Study of the formation of a solid electrolyte interphase (SEI) in ionically crosslinked polyampholytic gel electrolytes

There is an urgent need to fulfill the hunger for energy in an efficient and a safer way. In this context, non-Lithium based energy storage devices have received a noticeable interest. A prime attention has been given on electrolytes as many of the devices are composed with liquid electrolytes which have been later identified as having various problems. Gel polymer electrolytes (GPEs) are considered as viable substituents for liquid electrolytes because they exhibit liquid like conductivities while retaining good mechanical stabilities. To further improve the safety aspects of GPEs, nowadays many studies have been performed with ionic liquids (ILs) in place of solvents on GPEs. The present study was carried out to fabricate a Zn rechargeable cell with natural graphite (NG) cathode and an IL based GPE. The IL, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (1E3MITF) with the polymer poly(vinyl chloride) (PVC) and the salt zinc trifluoromethanesulfonate (Zn(CF3SO3)2 – ZnTF) were used in preparing the GPE. The electrochemical properties of the GPE were evaluated using electrochemical impedance spectroscopy (EIS) and DC polarization test. The maximum room temperature conductivity of the electrolyte was 3.06 × 10-4 S cm-1. Conductivity of the electrolyte was highly contributed by ions. The cell of the configuration, Zn / GPE / NG was characterized using cyclic voltammetry (CV) test. Initial specific charge was about 2.25 mA h g-1 and during 50 cycles, it had reduced down to 2.15 mA h g-1. Moreover, satisfactory stability was observed during CV test. Further studies are in progress to improve the cell performance.

The demand for electric vehicles is increasing rapidly as the world is preparing for a fossil fuel-free future in the automotive field. Lithium battery technologies are the most effective options to replace fossil fuels due to their higher energy densities. However, safety remains a major concern in using lithium as the anode, and the development of non-volatile, non-flammable, high conductivity electrolytes is of great importance. In this dissertation, a gel polymer electrolyte (GPE) consisting of ionic liquid, lithium salt, and a polymer has been developed for their application in lithium batteries. A comparative study between GPE and ionic liquid electrolyte (ILE) containing batteries shows a superior cyclic performance up to 5C rate and a better rate capability for 40 cycles for cells with GPE at room temperature. The improvement is attributed to GPE’s improved stability voltage window against lithium as well as higher lithium transference number. The performance of the GPE in lithium-sulfur battery system using sulfur-CNT cathodes shows superior rate capability for the GPE versus ILE for up to 1C rates. Also, GPE containing batteries had higher capacity retention versus ILE when cycled for 500 cycles vii at C/2 rate. Electrochemical impedance spectroscopy (EIS) studies reveal interfacial impedances for ILE containing batteries grew faster than in GPE batteries. The accumulation of insoluble Li2S2/Li2S on the electrodes decreases the active material thus contributes to capacity fading. SEM imaging of cycled cathodes reveals cracks on the surface of cathode recovered from ILE batteries. On the other hand, the improved electrochemical performance of GPE batteries indicates better and more stable passivation layer formation on the surface of the electrodes. Composite GPE (cGPE) containing micro glass fillers were studied to determine their electrochemical performance in Li batteries. GPE with 1 wt% micro fillers show superior rate capability for up to 7C and also cyclic stability for 300 cycles at C/2 rate. In situ, EIS also reveals a rapid increase in charge transfer resistance in GPE batteries, responsible for lowering the capacity during cycling. Improved ion transport properties due to ion-complex formations in the presence of the micro fillers, is evidenced by improved lithium transference number, ionic conduction, and ion-pair dissociation detected using Raman spectroscopy.

In order to overcome the problems associated with the use of liquid electrolytes in dye-sensitized solar cells (DSSCs), a new system composed of liquid crystal embedded, polymer electrolytes has been developed. For this purpose, three types of DSSCs have been fabricated. The cells contain electrospun poly(vinylidene fluoride-co-hexafluoropropylene) (e-PVdF-co-HFP) polymer gel electrolyte, with and without doping with the liquid crystal E7 and with a liquid electrolyte. The morphologies of the newly prepared DSSCs were explored using field emission scanning electron microscopy (FE-SEM). Analysis of the FE-SEM images indicate that the DSSC composed of E7 embedded on e-PVdF-co-HFP polymer gel electrolyte has a greatly regular morphology with an average diameter. The ionic conductivity of E7 embedded on e-PVdF-co-HFP polymer gel electrolyte was found to be 2.9 × 10(-3) S/cm at room temperature, a value that is 37% higher than that of e-PVdF-co-HFP polymer gel electrolyte. The DCCS containing the E7 embedded, e-PVdF-co-HFP polymer gel electrolyte was observed to possess a much higher power conversion efficiency (PCE = 6.82%) than that of an e-PVdF-co-HFP nanofiber (6.35%). In addition, DSSCs parameters of the E7 embedded, e-PVdF-co-HFP polymer gel electrolyte (V(oc) = 0.72 V, J(sc) = 14.62 mA/cm(2), FF = 64.8%, and PCE = 6.82% at 1 sun intensity) are comparable to those of a liquid electrolyte (V(oc) = 0.75 V, J(sc) = 14.71 mA/cm(2), FF = 64.9%, and PCE = 7.17%, both at a 1 sun intensity).

The charge/discharge efficiency and the cycle life of lithium-ion batteries (LiBs) depends on a large number of competing interfacial processes. Probably the best-known interface in LiBs is the solid-electrolyte-interphase (SEI), which is a passivating layer formed at the negative electrode by electrolyte decomposition during the first cycles.[1–3] Since the SEI stability is a deciding factor with regards to cycle life, understanding and improving the SEI formation process is crucial. In order to yield an effective SEI, commercial battery cells undergo an extensive so-called formation procedure after cell assembly, which generally consist of multiple voltage-holds and current-steps at various temperatures.[4,5] During the first cycles of a LiB, the SEI is generated by the reduction products of the commonly used carbonate-based electrolyte components, such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).[6,7] Additionally, electrolyte additives are commonly added to form an SEI layer with increased cycling stability and reduced irreversible capacity loss during the first cycle.[6,8–10] Among a variety of additives, vinylene carbonate (VC) is the most prominent. Since the reduction of VC initiates at higher reduction potentials compared to alkyl carbonate solvents (e.g., EC), it is reduced preferentially during the first charge and thus strongly affects the SEI composition.[6,11] In the present study we investigated the effect of the formation temperature in a range between 10-60 °C on the performance characteristics of full-cells with a graphite anode (MAGE) and an NCM831205 cathode as well as of MAGE/Li half-cells, both with LP572 (1 M LiPF6 in EC:EMC 3:7wt/wt + 2wt% VC) electrolyte. These include the first-cycle irreversible capacity loss, the negative electrode impedance build-up, the gas evolution during formation, the rate capability, and the cycle life.Employing electrochemical impedance spectroscopy using a µ-reference electrode allowed us to determine the graphite intercalation resistance (R Int), representing the sum of the SEI resistance (R SEI) and of the charge transfer resistance (R CT), after two 0.1C formation cycles at a given temperature. The impedance spectrum was recorded at 25 °C and 40% SOC referenced to 190 mAh/gNCM reversible capacity. As depicted in Figure 1, both R Int of the graphite electrode in the MAGE/NCM83125 full-cell and the first-cycle irreversible Li-loss determined a MAGE/Li half-cell increase with increasing formation temperature.Additionally, on-line electrochemical mass spectrometry (OEMS) was applied to identify and quantify the gaseous reduction products for LP572 in a MAGE/NCM831205 full-cell during one 0.1C formation cycle at different temperatures. As displayed in Figure 1, the relative increase of the total gas evolved, which is the sum of hydrogen, ethylene, carbon monoxide, and carbon dioxide, follows the relative increase of the intercalation resistance. Next to the overall gas amounts, changes in the quantity of the individual gases give insights into the highly temperature dependent reduction mechanisms of VC. Acknowledgement: The authors thank BMW AG for their financial support.

Propylene carbonate-nitrile solvent blends for thermally stable gel polymer lithium ion battery electrolytes

Electrochemical double layer capacitors (EDLCs) have received a tremendous interest due to their suitability for diverse applications. They have been fabricated using different carbon based electrodes including activated carbons, single walled/multi walled carbon nano tubes. But, graphite which is one of the natural resources in Sri Lanka has not been given a considerable attention towards using for EDLCs though it is a famous carbon material. On the other hand, EDLCs are well reported with various liquid electrolytes which are associated with numerous drawbacks. Gel polymer electrolytes (GPE) are well known alternative for liquid electrolytes. In this paper, it is reported about an EDLC fabricated with a nano composite polyethylene oxide based GPE and two Sri Lankan graphite based electrodes. The composition of the GPE was [{(10PEO: NaClO4) molar ratio}: 75wt.% PC] : 5 wt.% TiO2. GPE was prepared using the solvent casting method. Two graphite electrodes were prepared by mixing 85% graphite and 15% polyvinylidenefluoride (PVdF) in acetone and casting n fluorine doped tin oxide glass plates. GPE film was sandwiched in between the two graphite electrodes. A non faradaic charge discharge mechanism was observed from the Cyclic Voltammetry study. GPE was stable in the potential windows from (-0.8 V-0.8 V) to (-1.5 V-1.5 V). By increasing the width of the potential window, single electrode specific capacity increased. Impedance plots confirmed the capacitive behavior at low frequency region. Galvanostatic charge discharge test yielded an average discharge capacity of 0.60 Fg-1.

Traditional ethylene carbonate (EC)-based electrolytes constrain the applications of silicon carbon (Si-C) anodes under fast-charging and low-temperature conditions due to sluggish Li+ migration kinetics and unstable solid electrolyte interphase (SEI). Herein, inspired by the efficient water purification and soil stabilization of aquatic plants, a stable SEI with a 3D desolvation interface is designed with gel polymer electrolyte (GPE), accelerating Li+ desolvation and migration at the interface and within stable SEI. As demonstrated by theoretical simulations and experiment results, the resulting poly(1,3-dioxolane) (PDOL), prepared by in situ ring-opening polymerization of 1,3-dioxolane (DOL), creates a 3D desolvation area, improving the Li+ desolvation at the interface and yielding an amorphous GPE with a high Li+ ionic conductivity (5.73 mS cm-1). Furthermore, more anions participate in the solvated structure, forming an anion-derived stable SEI and improving Li+ transport through SEI. Consequently, the Si-C anode achieves excellent rate performance with GPE at room temperature (RT) and low temperature (-40°C). The pouch full cell coupled with LiFePO4 cathode obtains 97.42 mAh g-1 after 500 cycles at 5 C/5 C. This innovatively designed 3D desolvation interface and SEI represent significant breakthroughs for developing fast-charging and low-temperature batteries.

Electronic devices have been advanced rapidly over the last few decades with improved performance through high-level integration. Lithium ion batteries (LIB), which is representative rechargeable energy storage system, is increasingly demanding to high-energy-density storage system for the stable operation of high performance devices like mobile, laptop, etc. In addition, a recent quantum leap in electronic vehicles (EVs) require to energy storage system with long cycling stability incomparable to previous electronic devices. To achieve high performances and long lifetime, the use of lithium metal is one of attractive anode for as LIBs, because Li metal has high theoretical specific capacity of 3860 mAh/g, low redox potential of -3.04V (verse the standard hydrogen electrode) and low gravimetric density of 0.59 g/cm3. Nevertheless, since Li metal anode has the lithiation-delithiation mechanism of the ‘hostless’ electrochemical plating/stripping unlike the conventional graphite anode, it is confronted with serious problems occurred from uncontrollable interfacial reaction between surface of lithium metal anode and electrolyte applying to traditional LIB system; that is (1) the uncontrollable formation of solid electrolyte interphase (SEI) layer limitless volume expansion of Li metal anode (2) the dendritic growth and isolation of Li interface derived from inhomogeneous deposition of Li+ ion flux (3) the Li surface fracture induced by heterogeneously plated Li+ ion. Through these process, repetitive formation of undesirable SEI layer at surface of Li anode make fresh electrolyte dissipation and concurrently the anode surface becomes more distorted. These side-phenomena strongly correlate with low coulombic efficiency and decreasing lifetime of lithium metal battery (LMB) system where lithium metal is used as anode. In this works, in a dimensional aspect, to solve such problem mentioned above through stabilizing of surface of lithium metal anode, we introduce Janus-faced protective layer (JPL) as gel polymer electrolyte. JPL was composed with dual layer of Ni (3~7μm) embedded poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) and bare PVdF-HFP through simple casting method. The process of preparing JPL membrane is so simple to instantly bring in existing roll-to-roll process, and it is also possible to make a low cost process considered relatively low nickel price. In terms of mechanical property, it is based on flexible polymer and shows the merit of being foldable enough to able origami. In addition, JPL play a role of protection of Li metal anode. JPL has two functionalities. The metal part of JPL directly contacted with anode, and this layer was frameworks for preventing irregular SEI layer formation through inward plating of Li ion, the polymer part is stable state enough to be used as a gel polymer electrolyte. When lithium metal anode is subjected to cycle-test while keeping in contact with the nickel side of JPL membrane, Scanning electron microscope (SEM) image showed that the surface of lithium metal anode was very clean without any fluctuation compared to the case where only polypropylene (PP) separator was used. (Figure. 1) Moreover, as shown in Figure 2., the LMB system applied JPL show high stability of tolerable overvoltage within 5mV, 15mV even at high-current of 10mA/cm2, 20mA/cm2, respectively, in Li//JPL/JPL//Li symmetric cell. These results are attributable to effective function of each layer as mentioned earlier. JPL consists of two layer, the tough and adhesive nature of PVdF-HFP prevents the polymer part from being damaged or torn by dendritic growth and simultaneously ensure that metal part functions as robust framework for SEI layer formation without escaped particle from the membrane. This frameworks induce the SEI layer, which can act as a resistive element, to grown in the metal part while surface of Li metal anode become get out of main subject from the SEI layer formation. By virtue of that property, JPL has not allow forming resistive elements on the surface of Li metal anode, preventing the voltage rising up due to the formation of resistive elements at surface, so tolerability of overvoltage with clean surface of anode is maintained. In conclusion, the model study of nickel micro-particle/PVdF-HFP composite suggests the straightforward direction of protective layer to apply lithium metal as anode in LIB system with alleviating the problem caused by the ‘hostless’ property of lithium metal anode. Thus, when JPL is introduced to a LIB system, it shows a fairly positive possibility for high performance Lithium metal battery. Figure 1

Novel polymer gel electrolytes (PGE) based on a composition of oligourethane methacrylate, polypropylene glycol monomethacrylate, and 1 M solutions of LiClO4 or γ in LiBF4-butyrolactone or LiPF6 in ethylene carbonate/dimethyl carbonate (1 : 1 by volume) are synthesized and studied. The electrolyte films are solidified from liquid PGE solutions by irradiating the latter with UV light. Impedance spectroscopy shows PGE to have conductivity of about 3.0 × 10–3 S cm–1 at room temperature. To enhance mechanical strength of PGE, the electrolyte solutions are applied onto a fiber polypropylene separator and then photosolidified. The temperature dependences of the bulk conductivity and the charge transfer resistance through the Li/PGE interface are studied. The best PGE is that based on a 1 M LiClO4 solution in γ-butyrolactone containing 20 wt % of the polymer composite.

Characterization of gel polymer electrolyte for suppressing deterioration of cathode electrodes of Li ion batteries on high-rate cycling at elevated temperature

Influence of solvent on the poly (acrylic acid)-oligo-(ethylene glycol) polymer gel electrolyte and the performance of quasi-solid-state dye-sensitized solar cells

Super capacitors have emerged as a promising substitute for batteries and conventional capacitors which have played a key role as energy storage devices. The continuous concerns over green and low-cost concepts have motivated the consideration of natural materials as they are non-toxic and low cost. On the other hand, the realization of the dangers of liquid electrolytes has promoted the attention on gel polymer electrolytes to be used for devices. The prime objective of the present study is fabricating an electrochemical double-layer capacitor (EDLC) which is a type of super capacitor make using natural graphite and a gel polymer electrolyte. The electrolyte having the composition 0.5 PVdF-Co-HFP:0.70 MgTF:1 EC:1 DEC showed an optimum room temperature conductivity of 3.45 × 10−3 S cm−1. It was purely an ionic conductor having improved amorphous nature. EDLCs were fabricated with two identical natural graphite electrodes. Evaluation of EDLC was carried out using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge test. The proposed electrode/electrolyte combination seemed to be suitable for the application in EDLCs. Specific capacitance values obtained are satisfactory to carry forward further investigations to improve performance.

Although lithium ion batteries (LIBs) as an energy storage device have been identified as the most promising candidate for large scale automotive and aerospace applications, safety issues still remain to be secured. Hence in order to improve the safety of LIBs, we introduce a new gel polymer electrolyte (GPE) containing the frame retardant material, magnesium hydroxide (Mg(OH)2). First, a dense polymer film of the thickness of 20 µm is prepared by casting the precursor solution consisting of poly(vinylidene fluoride-co-hexafluoropropylene) (P(VdF-co-HFP)) as a polymer matrix, dibutyl phthalate (DBP) as a pore generation agent, and Mg(OH)2 as a flame retardant agent, in acetone. And then, DBP is extracted in the methanol bath to make pores in the polymer film. They are gelled with additional liquid electrolyte, 1.15M LiPF6 in ethylene carbonate/ethyl methyl carbonate (EC/EMC, 3/7 by vol). Mg(OH)2 composited GPE exhibits a distinguished fire-retardant properties, which results in a relevant self-extinguishing time (SET) reduction. The ionic conductivity is also greatly enhanced in the GPEs with increasing ratio of Mg(OH)2. Due to the hydrophilic properties of Mg(OH)2, unit cells employing LiCoO2 cathode, the composite GPE, and graphite anode achieve significant improvements in rate capability and cycle performance that were closely associated with their improved uptake for liquid electrolyte. The electrochemical stability of the GPE with Mg(OH)2 is comparable to that without Mg(OH)2. This results imply that Mg(OH)2composited GPE can provide the large-format LIBs with greatly improved safety. Acknowledgements This work was supported by the Energy Technology Program of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20142010102980) Refernces 1. Daeyong Yeon , Yunju Lee , Myung-Hyun Ryou , Yong Min Lee, Electrochimica Acta 157 (2015) 282-289 2. Myung-Hyun Ryou, Yong Min Lee, Jang Wook Choi, Jung-Ki Park, Electrochemistry Communications 17 (2012) 18-21 3. Yong Min Lee, Dong-Hyun Ko, Jun Young Lee, Jung-Ki Park, Electrochimica Acta 52 (2006) 1582–1587

The possible use of polymer gel electrolytes in nickel/metal hydride battery

Si has drawn much attention as a promising next-generation negative electrode material for lithium ion batteries (LIBs) because Si has a high theoretical capacity of ca. 4200mAh g-1, which is approximately 11 times higher than that of graphite. However, several technical issues, such as large volumetric changes of Si upon alloying/dealloying with Li and a large irreversible capacity particularly in the initial cycles, remain to be solved for practical use. Hence, the morphological changes need to be controlled to improve the charge/discharge performance. In this study, an amorphous Si thin-film was used as a model electrode and its morphological changes, which originate from the formation of solid electrolyte interphase (SEI) and volumetric changes upon alloying/dealloying with Li, was observed by in situ atomic force microscopy (AFM). The effects of electrolyte additives on morphological changes of Si were discussed. Amorphous Si thin-films were deposited on 500-μm-thick Cu sheet by electron beam evaporation. The thickness of Si was ca. 100 nm. The electrochemical properties were investigated by cyclic voltammetry (CV) (SP-300, Biologic) at a sweep rate of 0.5 mV s-1 between 3.0 and 0.02 V, using a laboratory-made three-electrode cell. The effective electrode surface area was limited to 1.77 cm2 by an O-ring. Both reference and counter electrodes were Li wire. All potentials are referred to as V vs. Li+/Li. The electrolyte solution used was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and ethylmethylcarbonate (EMC) (1:1, by volume) with and without 10 wt.% vinylene carbonate (VC). Electrochemical AFM observations coupled with CV were conducted using a 5500 AFM/SPM Microscope (Agilent) in a contact mode with pyramidal silicon nitride tips (spring constant: 0.02 N m-1, Olympus). The scan area was set to 10 × 10 μm2. AFM observation was carried out at a room temperature of about 25 oC in an Ar-filled glovebox with a dew point of less than −60 oC. Figure. 1 shows AFM images of amorphous Si thin-film electrodes during the potential scan from 0.48 to 0.02 V in 1 M LiPF6 / EC+EMC (a) without and (b) with 10 wt.% VC additives. In Fig.1a, a very large blister appeared on the Si surface at potentials below 0.35 V. Li1.7 1Si is known to form at around 0.35 V [1], and hence the formation of the blisters should be closely related to alloying reaction of Li-Si. The AFM observation was interrupted below 0.20 V due to the significant morphological changes. The blisters appeared below 0.35 V even when VC was used, while the sizes was relatively small and they were uniformly distributed. We observed the fact that a very thin surface film was formed evenly on a Si electrode at higher potentials. The alloying/dealloying reactions with Li seem to proceed in a relatively uniform way on the whole surface of a Si thin-film electrode through the VC-derived effective surface film. This assumption was proved by ex situ Raman spectroscopy. Figure 2 shows Raman spectra of an amorphous Si thin-film electrode after charging (lithiation) in VC-containing electrolyte solution, together with that of an as-prepared film as a reference. The line that peaks at around 480 cm-1, which is identified as an amorphous Si, decreased upon charging particularly at a blister (B in Fig.2), as compared with a smooth region (A in Fig.2). These results indicate that the blister consisted of Li-Si alloy; Si significantly expands upon alloying with Li and its mechanical strength lowers with an increase in Li content [2], and hence the blister should form. Such non-uniform morphological changes of Si upon lithiation can be suppressed by using VC additives, which results in high cycleability of Si electrode.