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RIKEN team develops high-performance lithium-iodine battery system with higher energy density than conventional Li-ion

The working concept of I3/I redox reaction in the aqueous Li-I2 battery. Zhao et al. Click to enlarge.

A team from Japan’s RIKEN, led by Hye Ryung Byon, has developed a lithium-iodine (Li-I2) battery system with a significantly higher energy density than conventional lithium-ion batteries. RIKEN is Japan’s largest research organization, with institutes and centers in locations throughout Japan.

In a paper published in Nature Communications, they report that aqueous lithium-iodine batteries based on the triiodide/iodide redox reaction show high battery performance. The high solubility of triiodide/iodide redox couples results in an energy density of ~ 0.33 kWh kg−1. The reversible redox reaction without the formation of resistive solid products promotes rechargeability, demonstrating 100 cycles with negligible capacity fading.

They suggest that a low cost, non-flammable and heavy-metal-free aqueous cathode can contribute to the feasibility of scale-up of lithium-iodine batteries for practical energy storage.

According to the Battery Roadmap 2010 announced by the New Energy and Industrial Technology Development Organization in Japan, the main target for rechargeable batteries is an improvement of the energy density up to 0.5 kWh kg-1cell (1.0 kWh l-1cell) by 2030 to enable electric vehicles to extend the driving range to a comparable level with gasoline-powered internal combustion engine vehicles (ca. 500 km) [311 miles]. However, achieving more than three times the energy density raised from the currently employed battery systems (<0.2 kWh kg-1cell) is an exceptional challenge, because the current battery technology has almost reached its performance limitation. Accordingly, new battery systems using new chemistries and system configurations are needed, which are capable of achieving higher energy density than the current ones.

...The alternative to make a less problematic storage system [than lithium-sulfur or lithium-O2] is aqueous lithium batteries. The aqueous solution has a high ionic conductivity in the presence of completely ionized substances, which leads to rapid redox reactions in electrochemical cells. The idea has been to employ these aqueous electrolytes in a cathode, referred to as the aqueous cathode using redox couple reactions that do not deteriorate the aqueous cathode and electrically conductive current collector, as well as not leave over any solid product and precipitation residue at the aqueous cathode/current collector interface. The aqueous cathode can, therefore, offer negligible polarization and volume expansion.

A promising aqueous cathode can be determined from the redox couples possessing high solubility and a suitable redox potential avoiding the electrolysis of water. The solubility is proportional to the energy density. In the diagram of redox couple solubility with respect to the standard reduction potential, the triiodide/iodide (I3/I) redox couple reaction shows a favorable solubility...The redox potential of the I3/I couples (0.536 V versus standard hydrogen electrode (SHE)) is also suitable to avoid water this work for the first time, we present the aqueous cathode operated by the I3/I redox couples and apply this for a lithium-iodine (Li-I2) battery.

The aqueous Li-I2 battery we demonstrate is noticeably different from either the conventional all-solid-state or non-aqueous electrolyte-based Li-I2 batteries, which have performed at extremely low discharge current rate or shown low Coulombic efficiency with the formation of a lithium iodide (LiI) layer. The I3/I redox reaction in aqueous cathode is rapid and performed up 12 mA cm-2 of discharge current rate without serious potential drop. The aqueous Li-I2 battery attains superior storage capacity (~98% of the theoretical capacity), Coulombic efficiency (>99.5%), and cyclic performance (>99.5% capacity retention for 100 cycles) which is, to the best of our knowledge, the best result among previous reports using new chemistries and system configurations.

—Zhao et al.

Schematic illustration of the aqueous Li-I2 battery. Zhao et al. Click to enlarge.

The aqueous Li-I2 batteries consist of a lithium anode (Cu mesh/Li metal/organic electrolyte/buffer layer), ceramic separator, the aqueous cathode, and current collector (Super P carbon/Ti foil). Li metal with 1 M of LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte was used for the anode. The Super P carbon-coated Ti foil was employed as the current collector in the aqueous cathode.

The Li-I2 batteries showed high energy density and excellent recharge ability.

  • Specific capacity was ~207 mAh g-1 at a current rate of 2.5 mA cm-2, which approaches ~98% of the theoretical capacity.

  • Energy density was ~0.35 kWh kg-1 calculated from the mass of aqueous cathode containing saturated I2 and KI and Li metal in anode and ~0.33 kWh kg-1 estimated from the experimental result—several times higher than previously reported for alkaline aqueous cathodes.

  • No significant capacity fading was observed during cycles; ~99.6% capacity retention and 99.5-100% Coulombic efficiency for 100 cycles is superior to Li-S and Li-O2 batteries, as well as other aqueous Li batteries using a solid Ni(OH)2 cathode.

  • The open circuit potential was ~3.8 V versus Li+/Li and the discharge and charge potentials were stable at 3.50 and 3.70 V versus Li+/Li on cycling, which resulted in overpotentials of 0.04 and 0.16 V, respectively. The overpotential value at such a high current rate is still comparable to Li-ion batteries, and superior to Li-S and Li-O2 batteries at current rates even an order of magnitude lower, demonstrating ~90% overall energy efficiency on cycling,

Byon and colleagues now plan to develop a three-dimensional, micro-structured current collector that could enhance the diffusion-controlled triiodide/iodide process and accelerate charge and discharge. They are also seeking to raise energy densities even further by using a flowing-electrode configuration using an external reservoir.

In summary, the I3/I redox reaction-operated aqueous Li-I2 battery exhibited excellent cyclic performance with considerable energy and power densities. Ideal Coulombic efficiency and capacity retention with no degradation of cathode establish the aqueous Li-I2 battery as one of the outstanding post-Li batteries. The specific energy density (~0.33 kWh kg-1) is fairly promising in comparison to the conventional lead-acid, nickel-metal hydride and Li-ion batteries, and can be further improved to be ~0.4 kWh kg-1 using NaI or LiI instead of aqueous KI...The aqueous cathode system can be also extendable to aqueous Na-I2 and K-I2 batteries. We believe that this energy storage system can be scaled up with a reasonably increased energy density by combining a flow-through-mode system.

—Zhao et al.


  • Zhao, Y., Wang, L. & Byon, H. R. (2013) High-performance rechargeable lithium-iodine batteries using triiodide/iodide redox couples in an aqueous cathode. Nature Communications 4, 1896 doi: 10.1038/ncomms2907



In my view the notion that batteries have improved in energy density by x amount per year, and so a similar rate of improvement is to be expected in the future, takes insufficient account of plateauing with particular technologies.

As the article says, to get to much higher densities a step change is needed, not incremental improvement, and when that might occur technically and economically can't be read off of a graph of previous battery energy density improvements.


Isn't the aqueous element problematic because water reacts with LiPF6 to make hydrogen fluoride?



you are right, but some here have problems to understand that simple fact, and still naively believe that battery progresses are driven by Moore-law like microprocessors...


But I can FEEL that breakthrough coming, I can feel it in my bones! Ok, that feeling's probably arthritis, but you gotta let me dream LOL



Sure breakthroughs happen and will happen, but they are not predictable, therefore trying to schedule how fast car electrification will grow is lost of time.

Bob Wallace

We can't predict breakthroughs, but we might be able to predict price drops for existing technologies to some extent.

Tesla has proved that we can build >200 mile range EVs. Two hundred is all we need.

Cost drops for present battery technology will allow rapid car electrification. A 200 mile range EV priced not much higher than a same-model ICEV will flip the market.


The mains difference between batteries and processors evolution is the slow rate of the former and the fast rate of the latter. Batteries are evolving at about 8%/year while processors are doing about 64%/year or about 8 times faster.

Why are processors evolving much faster. Probably not because it is easier or cheaper? Is it because processors get 8 times the R&D funds? It may very well be so. Majors like IBM, Intel, AMD, SEC, Samsung, Toshiba, Sony, LG, Texas Instruments and many others spend XX$B/year in ongoing R&D and have been doing so for 3+ decades.

Another reason may be the more important (wider) worldwide market for faster, cheaper, lighter and more energy efficient processors required for phones, tablets, computers, TVs, play consoles, cars, airplanes, satellites, etc.

As demand for batteries grows, more funds will become available for R & D and their evolution rate may pick up.


These numbers are very good for a system which may be put into a flow-battery configuration.

If flow-through cells can be designed, this may be the ideal stationary/grid battery. It seems to have very good cycling ability, scales well and very high round-trip efficiency.

In a flow configuration, this could scale to MWh systems easily.


Davemart / Treehugger the fact that current battery technology will plateau is obvious as any given chemical battery configuration is in the end defined by the laws of physics, and only optimizing all parameters will get you on the plateau.

I do believe however that we are just seeing the birth of battery development if you compare the amount of research money that goes into "fossile solutions" to "electric only" . If car builders really get serious.... I'm confident that 15% per year will continue to be the "battery improvement factor" (energy density, discharge rate, price) for some time to come.


@Dave and BobW,
I agree that graphs are limited for predicting future energy density, discharge/recharge cycle numbers, and cost/KwHr. It's just frustrating. We've all read for years about Silicon nano-wire this, carbon nanotube that, graphene something or other, Sulfur, Lithium-air, Zinc-Air, Flow-whatchamacallit, Prieto solid-state dohiccamajig, and much more. There were supposed to be better electrolytes, anodes, cathodes packaging and management.

What do we get for all of that? Tesla Model S goes with standard Lithium 18650 cells that were maybe 10% better than the Roadster's previous generation batteries. It just boggles the mind. How can so many research paths with so much theoretical upside all be unsolved after low these many years?


Good question HB. How can so many produce so little for so long? Is it due to lack of know how, lack of time, lack of funds, lack of desire to do better, lack of well defined objectives, lack of requirements, pressure from ICEVs, Oil, ethanol supporters or all of it?


If one compares battery evolution (from 35-45 wh/kg to 140-200 wh/kg after 120+ years i.e about 3X to 4.5X) with light bulbs evolution from (10 L/watt incandescent to over 200 L/Watt LEDs, i.e. about 20X) in about the same time frame; batteries are overdue for a better managed accelerated evolution.


The demand for better batteries has always existed, especially for military application in which portability and harsh environment were a constraint. The problem with batteries is that for every improvement you need to find a new material, designing and investigating new materials is tedious and costly, once you invent a new material you have to show that it is stable, safe, manufacturable, and it takes time and carry big risks. So progress in batteries are inherently slow, but still progress we see.a factor 4 improvement in 100years is not that bad at all, when you think of it


I don't buy the conspiracy theories. They require too many coincidences, when simpler explanations...this is hard...make as much or more sense. We're trying to optimize charge density, power density, safety, cost, long lifecycle, efficiency, and plentiful's not as simple as reducing the with of transistors and doubling the number on a chip. Still, you would think that one of these chemistries with a potential of 1000-2000wh/kg would get us at least to 500wh/kg once you make the compromises for the other variables.

Bob Wallace

We know that some new tech batteries are being tested by car companies now. They've made it out of the lab and into test vehicles at the car manufacturer.

Envia claims 400 Wh/kg and low cost. It's running around test tracks.

Electrovaya batteries were installed in Chrysler PHEV trucks. They didn't work for some more extreme conditions but should have no problems with regular EV/PHEV applications.

It's hard to know what other companies might be far enough along to appear in vehicles in the near future. Perhaps several, perhaps none. But I'd bet more than one.

Stuff is happening. Just not out in the open.

I suspect we forget how recently the widespread interest in EVs emerged. Nissan's LEAF and Tesla's Roadster were the first EVs to get serious attention. With a major vehicle manufacturer taking a strong EV stand and a new company blowing away all the perceived EV weaknesses. I suspect a lot of people realized that a better battery would make the developers wealthy.

And Bloomberg Financials reports that EV battery prices have dropped 40% since 2010.

Tesla has shown us that we would be OK with current battery technology. A significant price decrease would make longer range EVs affordable and it looks like prices are falling.

I expect the battery situation will be quite different two years from now. Both in capacity and cost.


Current battery technology is just fine for EVs. Cost reductions would be more important to the market. That cost reduction will come with volume. Except, most car companies build these things only to comply. What if in 5 years at 8% improvement we have also cut costs 50%? We'd have a 140% better battery at half the cost. The Volt would be able to reduce mass of the battery by 40% and charge half as much. This is where we are going. These are not hard numbers to hit.

Admittedly, 5 years if way off in the future for corporations that think only about the next quarter.


I've been following battery advancements for electric cars since I worked at the Physical Sciences Library in college in the 1980's. People ALWAYS say, "In 5 years..."

Five years is far enough away that big advancement sounds plausible, but not so far away that people feel hopeless about the length of the wait. We also thought that all the research in laptops and cell phone batteries, and economies of scale would get us cheap 300-mile battery packs that weighed less than half a ton. Not so, even with incremental improvements.

Don't get me wrong. The Tesla S is what it is because of all of that, and I admire it. I just want a car that costs one third as much with the same range. Do we need a Manhattan project, or what?


As battery technology improves, all sorts of vehicle economics look better with more electrification.

  • The stop-start micro-hybrid gets FE and driveability improvements by adding launch assist.
  • Cars with launch assist become full hybrids with more acceleration assist, more dynamic braking and all-electric accessories.
  • The full hybrid trends to a PHEV.
This is a one-way ratchet.  I'm averaging 115 MPG so far, which is more than 3x what my last car gave me.  Generalized to the whole USA, this amounts to a cut of almost 6 million bbl/d of gasoline and the better part of 2 mmbbl/d of diesel (most of the rest can be replaced by LNG).  The total is nearly equal to net US oil imports.

Kit P

Notice that E-P never tells how much gasoline he buys!

For example, since E-P bought a PEV I have bought 13.5 gallons for my commuter truck. I paid $1200 for it 15 years ago. This means I am about a $1000 ahead of E-P since his new way of saving money.

It cost nothing to drive less. Instead of being suckered at the dealer E-P should have found two people to carpool with so that he would truly be reducing energy consumption. This would also make his POS POV last longer.


I've bought about 20 gallons of gasoline in the last month, but only about 6.6 of that has been for personal use.  5 gallons of that was for the lawn tractor.


If I can find myself an old GE ElekTrak, I'll get rid of the gas that thing eats too.

Bob Wallace

"Don't get me wrong. The Tesla S is what it is because of all of that, and I admire it. I just want a car that costs one third as much with the same range. Do we need a Manhattan project, or what?"

I think we've already got a dispersed Manhattan Project up and running. A lot of people in a lot of labs are working hard to figure out better storage solutions. And I suspect we hear only about the startup ideas which are advertising for financing. I would imagine every large battery manufacturer has a an intense research effort going on, but out of sight.

According to Bloomberg EV battery prices have dropped 40% in price since 2010. I suspect existing technology batteries will reach "sweet prices" soon. What we most need is for a different technology that allows a battery pack the size/weight of the LEAF's to give us 2x the range.


"What do we get for all of that? Tesla Model S goes with standard Lithium 18650 cells that were maybe 10% better than the Roadster's previous generation batteries."

Roadster used 2.4ah cells, Model S uses 3.1ah cells, that's about 30% improvement. There are already 3.4ah cells available, and 4.0's in the works. Without any significant breakthroughs.



Thanks for the correction. Who is expected to release 4.0ah cells and on what approximate timeline?

Kit P

"If I can find myself an old GE ElekTrak"

You can buy a new electric mower if you were not full of BS.

It would be nice if I could talk my neighbors into into them. They all have riding mowers that require hearing protection. I just wait till it gets to noisy, then I go and mow while getting a little exercise.

When I lived in California, I had an electric mower. They are crap. Light duty jobs are great for motors and extension cords but ICE are the work horses.

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