A method for improving the performance of an oxide solid-state electrolyte by calcium doping

Abstract

The application discloses a method for improving the performance of oxide solid electrolyte by calcium doping, and belongs to the technical field of solid-state batteries. The method comprises the following steps: mixing raw materials containing a calcium source, a sodium source, a rare earth source and a phosphorus source to obtain a precursor; sintering the precursor to obtain a calcium-doped sodium superionic conductor precursor; and performing ion exchange on the precursor and a lithium salt to obtain a calcium-doped oxide solid electrolyte with a chemical formula of Li 3+ x La 1‑x Ca x (PO4)2, and 0 < x <= 0.1. By partially replacing lanthanum ions with calcium ions, lithium vacancies are introduced, the crystal lattice is relaxed, and the lithium ion migration path is optimized by using the charge compensation mechanism, so that the room-temperature ionic conductivity of the material is significantly improved. The method has low raw material cost, simple process and strong universality, can be expanded to various oxide solid electrolyte systems such as NASICON and garnet, and has good application prospect in the field of solid-state lithium ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of solid-state battery technology, specifically relating to a method for improving the ionic conductivity of oxide solid electrolytes through calcium doping, and more particularly to a method for preparing calcium-doped modified phosphate system oxide solid electrolytes. Background Technology

[0002] With the rapid development of portable electronic devices, electric vehicles, and energy storage systems, the demand for batteries with high safety and high energy density is increasing. Traditional liquid lithium-ion batteries have safety hazards such as flammability, leakage, and thermal runaway, while solid-state batteries are considered an important direction for the development of next-generation battery technology due to their high safety, long cycle life, and high energy density.

[0003] In solid-state batteries, the solid electrolyte is one of the core components. Oxide solid electrolytes have become a research hotspot due to their high ionic conductivity, good electrochemical stability, and excellent mechanical strength. For example, garnet-type (such as Li7La3Zr2O) 12 Lithium oxide (LLZO) and sodium superionic conductor (NASICON) type oxide electrolytes have been extensively studied. However, these materials still have some problems: the oxide crystal structure is dense and the bond energy is strong, making Li... + In the crystal lattice, lithium can only migrate through interstitial / vacancy sites, resulting in a high transition barrier. At the same time, the intrinsic concentration of lithium vacancies and interstitial lithium in pure phase oxides is low, which leads to low intrinsic ionic conductivity and affects its electrochemical performance.

[0004] To address the aforementioned problems, existing technologies often employ doping modification strategies, such as using Al... 3+ Ga 3+ Ta 5+ Doping with elements such as calcium (Ca) can improve ionic conductivity. However, these doping elements are costly or have a significant impact on the structural stability of the material. 2 + As a cheap and environmentally friendly divalent metal ion, its ionic radius is similar to that of La. 3+ The doping properties of calcium are close to those of lithium, and thus offer potential advantages. Therefore, developing a low-cost, efficient, and tunable calcium doping strategy to improve the overall performance of the novel oxide solid electrolyte Li3La(PO4)2 (LLPO) is of significant practical importance. Summary of the Invention

[0005] The purpose of this invention is to provide a method for improving the ionic conductivity of oxide solid electrolytes by calcium doping. By introducing calcium ions to lattice dope the oxide solid electrolyte, its crystal structure, ion migration path and interface characteristics are controlled, thereby significantly improving the ionic conductivity of the material.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A method for improving the ionic conductivity of oxide solid electrolytes by calcium doping includes the following steps:

[0008] The precursor is obtained by mixing raw materials containing calcium source, sodium source, rare earth source and phosphorus source;

[0009] The precursor is sintered to obtain a calcium-doped sodium superionic conductor precursor.

[0010] The calcium-doped sodium superionic conductor precursor was subjected to ion exchange with a lithium salt to obtain Li 3+ x La 1-x Ca x (PO4)2 calcium-doped oxide solid electrolyte; where x satisfies 0 < x ≤ 0.1.

[0011] The reaction mechanism of the method described in this invention is as follows: calcium ions (Ca 2+ ) partially replaces the lanthanum ions (La) located at the octahedral positions in the LLPO lattice. 3+ ), due to Ca 2+ Radius (approximately 1.00 Å) and La 3+ With similar radii (approximately 1.06 Å) and lower valence states, substitution introduces additional lithium vacancies or alters the local lithium-ion potential energy distribution in the crystal lattice. Specifically, each Ca... 2+ Replace a La 3+ To maintain charge balance, an additional lithium-ion vacancy is created in the crystal lattice, or some lithium ions migrate from interstitial sites to vacancy, thus effectively reducing the activation energy for lithium-ion migration. Meanwhile, Ca... 2+ The introduction of [something] slightly perturbs the lattice parameters, widens the migration channels of lithium ions, increases the transition frequency of lithium ions along one-dimensional or three-dimensional paths, and ultimately significantly improves the room temperature ionic conductivity.

[0012] In the preferred technical solution, the value of x ranges from 0.01 to 0.1, is more preferably from 0.03 to 0.07, and is most preferably 0.05.

[0013] The sintering temperature is 900°C to 1200°C, and the sintering time is 6 to 12 hours.

[0014] The calcium source is selected from at least one of calcium carbonate (CaCO3), calcium nitrate (Ca(NO3)2·4H2O), or calcium chloride (CaCl2); the sodium source is selected from at least one of sodium carbonate (Na2CO3) or sodium hydroxide (NaOH); the rare earth source is lanthanum oxide (La2O3); the phosphorus source is selected from at least one of ammonium dihydrogen phosphate (NH4H2PO4) or diammonium hydrogen phosphate ((NH4)2HPO4); and the lithium salt is selected from at least one of lithium bis(trifluoromethanesulfonylimide) (LiTFSI) or lithium hydroxide (LiOH).

[0015] Preferably, the precursor is subjected to ball milling before sintering, wherein the ball milling is performed using wet ball milling and the ball milling medium is anhydrous ethanol.

[0016] The sintering is carried out in an air atmosphere or an inert atmosphere, preferably an argon atmosphere.

[0017] The ion exchange process further includes steps of washing and drying the product, wherein the washing is performed using anhydrous ethanol.

[0018] The method of the present invention is also applicable to the preparation of NASICON-type oxide solid electrolytes or garnet-type oxide solid electrolytes.

[0019] Beneficial effects

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. Significantly improves ionic conductivity: through calcium ions (Ca... 2+ For La in the oxide solid electrolyte LLPO 3+ Partial substitution utilizes a charge compensation mechanism to introduce lithium vacancies, induce lattice relaxation, and optimize the lithium-ion migration path, effectively reducing the lithium-ion transition activation energy. As shown in Example 1, when the doping amount x = 0.05, the room temperature ionic conductivity decreases from 2.6 × 10⁻⁵ for the undoped sample. -4 S / cm increased to 1.3×10 -3 S / cm increased by approximately 5 times, reaching 10 -3 The S / cm range meets the practical application requirements of solid-state batteries.

[0022] 2. Low raw material cost and environmentally friendly: The calcium source used in this invention (such as calcium carbonate, calcium nitrate, etc.) is widely available and inexpensive, far lower than the Al commonly used in existing technologies. 3+ Ga 3+ Ta 5+ The raw material costs corresponding to the doping elements are also considered. Furthermore, calcium is non-toxic, and the preparation process does not produce harmful byproducts, meeting the requirements of green chemistry and sustainable development.

[0023] 3. Simple process and easy to scale up production: The method of this invention only requires conventional ball milling, high-temperature sintering and ion exchange steps, without the need for special equipment or harsh conditions (such as high pressure, vacuum, protective atmosphere, etc. are optional but not necessary). The process flow is short and easy to operate, and it is easy to realize the scale-up transformation from laboratory to industrial production.

[0024] 4. Strong applicability and can be extended to a variety of oxide systems: This invention is not only applicable to the LLPO system, as verified in Example 3, but can also be successfully applied to other typical oxide solid electrolyte systems such as NASICON type (such as LATP) and garnet type (such as LLZO). All of them show a significant improvement in ionic conductivity and maintenance of structural stability, indicating that this calcium doping strategy has good applicability and promotion value.

[0025] 5. Maintain the integrity of the crystal structure: as shown in the XRD results of Example 1 ( Figure 1 As shown in the figure, the crystal structure of LLPO material did not change after calcium doping, and no impurity phase was generated, which ensured the electrochemical stability and long-term cycle reliability of the electrolyte material in the battery. Attached Figure Description

[0026] Figure 1 The X-ray diffraction (XRD) patterns of LLPO materials before and after calcium doping show that calcium doping did not change the crystal structure of LLPO and no impurity phases were generated.

[0027] Figure 2 The electrochemical impedance spectroscopy (EIS) spectra of LLPO materials before and after calcium doping show that the impedance arc radius of the material is significantly reduced after calcium doping, indicating a significant increase in ionic conductivity. Detailed Implementation Plan

[0028] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The described examples are merely some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1 (LLPO system, x=0.05)

[0030] This embodiment provides a method for improving the performance of LLPO solid electrolyte through calcium doping, and the specific steps are as follows:

[0031] (1) According to the chemical formula Na 3.05 La 0.95 Ca 0.05 Weigh the raw materials for (PO4)2:

[0032] Sodium carbonate (Na2CO3): 1.525 mol (since 1 mol of Na2CO3 provides 2 mol of Na, 3.05 mol of Na requires 1.525 mol of Na2CO3);

[0033] Lanthanum oxide (La₂O₃): 0.475 mol (since 1 mol of La₂O₃ provides 2 mol of La, 0.95 mol of La requires 0.475 mol of La₂O₃);

[0034] Calcium carbonate (CaCO3): 0.05 mol;

[0035] Ammonium dihydrogen phosphate (NH4H2PO4): 2 mol.

[0036] (2) Place the above raw materials in a ball mill jar, add anhydrous ethanol as the ball milling medium, and perform wet ball milling for 5 hours to obtain uniform precursor powder.

[0037] (3) The precursor powder was sintered at 1200℃ in air for 10 hours, and then naturally cooled to obtain a calcium-doped sodium superionic conductor precursor with the chemical formula Na. 3.05 La 0.95 Ca 0.05 (PO4)2 (abbreviated as Ca-NLPO).

[0038] (4) The above Ca-NLPO ceramic powder was placed in a 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solution and stirred at 60°C for 12 hours for sodium-lithium ion exchange. After the exchange was completed, the powder was filtered, washed three times with anhydrous ethanol by centrifugation, and dried at 80°C for 12 hours to obtain a calcium-doped oxide solid electrolyte material with the chemical formula Li 3.05 La 0.95 Ca 0.05 (PO4)2.

[0039] (5) The obtained solid electrolyte material is ground into powder and pressed into a sheet sample with a diameter of 10 mm and a thickness of about 1 mm under a pressure of 200 MPa. The sample is sintered at 800℃ for 2 hours, and then gold electrodes are sputtered on both sides of the sheet sample to perform electrochemical performance testing.

[0040] Results analysis:

[0041] like Figure 1 The XRD pattern shows that the diffraction peak positions of the calcium-doped sample (Ca-LLPO) are consistent with those of the undoped LLPO standard card, and no impurity peaks are observed, indicating that calcium ions were successfully incorporated into the lattice without destroying the monoclinic phase structure of LLPO.

[0042] like Figure 2As shown in the EIS plot, the impedance arc radius of the calcium-doped sample is much smaller than that of the undoped sample, and the calculated ionic conductivity at room temperature (25℃) is 1.3 × 10⁻⁶. -3 The ionic conductivity was S / cm, while the ionic conductivity of the undoped control sample was approximately 2.6 × 10⁻⁶. -4 The ionic conductivity increased by approximately 5 times (S / cm).

[0043] Example 2 (Comparison of different doping levels)

[0044] Using essentially the same method as in Example 1, only adjusting the amount of calcium carbonate added, samples with x = 0.01, 0.03, 0.07, and 0.10 (i.e., chemical formula Li) were prepared. 3+x La 1-x Ca x (PO4)2 with x values ​​of 0.01, 0.03, 0.07, and 0.10 respectively, and an undoped sample (x=0) was prepared as a control.

[0045] The room temperature ionic conductivity and XRD structure analysis of each sample were performed, and the results are shown in the table below:

[0046]

[0047] The results show that when x = 0.05, the ionic conductivity reaches its maximum value, and the crystal structure remains pure phase. When x exceeds 0.07, excess calcium cannot completely enter the crystal lattice and forms an impurity phase, resulting in a decrease in ionic conductivity. Therefore, the preferred calcium doping amount x is 0.03~0.07, with 0.05 being optimal.

[0048] Example 3 (Method extended to other oxide systems)

[0049] This embodiment verifies the applicability of the method of the present invention in other oxide solid electrolyte systems.

[0050] (1) Example of NASICON type system

[0051] The calcium doping strategy of this invention is applied to the NASICON-type solid electrolyte Li 1.3 Al 0.3 Ti 1.7 (PO4)3 (LATP). According to the chemical formula Li... 1.3+x A l0.3 Ti 1.7-x Ca x(PO4)3 (taking x=0.05 as an example) raw materials are weighed: lithium carbonate (Li2CO3), alumina (Al2O3), titanium dioxide (TiO2), calcium carbonate (CaCO3), and ammonium dihydrogen phosphate (NH4H2PO4). Using a ball milling and sintering process (950℃ / 8h) similar to Example 1, a calcium-doped NASICON-type solid electrolyte is directly obtained (this system does not require subsequent ion exchange). Test results show that the room temperature ionic conductivity after calcium doping increases from 3.5 × 10⁻⁶ to 3.5 × 10⁻⁶. -4 S / cm increased to 8.1×10 -4 S / cm. 3

[0052] (2) Examples of garnet-type systems

[0053] The calcium doping strategy of this invention is applied to the garnet-type solid electrolyte Li7La3Zr2O. 12 (LLZO). To maintain charge balance, each Ca... 2+ Replace a La 3+ One Li needs to be reduced. + The chemical formula is corrected to Li 7-x La 3-x Ca x Zr2O 12 (Taking x=0.05 as an example, i.e., Li) 6.95 La 2.95 Ca 0.05 Zr2O 12 Raw materials were weighed: lithium carbonate (Li₂CO₃, 10% excess to compensate for high-temperature volatilization), lanthanum oxide (La₂O₃), zirconium oxide (ZrO₂), and calcium carbonate (CaCO₃). After mixing using high-energy ball milling, the mixture was sintered in air at 1100℃ for 12 hours to obtain calcium-doped LLZO. XRD showed that the sample maintained a cubic phase structure, and the room-temperature ionic conductivity reached 4.2 × 10⁻⁶. -4 S / cm, compared to the undoped sample (2.5×10 -4 The S / cm ratio increased by approximately 70%.

[0054] The above results show that the calcium doping method proposed in this invention can be effectively extended to a variety of oxide solid electrolyte systems and has good universality.

[0055] Explanation of the reaction mechanism

[0056] Based on the results of Examples 1 to 3, the present invention proposes the following mechanism for calcium doping to improve the ionic conductivity of oxide solid electrolytes:

[0057] Charge compensation mechanism: divalent Ca 2+ Alternatives to trivalent La 3+After the addition of (or other trivalent cations), the positive charge in the crystal lattice decreases. To maintain electroneutrality, lithium ion vacancies will be spontaneously generated within the material (for LLPO and NASICON systems) or the number of lithium ions will be reduced (for LLZO systems). The increase in lithium vacancies significantly lowers the activation energy required for lithium ion transitions, thereby increasing the ion migration rate.

[0058] Lattice relaxation effect: Ca 2+ The ionic radius (approximately 1.00 Å) is slightly smaller than that of La. 3+ (Approximately 1.06 Å), but the difference is small. Substitution causes slight local lattice shrinkage or distortion. This microstructural perturbation can increase the bottleneck size of lithium-ion migration channels while reducing the potential barrier between adjacent lattice sites, making it easier for lithium ions to jump.

[0059] Migration path optimization: In the crystal structure of LLPO, lithium ions originally migrate along one-dimensional channels. After calcium doping, due to the synergistic effect of charge defects and lattice distortion, some lithium ions can "jump" using new vacancy paths, effectively increasing the dimensionality and connectivity of the migration path.

[0060] In summary, this invention achieves synergistic regulation of the microstructure and defect chemistry of oxide solid electrolytes through calcium doping, thereby significantly improving ionic conductivity.

[0061] The calcium doping modification method provided by this invention uses inexpensive and readily available raw materials, has a simple process, and is easy to scale up for production. The solid electrolyte prepared by this invention has broad application prospects in solid-state lithium-ion batteries.

[0062] The above embodiments are merely preferred embodiments of the present invention, used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Any modifications, improvements, or equivalent substitutions made to the present invention by those skilled in the art without departing from the technical concept and solutions of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for improving the ionic conductivity of oxide solid electrolytes by calcium doping, characterized in that, Includes the following steps: The precursor is obtained by mixing raw materials containing calcium source, sodium source, rare earth source and phosphorus source; The precursor is sintered to obtain a calcium-doped sodium superionic conductor precursor. The calcium-doped sodium superionic conductor precursor was subjected to ion exchange with a lithium salt to obtain Li 3+x La 1-x Ca x (PO4)2 calcium-doped oxide solid electrolyte; Where x satisfies 0 < x ≤ 0.

1.

2. The method according to claim 1, characterized in that, The value of x ranges from 0.01 to 0.

1.

3. The method according to claim 2, characterized in that, The value of x ranges from 0.03 to 0.

07.

4. The method according to claim 3, characterized in that, The value of x is 0.

05.

5. The method according to claim 1, characterized in that, The sintering temperature is 900°C to 1200°C, and the sintering time is 6 to 12 hours.

6. The method according to claim 1, characterized in that, The calcium source is selected from at least one of calcium carbonate, calcium nitrate, or calcium chloride; the sodium source is selected from at least one of sodium carbonate or sodium hydroxide; the rare earth source is lanthanum oxide; the phosphorus source is selected from at least one of ammonium dihydrogen phosphate or diammonium hydrogen phosphate; and the lithium salt is selected from at least one of lithium bis(trifluoromethanesulfonyl)imide or lithium hydroxide.

7. The method according to claim 1, characterized in that, It also includes a step of ball milling the precursor before sintering; the ball milling is performed using wet ball milling and the ball milling medium is anhydrous ethanol.

8. The method according to claim 1, characterized in that, The sintering is carried out in an air atmosphere or an inert atmosphere; after the ion exchange, the product is further cleaned and dried, and the cleaning is performed using anhydrous ethanol.

9. The method according to claim 1, characterized in that, The method is also applicable to the preparation of NASICON-type oxide solid electrolytes or garnet-type oxide solid electrolytes.

10. The method according to claim 1, characterized in that, The calcium-doped oxide solid electrolyte prepared by the method has a room-temperature ionic conductivity of 10. -4 S / cm to 10 -3 S / cm.

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