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.
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.—Zu et.al
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.
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