Preparation and Electrochemical Characterization of Y-Doped Li_1.3Al_0.3Ti_1.7(PO₄)₃ Solid Electrolytes for Lithium-Metal Batteries

Abstract

Lithium-conducting NASICON materials have emerged as a promising alternative to organic liquid electrolytes for high-energy-density Li-metal batteries, owing to their superior ionic conductivity and excellent air stability. However, their practical application is hindered by poor sintering characteristics and high grain boundary resistance. In this investigation, Li_1.3Al_0.3−xY_xTi_1.7(PO₄)₃ (LAYTP-x, x = 0.00, 0.01, 0.03, 0.05, and 0.07) were successfully synthesized via conventional solid-state reaction to explore the impact of Y³⁺ on both ionic conductivity and chemical stability. The structural, morphological, and transport properties of the samples were comprehensively characterized in order to identify the optimal doping concentration. All samples exhibited a NASICON structure with a uniform distribution of Y elements within the electrolyte. Due to its highest relative density (95.8%), the LAYTP-0.03 electrolyte demonstrated the highest total conductivity of 2.03 × 10⁻⁴ S cm⁻¹ with a relatively low activation energy of 0.33 eV, making it suitable for solid-state batteries. When paired with the NCM811 cathode, the Li/LAYTP-0.03/NCM811 cell exhibited outstanding electrochemical performance: a high capacity of 155 mAh/g was achieved at 0.2C after 50 cycles with a Coulombic efficiency of approximately 100%, indicating highly reversible lithium plating/stripping facilitated by the LAYTP-0.03 electrolyte. These results suggest that the LAYTP-0.03 ceramic electrolyte could be a promising alternative for developing safe solid-state Li-metal batteries.

1. Introduction

New energy electric vehicles are smarter and cheaper than fuel vehicles, but the battery is still a big problem, such as battery life, density, weight, price, and safety issues. The current power batteries are roughly the following types, which are ternary lithium batteries, lithium iron phosphate batteries, lithium cobalt acid batteries, nickel-metal hydride batteries, solid-state batteries, etc. [1,2]. Among them, the rapid advancement of portable electronics and stationary energy storage systems has established lithium-ion batteries (LIBs) as the predominant power source due to their high energy densities and low environmental impact [3,4,5,6]. However, the limited thermal stability of polyolefin membranes and flammable organic electrolytes hinder the large-scale development of lithium-ion batteries with high energy and safety standards [7,8]. Compared with liquid batteries, solid-state batteries (SSBs) have some advantages such as long driving range, low cost, high safety, and longer life. At present, the range of most liquid batteries is only between 400 km and 600 km, and the range of solid-state batteries can easily break through 1000 km, and the charging speed is more than three times faster than that of liquid batteries [9,10,11,12]. As one of the key materials of SSBs, solid-solid electrolytes (SSEs) demonstrated stable ion transport, a wide electrochemical window, and uniform lithium-ion deposition, thereby promoting battery system stability [13,14]. Furthermore, the solid characteristics of solid electrolytes effectively inhibit side reactions and dendrite growth in metal-negative electrodes while eliminating vaporization, flammability, and explosion risks associated with organic liquid electrolyte lithium-ion batteries, significantly enhancing overall battery safety [15].

With further development and optimization, solid-state electrolytes (SSEs) can be categorized into oxide, polymer, and sulfide electrolytes [14,16]. Among these, oxide-based ceramics have demonstrated high mechanical strength, wide electrochemical windows, and high ionic conductivity, making them easier to prepare on a large scale to meet the requirements of large-size batteries [5,17]. One typical oxide electrolyte is the NASICON-type Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) electrolyte, which has garnered widespread attention due to its stability under ambient conditions, low density, high ionic conductivity, and stability at high voltages when paired with cathodes such as Ni-rich NMC [18,19,20]. Despite LATP grains exhibiting a high Li⁺ conductivity of 10⁻⁴ S cm⁻¹, the overall ionic conductivity is still insufficient for the development and application of solid-state batteries [21]. Currently, various strategies have been employed to enhance the lithium-ion conductivity of LATP solid electrolyte for all-solid-state lithium batteries through methods such as adding sintering aids [22,23] or employing special sintering techniques [24,25,26] and elements doping [13,27,28].

Element doping is an effective method for enhancing ionic conductivity by adjusting lattice parameters or controlling bottleneck size [29]. Specifically, heteroatom doping with trivalent atoms can lead to the production of highly dense ceramics in LATP crystals. Yttrium (Y) doping has been found to alter the structural phase transition temperatures in various inorganic compounds by influencing structural distortion levels. For instance, Y³⁺ has been shown to stabilize the α phase of Li_1.15Y_0.15Zr_1.85(PO₄)₃ (LYZP), resulting in a significant increase in total Li-ion conductivity (7 × 10⁻⁵ S cm⁻¹) [30]. Additionally, the substitution of Al³⁺ with trivalent ions such as Y³⁺, Gd³⁺, and Sc³⁺ has been reported to improve the ionic conductivity of LATP by enhancing bond effects at grain boundaries and generating non-lithium conductive phases [31]. Using sol-gel-vacuum drying and solid-phase sintering, Zhao et al. [32] obtained an LAYTP system with excellent electrochemical properties, and the ionic conductivity was as high as 7.8 × 10⁻⁴ S cm⁻¹. In addition to aliovalent doping, microstructure, density, and grain size also play crucial roles in determining ionic conductivity. So, further research is needed to establish the relationships among density, grain size, and conductivity. In this study, we synthesized a series of Y-doped LATP samples Li_1.3Al_0.3−xY_xTi_1.7(PO₄)₃ (LAYTP-x, x = 0.00–0.07) using a traditional solid-state reaction method. A high relative density and excellent electrochemical properties were recorded, and the structure-property relationships and possible mechanism of Y doping in LATP are additionally discussed.

2. Experimental Details

Generally, the preparation process of ceramic solid electrolyte must first be weighed according to the experimental formula, and then through the mixing, roasting, secondary mixing, sintering, and other processes, and finally obtain the electrolyte powder or ceramic sheet. In this work, Li_1.3Al_0.3−xY_xTi_1.7(PO₄)₃ (LAYTP-x, x = 0.00, 0.01, 0.03, 0.05 and 0.07) solid-electrolyte ceramic electrolytes were prepared by conventional solid-state reaction method using Li₂CO₃ ( 99%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China), Al₂O₃ (99%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China), TiO₂ (99.8%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China), NH₄H₂PO₄ (99.95%, Shanghai McLean Chemical Reagent Co., Ltd, Shanghai, China), and Y₂O₃ (99.95%, Shanghai Sinopharm Chemical Reagent Co., Ltd., Shanghai, China). All raw materials were accurately weighed according to the stoichiometric ratio, of which 10 wt% Li₂CO₃ is weighed to make up for the loss of lithium during the sintering process. Then, the weighed raw material was transferred to the agate ball mill tank, and anhydrous ethanol was used as the solvent for ball milling at 300 r/min for 4 h. After drying, the mixed powders were calcined at 800 °C for 4 h. After calcination, the mixture was milled again for 3 h. After drying, a certain amount of LATP powder was weighed, and 5 wt% PVA was added, and the powder was pressed into a round sheet with a diameter of 15 mm and a thickness of about 1.2 mm by a tablet press. After that, the green ceramic disc was kept at 600 °C for 2 h to remove the added PVA impurities, and then was sintered at 950 °C for 6 h to obtain white regular LATP ceramic sheets. In addition, after polishing the LATP ceramic sheet, a layer of Ag electrodes was fired on each side for subsequent electrochemical testing.

The apparent density of the sintered pellets was obtained from the weight and the dimensions (diameter and thickness) measured using a screw gauge. The thickness was averaged from several measurements at different locations to consider possible inhomogeneity of the pellet shape. The crystal structure of the samples was determined by X-ray diffractometer (XRD) analysis (Bruker D8) using Cu radiation (λ: Kα₁ = 1.54060 Å, Kα₂ = 1.54440 Å). The surface morphologies of electrolytes were observed by using field emission scanning electron microscopy (FESEM) (SU-8010, HITACH Co., Ltd., Tokyo, Japan). Electrochemical impedance spectroscopy (EIS) of the electrolytes was measured on an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) from 10⁻¹ Hz to 10⁶ Hz. The ionic conductivity (σ, S·cm⁻¹) of the electrolyte was calculated using Equation (1): σ = L/(R_b × A), in which L (cm) is the thickness of the electrolytes determined by a digital micrometer, the area of the electrode (cm²), and R_b the value of bulk resistance corresponding to the intercept of low-frequency signal in the Nyquist plot with real axis by EIS, respectively.

NCM811 (NCM811 = LiNi_0.8Co_0.1Mn_0.1O₂) cathodes and Li metal anodes were employed to assemble the Li–NCM811 batteries. NCM811 as the active material, Super P (carbon black) as the conducting agent, and poly(vinylidene fluoride) (PVDF–LiTFSI) as the binder were added to N-methylpyrrolidone (NMP) at a ratio of 8:1:1. The active material load was controlled to 2–3 mg/cm². The Li/LAYTP-0.03/NCM811 are assembled in an Ar-filled glove box (O² < 5 ppm, H₂O < 5 ppm), where LAYTP-0.03 pellet acts as the separator and electrolyte. The Celgard 2400 soaked by a liquid electrolyte (~5 μL 1M LiPF₆/EC-DMC(1:1)) was sandwiched between LATP pellet and Li metal to prevent the reaction between Li and LATP in CR2032.

3. Results and Discussion

Figure 1a depicts the XRD patterns of LAYTP-x (x = 0.00, 0.01, 0.03, 0.05, 0.07) ceramic pellets sintered at 950 °C. It is evident from the data that the diffraction peaks of all samples correspond to NASICON-type LTP (PDF#35–0754) [23,33]. The peak positions closely align with theoretical values, exhibiting only minor shifts. Furthermore, the peak intensity and width remain consistent across all samples, indicating that the addition of Y³⁺ does not impact the structural composition of LATP and Nasicon-structure samples have been successfully synthesized. Impurity phases such as LiTiPO₅ are observed in the XRD patterns, likely resulting from either LATP phase decomposition during sintering [23,34,35,36]. In addition, other small impurity peaks could belong to YPO₄, and the amount of this impurity increases with the increase in the addition of Y³⁺. The formation of YPO₄ in the Y doping LATP systems can be due to the fact that Y cannot fully enter the LATP lattice because the ionic radii of Y³⁺ (0.93 Å) are larger than that of Al³⁺ (0.535 Å) as well as Ti⁴⁺ (0.60 Å). Similar phenomena have been reported in other literature [32,37,38]. To further validate the crystallographic evolution of LATP-x ceramics, lattice parameters were calculated and presented in Figure 1b. Notably, it was observed that lattice parameters (a) and (c) varied with increasing x values; this suggests that Y enters the LATP lattice and reduces lattice distortion in LATP-based ceramics—an effect attributed to Al³⁺ replacement by Y³⁺, potentially influencing electrical properties accordingly.

Figure 2a–f display SEM images and relative density histograms of LATP and LAYTP-x (x = 0.01, 0.03, 0.05, 0.07). With the increase in the Y³⁺ additive, the surface SEM image reveals a denser grain structure with more rectangular grains for the LAYTP-x samples. The grain size initially increased and then exhibited more regular growth, leading to a reduction in grain boundaries and defects, ultimately enhancing the density and electrochemical performance of LATP ceramics [39,40]. As depicted in Figure 2f and

Table 1. Lattice parameters a and c, relative density (ρ_rd), total conductivity (σ_total), and activation energy (E_a) of Li_1.3Al_0.3−xY_xTi_1.7(PO₄)₃ ceramic solid electrolytes.

Scheme 1.	Lattice Parameter a (Å)	Lattice Parameter c (Å)	ρrd (%)	σtotal (S·cm−1)	Ea (eV)
LATP	8.5006	20.6805	92.3	1.08 × 10−4	0.38
LAYTP-0.01	8.4887	20.6966	93.3	1.48 × 10−4	0.30
LAYTP-0.03	8.4913	20.5412	95.8	2.03 × 10−4	0.33
LAYTP-0.05	8.4921	20.7505	92.5	1.91 × 10−4	0.35
LAYTP-0.07	8.4908	20.6012	93	0.77 × 10−4	0.41

, the relative density of LAYTP increases from 90.3% for LATP to 95.8% for LAYTP-0.03 due to larger grain size; however, further increase in Y³⁺ content (x = 0.05 and 0.07) resulted in a slight decrease in relative density to 92.5% and 93%, possibly attributed to excessive formation of secondary LiTiPO₅ or YPO₄ phases across grain boundaries. Despite this decline, as indicated by results presented in Table 1, all samples exhibited high relative density ρ_rd (>92%), suggesting that the LAYTP-x ceramics were well sintered, which is a prerequisite to achieving high conductivity because tightly connected grains support the continuous transport path for lithium ions. The energy-dispersive X-ray spectroscopy (EDS) elemental mapping results of each element in LAYTP-0.03 are shown in Figure 2(c1–c4), revealing a uniform distribution of Y elements within the electrolyte area.

Figure 3a presents the Nyquist plots obtained from EIS measurements to verify the resistance of LAYTP-x pellets in relation to the formation of grain boundaries and crystallinity. The impedance spectra selected are representative of the prepared materials, displaying one semicircle and one tail in the high- and low-frequency spectrum regions, which were effectively fitted by the equivalent circuit depicted in Figure 3a. The straight line is accurately described by a constant phase element (CPE), reflecting charge accumulation on blocking electrodes [13]. CPE is commonly utilized instead of a capacitor due to factors such as electrode porosity and rough interfaces between electrodes and ceramic electrolytes [23,41]. The semicircle corresponds to grain boundary resistance (R_gb) and can be characterized by a parallel connection of resistor R_gb and constant phase element CPE_gb. The bulk resistance (R_b) is determined by the distance between the origin of the axes and the intercept with the Z axis at high frequencies [23,35]. A comparison of measured resistance values revealed that R_gb was approximately 10 times higher than R_b. This implies that manipulation of the microstructure has the potential to enhance overall conductivity, for example, by increasing grain size and minimizing the number of grain boundaries. The low-frequency intercept of the semicircle with the Z axis represents total resistance (R_t). It is noteworthy that the total resistance of the LATP solid electrolytes decreased significantly with the addition of Y³⁺ from 0 to 0.03 and then slightly increased from 0.03 to 0.07. The lowest total resistance was 7.28 × 10² Ω in LAYTP-0.03 due to its high relative density ρ_rd (95.8%).

Figure 3b and Table 1 present the data on ionic conductivity for LAYTP-x pellets. According to the data, the mathematical model of ion conductivity and Y ion doping amount can be concluded as σ_T = −0.0098x³ + 0.0617x² − 0.0735x + 0.1293. Specifically, total ionic conductivity increased at x = 0.01–0.03 and then decreased with Y³⁺ addition, reaching a peak at an addition amount of Y³⁺ at 0.03, resulting in an ionic conductivity value of 2.03 × 10⁻⁴ S cm⁻¹, which is about 1.87 times as high as that of un-doped electrolyte and even comparable to that reported by many other groups (

Table 2. Comparison of the total conductivities for LATP electrolytes, measured at room temperature.

Methods	Sintering Temperature	Relative Density (%)	σ (S/cm)	Activation Energy Ea (eV)	Reference
Melt-quenching method	/	/	1.67 × 10−6 (30 °C)	0.24	[42]
Rapid thermal annealing	750	/	2.7 × 10−6	0.431	[43]
Modified solid-state method	850	95.2	7.8 × 10−4	0.17	[32]
Solid state method	/	91.8	5.06 × 10−7 (30 °C)	0.32	[44]
Solution method	800	92.5	1.21 × 10−3	0.26	[45]
Sol-gel route	1000	86~90	(3–4) × 10−4	0.21	[46]
Solid state method	950	95.8	2.03 × 10−4	0.33	Present work

), indicating the LAYTP-0.03 could prove a most advantageous in terms of electrochemical performance. The increase in relative density led to a twofold increase in conductivity compared to unmodified LATP solid electrolytes. Therefore, the sintering of LATP was significantly improved by adding Y³⁺, which enhanced its relative density and consequently its ionic conductivity. Furthermore, as an impurity with extremely low ionic conductivity, LiTiPO₅, and YPO₄ will hinder lithium-ion transmission across grain boundaries and thus induce grain boundary ionic resistivity [38]. On the basis of the above discussions, we conclude the following: (i) ions are successfully added to the LATP sample lattice; (ii) an appropriate amount of Y ion doping (0.03) can increase the grain size, and a proper amount of impurity phases (LiTiPO₅ or YPO₄) is necessary to increase the ceramic density, thereby increasing the ionic conductivity; (iii) as the non-lithium-ion conductor, the excessive interfacial phases (0.05–0.07) not only inhibited the growth of grain, but also were not conducive to ion conduction, thus reducing the ionic conductivity of ceramic electrolyte.

The electrochemical impedance spectroscopy (EIS) curves of LAYTP electrolyte pellets were examined over a temperature range of 25 to 70 °C. The activation energy (E_a) is a crucial property of LATP electrolytes, reflecting the lithium-ion migration rate and free sites, and can be determined using the Arrhenius Equation (2): σ_total = Aexp (−E_a/kT), where A is the pre-exponential factor, k is the Boltzmann constant, and T is the ambient temperature [35,47]. The temperature dependence of ionic conductivity and corresponding activation energy values calculated from the Arrhenius plots for pristine and doped samples are depicted in Figure 3c. For LATP, LATP-0.01, LATP-0.03, LATP-0.05, and LATP-0.07, the activation energy barriers are computed as 0.38 eV, 0.30 eV, 0.33 eV, 0.35 eV, and 0.41 eV, respectively (Table 1). These findings indicate that optimal levels of Y³⁺ doping result in enhanced ion conductivity with lower activation energies; suggesting that lower energy barriers facilitate improved charge ion diffusion rates [13].

In order to demonstrate the feasibility of the LAYTP electrolyte, Li/LAYTP-0.03/NCM811 solid-state batteries were assembled. A schematic of the solid-state batteries is shown in Figure 4a, with LAYTP-0.03 serving as both electrolyte and separator. The cell exhibits discharge-specific capacities of 155, 130, 112, and 94 mAh g⁻¹ at current densities of 0.1, 0.3, 0.5, and 1C, respectively (Figure 4b). When the current density is reduced to 0.2C, the average reversible capacity can be restored to about 150 mAh g⁻¹, demonstrating excellent rate capability. The high capacity of 155 mAh g⁻¹ achieved at a current density of 0.2C after 50 cycles at a temperature of 25 °C indicates superior electrochemical performance due to the high ion conductivity (Figure 4c). These results indicate that the introduction of Y³⁺ effectively inhibits the reaction between the active materials. Moreover, the corresponding Coulombic efficiencies of the Li/LAYTP-0.03/NCM811 cell approach 100% during 50 cycles, revealing a highly reversible lithium plating/stripping enabled by the LAYTP-0.03 electrolyte. Figure 4d described the smooth charge-discharge profiles of Li/LAYTP-0.03/NCM811 cells at different cycles, illustrating that no side reactions occurred in the cell [48]. Such an outstanding electrochemical performance verifies that the NCM811 cathode and lithium metal have good compatibility with the LAYTP-0.03 ceramic electrolyte, which proves the possibility of the LAYTP-0.03 ceramic electrolyte applied in lithium-metal batteries.

4. Conclusions

In this work, we present the successful fabrication of enhanced performance Li_1.3Al_0.3−xY_xTi_1.7(PO₄)₃ (LAYTP-x, x = 0~0.07) solid electrolyte by solid-state synthesis. The NASICON structure of LATP is well maintained in LAYTP-x. With Y³⁺ substitution increasing, the grain size of LAYTP-0.03 generally exhibits an upward trend and Y elements are uniformly distributed in the electrolyte. The highest total ion conductivity of 2.03 × 10⁻⁴ S·cm⁻¹ was obtained in LAYTP-0.03, with a relative density of 95.8%, and activation energy as low as 0.33 eV for the total conductivity. The discharge capacity and capacity retention rate of Li/LAYTP-0.03/NCM811 are higher: a stable cycling performance with a discharge capacity of 155 mAh·g⁻¹ and a Coulombic efficiency of approximately 100% was obtained at 0.2C for the full cell fabricated with LATP ceramics. These results indicate that anion doping strategies can create dense conductive interfaces, providing inspiration for researchers to solve the problem of low grain boundary conductivity and promoting the development of solid-state batteries. Finally, the next focus of research should focus on the interface problem between electrolyte and electrode, and the main methods include electrolyte interface modification, adding polymer electrolyte excess layer, and so on.