Industrial Applications Research Frontiers 2022 108 Operando X-ray diffraction and X-ray absorption spectroscopy of zero-strain lithium-ion battery material Li[Li 1/3 Ti 5/3 ]O 4 For lithium insertion materials for lithium-ion batteries (LIBs), the lattice dimensions are usually altered during the charge and discharge reactions owing to changes in the ionic radius of the redox species, Coulomb repulsion between O 2 - ions, etc. Among various lithium insertion materials, Li[Li 1/3 Ti 5/3 ]O 4 (LTO) and related compounds are regarded as zero-strain materials because their cubic lattice parameter ( a c ) is maintained at ~8.36 Å throughout the electrochemical reaction [1,2]. This zero-strain character is essential for long-cycle- life LIBs and is currently employed in all-solid-state LIBs, which require a robust interface between the electrodes and electrolyte. However, because of this zero-strain character, the reaction mechanisms of LTO, especially those underlying detailed structural transformations, have not been fully understood for approximately the past thirty years [1]. In contrast to conventional ex situ analyses, state-of-the-art in situ and operando techniques are powerful tools for monitoring electrode materials under operating conditions. Since the early 2010s, we have been developing both a Li cell and an experimental setup to perform a series of operando X-ray diffraction (XRD) and X-ray absorption (XAS) spectroscopy at the Toyota beamline, SPring-8 BL33XU [3,4]. The operando XRD/XAS system consists of a specially designed tapered undulator and servo-motor-driven Si channel-cut monochromator, which enables rapid switching to the desired X-ray energy within ~20 s. We applied the operando XRD/XAS technique to LTO together with chemometric analyses and the multivariate curve resolution by alternating least squares (MCR-ALS) method. Figure 1(a) shows the Rietveld analysis results of the pristine LTO sample. The crystal structure can be assigned to a spinel structure with the Fd – 3 m space group, in which Li + ions occupy tetrahedral 8 a and octahedral 16 d sites and Ti 4+ ions occupy the 16 d site (see the inset of Fig. 1(a)). Figure 1(b) shows the discharge and charge curves of the LTO/Li cell for the operando XRD/XAS measurements. The cell voltage ( E ) rapidly drops from ~2.8 V to ~1.5 V at the beginning of the discharge reaction and then remains constant (~1.5 V) up to a capacity ( Q ) of 150 mAh·g - 1 . The flat E is the signature of a two-phase reaction scheme, although only a linear change in a c was observed in previous ex situ XRD analyses [1]. These electrochemical properties are consistent with those of previous studies [1,2], confirming a homogeneous electrochemical reaction even in the LTO/Li cell for operando XRD/XAS measurements. Figure 2(a) shows the bond length between Ti and O atoms during the discharge and charge reactions, which were determined by operando XRD measurements ( d Ti-O XRD ). During the discharge reaction, d Ti-O XRD monotonically decreases from ~1.986 Å at 0 mAh·g - 1 to ~1.984 Å at ~170 mAh·g - 1 a n d t h e n r a p i d l y i n c r e a s e s t o ~ 1 . 9 9 0 Å a t ~220 mAh·g - 1 . During the charge reaction, d Ti-O XRD almost traces with d Ti-O XRD during the discharge r e a c t i o n a t Q ≥ 1 7 0 m A h · g - 1 . H o w e v e r, t h e difference in d Ti-O XRD between the discharge and charge reactions ( d d Ti-O XRD ) clearly appears below 170 mAh g - 1 . By contrast, the bond lengths between Ti and O atoms determined by operando XAS measurements ( d Ti-O XAS ) are almost the same between the discharge and charge reactions (Fig. 2(b)), i.e., d Ti-O XAS linearly increases from ~1.95 Å at 0 mAh·g - 1 to ~2.15 Å at ~170 mAh·g - 1 , corresponding to the change in the ionic radius of Ti ions. Other differences in the results between XRD and XAS measurements are also observed in the normalized concentration ( c ) profile obtained by the MCR-ALS analyses. For the operando XRD measurements shown in Fig. 2(c), the c profile of LTO-1 (or LTO-2) during the discharge reaction differs from that during the charge reaction, whereas for the operando XAS measurements shown in Fig. 2(d), the c profile of LTO-1 (or LTO-2) during the discharge reaction is similar to that during the charge reaction. Here, LTO-1 and LTO-2 represent Li[Li 1/3 Ti 5/3 ]O 4 and Li 2 [Li 1/3 Ti 5/3 ]O 4 , respectively. The difference in c between the discharge and charge reactions ( d c ) is related to d d Ti-O XRD . To summarize the differences in the results between XRD and XAS measurements, we schematically illustrate the reaction mechanisms occurring in LTO Fig. 1. (a) Rietveld analysis results of the pristine LTO sample and a schematic of the crystal structure (inset) and (b) discharge and charge curves of the Li/LTO cell for the operando XRD/XAS measurements. 10 20 30 40 50 60 70 80 90 2 θ θ (degree) (a) 8 a :Li + 16 d :Li + , Ti 4+ 0.0 0.5 1.0 1.5 2.0 2.5 3.0 (b) 0 50 100 150 200 250 Discharge Charge Q (mAh · g – 1 ) E (V vs Li + /Li) [ λ =0.79997(2) Å] Research Frontiers 2022 109 at both the micro- (XRD) and atomic (XAS) scales (Figs. 3(a) and 3(b), respectively). The presence of d d Ti-O XRD and d c indicates the difference in the distribution of the LTO-1 (or LTO-2) phase in an LTO particle. During the discharge reaction, Li + ions are inserted into the surface, forming a core–shell structure, with the core and shell being LTO-1 and LTO-2, respectively (Fig. 3(a)). During the charge reaction, Li + ions are extracted from the surface, forming the opposite core–shell structure. Because the XRD measurements correspond to the core (bulk) structure, d d Ti-O XRD and d c were observed below ~170 mAh·g - 1 . On the other hand, at the atomic scale (Fig. 3(b)), Li insertion/extraction proceeds along the two-phase reaction scheme, wherein the amount of LTO-1 (or LTO-2) phase linearly decreases/increases during the discharge and charge reactions. As described above, we first unveiled the dynamic structural transformations of LTO using this operando XRD/XAS technique. The present results would be useful for not only further improving the rate capability of LTO but also clarifying the reaction mechanisms of LiFePO 4 and Li[Ni 1 / 2 Mn 3 / 2 ]O 4 , both of which undergo two-phase reactions. Such studies are underway in our laboratory, and the results will be presented in a forthcoming paper. Fig. 2. (a) d Ti-O XRD and (b) d Ti-O XAS during the discharge and charge reactions. Normalized c profiles of the LTO-1 (Li[Li 1/3 Ti 5/3 ]O 4 ) a n d LTO - 2 ( L i 2 [ L i 1 / 3 Ti 5 / 3 ] O 4 ) c o m p o n e n t s o b t a i n e d b y t h e MCR-ALS analyses of operando (c) XRD and (d) XAS measurements. Kazuhiko Mukai*, Takeshi Uyama and Takamasa Nonaka Toyota Central R&D Labs., Inc. *Email: e1089@mosk.tytlabs.co.jp References [1] T. Ohzuku et al. : J. Electrochem. Soc. 142 (1995) 1431. [2] K. Mukai: Inorg. Chem. 58 (2019) 10377. [3] T. Nonaka et al. : Rev. Sci. Instrum. 83 (2012) 083112. [4] K. Mukai et al. : Inorg. Chem. 59 (2020) 16882. [5] K. Mukai, T. Nonaka and T. Uyama: Energy Storage Mater. 44 (2022) 547. 1.85 1.90 1.95 2.00 2.05 2.10 2.15 2.20 1.982 1.984 1.986 1.988 1.990 1.992 (b) 0 20 40 60 80 100 Normalized c (%) 0 20 40 60 80 100 Normalized c (%) d XRD (Å) Ti-O d XAS (Å) Ti-O (c) (a) (d) 0 50 100 150 200 0 50 100 150 200 0 50 100 150 200 0 50 100 150 200 Discharge Charge Discharge LTO-1 LTO-2 Charge LTO-1 LTO-2 XAS XAS Discharge Charge XRD XRD Discharge LTO-1 LTO-2 Charge LTO-1 LTO-2 Q (mAh·g – 1 ) Q (mAh·g – 1 ) Q (mAh·g –1 ) Q (mAh·g – 1 ) (a) (b) Micro scale + +Li Li + + LTO-1 (Li[Li 1/3 Ti 5/3 ]O 4 ) LTO-1 LTO-1 (Li[Li 1/3 Ti 5/3 ]O 4 ) LTO-2 LTO-2 (Li Li 2 2 [Li 1/3 Ti 5/3 ]O 4 ) LTO-2 (Li Li 2 2 [Li 1/3 Ti 5/3 ]O 4 ) Li 2- β [Li 1/3 Ti 5/3 ]O 4 Li 1+ α [Li 1/3 Ti 5/3 ]O 4 Li Li 0.21 0.21 Li 2 [Li 1/3 Ti 5/3 ]O 4 Li Li 0.21 0.21 Li 2 [Li 1/3 Ti 5/3 ]O 4 + +Li Li + + + +Li Li + + + +Li Li + + + +Li Li + + x = = 1 1– – β β x = α x = 1 x = 0 x = 0 x = 1 x = 1.21 x = 1.21 + +Li Li + + + +Li Li + + – –Li Li + + – –Li Li + + – –Li Li + + + +Li Li + + + +Li Li + + – –Li Li + + – –Li Li + + Two-phase Two-phase One-phase One-phase – –Li Li + + – –Li Li + + – –Li Li + + – –Li Li + + Initial Initial Atomic scale Discharged (≥1.0 V) Discharged (≥1.0 V) Discharged (<1.0 V) Discharged (<1.0 V) Fig. 3. Schematics of the reaction scheme of LTO at the (a) micro- (< 120 nm) and (b) atomic scale (< 1 nm) scales. For both scales, the two-and one-phase reaction models are applicable in the x ranges of 0 ≤ x ≤ 1 and 1 < x ≤ 1.21, respectively. At the microscale, the Li 1+ a [Li 1/3 Ti 5/3 ]O 4 (or Li 2− b [Li 1/3 Ti 5/3 ]O 4 ) phase is distributed differently in the particle between the discharge and charge reactions, providing d a c ( d d Ti-O XRD ) and d c .
home
- JP
- EN