Synergistic effect of cation ordered structure and grain boundary engineering on long-term cycling of Li0.35La0.55TiO3-based solid batteries

Journal of the European Ceramic Society 39 (2019) 3332–3337

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Synergistic effect of cation ordered structure and grain boundary engineering on long-term cycling of Li0.35La0.55TiO3-based solid batteries

T

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Qian Wanga,b, Jinhua Zhanga,b, Xiaoning Hea,b, Guoqin Caoa,b, Junhua Hua,b, , Jimin Pana, ⁎ Guosheng Shaoa,b, a

School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China State Center for International Cooperation on Designer Low-Carbon & Environmental Materials (CDLCEM), Zhengzhou University, 100 Kexue Avenue, Zhengzhou 450001, China

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Microstructure engineering Ordered structure Grain boundary conductivity Solid state battery

In this study, Li0.35La0.55TiO3 (LLTO) was coupled with Al-doped lithium lanthanum zirconate (LLZO) to improve the grain boundary and total conductivity. The obtained ceramic pellets (LLTZO) demonstrated a recordable grain boundary and total conductivity of 3.41 × 10−4 and 3.03 × 10−4 S/cm, respectively. The obtained results establish that the heteroatoms can perturb the cation ordered structure and improve the 3D conductivity in grain bulk. In addition, the residual Al-doped LLZO on the grain boundary led to a decline in the boundary resistance. An LiFeCoPO4 |Li cell was adopted to demonstrate the enhanced conductivity of LLTO. The solid state battery rendered a specific capacity of over 101.2 mAhg−1 after 300 cycles at a relatively high rate of 0.5C. It is established from the experiments that manufacturing a solid battery using the all-coating technique provides a promising approach to achieve a practical application.

1. Introduction Lithium ion batteries (LIBs) are widely employed as power sources for the emerging applications involving electric vehicles, consumer electronics, and large-scale energy storage systems for the grid. However, commercial LIBs with organic solvent-based electrolytes demonstrate the disadvantage of high flammability, which results in severe safety issues [1]. Solid secondary batteries based on ceramic electrolytes demonstrate the advantage of high mechanical strength, good chemical stability, and excellent safety performance [2]. Ceramic electrolytes contain two important material systems, such as the sulphide-based and oxide-based ceramics. Currently, most research on sulphide electrolytes concentrate on their materials chemistry and molecular modelling [3,4]. Several studies have been conducted on the battery performance of sulphide electrolytes as additives to cathodes or composite electrolytes with limited content [5–7]. In contrast, the oxide-based electrolytes, such as the NASICON (sodium super ionic conductor)-type phosphates, garnet-type zirconates, and lithium phosphorus oxynitride, demonstrate several advantages over the sulphides. They exhibit a wide electrochemical window (> 8 V), good stability in dry and humid atmosphere, and an extensive operating temperature scope [8–10]. Among these electrolytes, perovskite Li3xLa2/3–xTiO3

⁎

exhibits a high Li ion conductivity of grain interior (GI) at room temperature. The major drawback of LLTO is its high grain boundary (GB) resistance. Although the GI conductivity (10−4–10-3 S/cm) is sufficient for practical applications, the total conductivity is ultimately determined by the order of magnitude of the GB conductivity. Modification of the crystalline structure by doping various ions is the usual approach to improve the conductivity [11,12]. However, the doping effect attributes more to grain bulk than boundary conductivity [13,14]. Alternatively, other fast ion conductive materials were introduced on the GB to optimise the GB transportation properties. The composite thus obtained exhibited a high total conductivity of approximately 10−5 S/cm [15,16]. Amorphous silica and alumina were added on the GB to closely bond the separated grains and compensate for the Li diffusion ability across the boundary [17,18]. The optimised GB conductivity attains a value of approximately 10−4 S/cm. Gaining deeper insights into the GB defect chemistry is necessary to incorporate further improvements. Recently, some atomic-scale characterisation was conducted to reveal the origin of high GB resistance. Except for the actual GBs on a nano to sub-micron scale, a region of a structure and a stoichiometry deviation from LLTO adjacent to the GB on the scale of several nanometres was proposed as the origin of poor GB conductivity [3,19]. The established sintering temperature for the cubic LLTO is

Corresponding authors at: School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China. E-mail addresses: [email protected] (J. Hu), [email protected] (G. Shao).

https://doi.org/10.1016/j.jeurceramsoc.2019.04.045 Received 29 January 2019; Received in revised form 19 April 2019; Accepted 24 April 2019 Available online 28 April 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.

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always higher than 1200 °C [20]. It was inevitable for LLTO to form a Li-free Ti-O layer near the GB in perovskite LLTO, which is a carrierdeficient shell on the Li diffusion path. Introducing second phase and doping atoms to such an atomic scale region is considered difficult [3]. The report on the LLTO-based solid batteries is still limited due to the aforementioned issues [21–23]. In this work, a composite LLTZO was fabricated with a recordable GB conductivity. A synergistic optimisation of the GB and cation ordered structure was established. A solid battery with significantly enhanced cycling and rate performance established the high conductivity of LLTZO. This battery was fabricated using the all-coating technology, which is highly compatible with the current manufacturing line of the Li-ion battery. Fig. 1. a) XRD patterns of LLTZO with different LLZO percentages (minor peak of the La2Zr2O7 impurity is indicated by ◆) (b) The shift of dominant peaks indicates the doping effect. The selected 2θ scope of dominant peak was shown as a dotted rectangle.

2. Experiment 2.1. Material fabrication Li6.25Al0.25La3Zr2O12 (LLZO) was used as an additive in LLTO due to its stabilised garnet structure obtained through Al-doping [24]. LLZO was introduced in LLTO to enhance the boundary conductivity. The raw materials of both LLZO and LLTO were fabricated, as stated in the electronic supplementary information (ESI). The as-prepared LLZO and LLTO powders were mixed in ethanol via ball milling for 5 h, dried, and then pressed into pellets of 16 mm diameter at 20 MPa. These pellets were sintered at 1280 °C and 1350 °C for 10 h. The addition percentage of LLZO in these pellets were 0, 2.5, 5, 10, and 12.5 wt%. The optimised sintering process is shown in Fig. S3. After sintering, the pellet was crushed again by ball-milling (QM-QX2L, Changsha, China) for 5 h at 350 rpm. The sintering was performed on the powder bed of target materials. The resultant powder was termed LLTZO according to the weight percentage value of LLZO, for example, LLTZO-10. The mentioned reagent was in the AR (analytically pure) grade.

directly. The cell was assembled in a CR2025 type. A porous polypropylene (PP) sheet was attached between the Li metal and electrolyte to prevent their direct contact. A drop of 1 M LiClO4/propylene carbonate solution (˜ 60 μl) was dropped on the PP before the cell assembly to enhance its interface contact. The battery was cycled in the galvanostatic mode with a voltage range of 2.5–4.0 V at 35 °C. 3. Results and discussion The fabrication processes of both LLZO and pure LLTO were performed, as described in the ESI. After combining LLZO and LLTO, the sintering of target LLTZO was conducted at 1280 °C and 1350 °C. As shown in Fig. S4, the formation of the impurity phase (La2Zr2O7) is observed at 1280 °C. By elevating the sintering temperature to 1350 °C (Fig. 1), the single perovskite phase was fabricated (Fig. 1a). The pronounced shift to the low angle of dominant peaks at 2θ = 32.6° is observed from Fig. 1b. This shift was ascribed to the doping effect by hetero-atoms [16,25]. It is reasonable to conclude that LLZO would partially diffuse into the matrix during the sintering. Li and La can occupy the lattice position of the A sites in LLTO [26]. LLZO does not exhibit a peak, even when its weight percentage was 12.5 wt%. The excessive addition of LLZO (12.5 wt%) induces the formation of La2Zr2O7 (Fig. 1a). The formation of the superstructure, which is typical of the tetragonal LLTO, is indicated by the symbol ▽ [27]. A control group experiment was performed, which is shown in the top panel in Fig. 1a. No doping effect would result, if Li2CO3, La2O3, and ZrO2 were used with the addition percentage of 12.5% (Li2CO3: La2O3: ZrO2 = 7:3:4) to composite with LLTO instead of LLZO. It implies that the chemical state of the elements in LLZO contributed to the doping of LLTO. Fig. 2a shows the typical SEM image of LLTO. The measured primary grain sizes with standard deviation are shown in Fig. 2b. It is observed that the grain size declines marginally and turns uniform with doping. The grain size of LLTZO-12.5 decreases much compared with others, as the formation of the second phase (Fig. 1) can further depress the coarsening of grains [28,29]. Fig. 2c shows a conjoint region of the LLTZO grains. The LLTO as well as the residual LLZO grains can be observed in this figure. The LLZO grain with the size of 6 nm was as-

2.2. Material characterisation The crystal structures of the synthesised material were characterised using powder X-ray diffraction (XRD, Rigaku Ultima IV) equipped with the Cu Kα radiation (1.54056 Å) in the 2θ range of 10–80°. The morphology and microstructure were examined using field emission scanning electron microscopy (FESEM; JSM-7001F, JEOL) in the secondary electrons mode. Before observation, the samples were coated with the Au ion sputter for 100 s. As the surface morphology is usually not representative of the bulk microstructure, the samples were polished for the SEM observation. The grain size was measured using NANO MEASUREMENT. The analysis of the second phase was performed using high resolution transmission electron microscopy (HRTEM; FEI Tecnai G2 F20). The Raman spectroscopy (HORIBA JOBIN YVON, Lab RAMHR Evolution) was used to characterise the ordered structure evolution in LLTZO. The incident laser possessed a wavelength of 532 nm. The band at 480–600 cm−1 was deconvoluted into Lorentzian components by Peak Fit. The degree of fitting (R2) of LLTZO-2.5, LLTZO-5 and LLTZO10 was 0.998, 0.996 and 0.996, respectively. 2.3. Electrochemical and battery tests The cathode and electrolyte pieces were fabricated by the knife coating process during the cell assembly. LiFePO4 (LFP) cathodes and Li metal anodes were employed to assemble the solid-state batteries. LiFePO4 powders (Hefei Kejing Materials Technology Co., Ltd.), super P carbon blacks, LLTZO powder, and the polyvinylidene fluoride binder with a weight ratio of 7:1:1:1 were blended in N-methyl-2-pyrrolidone solution. The obtained slurry was coated on the Al foil and dried for 12 h at 80 °C in a drying oven. The LLTZO powder and polyvinylidene fluoride with a weight ratio of 92:8 were blended for 12 h in N-methyl2-pyrrolidone into a slurry. This slurry was coated on the cathode

¯

signed with the incident direction of [021]. The IFFT images of these grains treated by the DigitalMicrograph software are also demonstrated in this figure. As shown in Fig. S2c, the fabricated LLZO demonstrates a size of several hundred nanometres and an irregular shape. After sintering, it diminished in size and exhibited a spherical morphology. It is established that LLZO partially diffuses into the LLTO matrix and contributes to the doping effect demonstrated in Fig. 1b. The influence of the heteroatoms on the defect chemistry was further analysed by Raman spectroscopy. Fig. 3a presents similar Raman spectra of LLTZO 3333

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Fig. 2. a) SEM image of LLTO. For clear observation, the surface of ceramic pellet was heat etched at 1200 °C for 2 h. (b) Average grain size and standard deviation with different addition of LLZO. (c) TEM image of the nanoparticles of LLZO on the boundary of LLTZO-10. Fig. 3. a) Raman spectra of LLTZO with LLZO contents of 2.5, 5, and 10 wt%. The inset shows the selected point of sample LLTZO-10 under the Raman optical microscope. (b) Deconvolution into Lorentzian components of the band at 480–600 cm−1 for LLTZO-5 (top) and LLTZO-10 (bottom). (c) HRTEM images from the well-ordered regions in LLTZO-10. (d) HRTEM images from the perturbed regions in LLTZO-10. (e) Decline of intensity of the E component (f) Intensity change of the counterbalanced couple A and F (A: well- ordered region; F: loss of cation order region).

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Fig. 4. Shift of dominant peaks of the superstructure in LLTZO.

with different LLZO contents. They demonstrate the six typical Raman active modes for the tetragonal unit cells [30,31]. It is reported that the band at 480–600 cm−1 comprises three components [30,32]. After deconvolution into Lorentzian components (Fig. 3b), the bands at 525, 560, and 583 cm−1 that originate due to the vibration of oxygen were termed E (Eg), F(A1g), and A(A1g), respectively. The lower frequency component at 525 cm−1 is of the Eg type, which corresponds to the oxygen vibration within the X-Y plane. The presence of the nearby Li (or other heteroatoms) may perturb the oxygen atom and decrease the intensity of E (Fig. 3e). It is noted that the increase in Li in the octahedron interstitial would increase the carrier concentration and contribute positively to the conductivity of Li [33]. The components A and F originate from the well-ordered (Fig. 3c) and perturbed lattice regions (Fig. 3d), respectively [30,34]. Therefore, they are a counterbalanced couple. In Fig. 3f, the increase in F (in blue) and decrease in A (in green) both indicate the loss of cation order in the lattice. Although the loss of cation order does not eliminate the super-structure (Fig. 1a), the perturbed ordered structure causes the shrinkage of the average plane spacing along the c-axis (Fig. 3d). As a result, the dominant peaks of the superstructure in LLTZO also demonstrate an evident negative shift (Fig. 4). Therefore, the 3D (three dimensional) diffusion of Li ion will be enhanced. The loss of cation order would result in the 3D transportation of Li. After coupling with LLZO, the electrical impedance spectra of LLTZO were recorded (Fig.5a). The spectra include two semicircles, one at the high frequency (0.3–20 MHz) and another at the middle frequency. They denote the contribution of GI and GB resistances, respectively. The slope in the low frequency region demonstrates the superficial kinetics of the Li migration [35]. From the inset (Fig. 5a), by increasing the LLZO content, the semicircle decreases noticeably, which indicates the depressed GI resistance. LLTZO-12.5 shows a larger semicircle than the others. It may be due to the formation of the impurity phase La2Zr2O7 (XRD, Fig. 1a) [36]. The impedance spectra were fitted with the Z-view software. The equivalent circuit is shown in Fig. S5. The obtained conductivity at 35 °C is shown in Fig. 5b. The actual values with respect to the Li ion conductivity for the LLTZO samples are listed in Table S1. The error range provided by the Z-view is listed in Table S2. When the addition percentage of LLZO attained 10 wt%, a recordable GB conductivity was obtained. It is observed from Table S1 that compared to those of the pure LLTO, the GB conductivity of LLZO increases from 6.40 × 10−5 to 3.41 × 10-4 S /cm, and the GI conductivity increases from 6.0 × 10-4 to 2.78 × 10-3 S /cm. As a result, the total conductivity is estimated at 3.03 × 10-4S /cm. The recorded conductivity data of LLTO are listed in Table 1. A recordable GB conductivity was obtained in this experiment. It should be mentioned that the intrinsic garnet LLZO was employed as an additive to enhance the conductivity of LLTO in [16,43]. The

Fig. 5. a) Complex-plane impedance spectra collected at 35 °C for the Au/ LLZTO/Au structures with different LLZO contents. (b) Dependence of GB, GI, and total conductivity of LLTZO on the content of LLZO. (c) Arrhenius plots for lithium-ion conductivity in the GB and GI for LLTZO-10 and pure LLTO.

Table 1 Li ion conductivity (S/cm) of LLTZO samples. Sample

σGI×10−3

σGB×10−4

σtotal×10−4

Ref.

LLTO-Li7La3Zr2O12 Li0.5La0.5TiO3 /Li3PO4 LLTO-Al2O3 LLTO-Li2O-B2O3 LLTO-LiO2/SiO2/B2O3 LLTO- SiO2 LLTO + Li7La3Zr2O12 LLTZO-10

0.11 0.016 0.933 0.106 1.12 0.125 0.58 2.78

0.5 0.89 0.0238 0.291 0.0055 0.61 1.5 3.41

0.95 0.14 / / / 1.2 1.2 3.03

[16] [38] [39] [40] [41] [42] [43] In this work

distribution of Zr indicated that the enhancement in conductivity originated from the doping effect provided by Zr. In this work, LLZO (Li6.25Al0.25La3Zr2O12) was further stabilised by Al. The residual LLZO is identified on the GB, as shown in Fig. 2f. This work also includes 3335

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achieving the mechanism of enhancing the Li conductivity based on the order structure engineering (Fig. 3). Jong-Won Lee [3] proposed another microstructural engineering strategy by controlling the Li compositing and sintering process. A superior Li diffusivity was also obtained during this process. Fig. 5c shows the temperature dependence of the conductivity of LLTZO. From room temperature to 100 °C, the conductivity (log σ) shows a linear relationship with the reciprocal of temperature. From Arrhenius plots, the apparent activation energy values for the GI and GB are 0.31 and 0.46 eV, respectively. After coupling with LLZO, they declined to 0.24 and 0.32 eV, respectively. The decline in the activation energy due to the addition of LLZO is a synergistic result of optimisation of the boundary (Fig. 2e) and cation ordered structure (Fig. 3). It is significant to point out that the activation energy on the GB, which is 0.32 eV, is lower than those of the LLTO GI reported by other studies [44,45]. The improvement of the Li ion conductivity can also be evaluated using the symmetrical Li|Li cells, which is a indicator of the long-term cycling performance of the ceramic pellets > [37]. The symmetrical cell was assembled without the addition of a separator or liquid electrolyte on the interface. The cell with the LLTZO-10 electrolyte exhibited excellent stability with a nearly constant voltage polarisation of 10 mV during a 200 h cycle (Fig. 6). In contrast, the pure LLTO demonstrated serious polarisation profiles within 30 cycles due to poor conductivity [21]. The solid cell of LiFePO4 |Li with improved Li conductivity was adopted to demonstrate the cycling performance. The strategy of “polymer in ceramic” was adopted to fabricate the solid battery [46]. The LLTZO pellets were mechanically crushed into a powder again to further destroy the GB (Fig. S6) and diminish its influence. The long-term cycling at 0.5C of LLTZO-10 is demonstrated in Fig. 7a. It is observed from this figure that the initial discharging capacity of LLTZO-10 is ˜117.8 mAhg−1. After 300 cycles, it is 101.2 mAhg−1 and a coulombic efficiency of approximately 1 is retained. The comparison of the cell performance of pure LLTO with those of the other cells is shown in Fig. 7a. After 190 cycles, the charging and discharging capacity of LLTO is approximately 61.6 and 61.1 mAhg−1, respectively. The LLTZO-5 battery demonstrates a moderate capacity retention between LLTZO-10 and LLTO. The voltage drops (ΔV) between the midpoint voltage of the charge and discharge were regarded as the polarisation extent of the battery during the cycle [47,48]. Fig. 7b summarises the ΔV of the cells across 160 cycles. At 0.5C, ΔV is almost constant for LLZTO-10 and maintains a value of less than 0.28 V. The pristine LLTO shows a gradual increase in polarisation and attains a value of 0.51 V in the 160th cycle. The variations in ΔV of LLTO (Fig. 7b) agrees well with those of the capacity retention (Fig. 7a). The rate performance improves due to the low polarisation of the LLTZO solid battery, as shown in Fig. 8 [49–51]. In the first 10 cycles, the current density increases from 0.05–0.1 C, and the capacity decreases

Fig. 7. a) Cycling performances and coulombic efficiency at the rate of 0.5C (1.0 C = 170 mA g−1). (b) Voltage drop (ΔV) between midpoint voltage with respect to the charge and discharge curves at the rate of 0.5C. The corresponding galvanostatic discharge/charge profiles are shown in Fig. S7.

Fig. 8. Rate performance of the LLTZO-10 battery.

from 170–159 mAhg−1. After 30 cycles of rate test, the current density attains a value of 0.1 C and the capacity remains at 158 mAhg−1. Thus, the rate test establishes a stable and reversible rate performance. 4. Conclusion In this study, the cation ordered structure and GB engineering were employed to further optimise the Li transportation in LLTO by incorporating the Al-doped LLZO. When the addition percentage of LLZO attained a value of 10 wt%, the GB conductivity increased from 6.40 × 10−5 S/cm to a recordable value of 3.41 × 10-4 S/cm. As a result, the total conductivity was observed to be 3.03 × 10-4S/cm. An LFP|Li metal solid battery with “polymer in ceramic” electrolyte was employed to demonstrate the superior conductivity of LLTZO-10. At a high rate of 0.5C, the initial capacity was observed to be 117.8 mAhg-1, which remained at 101.2 mAhg-1 after 300 cycles. A coulombic efficiency of approximately 1 was retained. The polarisation (ΔV) achieved in 160 cycles at the rate of 0.5C was less than 0.28 V. Therefore, it is established that the improvement in the electrochemical performance is ascribed to the enhancement of Li conductivity, which is induced by the synergistic effect of microstructure engineering. Acknowledgements

Fig. 6. Symmetric batteries of LLTO and LLTZO-10. The current density is 0.1 mA cm−2.

We acknowledge the financial support from the National Natural Science Foundation of China [grant numbers: 51571182, 51001091], 3336

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Fundamental Research Program from the Ministry of Science and Technology of China [2014CB931704], and Program for Science & Technology Innovation Talents in the universities of Henan Province [18HASTIT009]. This work was also supported partially by the Henan province [2017GGJS001, 182102310815] and the International Joint Program of Henan (172102410023).

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