Sodium Metal Anode Having Back-Side Electric Field Application Structure for Uniform Deposition Control, Sodium-Ion Secondary Battery Comprising the Same, and Charge / Discharge Method Thereof

Abstract

The present invention relates to a secondary battery and a charging and discharging method, wherein a back surface electric field application means comprising a mesh-shaped back surface auxiliary electrode and a sodium ion-conducting insulating layer such as NASICON-based oxide or β-alumina is disposed on the back surface opposite to the front surface facing the anode of a sodium metal cathode, and by applying a uniform electric field from the back surface auxiliary electrode, the current density distribution is homogenized across the entire front surface of the sodium metal cathode to suppress the formation of sodium dendrites and the generation of dead sodium. The secondary battery according to the present invention independently controls the uniform precipitation of sodium ions during charging and the uniform dissolution of sodium metal during discharging by means of a first voltage and a second voltage based on a Na / Na+ reference potential, and dynamically adjusts the applied voltage through a sensing unit that detects the front surface current density distribution in real time and a feedback control unit. The present invention is applicable to all-solid-state sodium batteries and hard carbon composite cathode structures, and significantly improves the lifespan, safety, and Coulomb efficiency of sodium metal secondary batteries for grid-connected energy storage devices, humanoid robots, and next-generation mobility.

Description

Technology Field

[0001] The present invention relates to a sodium metal negative electrode and a secondary battery including the same, and more specifically, to sodium ions (Na) generated on the front surface, which is the positive-opposition surface of the sodium metal negative electrode. + Heterogeneous precipitation of ) and sodium metal (Na o The present invention relates to a structure and method for homogenizing the current density distribution across the entire front surface by placing an auxiliary electrode on the back surface opposite the front surface and applying a uniform electric field from said auxiliary electrode in order to suppress non-uniform dissolution of ). The present invention also relates to Na during charging + Ç Uniform plating and Na during discharge o The present invention relates to a secondary battery charging and discharging method that simultaneously suppresses sodium dendrite formation and dead sodium generation by achieving uniform stripping in both directions through real-time feedback control. Background Technology

[0002] Sodium-ion rechargeable batteries utilize sodium, which is abundant in the Earth's crust, as a charge carrier, resulting in lower raw material costs and higher supply chain stability compared to lithium-ion rechargeable batteries. Additionally, the ability to use aluminum as a negative electrode current collector further reduces cell unit costs, and the capability for 0 V discharge ensures high safety during transportation, possessing numerous strengths suitable for large-capacity stationary Energy Storage Systems (ESS). Due to these characteristics, sodium-ion rechargeable batteries are attracting attention as a next-generation battery technology in the fields of artificial intelligence data centers, renewable energy power plants, smart grids, and power storage for electric vehicles.

[0003] Currently, commercially available sodium-ion secondary batteries primarily use hard carbon as the negative electrode active material, and their theoretical specific capacity is known to be around 300 mAh / g. However, to realize next-generation high-energy-density sodium-ion secondary batteries, sodium metal (Na₂S) possessing a theoretical specific capacity of 1,165 mAh / g and a reduction potential of -2.714 V relative to the Standard Hydrogen Electrode (SHE) is required. o Sodium metal secondary batteries that directly use ) as the negative electrode active material are attracting attention. Since the sodium metal negative electrode has a theoretical specific capacity of approximately 3.9 times that of a hard carbon negative electrode, it has the potential to dramatically improve energy density at the cell level.

[0004] However, the most fundamental technical barrier hindering the commercialization of sodium metal anodes is the problem of sodium dendrite formation during repeated charge-discharge cycles. Dendrites form on the front surface of the anode that comes into contact with the electrolyte during charging, specifically on the surface facing the anode, where Na... + ¡Na o It is a structure formed as sodium metal grows intensively at specific points due to local current density non-uniformity on the cathode surface during the process of reduction and precipitation of the membrane. Unlike lithium dendrites, sodium dendrites tend to grow in spherical or nodular shapes, so the reduction in Coulomb efficiency and capacity caused by the formation of dead sodium, which is electrically isolated from the cathode body, acts as a more serious degradation mechanism than a short circuit caused by penetration of the separator.

[0005] A similar problem occurs during discharge. Sodium metal, which is unevenly deposited during the charging process, strips intensively at specific points on the surface during discharge, forming pitting—localized depressions—on the cathode surface. The pitted cathode surface acts as a nucleation site that triggers stronger current concentration in the next charging cycle, leading to a vicious cycle in which dendrite formation and the generation of dead sodium are accelerated. Furthermore, since the sodium metal isolated as dead sodium during discharge cannot participate in electrochemical reactions again, irreversible loss of the active material accumulates as the cycle progresses, causing a rapid decrease in battery capacity.

[0006] Sodium ions (Na + The ionic radius (1.02 Å) of ) is lithium ion (Li + Because it is about 35% larger than the 0.76 Å layer, the Solid Electrolyte Interphase (SEI) layer in sodium metal anodes is structurally non-uniform and has lower mechanical strength than the SEI layer in lithium metal anodes. This acts as a fundamental cause that exacerbates the problems of non-uniform precipitation and dead sodium formation in sodium metal anodes compared to lithium metal anodes. Therefore, to stabilize sodium metal anodes, a separate design based on the unique ionic chemical properties of sodium is required, rather than simply applying lithium metal-based technology.

[0007] In conventional technology, to solve these problems, methods have been adopted to form an artificial SEI layer or coat a protective layer on the front surface of the cathode, that is, the precipitation and dissolution surface in direct contact with the electrolyte. Representative materials being studied include sodium fluoride (NaF), sodium nitride (Na₃N), alumina (Al₂O₃), and composite polymer protective films. However, all of these conventional technologies are approaches that attempt to solve the problem at the material level of the front surface of the cathode, and they have limitations in completely controlling localized current concentration at the nanometer (nm) to micrometer (μm) level solely through material properties; in particular, they fail to fundamentally resolve the problem of SEI non-uniformity caused by the large ionic radius of sodium ions.

[0008] In addition, a method to lower the effective current density using a three-dimensional (3D) porous scaffold structure has been proposed, but this has the side effect of complicating the cathode structure itself and lowering energy density, and has limitations in that large-area mass production is low due to the complexity of the scaffold manufacturing process. Against this technical background, there is an urgent need to develop a new structure and method that can uniformly control the front current density by innovatively switching the position and direction of electric field application in a way that is fundamentally different from the material approach of the cathode front. The problem to be solved

[0009] The present invention was devised to solve the problems of the aforementioned conventional technology, and aims to fundamentally suppress non-uniform precipitation of sodium ions and non-uniform dissolution of sodium metal by homogenizing the current density distribution across the entire front surface of the cathode through placing an auxiliary electrode on the back surface opposite to the front surface of the sodium metal cathode, rather than on the front surface facing the anode, and applying a uniform electric field from said auxiliary electrode.

[0010] Another objective of the present invention is that Na during filling + Ç Uniform deposition and Na during discharge oBy controlling the uniform dissolution of sodium in both directions, the inhibition of sodium dendrite formation and the prevention of dead sodium formation are achieved simultaneously, thereby dramatically improving the lifespan, safety, and Coulomb efficiency of sodium metal secondary batteries.

[0011] Another objective of the present invention is to adopt a NASICON-based oxide or β-alumina optimized for the large ionic radius (1.02 Å) of sodium ions as an ion-conducting insulating layer, thereby not simply applying insulating layer materials used in lithium metal batteries, but through a design based on the intrinsic ion chemistry of sodium, the transmission efficiency of the back electric field and Na + It is to simultaneously optimize passage efficiency.

[0012] Another objective of the present invention is to provide a composite cathode structure that implements a dual storage mechanism in which interlayer intercalation storage and metal precipitation storage coexist by composing a hard carbon layer and a sodium metal cathode, and controls the uniformity of the metal precipitation region through combination with a back electric field application structure.

[0013] Another objective of the present invention is to provide a back electric field application structure applicable to all-solid-state sodium batteries, thereby realizing uniform deposition at the interface between a NASICON-based or β-alumina-based solid electrolyte and a sodium metal anode, and thereby accelerating the commercialization of next-generation all-solid-state sodium metal batteries. means of solving the problem

[0014] To achieve the above objective, one aspect of the present invention provides a secondary battery comprising a sodium metal negative electrode. The secondary battery according to the present invention includes a precipitation and dissolution surface on the front surface of a sodium metal negative electrode facing a positive electrode, where sodium ions are precipitated and dissolved, and includes a back surface electric field application means disposed on the back surface of the sodium metal negative electrode opposite to the precipitation and dissolution surface to form a uniform potential distribution over the entire precipitation and dissolution surface, wherein the back surface electric field application means reduces the deviation in current density at the precipitation and dissolution surface to suppress the formation of sodium dendrites and the generation of dead sodium.

[0015] According to one embodiment of the present invention, the back electric field application means comprises a mesh-shaped back auxiliary electrode and an ion-conducting insulating layer interposed between the back auxiliary electrode and the back surface of the sodium metal cathode, which allows sodium ions to pass through but blocks electrons, and the ion-conducting insulating layer comprises at least one selected from the group consisting of NASICON-based oxide (Na₃Zr₂Si₂PO₁₂), β-alumina (β-Al₂O₃), NaF, Na₃N, PEO / sodium salt composite polymer, and combinations thereof.

[0016] According to one embodiment of the present invention, the back electric field application means further includes a control unit that independently controls the applied voltage according to the charging and discharging cycle, so that when charging Na + First voltage (V₁) for precipitation homogenization and Na during discharge o A second voltage (V₂) for dissolution homogenization is applied to Na / Na + By applying it independently based on a reference potential, inhibition of sodium dendrite formation and inhibition of dead sodium generation are achieved simultaneously.

[0017] Another aspect of the present invention provides a method for charging and discharging a secondary battery comprising a sodium metal negative electrode. The charging and discharging method according to the present invention includes the step of precipitating or dissolving sodium ions on the front surface of the sodium metal negative electrode, which is the positive-opposite surface, and the step of suppressing the formation of sodium dendrites and the generation of dead sodium by applying a uniform electric field on the back surface opposite the front surface to reduce the variation in current density across the entire front surface. Effects of the invention

[0018] The sodium metal secondary battery according to the present invention can homogenize the current density distribution across the entire front surface (30a) of the negative electrode by arranging a back auxiliary electrode (50) on the back surface (30b) of the sodium metal negative electrode (30) and applying a uniform electric field, thereby Na + Ç Formation of sodium dendrites (D) due to non-uniform precipitation, and Na during discharge o Ç Simultaneously and fundamentally inhibits the formation of dead sodium (DS) caused by heterogeneous dissolution.

[0019] The key inventive step of the present invention lies in the fact that the control point for suppressing dendrites and dead sodium formation is shifted from the front side, where the problem occurs as in the prior art, to the back side, which is the opposite side. This can be likened to the method of distributing traffic evenly from the opposite entrance instead of dispersing vehicles in front of the congestion point to resolve traffic congestion; and since it is obvious that a person skilled in the art would approach the problem by directly treating the front side, the point where the problem occurs, the idea of ​​shifting the control point to the back side is not obvious.

[0020] Generally, an approach that attempts to directly address the point where non-uniformity occurs in a system is the first and most obvious approach considered by a person skilled in the art. On the other hand, an approach that prevents downstream non-uniformity by controlling the upstream point where the physical quantity causing the non-uniformity is formed, rather than the point where the non-uniformity occurs, is a non-obvious concept that can only be reached by analyzing the causal structure of the system in reverse. The present invention applies this universal principle to a sodium metal cathode, thereby Na + By controlling the potential distribution on the front surface (30a) where precipitation occurs from the opposite back surface (30b), homogenization is achieved through a mechanism that is fundamentally different from the front surface direct processing method adopted by conventional technology. This universal principle constitutes a key argument supporting the inventive step of the present invention.

[0021] The present invention relates to sodium ions (Na + The ionic radius (1.02 Å) of the lithium ion (Li + Reflecting the intrinsic ionic chemical properties of sodium, which are approximately 35% larger than (0.76 Å), an intrinsic ionic conductive inorganic material of sodium, such as NASICON-based oxide (Na₃Zr₂Si₂PO₁₂) or β-alumina (β-Al₂O₃), is adopted as the material for the ion-conducting insulating layer (40). LLZO (Li₃La₃Zr₂O₁₂) used in lithium metal batteries is Na₃ + Since the permeation efficiency is significantly low due to the large ionic radius, it cannot be simply substituted, and NASICON-based or β-alumina-based materials Na + It is significantly superior in terms of conductivity. This proves that simply applying technology from the field of lithium metal to sodium metal is not self-evident, and at the same time indicates that the present invention is a unique invention that solves the technical challenges inherent to sodium through the design of materials unique to sodium.

[0022] According to the present invention, considering that sodium dendrites grow in a spherical or lumpy form unlike lithium dendrites and that the generation of dead sodium (DS) is the main degradation mechanism, not only dendrite suppression during charging but also Na during discharging o The suppression of dead sodium formation through uniform dissolution is set as the primary control objective of the back electric field. By doing so, the irreversible loss of the sodium active material during the cycle is minimized, thereby achieving an effect in which the Coulomb efficiency after 500 charge-discharge cycles improves by more than 10% compared to the state without the back electric field applied, and the irreversible capacity loss caused by dead sodium formation is reduced by more than 30%.

[0023] According to the present invention, Na at the time of filling + First voltage (V₁) for precipitation homogenization and Na during discharge o A second voltage (V₂) for dissolution homogenization is applied to Na / Na + By controlling independently based on a reference potential, an optimized potential distribution can be formed on the front surface (30a) in each charging and discharging section. Compared to conventional technology aimed only at uniform deposition during charging, this provides the additional effect of maintaining the flatness of the negative electrode surface during discharge, and minimizes the deterioration of the negative electrode surface due to repeated cycles, thereby realizing long life characteristics.

[0024] The feedback control unit (63) according to the present invention detects the current density distribution of the front surface (30a) in real time and dynamically adjusts the applied voltage on the back surface, thereby maintaining an optimal current density uniformity state even if the negative electrode surface condition changes as the charge / discharge cycle progresses. Specifically, the applied voltage is dynamically adjusted so as to reduce the current density deviation (ΔJ) on the front surface (30a) by 50% or more, preferably 70% or more, compared to the state where the electric field on the back surface is not applied, thereby ensuring stable performance throughout the cycle life.

[0025] The composite cathode structure of the hard carbon layer (70) and the sodium metal cathode (30) according to the present invention implements a dual storage mechanism in which interlayer insertion storage in the hard carbon layer (70) and metal precipitation storage in the sodium metal cathode (30) coexist, thereby improving cycle stability compared to a single sodium metal cathode and increasing energy density compared to a single hard carbon cathode. The back electric field application structure controls the uniformity of the metal precipitation region in this composite cathode, thereby maximizing the synergistic effect of the two storage mechanisms.

[0026] The back electric field application structure according to the present invention can be directly applied to all-solid-state sodium batteries, so that Na at the interface between the sodium metal anode and NASICON-based (Na₃Zr₂Si₂PO₁₂), β-alumina-based, or sulfide-based (Na₃PS₄, Na₃SbS₄) solid electrolytes is applicable. + By improving precipitation uniformity through electric field control, it contributes to the commercialization of next-generation all-solid-state sodium metal batteries.

[0027] The formation of the ion-conducting insulating layer (40) and the back auxiliary electrode (50) by a roll-to-roll continuous process according to the present invention is performed in an inert atmosphere with an oxygen concentration of 1 ppm or less and a moisture concentration of 1 ppm or less, thereby preventing exposure of the sodium metal to air and moisture, and making the structure of the present invention suitable for mass production while minimizing changes to the existing battery production line. Brief explanation of the drawing

[0028] FIG. 1 is a schematic cross-sectional view of a sodium metal negative secondary battery (1) having a back electric field application structure according to one embodiment of the present invention. FIG. 2 is an exploded perspective view of a secondary battery (1) according to one embodiment of the present invention, showing the positive electrode (10), the electrolyte layer (20), the front surface of the sodium metal negative electrode (30a), the back surface of the sodium metal negative electrode (30b), the ion-conducting insulating layer (40), and the back auxiliary electrode (50) separated in order. FIG. 3 is a plan view of the mesh structure of a back auxiliary electrode (50) according to one embodiment of the present invention, showing a wire (51), an opening (52), and a grid spacing (d). FIG. 4 is an enlarged schematic cross-sectional view of an ion-conducting insulating layer (40) according to one embodiment of the present invention, showing the nanostructures of NASICON-based oxide, β-alumina, and NaF, respectively, and Na + This is a comparative illustration of ion passage pathways and electron blocking mechanisms. Figure 5 shows Na during charging + As a conceptual diagram comparing the movement paths, (a) shows the process of non-uniform precipitation and sodium dendrite (D) formation due to current concentration when the back electric field is not applied, and (b) shows the process of uniform precipitation and dendrite suppression due to current dispersion when the back electric field is applied. Figure 6 shows Na during discharge o As a conceptual diagram comparing the dissolution paths, (a) shows the process of pitting (P) and dead sodium (DS) generation due to local dissolution when the back electric field is not applied, and (b) shows the process of maintaining flatness of the front surface (30a) and suppressing the generation of dead sodium (DS) due to uniform dissolution when the back electric field is applied. FIG. 7 is a block diagram of a power control unit (60) according to an embodiment of the present invention, illustrating the interconnection relationship and signal flow of a sensing unit (61), a deviation calculation unit (62), a feedback control unit (63), and a voltage application unit (64). FIG. 8 is a waveform diagram of the back-applied voltage for each charge / discharge cycle, wherein the independent application waveforms of the first voltage (V₁) in the charging section and the second voltage (V₂) in the discharging section are Na / Na + It is plotted based on the reference potential and compares the DC and pulse waveforms. FIG. 9 is a comparison diagram of the front (30a) current density distribution simulation depending on whether a back electric field is applied, where (a) shows a non-uniform distribution with a large current density deviation (ΔJ) when not applied, and (b) shows a uniform distribution with a current density deviation reduced by more than 50% when applied. FIG. 10 is a graph of potential uniformity (%) according to the change in grid spacing (d) of the back auxiliary electrode (50), showing the optimal range in which potential uniformity of 90% or more is maintained in the grid spacing range of 1 μm to 500 μm. FIG. 11 shows Na according to the material of the ion-conducting insulating layer (40). + As a comparison graph of ionic conductivity, Na at different temperatures for each material: NASICON-based oxide, β-alumina, NaF, and PEO / sodium salt composite polymer. + It compares ionic conductivity (S / cm) and indicates the difference in conductivity with the lithium version insulating layer material (LLZO). Figure 12 is a graph comparing Coulomb efficiency (%) by charge / discharge cycle depending on whether a back electric field is applied, showing the comparison of four curves over 500 cycles: Example 1 (NASICON insulating layer), Example 2 (β-alumina insulating layer), Comparative Example 1 (no back electric field applied), and Comparative Example 2 (front artificial SEI coating). FIG. 13 is a schematic scanning electron microscope (SEM) diagram comparing the growth pattern of sodium dendrites (D) and the generation of dead sodium (DS) depending on whether a back electric field is applied, where (a) is the state after 50 cycles when not applied, and (b) is the state of the front (30a) after 50 cycles when applied. FIG. 14 is a graph showing the change in the front (30a) current density deviation (ΔJ) for each charge / discharge cycle, comparing the ΔJ values ​​when a back electric field is applied (Example 1) and when it is not applied (Comparative Example 1) according to the number of cycles, and showing the dynamic voltage adjustment effect by the feedback control unit (63). FIG. 15 is a graph comparing (a) electron blocking efficiency (%) and (b) back electric field transmission efficiency (%) according to the change in thickness of the ion-conducting insulating layer (40), showing the critical significance of the optimal thickness range that satisfies both characteristics simultaneously. FIG. 16 is a schematic cross-sectional view of an all-solid sodium battery, showing an integrated structure of a solid electrolyte layer (20a, NASICON-based), a sodium metal negative electrode (30), an ion-conducting insulating layer (40), and a back auxiliary electrode (50). FIG. 17 is a schematic cross-sectional view of a composite cathode structure of a hard carbon layer (70) and a sodium metal cathode (30), wherein a dual storage mechanism in which interlayer insertion storage in the hard carbon layer (70) and metal precipitation storage in the sodium metal cathode (30) coexist Na + The structure is illustrated as a moving path, with the back auxiliary electrode (50) and the ion-conducting insulating layer (40) placed on the back (30b) of the sodium metal cathode (30). FIG. 18 is a conceptual diagram of a roll-to-roll continuous process, illustrating the arrangement of a supply roll (101), a deposition zone (102), and a winding roll (103) within a vacuum chamber (100), and the sequential formation process of an ion-conducting insulating layer (40) and a back auxiliary electrode (50). Figure 19 is an optimization graph of the back electric field applied voltage (V₁, V₂), where Na / Na + The reduction rate (%) of the current density deviation (ΔJ) according to the change in the range of the first voltage (V₁) and the second voltage (V₂) relative to the reference potential is plotted, and the voltage ranges in which a reduction rate of 50% or more and 70% or more is achieved are indicated. FIG. 20 is an example of application of a grid-linked energy storage device (500), a humanoid robot (300), and an urban air mobility (400) equipped with a sodium metal secondary battery (1) according to the present invention, illustrating the hierarchical structure of a battery cell (200), a module (201), and a pack (202), and the integrated configuration for each application field. Specific details for implementing the invention

[0029] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The terms used in this specification have been selected to be as widely used as possible, taking into account their functions in the present invention; however, these terms may vary depending on the intent of those skilled in the art, case law, or the emergence of new technologies. Additionally, the size and shape of each component in the drawings may be exaggerated or simplified for clarity and convenience of explanation.

[0030] 1. Overview of the overall structure (see Figs. 1 and 2)

[0031] Referring to FIG. 1, a secondary battery (1) according to one embodiment of the present invention comprises a positive electrode (10), an electrolyte layer (20), a sodium metal negative electrode (30), an ion-conducting insulating layer (40), a back auxiliary electrode (50), and a power control unit (60). The sodium metal negative electrode (30) has a front surface (30a) facing the positive electrode (10) and a back surface (30b) located on the opposite side. The front surface (30a) is in contact with the electrolyte layer (20) and, when charged, Na + Ga Na o The film is reduced and precipitated, and Na is discharged. o Ì Na + It functions as a precipitation and dissolution surface that is oxidized and dissolved.

[0032] Referring to FIG. 2, the stacked structure of the secondary battery (1) is stacked in the order from the uppermost positive electrode (10) downwards to the electrolyte layer (20), the front surface (30a) of the sodium metal negative electrode (30), the main body of the sodium metal negative electrode (30), the back surface (30b) of the sodium metal negative electrode (30), the ion-conducting insulating layer (40), and the back auxiliary electrode (50). The back auxiliary electrode (50) is connected to an external power control unit (60) so that voltage is applied independently.

[0033] 2. Anode (10)

[0034] The positive electrode (10) includes a positive electrode current collector (11) and a positive electrode active material layer (12). The positive electrode current collector (11) may be composed of an aluminum (Al) thin film, and the positive electrode active material layer (12) may include a layered transition metal oxide-based positive electrode active material usable in a sodium-ion secondary battery, specifically a layered oxide such as NaxMnO₂, NaxNi₁ / ₃Fe₁ / ₃Mn₁ / ₃O₂, a Prussian white analog, a polyanionic compound (e.g., Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃), or at least one combination thereof. When charging, the positive electrode (10) Na + It functions as a sodium source that discharges and supplies to the front surface (30a) of the sodium metal cathode through the electrolyte layer (20), and during discharge, dissolved Na from the front surface (30a) + It plays the role of accepting.

[0035] 3. Electrolyte layer (20)

[0036] The electrolyte layer (20) is interposed between the anode (10) and the front surface (30a) of the sodium metal cathode (30) and Na +Ç provides a movement path. The electrolyte layer (20) may include any one of a liquid electrolyte, a gel polymer electrolyte, or a solid electrolyte. In the case of a liquid electrolyte, a sodium salt such as NaPF6, NaFSI (sodium bis(fluorosulfonyl)imide), NaTFSI (sodium bis(trifluoromethanesulfonyl)imide), NaBF₄, NaClO₄, etc., dissolved in ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), propylene carbonate (PC), ethylmethyl carbonate (EMC), diglyme, or a mixture thereof at a concentration of 0.5 M or more and 3 M or less may be used. In addition, the electrolyte may contain at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), and phosphorus-based additives in a ratio of 0.5% to 10% by weight, and these additives promote the formation of a NaF-containing SEI layer on the front surface (30a) and work synergistically with the back surface electric field application structure.

[0037] In an embodiment applied to a solid-state battery, the electrolyte layer (20) is sulfide-based (e.g., Na₃PS₄, Na₃SbS₄, Na₃PS₄, Na₃SbS₄) 1o It can be replaced with a solid electrolyte layer (20a) comprising a solid electrolyte of GeP₂S₁₂), NASICON-based (e.g., Na₃Zr₂Si₂PO₁₂), β-alumina-based, or PEO / sodium salt composite polymer-based. In an all-solid-state battery, the electrolyte layer (20) also serves as a separator.

[0038] 4. Sodium metal cathode (30)

[0039] The sodium metal cathode (30) is pure sodium metal (Na oIt is composed of a thin film and has characteristics of a theoretical specific capacity of 1,165 mAh / g and a reduction potential of -2.714 V relative to a standard hydrogen electrode. The thickness of the sodium metal cathode (30) can be set to 5 μm or more and 500 μm or less, preferably 10 μm or more and 100 μm or less. The sodium metal cathode (30) may include a current collector (31), and the current collector (31) may be composed of an aluminum (Al) thin film or a copper (Cu) thin film. When aluminum is used as the current collector, electrochemical stability is ensured because sodium does not undergo an alloy reaction with aluminum. The front surface (30a) of the sodium metal cathode (30) is in direct contact with the electrolyte layer (20), and the back surface (30b) is in contact with the ion-conducting insulating layer (40).

[0040] The sodium metal cathode (30) is Na on the front surface (30a) during the charging and discharging process. + Precipitation and Na o It has the characteristic of periodically changing thickness due to melting. Sodium metal has lower mechanical strength and higher reactivity than lithium metal, and since it oxidizes rapidly when exposed to air and moisture, all manufacturing processes must be carried out in an inert atmosphere.

[0041] 5. Ion-conducting insulating layer (40) (see FIG. 4)

[0042] An ion-conducting insulating layer (40) is interposed between the back surface (30b) of the sodium metal cathode (30) and the back surface auxiliary electrode (50), so that Na + It performs a selective permeability function that allows passage but blocks the movement of electrons. The ion-conducting insulating layer (40) has electronic insulation and Na + It is formed from a material that simultaneously possesses ionic conductivity.

[0043] As the material for the ion-conducting insulating layer (40), at least one of NASICON-based oxide (Na₃Zr₂Si₂PO₁₂), β-alumina (β-Al₂O₃), NaF (sodium fluoride), Na₃N (sodium nitride), PEO (polyethylene oxide) / sodium salt composite polymer, and composites thereof may be used. NASICON-based oxide (Na₃Zr₂Si₂PO₁₂) is Na + Ionic conductivity is about 10 at room temperature - It is one of the most desirable materials in this invention due to its excellent performance at the ø S / cm level and high chemical stability. β-alumina (β-Al₂O₃) is Na + It is a highly reliable material with ionic conductivity comparable to NASICON systems that has been used for a long time as a solid electrolyte in the field of sodium secondary batteries. Unlike LLZO (Li7La₃Zr₂O₁₂) used in lithium metal batteries, these materials use Na + Since it possesses an ion conduction pathway optimized for the ionic radius (1.02 Å), the Na that occurs when LLZO is applied as is + It resolves the problem of reduced transmission efficiency.

[0044] It is preferable that the thickness of the ion-conducting insulating layer (40) be set to be 1 nm or more and 10 μm or less. If the thickness is less than 1 nm, the electronic insulation function is insufficient, and the risk of an electrical short circuit between the back auxiliary electrode (50) and the sodium metal cathode (30) may increase; if it exceeds 10 μm, not only is the transmission efficiency of the back electric field into the sodium metal cathode (30) body reduced, but Na + The ion permeability resistance may increase, thereby unnecessarily raising the internal resistance of the secondary battery. The ion-conducting insulating layer (40) can be formed by chemical vapor deposition (CVD), atomic layer deposition (ALD), sputtering, or solution coating methods, and the ALD method is particularly advantageous for ensuring thickness uniformity at the nanometer level.

[0045] 6. Back auxiliary electrode (50) (see FIG. 3)

[0046] The back auxiliary electrode (50) is positioned with the ion-conducting insulating layer (40) in between, that is, from the outer surface of the ion-conducting insulating layer (40), i.e., the back surface (30b) of the sodium metal cathode (30). The back auxiliary electrode (50) has a mesh structure and consists of conductive wires (51) arranged in a grid shape and openings (52) between the wires (51). The reason for adopting a mesh structure is that, in the case of a complete plate electrode, the electric field is formed only on the surface of the auxiliary electrode, making it difficult to form a uniform potential distribution over the entire cathode area and resulting in a decrease in energy density, whereas the mesh structure optimizes the spatial distribution of the electric field through the openings (52), making it advantageous to form a uniform potential distribution over the entire front surface (30a) of the sodium metal cathode.

[0047] The material of the back auxiliary electrode (50) may include at least one of copper mesh (Cu mesh), aluminum mesh (Al mesh), carbon mesh, stainless mesh, and ITO (Indium Tin Oxide). Copper mesh is preferred because it has excellent electrical conductivity, high processability, and is stable in a sodium electrochemical environment. Aluminum mesh is stable in a sodium battery environment because sodium does not undergo an alloy reaction with aluminum, and it is advantageous in terms of energy density due to its lightweight properties. Carbon mesh has excellent electrochemical stability, is lightweight, and is flexible, so it can function stably even when applied to a solid-state battery.

[0048] It is preferable that the grid spacing (d) of the back auxiliary electrode (50), that is, the width of the opening (52) between adjacent wires (51), be set to be 1 μm or more and 500 μm or less. If the grid spacing (d) is less than 1 μm, the mesh structure becomes substantially closer to a plate-shaped electrode, increasing manufacturing costs and reducing the spatial optimization benefits of the electric field distribution, and if it exceeds 500 μm, the potential may be lowered at the midpoint between the grids, thereby reducing the potential uniformity of the front surface (30a). The grid spacing (d) is optimized through simulation or experiment, taking into account the thickness of the sodium metal cathode (30) and the thickness of the ion-conducting insulating layer (40), so that the potential uniformity on the cathode front surface (30a) is 90% or more.

[0049] 7. Power control unit (60) (see FIG. 7, FIG. 8)

[0050] The power control unit (60) controls the voltage applied to the back auxiliary electrode (50) and includes a sensing unit (61), a deviation calculation unit (62), a feedback control unit (63), and a voltage application unit (64).

[0051] The sensing unit (61) detects the current density distribution on the front surface (30a) of the sodium metal cathode (30) in real time. The sensing unit (61) may be composed of a plurality of microcurrent sensors or potential sensors arranged on the front surface (30a), or it may be implemented in a manner that indirectly measures the potential distribution formed locally on the front surface (30a) by the principle of electrochemical impedance spectroscopy (EIS). Since current density non-uniformity occurs during the early stages of sodium dendrite growth or when dead sodium generation begins, it is desirable that the response speed of the sensing unit (61) be within the millisecond (ms) level.

[0052] The deviation calculation unit (62) analyzes the current density distribution data received from the sensing unit (61) and calculates the current density deviation (ΔJ) across the entire front surface (30a) in real time. The deviation calculation unit (62) calculates the reference current density (J oThe deviation value is calculated by comparing the measured current density distribution with the ) and transmitted to the feedback control unit (63).

[0053] The feedback control unit (63) dynamically adjusts the voltage applied to the back auxiliary electrode (50) through the voltage application unit (64) when the current density deviation (ΔJ) received from the deviation calculation unit (62) exceeds a preset reference value (ΔJ_th). The feedback control unit (63) can independently set and apply a first voltage (V₁) and a second voltage (V₂) during charging and discharging. The first voltage (V₁) and the second voltage (V₂) are Na / Na + It is set based on the reference potential, which is the Li / Li ratio of lithium metal batteries. + It is distinguished from reference potential-based control. The feedback control unit (63) controls the applied voltage to reduce the current density deviation (ΔJ) by more than 50%, preferably more than 70%, compared to the state where the back electric field is not applied.

[0054] The voltage application unit (64) applies voltage to the back auxiliary electrode (50) in a direct current (DC) or pulse manner according to the control signal of the feedback control unit (63). When applying voltage in a pulse manner, the pulse period and duty cycle can be adjusted to disperse the point where localized current concentration occurs in real time.

[0055] 8. Mechanism of action of the back electric field (see Figs. 5, 6, and 9)

[0056] Referring to FIG. 5 (a), in a conventional structure where a back electric field is not applied, Na supplied from the positive electrode (10) during charging + When passing through the electrolyte layer (20) and reaching the front surface of the cathode (30a), current is concentrated at a specific point due to fine physical or chemical non-uniformity on the cathode surface or non-uniformity in the ion concentration gradient of the electrolyte. As a result, at that point Na oÇ Precipitation occurs preferentially, and the formed protrusions induce a stronger current concentration and grow into sodium dendrites (D).

[0057] Referring to FIG. 5 (b), in the back electric field application structure according to the present invention, a uniform potential distribution is formed from the back auxiliary electrode (50) into the sodium metal cathode (30) through the ion-conducting insulating layer (40). This uniform potential distribution is transmitted to the front surface (30a) through the cathode body, thereby creating a uniform potential environment across the entire front surface (30a). As a result, Na + Regardless of where it reaches on the front surface (30a), it experiences the same reduction potential, resulting in a uniform Na o Precipitation occurs and dendrite formation is inhibited.

[0058] Referring to FIG. 6 (a), in a conventional structure where a back electric field is not applied, during discharge, Na at a specific point is affected due to the uneven surface condition of the front surface (30a). o Ç Intensive dissolution occurs, forming pitting (P) and generating electrically isolated dead sodium (DS). Since dead sodium (DS) cannot function as an active material in subsequent cycles, irreversible capacity loss accumulates as the cycle progresses.

[0059] Referring to FIG. 6 (b), in the structure according to the present invention, during discharge, the potential distribution of the front surface (30a) is homogenized by the second voltage (V₂) applied from the back auxiliary electrode (50), and Na o Ç Dissolution is carried out uniformly across the entire front surface (30a). This suppresses the formation of pitting (P) and prevents the generation of dead sodium (DS), and maintains the flatness of the cathode front surface (30a) even after discharge, thereby continuously ensuring uniform deposition conditions in the next cycle.

[0060] 9. Composite cathode structure with hard carbon layer (70) (see FIG. 17)

[0061] Referring to FIG. 17, a composite cathode structure according to one embodiment of the present invention includes a structure in which a hard carbon layer (70) is additionally formed on the front surface (30a) of a sodium metal cathode (30). The hard carbon layer (70) includes a disordered carbon skeleton, micropores, and a (002) plane having an interlayer spacing of 0.37 nm or more, and Na + It functions as a negative electrode active material capable of interlayer intercalation storage. During charging, Na + Some of the Ç is stored in the hard carbon layer (70) in an interlayer insertion manner, and the remaining Na + It is stored in the sodium metal cathode (30) by a metal deposition method. During discharge, Na in the two storage regions is stored in reverse order. + is released.

[0062] This dual storage mechanism is advantageous in that it improves cycle stability compared to a single sodium metal cathode and increases energy density compared to a single hard carbon cathode. The back electric field application structure controls the uniformity of metal deposition in the sodium metal cathode (30), thereby improving the interfacial stability between the hard carbon layer (70) and the sodium metal cathode (30) and suppressing the formation of dendrites and dead sodium in the metal deposition region.

[0063] 10. Example of application of all-solid-state sodium battery (see FIG. 16)

[0064] Referring to FIG. 16, an all-solid-state battery (1a) according to another embodiment of the present invention comprises a solid electrolyte layer (20a) as an electrolyte layer (20). The solid electrolyte layer (20a) may comprise at least one of a sulfide-based solid electrolyte (e.g., Na₃PS₄, Na₃SbS₄), a NASICON-based solid electrolyte (e.g., Na₃Zr₂Si₂PO₁₂), a β-alumina-based solid electrolyte, or a polymer-based solid electrolyte (e.g., PEO / NaTFSI-based). In the all-solid-state battery (1a), a sodium metal negative electrode (30) is in direct contact with the solid electrolyte layer (20a), and the interfacial resistance and Na at the interface +Precipitation uniformity is a key factor determining battery performance. The back electric field application structure of the present invention is applied identically to an all-solid-state battery (1a), so that Na at the interface between the solid electrolyte layer (20a) and the negative electrode front surface (30a) + Precipitation uniformity is improved by electric field control.

[0065] 11. Example of a roll-to-roll continuous process (see FIG. 18)

[0066] Referring to FIG. 18, in a manufacturing method according to another embodiment of the present invention, an ion-conducting insulating layer (40) and a back auxiliary electrode (50) are formed on the back side (30b) of a sodium metal cathode (30) by a roll-to-roll (R2R) continuous process. The roll-to-roll process is performed continuously within a vacuum chamber (100), and proceeds in such a manner that a thin film of the sodium metal cathode (30) is unwound from a supply roll (101), passes through a deposition zone (102), and is then wound onto a winding roll (103). Since sodium metal is more reactive to air and moisture than lithium metal, the roll-to-roll process must be performed in an inert atmosphere with an oxygen concentration of 1 ppm or less and a moisture concentration of 1 ppm or less.

[0067] The present invention will be explained in more detail below through specific embodiments and comparative examples. However, the following embodiments are intended only to illustrate the present invention and do not limit the scope of the present invention.

[0068] <Example 1> Sodium metal secondary battery with NASICON insulation layer

[0069] As the sodium metal cathode (30), a sodium metal thin film with a thickness of 50 μm was formed by rolling it onto an aluminum current collector (31) (thickness 15 μm). An ion-conducting insulating layer (40) was formed by depositing a NASICON-based oxide (Na₃Zr₂Si₂PO₁₂) thin film with a thickness of 50 nm on the back surface (30b) of the sodium metal cathode (30) using an ALD method. A back auxiliary electrode (50) made of a copper mesh (wire diameter 20 μm, grid spacing (d) 100 μm) was attached to the ion-conducting insulating layer (40) by vacuum compression. 1 M NaPF6in EC / DEC (1:1 v / v) + 5 wt% FEC was used as the electrolyte, and Na₃V₂(PO₄)₂F₃ was used as the anode. The power control unit (60) has a first voltage (V₁) = 0.05 V (vs. Na / Na) when charging. + ), second voltage (V₂) during discharge = 0.03 V (vs. Na / Na + It was set to ) and operated in a feedback control manner.

[0070] <Example 2> Sodium metal secondary battery with β-alumina insulating layer

[0071] A sodium metal secondary battery was fabricated in the same manner as in Example 1, except that the material of the ion-conducting insulating layer (40) in Example 1 was changed to a β-alumina (β-Al₂O₃) thin film (thickness 80 nm) instead of a NASICON-based oxide.

[0072] <Example 3> Sodium metal secondary battery with NaF insulating layer

[0073] A sodium metal secondary battery was fabricated in the same manner as in Example 1, except that the material of the ion-conducting insulating layer (40) in Example 1 was changed to a NaF thin film (thickness 30 nm) and deposited by a sputtering method.

[0074] <Example 4> Application of Hard Carbon Composite Cathode

[0075] A sodium metal secondary battery was manufactured in the same manner as in Example 1, except that a hard carbon layer (70) (thickness 30 μm, specific surface area 18 m² / g) was additionally formed on the front surface (30a) of the sodium metal negative electrode (30) in Example 1 to form a composite negative electrode structure.

[0076] <Example 5> Application of All-Solid Sodium Battery

[0077] The battery was manufactured in the same manner as in Example 1, except that a NASICON-based solid electrolyte (Na₃Zr₂Si₂PO₁₂, thickness 200 μm) was used instead of a liquid electrolyte in Example 1 to form an all-solid-state battery (1a).

[0078] <Example 6> Application of auxiliary electrode on back surface of aluminum mesh

[0079] A sodium metal secondary battery was fabricated in the same manner as in Example 1, except that the back auxiliary electrode (50) in Example 1 was changed to an aluminum mesh (wire diameter 25 μm, grid spacing (d) 100 μm) instead of a copper mesh.

[0080] <Example 7> Pulse-type voltage application

[0081] A sodium metal secondary battery was manufactured in the same manner as in Example 1, except that the voltage application unit (64) in Example 1 was set to a pulse mode and the first voltage (V₁) and second voltage (V₂) were applied with a pulse period of 10 ms and a duty cycle of 50%.

[0082] <Comparative Example 1> Back electric field not applied

[0083] A sodium metal secondary battery was manufactured in the same manner as in Example 1, except that the ion-conducting insulating layer (40), the back auxiliary electrode (50), and the power control unit (60) were not applied in Example 1.

[0084] <Comparative Example 2> Application of full artificial SEI coating

[0085] A sodium metal secondary battery was fabricated in the same manner as in Example 1, except that instead of the back electric field application structure in Example 1, a NaF artificial SEI coating layer (thickness 50 nm) was formed on the front surface (30a) of the sodium metal negative electrode (30) using the ALD method.

[0086] <Comparative Example 3> Application of LLZO insulating layer (simple application of lithium version material)

[0087] A sodium metal secondary battery was fabricated in the same manner as in Example 1, except that the material of the ion-conducting insulating layer (40) in Example 1 was simply substituted with an LLZO (Li7La₃Zr₂O₁₂) thin film (thickness 50 nm) commonly used in lithium metal batteries instead of a NASICON-based material.

[0088] <Comparative Example 4> Grid spacing 1,000 μm (exceeds claims)

[0089] A sodium metal secondary battery was fabricated in the same manner as in Example 1, except that the grid spacing (d) of the back auxiliary electrode (50) in Example 1 was changed from 100 μm to 1,000 μm.

[0090] <Evaluation Example 1> Electrochemical Performance Evaluation

[0091] The charge-discharge performance of the sodium metal secondary batteries prepared in Examples 1 to 7 and Comparative Examples 1 to 4 was evaluated in coin-type half-cell (CR2032) and full-cell forms. The charge-discharge conditions were a current density of 0.1 C to 1 C and a voltage range of 0.01 V to 4.2 V (vs. Na / Na + It was performed for 500 cycles in ).

[0092] As a result, Example 1 (NASICON insulating layer) showed a Coulomb efficiency relative to initial capacity after 500 cycles improved by approximately 12% compared to Comparative Example 1 (unapplied), and irreversible capacity loss due to dead sodium formation was reduced by approximately 34%. Example 2 (β-alumina insulating layer) exhibited a performance improvement similar to that of Example 1. Example 3 (NaF insulating layer) compared to Example 1 Na + Although the transmission efficiency was somewhat low, resulting in a Coulomb efficiency improvement of approximately 8%, it was superior to Comparative Example 2 (full-surface artificial SEI). Comparative Example 3 (LLZO insulating layer) is LLZO's Na + Due to the low transmission efficiency, the effect of applying a back electric field was not sufficiently manifested, resulting in a Coulomb efficiency improvement of only about 3% compared to Comparative Example 1, which fell significantly short of NASICON-based Example 1. This result experimentally demonstrates the importance of selecting an insulating layer material optimized for the intrinsic ion chemistry of sodium.

[0093] <Evaluation Example 2> Effect of reducing current density deviation

[0094] As a result of measuring the current density deviation (ΔJ) of the front surface (30a) through electrochemical impedance spectroscopy (EIS) and finite element analysis (FEA) simulations for Example 1 and Comparative Example 1, it was confirmed that in Example 1, when a back electric field was applied, the ΔJ was reduced by about 65% compared to the unapplied state, and in the pulse method (Example 7), a reduction effect of about 72% was confirmed.

[0095] <Evaluation Example 3> Hard Carbon Composite Cathode Performance

[0096] Example 4 (hard carbon composite cathode) showed an additional capacity retention rate of about 8% after 500 cycles compared to a single sodium metal cathode (Example 1), and an energy density of about 42% compared to a single hard carbon cathode. This is analyzed to be due to the synergistic effect of the dual storage mechanism and the back electric field application structure.

[0097] Although embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various embodiments that can be modified by a person skilled in the art within the scope that does not deviate from the essence of the technical concept defined in the claims of the present invention should all be interpreted as falling within the scope of the present invention. Explanation of the symbols

[0098] 1: Secondary battery 1a: Solid-state battery 10: Anode 11: Positive current collector 12: Positive active material layer 20: Electrolyte layer 20a: Solid electrolyte layer 30: Sodium metal cathode 30a: Front surface (precipitation / dissolution surface) 30b: Back 31: The whole house 40: Ion-conductive insulating layer 50: Back auxiliary electrode 51: Wire 52: Opening 60: Power control unit 61: Sensing unit 62: Deviation calculation unit 63: Feedback control unit 64: Voltage application section 70: Hard carbon layer 100: Vacuum chamber 101: Supply Roll 102: Deposition Zone 103: Rolling Roll 200: Battery cell 201: Module 202: Pack 300: Humanoid Robot 400: Urban Air Mobility (UAM) 500: Grid-connected Energy Storage System (ESS) D: Sodium dendrite DS: dead sodium P: Pitting d: grid spacing V₁: First voltage during charging (Na / Na + standard) V₂: Second voltage during discharge (Na / Na + standard) J o : Reference current density ΔJ: Current density deviation ΔJ_th: Reference current density deviation value

Claims

Claim 1 A secondary battery comprising a sodium metal negative electrode, wherein the secondary battery includes a precipitation-dissolution surface on the front surface of the sodium metal negative electrode facing the positive electrode, wherein sodium ions are precipitated and dissolved, and a back surface electric field application means disposed on the back surface of the sodium metal negative electrode opposite to the precipitation-dissolution surface to form a uniform potential distribution over the entire precipitation-dissolution surface, wherein the back surface electric field application means reduces the deviation in current density at the precipitation-dissolution surface to suppress the formation of sodium dendrites and the generation of dead sodium. Claim 2 A secondary battery according to claim 1, wherein the back electric field application means comprises: a back auxiliary electrode in the form of a mesh; and an ion-conducting insulating layer interposed between the back auxiliary electrode and the back surface of the sodium metal negative electrode, which allows sodium ions to pass through but blocks electrons. Claim 3 A secondary battery according to claim 2, wherein the ion-conducting insulating layer comprises at least one selected from the group consisting of NASICON-based oxide (Na₃Zr₂Si₂PO₁₂), β-alumina (β-Al₂O₃), NaF, Na₃N, PEO / sodium salt composite polymer, and combinations thereof. Claim 4 A secondary battery according to claim 2, characterized in that the thickness of the ion-conducting insulating layer is 1 nm or more and 10 μm or less. Claim 5 A secondary battery according to paragraph 2, wherein the back auxiliary electrode comprises at least one of a copper mesh, an aluminum mesh, a carbon mesh, a stainless steel mesh, and ITO. Claim 6 A secondary battery according to claim 2, wherein the grid spacing of the back auxiliary electrode is 1 μm or more and 500 μm or less, and the grid spacing is designed such that the potential uniformity of the precipitation / dissolution surface relative to the thickness of the sodium metal negative electrode is 90% or more. Claim 7 A secondary battery according to claim 1, wherein the back electric field application means further comprises a control unit that independently controls the applied voltage according to a charging and discharging cycle, and the control unit performs the steps of: applying a first voltage for homogenizing sodium ion precipitation during charging; and applying a second voltage controlled independently of the first voltage for homogenizing sodium metal dissolution during discharging, thereby simultaneously achieving suppression of sodium dendrite formation and suppression of dead sodium generation. Claim 8 A secondary battery according to claim 7, wherein the control unit comprises: a sensing unit that detects the front current density distribution of the sodium metal cathode in real time; a deviation calculation unit that calculates a current density deviation from the detected current density distribution; and a feedback control unit that dynamically adjusts the voltage applied to the back auxiliary electrode when the current density deviation exceeds a reference value. Claim 9 A secondary battery according to claim 8, wherein the feedback control unit controls the applied voltage to reduce the front current density deviation by 50% or more compared to the state where the back electric field is not applied. Claim 10 A secondary battery according to claim 7, wherein the control unit applies the first voltage and the second voltage in a direct current or pulse manner, respectively, and, when applying the pulse manner, adjusts the pulse period and duty cycle to disperse the point where local current concentration occurs in real time. Claim 11 A secondary battery according to claim 1, wherein the thickness of the sodium metal negative electrode is 5 μm or more and 500 μm or less, and the sodium metal negative electrode is characterized by having a sodium metal thin film formed by rolling or electrodeposition on one surface of an aluminum or copper current collector. Claim 12 A secondary battery according to claim 1, characterized in that, by the means for applying a back electric field, the Coulomb efficiency after 500 charge / discharge cycles is improved by at least 10% compared to the state without applying a back electric field, and the irreversible capacity loss due to dead sodium generation is reduced by at least 30% compared to the state without applying a back electric field. Claim 13 A secondary battery according to claim 1, wherein the electrolyte of the secondary battery comprises at least one selected from the group consisting of fluoroethylene carbonate (FEC), NaFSI, NaPF6, and combinations thereof, and the current density homogenization by the back electric field application means and the formation of NaF-containing SEI by the electrolyte act synergistically to increase the sodium dendrite suppression effect. Claim 14 The secondary battery according to claim 1, wherein the secondary battery is an all-solid-state battery comprising a sodium ion-conducting solid electrolyte, and the solid electrolyte comprises at least one of a sulfide-based (Na₃PS₄, Na₃SbS₄), a NASICON-based (Na₃Zr₂Si₂PO₁₂), a β-alumina-based and a PEO / sodium salt composite polymer-based system, and the means for applying a back electric field controls the uniformity of sodium ion precipitation at the interface between the solid electrolyte and the sodium metal negative electrode. Claim 15 A secondary battery according to claim 2, wherein the ion-conducting insulating layer and the back auxiliary electrode are sequentially formed on the back surface of the sodium metal negative electrode by a roll-to-roll continuous process in a vacuum environment, and the roll-to-roll process is performed in an inert atmosphere with an oxygen concentration of 1 ppm or less and a moisture concentration of 1 ppm or less. Claim 16 A method for charging and discharging a secondary battery comprising a sodium metal negative electrode, characterized by comprising: a step of precipitating or dissolving sodium ions on a front surface opposite to the positive electrode of the sodium metal negative electrode; and a step of suppressing sodium dendrite formation and dead sodium generation by applying a uniform electric field on a back surface opposite to the front surface to reduce the current density deviation across the entire front surface. Claim 17 In claim 16, the step of applying the uniform electric field comprises a first voltage for homogenizing sodium ion precipitation during charging, Na / Na + A secondary battery charging and discharging method characterized by comprising: a step of applying based on a reference potential; and a step of applying a second voltage controlled independently of the first voltage to homogenize the dissolution of sodium metal during discharge. Claim 18 A secondary battery charging and discharging method according to claim 16, wherein the step of applying a uniform electric field further comprises: a step of detecting a current density distribution on the front surface in real time; a step of calculating a current density deviation from the detected current density distribution; and a step of dynamically adjusting the voltage applied to the back auxiliary electrode when the current density deviation exceeds a reference value. Claim 19 A secondary battery according to claim 1, wherein a hard carbon layer is additionally formed on the front surface of the sodium metal negative electrode, the hard carbon layer stores a portion of sodium ions in an interlayer intercalation manner and the remaining sodium ions are stored in the sodium metal negative electrode in a metal precipitation manner, and the means for applying a back electric field controls the uniformity of metal precipitation in the sodium metal negative electrode. Claim 20 An energy storage system comprising a secondary battery according to any one of claims 1 to 19, characterized by being mounted on a grid-connected energy storage device, a humanoid robot, an electric vehicle, or an urban air mobility.

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