Lithium metal protection layer for sulfide solid-state battery and preparation method and application thereof

Abstract

The application provides a lithium metal protection layer for a sulfide solid-state battery and a preparation method and application thereof. The preparation method comprises the following steps: S1, under an inert gas, high-energy ball milling of silicon and a metal oxide to obtain a mixed powder; S2, adding a binder to the mixed powder and roll pressing to obtain a protection film; and S3, high-temperature pressing of the protection film on the surface of lithium metal to obtain the lithium metal protection layer. The protection layer can effectively inhibit the growth of lithium dendrites and the volume expansion in the lithium intercalation / delithiation process, improve the negative electrode conductivity, lithium ion transport property and the capacity of the solid-state battery, has excellent battery charge / discharge first efficiency and cycle stability, and meanwhile, the preparation process is simple and fast, and the protection layer can be prepared on a large scale.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of new energy batteries, and relates to a lithium metal protective layer for a sulfide solid-state battery and a preparation method and application thereof. BACKGROUND

[0002] With the development of science and technology, the existing liquid lithium ion battery cannot meet the social demand, and the flammable and explosive characteristics make the society urgently need a low-carbon, environmentally friendly, efficient and safe new energy system. Based on the huge potential of solid-state batteries in energy density, safety and cycle life, all-solid-state lithium batteries use solid-state electrolyte instead of liquid electrolyte, which has become the most important development direction of lithium batteries in the international energy field.

[0003] Sulfide electrolyte has high lithium ion conductivity and excellent mechanical properties, and is recognized as a promising material system for commercializing solid-state batteries. Although the currently successful commercialized graphite negative electrode has the advantages of low cost and high stability, the low theoretical capacity has already failed to meet the current energy demand. Lithium metal has become a research hotspot due to its low oxidation potential and high theoretical capacity. However, the high activity of lithium metal causes high instability at the interface with the sulfide electrolyte, resulting in problems such as decreased coulombic efficiency, rapid capacity decay, and short circuit of the battery, which is mainly attributed to lithium dendrite growth, serious interfacial side reactions, and large volume expansion. At high current density, the stripping rate of lithium is greater than the deposition rate, resulting in voids, and a large number of electrons accumulate at the tip of the voids, inducing lithium ion deposition, thereby generating lithium dendrites, which further deteriorate and eventually penetrate the electrolyte, causing short circuit of the battery. In addition, a reaction occurs between the sulfide electrolyte and lithium metal, generating a passivation layer at the interface. Ideally, the passivation layer will inhibit interfacial side reactions and lithium dendrite growth. However, the composition of the interfacial layer is complex, and the thickness is usually too large, resulting in a significant increase in battery internal resistance and a decrease in performance. Currently, people mainly optimize solid-state lithium metal batteries by introducing artificial protective layers, lithium metal pretreatment, and lithium metal substitutes.

[0004] Introducing an artificial protective layer is a relatively simple and fast method for protecting lithium metal. However, the introduced protective layer faces problems such as low electrical conductivity and reduced battery capacity, and also needs to consider mechanical and physical properties. In the prior art, protective layers are prepared using silicon monoxide and metal elements as raw materials, which can protect the lithium metal negative electrode to some extent, but the film-forming property of the mixture of silicon monoxide and metal elements is poor, resulting in low cycle stability of the battery.

[0005] Therefore, there is an urgent need for a protective layer that balances cycle stability and protection of the lithium metal negative electrode. SUMMARY

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a lithium metal protective layer for sulfide solid-state batteries, its preparation method, and its application. The present invention uses silicon and non-lithium metal oxides as raw materials, and converts mechanical energy into chemical energy through high-energy ball milling. This ensures thorough mixing of the raw materials while initiating a preliminary reaction, resulting in a mixed powder (Si-M...) x O y The premixing process (where M is a metallic element) enhances the film-forming properties of the mixed powder. A binder is then added to further improve these properties, synergistically improving the battery's cycle stability. The mixed powder is then rolled to create a large-area protective film, suitable for various solid-state battery sizes. Finally, the protective film is subjected to high-temperature pressing to promote complete reaction of the mixed powder, generating a lithium metal protective layer containing silicon suboxide and metallic elements. This layer adheres tightly to the lithium metal anode surface, preventing the lithium metal from losing contact with the protective layer during battery cycling due to significant volume expansion, which would otherwise increase battery impedance and decrease stability.

[0007] The objective of this invention can be achieved through the following methods:

[0008] In a first aspect, the present invention provides a method for preparing a lithium metal protective layer for a sulfide solid-state battery, comprising the following steps:

[0009] S1. Under inert gas, silicon and metal oxide are subjected to high-energy ball milling to obtain a mixed powder;

[0010] S2. Add a binder to the mixed powder and roll it to obtain a protective film;

[0011] S3. Press the protective film onto the surface of the lithium metal sheet at high temperature to obtain a lithium metal protective layer.

[0012] As one embodiment of the present invention, in step S1, the silicon includes one or more of nano-sized silicon powder and micro-sized silicon powder.

[0013] In one embodiment of the present invention, in step S1, the metal oxide includes one or more of silver oxide, tin oxide, magnesium oxide, and aluminum oxide. Preferably, the metal oxide is magnesium oxide.

[0014] In one embodiment of the present invention, in step S1, the mass ratio of silicon to metal oxide is 0.4 to 2:1. Due to the incompleteness of the chemical reaction, a small amount of Si and metal oxide remain within the protective layer. These substances are uniformly dispersed within the protective layer and continue to react during battery cycling, thereby replenishing the main components lacking within the protective layer. If excessive silicon powder is used, the reaction may be incomplete, causing the remaining silicon to react with lithium metal to form a Li-Si alloy. However, the Li-Si alloy results in a significant volume change in the protective layer; therefore, a suitable ratio needs to be selected.

[0015] In one embodiment of the present invention, in step S1, the ball-to-material ratio of the high-energy ball mill is 20–50:1, and the rotation speed is 200–500 rpm. The high-energy ball milling time is 4–10 hours.

[0016] The high-energy ball milling of the present invention is based on the fact that the centrifugal force generated by ordinary ball milling after exceeding a certain speed will exceed the gravity of the grinding balls. At this time, the grinding balls and the grinding jar move at the same time, which is prone to grinding failure. High-energy ball milling will promote the balance of multiple forces, maintain effective grinding, and can produce protective layer precursors in large quantities.

[0017] In some embodiments, the inert gas is argon.

[0018] As one embodiment of the present invention, in step S2, the adhesive includes one or more of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polyacrylonitrile (PMA).

[0019] In one embodiment of the present invention, in step S2, the amount of the binder is 0.5-3% of the weight of the mixed powder. In some embodiments, after adding the binder to the mixed powder, grinding is performed for 0.5-2 hours.

[0020] In one embodiment of the present invention, in step S2, the rolling process involves multiple rolling operations using a rolling mill. In this invention, silicon exhibits significant volume change; reducing its particle size effectively suppresses its volume expansion. By adding a binder and rolling with a rolling mill, a large-area protective layer can be prepared, adaptable to various sizes of solid-state batteries. Simultaneously, after mechanical pressing, the protective layer possesses a dense porous structure, which is beneficial for improving Li... + It plays a role in transmission and mitigating volume expansion.

[0021] In one embodiment of the present invention, in step S2, the pressure of the high-temperature pressing is 300MPa to 400MPa. The temperature of the high-temperature pressing is 160 to 180°C, and the holding time is 15 to 20 minutes.

[0022] Furthermore, the high-temperature pressing process includes: placing the protective layer on lithium metal (negative electrode sheet), then placing it in a pressing mold, heating the upper and lower stainless steel parts of the mold to a specified temperature, and pressing it into a lithium metal protective layer.

[0023] Secondly, the present invention provides a lithium metal protective layer obtained by the preparation method described above.

[0024] Thirdly, the present invention provides an application of the lithium metal protective layer in the preparation of sulfide all-solid-state batteries.

[0025] As one embodiment of the present invention, the sulfide all-solid-state battery includes a lithium metal sheet, a lithium metal protective layer, a sulfide solid electrolyte, and a ternary material composite positive electrode sheet arranged sequentially.

[0026] Furthermore, the sulfide solid electrolyte includes Li2S-P2S5 and Li7P3S. 11 Li6PS5Cl, Li 5.5 PS 4.5 Cl 1.5 Li6PS5Br, Li6PS5I, Li 11 Si2PS 12 Li 9.54 Si 1.74 P 1.44 S 11.7 Cl、Li 10 SnP2S 12 Li 10 GeP2S 12 One or more of them.

[0027] Furthermore, the ternary composite positive electrode sheet comprises a positive electrode active material, a sulfide electrolyte, and a conductive agent; the positive electrode active material includes one or more of NCM111, NCM424, NCM523, NCM622, and NCM811; the sulfide electrolyte is a relatively hard sulfide electrolyte with added metal elements. The battery is press-formed into an all-solid-state battery in the order of positive electrode-electrolyte-negative electrode. This battery effectively improves safety, interfacial resistance, capacity, cycle life, and rate of change.

[0028] Traditional methods of directly mixing silicon suboxide and non-lithium metals result in poor film-forming properties of the mixed powder, affecting the cycle stability of the battery. Even when silicon suboxide and non-lithium metals are mixed using high-energy ball milling, the premixing effect described in this invention cannot be achieved, offering limited improvement in film-forming properties. This invention does not directly utilize silicon suboxide and non-lithium metals as raw materials. Instead, it creatively employs high-energy ball milling on silicon and metal oxides to achieve a premixing effect in the initial reaction, thereby improving subsequent film-forming properties and ultimately enhancing the battery's cycle life.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1. The protective layer prepared in this invention is mainly composed of silicon suboxide and non-lithium metal elements. Silicon suboxide exhibits high stability with the sulfide electrolyte, enabling the protective layer to isolate the lithium metal anode from the sulfide electrolyte, preventing side reactions and inhibiting lithium dendrite growth, thereby improving the battery's specific capacity and coulombic efficiency. Furthermore, silicon suboxide and non-lithium metal elements are relatively stable. In addition, during battery cycling, the non-lithium metal elements in the protective layer form an alloy with lithium, increasing the conductivity within the protective layer and reducing battery impedance, which is beneficial for improving battery cycle stability. This avoids the problem of high interfacial impedance in the protective layer caused by the low conductivity of silicon suboxide.

[0031] 2. This invention creatively utilizes high-energy ball milling to ensure that the raw materials (silicon and non-lithium metal oxides) are fully mixed while a preliminary reaction occurs, so that the mixed powder obtained from the preliminary reaction produces a premixing effect, which is beneficial to improving the film-forming properties of the mixed powder. Then, a binder is added to further enhance the film-forming properties of the mixed powder, thereby synergistically improving the cycle stability of the battery.

[0032] 3. The binder added in this invention can also improve the extensibility of the mixed powder, prepare a large-area protective film, and is beneficial for adapting to solid-state batteries of various sizes.

[0033] 4. This invention promotes the complete reaction of mixed powders through high-temperature pressing to generate a lithium metal protective layer containing silicon suboxide and elemental metals. This layer adheres tightly to the surface of the lithium metal anode, preventing the lithium metal from losing contact with the protective layer during battery cycling (volume expansion during lithium insertion / extraction), which would otherwise increase battery impedance and decrease stability. Furthermore, the pressed protective layer has a porous structure, offering better lithium-ion transport performance compared to traditional protective layers, and the pores within the protective layer effectively mitigate volume expansion.

[0034] 5. The protective layer of this invention has the advantages of excellent first charge and discharge efficiency, stability with sulfide electrolyte, low volume change, high film-forming properties and ductility, and can cope with lithium metal sheets of different sizes; at the same time, the preparation process is simple and quick, and can be prepared on a large scale. Attached Figure Description

[0035] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0036] Figure 1 This is a microscopic morphology diagram of the lithium metal protective layer in Example 1. Detailed Implementation

[0037] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. The following examples are implemented under the premise of the technical solution of the present invention, providing detailed implementation methods and specific operating procedures, which will help those skilled in the art to further understand the present invention. It should be noted that the scope of protection of the present invention is not limited to the following embodiments; any adjustments and improvements made under the concept of the present invention are all within the scope of protection of the present invention.

[0038] The nano-sized or micron-sized silicon powder used in this embodiment and comparative example is composed of elemental silicon.

[0039] Example 1

[0040] In an argon-filled glove box, approximately 0.2 g of nano-sized silicon powder and 0.35 g of silver oxide were placed in a zirconia ball mill jar, along with approximately 16.5 g of grinding beads. The jar was sealed tightly with adhesive tape, removed from the glove box, and subjected to high-energy ball milling for 4 hours at a speed of 450 rpm. The milled mixture was then placed back into the glove box. 500 mg of the powder was weighed out, and 1% PTFE was added. The mixture was ground for 1 hour to ensure thorough mixing and a certain degree of film-forming properties. Subsequently, the powder was repeatedly rolled in a roller press to form a film of uniform thickness. A 10 mm punch was used to cut circular pieces, which, along with lithium metal sheets of the same size, were placed in a 10 mm stainless steel pressing mold for high-temperature pressing. When the stainless steel reached 160°C, the film was pressed into sheets. After demolding, a mixture of lithium metal sheets and a protective layer (microscopic morphology as shown) was obtained. Figure 1 As shown, a lithium metal anode sheet with a porous structure was pressurized at 300 MPa for 15 minutes. 10 mg of a composite cathode (70% NCM811-29% Li6PS5Cl-1% VGCF) (all mass ratios) was uniformly pressed onto the surface of 100 mg of Li6PS5Cl electrolyte at a pressing pressure of 300 MPa, and then demolded. The composite cathode / sulfide electrolyte / lithium metal anode sheet was assembled into a pressure cell to obtain a sulfide all-solid-state battery.

[0041] Example 2

[0042] The difference between this embodiment and Embodiment 1 is that silver oxide is replaced with tin oxide.

[0043] The specific method is as follows: in a glove box filled with argon gas, a sulfide all-solid-state battery is prepared according to the method in Example 1.

[0044] Example 3

[0045] The difference between this embodiment and Embodiment 1 is that silver oxide is replaced with magnesium oxide.

[0046] The specific method is as follows: in a glove box filled with argon gas, a sulfide all-solid-state battery is prepared according to the method in Example 1.

[0047] Example 4

[0048] The difference between this embodiment and Embodiment 1 is that silver oxide is replaced with aluminum oxide.

[0049] The specific method is as follows: in a glove box filled with argon gas, a sulfide all-solid-state battery is prepared according to the method in Example 1.

[0050] Example 5

[0051] The preparation method of this embodiment is basically the same as that of Example 1, except that the ball milling time is replaced by 10 hours instead of 4 hours, the high temperature pressing temperature is replaced by 180°C instead of 160°C, and the holding time is replaced by 20 minutes instead of 15 minutes, so as to prepare a sulfide all-solid-state battery.

[0052] Comparative Example 1

[0053] The difference between this comparative example and Example 1 is that no protective layer is added.

[0054] The specific method is as follows: in a glove box filled with argon gas, a sulfide all-solid-state battery is prepared according to the method in Example 1.

[0055] Comparative Example 2

[0056] The difference between this comparative example and Example 1 is that no binder is added.

[0057] The specific method is as follows: In a glove box filled with argon gas, silicon powder and metal oxide are mixed using the method in Example 1, and then sulfide all-solid-state batteries are prepared according to the method in Example 1.

[0058] Comparative Example 3

[0059] The difference between this comparison and Example 1 is that no metal oxide is added.

[0060] The specific method is as follows: In a glove box filled with argon, silicon and binder are ground together, and then sulfide all-solid-state batteries are prepared according to the method in Example 1.

[0061] Comparative Example 4

[0062] The difference between this comparison and Example 1 is that no silicon is added.

[0063] The specific method is as follows: In a glove box filled with argon gas, metal oxides and binders are mixed using the method in Example 1, and then sulfide all-solid-state batteries are prepared according to the method in Example 1.

[0064] Comparative Example 5

[0065] The preparation method of this comparative example is basically the same as that of Example 1, except that high-energy ball milling is replaced by ordinary mixing; the ordinary mixing is carried out using a powder mixer to prepare sulfide all-solid-state batteries.

[0066] Comparative Example 6

[0067] The preparation method of this comparative example is basically the same as that of Example 1, except that the high-energy ball milling time is replaced with 2 hours to prepare a sulfide all-solid-state battery.

[0068] Comparative Example 7

[0069] The preparation method of this comparative example is basically the same as that of Example 1, except that the high temperature pressing temperature is replaced with 100°C and the holding time is 8 minutes.

[0070] Performance testing

[0071] Initial efficiency: The Newway Battery Testing System, model CT-4000, was used to test the all-solid-state battery. The 0.1C / 0.1C charge and discharge steps were used to test the initial charge and discharge capacity of the solid-state lithium battery. Initial efficiency % = initial discharge capacity / initial charge capacity × 100%.

[0072] Cycle stability of the entire battery: The Newway Battery Testing System (model: CT-4000) was used to test the all-solid-state battery using a 0.5C / 0.5C charge / discharge cycle. The capacity retention rate after each cycle was measured. Capacity retention rate % = (100-cycle discharge capacity / initial discharge capacity) × 100%.

[0073] Table 1. Performance test results of Examples 1-5 and Comparative Examples 1-7

[0074]

[0075] The test results in Table 1 show that the sulfide all-solid-state batteries in Examples 1-4 exhibit high initial charge-discharge specific capacity and efficiency, and good cycle stability under high current. Silicon provides high capacity to the battery while stabilizing the interface between the negative electrode and the electrolyte; the formation of an SEI layer mainly composed of SiO2 and Li2O effectively suppresses lithium dendrite growth and improves battery stability; the addition of a binder increases the lithium-ion conductivity of the protective layer and reduces interface impedance. Among them, Example 3 has the highest specific capacity and capacity retention, indicating that magnesium oxide provides the best protection for the lithium metal negative electrode. Example 5 improved the ball milling time, high-temperature pressing temperature, and holding time, further enhancing the initial efficiency of the lithium metal negative electrode.

[0076] In Comparative Example 1, without the addition of a protective layer, lithium dendrites grew and penetrated the electrolyte, causing the battery to short-circuit.

[0077] Comparative Example 2, without the addition of binder, has a low conductivity of the protective layer and a high interface impedance, which is not conducive to battery discharge and makes the battery stability worse.

[0078] In Comparative Example 3, no metal oxides were added. Although silicon provided high battery capacity, the volume expansion of silicon and the penetration of lithium dendrites deteriorated the battery stability due to the lack of an effective SEI layer.

[0079] Comparative Example 4, without silicon, has a battery with good stability, but due to the low capacity of the metal oxide itself and the reaction of most of the lithium metal with the metal oxide, the battery capacity is greatly reduced. At high rates, the specific capacity is less than 100mAh / g, which cannot achieve the high capacity of an all-solid-state battery.

[0080] Comparative Example 5 involved simply mixing the powders without sufficient kinetic energy to initiate a reaction, resulting in low initial performance and significant capacity decay. Similarly, Comparative Example 6, despite undergoing high-energy ball milling, showed a slight improvement compared to Comparative Example 5, but the reaction time was insufficient, leading to incomplete reaction, low capacity, and short lifespan.

[0081] Comparative Example 7 shortened the pressing temperature and time. Although high-energy ball milling was performed, the pressing temperature and time were insufficient. The protective layer still contained a large amount of silicon and metal oxides. These components are not conducive to lithium-ion transport, resulting in low capacity and severe degradation.

[0082] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a lithium metal protective layer for sulfide solid-state batteries, characterized in that, Includes the following steps: S1. Under inert gas, silicon and metal oxide are subjected to high-energy ball milling to obtain a mixed powder; The metal oxide includes one or more of silver oxide, tin oxide, magnesium oxide, and aluminum oxide; The mass ratio of silicon to metal oxide is 0.4~2:1; The high-energy ball milling time is 4~10 h; S2. Add a binder to the mixed powder and roll it to obtain a protective film; S3. Press the protective film onto the surface of the lithium metal sheet at high temperature to obtain a lithium metal protective layer; The high-temperature pressing pressure is 300MPa~400MPa; the high-temperature pressing temperature is 160~180℃; and the holding time is 15~20 min.

2. The preparation method according to claim 1, characterized in that, In step S1, the silicon includes one or more of nanoscale silicon powder and microscale silicon powder.

3. The preparation method according to claim 1, characterized in that, In step S1, the ball-to-material ratio of the high-energy ball mill is 20-50:1, and the rotation speed is 200-500 rpm.

4. The preparation method according to claim 1, characterized in that, In step S2, the adhesive includes one or more of polytetrafluoroethylene, polyvinylidene fluoride, and polyacrylonitrile.

5. The preparation method according to claim 1, characterized in that, In step S2, the amount of the binder is 0.5-3% of the weight of the mixed powder.

6. A lithium metal protective layer obtained by the preparation method according to any one of claims 1-5.

7. The application of the lithium metal protective layer as described in claim 6 in the preparation of sulfide all-solid-state batteries.

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