A new direction for the performance improvement of rechargeable lithium/sulfur batteries
Highlights
► A new direction for improving Li/S batteries by in situ protecting Li anode. ► LiNO3 promotes formation of a highly protective passivation film on Li surface. ► Li/Li2Sx liquid cell offers better capacity retention than conventional Li/S cells. ► LiNO3 increases specific capacity and cycling efficiency of Li/S cells.
Introduction
Lithium/sulfur (Li/S) batteries have attracted increasing interest in developing high density energy storage devices due to their high theoretical capacity. Based on the complete reduction of elemental sulfur to lithium sulfide (Li2S), Li/S batteries can deliver a specific capacity as high as 1675 mAh g−1 sulfur. However, the specific capacity of a practical cell is lower than the theoretical value and the cell suffers low charging efficiency, high self-discharge and short cycle life [1], [2]. All these problems are known to be related to the high solubility of lithium polysulfides, the series of sulfur reduction intermediates, in organic electrolyte solutions. Dissolution of lithium polysulfides not only results in the loss of sulfur active materials from the cathode, but also causes serious “redox shuttle” reactions between polysulfide anions in the electrolyte and the Li metal anode. Recently, a number of publications have reported a reduction in the dissolution of lithium polysulfides by making sulfur-carbon composite materials [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Based on physical adsorption, these composites in different contexts reduce the dissolution of lithium polysulfides from the cathode. However, these approaches are fundamentally ineffective since polysulfide anions carry negative charges, in discharging the electric field between two electrodes will drive polysulfide anions migrating toward Li anode. Furthermore, the incorporation of electrochemically inert carbons reduces the gravimetric energy density of Li/S batteries. In addition, we have noticed that most of decent capacities reported previously were obtained through two lows, that is, low sulfur content in composition and low sulfur loading in cathode. In many cases, the total sulfur content in the cathode is not more than 65% by weight and the sulfur loading is not higher than 2 mg sulfur per cm2 of cathode [4], [5], [6], [7], [14], [15], [16], [17], [18].
Since dissolution of lithium polysulfides (Li2Sx, x > 2) in organic electrolytes is inevitable, in this work we propose a different approach for the performance improvement of rechargeable Li/S batteries by employing a liquid electrolyte that is able to promote the formation of a highly protective passivation film on lithium surface in lithium polysuifide solutions. We expect that the resulting passivation film not only protects lithium metal from chemical reaction with the polysulfide anions but also prevents polysulfide anions from electrochemical reduction on the Li anode. Our effort will be focused on increasing Li cycling efficiency in highly concentrated lithium polysulfide solutions. To examine our idea, we selected Li/Li2S9 liquid cell [19], [20], instead of the conventional Li/S cell, as the testing vehicle by employing a porous carbon electrode as the cathode current collector and a Li2S9 solution as the catholyte. Due to the known ability of LiNO3 in facilitating the formation of a better passivation film on Li metal surface [21], [22], in this work we study the effect of LiNO3 on cycling performance of Li/Li2S9 liquid cells and on cycling efficiency of Li metal in Li2S9 catholyte solutions by adding LiNO3 as a co-salt of the Li2S9 catholyte.
Section snippets
Experimental
Elemental sulfur (S8, 99.5%), lithium sulfide (Li2S, 99%), and lithium nitrate (LiNO3, 99.99%) were purchased from Aldrich and used as received. Lithium bis(trifluoromethane sulfone)imide (LiN(SO2CF3)2, LiTFSI, 3M Company) was dried at 110 °C under vacuum for 10 h and triethylene glycol dimethyl ether (TG3, 99%, Aldrich) was dried over 4 Å molecular sieves for a week. For conventional Li/S cells, a liquid electrolyte was prepared by dissolving 0.2 m (molality) LiTFSI in TG3 in Ar-filled glove-box
Li/S cell vs. Li/Li2S9 liquid cell
Fig. 1 shows voltage profiles of the first and fifth cycles of a conventional Li/S cell and a Li/Li2S9 liquid cell, respectively. For the conventional Li/S cell (Fig. 1a), the initial discharging consists of three voltage regions: (1) a short plateau at 2.3 V as indicated by the arrow, (2) a linear sloping decline, and (3) a long plateau at ∼2.0 V until the end of discharge. Combining the conclusions of previous publications [16], [26], [27], [28], [29], we ascribe these three discharging voltage
Conclusions
This work demonstrates an alternative approach for the performance improvement of rechargeable Li/S batteries. While dissolution of lithium polysulfides (Li2Sx, x > 2) in organic electrolytes is inevitable, research efforts focusing on the protection of the lithium anode to increase Li cycling efficiency in highly concentrated polysulfide solutions may be more feasible. LiNO3 is excellent in promoting the formation of a denser and more protective passivation film on the Li surface. The film
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