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High capacity manganese layered-perovskite cathode for fluoride ion batteries involving cationic and anionic redox reaction

Developing electrochemical high-energy storage systems is of crucial importance towards a green and sustainable energy supply. A promising candidate is fluoride ion batteries (FIBs), which can deliver a higher energy density than is possible with lithium ion batteries 1,2 . However, conversion-type reactions with metal fluorides causes a poor electrochemical reversibility 1,3,4 . Recently, layered perovskite oxides such as LaSrMnO 4 have been shown to undergo topotactic electrochemical (de)fluorination, but they have low reversible discharge capacities (25 ~ 100 mAh/g) and poor rate capabilities. Here we show that a double-layered perovskite oxyfluoride La 1.2 Sr 1.8 Mn 2 O 7– δ F 2 exhibits topotactic (de)intercalation reaction inside the rock-salt slabs, achieving a large reversible capacity of 535 mAh/cm 3 (0 ≤ x ≤ 2 in La 1.2 Sr 1.8 Mn 2 O 7– δ F x ), with excellent cycle stability and rate capability. Surprisingly, despite the close-packed perovskite-based structure, two extra fluoride ions are (de)intercalated beyond x = 2, leading to a reversible capacity of 1168 mAh/cm 3 (0 ≤ x ≤ 4). During the further intercalation, oxygen molecules are formed in the perovskite layer, as in Na 0.75 [Li 0.25 Mn 0.75 ]O 25 , which is responsible for the charge compensation (i.e. anion redox) 5,6 and the concomitant formation of oxygen vacancies that allow the incorporation of the excess fluoride ions. These results highlight the layered perovskite oxide/oxyfluorides as a new class of active materials for the construction of high-performance FIBs. More generally, the concept of anion-intercalation through O 2 formation in the mixed-anion perovskite materials can be used to develop new functionalities.


Introduction
Considerable efforts have been devoted to developing efficient energy storage systems in order to achieve a green and sustainable energy society in the future. The widespread success of lithium-ion rechargeable batteries (LIBs) is attributed to their high energy density, light weight, long-term durability, and the presence of a large number of available host structures 7 . In recent years, special attention has been paid to enhancing the capacity of cathode materials through the redox of oxide anions (including the formation of O n-, O2 2-, O2 -, and even O2), in addition to the redox of transition metal cations 5,6,[8][9][10][11][12][13][14][15] . However, there is a growing demand for electric vehicles with longer mileage and grid energy storage, for which developing larger power source is required.
In this study, we demonstrate that La1. 2Sr1.8Mn2O7F2 is able to electrochemically deintercalate (and intercalate) two fluoride ions by fully utilizing the interstitial site in the rock-salt slab, with excellent cycle performance (80% capacity retention after 50 cycles) and rate capability (60 mAh/g at 100 mA/g). Unexpectedly, La1. 2Sr1.8Mn2O7F2 can be further topotactically fluorinated to a capacity density of 1168 mAh/cm 3 , allowing the development of FIBs with high energy density and durability. Interestingly, these excess fluoride ions are introduced into the perovskite slabs, where the charge compensation and space creation is achieved through the formation of molecular O2 (i.e., anion redox), as found in LIBs and sodium ion batteries 5,6,8,12,37,38 . Given the abundance of perovskite compounds, these results are expected to facilitate the development of cathode materials for FIBs, as mixed-anion compounds with anion redox could be applied to active materials.

Electrochemical intercalation in rock-salt slabs of
A cathode mixture was prepared by ball-milling La1. 2Sr1.8Mn2O7F2 with La0. 9Ba0.1F2.9 and Vapor-Grown Carbon Fiber (VGCF), as often used in FIBs 34,35 . A three-electrodes cell was assembled using the mixture, with Pb wire as a reference electrode by inserting its tip into the solid electrolyte not to contact with each electrode layer. Figure   1a shows discharge-charge profiles in a voltage range of -1.5~2.0 V at 10 mA/g. Upon the initial discharge, the capacity reached 91.2 mAh/g, which is equivalent to the capacity of two Fanions (91.7 mAh/g) and suggests an almost complete Fextraction from the La1. 2Sr1.8Mn2O7F2. Subsequent charge process yielded a capacity of 86.8 mAh/g, corresponding to a discharge-charge efficiency of 95.2%.  It is remarkable that the electrode La1. 2Sr1.8Mn2O7F2 exhibited excellent cycle stability and rate capability. It maintained capacity retention at 80% of the initial capacity after 50 cycles, with the coulombic discharge-charge efficiency of almost 100% during the cycles ( Figure 2a). Moreover, even at 100 mA/g, this electrode has a capacity of 60 mAh/g (corresponding to 65% of the discharge capacity at 10 mA/g), despite the relatively large particle size of 1~3 μm (Figure 2b). These properties are distinct from conversion-type active materials (e.g., CuF2, BiF3) whose capacity drops rapidly; in the case of BiF3, it is less than 50% of the initial discharge capacity after 10 We also conducted electrochemical measurements using La1. 2Sr1.8Mn2O7 as a starting material and obtained similar charge/discharge curves with excellent cycle stability (Supplementary Figure 7). Compared with La1. 2Sr1.8Mn2O7F2, XRD and STEM-EELS data (Supplementary Figure 8) show a difference in the 1st charge process, while subsequent cycles are reversible (Supplementary Figure 7), with the charge/discharge profiles similar to those of La1. 2Sr1.8Mn2O7F2. We also found that the Mn K-edge shifts to lower energy after the 1st cycle than that of pristine La1.      The efficiency was calculated as follows: the n-th discharge capacity divided by the nth charge capacity. b, Discharge curves with different current rates for La1. 2Sr1.8Mn2O7-δF2 after 15 cycles. The same conditions were adopted for charging, with an upper limit potential of 3.0 V and a current rate of 10 mA/g. c, Plots of the volumetric/gravimetric capacities for La1. 2Sr1.8Mn2O7-δF2 and cathode materials reported in LIBs 8,12,39,40 . cycles without obvious decrease in capacity, and the reversible capacity amounts to 1168 mAh/cm 3 . As shown in Figure 4c, this value is much higher than that of typical LIB cathode materials and, in terms of volumetric energy density, is comparable to that of recently investigated active materials using anion redox 8,12 . Figure 4b shows that our material with a fluorine content varying between x = 0 and 4 has a relatively high rate capability (200 mAh/g at 100 mA/g), which is possibly related to the rapid fluoride ion diffusion in the bulk.

Anionic redox reactions
The charge compensation mechanisms of (de)fluorination were examined by synchrotron hard/soft X-ray absorption spectroscopy (XAS) ( The frequency of the first vibrational level was approximately 1600 cm -1 , similar to the gaseous molecular O2 bound with Mn 5 , and differed from 1108cm -1 in O2 -and 743cm -1 in O2 49 . Since the RIXS measurements were performed under high vacuum, this oscillatory signal is attributed most likely to O2 molecules in La1. 2Sr1.8Mn2O7-δFx, rather than those in the gas phase or absorbed on the cathode surface. Such O2 formation has recently been observed in Na0. 75[Li0.25Mn0.75]O2 during charge process (i.e., Nadeintercalation) 5 . The greater polarization of the anion redox region (x > 2) of La1. 2Sr1.8Mn2O7-δFx (Figure 3a) and Na0. 75[Li0.25Mn0.75]O2 5 compared to Li2Ru0. 75Sn0.25O3 (in which redox via O2 -occurs) 50 is also in line with the formation/annihilation of O2.
In the pre-edge region of F K-edge spectra (Figure 5d and Supplementary Figure   15), a small peak attributed to metal-fluorine bond 51

Outlook for intercalation-type active material
We demonstrated electrochemical intercalation of fluoride ions with excellent reversibility, cyclability and rate capability, using the double-layered Ruddlesden-Popper type perovskite oxyfluoride La1. 2Sr1.8Mn2O7-δFx. Interestingly, in addition to the conventional Mn redox at 0 < x < 2, this oxyfluoride can incorporate excess fluoride ions (2 < x < 4) into the perovskite blocks by forming molecular O2 species (anion redox). It should be emphasized that, like the LIB, the current FIB has room for further improvement in terms of electrochemical properties such as cycle stability, rate characteristics, and fluoride ion diffusion; available strategies include chemical substitution with other transition metals, optimization of composition in an electrode mixture and tuning particle morphologies, in addition to modification of solid-solid interfaces with solid electrolytes (e.g., surface coating, orientation control).
Given the variety of structures and compositions known for conventional perovskite materials, the ability to capture excess fluorine ion through the formation of O2 molecules adds a new dimension to perovskite engineering not only for battery research but also for other disciplines. In fact, formation of molecules at the anion site in perovskite ABO3 with the 'closed packed' structure is not obvious, as seen in the theoretically predicted H2 molecule in SrTiO3 52 , and could be an interesting research topic in general. Furthermore, this study sufficiently raises awareness that electrochemistry is a powerful tool for obtaining novel oxyfluorides and, more generally, mixed-anion compounds 53