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Abstract: The Li–O2 chemistry in nonaqueous liquid carbonate electrolytes and the underlying reason for its limited reversibility was systematically investigated. X-ray diffraction data showed that regardless of discharge depth lithium alkylcarbonates (lithium propylenedicarbonate (LPDC), or lithium ethylenedicarbonate (LEDC), with other related derivatives) and lithium carbonate (Li2CO3) are constantly the main discharge products, while lithium peroxide (Li2O2) or lithium oxide (Li2O) is hardly detected. These lithium alkylcarbonates are generated from the reductive decomposition of the corresponding carbonate solvents initiated by the attack of superoxide radical anions. More significantly, in situ gas chromatography/mass spectroscopy analysis revealed that Li2CO3 and Li2O cannot be oxidized even when charged to 4.6V vs. Li/Li+, while LPDC, LEDC and Li2O2 are readily oxidized, with CO2 and CO released from LPDC and LEDC and O2 evolved from Li2O2. Therefore, the apparent reversibility of Li–O2 chemistry in an organic carbonate-based electrolyte is actually an unsustainable process that consists of (1) the formation of lithium alkylcarbonates through the reductive decomposition of carbonate solvents during discharging and (2) the subsequent oxidation of these same alkylcarbonates during charging. Therefore, a stable electrolyte that does not lead to an irreversible by-product formation during discharging and charging is necessary for truly rechargeable Li–O2 batteries. [Copyright &y& Elsevier]

Abstract: The charging process of Li2O2-based air electrodes in Li–O2 batteries with organic carbonate electrolytes was investigated using in situ gas chromatography/mass spectroscopy (GC/MS) to analyze gas evolution. A mixture of Li2O2/Fe3O4/Super P carbon/polyvinylidene fluoride (PVDF) was used as the starting air electrode material, and 1-M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in carbonate-based solvents was used as the electrolyte. We found that Li2O2 was actively reactive to 1-methyl-2-pyrrolidinone and PVDF that were used to prepare the electrode. During the first charging (up to 4.6V), O2 was the main component in the gases released. The amount of O2 measured by GC/MS was consistent with the amount of Li2O2 that decomposed during the electrochemical process as measured by the charge capacity, which is indicative of the good chargeability of Li2O2. However, after the cell was discharged to 2.0V in an O2 atmosphere and then recharged to ∼4.6V, CO2 was dominant in the released gases. Further analysis of the discharged air electrodes by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy indicated that lithium-containing carbonate species (lithium alkyl carbonates and/or Li2CO3) were the main discharge products. Therefore, compatible electrolytes and electrodes, as well as the electrode-preparation procedures, need to be developed for rechargeable Li-air batteries for long term operation. [Copyright &y& Elsevier]

Suppressing lithium (Li) dendritegrowth is one of the most critical challenges for the developmentof Li metal batteries. Here, we report for the first time the growthof dendrite-free lithium films with a self-aligned and highly compactednanorod structure when the film was deposited in the electrolyte consistingof 1.0 M LiPF6in propylene carbonate with 0.05 M CsPF6as an additive. Evolution of both the surface and the cross-sectionalmorphologies of the Li films during repeated Li deposition/strippingprocesses were systematically investigated. It is found that the formationof the compact Li nanorod structure is preceded by a solid electrolyteinterphase (SEI) layer formed on the surface of the substrate. Electrochemicalanalysis indicates that an initial reduction process occurred at ∼2.05V vs Li/Li+before Li deposition is responsible for theformation of the initial SEI, while the X-ray photoelectron spectroscopyindicates that the presence of CsPF6additive can largelyenhance the formation of LiF in this initial SEI. Hence, the smoothLi deposition in Cs+-containing electrolyte is the resultof a synergistic effect of Cs+additive and preformed SEIlayer. A fundamental understanding on the composition, internal structure,and evolution of Li metal films may lead to new approaches to stabilizethe long-term cycling stability of Li metal and other metal anodesfor energy storage applications. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we report the electrochemical characteristics of a novel cathode material, Li2CoPO4F, prepared by solid-state reactions. The solid-state reaction mechanism involved in synthesizing the Li2CoPO4F also is analyzed in this paper. When cycled between 2.0V and 5.0V during cyclic voltammetry measurements, the Li2CoPO4F samples present one, fully reversible anodic reaction at 4.81V. When cycled between 2.0V and 5.5V, peaks occurring at 4.81V and 5.12V in the first anodic scan evolved to one broad oxidative, mound-like pattern in subsequent cycles. Correspondingly, the X-ray diffraction (XRD) pattern of the Li2CoPO4F electrode discharged from 5.5V to 2.0V is slightly different from the patterns exhibited by a fresh sample and the sample discharged from 5.0V to 2.0V. This difference may correspond to a structural relaxation that appears above 5V. In the constant current cycling measurements, the Li2CoPO4F samples exhibited a capacity as high as 109mAhg−1 and maintained a good cyclability between 2.0V and 5.5V vs. Li/Li+. XRD measurements confirmed that the discharged state is Li2CoPO4F. Combining these XRD results and electrochemical data proved that up to 1mol Li+ is extractable when charged to 5.5V. [ABSTRACT FROM AUTHOR]

Rechargeable lithium (Li) metal batteries with conventional LiPF 6 -carbonate electrolytes have been reported to fail quickly at charging current densities of about 1.0 mA cm −2 and above. In this work, we demonstrate the rapid charging capability of Li||LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) cells can be enabled by a dual-salt electrolyte of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(oxalato)borate (LiBOB) in a carbonate solvent mixture. The cells using the LiTFSI-LiBOB dual-salt electrolyte significantly outperform those using the LiPF 6 electrolyte at high charging current densities. At the charging current density of 1.50 mA cm −2 , the Li||NCA cells with the dual-salt electrolyte can still deliver a discharge capacity of 131 mAh g −1 and a capacity retention of 80% after 100 cycles. The Li||NCA cells with the LiPF 6 electrolyte start to show fast capacity fading after the 30th cycle and only exhibit a low capacity of 25 mAh g −1 and a low retention of 15% after 100 cycles. The reasons for the good chargeability and cycling stability of the cells using the LiTFSI-LiBOB dual-salt electrolyte can be attributed to the good film-formation ability of the electrolyte on the Li metal anode and the highly conductive nature of the sulfur-rich interphase layer. [ABSTRACT FROM AUTHOR]

The practical application of lithium (Li) metal anode in rechargeable Li batteries is hindered by both the growth of Li dendrites and the low Coulombic efficiency (CE) during repeated charge/discharge cycles. Recently, we have discovered that CsPF 6 as an electrolyte additive can significantly suppress Li dendrite growth and lead to highly compacted and well aligned Li nanorod structures during Li deposition on copper substrates. In this paper, the effect of CsPF 6 additive on the performance of rechargeable Li metal batteries with lithium iron phosphate (LFP) cathode is further studied. Li|LFP coin cells with CsPF 6 additive in electrolytes show well protected Li anode surface, decreased resistance, enhanced rate capability and extended cycling stability. In Li|LFP cells, the electrolyte with CsPF 6 additive shows excellent long-term cycling stability (at least 500 cycles) at a charge current density of 0.5 mA cm −2 without internal short circuit. At high charge current densities, the effect of CsPF 6 additive becomes less significant. Future work needs to be done to protect Li metal anode, especially at high charge current densities and for long cycle life. [ABSTRACT FROM AUTHOR]