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 LiNi_0.8Mn_0.1Co_0.1O₂ 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 LiNi_0.8Mn_0.1Co_0.1O₂ (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 450^th 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