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Researchers develop non-flammable fluorinated electrolyte for Li-metal anodes with aggressive cathode chemistries; toward a 500 Wh/kg goal

Researchers at the University of Maryland (UMD), the US Army Research Laboratory (ARL), and Argonne National Laboratory (ANL) have developed a non-flammable fluorinated electrolyte that supports the most aggressive and high-voltage cathodes in a Li-metal battery.

In a paper in the journal Nature Nanotechnology, they report that a battery with their electrolyte shows high cycling stability, as evidenced by the efficiencies for Li-metal plating/stripping (99.2%) for a 5 V cathode LiCoPO4 (~99.81%) and a Ni-rich LiNi0.8Mn0.1Co0.1O2 cathode (~99.93%). At a loading of 2.0 mAh cm−2, the full cells retain ~93% of their original capacities after 1,000 cycles.

Surface analyses and quantum chemistry calculations show that stabilization of these aggressive chemistries at extreme potentials is due to the formation of a several-nanometer-thick fluorinated interphase.

Increasing the capacity of LIBs to 500 Wh kg−1, a new goal set by automotive applications for a longer driving range with a single charge, will have to resort to more aggressive chemistries such as conversion-reaction or high-voltage/ high-capacity intercalation cathodes, all of which involve Li metal as the anode.

Li metal offers one of the highest specific capacities (3,860 mAh g−1) and the lowest redox potential (−3.04 V versus standard hydrogen electrode (SHE)). Its coupling with a high-voltage/ high-capacity cathode such as LiNi0.8Mn0.1Co0.1O2 (NMC811) or LiCoPO4 (LCP) would create a high-energy density cell that meets the 500 Wh kg−1 goal4. However, numerous fundamental challenges, arising from the highly reactive nature of both the Li-metal anode and these aggressive cathodes, preclude the practical realization of rechargeable Li-metal batteries (LMBs). Because of their high reactivity, LMBs constantly operate with low Coulombic efficiency, signalling the rapid consumption of both electrolyte and Li and leading to a short cycle life. Though the instantaneous reactions between electrolytes and Li create a passivation layer called the solid electrolyte interphase (SEI), its inhomogeneous composition and morphology induce Li dendritic growth, which compromises LMB cycle life and safety. On the cathode side, electrolyte oxidation leads to a cathode electrolyte interphase (CEI), which at high voltage does not sufficiently stabilize the electrolyte from sustained oxidation.

… Here, we report a non-flammable electrolyte that demonstrates excellent stability toward both a Li-metal anode and high-voltage/ high-capacity cathodes. It consists of 1M lithium hexafluorophosphate (LiPF6) in a mixture of fluoroethylene carbonate/3,3, 3-fluoroethylmethyl carbonate/1,1,2,2-tetrafluoroethyl-2′,2′, 2′-trifluoroethyl ether (FEC:FEMC:HFE, 2:6:2 by weight). Unlike the previously reported fluorinated electrolytes, which suffered from increasing impedance at the anode side, this all-fluorinated electrolyte enables a high Li plating/stripping Coulombic efficiency of 99.2% and suppresses dendrites without raising the interfacial impedance. It also supports the stable cycling of NMC811 (Coulombic efficiency of ~99.93%) and LCP (Coulombic efficiency of ~99.81%) cathodes by forming a highly fluorinated interphase with thickness of 5–10nm that is responsible for the effective inhibition of electrolyte oxidation and transition metal dissolution. Unprecedented cycling stabilities were obtained for both Li||NMC811 (90% retention at the 450th cycle) and Li||LCP cells (93% retention at the 1,000th cycle).

—Fan et al.

Jang Wook Choi, an associate professor in chemical and biological engineering at Seoul National University in South Korea, who was not involved with the research said that the cycle lives achieved with the given electrode materials and operation voltage windows sound “unprecedented.”

This work is a [sic] great progress forward in the battery field in the direction of increasing the energy density, although further tuning might be needed to meet various standards for commercialization.

—Jang Wook Choi

The team demonstrated the batteries in coin-cells and is working with industry partners to use the electrolytes for a high voltage battery.

Chunsheng Wang, professor in the University of Maryland’s Clark School’s Department of Chemical and Biochemical Engineering, collaborated with Kang Xu at ARL and Khalil Amine at ANL on these new electrolyte materials. Since each element on the periodic table has a different arrangement of electrons, Wang studies how each permutation of chemical structure can be an advantage or disadvantage in a battery.

He and Xu also head up an industry-university-government collaborative effort called the Center for Research in Extreme Batteries, which aims to unite companies that need batteries for unusual uses with the researchers who can invent them.

The aim of the research was to overcome the capacity limitation that lithium-ion batteries experience. We identified that fluorine is the key ingredient that ensures these aggressive chemistries behave reversibly to yield long battery life. An additional merit of fluorine is that it makes the usually combustible electrolytes completely unable to catch on fire.

—Chunsheng Wang

The high population of fluorine-containing species in the interphases is the key to making the material work, even though results have varied for different researchers in the past regarding the fluorination.

You can find evidences from literature that either support or disapprove fluorine as good ingredient in interphases. What we learned in this work is that, in most cases it is not just what chemical ingredients you have in the interphase, but how they are arranged and distributed.

—Kang Xu

Resources

  • Xiulin Fan, Long Chen, Oleg Borodin, Xiao Ji, Ji Chen, Singyuk Hou, Tao Deng, Jing Zheng, Chongyin Yang, Sz-Chian Liou, Khalil Amine, Kang Xu & Chunsheng Wang (2018) “Non-flammable electrolyte enables Li-metal batteries with aggressive cathode chemistries” Nature Nanotechnology doi: 10.1038/s41565-018-0183-2

Comments

Davemart

This work shows how far we are from 500Wh/kg in EVs.

We are now getting to grips with the basics of the chemistry, and are not yet even at the stage of knowing exactly what we should do to, at some stage, make price insensitive button batteries using it, let alone mass producing such devices.

And that is still a long, long way from mass production for cars.

We are talking about 2030 plus.

If they ever get there.

SJC

If better batteries were easy, they would have been done by now.

Engineer-Poet
The team
  • demonstrated the batteries in coin-cells and
  • is working with industry partners to use the electrolytes for a high voltage battery.
Here by 2021.  If it works over temperature, in vehicles by 2024.
electric-car-insider.com

Every battery breakthrough provides more support for confidence that a 500Wh/kg is achievable. Whether the time frame for commercialization is 3, 7 or 12 years, it's a clear signal that the path forward for transportation is battery electric. More complex and expensive drive train solutions will have increasing difficulty obtaining funding within that window.

Davemart

Perhaps a rationale for how this technology is to progress at several times the speed of previous introductions would be good.

Sony batteries were catching fire for years before lithium batteries made it into cars, and they were the result of a lot more advanced from the lab bench development than anything this theoretical work can show.

I am not dissing the technology, but improbable time lines.

I was on this forum in 2009 when Nissan were talking about a double energy battery, supposed to arrive in 2015, and which is not here yet, maybe next year.

And that was nowhere near as radical a change as this.

Arnold

time frames for industry mass production still remain a human scale limited logistic and development problem. The problems are often unforeseen and can only be resolved at humans pace. I think we have become for too used to seeing apparently instant results in our ordinary lives while things continue to take the time they take remains constant.
On the bright side, the available technological tools and the understanding of their application have been breaking new ground in battery development and delivering at the exponential rate.
This is the important change as without solid science, the factories can only produce 'second rate ' no matter how fast or how many it is the understanding that will deliver the results we want and that understanding is occurring much faster than the doubters want to believe.


Engineer-Poet

Arnold, don't forget step-changes in progress due to technological discontinuities.  ENIAC and several generations of successors were based on, and limited by, vacuum-tube technology.  When tubes were replaced by transistors, there was a step-change in all sorts of figures of merit.

Replacement of ionic electrodes by metallic electrodes is a similar change.  The limit is the specs of primary (non-rechargeable) lithium cells.

Gasbag

“was on this forum in 2009 when Nissan were talking about a double energy battery, supposed to arrive in 2015, and which is not here yet, maybe next year.”

The 2010 Leaf was rated at 73 miles EPA,The 2018 Leaf 150, the 2019 Leaf is expected to be 225 miles EPA. Roughly triple the range and double the charging rate in a span of nine years is not fast enough for many but we’re getting close.

HarveyD

Unlike computer chips, increasing secondary (rechargeable) batteries performances while lowering mass production price has been a progressive endeavour going at about 5% to 8% per year for the last 20 to 30 years.

More drastic changes to the technologies used are required to change/increase the development rate but it will continue to take up to 10+ years to go from Labs to mass production.

Going from 2X to 3X batteries by 2025/2030, from 3X to 4X by 2035/2040 and from 4X to 5X batteries by 2045/2050 or so is to be expected unless...?

SJC

I don't know why people equate batteries with computer chips, the tech is totally different.

Arnold

The discontinuity as see it is the use of synchotron or particle beam microscopes as well as other real time imaging tools over the last five years or so.
These tools have been about for several decades but only recently has the technique been attempted for the lithium secondary cell studies.

By looking inside a cell as it charges and discharges researchers can see especially dendrite formation and other structural changes that have previously taken months of accelerated ageing to understand the end result.

These tools require sophisticated understanding and for commercial application or public interest there needs to be a strong case.

As both these requirements are easily met for Li cells and researchers are active in the area we can reliably predict massive improvements in both longevity and capacity over the coming years.

HarveyD

Increasing demands for EV batteries will attract more R&D funds and (also) contribute to quicker development such as improved capacity/density and longevity together with lower prices.

Will accelerated development and testing require less than the curent 10 years to go from one generation to the next?

Gasbag

“lowering mass production price has been a progressive endeavour going at about 5% to 8% per year for the last 20 to 30 years.”

You’re going back to far to get a rate that suits you. Going back 30 years means you’re looking at NiMh and NiCad and yes for those we had increases in specific energy density of 4-5% but there wasn’t a lot of demand. Your relevant time line should begin around 2010 when the Tesla MS and Nissan Leaf started shipping in significant numbers relative to the advanced battery market.

liBs specific energy density has increased consistently at 8% per year. That means doubling every 9 years. Although that is improving the real factors holding back EVs are the price per kWh and the recharge rate. Battery prices have declined at a much higher rate but have been less consistent.

Gasbag

Here is a link on specific energydensity increases of batteries. With respect to EVs the relevant chemistries are NCA and NMC. Lead acid, NiMH and NiCads don’t factor into the EV equation.

https://www.quora.com/Is-it-true-that-battery-energy-density-improves-5-8-per-year-Does-this-represent-an-average-or-is-it-a-consistent-trend-each-year-Do-these-improvements-increase-the-cost-What-has-been-the-trend-if-any-regarding-energy-to-weight-ratio

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