KR20210092928A - Method of Preparing Single Ion Conductive Organic and Inorganic Composite Protection Layer for Lithium Metal Battery - Google Patents

Abstract

The present invention relates to a method for manufacturing a single ion conductive organic and inorganic composite protection film for a lithium metal battery. More specifically, single ion conductive polymer is synthesized by using single ion conductive inorganic particles and single ion conductive monomer so that uniform flux induction of lithium ions is formed during high current charging and discharging and uniform lithium electroplating is possible, thereby having an effect of drastically improving performance of the battery.

Description

Method of Preparing Single Ion Conductive Organic and Inorganic Composite Protection Layer for Lithium Metal Battery

The present invention relates to a method for manufacturing a single ion conductive organic-inorganic composite protective film for a lithium metal battery, and more particularly, by synthesizing a single ion conductive polymer using a single ion conductive inorganic particle and a single ion conductive monomer, lithium ions during high current charging and discharging It relates to a method of manufacturing a single-ion conductive organic-inorganic composite protective film for a lithium metal battery that forms a uniform flux induction and allows uniform lithium electrodeposition.

Rechargeable lithium ion batteries (LIBs) have been gradually replaced for conventional energy storage batteries such as alkaline, Ni-Cd and lead acid batteries due to their high energy densities of 150 Wh/kg and 250 wh/L at the pack level. However, current LIBs platforms with graphite anodes with metal oxide cathode electrodes cannot actually meet the US Department of Energy (DOE) electric vehicle targets of 235 Wh/kg and 500 Wh/L, respectively. To dramatically increase the energy density of LIBs, replacement of conventional graphite anode and metal oxide cathode materials with alternative materials such as Li-metal or high nickel-containing NCMs (redox potentials greater than 4.3 V) should be considered. In particular, conventional anode materials such as graphite are based on an insertion mechanism, and the weight and volume of the host are obstacles to overcome the energy density.

Lithium metal is a promising anode material for next-generation high-energy battery batteries. However, dendrite deposits and low coulombic efficiency during cycling act as major disadvantages when using Li-metal as the anode material. In particular, the continuous electrolyte decomposition at the Li-metal interface resulted in a resistive porous layer due to the unstable interface between the Li-metal and the electrolyte.

Korean Patent No. 10-1575455, Korean Patent No. 10-2038624, and Korean Patent Publication No. 2019-0094139 disclose a method for manufacturing a lithium battery and a lithium ion conductive protective film thereof. Here, the reason why Li-metal attracts attention is because of its high capacity (3860 mAh/g) and low redox potential (-3.04V vs. standard hydrogen electrode). Replacing graphite anodes with Li-metal anodes increases the gravimetric and volumetric energy density by 35% and 50% at the battery pack level. However, when using a Li-metal anode instead of graphite, there are several obstacles such as dendrite Li deposition and low cycle maintenance characteristics due to the low coulombic efficiency resulting from the high reactivity of the Li-metal to the electrolyte. The inevitable and irreversible reaction between Li-metal and electrolyte is ongoing due to the universal Li dendrite growth behavior.

The origin of Li dendrite growth comes from 1) non-uniform current density distribution and 2) concentration gradient of Li-ions. The non-uniform current density is due to the irregular formation of the solid electrolyte interface (SEI). The SEI layer is spontaneously induced on the anode surface due to the lowest redox potential of Li-metal. In this process, continuous electrolyte decomposition occurs, which lowers the coulombic efficiency of the battery. Therefore, there are many studies on additive research to make stable SEI layers such as FEC, VC, LiNO _{3 and LiBOB.} These additives create a stable SEI layer, preventing further electrolytic degradation processes and uniform Li ion flux to the electrode. In particular, the SEI layer containing excess LiF (10 ^-7 to 10 ^-10 S/cm) induces redistribution of lithium, resulting in more uniform electrodeposition on the lithium metal surface. However, this spontaneously generated SEI layer cannot completely prevent the dendrite Li deposition behavior during repeated stripping/plating processes due to its low mechanical modulus. To provide sufficient mechanical stress for dendrite growth, the artificial SEI layer is demonstrated as a composite protection layer (CPL) by casting a mixture _{of rigid Al 2} O _{3 and soft PVdF-HFP polymer.} The Al ₂ O ₃ particles have sufficient mechanical modulus (20 Gpa) with a mechanical modulus of at least 6.2 GPa, which is theoretically necessary to prevent dendrite Li deposit growth. However, non-conductive Al ₂ O ₃ cannot provide a Li-ion conductive pathway where most of the dendrite Li deposits are observed on the polymer when the current density is increased.

To overcome this obstacle, a single ion conductive organic/inorganic protective layer is introduced to induce a uniform Li deposition and prevent the vertically growing Li deposition behavior. This single ion conductive layer has two advantages. One prevents Li ion depletion at the anode and the other provides sufficient mechanical toughness to suppress vertical Li deposition. According to Chazalviel's electromigration limiting model, the Li ⁺ cation concentration drops to zero at the anode surface with high current density. Therefore, the non-uniform interface of Li ions is observed by the local electric field causing Li-dendrite growth. According to this theory, in order to avoid zero concentration of Li ions on the Li-metal surface, a large number of electrolytes must be considered. From this point of view, since the single ion conductive protective layer has a ^{high Li +} ion transition number, the Li depletion phenomenon can be excluded. Therefore, it is possible to induce a uniform Li ion flux and suppress the random growth of Li dendrites by mechanical inhibition of a single ion conductive protective layer.

In addition, Rui Xu et al. describe a monoion pathway interface in a working battery of robust two-phase lithium metal (Rui Xu et al., (Adv. Mater. 2019, 31, 1808392). Since the described protective film has a low ionic conductivity with respect to lithium ions of 1x10 ^-5 S/cm, there is a problem in that it cannot be driven during high current charging and discharging.

Accordingly, the present inventors have made diligent efforts to solve the above problems. As a result, when a single ion conductive organic-inorganic composite protective film is produced by synthesizing a single ion conductive polymer using a single ion conductive inorganic particle and a single ion conductive monomer, physically hard inorganic The particles interfere with the vertical growth of dendrites, and both inorganic particles and polymers induce a uniform flux of lithium ions during high current charging and discharging, enabling uniform lithium electrodeposition. to improve the performance, and in particular, it was confirmed that it can be driven under high current charging and discharging conditions, and the present invention was completed.

An object of the present invention is to prevent the vertical growth of dendrites in a lithium metal electrode and form a uniform flux induction of lithium ions during high current charging and discharging to enable uniform lithium electrodeposition, and the presence or absence of single ion conductivity with improved lithium ion delivery number An object of the present invention is to provide a method for manufacturing a base composite protective film and a single ion conductive organic-inorganic composite protective film prepared by the method.

All of the above objects of the present invention can be achieved by the present invention described below.

In order to achieve the above object, the present invention comprises the steps of (a) mixing a single ion conductive inorganic particle, a single ion conductive monomer and a crosslinking agent and dispersing in an organic solvent to prepare a dispersion solution; (b) coating the dispersion solution on a lithium metal surface; and (c) heating the lithium metal coated with the dispersion solution to synthesize a single ion conductive polymer.

The present invention also provides a single ion conductive organic-inorganic composite protective film prepared by the above method, characterized in that the lithium metal surface is coated with a single ion conductive polymer so that the lithium transference number value is less than 0.3.

In the present invention, the method for manufacturing a single ion conductive organic-inorganic composite protective film can reduce the resistance at the interface by manufacturing a protective film by combining a polymer material having single ion conductivity with respect to lithium ions and inorganic particles (all-solid electrolyte, LLZO). , can induce a uniform lithium ion flow to the lithium metal surface.

Inorganic particles with strong rigidity physically inhibit the vertical growth of lithium dendrites and induce the growth of lithium dendrites in the horizontal direction, thereby inhibiting the growth of dendrites through the protective film. Since the single ion conductive polymer used here has flexibility, it can serve to increase the resistance to large volume changes of the protective film.

In addition, it can be applied in various fields because it can be applied to lithium sulfur, lithium air, and lithium metal batteries, which are batteries using lithium anode metal.

1 is a diagram schematically illustrating a Li deposition behavior and a Li conduction path in an organic-inorganic composite protective film according to an embodiment of the present invention and the related art.
Figure 2 is a chemical structure of (a) single-ion conductor triblock copolymer P (STFSILi)-b-PEO-bP (STFSILi) according to an embodiment of the present invention, (b) a photograph of a dispersed monoion conductive monomer , (c) a polymerized monoion conducting polymer (d) a dispersed monoion conducting monomer having a LLZO complex, and (e) a photo showing a polymerized monoion conducting monomer having a LLZO complex in an EC/DEC organic solvent.
3 is (ab) bare Li-metal, (c) SICP-coated Li-metal surface, (d) SICP-coated Li-metal surface, (e) SICP/LLZO composite coated surface according to an embodiment of the present invention; Li-metal surface, (f) SEM picture below the SICP/LLZO composite coated Li-metal surface after Li deposition.
4 is a time-current instantaneous curve for (a) bare Li-metal, SICP-coated Li-metal and LLZO/SICP-coated Li-metal chronoamperometry measurements according to an embodiment of the present invention, (b) ) Non-dimensional curves of chronoamperometric reactions, plots showing the growth of Li-deposition behavior for (c) bare lithium-metal anodes and (d) SICP/LLZO protected lithium-metal anodes, respectively.
5 is a Li-symmetric cell having a thickness of 40 μm Li-metal according to an embodiment of the present invention (a) Al ₂ O ₃ /Kynar composite protective layer, (b) LLZO/Nafion composite protective layer, (c) A diagram showing the electrochemical cell performance of the SICP protective layer and (d) 5mA/cm ² Li-metal and 2mAh/cm ^{2 LLZO protective layer.}
6 is a diagram comparing the electrochemical performance of a Li symmetric cell having a Li metal having a thickness of 40 μm according to an embodiment of the present invention.
Figure 7 is the electrochemical performance of the Li-LCO fully tested with ^{(a) 1.5mA / cm 2 (} = 1C) than Li source (150μm) of lower than, according to an embodiment of the invention, (b) 4.5mA / cm ² A diagram showing the electrochemical performance of a Li-LCO full cell with a Li source (40 μm) limited to less than (= 3C).
8 is a SEM image of Li deposition penetration through the CPL protective layer after 10 cycles at ^{5.0 mA/cm 2} , 2.0 mAh/cm ² according to an embodiment of the present invention.
9 is a SEM image of a cross section of (a) SICP/LLZO coated Li-metal, (b) SICP/LLZO coated Li-metal surface, and (c) SICP/LLZO coated Li-metal according to an embodiment of the present invention.
10 is a view showing the chronoamperometric (CA) measurement results for various contents of _{SiO 2} in the electrolyte according to an embodiment of the present invention.
11 is a view comparing the mechanical coefficients by the mass ratio of the SICP / LLZO composite protective layer according to an embodiment of the present invention.
12 is a photograph showing the Li-metal anode after the 100th cell cycle at ^{1.5mA/cm 2} according to an embodiment of the present invention. (a) bare lithium metal anode, (b) SICP/LLZO (2/1) coated lithium metal anode, (c) SICP/LLZO (1/1) coated lithium metal anode, (d) SICP/LLZO (1/2) ) coated lithium metal anode.
13 is a diagram illustrating cycling of a Li-NCM811 cell with an LLZO/SICP protective layer according to an embodiment of the present invention.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In general, the nomenclature used herein is those well known and commonly used in the art.

In the present invention, when a single ion conductive organic-inorganic composite protective film is produced by synthesizing a single ion conductive polymer using a single ion conductive inorganic particle and a single ion conductive monomer, physically hard inorganic particles interfere with the vertical growth of dendrites and It was confirmed that both inorganic particles and polymers induce a uniform flux of lithium ions during high current charging and discharging, allowing uniform lithium electrodeposition and improving battery performance by improving the lithium ion transfer number.

Accordingly, in one aspect, the present invention comprises the steps of: (a) mixing a single ion conductive inorganic particle, a single ion conductive monomer and a crosslinking agent and dispersing in an organic solvent to prepare a dispersion solution; (b) coating the dispersion solution on a lithium metal surface; and (c) heating the lithium metal coated with the dispersion solution to synthesize a single ion conductive polymer.

In addition, in another aspect of the present invention, a single ion conductive organic-inorganic composite protective film prepared by the above method and characterized in that the lithium metal surface is coated with a single ion conductive polymer so that the lithium transference number value is less than 0.3. it's about

Hereinafter, the present invention will be described in detail.

The present invention relates to a method for manufacturing a composite protective film of an organic/inorganic material for a lithium metal negative electrode, characterized in that an organic material and an inorganic material (all-solid electrolyte, LLZO) having a lithium ion transfer number value of 1 are used. do. In particular, the lithium transfer water value for the protective film is theoretically 1, and experimentally, it has a high value of 0.73, compared to that of the conventional composite protective film, which has a lithium ion transfer number of about 0.35 similar to that of most liquid electrolytes, resulting in a high current. It may have an advantage during high-capacity charging and discharging. Since this organic-inorganic composite protective film has high lithium ion conductivity, it can induce uniform electrodeposition and desorption of lithium ions onto the lithium metal surface. The Li-metal interface can be dynamically stabilized through the mechanical rigidity of inorganic particles and the flexibility of polymers.

1 compares the Li deposition behavior and Li ion concentration distribution near the Li-metal anode during the Li ion plating process in a conventional protective layer and a single ion conductive inorganic/organic composite protective layer. A conventional protective layer, called a ceramic/polymer composite layer (CPL), consists of tough ceramic particles that provide sufficient mechanical strength to prevent Li dendrite deposition growth and polymers from conducting Li ion pathways. acts as (Fig. 1(a)). However, as the current density increases, the Li ion flux concentrates dramatically on the polymer phase, where most of the Li deposits grow vertically through the polymer phase. As a result, the protective layer collapse phenomenon is generally observed in high current density cell operation (FIG. 8). In addition, due to the diffusion limit of Li ions, the concentration of Li ions is lowered, accelerating the dendrite deposition morphology according to Chazelvial theory. To overcome the previously reported limitations of CPL, a novel protective layer is designed to conduct Li ions through all phases and maintain a high Li ion concentration at the Li metal surface. The protective layer was a _{single ion conductive ceramic with Li 6.75} La ₃ Zr _1.75 Ta _0.25 O ₁₂ (LLZO) and a single ion conductive polymer (SICP) allowing Li ion access to the entire surface of the Li-metal anode (Fig. 1(b)). ) is made by mixing the compound. In particular, this single ion conductive protective layer can induce a uniform Li flux under high current density cell operating conditions. Further, it is possible to increase the single-ion conducting phase Li ⁺ delivery number (Li ⁺ transference number, t _{Li +)} to promote a number of Li ⁺ ion diffusion in the electrode surface.

로 가열한다. 그리고 SICP/LLZO 복합 전해질을 합성하기 위해, 먼저 단량체와 LLZO 분말을 EC/DEC (50/50, v/v) 유기용매에 잘 용해시킨 다음 혼합물 용매를 하루 동안 70 ℃로 가열한다(도 2(d)). 그런 다음 유기/무기 복합 전해질(SICP/LLZO)도 도 2(e)에서 나타낸 바와 같이 고형화된다. Li-금속 표면에 SICP/LLZO 복합재 층을 균일하게 코팅하기 위해, 잘 분산된 단량체 및 LLZO 혼합물 용매를 닥터 블레이드에 의해 Li-금속 상에 캐스팅한 다음 전극을 70

에서 하룻동안 가열한다. 보호층 두께는 5 ㎛이며 중합 중 고분자 수축으로 인해 복합층의 다공성 구조가 관찰된다(도 9).According to a preferred embodiment of the present invention, a single ion conductor can be synthesized using a Bis trifluoromethane sulfonimide (TFSI) anion that enables delocalization of anion charge. Therefore, the anionic functional group of TFSI can be dissociated for ion conduction by coordinating with Li+ ions (Fig. 2(a)). In order to improve the mechanical strength, a crosslinking agent of poly(ethylene glycol) diacrylate is added to the polymerization process. To synthesize SICP, the monomer and cross-linking agent are dissolved in an EC/DEC (50/50, v/v) organic solvent having a liquid state in FIG. 2(b). In order to polymerize the monomer, the mixed solution to solidify the solution as the gel polymer in Fig. 2(c) was mixed with 70 overnight.

heated with And in order to synthesize the SICP/LLZO composite electrolyte, first, the monomer and LLZO powder are well dissolved in an EC/DEC (50/50, v/v) organic solvent, and then the mixture solvent is heated to 70 ℃ for one day (Fig. d)). Then, the organic/inorganic composite electrolyte (SICP/LLZO) is also solidified as shown in FIG. 2(e). In order to uniformly coat the SICP/LLZO composite layer on the Li-metal surface, a well-dispersed monomer and LLZO mixture solvent was cast on the Li-metal by a doctor blade, and then the electrode was

heated for one day in The thickness of the protective layer was 5 μm, and the porous structure of the composite layer was observed due to polymer shrinkage during polymerization (FIG. 9).

In the present invention, the _{monoion conductive inorganic particles are Li 6.75} La ₃ Zr _1.75 Ta _0.25 O ₁₂ , Li ₇ La ₃ Zr ₂ O ₁₂ (LLZO), Li ₅ La _{3 Ta2O 12} , Li _0.33 La _0.55 TiO ₃ (LLT) ), Li ₃ PO ₄ (LiPON), Li ₄ SiO ₄ , Li ₃ BO ₃ , Li _1.5 Al _0.5 Ge _1.5 P ₃ O ₁₂ (LAGP), Li _1.3 Al _0.3 Ti _1.7 P ₃ O ₁₂ (LATP), Li ₇ At least one selected from the group consisting of _{La 3} Zr _2-x Ta _x O ₁₂ (LLZ-Ta) Li ₇ La ₃ Zr _2-x Nb _x O ₁₂ (LLZ-Nb) and Li ₄ SiO ₄ -Li ₃ PO _{4 .} may be, and preferably, _{Li 6.75} La ₃ Zr _1.75 Ta _0.25 O ₁₂ (LLZO) doped with Ta having an ionic conductivity ^{of 10 -5} to 10 ^-6 S/cm ² may be used.

In the present invention, the monoion conductive monomer may be a compound represented by the formula (2).

[Formula 2]

In the present invention, the crosslinking agent may be a compound represented by the formula (3).

[Formula 3]

In the present invention, the organic solvent may be a cyclic carbonate, a chain carbonate, or a mixture of a cyclic carbonate and a chain carbonate, the cyclic carbonate is ethylene carbonate (ethyl carbonate, EC), propylene carbonate ( It may be one or more selected from the group consisting of propylene carbonate, PC), butylene carbonate, BC, vinylene carbonate, VC, γ-butyrolactone, and mixtures thereof. , The chain carbonate is dimethyl carbonate (dimethyl carbonate, DMC), diethyl carbonate (DEC), dipropyl carbonate (dipropyl carbonate, DPC), methylpropyl carbonate (methylpropyl carbonate, MPC), ethylpropyl carbonate (ethylpropyl) carbonate, EPC) and ethylmethyl carbonate (EMC) may be at least one selected from the group consisting of, but is not limited thereto.

In the present invention, in the step (b), the dispersion solution may be coated on the surface of the lithium metal to a thickness of 2 to 50 µm, preferably 2 to 10 µm, more preferably 3 to 6 µm.

In the present invention, it may be heated at a temperature of 60 ~ 120 ℃ in step (c).

The present invention provides a single ion conductive organic-inorganic composite protective film prepared by the above method, characterized in that the lithium metal surface is coated with a single ion conductive polymer so that the lithium transference number value is less than 0.3.

In the present invention, the single ion conductive polymer may be represented by the formula (1).

[Formula 1]

In Formula 1, x is an integer from 1 to 20, y is an integer from 1 to 20, and z is an integer from 1 to 20.

In addition, in the present invention, the single ion conductive polymer may be represented by Formula 4 or Formula 5.

[Formula 4]

In Formula 4, x and z represent an integer of 1 to 10, and y represents an integer greater than 0 and less than 1000.

[Formula 5]

1, x+y=1을 만족하는 임의의 수를 나타낸다.In Formula 5, x and y are 0<x, y

1, represents an arbitrary number satisfying x+y=1.

The ionic conductivity of the composite SICP and SICP / LLZO complexes respectively ^{1.2X10 -4 S / cm, 1.3X10 -4} S / cm , and EC / DEC: is expanded by the (1 1, v / v) a co-solvent. And the Li ion transport number (transport number) of the SICP and SICP / LLZO material was measured using impedance spectroscopy and chronoamperometric method in FIG. 10 . _{The lithium ion transport number (1M LiPF 6} with EC/DEC (1:1, v/v)) of a typical electrolyte is: About 0.41 is almost similar to the previous report, and the SICP and SICP/LLZO composites with EC/DEC cosolvent (without Li salt) reach values greater than 0.85. When measuring the Li ion transport number of LLZO coated Li-metal, SICP coated SICP/LLZO coated Li-metal with liquid electrolyte containing Li salt is about 0.55, 0.65 and 0.73, respectively, due to the mobile anion salt. . However, the high 0.73 value for the SICP/LLZO protective layer is much higher than that of bare lithium-metal due to the characteristics of the single ion conductive protective layer. In addition, as a result of measuring the physical strength of the organic-inorganic protective film, the mechanical coefficient of the monoion conductive polymer was about 3 MPa (FIG. 11). When the inorganic particle ratio of the organic-inorganic composite protective film increased by 50% or more, the mechanical coefficient increased by 15 Gpa. Theoretically, the organic-inorganic composite protective film has sufficient mechanical strength because the physical strength of 6.3 Gpa or more is a value capable of inhibiting the growth of lithium dendrites.

According to the present invention, a single ion conductive organic/inorganic protective layer (LLZO/SICP) can be prepared to inhibit Li-dendrite growth and improve the cycleability of a Li metal battery. The composite protective layer has a high transfer number (0.73) and high mechanical strength (15 Gpa) leading to planar growth of Li deposition. In addition, the relatively high ionic conductivity (3.7×10 ⁻⁴ S/cm) with a high transfer number can promote fast Li ion flow, which can operate at ^{5 mA/cm 2 current density.} Therefore, the practical applicability of LMB with a Li metal anode and NMC811 cathode (5.0 mAh cm ^{−2 ) with a thickness of 40 μm was successfully demonstrated.}

Hereinafter, the present invention will be described in more detail through examples. These examples are only for illustrating the present invention, and it will be apparent to those of ordinary skill in the art that the scope of the present invention is not to be construed as being limited by these examples.

[Example]

Example 1: Preparation of a single ion conductive organic-inorganic composite protective film

As shown in the reaction scheme of Figure 2(a), monoion conductive inorganic particles (LLZO, Li _6.75 La ₃ Zr _1.75 Ta _0.25 O ₁₂ ), single ion conductive monomer (SICP, Formula 2), and a crosslinking agent (Formula 3) After mixing, it was dispersed in an organic solvent (EC/DEC, 1:1, v/v). At this time, the mass ratio of LLZO:SICP:crosslinking agent was 1:1:0.1.

The dispersion solvent was applied to the lithium metal surface and then coated with a thickness of 5 μm using a casting bar. Lithium metal was heated on a hot plate at 90 °C in a glove box to crosslink single ion conductive monomers to synthesize a single ion conductive polymer.

In the present invention, a single ion conductor can be synthesized using a Bis trifluoromethane sulfonimide (TFSI) anion that enables delocalization of anion charge. Therefore, the anionic functional group of TFSI can be dissociated for ion conduction by coordinating with Li+ ions (Fig. 2(a)). In order to improve the mechanical strength, a crosslinking agent of poly(ethylene glycol) diacrylate is added to the polymerization process. To synthesize SICP, the monomer and cross-linking agent are dissolved in an EC/DEC (50/50, v/v) organic solvent having a liquid state in FIG. 2(b). In order to polymerize the monomer, the mixed solution in which the solution is solidified like the gel polymer in Fig. 2(c) is heated to 70 DEG C for one day. And in order to synthesize the SICP/LLZO composite electrolyte, first, the monomer and LLZO powder are well dissolved in an EC/DEC (50/50, v/v) organic solvent, and then the mixture solvent is heated to 70 ℃ for one day (Fig. d)). Then, the organic/inorganic composite electrolyte (SICP/LLZO) is also solidified as shown in FIG. 2(e). In order to uniformly coat the SICP/LLZO composite layer on the Li-metal surface, a well-dispersed monomer and LLZO mixture solvent were cast on the Li-metal by a doctor blade and then the electrode was heated at 70 °C for one day. The thickness of the protective layer was 5 μm, and the porous structure of the composite layer was observed due to polymer shrinkage during polymerization (FIG. 9). FIG. 9(a) is a photograph in which an organic-inorganic composite protective film is coated on a lithium metal surface, FIG. 9(b) is an SEM photograph of the lithium metal surface, and FIG. 9(c) is a lithium metal cross-section SEM photograph.

Example 2: Characterization of single ion conductive organic-inorganic composite protective film

Electrochemical measurement

Electrochemical performance measurements were performed using a 2032 coin-type cell. The lithium metal battery was assembled in an argon-charged glove box consisting of an electrode, a polypropylene separator (PP2400, Celgard, USA) and an electrolyte (PANAX ETEC Co. Ltd, Korea). _{100 μl of lithium hexafluorophosphate (LiPF 6} , 1 M) in ethylene carbonate and diethyl carbonate (EC/DEC, 1:1 by volume) was used as electrolyte for all electrochemical cell tests. LiCoO ₂ (LCO) cathode is 90 wt. % LCO (Umicore Co. Ltd., Korea), 5 wt. % Super-P carbon black (Switzerland), 5 wt. % polyvinylidene fluoride (PVdF) , Sigma-Aldrich, USA) using binder and N-methyl-2-pyrrolidone (NMP) as a solvent on Al foil. The mass loading density per area of the LCO cathode is 1.5 mAh cm ^-2 . The galvanostatic test was performed using a battery cycler (WBCS 3000, WonAtech, Korea), and the cells were placed in an environmental chamber controlled at a temperature of 25°C. To examine the performance of a Li / LCO cells, then the cell assembly during the charge and discharge cycle forming operation at a current density of 0.15mA cm ^-2 was circulated to 1.5mA cm ^-2 conditions. The cut-off voltages for charging and discharging were 4.2V and 3.0V vs Li/Li ⁺ . Electrochemical impedance spectroscopy (EIS) measurements were performed using a Solartron 1255 (UK Solartron Analysis, FRA) Frequency Response Analyzer (FRA) and a Solartron 1287 Electrochemical Workstation in the frequency range from 1 MHz to 0.1 Hz with a perturbation amplitude of 10 mV. carried out For the Li||NMC811 cell, a thin Li metal (40 μm, Honjo metal, Japan) was used with an automated machine-made NMC811 cathode laminate (4.2 mAhcm ^-2 , LG Chem). All Li||NMC811 full cells were formed with one cycle of C/10 in the voltage range of 3.0-4.3 V versus Li/Li ^{+ .} Then, long-term cycling tests were performed at C/5 and C/2 for the charging and discharging processes, respectively.

Charonoamperometery

Electrochemical impedance spectroscopy (EIS; Solartron 1255, frequency range from 1 MHz to 0.1 Hz) was measured during charge-discharge cycling at room temperature. To determine the t _Li+ and D _Li+ values of the electrolyte, for a Swagelok-type Li/Li symmetric cell, a potentiostatic polarization experiment was performed at an applied voltage of 5 mV for 15 min, and the impedance (1-0.1 Hz) The frequency range of ) was measured before and after polarization measurement. The nucleation mode of ^{Li +} ions on the Li-metal surface was investigated by chronoamperometric (CA) analysis using a coin-type Li/Li symmetric cell. The step potential was set to 5 mV and the reaction current was monitored during the initial nucleation phase for 5 s (VSP Biologics). Transient current results were analyzed using a mathematical model for 3D nucleation proposed by Scharifker and Hills (SH equation) to explain the nucleation mechanism of Li electrodeposition.

Morphological characterizations

The morphology of the Li metal electrode was investigated by field-emission scanning electron microscopy (FE-SEM, Sirion, FEI). After electrochemical irradiation, the Li metal electrode was carefully separated from the decomposed cell in a dry glove box filled with argon. The obtained electrode was washed with 1,2-dimethoxyethane (DME, anhydrous, 99.5 %, Sigma Aldrich, USA) to remove residual electrolyte and vacuum dried overnight. For safe transport of the samples, they were vacuum sealed in polypropylene bottles without contamination and exposure to air during sample loading.

Determination of ionic conductivity and Li ion transport water

The ionic conductivity of the electrolyte was determined from the AC impedances of the SUS/PP separator/SUS symmetric cell containing the electrolyte. The thickness of the PP separator was 20 μm and the diameter of the SUS disk electrode was 0.8 cm. Then, an excess of electrolyte was injected into the 2032 type coin battery.

The Li deposition morphologies of SICP-coated Li-metal and SICP/LLZO composite coated Li-metal were compared in FIG. 3 . The relationship between the protective layer and the resulting Li morphology is not yet clear, but the transport number and mechanical properties of the Li protective layer are important for controlling the Li metal morphology. The high ionic conductivity and elastic protective layer allows for Li deposition under the protective layer and can withstand some of the resulting volume expansion, resulting in a round shaped Li deposit. After Li deposition at 1 mAcm ^-2 and 1 mAhcm ^-2 , the Li/Li symmetric cell was disassembled and the surface of the cycled Li metal electrode was observed using SEM. As shown in Fig. 3(a), in the case of the control electrolyte, sub-micron-sized moss and needle-shaped Li dendrites were observed. Such Li deposition should reduce the coulombic efficiency by causing the Li surface to expand and accelerating the reaction between the electrolyte and the Li metal. When the Li-metal was coated with only SICP, a round Li-deposited form was observed ( FIGS. 3(b) and 3(c)). And the morphology of Li deposition with LLZO/SICP coated Li-metal electrodes also showed a round shape in Fig. 3(e). These morphological features can be understood by considering the uniform Li ion flux with mechanical pressing by the LLZO inorganic particles. The Li ion conductive LLZO component allows Li ion transport through the protective layer and the elastic SICP accommodates the volume expansion of Li dendrites.

To further understand the relationship between the protective layer and the Li deposition morphology, the surface diffusion behavior of Li to the nucleus was investigated using electrochemical analysis by chronoamperometric (CA) analysis. Two kinds of nucleation models proposed by Scharifker-Hills (SH) are instantaneous nucleation models and progressive nucleation models. First, the instantaneous nucleation model follows that all nuclei are generated at t=0 with the same nucleation activation energy. Thus, almost all nuclei are generated in the initial filling phase, which means that the number of nucleation sites is constant. Second, the progressive nucleation model follows that the nucleation sites increase over time because the active sites for nucleation have different activation energies. As shown in FIG. 4( a ), this graph shows the change in current with time according to an overpotential of 10 mV. Change of current values with time for three samples of three bare lithium-metal with liquid electrolyte (control), SICP coated lithium-metal (SICP) and SICP/LLZO coated lithium-metal (SICP/LLZO) was observed. The current value tends to increase over time and then decrease back to a negative value. Increasing the current value up to t _max is the growth of lithium nuclei, followed by overlapping of the nuclei. Showing a non-dimensional plot of the chronoamperometry response shows that three samples of SICP, SICP/LLZO controls follow an immediate mechanism (Fig. 4(b)). As shown in FIG. 11 , the diffusion film ( D _Li+ ) lithium ion was increased more than 3 times by the introduction of the protective film (10 mV). Enhanced Li diffusion by the SICP/LLZO protective layer can facilitate the Li deposition process even after a long period of time. Based on Sand's time equation, τ=ð D _Li+ ( eC0/(2Jt _a )), where τ is the time of the sand, the expected generation time of the dendrite, D, the ion concentration C0, and the current density J and thus, the anion transfer water, t _a =1- t _Li+ ), a single ion conductive protective layer can effectively inhibit the growth of Li dendrites by simultaneously improving t _Li+ and D _{Li+ .} When no protective film was introduced, lithium dendrites grow randomly in three dimensions due to uneven lithium diffusion (Fig. 4(c)). On the other hand, when a single ion conductive SICP/LLZO protective layer is introduced, lithium dendrite growth is induced in the planar direction due to the high transfer number and strong physical stiffness of the vertically coated protective layer (Fig. 4(d)). Therefore, the morphology-like bundle of Li deposits is consistent with the SEM image of the lithium electrodeposited shape in Fig. 3(f).

A lithium symmetric cell test was performed to verify the lithium efficiency of stripping plating of previously reported protective films, which are CPL (Al ₂ O _{3 +Kynar), LLZO/Nafion, SICP and LLZO.} The battery test was performed under harsh conditions using a 40 μm thick Li metal anode at a current density of 5 mA/cm ² and a capacity of ^{2 mAh/cm 2 .} The previously reported protective layer of CPL showed uneven voltage stagnation at first, and cell degradation after 70 cycles due to uneven Li deposition and collapse of the protective layer (Fig. 5(a)). Although the LLZO/Nafion composite protective film has a high mechanical modulus and a high lithium ion transfer number (0.82), cell degradation is still observed during charge/discharge cycles of high current density (Fig. 5(b)). Due to the very low ionic conductivity (1.2X10 ^-5 S/cm) of the protective film, lithium ions cannot be rapidly supplied to the lithium metal surface under high charge/discharge driving conditions.

The single ion conductive polymer (SICP) protective layer synthesized by the present invention initially showed stable charging and discharging operation, but after 90 cycles, the cell deteriorated due to poor mechanical properties (3 Gpa) (Fig. 5(c)). ). Finally, the LLZO ceramic particles are coated onto the Li-metal surface by a roll press machine. A low overdislocation is observed in the LLZO sample as the new Li-metal surface is exposed by the roll press. However, lithium dendrites grew between LLZO particles to achieve cycle performance of less than 50 cycles due to the breakdown of the protective film (Fig. 5(d)).

Table 1 shows the measurement results of lithium ion conductivity and lithium ion delivery water.

[Table 1]

In general electrolyte, the value of lithium ion transfer water was as low as 0.45, but increased to 0.73 when single ion conductive organic-inorganic composite protective film (SICP/LLZO) was introduced. Thus, it was confirmed that a uniform lithium ion flux could be induced to the lithium metal surface.

Since the Li transfer number of the inorganic particles (LLZO) in Table 1 is theoretically 1, and the SICP transfer number value is 0.93, the difference between the lithium transfer number values of 1 and 0.93 is only 0.07, so the lithium transfer number between the polymer and the inorganic particles is only 0.07. Reduced resistance during transmission.

The protective effect was confirmed by comparing ^{the reversibility of lithium from 2mAh/cm 2} to 4mAh/cm ² through a lithium symmetric cell test according to the Li usage rate at a current density of 5mA/cm ^{2 .} At a capacity of 2 mAh/cm ^{2 ,} the control sample without protective film showed unstable charge-discharge behavior at 30 cycles, followed by cell degradation at 70 cycles. However, in the sample with a protective film, cell deterioration occurred at 180 cycles while operating stably in the initial cycle (FIG. 6(a)). In particular, the SICP/LLZO-based protective film can operate under high current cell operating conditions (5mA/cm ² ). This is because the ion conductivity (3.7x10 ^-4 S/cm) and lithium ion transport water (0.73) are rather high compared to other protective layers. This is believed to be a result of the inhibited concentration gradient formation at the interface and ^{the highly efficient transport of Li + during long-term cell operation.} The Nafion LLZO previously reported based protective layer (Fig. 5 (b)) has a high lithium gatjiman transmission number (0.87), low ionic conductivity due to (3.0x10-5 S / cm) 5mA / cm 2 current density in cell operation The ability to rapidly deliver lithium ions is limited. At a capacity of 3 mAh/cm ² , denaturation was observed after 43 cycles in the control sample, but the effect of the LLZO/SICP protective layer was still present for 92 cycles (Fig. 6(b)). Even at a higher capacity of 4 mAh/cm ² , the protective layer extended cycle retention up to 86 cycles, twice that of the control sample (Fig. 6(c)).

To demonstrate the applicability of the LLZO/SICP protective layer for lithium metal batteries, a Li metal/LCO cell with 1 M LiPF6 EC/DEC (1:1, v/v) electrolyte was used (Fig. 7(a)). The discharge capacity of Li/LCO cells was compared with control and SICP/LLZO protective layer coated samples. Li/LCO cells with control electrolyte showed poor capacity retention; The discharge capacity decreased sharply after the 60th cycle at 1.5 mAcm ^{-2 current density (1C).} This low cycle maintenance is due to the growth of a highly resistant porous layer due to the growth of lithium dendrites on the lithium metal surface (Fig. 12). LLZO/SICP samples showed improved capacity retention compared to control cells. However, samples with 1/2 ratio of LLZO and SICP show a sharp decrease in capacity after 200 cycles. This is because the high polymer ratio does not provide sufficient mechanical properties. On the other hand, samples with a 2/1 ratio of LLZO and SICP with high LLZO content also exhibited low cycle retention properties because the low polymer content could not sufficiently bind to the protective film. Samples with a 1/1 ratio of LLZO and SICP were run over 300 cycles because they effectively protected the Li-metal anode by providing mechanical inhibition and a uniform Li ion flux. In addition, an electrochemical performance test of Li-LCO (3C = 4.5 mAh/cm ² ) with a thickness of 40 μm limited to a Li source was performed (FIG. 7(b)). To operate the battery under more severe conditions, an electrolyte containing many FEC electrolytes (1M LiPF ₆ FEC/DEC, (3:7, v/v)) was used for interfacial stability of thin Li-metals. When the control and content of the LLZO (LLZO/SICP=1/2) sample is small, the capacity is largely lost due to the unstable interface between the Li-metal and the electrolyte. However, the high LLZO protective layer (1:1, 2:1 of LLZO/SICP) showed high performance with a capacity retention rate of 62% or more for 400 cycles. In practice, high cathode loading (NCM 811, 4.2 mAh/cm ² ) and thin Li anode (40 μm) are used for high energy density. As shown in FIG. 13 , the Li-NCM cell using the LiPF6-FEC/DEC (30:70, v/v) electrolyte showed capacity fading after 30 cycles without a protective layer. On the other hand, Li-NCM with LLZO/SICP protective layer showed improved capacity retention compared to the control cell with 96% cycle retention up to 95 cycles. Therefore, it was confirmed that the synthesized LLZO/SICP composite protective layer had a sufficient protective effect even when a very thin lithium metal and a high-capacity cathode material were used.

As the specific parts of the present invention have been described in detail above, it will be apparent to those of ordinary skill in the art that these specific descriptions are only preferred embodiments, and the scope of the present invention is not limited thereby. will be. Accordingly, it is intended that the substantial scope of the invention be defined by the claims and their equivalents.

Claims (11)

A method for producing a single ion conductive organic-inorganic composite protective film comprising the following steps:
(a) preparing a dispersion solution by mixing single ion conductive inorganic particles, a single ion conductive monomer and a crosslinking agent and dispersing them in an organic solvent;
(b) coating the dispersion solution on a lithium metal surface; and
(c) synthesizing a single ion conductive polymer by heating the lithium metal coated with the dispersion solution.
According to claim 1, wherein the _monoion conductive inorganic particles are Li 6.75 La ₃ Zr _1.75 Ta _0.25 O ₁₂ , Li ₇ La ₃ Zr ₂ O ₁₂ , Li ₅ La ₃ Ta2O ₁₂ , Li _0.33 La _0.55 TiO ₃ , Li ₃ PO ₄ , Li ₄ SiO ₄ , Li ₃ BO ₃ , Li _1.5 Al _0.5 Ge _1.5 P ₃ O ₁₂ , Li _1.3 Al _0.3 Ti _1.7 P ₃ O ₁₂ , Li ₇ La ₃ Zr _2-x Ta _x O ₁₂ , Li ₇ La ₃ Zr _2-x Nb _x O ₁₂ and Li ₄ SiO ₄ -Li ₃ PO _{4 A} method of manufacturing a single ion conductive organic-inorganic composite protective film, characterized in that at least one selected from the group consisting of.
The method of claim 1, wherein the single ion conductive monomer is a compound represented by Formula 2, wherein the single ion conductive organic-inorganic composite protective film is prepared.
[Formula 2]

The method according to claim 1, wherein the crosslinking agent is a compound represented by Formula 3, wherein the single ion conductive organic-inorganic composite protective film is prepared.
[Formula 3]

The method of claim 1, wherein the organic solvent is a cyclic carbonate, a chain carbonate, or a mixture of a cyclic carbonate and a chain carbonate.
According to claim 5, wherein the cyclic carbonate is ethylene carbonate (ethyl carbonate, EC), propylene carbonate (PC), butylene carbonate (butylene carbonate, BC), vinylene carbonate (vinylene carbonate, VC), γ- A method for manufacturing a single ion conductive organic-inorganic composite protective film, characterized in that at least one selected from the group consisting of butyrolactone (γ-butyrolactone) and mixtures thereof.
According to claim 5, wherein the chain carbonate is dimethyl carbonate (dimethyl carbonate, DMC), diethyl carbonate (DEC), dipropyl carbonate (dipropyl carbonate, DPC), methylpropyl carbonate (methylpropyl carbonate, MPC), A method of manufacturing a single ion conductive organic-inorganic composite protective film, characterized in that at least one selected from the group consisting of ethylpropyl carbonate (EPC) and ethylmethyl carbonate (EMC).
The method according to claim 1, wherein in step (b), the dispersion solution is coated on the lithium metal surface to a thickness of 2-50 µm.
The method of claim 1, wherein in step (c), heating is performed at a temperature of 60 to 120 °C.
10. A single ion conductive organic-inorganic composite protective film prepared by the method of any one of claims 1 to 9, characterized in that the lithium metal surface is coated with a single ion conductive polymer so that the lithium transference number value is less than 0.3. .
The single ion conductive organic-inorganic composite protective film according to claim 10, wherein the single ion conductive polymer is represented by Chemical Formula 1, Chemical Formula 4 or Chemical Formula 5.
[Formula 1]

In Formula 1, x is an integer from 1 to 20, y is an integer from 1 to 20, and z is an integer from 1 to 20.
[Formula 4]

In Formula 4, x and z represent an integer of 1 to 10, and y represents an integer less than 1000.
[Formula 5]

In Formula 5, x and y represent arbitrary numbers satisfying 0<x, y≤1, and x+y=1.

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