High‐Performance Low‐Temperature Li⁺ Intercalation in Disordered Rock‐Salt Li–Cr–V Oxyfluorides

Abstract

Li-ion batteries have driven the technological evolution of portable electronics, automotive, and stationarya pplications over the past decades.[1] However, the poor low-temperature performance of Li-ion batteries impedes their specific applications, such as fora erospace, defense, and transportation systems.[2] When operating at reduced temperatures, especially below 0 8C, the kinetic performance of the employed materials, governed by the intrinsic transport property for electrons and Li ions, becomes ac ritical factor determining the deliverable capacity.[3] Classic Li-ion battery cathode materials suffer from substantial capacity loss, [4] severe voltage hysteresis, [5] and change in voltage profiles.[6] As reported, from 25 to 0 8C, ac ommercialL iFePO 4 -based cell has ac apacity retention of only 42 %. [7] The sluggish kinetics of Li + transport and the reduced electronic conductivity in the solid LiFePO 4 phase at low temperatures leads to the voltage profiles evolving from ap lateau-like to sloping regime.[8] For layered Ni-Mn-Co oxides, [9] ad ischarge voltage downshift of 0.5 Vh as been observed when reducing the temperature from 25 to À20 8C. [6] In addition, the performance deteriorationo fa node materials and the appliede lectrolytes at low temperatures contributet ot he poor overall battery performance. [2,10] At sub-ambient temperatures, Li platingt ends to occur at the graphite anode,o wing to the reduced Li + intercalation into graphite.[11] At temperatures below À10 8C, the ionic mobility in the electrolyte solution and conductivity of the conventional binary carbonate electrolyte fall rapidly.[12] The optimization of the electrolyte formulations for different electrode materials at low temperatures still remains to be solved. Nevertheless, exploring new cathodem aterials with well-retaineds pecific energiesa tr educed temperatures is am ore decisive step towards broad applications of Li-ion batteries.We have recentlyr eportedt hat an ew cathode material, Li 2 VO 2 F, with ad isordered rock-salt structure can deliver ah igh initial intercalation capacity when cycling at al ow current rate.[13] However,L i 2 VO 2 Fs uffers from capacity fading whilst cycling. It has been found that the poor cycling performance of Li 2 VO 2 Fc an be improved by substituting Cr forVi nt his system.[14] Moreover,t he cyclability of the mixed V/Cr oxyfluorides depends on the applied chargecutoff voltages and operation temperature. Af acile Li + chemical diffusion in such ac lass of materials favors the kinetic properties.[15] Herein,w ee xplore the composition-dependent, low-temperature properties of the oxyfluorides and examine the structural features responsible for the cycling performance.The low-temperature rate capabilities of Li 2 VO 2 F, Li 2 CrO 2 F, and Li 2 Cr 0.2 V 0.8 O 2 Fw ere first evaluated at À10 8Ca tv arious current densities from 26 to 1050 mA g À1 ,a nd then back again at 26 mA g À1 (Figures 1a-c

Design principles for high transition metal capacity in disordered rocksalt Li-ion cathodes

“…When F is directly bonded to Mn or V, the paramagnetic interactions are so strong that the resulting signals are too broad to be observed and are lost in the background noise. Hence, the relatively weak 19 F signal obtained for all three LMVF20 compounds suggests that a significant fraction of the F is bonded to at least one transition metal, consistent with the predicted fluorination behavior of disordered rocksalt materials. 5 These invisible paramagnetic F sites prevent us from quantifying the fraction of F in LiF-like domains or particles in the pristine cathode samples, evidenced by the LiF-like signals (centered at À204 ppm) present in all the spectra.…”

“…However, previous studies have demonstrated that, at least in stoichiometric layered oxide cathodes, LiF may form electrochemically inactive coatings. 51,52 We rely on 19 F solid-state nuclear magnetic resonance spectroscopy (ssNMR) to confirm that, in our case, the fluorine is not segregated. The 19 F NMR spectra collected for ST-LMVF20, MR-LMVF20 and LR-LMVF20, shown in Fig.…”

“…The 19 F NMR spectra collected for ST-LMVF20, MR-LMVF20 and LR-LMVF20, shown in Fig. 2b, 6 In addition, some of our ongoing work on related paramagnetic cation-disordered oxyfluorides indicates that observable paramagnetic 19 F NMR signals can be assigned to F nuclei surrounded by 6 Li in their first coordination shell, with paramagnetic species (here Mn and V) in their second and/or third metal coordination shells (see ESI, † Fig. S2a in Lee et al 6 for a schematic diagram of coordination shells).…”

“…Solid-state nuclear magnetic resonance (NMR) spectroscopy 19 F NMR data were acquired for the as-synthesized ST-, MR-and LR-LMVF20 powder samples at room temperature using a Bruker Avance500 WB spectrometer (11.7 T), at a Larmor frequency of À470.7 MHz. The data were obtained under 60 kHz magic angle spinning (MAS) using a 1.3 mm double-resonance probe, and chemical shifts were referenced against lithium fluoride powder (LiF, d iso ( 19 F) = À204 ppm).…”

“…The data were obtained under 60 kHz magic angle spinning (MAS) using a 1.3 mm double-resonance probe, and chemical shifts were referenced against lithium fluoride powder (LiF, d iso ( 19 F) = À204 ppm). Because the resonant frequency range of the 19 F nuclei in the as-synthesized cathodes is larger than the excitation bandwidth of the radio frequency (RF) pulse used in the NMR experiment, seven spin echo spectra were collected for each sample, with the irradiation frequency varied in steps equal to the excitation bandwidth of the RF pulse (330 ppm or 155 kHz) from À1230 to 750 ppm. The individual sub-spectra were processed using a zero order phase correction so that the on-resonance signal was in the absorption mode.…”