Partially fluorinated solvent as a co-solvent for the non-aqueous electrolyte of Li/air battery

In this work we study methyl nonafluorobutyl ether (MFE) and tris(2,2,2-trifluoroethyl) phosphite (TTFP), respectively, as a co-solvent for the non-aqueous electrolyte of Li-air battery. Results show that in certain solvent ratios, both solvents are able to increase the specific capacity of carbon in Li/O₂ and Li/air cells. More interestingly, the improvement in discharge performance of the Li/air cells increases with discharge current density. These results cannot be explained by the ionic conductivity and viscosity data of the electrolytes since the participation of fluorinated co-solvents hardly changes viscosity of the solvent blends while reversely reduces ionic conductivity of the electrolyte. In particular, we find that a 30 wt.% (vs. solvent) addition of TTFP into a 0.2 m (molality) LiSO₃CF₃ PC electrolyte can significantly improve the discharge performance of Li/air cells, and that the resultant electrolyte is able to support long-term operation of Li/air cells in dry ambient environments due to its low volatility. We believe that the observed performance improvement is associated with the increased dissolution kinetics and solubility of oxygen in fluorinated solvent containing electrolyte.     

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.     

Ambient operation of Li/Air batteries

In this work, Li/air batteries based on nonaqueous electrolytes were investigated in ambient conditions (with an oxygen partial pressure of 0.21 atm and relative humidity of ∼20%). A heat-sealable polymer membrane was used as both an oxygen-diffusion membrane and as a moisture barrier for Li/air batteries. The membrane also can minimize the evaporation of the electrolyte from the batteries. Li/air batteries with this membrane can operate in ambient conditions for more than one month with a specific energy of 362 Wh kg⁻¹, based on the total weight of the battery including its packaging. Among various carbon sources used in this work, Li/air batteries using Ketjenblack (KB) carbon-based air electrodes exhibited the highest specific energy. However, KB-based air electrodes expanded significantly and absorbed much more electrolyte than electrodes made from other carbon sources. The weight distribution of a typical Li/air battery using the KB-based air electrode was dominated by the electrolyte (∼70%). Lithium metal anodes and KB-carbon account for only 5.12% and 5.78% of the battery weight, respectively. We also found that only ∼20% of the mesopore volume of the air electrode was occupied by reaction products after discharge. To further improve the specific energy of the Li/air batteries, the microstructure of the carbon electrode needs to be further improved to absorb much less electrolyte while still holding significant amounts of reaction products.     

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 novel structure of ceramics electrolyte for future lithium battery

In order to fabricate large scale all-solid-state Li battery, we suggested a novel structure of solid electrolyte, which is composed of porous electrolyte supported by honeycomb-type electrolyte. A possibility of fabrication of the honeycomb-supported porous electrolyte and a compatibility of this structure with all-solid-state battery were examined using LLT (Li_0.35La_0.55TiO₃) solid electrolyte which is one of the anticipated solid electrolytes due to its high Li ion conductivity. A porous layer membrane with 3 dimensionally ordered (3DOM) macroporous structure was prepared by a colloidal crystal templating method. The porous honeycomb was fabricated by pushing the membrane into holes of honycomb using a needle followed by calcination. The 3DOM membrane and honeycmb electrolyte were sintered well each other. After filling the 3DOM pores with LiMn₂O₄ cathode material, the compatibility of this novel porous honeycomb electrolyte with all-solid-state battery was examined. The LiMn₂O₄/porous honeycomb cell clearly demonstrated charge and discharge behaviors, indicating the porous honeycomb structure can be applied to the all-solid-state battery. The discharge capacity was 71 mA h g⁻¹ (1.3 mA h cm⁻²) at 30 °C.     

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.     

A novel flame retardant and film-forming electrolyte additive for lithium ion batteries

Allyl tris(2,2,2-trifluoroethyl) carbonate (ATFEC) was synthesized as a bi-functional additive of flame retardant and film former in electrolytes for lithium ion batteries (LIBs). The flame retardancy of the additive was characterized with differential scanning calorimetry (DSC) and self-extinguishing time (SET). It is shown that adding 1 vol.% ATFEC in 1 M LiPF₆/propylene carbonate (PC) can effectively enhance the thermal stability of the electrolyte and suppress the co-intercalation of PC into the graphitic anode. Further evaluation indicates that the additive hardly affect the conductivity of electrolyte. These support the feasibility of using ATFEC as an additive on formulating an electrolyte with multiple functions such as film-forming enhancement, high thermal stability and high ionic conductivity.     

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 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.     

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).     

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.     

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.     

Effect of co-doping nano-silica filler and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide into polymer electrolyte on Li dendrite formation in Li/poly(ethylene oxide)-Li(CF3SO2)2N/Li

Lithium metal dendrite growth in Li/poly (ethylene oxide)-lithium bis (trifluoromethanesulfonyl) imide (PEO₁₈LiTFSI), nano-silica, and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13TFSI) composite solid polymer electrolyte/Li was investigated by direct in situ observation. The dendrite onset time decreased with increasing current density and deviated from Sand's law in the current density range of 0.1-0.5 mA cm⁻² at 60 °C. Lithium dendrite formation was not observed until 46 h of polarization at 0.5 mA cm⁻² and 60 °C, which is a significant improvement compared to that observed in Li/(PEO₁₈LiTFSI)/Li, where the dendrite formation was observed after 15 h polarization at 0.5 mA cm⁻² and 60 °C. The suppression of dendrite formation could be explained by the electrical conductivity enhancement and decrease of the interface resistance between Li and the polymer electrolyte by the introduction of both nano-SiO₂ and PP13TFSI into PEO₁₈LiTFSI. The electrical conductivity of 4.96 × 10⁻⁴ S cm⁻¹ at 60 °C was enhanced to 7.6 × 10⁻⁴ S cm⁻¹, and the interface resistance of Li/PEO₁₈LiTFSI/Li of 248 Ω cm² was decreased to 74 Ω cm² by the addition of both nano-SiO₂ and PP13TFSI into PEO₁₈LiTFSI.     

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.     

Characterization of electrode/electrolyte interface for lithium batteries using in situ synchrotron X-ray reflectometry—A new experimental technique for LiCoO2 model electrode

A new experimental technique was developed for detecting structure changes at electrode/electrolyte interface of lithium cell using X-ray reflectometry and two-dimensional model electrodes with a restricted lattice-plane. The electrodes were constructed with an epitaxial film of LiCoO₂ synthesized by pulsed laser deposition method. The orientation of the epitaxial film depends on the substrate plane; the 2D layer of LiCoO₂ is parallel to the SrTiO₃ (1 1 1) substrate (_(0 0 3)LiCoO₂//_(1 1 1)SrTiO₃)$({(003)}_{\text{LiCo}{\text{O}}_2}//{(111)}_{\text{SrTi}{\text{O}}_3})$, while the 2D layer is perpendicular to the SrTiO₃ (1 1 0) substrate (_(1 1 0)LiCoO₂//_(1 1 0)SrTiO₃)$({(110)}_{\text{LiCo}{\text{O}}_2}//{(110)}_{\text{SrTi}{\text{O}}_3})$. The anisotropic properties were confirmed by electrochemical measurements. Ex situ X-ray reflectivity measurements indicated that the impurity layer existed on the as-grown LiCoO₂ was dissolved and a new SEI layer with lower density was formed after soaking into the electrolyte. In situ X-ray reflectivity measurements indicated that the surface roughness of the intercalation (1 1 0) plane increased with applying voltages, while no significant changes in surface morphology were observed for the intercalation non-active (0 0 3) plane during the pristine stage of the charge–discharge process.     

A novel titania nanorods‐filled composite solid electrolyte with improved room temperature performance for solid‐state Li‐ion battery

Solid‐state batteries (SSBs) with room temperature (RT) performances had been one of the most promising technologies for energy storage. To achieve a chemical stable and high ionic conductive solid electrolyte, herein, a titania (TiO₂) (B) nanorods‐filled poly(propylene carbonate) (PPC)‐based organic/inorganic composite solid electrolyte (CSE) was prepared for the first time. It was found that by using TiO₂(B) nanorods, the ionic conductivity of the CSE membrane could be improved to 1.52 × 10^?4 S/cm, the electrochemical stable window was more than 4.6 V, and the tensile strength reaches 27 MPa with a strain less than 6%. The CSE was applied for SSB and showed excellent room temperature electrochemical performances. At 25°C, the LiFePO₄/CSE/Li SSB with 3%TiO₂‐filled CSE had the first cycle specific discharge capacity of 162 mAh/g with a capacity retention of 93% after 100 cycles at 0.3C. While the NCM622/CSE/Li SSB with 3%TiO₂‐filled CSE had the first specific discharge capacity of 165 mAh/g with a capacity retention of 88% after 100 cycles at 0.3C. The enhancement effect of TiO₂(B) nanorods could be ascribed that the rod‐like fillers provide more continuous Li‐ion transport path compared with nano particles, and the surface porosity and composition of TiO₂(B) nanorods could also improve the interfacial contact and Lewis acid‐base reaction sites between polymer and fillers. The TiO₂(B) nanorods‐filled CSE with high chemical stability, potential window, and ionic conductivity was promising to meet the requirements of SSBs.     

On the thermal behavior of model Li–LixCoO2 systems containing ionic liquids in standard electrolyte solutions

We report herein on the possibility of using ionic liquids (ILs) as additives to conventional electrolyte solutions, based on alkyl carbonates and LiPF₆ for attenuating thermal reactions in Li battery systems. As a model, a Li–Li_0.5CoO₂ system was used. The ionic liquids chosen included cations based on derivatives of pyrrolidinium and imidazolium, and the anions bioxalato borate (C₄O₈B⁻, BOB), (CH₃SO₂)₂N⁻ (TFSI), and PF₃(C₂S₅)₃⁻ (FAP). The thermal behavior of solutions alone, solutions with Li metal, Li_0.5CoO₂ and Li metal + Li_0.5CoO₂ was studied. It was found that the presence of 10% of ILs, with derivatives of pyrrolidinium cations and FAP or TFSI anions in standard EC–DMC/LiPF₆ solutions, improves considerably the thermal stability of Li_0.5CoO₂ in electrolyte solutions. The onset temperatures of the thermal reactions of Li_0.5CoO₂ with solution species are higher and their heat evolution is considerably lower, when they contain these ionic liquids as additives. This finding opens the door for further studies and optimization of the use of selected ILs as additives that may improve the safety features of Li-ion batteries.     

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.