## Li-S battery with novel solid-state electrolyte shows capacity approaching theoretical value and high Coulombic efficiency

##### 30 January 2015
 Voltage profiles of charge-discharge cycles of the solid-state Li-S battery. Current density of 0.05 C). The specific capacity is given per g of sulfur. Yamada et al. Click to enlarge.

A team from Samsung R&D and the University of Rome “La Sapienza” have fabricated a novel all solid-state Li-S battery that exhibits a capacity (∼ 1600 mAhg−1) approaching the theoretical value and an initial charge-discharge Coulombic efficiency approaching 99% (the average in ten cycles was 98%). An open access paper on their work is published in the Journal of The Electrochemical Society.

In addition to these and its other favorable properties (ie.e, smooth stripping-deposition of lithium), the activation energy of the charge transfer process was 44.5 kJmol−1—much smaller than that of a corresponding liquid electrolyte Li-S cell. These results, the team concluded, “are convincing in demonstrating that the solid electrolyte is very effective in physically preventing polysulfide migration.

Overcoming the polysulfide shuttle is a significant advantage since it is a major drawback for a typical liquid electrolyte based Li-S battery. Further work is in progress in our laboratories to elucidate the behavior of our battery, and also to improve its construction. Nevertheless, we believe that the data here reported, even if still at a preliminary stage, are of importance for the progress of the lithium-sulfur battery technology.

While Li-sulfur batteries are of keen interest as a high energy density successor to current Li-ion technology, there are significant challenges to overcome, including the red-ox shuttle of polysulfides originating originates from the dissolution of the sulfur cathode material into the organic electrolyte and poor lithium cycle performance due to the consumption of lithium metal during the charge-discharge process.

Solid-state electrolytes, based on both inorganic and organic compounds, are valid alternatives to develop lithium batteries with high safety and long cycle life, as in fact practically demonstrated. … It is important to point out that these “solid-state batteries” have the favorable characteristic of avoiding lithium dendrite deposition and hence, of preventing short circuits, in cells using lithium metal as the anode active material. In addition, solid electrolytes are expected to be safer than common non-aqueous electrolyte media, because of their low or negligible vapor pressure.

… In a previous work, we have demonstrated a solid-state Li-S battery based on 0.8Li2S-0.2P2S5 electrolyte. However, even cycling under shallow depth of discharge (DOD), dendrite short circuits were indeed observed. In this work we have extended the investigation, by developing and testing a solid-state Li-S battery using a stoichiometric composition of 0.75Li2S-0.25P2S5, Li3PS4 as the electrolyte.

The team prepared the sulfide solid electrolyte using a high energy ball milling method. A cathode composite was made from mixing sulfur powder and carbon nanofibers in the ratio of 3:1 (w/w). A lithium foil served as the anode.

In electrochemical testing, the Li/Li3PS4/S solid-state cell achieved the high specific capacity at both 25 ˚C and 80 ˚C. The team concluded that the high value of the coulombic efficiency was clear evidence that polysulfide shuttle was prevented by the solid electrolyte layer.

Another important remark is that the discharge plateau typically reported for Li-S batteries was not seen at 25 ˚C where a large discharge-charge polarization was also observed. The electrochemical reaction was significantly accelerated at 80 ˚C, where the cycling nearly evolved along the expected plateau. Although the reaction in the solid electrolyte Li-S cell is still unclear, its kinetics are expected to be much slower in the liquid electrolyte cell.

… Realistically, the risk of dendrite formation and of cathode and electrolyte degradation cannot be excluded and they might shorten the cycle life of the battery. Long cycle life and high coulombic efficiency have been reported for thin film lithium batteries where the electrolyte layers are generally prepared by a vapor deposition process and are quite dense. This suggests that a dense solid electrolyte is a key for making a high performance solid state battery.

Let's not forget that batteries' cost will go down (below $100/kWh) and performance will go up to over 4X by early 2020s. Lighter and lower cost batteries will translate into much lighter, lower cost ($20K to $30K) extended range BEVs before 2025. Fossil and bio-fuels and H2 will always cost more than clean Hydro, Solar and Wind electricity. People living in sunny places may produce enough clean electricity with future higher performance solar cells to meet 90+% of the needs for two EVs plus the 10-15 kWh/day for the house. The batteries from the two extended range EVs could help during cloudy-rainy days. Of course, the in house solar systems will have their own fixed batteries. Let's remember that in order to get down to the magic$100 per kWh at the cell level, a single 18650 cell, of the type used to make up the Tesla packs (i.e. 46 g, 12 Wh each), needs only to come down in cost to $1.20 per cell. Industry insiders suggest that Tesla is buying them currently at$1.90 per cell, so \$100 per kWh at the cell level is not too far away. At the pack level, Telsa are making big progress cutting the additional cost / weight of the non-cell pack components too.