Synthesis and Characterization of Lithium-Ion Conductive LATP-LaPO4 Composites Using La2O3 Nano-Powder

LATP-based composite electrolytes were prepared by sintering the mixtures of LATP precursor and La2O3 nano-powder. Powder X-ray diffraction and scanning electron microscopy suggest that La2O3 can react with LATP during sintering to form fine LaPO4 particles that are dispersed in the LATP matrix. The room temperature conductivity initially increases with La2O3 nano-powder addition showing the maximum of 0.69 mS∙cm−1 at 6 wt.%, above which, conductivity decreases with the introduction of La2O3. The activation energy of conductivity is not largely varied with the La2O3 content, suggesting that the conduction mechanism is essentially preserved despite LaPO4 dispersion. In comparison with the previously reported LATP-LLTO system, although some unidentified impurity slightly reduces the conductivity maximum, the fine dispersion of LaPO4 particles can be achieved in the LATP–La2O3 system.


Introduction
The popularization of electric vehicles and mobile devices is calling for an advance in battery technology to meet the requirement on the battery reliability and higher energy density.Solid-state electrolytes (SSEs), with wider electrochemical window, nonflammability and low-temperature stability in comparison with the liquid counterparts, is a key component for the all-solid-state battery (ASSB) that is safer to use and allows more compact designs [1][2][3][4].In recent decades, research has focused on the improvement of room temperature conductivities for SSEs, mainly through the development of new lithium-ion conductors or the improvement of currently available SSEs by means of doping or lattice tuning [1,3,[5][6][7][8][9].
Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) is an oxide-based solid-state electrolyte with a rhombohedral NASICON-type structure that is composed of corner-sharing MO 6 (M = Ti or Al) octahedra and PO 4 tetrahedra, forming a three-dimensional diffusion network for lithium-ions within the lattice [1,3].We have previously achieved 3 times improvement in room temperature conductivity by introducing Li 0.348 La 0.55 TiO 3 (LLTO) particles into the LATP matrix.The introduced LLTO reacted with the LATP matrix during the sintering process, forming fine LaPO 4 which act as insulative particles [40].However, the direct introduction of LaPO 4 into LATP did not enhance the conductivity due to the growth of LaPO 4 particles [42].In order to disperse the LaPO 4 particles finely through a simplified reaction, La 2 O 3 nano-powder is selected as a more direct lanthanum source rather than LLTO particles.In this work, LATP-LaPO 4 composites are prepared by employing La 2 O 3 nano-powder to compare with the results of the previous LLTO added system.

Synthesis of the LATP Precursor
Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) precursor was prepared by the solid-state reaction method.Stoichiometric amounts of Li 2 CO 3 (99.0%Wako Pure Chem., Osaka, Japan, with 10 wt.% excess), γ-Al 2 O 3 (97.0%Stream Chemical, Newburyport, MA, USA), TiO 2 (rutile, 99.9% High Purity Chem., Saitama, Japan) and NH 4 H 2 PO 4 (99.0%Wako Pure Chem., Osaka, Japan) were mixed in an automatic grinder for 5 h with an aid of ethanol.After drying for 24 h, the mixture was uniaxially pressed to form the green compact which was then calcined at 700 • C for 2 h.To form fine LATP precursor, the calcined product was crushed and ball-milled in zirconia pot with ethanol and zirconia balls for 5 h at 400 RPM (Pulverisette7 Premium Line, Fritsch, Idar-Oberstein, Germany).

Synthesis of the LATP-La 2 O 3 Composite
To fabricate LATP-La 2 O 3 composite pellets, the fine LATP precursor was mixed with La 2 O 3 nano-powder (<100 nm, 99% Sigma-Aldrich, Hesse, Germany) by ball milling (zirconia balls and pot, Pulverisette7 Premium Line, Fritsch) with the aid of a small amount of ethanol for 1.5 h at 400 RPM.After drying, the powder mixture was isostatically pressed to form cylindrical pellets at 200 MPa followed by sintering at 1000 • C for 4 h.The sintering time was optimized according to the preliminarily examined sintering time dependence, as represented in Figures S1-S3 in the Supplementary Materials.In this work, the introduced La 2 O 3 nano-powders were weighted 2, 4, 6, 8, 12 and 16 wt.% of the total weight (LATP + La 2 O 3 mixture).Herein, the samples are referred as LATP-x wt.% La 2 O 3 , based on the amount of added La 2 O 3 .

Characterizations and Electrochemical Properties
The obtained crystalline phases were investigated by powder XRD on the Ultima VI diffractometer (Rigaku, Tokyo, Japan) using a CuKα radiation source (40 kV, 40 mA).The microstructure and particle distribution of the samples were observed by scanning electron microscopy under the back-scattering electron mode (SEM, SU6600, Hitachi, Tokyo, Japan).The sample pellets with a 6 mm diameter and 3 mm thickness were polished on both sides and sputtered with gold to form electrodes. To investigate the temperature variation of electrochemical impedance, the samples were clamped in a 4-electrode test apparatus in a temperature-controlled tubular furnace.An amount of 0.5 V of AC potential was applied to the sample pellets using an LRC meter (3531 Z Hitester, Hioki, Japan) in a frequency range of 130 Hz-1.3 MHz and a temperature range of 25-200 • C. The conductivities were calculated by the equivalent circuit fitting from the impedance spectroscopies using ZView ® software (Scribner, New York, NA, USA) [43].

Results and Discussions
Powder XRD pattern of LATP (Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 ) and LATP-x wt.% La 2 O 3 composites are shown in Figure 1, where the major peaks are associated with LATP that is isostructural with LiTi 2 (PO 4 ) 3 .The existence of LaPO 4 (labelled by solid inverted triangle) suggests a solid-state reaction between the LATP matrix and introduced La 2 O 3 during sintering.LaPO 4 formation at the sintering also occurred in LATP-LLTO and LAGP-LLTO systems in the previous works [40,41].In addition to LaPO 4 formation, a LiTiPO 5 phase and an unidentified impurity were also observed in the powder XRD patterns, as labelled by hollow diamonds and hollow inverted triangles in Figure 1.The small amount of LiTiPO 5 phase is believed to be formed during sintering when the LATP matrix donates phosphorus to form LaPO 4 .The LiTiPO 5 and unidentified impurities constantly remained despite prolonged sintering, as observed in Figure S1, for the LATP-8 wt.% La 2 O 3 system.

Results and Discussions
Powder XRD pattern of LATP (Li1.3Al0.3Ti1.7(PO4)3)and LATP-x wt.% La2O3 co sites are shown in Figure 1, where the major peaks are associated with LATP t isostructural with LiTi2(PO4)3.The existence of LaPO4 (labelled by solid inverted tria suggests a solid-state reaction between the LATP matrix and introduced La2O3 durin tering.LaPO4 formation at the sintering also occurred in LATP-LLTO and LAGP-L systems in the previous works [40,41].In addition to LaPO4 formation, a LiTiPO5 and an unidentified impurity were also observed in the powder XRD patterns, as lab by hollow diamonds and hollow inverted triangles in Figure 1.The small amou LiTiPO5 phase is believed to be formed during sintering when the LATP matrix do phosphorus to form LaPO4.The LiTiPO5 and unidentified impurities constantly rem despite prolonged sintering, as observed in Figure S1, for the LATP-8 wt.% La2O3 sy Figure 2 presents SEM images of pristine LATP and composite samples capture der back-scattered electron mode, where the bright spots represent the lanthanum taining particles due to the heaver atom.For relatively smaller La2O3 addition be wt.%, the dispersed particles are isolated, keeping the similar sizes, as shown in F 2b-d.At higher La2O3 additions such as 12 or 16 wt.%,the particles are aggregat break the percolation of LATP matrix, as shown in Figure 2e,f.Figure 2 presents SEM images of pristine LATP and composite samples captured under back-scattered electron mode, where the bright spots represent the lanthanum-containing particles due to the heaver atom.For relatively smaller La 2 O 3 addition below 8 wt.%, the dispersed particles are isolated, keeping the similar sizes, as shown in Figure 2b-d  The Nyquist plots of electrochemical impedance spectroscopies for pristine L and composite samples are shown in Figure 3. Owing to the limited frequency range impedance spectra are fitted by using a conventional equivalent circuit in the inset t tain the right side of the semi-circles as the total resistivity.The room temperature ductivities of the samples are presented as a function of La2O3 addition in Figure 4, w the highest conductivity of 0.69 mS•cm −1 is achieved at 6 wt.% of La2O3 addition.This gests that the addition of La2O3 nano-powder can form LaPO4 particles in LATP m From 6 wt.% up to 16 wt.% of La2O3 introduction, the conductivity decreases with La2O3 addition.This is caused by the aggregation of the insulative particles, whic verely block the migration of the lithium-ions in the LATP matrix to reduce the total ductivity.The Nyquist plots of electrochemical impedance spectroscopies for pristine LATP and composite samples are shown in Figure 3. Owing to the limited frequency range, the impedance spectra are fitted by using a conventional equivalent circuit in the inset to obtain the right side of the semi-circles as the total resistivity.The room temperature conductivities of the samples are presented as a function of La 2 O 3 addition in Figure 4, where the highest conductivity of 0.69 mS•cm −1 is achieved at 6 wt.% of La 2 O 3 addition.This suggests that the addition of La 2 O 3 nano-powder can form LaPO 4 particles in LATP matrix.From 6 wt.% up to 16 wt.% of La 2 O 3 introduction, the conductivity decreases with the La 2 O 3 addition.This is caused by the aggregation of the insulative particles, which severely block the migration of the lithium-ions in the LATP matrix to reduce the total conductivity. For comparison, the conductivity of previous LATP-LLTO composites [40] are also plotted in Figure 4 (hollow triangles).The weight percentage of LLTO is converted to the equivalent amount of La 2 O 3 based on the lanthanum content in additives.Although the highest conductivity in this work is slightly smaller than the previously observed 0.76 mS•cm −1 in LATP-4 wt.% LLTO [40], about three-fold enhancement from the pristine can be achieved.The slightly smaller conductivity might be due to the unidentified impurity, which could block the LATP matrix/LaPO 4 particle interface.It should be noted that the maximum conductivity occurs at higher lanthanum content in comparison with the previous LATP-LLTO system, indicating that La 2 O 3 nano-powder is effective in forming finely dispersed LaPO 4 particles without aggregation.Suppressing the formation of unidentified impurity should be critical for further enhancement in conductivity.
The conductivities are plotted against inverse temperature, as shown in Figure 5a, which can be linearly fitted to the Arrhenius equation σ T T = σ 0 exp(−E a /kT), where σ T , σ 0 and E a denote the total conductivity, pre-exponential term and the activation energy, respectively.The deduced activation energy is plotted as a function of La 2 O 3 addition in Figure 5b.The activation energies are similar to pristine LATP or slightly increased with the introduction of La 2 O 3 nano-powder, suggesting that the lithium migration mechanism of composite is essentially consistent with that of pristine LATP.For comparison, the conductivity of previous LATP-LLTO composites [40] are also plotted in Figure 4 (hollow triangles).The weight percentage of LLTO is converted to the equivalent amount of La2O3 based on the lanthanum content in additives.Although the highest conductivity in this work is slightly smaller than the previously observed 0.76 mS•cm −1 in LATP-4 wt.% LLTO [40], about three-fold enhancement from the pristine can This work LATP -LLTO [40]   s / mS×cm   In summary, by adding La2O3 nano-powder into the LATP precursor, LaPO4 particle can be dispersed into the LATP matrix through solid-state reaction during sintering pro cess.A three-fold enhancement in conductivity is observed in the LATP-6 wt.% La2O  In summary, by adding La 2 O 3 nano-powder into the LATP precursor, LaPO 4 particles can be dispersed into the LATP matrix through solid-state reaction during sintering process.A three-fold enhancement in conductivity is observed in the LATP-6 wt.% La 2 O 3 sample, while the activation energy of the composite is not largely different from the pristine LATP.In further study, characterizations such as 7 Li solid-state NMR spectroscopy and high-resolution transmission electron microscopy are required to scrutinize the lithium-ion conduction mechanism and microstructural features at the LATP matrix/LaPO 4 particle interface.

Conclusions
In this work, LATP-based composite electrolytes were synthesized by adding La 2 O 3 nano-powder into an LATP precursor.Powder XRD and back-scattered SEM prove that LaPO 4 particles were formed to disperse in the sintered samples during sintering.The aggregation of particles is observed at higher lanthanum introduction.The room temperature conductivity of the composite electrolytes increases with the La 2 O 3 addition until 6 wt.%,where the maximum conductivity of 0.69 mS•cm −1 is achieved, which is ascribed to the insulative particle dispersion effect.In comparison with the previous study on the LATP-LLTO composites [40], the maximum conductivity is observed at the higher lanthanum content, although the maximum conductivity is inferior to the previous one.Further improvement is expected through the elimination of impurities.The compositional dependence of activation energies of conductivity suggests that the present LATP-La 2 O 3 system possesses a similar conduction mechanism to the previous LATP-LLTO system.
Figure2presents SEM images of pristine LATP and composite samples captured under back-scattered electron mode, where the bright spots represent the lanthanum-containing particles due to the heaver atom.For relatively smaller La 2 O 3 addition below 8 wt.%, the dispersed particles are isolated, keeping the similar sizes, as shown in Figure2b-d.At higher La 2 O 3 additions such as 12 or 16 wt.%,the particles are aggregated to break the percolation of LATP matrix, as shown in Figure2e,f.

Figure 3 .
Figure 3. Nyquist plots of pristine LATP and composite samples with fitted curves.The related equivalent circuit is shown in the inset.

Figure 3 .
Figure 3. Nyquist plots of pristine LATP and composite samples with fitted curves.The related equivalent circuit is shown in the inset.Materials 2021, 14, x FOR PEER REVIEW 6 of 10

Figure 4 .
Figure 4. Room temperature conductivity of LATP-x wt.% La2O3 as a function of La2O3 addition, in comparison with the results in LATP-y wt.% LLTO from the previous work[40].

Figure 4 .
Figure 4. Room temperature conductivity of LATP-x wt.% La 2 O 3 as a function of La 2 O 3 addition, in comparison with the results in LATP-y wt.% LLTO from the previous work [40].