Tuning the Interfacial Chemistry for Stable and High Energy Density Aqueous Sodium-Ion/Sulfur Batteries

With increasing energy storage demand, research on high energy density and stable battery became essential. Among different anode materials for lithium batteries, lithium metal is an ideal anode material as it has low redox potential and high specific capacity. Therefore, for post-lithium ion battery with high energy density cannot avoid using lithium metal as an anode. However, lithium metal anode has stability and safety issues due to dendritic growth. Lithium metal in contact with organic electrolyte reacts with the electrolyte to form solid electrolyte interface (SEI). SEI prevents further electrolyte consumption, however presence of unstable SEI causes uneven lithium ion diffusion through the SEI layer and induces lithium dendrite growth. Therefore, uniform deposition of lithium and stable SEI is important to operate lithium metal anode safely.The application of nitrate additives in carbonate electrolyte has been very limited due to poor solubility. However, nitrate containing polymer interlayer can release additive constantly enabling nitrate act as an electrolyte additive. Herein, AgNO3 synthesized with PAN nanofibers (AgPAN) is used as an additive to induce uniform lithium deposition and stable SEI formation. In the symmetric cell test, life time of 20 µm thick lithium foil enhanced from 140 hr to 300 hr with AgPAN. Lithium nucleation overpotential disappeared and overall overpotential is reduced. Originally, plane lithium foil had the sparsely deposited dendrite shaped lithium, but with AgPAN lithium was evenly deposited and grow in spherical shape. Ag+ reduces on lithium metal surface acting as a lithium nucleation seed helping uniform lithium deposition and NO3 - reacts with lithium to form stable inorganic SEI layer (Li2O, Li3N, LiNxOy, and etc) resulting in stable cycling of lithium metal anode.

In these days, IT devices such as smart card, flexible display and wearable devices requires flexible and high performances. To satisfy these demands, the battery also must be flexible and have high energy density. Lithium ion batteries(LIBs) with high energy density and long cycle performance are most promising candidate for wearable devices. However, the current collector which made of metal(Cu, Al) is an obstacle to flexibility of LIBs. For these reasons, many researches that replace metal current collector with flexible current collector had conducted. Nanocarbon materials such as carbon nanotube(CNT) and graphene have much attentions for using current collector of LIBs due to their high electrical conductivity. In addition, CNT and graphene also can perform as anode material of LIBs. Although nanocarbon materials have many advantages, they are suffered for the huge irreversible capacity and low coulombic efficiency at the initial cycling. This phenomenon is usually because of the formation of unstable solid electrolyte interface(SEI) due to nanocarbon materials’ large surface area. In this study, we tried to improve LIB’s reversible capacity and coulombic efficiency by forming stable SEI on CNT film’s surface. The CNT films were synthesized via direct spinning method. Stable SEI was formed by directly contacting CNT film with Li metal in electrolyte (1M LiPF6 solution in ethyl carbonate(EC) and diethyl carbonate(DEC) (1:1 v/v)) for 1h before cell assembled. The prelithiated CNT film was assembled to coin type half-cell without polymer binder and current collector under argon environment. According to the electrochemical analysis of the cells, directly prelithiated CNT film electrode didn’t show the SEI peak in the CV curve and very low surface resistance was measured in impedance test. After stabilization of a few initial cycles, the capacity increased up to 1300mAh/g at 300th cycle, which was higher than theoretical capacity of CNT. Pouch type cell assembled with the prelithiated CNT film was normally worked and maintained stable OCV during crumpled.

Building stable solid electrolyte interphases (SEI) for microsized silicon anode and 5V-class cathode with salt engineered nonflammable phosphate-based lithium-ion battery electrolyte

We had reported that an amorphous silicon flake powder (Si LeafPowder®, Si-LP), of which the lateral dimension and the thickness were ~4 μm and 100 nm, respectively, demonstrated superior cycle performances.[1, 2] Cyclability of the Si-LP electrodes was successfully improved by an addition of solid electrolyte interface (SEI)-forming additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC). However, the capacity-fading mechanism of the Si-LP electrodes has not been clarified. In order to understand the capacity fading of the Si-LP and effects of additives, variations of impedance components and the morphology changes were investigated. Test electrodes were composed of Si-LP (83.3 wt.%), Ketjen Black (5.6 wt.%) and carboxymethyl cellulose sodium salt (11.1 wt.%), and coated on a Cu foil. The loading of the Si-LP composite was approximately 0.4 mg cm-2. Electrolytes were 1 M LiPF6 dissolved in a mixture of ethylene carbonate and diethyl carbonate (EC+DEC, 1:1 by vol.) with and without additive of 10 wt.% VC or FEC. Charge and discharge tests were conducted at C/2 rate in the CC-CV mode between 1.5 and 0.02 V using a two-electrode coin-type cell. The electrochemical impedance spectroscopy (EIS) was performed using a three-electrode cell in which impedance spectra were obtained by applying an AC voltage of 10 mV over the frequency range of 0.03 Hz to 300 kHz. Surface and cross-sectional morphologies of the Si-LP electrodes after cycling were observed by a scanning electron microscope (SEM) without air exposure. Cyclability of the Si-LP electrodes in the electrolyte with and without FEC is shown in Fig. 1(a). Capacity retention for Si-LP electrode was substantially improved by the FEC-addition, although the discharge capacity at the 100th cycle decreased to 50% of the initial discharge capacity for the Si-LP electrode without FEC. To understand effects of the FEC-addition, EIS measurements were conducted and the SEI resistance (R SEI) was derived from the impedance spectra obtained at 0.1 V in the charging process. The SEI resistance for the Si-LP without FEC increased with increase in cycle number as shown in Fig. 1(b). By the FEC-addition, the SEI resistance was decreased and its increment with cycle number was substantially suppressed in comparison with the case of additive-free, indicating that reductive decomposition of the electrolyte on the Si-LP electrode was suppressed with a stable SEI film derived from FEC. Cross-sectional SEM images of the Si-LP electrodes after 10 and 30 cycles in the additive-free electrolyte are shown in Fig 1(c) and (d). Deposits of reductive products of the electrolyte were confirmed on Si-LPs. The amount of the deposition increased with cycle number, indicating a continuous decomposition of the electrolyte and a growth of SEI layer. The growth of the SEI layer lades an increment of the SEI resistance. Moreover, we found that the thickness of the Si-LP composite electrode was increased drastically by the growth of SEI layer and bending of Si-LPs, leading to a loss of the electric contact between the Si-LPs. Although Si-LPs bended after cycling, pulverization of Si-LPs was not observed. On the other hand, thinner SEI layer was observed on Si-LPs with the FEC-addition, and the expansion of the composite electrodes was suppressed. The stable SEI layer suppresses increment of the SEI resistance. Ensuring the electric-conducting path between active materials by the stable SEI layer is necessary to achieve a superior cycle performance of Si anodes. Acknowledgments: This work was supported by JST-ALCA-SPRING and JSPS-KAKENHI Grant Number 25820335 and 16H04649.

Silicon materials have been recognized as one of the new electrode materials in the next generation of lithium ion batteries (LIBs) with high energy density and large specific capacity. However, there is still a significant limitation to use the silicon based composite electrodes since the fracture occurs as a result of the huge volume change and fast lithium diffusion during the electrochemical lithiation process. A stable solid electrolyte interface (SEI) can help prevent fraction. However, the lithium diffusion inside SEI layer is still not fully understood. In this study, we focus on two dimensions (2-D) lithium diffusion simulation inside SEI layer with considering its morphology evolution. Our previous research (1) has proven that SEI morphology evolution can be regarded as a solification process, and phase field method shows great advantage to deal with the solification problem. The phase field variable is applied to simulate the SEI morphology evolution without tracking the multiple interfaces directly. The detailed chemistry of SEI species formation reaction has not been considered in the phase field modelling. The simulation assumes that SEI is composed of hydrocarbons, PEO-type oligomers, LiF, LixPFy, and LixPFyOz products based on the XPS experiments in the literature. We apply different formation contact angles to represent the different surface energy of different SEI specis. We are able to track different material species in SEI layer with defining the properties of each species for lithium diffusion simulation. In this work, a new model will be developed to simulate lithium diffusion during SEI growth and its morphology evolution based on our previous research and the computational method reported Han’s work (2). The concentration field can be regarded as a conserved property during the long-range diffusion. Therefore, the Cahn-Hilliard equations can be applied to formulate the phase transformations and description of diffusion inside the silicon based electrode. The chemical potential can be achieved by driving the total free energy equation. And the flux, which is assumed to be proportional to the gradient of the diffusion potential, can be presented by Fick’s Law. With the combination of the Cahn-Hilliard formulation and Fick’s Law, we are able to predict the lithium diffusion coefficient and lithium concentration distribution inside SEI layer. With the improvement of the current model, it is expected that the understanding on lithium diffusion inside SEI layer could be advanced to more accurately predict the fracture of silicon based composite electrodes during the electrochemical lithiation process. In addition, the developed model could be applied to other interface region problems and it has great potential to be extended to three-dimensions. 1. P. Guan, X. Lin and L. Liu, ECS Transactions, 61, 29 (2014). 2. B. C. Han, A. Van der Ven, D. Morgan and G. Ceder, Electrochimica Acta, 49, 4691 (2004).

State of the art lithium-ion batteries containing a transition metal cathode and a graphite anode have nearly reached their energy density limit. Due to its high theoretical gravimetric capacity (3590 mAh g-1 Li15Si4) and its low working potential, silicon is an attractive candidate to at least partially substitute the graphite anode [1]. Lithium-sulfur batteries are a promising battery system because of the high capacitive sulfur cathode (1672 mAh g-1) and low material costs. But current cells mainly suffer from the instable lithium anode. Lithiated silicon could be a more stable alternative preventing dendrite formation and electrolyte decomposition for that next generation battery cell type [2]. During the lithiation process of silicon a large undesirable volume expansion occurs. This volume change leads to pulverisation of the silicon and a steady reformation of the solid electrolyte interface (SEI). The electrolyte is consumed and the capacity declines rapidly. This is why silicon anodes with high quantities of silicon are hindered so far. To overcome these obstacles nanostructured silicon carbon composite anodes are very promising. The volume change is potentially compensated by the free volume between silicon core and carbon shell and a stable SEI is ensured at the surface of the carbon shell preventing electrolyte consumption during cycling. These void structures are mainly generated using silica templates which deliver a precise control of the void structure, but need to be removed by toxic hydrofluoric acid and laborious washing steps. In this work an easily scalable process without etching treatment is presented. The void structure results from the removal of a sacrificial template layer on the surface of commercially available silicon particles during the carbonization of the silicon carbon composite within a concerted process. For general electrochemical characterization the silicon carbon composite electrodes are galvanostatic cycled vs. metallic lithium in half cells with different electrolyte systems on coin cell as well as on pouch cell level. The silicon carbon composite electrodes reveal a much higher capacity and cycle stability compared to bare silicon nanoparticles and Si-C composites without void structure. High area capacities above 2.5 mAh cm-2 were reached. In sulfur-lithiated-silicon (SLS) full cells the cycle stability with low lithium excess and the enhanced volumetric energy densities using lithiated silicon anodes was analysed. The electrochemical performance of SLS cells with silicon carbon void structures are compared to that with hierarchical columnar silicon anodes, recently introduced by our group [3]. The feasibility of both SLS cell concepts was successfully demonstrated in coin and in pouch cells. A new electrolyte composition is introduced to enhance the cycle life by stabilization of the SEI and prevention of the electrolyte depletion [4]. Finally, prospects, limits and suggestion for further research directions for SLS cells are envisioned, based on the estimation of the expectable energy density. [1] J. K. Lee, C. Oh, N. Kim, J.-Y. Hwang, Y.-K. Sun, J. Mater. Chem. A 2016, DOI: 10.1039/c6ta00265j [2] J. Brückner, S. Thieme, F. Boettger-Hiller, I. Bauer, H. T. Grossmann, P. Strubel, H. Althues, S. Spange, S. Kaskel, Adv. Funct. Mater. 2014, DOI: 10.1002/adfm.201302169 [3] M. Piwko, T. Kuntze, S. Winkler, S. Straach, P. Härtel, H. Althues, S. Kaskel, J. Power Sources 2017 DOI: 10.1016/j.jpowsour.2017.03.080 [4] M. Piwko, S. Thieme, C. Weller, H. Althues, S. Kaskel, J. Power Sources 2017 DOI: 10.1016/j.jpowsour.2017.07.046

Hierarchical-structure and high-modulus aramid nanofiber membrane protective layer achieving high-performance lithium metal anode

The solid electrolyte interface (SEI) significantly affects alkaline metal ion battery performance in terms of reversible capacity, Coulombic efficiency, and cycling stability. However, intrinsic properties of SEI layer in potassium ion batteries (KIBs), including structures, components, formation mechanism, and corresponding K+ storage behavior, are poorly understood. Here, we focus on the effect of electrolyte on SEI formation and K+ storage behavior in self-supported nitrogen-doped graphite foams (NGFs). Two types of organic electrolytes, KPF6 and KN(SO2F)2 (KFSI) salt in EC/DEC solution, were carefully selected and compared in detail to reveal the effect of SEI on the K+ ion storage mechanism. The experimental results, including in situ electrochemical evaluations and depth-profiling XPS analysis, demonstrate that the salts of KFSI result in a more uniform, stable, and thinner SEI layer compared with the SEI induced by KPF6. Particularly, the KFSI-induced SEI is rich in stable and uniformly distributed inorganic species and polycarbonates, whereas the KPF6-induced SEI is mainly composed of instable alkyl carbonates. This could be attributed to the larger FSI– size over PF6– and lower LUMO levels than solvents according to theoretical calculations, which effectively prevent SEI from co-intercalation damage, thus leading to high stability of the as-obtained SEI layer. In general, the above-mentioned features could ensure high reversibility and good cycling stability of the self-supported NGFs electrode in KFSI-based electrolyte.

Sluggish kinetics severely limit the development of potassium-ion hybrid capacitors (PIHCs). Exposing active sites is recognized as an ideal strategy to resolve this issue, but the corresponding ma...

There is currently a considerable interest in the development of high capacity energy storage technologies. It is driven by the need in transport applications, such as electric vehicles (EV) where a capacity to go more than 500 km on one charge would be a desirable goal, as well as by renewable energy applications based on intermittent sources. The state-of-the-art for commercial rechargeable energy storage systems is the lithium ion battery (LIB) which has revolutionized the market for portable electronics. However, neither their capacity nor their price is currently matching the needs from the automotive or the renewable energy sectors. In addition, there is a need to improve the sustainability of the battery when considering mass implementation of large-scale energy storage systems. Li-Sulfur (LiS) technology has great potential for large-scale energy storage applications due to the very high theoretical specific capacity (1675 mAh g-1) and high energy density (2500 Wh kg-1) [1]. In addition the technology has potential for being a sustainable solution due to the abundance, low price and low toxicity of sulfur. However, a much lower practical capacity, limited cyclability, and low rate capability prevent commercialization of the technology. The origin of many of the problems can be traced to the intrinsic electrochemical process of the Li-Sulfur cell. During discharge sulfur is converted to Li2S through a series of steps involving the creation of lithium polysulfides (Li2Sx, 2<x≤8). A major issue is that these polysulfides have a relatively large solubility in standard electrolytes. In addition, the dissolved polysulfides cause degradation of the anode through shuttle reactions when they migrate through the electrolyte, forming insoluble layers on the anode surface. This leads to active material loss during the discharge-charge process gradually reducing the capacity and decreased Coulombic efficiency of the cell [7-10]. In addition, conducting material has to be added to sulfur (more than 30 wt%) when forming the cathode since sulfur has low electronic conductivity Consequently, the amount of active material in the cathode is further decreased, as is consequently the practical energy density of the cell. The problems in the Li-Sulfur battery can be addressed by new materials concepts for both the electrolyte and the cathode. For the development of the electrolyte promising approaches include moving away from organic solvents, improving both performance and safety, and the use of polymer gel or solid polymer electrolytes or ceramic electrolytes [2]. It has also been shown that by deliberately adding polysulfides to the electrolyte a more stable cycling, high capacity and good utilization of active material can be obtained [3]. To mitigate the polysulfide dissolution, and improve the conductivity of the cathode at the same time, nanostructured porous-carbon/sulfur composites have been applied [1]. Even though these approaches have proven rather successful in preventing polysulfide dissolution the practical energy density of these cells remain limited due to rather low active material (sulfur) loading. In this contribution we present different approaches towards Li-Sulfur cells with improved stability, safety, and high energy density. These include both new electrolyte and cathode solutions [2]. In particular we will discuss the application of the catholyte concept, where the active material is dissolved in the electrolyte. We show that by using a nanostructured porous-carbon cathode we can obtain a very high practical energy density, high sulfur utilization, and stable cycling. We also discuss the role of the presence of polysulfides in the electrolyte for the stabilization of the Li-metal anode in this cell [4].

In this article the polypropylene (PP) separator grafted by methyl methacrylate (MMA) functional groups with plasma technique has been primarily investigated. The MMA groups are expected to form a stable interface with lithium metal electrode. In addition, the effect of the surface functional group MMA on the graphite and the LiFePO4 electrode has been also primarily examined. The cyclic voltammetry (CV) tests have exhibited faster deposition/dissolution reaction kinetics of lithium on stainless steel electrode. The impedance measurements have shown a smaller interface resistance between the electrolyte (PP-MMA separator) and the lithium metal electrode. The interface reaction kinetics of the electrolyte (separator)/lithium electrode can be effectively enhanced by the MMA surface functional groups. It is also found that the existence of the MMA-groups at the separator surface can increase the reversible capacity of both LiFePO4 and graphite electrode. Rechargeable secondary batteries with lithium metal as the negative electrode have a relatively high energy density in comparison with those with carbon-based materials as the negative electrode due to the high theoretical capacity of 3860 mAh/g. In the early stage of the researches on lithium battery systems, many attempts had been made to improve the cycle life of the lithium metal electrode. Later researchers paid their attention on the splendid appearance of the graphite-based carbon materials. As well-known, the carbon-based materials have been successfully used as the negative electrode in the commercial rechargeable Li-ion batteries nowadays. In fact, lithium metal electrode still has potential in the rechargeable lithium-based micro-batteries and the all-solid-state lithium batteries, etc. 1–6 For some devices such as the implantable medicine devices and microchips, the output power of the batteries is relatively small, but the cycle life must be long enough. Safety is the most important factor to the commercial applications; the solution leakage is absolutely not allowed in such systems. This type of battery is named as energy-type battery. The issue that the lithium metal anode faces is that the formation of the lithium dendrites leads to two main consequences. Firstly, the electrically isolated lithium is not to be used and will shed. Secondly, the penetration of fine lithium particles into the separator will cause the internal short. There are three main strategies that are used to improve the cyclic performance of the lithium metal electrode. The first strategy has been extensively investigated by researchers and is just on Li-alloys and Li-intermetallics. 2 The improvement of the cycle stability of the lithium electrode is expected by modifying the solid electrolyte interface (SEI) or by buffering the volume expansion. The second one is focused on the electrolyte additives, such as HF, CO2, sulfides and surfactants. 7 The additives play the role of decreasing the content of the deleterious impurities in SEI layers, stabilizing the SEI, modifying the morphology of SEI and weakening or delaying the formation of the lithium dendrites by the specific adsorption of these surfactants on the convex points of the surface that has a relatively high surface energy. Ishikawa and Morita 8 have compiled the related results. We thought that the first strategy came from the lithium metal itself; the second one originated from the electrolyte. There also exists the third strategy which is on the separator. After being assembled, the lithium metal electrode is in close contact with the separator. A more stable SEI layer can be constructed by modifying the surface of the separator and by grafting some functional groups into the separator surface. These functional groups are expected to improve the stability of the interface between the liquid electrolyte (separator) and the lithium metal electrode. In commercial lithium ion batteries, the separator is also in contact with the positive

Mitigating PTFE decomposition in ultra thick dry-processed anodes for high energy density lithium-ion batteries

Lithium ion batteries are widely used as energy storage devices in a variety of products such as smartphones, tablets, laptops and other portable electronics. Thanks to their high energy density and cyclability, they are currently being used by and developed for electric vehicles. There is a growing need for cost reduction; increase in energy density; wider operating temperature range; and improved safety characteristics of the batteries. Organic carbonates are the primary solvents used in lithium-ion battery electrolytes along with electrolyte additives. The reversibility of current lithium-ion batteries is dependent upon the electrolyte used in the batteries. During the initial charging cycles of the cell, a solid electrolyte interface (SEI) is formed by reduction of organic carbonates, electrolyte salts and/or electrolyte additives on the surface of the graphitic anode in lithium-ion batteries. The generation of a stable anode SEI prevents continuous electrolyte reduction on the surface of the anode. The SEI functions as a Li ion conductor but an electrical insulator. The reduction reactions of the electrolytes on the graphitic anode surface have been investigated for many years and it been proposed to contain a complicated mixture of products including lithium oxalate, lithium alkoxides, and lithium oxide from the carbonate solvents and LiF and lithium fluorophosphates from the reduction of LiPF6. Similar ambiguity exists about the components of SEI formed from electrolyte additives and other electrolyte salts. Despite the extensive investigations, the structure, formation mechanisms and evolution of the SEI are poorly understood. Understanding the mechanisms of the reduction reactions of organic carbonates, electrolyte salts and electrolyte additives along with the products of the reactions which result in the generation of the SEI is essential for the development of safer lithium-ion batteries with wider operating temperature range. Lithium naphthalenide has been investigated as a one electron reducing agent for organic carbonates solvents, some of the most robust additives and salts used in lithium ion battery electrolytes. The reaction precipitates have been analyzed by IR-ATR, XPS and solution NMR spectroscopy. The evolved gases and the volatile components have been analyzed by GC-MS. The reduction products of ethylene carbonate and propylene carbonate are lithium ethylene dicarbonate (LEDC) and ethylene and lithium propylene dicarbonate (LPDC) and propylene, respectively. The reduction products of diethyl and dimethyl carbonate are lithium ethyl carbonate (LEC) and ethane and lithium methyl carbonate(LMC) and methane, respectively. Electrolyte additives, FEC and VC reductively decompose to HCO2Li, Li2C2O4, Li2CO3, and polymerized VC.

Introducing nanodiamond-modified electrolyte to realize high capacity and rate performance of sodium ion batteries

Metallic Sb‐stabilized porous silicon with stable SEI and high electron/ion conductivity boosting lithium‐ion storage performance