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
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| (A) Comparison of different Li-ion cathode materials. Numbers in parentheses are the specific energy of a battery made of the cathode and a silicon anode with a specific capacity of 2000 mAh/g and potential of 0.45 V vs Li/Li+. (B) Schematic diagram showing the effect of applying a high cutoff to activate Li2S. After overcoming the initial barrier, a polysulfide phase is formed and Li2S becomes active. Credit: ACS, Yang et al. Click to enlarge. |
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.
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| Summary of the model for the initial charging of Li2S. Before reaching the top of the potential barrier, Li2−xS exists as a single phase with a lithium-poor shell on the surface. In step 2, the shell is highly lithium deficient while the core remains in near stoichiometry. In step 3, soluble polysulfides are formed after overcoming the initial barrier, shown as the yellow part around the solid Li2S particle. Consequently, the kinetics is significantly improved. At the end of charging, only the polysulfide phase exists with a fast kinetics. Click to enlarge. Credit: ACS, Yang et al. |
Resources
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
" ... a capacity decay rate of only 0.25% per cycle."
On first reading this is not that great since this implies that the capacity after 10 cycles is 97.5%; after 100 cycles, 77.9%; and, after 1000 cycles, 8.2%. Did I miss something?
Posted by: NorthernPiker | 11 September 2012 at 05:32 AM
Oh my...that's a promising result! An energy density of 1000 watts per kg is more than twice the capacity of current Li batteries.
I know that if a leaf could travel 150-200 miles on a charge, that would pretty much be the game changer. Imagine all the electric cars that would be rolling off the line with the range limit removed!
The only problem I now see is a short supply of Lithium. There would be a new Saudi Arabia and it's name is Bolivia.
Posted by: EVryman | 11 September 2012 at 05:34 AM
Five year exhale http://www.greencarcongress.com/2007/12/researchers-exp.html
Posted by: kelly | 11 September 2012 at 06:35 AM
Future lithium and post-lithium batteries will have energy density of more than 1000 Wh/Kg and EVs equipped with light weight 120+ KWh batteries will travel 800 to 1200 Km per charge.
Those EVs will not really need quick charges because after 1000 Km drivers and passengers will have/want to stop for an overnight rest while their BEV in on a relatively slow charger.
Of course, all road side hotels/motels will be equipped with adequate (money making) charging facilities. The days of the smelly adjacent gas stations are numbered.
Posted by: HarveyD | 11 September 2012 at 07:01 AM
Harvey
you are the most predictable person ever seen on the web, always repeating the same things...
Posted by: Treehugger | 11 September 2012 at 08:09 AM
The main problem with Li2S is the formation of polysulfides, which are soluble in the electrolyte and migrate to the anode and contaminate it. So, the 0.22% decay per cycle is the problem. I do believe this is the material, because of the high capacity and low cost that will couple with Si anodes to improve cell capacity and cost. This study does nothing to resolve the real issue however. How do you keep the polysulfides on the cathode side? Answer that, and you have a solution. Until then you have a research problem, which by the way, is a very good way to make a living, being a researcher that is.
Posted by: Brotherkenny4 | 11 September 2012 at 08:22 AM
@ EVryman
Lithium supplies should not be a long-term issue since there is no shortage at current prices and it a relatively abundant element - 20 ppm in the earth's crust and 0.17 ppm in seawater.
Yes, there may be a temporary price spike if Li-ion battery demand soars but the market will stabilize in a year or so just like silicon did a couple of years ago when solar cell production took off.
Furthermore, lithium does not represent even 1% of the material cost of current Li-ion batteries. This is why Li-ion batteries are typically re-cycled to recover heavy or expensive metals like cobalt and nickel.
See this recent comprehensive paper, "Resource Constraints on the Battery Energy Storage Potential for Grid and Transportation Applications", by Albertus et al.
http://cyruswadia.com/prof/Publications_files/Wadia%20et.al.%20Resource%20Constraints%20on%20Battery%20Storage.pdf
Posted by: NorthernPiker | 11 September 2012 at 10:14 AM
Tree....e-storage technologies will evolve and their energy density will increase at 8%/year rate and up to 12%/year with the arrival of new materials and technologies. The current 300 Wh/Kg best case will increase to 500 Wh/Kg best case by 2017 and to 1000 Wh/Kg by 2022 or so. That is normal evolution without major breakthroughs.
By 2022 or so, 1000Wh/Kg (120 KWh) light weight battery packs will extend lighter BEVs range close to 1000 Km. Quick recharges en-route will no longer be required. People will drive 1000 Km and recharge overnight while resting.
Current worries about electrified vehicles insufficient e-range, quick charging facilities availability and batteries high cost will progressively fade away in the next 10 years or so.
Posted by: HarveyD | 11 September 2012 at 03:53 PM
Harvey, a compact BEV travelling 1000 km will consume over 125 kWh. So, an overnight recharge (~10 hours) will require at least a 60 amp, 230 volt circuit (13.8 kW), or a 25 amp, 550 volt circuit (13.75 kW). Not standard circuits at the Shady Rest Motel.
Posted by: NorthernPiker | 11 September 2012 at 06:58 PM
440v 3 phase
Posted by: SJC | 11 September 2012 at 07:49 PM
No, Shady rest motel doesn't have that kind of service...yet...but if they want EV travellers, they will......! :)
Infrastructure definitely needs to be beefed up. I've often wanted to provide an at cost charge depot for EVs at my home...you know, for the odd EV enthusiast who needs a quick rescue charge. If a network of individuals did this, travelling on EV adventures could be a new pass-time!
I could definitely convince my family to go on an electric road adventure.
Posted by: EVryman | 11 September 2012 at 09:40 PM
Overnight Hotel/Motel charging facilities will quickly become an added revenue item for owners/operators. The investment required will be recovered very quickly with captive customers. The only losers will be the vacant adjacent gas stations.
Picking up 120 KWh over 12 hours (10 KW/h) is not much of a challenge. Our house consumes more than that for heating (alone) on very cold days + 4.5 KW for the hot water tank + 3.5Kw for the dryer + 3+ Kw for the electric range etc.
Why are so many posters worried about overnight charging of a few million EVs. Most of the energy is already available. Wind converters could supply 10X to 100X of the energy required. Solar energy could do much more.
Posted by: HarveyD | 12 September 2012 at 09:11 AM
By the way, most homes in our area are equipped with 400 Amps @ 230 VAC = 92KW. Charging one or two EVs overnight @ 10 KW (each) rate is very possible without any changes other than added low cost 230 VAC outlets, similar to the Dryer and/or e-range.
Posted by: HarveyD | 12 September 2012 at 09:17 AM
If 70+% of our current car/light truck fleet was magically changed over to EVs the current grid has the generation and transmission capacity to charge them right now.
Our grid is built to avoid power outages on the very hottest summer afternoons when we experience peak-peak demand. Lots of that capacity goes unused at night.
All that is missing in terms of infrastructure are 'reachable outlets' for the ~40% who don't park near one and an adequate number of rapid charge points along our major travel routes.
Posted by: Bob Wallace | 12 September 2012 at 03:02 PM
So rightly said BW.
Posted by: HarveyD | 13 September 2012 at 01:50 PM
Even non-believers, naysayers and fossil fuel fans will have to face reality. Overnight charging 2 or 3 BEVs per family will normally require nothing more than adding 2 or 3 ... 220/240 VAC outlets and low cost timers to benefit from night low tariffs.
Apartment dwellers will have to invest a bit more to add and cable charging facilities but many subsidies will help.
Road side hotels/motels/restaurants etc will welcome the new business opportunity. It will be an easy way to make a few more $ per night per room/meal/customer etc.
Electric energy suppliers will also welcome the added business, specially all those overnight chargers using available/unused energy.
Posted by: HarveyD | 13 September 2012 at 02:04 PM
The EV grid already exists...you just have to put a sign on the front of the outlet.
Posted by: EVryman | 13 September 2012 at 05:56 PM
Just ran across an interesting blog by a guy who is driving his Tesla Model across the continent.
He's doing some of his charging in RV parks. They're already set up for people to plug and pay.
http://teslamodelsxc.wordpress.com/
And they are driving a luxury car while tent camping, which I find a hoot. I love people who aren't locked into "this is how you do it" patterns.
Makes me think of the first people to drive cross country in Model Ts. Except the roads are a lot better....
Posted by: Bob Wallace | 14 September 2012 at 11:16 AM