Ternary Metal Fluorides As New Cathodes of Rechargeable Lithium Batteries with Ultrahigh Energy Density

Some of transition metal fluorides (MF_x) have been shown promising for use as high-capacity cathodes in rechargeable Li-ion batteries for large-scale applications, such as electric vehicles and grid-scale storage, but energy-efficiency and kinetics related issues remain a major hurdle to their commercial use. Cu based fluorides, such as CuF₂ or the composites are particularly attractive due to the 3.55 V redox potential and extraordinarily high specific energy (1874 Wh/kg), but due to the irreversibility of Cu redox they were used only in primary batteries.  Novel ternary metal fluorides M¹_yM²_1-yF_x (M¹, M² = transition metal, 0 ≤ y ≤ 1), of varying metal species and stoichiometry, were synthesized by cost-effective mechanochemical process. Due to incorporation of a second cation in the same lattice, this composite system exhibits exceptional electrochemical properties, shown as significantly reduced 1^st discharge polarization, cycling hysteresis and faster reaction kinetics than the binary metal counterparts. And strikingly high reversibility of Cu redox (Cu²⁺/Cu⁰) was found in the Cu_yFe_1-yF₂ system by electrochemical measurements along with confirmation via x-ray absorption spectroscopy. The results from this study demonstrated, for the 1^st time, the feasibility of using Cu-based reversible conversion cathodes that will provide 3 times higher energy density than conventional intercalation cathodes.

The Li storage/release mechanisms and limits to cycling stability of Cu_yFe_1-yF₂ were investigated by combining electrochemical measurement with comprehensive structural and chemical analysis using in-situ X-ray absorption spectroscopy, X-ray diffraction, and transmission electron microscopy-electron energy loss spectroscopy (TEM-EELS). The lithium reaction process is much more complicated in Cu_yFe_1-yF₂ than the binary metal counterparts (i.e. FeF₂, CuF₂)[1, 2], and it was found that there are two separate conversion processes in Cu_yFe_1-yF₂ during lithiation, a reduction of Cu component into metallic Cu⁰ and concomitant formation of disordered FeF₂ at higher potentials, followed by Fe^2+/0 reduction. During delithiation process, Fe⁰ was over-oxidized to Fe^2+/3+ states, which leads to LiF deficiency and subsequently partial reconversion of Cu. Some of the recent results on synthesis, structural and electrochemical characterization of ternary metal fluorides M¹_yM²_1-yF_x will be presented. Detailed lithium reaction mechanisms, and Cu-loss related issues along with possible remedy solutions in the Cu_yFe_1-yF₂ system, will be discussed.  [1] Wang et al., Conversion Reaction Mechanisms in Lithium Ion Batteries: Study of the Binary Metal Fluoride Electrodes, J. Am. Chem. Soc., 133 18828 (2011); [2] Wang et al., “Tracking of Li Transport and electrochemical reaction in nanoparticles”, Nat. Comm., 3 (2012) 1201.

Acknowledgement This research is supported as part of the Northeastern Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC0001294.Some of transition metal fluorides (MF_x) have been shown promising for use as high-capacity cathodes in rechargeable Li-ion batteries for large-scale applications, such as electric vehicles and grid-scale storage, but energy-efficiency and kinetics related issues remain a major hurdle to their commercial use. Cu based fluorides, such as CuF₂ or the composites are particularly attractive due to the 3.55 V redox potential and extraordinarily high specific energy (1874 Wh/kg), but due to the irreversibility of Cu redox they were used only in primary batteries.  Novel ternary metal fluorides M¹_yM²_1-yF_x (M¹, M² = transition metal, 0 ≤ y ≤ 1), of varying metal species and stoichiometry, were synthesized by cost-effective mechanochemical process. Due to incorporation of a second cation in the same lattice, this composite system exhibits exceptional electrochemical properties, shown as significantly reduced 1^st discharge polarization, cycling hysteresis and faster reaction kinetics than the binary metal counterparts. And strikingly high reversibility of Cu redox (Cu²⁺/Cu⁰) was found in the Cu_yFe_1-yF₂ system by electrochemical measurements along with confirmation via x-ray absorption spectroscopy. The results from this study demonstrated, for the 1^st time, the feasibility of using Cu-based reversible conversion cathodes that will provide 3 times higher energy density than conventional intercalation cathodes.

The Li storage/release mechanisms and limits to cycling stability of Cu_yFe_1-yF₂ were investigated by combining electrochemical measurement with comprehensive structural and chemical analysis using in-situ X-ray absorption spectroscopy, X-ray diffraction, and transmission electron microscopy-electron energy loss spectroscopy (TEM-EELS). The lithium reaction process is much more complicated in Cu_yFe_1-yF₂ than the binary metal counterparts (i.e. FeF₂, CuF₂)[1, 2], and it was found that there are two separate conversion processes in Cu_yFe_1-yF₂ during lithiation, a reduction of Cu component into metallic Cu⁰ and concomitant formation of disordered FeF₂ at higher potentials, followed by Fe^2+/0 reduction. During delithiation process, Fe⁰ was over-oxidized to Fe^2+/3+ states, which leads to LiF deficiency and subsequently partial reconversion of Cu. Some of the recent results on synthesis, structural and electrochemical characterization of ternary metal fluorides M¹_yM²_1-yF_x will be presented. Detailed lithium reaction mechanisms, and Cu-loss related issues along with possible remedy solutions in the Cu_yFe_1-yF₂ system, will be discussed.  [1] Wang et al., Conversion Reaction Mechanisms in Lithium Ion Batteries: Study of the Binary Metal Fluoride Electrodes, J. Am. Chem. Soc., 133 18828 (2011); [2] Wang et al., “Tracking of Li Transport and electrochemical reaction in nanoparticles”, Nat. Comm., 3 (2012) 1201.

Acknowledgement This research is supported as part of the Northeastern Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC0001294.