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Enhancing the cycling stability of all-solid-state lithium-ion batteries assembled with Li1.3Al0.3Ti1.7(PO4)3 solid electrolytes prepared from precursor solutions with appropriate pH values
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Enhancing the cycling stability of all-solid-state lithium-ion batteries assembled with Li1.3Al0.3Ti1.7(PO4)3 solid electrolytes prepared from precursor solutions with appropriate pH values Zhiyan Kou, Chang Miao, Ping Mei, Yan Zhang, Xuemin Yan, Yu Jiang, Wei Xiao∗ College of Chemistry and Environmental Engineering, Yangtze University, Jingzhou, 434023, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Li1.3Al0.3Ti1.7(PO4)3 NASICON pH value All-solid-state electrolytes Lithium-ion batteries
NASICON-type solid electrolyte powders Li1.3Al0.3Ti1.7(PO4)3 (LATP) are synthesized by spray-drying and assisted sintering processes. The effects of different pH values of the spray-dried precursor solution on the crystallinity, morphology, ionic conductivity, compaction density and cycling stability of the LATP solid electrolyte sheets are investigated. The LATP-6.0 solid electrolyte sheets present the most perfect microstructure with a compaction density of 2.968 g cm−1 and the highest ionic conductivity of 1.182 E−4 S cm−1 with an activation energy of 0.273 eV at room temperature when the pH value of the precursor solution is adjusted to 6.0. Additionally, the assembled LiCoO2/LATP-6.0/Li all-solid-state coin cell delivers commendable cycling stability with a high discharge specific capacity of 150.5 mAh g−1 at 0.1 C after five activation cycles and retains a specific capacity retention rate of 94.28% after 300 cycles with various current densities at room temperature. Moreover, the LATP-6.0 solid electrolyte sheets maintain an intact morphology and uniformly-distributed elements composition after cycling. Based on these excellent results, the use of the LATP-6.0 sheets as solid electrolytes in high-performance all-solid-state lithium-ion batteries is a promising strategy.
1. Introduction In past few years, lithium-ion batteries, which have the outstanding advantages of a high energy density and long service life, have been developed into leading energy storage devices in the field of rechargeable batteries [1–4]. Relevant investigations are continuously provoked by the dramatic growth of the market of electric vehicles and stationary energy storage facilities [5,6]. Although many studies have reported some achievements in the research field, lithium-ion batteries are likely to be assembled into large-scale automobiles and aerospace vehicles in the future, thus requiring a greater capacity and energy storage with a lower self-discharge rate when they are not in use [7–9]. Unfortunately, critical challenges in the development of lithium-ion batteries have been noted, particularly when flammable organic electrolytes are used an integral component of commercial batteries. Moreover, the formation of lithium dendrites during cycles can impale the electrolyte membrane and cause short circuits to destroy the whole battery system [10–12]. Polymer electrolytes have been gradually employed in the laboratory to inhibit the formation of lithium dendrites during charging and discharging cycles, but problems related to flammability and explosive safety persist due to the entrapped organic
∗
liquid electrolytes or plasticizers in the polymer matrix [13–15]. Moreover, the inferior machinability of the plasticized polymer matrix is an obstacle for practical applications. In contrast, the all-solid-state lithium-ion batteries that employ inorganic solid electrolytes to completely replace organic electrolytes are receiving increasing attention in terms of the security and stability of the battery system, in which a single-ion conduction mechanism decreases the concentration polarization and reduces electrolyte decomposition or dendrite growth, thereby achieving a high energy density [16–19]. Among the studied solid electrolytes with various crystal structures, superionic oxides are preferred candidates for applications in all-solid-state lithium-ion batteries, which contain A3B2(XO4)3 garnet-type materials (A = Ca, Mg, Y, La; B]Al, Fe, Mn, Ga, Ni, V, Ge; X = Si, Ge, Al) [20–22], ABO3 perovskite structures (A = Li, La; B]Ti) [23–25] and NASICON-type structures (Li1+xAlxTi2-x(PO4)3 (LATP) [26], Li1+xAlxGe(PO4)3 (LAGP)) [27]. LATP solid electrolytes with a NASICON-type structure have received increasing attention from researchers due to their high stability in air and water [28,29]. Nevertheless, the practical applications of LATP solid electrolytes in all-solid-state lithium-ion batteries are still limited by their intrinsic shortcomings, including a high grain boundary resistance, poor electrochemical compatibility with electrodes and
Corresponding author. E-mail address: [email protected] (W. Xiao).
https://doi.org/10.1016/j.ceramint.2019.12.229 Received 8 December 2019; Received in revised form 24 December 2019; Accepted 26 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhiyan Kou, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.229
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LATP/Ag symmetrical blocking cells at different temperatures by using a CHI660E electrochemical workstation (Shanghai Chenhua Co., Ltd.) with an amplitude of 10 mV and a frequency ranging from 0.01 to 105 Hz. The ionic conductivity of solid electrolytes is calculated using Eq. (2), where σ, R and S represent the ionic conductivity, total impedance and cross sectional area of the solid electrolyte sheets, respectively. Cyclic voltammetry (CV) measurements are performed on a CHI660E instrument using the three-electrode system with a sweep rate of 0.1 mV s−1. The electrochemical performance of the assembled LiCoO2/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Li CR-2032 coin cells is evaluated by galvanostatic charging and discharging on a Land Battery System (Wuhan Land Electronic Co., Ltd.) at different current densities (0.1, 0.2, 0.5 and 1.0 C) with the cut-off voltage ranging from 2.75 to 4.25 V.
weak long-term cycling stability [30–32]. Therefore, studies aiming to address these issues of LATP solid electrolytes are important for generating a more effective lithium-ion battery technology. The preparation of LATP solid electrolytes with a better morphology will address these problems because more uniform and perfect morphology will increase the compaction density of solid electrolyte sheets to reduce the grain boundary resistance, thereby improving the ionic conductivity. In addition, the perfect morphology will provide a greater surface area to achieve better contact between the solid electrolyte sheets and electrodes, which is very beneficial for the transfer of lithium ions during charging-discharge processes of the coin cell. The crucial synthesis parameters, such as the pH value, exert a substantial effect on the morphology of solid electrolytes. Therefore, in the present study, powders of LATP solid electrolytes are obtained from LATP precursor solutions with different pH values by using spray-drying and sintering methods. Further, the effects of different pH values on the crystallinity, morphology and electrochemical properties of solid electrolytes are evaluated in detail.
ρ=
4m Sh
(1)
ρ=
4l SR
(2)
2. Experimental 3. Results and discussion
2.1. Material synthesis
Fig. 1 exhibits the XRD spectra of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte powders at annealing temperatures of 900 °C for 6 h. It is well known that inorganic solid electrolyte powders have perfect crystallinity. As shown in Fig. 1, the main characteristic diffraction peaks of LATP powders at different pH values are well indexed to the standard spectra of the LiTi2(PO4)3 phase (JCPDS#35–0754). The crystal structure of LATP solid electrolyte powders is diamond-shaped with a rhombohedral space group (R–3(—) C). In addition, a secondary phase is not observed, indicating that pure single-phase LATP solid electrolyte powders are successfully prepared via this simple process. Notably, the LATP powders prepared from precursor solutions with different pH values possess almost identical characteristic diffraction peaks, except for the intensity. The crystallization intensity of LATP powders gradually increases and then decreases as the pH value of the LATP precursor solution increases. The strongest peak intensity of the LATP solid electrolyte powders is observed when the pH value reaches 6.0, indicating that the LATP-6.0 solid electrolytes may present a perfect morphology and excellent electrochemical properties. In addition, the composition and content of LATP samples are confirmed by the results of the ICP-AES analysis shown in Table 1, which are greatly
Li1.3Al0.3Ti1.7(PO4)3 (LATP) solid electrolyte powders are fabricated through a spray-drying method using LiNO3·H2O, Al(NO3)·9H2O, Ti (OC4H9)4, H3PO4 with a molar ratio of Li:Al:Ti:P = 1.3:0.3:1.7:3 as the starting materials. First, the weighted chemicals are dissolved in a mixture of deionized water and alcohol (1:1, v/v) with citric acid (H3Cit) as a complexing agent and thoroughly stirred at room temperature, resulting in a uniform white precursor solution. The precursor solution is continuously stirred at 80 °C for 2 h, during which time it is adjusted to different pH values (5.8, 6.0 and 6.3; the pH value of the original solution is 1.2) with ammonium hydroxide. The desired mixed solution is pumped into a spray-dryer (YC-015, Shanghai Pilotech Instrument Equipment Co., Ltd.) operated at a speed of 1500 mL h−1 and a pressure of 0.2 MPa to obtained the LATP precursor powders. The targeted LATP solid electrolyte powders are successfully prepared by sintering at 900 °C in an air atmosphere for 6 h, and LATP-x (x = 1.2, 5.8, 6.0 and 6.3) is used to label the name of the samples with different pH values in subsequent experiments. In addition, the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte sheets are prepared using polyvinyl alcohol as a binder in a tablet machine at a pressure of 30 MPa for 5 min and then placed in a tube furnace for secondary sintering at the same sintering temperature as described above, in which the binder can be completely volatilized and the compaction density of electrolyte sheets can be significantly enhanced during the second sintering process. All the experimental chemicals are analytical grade and purchased from Shanghai Sinopharm Co., Ltd.. 2.2. Properties characterization X-ray diffraction (XRD, EMPYREAN) with CuKα radiation is used to identify the crystalline phase composition of solid electrolyte powders. The test conditions include a current of 40 mA, tube voltage of 45 V, diffraction angle ranging from 10-90° and scan rate of 4° s−1. The morphologies of solid electrolyte powders and sheets are observed by using a field emission scanning electron microscope (FESEM, MIRA3 TESCAN). The chemical compositions of LATP solid electrolytes are detected by using inductively coupled plasma atomic emission spectrometry (ICP-AES) after dissolving the powder samples in nitric acid solution. The compaction density is calculated by the following Eq. (1), in which ρ, m, S and h are expressed as the compaction density, weight, cross sectional area, and thickness of the solid electrolyte sheets, respectively. The interfacial impedances of solid electrolyte sheets between two Ag electrodes are determined with sandwich structure composed of Ag/
Fig. 1. XRD patterns of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte powders at annealing temperature of 900 °C for 6 h. 2
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appear as uniform and smooth cubic particles, the LATP-6.0 powders appear as spheres composed of cubic particles, and the LATP-6.3 powders present a globular shape with a particularly rough surface. These phenomena can be explained using Eq. (3) based on two aspects. On the one hand, (PO4)3- ions exist in the form of (H2PO4)- ions in the presence of a low pH value, which prevents (PO4)3- and Al3+ ions from mixing well with each other, and the nucleation rate dramatically decreases to yield smaller LATP particle crystals [33]. However, the nucleation rate gradually increases as the pH value increases, easily allowing LATP particles to produce larger crystals. On the other hand, the as-formed Al(OH)4 precipitates are easily dissolved in H3Cit, which provides sufficient Al3+ ions for subsequent reactions. Therefore, the complete mixing of Al3+ and (PO4)3- ions in the precursor solution is the main explanation for the formation of LATP powders with a globular shape. In addition, Fig. 2(E–H) display the SEM images of LATP solid electrolyte sheets with different pH values at the secondary heat treatment temperature of 900 °C. It can be clearly observed that the microstructure of the sheets is appropriately the same as solid electrolyte powders, indicating that the secondary sintering process exerts a slight effect on the microstructure of the samples. As shown in Fig. 2(E–H), the LATP-6.0 solid electrolyte sheets possess a relatively
Table 1 ICP-AES results of different elements in the as-prepared LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte powders. Type
Li
Al
Ti
P
LATP LATP-1.2 LATP-5.8 LATP-6.0 LATP-6.3
1.300 1.138 1.144 1.137 1.141
0.300 0.297 0.293 0.299 0.318
1.700 1.620 1.628 1.620 1.618
1.700 1.619 1.628 1.620 1.618
consistent with the stoichiometry of the targeted samples. It is worth noting that the content of Li element deviates slightly from the expected value, which can be mainly attributed to the light mass and easily volatilization attributes of Li element at a high temperature. Comparisons of SEM images of LATP powders prepared from precursor solutions with different pH values at annealing temperatures of 900 °C are demonstrated in Fig. 2(A–D). It should be noted from Fig. 2(A–D) that the microstructures of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) powders present substantial differences. The LATP-1.2 powders consist of many uneven, small particles, the LATP-5.8 powders
Fig. 2. SEM images of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte powders (A–D) and LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte sheets (E–H). 3
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and 6.3)/Ag simulated cells are displayed in Fig. 3. As shown in Fig. 3, the impedance profiles of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte sheets consist of an incomplete semicircle in the high and medium frequency ranges and an oblique line in the low frequency range, which are mainly attributed to the grain boundary migration resistance and the lithium ion diffusion in the solid electrolyte system, respectively. More specifically, the left intercept of the semicircle on the real axis represents the bulk resistance (Rb), the diameter of the semicircle is defined as the grain boundary resistance (Rg), and the oblique line represents the Warburg resistance (W) of samples. The change in the total impedance R, including Rb and Rg, of solid electrolytes mainly depends on Rg because the Rb of LATP solid electrolytes is consistent at the same temperature. Thus, a reduction in Rg of the solid electrolyte is feasible approach to enhance the interfacial properties of the battery system. It can be obviously seen from Fig. 3(A) that the corresponding impedance value changes as the value of pH increases, in which the tested R value is estimated to be approximately 1316, 839.7, 419.9 and 1165 Ω when the pH value is 1.2, 5.8, 6.0 and 6.3, respectively. The LATP-6.0 solid electrolyte sheets exhibit a quite lower resistance value of 419.9 Ω compared with the other samples, which may be related to the microstructure of solid electrolyte sheets. Therefore, the perfect morphology effectively reduces the grain boundary resistance by improving the compaction density of the solid electrolyte sheets to decrease the total resistance. The ionic conductivity σ of samples is calculated using Eq. (2), where R is determined from Fig. 3(A). As displayed in Table 2, the highest value for the ionic conductivity of the LATP-6.0 solid electrolyte sheets is approximately 1.182 E−4 S cm−1 when the pH value is 6.0. The explanations for these findings are mainly due to the perfect microstructure and the excellent compaction density of the as-prepared solid electrolyte sheets. The impedance profiles of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte sheets at different temperatures (30, 40, 50, 60 and 70 °C) are tested and presented in Fig. 3(B-E) to further explore the conduction mechanism of lithium ions in solid electrolytes. Based on the data, the resistance values gradually decrease as the test temperature increases, which is due to the increase in the lithium ion diffusion
Table 2 Ionic conductivity, compaction density and activation energy of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolytes at room temperature. Type
Ionic conductivity (S cm−1)
Compaction density (g cm−1)
Ea (eV)
LATP-1.2 LATP-5.8 LATP-6.0 LATP-6.3
9.600 2.121 1.182 3.089
2.734 2.816 2.968 2.767
0.291 0.288 0.273 0.292
E−5 E−4 E−4 E−4
good compaction density with few gaps compared to other sheets, which may provide more efficient and rapid pathways for lithium ion transfer, thus leading to an improvement in the cycling performance. Based on these results, the LATP solid electrolyte sheets achieve the most perfect and densest microstructure when the pH value is adjusted to 6.0, which potentially increases ionic conductivity and electrochemical properties by improving the compaction density of the solid electrolyte sheets. Furthermore, the values of the compaction density of solid electrolyte sheets at room temperature are calculated using Eq. (1) and listed in Table 2. As shown in Table 2, the compaction density of the LATP solid electrolyte sheets is closely related to the pH value, in which the value of the compaction density first increases and then decreases as the pH value changes. The LATP solid electrolyte sheets have the highest compaction density of 2.968 g cm−1 when the pH value is maintained at 6.0, which can be mainly ascribed to the smooth and uniform distribution morphology of LATP solid electrolyte powders prepared with annealing temperatures of 900 °C. These results are consistent with the SEM images of solid electrolytes.
1.7Ti(OC4 H9) 4 + 0.3Al(NO3)⋅9H2 O+1.3LiNO3⋅H2 O+ 3H3 PO4 + 42.25O2 + 6.8H3 Cit→ Li1.3Al 0.3Ti1.7 (PO4)3 + 1.6NO2 ↑ + 42.5H2 O+27.2CO2 ↑ + 6.8H3 Cit− (3) The ionic conductivity is an indispensable factor for evaluating the performance of solid electrolytes in all-solid-state lithium-ion batteries. The impedance profiles of the assembled Ag/LATP-x (x = 1.2, 5.8, 6.0
Fig. 3. Impedance profiles (A) of the Ag/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Ag simulated cells at room temperature, impedance profiles (B–E) and the corresponding Arrhenius fitting curves (F). 4
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rate at high temperatures. Furthermore, the corresponding fitting curves of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte sheets at different temperatures are displayed in Fig. 3(F), from which it can be observed that the curves fitted by Arrhenius equation display a good match with the experimental data. Based on these results, the transfer of lithium ions in the system mainly depends on ion-hopping conduction, which is consistent with our previous results [15]. As shown in Fig. 3(F), the ionic conductivity values of the LATP-6.0 solid electrolyte sheets are all higher than the other samples throughout the temperature range, which agrees with the results of the analyses described above. The activation energy of solid electrolytes is calculated by using Eq. (4), and the corresponding results are shown in Table 2, where K, A, Ea and R represent the temperature, pre-exponential factor, activation energy and molar gas constant, respectively. It can be clearly observed from Table 2 that the LATP-6.0 solid electrolytes possess the minimum activation energy of approximately 0.273 eV compared with other solid electrolytes. Based on the data, the lithium ions can easily transfer when the LATP-6.0 solid electrolytes are working in the system, which is a great benefit for improving the cycling performance of all-solidstate lithium-ion batteries.
σ = Aexp
−Eα kT
were evaluated and are displayed in Fig. 4. The shapes of the curves are completely comparable to the traditional liquid lithium-ion battery, indicating that no change in the charge-discharge mechanism is observed when the LATP solid electrolyte sheets are used in the all-solidstate battery system. As shown in Fig. 4, the initial discharge specific capacities of the cells are 123.3, 142.2, 144.4 and 139.7 mAh g−1 at 0.1 C when the pH values are 1.2, 5.8, 6.0 and 6.3, respectively. These profiles provide solid evidence for the effectiveness of the LATP solid electrolytes in all-solid-state lithium-ion batteries. In particular, it can be distinctly observed from the insert diagram in Fig. 4 that the discharge specific capacities continue to increase and finally stabilize at 139.9, 148.7, 150.5 and 142.9 mAh g−1 after five activation cycles at room temperature. Thus, a beneficial interphase between the solid electrolytes and electrodes forms during the activation process that is based on the improvement in interphase compatibilities between the LATP solid electrolyte sheets and electrodes during the activation process. The discharge specific capacities of the cells with LATP solid electrolytes are comparable to a conventional liquid electrolyte. The obtained results are mainly attributed to the high ionic conductivity and favorable potential stability of LATP solid electrolytes against Li electrodes, which effectively inhibit the adverse reactions and improve interface contacts between solid electrolytes and electrodes. Comparatively, the assembled coin cell with LATP solid electrolytes exhibits a high discharge specific capacity of 150.5 mAh g−1 when the pH value is adjusted to 6.0, which is superior to other coin cells at different pH values. Clearly, the morphology of solid electrolyte powders exerts a substantial effect on improving the discharge specific capacity,
(4)
To investigate the electrochemical performance of the solid electrolytes in the all-solid-state lithium-ion batteries, the galvanostatic charge-discharge curves of the assembled LiCoO2/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Li coin cells in first five cycles between 2.75 and 4.25 V
Fig. 4. Charge-discharge curves of the assembled LiCoO2/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Li coin cells in first five cycles at 0.1C. 5
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Fig. 5. Cycling performance and coulombic efficiency curves of the LiCoO2/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Li coin cells at various current densities.
under a high tested current density, which would ultimately decrease the electrochemical performance of the battery. Notably, the microstructure of the LATP samples exerts a major effect on electrochemical performance; specifically, the LATP-6.0 samples are strongly preferred over the other samples. Additionally, the use of the LATP-6.0 materials as solid electrolytes may be effective, and these materials could function well in practical applications in all-solid-state lithium-ion batteries. Cyclic voltammograms (CV) curves at 0.1 mV s−1 are recorded to elucidate the electrochemical mechanism of the LATP solid electrolytes when applied to the all-solid-state lithium-ion batteries during the charge-discharge process, and the corresponding results are shown in Fig. 6. Similar results were obtained to publish studies using LiCoO2 as anode materials in the liquid electrolyte system [16]. It is noteworthy that the small platforms caused by reversible reactions of LiCoO2 are observed at the beginning of the charging process and the end of the discharging process are shown in Figs. 4 and 6, indicating that the transfer of Li+ between LiCoO2 materials and LATP-6.0 solid electrolytes is feasible. An increase in the initial discharge capacity is also obviously found in the inset of Fig. 6, which is consistent with the results shown in Fig. 4. In the charge-discharge process, the first scan curves differ from the subsequent curves, which is explained by the activation of the electrode materials during the first lithiation process. The oxidation peaks at 3.84, 4.05 and 4.15 V and the reduction peaks at 4.03, 4.09 and 4.19 V are detected in Fig. 6, which are potentially attributed to Li+ intercalation and deintercalation processes, respectively. Furthermore, since the reversibility of Li+ intercalation and deintercalation in the LATP-6.0 solid electrolytes is so high, the curves depicting the charge-discharge process basically overlap after only two cycles. Based on these findings, the LATP-6.0 solid electrolyte sheets with a perfect morphology are suitable for application in the all-solidstate lithium-ion batteries. To better explain the excellent electrochemical performance of the
increasing the utilization of active material of LiCoO2 in coin cells and thereby promoting the electrochemical performance by enhancing the compatibilities between electrodes and solid electrolytes. Further cycling tests are performed at various current densities (0.1, 0.2, 0.5, 1.0 and 2.0 C) to examine the cycling performance and coulombic efficiency of the assembled LiCoO2/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Li coin cells. As shown in Fig. 5, the coulombic efficiency of the cells continuously stabilizes at 100% as the current density increases, except for the cell with the LATP-1.2 solid electrolyte sheets, which is potentially due to its relatively poor morphology and low compaction density. Moreover, the specific capacity of the LiCoO2/ LATP-1.2/Li cell decreases sharply during cycles among all the assembled coin cells analyzed at the same rate. Compared to the LiCoO2/ LATP-1.2/Li cell, the assembled LiCoO2/LATP-x (x = 5.8, 6.0 and 6.3)/ Li coin cells present a favorite cycling property and deliver a discharge specific capacity of 150.5, 147.2, 147, 141.3 and 130.3 mAh g−1 at 0.1, 0.2, 0.5, 1.0 and 2.0 C, respectively. The cycling stability and discharge specific capacity are more stable when the pH value is adjusted to 6.0. These results are attributed to the excellent solid electrolytes, and are potentially explained by two factors. On the one hand, the superior morphology of solid electrolyte sheets may not lose contacts easily during cycling because the powders adequately accommodate changes in volume. On the other hand, the high compaction density of solid electrolyte sheets reduces the length of the ion diffusion path. Moreover, the cell delivers a discharge specific capacity of 136.8 mAh g−1 after 300 cycles when the current density is abruptly returned to its initial rate of 0.1 C, which may be mainly due to the lack of serious exfoliation of active LiCoO2 anode materials from Al current collectors. These results suggest an excellent ability to inhibit lithium dendrite deposition. The initial specific capacity is lower at 0.1 C, and the cell exhibits an inferior coulombic efficiency of 94.28% when it returns to the initial current density, indicating that the cell may be rapidly aged 6
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cycling stability. Comparisons of the XRD patterns of the solid electrolyte sheets before and after cycling are displayed in Fig. 7(B). All the characteristic diffraction peaks of solid electrolyte sheets after cycling are still well indexed to the NASICON structure LiTi2(PO4)3 (JCPDS#35–0754), confirming that the samples have a superior stability and that the crystal structure is not damaged before and after cycling. Thus, the LATP-6.0 solid electrolyte sheets exhibit excellent electrochemical performance when applied in all-solid-state lithium-ion batteries.
4. Conclusions Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte powders with a NASICON structure and compact morphology are prepared using spray drying and sintering methods. The LATP-6.0 solid electrolyte sheets exhibit an excellent microstructure compared to other electrolyte sheets when the pH value of the precursor solution is adjusted to 6.0, resulting in a higher ionic conductivity and better compaction density at room temperature. In addition, the cell assembled with the LATP-6.0 solid electrolytes delivers a high specific discharge capacity and improved cycling performance. Therefore, these outstanding properties indicate that the LATP-6.0 solid electrolytes with an excellent microstructure have promise for applications in all-solid-state lithium-ion batteries.
Fig. 6. Cyclic voltammograms curves of the LiCoO2/LATP-6.0/Li coin cell at 0.1 mV s−1.
cell with the LATP-6.0 solid electrolyte sheets, the morphology and crystalline phase composition of the LATP-6.0 solid electrolyte sheets before and after cycling are evaluated, and the results are shown in Fig. 7. Obvious differences occur in the compaction density, which manifests as the presence of many holes in the solid electrolyte sheets after cycling, indicating that the compaction density decreases significantly. Moreover, the LATP-6.0 solid electrolyte powders appear to lack sharp angles compared with the fresh solid electrolyte particles. These phenomena are attributed to the structural collapse of the solid electrolytes after many charging-discharging cycles. However, the surface morphology of solid electrolyte sheets after cycling is still smooth and uniformly distributed, similar to the morphology observed before cycling, further indicating that the solid electrolyte exhibits high
Declaration of competing interest No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
Fig. 7. SEM images (A–B) and XRD patterns (C) of the LATP-6.0 solid electrolyte sheets before and after cycles. 7
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Acknowledgments
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This work was financially supported by the National Natural Science Foundation of China (No. 51874046 and 51404038), the Project of Scientific Research of Jingzhou (No. 2018029) and the Yangtze Youth Talents Fund (No. 2016cqr05). References [1] S. Choudhury, A. Agrawal, S. Wei, E. Jeng, L.A. Archer, Hybrid hairy nanoparticle electrolytes stabilizing lithium metal batteries, Chem. Mater. 28 (7) (2016) 2147–2157. [2] A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, Y.-S. Su, Rechargeable lithium-sulfur batteries, Chem. Rev. 114 (23) (2014) 11751–11787. [3] J.C. Bachman, S. Muy, A. Grimaud, H.-H. Chang, N. Pour, S.F. Lux, O. Paschos, F. Maglia, S. Lupart, P. Lamp, L. Giordano, Y. Shao-Horn, Inorganic solid-state electrolytes for lithium batteries: mechanisms and properties governing ion conduction, Chem. Rev. 116 (1) (2016) 140–162. [4] R. Fang, W. Xiao, C. Miao, P. Mei, Y. Zhang, X. Yan, Y. Jiang, Fabrication of SiSiO2@Fe/NC composite from industrial waste AlSiFe powders as high stability anodes for lithium ion batteries, Electrochim. Acta 324 (2019) 134860, https://doi. org/10.1016/j.electacta.2019.134860. [5] D. Lin, D. Zhuo, Y. Liu, Y. Cui, All-integrated bifunctional separator for Li dendrite detection via novel solution synthesis of a thermostable polyimide separator, J. Am. Chem. Soc. 138 (34) (2016) 11044–11050. [6] R. Fang, W. Xiao, C. Miao, P. Mei, Y. Zhang, X. Yan, Y. Jiang, Enhanced lithium storage performance of core-shell structural Si@TiO2/NC composite anode via facile sol-gel and in situ N-doped carbon coating processes, Electrochim. Acta 317 (2019) 575–582. [7] R. Li, W. Xiao, C. Miao, R. Fang, Z. Wang, M. Zhang, Sphere-like SnO2/TiO2 composites as high-performance anodes for lithium ion batteries, Ceram. Int. 45 (10) (2019) 13530–13535. [8] R. Fang, C. Miao, H. Mou, W. Xiao, Facile synthesis of Si@TiO2@rGO composite with sandwich-like nanostructure as superior performance anodes for lithium ion batteries, J. Alloy. Comp. (2019) 152884, https://doi.org/10.1016/j.jallcom.2019. 152884. [9] R. Li, C. Miao, M. Zhang, W. Xiao, Novel hierarchical structural SnS2 composite supported by biochar carbonized from chewed sugarcane as enhanced anodes for lithium ion batteries, Ionics (2019), https://doi.org/10.1007/s11581-019-03288-8. [10] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, High-power all-solid-state batteries using sulfide superionic conductors, Nat. Energy 1 (4) (2016) 16030, https://doi.org/10.1038/nenergy.2016.30. [11] J. Schnell, T. Günther, T. Knoche, C. Vieider, L. Köhler, A. Just, M. Keller, S. Passerini, G. Reinhart, All-solid-state lithium-ion and lithium metal batteriespaving the way to large-scale production, J. Power Sources 382 (2018) 160–175. [12] F. Han, Y. Zhu, X. He, Y. Mo, C. Wang, Electrochemical stability of Li10GeP2S12 and Li7La3Zr2O12 solid electrolytes, Adv. Energy Mater. 6 (8) (2016) 1501590, https:// doi.org/10.1002/aenm.201501590. [13] Y. Liu, D. Yang, W. Yan, Q. Huang, Y. Zhu, L. Fu, Y. Wu, Synergy of sulfur/polyacrylonitrile composite and gel polymer electrolyte promises heat-resistant lithiumsulfur batteries, iScience 19 (2019) 316–325. [14] A. Manthiram, X. Yu, S. Wang, Lithium battery chemistries enabled by solid-state electrolytes, Nat. Rev. Mater. 2 (4) (2017) 16103, https://doi.org/10.1038/
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Enhancing the cycling stability of all-solid-state lithium-ion batteries assembled with Li1.3Al0.3Ti1.7(PO4)3 solid electrolytes prepared from precursor solutions with appropriate pH values Zhiyan Kou, Chang Miao, Ping Mei, Yan Zhang, Xuemin Yan, Yu Jiang, Wei Xiao∗ College of Chemistry and Environmental Engineering, Yangtze University, Jingzhou, 434023, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Li1.3Al0.3Ti1.7(PO4)3 NASICON pH value All-solid-state electrolytes Lithium-ion batteries
NASICON-type solid electrolyte powders Li1.3Al0.3Ti1.7(PO4)3 (LATP) are synthesized by spray-drying and assisted sintering processes. The effects of different pH values of the spray-dried precursor solution on the crystallinity, morphology, ionic conductivity, compaction density and cycling stability of the LATP solid electrolyte sheets are investigated. The LATP-6.0 solid electrolyte sheets present the most perfect microstructure with a compaction density of 2.968 g cm−1 and the highest ionic conductivity of 1.182 E−4 S cm−1 with an activation energy of 0.273 eV at room temperature when the pH value of the precursor solution is adjusted to 6.0. Additionally, the assembled LiCoO2/LATP-6.0/Li all-solid-state coin cell delivers commendable cycling stability with a high discharge specific capacity of 150.5 mAh g−1 at 0.1 C after five activation cycles and retains a specific capacity retention rate of 94.28% after 300 cycles with various current densities at room temperature. Moreover, the LATP-6.0 solid electrolyte sheets maintain an intact morphology and uniformly-distributed elements composition after cycling. Based on these excellent results, the use of the LATP-6.0 sheets as solid electrolytes in high-performance all-solid-state lithium-ion batteries is a promising strategy.
1. Introduction In past few years, lithium-ion batteries, which have the outstanding advantages of a high energy density and long service life, have been developed into leading energy storage devices in the field of rechargeable batteries [1–4]. Relevant investigations are continuously provoked by the dramatic growth of the market of electric vehicles and stationary energy storage facilities [5,6]. Although many studies have reported some achievements in the research field, lithium-ion batteries are likely to be assembled into large-scale automobiles and aerospace vehicles in the future, thus requiring a greater capacity and energy storage with a lower self-discharge rate when they are not in use [7–9]. Unfortunately, critical challenges in the development of lithium-ion batteries have been noted, particularly when flammable organic electrolytes are used an integral component of commercial batteries. Moreover, the formation of lithium dendrites during cycles can impale the electrolyte membrane and cause short circuits to destroy the whole battery system [10–12]. Polymer electrolytes have been gradually employed in the laboratory to inhibit the formation of lithium dendrites during charging and discharging cycles, but problems related to flammability and explosive safety persist due to the entrapped organic
∗
liquid electrolytes or plasticizers in the polymer matrix [13–15]. Moreover, the inferior machinability of the plasticized polymer matrix is an obstacle for practical applications. In contrast, the all-solid-state lithium-ion batteries that employ inorganic solid electrolytes to completely replace organic electrolytes are receiving increasing attention in terms of the security and stability of the battery system, in which a single-ion conduction mechanism decreases the concentration polarization and reduces electrolyte decomposition or dendrite growth, thereby achieving a high energy density [16–19]. Among the studied solid electrolytes with various crystal structures, superionic oxides are preferred candidates for applications in all-solid-state lithium-ion batteries, which contain A3B2(XO4)3 garnet-type materials (A = Ca, Mg, Y, La; B]Al, Fe, Mn, Ga, Ni, V, Ge; X = Si, Ge, Al) [20–22], ABO3 perovskite structures (A = Li, La; B]Ti) [23–25] and NASICON-type structures (Li1+xAlxTi2-x(PO4)3 (LATP) [26], Li1+xAlxGe(PO4)3 (LAGP)) [27]. LATP solid electrolytes with a NASICON-type structure have received increasing attention from researchers due to their high stability in air and water [28,29]. Nevertheless, the practical applications of LATP solid electrolytes in all-solid-state lithium-ion batteries are still limited by their intrinsic shortcomings, including a high grain boundary resistance, poor electrochemical compatibility with electrodes and
Corresponding author. E-mail address: [email protected] (W. Xiao).
https://doi.org/10.1016/j.ceramint.2019.12.229 Received 8 December 2019; Received in revised form 24 December 2019; Accepted 26 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhiyan Kou, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.229
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LATP/Ag symmetrical blocking cells at different temperatures by using a CHI660E electrochemical workstation (Shanghai Chenhua Co., Ltd.) with an amplitude of 10 mV and a frequency ranging from 0.01 to 105 Hz. The ionic conductivity of solid electrolytes is calculated using Eq. (2), where σ, R and S represent the ionic conductivity, total impedance and cross sectional area of the solid electrolyte sheets, respectively. Cyclic voltammetry (CV) measurements are performed on a CHI660E instrument using the three-electrode system with a sweep rate of 0.1 mV s−1. The electrochemical performance of the assembled LiCoO2/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Li CR-2032 coin cells is evaluated by galvanostatic charging and discharging on a Land Battery System (Wuhan Land Electronic Co., Ltd.) at different current densities (0.1, 0.2, 0.5 and 1.0 C) with the cut-off voltage ranging from 2.75 to 4.25 V.
weak long-term cycling stability [30–32]. Therefore, studies aiming to address these issues of LATP solid electrolytes are important for generating a more effective lithium-ion battery technology. The preparation of LATP solid electrolytes with a better morphology will address these problems because more uniform and perfect morphology will increase the compaction density of solid electrolyte sheets to reduce the grain boundary resistance, thereby improving the ionic conductivity. In addition, the perfect morphology will provide a greater surface area to achieve better contact between the solid electrolyte sheets and electrodes, which is very beneficial for the transfer of lithium ions during charging-discharge processes of the coin cell. The crucial synthesis parameters, such as the pH value, exert a substantial effect on the morphology of solid electrolytes. Therefore, in the present study, powders of LATP solid electrolytes are obtained from LATP precursor solutions with different pH values by using spray-drying and sintering methods. Further, the effects of different pH values on the crystallinity, morphology and electrochemical properties of solid electrolytes are evaluated in detail.
ρ=
4m Sh
(1)
ρ=
4l SR
(2)
2. Experimental 3. Results and discussion
2.1. Material synthesis
Fig. 1 exhibits the XRD spectra of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte powders at annealing temperatures of 900 °C for 6 h. It is well known that inorganic solid electrolyte powders have perfect crystallinity. As shown in Fig. 1, the main characteristic diffraction peaks of LATP powders at different pH values are well indexed to the standard spectra of the LiTi2(PO4)3 phase (JCPDS#35–0754). The crystal structure of LATP solid electrolyte powders is diamond-shaped with a rhombohedral space group (R–3(—) C). In addition, a secondary phase is not observed, indicating that pure single-phase LATP solid electrolyte powders are successfully prepared via this simple process. Notably, the LATP powders prepared from precursor solutions with different pH values possess almost identical characteristic diffraction peaks, except for the intensity. The crystallization intensity of LATP powders gradually increases and then decreases as the pH value of the LATP precursor solution increases. The strongest peak intensity of the LATP solid electrolyte powders is observed when the pH value reaches 6.0, indicating that the LATP-6.0 solid electrolytes may present a perfect morphology and excellent electrochemical properties. In addition, the composition and content of LATP samples are confirmed by the results of the ICP-AES analysis shown in Table 1, which are greatly
Li1.3Al0.3Ti1.7(PO4)3 (LATP) solid electrolyte powders are fabricated through a spray-drying method using LiNO3·H2O, Al(NO3)·9H2O, Ti (OC4H9)4, H3PO4 with a molar ratio of Li:Al:Ti:P = 1.3:0.3:1.7:3 as the starting materials. First, the weighted chemicals are dissolved in a mixture of deionized water and alcohol (1:1, v/v) with citric acid (H3Cit) as a complexing agent and thoroughly stirred at room temperature, resulting in a uniform white precursor solution. The precursor solution is continuously stirred at 80 °C for 2 h, during which time it is adjusted to different pH values (5.8, 6.0 and 6.3; the pH value of the original solution is 1.2) with ammonium hydroxide. The desired mixed solution is pumped into a spray-dryer (YC-015, Shanghai Pilotech Instrument Equipment Co., Ltd.) operated at a speed of 1500 mL h−1 and a pressure of 0.2 MPa to obtained the LATP precursor powders. The targeted LATP solid electrolyte powders are successfully prepared by sintering at 900 °C in an air atmosphere for 6 h, and LATP-x (x = 1.2, 5.8, 6.0 and 6.3) is used to label the name of the samples with different pH values in subsequent experiments. In addition, the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte sheets are prepared using polyvinyl alcohol as a binder in a tablet machine at a pressure of 30 MPa for 5 min and then placed in a tube furnace for secondary sintering at the same sintering temperature as described above, in which the binder can be completely volatilized and the compaction density of electrolyte sheets can be significantly enhanced during the second sintering process. All the experimental chemicals are analytical grade and purchased from Shanghai Sinopharm Co., Ltd.. 2.2. Properties characterization X-ray diffraction (XRD, EMPYREAN) with CuKα radiation is used to identify the crystalline phase composition of solid electrolyte powders. The test conditions include a current of 40 mA, tube voltage of 45 V, diffraction angle ranging from 10-90° and scan rate of 4° s−1. The morphologies of solid electrolyte powders and sheets are observed by using a field emission scanning electron microscope (FESEM, MIRA3 TESCAN). The chemical compositions of LATP solid electrolytes are detected by using inductively coupled plasma atomic emission spectrometry (ICP-AES) after dissolving the powder samples in nitric acid solution. The compaction density is calculated by the following Eq. (1), in which ρ, m, S and h are expressed as the compaction density, weight, cross sectional area, and thickness of the solid electrolyte sheets, respectively. The interfacial impedances of solid electrolyte sheets between two Ag electrodes are determined with sandwich structure composed of Ag/
Fig. 1. XRD patterns of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte powders at annealing temperature of 900 °C for 6 h. 2
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appear as uniform and smooth cubic particles, the LATP-6.0 powders appear as spheres composed of cubic particles, and the LATP-6.3 powders present a globular shape with a particularly rough surface. These phenomena can be explained using Eq. (3) based on two aspects. On the one hand, (PO4)3- ions exist in the form of (H2PO4)- ions in the presence of a low pH value, which prevents (PO4)3- and Al3+ ions from mixing well with each other, and the nucleation rate dramatically decreases to yield smaller LATP particle crystals [33]. However, the nucleation rate gradually increases as the pH value increases, easily allowing LATP particles to produce larger crystals. On the other hand, the as-formed Al(OH)4 precipitates are easily dissolved in H3Cit, which provides sufficient Al3+ ions for subsequent reactions. Therefore, the complete mixing of Al3+ and (PO4)3- ions in the precursor solution is the main explanation for the formation of LATP powders with a globular shape. In addition, Fig. 2(E–H) display the SEM images of LATP solid electrolyte sheets with different pH values at the secondary heat treatment temperature of 900 °C. It can be clearly observed that the microstructure of the sheets is appropriately the same as solid electrolyte powders, indicating that the secondary sintering process exerts a slight effect on the microstructure of the samples. As shown in Fig. 2(E–H), the LATP-6.0 solid electrolyte sheets possess a relatively
Table 1 ICP-AES results of different elements in the as-prepared LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte powders. Type
Li
Al
Ti
P
LATP LATP-1.2 LATP-5.8 LATP-6.0 LATP-6.3
1.300 1.138 1.144 1.137 1.141
0.300 0.297 0.293 0.299 0.318
1.700 1.620 1.628 1.620 1.618
1.700 1.619 1.628 1.620 1.618
consistent with the stoichiometry of the targeted samples. It is worth noting that the content of Li element deviates slightly from the expected value, which can be mainly attributed to the light mass and easily volatilization attributes of Li element at a high temperature. Comparisons of SEM images of LATP powders prepared from precursor solutions with different pH values at annealing temperatures of 900 °C are demonstrated in Fig. 2(A–D). It should be noted from Fig. 2(A–D) that the microstructures of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) powders present substantial differences. The LATP-1.2 powders consist of many uneven, small particles, the LATP-5.8 powders
Fig. 2. SEM images of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte powders (A–D) and LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte sheets (E–H). 3
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and 6.3)/Ag simulated cells are displayed in Fig. 3. As shown in Fig. 3, the impedance profiles of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte sheets consist of an incomplete semicircle in the high and medium frequency ranges and an oblique line in the low frequency range, which are mainly attributed to the grain boundary migration resistance and the lithium ion diffusion in the solid electrolyte system, respectively. More specifically, the left intercept of the semicircle on the real axis represents the bulk resistance (Rb), the diameter of the semicircle is defined as the grain boundary resistance (Rg), and the oblique line represents the Warburg resistance (W) of samples. The change in the total impedance R, including Rb and Rg, of solid electrolytes mainly depends on Rg because the Rb of LATP solid electrolytes is consistent at the same temperature. Thus, a reduction in Rg of the solid electrolyte is feasible approach to enhance the interfacial properties of the battery system. It can be obviously seen from Fig. 3(A) that the corresponding impedance value changes as the value of pH increases, in which the tested R value is estimated to be approximately 1316, 839.7, 419.9 and 1165 Ω when the pH value is 1.2, 5.8, 6.0 and 6.3, respectively. The LATP-6.0 solid electrolyte sheets exhibit a quite lower resistance value of 419.9 Ω compared with the other samples, which may be related to the microstructure of solid electrolyte sheets. Therefore, the perfect morphology effectively reduces the grain boundary resistance by improving the compaction density of the solid electrolyte sheets to decrease the total resistance. The ionic conductivity σ of samples is calculated using Eq. (2), where R is determined from Fig. 3(A). As displayed in Table 2, the highest value for the ionic conductivity of the LATP-6.0 solid electrolyte sheets is approximately 1.182 E−4 S cm−1 when the pH value is 6.0. The explanations for these findings are mainly due to the perfect microstructure and the excellent compaction density of the as-prepared solid electrolyte sheets. The impedance profiles of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte sheets at different temperatures (30, 40, 50, 60 and 70 °C) are tested and presented in Fig. 3(B-E) to further explore the conduction mechanism of lithium ions in solid electrolytes. Based on the data, the resistance values gradually decrease as the test temperature increases, which is due to the increase in the lithium ion diffusion
Table 2 Ionic conductivity, compaction density and activation energy of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolytes at room temperature. Type
Ionic conductivity (S cm−1)
Compaction density (g cm−1)
Ea (eV)
LATP-1.2 LATP-5.8 LATP-6.0 LATP-6.3
9.600 2.121 1.182 3.089
2.734 2.816 2.968 2.767
0.291 0.288 0.273 0.292
E−5 E−4 E−4 E−4
good compaction density with few gaps compared to other sheets, which may provide more efficient and rapid pathways for lithium ion transfer, thus leading to an improvement in the cycling performance. Based on these results, the LATP solid electrolyte sheets achieve the most perfect and densest microstructure when the pH value is adjusted to 6.0, which potentially increases ionic conductivity and electrochemical properties by improving the compaction density of the solid electrolyte sheets. Furthermore, the values of the compaction density of solid electrolyte sheets at room temperature are calculated using Eq. (1) and listed in Table 2. As shown in Table 2, the compaction density of the LATP solid electrolyte sheets is closely related to the pH value, in which the value of the compaction density first increases and then decreases as the pH value changes. The LATP solid electrolyte sheets have the highest compaction density of 2.968 g cm−1 when the pH value is maintained at 6.0, which can be mainly ascribed to the smooth and uniform distribution morphology of LATP solid electrolyte powders prepared with annealing temperatures of 900 °C. These results are consistent with the SEM images of solid electrolytes.
1.7Ti(OC4 H9) 4 + 0.3Al(NO3)⋅9H2 O+1.3LiNO3⋅H2 O+ 3H3 PO4 + 42.25O2 + 6.8H3 Cit→ Li1.3Al 0.3Ti1.7 (PO4)3 + 1.6NO2 ↑ + 42.5H2 O+27.2CO2 ↑ + 6.8H3 Cit− (3) The ionic conductivity is an indispensable factor for evaluating the performance of solid electrolytes in all-solid-state lithium-ion batteries. The impedance profiles of the assembled Ag/LATP-x (x = 1.2, 5.8, 6.0
Fig. 3. Impedance profiles (A) of the Ag/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Ag simulated cells at room temperature, impedance profiles (B–E) and the corresponding Arrhenius fitting curves (F). 4
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rate at high temperatures. Furthermore, the corresponding fitting curves of the LATP-x (x = 1.2, 5.8, 6.0 and 6.3) solid electrolyte sheets at different temperatures are displayed in Fig. 3(F), from which it can be observed that the curves fitted by Arrhenius equation display a good match with the experimental data. Based on these results, the transfer of lithium ions in the system mainly depends on ion-hopping conduction, which is consistent with our previous results [15]. As shown in Fig. 3(F), the ionic conductivity values of the LATP-6.0 solid electrolyte sheets are all higher than the other samples throughout the temperature range, which agrees with the results of the analyses described above. The activation energy of solid electrolytes is calculated by using Eq. (4), and the corresponding results are shown in Table 2, where K, A, Ea and R represent the temperature, pre-exponential factor, activation energy and molar gas constant, respectively. It can be clearly observed from Table 2 that the LATP-6.0 solid electrolytes possess the minimum activation energy of approximately 0.273 eV compared with other solid electrolytes. Based on the data, the lithium ions can easily transfer when the LATP-6.0 solid electrolytes are working in the system, which is a great benefit for improving the cycling performance of all-solidstate lithium-ion batteries.
σ = Aexp
−Eα kT
were evaluated and are displayed in Fig. 4. The shapes of the curves are completely comparable to the traditional liquid lithium-ion battery, indicating that no change in the charge-discharge mechanism is observed when the LATP solid electrolyte sheets are used in the all-solidstate battery system. As shown in Fig. 4, the initial discharge specific capacities of the cells are 123.3, 142.2, 144.4 and 139.7 mAh g−1 at 0.1 C when the pH values are 1.2, 5.8, 6.0 and 6.3, respectively. These profiles provide solid evidence for the effectiveness of the LATP solid electrolytes in all-solid-state lithium-ion batteries. In particular, it can be distinctly observed from the insert diagram in Fig. 4 that the discharge specific capacities continue to increase and finally stabilize at 139.9, 148.7, 150.5 and 142.9 mAh g−1 after five activation cycles at room temperature. Thus, a beneficial interphase between the solid electrolytes and electrodes forms during the activation process that is based on the improvement in interphase compatibilities between the LATP solid electrolyte sheets and electrodes during the activation process. The discharge specific capacities of the cells with LATP solid electrolytes are comparable to a conventional liquid electrolyte. The obtained results are mainly attributed to the high ionic conductivity and favorable potential stability of LATP solid electrolytes against Li electrodes, which effectively inhibit the adverse reactions and improve interface contacts between solid electrolytes and electrodes. Comparatively, the assembled coin cell with LATP solid electrolytes exhibits a high discharge specific capacity of 150.5 mAh g−1 when the pH value is adjusted to 6.0, which is superior to other coin cells at different pH values. Clearly, the morphology of solid electrolyte powders exerts a substantial effect on improving the discharge specific capacity,
(4)
To investigate the electrochemical performance of the solid electrolytes in the all-solid-state lithium-ion batteries, the galvanostatic charge-discharge curves of the assembled LiCoO2/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Li coin cells in first five cycles between 2.75 and 4.25 V
Fig. 4. Charge-discharge curves of the assembled LiCoO2/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Li coin cells in first five cycles at 0.1C. 5
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Fig. 5. Cycling performance and coulombic efficiency curves of the LiCoO2/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Li coin cells at various current densities.
under a high tested current density, which would ultimately decrease the electrochemical performance of the battery. Notably, the microstructure of the LATP samples exerts a major effect on electrochemical performance; specifically, the LATP-6.0 samples are strongly preferred over the other samples. Additionally, the use of the LATP-6.0 materials as solid electrolytes may be effective, and these materials could function well in practical applications in all-solid-state lithium-ion batteries. Cyclic voltammograms (CV) curves at 0.1 mV s−1 are recorded to elucidate the electrochemical mechanism of the LATP solid electrolytes when applied to the all-solid-state lithium-ion batteries during the charge-discharge process, and the corresponding results are shown in Fig. 6. Similar results were obtained to publish studies using LiCoO2 as anode materials in the liquid electrolyte system [16]. It is noteworthy that the small platforms caused by reversible reactions of LiCoO2 are observed at the beginning of the charging process and the end of the discharging process are shown in Figs. 4 and 6, indicating that the transfer of Li+ between LiCoO2 materials and LATP-6.0 solid electrolytes is feasible. An increase in the initial discharge capacity is also obviously found in the inset of Fig. 6, which is consistent with the results shown in Fig. 4. In the charge-discharge process, the first scan curves differ from the subsequent curves, which is explained by the activation of the electrode materials during the first lithiation process. The oxidation peaks at 3.84, 4.05 and 4.15 V and the reduction peaks at 4.03, 4.09 and 4.19 V are detected in Fig. 6, which are potentially attributed to Li+ intercalation and deintercalation processes, respectively. Furthermore, since the reversibility of Li+ intercalation and deintercalation in the LATP-6.0 solid electrolytes is so high, the curves depicting the charge-discharge process basically overlap after only two cycles. Based on these findings, the LATP-6.0 solid electrolyte sheets with a perfect morphology are suitable for application in the all-solidstate lithium-ion batteries. To better explain the excellent electrochemical performance of the
increasing the utilization of active material of LiCoO2 in coin cells and thereby promoting the electrochemical performance by enhancing the compatibilities between electrodes and solid electrolytes. Further cycling tests are performed at various current densities (0.1, 0.2, 0.5, 1.0 and 2.0 C) to examine the cycling performance and coulombic efficiency of the assembled LiCoO2/LATP-x (x = 1.2, 5.8, 6.0 and 6.3)/Li coin cells. As shown in Fig. 5, the coulombic efficiency of the cells continuously stabilizes at 100% as the current density increases, except for the cell with the LATP-1.2 solid electrolyte sheets, which is potentially due to its relatively poor morphology and low compaction density. Moreover, the specific capacity of the LiCoO2/ LATP-1.2/Li cell decreases sharply during cycles among all the assembled coin cells analyzed at the same rate. Compared to the LiCoO2/ LATP-1.2/Li cell, the assembled LiCoO2/LATP-x (x = 5.8, 6.0 and 6.3)/ Li coin cells present a favorite cycling property and deliver a discharge specific capacity of 150.5, 147.2, 147, 141.3 and 130.3 mAh g−1 at 0.1, 0.2, 0.5, 1.0 and 2.0 C, respectively. The cycling stability and discharge specific capacity are more stable when the pH value is adjusted to 6.0. These results are attributed to the excellent solid electrolytes, and are potentially explained by two factors. On the one hand, the superior morphology of solid electrolyte sheets may not lose contacts easily during cycling because the powders adequately accommodate changes in volume. On the other hand, the high compaction density of solid electrolyte sheets reduces the length of the ion diffusion path. Moreover, the cell delivers a discharge specific capacity of 136.8 mAh g−1 after 300 cycles when the current density is abruptly returned to its initial rate of 0.1 C, which may be mainly due to the lack of serious exfoliation of active LiCoO2 anode materials from Al current collectors. These results suggest an excellent ability to inhibit lithium dendrite deposition. The initial specific capacity is lower at 0.1 C, and the cell exhibits an inferior coulombic efficiency of 94.28% when it returns to the initial current density, indicating that the cell may be rapidly aged 6
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cycling stability. Comparisons of the XRD patterns of the solid electrolyte sheets before and after cycling are displayed in Fig. 7(B). All the characteristic diffraction peaks of solid electrolyte sheets after cycling are still well indexed to the NASICON structure LiTi2(PO4)3 (JCPDS#35–0754), confirming that the samples have a superior stability and that the crystal structure is not damaged before and after cycling. Thus, the LATP-6.0 solid electrolyte sheets exhibit excellent electrochemical performance when applied in all-solid-state lithium-ion batteries.
4. Conclusions Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte powders with a NASICON structure and compact morphology are prepared using spray drying and sintering methods. The LATP-6.0 solid electrolyte sheets exhibit an excellent microstructure compared to other electrolyte sheets when the pH value of the precursor solution is adjusted to 6.0, resulting in a higher ionic conductivity and better compaction density at room temperature. In addition, the cell assembled with the LATP-6.0 solid electrolytes delivers a high specific discharge capacity and improved cycling performance. Therefore, these outstanding properties indicate that the LATP-6.0 solid electrolytes with an excellent microstructure have promise for applications in all-solid-state lithium-ion batteries.
Fig. 6. Cyclic voltammograms curves of the LiCoO2/LATP-6.0/Li coin cell at 0.1 mV s−1.
cell with the LATP-6.0 solid electrolyte sheets, the morphology and crystalline phase composition of the LATP-6.0 solid electrolyte sheets before and after cycling are evaluated, and the results are shown in Fig. 7. Obvious differences occur in the compaction density, which manifests as the presence of many holes in the solid electrolyte sheets after cycling, indicating that the compaction density decreases significantly. Moreover, the LATP-6.0 solid electrolyte powders appear to lack sharp angles compared with the fresh solid electrolyte particles. These phenomena are attributed to the structural collapse of the solid electrolytes after many charging-discharging cycles. However, the surface morphology of solid electrolyte sheets after cycling is still smooth and uniformly distributed, similar to the morphology observed before cycling, further indicating that the solid electrolyte exhibits high
Declaration of competing interest No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
Fig. 7. SEM images (A–B) and XRD patterns (C) of the LATP-6.0 solid electrolyte sheets before and after cycles. 7
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Acknowledgments
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