No abstract

No abstract

Owing to the natural abundance of sodium resources and their low price, next-generation batteries employing an Na metal anode, such as Na-O and Na-S systems, have attracted a great deal of interest. However, the poor reversibility of an Na metal electrode during repeated electrochemical plating and stripping is a major obstacle to realizing rechargeable sodium metal batteries. It mainly originates from Na dendrite formation and exhaustive electrolyte decomposition due to the high reactivity of Na metal. Herein, we report a free-standing composite protective layer (FCPL) for enhancing the reversibility of an Na metal electrode by mechanically suppressing Na dendritic growth and mitigating the electrolyte decomposition. A systematic variation of the liquid electrolyte uptake of FCPL verifies the existence of a critical shear modulus for suppressing Na dendrite growth, being in good agreement with a linear elastic theory, and emphasizes the importance of the ionic conductivity of FCPL for attaining uniform Na plating and stripping. The Na-Na symmetric cell with an optimized FCPL exhibits a cycle life two times longer than that of a bare Na electrode.

Interfacial stability is one of the crucial factors for long-term cyclability of lithium (Li) metal batteries (LMBs). While cross-contamination phenomena have been well-studied in Li-ion batteries (LIBs), similar phenomena have rarely been reported in LMBs. Here, we investigated cathode failure triggered by chemical crossover from the anode in LMBs. In contrast to LIBs, the cathode in LMBs suffers more significant capacity fading, and its capacity cannot be fully recovered by replacing the Li anode. In-depth surface characterization reveals severe deterioration related to the accumulation of highly resistive polymeric components in the cathode–electrolyte interphase. The soluble byproducts generated by extensive electrolyte decomposition at the Li metal surface can diffuse toward the cathode side, resulting in severe deterioration of the cathode and separator surfaces. A selective Li-ion permeable separator with a polydopamine coating has been developed to mitigate the detrimental chemical crossover and enhance the cathode stability.

Uncontrolled growth of insulating lithium sulfide leads to passivation of sulfur cathodes, which limits high sulfur utilization in lithium-sulfur batteries. Sulfur utilization can be augmented in electrolytes based on solvents with high Gutmann Donor Number; however, violent lithium metal corrosion is a drawback. Here we report that particulate lithium sulfide growth can be achieved using a salt anion with a high donor number, such as bromide or triflate. The use of bromide leads to ~95 % sulfur utilization by suppressing electrode passivation. More importantly, the electrolytes with high-donor-number salt anions are notably compatible with lithium metal electrodes. The approach enables a high sulfur-loaded cell with areal capacity higher than 4 mA h cm−2 and high sulfur utilization ( > 90 %). This work offers a simple but practical strategy to modulate lithium sulfide growth, while conserving stability for high-performance lithium-sulfur batteries.

Despite the theoretically high energy density, the practical energy density of Li-S batteries at the moment does not meet the demand due to low sulfur (S) loading (<2 mg cm −2 ), large electrolyte amount (electrolyte/sulfur ratio >20 µL mg −1 ), and excess lithium (Li) metal use (>10 times excess). [5] In particular, large electrolyte usage (flooding) greatly diminishes the practical energy density of Li-S batteries. Due to the intrinsic solution-based redox chemistry, however, many of the challenges arise from minimizing the electrolyte/ sulfur ratio (E/S ratio). Since soluble lithium polysulfide (LiPS, Li 2 S x when 2 < x ≤ 8) intermediates are self-redox mediating, the decrease in the LiPS dissolution causes a sluggish sulfur conversion and high polarization. [6] Next, the morphology of lithium sulfide (Li 2 S) electrodeposition and the kinetics of the re-oxidation are affected by the sulfur species solubility as well. [7] Hence, uncontrolled precipitation and continual accumulation of Li 2 S limit the discharge capacity and further passivate the cathode interface throughout the cycling. [8] Reducing the electrolyte volume exacerbates not only the cathode performance but also the anode stability. A high reactivity and an infinite volume change of the Li metal anode cause the incessant decomposition of the electrolyte. Therefore, the lean electrolyte condition accelerates the increase of the cell resistance and provokes earlier performance failure compared to the flooding electrolyte system. [9] Manipulating electrolyte materials (solvents, salt anions, and additives) has a considerable impact on the electrochemical performance of Li-S batteries. There have been studies in which solvents with high Gutmann donor numbers (DNs) form strong interactions with lithium ions (Li + ) and promote the solvation of polysulfide (PS) anions. The increased LiPS solubility facilitates the solution-mediated reaction pathway, enabling fast reaction kinetics and high sulfur utilization. [10] Furthermore, the same merits can also be achieved with salt anions having high-DNs [11] or additives promoting ionic solvation. [12] Under a lean electrolyte regime, the role of highly solvating electrolytes becomes more prominent because of the limited solubility of sulfur species. For example, high-DN solvents can enhance the sulfur utilization under the reduced electrolyte amount by promoting the charge/discharge reactions. [13] Despite this fact, the Minimizing electrolyte use is essential to achieve high practical energy density of lithium-sulfur (Li-S) batteries. However, the sulfur cathode is more readily passivated under a lean electrolyte condition, resulting in low sulfur utilization. In addition, continuous electrolyte decomposition on the Li metal anode aggravates the problem, provoking rapid capacity decay. In this work, the dual functionalities of NO 3 − as a high-donor-number (DN) salt anion is presented, which improves the sulfur utilization and cycling stability of lean-electrolyte Li-S batteries. The NO 3 − anion eleva...

No abstract

No abstract