78 Chemical Science Research Frontiers 2024 Lithium-ion batteries (LIBs) are widely used in various electronic devices owing to their high energy density, high power density, and long-term durability. However, with the growing demand for longer-range electric vehicles, there is a need to develop high- energy-density batteries that do not rely on lithium. One strategy to surpass the energy density of current LIBs is to utilize electrode materials that enable multielectron reactions. However, using polyvalent ions such as magnesium ions (Mg 2+ ) as carriers presents kinetic disadvantages, such as slow diffusion within solid electrodes. In contrast, fluoride ions (F – ), which are monovalent and have a small ionic radius (1.33 Å), can enable multielectron reactions with fast ionic conduction. Owing to these characteristics, all- solid-state fluoride-ion batteries (FIBs) that use F – as a carrier have attracted attention for their potential to achieve high energy and power densities [1,2]. While metal/metal fluorides have been developed as typical positive electrodes, they suffer from poor cyclability and power density due to large volume changes during charging and discharging. To address these issues, electrode materials based on topotactic F – intercalation reactions have been developed. Compared to metal/metal fluorides, these materials significantly improve cyclability and power density. However, their usable capacity is limited because of their relatively large chemical formula weight. In this study, to increase the capacity of intercalation-type positive electrode materials, anion redox, as reported in LIB cathodes [3], was applied to a double-layered perovskite La 1.2 Sr1.8 Mn 2 O 7– δ F 2 [4]. The La 1.2 Sr 1.8 Mn 2 O 7– δ F 2 delivered a charge capacity of approximately 250 mA·h/g (Fig. 1(a)) after the initial discharge. The subsequent discharge yielded a large capacity of 190 mA·h/g, nearly double the initial discharge. Excess electrochemical fluorination was confirmed to be topotactic through X-ray diffraction (XRD), scanning transmission electron microscopy (STEM) and atomic resolution electron energy loss spectroscopy (EELS). The XRD pattern of the 3.0 V charged La 1.2 Sr 1.8 Mn 2 O 7– δ F 2 remained similar to that of the pristine state (Fig. 1(b)), while STEM imaging indicated regular cation arrangements (Fig. 1(c)). EELS mapping of the 3.0 V charged La 1.2 Sr 1.8 Mn 2 O 7– δ F 2 (Fig. 1(c)) revealed the presence of excess F – in the perovskite block alongside the interstitial site within the rock-salt slabs. Upon discharge, the defluorinated Double-layered perovskite positive electrode with high capacity involving O–O bond formation for all-solid-state fluoride-ion batteries Fig. 1. (a) Charge/discharge curves of La 1.2 Sr 1.8Mn 2O 7− δ F 2 . (b) XRD patterns of cathode composites containing La 1.2Sr 1.8Mn 2O 7− δ F 2 during charge/discharge. Red and black broken lines correspond to La 1.2Sr 1.8Mn 2O 7− δ F 2 and La 1.2 Sr 1.8 Mn 2 O 7− δ , respectively. Asterisks denote the La 0.9 Ba 0.1 F 2.9 electrolyte. (c) Atomic resolution STEM- EELS mapping images of La 1.2 Sr 1.8 Mn 2O 7− δ F 2 during charge/discharge along the [010] crystallographic axis. (a) (c) (b) Capacity (mA·h·g – 1 ) Cell Voltage (V) 0 Intensity (arb. units) 28 32 36 40 44 48 0 1.0 Intercalation to rocksalt layer 2nd Initial discharge 1st 2.0 3.0 4.0 5.0 6.0 – 2.0 – 1.0 0 1.0 2.0 3.0 50 100 150 200 250 300 F-K 1st discharged ( – 1.5 V) 1st charged (3.0 V) x in La 1.2 Sr1.8Mn 2 O 7– δ δ F x O-K Mn-L La-M Sr-L ADF F-K 1st discharged – 1.5 V 105 * * * * 105 107 110 110 116 116 1st charged 3.0 V Discharged – 1.5 V Pristine La 1.2 Sr 1.8 Mn 2 O 7 – δ F 2 O-K Mn-L La-M Sr-L ADF 0.5 nm 0.5 nm F-K O-K Mn-L La-M Sr-L ADF 0.5 nm Pristine (La 1.2 Sr 1.8 Mn 2 O 7 – δ δ F 2 ) 2 θ θ (deg.) 79 Research Frontiers 2024 La 1.2 Sr 1.8 Mn 2 O 7– δ , phase was identified by both XRD and EELS (Figs. 1(b,c)). The charge compensation mechanism during (de)fluorination was analyzed using soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) (Fig. 2). The Mn L -edge spectrum of La 1.2 Sr 1.8 Mn 2 O 7– δ F 2 (Fig. 2(a)) showed a shift to higher energy from x = 0 to x = 2.0 but no further shift was observed from x = 2.0 to x = 4.8. This behavior indicates that Mn ions are oxidized from Mn 3+ to Mn 4+ during the initial charging phase and remain as Mn 4+ in the later stage of charge. In the O K -edge spectrum, the intensity at 529 eV increased during the early stage of charging ( x < 2) (Fig. 2(b)) owing to the oxidation of Mn 3+ to Mn 4+ . However, as charging progressed beyond x ~ 2, a new O K -edge peak emerged at ~530.8 eV. The intensity of this peak increased with further charging and reversibly disappeared upon discharge. To investigate the origin of this new peak, RIXS measurements were performed at excitation energies between 528.5 and 533.0 eV. The RIXS spectra at 530.8 eV (Fig. 2(c)) revealed discrete energy loss peaks near the elastic line from 5 to 0 eV. The vibrational frequency of the first level was measured at 1591 cm –1 , closely matching the molecular O 2 (~1600 cm –1 ), observed in charged Na 0.75 [Li 0.25 Mn 0.75 ]O 2 by RIXS [5]. This result indicates the formation of an O–O bond in charged La 1.2 Sr 1.8 Mn 2 O 7– δ F 2 . Upon discharge, this vibration signature disappeared, confirming the reversible nature of O–O bond formation/breaking. These results demonstrate that La 1.2 Sr 1.8 Mn 2 O 7– δ F 2 undergoes two distinct fluoride (F – ) intercalation processes: initial (de)intercalation into rock-salt slabs with conventional Mn redox and subsequent (de)intercalation of excess F – into the perovskite layer involving the formation of an oxygen–oxygen bond (anion redox) (Fig. 3). This study is the first to demonstrate the introduction of electronic holes and the associated formation of O–O bonds following the electrochemical intercalation of an anionic species. Given the abundance of perovskite compounds, these findings are expected to advance the development of cathode materials for FIBs, where mixed-anion compounds with anion redox reactions may serve as effective active materials. Kentaro Yamamoto Faculty of Engineering, Nara Women’s University Email: k.yamamoto@cc.nara-wu.ac.jp References [1] M. Anji Reddy and M. Fichtner: J. Mater. Chem. 21 (2011) 17059. [2] D. Zhang et al. : ACS Appl. Mater. Interfaces 13 (2021) 30198. [3] M. Sathiya et al. : Nat. Mater. 12 (2013) 827. [4] H. Miki, K. Yamamoto, H. Nakaki, T. Yoshinari, K. Nakanishi, S. Nakanishi, H. Iba, J. Miyawaki, Y. Harada, A. Kuwabara, Y. Wang, T. Watanabe, T. Matsunaga, K. Maeda, H. Kageyama, Y. Uchimoto: J. Am. Chem. Soc. 146 (2024) 3844. [5] R. A. House et al. : Nature 577 (2020) 502. Fig. 3. Discharge/charge scheme of La 1.2Sr 1.8Mn 2O 7– δ F 2 . The specific locations of the O–O bond and excess F – in the charged La 1.2Sr 1.8Mn 2 O 7– δ F 2 are not clear. (a) (c) Photon Energy (eV) Energy Loss (eV) Energy Loss (eV) (b) 632 636 640 644 648 652 Mn L -edge O K -edge MnO 2 Mn 3+ discharged x = 4.8 x = 3.6 x = 2.4 x = 2.0 x = 1.5 x = 1.0 x = 0.5 x = 0 discharged New peak x = 4.8 x = 3.6 x = 2.4 x = 2.0 x = 1.5 x = 1.0 x = 0.5 x = 0 Mn 4+ MnCO 3 Mn 2 O 3 656 660 Photon Energy (eV) 524 pristine charge discharge Excitaion Energy 530.8 eV 0.2 eV . =. 1600 cm –1 530.2 eV 526 528 530 532 534 536 Intensity (arb. units) Intensity (arb. units) 15 10 5 0 2 1 0 Intensity (arb. units) c Charge Discharge Discharge Charge a b La 1.2 Sr 1.8 Mn 2 O 7– δ La 1.2 Sr 1.8 Mn 2 O 7– δ F 2 La 1.2 Sr 1.8 Mn 2 O 7– δ F 4 2F – F 2F – MnO 6 La/Sr O/F O–O F Fig. 2. (a) Mn L -edge and (b) O K -edge XAS spectra collected at SPring-8 BL27SU of La 1.2 Sr 1.8 Mn 2 O 7– δ F 2 during charge/discharge. The parameter x in the figure represents the fluoride content ( x in La 1.2 Sr 1.8 Mn 2 O 7– δ F x ). (c) O K -edge RIXS spectra recorded at excitation energies of 530.2 and 530.8 eV at SPring-8 BL07LSU for the pristine, charged (3.0 V), and discharged (–1.5 V) states, respectively.
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