Superior performance of silicon based anodes using tanic acid binders

Silicon has been the focus of many research studies as the next generation high-capacity anode material for lithium-ion batteries. However, the mechanical stability of the electrode remains a bottleneck to the commercialization of the material. Many studies were devoted to nanostructured silicon composites with voids to accommodate the volume expansion [1]. Yet, full capability of silicon cannot be utilized because of the low volumetric energy density of these nanostructures. To increase the volumetric energy density compared to graphite, dense silicon electrodes are needed. Volume expansion within the electrode becomes an important factor affecting its cycle stability. In reality, how much is the volume expansion? What is the mechanism during the charge and discharge? Few studies have addressed these issues. In this work, we employed electrochemical dilatometry to measure the thickness change of Si electrodes during charge and discharge to understand the behavior with time. We demonstrate that the amount of electrode expansion and contraction is non-linear with the amount of lithium, which is explained by a model. We identify binder breakdown as a one of the causes of capacity degradation. Better reversibility in thickness change is achieved by using a more flexible binder such as polyimide, resulting in better cycle stability. Bulk silicon particles with a size of 10-20 μm from Sigma Aldrich was used to make the electrodes. The powder was mixed with acetylene black (Alfa Aesar) and binder to make an electrode. Electrode composition and type of binder were varied to study their effects on electrode volume change and capacity during charge-discharge. Typical electrode thickness is between 20-25 μm with 60wt% active material and a packing density of about 1.2 g cm-3. This corresponds to an electrode loading of about 1.5 mg silicon cm-2. The electrodes were assembled in an electrochemical dilatometer (ECD-1 from EL-Cell) with Li metal as counter electrodes for the thickness measurement. Similar device has been used by others to measure thickness change of various electrodes [2,3]. Fig. 1 shows the change in thickness with respect to cumulative capacity of a Si electrode with 20% carbon black and 20% carboxymethyl cellulose during the first three cycles. As expected, an increase in electrode thickness is observed during lithiation (increase in capacity), and a decrease in thickness during delithiation (decrease in capacity). The increase and decrease in electrode thickness are however not linear. A three-stage expansion model is used to describe the observation. At the beginning of lithiation (stage I), the electrode thickness change is small, as the composite electrode contains space between the particles that can accommodate the volume expansion. Beyond a certain point, the particles impinge on each other and the volume expansion of the particles lead to an overall increase in the film thickness (stage II). The amount of increment during stage II is similar to the theoretical increase in volume (dotted line in Fig. 1) for alloying Li with Si, indicating that it is due to structural change within the particle. Further incorporation of lithium into the electrode leads to an accelerated increase in thickness (stage III). The onset of stage III expansion depends on the type of binder used in the electrode, which suggests that it is affected by the ability of the binder to hold the particles together. During delithiation (stage IV), the contraction behavior is different from that during expansion. This is partly because the particle can contract in all three directions, as opposed to one direction during lithiation. Electrodes with polyimide (PI) binder show better cycling stability than those with polyvinylidene fluoride and carboxymethyl cellulose, which is attributed to the ability of the PI binder to hold the particles together after expansion. The results show that binder breakdown is one of the main causes of electrode degradation for Si electrodes. More experimental details and results will be shown during the presentation. Acknowledgement This research is sponsored by the GRF/ECS Scheme (21202014) managed by the Research Grants Council, the Government of the Hong Kong SAR. References Wu, H. & Cui, Y. Nano Today 7, 414-420 (2012). Hantel, M. M. et al. J. Electrochem. Soc. 159, A1897-A1903 (2012). Kim, J. S. et al. J. Power Sources 244, 521-526 (2013). Figure 1

The rapid development of mobile electronics, electric-vehicles and intermittent power generation relies on the availability of advanced electrochemical energy storage systems such as high capacity lithium ion batteries (LIBs). Although the state-of-the-art commercial LIBs have impressive electrochemical performance, they are suffering from relative low capacities due to the usage of host materials for storing lithium ions safely and reproducibly. The host material for LIB anode is dominated by graphite with a theoretical capacity of 372 mAh g-1. To further improve the capacity and energy density of LIBs, revolutionary changes in electrode materials, electrolytes, additives, binders and cell design are much needed. Alloying anode materials like Si, Ge and Sn have much higher theoretical capacities (4200, 1600 and 990 mAh g-1 for Si, Ge and Sn, respectively) as compared to graphite. However, a high capacity inevitably leads to a large electrode volume change during lithiation and de-lithiation process (e.g. 300% for Si-based anodes). Such a huge volume change can cause electrode pulverization, detachment of active materials from conductive additives, unstable solid-electrolyte interphase, and eventually result in a rapid, permanent capacity loss. Although Si-based anode has caught much research attention, formidable challenges remain to develop an electrode fabrication method that is efficient, low-cost, scalable, highly reproducible, and self-assembled, thus boosting the commercialization potential of Si-based anodes. As inspired by the hollow fiber membrane technology that has been successfully commercialized in the last decades to provide billions of gallons of purified drinking water worldwide, we report the use of carbonized asymmetric membrane electrodes (fiber and thin film types) that contain silicon, silicon monoxide, germanium, tin dioxide or vanadium (V) oxide to significantly increase LIB electrode capacity while maintaining a long cycling life and an excellent rate performance. These asymmetric membrane electrodes are fabricated using a modified reverse-osmosis membrane technology, the phase-inversion method combined with dip-coating and carbonization. The asymmetric membrane electrodes have a thin, nanoporous top layer (up to several microns) and a thick, macroporous bottom layer (up to 200 µm). The top layer can prevent fractured anode materials from falling away, while the bottom layer can efficiently accommodate the large volume changes of high capacity alloying anodes. 90% capacity of thin film asymmetric membrane at 1000 mAh g-1 containing Si nanoparticles can be retained after 200 cycles with an initial capacity loss of ~ 30%. Combining phase inversion with sol-gel chemistry, this method can be extended to obtain SnO2 and V2O5 asymmetric membranes. Thin film SnO2 asymmetric membrane electrode demonstrates a specific capacity of 500 mAh g-1 a current density of 280 mA g-1 (~0.5C) with >96% capacity retention after 400 cycles. When the current density is increased from 28 to 560 mA g-1, more than 65% capacity can be retained. The V2O5 electrode can be cycled at 0.5C with a capacity of 160 mAh g-1 for 380 times with ~100% capacity retention. The same method can be employed to stabilize micron-size Ge and Si alloying anodes with >80% capacity retention in 100 cycles. Most recently, our group has successfully fabricated various fibrous Si and Sn asymmetric membrane electrodes whose electrochemical performance is compared to thin film Si and Sn asymmetric membrane electrodes. e.g. ~90 % initial capacity at ~1,200 mAh g-1 can be maintained in 300 cycles applying a current density of 400 mA g-1 with an initial capacity loss less than 15%. These asymmetric membrane electrodes have been systematically characterized using extensive instruments spanning from scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy, powder X-ray diffractometer (PXRD), thermogravimetric analyzer (TGA), surface area analyzer (BET method) to X-ray photoelectron spectroscopy (XPS), as well as various electrochemical tests such as cycling and rate performance, cyclic voltammetry, and electrochemical impedance spectroscopy, purposing to understand the relationship between structure, chemical composition and electrochemical properties for these asymmetric membrane electrodes.

The excellent performance battery with high power and high energy density is required for automobile application.To realize such batteries, it is necessary to comprehend the internal resistance of battery. In this study the novel electrochemical analysis method are applied to porous electrode of Lithium-ion battery by using transmission line model.To study on the internal resistances of porous electrodes of Lithium-ion batteries in more detail, we examined how to separate constituent of resistance in porous electrode, such as electric resistance (Re), ionic resistance inside pore (Rion), and charge-transfer resistance (Rct). The real electrode of Lithium-ion batteries is composed of insertion material (improved NCA based cathode material), conductive carbon, binder and pore filled with the electrolyte1, 2. We investigated electrochemical impedance spectroscopy using symmetric cell and treated the electrolyte/porous electrode interface by using transmission line model with or without charge transfer resistance3. From transmission line model4, the overall impedance (Z) is derived as Eq.1 without charge transfer reaction. (L: pore radius, X: pore length, CdL:double-layer capacitance)The overall impedance is derived as Eq.2 with charge transfer reaction.Fig.1 shows the experimental impedance spectra and the spectra calculated by inputting each parameter into Eq.1 and Eq.2. The experimental results of the impedance spectra agree well with that of the theoretical calculations. This result shows that the resistance constituents inside electrode are separated well by this method.To investigate the factors that affect Rion and Rct inside electrode, the analysis methods as mentioned above were applied to various kinds of electrode of Lithium-ion battery. Fig.2 shows the relation between the resistance constitution (Ri on and Rct) and thickness of electrode with different composition ratio of the insertion material, conductive carbon and binder. (The area of the electrodes in symmetric cell is constant.) The value of Rion increases with increasing in thickness of electrode, and the Rion is directly proportional to thickness. The value of Rct decreases with the increase in thickness, and the value of Rct are inversely proportional to thickness. Similar results are derived from Eq.1 and Eq.2. These results mean that the rate of Rion becomes larger among total resistance in the thick electrodes. Correlation between the Rion and the thickness vary depending on composition of the electrodes, whereas the Rct does not vary so much. The results show that the ionic resistance was dependents on the structure of electrode, and the charge transfer resistance dependents on state of the insertion material. We conclude that these results are very important. These results indicate that the decrease of the Rion is key point in the high energy density battery with thick electrode for excellent performance battery with high power and high energy density. The Rion represents the ease of movement of the lithium-ion inside pore filled with the electrolyte, and the pore size and distribution inside electrode are closely related.In the following, the values of each resistance were measured in a wide temperature range and the values of activation energies were calculated. Especially the value of Rct increased as the low temperature and the activation energy of Rct is greater than that of Rion. The results show that the decrease of Rct was important under low temperature. In summary, the Rion is more important on thick electrode in normal temperature. It is necessary to improve electrode structure for excellent performance battery in this case. The Rct is more important on thin electrode under low temperature. It is necessary to improve insertion material for excellent performance battery in that case.These analytical tools as mentioned above can provide very useful information in electrode design for excellent performance battery. More quantitative analyses and applications will be discussed in the presentation. Reference 1. Y. Itou and Y. Ukyo, J. Power Sources, 146, 39 (2005)2. H. Kondo and Y. Itou, J. Power Sources, 174 (2007) 11313. N. Ogihara and Y. Itou, J. Electrochem. Soc., 159(7) A1034 (2012)4. J. H. Jang and S. M. Oh, J. Electrochem. Soc., 151 A571 (2004)

Silicon (Si), with its extremely high theoretical specific capacity (up to 4200 mAhg−1), natural abundance and low toxicity, is considered as one of the most promising anode material candidates for next generation high energy density lithium-ion batteries. However, large volume expansion (~300 %) upon lithiation/delithiation makes it difficult for Si-based anode to maintain its capacity over extended cycles. Several possible mechanisms have been proposed to explain the rapid capacity fade in Si-based anode such as pulverization of the active materials, loss of electrical pathways caused by rearrangement of electrode structure, instability of the electrode/electrolyte interface or some combinations of these. While application of nano-structured Si (nanoparticles, nanowires) as anode largely resolved the pulverization issue, the use of nano-silicon inevitably increases the reactive surface between active materials and the electrolyte. Additionally, the continuous growth of solid-electrolyte-interphase (SEI) on the particle surface remains a significant cause for performance deterioration. Therefore, an increasing number of research has focused on the correlation between surface functionality of Si NPs and their electrochemical performance. Previous studies in this area mainly focused on the study of the inherent SiOx layers on Si NPs or the effect of electrolyte additives on the surface properties of Si NPs. We take a different strategy to address the Si anode issue. We propose to chemically modify the surface of Si NPs and study how the chemical modification would affect the electrochemical performance of the Si-based anode. Commercially available Si NPs usually bear a native layer of silicon oxide on the surface of the particle. The first step in the surface modification of Si NPs is to enrich the surface silanol (Si-OH) groups as the new working platform. The silanol enriched surfaces will be converted to a variety of functional groups on the Si NPs. (Scheme 1). In this paper, we will present some preliminary results on the surface-modified Si NPs with methacrylate group and amine group and discuss the effects of the functional groups on the electrochemical performance of the Si anode. Scheme 1. Chemical surface modification of Si NPs via silane chemistry. Figure 1

Si-based anode materials are considered as potential materials for high-energy lithium-ion batteries (LIBs) with the advantages of high specific capacities and low operating voltages. However, significant initial capacity loss and large volume variations during cycles are the primary restrictions for the practical application of Si-based anodes. Herein, we propose an affordable and scalable synthesis of double-layered SiOx/Mg2SiO4/SiOx composites through the magnesiothermic reduction of micro-sized SiO with Mg metal powder at 750 °C for 2 h. The distinctive morphology and microstructure of the double-layered SiOx/Mg2SiO4/SiOx composite are beneficial as they remarkably improve the reversibility in the first cycle and completely suppress the volume variations during cycling. In our material design, the outermost layer with a highly porous SiOx structure provides abundant active sites by securing a pathway for efficient access to electrons and electrolytes. The inner layer of Mg2SiO4 can constrain the large volume expansion to increase the initial Coulombic efficiency (ICE). Owing to these promising structural features, the composite prepared with a 2:1 molar ratio of SiO to Mg exhibited initial charge and discharge capacities of 1826 and 1381 mA h g-1, respectively, with an ICE of 75.6%. Moreover, it showed a stable cycle performance, maintaining high capacity retention of up to >86.0% even after 300 cycles. The proposed approach provides practical insight into the mass production of advanced anode materials for high-energy LIBs.

The effect of polymer binder on the nature and location of solid-electrolyte interphase (SEI) formation on Si active material was investigated. Thin layers of polymeric binder (polyvinylidene fluoride and sodium carboxymethyl cellulose) were spin-coated onto polished single crystal Si wafers. The samples were cycled against a Li counter/reference electrode under various electrochemical conditions in a coin cell configuration. The electrolyte was 1 M of LiPF6 in 1:1:1 wt% EC:DEC:DMC. After cycling, the samples were extracted from the coin cell under inert conditions and X-ray photoelectron spectroscopy (XPS) was performed. Electron microscopy and atomic force microscopy indicate that the binder films are smooth and continuous across the wafer surface, and no impact on the electrochemical behavior was observed. However, notable differences were detected using XPS, which revealed both difference in SEI composition and location relative to the binder-electrolyte and binder-substrate interfaces. These observations and insights will be useful in cell designs, binder selection, and reliability estimates because the location of SEI formation may influence cyclic performance of electrodes.

Since Li-oxygen (Li-O2) battery was first roported in 1996 by Abraham and Jiang 1, it had been attracting increasingly attention due to its remarkably high theoretical specific energy of 11400 Wh/kg, which is very close to that of gasoline (13000 Wh/kg) and is regarded as the most promising alternative to fossil fuel based energy2. However, the practical capacity of Li-O2 battery is always far below the theoretical value. The prime reason is that, up discharging, the insoluble discharge product, namely Li2O2, will block the paths for both oxygen and the electrolyte and result in an early termination of the discharge process. So the specific capacity of Li-O2 electrode strongly depends on the porosity of the electrode. And it is very crucial to build porous air electrodes with plenty of voids to accommodate Li2O2 and to facilitate the transportation of oxygen and electrolyte. Up to now, various mesoporous materials or nano-structured materials have been explored to make porous electrodes for Li-O2batteries and have promoted the electrochemical performance significantly. However, present electrode manufacturing prefer to use polyvinyliden-di-fluoride (PVdF) as the binder and use volatile organic solvents such as N-methyl-pyrrolidone (NMP). Most of these solvents cost a lot and lead to the harmful environment pollution during the electrode manufacturing process. In addition, PVdF is also a relatively costly polymer and is necessary to be substitued by an economic binder. Herein, we proposed a green method to build a porous air electrode for Li-O2 batteries by a freeze-dry process with carboxymethyl cellulose (CMC) as the binder. The electrode is characterized with abundant voids for discharge product, which also provide plenty of effective passages for both oxygen and electrolyte. This electrode demonstrate not only a high specific capacity but also an excellent rechargeable ability by combination with high conductive MWCNT. As shown in Figure 1a, at a current density of 100 mA g-1, the porous air cathode demonstrates an initial discharge capacity of 6,702 mAh g-1 with an average discharge potential of about 2.70 V. The increased discharge capacity can be ascribed to the fact that the plenty of room in the porous electrode accommodate more discharge product. Meanwhile, the abundant space facilitates the transportation of oxygen and electrolyte, thus extending the discharge process. The high conductivity of MWCNT also promotes the electrochemical performance of Li-O2batteries. By all the virtues above, the porous air cathode shows an enhanced cycling life as high as 53cycles (Figure 1b). Acknowledgement This work was supported by Science Foundation of Ministry of Education of China (grant 413064), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. Reference 1. K. M. Abraham, J. Electrochem. Soc. 1996, 143, 1-5. 2. Z. Q. Peng, S. A. Freunberger, Y. H. Chen, P. G. Bruce, Science, 2012, 337, 563 – 566. Figure 1

Graphite is known as state-of-the-art anode material in the contemporary commercial lithium-ion batteries (LIBs), and is attributed to its low cost and superior long cycle life. However, its low theoretical specific capacity (372 mAh g-1, based LiC6) limits its usage for the emerging innovative applications such as in the electro mobility (electric vehicles, xEVs), efficient integration of renewable energy sources and deployment of large-scale smart grid/utility storage devices. 1 With extra-high capacity (4200 mAh g-1, based on Li22Si4), suitable operating potential (0.2-0.4 V vs. Li/Li+), high abundance (2nd richest element in the earth crust), low cost, negligible toxicity, superior safety and environment-friendliness, Silicon (Si) is considered as one of the most promising alterative anode material to revolutionize next-generation LIBs. However, despite all these resilient features, intrinsic drawbacks such as low electronic conductivity (~10-3 S cm-1) and Li diffusion coefficient (10-14-10-13 cm2S-1), colossal volume change (200-400% vs. 8-12% for graphite), low initial Coulombic/energy efficiency (ca. 65-85% vs. 90-94% for graphite), unstable solid electrolyte interphase (SEI), electrode swelling and electrolyte drying are among the seemingly challenges hampering its commercialization. 2–4 Among the various strategies, the use of graphite (G)-silicon (Si) composite along with designer polymeric binders represents one of the most enabling approach to overcome the above-mentioned challenges. Herein, we report on highly robust, new, naturally available, green (aqueous solvent based) and multi-functional polymeric binders for use with an optimized G-Si composite electrodes. Firstly, the composition of G-Si composite was optimized using state-of-the-art binders used in Si-based anode such as carboxymethyl cellulose (CMC) and poly (acrylic acid) (PAA). Following the optimization, highly performing new binders have been developed and investigated. An in-depth investigation of the electrochemical performance (long-term cycling, Coulombic/energy efficiency, and rate capability), surface characterization using X-ray photoelectron spectroscopy (XPS), and thermal reactivity via differential scanning calorimeter (DSC) evidenced the huge potentiality of the new polymeric binders and optimized G-Si composition.

From structures to properties: The impacts of two modification methods on pectin/sodium carboxymethyl cellulose composite film for advanced packaging materials.

Li4Ti5O12 microspheres composed of nanoflakes were synthesized within 1 h by a combination of a microwave-assisted hydrothermal method and a microwave postannealing process. The Li4Ti5O12 microspheres were characterized by X-ray diffraction, Brunauer–Emmett–Teller N2 adsorption, scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. Sodium carboxymethyl cellulose (CMC) was also investigated as a low-cost green binder. The electrochemical tests, including constant current charge–discharge, cyclic voltammetry, and electrochemical impedance spectroscopy, demonstrated that the electrode using CMC as binder had better high-rate capability than the one with polyvinylidene fluoride (PVDF) binder. The electrode using CMC and PVDF as binder had the same lithium diffusion coefficient. The electrode using CMC as binder showed much lower charge transfer resistance, lower apparent activation energy, and lower apparent diffusion activation energy than for the electrode using PVDF as the binder. Apparent activation energies of Li4Ti5O12 microsphere electrodes using CMC and PVDF as binder were calculated to be 26.8 and 33.6 kJ mol–1, respectively.

pH triggered on-demand tannic acid release via “open-close” of soybean isolate proteins spatial structure for shrimp preservation

Porous conductive electrode for highly sensitive flexible capacitive pressure sensor over a wide range

Polymer brushes refer to polymer chains that are end-grafted to surfaces or interfaces at high grafting density. Polymer brushes are able to make surface self-assembly or surface coassembly with free block copolymer (BCP) chains, and surface nanostructures with different morphologies are formed on the surfaces. The formation of the surface nanostructures endows materials with new surface properties. To develop a simple and versatile approach to the synthesis of polymer brushes and surface nanostructures, the synthesis of polymer brushes and surface nanostructures on tannic acid (TA) coatings was investigated in this research. TA molecules were coated on the surfaces of amino-modified silica particles (SiO2-NH2) through noncovalent bonds. Quaternized poly(2-(dimethylamino)ethyl methacrylate)-block-polystyrene (qPDMAEMA-b-PS) chains were anchored onto the TA coatings through electrostatic interaction between qPDMAEMA blocks and TA coatings, and PS brushes are formed on the particle surfaces. Thermo gravimetric analysis demonstrates the grafting of the BCP chains on the TA coatings, and the calculation results indicate that the PS chains on the silica particles are in the polymer brush regime. PS brushes on TA coatings are able to make surface coassembly with free BCP chains, and surface nanostructures with different sizes and morphologies are produced on TA coatings. The grafting density of PS blocks and BCP concentration both exert significant influences on the formation of the surface nanostructures. The surface nanostructures can be removed from the particle surfaces in the aqueous solution of sodium chloride. This research provides a simple and versatile method for the fabrication of polymer brushes and removable surface nanostructures on any solid substrates, which have specific interactions with TA molecules.

Carbon-coated LiTi2(PO4)3 composites synthesized through tannic acid with high rate performance for aqueous lithium-ion batteries

Porous nickel electrodes in water electrolysis 1. Electrode preparation and polarization studies in strong alkali