Lithium-sulfur cells exhibit poor cycle life, due to the well-known 'polysulfide shuttle' enabled by the dissolution of the sulfur reduction products in organic electrolyte. Different strategies have been implemented to reduce the shuttle effects, with limited success, especially through use of low sulfur loadings (1-2 mg/cm 2 ). Dense electrodes with high sulfur loadings are essential for high energy cells, however, such electrodes experience more serious polysulfide effects. In this paper, we describe the benefits of blending sulfur with a transition metal sulfide (here, TiS 2 and MoS 2 ) to form dense composite cathodes with enhanced conductivity. There is an improvement in both the initial capacity from sulfur utilization (∼800 mAh/g based on sulfur content), the coulombic efficiency (>96%) and also in cycle life upon blending with the metal sulfide. High sulfur loadings (>12 mg/cm 2 or ∼6 mAh/cm 2 per side) were demonstrated to display high sulfur utilization in Li-S cells containing the metal sulfide blends either with or without coatings over the sulfur cathode. XRD studies were carried out to understand the redox behavior of the metal sulfide additive during charge/discharge cycling of the sulfur cathodes. DC polarization and Potentiometric Intermittent Titration Technique (PITT) measurements were made on sulfur cathodes with and without metal sulfide blends to determine the charge transfer and diffusional kinetics. Since their inception in 1991, Li-ion batteries 1 have progressed at a rapid pace, with a three-fold enhancement in their performance achieved through many advances in both electrode materials and electrolytes.2-9 Current Li-ion cells from commercial vendors (e.g., Panasonic, LG and Samsung) provide an impressive specific energy of >250 Wh/kg and energy density of >600 Wh/l which are benefiting a wide range of applications including portable electronic devices, electric vehicles and aerospace needs. Yet, many of the emerging markets such as electric vehicles and renewable energy technologies place even higher demands on the battery technologies, both in terms of performance and cost. It is believed that the performance of lithium-ion technologies has reached a plateau, and any future improvements will only be marginal. Replacement of graphite anodes with Si, and conventional 4 V cathodes with the high voltage Li-rich layered-layered composite cathodes, 10,11 has not yet successfully been applied to commercial batteries. Accordingly, any gains in specific energy and energy density have been modest so far after a decade of development. 12,13 There is, thus, a pursuit for more energetic battery technologies beyond Li-ion batteries. This has led to a renewed interest in the lithiumsulfur system, which has the highest theoretical specific energy of all the known rechargeable systems (due to the high capacity of sulfur, 1672 mAh/g, ∼ 6-10x of Li-ion cathodes), with the notable exception of Li-O 2 which itself has several serious fundamental hurdles that are not close to being overcome.14-16 The spe...

Europa is a premier target for advancing both planetary science and astrobiology, as well as for opening a new window into the burgeoning field of comparative oceanography. The potentially habitable subsurface ocean of Europa may harbor life, and the globally young and comparatively thin ice shell of Europa may contain biosignatures that are readily accessible to a surface lander. Europa’s icy shell also offers the opportunity to study tectonics and geologic cycles across a range of mechanisms and compositions. Here we detail the goals and mission architecture of the Europa Lander mission concept, as developed from 2015 through 2020. The science was developed by the 2016 Europa Lander Science Definition Team (SDT), and the mission architecture was developed by the preproject engineering team, in close collaboration with the SDT. In 2017 and 2018, the mission concept passed its mission concept review and delta-mission concept review, respectively. Since that time, the preproject has been advancing the technologies, and developing the hardware and software, needed to retire risks associated with technology, science, cost, and schedule.

Exploration missions to the moons of the outer planets (such as Europa) pose unique challenges regarding the design of the spacecraft power source. Current aerospace qualified primary battery technologies cannot adequately meet the mass and volume requirements of proposed missions. Although they have not been used in prior deep space landed missions, lithium carbon-fluoride (Li/CF x ) technologies were identified as a potentially viable option, both with and without blends of manganese dioxide (MnO 2 ). To meet the performance requirements over the intended operating conditions of future NASA missions requires further development of this technology, in particular in the delivery of a high specific energy at moderate to low temperatures, and low discharge rates. A cell development effort was therefore pursued with an industrial battery cell manufacturer. Low (50 mA) and medium (250 mA) discharge rates were used to assess the performance of D-size cells under mission relevant conditions, between 0 • C and −40 • C. Select AA-size and C-size cells were also evaluated using similar rates scaled to the lower cell capacities. Developmental Li/CF x -MnO 2 D-size cells designed for higher specific energy over these conditions were fabricated and tested, targeting operation between 0 and −40 • C and a 50 mA constant discharge current, as the baseline operating condition.

The effect of additives and electrolyte composition was investigated for Li CF x and Li CF x -MnO 2 cells operated at low (ca. −40 • C) temperature. Electrochemical impedance spectroscopy (EIS) and Tafel measurements indicated that the anode is far more resistive than the cathode at low temperatures. Adding a small amount of fluoroethylene carbonate (FEC) to a propylene carbonate (PC) + 1,2-dimethoxyethane (DME) + tetrahydrofuran (THF) ternary blend was found to have a beneficial effect on the anode of a cell when using 0.5 M LiClO 4 as the electrolyte salt. Although the cathode impedance did increase with the addition of FEC, the overall cell impedance dropped dramatically, particularly at −40 • C. Capacity at low temperature was not significantly improved with FEC addition, however, indicating that static (i.e. EIS) measurements may not be the best tool to understand the cell under dynamic (discharge) conditions. 15-crown-5 (as an additive) proved to have severe negative effects on specific capacity.

Lithium-sulfur cells exhibit poor cycle life, due to the 'polysulfide shuttle' enabled by the dissolution of sulfur reduction products in organic electrolyte. Different strategies have been implemented to reduce the shuttle effects, including novel cathode designs to sequester polysulfides within the cathode, new electrolytes with reduced polysulfide solubility, and blocking layers to hinder polysulfide migration towards the anode. However, these are less successful in the case of high-energy Li/S cells with cathodes featuring substantial sulfur loadings, as proportionately greater polysulfide dissolution rapidly degrades performance. Additionally, the electrolyte/sulfur ratio is a limiting factor in high-energy cells, indicating that a minimum polysulfide solubility is required. Herein we describe the benefits of modified separators with the objective of blocking polysulfide migration. Specifically, we demonstrate improved discharge performance in laboratory Li/S cells with ceramic-coated separators, including single-sided and double-sided Al 2 O 3 -coated polyolefin material from commercial sources. We also developed a new AlF 3 coating on a polyolefin separator that showed even greater improvements: higher initial specific capacity (∼800 mAh/g S ), coulombic efficiency (>96%), and cycle life (>100 cycles). Detailed analytical study via SEM and EDS of separators harvested from cycled cells indicated an approximately uniform sulfur distribution across the ceramic-coated separator. Finally, concentrated electrolytes, which reportedly minimize polysulfide diffusion and lithium dendrite growth, were tested and shown to yield high coulombic efficiency although with low sulfur utilization.