U Texas Austin team finds P2S5 electrolyte additive enables use of Li2S bulk particles for high-capacity cathodes in lithium-sulfur batteries; ~800 mAh/g

19 December 2014

Researchers at the University of Texas at Austin, led by Prof. Arumugam Manthiram, have found that using phosphorus pentasulfide (P₂S₅) as an electrolyte additive
enables the direct use of commercially available bulk Li₂S particles as high-capacity cathode materials for rechargeable Li−S batteries, without intricate synthesis or application of a high charging cut-off voltage that deteriorates the electrolyte stability and safety.

The ability to use commercially available bulk particles could significantly decrease the manufacturing cost of Li−S batteries with a LiS cathode. In a paper published in the ACS Journal of Physical Chemistry Letters, the team suggested that this strategy is both of significance for the safe and effective use of Li₂S as a cathode material and as a promising step toward the low-cost fabrication of metallic-lithium-free Li−S batteries.

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While Lithium-sulfur (Li-S) batteries offer the promise of a high theoretical energy of ∼2500 Wh kg⁻¹, they require the use of a lithium-metal anode, the degradation of which can prove problematic for safety and performance. A possible alternative to the straight Li-S battery is to combine lithium-free anodes (such as tin or silicon) with a lithium sulfide (Li₂S) cathode (∼1166 mAh g⁻¹). (Earlier post.)

Researchers have identified that one issue with the use of Li₂S is a large potential barrier (∼1 V) on the initial charge of insulating bulk Li₂S particles, believed to relate to the nucleation of a new polysulfide phase.

The large voltage barrier can be overcome by applying a high initial charging cut-off voltage of ∼4.0 V—but that causes instabilities in the commonly used ether-based electrolytes and deteriorates the electrochemical performance.

It has recently been reported that P₂S₅ reacts with Li₂S in ether-based electrolytes. In this work, we systematically investigate the interaction between P₂S₅ and Li₂S and identify the possibility of using P₂S₅ as an electrolyte additive to activate commercially available bulk Li₂S particles for their direct use in room-temperature rechargeable Li−S batteries without apply-ing a high charging cut-off voltage. It is found that the electrolyte with the P₂S₅ additive effectively enhances the electrochemical activity of Li₂S. The activated Li₂S cathode exhibits a greatly lowered initial charging voltage plateau, indicating effective oxidation of Li₂S to polysulfides. Furthermore, the activation correlates to the formation of intermediate sulfur- and phosphorus-containing species on the surface of Li₂S, assisting the surface charge transfer.

The researchers examined the performance of the Li₂S cathode in electrolyte with P₂S₅ with CR2032 coin cells. Among the results were reversible discharge capacities of ∼800 mAh g (Li₂S)⁻¹ and capacity retention as high as 83% after 80 cycles. Coulombic efficiency remained near 100% with cycling.

They concluded that the interaction between P₂S₅ and Li₂S results in reduced cell resistance and enhanced surface oxidation of Li₂S to polysulfides, significantly lowering the initial charging voltage plateau.

The most effective electrochemical activation occurred when the molar ratio between Li₂S and P₂S₅ is 7:1, before a thick solid electrolyte layer forms on the surface of Li₂S. Because the core structure of the P₂S₅-activated, micron-sized Li₂S is retained when the most effective activation occurs, the activation is mainly a surface effect, they concluded.

Resources

• Chenxi Zu, Michael Klein, and Arumugam Manthiram (2014) “Activated Li₂S as a High-Performance Cathode for Rechargeable Lithium–Sulfur Batteries,” The Journal of Physical Chemistry Letters 2014 5 (22), 3986-3991 doi: 10.1021/jz5021108