CN115036454B

CN115036454B - Safe and stable activated negative electrode

Info

Publication number: CN115036454B
Application number: CN202210706751.XA
Authority: CN
Inventors: 吴凡; 闫汶琳; 李泓
Original assignee: Yangtze River Delta Physics Research Center Co ltd; Institute of Physics of CAS; Tianmu Lake Institute of Advanced Energy Storage Technologies Co Ltd
Current assignee: Yangtze River Delta Physics Research Center Co ltd; Institute of Physics of CAS; Tianmu Lake Institute of Advanced Energy Storage Technologies Co Ltd
Priority date: 2022-06-21
Filing date: 2022-06-21
Publication date: 2023-06-02
Anticipated expiration: 2042-06-21
Also published as: CN115036454A

Abstract

The invention provides a safe and stable activated negative electrode, which is prepared by introducing Li-rich phase Li into an intangible LiSi alloy negative electrode ₁₅ Si ₄ 、LiC ₆ The lithium conductive network is woven into a three-dimensional texture, so that the active area of the electrode is effectively increased, and the dynamic performance of the electrode is optimized. The invention effectively improves the long-cycle performance of the battery, further optimizes the proportion parameters of alloy and hard carbon, and successfully obtains the all-solid-state battery with high load, large current and long-cycle capability.

Description

Safe and stable activated negative electrode

Technical Field

The invention relates to the technical field of battery materials, in particular to a safe and stable activated negative electrode, a preparation method thereof and a battery adopting the negative electrode.

Background

At present, the solid-state full battery has great difficulty and challenge in realizing the load and the surface capacity of a commercial lithium ion battery and realizing stable long cycle, and high load (the surface capacity is not less than 1mAh/cm ² ) The negative electrode side is mainly made of lithium metal, liIn alloy, graphite, li ₄ Ti ₅ O ₁₂ And silicon-containing cathodes, the number of cycles is not more than 1000 weeks, and it is difficult to achieve commercial levels. When lithium metal is used as a negative electrode, the problem of short-circuit failure of the battery caused by dendrite growth exists, and the ways of electrolyte structural design, lithium metal protection, no lithium negative electrode and the like are researched and tried, but stable long circulation under high current density of a high-load battery cannot be realized, for example, three stars realize 6.8mAh/cm by using an Ag-C negative electrode ² Full cell loaded and at 3.4mA/cm ² Circulation is carried out for 1000 weeks under the current density, but the preparation process is complex; xing Zhang et al observed a high load (4 mAh/cm ² ) Full cell high current density at room temperature (3.8 mA/cm) ² ) Dendrite growth during the lower cycle, after 890 weeks of cycle, the cell failed in short circuit. When graphite is used as a negative electrode, the service life of the high-load battery is not more than 200 weeks, the specific capacity of the graphite is low, when the graphite is matched with a high-load positive electrode, the electrode is too thick, the electrode dynamics greatly limit the performance of the battery, and the energy density is reduced. LTO holds great promise for high rate, long cycling (zero strain materials), but high potential limits the voltage and energy density of the cell. The potential of the silicon-containing anode is proper, the problem of dendrite growth of the lithium metal anode can be effectively avoided, the interface between electrolytes is relatively stable, and the specific capacity is high (3579 mAh/g Li) ₁₅ Si ₄ ) High load, high energy density, high magnification are most promising, but long cycle capacity is limited due to their large volume change.

Together, silicon and sulfide solid state electrolytes represent the next generation lithium ion batteriesIs developed in the direction of Si electron conduction (10 ^-3 S/m) and lithium diffusion coefficient (10 ^-14 -10 ^-13 cm ² S), the SEI repeatedly generates consumed active lithium in a liquid battery system, is a main reason for the capacity decay of a full battery, has very fast capacity decay of the liquid full battery matched with a silicon negative electrode, and is difficult to meet the requirements of commercial batteries. Silicon is used for the sulfide all-solid state battery system, and a large amount of interface reaction can be avoided. The silicon cycling process has large volume variation, and the initial perimeter surface reaction loses a large amount of lithium, exacerbating the full cell capacity fade.

Compared with a pure silicon negative electrode, the lithium silicon alloy negative electrode (LixSi) has obviously improved electron conduction capability and lithium conduction capability. Lithium silicon alloy hardness (1.3-1.5 GPa, li) _3.75 Si) hardness is smaller than Si (10.0-11.6 GPa), stress concentration degree of the LixSi alloy cathode in the solid-state battery is smaller than that of pure silicon, the electrode gradually forms a whole in the circulation process, generation of pores is reduced, and material transmission in the electrode is further promoted. Meanwhile, lithium in the LixSi alloy can supplement lost lithium in the circulation process, so that the circulation is more stable, the volume expansion is smaller than that of pure silicon, and the LixSi alloy is possibly more suitable for sulfide all-solid-state batteries than the pure silicon.

Disclosure of Invention

Aiming at the defects of the lithium silicon alloy in the prior art, the invention avoids the occurrence of short circuit phenomenon by adding hard carbon into the negative electrode, forms a three-dimensional lithium conductive network woven by a lithium-rich phase in the electrode, successfully obtains a safe and stable activated negative electrode, effectively improves the long cycle performance of the battery, further optimizes the ratio parameters of the alloy and the hard carbon, and successfully obtains the all-solid-state battery with high load, high current and long cycle capacity.

The safe and stable activated negative electrode provided by the invention has the characteristics of high lithium content Li phase ₁₅ Si ₄ 、LiC ₆ And weaving a three-dimensional textured lithium conductive network.

As an optimized alternative, the lithium conductive network in a three-dimensional texture is distributed in the amorphous lithium silicon alloy.

As an optimized alternative, the Li/Si capacity ratio of the metal lithium and silicon contained in the negative electrode is preferably 0.6 or more, more preferably 0.8.+ -. 0.1.

As an optimized alternative, the load of the cathode can be 1mAh/cm ² The above can reach 2mAh/cm ² Above, 3.6mAh/cm ² Above, 4mAh/cm ² Above, 5mAh/cm ² Above or even 6mAh/cm ² The above.

The invention also provides a preparation method of the safe and stable activated negative electrode, which comprises the steps of mixing and grinding silicon particles and hard carbon particles, and mixing the silicon particles and the hard carbon particles with a binder to prepare a silicon-containing layer; and setting the silicon-containing layer and the lithium sheet adjacent to each other as a negative electrode, and activating the assembled battery.

As an optimized alternative, the ratio of the mass of silicon in the negative electrode to the total mass of silicon and hard carbon may be 20% -80%, preferably 30% -60%, most preferably 40% ± 5%.

As an optimized alternative, the raw materials silicon and hard carbon in the negative electrode are micron-sized particles with average particle diameters below 8 μm.

As an optimized alternative, the average particle size of silicon in the negative electrode after activation is 1.5±1.2 μm, and the average particle size of hard carbon is 2±1.5 μm.

As an optimized alternative scheme, the specific capacity of the silicon-containing layer is 900-3000 mAh/g, and the thickness is 5-100 mu m; the preferable specific capacity is 1500-2500 mAh/g, and the thickness is 15-40 mu m.

As an optimized alternative, the pressure level during battery assembly is above 5t, preferably 7t-10t.

As an optimized alternative, the cyclic activation is performed at a rate of 0.1C-0.5C for at least 1 week, preferably 1-2 weeks.

As an optimized alternative, battery assembly matched positive electrodes include, but are not limited to, ternary systems (NCM, NCA), lithium cobalt oxide systems (LCO), lithium iron phosphate systems (LFP), lithium manganate systems (LMO), and the like.

As an optimized alternative, the load of the matched anode can be 6mAh/cm ² The above can reach 12mAh/cm ² Above, 16mAh/cm ² The above steps,Or even 20mAh/cm ² The above.

As an optimized alternative, the stable operating temperature of the battery is 5-75 ℃.

According to the invention, the LiSH46 negative electrode is prepared by introducing hard carbon into the LiSi alloy negative electrode, si and adjacent Si and C with suspension bonds at the edges of HC are bonded in a cyclic process of the battery, so that Si-Si bonds and Si-C bonds are generated, part of grain boundaries are eliminated, a uniform and compact electrode is formed, in the subsequent cyclic process, the electrode is subjected to continuous plastic deformation under pressure, the electrode is driven to form a uniform and almost stress-concentration-free continuous body, and the stability of the electrode is enhanced. Lithium-rich phase (Li) ₁₅ Si ₄ 、LiC ₆ ) The woven three-dimensional lithium conductive network can effectively increase the active area of the electrode and optimize the dynamic performance of the electrode.

LiSH46 anode can realize high load (3.6 mAh/cm) ² ) High current density (6 mA/cm) ² ) Stable long cycle (3000 weeks capacity kept 70%), thickness variation before and after battery cycle was only 2.2%; ultra-high current density (14.64 mA/cm) ² ) Ultra-long cycles down (24000 weeks capacity without any decay relative to the first week); can be at 122.1mA/cm ² The silicon carbon layer can effectively inhibit the growth of lithium dendrite after discharging, thereby realizing the matching of the ultra-high surface capacity (16.92 mAh/cm) ² ) The energy density of the battery layer surface can reach 263Wh/kg; matching LCO positive electrode at 20C (14.64 mA/cm ² ) Cycling 24000 weeks capacity did not decay; has a very practical prospect.

The method takes micron silicon and hard carbon materials as starting points, obtains SH materials through simple ball milling, forms a LiSH anode in the process of assembling the battery, controls the production cost in the whole process, improves the safety (spontaneous combustion of Li-Si alloy in air), and provides a new solution idea for the trend of solid-state batteries to practical application.

Drawings

FIG. 1 shows a NMC811|Si full cell with a theoretical specific capacity of NMC811 of 6mAh/cm ² : (a) cycling at 1C; (B) rate testing; (C) charge-discharge curves at different rates.

Fig. 2 shows the ionic conductivity and electronic conductivity of different silicon-carbon ratio materials. (A) Different silicon-carbon ratios LiSH, si, SH and lithium diffusion coefficients in the lithium intercalation process; (B) Lithium diffusion coefficients of different silicon-carbon ratios LiSH and Si in the lithium removal process; (C) electrode electron conductance as measured by voltammetry.

Fig. 3 is XRD of the LiSi negative electrode made with different Li/Si capacity ratios.

FIG. 4 shows NMC811|LiSi full cell at 1C (6 mA/cm ² ) The lower cycle, li/Si capacity ratios were 0.4, 0.6 and 0.8, respectively.

Fig. 5 is a surface morphology of Si negative electrode and LiSi negative electrode: (A) Si; (B) LiSi.

Fig. 6 is electrical properties of LiSC electrodes made by adding graphite, soft carbon, and hard carbon to LiSi negative electrode, respectively, and making full cells: (a) performing a cyclic test at 1C; (B) rate testing; (C) NMC811|LiSG46 full battery different multiplying power charge-discharge curve; (D) NMC811|liss46 full battery different rate charge-discharge curves; (E) NMC811|LiSH46 full cell different rate charge-discharge curves.

Fig. 7 is a full cell of NMC811 positive electrode matching LiSH negative electrodes of different Si and hard carbon ratios: (a) cycle testing; (B) rate testing; (C) charge-discharge curves at 0.1C and 1C.

Fig. 8 is a characterization of LiSH46 negative electrode: (a) SEM-EDS characterization of SH46 powder; (B) particle size distribution of HC; (C) particle size distribution of Si.

Fig. 9 is a SH46 and LiSH46 negative SEM: (a) SH46 film surface SEM; (B) LiSH46 negative electrode surface SEM.

Fig. 10 is a cross-sectional view of the LiSH46 anode: (a) an initial state; (B) after activation; (C) cycle at 1C for 10 weeks.

FIG. 11 is a structural characterization of LiSH 46: (A) XRD pattern of LiSH46, compared with LiHC, liSi, SH. XPS spectrum of LiSH46 negative electrode: (B) C1 s, (C) Li 1s.

Fig. 12 is an AES characterization of LiSH46 electrodes. (a) secondary electron imaging; (B) Li element distribution; (C) distribution of C elements; (D) Si element distribution.

Fig. 13 shows XPS component analysis of the interface between the LiSH46 anode and the electrolyte: (a) S2 p; (B) Cl 2p; (C) Li 1s.

FIG. 14 shows NMC811|LiSH46 full cell at 1C (5.86 mA/cm ² ) Electric powerCirculation under stream density: (a) full cell cycle testing; and (B) charging and discharging curves with different cycle numbers.

FIG. 15 shows that the LiSH46|NMC811 full cell and the surface capacity is higher than 1mAh/cm in the present work ² The number of cycles of the all-solid-state battery.

Fig. 16 is lco|lish46 full cell: cycling (a) the number of cycles at 20C with the corresponding specific capacity and coulombic efficiency; (B) a charge-discharge curve; cycling (C) the number of cycles at 30C with the corresponding specific capacity and coulombic efficiency; (D) charging and discharging curves with different turns.

Fig. 17 shows the lco|lish46 battery after 30000 weeks of battery cycling, after 20C charging and 0.2C discharging, the battery was returned to 20C cycling.

Fig. 18 shows the comparison of lco|lish46 full cell in this work with full solid state cell cycling over 1000 cycles.

Fig. 19 is NMC811|lish46 full cell: (A) Multiplying power test (B) NMC811 theoretical surface capacity of 6.48mAh/cm ² The method comprises the steps of carrying out a first treatment on the surface of the (C) NMC811|lish46 full cell fast-discharge test; (D) NMC811|lish46 full cell fast discharge current density is compared to full solid state cells that exceed the traditional lithium ion cell load.

FIG. 20 shows a full cell rate test of LCO|LiSH46 with a LCO theoretical surface capacity of 1.43mAh/cm ² : (A) The charge-discharge capacity and coulomb efficiency vary with the number of cycles; (B) charge-discharge curves at different current densities.

Fig. 21 is a LiSH46 negative electrode matching ultra-high load NMC811 full cell: (A) The surface loads are 59.83mg/cm respectively ² ,77.17mg/cm ² And 100.12mg/cm ² A charge-discharge curve of NMC811|lish46 full cell at 0.05C; (B) The surface capacity is 10.54mAh/cm ² The NMC811|lish46 full cell of (C) was charged at 0.05C, increasing the discharge rate from 0.1C to 7.5C; (C) 12mAh/cm ² High load NMC811 matches LiSH46 full cell cycling at 0.1C; (D) 12mAh/cm ² The high load NMC811 was cycled at 0.1C matching a pure Si negative full cell.

Fig. 22 is a comparison of NMC811|lish46 full battery load with literature report solid state battery load.

Fig. 23 is a view showing the high-load battery operation temperature range of lish46|nmc 811.

Detailed Description

In order that the invention may be understood more fully, a more particular description of the invention will be rendered by reference to preferred embodiments thereof. It should be understood that these examples are for the purpose of more detailed description only and should not be construed as limiting the invention in any way, i.e., not intended to limit the scope of the invention.

1. Preparation of the Material

Preparation of a negative electrode active material: micrometer Si (3-5 μm, supplied by Tianmu lead company) and hard carbon are respectively weighed according to different mass ratios, such as the mass ratios of 10:0, 8:2, 6:4, 4:6, 2:8 and 0:10 respectively, the ball milling is carried out for 10 hours in a sealing way, the obtained samples can be respectively expressed as Si, SH82, SH64, SH46, SH28 and HC, and the argon atmosphere is kept in the whole process. The Si to carbon mass ratio was controlled to 4:6, hard carbon was replaced with soft carbon and graphite, and comparative samples were made, which were denoted as SS46 and SG46, respectively. With these samples of different silicon and carbon materials made at different mass ratios, further made cells are denoted as LiSi, liSH82, liSH64, liSH46, liSH28, liHC, liSS46 and LiSG46, respectively. In the invention, soft carbon and hard carbon are purchased from the Ministry of sea na, and D50 is 5-10 mu m.

Preparing a silicon-containing layer: mixing the sample obtained by ball milling in the previous step with PTFE in a mass ratio of 95:5, and rolling by a roll squeezer to obtain a film layer which is the silicon-containing layer. In high load NMC cells, the quality of the silicon-containing layer was controlled such that the capacities were substantially uniform, and the specific capacity of each sample was also measured from half cells (table 1), the assembly and testing of which see electrochemical testing. In the low-load LCO battery, the mass of the silicon-containing layer is controlled to be about 1.7 mg.

TABLE 1 specific capacities of half-cells with different Si/C ratios and masses of corresponding Si-containing layers

NMC811 positive electrode preparation: the NMC811@LATP, LPSCl and VGCF are fully and uniformly mixed according to the mass ratio of 80:17:3, PTFE is added into the mixed anode according to the mass ratio of 400:3, after mixing, the anode film with uniform thickness is obtained after rolling and folding for many times at 80 ℃, and the sheet with the diameter of 8mm is punched for standby.

LCO positive electrode preparation: LCO and LPSCl are fully and uniformly mixed according to the mass ratio of 60:40, PTFE is added into the mixed anode, the mass ratio is 100:0.5, after mixing, the anode film with uniform thickness is rolled, and the anode film is punched into 9mm sheets for standby.

2. Structure and characterization of materials

The phase representation of the material adopts an X-ray powder diffractometer, the model is XD-2X of Beijing general purpose instrument responsibility Co., ltd, the test angle is 10-80 degrees, and the sweeping speed is 8 degrees/min.

SEM characterization of the material used was a scanning electron microscope from Hitachi, japan, model SU8100.

XPS characterization of the material used PHI5000 Versamrobe III from ULVAC-PHI. Monochromatic alkα radiation was used to study the chemical composition of the negative electrode.

AES characterization of the material adopts JEOL JAMP-9510F to study the distribution of Li, si and C elements of the section of the anode.

Electrochemical characterization of the material was accomplished by the LAND CT2001A battery test system.

3. Battery assembly and electrochemical testing

Preparation of a composite silicon negative electrode: the ball-milled negative electrode active material and LPSCl (Li ₆ PS ₅ Cl), and then adding a conductive agent VGCF, and uniformly mixing to obtain the mixed electrode, wherein the mass ratio of Si, LPSCl and VGCF is 50:45:5.

Solid-state half-cell assembly and electrochemical testing: and pre-pressing 80mg of electrolyte powder at 1t, uniformly dispersing 2mg of composite negative electrode powder at one side, maintaining the pressure at 7t for 3min, sequentially attaching In foil and Li foil at the other side, applying a pressure of 1t while screwing a screw, and thus obtaining the solid half-cell. The battery is tested at a set temperature (e.g., 55 ℃) with a test voltage range of-0.615-0.88V and a test current density of 400mA/g.

Solid state full battery assembly: taking 80mg of electrolyte powder, adding a positive electrode on one side, pre-pressing at 1t, sequentially putting a silicon-containing layer and a lithium sheet on the other side, and screwing down a screw of the battery at 7t pressure to obtain the solid-state full battery.

The cell was tested at a set temperature (e.g., 55 ℃) and the test voltage range was set to 2.5-4.2V.

Electrochemical testing: the load is 30mg/cm ² The NMC811 cells of (C) were all activated by cycling at 0.1C for 2 weeks, and then cycle and rate testing were performed at a rate of 1C (-6 mA/cm) ² ) The rate tests were cycled at 0.2C, 0.5C, 1C, 1.25C, 1.5C, 1.75C, 2C, 0.1C for 5 weeks, respectively. The ultra high load NMC811 battery test rate was 0.05C and the cycle test was performed at 0.1C. LCO batteries are cycled for one week at 0.2C and 0.5C, and then are subjected to cycle and multiplying power test, wherein the multiplying power of the cycle test is 20C (14.64 mA/cm ² )、30C(20.45mA/cm ² ) The magnification test was carried out for 5 weeks at 1C, 5C, 10C, 15C, 20C, 30C, 40C, 50C, and 1C.

PITT battery assembly: 80mg of electrolyte powder is pre-pressed at 1t, a silicon-containing layer and a Li sheet are sequentially placed on one side, pressure is maintained for 3min at 7t, an In sheet and a Li sheet are sequentially placed on the other side, and screws of the battery are screwed.

PITT test: the battery is kept stand for 2 hours, constant voltage is started from open circuit voltage, the voltage interval is set to be 5mV, the constant voltage to the current is smaller than 2 mu A or 30min, and the voltage interval is-0.62-0.88V. The calculation formula is as follows:

l is the electrode thickness (m); i: current (mA) at constant voltage charge and discharge; t: constant pressure time(s).

4. Result verification and analysis

4.1 verification of the Performance optimization results of Si, liSi, liSC negative electrode

As shown in fig. 1, the capacity of the full cell matched with the Si negative electrode decays rapidly, the rate performance is poor (fig. 1A), the capacity of the LiSi alloy as the negative electrode hardly decays, and micro short circuit occurs after a few weeks of circulation. The battery is subjected to multiplying power test (figure 1B), when the current density is increased to 0.5C, serious micro short circuit occurs to the battery, and the micro short circuit phenomenon can be effectively relieved by introducing Hard Carbon (HC) LiSC into LiSi; while as in fig. 1c, lisc also maintains a stable long cycle of the battery at high load, high current density.

The experiment initially proves that the formation of the lithium silicon alloy effectively improves the conductive lithium conducting capacity of the pure silicon negative electrode, and is a basic guarantee for keeping higher reversible capacity. The lithium silicon alloy negative electrode is obtained by spontaneous reaction after lithium is intercalated into the silicon negative electrode, a lithium source can be obtained from a metal lithium sheet, and the lithium silicon alloy negative electrodes with different Li/Si capacity ratios can be obtained by adjusting the quality of the metal lithium sheet and the Si content in the silicon-containing layer. Li/Si capacity ratio= (mass of Li)/(mass of Si) = (mass of Li)/(mass of Si) of Si containing layer) = Li/Si containing layer capacity ratio, wherein the specific capacities can be respectively taken as: the specific capacity of Li is 3860mAh/g, and the specific capacity of Si is 3300mAh/g. The lithium silicon alloy negative electrode is denoted as LixSi or LiSi in the present invention.

The volume change of the pure silicon negative electrode in the lithium removal process is large (Li) ₁₅ Si ₄ The volume expansion is 280%), a large number of cracks are easy to generate in the electrode, partial contact loss is caused, meanwhile, the passivation layer of the electrode electrolyte interface is continuously broken and generated due to the volume change, the passivation layer is thickened, partial active silicon is insulated, a large amount of lithium is lost, electrode polarization is increased, and the battery capacity is continuously attenuated. From the dynamics, as shown in FIG. 2, the lithium diffusion coefficient is the smallest when pure silicon intercalates lithium, 10 ^-15 m ² /s, the electron conductance is also only 10 ^-6 S/m, more overpotential is needed to drive the lithium intercalation reaction, electrode polarization is larger, and the reversible capacity of the pure silicon negative electrode under high current is obviously lower than that of LiSi and LiSH46. The capacity fade of the full-cell rate test matched with the pure Si negative electrode is obviously faster than that of the cyclic test, because the SOC of small current is larger than that of large current, the volume change degree of silicon is larger than that of large current, more cracks can be generated in the electrode, and the capacity fade is aggravated.

Si and Li willSpontaneous reaction to form amorphous LixSi alloy, li being converted into crystalline state upon full intercalation ₁₅ Si ₄ Young's modulus after Si intercalates lithium is reduced, li is fully intercalated ₁₅ Si ₄ Has a hardness of 1.3-1.5GPa, about one order of magnitude less than Si (10-11.6 GPa), li ₁₅ Si ₄ The stability of the electrode can be improved. Li (Li) ₁₅ Si ₄ Is 10 ^-10 -10 ^-12 m ² S, exceeds Li ₆ PS ₅ Cl (Li with ionic conductivity of 3.9 mS/cm) ₆ PS ₅ Cl has a lithium ion diffusion coefficient of 2.5X10 ^-12 m ² /s). The electron conductance of the LiSi alloy is also obviously improved compared with that of Si, li ₁₅ Si ₄ The electrode dynamics can be significantly improved. As shown in FIG. 2, the electron conductance (0.87S/cm) of the LiSi anode was higher than that of pure silicon (8.8X10) ^-6 S/cm) of 5 orders of magnitude, and the diffusion coefficient of lithium ions (10 ^-14 m ² S) are also raised (about 2 times). In the process of battery pressing, si and lithium sheets are in short circuit contact and react to generate Li ₁₅ Si ₄ . As shown in FIG. 3, when the Li/Si capacity ratio is 0.6, li ₁₅ Si ₄ Is significantly broader and has also a distinct Si peak, li at a Li/Si capacity ratio of 0.8 ₁₅ Si ₄ The peaks of Si are more pronounced and the peaks of Si are less pronounced, indicating that the crystal structure of Si in the electrode is destroyed and no larger, intact Si particles are present.

From XRD results, it is presumed that when the Li/Si capacity ratio is not less than 0.8, the electrode is composed mainly of Li ₁₅ Si ₄ The electrode is more easy to realize stable long cycle because large particles without Si cause electrode pulverization or repeated generation of an interface passivation layer. LiSi alloy cathodes with Li/Si capacity ratios of 0.4, 0.6 and 0.8 are respectively used for matching 6mAh/cm ² Full cell of NMC811 and at 1C (-6 mA/cm) ² ) As shown in FIG. 4, when the Li/Si capacity ratio was 0.8, the reversible capacity of the full cell was about 120mAh/g, and the cycle was conducted for 27 weeks with little attenuation, and a short circuit occurred at the 27 th week. Whereas electrodes with Li/Si capacity ratios of 0.4 and 0.6 exhibited rapid capacity fade for full cells. Therefore, the Li/Si capacity ratio in the negative electrode of the present invention is preferably 0.6 or more, more preferably 0.8.+ -. 0.1. When a silicon-carbon composite silicon-containing layer is usedWhen the negative electrode is used in the present invention, the silicon-containing layer may be designed to have the same capacity, i.e., the capacity ratio of Li to silicon-containing layer is preferably 0.6 or more, more preferably 0.8.+ -. 0.1, in order to maintain the same cycle ability.

The reasons for the short circuit after the LiSi negative electrode cycle may be: 1) The lithium diffusion coefficient of the LiSi anode is limited, and when a large current is applied to the electrode, lithium may be deposited at the electrode electrolyte interface instead of diffusing into the inside of the electrode. 2) As shown in fig. 5, the surface of the LiSi negative electrode is dense, there is no pore like Si thin film, the volume expands after the Si lithiation, the pore of the electrode is reduced, and excessive lithium cannot be contained, and during the battery cycle, lithium gradually accumulates, penetrates through the electrolyte, and causes a short circuit of the battery. 3) Lithium starts to precipitate after the silicon is fully intercalated, the nucleation potential of the silicon surface is higher, and large dendrites tend to be formed, so that the LiSi alloy anode is easy to generate micro short circuit.

The carbon material can promote electrode dynamics, can provide defects and excessive lithium stored in pores, and is expected to solve the problem of short circuit of the LiSi negative electrode. Carbon materials are of a wide variety including graphite, soft carbon and hard carbon. Wherein, the hard carbon is carbon which can not be graphitized at the temperature of more than 2500 ℃, and is mainly prepared by resin-based, pitch-based and biomass-based precursors; soft carbon refers to carbon capable of graphitizing above 2500 c, and its soft carbon precursors may include petroleum coke, petroleum pitch, and condensed aromatic compounds. Graphite, soft carbon and hard carbon all have obvious differences in structural order, interlayer spacing, structural strain capacity and the like.

After different carbon materials are introduced into the LiSi negative electrode, the prepared LiSiC negative electrode is used for 6mAh/cm ² NMC811 was fully charged and the results are shown in fig. 6. Graphite and soft carbon are introduced, the high reversible capacity of the full battery cannot be maintained under high current, micro short circuit exists, the Hard Carbon (HC) can stabilize the lithium silicon negative electrode, stable long cycle under high current density is realized, and when the current density reaches 12.96mA/cm ² (2C) When the battery did not develop a significant micro-short. HC has rich defects and holes, the lithium intercalation voltage range is 0.8-0.001V and is close to 0V, lithium precipitated after full intercalation of LiSi alloy can be intercalated into HC, and then precipitated from the HC surface, and due to small potential difference between full intercalation HC and metal lithium, the metal is dopedThe lithium precipitation rate is slower, the precipitated lithium tends to be uniformly deposited on the surface of the electrode, HC can effectively inhibit the growth of lithium dendrite, HC can play a supporting role, HC has little volume change in the lithium intercalation process, and can buffer the volume change of Si and maintain the stability of the electrode. HC can effectively promote electrode dynamics, and from the PITT test result (figure 2), the addition of HC obviously promotes the lithium diffusion coefficient of the electrode, which is at least one order of magnitude higher than that of Si and LiSi, and increases with the increase of the HC ratio, but the thickness of the electrode increases (table 1), the migration path of lithium becomes longer, and the diffusion time (t ≡L ² ) And (3) increasing.

Effect of Si and HC ratio on battery performance as shown in fig. 7 (a-C), the LiSH46 anode performs optimally for all solid state batteries at 1C (6 mA/cm ² ) The reversible capacity of 123.6mAh/g was maintained at 2C (12.96 mA/cm ² ) No micro-shorting occurs below. When Si is excessive, the battery faces micro short circuit under large current, similar to a LiSi negative electrode, lithium diffusion coefficient is insufficient to enable lithium to fully diffuse into the electrode under large current, so that lithium grows lithium dendrite through deposition of an interface, the coulomb efficiency of the LiSH82 negative electrode is lower, obvious micro short circuit continuously occurs, the electrode deposited lithium continuously reacts with electrolyte, interface impedance continuously increases, battery capacity rapidly decays, si volume of the LiSH64 negative electrode is greatly changed in a circulating process, on one hand, the interface is unstable, SEI is repeatedly generated, a large amount of active lithium is consumed, on the other hand, electrode dynamics is gradually deteriorated, lithium dendrite is gradually grown, the lithium reacts with the electrolyte, more active lithium is consumed, and battery capacity rapidly decays after a plurality of weeks of stable circulation. When HC is excessive, although the lithium diffusion coefficient of the electrode is improved, the thickness of the electrode is remarkably increased (LiHC-170 μm, liSH 28-70 μm), the diffusion time of lithium in the electrode is remarkably increased, the electrode utilization rate is reduced under high current, and the reversible capacity of the battery is low. The electrode is susceptible to diffusion-controlled lithium trapping phenomena, resulting in a large amount of lithium being trapped inside the electrode, and the battery capacity continues to decay. Therefore, optimizing the ratio of Si and HC can further improve the high current and long cycle capability of the lithium silicon alloy anode. Si is of the mass ofThe ratio of Si to HC in the total mass may be 20% -80%, preferably 30% -60%, and most preferably 40% + -5%.

4.2 characterization and optimization theory analysis of hard carbon stable lithium silicon alloy negative electrode

The LiSH46 cathode is formed by rolling SH46 powder and PTFE into an SH46 film, then making short-circuit contact with a Li sheet and reacting under pressure. As shown in FIG. 8, the morphology of SH46 powder was shown, HC and Si were both micron-sized particles of 8 μm or less, the average particle size of HC was 2.39 μm, and the average particle size of Si was 1.21. Mu.m. SH46 film with thickness of 33 μm is prepared, and the particles show crystal self-normative characteristics and have a more regular structure as shown in figure 9. When the SH46 thin film and 35 μm lithium sheet produced a LiSH46 negative electrode, the electrode thickness became 45 μm (fig. 10A), liSH46 became denser than SH46, and part of grain boundaries and voids disappeared, approaching the phenomenon of the LiSi negative electrode. LiSH46 was activated at 0.1C for 2 weeks, the electrode thickness became 46 μm, and the electrode did not have significant transverse cracks as seen in the cross section of the electrode (FIG. 10B), indicating the stability of the LiSH46 structure.

LiSH46 anode contains Li ₁₅ Si ₄ And LiC ₆ As shown in fig. 11A, li is generated in the LiSH46 anode ₁₅ Si ₄ Phase, compared with LiHC negative electrode, li ₁₅ Si ₄ Main peak of (2) and LiC ₆ Is coincident with the main peak of (a). By peak-splitting fitting of XPS C1 s spectra of LiSH46 negative electrode (FIG. 11B), it was demonstrated that LiS 46 negative electrode generated LiC ₆ (285.2 eV). Peak-splitting fitting of the Li 1s spectra (fig. 11C) also demonstrated the formation of Li-C (56.2 eV) and Li-Si (55.4 eV). The existence of the lithium-rich phase greatly improves the electron conductivity and the lithium diffusion coefficient of the electrode, improves the electrode dynamics, and remarkably improves the lithium diffusion coefficient and the electron conductivity of the LiSH46 cathode compared with the pure Si and LiSi cathode as shown by the PITT test and the voltammetry test results of FIG. 2.

And carrying out AES characterization on the activated LiSH46 electrode, analyzing the distribution of Li, si and C elements of the cross section, and as shown in figure 12, the Si and C elements lose the original regular morphology and become texture, the enrichment sites of the C and Si elements are adjacent, and the Li element is distributed in the electrode and basically coincides with the enrichment positions of the Si and C elements, so that the lithium-rich phase in the LiSH46 anode is woven into a three-dimensional conductive lithium-conducting network. The battery circulates under the external pressure, si is amorphous and generates plastic deformation when lithium is intercalated, and along with volume expansion, the graphite-like layer of HC particles is pushed to migrate, the strong binding capacity of HC particles relative to SC also relieves microcrack generation caused by the volume expansion of Si, si bonds with adjacent Si and C with suspension bonds at the HC edge in the delithiation process, si-Si bonds and Si-C bonds are generated, part of crystal boundaries are eliminated, a uniform and compact electrode is formed, the electrode continuously generates plastic deformation under the pressure in the subsequent circulation process, the electrode is driven to form a uniform and almost stress-free continuous body, and the stability of the electrode is enhanced. The three-dimensional lithium conductive network woven by the lithium-rich phase can effectively increase the active area of the electrode and optimize the dynamic performance of the electrode.

The stability of the interface between the LiSH46 anode and the electrolyte was also confirmed by XPS, and as shown in FIG. 13, when the battery was fabricated, a small amount of electrolyte on the surface of the LiSH46 anode was decomposed, and Li was found from the S2 p region peak-split fitting result ₂ S (159.9 eV) generated, PS of electrolyte ₄ ^3- (161.2 eV) is also the main phase, and is not obviously reduced after activation, which indicates that a relatively stable interface passivation layer is generated on the surface of the LiSH46 anode. Peaks after Cl 2p activation and in the initial state represent Cl ^- (198.4 eV) there was no difference. The P2P peak-to-peak fitting results showed only a small amount of PS ₄ ^3- Is reduced to P ₂ S ₇ ^4- . And the electrolyte-containing composite negative electrode or lithium metal negative electrode reported in the literature, the electrolyte decomposition degree of the surface of LiSH46 is weaker, and the interface stability of the LiSH46 negative electrode and the electrolyte is demonstrated.

The above results and analysis prove that the hard carbon material provides a rich storage space for the intercalation and deposition of metallic lithium, is favorable for fully accommodating the metallic lithium to avoid lithium dendrites, has better supporting effect relative to the soft carbon material while being favorable for rapid ion diffusion, and is favorable for relieving the huge volume expansion deformation of Si through larger migration restriction between layers. The amorphous LiSi alloy promotes refinement of Si particles, improves contact between particles, and precipitates and derivatizes nearby lithium-rich phases, thereby forming a negative electrodeThe three-dimensional conductive lithium-conductive network woven by the lithium-rich phase effectively increases the ion conductivity of the cathode and improves the electrode dynamics. It can thus also be concluded that large-particle crystalline Si is not expected to be retained too much in the negative electrode system, that we expect its more transformation to amorphous form and partial transformation to Li ₁₅ Si ₄ And LiC ₆ To build an effective three-dimensional ion-conducting network in the interface, other means of bringing about such transformation can be utilized for the same reasons and to achieve substantially the same desired effect. For example, we can introduce amorphous LiSi alloys, li, directly into hard carbon particles in the desired stoichiometric ratio ₁₅ Si ₄ And LiC ₆ Coating is carried out, so that a three-dimensional ion conductive network is formed more directly.

4.3 electrochemical performance improvement verification under high load, high current and high multiplying power of full battery

To further characterize the performance of the LiSH46 electrode in a full cell, this work matched NMC811 and LCO positive electrode, respectively, demonstrating superior electrochemical performance of LiSH46. The NMC811 composite cathode is used for constructing a high-load full battery, and the active material accounts for 80 percent. The LCO composite anode is used for testing the performance of the LiSH46 anode under high current and high multiplying power by eliminating the influence of diffusion length under a smaller load, and the active material accounts for 70%.

Matching high load (5.89 mAh/cm) ² ) NMC811 electrode with high active material ratio (80%) at 1C (5.89 mA/cm ² ) With cycling of current density (FIG. 14), reversible capacity can reach 123.6mAh/g (3.6 mAh/cm) ² ) The level of the commercial lithium battery is reached, the cycle life reaches 1033 circles, the reversible capacity of 70% is still kept after 3000 circles, the capacity attenuation of the battery is mainly in the first 100 weeks, the capacity attenuation is 10%, and the capacity is kept 80% after 3000 circles compared with the 100 th week. The cycle number of the battery is higher than 1mAh/cm in the surface capacity reported in the current literature ² The longest of all-solid-state batteries (FIG. 15) exceeded the previously reported μm-Si negative electrode (surface capacity of 2mAh/cm ² At 5mA/cm ² Lower cycle 500 weeks), ag-C negative electrode (surface capacity 4.62mAh/cm ² At 3.4mA/cm ² Lower cycle 1000 weeks)LiIn anode (surface capacity of 4 mAh/cm) ² At 3.8mA/cm ² Lower cycle 897 turns, short circuit failure occurred). The excellent cycling stability of the LiSH46 can be attributed to the structural stability, the lithium-rich conductive sub-network of the LiSH46 cathode is easy to generate plastic deformation under stress, stress concentration in the electrode is reduced, stability of the electrode is maintained, as shown in fig. 10 (C), after the electrode is cycled for 10 weeks under 1C, no obvious transverse crack exists, the structural stability is obviously superior to that of μm-Si reported by Meng and the like, and the lithium-rich conductive sub-network of the LiSH46 cathode shows very obvious transverse crack after first charge and discharge. The structure of the LiSH46 anode is stable while the stability of the two-dimensional passivation layer between the electrode electrolytes is maintained. The electrode exhibits excellent cycling stability.

TABLE 2 NMC811 LiSH46 cells working with surface capacities higher than 1mAh/cm reported in the literature ² Solid state battery cycle number comparison

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Under high load, the existence of too long diffusion path limits the battery performance, in order to further prove the cycling stability of LiSH46 anode, we reduce the thickness of LiSH46, match it with 5mg/cm ² The left and right LCO anodes were subjected to long cycles at 20C and 30C, respectively, for the anodes (FIG. 16), and the current densities were 14.64mA/cm, respectively ² And 20.45mA/cm ² The initial reversible capacity at 20C is 76mAh/g, the capacity gradually rises to 87mAh/g in the circulation process, then falls back to 80mAh/g, the capacity starts to be obviously attenuated from 24000 circles, the circulation is carried out until 30000 circles, and the capacity is maintained to be 72.1% of the initial reversible capacity. The initial reversible capacity at 30C is 52.06mAh/g, and the reversible capacity is increased to 73mAh/g (ultrahigh) in the circulation process10000 weeks) and then the capacity continues to decay to 80% of the initial capacity at 15000 turns. As shown in fig. 16B and (D), as the battery cycle proceeds, the polarization of the battery decreases and the battery capacity increases, possibly for two reasons: 1) In the circulation process, si-Si and Si-C bonds eliminate a part of grain boundaries, more lithium diffusion channels are activated, the utilization rate of the electrode is increased, a lithium-rich phase conductive network structure is formed, and the active area of the electrode is increased; 2) Joule heat is generated under high current, lithium diffusion and electron conduction in the electrode are promoted, and the battery capacity is continuously increased.

The capacity of the battery was then decayed, in part due to diffusion-controlled lithium capture, as shown in fig. 17, by 0.2C discharge for a battery cycled 30000 weeks at 20C with a discharge capacity of 99.11mAh/g, lithium captured in the LiSH46 negative electrode was stripped off, then a constant current charge-discharge was resumed at 20C, the capacity of the battery was increased from 55mAh/g at 30000 weeks to 66.12mAh/g, and a partial capacity was resumed, indicating that lithium captured in the negative electrode resulted in a partial capacity loss. The major capacity loss may also be due to failure of the electrode structure, requiring further investigation. The cycle life of the lco|lish46 full cell as shown in fig. 18 exceeds that of all the full solid state cells.

Table 3 lco|lish46 cell of the present work compared to the solid state cell cycle number reported in the literature for cycles exceeding 1000 cycles

Full cells matching the LiSH46 negative electrode exhibit very excellent stability at ultra-high current densities. As shown in fig. 19, a high load (6.48 mAh/cm ² ) NMC811 electrode with high active material ratio (80%) at 2C (12.96 mA/cm ² ) Charging and discharging were carried out at a current density exceeding that of μm-Si (5 mA/cm) ² ) Or lithium metal negative electrode (Ag-C3.4 mA/cm) ² ，LiIn 3.8mA/cm ² ) Is a full cell of (a). The battery can realize discharge at 20 ℃ at maximum, and the current density reaches 122.1mA/cm ² Is that the current surface capacity exceeds 1mAh/cm ² Most of all solid-state batteries of (a)High discharge current. The full battery matched with the LiSH46 cathode can realize ultrahigh current density mainly because of a three-dimensional lithium-rich lithium-conducting network in the electrode, compared with a two-dimensional reaction interface of the Si cathode reported by Meng and the like, the three-dimensional lithium-conducting network in the LiSH46 greatly increases the active area of the electrode, the electrode dynamics is obviously improved, and lithium is diffused into the electrode at a faster speed instead of being deposited on an electrode electrolyte interface.

Table 4NMC811|LiSH46 full cell current density and literature reported surface capacity above 1mAh/cm ² Is a solid state battery of (c).

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To exclude the effect of diffusion paths, further demonstrating the performance of LiSH46 at ultra-high current densities, we reduced the thickness of LiSH46, matching it to 10mg/cm ² The left and right LCO positive electrodes can be used for the positive electrode 50C (71.5 mA/cm ² ) Charging and discharging were achieved without micro-short circuit (FIG. 20), which is the highest current density (8.6 mA/cm ² ) 8.3 times of (3). At the corresponding positive electrode 10C (6 mA/cm ² ) The lower full battery maintains the reversible capacity of 103mAh/g (the reversible capacity of 0.2C is 137 mAh/g), and the excellent rate capability and the stability under the ultra-high current density of LiSH46 are fully proved.

To verify the lithium metal resistance of the LiSH46 negative electrode, the present work used 6mAh/cm ² The LiSH46 negative electrode of the full cell matches the ultra-high load NMC811 positive electrode, as shown in FIG. 21, when excessive lithium migrates to the negative side, the negative side is transformed into a lithium metal protection layer, and a large amount of lithium is deposited on the negative side without micro short circuit, 6mAh/cm ² LiSH46 can withstand at least 20mAh/cm ² Reversible charge and discharge of high-load positive electrode NMC811, illustrating lithium dendrite growth by LiSH46And the lithium dendrite growth of the LiSH46 cathode is not easy to occur in the actual use process. We used 6mg/cm ² LiSH46 negative electrode realizes highest reversible surface capacity of 16.92mAh/cm reported by current all-solid-state battery ² (FIG. 22), the capacity of the system has a further improvement space, the anode with higher load can be matched by improving the mass of the cathode and increasing the N/P ratio, the active material ratio of the anode can be reduced, and the ion transport in the electrode is optimized to further improve the capacity of the battery. As shown in FIG. 21C, the positive electrode load was 12mAh/cm ² The LiSH46|NMC811 full cell of (C) can be 1.22mA/cm ² Realize stable circulation and keep close to 9mAh/cm ² The coulombic efficiency was higher than 90%, while the capacity of the pure silicon negative electrode full cell of the same load decayed rapidly, as shown in fig. 21D, the capacity at week 10 remained only 3mAh/cm ² Coulomb efficiency was low and micro-shorting occurred by cycling to week 12. As shown in Table 6, the energy density of the battery layer of the NMC811|LiSH46 full cell is close to the liquid cell level, 20mAh/cm ² The energy density of the battery layer (positive electrode + negative electrode + electrolyte layer) of the high-load NMC 811/LiSH 46 full battery is up to 263Wh/kg, the battery uses an electrolyte layer with the thickness of 650 mu m, the energy density can be further improved by thinning the electrolyte layer, if the electrolyte layer with the thickness of 20 mu m is used, the energy density of the battery can be improved to 450Wh/kg, and the battery system has great application prospect.

Table 5 NMC811|LiSH46 full cell load and literature report full cell comparison

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Table 6 summary of battery energy densities for lish46|nmc811 with different loads

The LiSH46|NMC811 high-load battery can stably work at 5-75 ℃, even if the temperature is as low as 5 ℃, the battery still has no micro short circuit, and the lower capacity of the battery at low temperature is that the positive electrode active material is excessively high (80%), the conductivity of the conductive path in the electrode is obviously reduced at low temperature, so that the positive electrode capacity is difficult to develop. From the discharge curve, as the temperature decreases, the specific gravity of the dissolution plateau capacity of lithium to the total capacity increases, indicating that lithium intercalation by LiSH46 is somewhat limited at possible low temperatures.

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The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, alternatives, and improvements that fall within the spirit and scope of the invention.

Claims

1. A safe and stable activated negative electrode is characterized in that the negative electrode has a lithium-rich phase Li ₁₅ Si ₄ 、LiC ₆ Braiding a lithium conductive network in a three-dimensional texture shape; the lithium conductive network in a three-dimensional texture shape is distributed in the amorphous lithium silicon alloy; the Li/Si capacity ratio of the metal lithium and silicon contained in the negative electrode is 0.6 or more; the load of the negative electrode is 1mAh/cm ² The above.

2. The safe and stable activated negative electrode according to claim 1, characterized in that the Li/Si capacity ratio of metal lithium and silicon contained in the negative electrode is 0.8±0.1.

3. The safe and stable activated negative electrode according to claim 1, wherein the load of the negative electrode is 2mAh/cm ² The above.

4. The safe and stable activated negative electrode according to claim 1, characterized in that the load of the negative electrode is 3.6mAh/cm ² The above.

5. The safe and stable activated negative electrode according to claim 1, wherein the load of the negative electrode is 4mAh/cm ² The above.

6. The safe and stable activated negative electrode according to claim 1, wherein the load of the negative electrode is 5mAh/cm ² The above.

7. The safe and stable activated negative electrode according to claim 1, wherein the load of the negative electrode is 6mAh/cm ² The above.

8. The method for preparing a safe and stable activated negative electrode according to any one of claims 1 to 7, characterized in that silicon particles and hard carbon particles are mixed and ground and then mixed with a binder to prepare a silicon-containing layer; and setting the silicon-containing layer and the lithium sheet adjacent to each other as a negative electrode, and activating the assembled battery.

9. The method for producing a safe and stable activated negative electrode according to claim 8, wherein the silicon mass in the negative electrode is 20 to 80% in terms of the ratio of the total mass of silicon and hard carbon.

10. The method for producing a safe and stable activated negative electrode according to claim 8, wherein the silicon mass in the negative electrode is 30 to 60% in terms of the ratio of the total mass of silicon and hard carbon.

11. The method for producing a safe and stable activated negative electrode according to claim 8, wherein the silicon mass in the negative electrode is 40% ± 5% of the total mass of silicon and hard carbon.

12. The method for producing a safe and stable activated negative electrode according to claim 8, wherein the silicon and the hard carbon as raw materials in the negative electrode are micron-sized particles having an average particle diameter of 8 μm or less.

13. The method for producing a safe and stable activated negative electrode according to claim 8, wherein the average particle diameter of silicon in the negative electrode after activation is 1.5±1.2 μm and the average particle diameter of hard carbon is 2±1.5 μm.

14. The method for producing a safe and stable activated negative electrode according to claim 8, wherein the specific capacity of the silicon-containing layer is 900 to 3000mAh/g and the thickness is 5 to 100 μm.

15. The method for producing a safe and stable activated negative electrode according to claim 8, wherein the specific capacity of the silicon-containing layer is 1500 to 2500mAh/g and the thickness is 15 to 40 μm.

16. The method for producing a safe and stable activated negative electrode according to claim 8, wherein the pressure during the battery assembly is 5t or more.

17. The method for producing a safe and stable activated negative electrode according to claim 8, wherein the pressure during the battery assembly is 7t to 10t.

18. The method for producing a safe and stable activated negative electrode according to claim 8, wherein the cyclic activation is performed at a rate of 0.1C to 0.5C for at least 1 week.

19. The method for producing a safe and stable activated negative electrode according to claim 8, wherein the cyclic activation is performed at a rate of 0.1C to 0.5C for 1 to 2 weeks.

20. A battery comprising the safe and stable activated negative electrode of any one of claims 1-7.