Reduced Sintering Temperatures of Li + Conductive Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 Ceramics

: All-solid-state batteries (ASSB) are considered promising candidates for future energy storage and advanced electric mobility. When compared to conventional Li-ion batteries, the substitution of Li-ion conductive, flammable liquids by a solid electrolyte and the application of Li-metal anodes substantially increase safety and energy density. The solid electrolyte Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) provides high Li-ion conductivity of about 10 − 3 S / cm and is considered a highly promising candidate for both the solid electrolyte-separator and the ionically conductive part of the all-solid state composite cathode, consisting of the cathode material, the solid electrolyte, and an electron conductor. Co-sintering of the composite cathode is a sophisticated challenge, because temperatures above 1000 ◦ C are typically required to achieve the maximum ionic conductivity of LATP but provoke reactions with the cathode material, inhibiting proper electrochemical functioning in the ASSB. In the present study, the application of sintering aids with di ﬀ erent melting points and their impact on the sinterability and the conductivity of LATP were investigated by means of optical dilatometry and impedance spectroscopy. The microstructure of the samples was analyzed by SEM. The results indicate that the sintering temperature can be reduced below 800 ◦ C while maintaining high ionic conductivity of up to 3.6 × 10 − 4 S / cm. These insights can be considered a crucial step forward towards enable LATP-based composite cathodes for future ASSB. appropriate electrical equivalent circuit. The ﬁt results are used to calculate the ionic conductivity, taking into account the geometry of the samples. The calculated values were not corrected for porosity.


Introduction
All-solid-state batteries (ASSB) are considered as promising candidates for future energy storage and advanced electric mobility. When compared to conventional Li-ion batteries, the substitution of Li-ion conductive, flammable liquids by a solid electrolyte and the application of Li-metal anodes substantially increase safety and energy density [1]. When compared to common Li-ion batteries (LIB) currently used in mobile electronic devices and electric vehicles, the liquid electrolyte is substituted by a solid Li-ion conductor in the ASSB [2]. This includes a solid electrolyte-separator and the ionically conductive part of the all-solid state composite cathode, consisting of a cathode material, the solid electrolyte, and an electron conductor. Besides advantages regarding energy density, the substitution of the flammable liquid electrolyte will improve the safety significantly. Three types of materials have proven to be promising for this approach: polymers, sulfides, and oxides [3]. The polymers are easy to handle under dry air and production is scalable, but the Li-ion conductivity at room temperature is limited to the range of 10 −6 -10 −5 S/cm and can be increased by heating at 60 • C to >10 −4 S/cm [4]. In contrast, sulfides exhibit high Li-ion conductivity of~10 −3 S/cm at room temperature, but require extreme pressures [5]. Furthermore, sulfides are very sensitive to moisture, which necessitates handling under inert conditions. Oxides are not affected by these restrictions and Figure 1 shows the experimental workflow of preparing Li + conductive LATP ceramics. Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) powder with high phase purity was prepared by sol-gel synthesis as reported previously [15]. After milling, the average particle size was determined to be d 50 = 0.76 µm with laser diffraction in a particle size range between 20 nm and 2000 µm (Mastersizer 2000, Malvern Instruments Ltd., Malvern, UK). The powder was mixed with 15 mol% sintering aid by wet-milling in ethanol. Table 1 lists the sintering aids used together with their respective melting points. Intension for addition of sintering aids with melting points below the sintering temperature of LATP was to induce the liquid phase sintering and in this way reduce the sintering temperature. The milling step was performed for two hours in a planetary micro mill (Pulverisette 7, Fritsch, Germany) with milling balls and cups made of zirconia. The suspension was dried in air for 12 h. After grinding in a mortar, the powder was pressed into cylindrical discs of 8 mm in diameter and 2 mm in height. The discs were sintered by application of a heating ramp (3 K/min) followed by 1 h dwell time. The final temperature was varied between 700 and 1100 • C. After sintering, the density was calculated by the geometrical data and the mass of each specimen. The density value was determined as an average of three samples with a standard deviation of ≤0.08 g/cm 3 . The relative densities were determined using the experimentally obtained densities and the theoretical density of bulk LATP (2.94 g/cm 3 ).
Crystals 2020, 10, x FOR PEER REVIEW 3 of 12 Figure 1 shows the experimental workflow of preparing Li + conductive LATP ceramics. Li1.3Al0.3Ti1.7(PO4)3 (LATP) powder with high phase purity was prepared by sol-gel synthesis as reported previously [15]. After milling, the average particle size was determined to be d50 = 0.76 µm with laser diffraction in a particle size range between 20 nm and 2000 µm (Mastersizer 2000, Malvern Instruments Ltd., Malvern, UK). The powder was mixed with 15 mol% sintering aid by wet-milling in ethanol. Table 1 lists the sintering aids used together with their respective melting points. Intension for addition of sintering aids with melting points below the sintering temperature of LATP was to induce the liquid phase sintering and in this way reduce the sintering temperature. The milling step was performed for two hours in a planetary micro mill (Pulverisette 7, Fritsch, Germany) with milling balls and cups made of zirconia. The suspension was dried in air for 12 h. After grinding in a mortar, the powder was pressed into cylindrical discs of 8 mm in diameter and 2 mm in height. The discs were sintered by application of a heating ramp (3 K/min) followed by 1 h dwell time. The final temperature was varied between 700 and 1100 °C. After sintering, the density was calculated by the geometrical data and the mass of each specimen. The density value was determined as an average of three samples with a standard deviation of ≤ 0.08 g/cm³.The relative densities were determined using the experimentally obtained densities and the theoretical density of bulk LATP (2.94 g/cm 3 ).  For analyzing the shrinkage behavior by optical dilatometry, the powder mixtures were pressed into cylindrical shapes with 2 mm diameter and 2 mm height and placed in the EM301 heating microscope (Hesse Instruments, Osterode, Germany). The shrinkage behavior was measured during application of a heating ramp (5 K/min) and a final temperature of 1200 °C. The linear shrinkage was determined by EMI III heating microscope software (Hesse Instruments, Osterode, Germany). Based on these data, the shrinkage rate was calculated as first derivation.

Materials and Methods
The ionic conductivity of the LATP ceramics was determined by impedance spectroscopy. For this purpose, the samples were sputtered with gold as ion-blocking electrodes. The impedance spectra were measured at room temperature using a potentiostat (VMP3, Biologic) in a frequency range from 10 6 to 10 −2 Hz and a p-p amplitude of 20 mV. The impedance data were fitted to an appropriate electrical equivalent circuit. The fit results are used to calculate the ionic conductivity, taking into account the geometry of the samples. The calculated values were not corrected for porosity.  For analyzing the shrinkage behavior by optical dilatometry, the powder mixtures were pressed into cylindrical shapes with 2 mm diameter and 2 mm height and placed in the EM301 heating microscope (Hesse Instruments, Osterode, Germany). The shrinkage behavior was measured during application of a heating ramp (5 K/min) and a final temperature of 1200 • C. The linear shrinkage was determined by EMI III heating microscope software (Hesse Instruments, Osterode, Germany). Based on these data, the shrinkage rate was calculated as first derivation.
The ionic conductivity of the LATP ceramics was determined by impedance spectroscopy. For this purpose, the samples were sputtered with gold as ion-blocking electrodes. The impedance spectra were measured at room temperature using a potentiostat (VMP3, Biologic) in a frequency range from 10 6 to 10 −2 Hz and a p-p amplitude of 20 mV. The impedance data were fitted to an appropriate electrical equivalent circuit. The fit results are used to calculate the ionic conductivity, taking into account the geometry of the samples. The calculated values were not corrected for porosity.  Figure 2 compares the shrinkage behaviors of pure LATP and the different mixtures of LATP and sintering aids. The sintering temperature was significantly reduced and the maximum shrinkage rate was enhanced for all mixtures of LATP with sintering aids when compared to pure LATP. The lowest sintering temperature of about 730 • C was reached with the mixture of Li 2 CO 3 and LiBO 2 (forming Li 3 BO 3 with eutectic melting point of T = 665 • C). The addition of Li 2 CO 3 as well as Li 3 PO 4 resulted in slightly higher sintering temperatures of approximately 760 • C (cf. Figure 2a). LiBO 2 and LiOH reduced the sintering temperature of LATP to approximately 800 • C (cf. Figure 2b). Mixtures with LiCl salt as additive showed the smallest impact on both the sintering temperature (840 • C) and the shrinkage rate. Interestingly, the influence of the different additives on the sintering temperature and the maximum shrinkage rate did not correlate with their specific melting points, as depicted in Figure 3 by plotting the temperature of the maximum shrinkage rate against the melting point of the sintering aids. and sintering aids. The sintering temperature was significantly reduced and the maximum shrinkage rate was enhanced for all mixtures of LATP with sintering aids when compared to pure LATP. The lowest sintering temperature of about 730 °C was reached with the mixture of Li2CO3 and LiBO2 (forming Li3BO3 with eutectic melting point of T = 665 °C). The addition of Li2CO3 as well as Li3PO4 resulted in slightly higher sintering temperatures of approximately 760 °C (cf. Figure 2a). LiBO2 and LiOH reduced the sintering temperature of LATP to approximately 800 °C (cf. Figure 2b). Mixtures with LiCl salt as additive showed the smallest impact on both the sintering temperature (840 °C) and the shrinkage rate. Interestingly, the influence of the different additives on the sintering temperature and the maximum shrinkage rate did not correlate with their specific melting points, as depicted in Figure 3 by plotting the temperature of the maximum shrinkage rate against the melting point of the sintering aids.

Results and Discussions
The investigated sintering additives considerably improved the shrinkage behavior of LATP and reduced the sintering temperature by more than 200 K. We explain these phenomena by the solution and precipitation process of the Li salts on the surface of the LATP particles during heating, so that local melting and disorder accelerates the surface diffusion as a driving force for sintering.
However, it has to be considered that the sintering behavior and associated densification are also affected by the homogeneity of the mixture, the mobility of the ions, and the wetting behavior. Most probably, these properties vary with the type of sintering aid, resulting in the different behaviors observed in Figure 2.   Mixtures of LATP with Li3BO3, Li3PO4, and Li2CO3, respectively, were selected for further investigation because they showed the most promising sintering behavior. The powder mixtures were pressed into discs and sintered with short dwell times followed by comprehensive materials characterization. Figure 4 depicts the relative density of sintered LATP ceramics with and without sintering aids. As expected from the analysis of the shrinkage behavior, the densification of the mixtures with sintering aids occurred between 700 and 800 °C (cf. Figure 4). In contrast, densifying the pure LATP required temperatures higher than 1000 °C. It has to be noted that for sintering at T = 1100 °C, cracking of the samples was observed. This effect is known for sintering of LATP. It is related The investigated sintering additives considerably improved the shrinkage behavior of LATP and reduced the sintering temperature by more than 200 K. We explain these phenomena by the solution and precipitation process of the Li salts on the surface of the LATP particles during heating, so that local melting and disorder accelerates the surface diffusion as a driving force for sintering.
However, it has to be considered that the sintering behavior and associated densification are also affected by the homogeneity of the mixture, the mobility of the ions, and the wetting behavior. Most probably, these properties vary with the type of sintering aid, resulting in the different behaviors observed in Figure 2.
Mixtures of LATP with Li 3 BO 3 , Li 3 PO 4 , and Li 2 CO 3 , respectively, were selected for further investigation because they showed the most promising sintering behavior. The powder mixtures were pressed into discs and sintered with short dwell times followed by comprehensive materials characterization. Figure 4 depicts the relative density of sintered LATP ceramics with and without sintering aids. As expected from the analysis of the shrinkage behavior, the densification of the mixtures with sintering aids occurred between 700 and 800 • C (cf. Figure 4). In contrast, densifying the pure LATP required temperatures higher than 1000 • C. It has to be noted that for sintering at T = 1100 • C, cracking of the samples was observed. This effect is known for sintering of LATP. It is related to an anisotropic grain growth at high temperature and associated tensions in the microstructure, which result in cracking [9,26]. In this case, a reliable determination of the density was not possible. Mixtures of LATP with Li3BO3, Li3PO4, and Li2CO3, respectively, were selected for further investigation because they showed the most promising sintering behavior. The powder mixtures were pressed into discs and sintered with short dwell times followed by comprehensive materials characterization. Figure 4 depicts the relative density of sintered LATP ceramics with and without sintering aids. As expected from the analysis of the shrinkage behavior, the densification of the mixtures with sintering aids occurred between 700 and 800 °C (cf. Figure 4). In contrast, densifying the pure LATP required temperatures higher than 1000 °C. It has to be noted that for sintering at T = 1100 °C, cracking of the samples was observed. This effect is known for sintering of LATP. It is related to an anisotropic grain growth at high temperature and associated tensions in the microstructure, which result in cracking [9,26]. In this case, a reliable determination of the density was not possible.   (Figure 5b). All spectra show a semi-circle at high frequencies and a straight line at medium to low frequency ranges. The latter is known to be related to the capacitive charging of the interface between the sample and the ion-blocking electrode. The semi-circle is attributed to a    (Figure 5b). All spectra show a semi-circle at high frequencies and a straight line at medium to low frequency ranges. The latter is known to be related to the capacitive charging of the interface between the sample and the ion-blocking electrode. The semi-circle is attributed to a relaxation process of charge carriers in the LATP sample. Figure 5 indicates that both the sintering temperature and the type of sintering aid affect the impedance of the samples.
Typically, the impedance spectra of ionic conducting ceramics exhibit two distinct features attributable to bulk and grain boundary impedance (c.f. Figure 6a) with typical capacitances of C b~1 0 −12 and C gb~1 0 −11 -10 −8 F/cm, respectively [27]. In this case, the impedance was successfully fitted by the equivalent circuit shown in Figure 6a. The bulk phase had a very low time constant (τ b = R b C b ) and the relaxation frequency of the corresponding semi-circle was located in the MHz range. For this reason, this semi-circle was not visible in our impedance spectra. Nevertheless, the impedance data fit the semi-circle related to the grain boundary very well (χ 2 /|Z| < 10 −3 ). Consequently, to fit the whole impedance spectra, a simplified equivalent circuit can be used (R b + R gb |Q gb + Q int ), with Q being a constant phase element. With this procedure, the specific capacitance of the bulk contribution could not be determined, but the resistance associated with bulk conductivity was estimated with sufficient accuracy. Figure 6b shows the total ionic conductivity of the LATP samples and the individual contributions of the bulk material and the grain boundaries calculated from the obtained R b and R gb values and plotted against the sintering temperature. The bulk conductivity increased slightly with increasing sintering temperature or relative density and was almost constant and close to the values reported for single crystals for samples with relative density > 90% exhibit similar grain boundary capacitance as for high temperature sintered pure LATP (see Figure 7b), which supports the accuracy of the impedance analysis. The conductivity of the grain boundaries, which limits the total ionic conductivity, increased strongly with higher sintering temperature for all samples containing sintering aids. The porosity of the samples decreased with increasing sintering temperature and had a positive impact on R gb . Comparing the results to those obtained on samples with relative density >90%, the highest conductivity of the grain boundaries was obtained with Li 2 CO 3 as the sintering additive.
Crystals 2020, 10, x FOR PEER REVIEW 6 of 12 relaxation process of charge carriers in the LATP sample. Figure 5 indicates that both the sintering temperature and the type of sintering aid affect the impedance of the samples. Typically, the impedance spectra of ionic conducting ceramics exhibit two distinct features attributable to bulk and grain boundary impedance (c.f. Figure 6a) with typical capacitances of Cb ~ 10 −12 and Cgb ~ 10 −11 − 10 −8 F/cm, respectively [27]. In this case, the impedance was successfully fitted by the equivalent circuit shown in Figure 6a. The bulk phase had a very low time constant ( = ) and the relaxation frequency of the corresponding semi-circle was located in the MHz range. For this reason, this semi-circle was not visible in our impedance spectra. Nevertheless, the impedance data fit the semi-circle related to the grain boundary very well (χ 2 /|Z| < 10 −3 ). Consequently, to fit the whole impedance spectra, a simplified equivalent circuit can be used (Rb + Rgb|Qgb + Qint), with Q being a constant phase element. With this procedure, the specific capacitance of the bulk contribution could not be determined, but the resistance associated with bulk conductivity was estimated with sufficient accuracy. Figure 6b shows the total ionic conductivity of the LATP samples and the individual contributions of the bulk material and the grain boundaries calculated from the obtained Rb and Rgb values and plotted against the sintering temperature. The bulk conductivity increased slightly with increasing sintering temperature or relative density and was almost constant and close to the values reported for single crystals for samples with relative density > 90% exhibit similar grain boundary capacitance as for high temperature sintered pure LATP (see Figure 7b), which supports the accuracy of the impedance analysis. The conductivity of the grain boundaries, which limits the total ionic conductivity, increased strongly with higher sintering temperature for all samples containing sintering aids. The porosity of the samples decreased with increasing sintering temperature and had a positive impact on Rgb. Comparing the results to those obtained on samples To achieve insight regarding the impact of sintering aids and temperature on the properties of the LATP samples, we analyzed the specific capacitance of the grain boundaries determined by fitting the impedance data. Figure 7a shows that specific capacitance of the grain boundaries clearly increased with the sintering temperature for samples prepared with sintering aids. Generally, the specific capacitance of the grain boundaries was strongly affected by microstructural properties, such as pores, grain sizes, the thickness of the intergranular regions, and the homogeneity of the grain boundary phase. Moreover, the relative permittivity of the grain boundary phase was affected by the composition, which might vary for the different sintering aids and temperatures. In the case of completely dense sintered materials, the capacitance was expected to increase for a narrowing of the intergranular regions [27]. Although this was not fully achieved in the present case, it was found that samples with Li 2 CO 3 (relative density > 90%) exhibited similar grain boundary capacitance as that of high temperature sintered pure LATP (see Figure 7b), which indicates that similar sintering quality can be achieved at temperatures as low as 800 • C by applying sintering aids. The impact of the different sintering aids was comparably strong for Li 2 CO 3 , Li 3 PO 4 , and Li 3 BO 3 , nevertheless, the Li 2 CO 3 seemed to have the most positive effect on the densification and the conductivity of the grain boundaries and bulk.
Without sintering additives, the Li-ion conductivity of the LATP reached a maximum value of 3.8 × 10 −4 S/cm after sintering at a temperature of 1080 • C (Figure 6b, Table 1). Comparable values Crystals 2020, 10, 408 7 of 11 were obtained for LATP with Li 3 PO 4 and Li 2 CO 3 sintered at much lower temperatures. For example, in the case of Li 2 CO 3 , a conductivity of 3.6 × 10 −4 S/cm was achieved for sintering at T = 840 • C. By comparison, the conductivities of samples prepared with Li 3 PO 4 as the sintering aid were slightly lower. The differences found for the different sintering aids correlated with differences observed in samples' microstructures ( Figure 8). In the case of LATP with Li 2 CO 3 as the sintering aid, the open porosity was progressively reduced with increased sintering temperature, whereby the sintering additive seemed to be located in the triple point between the grains or was dissolved in the LATP lattice. At a density of 96%, the microstructure was densified homogeneously with only slight amounts of residual pores between and inside the grains. In this final sintering stage, the Li 2 CO 3 seemed to be decomposed or dissolved in the structure and was not detectable by SEM-EDX. In the case of Li 3 PO 4 as the sintering aid, the microstructure with density of~93% also showed residual porosity but a higher amount of the additive in the triple points between the grains or glassy filled pores (Figure 9a). This local distribution of the sintering aid between the grains might cause the slightly decreased conductivity when compared to that of Li 2 CO 3 . Cross-sectional preparation of the LATP samples must be conducted under inert atmosphere or vacuum to avoid unwanted reactions, e.g., with moisture. Unfortunately, cross-sectional preparation by ion beam polishing does not etch the grain boundaries to visualize the microstructure more clearly (Figure 9). Therefore, the exact position of the sintering aids (grain boundaries or triple points) could not be elucidated in more detail in the current stage of development, but is the subject matter of ongoing work.
Crystals 2020, 10, x FOR PEER REVIEW 7 of 12 with relative density > 90%, the highest conductivity of the grain boundaries was obtained with Li2CO3 as the sintering additive. To achieve insight regarding the impact of sintering aids and temperature on the properties of the LATP samples, we analyzed the specific capacitance of the grain boundaries determined by fitting the impedance data. Figure 7a shows that specific capacitance of the grain boundaries clearly increased with the sintering temperature for samples prepared with sintering aids. Generally, the specific capacitance of the grain boundaries was strongly affected by microstructural properties, such as pores, grain sizes, the thickness of the intergranular regions, and the homogeneity of the grain boundary phase. Moreover, the relative permittivity of the grain boundary phase was affected by the  Without sintering additives, the Li-ion conductivity of the LATP reached a maximum value of 3.8 × 10 −4 S/cm after sintering at a temperature of 1080 °C (Figure 6b, Table 1). Comparable values were obtained for LATP with Li3PO4 and Li2CO3 sintered at much lower temperatures. For example, in the case of Li2CO3, a conductivity of 3.6 × 10 −4 S/cm was achieved for sintering at T = 840 °C. By comparison, the conductivities of samples prepared with Li3PO4 as the sintering aid were slightly lower. The differences found for the different sintering aids correlated with differences observed in samples' microstructures ( Figure 8). In the case of LATP with Li2CO3 as the sintering aid, the open porosity was progressively reduced with increased sintering temperature, whereby the sintering additive seemed to be located in the triple point between the grains or was dissolved in the LATP lattice. At a density of 96%, the microstructure was densified homogeneously with only slight amounts of residual pores between and inside the grains. In this final sintering stage, the Li2CO3 seemed to be decomposed or dissolved in the structure and was not detectable by SEM-EDX. In the case of Li3PO4 as the sintering aid, the microstructure with density of ~93% also showed residual porosity but a higher amount of the additive in the triple points between the grains or glassy filled pores (Figure 9a). This local distribution of the sintering aid between the grains might cause the slightly decreased conductivity when compared to that of Li2CO3. Cross-sectional preparation of the LATP samples must be conducted under inert atmosphere or vacuum to avoid unwanted reactions, e.g., with moisture. Unfortunately, cross-sectional preparation by ion beam polishing does not etch the grain boundaries to visualize the microstructure more clearly (Figure 9). Therefore, the exact position of the sintering aids (grain boundaries or triple points) could not be elucidated in more detail in the current stage of development, but is the subject matter of ongoing work. In Figure 10, the phase composition of the LATP ceramics without sintering aids and with Li3BO3, Li3PO4, and Li2CO3 addition analyzed by XRD is shown. The sample without sintering aids had a density of 89.8% and consisted of LATP and a small amount of rutile (TiO2). Whereas, the diffraction pattern of samples with sintering aids showed LATP as the main phase with small amounts of rutile, Li(Ti,Al)OPO4, and Li2P4O7 (only for Li3PO4 addition). In all cases, the sintering additives were not detected in the final sintered material, which suggests a solution and phase separation process during heat treatment. The solubility of the sintering additives seems not to be high enough, so that the Li(Al,Ti)OPO4 phase as the second crystalline phase in the Li2O-TiO2-P2O5 system with higher Li2O content was formed. The amount of the secondary phase seems to be low and cannot explain the difference of one order of magnitude in Li-ion conductivity between ceramics sintered with Li3BO3 and those with Li2CO3. The most reasonable explanation for this difference is the unfavorable composition of the grain boundary phase in the case of Li3BO3, whose composition In Figure 10, the phase composition of the LATP ceramics without sintering aids and with Li3BO3, Li3PO4, and Li2CO3 addition analyzed by XRD is shown. The sample without sintering aids had a density of 89.8% and consisted of LATP and a small amount of rutile (TiO2). Whereas, the diffraction pattern of samples with sintering aids showed LATP as the main phase with small amounts of rutile, Li(Ti,Al)OPO4, and Li2P4O7 (only for Li3PO4 addition). In all cases, the sintering additives were not detected in the final sintered material, which suggests a solution and phase separation process during heat treatment. The solubility of the sintering additives seems not to be high enough, so that the Li(Al,Ti)OPO4 phase as the second crystalline phase in the Li2O-TiO2-P2O5 system with higher Li2O content was formed. The amount of the secondary phase seems to be low In Figure 10, the phase composition of the LATP ceramics without sintering aids and with Li 3 BO 3 , Li 3 PO 4 , and Li 2 CO 3 addition analyzed by XRD is shown. The sample without sintering aids had a density of 89.8% and consisted of LATP and a small amount of rutile (TiO 2 ). Whereas, the diffraction pattern of samples with sintering aids showed LATP as the main phase with small amounts of rutile, Crystals 2020, 10, 408 9 of 11 Li(Ti,Al)OPO 4 , and Li 2 P 4 O 7 (only for Li 3 PO 4 addition). In all cases, the sintering additives were not detected in the final sintered material, which suggests a solution and phase separation process during heat treatment. The solubility of the sintering additives seems not to be high enough, so that the Li(Al,Ti)OPO 4 phase as the second crystalline phase in the Li 2 O-TiO 2 -P 2 O 5 system with higher Li 2 O content was formed. The amount of the secondary phase seems to be low and cannot explain the difference of one order of magnitude in Li-ion conductivity between ceramics sintered with Li 3 BO 3 and those with Li 2 CO 3 . The most reasonable explanation for this difference is the unfavorable composition of the grain boundary phase in the case of Li 3 BO 3 , whose composition however cannot be determined by XRD.
Li3BO3, Li3PO4, and Li2CO3 addition analyzed by XRD is shown. The sample without sintering aids had a density of 89.8% and consisted of LATP and a small amount of rutile (TiO2). Whereas, the diffraction pattern of samples with sintering aids showed LATP as the main phase with small amounts of rutile, Li(Ti,Al)OPO4, and Li2P4O7 (only for Li3PO4 addition). In all cases, the sintering additives were not detected in the final sintered material, which suggests a solution and phase separation process during heat treatment. The solubility of the sintering additives seems not to be high enough, so that the Li(Al,Ti)OPO4 phase as the second crystalline phase in the Li2O-TiO2-P2O5 system with higher Li2O content was formed. The amount of the secondary phase seems to be low and cannot explain the difference of one order of magnitude in Li-ion conductivity between ceramics sintered with Li3BO3 and those with Li2CO3. The most reasonable explanation for this difference is the unfavorable composition of the grain boundary phase in the case of Li3BO3, whose composition however cannot be determined by XRD.
Aono et al. [18] reported decreased sintering temperatures and improved conductivity for LiTi2(PO4)3 with Li3PO4 and Li3BO3 as sintering aids. The present systematic study revealed that in the case of LATP, the lowest sintering temperature was achieved by adding Li3BO3. However, the Liion conductivity (9.8 × 10 −5 S/cm) was reduced in comparison to the other sintering aids investigated. 20   Aono et al. [18] reported decreased sintering temperatures and improved conductivity for LiTi 2 (PO 4 ) 3 with Li 3 PO 4 and Li 3 BO 3 as sintering aids. The present systematic study revealed that in the case of LATP, the lowest sintering temperature was achieved by adding Li 3 BO 3 . However, the Li-ion conductivity (9.8 × 10 −5 S/cm) was reduced in comparison to the other sintering aids investigated.
In this study, the highest total Li-ion conductivity of 3.8 × 10 −4 S/cm for Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 sintered with Li 2 CO 3 was about one magnitude lower compared to 5 × 10 −3 S/cm of single crystals [14] with the same composition. The bulk conductivity of crystalline phase obtained from EIS (2 × 10 −3 S/cm, see Figure 6b) showed values close to those of single crystals. The microstructural and phase analyses suggest that for sintered samples with high density, the amount, composition, thickness, and distribution of grain boundary phase in the microstructure (i.e., at triple points or along the grains); secondary phases (rutile and Li(Ti,Al)OPO 4 phases); and residual porosity are the main factors influencing the ionic conductivity.

Conclusions
Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) solid electrolyte sintered at 1080 • C provided an excellent Li-ion conductivity of 3.8 × 10 −4 S/cm, which appears promising for the application in an all-solid-state composite cathode consisting of an active storage material, the solid electrolyte, and an electron conductor. However, co-sintering of these components required much lower temperatures to avoid chemical reactions that led to poor electrochemical performance. The sintering activity of Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) at low temperature can be improved significantly by adding sintering aids.
The sintering temperature, associated with the maximum shrinkage rate, ranged in the following order for adding 15 mol% of the respective sintering aids: Li 3 BO 3 (730 • C) < Li 2 CO 3 = Li 3 PO 4 (760 • C) < LiBO 2 (790 • C) < LiOH (810 • C) < LiCl (840 • C). Interestingly, the respective impact on the shrinkage behavior did not correlate with the melting point of the aids. Li 3 BO 3 had the most significant impact on the sintering temperature. The highest Li-ion conductivity of 3.6 × 10 −4 S/cm was achieved by adding Li 2 CO 3 as the sintering aid. In this case, the densification of the microstructure was improved even at temperatures as low as 840 • C, which was caused by the lowest grain boundary resistance observed in well-densified samples. The reason for low grain boundary resistance is the decomposition and evaporation or dissolution of the additive into the LATP structure in the final sintering stage. This leads to similar grain boundary resistance as that in material sintered without sintering aids. These novel insights concerning the reduction of the sintering temperature of LATP-based solid electrolytes are a first but important step towards co-sintered composite cathodes for all-solid-state Li-ion batteries.