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Stanford team develops method enabling use of lithium sulfide as cathode material for high specific energy batteries; a simpler approach rivaling lithium sulfur
11 September 2012
A team of Researchers at Stanford University and SLAC National Accelerator Laboratory, led by Stanford’s Dr. Yi Cui, has developed a simple and scalable approach to utilizing Li2S (lithium sulfide) as the cathode material for rechargeable lithium-ion batteries with high specific energy.
The results, reported in a paper published in the Journal of the American Chemical Society, could potentially lead to rechargeable batteries with specific energies of about 4 times that of current technology and approaching those of lithium-sulfur (LiS) systems currently under intensive study, while avoiding some of the issues with those systems.
Li2S has a theoretical capacity of 1166 mAh/g—nearly 1 order of magnitude higher than traditional metal oxides/phosphates cathodes. If paired with Si anodes with 2000 mAh/g capacity, the specific energy of a Li2S-based lithium-ion battery could be 60% higher than the theoretical limit of metal oxide/phosphate counterparts, according to the team, and three times that of the current prevailing LiCoO2/graphite system.
Moreover, they note, Li2S could be paired with a lithium-free anode, preventing safety concerns and low Coulomb efficiency of lithium metal in Li/S batteries. However:
The main hindrance for utilizing Li2S is that it is both electronically and ionically insulating. Therefore, Li2S was considered electrochemically inactive. Recently, significant progress has been made to activate Li2S.
...In this work, we show that there is a potential barrier of ∼1 V at the beginning of the first charging of Li2S. By simply applying a higher voltage cutoff to overcome this barrier, Li2S can be oxidized to polysulfides and rendered active. After this activation process, the barrier does not appear again in subsequent cycling.—Yang et al.
The formation of the polysulfide phase dramatically improves the kinetics of Li2S, such as the charge transfer process, they found. Subsequent cycling showed that the material behaves similar to common sulfur cathodes with high energy efficiency. They observed an initial discharge capacity higher than 800 mAh/g; after 10 cycles, the capacity is stabilized around 500–550 mAh/g with a capacity decay rate of only 0.25% per cycle.
With either polysulfide or LiNO3 additives in the electrolyte, the cycle retention was improved to 85−88% from the 11th to the 50th cycles with a specific capacity of 500−550 mAh/g. The decay rate is only 0.22% per cycle between the 10th and the 100th cycles for the sample with polysulfide additive.
Results above show that Li2S is a promising candidate as a high-capacity cathode for Li-ion batteries. Along with studies on Li2S, a high-energy Li/S battery is currently an active field and plenty of progress has been achieved in improving its performance. Thus, it is meaningful to compare the characteristics of these two systems.
The theoretical specific energy of the Li/S system is 2600 Wh/kg, 70% higher than the Li2S/silicon system (1550 Wh/kg). However, practically, significantly more lithium is required in Li/S batteries due to formation of mossy lithium and the low Coulomb efficiency of lithium. Consequently, the practical specific energy of Li2S/ silicon (930 Wh/kg) is close to that of the Li/S battery (1000 Wh/kg). The Li2S/silicon system also avoids the safety issue in Li/S batteries.
...by applying a high voltage cutoff in the initial charging, we demonstrated a simple and scalable method for activating Li2S, especially given the fact that this material is air sensitive. No extra processing, such as lithiation or high- temperature processing to form carbon/Li2S composite, is needed. Moreover, our approach is also compatible with conventional liquid electrolyte and room temperature operation. To our knowledge, this activation behavior is novel and has not been observed in other battery systems.—Yang et al.
They discovered that the origin of the initial barrier is the phase nucleation of polysulfides, with the amplitude of barrier is mainly due to two factors: (a) charge transfer directly between Li2S and electrolyte without polysulfide and (b) lithium-ion diffusion in Li2S.
Yuan Yang, Guangyuan Zheng, Sumohan Misra, Johanna Nelson, Michael F. Toney, and Yi Cui (2012) High-Capacity Micrometer-Sized Li2S Particles as Cathode Materials for Advanced Rechargeable Lithium-Ion Batteries. Journal of the American Chemical Society doi: 10.1021/ja3052206
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