great challenge for implementing Si anode in commercial batteries. This work uses a simple water steam selective etching method to simultaneously engineer the pores and the confinement of commercial Si/SiOx in carbon paper electrodes, leading to a significant improvement in electrode flexibility and cycle life. The as-prepared freestanding lotus-seed-pod-like steam-etched Si/SiOx@CNF electrode shows a high capacity retention of 137% after 1000 cycles at 3 A g−1. It also possesses outstanding electrochemical performance in a flexible lithium-ion full battery with LiCoO2/steam-etched CNF as the cathode, even under bended condition. This simple approach may offer a pathway for the application of Si-based anode in commercialization and/or flexible energy storage devices.

Hierarchical Engineering of Porous P2‐Na2/3Ni1/3Mn2/3O2 Nanofibers Assembled by Nanoparticles Enables Superior Sodium‐Ion Storage Cathodes

AbstractLayered transition metal oxides (TMOs) are appealing cathode candidates for sodium‐ion batteries (SIBs) by virtue of their facile 2D Na+ diffusion paths and high theoretical capacities but suffer from poor cycling stability. Herein, taking P2‐type Na2/3Ni1/3Mn2/3O2 as an example, it is demonstrated that the hierarchical engineering of porous nanofibers assembled by nanoparticles can effectively boost the reaction kinetics and stabilize the structure. The P2‐Na2/3Ni1/3Mn2/3O2 nanofibers exhibit exceptional rate capability (166.7 mA h g−1 at 0.1 C with 73.4 mA h g−1 at 20 C) and significantly improved cycle life (≈81% capacity retention after 500 cycles) as cathode materials for SIBs. The highly reversible structure evolution and Ni/Mn valence change during sodium insertion/extraction are verified by in operando X‐ray diffraction and ex situ X‐ray photoelectron spectroscopy, respectively. The facilitated electrode process kinetics are demonstrated by an additional study using the electrochemical measurements and density functional theory computations. More impressively, the prototype Na‐ion full battery built with a Na2/3Ni1/3Mn2/3O2 nanofibers cathode and hard carbon anode delivers a promising energy density of 212.5 Wh kg−1. The concept of designing a fibrous framework composed of small nanograins offers a new and generally applicable strategy for enhancing the Na‐storage performance of layered TMO cathode materials.

Bipolymer-Cross-Linked Binder to Improve the Reversibility and Kinetics of Sodiation and Desodiation of Antimony for Sodium-Ion Batteries.

Although the volume of antimony tremendously expands during the alloying reaction with sodium, it is considered a promising anode material for sodium-ion batteries (SIBs). Repeated volume changes along the sodiation/desodiation cycles encourage capacity fading by triggering pulverization accompanying electrolyte decomposition. Additionally, the low cation transference number of sodium ions is another hindrance for application in SIBs. In this work, a binder was designed for the antimony in SIB cells to ensure bifunctionality and improve (1) the mechanical toughness to suppress the serious volume change and (2) the transference number of sodium ions. A cross-linked composite of poly(acrylic acid) and cyanoethyl pullulan (pullulan-CN) was presented as the binder. The polysaccharide backbone of pullulan-CN was responsible for the mechanical toughness, while the cyanoethyl groups of pullulan-CN improved the lithium-cation transfer. The antimony-based SIB cells using the composite binder showed improved cycle life with enhanced kinetics. The capacity was maintained at 76% of the initial value at the 200th cycle of 1C discharge following 1C charge, while the capacity at 20C was 61% of the capacity at 0.2C, implying that the composite binder significantly improved the sodiation/desodiation reversibility of antimony.

Additives for Cycle Life Improvement of High‐Voltage LNMO‐Based Li‐Ion Cells

AbstractIn this empirical study, five electrolyte additives, namely, lithium bis(oxalato) borate, lithium difluoro(oxalato) borate, 1‐vinyl‐1,2,4‐triazole, 1‐vinyl imidazole and dimethyl‐2,5‐dioxahexane dioate are described and compared regarding their effect in LNMO//graphite cells. The additives were selected from a preliminary study of 59 potential additives. The basis electrolyte mixture is DMC/EC+1 M LiPF6 (LP30) for all cells. All additives are able to enhance the cycle life at room temperature and at elevated temperatures (50 °C) significantly compared to the non‐additive electrolyte blend (basis electrolyte). The cell enhancement is discussed based on the solid‐electrolyte interface (SEI) and metal‐ion content on anode side. The aim of the study is to suggest promising already known as well as new additives that are able to overcome the issue of rapid capacity fading of LNMO‐based cells when the cells are cycled up to 4.8–5.0 V vs. Li/Li+, especially at higher temperatures.

Trifluoropropylene Carbonate‐Driven Interface Regulation Enabling Greatly Enhanced Lithium Storage Durability of Silicon‐Based Anodes

AbstractThe extremely high specific capacity of Si anodes is a double‐edged sword, bringing both high energy density and poor lifespan to Li‐ion batteries (LIBs). Despite recent advances in constructing nanostructured/composite‐Si anodes with an alleviated volume change and improved cycle life, daunting challenges still remain for Si anodes to suppress the irreversible capacity loss associated with the repeated rupture/reconstruction of the solid electrolyte interphase (SEI) layer. Herein, an electrolyte‐based optimization strategy is devised to in situ construct a thin, continuous, and mechanically stable SEI film on Si surface by using a trifluoropropylene carbonate (TFPC) cosolvent, targeting highly stable Si‐based anodes for LIBs. TFPC is featured with its low unoccupied molecular orbital energy, high reduction potential and outstanding film‐forming capability, outperforming those of the state‐of‐the‐art fluoroethylene carbonate additive. More importantly, TFPC plays a key role in regulating the structure and component of SEI layer. As such, 10 wt% TFPC addition promotes the formation of an optimal SEI film with appropriate amounts of polyolefins and LiF, endowing the SEI layer with enhanced rigidity and toughness as well as high ionic conductivity. Both the Si nanoparticle‐based and Si/C composite electrodes deliver a greatly enhanced cycling stability, rate capability, and overall structural integrity in such optimized electrolyte.

Garnet-Based Flexible Composite Sheet Electrolyte

Li-ion batteries are widely used in portable electronics, such as laptops and smartphones, since their first commercialization by Sony in 1991.1 However, Li-ion batteries are approaching the theoretical limit because of the relatively low theoretical specific capacity of the graphite anode material (372 mA h g-1). Larger scale applications, such as electric vehicles (EVs) with a range of 800 km, require substantial improvements in capacity, and safety of the state-of-the-art Li-ion battery technology. Recently, Li metal batteries have regained tremendous interest as promising candidates for energy-storage devices, because the metallic Li is an ideal anode material with a high theoretical specific capacity (3860 mA h g-1), and the lowest negative electrode potential (-3.04 V vs. Standard hydrogen electrode (SHE)).2 Solid state electrolytes could not only enable stable cycling of metallic Li anode, but could also reduce the risk of fires and explosions of combustible organic liquid electrolytes. Garnet-type Li7La3Zr2O12 (LLZO) has been widely regarded as the most promising solid oxide electrolyte because it has a high ionic conductivity (of the order of 10-4 S cm-1 25 ºC) and can also act as a physical barrier to prevent Li dendrite penetration.3 However, the high resistance between LLZO and electrode materials greatly limits the rate performance of all-solid-state batteries with LLZO electrolytes and Li metal anodes.4 Moreover, high-temperature sintering for large scale production of thin ceramic electrolytes is impractical, and the device integration of brittle ceramic electrolytes would also be difficult. Here, we report a room-temperature synthesis of a flexible composite Al-LLZO sheet electrolyte by tape-casting and ionic liquid (IL-impregnation. The IL was LiFSI/EMI-FSI, lithium dissolved in 1-ethyl-3- (EMI-FSI). The sheet electrolyte was highly flexible, mechanically robust, and highly conductive (as high as 6.5 ×10-4 S cm-1 25 ℃, higher than all those of sintered dense Al-LLZO pellets and LLZO/PEO flexible sheet electrolytes reported to date). The non-flammable and non-volatile IL impregnated in the sheet electrolyte (about 12 %) can not only increase the ionic conductivity of the sheet electrolyte, but can also effectively reduce the resistance. The synthesis requires no high-temperature sintering, and is easy to carry out and scale up, and would be beneficial to the development of practical Li metal batteries. Reference Yoshino, A., The Birth of the Lithium‐Ion Battery. Angewandte Chemie International Edition 2012, 51 24), 5798-5800., E., Wood, K. N., Dasgupta, N. P., Improved cycle life and stability of lithium metal anodes through atomic layer deposition surface treatments. Chemistry of Materials 2015, 27 18), 6457-6462.Cheng, E. J. A., Sakamoto, J., Intergranular Li metal propagation through Li6.25Al0.25La3Zr2O12 . Electrochimica Acta 2017, 223 , 85-91.Kotobuki, M., Munakata, H., Kanamura, K., Sato, Y., Yoshida, T., Compatibility of Li7La3Zr2O12 to all-solid-state battery using Li metal anode. Journal of The Electrochemical Society 2010, 157 (10), A1076-A1079.

Next Generation Materials for Lithium-Ion Space Batteries

EaglePicher has been supplying batteries for energy storage for satellites and other space applications since 1958. These batteries have achieved over 2.6 billion hours of cell operations with no failures in orbit. Over the decades the battery chemistry has changed amongst various chemistries with some overlap and some still being used today. Typical space battery chemistries include: silver-zinc, nickel-hydrogen, nickel-cadmium, and then lithium-ion. Even within the lithium-ion chemistry the formulations and packaging has evolved over time and continues to change. Lithium-ion space batteries continue to evolve today. New satellites and space applications are demanding increased energy, more power, and smaller volumes. This has led to improvements in increased energy, wider operating temperatures, more power and smaller packages. Every change in battery performance requires new or modified battery materials. For increased energy the incumbent cathode material for lithium-ion batteries is Lithiated Nickel Cobalt Oxide (NCO). This was changed to the Lithiated Nickel Cobalt Aluminum Oxide to increase the energy and improve cycle life. For wider operating temperatures the electrolyte formulation has evolved with new components and additives. Newer and next generation satellites are carrying state of the art instruments that demand high power or pulses of power. New lithium-ion battery chemistries have been developed to supply very high power discharges to supply the large currents. As small satellites are increasing in popularity, there is a push for smaller battery packs. This has led to a decrease in cell capacity and the introduction of commercial of the shelf (COTS) cells. These are often cylindrical in shape and typically 18650 in size. Transitioning to COTS cells has increased the ability to create modular space batteries, but has brought up new concerns on quality and reliability. Quality and reliability, plus safety, are top concerns for space batteries. Replacement of batteries in space is extremely difficult and expensive. As each of the next generation battery materials are developed, reliability and safety much be eminent. These materials and batteries are often expected to supply power for decades. The continued testing, evaluation and analysis of the new chemistries and designs provides assurances for long-term performance and reliability. As these batteries have the potential to power satellites, landers, rovers, capsules, and habitats, EaglePicher continues to develop, evolve, demonstrate, and produce new battery chemistries and designs to meet the mission requirements. Acknowledgements We would like to thank: NASA JPL, Lockheed Martin and the Unites States Government for supporting some of this effort, and many of the EaglePicher employees who worked on these projects.

Mechanofusion-Derived Si Alloy-Graphite Composite Electrode Material for Li-Ion Battery

Silicon-graphite composites are extremely promising negative electrode materials for Li-ion batteries. However, simple and effective methods to synthesize silicon-graphite composites with engineered structures are needed to realize this technology in practical applications. Here, we show that the environmentally friendly and low-cost dry mechanofusion (MF) process can effectively synthesize silicon alloy-graphite composites, in which silicon alloy particles can be well dispersed and embedded between flake graphite layers, as shown in Figure 1(a)-(b). This results in increased tap density and reduced surface area. The special structure provides a way to buffer volume expansion and contraction of the silicon alloy during lithiation and delithiation. As a result of this hierarchical arrangement, superior cyclability and rate capability are achieved compared to simple mixtures, with capacities of 950 mAh/g (i.e. 1473 Ah/L) and 900 mAh/g (i.e. 1432 Ah/L) at 2C and 4C, respectively (Figure 1(c)). Moreover, using this process, micron-sized silicon alloy particles can be well embedded within spherical natural graphite particles, as shown in Figure 1(d)-(e). The specially engineered structure created by the MF dry process leaves room for expansion and contraction of the silicon alloy during lithiation and delithiation process. In addition, the silicon alloy particles are protected within the graphite particle, which could be helpful to avoid excessive electrolyte exposure and improve cycle life. Figure 1

(Invited) Keeping It Simple: Anode Free Lithium Metal Cells with Liquid Electrolytes As a Path to Higher Energy Density

In the pursuit of higher energy density, both academia and industry are intensely researching many alternative rechargeable battery concepts beyond conventional lithium-ion cells. The most promising alternatives will be those that can achieve cell level energy density larger than that of state-of-the-art lithium ion batteries at a comparable or lower cost. Based on these criteria, anode free lithium metal cells emerge as potentially useful lithium-ion alternatives as their simple cell construction can enable both high energy density and low cost assembly. In anode free cells, the negative electrode begins as a bare copper current collector and the lithium metal anode is plated in-situ from the lithium inventory in the cathode. These cells can achieve energy density 40-50% greater than lithium-ion at the cell level and have a simpler cell construction that eliminates the need to handle lithium metal foils or coat active materials on the negative current collector. However, the cycling stability of these cells in conventional carbonate liquid electrolytes is often terrible due to the low CE of lithium metal and the lack of an excess lithium reservoir. In this way, anode free cells exaggerate the technical challenges of lithium metal which makes them excellent test vehicles for optimizing lithium metal cycling. In this presentation we will highlight our progress in improving lithium metal cycling efficiency in liquid electrolytes toward the development of safe, practical, high energy density anode free cells. Much of this work focuses on the development and characterization of anode free LiNi0.5Mn0.3Co0.2O2 (NMC 532)//Cu wound pouch cells. We demonstrate that the cycling stability and safety of these cells is directly correlated to the morphology of the lithium plated during charge. High surface area mossy lithium deposits in conventional liquid electrolytes lead to fast capacity fade and high anode reactivity. We will also show how many different factors including, cycling rate, charge protocol, applied pressure, temperature, and electrolyte composition affect lithium morphology. Our latest results will be highlighted which show that with the appropriate electrolyte chemistry and cycling conditions, smooth lithium morphology with densely packed columns 50 um in diameter can be achieved after cycling in liquid electrolytes. This excellent lithium morphology significantly improves the capacity retention (Figure 1) of anode free cells and limits the reactivity of the deposited lithium. Cell failure mechanisms will also be addressed with insight as to how these failures can be overcome to further improve cycle life. These results demonstrates that stable cycling of dendrite free lithium metal is possible in simple anode free cells with liquid electrolytes which represents a promising and practical avenue for the design of high energy density batteries. Figure 1

(Invited) Understanding Role of Li Substitution at Alkali-Site of NaX[Fe0.5Mn0.5]O2 As Cathode for Sodium Ion Batteries

Sodium ion batteries(SIB) have gained significant attention as next generation of rechargeable batteries based on natural abundance and low cost. Transition metal oxides with layered structure is well known electrode material that have been successfully commercialized in LIB system and have been reported to be one of the promising electrode material for SIBs. Extensive studies have been reported about substitution of various foreign element in the layered structure. Substitution of foreign element could be achieved by substitution at either transition metal site or alkali site. Many studies focused on the composition and substitution at the transition metal site of the layered structure. Here, this study focused on the substitution at the alkali site of layered structure as cathode materials for SIBs. Substitution at alkali site could not only affect the properties of layered structure, but also interfere with intercalation of sodium. In this study, Li was substituted on alkali site of NaXFe0.5Mn0.5O2 as cathode material for SIBs. Although Li have been reported to be substituted at transition metal site in previous literature, Li substitution at alkali site was achieved by varying the ratio between Fe and Mn. Rietveld refinement synchrotron XRD and neutron diffraction patterns, XANES analysis, and ICP analysis were conducted in order to confirm the substitution of Li at alkali site. When Li was substituted at alkali site, it showed improved cycle life and rate capability in spite of decreased interlayer spacing. In situ XRD analysis and DFT calculation was conducted in order to understand the role of Li in structure stability and Na intercalation behavior. The result showed that Li substitution at alkali site stabilized layered structure during charge/discharge process and that Li lowered the energy barrier of Na hopping, leading to improved cycle life and rate capability.

Lithium-Ion Exchange Polymer Membranes Limit Migration of Dissolved Polysulfide Species in Li-S Batteries

Conventional lithium-ion (Li-ion) batteries dominate development efforts in electrical vehicles due to their high open circuit voltage, desirable power density, and long cycle life. Despite these advantages, state-of-the-art Li-ion batteries barely meet the energy requirement of EV-based power trains due low energy density (180-200 Wh/kg cell) and high cost. Lithium sulfur (Li-S) batteries are promising for EV applications because they address both these barriers. Li-S batteries exploit a redox couple described by the reaction below, which has an open circuit potential of 2.2 V with respect to Li+/Li: 16 Li+ + S8 + 16e-→ 8 Li2S Assuming full conversion, the theoretical energy density and specific capacity of Li-S batteries is 2500 Wh/kg S and 1675 mAh/g S, respectively.1 In addition, sulfur, the key active cathode material is significantly less expensive than conventional transition metal oxide cathodes used in Li-ion batteries. However, Li-S batteries have not gained acceptance for practical applications because of their poor cycle life. This limitation originates from polysulfide diffusion from the cathode through the electrolyte to the anode which causes parasitic side-reactions and limits capacity and cycle retention. Our work aims to overcome the challenges of polysulfide diffusion by incorporating lithium-ion exchange (LIE) ionomers as polysulfide blocking barriers in Li-S cells. When exposed to electrolyte solvent, these LIE membranes provide a physical barrier that limits polysulfide diffusion, while also delivering high Li+ ion conductivity through swelling in electrolyte solvent. The LIE ionomers can be applied in a variety of ways including as: binders in the electrode formulation, coatings applied directly to the electrode surface, and as freestanding polymer electrolyte membranes. We have also developed a porous, dimensionally stable membrane (DSM) which can be impregnated with LIE ionomers to improve mechanically stability by limiting in-plane (X-Y direction) swelling. This general approach of incorporating LIE ionomers into Li-S cells has achieved improvements in discharge capacity, cycle life, coulombic efficiency, and energy density.

Atomic Layer Deposition of Ultrathin Glassy Lithium Borate-Carbonate Solid Electrolytes

Solid state electrolytes could enable significant improvements in energy density, cycle life, and safety of next-generation battery chemistries. The ability to fabricate thin electrolyte films with high ionic conductivity and excellent stability on complex architectures has been a bottleneck to realizing a wide range of 3D structured thin film and bulk batteries. Recent progress in ALD for solid state electrolytes has shown great promise, and enabling even higher conductivities and stability with Li metal anodes could dramatically enhance the energy and power density of ALD-based batteries. In addition, ALD films with high conductivity and good stability could be used for interfacial engineering of bulk-type solid and liquid based batteries to improve stability and safety. This work demonstrates a novel ALD process for glassy lithium borate-carbonate thin films with ionic conductivities above 10-6 S/cm at 298K. This represents a 6x improvement over the previous best reported value for an ALD solid electrolyte.1 The composition, structure, and stability of the films are characterized with X-ray photoelectron spectroscopy and a range of electrochemical measurements. These experiments are compared with those calculated with Density Functional Theory and Molecular Dynamics to elucidate the origins of the high ionic conductivity and excellent stability. The properties are studied as a function of deposition temperature showing tradeoffs between process conditions and performance, and demonstrating the precise control afforded by the ALD process.2 The optimized film remains an ionic conductor when in contact with metallic Li, with no measurable changes even over several weeks, and displays stable cycling when paired with a thin-film cathode and Li metal anode. References (1) Pearse, A. J.; Schmitt, T. E.; Fuller, E. J.; El-Gabaly, F.; Lin, C. F.; Gerasopoulos, K.; Kozen, A. C.; Talin, A. A.; Rubloff, G.; Gregorczyk, K. E.; Chem. Mater. 2017, 29, 3740–3753. (2) Kazyak, E.; Chen, K. H.; Davis, A. L.; Yu, S.; Sanchez, A. J.; Lasso, J.; Bielinski, A. R.; Thompson, T.; Sakamoto, J.; Siegel, D. J.; Dasgupta, N. P.; J. Mater. Chem. A 2018, 6, 19425–19437.