Composite positive electrode material for realizing sodium ion internal compensation based on electrochemical potential difference as well as preparation method and application of composite positive electrode material

Abstract

The invention discloses a composite positive electrode material for realizing sodium ion internal compensation based on electrochemical potential difference as well as a preparation method and application thereof, relates to a sodium ion battery composite positive electrode material as well as a preparation method and application thereof, and aims at solving the problems of polarization increase, capacity reduction and poor rate capability of the existing sodium ion battery positive electrode material at the discharge end and in a low-temperature environment. The composite positive electrode material is formed by compounding a main phase material A and an auxiliary phase material B with the electrochemical potential higher than that of the main phase material A in a physical mode; at the final stage of discharge or at low temperature and high rate, the auxiliary phase spontaneously releases sodium ions to the main phase based on the electrochemical potential difference of the two phases without the action of an external electric field, and the available capacity of the main phase is improved through internal compensation. The specific discharge capacity of the sodium vanadium fluorophosphate and sodium vanadium phosphate composite material is 119.2 mAh g <-1 >, the specific discharge capacities of the sodium vanadium fluorophosphate and sodium vanadium phosphate composite material are 76.4 mAh g <-1 > and 107.4 mAh g <-1 > at the multiplying power of 30 C and the multiplying power of-20 DEG C and the multiplying power of 0.1 C, and the sodium vanadium fluorophosphate and sodium vanadium phosphate composite material can be applied to the field of sodium ion batteries.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage technology, specifically to a sodium-ion battery composite cathode material and its application, and more particularly to a cathode material construction method with an internal sodium-ion compensation mechanism and electrochemical potential difference-driven migration, and its application in improving end-of-discharge capacity and low-temperature rate performance. Background Technology

[0002] Sodium-ion batteries, as a resource-friendly, low-cost, and highly safe energy storage technology, have shown broad potential in large-scale energy storage and cryogenic applications. Their basic working mechanism is similar to that of traditional lithium-ion batteries, both employing a "rocking chair" process: during charging, sodium ions are released from the positive electrode and migrate to the negative electrode; during discharging, sodium ions are re-inserted into the positive electrode material.

[0003] However, towards the end of the discharge phase, as sodium ions are gradually inserted, the number of sodium vacancies within the cathode material decreases, while the electrostatic repulsion between already inserted sodium ions intensifies, making subsequent sodium ion insertion more difficult. This structure-kinetic coupling effect often leads to problems such as a decrease in the sodium ion diffusion coefficient, increased diffusion resistance, intensified polarization, and a shortened voltage plateau, ultimately manifesting as capacity loss due to kinetic limitations. This problem is even more pronounced at low temperatures, where the sodium ion diffusion rate further decreases, making it difficult to release the material's inherent properties.

[0004] To address the aforementioned issues, current research often employs strategies such as heteroelement doping, surface coating with conductive phases, nano-particle size reduction, and vacancy manipulation to improve sodium ion migration pathways and interfacial properties, thereby enhancing the material's kinetic properties. While these methods are effective in certain specific material systems, they generally suffer from the following limitations:

[0005] 1. Modification methods are material-dependent and structure-specific; for example, some doping elements or coating agents are only sensitive to specific crystal types or valence states.

[0006] 2. The preparation process is complex and requires high process control, making it difficult to achieve stable and consistent results in large-scale production;

[0007] 3. It has a narrow scope of application, making it difficult to achieve universal optimization of most material systems using a single modification method.

[0008] Therefore, there is an urgent need to develop a novel construction strategy that has a clear mechanism, is easy to operate, has strong process compatibility, and is applicable to a variety of sodium-ion cathode materials, so as to effectively alleviate capacity loss at the end of discharge and limited diffusion at low temperatures. Summary of the Invention

[0009] This invention aims to address the technical problems of increased polarization, decreased capacity, and poor rate performance in existing sodium-ion battery cathode materials due to restricted sodium ion diffusion at the end of discharge and in low-temperature environments. It provides a composite cathode material based on electrochemical potential difference to achieve sodium ion internal compensation, along with its preparation method and applications. The method of this invention is a universal, simple, and doping-free composite cathode material design approach based on electrochemical potential difference-driven sodium ion migration, thereby achieving capacity compensation and performance enhancement of the main phase cathode material under kinetically constrained conditions.

[0010] The composite cathode material of the present invention, which achieves sodium ion internal compensation based on electrochemical potential difference, comprises a main phase material A and an auxiliary phase material B. The main phase material A is a sodium-ion cathode material in which sodium vacancies in the sodium-ion intercalation / deintercalation structure are easily saturated and diffusion rates are limited at the end of discharge. The auxiliary phase material B is a sodium source material with an electrochemical potential higher than that of the main phase material A, and this sodium source material has the ability to migrate sodium ions to the main phase during discharge. That is, the auxiliary phase material B itself can be used as a sodium-ion battery cathode material, possessing sodium ions that can be intercalated / deintercalated and vacancies to accommodate sodium ions. The electrochemical potential of the material is based on the Nernst equation. Determined, in the formula This represents the electrochemical potential of sodium ions within a material phase, and its unit is J mol. -1 or kJ mol -1 ; The standard chemical potential of sodium ions is expressed in J mol. -1 or kJmol -1 ; The open-circuit voltage is represented by V; F represents the Faraday constant, with a value of 96485 C mol. -1 , representing the amount of charge carried by 1 mol of electrons.

[0011] Furthermore, the main phase material A is sodium vanadium fluorophosphate Na3V2(PO4)2F3 (abbreviated as NVPF).

[0012] Furthermore, the auxiliary phase material B is sodium ferric fluorophosphate Na2FePO4F (NFPF), Prussian blue, or sodium vanadium phosphate Na3V2(PO4)3 (NVP).

[0013] Furthermore, the mass ratio of the main phase material A to the auxiliary phase material B is (2.3~19):1. This setting can reduce the energy density loss of the main phase material A caused by the introduction of the low-voltage auxiliary phase material B.

[0014] Furthermore, the mass ratio of the main phase material A to the auxiliary phase material B is (7~11):1.

[0015] Furthermore, the electrochemical potential of the auxiliary phase material B is higher than that of the main phase material A, that is, the discharge voltage of the auxiliary phase material B is lower than the discharge voltage at the end of the discharge of the main phase material A. In order to ensure that the composite material has a relatively high energy density to ensure its practicality, the difference between the discharge voltage of the auxiliary phase material B and the discharge voltage at the end of the discharge of the main phase material A is less than 500 mV.

[0016] The above-mentioned method for preparing composite cathode material with sodium ion internal compensation based on electrochemical potential difference is as follows: grinding and mixing the main phase material A and the auxiliary phase material to obtain composite cathode material with sodium ion internal compensation based on electrochemical potential difference.

[0017] Furthermore, the grinding and mixing time is 20-30 minutes.

[0018] The aforementioned application of the composite cathode material that achieves internal compensation of sodium ions based on electrochemical potential difference involves preparing the composite cathode material into an electrode sheet for use as the cathode of a sodium-ion battery.

[0019] The main and auxiliary phase materials of this invention are constructed into a composite structure through a simple physical method, rather than forming a single pure phase. In this sodium-ion battery composite cathode material with an internal sodium-ion compensation mechanism, during battery discharge, the sodium ion diffusion kinetics of the main phase material A are restricted due to reduced sodium vacancies and enhanced internal electrostatic repulsion, leading to a decrease in the diffusion coefficient and increased electrode polarization. Meanwhile, the auxiliary phase material B still contains releaseable sodium ions. The electrochemical potential difference between the two phases provides a driving force for the sodium ions to spontaneously migrate to the main phase A, thereby compensating for the main phase capacity, alleviating kinetic obstacles, and improving the capacity at the end of discharge and the overall rate performance.

[0020] The preparation process of the electrode using the composite cathode material for sodium ion compensation based on electrochemical potential difference of the present invention is similar to that of conventional electrode preparation processes. Specifically, the composite cathode material for sodium ion compensation based on electrochemical potential difference, a conductive agent, and a binder are mixed uniformly in a conventional proportion to obtain an electrode slurry; this slurry is then coated and dried to obtain an electrode sheet. The composite cathode material of the present invention does not require additional doping, coating, or nano-sizing treatment.

[0021] Furthermore, the mass ratio of the composite cathode material, conductive agent, and binder that achieves sodium ion internal compensation based on electrochemical potential difference is 7:2:1.

[0022] The advantages of the composite cathode material based on electrochemical potential difference for internal sodium ion compensation compared to pure-phase cathode materials are as follows:

[0023] (1) This invention constructs an electrochemical potential difference-driven mechanism between the main phase material and the auxiliary phase material by physically combining them, enabling sodium ions to spontaneously migrate from the auxiliary phase to the main phase without an external electric field, thereby achieving internal compensation of sodium ions during discharge. This structure can compensate for the capacity loss caused by the reduction of sodium vacancies and the increase in migration resistance in the main phase during discharge, especially showing more obvious performance advantages under conditions where migration resistance increases significantly, such as high rate or low temperature. Specifically, at room temperature (25℃) and 1C rate, a sodium-ion half-cell assembled with a composite cathode material of sodium vanadium fluorophosphate Na3V2(PO4)2F3 as the main phase material A and sodium vanadium phosphate Na3V2(PO4)3 as the auxiliary phase material B achieved a capacity of 119.2 mAh g. -1 The discharge specific capacity is higher than that of sodium vanadium fluorophosphate (114.5 mAh g⁻¹). -1 As the rate of reaction increases, polarization intensifies during the electrode reaction. Conventional materials suffer severe capacity loss due to limited sodium ion diffusion. However, in the material of this invention, sodium ions in the auxiliary phase can migrate to the main phase under the drive of electrochemical potential to participate in the reaction, thereby effectively compensating for this capacity loss. At a rate of 30C, the material of this invention can still output 76.4 mAh g⁻¹. -1 The discharge specific capacity is significantly better than that of the pure phase material (45.3 mAh g⁻¹). -1 Similarly, under low-temperature conditions (-20°C, 0.1C rate), the effective capacity release of traditional materials is significantly limited due to the substantial decrease in sodium ion migration rate. However, the material of this invention, relying on a sodium ion migration mechanism driven by an internal electrochemical potential difference, can still achieve 107.4 mAh g⁻¹. -1 The discharge specific capacity is higher than that of the pure phase material (98.6 mAh g). -1 This demonstrates superior low-temperature adaptability. When sodium ferric fluorophosphate is selected as the auxiliary phase material, the prepared composite material also exhibits excellent electrochemical performance. Specifically, a sodium-ion half-cell assembled with sodium vanadium fluorophosphate Na3V2(PO4)2F3 as the main phase material A and sodium ferric fluorophosphate Na2FePO4F as the auxiliary phase material B achieved a 116.7 mAh g⁻¹. -1 The discharge specific capacity is higher than that of sodium vanadium fluorophosphate (114.5 mAh g⁻¹). -1 When the rate of change is increased to 30C, the material of this invention can still output 66.8 mAh g. -1 The discharge specific capacity is significantly better than that of the pure phase material (45.3 mAh g⁻¹). -1 .

[0024] (2) This invention constructs a new sodium ion transport pathway by introducing an auxiliary phase with a higher electrochemical potential into the cathode material. Driven by the electrochemical potential, sodium ions can spontaneously migrate from the auxiliary phase to the main phase, achieving dynamic redistribution of sodium ions without the need for an electric field, thus enhancing the overall sodium ion migration flux. Especially at the end of the discharge, the diffusion coefficient of the main phase material usually decreases due to factors such as increased sodium ion embedding, reduced vacancies, and local concentration polarization, leading to obstruction of the transport process. The internal compensation mechanism constructed in this invention effectively alleviates the diffusion lag caused by the increased occupancy rate of the main phase by continuously releasing sodium ions from the auxiliary phase, thereby maintaining a high sodium ion diffusion rate at this stage. GITT (Gas-Constant Intermittent Titration) test results show that compared with the single pure-phase sodium vanadium fluorophosphate cathode, the composite cathode material of this invention exhibits a higher sodium ion diffusion coefficient at the end of the discharge, verifying the effectiveness of this mechanism in improving kinetic performance. At the same time, the introduction of this mechanism does not change the overall crystal structure or preparation process of the material, and has good versatility and scalability.

[0025] (3) The present invention adopts a strategy of physical composite of main and auxiliary phases, which eliminates the need for traditional modification methods with complex structures and difficult parameters, such as element doping, surface coating or high temperature atmosphere treatment. This avoids the destruction of the stability of the material crystal structure and the introduction of additional process flow, and significantly simplifies the preparation process of cathode material.

[0026] (4) This method is simple and can complete the preparation of composite cathodes through conventional mixing, paste preparation and coating steps. It is highly compatible with the existing sodium-ion battery electrode preparation system, does not rely on special equipment or exogenous additives, has good versatility and operational stability, is easy to scale up and apply directly on existing battery production lines, and has high industrial transformation potential. Attached Figure Description

[0027] Figure 1 This is a schematic diagram illustrating the working principle of the electrochemical potential-driven sodium ion internal compensation mechanism constructed in this invention.

[0028] Figure 2 The first charge-discharge curve of the pure-phase sodium vanadium fluorophosphate cathode material in Comparative Example 1 at a rate of 0.2 C is shown.

[0029] Figure 3 shows the first charge-discharge curves of the sodium-ion half-cell assembled in Example 1 at a rate of 0.2 C;

[0030] Figure 4 shows the differential capacity curve of the sodium-ion half-cell assembled in Example 1 at a rate of 0.2 C;

[0031] Figure 5 The rate performance of the sodium-ion half-cell assembled in Example 1 and Comparative Example 1 at 25 °C is shown in the graph.

[0032] Figure 6 The first charge-discharge curve of the composite cathode material in Example 2 at a rate of 0.2 C is shown.

[0033] Figure 7 The differential capacity curve of the composite cathode material in Example 2 at a rate of 0.2 C is shown.

[0034] Figure 8 The diagram shows the sodium ion diffusion coefficients calculated by GITT testing for Example 2 and Comparative Example 1.

[0035] Figure 9 The rate performance of the sodium-ion half-cell assembled in Example 2 and Comparative Example 1 at 25 °C is shown in the graph.

[0036] Figure 10 The first charge-discharge curve of the pure-phase sodium vanadium fluorophosphate cathode material in Comparative Example 1 at -20℃ is shown.

[0037] Figure 11 The first charge-discharge curve of the composite cathode material in Example 2 at -20°C is shown.

[0038] Figure 12 The differential capacity curve of the composite cathode material in Example 2 at -20℃ is shown.

[0039] Figure 13 The graph shows the rate performance of the sodium-ion half-cell assembled in Example 2 and Comparative Example 1 at -20°C. Detailed Implementation

[0040] The beneficial effects of the present invention will be verified using the following examples.

[0041] Example 1: The composite cathode material for sodium ion internal compensation based on electrochemical potential difference in this example is carried out according to the following steps:

[0042] Accurately weigh sodium vanadium fluorophosphate (NVPF) and sodium iron fluorophosphate (Na2FePO4F, NFPF) powders at a mass ratio of 9:1, place them in a mortar, and grind and mix them thoroughly for 30 minutes to obtain a composite cathode material that achieves internal compensation of sodium ions based on electrochemical potential difference.

[0043] The composite cathode material prepared in Example 1, which achieves internal compensation of sodium ions based on electrochemical potential difference, was used to prepare electrode sheets and sodium-ion half-cells. The specific steps are as follows:

[0044] The composite cathode material prepared in Example 1, which achieves sodium ion internal compensation based on electrochemical potential difference, conductive carbon black (Super P), and PVDF binder were thoroughly mixed at a mass ratio of 7:2:1. An appropriate amount of NMP solvent was added, and the mixture was magnetically stirred for 4 hours to form a slurry. This slurry was then uniformly coated onto a current collector aluminum foil and dried in a vacuum drying oven at 120 °C for 12 hours. The dried cathode sheet was then stamped into a 14 mm diameter disc for use as the cathode. The cathode sheet was then combined with a sodium metal anode, a glass fiber separator, and 1 mol L... -1 NaClO4 / EC:PC = 1:1 + 5% FEC was used to assemble a CR2025 type sodium-ion half-cell.

[0045] Example 2: The preparation method of the composite cathode material based on electrochemical potential difference to achieve internal compensation of sodium ions in this example is carried out according to the following steps:

[0046] Accurately weigh sodium vanadium fluorophosphate (NVPF) and sodium vanadium phosphate (NVP) powders at a mass ratio of 9:1, place them in a mortar, and grind and mix them thoroughly for 30 minutes to obtain a composite cathode material that achieves internal compensation of sodium ions based on electrochemical potential difference.

[0047] The composite cathode material prepared in Example 2, which achieves internal sodium ion compensation based on electrochemical potential difference, was thoroughly mixed with conductive carbon black (Super P) and PVDF binder at a mass ratio of 7:2:1. An appropriate amount of NMP solvent was added, and the mixture was magnetically stirred for 4 hours to form a slurry. This slurry was then uniformly coated onto a current collector aluminum foil and dried in a vacuum drying oven at 120 °C for 12 hours. The dried cathode sheet was then stamped into a 14 mm diameter disc for use as the cathode. The cathode sheet was then combined with a sodium metal anode, a glass fiber separator, and 1 mol / L... -1 NaClO4 / EC:PC = 1:1 + 5% FEC was used to assemble a CR2025 type sodium-ion half-cell.

[0048] Comparative Example 1: This comparative example differs from Example 1 in that it does not require the preparation step of composite cathode material. It only requires the use of pure phase sodium vanadium fluorophosphate as active material to prepare electrode sheet. Everything else is the same as Example 1.

[0049] The sodium-ion half-cells assembled in Examples 1, 2, and Comparative Example 1 were subjected to constant current charge-discharge tests with a voltage range of 2.5–4.5 V and test temperatures of 25 °C and -20 °C, respectively. GITT tests were conducted at room temperature (25 °C) under the following conditions: discharge at a rate of 0.1 C, pulse duration of 0.5 hours, and rest period of 2 hours.

[0050] Figure 1This diagram illustrates the working principle of the electrochemical potential-driven sodium-ion internal compensation mechanism constructed in this invention, using the battery of Example 2 as an example. During the discharge process of the sodium-ion battery, the main cathode material, sodium vanadium fluorophosphate (NVPF), undergoes a sodium-ion intercalation reaction. As the discharge progresses, sodium vacancies in the NVPF structure are gradually occupied, forming a main phase Na containing some sodium vacancies. 3-x The presence of V2(PO4)2F3 and the enhanced electrostatic repulsion between sodium ions significantly slows down the intercalation reaction kinetics, leading to increased material polarization and a rapid voltage drop to the voltage plateau region of the auxiliary phase sodium vanadium phosphate (Na3V2(PO4)3, abbreviated as NVP). At this point, the system first enters step 1: NaV2(PO4)3 in the auxiliary phase NVP undergoes an intercalation reaction with the participation of sodium ions to form Na3V2(PO4)3. This process is a normal discharge behavior, releasing the capacity corresponding to the voltage plateau. Then, the system enters the crucial step 2: because the electrochemical potential of Na3V2(PO4)3 is higher than that of the main phase Na, which still has some sodium vacancies... 3-x In the V2(PO4)2F3 system, a stable electrochemical potential is formed, driving some sodium ions in Na3V2(PO4)3 to spontaneously migrate into the NVPF structure, replenishing vacant sites in the host phase material that have not yet been inserted due to kinetic constraints. This step does not require an external electric field, demonstrating the internal compensation mechanism constructed in this invention. The overall reaction can be represented as follows:

[0051] Step 1: NaV2(PO4)3 + Na + +e - →Na3V2(PO4)3

[0052] Step 2: yNa3V2(PO4)3 + Na 3-x V2(PO4)2F3→yNaV2(PO4)3+ Na 3-x+2y V2(PO4)2F3

[0053] Furthermore, this mechanism is reversible; the material generated in step 2 can be repeatedly used in step 1, thereby increasing the overall discharge capacity, broadening the working platform, and compensating for capacity loss. Through this process, the auxiliary phase material NVP not only provides its own discharge capacity but also provides an additional source of sodium ions to the main phase under the influence of electrochemical potential, achieving "internal compensation" of capacity. This significantly alleviates the capacity decay caused by the obstruction of sodium ion diffusion at the end of the discharge, improving the overall discharge capacity release efficiency.

[0054] Figure 2 The first charge-discharge curves of the sodium-ion half-cell assembled in Comparative Example 1 at a rate of 0.2 C are shown. Figure 2 It can be seen that the sodium-ion half-cell assembled in Comparative Example 1 can provide 114.5 mAh g⁻¹ at a rate of 0.2 C.-1 The discharge specific capacity is close to its theoretical capacity, indicating that the material itself possesses good reversibility and structural stability. The charge-discharge curves show only two pairs of relatively stable voltage plateaus, located around 3.6 / 3.7 V and 4.1 / 4.2 V, respectively, corresponding to the VF value in the NVPF. 4+ / V 3+ The reversible redox reaction process was observed. No other voltage plateaus were observed, especially the characteristic discharge plateau of sodium vanadium phosphate in the region of about 3.4V, indicating that the sodium vanadium fluorophosphate (NVPF) material used in Comparative Example 1 is a single-phase composition and does not contain NVP phase impurities.

[0055] Figure 3 shows the first charge-discharge curves of the sodium-ion half-cell assembled in Example 1 at a rate of 0.2 C. As can be seen from the figure, the sodium-ion half-cell assembled in Example 1 can provide 116.7 mAh g⁻¹ during the first discharge. -1 The discharge specific capacity of the sample is higher than that of Comparative Example 1, indicating that the introduction of the auxiliary phase sodium fluorophosphate can provide capacity compensation during discharge. A plateau at low voltage can be observed in the curve, corresponding to the charge-discharge characteristic reaction of the auxiliary phase sodium fluorophosphate, which effectively improves the capacity decay of the main phase in the kinetically constrained stage.

[0056] Figure 4 shows the differential capacity curve of the sodium-ion half-cell assembled in Example 1 at a rate of 0.2 C. By integrating the charge and discharge peak areas of the auxiliary phase sodium fluorophosphate in the voltage range of the curve, its charging capacity was calculated to be 4.83 mAh g. -1 The discharge capacity is 6.97 mAh g. -1 The results showed that the sodium ferric fluorophosphate auxiliary phase exhibited a higher discharge capacity than charge capacity.

[0057] Figure 5 The graph shows the rate performance of the sodium-ion half-cells assembled in Example 1 and Comparative Example 1 at 25 °C. Thanks to the presence of the auxiliary phase sodium iron fluorophosphate, Example 1 exhibits superior rate performance. As shown in the figure, the specific discharge capacity of the sodium-ion half-cell assembled in Example 1 at 1 C, 15 C, 20 C, and 30 C rates is 113.3 mAh g⁻¹. -1 95.1 mAh g -1 88.6 mAh g -1 and 66.8 mAh g -1 The sodium-ion half-cell assembled in Comparative Example 1 had a discharge specific capacity of 112.7 mAh g⁻¹ at the same rate. -1 92.7 mAh g -1 79.6 mAh g -1 and 46.5 mAh g-1 The values ​​were all lower than those of Example 1. Especially at the extremely high rate of 50 C, the half-cell assembled in Comparative Example 1 had almost no discharge capacity, while Example 1 could still maintain 30.1 mAh g⁻¹. -1 Its discharge specific capacity exhibits excellent high-rate performance.

[0058] Figure 6 The first charge-discharge curve of the sodium-ion half-cell assembled in Example 2 at a rate of 0.2 C is shown. It can be seen that within a voltage range of 2.5–4.5 V, this composite electrode achieved a discharge rate of 119.3 mAh g⁻¹ during the first discharge. -1 The discharge specific capacity is significantly higher than that of the pure-phase NVPF in Comparative Example 1, which exhibits 114.5 mAh g⁻¹. -1 This indicates that the material of the present invention possesses superior initial capacity release capability. The positive electrode active material used in Example 2 is a physical composite phase of Na3V2(PO4)2F3 and Na3V2(PO4)3, and its theoretical specific capacity is calculated to be approximately 126.9 mAh g⁻¹. -1 Slightly lower than the theoretical capacity of pure-phase NVPF (approximately 128 mAh g). -1 However, in actual testing, the composite material achieved a higher discharge specific capacity, indicating that the composite design did not weaken capacity release but rather improved the actual capacity output, further verifying the significant role of the electrochemical potential-driven sodium ion internal migration mechanism for capacity compensation constructed in this invention. Furthermore, Figure 3 Three pairs of charge-discharge plateaus can be clearly observed. One pair, located in the approximately 3.3 / 3.4V region, corresponds to the typical sodium insertion / deintercalation reaction of Na3V2(PO4)3 material. This plateau accounts for approximately 10% of the capacity, consistent with the NVP mass ratio in the composite material. Compared to Example 1, Example 2 exhibits a relatively higher discharge capacity. This is mainly due to the difference in the electrochemical properties of the auxiliary phase itself. Sodium ferric fluorophosphate is slightly inferior to sodium vanadium phosphate in terms of conductivity and ion migration rate, resulting in a slightly weaker capacity compensation effect. However, the overall performance is still significantly better than the system without the introduction of the auxiliary phase.

[0059] Figure 7 The differential capacity curve of the sodium-ion half-cell assembled in Example 2 at a rate of 0.2 C is shown. The capacity contributed by sodium vanadium phosphate during charge and discharge can be calculated by integration. The results show that during charging at approximately 3.4 V, the peak area integral value corresponding to sodium vanadium phosphate is 10.75 mAh g⁻¹. -1 During the discharge process at approximately 3.3V, sodium vanadium phosphate contributed 11.11 mAh g of capacity. -1The discharge capacity is slightly higher than its corresponding charging capacity. In conventional battery systems, due to losses caused by irreversible reactions during the first cycle (such as active sodium consumption, solid electrolyte interphase (SEI) formation, side reactions, etc.), the discharge capacity is usually lower than the charging capacity. However, in Example 2, the discharge capacity of sodium vanadium phosphate exceeded its charging capacity, breaking the electrochemical capacity balance mode of traditional materials and fully verifying the effectiveness of the sodium ion internal migration compensation mechanism driven by electrochemical potential proposed in this invention. Specifically, at the end of the discharge, due to the reduction of sodium vacancies in the main phase sodium vanadium fluorophosphate and the limitation of electrochemical reaction kinetics, some sodium ions cannot be intercalated, resulting in capacity loss. At this time, the Na3V2(PO4)3 already formed in the auxiliary phase, driven by the electrochemical potential difference between itself and the main phase, migrates some sodium ions to the unsaturated structure of sodium vanadium fluorophosphate, compensating for the capacity loss of the main phase in the kinetic bottleneck region. The NaV2(PO4)3 formed after the migration still has good sodium ion storage capacity and can participate in the sodium intercalation reaction again, contributing to the discharge capacity, thus exhibiting a discharge capacity higher than the charging capacity. This phenomenon not only further corroborates the fact that the sodium vanadium phosphate discharge capacity exceeds its charging capacity, but also demonstrates the fact that the sodium vanadium phosphate discharge capacity exceeds its charging capacity. Figure 1 The internal compensation pathway shown is practically feasible and provides direct evidence from the perspective of electrochemical behavior, demonstrating that the auxiliary phase in the system of this invention not only provides its own capacity but also enhances the capacity release efficiency of the main phase through migration, significantly improving the overall energy utilization of the material. This is consistent with the results of the sodium fluorophosphate auxiliary phase in Example 1, further verifying the universality of the "electrochemical potential-driven sodium ion internal migration" mechanism proposed in this invention. This mechanism enables the auxiliary phase to not only participate in the sodium storage reaction itself during charge-discharge cycles but also to replenish sodium ions to the main phase during discharge, thereby achieving capacity compensation and improving the overall discharge specific capacity of the battery.

[0060] Figure 8The figures show the sodium ion diffusion coefficients calculated by GITT testing for Examples 2 and Comparative Example 1. As can be seen from the figures, the sodium ion diffusion coefficient of the pure-phase sodium vanadium fluorophosphate in Comparative Example 1 significantly decreases at the end of the discharge period (low voltage region). This is mainly attributed to the fact that in the later stages of discharge, as sodium ions continuously intercalate, the sodium ion vacancies in the sodium vanadium fluorophosphate crystal structure gradually decrease, while the electrostatic repulsion generated by the sodium ions already present in the intercalation channels increases, leading to hindered sodium ion migration and limited diffusion kinetics. In contrast, the composite cathode material used in Example 2 maintains a high sodium ion diffusion coefficient at the end of the discharge period, significantly better than Comparative Example 1. This phenomenon indicates that introducing an appropriate amount of auxiliary phase sodium vanadium phosphate effectively alleviates the diffusion bottleneck of the main phase in the high sodium occupancy state. As a three-dimensional framework structure with good sodium ion conductivity, this auxiliary phase, driven by the electrochemical potential difference, can provide additional migration paths to the main phase and promote the internal diffusion of sodium ions, thereby enhancing the overall sodium ion migration flux of the electrode and improving the diffusion performance at the end of the discharge period. The results above further confirm the effectiveness of the design concept of this invention, which enables the spontaneous migration of sodium ions from the auxiliary phase to the main phase by constructing an electrochemical potential-driven mechanism, in improving actual kinetics.

[0061] Figure 9 The graph shows the rate performance of the sodium-ion half-cells assembled in Example 2 and Comparative Example 1 at 25 °C. The results show that the specific discharge capacity of the sodium-ion half-cell assembled in Example 2 at 1 C, 10 C, 20 C, and 30 C rates is 119.2 mAh g⁻¹. -1 110.1 mAh g -1 96.9 mAh g -1 and 76.4 mAh g -1 The sodium-ion half-cell assembled in Comparative Example 1 had a discharge specific capacity of 112.7 mAh g⁻¹ at the same rate. -1 102.4 mAh g -1 79.6 mAh g -1 and 46.5 mAh g -1 The values ​​were all lower than those of Example 2. Especially at the extremely high rate of 50 C, the half-cell assembled in Comparative Example 1 had almost no discharge capacity, while Example 2 could still maintain 34.7 mAh g⁻¹. -1 The discharge specific capacity exhibits excellent high-rate performance. This phenomenon fully demonstrates that by introducing the auxiliary phase sodium vanadium phosphate, an electrochemical potential difference-driven mechanism is constructed, effectively alleviating the capacity decay caused by the limited sodium ion migration kinetics in the main phase during high-rate rapid charge and discharge. It should be noted that the auxiliary phase NVP accounts for only about 10% in Example 2, and its theoretical capacity contribution is approximately 11.7 mAh g. -1However, at a 50 C rate, the capacity improvement in Example 2 compared to Comparative Example 1 far exceeds this value. Therefore, it cannot be simply attributed to the self-discharge of sodium vanadium phosphate, but rather to its role as a "sodium ion buffer" to compensate for sodium ions in the main phase, thereby significantly enhancing high-rate performance. This result further corroborates the scientific validity and effectiveness of the electrochemical potential-driven sodium ion migration compensation mechanism proposed in this invention.

[0062] Figure 10 The first charge-discharge curves of the pure-phase sodium vanadium fluorophosphate cathode material in Comparative Example 1 are shown at -20°C. The test results show that the discharge specific capacity of this battery decreases significantly at low temperatures, reaching only 98.7 mAh g⁻¹. -1 Compared to its capacity decaying significantly at room temperature (25°C), and with the initial coulombic efficiency also decreasing to 82.35%, this further indicates a strong irreversible reaction in the system at low temperatures. The main reason for this phenomenon is the significant suppression of electrode reaction kinetics at low temperatures. On the one hand, the mobility of sodium ions in the electrolyte decreases, increasing diffusion resistance; on the other hand, sodium ion diffusion is restricted in the sodium vanadium fluorophosphate crystal structure, slowing down the sodium ion insertion process within the main phase. This leads to increased polarization and a shortened voltage plateau during charge and discharge, ultimately resulting in insufficient capacity release. Furthermore, the charge transfer impedance increases significantly at low temperatures, raising the activation energy of the interfacial reaction and further limiting the electrochemical reaction. Therefore, this figure fully reveals the kinetic bottleneck faced by pure-phase sodium vanadium fluorophosphate under low-temperature conditions, providing a comparative basis for introducing an electrochemical potential compensation mechanism in this invention.

[0063] Figure 11 The figure shows the initial charge-discharge curves of the composite cathode material in Example 2 at -20°C. As can be seen from the figure, Example 2 achieved a charge-discharge ratio of 107.3 mAh g⁻¹ within a voltage window of 2.5–4.5 V. -1 Its discharge specific capacity is significantly higher than that of other discharge capacities. Figure 10 Comparative Example 1 (pure phase NVPF) only showed 98.7 mAh g. -1 This demonstrates a significant improvement in low-temperature discharge performance. Compared to room temperature, the diffusion kinetics of sodium ions are significantly hindered at low temperatures, resulting in slower ion migration rates, increased charge transfer resistance, and more severe polarization, leading to a significant capacity reduction in traditional cathode materials at low temperatures. However, in this invention, by introducing the auxiliary phase sodium vanadium phosphate to construct a composite cathode material system, an electrochemical potential difference-driven mechanism is formed, allowing sodium ions embedded in Na3V2(PO4)3 to spontaneously migrate to the main Na phase. 3-xIn V2(PO4)2F3, the capacity loss caused by the inability of the main phase to fully intercalate sodium due to kinetic limitations is compensated. This mechanism has shown some effectiveness at room temperature, and its internal compensation advantage is further amplified in the low-temperature environment where kinetic limitations are more significant, resulting in a more pronounced improvement in overall discharge capacity, further verifying the effectiveness and universality of the mechanism of this invention. Furthermore, from... Figure 11 Three distinct charge-discharge plateaus can be observed in the charge-discharge curves. The low-voltage plateau at approximately 3.3 / 3.4V is attributed to the reaction characteristics of the auxiliary phase sodium vanadium phosphate. The discharge capacity exceeds the corresponding charging capacity, indicating that the auxiliary phase not only undergoes its own reaction but also participates in capacity output again through sodium ion migration. The curve characteristics further confirm the theoretical mechanism of "electrochemical potential-driven sodium ion internal compensation".

[0064] Figure 12 This figure shows the differential capacity curve of the composite cathode material in Example 2 at -20°C. This curve is used to further quantify the specific contribution of the auxiliary phase sodium vanadium phosphate to the overall capacity output at low temperatures and to analyze its capacity compensation role in the composite cathode system described in this invention. The integral calculation of the charge / discharge characteristic peaks in the 3.3 / 3.4V voltage range in the figure shows that the capacity contributed by the auxiliary phase sodium vanadium phosphate during charging is 6.71 mAh g⁻¹. -1 During the subsequent discharge process, its corresponding capacity output reached as high as 17.56 mAh g. -1 This is significantly higher than the charging capacity. Based on the approximately 10% sodium vanadium phosphate content in Example 2, it could ideally provide a maximum of approximately 11.7 mAh g⁻¹. -1 The actual test results showed that the discharge capacity exceeded the theoretical value by 50%, indicating that there was an additional source of capacity during the discharge process. Comparative analysis revealed that this additional capacity originated from the main phase Na. 3-x The electrochemical potential difference-driven mechanism between V2(PO4)2F3 and the auxiliary phase Na3V2(PO4)3 is as follows: During discharge, Na3V2(PO4)3 first releases sodium ions as the active species to generate NaV2(PO4)3. Under the drive of the electric field, the generated NaV2(PO4)3 can continue to act as a Na⁺ acceptor to undergo sodium intercalation, transforming itself back into Na3V2(PO4)3. This achieves the cyclic migration of sodium ions and multiple rounds of intercalation / deintercalation, effectively expanding the usable capacity. This spontaneous ion migration compensation mechanism overcomes the capacity bottleneck caused by kinetic limitations in traditional structures. Especially under the condition that the kinetic limitations are further amplified at low temperatures, the advantages of this compensation mechanism are fully amplified, resulting in an actual output far exceeding the theoretical capacity.

[0065] Figure 13The rate performance graphs of the sodium-ion half-cells assembled in Example 2 and Comparative Example 1 at -20°C are shown. As can be seen from the graphs, the discharge specific capacity of both sets of batteries decreases with increasing rate, reflecting the capacity decay caused by the limited kinetic processes at low temperatures. However, compared to Comparative Example 1, Example 1 exhibits a higher discharge specific capacity at all rates, demonstrating superior rate performance and low-temperature adaptability. Specifically, the discharge specific capacity of the sodium-ion half-cell assembled in Example 1 at rates of 0.1 C, 0.2 C, 1 C, 2 C, and 10 C is 106.6 mAh g⁻¹. -1 100.3 mAh g -1 83.4 mAh g -1 74.5 mAh g -1 and 39.5 mAh g -1 The sodium-ion half-cell assembled in Comparative Example 1 had a discharge specific capacity of 98.7 mAh g⁻¹ at the same rate. -1 89.5 mAh g -1 70.3 mAh g -1 59.8mAh g -1 and 30.9 mAh g -1 The performance was lower than that of Example 1. This performance advantage is attributed to the Na₂S₂ main phase constructed in this invention. 3-x The electrochemical potential difference driving mechanism between V2(PO4)2F3 and the auxiliary phase Na3V2(PO4)3, as described above, further verifies the rationality and universality of the mechanism proposed in this invention from the perspective of electrochemical response, indicating that this strategy has broad application prospects in dealing with complex operating conditions (such as low temperature and high rate).

[0066] The above results demonstrate that while the type of auxiliary phase in this invention has a certain impact on the performance of the composite cathode, as long as a suitable auxiliary phase material is selected, an electrochemical potential-driven system can be constructed, and the capacity loss due to kinetic limitations can be compensated by the internal migration of sodium ions. In other words, by selecting suitable auxiliary phase materials according to the screening principles of this invention, and through simple and easy experimental methods, battery performance can be improved.

[0067] This invention achieves spontaneous migration of sodium ions in the absence of an external electric field by constructing a composite cathode system with an electrochemical potential difference. This effectively mitigates the adverse effects of kinetic limitations on capacity release, exhibiting significant capacity compensation effects, especially under extreme conditions such as the end of discharge, high rate, and low temperature. Compared to traditional modification methods such as doping and coating, this invention features a simpler process, a more universal structural design, good compatibility with existing battery manufacturing processes, and is easy to scale up and apply, demonstrating significant practical value and industrialization prospects.

Claims

1. A composite cathode material based on electrochemical potential difference to achieve internal compensation of sodium ions, characterized in that, The material comprises a main phase material A and an auxiliary phase material B. The main phase material A is a sodium-ion cathode material with a sodium-ion intercalation / deintercalation structure where sodium vacancies are easily saturated and diffusion rates are limited at the end of discharge. The auxiliary phase material B is a sodium source material with an electrochemical potential higher than that of the main phase material A, and this sodium source material possesses the ability to migrate sodium ions to the main phase during discharge. The electrochemical potential of the material is based on the Nernst equation. Determined, in the formula This represents the electrochemical potential of sodium ions within a material phase, and its unit is J mol. -1 or kJ mol -1 ; The standard chemical potential of sodium ions is expressed in J mol. -1 or kJmol -1 ; This represents the open-circuit voltage, measured in volts (V). F represents the Faraday constant, which has a value of 96485 C mol. -1 , representing the amount of charge carried by 1 mol of electrons.

2. The composite cathode material for sodium ion internal compensation based on electrochemical potential difference according to claim 1, characterized in that, The main phase material A is sodium vanadium fluorophosphate Na3V2(PO4)2F 3。 3. A composite cathode material for sodium ion internal compensation based on electrochemical potential difference according to claim 1 or 2, characterized in that, The auxiliary phase material B is sodium ferric fluorophosphate Na2FePO4F, Prussian blue, or sodium vanadium phosphate Na3V2(PO4)3.

4. A composite cathode material for sodium ion internal compensation based on electrochemical potential difference according to claim 1 or 2, characterized in that, The mass ratio of the main phase material A to the auxiliary phase material B is (2.3~19):

1.

5. A composite cathode material for sodium ion internal compensation based on electrochemical potential difference according to claim 1 or 2, characterized in that, The mass ratio of the main phase material A to the auxiliary phase material B is (7~11):

1.

6. A composite cathode material for sodium ion internal compensation based on electrochemical potential difference according to claim 1 or 2, characterized in that, The electrochemical potential of the auxiliary phase material B is higher than that of the main phase material A, that is, the discharge voltage of the auxiliary phase material B is lower than the discharge voltage of the main phase material A at the end of the discharge, and the difference between the discharge voltage of the auxiliary phase material B and the discharge voltage of the main phase material A at the end of the discharge is less than 500 mV.

7. The method for preparing a composite cathode material based on electrochemical potential difference to achieve internal compensation of sodium ions as described in claim 1, characterized in that, The method involves grinding and mixing the main phase material A and the auxiliary phase material to obtain a composite cathode material that achieves internal compensation of sodium ions based on electrochemical potential difference.

8. The method for preparing a composite cathode material based on electrochemical potential difference to achieve internal compensation of sodium ions according to claim 7, characterized in that, The grinding and mixing time is 20-30 minutes.

9. The application of the composite cathode material for sodium ion internal compensation based on electrochemical potential difference as described in claim 1, characterized in that, This application involves fabricating the composite cathode material, which achieves internal compensation of sodium ions based on electrochemical potential difference, into an electrode sheet for use as the cathode of a sodium-ion battery.

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