Hybrid Electrolyte Engineering Enables Safe and Wide‐Temperature Redox Flow Batteries

Chapter 6 - Membranes for vanadium-air redox flow batteries

The mostly widely used lithium (Li) batteries, including lithium-ion (Li-ion) batteries, are activated via organic solvent based electrolytes that are highly flammable. The flammable electrolyte can potentially impose impediments to large-format battery device implementation because, under abuse conditions (e.g., internal short circuit, overcharge, crush, etc.), Li batteries can be hazardous due to heat generation which can potentially lead to toxic/flammable material release, fire and over-pressurization/explosion [1]. These hazards represent major concerns for power source platforms populated by personnel, sensitive equipment and/or vital structures. Recent high-profile incidents involving energy dense batteries (e.g., the Boeing 787 Dreamliner battery fire incident [2]) have increased public awareness of battery safety and resulted in intensified scrutiny for the processes used to develop, manufacture, certify and transport Li batteries. Here we present ADA efforts in the development of a Li battery technology comprised from a nonflammable, high performance, room temperature ionic liquid (RTIL)-based or hydrofluoroether (HFE)-based electrolytes and a nano-engineered, atomic layer deposition (ALD) functional material coated, high voltage, high energy density cathode (i.e., high voltage LiCoO2 or HV-LCO). We demonstrate that such combinations can afford a Li battery technology with an unprecedented high energy density (over state-of-the-art Li-ion or Li batteries), high rate capability, a wide operation temperature range, long cycle life at room temperature (RT) and temperature extremes and safety. Figure 1 shows electrolyte flammability test results. A lighter was used to ignite the electrolyte in a watch glass dish in open air. An organic solvent based, Li-ion battery baseline (conventional) electrolyte was ignited and self-burning when the ignitor was moved away. Each of the RTIL- and HFE-based electrolytes did not ignite and, no self-combustion was observed. Figure 2 shows discharge voltage profiles of Li/HV-LCO cells with RTIL- and HFE-based nonflammable electrolytes as a function of temperature. The cells were cycled between 3-4.6V. At RT, the cells delivered a high specific capacity/energy (> 200 mAh/g and > 800 Wh/kg, cathode active weight). At 50°C, the cells delivered a slightly higher (1-3%) specific capacity than RT. At -20°C, the cells were cycled at C/10 rate and delivered ~75% of the capacity at RT (~155 mAh/g). At -40°C, the cells were cycled at C/20 rate. The HFE-based electrolyte cell delivered a much higher specific capacity (66.5 mAh/g) than the RTIL-based (negligible). Figure 3 shows excellent rate capability of the RTIL-based electrolyte with 85% capacity retention at 10C rate. Figure 4(a) shows high voltage cycle life between 3-4.6V at RT for pristine LCO and HV-LCO based Li cells using the RTIL-based electrolyte. The pristine LCO cells experienced a rapid capacity fade while the HV-LCO cells demonstrated excellent cycle life stability with > 92% capacity retention after 100 cycles. We attribute the excellent HV-LCO high voltage cycle performance to the ALD functional material coating on LCO; this coating effectively suppresses the degradation root cause, i.e., the Co4+ dissolution and hexagonal to monoclinic phase transition [3]. We also evaluated cycle life at an extreme high temperature of 75°C. Figure 4(b) shows high voltage cycle life test data for pristine LCO and HV-LCO based lithium cells using RTIL-based electrolyte with additives. The cells were cycled between 3-4.5V. The pristine LCO based cell still faded much more quickly than the HV-LCO based cell; the latter exhibited an extraordinary high voltage high temperature cycle resiliency (97% capacity retention after 30 cycles, vs. 75% of the pristine LCO) even at the extreme high temperature of 75°C. We attribute the outstanding cycle stability at 75°C to the synergistic benefits of nano-engineered cathode and passivation protection from the electrolyte additives. Our study highlights the importance of combining high performance, safe electrolyte with nano-engineered cathode (e.g., ALD nano-engineered LCO) to render the resultant lithium battery with unprecedented energy density (gained from the high voltage operation), high rate capability, safety (nonflammability) and robust cycle life over a wide temperature range particularly at the extreme high temperatures (e.g., 75°C). We demonstrate that our Li battery technology is ideally suited for many high energy density and safety demanding applications, such as, electric drive vehicles, consumer electronics, medical, space and military applications. References Peter Roth and Christopher J. Orendorff, Electrochem. Soc. Interface, Summer 2012, page 44.“FAA Grounds Boeing 787 Dreamliner for Battery Problems”, Aviation International News, January 2013Akira Yano, Masahiro Shikano, Atsushi Ueda, Hikari Sakaebe, and Zempachi Ogumi, Electrochem. Soc., 164, A6116 (2017). Figure 1

Energy storage systems (ESSs) are required for stable electricity supply from power plants and efficient usage of generated electricity. ESSs are also needed to accommodate fluctuations in renewable energy generation. Redox flow batteries (RFBs) are considered as one of the reliable choices for ESSs. Since 1970s, RFB technologies have been developed mainly on the basis of aqueous electroytes, which show a critical drawback; limited working voltage (< 1.23 V) due to a narrow electrochemical stability window of water.1 This constraint can, however, be relieved if electrochemical stability window is widened by deliberately controlling the composition of non-aqueous electrolyte systems. Redox couple is the core component in non-aqueous electrolyte systems, which should satisfy at least the following requirements; (i) high working voltage, (ii) high solubility in non-aqueous solvents, (iii) stability for high couloumbic efficiency and cycle retention, and (iv) facile molecular diffusion in solutions. It must be additionally beneficial if it can be used for both catholyte and anolyte. When such a single electrolyte component is used for RFBs, the loss of redox couple by side reactions can be greatly suppressed. A metal-multidentate complex, (nickel(II)-1,4,8,11-tetracyclotetradecane, Ni(cyclam)2+) has been tested if or not it satisfies the above requirements. The nickel complex is reduced and oxidized at -1.78 V and 0.74 V (vs. Fc/Fc+), respectively (Fig. 1). Hence, this redox couple can be used as a single electrolyte component for non-aqueous RFBs. The expected working voltage is as high as 2.52 V, which is much larger than those for conventional aqueous RFBs. The maximum solubility is ~0.4 M at room temperature. This value is again higher than those for the conventional metal-ligand complexes (0.1 M).2 A much enlarged energy density is thus expected with this RFB since it shows larger values for both cell voltage and capacity. This redox couple also exhibits a stable cycle performances. As seen in Fig. 1, the current profiles are not changed even after 50 cycles. Galvanostatic charge-discharge cycling is carried out in a non-flowing static cell. The coulombic efficiency is ~ 75% after 3 cycles.

Developing new electrolytes that cater to the needs of lithium anode and Ni-rich cathodes is critical for next-generation battery technologies with high energy density and enhanced safety. Although significant advances have been made in non-aqueous electrolytes with wide electrochemical stability window (ESW), the performance of lithium battery is still not comparable to that of commercial Li-ion batteries (LIBs) due to the notorious reactivity of metallic lithium as well as the catalytic nature of high-voltage cathode. Such situations require rational design of advanced electrolytes and mechanistic understanding on interphase formation for stabilizing the electrolyte-electrode interfaces and supporting the next-generation battery chemistries beyond LIB technology.In the presentation, I will introduce the design and development of nonflammable fluorinated electrolytes (NFEs) in Prof. Chunsheng Wang group. We demonstrated that the NFEs with wide ESW are able to form thin fluorinated interphase on both Li anode and high-voltage Ni-rich cathodes, thus leading to high efficiencies of Li-metal plating/stripping (> 99.2%) for a 5 V cathode LiCoPO4 (~99.81%) and a Ni-rich LiNi0.8Mn0.1Co0.1O2 cathode (~99.93%).[1] Meanwhile, the NFEs with LiDFOB additive enables high-energy cobalt-free LiNiO2 (LNO) cathode to achieve a good cycle life.[2] The in-situ-formed F, B-rich interphase is proved to protect the LNO from continuous reactions with electrolyte and suppress the structural transformation from layered to disordered rock-salt (NiO) phase. Further study found that the non-polar fluorinated solvent could help maintain the high Li-ion conductivity of NFEs even under -125 oC, thus enable good cycling of all-temperature Li metal batteries operating from -80 to +70 oC.[3] The key findings on NFEs system provide a new avenue towards high-energy and safe Li metal batteries with wide operational temperature range.

n+InAs(Si) epitaxial thin films heavily doped by silicon and Hall effect magnetic field sensors based on this structures have been fabricated and studied. We have demonstrated the successful formation of highly doped InAs thin films (∼100 nm) with the different intermediate layer arrangement and appropriate electron mobility values. Hall sensors performance parameters have been measured in wide temperature range. Obtained sensitivity varied from 1 to 40 Ω/T, while the best linearity and lower temperature coefficient have been found in the higher doped samples with lower electron mobility. We attribute this to the electron system degeneracy and decreased phonon contribution to electron mobility and resistance.

Radiation induced degradation of widely used bipolar voltage comparators was investigated in wide temperature range from high to liquid nitrogen temperatures. It was obtained, that the parametric degradation is significantly greater at low temperature measurements.

Localized data conversions for sensors placed under harsh environmental conditions often require Analog to Digital Converters (ADC) capable of working under wide operating temperature range. Even though the fastest analog to digital converter, flash ADC is omitted from the high temperature regime due to the requirements of reference voltage source, reference voltage divider and a comparator array capable of providing stable operation under wide operating temperatures. A CMOS Schmitt inverter-based internal reference comparator array for flash ADC which is efficient in terms of area and power consumption, capable of working under wide temperature range is presented in this paper. The proposed idea replaces the reference voltage source, resistive reference voltage divider and differential comparator array of a conventional flash ADC by an array of suitably sized Schmitt inverters. The circuit was simulated for static and dynamic characteristics with gpdk 180 nm library using cadence virtuoso simulation suite. The voltage transfer characteristic curve (VTC) of the proposed comparator array was found invariable when a temperature sweep in the range of −40 to 170 degree Celsius was performed. Each of the comparator in the proposed array consumes a silicon area of 40.68 and the average dynamic power consumption per cycle is estimated as 103.1 uW.

Commercial perfluorosulfonic acid-based electrolyte membranes (PFSA, e.g., Nafion) are essential components in polymer electrolyte fuel cells (PEFCs) operated in the normal temperature range from 60 to 80 °C. Toward the widespread use of heavy-duty vehicles (HDVs) equipped with PEFCs, it will be necessary to approach higher temperature operation, up to approximately 120 °C, in addition to the normal temperature range, leading to the concept of so-called wide temperature range operation. We here propose a composite PFSA membrane with a hydrophilic filler of Ta-doped TiO2 (Ta-TiO2) for wide temperature range operation. A composite PFSA membrane (9 cm × 9 cm × 25 μmt) prepared by die coating was flexible, without pores or cracks. The hydrophilic Ta-TiO2 filler with a unique fused-aggregate network microstructure was well-dispersed in the composite membrane. The proton conductivity of the composite membrane was found to be 1.3 times higher than that of a commercial Nafion membrane at 80 °C over a wide humidity range, from 20% RH to 80% RH. Small-angle X-ray scattering (SAXS) profiles indicated a morphological change of the composite membrane compared with a commercial Nafion membrane and the preservation of a constant water cluster size over a wide range of humidity. The proton conductivity also increased, without an increase in water uptake, most likely due to enhanced hopping between the clusters. The mechanical strength of the composite membrane was similar to that of the commercial membrane. The current–voltage performance of a single cell using the composite membrane was higher than that for a cell using a commercial Nafion membrane at 120 °C, 20% RH, and was equivalent to that of a cell using Nafion at 80 °C, 100% RH. Thus, the newly developed composite membrane can be considered as an effective candidate to be applied in PEFCs operating over a wide temperature range.

Performance Testing of Lithion 8-Cell, 25 AHr Lithium-Ion Batteries for Future Aerospace Applications

The redox flow battery has received significant research interest as a large scale energy storage system for renewable-energy sources. Although the all-vanadium aqueous redox flow battery has been most developed, its energy density is not sufficient mainly because of narrow electrochemical stability window of water. Therefore, recent efforts have shifted to developing nonaqueous redox flow batteries because they can achieve higher voltage due to wider electrochemical stability window of nonaqueous electrolytes. However, nonaqueous redox flow batteries generally suffer from insufficient energy densities because of low solubility of organic active materials in nonaqueous electrolytes. In this study, we designed and synthesized two highly soluble organic active materials, benzoquinone 1 and naphthoquinone 2 having diethylene glycol mono methyl ether groups. Both 1 and 2 were highly soluble in propylene carbonate (PC). Charge-discharge properties of the batteries using 1 and 2 as organic active materials were measured in LiBF4/PC using a cell equipped with a carbon fiber anode, a lithium-metal anode, and a glass filter separator. The charge-discharge measurements showed that 1 and 2 exhibited high voltages of ca. 3 V and good charge-discharge properties. Notably, 2 showed a higher energy density than conventional all-vanadium aqueous redox flow batteries.

Redox flow batteries have been expected as the large scale storage battery. Redox flow batteries utilize the oxidation and reduction of two soluble redox couples for charging and discharging without the solid phase reaction. Therefore, the cycle lives of the redox flow batteries are expected to be much longer than those of rechargeable lithium batteries using insertion-type active materials. Currently, vanadium redox flow batteries have been put into practical use to stabilize the supply of renewable energy. Vanadium redox flow batteries have low energy density compared to other batteries. Since aqueous solution is used for the electrolyte, the electrochemical window of the water limits the cell voltage. Although there has been various studies using an organic electrolyte in order to enlarge the cell voltage, the flammability and volatility of the organic solvents can be disadvantageous from the point of view of safety. 1) Recently, aprotic ionic liquids have attracted attention as nonaqueous electrolytes. Especially, aprotic ionic liquids containing bis(trifluoromethylsulfonyl)amide (TFSA-) have been studied extensively in the electrochemical field because they show favorable features such as incombustibility, involatility and wide electrochemical potential window. We have already reported the redox reactions of various metal complexes in the ionic liquids and their applicability to redox flow batteries. 2) However, there are a few reports regarding the redox battery using the ionic liquids probably due to lack of the suitable ion exchange membrane that can move the career ion in the ionic liquids. In the present study, a new redox battery, namely “lithium-redox battery”, was proposed using a redox reaction of an iron complex for a cathode reaction, dissolution and deposition of lithium for an anode reaction, and LiTFSA – tetraglyme (G4) solvate ionic liquid for an electrolyte. In addition, lithium ion conductive solid electrolyte (LICGCTM) was used as the diaphragm, which prevents penetration of the redox species and enables transfer of lithium ions. Tris(2,2'-bipyridine)iron(II) complex ([Fe(bpy)2]2+) was found to be soluble and stable in the solvate ionic liquid. The redox reaction between [Fe(bpy)3]2+ and [Fe(bpy)3]3+ was confirmed to be reversible by cyclic voltammetry. The formal potential of [Fe(bpy)3]3+/2+ was 4.1 V vs. Li|Li(I). Reversible charge-discharge cycling of the lithium redox battery having an average cell voltage of 3.9 V was possible for 30 cycles with a coulombic efficiency of more than 95%. References 1) Y. Matsuda, K. Tanaka, M. Okada, Y. Takasu, and M. Morita, J. Appl. Electrochem., 18, 909 (1988). 2) Y. Katayama, M. Yoshihara, and T. Miura, J. Electrochem. Soc., 162, H501 (2015).

All‐solid‐state thin film lithium‐ion microbatteries (TFBs) are regarded as the ideal on‐board power sources for microelectronic devices. However, the indispensable high preparation temperature (≥500 °C) required for commonly used cathodes, coupled with the Li anode's low melting point and high chemical reactivity, restricts TFBs’ on‐chip integration and high‐temperature operational capabilities. In this work, a Li x MnO 2 /LiPON/Nb 2 O 5‐x configuration (LMN‐TFB), featuring a low preparation temperature (180 °C) and exceptional chemical and thermal stability, is proposed to simultaneously enable a wide operational temperature range and on‐chip multilayer integration. The LMN‐TFB exhibits a reversible capacity of 26.5 µAh cm −2 and a long cycle life (83.5% capacity retention after 1500 cycles) at room temperature, as well as adequate working ability in a wide temperature range between ‐20 and 210 °C. Moreover, the integration of multilayer stacked LMN‐TFBs on a chip allows for increased capacity (37.5 µAh cm −2 ) or higher working voltage (6.5 V) than the individual LMN‐TFB, resulting in increased areal energy densities within the limited footprint area without significantly increasing the volume or weight. This work presents a promising approach for constructing integratable and wide‐temperature TFBs, offering significant potential for on‐board microelectronic devices.

With the exponential growth of electricity generation using intermittent renewable power sources, there is a need for affordable energy storage. For grid level storage in past years, redox flow batteries (RFB) have drawn considerable attention. Most successful RFBs till now are all-vanadium RFB and all-iron RFB. But these RFBs have a few drawbacks associated with them like; First RFB developed by NASA[1], Iron-chromium RFB, had problems of ions crossing over; all-iron RFB [2] have a problem of hydrogen evolution reaction (HER) at the negative electrode which causes capacity decay; and the commercialized all-vanadium RFB's [3] is expensive and has limited vanadium reserves.In an effort to address these problems, we present here the initial development of an all-tungsten RFB. Its key advantage is no capacity fading with cycles due to ions crossing over. The tungsten salt used was phosphotungstic acid (PTA). As shown in cyclic voltammograms (CV) of PTA in Figure 1(a), it is clear that PTA ions exist in four different oxidation states (PTA3-, PTA4-, PTA5- and PTA6-). For all-PTA redox flow battery experiments, we used the same PTA5- active species for both negative as well as positive electrolyte. Prior to the all-PTA RFB tests, PTA3- was reduced to PTA5-. Since PTA3- is the only salt available, it was reduced to PTA5- by using it as a negative electrolyte for the iron-tungsten RFB using a procedure developed earlier [4].As shown in reaction 1, during charging on the negative side, PTA5- gains 2 electrons and reduces to PTA6- oxidation state. Simultaneously, on the positive side PTA5- loses 2 electrons and gets oxidized to PTA3-. Moreover, the maximum potential difference that can be achieved by using the highest reduced form of phosphotungstic acid as a single cell is +0.611 V vs. RHE (Eq. 4). Although we are getting half the potential when compared to an all-vanadium or an all-iron battery. But due to its extremely fast kinetics and large discharge currents makes all-PTA RFB a promising alternative. For the large capacity of the battery, one of the deciding factors is the solubility of its active species salts and the maximum solubility of PTA3- is 0.7 M.Flow cell experiments were conducted using an in-house constructed standard fuel cell setup [4]. Meanwhile, CV experiments were conducted in 3 electrode setup using glassy carbon as working electrode, platinum mesh as counter electrode and mercury/mercurous sulphate (Hg/Hg2SO4) as reference electrode. Hg/Hg2SO4 was calibrated in 0.5 M H2SO4 using an in house RHE. Also, all the potentials mentioned here are with respect to RHE. A CV (see Figure 1(a)) for phosphotungstic acid was obtained which showed 3 distinct peaks corresponding to the 3 reactions. Reactions 2 and 3 involve transfer of 1 electron each, while reaction 1 requires 2 electrons as proven from the current peaks of the CVs as well as limiting currents obtained from their RDEs [5]. The involvement of parasitic HER is clear from the 3rd reduction peak. Kinetic parameters such as rate constant (k0), Tafel slope, diffusion coefficients (Do) and transfer coefficient (α) were determined using RDE studies. From RDE analysis at different RPM, rate constant for PTA3- was found to be 4.49 x 10-3 cm s-1.Meanwhile, for the flow cell experiments we have taken electrolyte solution concentrations similar to that for the CVs. The flow rate (120 mL min-1) and volume (40 mL) for both the electrolyte were kept same. As shown in Figure 1(b), the all-PTA battery was charged and discharged using chronopotentiometry at a current density of 50 mA cm-2. Furthermore, to obtain the i-V performance curve the cell was discharged from 2 mA to 1000 mA for 10s. The potential at the end of 10th second was reported to eliminate the pseudo steady state [6]. i-V and power density performance curve was plotted from these current and potential values (see Figure 1(c)).From just 40mM concentration of PTA5- the maximum volumetric capacity we got is around 200 mAh L-1, the maximum power density obtained was 10 mW cm-2 and the maximum current that can be drawn was 85 mA cm-2. The above results strongly suggest that the all-PTA RFB could indeed be a promising alternative. Figure 1

Multi-adaptability supercapacitor electrolyte based on Na-MMT/PDADMAC and application in wide temperature range

Developing solid-state lithium metal batteries with wide operating temperature range is important in future. Polyethylene oxide (PEO)-based solid-state electrolytes are extensively studied for merits including superior flexibility and low glass transition temperature. However, ideal usage temperatures for conventional PEO-based solid-state electrolytes are between 60 and 65°C, and unequable temperature degrades their electrochemical performances at low and high temperatures (≤25°C and ≥80°C). Herein, modification methods of PEO electrolytes for low, high especially wide-temperature applications are reviewed based on detailed analyses of mechanisms involved in its modification at different temperatures. First, shortcomings of PEO solid electrolytes due to influence of temperature are pointed out. Second, existing modification strategies are summarized in detail from three aspects of high, low especially wide temperatures, including application of PEO derivatives or chain segment modification treatment of PEO, addition of fillers, and other modification methods such as reasonable regulation of lithium salts, introduction of functional layers and addition of metal-organic frameworks (MOFs) or covalent organic frameworks (COFs). Finally, a summary and description of PEO-based solid electrolyte research and development trends for wide-temperature applications are provided. The review aims to offer some guidance for the creation of PEO solid batteries with wider working temperature ranges.