Facile Synthesis of Nanosized Lithium-Ion-Conducting Solid

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Facile synthesis of nanosized lithium-ion-conducting solid electrolyte Li1.4Al0.4Ti1.6(PO4)3 and its mechanical nanocomposites with LiMn2O4 for enhanced cyclic performance in lithium ion battery Xingang Liu, Jiang Tan, Ju Fu, Ruoxin Yuan, Hao Wen, and Chuhong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16233 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 5, 2017

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ACS Applied Materials & Interfaces

Facile synthesis of nanosized lithium-ion-conducting solid electrolyte Li1.4Al0.4Ti1.6(PO4)3 and its mechanical nanocomposites with LiMn2O4 for enhanced cyclic performance in lithium ion battery Xingang Liu†, Jiang Tan†, Ju Fu, Ruoxin Yuan, Hao Wen, Chuhong Zhang* State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan University, Chengdu 610065, China Keywords: solid electrolyte, LATP nanoparticles, non-coating surface modification, mechanical LATP/ LiMn2O4 nanocomposites, lithium ion battery.

Abstract: Nanoparticles of fast lithium-ion-conducting solid electrolyte Li1.4Al0.4Ti1.6(PO4)3 (LATP) are prepared by a modified citric-acid-assisted sol–gel method that involves a two-step heat treatment in which the dry gel is calcined first in argon and then in air. The obtained LATP exhibits smaller particle size (down to 40 nm) with a narrower size distribution and less aggregation than LATP prepared by a conventional sol–gel method because of a polymeric network that preserves during LATP crystallization. It has a high relative density of 97.0% and a high room temperature conductivity of 5.9 × 10−4 S cm−1. The as-prepared superfine LATP is further used to composite with a spinel LiMn2O4 cathode in lithium ion batteries by simple

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grinding. This non-coating speckled layer over the LiMn2O4 particle surface has a minimal effect on the electrode’s electronic conductivity while offering excellent ionic conductivity. The cycling stability and rate capability of LiMn2O4 are greatly improved at both ambient and elevated temperatures. After 100 cycles at 25°C and 55°C, the capacity retention is 96.0% and 89.0%, respectively, considerably higher than the values of pristine LiMn2O4 (61.0% at 25°C, 51.5% at 55°C) and mechanical LiMn2O4 composite with LATP made by a conventional sol–gel method (85.0% at 25°C, 71.4% at 55°C).

1. Introduction The glass ceramic LiTi2(PO4)3, with a Na+ superionic conductor (NASICON)-type structure, exhibits a high ionic conductivity of 10−6 S cm−1 at room temperature 1, which can be further enhanced by more than two orders of magnitude when Ti4+ is partially substituted by Al3+, i.e., [Li1+xAlxTi2-x(PO4)3] (LATP, x = 0.3–0.5)

2–5

. Therefore, along with other advantages such as

excellent stability in air and water and high electrochemical oxidative potential (~6 V vs. Li/Li+), LATP has received much attention for applications in all-solid-state lithium ion batteries (LIBs) as well as lithium air batteries 6,7. In addition, it has also been recognized as a good candidate for surface modification of cathode materials in LIBs 8,9. Surface modification is currently one of the most widely used strategies for improving the cycling performance of cathode materials and is found to work particularly well on spinel LiMn2O4 10-12. Spinel LiMn2O4 is one of the most promising cathode materials for LIBs owing to its low cost, low toxicity, and a relatively high discharge potential of about 4.2 V (vs. Li/Li+) 1319

. However, its capacity fades dramatically during cycling, especially at elevated temperature, as

a result of Jahn–Teller distortion of Mn3+ and dissolution of Mn2+ in the electrolyte

20

. Surface

modification can effectively inhibit side reactions at the electrode/electrolyte interface, resulting


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in better cyclability. The surface modification materials are usually metal oxides, e.g., Al2O3 21-23, TiO2

24,25

, or carbon

26-28

. On the one hand, these materials form a protective layer between the

electrode and electrolyte; however, on the other hand, they behave as insulating barriers against the movement of lithium ions. Accordingly, considering that LATP is a good lithium ion conductor, it is expected to improve cell performance more significantly. Wu et al. has proved this by investigating the electrochemical properties for up to 40 cycles of LiMn2O4 coated with a layer of LATP (x = 0.3) via an in-situ wet chemical route 8. The ionic conductivity and morphology of LATP and its interaction with LiMn2O4 are obviously critical to the modification result, and new methods of effective surface modification using LATP are of great interest. Particle size plays an important role in conductivity, and some nanocrystalline lithium ion conductors show superior ionic conductivity

29,30

. There are various

methods of synthesizing LATP, such as solid-state reaction, sol–gel process, and melting– quenching method

31-35

. However, LATP particles prepared by these conventional synthetic

methods are generally larger than 100 nm, and the powders have to be subjected to vigorous ballmilling to obtain nanosized LATP 36. Here, we report for the first time a chemical synthesis route based on a modified sol–gel method with a two-step heat treatment in different atmospheres for preparation of LATP (x = 0.4) nanoparticles; this method is simple and reproducible and introduces no impurities, unlike ball-milling. The obtained nanosized LATP has higher conductivity than its bulk analog, and it is further applied to speckle over the particle surface of a LiMn2O4 cathode by simple grinding. The cell performance of the composite electrode is investigated and compared to those of electrodes modified by other LATPs with different particle sizes or morphologies. Advantages of this noncoating composition with cathode materials are discussed.


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2. Experimental 2.1. Preparation and characterization of LATP (x = 0.4) nanoparticles Reagent-grade LiNO3⋅H2O, Al(NO3)3⋅9H2O, Ti(OC4H9)4, and NH4H2PO4 were used as starting materials. Inspired by ref. [33], we used a modified citric-acid-assisted sol–gel method to prepare LATP (x = 0.4) nanoparticles. Typically, Ti(OC4H9)4, a 50 wt% citric acid solution, and ammonium hydroxide were mixed and stirred at 70°C for 1 h, forming a titanium citrate solution. Then stoichiometric LiNO3⋅H2O, NH4H2PO4, and Al(NO3)3⋅9H2O were dropped into the titanium citrate solution under constant stirring until the solution became clear. After the pH value was adjusted to 7, a certain volume of glycol was added. In this work, the molar ratio of [citric acid + glycol] to [Li+ + Al3+ + Ti4+] metal ions was 8:1, which was twice that used in ref. [33]. After heating at 80°C for about 8 h, the viscous gel was further dried in an oven at 130°C for 3 h and then subjected to further heat treatment as illustrated in Figure 1. First, under an argon flow, the dry gel was pyrolyzed at 500°C for 4 h and then annealed within a temperature range of 700– 950°C for another 2 h at a heating rate of 1°C min−1 and a cooling rate of 3°C min−1. Subsequently, the gas atmosphere was switched to air, and the fine powders were further calcined at the same annealing temperature for 2 h. LATP heat-treated as in the first step but directly in air was also prepared for comparison. We denote the samples calcined in two steps or directly in air as LATP2-T or LATP1-T, respectively, where T refers to the calcination temperature. Powder X-ray diffraction (XRD) measurements were performed on a Rigaku Smart Lab(3) instrument using Cu Kα radiation to identify the crystalline phase of the samples. Data were collected at 2θ = 10–80° with a step width of 0.06°. The morphology of the prepared powders was observed by scanning electron microscopy (SEM) (JEOL, JSM-6510) and transmission


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electron microscopy (TEM) (JEM-2010). The as-prepared powders were pelletized by cold pressing at 200MPa to a diameter and thickness of 13 mm and 1.3 mm, respectively. The pellets were sintered at 950°C for 6 h in air and sputtered with Au on both sides as blocking electrodes for ionic conductivity measurements. Note that sintering is a standard procedure for the ceramic materials to get a dense pellet for conductivity measurement. The Nyquist plots of LATP sintered at different temperatures indicated that a better lithium-ionic conductivity was obtained when sintering at 950 °C for 6 h, which was then chosen as the sintering condition for all the electrolyte samples (Figure S1). Each electrolyte pellet was placed into a two-electrode cell inside a sealed can. The can was then placed in an oil bath and the internal cell temperatures were monitored using K-type thermocouples. Ac impedance measurements were performed using an AutoLab 302N impedance analyzer with a perturbation voltage of 10 mV over the frequency range of 10−1–105 Hz. Before measurement was made at each temperature, a 2 h equilibration period was used after the internal cell temperature had reached a steady state.

2.2 Preparation and electrochemical measurements of nano-LATP/LiMn2O4 composites LiMn2O4 was purchased from Hejian Jinxin New Energy Ltd. To prepare active materials, LiMn2O4 was first ground with 1 wt%, 5 wt% and 10 wt% of the prepared LATP using a pestle and agate mortar at room temperature. The crystal structure and morphology of the nano-LATP/ LiMn2O4 composites were characterized by XRD, SEM and TEM. Energy dispersive X-ray spectrometry (EDS) was also applied to identify the distribution of elements in the composites. The working electrodes were fabricated from slurries of the active materials (80 wt%), acetylene black (10 wt%), and polyvinylidene fluoride (10 wt%) mixed in the solvent N-methyl-2pyrrolidone. The slurries were then cast onto an aluminum foil and dried at 120°C in vacuum


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overnight. Electrochemical characterizations were performed on a CR2032 coin-type cell with lithium metal as the counter electrode and 1 M LiPF6 in an ethylene carbonate:dimethyl carbonate (1:1 in volume) solution as the electrolyte. The cells were assembled in an argon-filled glove box. Galvanostatic charge/discharge measurements were carried out between 3.0 and 4.2 V (vs. Li/Li+) at room temperature and 55°C using a battery test system LAND CT2001A. Ac impedance measurements for the coin cells were carried out using an AutoLab 302N impedance analyzer. The applied voltage and frequency range were 10 mV and 10−1–105 Hz, respectively.

3. Results and Discussion 3.1 Structure, morphology, and conductivity analysis of nano-LATP (x = 0.4) XRD patterns of the powders prepared by the conventional sol-gel method and our modified citric-acid-assisted sol–gel method are shown in Figure 2a and b, respectively. The synthesis details are described in Section 2.1. The standard diffraction peaks of LiTi2(PO4)3 (JCPDF Card No. 00-35-0754) are also indicated in Figure 2. As observed in Figure 2a, NASICON-type LATP (x = 0.4) is successfully obtained at annealing temperatures between 800 and 900°C. Above 900°C, decomposition occurs, leading to the formation of an extra phase of AlPO4, whereas below 800°C, a Li4P2O7 impurity is visible. Therefore, for further experiments, the annealing temperature is set between 800 and 900°C. There is no obvious difference in the XRD patterns of the LATP prepared in air directly or through our two-step calcination in argon and air, although the crystallite size of LATP2-T is slightly smaller than that of LATP1-T (broader Bragg Peaks as manifested in the inset of Figure 2b). For example, the average crystallite size of LATP2-800 and LATP1-800 are about 50 nm and 57 nm calculated by using Scherrer’s equation, which are well consistent with the SEM results.


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Figure 3 shows SEM images and the corresponding particle size distribution histograms of the LATP powders obtained by the conventional and modified sol–gel methods at different calcination temperatures. It is clear that with increasing annealing temperature, the particles grow gradually in both cases. At the same temperature, however, LATP2-T always has a smaller particle size, narrower size distribution, and more uniform morphology than LATP1-T. For example, the particle size of LATP1-800 is between 50 and 100 nm (Figure 3a), but the majority are agglomerates, whereas the particle size of LATP2-800 is mostly 30–70 nm with less agglomeration and thus much better dispersion (Figure 3d). SEM-EDS analysis on the obtained LATP2-800 has revealed that the stoichiometric molar ratio of Al is 0.39, which is close to the theoretical x value of the LATP electrolyte formulated as Li1+xAlxTi2-x(PO4)3 (x=0.4). The polymeric network formed by esterification between excess citric acid and glycol (which has 8 times the molar mass of the metal ions here) plays a crucial role in determining the structure and morphology of the final LATP powders. First, the metal cations, Li+, Ti4+, and Al3+, are trapped throughout the polymer matrix homogeneously, so local stoichiometry can be maintained. As illustrated in Figure 1, when the dry gel is calcined directly in air, the polymeric network is pyrolyzed, and LATP crystallizes and grows easily with accompanying aggregation. The higher the temperature is, the bigger the LATP particles are, and the more severe aggregation occurs (Figure 3a–c). However, when the dry gel is calcined in argon, the polymeric network is only carbonized and can still restrain the growth of LATP crystals and prevent the particles from contacting each other and thus aggregating (Figure 3d–f). The density of the prepared LATPs is determined by the Archimedes method with ethanol as the immersion medium. The relative densities of the sample discs are about 96.5%, 97.0%, 96.1%, and 94.3% for LATP1-800, LATP2-800, LATP2-850, and LATP2-900, respectively. The


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decrease in density with increasing calcination temperature is consistent with the particle growth progress observed by SEM. Note that the LATP prepared by two-step-calcination is slightly denser than that calcined directly in air, suggesting important differences in their microstructure. Figure 4a displays room temperature Nyquist plots of the LATP prepared using the two methods at various temperatures. One semicircle at high frequencies along with an inclined spike at low frequencies are observed for all samples. The bulk resistance (Rb) and total resistance (Rb + Rgb) (Rgb refers to the grain boundary resistance) can be calculated from the left and right intercepts of the semicircle with the real axis, respectively. Thus one can obtain the bulk conductivity (σb) and total conductivity (σt) using equation: σ=l/R·S, where l and S are thickness and area of the disc, respectively. An equivalent circuit is indicated in the inset of Figure 4a, in which in addition to Rb and Rgb, CPE represents the grain boundary capacitance and ZW is the Warburg impedance. As can be seen from Figure 4a, the total ionic conductivity increases as the annealing temperature decreases from 900 to 800°C, and the total ionic conductivity of LATP2800

is found to be 5.9 × 10−4 S cm−1 at room temperature, which is almost twice that of LATP1-800

(3.1 ×10−4 S cm−1), while LATP2-850 and LATP2-900 have a total ionic conductivity of 2.8 × 10−4 S cm−1 and 1.9 × 10−4 S cm−1, respectively. For superfine materials, the grain boundaries dominate the total conductivity. The following reasons can probably account for the enhanced conductivity of LATP2-800, which has the finest particles: (1) In the intergranular region, more defects at the surface of smaller particles result in a larger concentration and higher mobility of ions 29; (2) The smaller particle size allows larger interfacial contact areas and thus a larger cross section for a continuous conduction path for Li+ transport 30; (3) The conductivity of ceramics also depends on the relative density 37, where the grain boundary conductivity increases with increasing relative density, although the bulk conductivity remains constant.


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The temperature dependence of the total conductivity of LATP2-800 at 303–383 K is shown in Figure 4b. The plots of log(σ) against 1000/T are linear and well fitted by the Arrhenius equation, σT = σ0exp(−Ea/RT), where Ea represents the activation energy of the ceramic electrolyte, and σ0 and R are the pre-exponential factor and gas constant, respectively. The Ea value of the LATP2-800, calculated from the slope of the linear fitted line, is 0.274 eV. NASICON-structured lithium electrolytes usually have a constant bulk Ea of about 0.3 eV, but the Ea value at the grain boundary varies depending on the M3+ content and degree of densification 38,39. The replacement of Ti4+ by Al3+ (x = 0.4) and the higher density of LATP2-800 might decrease the Ea value at the grain boundary. In Section 3.2, LATP calcined at 800°C (i.e., LATP2-800) will be used to fabricate a LiMn2O4 composite cathode because it has the highest conductivity and smallest particle size.

3.2 Mechanical mixed nano-LATP/LiMn2O4 composite as cathode material in LIB Electrochemical measurements were first performed on the LiMn2O4/LATP composite with different content of LATP2-800. As shown in Figure S2, LiMn2O4 electrodes with 5 wt% and 10 wt% LATP2-800 exhibit much better cyclic performance than the one with only 1 wt% LATP2-800. However, with increasing the content of LATP from 5 wt% to 10 wt%, the discharge capacity of the LiMn2O4 electrode rather deteriorates, suggesting that addition of excessive LATP is not beneficial to the cell performance of LiMn2O4. A balance between incorporating protective additive and maintaining good electronic conductivity of the electrode is important. Therefore, in the discussion below, we will focus on the LATP/LiMn2O4 composites mechanically mixed with 5 wt% LATP.


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Figure 5 shows the XRD patterns of the mechanically mixed LATP/LiMn2O4 composites with different LATPs. Apart from an observation of traces of LATP at 24.8°, the spinel structure of LiMn2O4 is not affected by the presence of LATP, and the crystal lattice parameters are unchanged. SEM and high-resolution TEM (HRTEM) images of pristine LiMn2O4 and LATP/LiMn2O4 composites are presented in Figure 6. The LATP1-800 powders on the LiMn2O4 surface are mainly agglomerates of small LATP particles (Figure 6b), whereas the LATP2-800 powders are much finer and disperse much more uniformly on the surface of LiMn2O4 (Figure 6c). Furthermore, the HRTEM image in Figure 6d clearly shows small particles with a lattice spacing of 0.4 nm on the surface of the spinel LiMn2O4; this spacing corresponding to the lattice distance between two [104] crystal planes of LATP. The average particle size of the LATP speckled on the surface of LiMn2O4 is estimated to be around 20 nm (Figure 6d). This value is slightly lower than that observed by FESEM as indicated in Figure 3d, because agglomeration of smaller nanoparticles can not be avoided in the bulk LATP and FESEM can not distinguish such small individual nanoparticles with limited resolution. However, after mechanical milling with LiMn2O4, the agglomerates of smaller LATP nanoparticles are re-dispersed and composited with LiMn2O4, which can then be clearly observed by HRTEM. Note that unlike the usual in-situ surface modification of cathode materials, the LiMn2O4 is not covered by a tight LATP coating layer; instead, it is partially protected by LATP nanoparticles. In order to investigate the dispersion of LATP on LiMn2O4 surface, energy dispersive X-ray spectrometry (EDS) element mapping analysis was carried out on the 5 wt% LATP2-800/LiMn2O4 composite. As shown in Figure 7, the Ti and Al elements associated with LATP, as well as the Mn element associated with LiMn2O4, demonstrate similar homogenous distribution within the whole sample, indicating a good


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dispersion of LATP nanoparticles over the surface of LiMn2O4. The amount of LATP2-800 in the composite electrode is calculated to be 4.97% from the EDS result, which is very close to the theoretical value. For the non-coating nano- LATP/LiMn2O4 composite electrodes, both LATP and acetylene black powder with PVDF binder modified the surface of LiMn2O4 particles, which function as conduction paths for both electrons and ions, and prevent the direct contact of LiMn2O4 particles with the liquid electrolyte in the cell. The type of surface modification will strongly influence the cell performance of the cathode materials. Figure 8 displays the 1st and 50th charge/discharge profiles of pristine LiMn2O4 and LATP/LiMn2O4 composite cathode electrodes at a charge/discharge rate of 0.2C (where 1C = one complete discharge in 1 h, i.e., 148 mA g−1). All the cathodes show two typical plateaus characteristic of spinel LiMn2O4. As shown in Figure 8a, in the 1st cycle, the charge and discharge curves of pristine LiMn2O4 and LATP2-800/LiMn2O4 almost overlap; however, a slightly lower discharge plateau and higher charge plateau are observed for LATP1-800/LiMn2O4. Similar to the latter observation, a larger polarization has also been found for LATP coated LiMn2O4 prepared through the wet chemical route, indicating a very likely lower electronic conductivity in association with a decrease in the initial discharge capacity 8. Therefore, partial coverage of LiMn2O4 with LATP nanoparticles speckled over the surface is evidently superior to full coating with an LATP layer on the surface in terms of the electronic conductivity of the cathode material. In addition, the particle size and morphology of LATP also play important roles in the cell performances. After 50 cycles, pristine LiMn2O4 exhibits a much larger polarization than the composite ones as a result of the inevitable side reactions between the electrolyte and the electrode upon cycling, and LATP2-800 well protected LiMn2O4 exhibits the highest discharge capacity demonstrating an excellent capacity retention capability (Figure 8b).


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Figure 9 shows the cycling performance of the LATP/LiMn2O4 composite electrodes for up to 100 cycles at a charge/discharge rate of 0.2C at 25 and 55°C. At 25°C, the capacity fading is much less pronounced for LiMn2O4 composited with our two-step-calcined LATP than for pristine or conventional one-step-calcined LATP composited LiMn2O4. Specifically, the capacity retention of LATP2-800/LiMn2O4 is 96.0% after 100 cycles, which is considerably higher than the values of 61.0% for unmodified LiMn2O4 and 85.0% for LATP1-800/LiMn2O4. The capacity retention difference is even more significant at elevated temperature. After 100 cycles at 55°C, the capacity retention of LATP2-800/LiMn2O4 remains as high as 89.0%, which is still considerably higher than that of pristine LiMn2O4 (51.5%) and LATP1-800/LiMn2O4 (71.4%). Figure 10 summarizes the cycling performance of the three electrodes at 25°C under various rates, where the current densities increase from 0.2C to 2C and then return to 0.2C. LATP2800/LiMn2O4

composite electrode delivers much higher discharge capacities than pristine

LiMn2O4 and LATP1-800/LiMn2O4, especially at high rates. It also exhibits much better rate capability than the other two samples. As the rate increases from 0.2C to 2C, LATP2-800/LiMn2O4 maintains 70.0% of the discharge capacity, whereas pristine LiMn2O4 and LATP1-800/LiMn2O4 preserve only 48.5% and 58.5%, respectively. In addition, when the rate is restored to the initial 0.2C, LATP2-800/LiMn2O4 and LATP1-800/LiMn2O4 can recover about 99.0% and 96.1% of their starting capacities respectively, whereas the pristine one can recover only about 90.0%. The electrode/electrolyte interfacial resistance was investigated using Ac impedance spectroscopy. The Nyquist plots of the coin cells made by pristine LiMn2O4 and LATP/LiMn2O4 composite electrode materials at room temperature are shown in Figure S3. Two overlapped depressed semi-circles at high frequency along with an inclined spike at low frequency are observed for all the spectra. The two semi-circles represent the solid electrolyte interphase (SEI)


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impedance and the charge-transfer impedance at the electrode/electrolyte interface respectively, whereas the straight line is associated with diffusion of Li+ through electrode material 40. As can be seen from Figure S3, the LATP2-800/LiMn2O4 cell shows the smallest resistance among the three, indicating a better conductivity for lithium ions due to the presence of the good lithium-ion conductor nano-LATP. In all the LATP modified LiMn2O4 electrodes, the LATP acts as a guardian for the LiMn2O4, protecting it from side reactions with the electrolyte. Various factors affect the protection ability, such as the ionic conductivity and morphology of LATP and the interaction between LATP and LiMn2O4. In this work, the LATP prepared by our modified two-step-calcination sol–gel method has a higher lithium ionic conductivity and better dispersion on the surface of LiMn2O4 than LATP prepared by a conventional one-step-calcination sol–gel method, resulting in better cycling performance. Furthermore, this mechanically mixed LATP/LiMn2O4 composite also has the advantage of minimizing the effect on the electronic conductivity of the electrode by uniformly dispersing nanoparticles rather than chemically coating an electrically insulating layer on the surface of LiMn2O4, which leaves spaces for direct contact between the active material and the electronic conducting component, such as carbon black. Unsurprisingly, the capacity retention capability of the LATP/LiMn2O4 is superior to most of LiMn2O4 coated by ionicinsulating metal oxides reported so far. For example, 10-ALD-layer Al2O3 coating LiMn2O4 prepared by atomic layer deposition (ALD)21 and 5% Al2O3 coating LiMn2O4 prepared by electrostatic attraction forces (EAF)22 can only retain 88.5% (1C rate) after 100 cycles and 70.8% (0.2C rate) after 25 cycles of the original capacity respectively, much lower than the value 96.0% (0.2C rate) of our LATP2-800/LiMn2O4. Very recently, three-dimensional hierarchical Al2O3 nanosheets wrapped LiMn2O4 with a well-designed flower-like structured Al2O3 layer has been


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reported to be able to improve the cycling retention ratio to 92.0% after 100 cycles and 89.8% after 800 cycles, which also benefit from the 3D porous coating 23. We first demonstrate in this work that a full coverage of LATP on the surface is not necessary in terms of improving cycling performance of spinel LiMn2O4.

4. Conclusions Nanosized lithium-ion-conducting solid electrolyte LATP (x = 0.4) is prepared by a modified two-step-calcination citric-acid-assisted sol–gel method. The polymeric network formed by esterification and polymerization between citric acid and glycol is carbonized and retained under calcination in argon, which protects the LATP particles from contact and further aggregation. Subsequently, after calcination in air, non-aggregated LATP nanoparticles with a smaller particle size and narrower size distribution are obtained. The minimum average particle size reaches 40 nm when the powders are calcined at 800°C. The LATP has a high relative density of 97.0% and a room temperature conductivity of 5.9 × 10−4 S cm−1 (almost twice that of LATP prepared by the usual sol–gel method). Furthermore, the prepared nano-LATP is used to composite with a spinel LiMn2O4 cathode material in LIBs. The superfine ionic conductive powder disperses well on the surface of LiMn2O4. This non-coating composition strategy, not only effectively prevents side reactions of LiMn2O4 particles with the liquid electrolyte, but also provides conduction paths for both electrons and ions rather than forming an electrically insulating layer on the surface of active material as the conventional surface coating modification usually does. Therefore, the cycling performance of LiMn2O4 is greatly improved at both ambient and elevated temperatures, with only 4.0% and 11.0% capacity loss after 100 cycles at 25 and 55°C, respectively. The rate


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capability of LiMn2O4 composited with the LATP obtained in this work is also superior to that of pristine LiMn2O4 and LiMn2O4 composited with LATP made by the conventional sol–gel method.

Supporting Information

Figure S1. Complex impedance plots of LATP pellets sintered at various temperatures for 6 h; Figure S2. Cycling performance of LATP2-800 /LiMn2O4 composite electrodes with different LATP content at a current density of 100 mAg-1; Figure S3. Complex impedance plots for the cells made by pristine LiMn2O4, LATP1-800/LiMn2O4 and LATP2-800/LiMn2O4 composite electrodes at room temperature.

Author Information Corresponding author * Prof. Chuhong Zhang E-mail: [email protected] Tel/ Fax: +86-28-85402819 Notes The authors declare no competing financial interest. † These authors contributed equally to this work.

Acknowledgements This work was financially supported by National Basic Research Program of China (973 program, No: 2013CB934700) and National Natural Science Foundation of China (No:


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51222305, 51673123) and Program for New Century Excellent Talents in University (No: NCET-12-0386).

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Figure captions Figure 1. Schematic illustration of preparation of LATP by conventional one-step-calcination and modified two-step-calcination sol–gel methods. Figure 2. XRD patterns of LATP powders annealed at different temperatures by (a) conventional one-step-calcination and (b) modified two-step-calcination sol–gel methods (inset: comparison of the characteristic peak at 24.8° of LATP1-800 and LATP2-800). Figure 3. SEM images and corresponding particle size distribution histograms of LATP prepared by (a)–(c) conventional one-step-calcination and (d)–(f) modified two-step-calcination sol–gel methods at 800, 850, and 900°C as indicated. Figure 4. (a) Room temperature complex impedance plots of LATP prepared by conventional one-step-calcination and modified two-step-calcination sol-gel method (inset: equivalent circuit used to describe the experimental data), and (b) Arrhenius plot of the LATP2-800 sample. Figure 5. XRD patterns of (a) pristine LiMn2O4, (b) LATP1-800/LiMn2O4 and (c) LATP2800/LiMn2O4

composites. (LATP content: 5%wt).

Figure 6. SEM images of (a) pristine LiMn2O4, (b) LATP1-800/LiMn2O4 and (c) LATP2800/LiMn2O4

composites, and (d) HRTEM image of LATP2-800/LiMn2O4 composite. (LATP

content: 5%wt). Figure 7. (a) SEM image of LATP2-800/LiMn2O4 composite (LATP content: 5%wt), and the corresponding EDS element mapping images of (b) Mn, (c) Ti and (d) Al elements. Figure 8. (a) 1st and (b) 50th charge and discharge profiles of pristine LiMn2O4, LATP1800/LiMn2O4

and LATP2-800/LiMn2O4 composite electrodes (LATP content: 5%wt).

Figure 9. Cycling behaviors of pristine LiMn2O4, LATP1-800/LiMn2O4 and LATP2-800/LiMn2O4 composite electrodes (LATP content: 5%wt) at different temperatures (rate: 0.2C).


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Figure 10. Comparison of cycling stability of pristine LiMn2O4, LATP1-800/LiMn2O4 and LATP2800/LiMn2O4

composite electrodes (LATP content: 5%wt) with various rates at 25°C.


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Figure(s)

Figure 1. Schematic illustration of preparation of LATP by conventional one-stepcalcination and modified two-step-calcination sol-gel methods.


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Figure 2. XRD patterns of LATP powders annealed at different temperatures by (a) conventional one-step-calcination and (b) modified two-step-calcination sol-gel method (inset: comparison of the characteristic peak at 24.8° of LATP1-800 and LATP2-800).


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Figure 3. SEM images and corresponding particle size distribution histograms of LATP prepared by: (a)-(c) conventional one-step-calcination and (d)-(f) modified two-stepcalcination sol-gel method at 800°C, 850°C, 900°C as indicated.


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Figure 4. (a) Room temperature complex impedance plots of LATP prepared by conventional one-step-calcination and modified two-step-calcination sol-gel method (inset: equivalent circuit used to describe the experimental data), and (b) Arrhenius plot of the LATP2-800 sample.


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Figure 5. XRD patterns of (a) pristine LiMn2O4, (b) LATP1-800/LiMn2O4 and (c) LATP2800/LiMn2O4

composites. (LATP content: 5%wt)


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Figure 6. SEM images of (a) pristine LiMn2O4, (b) LATP1-800/LiMn2O4 and (c) LATP2800/LiMn2O4

composites, and (d) HRTEM image of LATP2-800/LiMn2O4 composite. (LATP content: 5%wt)


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Figure 7. (a) SEM image of LATP2-800/LiMn2O4 composite (LATP content: 5%wt), and the corresponding EDS element mapping images of (b) Mn, (c) Ti and (d) Al elements.


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Figure 8. (a) 1st and (b) 50th charge and discharge profiles of pristine LiMn2O4, LATP1800/LiMn2O4 and LATP2-800/LiMn2O4 composite electrodes (LATP content: 5%wt).


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Figure 9. Cycling behaviors of pristine LiMn2O4, LATP1-800/LiMn2O4 and LATP2800/LiMn2O4

composite electrodes (LATP content: 5%wt) at different temperatures (rate: 0.2C).


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Figure 10. Comparison of cycling stability of pristine LiMn2O4, LATP1-800/LiMn2O4 and LATP2-800/LiMn2O4 composite electrodes (LATP content: 5%wt) with various rates at 25°C.


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Table of Contents Graphic


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Facile Synthesis of Nanosized Lithium-Ion-Conducting Solid

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