Polyolefin Separators Research Articles

Flexible, High-Wettability and Fire-Resistant Separators Based on Hydroxyapatite Nanowires for Advanced Lithium-Ion Batteries.

Separators play a pivotal role in the electrochemical performance and safety of lithium-ion batteries (LIBs). The commercial microporous polyolefin-based separators often suffer from inferior electrolyte wettability, low thermal stability, and severe safety concerns. Herein, a novel kind of highly flexible and porous separator based on hydroxyapatite nanowires (HAP NWs) with excellent thermal stability, fire resistance, and superior electrolyte wettability is reported. A hierarchical cross-linked network structure forms between HAP NWs and cellulose fibers (CFs) via hybridization, which endows the separator with high flexibility and robust mechanical strength. The high thermal stability of HAP NW networks enables the separator to preserve its structural integrity at temperatures as high as 700 °C, and the fire-resistant property of HAP NWs ensures high safety of the battery. In particular, benefiting from its unique composition and highly porous structure, the as-prepared HAP/CF separator exhibits near zero contact angle with the liquid electrolyte and high electrolyte uptake of 253%, indicating superior electrolyte wettability compared with the commercial polyolefin separator. The as-prepared HAP/CF separator has unique advantages of superior electrolyte wettability, mechanical robustness, high thermal stability, and fire resistance, thus, is promising as a new kind of separator for advanced LIBs with enhanced performance and high safety.

Solid State Batteries and the Mechanical Properties of Lithium

The current revival of electrified vehicles seen in recent years has been enabled by significant advances in lithium ion batteries. This technology has progressed steadily at approximately 8% per year improvement in specific energy (Wh/kg). The achievement and maintenance of this specific energy growth rate is the result of massive, global progress in battery research, development and manufacturing. However, to promote the rapid displacement of petroleum fuels from the transportation sector, the current evolutionary growth in battery energy may be insufficient. Amongst candidate next generation battery approaches, solid state (SS) is one of the most versatile and promising technologies. SS batteries can enable improvements in specific energy mainly by allowing for the use of high energy (due to its low electronegativity and high capacity) lithium metal anodes. A SS separator of comparable dimensions as a conventional polyolefin separator could lead to ~50% improvement in cell energy density by enabling metallic lithium anodes [1]. However, elemental lithium is pyrophoric & difficult to stabilize owing to its strongly reactive nature, and as a result its use as a reliable rechargeable battery electrode has proven challenging despite research efforts for more than 50 years. The development of a SS electrolyte material that could introduce lithium metal anodes would provide rechargeable batteries with a step change improvement in specific energy, leading to longer range EVs due to lighter/smaller batteries. The research and selection of a solid state material requires balancing many different and often competing requirements simultaneously, which no one material can satisfy currently. Additionally, some SS electrolytes have shown a tendency to allow for lithium dendrites to penetrate their structure once a critical charge density (CCD) has been reached [2]. The role lithium's mechanical properties play in the dendrite growth mechanism is poorly understood and is the focus of this work. The literature features limited reports of lithium's elastic modulus, with tension values as low as 1.9, commonly referencing bending values of 5.0 and single crystal calculations as high as 11.5 GPa [3,4,5]. This work will explore further the mechanical properties of lithium in context of the actual SS battery environment where additional properties such as creep may dominate. REFERENCES[1] McCloskey, B. et al. J Phys. Chem. Lett. 6 (2015) 4581-4588.[2] Sharafi, A. et al. J. Power Sources, 302 (2016) 135-139.[3] Schultz, R. Technical Report FERMILAB-TM-2191. (2002).[4] Bridgeman, P. Proc Am Acad Arts Sci., 57 (1922) 41-66.[5] Robertson, W. et al. Phys. Rev., 117 (1960) 440-442.

A low cost shutdown sandwich-like composite membrane with superior thermo-stability for lithium-ion battery

A separator plays the key role in solving the safety issues of lithium-ion batteries. The currently used polyolefin separators are seriously limited by their low thermal stability. In this paper, a sandwich-like composite membrane with super-high thermal stability was designed and fabricated. In the design, a porous polybenzimidazole (PBI) layer was introduced on both sides of a PE separator by a typical phase inversion method. The porous PBI layer on both sides of the PE ensured high ion conductivity, high thermal stability and the nonflammable property. Meanwhile, this PE separator also acted as a shut-down layer at an elevated temperature. As a result, the PBI/PE/PBI (referred to as PBIE) sandwich-like composite membrane exhibited no dimensional shrinkage up to 200°C. In addition, the prepared PBIE membrane could easily shut the battery down at about 140°C. Combing these features of shut-down function and high thermal stability, the PBIE membrane is expected to internally solve the safety issues, which have been extremely troublesome so far for lithium-ion batteries.

A comparative study on polypropylene separators coated with different inorganic materials for lithium-ion batteries

Coating commercial porous polyolefin separators with inorganic materials can improve the thermal stability of the polyolefin separators and hence improve the safety of lithium-ion batteries. Several different inorganic materials have been studied for the coating. However, there lacks a study on how different inorganic materials affect the properties of separators, in terms of thermal stability and cell performance. Herein, we present such a study on coating a commercial polypropylene separator with four inorganic materials, i.e., Al2O3, SiO2, ZrO2 and zeolite. All inorganic coatings have improved thermal stability of the separators although with differences. The coating layers add 28%–45% of electrical resistance compared with the pure polypropylene separator, but all the cells prepared with the coated polypropylene separators have the same electrical chemical performance as the uncoated separator in terms of rate capability and capacities at different temperatures.

A Sandwich PVDF/HEC/PVDF Gel Polymer Electrolyte for Lithium Ion Battery

A sandwich structure-like PVDF/HEC/PVDF polymer membrane is reported as host for matrix of gel polymer electrolytes for lithium ion batteries. The inner HEC (hydroxyethyl cellulose) layer is compact, and can avoid micro short-circuits. The producibility of the composite membrane is guaranteed due to the simple and efficient electro-spinning process for manufacturing the PVDF (poly(vinylidene fluoride)) layer and low-cost and eco-friendly HEC layer. This structure integrates the advantages of the HEC and PVDF, and it presents high uptake of liquid electrolyte (135.4%), excellent retention of liquid electrolyte, high ionic conductivity (0.88mScm−1 at ambient temperature), low activation energy for lithium movement (0.016eV), and high lithium ion transference number (t+=0.57), all properties are superior to those of the pure HEC membrane and commercial polyolefin separator (Celgard 2730). With the PVDF/HEC/PVDF gel polymer electrolyte as the electrolyte and separator, the assembled Li//LiFePO4 cell exhibits a notably high discharge capacity (140mAhg−1) over 140 cycles. The PVDF and HEC components are non-combustible, which is in sharp contrast with the combustible polymers for the commercial separators. These results show that this kind of sandwiched gel polymer electrolyte is promising for scale-up of lithium ion batteries especially those needing high safety and reliability.

A nano-silica modified polyimide nanofiber separator with enhanced thermal and wetting properties for high safety lithium-ion batteries

The commercial polyolefin separators still possess two well-known drawbacks: poor wettability and thermal stability. Herein, a thin and lightweight silica filled in polyimide (PI) nanofibers membrane is prepared by electrospinning. Without any binders, the nano-silica particles are firmly embedded in the PI nanofibers with high structure stability. The PI-SiO2 membrane with high porosity (90%) presents enhanced conductivity due to the excellent electrolyte wettability and large electrolyte uptake (about 2400%). In addition, the PI-SiO2 membrane displays good mechanical flexibility and enhanced thermal stability up to 250°C, which significantly improve the safety of lithium-ion batteries when used as a separator. The LiMn2O4/Li cell with the PI-SiO2 separator exhibits highly improved rate capability and cycling stability at different temperatures (25°C and 55°C), which make PI-SiO2 membrane as a promising secure separator candidate for high-performance and safety lithium-ion batteries.

Enhancement on the thermostability and wettability of lithium-ion batteries separator via surface chemical modification

The separator is one of the most important components of the battery, which is related to safety performance of the battery. The traditional polyolefin separators have disadvantages such as the poor thermal stability and weak surface hydrophilicity. Herein, we introduced a SiO2/PAM-grafted polypropylene (PP) separator by surface chemical modification. Compared with unmodified separator, the results showed that the modified separators had better thermal stability and superior wettability. The shrinkage of the bare PP and SiO2/PAM-grafted PP separators were 70% and 12% at 150°C for 30min, respectively. In addition, the surface contact angle of PP separator is reduced from 105° to 37° after grafting modification. Therefore, modified separator not only improves the thermal stability and the wettability but also thickness of separator increase slightly. The dis/charge capacity of different separators were investigated. It was showed that the cell with SiO2/PAM-grafted PP separator had better electrochemical performance than that with bare PP.

Exploring Rate Capabilities of Symmetric Nonaqueous Redox Flow Batteries

After decades of research, widespread implementation of redox flow batteries (RFBs) for grid electricity storage has only recently begun to appear feasible. With MWh-level installations in the U.S. and GWh-level installations planned in China [1], the aqueous vanadium RFB chemistry appears to be the market leader. These projects offer levelized cost of capacity lower than the large Li-ion storage systems offered by suppliers like Tesla. A recent US study, which used the state of Michigan’s electrical grid as a test case, found that RFBs reduced emissions per kWh in all scenarios, but also found that RFBs were only economically preferable to wind curtailment at curtailment levels above 66% [2]. So there remains a need to find alternative, less costly energy storage strategies to better compete with other carbon-intensity improvement strategies that are available to utilities. Symmetric RFBs offer a step-change reduction in reactor cost because their use of identical solutions in the discharged posolyte and negolyte mitigates the need for expensive ion-exchange-membrane separators. Many symmetric RFB chemistries exist: redox-active organometallic chemistries such as V(acac)3 ­[3], or redox-active organic chemistries such as polycyclic aromatic hydrocarbons [4]. These RFB chemistries operate at voltages that split water, requiring nonaqueous environments. Nonaqueous RFBs are often rejected out of hand, based on their expected inability to provide reasonable power. Such an expectation is based on a qualitative argument that nonaqueous solutions tend to have low conductivites, and consequently cannot support high-current operation. However, ion-exchange membranes in aqueous solutions also tend to have similarly low conductivities. Being freed from the need for costly, resistive ion-exchange membranes, symmetric chemistries can employ inexpensive porous polyolefin separators, which we will show can permit rate performance comparable to aqueous vanadium RFBs. Results were collected from a bespoke system involving a 6.25 cm2 flow-through reactor and 100 mL active-liquid storage vessels: the positive and negative flow-through regions contained carbon felt at 6% compression to fit the 3 mm depth. Flow cell designs of these nonaqueous systems are non-trivial since many of the materials used in these nonaqueous chemistries are incompatible with materials used for aqueous systems. We will discuss some design criteria and lessons learned in the fabrication of these nonaqueous flow cells. Figure 1 shows 20 cycles of a symmetric V(acac)3 cell operating at 20 mAcm–2 in a cell with a porous polyolefin separator. The voltage cutoff was not triggered during the charging procedure, allowing the charging cycles to achieve the full programmed capacity. The coulombic efficiency, voltage efficiency, and energy efficiency quickly stabilize to 83%, 87%, and 72%, respectively. Informed by similar data across a range of charge/discharge currents and flow rates, we will report more generally on the performance of nonaqueous symmetric RFB chemistries. [1] Unienergy Technologies. www.uetechnologies.com. Accessed Dec. 2016. [2] M. Arbabzadeh, et al. Appl. Energy 2015, 146, 397. [3] Q. Liu et al. Electrochem. Comm. 11 (2009): 2312. [4] J. Saraidaridis et al. ECS Meeting on Electrochemical Energy and Storage s. (2015): 685. Figure 1: 0.1 M V(acac)3/0.5 M TEABF4 in acetonitrile, 20 mAcm-2, polypropylene separator, 50% SOC charging cycles after 1st charge to 75%, 50% SOC 1st discharge and to 1 V cutoff on subsequent cycles, Argon atmosphere with <1ppm O2, H2O Figure 1

Polysulfide-Breathing/Dual-Conductive, Heterolayered Battery Separator Membranes Based on 0D/1D Mingled Nanomaterial Composite Mats.

Facile/sustainable utilization of sulfur active materials is an ultimate challenge in high-performance lithium-sulfur (Li-S) batteries. Here, as a membrane-driven approach to address this issue, we demonstrate a new class of polysulfide-breathing (capable of reversibly adsorbing and desorbing polysulfides)/dual (electron and ion) conductive, heterolayered battery separator membranes (denoted as "MEC-AA separators") based on 0D (nanoparticles)/1D (nanofibers) composite mats. The MEC-AA separator is fabricated through an in-series, concurrent electrospraying/electrospinning process. The top layer of the MEC-AA separator comprises close-packed mesoporous MCM-41 nanoparticles spatially besieged by multiwalled carbon nanotubes (MWNT) wrapped poly(ether imide) (PEI) nanofibers. The MCM-41 in the top layer shows reversible adsorption/desorption of polysulfides, and the MWNT-wrapped PEI nanofibers act as a dual-conductive upper current collector. Preferential deposition of the MWNTs along the PEI nanofibers and dispersion state of the separator components are elucidated theoretically using computational methods. The support layer, which consists of densely packed Al2O3 nanoparticles and polyacrylonitrile nanofibers, serves as a mechanically/thermally stable and polysulfide-capturing porous membrane. The unique structure and multifunctionality of the MEC-AA separator allow for substantial improvements in redox reaction kinetics and cycling performance of Li-S cells far beyond those achievable with conventional polyolefin separators. The heterolayered nanomat-based membrane strategy opens a new route toward electrochemically active/permselective advanced battery separators.

3D Printable Ceramic–Polymer Electrolytes for Flexible High‐Performance Li‐Ion Batteries with Enhanced Thermal Stability

This study establishes an approach to 3D print Li‐ion battery electrolytes with controlled porosity using a dry phase inversion method. This ink formulation utilizes poly(vinyldene fluoride) in a mixture of N‐methyl‐2‐pyrrolidone (good solvent) and glycerol (weak nonsolvent) to generate porosity during a simple drying step. When a nanosized Al2O3 filler is included in the ink, uniform sub‐micrometer pore formation is attained. In other words, no additional processing steps such as coagulation baths, stretching, or etching are required for full functionality of the electrolyte, which makes it a viable candidate to enable completely additively manufactured Li‐ion batteries. Compared to commercial polyolefin separators, these electrolytes demonstrate comparable high rate electrochemical performance (e.g., 5 C), but possess better wetting characteristics and enhanced thermal stability. Additionally, this dry phase inversion method can be extended to printable composite electrodes, yielding enhanced flexibility and electrochemical performance over electrodes prepared with only good solvent. Finally, sequentially printing this electrolyte ink over a composite electrode via a direct write extrusion technique has been demonstrated while maintaining expected functionality in both layers. These ink formulations are an enabling step toward completely printed batteries and can allow direct integration of a flexible power source in restricted device areas or on nonplanar surfaces.

Characterization of Polyolefin Separator Support Membranes with Hydrophilic Coatings

Characterization of Polyolefin Separator Support Membranes with Hydrophilic Coatings

Poly (ether ether ketone) (PEEK) porous membranes with super high thermal stability and high rate capability for lithium-ion batteries

Lithium-ion batteries are receiving intensive interest due to their promising prospect for electric vehicles. However, the safety issues and low rate capability of lithium-ion batteries limited their further development due to the poor dimensional thermo-stability and low electrolyte wettability of commercial polyolefin separators. Herein, we report a sponge-like porous poly (ether-ether-ketone) (PEEK) membrane with super high thermal stability and good rate capability for lithium ion batteries. The porous PEEK membrane showed no fusion deformation even at temperature of 350°C, showing excellent thermal stability. An electrolyte uptake of 251% was obtained for a porous PEEK membrane, which was more than twice than that of a commercial PE separator, showing very good electrolyte wettability. As a result, the lithium-ion battery with a porous PEEK membrane showed a discharge capacity of 124.1mAhg−1 at 5C, showing good rate performance. The battery performance of a PEEK membrane after treating at 350°C changed rarely. The PEEK membranes provide more options for cost-effective high power battery separators.

Sustainable and Superior Heat-Resistant Alginate Nonwoven Separator of LiNi0.5Mn1.5O4/Li Batteries Operated at 55 °C.

High-voltage lithium-ion batteries have become a major research focus. As a major part of lithium batteries, the separator plays a critical role in the development of high-voltage lithium batteries. Herein, we demonstrated a sustainable and superior heat-resistant alginate nonwoven separator for high-voltage (5 V) lithium batteries. It was demonstrated that the resultant alginate nonwoven separator exhibited better mechanical property (37 MPa), superior thermal stability (up to 150 °C), and higher ionic conductivity (1.4 × 10-3 S/cm) as compared to commercially available polyolefin (PP) separator. More impressively, the 5 V class LiNi0.5Mn1.5O4 (LNMO)/Li cell with this alginate nonwoven separator delivered much better cycling stability (maintaining 79.6% of its initial discharge capacity) than that (69.3%) of PP separator after 200 cycles at an elevated temperature of 55 °C. In addition, the LiFePO4/Li cell assembled with such alginate nonwoven separator could still charge and discharge normally even at an elevated temperature of 150 °C. The above-mentioned fascinating characteristics of alginate separator provide great probability for its application for high-voltage (5 V) lithium batteries at elevated temperatures.

Lithium Ion Batteries with Alumina Separator for Improved Safety

Lithium ion batteries with inorganic separators offer the advantage of safer and stable operation in a wider temperature range. In this work, lithium ion batteries in both half and full cell configuration with an alumina separator were fabricated by an improved method of blade coating α-Al2O3 slurry directly on either Li4Ti5O12 or LiNi1/3Mn1/3Co1/3O2 electrode followed by drying. The improved separator preparation involves a twice-coating process instead of coating the slurry all at once in order to obtain a thin (∼40 μm) and uniform coat. Unlike state-of-the-art polyolefin separators such as polypropylene (PP) which are selectively wettable with only certain electrolytes, the excellent electrolyte solvent wettability of α-Al2O3 allows the coated alumina separator to function with different electrolytes. The coated α-Al2O3 separator has a much higher resistance to temperature effects than its polyolefin counterparts as it retains its dimensional integrity at temperatures as high as 200°C, eliminating the possibility of a short circuit during thermal runaway. Lithium ion cells with coated α-Al2O3 separator exhibit electrochemical performance comparable to that of polymer separator at room temperature, but the former provide safer operation and shows better cycling performance under extreme temperatures in the range of -30°C to 60°C than the latter.

Tailored Metal Oxide Thin Film on Polyethylene Separators for Sodium-Ion Batteries

The development of polymer separators for Na+-ion batteries has not been of interest because conventional polyolefin (e.g., polyethylene or polypropylene) separators are not suitable for the solvation of Na+-ion-containing electrolytes. Here, we report a simple surface modification method based on chemical vapor deposition of SiO2 applied to a polyethylene separator for Na+-ion batteries. A thin SiO2 layer is coated uniformly onto a porous polymer separator with the negligibly increased total separator thickness. Improved wetting ability of the SiO2-film-coated polyethylene separators with a polar electrolyte based on ethylene carbonate (EC) and dimethyl carbonate (DMC) solvents is demonstrated, with superior electrochemical performance characteristics, such as initial specific capacity, C-rate and cyclic stability. In addition, the thin SiO2 coating film results in substantially suppressed thermal shrinkage, which may lead to improvements in the thermal and dimensional stability of Na+-ion batteries. Compared to a glass-fiber separator and the conventional PE separator, the metal-oxide-thin-film-coated polyethylene separator will accelerate the development of Na+-ion batteries for various electrochemical energy storage applications.

The Potential for the Creation of a High Areal Capacity Lithium-Sulfur Battery Using a Metal Foam Current Collector

A high areal capacity lithium-sulfur battery making use of mass produced aluminum metal foam as a current collector was investigated. A sulfur/Ketjenblack (KB) composite was filled and deposited into the aluminum foam current collector via a predetermined filling procedure, resulting in high sulfur loading. The value for this loading was found to be 17.7 mg sulfur/cm2 by using carboxymethyl cellulose and styrene butadiene rubber (CMC + SBR) as a binder. An operating single-layer pouch-type cell with an S/KB-CMC+SBR on Al foam cathode was created as a result of this synthesis and found to possess an unprecedentedly high areal capacity of 21.9 mAh/cm2. On the basis of the achieved areal capacity, the energy density of a theoretical lithium-sulfur battery was estimated with the assumption of an electrolyte/sulfur ratio of 2.7 μL/mg. This was calculated upon 100% of the pore volume in the S/KB-CMC + SBR on Al foam cathodes and polyolefin separator, along with the inclusion of the weights of the tabs for the current lead and pouch film packaging in the case of a seven-layer pouch-type battery. With this calculation, it was determined that the creation of a lithium-sulfur battery with an energy density of greater than 200 Wh/kg is plausible.

Enhanced separator wettability by LiTFSI and its application for lithium metal batteries

Separator wettability is vital to electrochemical performance of lithium metal batteries. This study demonstrates that the wettability of the polyolefin separator is enhanced by adding lithium bis(trifuoromethanesulfonyl)imide (LiTFSI) in the electrolyte and the good wettability has the positive impacts on the rate performance and lithium dendrite suppression of lithium metal batteries. Moreover, the underlying wetting mechanism of the LiTFSI has been investigated by the comparison with its cousin lithium bis(fluorosulfonyl)imide (LiFSI). It is revealed that perfluorinated alkyl (-CF3) groups in TFSI- anions can greatly reduce the surface tension of the electrolyte, and thus favor the good wettability of the separator. The better separator wettability of 1M LiTFSI/propylene carbonate (PC) than those of 1M LiFSI/PC and 1M LiPF6/PC contributes to the higher rate capability in Li||LiFePO4 batteries. Introduction of the LiTFSI into the LiPF6-based electrolyte can obtain comparable separator wettability with the LiTFSI-based electrolyte. Owing to the greatly enhanced wettability of the separator, the electrolyte of 0.4M LiPF6-0.6M LiTFSI/PC as well as 1M LiTFSI/PC has the positive effect on the lithium dendrite suppression in the lithium metal batteries.

Advanced Separators for Lithium-Ion and Lithium-Sulfur Batteries: A Review of Recent Progress.

Li-ion and Li-S batteries find enormous applications in different fields, such as electric vehicles and portable electronics. A separator is an indispensable part of the battery design, which functions as a physical barrier for the electrode as well as an electrolyte reservoir for ionic transport. The properties of the separators directly influence the performance of the batteries. Traditional polyolefin separators showed low thermal stability, poor wettability toward the electrolyte, and inadequate barrier properties to polysulfides. To improve the performance and durability of Li-ion and Li-S batteries, development of advanced separators is required. In this review, we summarize recent progress on the fabrication and application of novel separators, including the functionalized polyolefin separator, polymeric separator, and ceramic separator, for Li-ion and Li-S batteries. The characteristics, advantages, and limitations of these separators are discussed. A brief outlook for the future directions of the research in the separators is also provided.

High Performance Cross-Linked Composite Gel Polymer Electrolyte Using Mesoporous and Reactive SiO2 Nanoparticles for Lithium-Ion Polymer Cells

The rapidly expanding use of rechargeable lithium-ion batteries has led to intensive research on electrolyte systems with high electrochemical performance. However, current lithium-ion batteries have risks associated with leakage and fire hazards due to the high flammability of the organic solvents. In addition, the polyolefin separators used in lithium-ion batteries may shrink and even melt at elevated temperatures, which may cause a short circuit between the two electrodes in cases where unusually high heat is generated. Therefore, there is a pressing need for safer and more reliable electrolyte systems. In this work, a cross-linked composite gel polymer electrolyte was prepared and applied to lithium-ion polymer cells as a safer and more reliable electrolyte. Mesoporous SiO2 nanoparticles containing reactive methacrylate groups as cross-linking sites were synthesized and dispersed into the fibrous polyacrylonitrile membrane. They directly reacted with gel electrolyte precursors containing tri(ethylene glycol) diacrylate, resulting in the formation of a cross-linked composite gel polymer electrolyte with high ionic conductivity and favorable interfacial characteristics. The mesoporous SiO2 particles also served as HF scavengers to reduce the HF content in the electrolyte at high temperature. As a result, the cycling performance of the lithium-ion polymer cells with cross-linked composite gel polymer electrolytes employing methacrylate-functionalized mesoporous SiO2 nanoparticles was remarkably improved at elevated temperatures as well as ambient temperature.

In Situ SEM Study on Electrochemical Lithiation/Delithiation Behavior of Silicon Anodes with Polyimide Binder

Lithium-ion batteries (LIBs) currently outperform other competitive batteries, for instance, lead-acid and nickel–metal hydride, but still need extensive improvement to meet the requirements for automotive and large-scale energy storage applications. Silicon is one of the most promising anode materials owing to the high theoretical capacity (3579 mAh g– 1 vs. 372 mAh g–1 (conventional graphite)). However, the dramatic volume change (~300 %) experienced by Si upon alloying/dealloying with Li induces severe pulverization and subsequent loss of electrical connectivity. It inevitably results in fatal capacity decay and hampers practical applications of the Si anode. To address this issue, great research effort focused on the optimization of the electrode composition and structure. Ishikawa and co-workers have reported that micro-sized Si with a polyimide (PI) binder exhibits a stable cyclability over 300 cycles, while the one using other binder showed a rapid capacity decay.1,2 With the aim to clarify the role of binder, the charge-discharge behavior of the Si anode with a PI binder (Si-PI) was investigated by an in situ scanning electron microscope technique,3which was established by our group. Also we have designed a brand-new coin-cell type for easy access to the comparison of the data obtained by conventional electrochemical measurement and in situ SEM observation. To reproduce the practical battery testing condition during the in SEM observation, a new cell for the in situ observation was designed using a commonly-used CR2032 coin cell. The edge of coin cell was carefully cut to observe the cross-section of cell components. Silicon anode with a PI binder was prepared in accordance with previous reports.1,2 Electrochemical measurements were performed using a lithium cobalt oxide (LiCoO2) cathode, a polyolefin separator, a Si anode, and 1 mol L–1 Li[TFSA]–[C2mim][FSA] (Li[TFSA]: lithium bis(trifluoromethanesulfonyl)amide; [C2mim][FSA]: 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide) as the electrolyte. The cell was assembled in an Ar-filled glovebox and then immediately transferred into a SEM chamber. Cut-off voltage range was −3.895 V to −2.500 V vs. LiCoO2. For the initial cycle, the cell was charged/discharged at constant current density of 0.2 A g– 1, while constant current constant voltage mode was performed to complete the lithiation. After the second cycle the cells were charged/discharged with a constant current density of 1.0 A g–1. As shown in Fig. 1a, Si-PI anode assembled in the tailored coin cell showed a comparable capacity to previous report1 when tested under vacuum condition, confirming the feasibility of our cell design. At the initial cycle, a big irreversible capacity was recognized, but the charge–discharge curves became to show a better reversible behavior after several cycles. The discharge capacity exceeded 1,000 mAh g– 1 at the 20th cycle. In this investigation, two different positions of Si anodes, namely, edge and center, were observed by SEM. Fig. 1b shows in situ SEM images of the Si anode at the edge. Before the charge, a Si anode, a Cu current collector, and a separator can be clearly distinguished. But after the charge, the morphology is dramatically varied by the known lithiation reaction. Obvious volume expansion of the whole anode was recognized while contraction during the discharge was less noticeable, suggesting that the small contraction is closely linked to the big irreversible capacity observed in the charge–discharge curve at the initial cycle. This behavior is very similar to our previous result on the Si anode with a polyvinylidene difluoride (PVDF) binder.3Afterwards, reversible expansion and shrinkage of the electrode was observed for several cycles, and then the charge–discharge curves showed a good reversibility. Interestingly the expansion of Si anodes was clearly suppressed as compared to the case of our previous research using a PVDF binder. The in situ SEM observation revealed that the mechanical properties of binder play a critical role in electrochemical properties of Si anodes. Acknowledgement Part of this research was supported by the Grant-in-Aid for Scientific Research, Grant Numbers 15H03591, 15K13287, and 15H2202 from the Japanese Ministry of Education, Culture, Sports, Science and Technology and by the ALCA-SPRING program, Japan Science and Technology Agency.