Effect of Low-Cost Spodumene Sintering Aids on Electrical Properties for Li0.25La0.25NbO3 Ceramic

Abstract

In this work, a low-cost natural mineral material, spodumene is used as novel sintering aids for Li_0.25La_0.25NbO₃ ceramics. Li_0.25La_0.25NbO₃ (LLNO) with improved electrical properties are synthesized by the spodumene-assisted sintering via high temperature solid state method. The effects of spodumene sintering aids on structure, microstructure and electrical properties for LLNO solid electrolyte were investigated by using XRD, SEM and AC-impedance, respectively. LLNO sintered at 1000 °C exhibits a pure perovskite structure while the samples with added spodumene sintering aids contain some impurities. LLNO with 5 wt.% spodumene sintered at 1000 °C presents an ionic conductivity of 4.96 × 10^-5 S·cm^–1 at 25 °C with an activation energy of 0.32 eV. The conductivity of LLNO-5% spodumene sample is one times magnitude higher than that of pure LLNO. The results indicate that the introduction of spodumene successfully acts as a sintering aid for LLNO ceramic solid electrolytes.

Keywords:
Spodumene; solid electrolyte; electrical property; natural mineral material

1. Introduction

Spodumene (LiAl[Si₂O₆]) is a low-cost natural mineral material, which is used as additives for metallurgy, glass and ceramics¹. Spodumene can be used as a sintering aid to reduce ceramic sintering temperature, increase ceramic density, and thus improve ceramic properties²,³. Excessive sintering temperature is a huge challenge faced by oxide-type solid electrolytes for solid state lithium batteries. Co-sintering⁴ of solid electrolyte with electrodes is a solid-state battery manufacturing technology with development prospects. The manufacture temperature of traditional electrode for lithium battery is about 600~900 °C (such as LiMn₂O₄, LiFePO₄ cathode and Li₄Ti₅O₁₂ anode)⁵-⁷, as well as exceed 1100 °C for oxide-type solid electrolyte⁸. The high sintering temperature of solid electrolytes hinders the practical application of co-sintering technology in solid-state battery manufacturing⁹.

Li_0.25La_0.25NbO₃ ceramic is a typical solid electrolyte for solid lithium battery. LLNO ceramics exhibit promising ionic conductivity and stability, but their high sintering temperatures lead to excessive energy consumption, lithium volatilization, and increased grain boundary resistance. LLNO possesses the perovskite-type A-site-deficient structure, high intrinsic vacancies concentration and larger lattice volume. LLNO has the potential to achieve high lithium-ion conductivity. The grain conductivity of LLNO ceramic solid elelctrolyte is 10^-3 S·cm⁻¹ as high as at room temperature¹⁰.

The addition of sintering aids offers a viable pathway to simultaneously reduce sintering temperatures and enhance conductivity, particularly by mitigating grain boundary resistance¹¹-¹³. Liu et al.¹⁴ reported that the density of the electrolyte was improved after the addition of LiF. Specifically, at 500 °C, the addition of 4% LiF as a sintering flux resulted in a conductivity of 2.45 × 10^-3 S·cm^-1. At this point, the density and conductivity of the electrolyte were at their highest. Park et al.¹⁵ showed that Li₂CO₃ is an effective sintering aid in Ce_0.8Sm_0.2O_2−δ electrolytes. It improves the density of Ce_0.8Sm_0.2O_2-δ, thereby enhancing ionic conductivity and battery performance. Liu et al.¹⁶ synthesized a garnet-structured solid electrolyte Li_6.1Ga_0.3La₃Zr₂O₁₂ using a traditional solid-state method by introducing Al₂O₃ as a sintering aid. Adding a small amount of Al₂O₃ as a sintering aid reduced the sintering temperature from 1100 °C to approximately 1050 °C. At room temperature, the ionic conductivity can reach up to 1.28 × 10^-3 S·cm^-1. Spodumene presents a compelling alternative due to its ultra-low cost compared to synthetic sintering aids. Its efficacy in lowering sintering temperatures and improving properties in other ceramics²,³, combined with its intrinsic lithium content, suggests significant potential for enhancing LLNO processability and performance cost-effectively. However, The application specifically to LLNO has not been systematically investigated.

However, the total conductivty of LLNO ceramic is only 10^-5 S·cm^–1 at room temperature, which is almost two orders of magnitude lower than that of grain conductivity. So, there is a clear room for improvement of the total conductivity of LLNO ceramic solid electrolyte. Furthermore, the traditional sintering temperature of LLNO is exceed 1100 °C¹⁷. Such a high sintering temperature is unfavorable for the practical applications of LLNO ceramic solid electrolyte. Adding sintering aids is an effective method to reduce the sintering temperature of solid electrolytes and enhance their conductivity. The process of sintering is known to result in an improvement in the density of ceramic particles, thereby lowering the sintering temperature and enhancing electrical conductivity⁹,¹⁴. Currently, there have been limited studies on improving the performance of LLNO through the addition of sintering aids.

In this work,Li_0.25La_0.25NbO₃ ceramic solid electrolyte with different contents of spodumene as sintering aids were prepared by solid-state reaction method. The structure, microstructure and electrical properties of the LLNO ceramic solid electrolytes with or without spodumene sintering aids were investigated and compared by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and AC-impedance method, respectively. The effect of low-cost spodumene sintering aids on electrical properties for Li_0.25La_0.25NbO₃ solid electrolyte was disscused.

2. Materials and Methods

LLNO solid electrolytes were synthesized via a solid-state reaction method and then sintered at high temperature. The stoichiometric weighed of anhydrous La₂O₃ (99.9%, Sinopharm Chemical Reagent Co., Ltd), Li₂CO₃ (99.8%, Aladdin Chemical Reagent Co., Ltd), Nb₂O₅ (99.5%, Sinopharm Chemical Reagent Co., Ltd) and spodumene (Natural mineral powder, Fengfeng Shun stone powder factory) powder were used as the raw materials. The spodumene content was 0%, 1%, 3%, 5%, and 8%, and the corresponding ceramic samples were named LLNO-SP0, LLNO-SP1, LLNO-SP3, LLNO-SP5, and LLNO-SP8, respectively. 10 wt.% Li₂CO₃ powder were added into the raw materials to avoid the loss of lithium caused by the evaporation of Li₂O during high temperature synthesis. The raw materials were mixed by using ball-milling method for 8 hours. After ball-milling, the mixed powder was calcined at 900 °C for 12 h in air with an alumina crucible. After calcine, the powder was reground and pressed into pellets with 15 mm diameter at 10 MPa. Finally, all pellets were sintered at 1000-1100 °C for 12 h in air.

The phase composition of samples was obtained using X-ray diffraction (XRD) on a Bruker D8 device with Cu Kα radiation. The cross-section surface microstructure of pellets was characterized via scanning electron microscope (SEM) on a S-3000N device. The conductivity of the samples was determined by AC impedance spectroscopy (1 Hz~1 MHz, CHI660E) between room temperature and 100 °C. The electronic conductivity of samples were measured using an CHI660E electrochemical station device via DC-polarization method at room temperature. Before the electrical property characterization, the surface of samples was coated with soft graphite electrode as the ion-blocking electrode.

3. Results and Discussion

Figure 1 shows the XRD images of spodumene and ceramic samples. All solid electrolyte ceramic samples display the typical perovskite structure. The positions of the main diffraction peaks remain consistent. LLNO sintered at 1100 °C have a pure perovskite structure. Samples with added spodumene sintering aids at 1000 °C remain the perovskite structure but contain some impurities such as LiNbO₃, La₂O₃, and LaNbO₄. The characteristic peaks of spodumene disappeared in solid electrolyte ceramics. The main diffraction peaks of all samples remain in the same position, indicating that no doping occurred. The role of spodumene during sintering for LLNO ceramic is not dopants but sintering additives.

Figure 1
XRD pattern of ceramic pellets.

Figure 2 shows the SEM images of fracture surface for ceramic samples. The size of the sample was roughly measured with a ruler. Grain sizes of the ceramic pellets were determined to be 38 µm. All samples are relatively dense but same pores can be observed due to the loss of lithium caused by the evaporation of Li₂O during high temperature synthesis. The grain boundaries of LLNO samples are clear. There are obvious melting marks at the grain boundaries of LLNO-SP1, LLNO-SP3, LLNO-SP5 and LLNO-SP8 ceramics, which were caused by the assisting sintering effect of spodumene as sintering aids. This indicates that the introduction of spodumene acts as a sintering aid for ceramics. Wrinkle and crack were observed in all samples, which was caused by the thermal stress during rapid cooling process after sintering. The emergence and intensification of impurity peaks with increasing spodumene content are attributed to side reactions during sintering¹⁸. Spodumene (LiAlSi₂O₆) decomposes above 900 °C, releasing reactive Li₂O₃ and SiO₂-Al₂O₃ components. Li₂O + Li_0.25La_0.25NbO₃ → 2LiNbO₃ + La₂O₃ (lithium excess triggers LiNbO₃ precipitation), SiO₂ + La₂O₃ → La₂SiO₅ (silica consumes lanthanum, forming silicate intermediates), Al₂O₃ + Nb₂O₅ → LaNbO₄ (aluminum facilitates niobium oxide segregation). Higher spodumene loads amplify these reactions, leading to stronger impurity peaks in XRD. This aligns with observations in Al₂O₃-spodumene systems, where ≥5% additives induce secondary phases proportional to dosage. Spodumene undergoes partial melting above 1000 °C. Decomposed spodumene components react with LLNO or surface oxides, as confirmed by the emergence of LaNbO₄ and LiNbO₃.

Figure 2
SEM images of LLNO (a), LLNO-SP1 (b), LLNO-SP3 (c), LLNO-SP5 (d), LLNO-SP8 (e).

Figure 3(a) shows the AC-impedance spectra of LLNO while Figure 3(b) gives the AC-impedance spectra of LLNO-SP0, LLNO-SP1, LLNO-SP3, LLNO-SP5, and LLNO-SP8. The conductivity (σ) of samples were calculated by the following Formula 1.

$$\sigma =\frac{d}{R\times S}$$ (1)

where σ is the conductivity (S·cm^-1) of pellets, S is the surface area (cm²) and d is the thickness (cm) of ceramic pellet. The total resistance (R, Ω) of ceramic pellets is the intercept of semicircles as shown in Figure 3 on the real axis (Z´).

Figure 3
AC-impedance images of ceramic pellets.

According to the calculation results, LLNO without spodumene exhibits a low conductivity of 2.38×10^-6 S·cm^-1. The conductivity of samples increased after adding spodumene as sintering aids. LLNO-SP5 (5% spodumene) sintered at 1000 °C presents an ionic conductivity of 4.96 × 10^-5 S·cm^–1 at 25 °C. The conductivity of LLNO-5% spodumene sample is 10 times higher than that of pure LLNO sintered at 1100 °C without spodumene. All samples exhibit only one well-defined semicircle in the high-frequency region, indicating that grain boundary resistance is negligible compared to bulk resistance. This occurs because high-purity LLNO with relative density minimizes grain boundary scattering sites, merging grain and grain boundary contributions into a single semicircle. In Figure 3(a) and Figure 3(b), the intercept of the semicircle's left end on the Z´ axis defines R.

Figure 4 shows the temperature dependence of the conductivity (σ) of LLNO ceramics. The conductivity, electronic conductivity and activation energy of each sample were calculated and shown in Table 1. As can be seen, the activity energy deceased when spodumene was added as sintering additives. The activation energy of LLNO-SP5 is 0.32 eV, which was lower than pure LLNO. It is worth noting that the Arrhenius curve of LLNO-SP3 exhibits a scattered characteristic, which means that the conductivity activation energy of LLNO-SP3 does not extremely conform to the Arrhenius equation. These results indicate that adding spodumene is benefit to lithium conduction in LLNO ceramics. The relative density of LLNO increases and ion blockage interfaces caused by porosity are reduced when 5% spodumene is heated to 1000 °C¹⁹. The addition of sintering aid was inadequate, resulting in ineffectiveness. Conversely, an excess of combustion aid led to non-conductivity in the sample. Ying et al.²⁰ calculated that the conductance activation energy is directly related to the defect concentration and type introduced by the impurity. The sudden drop in the conductivity and Ea value of LLNO-SP8 could be caused by the sintering aids being added in excessive amounts²¹. When the amount of spodumene added exceeds a certain level, more impurities are produced through additional side reactions. As shown in Figure 1, excessive sintering aid affects the conductivity and Ea of the sample.

Figure 4
Arrhenius plots of ceramic samples.

Table 1
Conductivity and activation energy of the spodumene composite LLNO ceramics.

Sample	σ25°C/S·cm-1	σele/S·cm-1	Ea/eV
LLNO	2.38×10-6	3.69×10-9	0.42
LLNO-SP1	2.45×10-6	0.74×10-9	0.39
LLNO-SP3	8.96×10-7	2.95×10-9	non
LLNO-SP5	4.96×10-5	0.54×10-9	0.32
LLNO-SP8	3.81×10-5	0.90×10-9	0.38

Figure 5 shows the DC-polarization spectra of samples. The electronic conductivity of samples was calculated and listed in Table 1. As can be seen, the electronic conductivity calculated based on DC-polarization curves is about 4-orders lower than the corresponding ionic conductivity for all samples. This means that LLNO ceramics with spodumene sintering aids is still an ionic-conductor. Ionic conductivity arises from the migration of charged ions through lattice defects or grain boundaries. It dominates in solid electrolytes where ions are the primary charge carriers. Electronic conductivity involves the movement of electrons or holes through the material. While essential for electrodes, uncontrolled σ_e in electrolytes causes self-discharge and capacity loss in batteries by creating internal short circuits. A high σ_i/σ_e ratio (>10³) is crucial in solid-state electrolytes²². Our results meet this requirement, ensuring minimal electron leakage during operation. It is conceivable that fluctuations in the sodium feldspar composition of the samples may have precipitated the emergence of impurities. In the sample designated LLNO-SP5, the densification process that was found to be optimal resulted in the separation of the impurity phase, thereby reducing σ_e to a minimum²³. In the sample designated LLNO-SP8, the presence of excessive lithium feldspar resulted in the promotion of the formation of LaNbO₄. In this phase, the reduction of the aforementioned phase is exhibited through mixed ionic-electronic conductivity, which is evidenced by an increase in σ_e to 2.95×10^-9 S·cm^-1.

Figure 5
DC-polarization spectra of ceramic samples.

4. Conclusions

In conclusion, Li_0.25La_0.25NbO₃ ceramic solid electrolyte with different contents of spodumene as sintering aids were synthesized succesfully by solid-state reaction method. SEM images revealed distinct melting marks at the grain boundaries of ceramic pellets, attributed to the sintering-enhancing effect of spodumene. The sample LLNO-SP5 sintered at 1000 °C exhibited the best performance, achieving the highest ionic conductivity of 4.96 × 10^-5 S·cm^-1 at 25 °C, a significant increase from 2.38×10^-6 S·cm^-1 for the baseline material. Furthermore, LLNO-SP5 demonstrated a low activation energy of 0.32 eV. The electronic conductivity of LLNO-SP5 is four orders of magnitude lower than its ionic conductivity, demonstrating that LLNO-SP5 is a pure ionic conductor. Therefore, the natural spodumene mineral proves to be a feasible and effective sintering aid for enhancing the properties of LLNO solid electrolyte ceramics.

5. Acknowledgments

This work was supported by National & Local Joint Engineering Research Center for Mineral Salt Deep Utilization (SF202206), Foundation of Key Laboratory for Palygorskite Science and Applied Technology of Jiangsu Province (HPZ202201), The Natural Science Foundation of Jiangsu Higher Education Institutions of China (23KJB450001).

• Data Availability
  All data are provided within the manuscript.

6. References

• ¹ Lv S, Li M, Ni W, Tong S, Chen R, Li S, et al. Effect of spodumene particle size on properties of Al₂O₃-SiC-C castable. Refractories. 2023;57(4):337-42.
• ² Naga SM, Elmaghraby AA, Hassan AM. Mullite/ β-spodumene composites: preparation and characterization. Ceram Int. 2016;42(10):12161-6. http://doi.org/10.1016/j.ceramint.2016.04.150
  » http://doi.org/10.1016/j.ceramint.2016.04.150
• ³ Xie R, Zhu Y, Liu J, Li Y, Wang X, Shumin Z. Research status of spodumene flotation: A review. Miner Process Extr Metall Rev. 2021;42(5):321-34. http://doi.org/10.1080/08827508.2020.1776278
  » http://doi.org/10.1080/08827508.2020.1776278
• ⁴ Han F, Yue J, Chen C, Zhao N, Fan X, Ma Z, et al. Interphase engineering enabled all-ceramic lithium battery. Joule. 2018;987(73):497-508. http://doi.org/10.1016/j.joule.2018.02.007
  » http://doi.org/10.1016/j.joule.2018.02.007
• ⁵ Li J, Wang R. Recent advances in the interfacial stability, design and in situ characterization of garnet-type Li₇La₃Zr₂O₁₂ solid-state electrolytes based lithium metal batteries. Ceram Int. 2021;47(10):13280-90. http://doi.org/10.1016/j.ceramint.2021.02.034
  » http://doi.org/10.1016/j.ceramint.2021.02.034
• ⁶ Zhang N, Long X, Wang Z, Yu P, Han F, Fu J, et al. Mechanism study on the interfacial stability of a lithium garnet-type oxide electrolyte against cathode materials. ACS Appl Energy Mater. 2018;1(11):5968-76. http://doi.org/10.1021/acsaem.8b01035
  » http://doi.org/10.1021/acsaem.8b01035
• ⁷ Wu D, Chen L, Li H, Wu F. Solid-state lithium batteries-from fundamental research to industrial progress. Prog Mater Sci. 2023;139:101182. http://doi.org/10.1016/j.pmatsci.2023.101182
  » http://doi.org/10.1016/j.pmatsci.2023.101182
• ⁸ Liu Q, Zhao X, Yang Q, Hou L, Mu D, Tan G, et al. The progress in the electrolytes for solid state sodium-ion battery. Adv Energy Mater. 2023;8(7):2200822.
• ⁹ Yaroslavtsev AB. Solid electrolytes: main prospects of research and development. Russ Chem Rev. 2016;85(11):1255-76. http://doi.org/10.1070/RCR4634
  » http://doi.org/10.1070/RCR4634
• ¹⁰ Zheng J, Li Y, Yang R, Li G, Ding X. Lithium ion conductivity in the solid electrolytes (Li_0.25La_0.25)_1−xM_0.5xNbO₃ (M=Sr, Ba, Ca, x=0.125) with perovskite-type structure. Ceram Int. 2017;43(2):1716-21. http://doi.org/10.1016/j.ceramint.2016.08.144
  » http://doi.org/10.1016/j.ceramint.2016.08.144
• ¹¹ Zolhafizi J, Azmi MA, Rahman HA, Zakaria H, Hassan S, Mahzan S, et al. Samarium doped ceria (SDC) electrolyte modification by sintering aids addition to reducing sintering temperature: a review. Jurnal Kejuruteraan. 2023;35(1):65-76. http://doi.org/10.17576/jkukm-2023-35(1)-07
  » http://doi.org/10.17576/jkukm-2023-35(1)-07
• ¹² Zhang J, Han F, Li C, Zhang S. A-site deficient Sr_0.9Ti_0.3Fe_0.7O_3–δ perovskite: a high stable cobalt-free oxygen electrode material for solid oxide electrochemical cells with excellent electrocatalytic activity and CO₂ tolerance. J Eur Ceram Soc. 2022;42(13):5801-12. http://doi.org/10.1016/j.jeurceramsoc.2022.06.047
  » http://doi.org/10.1016/j.jeurceramsoc.2022.06.047
• ¹³ Zhang J, Lu X, Mao H, Tian Y, Zhang S. Effect of sintering additives on sintering behavior and conductivity of BaZr_0.1Ce_0.7Y_0.2O_3–δ electrolytes. J Inorg Mater. 2025;40(1):84-90. http://doi.org/10.15541/jim20240328
  » http://doi.org/10.15541/jim20240328
• ¹⁴ Liu Y, Zhao S, Wang L, Li Y, Zhou H, He Z, et al. Effect of LiF on the ion conductivity and sinterability of (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ solid electrolyte. J Alloys Compd. 2021;851:156337. http://doi.org/10.1016/j.jallcom.2020.156337
  » http://doi.org/10.1016/j.jallcom.2020.156337
• ¹⁵ Park G, Mo S, Kim J, Yun J. Characteristics of Li₂CO₃ as sintering aid for Ce_0.8Sm_0.2O_2−δ electrolyte in solid oxide fuel cells. Korean J Chem Eng. 2022;39(7):1796-804. http://doi.org/10.1007/s11814-022-1112-5
  » http://doi.org/10.1007/s11814-022-1112-5
• ¹⁶ Liu X, Jiang Y, Zhu XH. Preparation and characterization of LLZO-based solid electrolytes by Al₂O₃-assisted sintering. Adv Condens Matter Phys. 2022;11(1):1-11. http://doi.org/10.12677/CMP.2022.111001
  » http://doi.org/10.12677/CMP.2022.111001
• ¹⁷ Daugėla S, Kežionis A, Šalkus T, Orliukas AF, Belous AG, V’yunov OI, et al. Peculiarities of ionic conduction in Li_0.5-yNa_yLa_0.5Nb₂O₆ system at high temperatures. Solid State Ion. 2017;300:86-90. http://doi.org/10.1016/j.ssi.2016.12.011
  » http://doi.org/10.1016/j.ssi.2016.12.011
• ¹⁸ Ogiwara T, Noda Y, Kimura O. Low‐temperature sintering of β-spodumene ceramics using Li₂O-GeO₂ as a sintering additive. J Am Ceram Soc. 2013;96(8):2577-82. http://doi.org/10.1111/jace.12390
  » http://doi.org/10.1111/jace.12390
• ¹⁹ Cheng L, Chen W, Kunz M, Persson K, Tamura N, Chen G, et al. Effect of Sb-Ba codoping on the ionic conductivity of Li₇La₃Zr₂O₁₂ ceramic. Chem Mater. 2015;27(15):5495-504.
• ²⁰ Ying L, Jiang G, Zi T, Yu X, Zhong Z. Temperature-independent activation energy for ionic conduction of zirconia based solid electrolytes. Wuli Huaxue Xuebao. 2001;17(9):792-6. http://doi.org/10.3866/PKU.WHXB20010906
  » http://doi.org/10.3866/PKU.WHXB20010906
• ²¹ Yi E, Wang W, Kieffer J, Laine RM. Flame made nanoparticles permit processing of dense, flexible, Li⁺ conducting ceramic electrolyte thin films of cubic-Li₇La₃Zr₂O₁₂ (c-LLZO). J Mater Chem A Mater Energy Sustain. 2017;5(25):13147-59.
• ²² Thangadurai V, Narayanan S, Pinzaru D. Garnet-type solid-state fast Li ion conductors for Li batteries: critical review. Chem Soc Rev. 2014;43(13):4714-27. http://doi.org/10.1039/c4cs00020j
  » http://doi.org/10.1039/c4cs00020j
• ²³ Bruce PG, Freunberger SA, Hardwick LJ, Tarascon JM. Li–O₂ and Li–S batteries with high energy storage. Nat Mater. 2012;11(1):19-29. http://doi.org/10.1038/nmat3191
  » http://doi.org/10.1038/nmat3191

Edited by

• Associate Editor:
  Jose Eiras.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

All data are provided within the manuscript.

Publication Dates

• Publication in this collection
  20 Oct 2025
• Date of issue
  2025

History

• Received
  01 Apr 2025
• Reviewed
  23 June 2025
• Accepted
  14 Aug 2025