Stable Voltage Window Research Articles

Stable Performance of Aluminum‐Metal Battery by Incorporating Lithium‐Ion Chemistry

AbstractBased on the recent discovery of the ionic liquid involving the AlCl4−‐Al2Cl7− redox couple as an electrolyte, aluminum (Al) rechargeable batteries have received revamped interest. However, the corrosive nature of the chloride ion and Al2Cl7− makes it challenging to find suitable current collectors and cathode materials. Here, we screen various metals and carbon materials as current collectors, and indeed find that none of the metals commonly used for battery current collectors is stable against corrosion from the electrolyte. Only pyrolytic graphite is stable, but exhibits undesired AlCl4− intercalation at 1.7 V vs. Al/Al3+. The addition of lithium chloride (LiCl) salt to the electrolyte not only eliminates AlCl4− intercalation through Li−AlCl4− coordination, but also extends the stable voltage window to 2.3 V vs. Al/Al3+. Moreover, the incorporation of Li ions allows us to engage the established LiFePO4 olivine cathode in Li‐ion batteries. This Al−Li hybrid cell exhibits an operation voltage of 1.3 V with robust cyclability (83.4 % retention after 400 cycles) and minimal self‐discharge. This series of results suggest that proper employment of Li‐ion chemistry can improve the electrochemical performance and safety of Al rechargeable batteries.

Relevant Features of a Triethylene Glycol Dimethyl Ether-Based Electrolyte for Application in Lithium Battery.

Triethylene glycol dimethyl ether (TREGDME) dissolving lithium trifluoromethanesulfonate (LiCF3SO3) is studied as a suitable electrolyte medium for lithium battery. Thermal and rheological characteristics, transport properties of the dissolved species, and the electrochemical behavior in lithium cell represent the most relevant investigated properties of the new electrolyte. The self-diffusion coefficients, the lithium transference numbers, the ionic conductivity, and the ion association degree of the solution are determined by pulse field gradient nuclear magnetic resonance and electrochemical impedance spectroscopy. The study sheds light on the determinant role of the lithium nitrate (LiNO3) addition for allowing cell operation by improving the electrode/electrolyte interfaces and widening the voltage stability window. Accordingly, an electrochemical activation procedure of the Li/LiFePO4 cell using the upgraded electrolyte leads to the formation of stable interfaces at the electrodes surface as clearly evidenced by cyclic voltammetry, impedance spectroscopy, and ex situ scanning electron microscopy. Therefore, the lithium battery employing the TREGDME-LiCF3SO3-LiNO3 solution shows a stable galvanostatic cycling, a high efficiency, and a notable rate capability upon the electrochemical conditions adopted herein.

Electrochemical Redox Behavior of β-Li3PS4 lithium-Ion Conducting Solid Electrolyte

Lithium-ion conducting solid electrolytes are a promising alternative to conventional liquid electrolytes for making fire resistant and thus safer Li-ion batteries. Recent improvements in the conductivity of sulfide-based glass/ceramic electrolytes (> 0.1 mS/cm) have prompted further investigation into their material properties and interfacial characteristics [1]. Particularly, β-lithium thiophosphate (β-Li3PS4), owing to its sufficiently high conductivity (~0.2 mS/cm), scalable synthesis route, and favorable bulk mechanical properties, is a potential electrolyte candidate for solid-state Li-ion batteries [2]. Existing issues, however, surround the interface between β-Li3PS4 and conventional 4 V cathodes due to the limited electrochemical stability window of the electrolyte (1.7-2.31 V, theoretical [3]). The resulting solid electrolyte interface poses high impedance resulting in poor rate capability. While oxide-based coatings of cathode active material particles have shown improvement, long term mechanical reliability of the coating is in question [4]. In order to address the interfacial impedance issue with the β-Li3PS4 solid electrolyte, it is important to characterize its redox behavior outside its stable voltage window. We have developed a novel technique based on cyclic voltammetry, which treats the solid electrolyte as an active material electrode and uses internal redox-capable standards (in this case, elemental phosphorus and sulfur) to systematically determine the species makeup of β-Li3PS4 at the β-Li3PS4/carbon interface as a function of the applied potential. Through this technique, we show that even in the absence of active material, β-Li3PS4 undergoes decomposition at the solid electrolyte/carbon interface producing redox active species of phosphorus and sulfur. Further, as the cell voltage is cyclically swept between 0-5 V vs. Li/Li+ and the oxidation states of these species change as a consequence, the result is a constantly changing interfacial composition during typical cell operation. This is unlike in case of liquid electrolytes which decompose into electrochemically irreversible species under normal operating conditions. In addition, via ex-situ XPS surface analysis of the β-Li3PS4/C interface, we have observed the formation of elemental sulfur and P2S5 upon oxidation of β-Li3PS4 solid electrolyte to 5 V vs. Li/Li+, confirming DFT-based theoretical predictions [3]. Formation of elemental sulfur is shown via the developed cyclic voltammetry based technique to be the cause for the apparent reversible cyclability of the sulfide-based solid electrolytes reported in literature [5]. Through our analysis we highlight the dynamic nature of the interface between β-Li3PS4 solid electrolyte and a high voltage material. Our technique may be used to characterize other superconducting sulfide-based solid electrolytes such as Li7P3S11 and Li10GeP2S12. Determining the properties of this redox capable solid-electrolyte/cathode interface and developing methods to mitigate the formation of high impedance species is an essential next step for sulfide-based solid electrolyte research. Acknowledgments We gratefully acknowledge support from the US Department of Energy’s Office of Basic Energy Science for the Chemo-mechanics of Far-From-Equilibrium Interfaces (COFFEI) small group, through award number DE-SC0002633.

Hybrid Mg/Na Ion Battery with High Capacity and Ultra-Long Cycle Life

The development of new energy storage systems beyond lithium ion batteries is a central task for electric transportation and grid energy storage. Sodium-ion batteries (SIBs) have attracted great interest owing to the natural abundance and low cost of sodium[1]. However, emerging anode materials for SIBs suffer from reduced cycle stability and limited specific capacity. In contrast to Li and Na, metallic magnesium (Mg) can be safely used as an anode material in Mg-ion batteries (MIBs)[2]. But, the development of MIBs is hindered by a lack of suitable cathode materials and electrolytes and overall anode-electrolyte-cathode incompatibilities. In order to fully exploit the cost and safety benefits of SIBs and MIBs, we present herein a hybrid Na/Mg ion battery in which metallic Mg serves as the anode, commercial titanium disulfide (TiS2) as the cathode, and an electrolyte composed of 1.0 M NaBH4 + 0.1 M Mg(BH4)2 dissolved in diglyme. The stable voltage window of the chosen electrolyte is limited to 0.4 ~ 2.0 V vs. Mg2+/Mg, as confirmed by control experiments using a stainless steel current collector. Galvanostatic cycling shows that most of the discharge capacity was released in the voltage window of 0.8~1.2 V (Figure 1a). Good synergetic effect between the anode, cathode and the electrolyte is evidenced by high Columbic efficiency of ~ 100%. Rate capability test demonstrates that the hybrid ion battery shows excellent rate performance (Figure 1b). Cells were initially cycledat 200 mA g-1 (~ 1 C rate) obtaining a stable discharge capacity of 190 mAh g-1. Then the current was increased to 500, 1000, 2000 and 4000 mA g-1. Even at such high rates, the battery cells could deliver stable capacity of 160, 130, 110 and 60 mAh g-1, respectively. Long term charge-discharge cycling shows that the battery cell could deliver 90 mAh g-1 after 10000 cycles which is 80 % of its highest capacity at the 10 C rate (Figure 1c). Elemental analysis of the fully discharged cathode shows that both Na+ (~ 90 at.%) and Mg2+ (~ 10 at.%) ions are intercalated into the TiS2 cathode. Meanwhile, the (001) interlayer space is increased from 0.57 nm to 0.66 nm as determined by high transmission electron microscope (HRTEM). On the anode side, nano sized metallic Mg grains are uniformly deposited on the current collector after long term cycling, without formation of any Mg dendrites. The ionic diffusion coefficients of the TiS2 cathode are determined by galvanostatic intermittent titration technique (GITT), which are 10-9~10-10 cm2s-1 during the whole discharge process. It is believed that the large ionic diffusion coefficients results in high rate capability of the battery. Due to the above excellent electrochemical performance, our hybrid Na/Mg ion battery shows promising potential in stationary energy storage and grid energy storage. Figure 1. Charge-discharge voltage profiles (a), rate dependent cycling performance (b), and cycling performance of the hybrid Na/Mg ion battery (c). Acknowledgements This work was supported by the Ministry of Science and Technology of China under the grant No. 2015CB251103.

Degradation Mechanisms of Magnesium Metal Anodes in a (CF3SO2)2N- (TFSI)-Based Mg-Ion Electrolyte

Efficiently using anodes based on the deposition of light metals would be a breakthrough that would produce leaps in energy density of rechargeable batteries. Magnesium is one of such metal electrodes that are not susceptible to dendrite formation. This unique property stems from (1) the thermodynamic stability in Grignard-based electrolytes and (2) the inherent preference to form three-dimensional deposits. However, the use of Grignard-based electrolytes results in severe chemical reactions between the nucleophilic electrolytes and electrophilic cathodes such as oxide materials. Accordingly, a lot of effort has been paid to find non-nucleophilic electrolyte solutions that deposit and dissolve Mg metal reversibly. Among non-nucleophilic anions, bis(trifluoromethylsulfonyl)imide (CF3SO2)2N–, TFSI) anion has shown exceptional stability on Mg metal anode. And the ethereal solution of the magnesium salt (MgTFSI2) has shown quite reversible Mg metal deposition and dissolution with coulombic efficiency of ~90% and reasonable voltage stability window of about 3 V. The most important difference from the Grignard-based electrolytes is that the TFSI-based electrolyte solution is non-nucleophilic and possibly compatible with oxide cathodes, which can increase the voltage and energy density of Mg rechargeable batteries. Herein, we report the mechanisms of degradation in the Mg metal anode and electrolyte after hundreds of cycles at high current density of 2 mA/cm2. Although the Mg metal anode operated without a significant problem at current densities lower than 0.1 mA/cm2, degradation was evident after long-term cycling at high current density. Overall, the Mg metal became mechanically brittle, with microscopy showing extremely porous structures (Fig. 1a). In addition, large hemispherical deposits up to several hundred micrometers were scattered or aggregated at the Mg surface (Fig. 1b). In turn, the color of the electrolyte turned to dark brown upon cycling, indicating a change in the chemical structure of the solvent. The macroscopic consequence was the short circuiting of the cells after several cycles at a high current density. These results suggest that the lemma of dendrite-free metal deposition is possible when the system meets several conditions such as absolute chemical stability of the metal in the electrolyte, adequate level of current density and charge density, and other engineering factors such as organic or inorganic additives that complexes with the metal ion and lead to more uniform metal deposition. Those understandings will serve as step stones to get to non-nucleophilic electrolytes, in which metal anodes operate without dendrite formation. Figure 1

Flexible coaxial-type fiber solid-state asymmetrical supercapacitor based on Ni3S2 nanorod array and pen ink electrodes

Fiber supercapacitors have attracted public attentions because of their merits of relatively high capacitance density, flexibility and easy integration with various electronic devices. In this work, a flexible coaxial-type fiber solid-state asymmetrical supercapacitor (ASC) based on Ni3S2 nanorod array electrode and pen ink electrode was successfully fabricated. The Ni3S2 nanorod array electrode was synthesized by directly growing Ni3S2 nanorod arrays on a nickel wire through a facile hydrothermal method and the pen ink electrode as negative electrode was synthesized by a simple dip-coating method. The solid-state ASC device presents a stable voltage window of 1.4 V and behaves a high specific capacitance of 34.9 F g−1 (87.25 F cm−1) at a scan rate of 10 mV s−1. Compared with the symmetric supercapacitor (SSC) based on Ni3S2 electrodes, the ASC device provide an increased energy density of 8.2 Wh kg−1 (0.81 mWh cm−3) at the power density of 214.6 W kg−1 (21.12 mW cm−3). In addition, the ASC device also exhibits excellent electrochemical stability with 93.1% of initial specific capacitance after 3000 consecutive cycles and good mechanical stability. These encouraging results present its great potential in flexible solid-state energy storage devices.

A 1.8 V Aqueous Supercapacitor with a Bipolar Assembly of Ion-Exchange Membranes as the Separator

We introduce a novel bipolar assembly of ion-exchange membranes as the separator for aqueous supercapacitors. The new bipolar separator enables the positive electrode and the negative electrode to operate in acidic electrolyte and alkaline electrolyte, separately. The bipolar separator increases the theoretically stable voltage window from 1.23 to 1.76 V for the device when pH 1 and pH 10 are selected for the positive and negative electrode, respectively, based on the pH tolerance of commercial ion exchange membranes. By considering the results from comprehensive electrolyte stability investigation, we charge the cells to 1.8 V, which increases the maximum cell discharge voltage to 1.77 V. A specific energy of 12.7 Wh/kg is achieved based on the electrodes’ mass, which is twice the value of the comparative best performing single electrolyte. The new cell configuration with the three-compartment bipolar separator effectively prevents the acid/base electrolyte cross diffusion, where after 10,000 cycles, the capacitance retention is 97% with coulombic efficiency maintained above 99.6% all through cycling. The design here may open up a new avenue toward high-energy aqueous energy storage devices.

NH3 assisted photoreduction and N-doping of graphene oxide for high performance electrode materials in supercapacitors.

Nitrogen-doped graphene was synthesized by simple photoreduction of graphene oxide (GO) deposited on nickel foam under NH3 atmosphere. The combination of photoreduction and NH3 not only reduces the GO in a shorter time but also induces nitrogen doping easily. The nitrogen doped content of N-rGO@NF reaches a high of 5.99 at% with 15 min of irradiation. The nitrogen-doped graphene deposited on Ni foam (N-rGO@NF) can be directly used as an electrode for supercapacitors, without any conductive agents and polymer binders. In the electrochemical measurement, N-rGO@NF displays remarkable electrochemical performance. In particular, the N-rGO@NF irradiated for 45 min at a high current density of 92.3 A g(-1) retained about 77% (190.4 F g(-1)) of its initial specific capacitance (247.1 F g(-1) at 0.31 A g(-1)). Furthermore, the stable voltage window could be extended to 2.0 and 1.5 V by using Li2SO4 and a mixed Li2SO4/KOH electrolyte, and the maximum energy density was high up to 32.6 and 21.2 Wh kg(-1), respectively. The results show that compared to Li2SO4, a mixed electrolyte (Li2SO4/KOH) more efficiently balances the relationship between the high energy densities and high power densities.

Low-temperature sintering effects on NASICON-structured LiSn2P3O12 solid electrolytes prepared via citric acid-assisted sol-gel method

LiSn2P3O12 with sodium (Na) super ionic conductor (NASICON)-type rhombohedral structure was successfully obtained at low sintering temperature, 600 °C via citric acid-assisted sol-gel method. However, when the sintering temperature increased to 650 °C, triclinic structure coexisted with the rhombohedral structure as confirmed by X-ray diffraction analysis. Conductivity–temperature dependence of all samples were studied using impedance spectroscopy in the temperature range 30 to 500 °C, and bulk, grain boundary and total conductivity increased as the temperature increased. The highest bulk conductivity found was 3.64 × 10−5 S/cm at 500 °C for LiSn2P3O12 sample sintered at 650 °C, and the lowest bulk activation energy at low temperature was 0.008 eV, showing that sintering temperature affect the conductivity value. The voltage stability window for LiSn2P3O12 sample sintered at 600 °C at ambient temperature was up to 4.4 V. These results indicated the suitability of the LiSn2P3O12 to be exploiting further for potential applications as solid electrolytes in electrochemical devices.

Effect of cation on diffusion coefficient of ionic liquids at onion-like carbon electrodes

While most supercapacitors are limited in their performance by the stability of the electrolyte, using neat ionic liquids (ILs) as the electrolyte can expand the voltage window and temperature range of operation. In this study, ILs with bis(trifluoromethylsulfonyl)imide (Tf2N) as the anion were investigated as the electrolyte in onion-like carbon-based electrochemical capacitors. To probe the influence of cations on the electrochemical performance of supercapacitors, three different cations were used: 1-ethyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium and 1,6-bis(3-methylimidazolium-1-yl). A series of electrochemical characterization tests was performed using cyclic voltammetry (CV), galvanostatic cycling and electrochemical impedance spectroscopy (EIS). Diffusion coefficients were measured using EIS and correlated with quasielastic neutron scattering and molecular dynamics simulation. These three techniques were used in parallel to confirm a consistent trend between the three ILs. It was found that the IL with the smaller sized cation had a larger diffusion coefficient, leading to a higher capacitance at faster charge–discharge rates. Furthermore, the IL electrolyte performance was correlated with increasing temperature, which limited the voltage stability window and led to the formation of a solid electrolyte interphase on the carbon electrode surface, evident in both the CV and EIS experiments.

Development of Non-Aqueous Redox Flow Batteries at Pacific Northwest National Laboratory

Redox flow battery (RFB) is considered as one of the most promising large-scale stationary energy storage technologies and has attracted much attention both academically and industrially. The unique cell architecture renders RFB with attractive advantages including separation of energy and power, exceptional design flexibility, excellent scalability and modularity, long service life, high efficiency, etc.Therefore, RFB is a well-suitable option to stabilize the power grid and tackle the intermittency of the renewable energies. Conventional aqueous RFBs have achieved significant progress such as vanadium-vanadium, iron-vanadium, zinc-bromine, polysulfide-bromine systems. However, aqueous RFBs also are confined by two major intrinsic limitations: narrow voltage window to avoid water electrolysis and relatively low concentrations of the electroactive materials, making them generally low energy density systems.Non-aqueous electrolytes provide wider electrochemically stable voltage window and examples having cell potential of >2V have been demonstrated recently. The expansion of the operational cell potential has a direct impact on the energy density of the non-aqueous RFB system. Meanwhile, the wider voltage windows make more redox couples available as electroactive material choices and increase the possibility of using multiple electron transfer redox reactions. Therefore, a number of non-aqueous RFB systems have been proposed and investigated [1]. Most of these systems are based on organic active materials that have the advantages of convenient structural tunability and availability of different redox mechanisms.Here we report the development of non-aqueous RFBs at PNNL with the focus on several lithium-organic RFB systems, each adopting a different organic cathode material [2]. Chemical functionalization to the organic active materials was carried out to increase its solubility targeting high energy density energy storage systems. The flow cells of these systems demonstrated excellent electrochemical performance with impressive cell efficiencies, high operational current density, and remarkable cycling stability that exceeded those of other reported non-aqueous RFBs. Effective protection of the lithium anode employed in these lithium-organic RFB systems will be discussed.

Electrochemical performance of lithium ion capacitors using aqueous electrolyte at high temperature

Aqueous lithium ion capacitors (LICs) with LiFePO4 positive electrodes and activated carbon (AC) negative electrodes were developed and their electrochemical performances were investigated by cyclic voltammetry, galvanostatic charge/discharge, and ac impedance spectroscopy at various temperatures. It is found that the stable voltage window for the aqueous LICs is 0–1.7 V and the optimized mass ratio between the positive electrode and negative electrode is 1:1. The rate capability of the LIC is much higher than that of the LiFePO4 electrodes. The Ohmic resistance and electrochemical impedance of the LiFePO4 and AC electrodes decrease with increasing the temperature, leading to high rate capability of the LIC. The performance of the LIC is deteriorated when it is cycled at 70 °C which is caused by the reduction of Fe2+ in LiFePO4.

A novel 1,1′-bis[4-(5,6-dimethyl-1H-benzimidazole-1-yl)butyl]-4,4′-bipyridinium dibromide (viologen) for a high contrast electrochromic device

A new electrochromic viologen, 1,1′-bis-[4-(5,6-dimethyl-1H-benzimidazole-1-yl)-butyl]-4,4′-bipyridinium dibromide (IBV) was synthesized by di-quaternization of 4,4′-bipyridyl using 1-(4-bromobutyl)-5,6-dimethyl-1H-benzimidazole. X-ray photoelectron spectroscopy confirmed the formation of the IBV (viologen) salt as distinct signals due to quaternary nitrogen and neutral nitrogen, and ionic-bonded bromide were identified. An electrochromic device encompassing a dicyanamide ionic liquid based gel polymeric electrolyte with high ionic conductivity, a thermal decomposition temperature above 200°C, and a stable voltage window of ∼4V with the IBV viologen dissolved therein, was constructed. IBV is a cathodically coloring organic electrochrome and the device underwent reversible transitions between transparent and deep blue hues; the color change was accompanied by an excellent optical contrast (30.5% at 605nm), a remarkably high coloration efficiency of 725cm2C−1 at 605nm and switching times of 2–3s. Electrochemical impedance spectroscopy revealed an unusually low charge transfer resistance at the IBV salt/gel interface, which promotes charge propagation and is responsible for the intense coloration of the reduced radical cation state. The device was subjected to repetitive switching between the colored and bleached states and was found to incur almost no loss in redox activity, up to 1000cycles, thus ratifying its suitability for electrochromic window/display applications.

Hydrofluoroether electrolytes for lithium-ion batteries: Reduced gas decomposition and nonflammable

The optimum combination of high energy density at the desired power sets lithium-ion battery technology apart from the other well known secondary battery chemistries. However, this is besieged by thermal instability of the electrolyte. This “Achilles heel” still remains a significant safety issue and unless this propensity is improved the promise of widespread adoption of Li-ion batteries for Transportation application may not be realized. With this in mind we launched a systematic study to evaluate fluoro solvents that are known to be nonflammable, for thermal and electrochemical performances. We investigated hydro-fluoro-ethers (HFE) (1) 2-trifluoromethyl-3-methoxyperfluoropentane {TMMP} and (2) 2-trifluoro-2-fluoro-3-difluoropropoxy-3-difluoro-4-fluoro-5-trifluoropentane {TPTP} in Sandia-built cells. Thermal properties under near abuse conditions that exist in thermal runaway environment and the electrochemical characteristics for these electrolytes were measured. In the thermal ramp (TR) measurement, EC:DEC:TPTP-1M LiBETI (or TFSI or LiPF6) electrolytes exhibited no ignition/fire. Similar behavior was observed for the EC:DEC:TMMP-1M LiBETI. Further, in ARC studies the HFE electrolytes generated less gas by 50% compared to the EC:EMC-1.2M LiPF6 {CAR-1} electrolyte. Although in all cases the HFEs generated less gas, the onset of gas generation appears to depend on the salt. For the LiBETI and TFSI containing HFEs the onset is pushed out by ∼80°C and for the LiPF6 the onset is comparable to that of the CAR-1. The solution ionic conductivity of these HFE electrolytes was lower (4–5 times) than that of the CAR-1 electrolyte however, the electrochemical performance was comparable. For example, full cells in 2032 type coin cells containing LiMN0.33Ni0.33Co0.33O2 cathode and carbon anode showed around 5mAh capacity and the computed specific capacity was ∼154mAh for all the electrolytes. In half-cells against lithium the cathode and anode gave specific capacity on the order of 170mAh and 340mAh respectively. These electrolytes when tested in 18,650 cells containing the above cathode and anode also showed comparable capacity. Further, the voltage stability window was not compromised by the HFEs. ARC measurements on 18,650 full cells showed less gas generation for the HFE electrolytes compared to CAR-1 electrolyte.

Improved Porous Mixture of Molybdenum Nitride and Tantalum Oxide as a Charge Storage Material

High surface area γ‐molybdenum nitride has shown promise as a charge storage material. The addition of amorphous tantalum oxide to the molybdenum nitride system not only improves the film cohesion tremendously, but also widens the voltage stability window from 0.8 to 1.1 V. This occurs without adversely effecting the capacitance.

Stability domain of a complexed lithium salt-poly(ethylene oxide) polymer electrolyte

The redox stability domain of a polyethylene oxide-lithium trifluoromethanesulphonate polymer electrolyte has been investigated using low-sweep-rate voltammetry, in the range 100–170°C. Below 140°C, its voltage stability window is in excess of 3.3 volts, making it a candidate for use with practical electrochemical couples. The electrolyte is not, however, a pure cation conductor, and problems associated with the observed significant anion mobility must be overcome in order to improve the polymer's suitability as a solid-state electrolyte for battery applications.