A non-aqueous electrolyte for the operation of Li/air battery in ambient environment

In this work we report a non-aqueous electrolyte that supports long-term operation of the Li/air battery in dry ambient environments based on a non-hydrolytic LiSO₃CF₃ salt and a low volatility propylene carbonate (PC)/tris(2,2,2-trifluoroethyl) phosphate (TFP) solvent blend. By measuring and analyzing the viscosity of PC/TFP solvent blends, the ionic conductivity of electrolytes, and the discharge performance of Li/air cells as a function of the PC/TFP weight ratio, we determined the best composition of the electrolyte is 0.2 m (molality) LiSO₃CF₃ 7:3 wt. PC/TFP for Li/O₂ cells and 0.2 m LiSO₃CF₃ 3:2 wt. PC/TFP for Li/air cells. Discharge results indicate that Li/air cells with the optimized electrolyte are significantly superior in specific capacity and rate capability to those with baseline electrolytes. More interestingly, the improvement in discharge performance becomes more significant as the discharge current increases or the oxygen partial pressure decreases. These results agree neither with the viscosity of the solvent blends nor the ionic conductivity of the electrolytes. We consider that the most likely reason for the performance improvement is due to the increased dissolution kinetics and solubility of oxygen in TFP-containing electrolytes. In addition, the electrolyte has a 5.15 V electrochemical window, which is suitable for use in rechargeable Li/air batteries.     

Discharge characteristic of a non-aqueous electrolyte Li/O2 battery

Discharge characteristic of Li/O₂ cells was studied using galvanostatic discharge, polarization, and ac-impedance techniques. Results show that the discharge performance of Li/O₂ cells is determined mainly by the carbon air electrode, instead by the Li anode. A consecutive polarization experiment shows that impedance of the air electrode is progressively increased with polarization cycle number since the surfaces of the air electrode are gradually covered by discharge products, which prevents oxygen from diffusing to the reaction sites of carbon. Based on this observation, we proposed an electrolyte-catalyst “two-phase reaction zone” model for the catalytic reduction of oxygen in carbon air electrode. According to this model, the best case for electrolyte-filling is that the air electrode is completely wetted while still remaining sufficient pores for fast diffusion of gaseous oxygen. It is shown that an electrolyte-flooded cell suffers low specific capacity and poor power performance due to slow diffusion of the dissolved oxygen in liquid electrolyte. Therefore, the status of electrolyte-filling plays an essential role in determining the specific capacity and power capability of a Li/O₂ cell. In addition, we found that at low discharge currents the Li/O₂ cell showed two discharge voltage plateaus. The second voltage plateau is attributed to a continuous discharge of Li₂O₂ into Li₂O, and this discharge shows high polarization due to the electrically isolating property of Li₂O₂.     

Diphenyloctyl phosphate and tris(2,2,2-trifluoroethyl) phosphite as flame-retardant additives for Li-ion cell electrolytes at elevated temperature

The effect of diphenyloctyl phosphate (DPOF) and tris(2,2,2-trifluoroethyl) phosphite (TTFP) as flame-retardant (FR) additives in the liquid electrolyte of Li-ion cells is evaluated at both elevated temperature (40 °C) and room temperature (RT, 25 °C). The tested cells use mesocarbon microbeads (MCMB) and LiCoO₂ as the anode and cathode materials, respectively. Cell characteristics are investigated by means of electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The results of the cycle performance tests demonstrate the superior discharge capacity and capacity retention of the DPOF-containing cell compared will TTFP after cycling at both RT and 40 °C. Therefore, these results confirm the promising potential of DPOF as an FR additive for improving the electrochemical performance of Li-ion batteries.     

Diffusion-limited model for a lithium/air battery with an organic electrolyte

A diffusion-limited transient mathematical model for a lithium/air cell, with the air cathode pores flooded with an organic electrolyte, has been developed. During cell discharge, the cathode pore radius profile is reflective of the distribution of the lithium peroxide product in the cathode. The cathode pore radius profile has been predicted as a function of time, current density, oxygen gas pressure, and cathode thickness for an assumed initial porosity and average cathode pore size. Transient concentration profiles of the dissolved oxygen in the electrolyte were also determined. Capacities of the lithium/air cell were predicted and compared favorably with literature experimental results.     

Investigation of the gas-diffusion-electrode used as lithium/air cathode in non-aqueous electrolyte and the importance of carbon material porosity

The gas-diffusion-electrode used in a Li-air cell has been studied in a unique homemade electrochemical cell. Three major obstacles for the development of a feasible Li-air system were discussed with a focus on the development of a functional gas-diffusion-electrode in non-aqueous electrolytes and the way of avoiding the passivation of gas-diffusion-electrodes caused by the deposition of the reduction products. It is the first time that the importance of establishing the 3-phase electrochemical interface in non-aqueous electrolyte is demonstrated by creating air-diffusion paths and an air saturated portion for an air cathode. A model mechanism of electrode passivation by the reaction products was also proposed. Lithium oxides formed during O₂ reduction tend to block small pores, preventing them from further utilization in the electrochemical reaction. On the other hand, lithium oxides would accumulate inside the large pores during the reduction until the density of oxides becomes high enough to choke-off the mass transfer. Carbon materials with a high surface area associated with larger pores should be selected to make the gas-diffusion-electrode for Li-air battery. For the first time, a near linear relationship between the capacity of GDE in a non-aqueous electrolyte and the average pore diameter was demonstrated, which could be used to estimate the capacity of the GDE quantitatively.     

Electrochemical performance of gel polymer electrolyte with ionic liquid and PUA/PMMA prepared by ultraviolet curing technology for lithium-ion battery

A series of diethylethyletherylmethanamine bis(trifluoromethanesulfonyl)imide (DEEYTFSI) ionic liquid gel polymer electrolyte based polyurethane acrylate (PUA)/poly(methyl methacryltae) (PMMA) matrix with different contents of DEEYTFSI, PUA and LiTFSI were prepared via ultraviolet (UV) curing system. Electrochemical performances of the gel polymer were studied by electrochemical station and charge–discharge system. The gel polymer electrolyte with 19 wt.% DEEYTFSI obtained a maximum conductivity σ of 2.76 × 10^?4 S cm^?1 and the transference number t_Li+ of ～0.22 at room temperature. 19 wt.% DEEYTFSI caused the easier transferring of lithium ions due to less apparent activation energy E_a of 21.1 kJ mol^?1. The DEEYTFSI/LiTFSI/PUA/PMMA electrolyte had good compatibility with LiFePO₄ cathode. The DEEYTFSI/LiTFSI/PUA/PMMA electrolyte with the electrochemical window of 4.70 V was enough stability for being the electrolyte material of lithium battery. The Li/19 wt.% DEEYTFSI–LiTFSI–PUA–PMMA/LiFePO₄ coin-typed cell cycled at 0.1 C presented 95% efficiency on the 50th cycle.     

Tris(pentafluorophenyl) borane as an electrolyte additive for LiFePO4 battery

The effects of tris(pentafluorophenyl) borane (TPFPB) additive in electrolyte at the LiFePO₄ cathode on the high temperature capacity fading were investigated by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), cyclability, SEM and Fourier transform infrared (FTIR). According to the study results, tris(pentafluorophenyl) borane has the ability to improve the cycle performance of LiFePO₄ at high temperature. LiFePO₄ electrodes cycled in the electrolyte without the TPFPB additive show a significant increase in charge transfer resistance by EIS analysis. SEM and FTIR disclose evidence of surface morphology change and solid electrolyte interface (SEI) formation. FTIR investigation shows various functional groups are found on the cathode material surface after high temperature cycling tests. The results showed an obvious improvement of high temperature cycle performance for LiFePO₄ cathode material due to the TPFPB additive. The observed improved cycling performance and improved lithium ion transport are attributed to decreased LiF content in the SEI film.     

Air electrode design for sustained high power operation of Li/air batteries

The rapid development of portable electronic devices increasingly requires much more energy to support advanced functions. However, currently available batteries do not meet the high energy requirement of these devices. Metal/air batteries, especially Li/air batteries, have a much higher specific energy than most other available batteries, but their power rate is limited by the accumulation of reaction products in the air electrode. Several approaches to improve the power rate of Li/air batteries have been analyzed in this work, including adjustment of air electrode porosity and catalyst reactivity distributions to minimize diffusion limitations and maximize air electrode material utilization. An interconnected dual pore system (one catalyzed and one non-catalyzed) is proposed to improve oxygen transport into the inner regions of the air electrode, but this approach alone cannot supply high power for long term applications. A time-release multiple catalyst approach is analyzed to provide temporal release of reactivity in the air electrode. When coupled with the dual pore configuration and catalysts with high reactivities, the time-release catalyst concept can extend the duration of higher powers to longer times, and result in maximum utilization of air electrode materials.     

A study on lithium/air secondary batteries—Stability of the NASICON-type lithium ion conducting solid electrolyte in alkaline aqueous solutions

The stability of the high lithium ion conducting glass ceramics, Li_1+x+yTi_2−xAl_xSi_yP_3−yO₁₂ (LTAP) in alkaline aqueous solutions with and without LiCl has been examined. A significant conductivity decrease of the LTAP plate immersed in 0.057 M LiOH aqueous solution at 50 °C for 3 weeks was observed. However, no conductivity change of the LTAP plate immersed in LiCl saturated LiOH aqueous solutions at 50 °C for 3 weeks was observed. The pH value of the LiCl-LiOH-H₂O solution with saturated LiCl was in a range of 7-9. The molarity of LiOH and LiCl in the LiOH and LiCl saturated aqueous solution were estimated to be 5.12 and 11.57 M, respectively, by analysis of Li⁺ and OH⁻. The high concentration of LiOH and the low pH value of 8.14 in this solution suggested that the dissociation of LiOH into Li⁺ and OH⁻ is too low in the solution with a high concentration of Li⁺. These results suggest that the water stable LTAP could be used as a protect layer of the lithium metal anode in the lithium/air cell with LiCl saturated aqueous solution as the electrolyte, because the content of OH⁻ ions in the LiCl saturated aqueous solution does not increase via the cell reaction of Li + 1/2O₂ + H₂O → 2LiOH, and LTAP is stable under a deep discharge state.     

Electrochemical properties of the soluble reduction products in rechargeable Li/S battery

In this paper, the electrochemical behavior of the reduction products in solution for Li/S cell is studied by UV-visual spectroscopy and electrochemical impedance spectroscopy (EIS). The results tell that the redox process of the polysulfide intermediate contains five charge-transfer steps in the practical Li/S cell. The formation of final reduction product of Li₂S and the final re-oxidation product of S₈ is completely irreversible. The transform between polysulfide and Li₂S₂ is electrochemical sluggish. The peaks corresponding to transformation Li₂S_x ↔ Li₂S_y (2 < x < y ≤ 6) are still symmetrical in spite of an increasing polarization with the proceeding of CV scan. While the redox process corresponding to Li₂S_m ↔ Li₂S_n (4 < m < n ≤ 8) is reversible. The dissolution long-chain polysulfide and deposition of short-chain polysulfide contribute mostly to the electrode deterioration even electrode blockage. Therefore, homogeneous mixing element sulfur with conductive components and alleviating the polysulfide dissolution are equally important to improving the active material utilization and rechargeability for rechargeable Li/S battery.     

A low temperature electrolyte for primary Li/CFx batteries

In this work, a 1:1 by weight blend of acetonitrile (AN) and γ-butyrolactone (BL) was studied as the solvent of low temperature electrolyte for high energy density Li/CF_x batteries. Both visual observation and impedance analysis show that metallic Li is kinetically stable in a 0.5 m LiBF₄ 1:1 AN/BL electrolyte. This property is attributed to the formation of a protective passivation film on the surface of metallic Li, and it has been successfully used to develop the low temperature electrolyte for Li/CF_x cells. It is shown that the cell with such an electrolyte outperforms the control cell with 0.5 m LiBF₄ 1:1 (wt.) propylene carbonate (PC)/1,2-dimethoxyethane (DME) electrolyte in both power capability and low temperature discharge performance. Impedance analyses reveal that the improved discharge performance is attributed to the reduction in both the bulk resistance and cell reaction resistance of the Li/CF_x cell, which is related to the high ionic conductivity of the AN/BL electrolyte. Due to the chemical incompatibility between metallic Li and AN at high temperatures, the storage and operation temperature for the Li/CF_x cells with 0.5 m LiBF₄ 1:1 AN/BL electrolyte is limited to or below ambient temperature (30 °C).     

Ag/C nanoparticles as an cathode catalyst for a zinc-air battery with a flowing alkaline electrolyte

The cyclic voltammetry indicated that the oxygen reduction reaction (ORR) proceeded by the four-electron pathway mechanism on larger Ag particles (174 nm), and that the ORR proceeded by the four-electron pathway and the two-electron pathway mechanisms on finer Ag particles (4.1 nm), simultaneously. The kinetics towards ORR was measured at a rotating disk electrode (RDE) with Ag/C electrode. The number of exchanged electrons for the ORR was found to be close to four on larger Ag particles (174 nm) and close to three on finer Ag particles (4.1 nm). The zinc-air battery with Ag/C catalysts (25.9 nm) was fabricated and examined.     

Unveiling the roles of alumina as a sintering aid in Li-Garnet solid electrolyte

The superiority of garnet-type solid electrolyte makes it one of most promising candidates for all-solid-state lithium batteries. Several studies show that introduction of alumina during synthesis can greatly improve the density and ionic conductivity of garnet electrolyte Li₇La₃Zr₂O₁₂ (LLZO), but the reason of poor sinterability of LLZO is still unclear. In this study, we reveal that lithium carbonate, which has a high decomposition temperature and covers on the particle surface of LLZO, is the underlying reason that handicaps the sinterability of Li-Garnet electrolyte in air. The addition of alumina promotes the decomposition of Li₂CO₃ (down to 400°C) and the concomitant product LiAlO₂, as a fast Li-ion conductor, facilitates the sintering process and bulids a fast Li-ion conducting network along the grain-boundaries, significantly increasing the ionic conductivity of Li-Garnet electrolyte. By the conventional solid state sintering, the 10 mol% Al₂O₃ modified Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZT-10Al₂O₃) electrolyte reaches a relative density of 96% and shows a conductivity of 0.31 mS cm⁻¹ at room temperature. The prepared LLZT-10Al₂O₃ electrolyte exhibits a good wetting property toward metallic Li electrode with an interfacial resistance of 59 Ω cm² compared to 1270 Ω cm² for LLZT/Li. This work provides a fundamental understanding and a valuable strategy for developing high performance garnet-type electrolyte for all-solid-state lithium batteries.     

Carbothermal treatment for the improved discharge performance of primary Li/CFx battery

Carbothermal treatment was used to improve the discharge rate performance of primary lithium/carbon monofluoride (Li/CF_x with x = 1) batteries. The treatment was carried out by heating a mixture of CF_x and carbon black (CB) just below the decomposition temperature of CF_x under nitrogen for 2 h. In the treatment, poly(vinylidene fluoride-co-hexafluoropropylene) (Kynar) was used as a fluorinated polymer binder to press the CF_x/CB mixture into pellets. It was shown that the content of Kynar significantly affected the discharge performance of the resulting treated-CF_x (T-CF_x). This can be attributed to the catalytic effect of HF formed by the pyrolysis of Kynar on the decomposition of CF_x and on the reaction of CB with the volatile fluorocarbons formed by the decomposition of CF_x. The discharge performance of T-CF_x cathode was also affected by the temperature of carbothermal treatment and by the ratio of CF_x to CB. In this work the best result was obtained from a treatment conducted at 470 °C on a 87CF_x/10CB/3Kynar (by weight) mixture. In the discharge condition of C/5 and 20 °C, the Li/CF_x cell with such-obtained T-CF_x cathode showed about 95 mV higher voltage than the control cell while retaining nearly the same specific capacity. Impedance analyses indicate that the improved discharge performance is mainly attributed to a reduction in the cell reaction resistance (R_cr) that includes an ohmic resistance related to the ionic conductivity of the discharge product shell and a Faradic resistance related to the processes of charge-transfer and Li⁺ ion diffusion in the CF_x reaction zone.     

Low temperature pyrolyzed cobalt tetramethoxy phenylporphyrin catalyst and its applications as an improved catalyst for metal air batteries

As an alternative catalyst to replace the expensive platinum-based catalysts for oxygen reduction reaction (ORR) in both proton exchange membrane (PEM) fuel cells and metal-air batteries, tetra methoxyphenyl porphyrin cobalt complex, CoTMPP, has drawn much attention in the areas of ORR catalyst synthesis and characterization. In the present work, CoTMPP is investigated at varying temperatures of pyrolysis, ranging from 410 to 810 °C, to determine the optimal temperature of pyrolysis to produce catalysts with desirable properties. Carbon supported pyrolyzed CoTMPP catalyst (CoTMPP/C) was assessed using X-ray diffraction and Raman, a number of varied electrochemical tests, as well as a single cell test. Through these, it has been found that the CoTMPP/C, pyrolyzed at 410 °C, demonstrates superior catalytic activity towards ORR, and has a higher fuel cell performance than the catalysts pyrolyzed at 800 °C, the industry standard pyrolysis temperature. This paper also utilizes scanning auger microscopy (SAM) to accredit the origin of the increased activity to the bond N-C(O), which plays a major role in catalysis and is decomposed at higher temperatures.     

Influence of Ag nanoparticles on state of the art MnO2 nanorods performance as an electrocatalyst for lithium air batteries

The need for an alternative and efficient electrocatalyst to replace Pt-based noble materials is a goal of prime importance in Li– air battery technology. In this work, novel silver nanoparticles-incorporated MnO₂ nanorods as an air electrode bifunctional catalyst have been synthesized by a simple polyol method. The physical characteristics of the thus prepared materials are analyzed by X-ray diffraction (XRD), SEM, and Brunauer–Emmett–Teller (BET) techniques. These analyses confirmed the successful synthesis of 20 to 25 nm-sized different weight % Ag nanoparticles incorporated on α-MnO₂ nanorods. Linear sweeping voltammetric results of AgMnO₂ showed improved ORR performance as compared to α- MnO₂ nanorods in terms of the onset potential, half wave potential and limiting current. The addition of catalysts has significantly increased the discharge capacity and overall performance of the cells. The first discharge curve of 5 wt% Ag MnO₂ sample reached a maximum capacity of 3500 mAhg-1 at 2.0 V with a current density of 0.1 mA cm^?2 with a plateau between 2.7 and 2.6 V. Long term stability of increasing weight percentage of Ag nanoparticles on MnO₂ samples is increased.     

Elucidation of the role of lithium iodide as an additive for the liquid-based synthesis of Li7P2S8I solid electrolyte

Liquid-phase synthesis for a sulfide-based solid electrolyte has been widely studied due to its great advantage of being a simpler and more cost-effective method compared with the conventional solid-phase synthesis, even it could induce homogeneous reactions in the solution. However, the physically and chemically stable phosphorus pentasulfide (P₂S₅) is barely soluble in various solvents; this has been a major problem for achieving a pure solution-phase dissolving solid electrolyte. Therefore, exploring an effective additive for liquid-phase synthesis would be worthwhile and could potentially lead to the discovery of new chemicals to produce qualified solid electrolytes. In this paper, lithium iodide's (LiI) dual role as a strong nucleophile as well as a major reactant source for producing Li₇P₂S₈I is first investigated. The nucleophilic additive of LiI has been proven to break the P-S bonds of P₂S₅, driving the insoluble P₂S₅ to become the soluble intermediate complex. Also, it is demonstrated that controlling the reaction times between the LiI and the P₂S₅ is key to achieving solution-based synthesis, and the role of LiI is investigated by conducting bonding analysis.     

Synthesis and electrochemical studies of the 4 V cathode, Li(Ni2/3Mn1/3)O2

Layered Li(Ni_2/3Mn_1/3)O₂ compounds are prepared by freeze-drying, mixed carbonate and molten salt methods at high temperature. The phases are characterized by X-ray diffraction, Rietveld refinement, and other methods. Electrochemical properties are studied versus Li-metal by charge–discharge cycling and cyclic voltammetry (CV). The compound prepared by the carbonate route shows a stable capacity of 145 (±3) mAh g⁻¹ up to 100 cycles in the range 2.5–4.3 V at 22 mA g⁻¹. In the range 2.5–4.4 V at 22 mA g⁻¹, the compound prepared by molten salt method has a stable capacity of 135 (±3) mAh g⁻¹ up to 50 cycles and retains 96% of this value after 100 cycles. Capacity-fading is observed in all the compounds when cycled in the range 2.5–4.5 V. All the compounds display a clear redox process at 3.65–4.0 V that corresponds to the Ni^2+/3+–Ni^3+/4+ couple.     

An improved thermal and electrical model for a solar photovoltaic thermal (PV/T) air collector

In this paper, an attempt is made to investigate the thermal and electrical performance of a solar photovoltaic thermal (PV/T) air collector. A detailed thermal and electrical model is developed to calculate the thermal and electrical parameters of a typical PV/T air collector. The thermal and electrical parameters of a PV/T air collector include solar cell temperature, back surface temperature, outlet air temperature, open-circuit voltage, short-circuit current, maximum power point voltage, maximum power point current, etc. Some corrections are done on heat loss coefficients in order to improve the thermal model of a PV/T air collector. A better electrical model is used to increase the calculations precision of PV/T air collector electrical parameters. Unlike the conventional electrical models used in the previous literature, the electrical model presented in this paper can estimate the electrical parameters of a PV/T air collector such as open-circuit voltage, short-circuit current, maximum power point voltage, and maximum power point current. Further, an analytical expression for the overall energy efficiency of a PV/T air collector is derived in terms of thermal, electrical, design and climatic parameters. A computer simulation program is developed in order to calculate the thermal and electrical parameters of a PV/T air collector. The results of numerical simulation are in good agreement with the experimental measurements noted in the previous literature. Finally, parametric studies have been carried out. Since some corrections have been down on thermal and electrical models, it is observed that the thermal and electrical simulation results obtained in this paper is more precise than the one given by the previous literature. It is also found that the thermal efficiency, electrical efficiency and overall energy efficiency of PV/T air collector is about 17.18%, 10.01% and 45%, respectively, for a sample climatic, operating and design parameters.     

Electrochemical study of the formation of a solid electrolyte interface on graphite in a LiBC2O4F2-based electrolyte

The formation of a solid electrolyte interface (SEI) on the surface of graphite in a LiBC₂O₄F₂-based electrolyte was studied by galvanostatic cycling and electrochemical impedance spectroscopy (EIS). The results show that a short irreversible plateau at 1.5–1.7 V versus Li⁺/Li was inevitably present in the first cycle of graphite, which is attributed to the reduction of –OCOCOO⁻ pieces as a result of the chemical equilibrium of oxalatoborate ring-opening. This is the inherent property of LiBC₂O₄F₂ and it is independent of the type of electrode. EIS analyses suggest that the reduced products of LiBC₂O₄F₂ at 1.5–1.7 V participate into the formation of a preliminary SEI. Based on the distribution of the initial irreversible capacity and the correlation of the SEI resistance and graphite potential, it was concluded that the SEI formed at potentials below 0.25 V during which the lithiation takes place is most responsible for the long-term operation of the graphite electrode in Li-ion batteries. In addition, the results show that the charge-transfer resistance reflects well the kinetics of the electrode reactions, and that its value is in inverse proportion to the differential capacity of the electrode.