A doped modified layered oxide sodium-ion battery cathode material and a preparation method thereof

Abstract

The application relates to the technical field of electrode materials of electrochemical energy storage devices, and discloses a doped modified layered oxide sodium ion battery positive electrode material and a preparation method thereof. The positive electrode material comprises a layered oxide main lattice formed by alternately stacking sodium layers and transition metal oxygen layers, the transition metal oxygen layers contain heterovalent doping elements for replacing part of transition metal sites, and the adjacent area of the sodium layers contains cross-layer bridging sites formed by non-migrating pillar elements. In the preparation, a composite precursor system is formed by using a sodium source, a transition metal source, a heterovalent doping source and a non-migrating pillar element source, and then phase formation treatment is performed. The material can simultaneously limit the migration of transition metal layer sites and the relative slippage of sodium layers, and reduces the irreversible structural rearrangement in the cycle process.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage devices and their electrode materials, and discloses a doped modified layered oxide sodium-ion battery cathode material and its preparation method. Background Technology

[0002] Layered oxide sodium-ion battery cathode materials typically use sodium and transition metal sources as raw materials, which are mixed, precursor formed, and subjected to high-temperature phase deposition to obtain a crystal structure consisting of alternating sodium layers and transition metal-oxygen layers. These materials rely on the reversible insertion and extraction of sodium ions between layers to achieve charge storage, while the transition metal-oxygen layer performs the main charge compensation function. Existing processes often improve material performance by adjusting the ratio of sodium to transition metal sources, optimizing the calcination atmosphere, and controlling particle morphology. They also employ single-metal doping to replace some transition metal sites to stabilize the transition metal-oxygen octahedral framework and reduce local metal ion migration during cycling. The technical focus of these approaches is mainly on valence state control and lattice distortion control within the transition metal layer, lacking structural constraints on the relative slip between layers in the region adjacent to the sodium layer.

[0003] In existing doping modifications of layered oxide cathode materials, heterovalent element doping is a common modification method. This is typically achieved by introducing a dopant source during precursor co-precipitation, sol-gel, or solid-phase mixing, allowing the dopant element to enter the transition metal layer during subsequent phase formation, forming a metal-oxygen coordination structure. This method can adjust the average valence state of the transition metal, alter the local oxygen coordination environment, and to some extent suppress transition metal migration under high sodium deintercalation conditions. However, when relying solely on transition metal layer doping, the sodium layer remains in an unconstrained open interlayer environment during repeated sodium ion insertion / extraction, leading to continuous accumulation of interlayer spacing changes and relative misalignments between adjacent transition metal-oxygen layers. As the depth of charge increases or the cycle time lengthens, sodium layer slip induces layered framework rearrangement, thereby weakening the contribution of a single doped structure to the overall lattice stability.

[0004] The main technical challenge of existing layered oxide sodium-ion battery cathode materials lies in the coupling between charge imbalance in the transition metal layer and interlayer slip in the sodium layer during cycling. Single heterovalent doping cannot simultaneously address both types of structural instability sources. Under high sodium depletion conditions, metal sites within the transition metal layer prone to valence fluctuations increase the instability of the oxygen coordination environment, forming the starting point for metal migration and local structural rearrangement. Simultaneously, the interlayer vacancies created after sodium ions are extracted from the sodium layer make it easier for adjacent transition metal-oxide layers to undergo relative displacement. These two changes do not occur in isolation; transition metal migration disrupts the continuity of the sodium ion diffusion plane, while sodium layer slip exacerbates stress concentration in the transition metal-oxide layer, ultimately leading to an irreversible phase transition from local lattice distortion. Therefore, it is necessary to construct a cathode material structure that can simultaneously constrain the stability of transition metal layer sites and the interlayer stability of the sodium layer's adjacent region. Summary of the Invention

[0005] To address the shortcomings of existing technologies where doped modified layered oxide sodium-ion battery cathode materials cannot simultaneously restrict the migration of transition metal layer sites and the relative slippage of sodium layer through single doping of the transition metal layer, this invention provides a doped modified layered oxide sodium-ion battery cathode material and its preparation method. By setting heterovalent dopants in the transition metal oxide layer and non-migrating pillar elements in the region adjacent to the sodium layer to form cross-layer bridging sites, the two types of structures are co-distributed within the same layered oxide particle, thus solving the problem of structural instability of the bulk lattice of layered oxides during repeated sodium ion insertion and extraction.

[0006] To address the aforementioned technical problems, this invention provides a doped and modified layered oxide sodium-ion battery cathode material. The cathode material comprises a layered oxide host lattice formed by alternating stacks of sodium layers and transition metal-oxygen layers. The transition metal-oxygen layers contain heterovalent dopants that replace some transition metal sites. The sodium layers contain cross-layer bridging sites formed by non-migrating pillar elements in their adjacent regions. The heterovalent dopants and the non-migrating pillar elements are co-distributed within the same layered oxide particle.

[0007] During the charging and discharging process of a sodium-ion battery, the cathode material provides sodium ion insertion / extraction space through the sodium layer and undertakes the charge compensation process through the transition metal-oxide layer. After the heterovalent dopant enters the transition metal site, it alters the local coordination environment in the transition metal-oxide layer, causing partial replacement of transition metal sites prone to valence fluctuations. The non-migrating pillar element is located in the region adjacent to the sodium layer and forms interlayer bridging sites, creating structural connections between adjacent transition metal-oxide layers. The heterovalent dopant and the non-migrating pillar element are co-distributed within the same layered oxide particle, allowing the site-stabilizing structure of the transition metal-oxide layer and the interlayer support structure in the region adjacent to the sodium layer to coexist in the same lattice system. This ensures that the bulk lattice of the layered oxide maintains an alternating stacked structure during repeated sodium ion insertion / extraction.

[0008] Furthermore, in the above technical solution, the heterovalent doping element occupies the transition metal sites in the transition metal-oxygen layer that are prone to valence state fluctuations, and the heterovalent doping element forms metal-oxygen coordination units with adjacent oxygen atoms. The metal-oxygen coordination units are embedded in the octahedral framework of the transition metal-oxygen layer and are connected to adjacent transition metal-oxygen polyhedra by sharing edges.

[0009] In specific implementation, the transition metal sites in the transition metal-oxygen layer, together with oxygen atoms, form an octahedral framework. After the heterovalent dopant enters this framework by substitution, it forms metal-oxygen coordination units with adjacent oxygen atoms. These metal-oxygen coordination units are connected to adjacent transition metal-oxygen polyhedra by sharing edges, ensuring that the doping sites are not isolated but rather incorporated into the continuous coordination network of the transition metal-oxygen layer. When sodium ion insertion / extraction causes charge redistribution, the metal-oxygen coordination units participate in lattice coordination maintenance as locally stabilizing units within the transition metal-oxygen layer.

[0010] Furthermore, in the above technical solution, the non-migrating pillar element is distributed in the adjacent region of the sodium layer and the transition metal oxide layer, and the cross-layer bridging site includes a bridging coordination structure connecting the upper and lower adjacent transition metal oxide layers. The bridging coordination structure is misaligned with the sodium ion migration channel in the sodium layer and forms an intermittent support framework within the layered oxide host lattice.

[0011] In specific implementation, the sodium layer is the main region for sodium ion insertion / extraction and migration. The non-migrating pillar element does not discontinuously occupy the sodium ion migration channels, but instead forms a bridging coordination structure in the adjacent region between the sodium layer and the transition metal oxide layer. The bridging coordination structure connects the upper and lower adjacent transition metal oxide layers, creating spaced support between adjacent layers. During charging and discharging, the sodium ion migration channels within the sodium layer retain continuous space, while the bridging coordination structure restricts excessive relative displacement between adjacent transition metal oxide layers.

[0012] Furthermore, in the above technical solution, the heterovalent dopant elements are distributed in a gradient extending from the surface to the bulk phase in the layered oxide particles, and the non-migrating pillar elements are distributed in a gradient matching the heterovalent dopant elements in the region adjacent to the sodium layer, and both have a continuous transition distribution interface in the grain boundary region and the intragranular region of the layered oxide particles.

[0013] In practice, the surface, grain boundary, and intragranular regions of the layered oxide particles undergo different local structural changes during phase formation and charge / discharge processes. The heterovalent dopants extend from the surface into the bulk phase, while the non-migrating pillar elements form a matching gradient distribution in the region adjacent to the sodium layer, allowing for a continuous spatial transition between the two types of doped structures. This continuous transition reduces abrupt compositional changes between different regions within the particle, ensuring that both the grain boundary and intragranular regions contain doped structures connected to the bulk lattice of the layered oxide.

[0014] Furthermore, in the above technical solution, the layered oxide host lattice also contains charge compensation sites induced by the heterovalent doping elements. The charge compensation sites are located within the transition metal oxygen layer, and the interlayer bridging sites are located in the adjacent region of the sodium layer. At the unit cell scale, the two correspond to the transition metal layer sites and the interlayer sites, respectively.

[0015] In specific implementation, the charge compensation sites are located within the transition metal oxide layer, corresponding to the local charge environment after the heterovalent dopant enters the transition metal site; the interlayer bridging sites are located in the adjacent region of the sodium layer, corresponding to the interlayer connections between adjacent transition metal oxide layers. These two structures are respectively arranged at the transition metal layer sites and interlayer sites, ensuring that the charge compensation structure and the interlayer support structure do not substitute for each other. During sodium ion insertion / extraction, the charge compensation sites correspond to changes in the charge of the transition metal oxide layer, and the interlayer bridging sites correspond to changes in the interlayer position in the adjacent region of the sodium layer.

[0016] Furthermore, in the above technical solution, the heterovalent doping element is selected from metal elements that can enter the transition metal-oxygen layer and maintain the electrical neutrality of the layered oxide lattice. The metal element and the main transition metal in the transition metal-oxygen layer together form a mixed metal-oxygen octahedral array, and the mixed metal-oxygen octahedral array is continuously arranged along the sodium ion diffusion plane.

[0017] In specific implementation, after the metal element enters the transition metal-oxygen layer, it participates in the formation of metal-oxygen octahedra together with the host transition metal, transforming the transition metal-oxygen layer from a single host metal-oxygen octahedral arrangement into a mixed metal-oxygen octahedral array. The mixed metal-oxygen octahedral array is continuously arranged along the sodium ion diffusion plane, avoiding the doping sites from disrupting the planar structure of the transition metal-oxygen layer. During charging and discharging, the continuously arranged mixed metal-oxygen octahedral array maintains an adjacent relationship with the sodium ion diffusion plane of the sodium layer, ensuring that the doped structure matches the sodium ion migration space.

[0018] Furthermore, in the above technical solution, the non-migrating pillar element is selected from a metal element with a fixed coordination tendency. The metal element with the fixed coordination tendency forms discrete pillar sites in the region adjacent to the sodium layer. The discrete pillar sites discontinuously occupy the main migration path of the sodium layer and form stable coordination connections with oxygen atoms in the transition metal oxygen layer.

[0019] In specific implementation, the metal element with the fixed coordination tendency forms discrete strut sites in the region adjacent to the sodium layer. These discrete strut sites are coordinated and connected to oxygen atoms in the transition metal-oxygen layer, fixing the strut structure in the interlayer adjacency region. Because the discrete strut sites discontinuously occupy the main migration path of the sodium layer, sodium ions can still move along the main migration path within the sodium layer. During charging and discharging, the discrete strut sites exist as local interlayer connection points, and their spatial distribution is offset from the sodium ion migration space.

[0020] Furthermore, in the above technical solution, the continuously transitioning distribution interface includes a doping distribution structure composed of a particle surface enrichment region, a transition diffusion region, and a bulk solid solution region. The heterovalent dopant element enters the transition metal site in the bulk solid solution region, and the non-migrating pillar element forms interlayer pillar sites in the particle surface enrichment region and the transition diffusion region.

[0021] In specific implementation, the enriched region on the particle surface, the transition diffusion region, and the bulk solid solution region constitute a doping distribution structure from the outside to the inside of the particle. The heterovalent dopant enters the transition metal sites in the bulk solid solution region, giving the transition metal oxygen layer inside the particle a doped coordination structure; the non-migrating pillar element forms interlayer pillar sites in the enriched region and the transition diffusion region on the particle surface, giving the adjacent sodium layer near the surface and grain boundary transition regions interlayer connectivity. This doping distribution structure ensures that the doping morphology in different regions corresponds to its lattice position.

[0022] Furthermore, in the above technical solution, the charge compensation site and the interlayer bridging site are interconnected through a shared oxygen coordination environment. The shared oxygen coordination environment includes an oxygen coordination network connecting the heterovalent dopant, the host transition metal, and the non-migrating pillar element, and the oxygen coordination network maintains an alternating layered arrangement of the sodium layer and the transition metal oxygen layer.

[0023] In specific implementation, the shared oxygen coordination environment enables charge compensation sites and interlayer bridging sites to form structural associations through the coordination network where oxygen atoms reside. This oxygen coordination network connects heterovalent dopants, the host transition metal, and non-migrating pillar elements, allowing dopants located within the transition metal oxide layer and pillar sites located in the adjacent region of the sodium layer to be incorporated into the same layered oxide host lattice. During sodium ion insertion / extraction, the oxygen coordination network maintains the alternating layered arrangement of the sodium layer and the transition metal oxide layer, enabling the two types of sites to work collaboratively within the same lattice framework.

[0024] Furthermore, the present invention also provides a method for preparing a doped modified layered oxide sodium-ion battery cathode material, comprising: mixing a sodium source, a transition metal source, a heterovalent dopant source, and a non-migrating pillar element source to form a composite precursor system, wherein the heterovalent dopant source and the transition metal source form a transition metal layer precursor structure, and the non-migrating pillar element source is distributed in the sodium layer formation region; and performing phase formation treatment on the composite precursor system to obtain a layered oxide sodium-ion battery cathode material containing a heterovalent doped transition metal oxygen layer and interlayer bridging sites.

[0025] In specific implementation, the preparation method simultaneously introduces a sodium source, a transition metal source, a heterovalent dopant source, and a non-migrating pillar element source in the composite precursor system stage. This allows the heterovalent dopant source to form an adjacent distribution base with the transition metal source in the transition metal layer precursor structure, and the non-migrating pillar element source to be pre-distributed in the sodium layer formation region. During the phase formation process, the transition metal layer precursor structure transforms into a transition metal-oxygen layer, and the heterovalent dopant source enters some transition metal sites within the transition metal-oxygen layer; the non-migrating pillar element source forms interlayer bridging sites in the region adjacent to the sodium layer. This yields a layered oxide host lattice structure corresponding to the doped and modified layered oxide sodium-ion battery cathode material.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This doped and modified layered oxide sodium-ion battery cathode material incorporates heterovalent dopants that partially replace transition metal sites within the same layered oxide particle. Non-migrating pillar elements form interlayer bridging sites in the region adjacent to the sodium layer, ensuring that both the transition metal-oxygen layer and the sodium layer are structurally constrained. After entering the transition metal-oxygen layer, the heterovalent dopants form metal-oxygen coordination units embedded in the octahedral framework with adjacent oxygen atoms, partially replacing transition metal sites prone to valence fluctuations with stable heterovalent sites. The non-migrating pillar elements form bridging coordination structures connecting adjacent transition metal-oxygen layers in the region adjacent to the sodium layer, and are misaligned with the sodium ion migration channels, preventing the interlayer support structure from directly blocking the sodium ion migration plane. With both types of structures co-distributed within the same particle, the local charge compensation sites of the transition metal layer and the interlayer bridging sites form continuous lattice constraints through the oxygen coordination environment. During repeated sodium ion insertion / extraction, the material simultaneously reduces transition metal layer site migration and relative sodium layer slippage, making it easier to maintain the alternating arrangement of the layered framework and reducing irreversible structural rearrangement during cycling.

[0027] 2. Gradiently distributed heterovalent dopants and non-migrating pillar elements enable the formation of continuous doped interfaces between the particle surface, transition regions, and bulk regions, reducing grain boundary stress concentration caused by abrupt compositional changes. Heterovalent dopants enter transition metal sites in the bulk solid solution region, while non-migrating pillar elements form interlayer pillar sites in the enriched region and transition diffusion region of the particle surface, ensuring that different regions of the particle have stable units adapted to their structural state. Charge compensation sites and interlayer bridging sites are interconnected through a shared oxygen coordination environment, forming an oxygen coordination network connecting heterovalent dopants, the main transition metal, and non-migrating pillar elements. This network maintains the alternating layered arrangement of the sodium layer and the transition metal oxygen layer. Because the pillar sites are discretely distributed and do not discontinuously occupy the main migration path of the sodium layer, the material retains the continuity of sodium ion diffusion channels while obtaining interlayer support, thereby reducing the risk of ion transport obstruction caused by structural reinforcement. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to embodiments. Those skilled in the art can reproduce the technical solution of the present invention and achieve its claimed technical effects based on the content disclosed in this specification. It should be noted that the following embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention. Any non-substantial improvements and adjustments made based on the core concept of the present invention should fall within the scope of protection of the present invention.

[0029] In the following embodiments, the sodium source is calculated as sodium element, and the transition metal source, heterovalent dopant source, and non-migrating pillar element source are calculated as their respective metal elements. The total molar fraction of the transition metal layer host cation, heterovalent dopant element, and non-migrating pillar element is 1.00. The preparation of the layered oxide sodium-ion battery cathode material revolves around the formation and phase formation of the composite precursor system.

[0030] Example 1: The sodium source is sodium carbonate, with a sodium molar content of 1.03; the transition metal sources include nickel oxide, iron oxide, and manganese oxide, with nickel molar content of 0.32, iron molar content of 0.32, and manganese molar content of 0.31; the anisovalent doping source is magnesium oxide, with a magnesium molar content of 0.03; and the non-migratory pillar element source is titanium dioxide, with a titanium molar content of 0.02.

[0031] The preparation method is as follows: Nickel oxide, iron oxide, manganese oxide, and magnesium oxide are weighed according to the above formula and placed in a ball mill jar for the first stage of mixing. The mixture is ball-milled for 6 hours to obtain a transition metal layer precursor mixture containing nickel, iron, manganese, and magnesium. Sodium carbonate and titanium dioxide are added to the transition metal layer precursor mixture for the second stage of mixing. The mixture is ball-milled for 3 hours to obtain a composite precursor system, where magnesium oxide and the transition metal source together constitute the transition metal layer precursor structure, and titanium dioxide and the sodium source are distributed together in the sodium layer formation region. The composite precursor system is placed in an alumina crucible and heated to 500°C at 5°C per minute in air atmosphere, held for 5 hours, cooled, and then ground. The temperature is then increased to 850°C at 5°C per minute and held for 12 hours, followed by cooling to room temperature at 3°C ​​per minute. The mixture is then ground and passed through a 200-mesh sieve to obtain a doped modified layered oxide sodium-ion battery cathode material. In the resulting material, magnesium enters the transition metal-oxygen layer and forms metal-oxygen coordination units, while titanium forms translayer bridging sites in the region adjacent to the sodium layer. Both are distributed together within the same layered oxide particle.

[0032] Example 2: The difference between this example and Example 1 is that the amount of magnesium oxide, the heterovalent dopant source, is adjusted to 0.02 moles of magnesium, 0.32 moles of manganese, 0.32 moles of nickel, and 0.32 moles of iron. The non-migrating pillar element source, titanium dioxide, is still added at 0.02 moles of titanium, and the moles of sodium remain at 1.03 moles. All other preparation conditions are the same as in Example 1. This example is used to verify the structural preservation of the coexistence of metal-oxygen coordination units in the transition metal-oxygen layer and interlayer bridging sites in the adjacent region of the sodium layer when the proportion of heterovalent dopant elements is relatively low.

[0033] Example 3: The difference between this example and Example 1 is that the amount of magnesium oxide, the heterovalent dopant source, is adjusted to 0.04 moles of magnesium, 0.30 moles of manganese, 0.32 moles of nickel, and 0.32 moles of iron. The non-migrating pillar element source, titanium dioxide, is still added at 0.02 moles of titanium, and the moles of sodium remain at 1.03 moles. All other preparation conditions are the same as in Example 1. This example is used to verify the effect of magnesium doping sites on the continuity of the octahedral framework of the transition metal oxygen layer when the proportion of heterovalent dopant elements is relatively high.

[0034] Example 4: The difference between this example and Example 1 is that the heterovalent dopant source is replaced with aluminum oxide instead of magnesium oxide, with an aluminum molar fraction of 0.03, nickel molar fraction of 0.32, iron molar fraction of 0.32, and manganese molar fraction of 0.31. The non-migrating pillar element source, titanium dioxide, is still added at a titanium molar fraction of 0.02, and the sodium molar fraction remains 1.03. All other preparation conditions are the same as in Example 1. This example is used to verify the adaptability of a metal element capable of entering the transition metal-oxygen layer and maintaining the electrical neutrality of the layered oxide lattice as a heterovalent dopant element.

[0035] Example 5: The difference between this example and Example 1 is that the non-migratory pillar element source is replaced with zirconium dioxide instead of titanium dioxide. The molar fraction of zirconium is 0.02; the molar fractions of magnesium, nickel, iron, and manganese are 0.32 and 1.03 respectively. All other preparation conditions are the same as in Example 1. This example is used to verify the adaptability of a metal element with a fixed coordination tendency as a non-migratory pillar element in forming discrete pillar sites in the region adjacent to the sodium layer.

[0036] Example 6: The difference between this example and Example 1 is that the heterovalent doping source is replaced with zinc oxide instead of magnesium oxide, with a zinc molar content of 0.03, nickel molar content of 0.32, iron molar content of 0.32, and manganese molar content of 0.31. The non-migrating pillar element source, titanium dioxide, is still added at a titanium molar content of 0.02, and the sodium molar content remains 1.03. All other preparation conditions are the same as in Example 1. This example is used to verify the stability of the continuous arrangement of the mixed metal-oxygen octahedral array along the sodium ion diffusion plane after changing the type of heterovalent doping element.

[0037] Example 7: The difference between this example and Example 1 is that the non-migratory support element source uses a combination of titanium dioxide and zirconium dioxide, with titanium having a molar fraction of 0.01, zirconium having a molar fraction of 0.01, magnesium having a molar fraction of 0.03, nickel having a molar fraction of 0.32, iron having a molar fraction of 0.32, manganese having a molar fraction of 0.31, and sodium having a molar fraction of 1.03. The remaining preparation conditions are the same as in Example 1. This example is used to verify the continuity of the main migration path of the sodium layer when two metal elements with fixed coordination tendencies jointly form discrete support sites.

[0038] Example 8: The difference between this example and Example 1 is that the molar amounts of nickel, iron, and manganese in the transition metal source are adjusted to 0.28, 0.34, and 0.33, respectively. The heterovalent dopant source, magnesium oxide, is still added at a molar amount of 0.03, the non-migrating pillar element source, titanium dioxide, is still added at a molar amount of 0.02, and the sodium molar amount remains 1.03. All other preparation conditions are the same as in Example 1. This example is used to verify the adaptability of heterovalent dopant elements and non-migrating pillar elements to co-distribution within the same layered oxide particles when the proportion of the main transition metal changes.

[0039] Example 9: The difference between this example and Example 1 is that the sodium source is replaced with sodium bicarbonate instead of sodium carbonate. The molar amount of sodium remains 1.03; the molar amounts of nickel, iron, manganese, magnesium, and titanium are 0.32, 0.32, 0.31, 0.03, and 0.02, respectively. All other preparation conditions are the same as in Example 1. This example is used to verify the adaptability of the composite precursor system to form an alternating stacked structure of sodium and transition metal-oxygen layers after phase formation treatment following sodium source replacement.

[0040] Example 10: This example has the same formulation as Example 1, except that the mixing times of the first and second stages in the formation of the composite precursor system are adjusted. The first stage mixing time is adjusted from 6 hours to 4 hours, and the second stage mixing time is adjusted from 3 hours to 4 hours; the remaining phase formation conditions are the same as in Example 1. This example is used to verify the process adaptability of heterovalent doping sources and transition metal sources in forming a transition metal layer precursor structure and the distribution of non-migrating pillar element sources in the sodium layer formation region when the mixing time is varied.

[0041] Example 11: This example has the same formulation as Example 1, only the pre-phase treatment conditions are adjusted. The 500℃ holding time for 5 hours in Example 1 is changed to 480℃ holding time for 6 hours; the final phase formation treatment remains 850℃ holding time for 12 hours, and the other conditions are the same as in Example 1. This example is used to verify the stability of the subsequent formation of heterovalent doped transition metal oxygen layer and interlayer bridging sites when the degree of thermal decomposition of the composite precursor system changes during the pre-phase stage.

[0042] Example 12: This example has the same formulation as Example 1, only the final phase formation treatment conditions are adjusted. The holding time of 850℃ for 12 hours in Example 1 is changed to 870℃ for 8 hours; the first stage mixing, second stage mixing, and pre-phase formation treatment conditions are the same as in Example 1. This example is used to verify the formation stability of the layered oxide host lattice and the discrete support sites in the adjacent region of the sodium layer when the final phase formation temperature and holding time are changed.

[0043] Comparative Example 1: This comparative example is a comparison example lacking the core component. Compared with Example 1, no heterovalent dopant source, magnesium oxide, was added, and the molar amount of magnesium was 0; to maintain the total molar amount of metal elements at 1.00, the molar amount of manganese was adjusted from 0.31 to 0.34; the molar amounts of nickel and iron were 0.32; the non-migrating pillar element source, titanium dioxide, was still added at a molar amount of titanium (0.02); and the molar amount of sodium remained at 1.03. The remaining preparation conditions were the same as in Example 1. This comparative example is used to verify the influence of the interlayer bridging sites in the adjacent region of the sodium layer on the bulk lattice of the layered oxide when the heterovalent dopant element of the transition metal oxide layer is missing.

[0044] Comparative Example 2: This comparative example is a conventional single-doped comparative example. Compared with Example 1, the non-migratory pillar element source titanium dioxide was not added, and the molar fraction of titanium was 0; to maintain the total molar fraction of metal elements at 1.00, the molar fraction of manganese was adjusted from 0.31 to 0.33; the molar fraction of magnesium remained at 0.03, the molar fraction of nickel at 0.32, the molar fraction of iron at 0.32, and the molar fraction of sodium at 1.03. The remaining preparation conditions were the same as in Example 1. This comparative example corresponds to the conventional modification method in the prior art, which only replaces part of the transition metal sites with heterovalent doping elements, and is used to verify the structure retention capability when interlayer bridging sites in the adjacent region of the missing sodium layer are missing.

[0045] Comparative Example 3: This comparative example shows a deviation of the preparation method parameters from the comparative example. The formulation is completely consistent with Example 1, and the conditions for the first stage mixing, the second stage mixing, and the pre-phase formation treatment are all the same as in Example 1. Only the final phase formation treatment conditions are adjusted to be 950℃ for 12 hours. All other conditions are the same as in Example 1. This comparative example is used to verify the changes in sodium source volatilization, disturbance of the sodium layer formation region, and preservation of the layered oxide lattice when the final phase formation temperature deviates from the suitable phase formation range.

[0046] Comparative Example 4: This comparative example is for a missing preparation method. The formulation is completely identical to that of Example 1, but the staged mixing method in Example 1 is omitted. Sodium carbonate, nickel oxide, iron oxide, manganese oxide, magnesium oxide, and titanium dioxide are added to a ball mill jar all at once and mixed for 9 hours. Subsequently, the pre-phase formation treatment and final phase formation treatment conditions of Example 1 are followed. The remaining conditions are the same as in Example 1. This comparative example is used to verify the differences in material structure when the heterovalent dopant source is not preferentially combined with the transition metal source to form a transition metal layer precursor structure, and when the non-migrating pillar element source is not preferentially distributed in the sodium layer formation region.

[0047] Test method: The cathode materials obtained in Examples 1 to 12 and Comparative Examples 1 to 4 were respectively prepared as cathode sheets.

[0048] The positive electrode active material, conductive agent and binder are mixed in the same mass ratio to form a slurry, which is then coated onto the current collector and dried and pressed into tablets. Coin cells were assembled using sodium metal sheets as the counter electrode, and charge-discharge tests were conducted under the same voltage range and the same rate regime.

[0049] X-ray diffraction analysis was used to determine the retention rate of the lamellar phase after cycling. Electrochemical impedance spectroscopy was used to test the changes in charge transfer impedance before and after cycling. The proportion of cracked particles after cycling was statistically analyzed by observing the cross-sectional morphology.

[0050] The test results are shown in Table 1.

[0051] Table 1 Performance test results of each embodiment and comparative example

[0052] Example 1 uses magnesium oxide as a heterovalent dopant source and titanium dioxide as a non-migrating pillar element source. Through staged mixing, magnesium oxide preferentially forms a transition metal layer precursor structure with the transition metal source, allowing titanium dioxide and sodium sources to co-distribute in the sodium layer formation region. Test results show that Example 1 exhibits a 91.6% capacity retention after 200 cycles, a 96.2% layered phase retention after 200 cycles, a 24.5% increase in charge transfer impedance, a 2.8% cation mixing ratio after cycling, and a 5.4% proportion of cracked particles after cycling. These data indicate that when heterovalent dopant elements and non-migrating pillar elements are co-distributed within the same layered oxide particles, metal-oxygen coordination units within the transition metal-oxygen layer and interlayer bridging sites in the adjacent sodium layer can coexist. This results in a relatively intact layered structure after cycling, low impedance accumulation, and fewer particle cracks.

[0053] In Examples 2 and 3, the amount of magnesium oxide added was adjusted. Compared to Example 1, the molar amount of magnesium in Example 2 was reduced, resulting in a 200-cycle capacity retention of 88.7% and a cation mixing ratio of 3.4%. In Example 3, the molar amount of magnesium was increased, resulting in a 200-cycle capacity retention of 89.4% and a cation mixing ratio of 3.1%. These two sets of data demonstrate that, regardless of the proportion of heterovalent dopants, as long as the heterovalent dopants can still enter the transition metal oxide layer and coexist with the interlayer bridging sites formed by the non-migrating pillar elements, the material can still maintain high cycling stability. The cation mixing ratio in Example 3 was slightly lower than in Example 2, indicating that a higher magnesium doping amount has a stronger constraint on the stability of sites within the transition metal oxide layer; however, its first-cycle discharge specific capacity was lower than that of Example 1, indicating that excessive inactive doping sites would occupy some transition metal sites that could participate in charge compensation.

[0054] In Examples 4 and 6, the heterovalent dopant sources were replaced with alumina and zinc oxide, respectively. In Examples 5 and 7, the non-migrating pillar element source was replaced with zirconium dioxide or a combination of titanium dioxide and zirconium dioxide, respectively. The capacity retention rate after 200 cycles was maintained above 86.8% in all replacement examples, and the layered phase retention rate after 200 cycles was maintained above 92.9%, indicating that metal elements capable of entering the transition metal-oxygen layer and maintaining the electrical neutrality of the layered oxide lattice can be used as heterovalent dopant elements, and metal elements with a fixed coordination tendency can be used as non-migrating pillar elements. The charge transfer impedance growth rate in Example 7 was 29.1%, lower than the 30.6% in Example 5, indicating that the discrete pillar sites formed by the combination of titanium and zirconium can maintain interlayer connectivity even when discontinuously occupying the main migration path of the sodium layer.

[0055] Example 8 changed the proportions of the main transition metals (nickel, iron, and manganese), and Example 9 changed the type of sodium source. The 200-cycle capacity retention rate of Example 8 was 86.2%, and that of Example 9 was 87.0%, both higher than all comparative examples. This result indicates that even with changes in the proportions of the main transition metals and the type of sodium source, the composite precursor system, after phase formation treatment, can still form a layered oxide lattice with alternating sodium and transition metal oxygen layers, and can accommodate the co-distribution of heterovalent dopants and non-migrating pillar elements. The first-cycle discharge specific capacity of Example 8 was lower than that of Example 1, mainly due to the change in the electrochemical active site composition after the change in the proportion of the main transition metals; however, its layered phase retention rate was still 92.4%, indicating that the dual-site structure still has reproducibility in lattice retention.

[0056] Examples 10 to 12 only changed the mixing time, pre-phase treatment conditions, or final phase formation treatment conditions in the preparation method. The 200-cycle capacity retention rate of Example 10 was 88.4%, Example 11 was 88.8%, and Example 12 was 89.0%, all within the same performance range as Example 1. This result shows that the preparation method does not rely on a single fixed process point to form the target structure; when there are reasonable changes in the formation of the composite precursor system and the phase formation treatment conditions, the heterovalent doping source can still form a transition metal layer precursor structure with the transition metal source, the non-migrating pillar element source can still be distributed in the sodium layer formation region, and the final material can still form a layered oxide structure containing a heterovalent doped transition metal oxygen layer and interlayer bridging sites.

[0057] Comparative Example 1 removed the heterovalent dopant source magnesium oxide, retaining only the non-migrating pillar element source titanium dioxide. Its 200-cycle capacity retention decreased to 72.3%, and after 200 cycles, the layered phase retention decreased to 79.4%, while the cation mixing ratio increased to 8.7%. This result indicates that without the heterovalent dopant, the transition metal sites in the transition metal-oxygen layer, which are prone to valence state fluctuations, lack corresponding metal-oxygen coordination units for constraint. Even if interlayer bridging sites exist in the region adjacent to the sodium layer, it is difficult to restrict cation mixing within the transition metal-oxygen layer. Comparative Example 2 used a conventional single heterovalent doping method without adding the non-migrating pillar element source titanium dioxide. Its 200-cycle capacity retention was 78.1%, and after 200 cycles, the layered phase retention was 83.6%, with a crack particle ratio of 18.3% after cycling. This result indicates that when relying solely on heterovalent dopant to replace the transition metal sites, the region adjacent to the sodium layer lacks interlayer bridging sites, and interlayer relative slip during charge and discharge processes still accumulates.

[0058] Comparative Example 3, with the same formulation, had its final phase formation treatment adjusted to 950℃ for 12 hours. Its first-cycle discharge specific capacity was 108.7 mAh / g, the 200-cycle capacity retention was 63.8%, the charge transfer impedance growth rate was 102.4%, and the proportion of cracked particles after cycling was 34.7%. These results indicate that when the final phase formation treatment temperature deviates from the suitable phase formation range, the sodium layer formation region is disturbed, the integrity of the layered oxide lattice decreases, and the heterovalent doped transition metal oxygen layer and interlayer bridging sites are difficult to form in a coordinated state. Comparative Example 4, although having the same formulation as Example 1, omitted the staged mixing method and used a one-time mixing of all raw materials. Its 200-cycle capacity retention was 80.4%, the layered phase retention was 86.2%, and the cation mixing ratio was 6.4%. These results indicate that identical components alone are insufficient to obtain the same structural effect; preferentially forming a transition metal layer precursor structure with the heterovalent dopant source and preferentially distributing non-migrating pillar element sources in the sodium layer formation region are important preparation conditions for obtaining a synergistic distribution of dual structures within the same particle.

[0059] The comparison between the examples and the comparative examples reveals a synergistic relationship between the metal-oxygen coordination units formed by heterovalent doped elements within the transition metal-oxygen layer and the interlayer bridging sites formed by non-migrating pillar elements in the adjacent region of the sodium layer. The absence of either structure results in either increased cation mixing or accumulation of interlayer slip. Improper phase formation treatment or an imbalance in the distribution of the composite precursor system leads to decreased structural stability even with the same formulation. Examples 1 to 12 outperform the comparative examples under different formulation gradients, element substitutions, and process conditions, demonstrating that the doped modified layered oxide sodium-ion battery cathode material and its preparation method can stably form heterovalent doped transition metal-oxygen layers and interlayer bridging sites within the same layered oxide particle, thereby achieving unexpected structural retention and cycle stability.

Claims

1. A doped and modified layered oxide cathode material for sodium-ion batteries, characterized in that, The cathode material comprises a layered oxide host lattice formed by alternating stacking of sodium layers and transition metal oxide layers. The transition metal oxide layers contain heterovalent dopants that replace some transition metal sites. The sodium layer adjacent region contains cross-layer bridging sites formed by non-migrating pillar elements. The heterovalent dopants and the non-migrating pillar elements are co-distributed within the same layered oxide particle.

2. The doped modified layered oxide sodium-ion battery cathode material according to claim 1, characterized in that, The heterovalent dopant occupies the transition metal sites in the transition metal-oxygen layer that are prone to valence fluctuations, and the heterovalent dopant forms metal-oxygen coordination units with adjacent oxygen atoms. The metal-oxygen coordination units are embedded in the octahedral framework of the transition metal-oxygen layer and are connected to adjacent transition metal-oxygen polyhedra by sharing edges.

3. The doped modified layered oxide sodium-ion battery cathode material according to claim 1, characterized in that, The non-migrating pillar element is distributed in the adjacent region of the sodium layer and the transition metal oxide layer. The cross-layer bridging site includes a bridging coordination structure connecting the upper and lower adjacent transition metal oxide layers. The bridging coordination structure is misaligned with the sodium ion migration channel in the sodium layer and forms an intermittent support framework within the layered oxide host lattice.

4. The doped modified layered oxide sodium-ion battery cathode material according to claim 1, characterized in that, The heterovalent dopant elements are distributed in a gradient extending from the surface to the bulk phase in the layered oxide particles, and the non-migrating pillar elements are distributed in a gradient matching the heterovalent dopant elements in the region adjacent to the sodium layer. Both have a continuous transition distribution interface in the grain boundary region and the intragranular region of the layered oxide particles.

5. The doped modified layered oxide sodium-ion battery cathode material according to claim 1, characterized in that, The layered oxide host lattice also contains charge compensation sites induced by the heterovalent dopant elements. The charge compensation sites are located within the transition metal oxide layer, and the interlayer bridging sites are located in the region adjacent to the sodium layer. At the unit cell scale, they correspond to the transition metal layer site and the interlayer site, respectively.

6. The doped modified layered oxide sodium-ion battery cathode material according to claim 2, characterized in that, The heterovalent doping element is selected from metal elements that can enter the transition metal-oxygen layer and maintain the electrical neutrality of the layered oxide lattice. The metal element and the main transition metal in the transition metal-oxygen layer together form a mixed metal-oxygen octahedral array, and the mixed metal-oxygen octahedral array is continuously arranged along the sodium ion diffusion plane.

7. The doped modified layered oxide sodium-ion battery cathode material according to claim 3, characterized in that, The non-migrating strut element is selected from metal elements with a fixed coordination tendency. The metal elements with a fixed coordination tendency form discrete strut sites in the region adjacent to the sodium layer. The discrete strut sites discontinuously occupy the main migration path of the sodium layer and form stable coordination connections with oxygen atoms in the transition metal oxygen layer.

8. The doped modified layered oxide sodium-ion battery cathode material according to claim 4, characterized in that, The continuous transition distribution interface includes a doping distribution structure consisting of a particle surface enrichment region, a transition diffusion region, and a bulk solid solution region. The heterovalent dopant element enters the transition metal site in the bulk solid solution region, and the non-migrating pillar element forms interlayer pillar sites in the particle surface enrichment region and the transition diffusion region.

9. The doped modified layered oxide sodium-ion battery cathode material according to claim 5, characterized in that, The charge compensation sites and the interlayer bridging sites are interconnected through a shared oxygen coordination environment, which includes an oxygen coordination network connecting the heterovalent dopant, the host transition metal, and the non-migrating pillar element, and the oxygen coordination network maintains an alternating layered arrangement of the sodium layer and the transition metal oxygen layer.

10. A method for preparing a doped and modified layered oxide sodium-ion battery cathode material, characterized in that, include: A composite precursor system is formed by mixing a sodium source, a transition metal source, a heterovalent doped source, and a non-migrating pillar element source, so that the heterovalent doped source and the transition metal source form a transition metal layer precursor structure, and the non-migrating pillar element source is distributed in the sodium layer formation region. The composite precursor system was subjected to phase formation treatment to obtain a layered oxide sodium-ion battery cathode material containing a heterovalent doped transition metal oxygen layer and interlayer bridging sites.

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